Developmental Downregulation of Histone Posttranslational Modifications Regulates Visual Cortical Plasticity

Neuron

Article Developmental Downregulation of Histone Posttranslational Modifications Regulates Visual Cortical Plasticity Elena Putignano,1 Giuseppina Lonetti,2 Laura Cancedda,1 Gianmichele Ratto,2 Mario Costa,2 Lamberto Maffei,1,2 and Tommaso Pizzorusso2,3,* 1

Scuola Normale Superiore, Neurobiology Laboratory, Area Ricerca CNR, via Moruzzi, 1 Pisa 56125, Italy Istituto Neuroscienze CNR, Area Ricerca CNR, via Moruzzi, 1 Pisa 56125, Italy 3 University of Florence, Department of Psychology, via S. Niccolo`, 93 50125 Florence, Italy *Correspondence: [email protected] DOI 10.1016/j.neuron.2007.02.007 2

SUMMARY

The action of visual experience on visual cortical circuits is maximal during a critical period of postnatal development. The long-term effects of this experience are likely mediated by signaling cascades regulating experiencedependent gene transcription. Developmental modifications of these pathways could explain the difference in plasticity between the young and adult cortex. We studied the pathways linking experience-dependent activation of ERK to CREB-mediated gene expression in vivo. In juvenile mice, visual stimulation that activates CREB-mediated gene transcription also induced ERK-dependent MSK and histone H3 phosphorylation and H3-H4 acetylation, an epigenetic mechanism of gene transcription activation. In adult animals, ERK and MSK were still inducible; however, visual stimulation induced weak CREB-mediated gene expression and H3-H4 posttranslational modifications. Stimulation of histone acetylation in adult animals by means of trichostatin promoted ocular dominance plasticity. Thus, differing, experiencedependent activations of signaling molecules might be at the basis of the differences in experience-dependent plasticity between juvenile and adult cortex. INTRODUCTION The visual cortex displays various forms of anatomical and functional plasticity (Gilbert et al., 1990; Katz and Shatz, 1996; Desai et al., 2002; Hofer et al., 2006; Shuler and Bear, 2006). For example, manipulations of visual experience such as dark rearing (dr) or monocular deprivation (MD) performed during early development cause modifications in the typical response pattern and connectivity of visual cortical neurons (Berardi et al., 2004; Hensch,

2004). Although some forms of plasticity are present in the adult visual cortex (Sawtell et al., 2003; Pham et al., 2004; Tagawa et al., 2005; He et al., 2006; Karmarkar and Dan, 2006; Pizzorusso et al., 2006), the impact of visual deprivation on the functional organization of the visual cortex, and the ability of the cortex to recover from these effects, is maximal during a critical period of postnatal development (Gordon and Stryker, 1996; Berardi et al., 2000; Prusky et al., 2000; Pizzorusso et al., 2002, 2006; Hensch, 2005; McGee et al., 2005; Tohmi et al., 2006). Several large-scale analyses of gene expression in the visual cortex of visually deprived mice, which had been either dark reared or monocularly deprived, have shown modifications of the expression levels of many genes (Prasad et al., 2002; Lachance and Chaudhuri, 2004; Ossipow et al., 2004; Majdan and Shatz, 2006; Tropea et al., 2006). This suggests that the consequences of visual deprivations on cortical circuits are at least partially mediated by modifications in the transcriptional levels of sets of experience-regulated genes. The ensemble of genes regulated by visual experience during the critical period is overlapping, but distinct from, the ensemble observed in adult mice, suggesting the presence, at a transcriptional level, of a molecular signature of the action of visual experience that is specific to juvenile animals (Majdan and Shatz, 2006). Thus, it is important not only to identify the signaling mechanisms that translate visual experience into regulation of gene transcription, but also to understand whether these signaling mechanisms are differentially regulated between the critical period and adulthood. The information available about these mechanisms and their developmental regulation is limited. It has been shown that a key mediator of experience-dependent plasticity in the visual cortex during the critical period is the kinase ERK (Di Cristo et al., 2001). ERK is promptly and potently activated by visual experience in visual cortical neurons, and its pharmacological inhibition prevents synaptic plasticity as well as the effects of MD on ocular dominance in the developing visual cortex (Kaminska et al., 1999; Di Cristo et al., 2001; Cancedda et al., 2003; Suzuki et al., 2004). In addition, ERK action is necessary to mediate the effects of visual experience on the transcription of several genes (Majdan and Shatz, 2006). However, little is

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known about how visually activated ERK regulates experience-dependent gene transcription in vivo. One possibility is that ERK acts through the transcription factor CREB. Indeed, ERK is required for visually stimulated transcription mediated by CREB (Cancedda et al., 2003), and CREB plays a pivotal role in various forms of plasticity in visual cortex (Pham et al., 1999, 2004; Mower et al., 2002) and other brain structures (Lonze and Ginty, 2002). Activation of specific transcription factors might be only one among many possible ways by which visual experience can regulate gene expression. For example, neural activity was shown to induce posttranslational modifications of histones which, by rendering chromatin accessible to transcription machinery, resulted in changes in gene expression (Crosio et al., 2003; Korzus et al., 2004). Activity-dependent regulation of gene expression by means of posttranslational modifications of histones has been proposed to be important for hippocampal synaptic plasticity and learning and memory (Colvis et al., 2005; Levenson and Sweatt, 2005). Whether this mechanism is also at work in mediating the action of visual experience in developing visual cortical neurons, and if it differs between animals analyzed before or after the end of the critical period, is still unknown. We investigated whether an episode of visual experience during the critical period activates ERK-dependent posttranslational modifications of histones associated with chromatin rearrangement and CREB activation. We found that, during the critical period, exposure to light after 3 days of dr triggered ERK-dependent histone H3 acetylation and phosphorylation, and resulted in activation of MSK, a CREB and H3 kinase, and CREB. Histone H4 acetylation was also induced. The same visual stimulation in adult animals induced different effects: ERK and MSK were still responsive to visual experience, but their nuclear targets CREB, H3, and H4 showed very little regulation by visual experience, resulting in downregulation of experience-dependent CREB-mediated gene expression. To analyze the functional consequences of the developmental reduction of experience-dependent histone acetylation, we analyzed ocular dominance plasticity in adult animals treated with trichostatin (TSA), a molecule that increases histone acetylation by inhibiting deacetylases. We found that TSA treatment promotes ocular dominance plasticity in the adult. RESULTS Visual stimulation was achieved by exposing animals to a normally lightened environment for various times after 3 days of dr. Light exposure began at P24–P25 (critical period group) or at P80–P100 (adult group). Visual Experience during the Critical Period Causes ERK-Dependent Activation of MSK Previous data showed that visual stimulation during the critical period results in an activation of gene expression mediated by CREB and that activation of ERK is neces-

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sary for this effect of visual stimulation (Cancedda et al., 2003). Moreover, it has been proposed that MSK, a nuclear kinase activated by ERK, is a major mediator of the action of ERK on CREB in neuronal cells (Arthur et al., 2004). Thus, we sought to determine whether MSK is activated by visual stimulation during the critical period by studying MSK’s phosphorylation at thr 581, a site phosphorylated by ERK that is necessary for MSK activity (McCoy et al., 2005). P27–P28 mice were exposed to light for 15 and 40 min after 3 days of dr and compared with mice sacrificed in darkness. MSK phosphorylation was assessed by means of immunohistochemistry with an antibody specific for thr 581 phosphorylated MSK (pMSK). Figure 1 (inset) shows the typical nuclear staining of pMSK. The number of positive cells was significantly increased after 15 min of light, and returned to dr levels at 40 min (Figure 1A), a transient kinetics similar to that observed for visually induced ERK phosphorylation (Cancedda et al., 2003). Also, the spatial distribution of pMSK-positive cells was similar to that previously described for pERK, being maximal in superficial and deep cortical layers but weak in intermediate layers (Figure 1B). To further investigate whether activation of MSK was consequent to activation of ERK, we performed two experiments: we first studied whether intracortical infusion of UO126, an inhibitor of the ERK upstream kinase MEK, into the visual cortex of one hemisphere could prevent MSK phosphorylation induced by 15 min of visual stimulation. As shown in Figure 1C, pMSK levels were strongly reduced in the UO126-treated cortex of visually stimulated mice with respect to the contralateral cortex. In contrast, equal numbers of pMSK-positive cells were observed in the two cortices of mice that received the inhibitor vehicle in one cortex as a control. We next costained visual cortical sections of mice stimulated for 15 min for pMSK and pERK. We found that 86.6% of the labeled cells were costained for pMSK and pERK (Figure 1D). Taken together, these results show that pMSK is activated in the same cells where ERK is activated and that ERK inhibition completely prevents MSK activation. A crucial step in CREB activation is phosphorylation of Ser 133 (Mayr and Montminy, 2001). Since MSK can phosphorylate CREB at Ser 133 (Deak et al., 1998; Arthur et al., 2004), we investigated, by means of western blot, whether phosphorylation of CREB Ser 133 is activated by visual stimulation. Antibody specificity for phosphorylated CREB was tested on total cell extracts from SK-N-MC cells stimulated with IBMX and forskolin (positive control) or left untreated (negative control). A band was present only in the lane corresponding to the stimulated cells (data not shown). The analysis of the visual cortex samples showed that exposure to light induced an increase in CREB Ser 133 phosphorylation that is significant after 40 min of stimulation (Figure 2). Thus, visual stimulation induces sustained phosphorylation of Ser 133 of CREB, a phosphorylation site targeted by MSK.

Neuron Molecular Correlates of Visual Cortical Plasticity

Figure 1. Visual Stimulation Induces MSK Thr 581 Phosphorylation (A) The number of pMSK-positive cells is increased by 15 min of visual stimulation. The activation is transient and returns to baseline at 40 min (dr, n = 7; 15 min, n = 11; 40 min, n = 4; oneway ANOVA, p = 0.003; post hoc Holm-Sidak: 15 min versus dr, p < 0.05; 15 min versus 40 min, p < 0.05; 40 min versus dr, p = 0.53). Hollow symbols represent data from single animals; filled symbols report average ± SEM. Data are normalized to the average of dr animals. (B) Examples of pMSK staining in visual cortex of dr and 15 min mice. Cortical layers are indicated on the right. The inset shows the nuclear staining of pMSK as shown by a double staining for Neurotrace. Scale bar = 70 mm (10 mm for the inset). (C) MSK phosphorylation induced by visual stimulation is blocked by inhibition of ERK. Average (±SEM) number of pMSK-positive cells (normalized to the dr value, dotted line) in visual cortex of mice visually stimulated for 15 min and treated with UO126. Significantly fewer positive cells are present in the treated cortex with respect to the contralateral untreated cortex (n = 6, paired t test, p = 0.01). The number of pMSK-positive cells in the cortex treated with the inhibitor vehicle is not different from that in the untreated contralateral cortex (n = 6, paired t test, p = 0.48). On the right, pMSK staining from a mouse visually stimulated for 15 min with one cortex infused with UO126. Scale bar = 60 mm. (D) pMSK (red) and pERK (green) staining coexist in the same cells. Cells (n = 602) were classified as single-stained for pERK, singlestained for pMSK, or pMSK/pERK doublestained after inspection of z-stacks of confocal images. Scale bar = 14 mm.

Visual Experience during the Critical Period Causes Histone H3 Phosphoacetylation Combinatorial phosphorylation of Ser 10 and acetylation of Lys 9 of histone H3 is known to correlate with active gene transcription of plasticity-related genes (Chwang et al., 2006; Wood et al., 2006). To assess whether these posttranslational modifications of histones are regulated during the critical period by visual experience, we analyzed their induction by light exposure in the visual cortex. To assess phosphorylation of H3 Ser 10, we ran immunohistochemical experiments using a phosphospecific antibody (pH3). Figure 3 shows examples of staining in the binocular part of the visual cortex of mice sacrificed in darkness or exposed to light for 15 or 40 min prior to sacrifice. In agreement with the known localization of H3, pH3

staining appeared exclusively nuclear (Figure 3, inset). The number of cells positive for pH3 was strongly increased by visual stimulation at both the 15 min and 40 min time points (Figure 3). Ser 10 of H3 is a substrate of MSK (Thomson et al., 1999; Soloaga et al., 2003) and therefore could become phosphorylated after visually induced activation of ERK and MSK. To test this hypothesis, we analyzed H3 phosphorylation in animals infused unilaterally with UO126 in the visual cortex and exposed to light for 15 or 40 min. Figure 4 shows that ERK inhibition prevented the increase in pH3 stained cells induced by visual stimulation at both time points, indicating that H3 phosphorylation requires ERK activation. Normal induction of H3 phosphorylation was observed in the untreated cortex or in the cortex of

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Figure 2. Visual Stimulation Induces Sustained CREB Ser 133 Phosphorylation pCREB/CREB ratio (average ± SEM) was significantly increased by visual stimulation (one-way ANOVA, p < 0.05; post hoc Dunnett test, 40 min versus dr, p < 0.05; 15 min versus dr, p > 0.05; four samples for each experimental group).

control mice treated with vehicle (Figure 4A). To investigate whether visually induced H3 phosphorylation occurs within the same cells in which visual stimulation activates ERK, we studied the cellular colocalization of pH3 and pERK. We analyzed pERK-pH3 costaining only in mice exposed to light for 15 min, because, unlike pH3 labeling, pERK staining returns to baseline at 40 min (Cancedda et al., 2003). Figure 4B shows that the great majority of the cells stained for pERK at 15 min were also positive for pH3, suggesting that activation of ERK is followed by phosphorylation of H3 within the same cell. To investigate H3 acetylation, we used Lys 9 acetyl-H3 (AcH3)-specific antibodies in western blot experiments (Levenson et al., 2004). Specificity of the antibody was tested analyzing histone acetylation in cultures of NIH3T3 fibroblasts stimulated with TSA (300 ng/ml) for 12 hr. The AcH3 antibody produced a band of strong intensity at the expected molecular weight in the treated cells (leftmost lane, Figure 5A), but no band in the unstimulated cells (data not shown). To assess the effect of visual stimulation on H3 acetylation in visual cortex, we prepared samples from visual cortices of mice exposed to light for 40 and 90 min; these are time intervals compatible with previous data on the time course of histone acetylation induced by neural activity in brain structures other than the visual cortex (Levenson et al., 2004). We found that visual stimulation induced a pronounced increase in H3 acetylation that was present at both time intervals, but larger after 90 min than after 40 min (Figure 5A). Furthermore, the increase in histone acetylation was dependent on ERK activation since it was blocked by treat-

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Figure 3. Visual Stimulation Induces H3 Ser 10 Phosphorylation (A) Examples of pH3 staining in visual cortex of dr and visually stimulated mice. The inset shows the nuclear staining of pH3. Cortical layers are indicated on the left of the 40 min picture. Scale bar = 80 mm (10 mm for the inset). (B) The number of pH3-positive cells is increased after 15 and 40 min of visual stimulation (dr n = 12, 15 min n = 10, 40 min n = 17; one-way ANOVA on ranks p < 0.001; post hoc Dunn’s test: 15 min versus dr, p < 0.05; 40 min versus dr, p < 0.05; 40 min versus 15 min, p > 0.05). Hollow symbols represent data from single animals; filled symbols report average ± SEM. Data are normalized to the average of dr animals.

ment with the ERK inhibitor SL237 (Figure 5B). Taken together, these data demonstrate that visual stimulation during the critical period causes sustained, ERK-dependent, epigenetic tagging of H3 at sites that influence gene expression.

Neuron Molecular Correlates of Visual Cortical Plasticity

Figure 4. Visually Induced H3 Phosphorylation Is ERK-Dependent (A) H3 phosphorylation induced by visual stimulation is blocked by inhibition of ERK. The top graph shows the average (±SEM) number of pH3-positive cells in the visual cortex of mice visually stimulated for 15 and 40 min and treated with the MEK inhibitor UO126. Data are normalized to the dr value (dotted line). Gray bars represent data for the UO126treated cortex (40 min, n = 7; 15 min, n = 5) and black bars represent values of the contralateral untreated cortex of the same animals (paired t test, p < 0.01 at both time points). The number of pH3-positive cells in the cortex treated with only the inhibitor vehicle (white bars in bottom graph; 40 min, n = 6; 15 min, n = 7) is not different from that of the untreated contralateral cortex of the same animals (black bars; paired t test, p = 0.89 at 40 min and p = 0.14 at 15 min). (Right) Examples of pH3 staining in the cortex infused with UO126 and in the contralateral untreated cortex from mice visually stimulated for 15 and 40 min. Scale bar = 70 mm. (B) pH3 (red) and pERK (green) staining coexist in the same cells. Cells (n = 689) were classified as single-stained for pERK, single-stained for pH3, or pH3/pERK double-stained after inspection of z-stacks of confocal images. Scale bar = 15 mm.

Phosphorylation of ERK and Activation of CREBMediated Gene Expression by Visual Experience Are Differentially Regulated during Development Visual experience during the critical period activates a molecular cascade, involving ERK, MSK, CREB, and histones, that regulates CREB-mediated gene expression. Is this cascade also activated by visual experience after the end of the critical period? We analyzed ERK activation and CREB-mediated gene expression after visual stimulation in adult mice (P80–P100) and compared the results with those previously obtained (Cancedda et al., 2003) from juvenile mice within the critical period. To study the effects of visual stimulation on ERK activation and CREB-activated gene expression in the adult, we choose durations of stimulation (15 min for ERK and 12 hr for CREB-mediated gene expression) that produced maximal activation in juvenile mice. CREB-mediated gene expression was studied using a transgenic model expressing the reporter gene lacZ under the control of the CRE sequences bound by CREB (Impey et al., 1996). We found that 15 min of light exposure equally activated ERK during or after the critical period (Figure 6A). In contrast, while 12 hr of visual stimulation during the critical period caused a strong increase in the number of cells positive for b-galactosidase, the gene product of lacZ, CRE-dependent gene expression was induced at much lower levels in the adult (Figure 6B). This effect was not due to a decrease in neuronal density between young

and adult animals, since a similar number of cells stained for the neuronal marker NeuN were observed in cortical fields from young and adult visual cortex (see Figure S1 in the Supplemental Data). These data suggest that, after the end of the critical period, the molecular chain of events that links experience-dependent ERK activation to CREB-dependent gene expression is broken. Downregulation of Experience-Dependent Posttranslational Modifications of CREB, H3, and H4, but Not MSK, in Adult Mice To probe at the level at which the disconnection between experience-dependent ERK activation and CREB-dependent gene expression could occur, we analyzed the phosphorylation of MSK and CREB and the induction of posttranslational modifications of histones in the adult visual cortex. As shown in Figure 7A, MSK activation by visual experience was similar in adult and juvenile animals. As in juvenile animals, visually activated MSK was localized in the nucleus (Figure S2). In contrast, visually induced phosphorylation of H3 was strongly downregulated in adult animals, showing a reduction in amplitude and a transient kinetics that returned to baseline levels at 40 min (Figure 7B). A developmental decrease of experience-dependent H3 posttranslational modification was confirmed by also analyzing H3 acetylation. After 40 min or 90 min of light exposure, western blot analysis did not

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Figure 6. Comparison between Activation of ERK and CREMediated Gene Expression in Juvenile and Adult Mice For juvenile animals, we report data obtained in our laboratory and previously published in Cancedda et al. (2003). (A) Visual experience (15 min) caused ERK phosphorylation in the visual cortex of adult mice (dr, n = 4; adults, n = 7). Two-way ANOVA showed no effect of the age of the animals (p = 0.94) and a significant effect of visual experience (p < 0.001). Post hoc Hom-Sidak test (p < 0.05) showed that 15 min differed significantly from dr. Open symbols report single animal data; filled symbols report averages ± SEM. (B) Developmental downregulation of CRE-mediated gene expression induced by visual experience. In adult animals, 12 hr of visual experience caused only a modest increase of X-gal-positive cells compared with that in dr mice (t test, p < 0.05). The number of X-gal-positive cells in visually stimulated juvenile mice is much higher than in visually stimulated adult mice (t test, p < 0.01).

Figure 5. Visual Experience Induces ERK-Dependent H3 Acetylation (A) Visual stimulation induces sustained H3 Lys 9 acetylation. The ratio between the intensity of the bands of AcH3 and total H3 was taken as an index of the amount of acetylated H3. TSA lane corresponds to NIH3T3 cells treated with TSA (positive control). The other lanes correspond to samples obtained from dr or from animals visually stimulated for 40 or 90 min. AcH3/H3 ratio was significantly increased by visual stimulation (one-way ANOVA on ranks p = 0.014; post hoc Dunn’s test: 90 min versus dr, p < 0.05; 40 min min versus dr, p > 0.05; and 40 min versus 90 min, p > 0.05; dr, n = 6 samples; 40 min, n = 6 samples; 90 min, n = 7 samples; error bars = SEM). (B) Histone Lys 9 acetylation induced by visual stimulation is blocked by the inhibitor of ERK activation SL237. Western blot of visual cortex samples of dr (n = 6) and visually stimulated mice treated with the ERK inhibitor (n = 5 samples) or vehicle (n = 5 samples). In control animals visual stimulation significantly increased the AcH3/H3 ratio with respect to dr, but this effect was completely prevented by SL237 (one-way ANOVA, p = 0.004; post hoc Holm-Sidak test: vehicle versus dr, p = 0.005; vehicle versus SL237, p = 0.002; dr versus SL237, p = 0.49; error bars = SEM).

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detect any significant increase of H3 acetylation in the adult visual cortex either (Figure 7C). It has been suggested that different forms of plasticity might involve selective acetylation of different histones (Levenson et al., 2004). To investigate this issue, we studied the developmental regulation of visually induced acetylation of histone H4. We found that, similarly to H3 acetylation, H4 acetylation was induced by visual experience in the visual cortex of juvenile animals, but visual induction was not present in adult animals (Figure S3). Finally, we found a developmental downregulation of the action of visual experience on CREB phosphorylation as well (Figure 7). In summary, while ERK and MSK activation is equal in adult and juvenile animals, the action of visual experience on factors regulating gene transcription, like CREB or histones, is reduced in the adult visual cortex.

Neuron Molecular Correlates of Visual Cortical Plasticity

The Deacetylase Inhibitor TSA Promotes Ocular Dominance Plasticity in the Adult Mouse To assess whether the developmental downregulation of experience-dependent histone acetylation could have a functional consequence on visual cortical plasticity, we analyzed the effects of MD in adult mice (3 days of deprivation beginning at P80–P110) treated with TSA or its vehicle. TSA is a deacetylase inhibitor that has been used to increase histone acetylation in the brain (Korzus et al., 2004; Levenson et al., 2004). This compound does not activate ERK (Levenson et al., 2004) or MSK (Figure S4), but potentiates CRE-mediated gene transcription (Chang et al., 2006). To assess ocular dominance plasticity, we recorded visual evoked potentials (VEPs) elicited by stimulation of either eye with gratings (0.1 c/deg). VEPs were recorded by means of a micropipette inserted into the primary visual cortex contralateral to the deprived eye. The effects of MD vary with the eccentricity of the receptive fields of the recorded cells, and tend to be maximal for cells located close to the vertical meridian (Gordon and Stryker, 1996). Therefore, we analyzed MD effects at cortical locations corresponding to the representation of the vertical meridian. To locate the receptive field of the cells generating the VEP response, we stimulated with a narrow grating (width of 20 deg) that could be positioned at different azimuths in the visual field (Figure 8A). Ocular dominance (Contra/Ipsi ratio of VEP response amplitude) was measured at the electrode location for which the response to the narrow grating was maximal with the stimulus positioned around the vertical meridian. In agreement with previous data (Rossi et al., 2001), Contra/Ipsi ratio of normal mice at this azimuth was 2.19 (SEM 0.25, n = 8). In control mice, 3 days of MD were ineffective in altering the Contra/Ipsi ratio (Sawtell et al., 2003; Hofer et al., 2006); by contrast, in mice treated with TSA, the ratio between the amplitude of the responses to the stimulation of

Figure 7. Downregulation of Experience-Dependent Modifications of CREB and H3, but Not MSK, in Adult Mice (A) Visual experience induces MSK phosphorylation in adult animals. Average number of pMSK-positive cells in visual cortex of adult (gray bars) mice in dr mice (n = 12) and in mice visually stimulated for 15 (n = 13) and 40 min (n = 11). Error bars = SEM. For comparison, the average of the pMSK-positive cells activated in juvenile mice (data of Figure 1) is reported. No difference is present between activation of pMSK in adult animals and in juvenile animals (Two-way ANOVA, effect of age p = 0.86; effect of duration of visual stimulation, p = 0.004; interaction time 3 age, p = 0.52; post hoc Holm-Sidak tests show that 15 min differs from both dr and 40 min mice, p < 0.03). (Right) Examples of pMSK staining in adult dr, dr + 15 min light, and dr + 40 min light. Scale bar = 70 mm. (B) Reduced experience-dependent activation of H3 phosphorylation in adult animals. Average number (error bars = SEM) of pH3-positive cells in visual cortex of adult mice visually stimulated for 15 and 40 min or in dr (gray bars). One-way ANOVA shows a significant effect of visual stimulation (p = 0.029). Post hoc Holm-Sidak comparisons show that only the 15 min time point differs from dr (p = 0.02). For com-

parison, the average of the pH3-positive cells activated during the critical period (data of Figure 4) is reported. pH3 induction by visual experience is significantly different in adult and juvenile mice (two-way ANOVA: effect of age, p < 0.001; effect of duration of visual stimulation, p = 0.002; interaction time 3 age, p = 0.02; post hoc Holm-Sidak tests show that adults differ from juvenile mice both at 15 min and 40 min, p < 0.03). (Right) Examples of pH3 staining in adult dr, dr + 15 min light, and dr + 40 min light. Same calibration as in (A). (C) Visual experience does not induce H3 acetylation in the adult visual cortex. The AcH3/H3 ratio (error bars = SEM) does not vary significantly after 40 or 90 min of visual experience with respect to dr (six samples for each group, one-way ANOVA, p = 0.41). The data for juvenile mice shown in Figure 6 are reported for comparison (white bars). (Right) Example of a western blot experiment on samples of adult visual cortex. (D) Visual experience does not activate CREB phosphorylation in adult animals. pCREB/CREB ratio is not increased by visual stimulation at any time point (one-way ANOVA on ranks p = 0.375; nine samples for each group; error bars = SEM). For comparison, the time course of CREB phosphorylation observed in juvenile animals and shown in Figure 3 is reported. (Right) Example of a western blot experiment on samples of adult visual cortex.

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Figure 8. Inhibition of Histone Deacetylases Promotes Visual Cortical Plasticity in the Adult (A) Example of a VEP response to stimulation of the contralateral eye. (B) Example of localization of the receptive fields of the neurons generating the VEP response obtained in two tracks from the same nondeprived animal. In track 1, the response peaks with the stimulus positioned on the vertical meridian (i.e., 0 azimuth). In track 2, the response is maximal for stimuli placed more laterally in the visual field. Track 1 corresponded to a Contra/Ipsi ratio of 2.3; in track 2, Contra/Ipsi ratio was 3.6. (C) TSA promoted ocular dominance plasticity in adult mice after 3 days of MD. Shaded area corresponds to the mean Contra/Ipsi ratio of the normal mice ± SEM (mean ratio = 2.19 ± 0.25, n = 8). Monocularly deprived mice receiving control treatment (Veh, n = 5) showed values similar to those of normal animals. In contrast, Contra/Ipsi ratio was significantly decreased in TSA-treated animals. (n = 6; one-way ANOVA, p = 0.007; post hoc Tukey test shows that TSA group is different from either the normal group [p = 0.012] or the VEH group [p = 0.016]. No difference is present between the normal group and the VEH group [p = 0.97]). For both groups recordings were performed from the cortex contralateral to the deprived eye. Hollow symbols report single animal data; filled symbols report the group average.

the contralateral, monocularly deprived eye and the ispilateral, nondeprived eye was reduced to 1.19 (SEM 0.16, n = 5; Figure 8B). Thus, pharmacological augmentation of histone acetylation promotes ocular dominance plasticity in the adult mouse. DISCUSSION Our results show that visual experience differently activates intracellular signaling pathways that control gene expression in the visual cortex of juvenile and adult mice, and that this developmental downregulation could regulate the developmental reduction of plasticity occurring in the adult visual cortex. During the critical period ERK is strongly activated by visual experience, and its activation is required for the induction of a molecular cascade that involves MSK, CREB, and histones H3 and H4. In the adult, ERK and MSK are still fully responsive to visual stimulation, but visual activation of CREB-mediated gene expression, CREB phosphorylation, H3 phosphoacetylation, and H4 acetylation is downregulated. Increasing histone acetylation pharmacologically in adult animals promoted experience-dependent plasticity of the visual cortex. Visual Experience during the Critical Period Induces ERK-Dependent Histone Acetylation and Phosphorylation The existence of mechanisms by which neurons modify gene expression in response to neural activity is thought

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to be critical in explaining how neuronal circuits can display long-term modifications in response to experience. These mechanisms are likely involved in mediating the action of visual experience on the development of the visual cortex. Indeed, inhibition of the synthesis of new proteins inhibits the effects of MD on ocular dominance of visual cortical neurons (Taha and Stryker, 2002), and many studies have shown dramatic changes in gene transcription in visual cortical neurons in response to visual stimulation or visual deprivations (Prasad et al., 2002; Lachance and Chaudhuri, 2004; Ossipow et al., 2004; Majdan and Shatz, 2006; Tropea et al., 2006). These studies have also found that the effects on gene transcription were specific for the different type of manipulation of visual experience, and for the age at which the deprivation was performed. For example, different sets of genes were activated by dr and MD, and while some genes were activated by MD at all ages, other genes were activated only when MD was performed during the critical period (Majdan and Shatz, 2006). This selective action of visual experience on gene expression requires specific signaling mechanisms that regulate experience-dependent gene transcription. These mechanisms should be at the core of the molecular events underlying visual cortical plasticity. It is increasingly clear that regulation of gene transcription requires not only activation of transcription factors, but also recruitment of multifunctional coactivators that stimulate or repress transcription (Rosenfeld and Glass, 2001). Part of the role of these coactivators is to induce dynamic changes in the organization of chromatin directing

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gene expression. Histone acetylation and phosphorylation are mechanisms for the local and global control of chromatin structure. For instance, it has been shown that histone acetylation in a region of active transcription is necessary for high levels of transcription (Neely and Workman, 2002; Fischle et al., 2003). Histone acetylation exerts its effects either by physical remodeling of chromatin structure or by further recruitment of signaling complexes that drive or repress transcription (Jacobson et al., 2000; Peterson and Laniel, 2004). Several studies have demonstrated that regulation of gene expression through posttranslational modifications of histones is also present in neuronal cells. Indeed, stimuli that reset the circadian rhythms induce phosphorylation of H3 in the suprachiasmatic nucleus (Crosio et al., 2000), and acetylation of H3 and H4 during the transcriptional activation phase of the circadian rhythm has been described (Etchegaray et al., 2003); histone phosphoacetylation in the striatum is involved in cocaine-induced neural and behavioral plasticity (Kumar et al., 2005); and histone acetylation, together with DNA methylation, is involved in mediating the influence of postnatal environment on brain response to stress (Meaney and Szyf, 2005). Histone acetylation also controls transcription of genes required for consolidation of long-term memory and LTP (Levenson and Sweatt, 2005). In particular, it has been shown that the histone acetyltransferase activity of CREB binding protein (CBP) is necessary for synaptic plasticity in the hippocampus and activity-dependent gene expression (Alarcon et al., 2004; Korzus et al., 2004). CBP is a transcriptional coactivator important for the action of several transcription factors, including CREB, suggesting that histone acetylation is an important regulatory step of gene transcription mediated by CREB (Mayr and Montminy, 2001; Vo and Goodman, 2001). Our data indicate that experience-dependent regulation of chromatin structure could be an important mechanism of regulation of gene expression also in the developing visual cortex. Indeed, we found that visual experience activates mechanisms regulating gene transcription at different levels: an episode of visual stimulation in juvenile mice activated CREB-mediated gene expression and induced phosphorylation of the transcription factor CREB; in parallel, visual experience also induced histone phosphorylation and acetylation. Interestingly, this result has been obtained without prior immunoprecipitation of specific segments of chromatin, suggesting that visual experience exerts a global control over chromatin organization in visual cortical neurons at many sites. This is in agreement with the high number of genes regulated by visual experience in developing visual cortex (Prasad et al., 2002; Lachance and Chaudhuri, 2004; Ossipow et al., 2004; Majdan and Shatz, 2006; Tropea et al., 2006). The action of visual experience on CREB and H3 was blocked by inhibitors of ERK activation, suggesting that ERK mediates visually stimulated biochemical modifications of CREB and H3. ERK does not directly phosphorylate CREB; therefore, an intermediate molecular step is

required to explain the action of ERK on CREB. The action of ERK on H3 phosphorylation can be mediated by various kinases, including MSK (Bode and Dong, 2005; Chwang et al., 2006). MSK is required for CREB and H3 phosphorylation in neuronal cells in response to various stimuli, such as neurotrophins and cocaine (Arthur et al., 2004; Brami-Cherrier et al., 2005), and many studies on cell lines have proven MSK’s ability to phosphorylate CREB and H3 (Deak et al., 1998; Thomson et al., 1999; Soloaga et al., 2003). Our data show that MSK is activated by visual stimulation and that its activation is prevented by ERK inhibitors in visual cortical neurons in vivo. The existence of an ERK-MSK-H3 pathway activated by visual experience in the developing visual cortex is also indicated by the observation that cells positive for activated ERK were also positive for activated MSK and phosphorylated H3, suggesting that these biochemical events occurred in the same cell. Thus, MSK could bridge the action of visually activated ERK with CREB and H3 phosphorylation. Several studies suggest that the kinase RSK2 could be another mediator of CREB and H3 phosphorylation (Xing et al., 1996; Sassone-Corsi et al., 1999). Experiencedependent RSK2 phosphorylation has been observed in various brain structures (Sananbenesi et al., 2002; Butcher et al., 2004). In particular, RSK2 is activated in the monocular subfield of visual cortex contralateral to the nondeprived eye of monocularly deprived mice (Suzuki et al., 2004). Thus, RSK2 could participate in visually induced phosphorylation of CREB and H3. However, the role of RSK2 in mediating CREB and H3 phosphorylation has been recently called into question. Indeed, RSK2 is dispensable for CREB phosphorylation in various cell types (Bruning et al., 2000; Wiggin et al., 2002), including cortical neurons (Arthur et al., 2004), and it is also dispensable for H3 phosphorylation in embryonic fibroblasts (Soloaga et al., 2003). Nevertheless, it is possible that visually activated RSK2 might act on substrates important for plasticity (Thomas et al., 2005) distinct from H3 or CREB. What could be the possible mediators of the effects of visual stimulation on H3 acetylation? Although additional studies are required to outline the molecular cascade linking ERK to histone acetylation, it is possible that histone acetylation is due to the activity-dependent recruitment of transcriptional coactivators endowed with acetyltransferase activity, such as CBP, to chromatin. ERK can phosphorylate CBP in vitro (Janknecht and Nordheim, 1996); however, a direct action of ERK on CBP seems dispensable for NMDA-induced CBP phosphorylation (Impey et al., 2002). Thus, ERK could be required for histone acetylation because of its role in regulating the formation of CREB-CBP transcriptional complex that recruits CBP to chromatin and allows histone acetylation by CBP. The requirement of ERK for visually stimulated histone acetylation does not exclude other kinases from contributing to this signaling pathway by interacting with ERK-regulated mechanisms. For instance, CBP phosphorylation by CaMKIV is crucial for activity-dependent gene transcription in cortical cultures (Impey et al., 2002). PKA also plays an

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important role in synaptic plasticity of the visual cortex and regulates CREB-mediated gene expression (Beaver et al., 2001; Cancedda et al., 2003). Previous studies showed that inhibition of PKA substantially reduces visually stimulated activation of ERK, suggesting that at least part of the effects of PKA on visual cortical plasticity could be mediated by ERK (Cancedda et al., 2003). Developmental Regulation of Visually Stimulated Gene Transcription Defective visual experience during a critical period of postnatal development causes dramatic alterations in the development of the functional organization of the visual cortex, resulting in poor vision of the affected eye (amblyopia). This observation indicates that visual experience is an important player in the mechanisms underlying development of cortical circuits. Although experiencedependent plasticity is present in the adult visual cortex (Karmarkar and Dan, 2006), the effects of manipulations of visual input are less readily inducible in adult animals and might be mediated by different mechanisms (Sawtell et al., 2003). The reduced level of plasticity present in visual cortex of adult animals could also contribute to the reduced ability of adult animals to recover normal vision after periods of deprivation initiated during the critical period (Prusky et al., 2000; Pizzorusso et al., 2006). Several mechanisms have been proposed to contribute to the closure of the critical period. These include modifications of extracellular matrix composition and maturation of intracortical myelination, which could inhibit structural plasticity (Pizzorusso et al., 2002; McGee et al., 2005), and maturation of inhibitory circuitry (Huang et al., 1999; Hensch, 2005), which could make the excitatory-inhibitory balance less favorable for plasticity. It is possible that the developmental regulation of additional factors mediating intracellular signaling activated by visual experience could contribute to the closure of the critical period. Monocular enucleation activates genetic transcription of different sets of genes in the visual cortex of adult mice with respect to juvenile mice (Majdan and Shatz, 2006), suggesting that visual experience triggers different mechanisms of regulation of gene transcription before and after the end of the critical period. Our data indicate a possible molecular substrate of this developmental regulation of experience-dependent gene transcription; indeed, we observed a substantial reduction of the effects of visual stimulation on CREB phosphorylation and on CRE-mediated gene expression in adult animals. Previous studies have shown that, during the critical period, MD activates CRE-mediated gene transcription in the nondeprived cortex, and that this effect is not present in adult mice (Pham et al., 1999). Thus, a reduction in experience-dependent regulation of CREB-mediated gene transcription could be involved in regulating the closure of the critical period. This possibility is also supported by the observation that MD in adult mice triggers a labile form of plasticity that can be rendered persistent by the expression of a constitutively active CREB mutant

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(Pham et al., 2004). As opposed to CREB, the effects of visual stimulation on ERK and MSK were similar in juvenile and adults, in agreement with the persistence of ERKdependent transcription of several genes at all ages (Majdan and Shatz, 2006). Therefore, the developmental reduction of experience-dependent CREB-mediated gene expression is not due to reduced activation of MSK or ERK. Our data indicate that closure of the critical period is also associated with a decrease in the ability of visual experience to drive changes in histone phosphorylation and acetylation that control gene transcription. It has been shown that regulation of transcription at the level of histone posttranslational modifications is involved in regulation of the production of transcripts necessary for consolidation of long-term changes in synaptic efficacy in the hippocampus (Alarcon et al., 2004; Korzus et al., 2004; Levenson et al., 2004; Wood et al., 2006). This mechanism might be important for plasticity in the visual cortex during the critical period, and its downregulation could be involved in the closure of the critical period. Indeed, our data show that increasing histone acetylation in the adult visual cortex promotes ocular dominance plasticity in the adult visual cortex. Thus, multiple molecular mechanisms acting at different levels—extracellularly, on the cell membrane, and intracellularly—might contribute to the developmental downregulation of plasticity occurring in coincidence with the closure of the critical period. EXPERIMENTAL PROCEDURES Animal Treatment Mice (P27–P28 or P80–P100) were sacrificed in darkness or at various time points upon light exposure after 3 days of dr. A group of animals was implanted, in one cortex, with osmotic minipumps (ALZET 1007D, 0.5 ml/hr) connected to a cannula (gauge 30, model 1007D; Alzet, Palo Alto, CA) via a PVC tubing at a location 4 mm anterior and 3 mm lateral to lambda. Minipumps were filled with 250 mM UO126 (Promega) or vehicle (DMSO 0.5% in saline). Another group of mice received an intraperitoneal injection (100 ml/kg i.p.) of SL327 (Bristol-Myers Squibb, Princeton, New Jersey, USA) before light exposure, while control mice received the same injection of vehicle (DMSO). The effectiveness of ERK inhibition was controlled by immunohistochemistry for pERK (Figure S5). Sample Preparation and Western Blotting A total of 136 C57BL6/J mice were killed by decapitation, and brains were removed rapidly and frozen on dry ice. Cortices were then homogenized in a hypotonic lysis buffer containing 10 mM Tris (pH 7.5), 1 mM EDTA, 2.5 mM sodium pyrophosphate, 1 mM b-glycerophosphate, 1 mM sodium orthovanadate, 1 mM phenylmethylsulfonylfluoride, 10 mg/ml Aprotinin, 10 mg/ml Leupeptin (Sigma, Italy), and 1% Igepal CA-630. Histones were extracted from the nuclear fraction by the addition of five volumes of 0.2 M HCl and 10% glycerol, and the insoluble fraction was pelleted by centrifugation (18,000 3 g; 30 min; 4 C). Histones in the acid supernatant were precipitated with ten volumes of ice-cold acetone followed by centrifugation (18,000 3 g; 30 min; 4 C). The histone pellet was then resuspended in 9 M urea (Swank and Sweatt, 2001). Protein concentration was determined by Biorad assay (Biorad, Italy). Each sample, consisting of pooled cortices of 1–3 animals, was boiled, and 30 mg/lane was loaded into 12% acrylamide gels using the Precast Gel System (Biorad, Italy). Samples were

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blotted onto nitrocellulose membrane (Amersham, Bucks, UK), blocked in 4% nonfat dry milk in Tris-buffered saline for 1 hr, and then probed with the following antibodies: AcH3, H3, AcH4, H4, pCREB (Upstate, NY), and CREB (Cell Signaling, Beverly, MA). All antibodies were diluted in TTBS and 2% milk or 2% BSA and incubated overnight at 4 C. Blots were then rinsed for 20 min in TTBS, incubated in HRP-conjugated anti-mouse or anti-rabbit (1:3000 Biorad, Italy, in 2% milk or 2% BSA and TTBS), rinsed, incubated in enhanced chemiluminescent substrate (Biorad, Italy), and exposed to film (Hyperfilm, Amersham Biosciences, Europe). Films were scanned, and densitometry was analyzed through Metamorph Offline software. To minimize variability, each sample was loaded in parallel in two lanes and two gels were run simultaneously on the same apparatus. For each gel, the corresponding filters obtained after blotting were cut in two in order to obtain in each filter a complete series of samples. One of the two filters was reacted with an antibody for the modified protein (AcH3, AcH4, or pCREB) and the other with an antibody insensitive to the target protein modifications (H3, H4, or CREB). The densitometric quantification of the band corresponding to the modified protein was then normalized to the value obtained for the total amount of protein from the same gel. Each sample was analyzed four times using this procedure. Immunohistochemistry A total of 102 C57BL6/J mice were employed. Transcardial perfusion was executed with ice-cold 4% paraformaldehyde in 0.1 M TBS and sodium orthovanadate 1 mM (pH 7.4) (TBSV). Brains were quickly removed and cryoprotected in 30% sucrose overnight, and 35 mm coronal sections were cut on microtome and processed for pERK, pMSK, pH3, and NeuN immunohistochemistry. Free-floating sections were subjected to a 1–2 hr block (TBSV containing 10% BSA and 0.3% Triton X-100) followed by incubation with pERK antibody (1:1000, Sigma; in TBSV containing 5% BSA and 0.1% Triton), pMSK1 (1:50 MBL; in PBS containing 5% BSA and 0.1% Triton), or pH3 (1:750 Cell Signaling). The reaction was completed with biotinylated secondary antibody (1:200, Vector Laboratories, Burlingame, CA) followed by ExtravidinCy3 (1:500, Sigma). Some sections were counterstained with Neurotrace (Molecular Probes). For double immunostaining, pERK was added after the end of pMSK and pH3 immunohistochemistry. pERK was revealed using Alexa 488 labeled secondary antibodies (Molecular Probes 1:400). Slices were coded and, for each animal, confocal images (Olympus FV-300) of at least six representative fields (706 3 706 mm) of the binocular part of the primary visual cortex were acquired by an operator blind to the treatment. Images were analyzed through Metamorph software to count positive neurons and to measure their fluorescence intensity. The code was broken only at the end of the analysis process. A total of 20,350 cells were counted for pMSK, 48,353 cells for pH3, 4284 cells for pERK, and 1054 cells for LacZ. X-gal histochemistry was performed as described (Cancedda et al., 2003) on adult CRE-LacZ transgenic mice (Impey et al., 1996). TSA Treatment and VEP Recordings TSA (5 mg/ml in DMSO) was administered by means of daily i.p. injections at a dose of 2.5 mg/g (Korzus et al., 2004) beginning from the day before the start of MD. Each animal received four injections of either TSA or the same volume of vehicle. MD was performed as described (Huang et al., 1999). VEP recordings were performed using a micropipette inserted into the binocular part of the primary visual cortex (depth 400 mm) in urethane-anesthetized mice as described (Rossi et al., 2001). Eyes were not restrained in a fixed position, but their position remained stable throughout the experiment and was similar between different mice (Schuett et al., 2002). Stimuli consisted of full-field computer-generated horizontal gratings (0.1 c/deg, mean luminance 15 cd/m2, 95% contrast, 90 3 72 deg) alternating in time (1 Hz) presented at 20 cm from the animal. To measure VEP receptive field, grating width was 20 , its position was changed by 20 steps along the horizontal meridian, and VEP amplitude was recorded. Contra/Ipsi ratio

was assessed by acquiring VEP responses to interleaved stimulation of each eye in electrode tracks with receptive field centered on the vertical meridian. Supplemental Data The Supplemental Data for this article can be found online at http:// www.neuron.org/cgi/content/full/53/5/747/DC1/. ACKNOWLEDGMENTS This work was supported by MIUR COFIN and FIRB. Received: August 7, 2006 Revised: December 22, 2006 Accepted: February 8, 2007 Published: February 28, 2007 REFERENCES Alarcon, J.M., Malleret, G., Touzani, K., Vronskaya, S., Ishii, S., Kandel, E.R., and Barco, A. (2004). Chromatin acetylation, memory, and LTP are impaired in CBP+/ mice: a model for the cognitive deficit in Rubinstein-Taybi syndrome and its amelioration. Neuron 42, 947–959. Arthur, J.S., Fong, A.L., Dwyer, J.M., Davare, M., Reese, E., Obrietan, K., and Impey, S. (2004). Mitogen- and stress-activated protein kinase 1 mediates cAMP response element-binding protein phosphorylation and activation by neurotrophins. J. Neurosci. 24, 4324–4332. Beaver, C.J., Ji, Q., Fischer, Q.S., and Daw, N.W. (2001). Cyclic AMPdependent protein kinase mediates ocular dominance shifts in cat visual cortex. Nat. Neurosci. 4, 159–163. Berardi, N., Pizzorusso, T., and Maffei, L. (2000). Critical periods during sensory development. Curr. Opin. Neurobiol. 10, 138–145. Berardi, N., Pizzorusso, T., and Maffei, L. (2004). Extracellular matrix and visual cortical plasticity: freeing the synapse. Neuron 44, 905–908. Bode, A.M., and Dong, Z. (2005). Inducible covalent posttranslational modification of histone H3. Sci. STKE 2005, re4. 10.1126/stke. 2812005re4. Brami-Cherrier, K., Valjent, E., Herve, D., Darragh, J., Corvol, J.C., Pages, C., Arthur, S.J., Girault, J.A., and Caboche, J. (2005). Parsing molecular and behavioral effects of cocaine in mitogen- and stressactivated protein kinase-1-deficient mice. J. Neurosci. 25, 11444– 11454. Bruning, J.C., Gillette, J.A., Zhao, Y., Bjorbaeck, C., Kotzka, J., Knebel, B., Avci, H., Hanstein, B., Lingohr, P., Moller, D.E., et al. (2000). Ribosomal subunit kinase-2 is required for growth factor-stimulated transcription of the c-Fos gene. Proc. Natl. Acad. Sci. USA 97, 2462–2467. Butcher, G.Q., Lee, B., Hsieh, F., and Obrietan, K. (2004). Light- and clock-dependent regulation of ribosomal S6 kinase activity in the suprachiasmatic nucleus. Eur. J. Neurosci. 19, 907–915. Cancedda, L., Putignano, E., Impey, S., Maffei, L., Ratto, G.M., and Pizzorusso, T. (2003). Patterned vision causes CRE-mediated gene expression in the visual cortex through PKA and ERK. J. Neurosci. 23, 7012–7020. Chang, J.H., Vuppalanchi, D., van Niekerk, E., Trepel, J.B., Schanen, N.C., and Twiss, J.L. (2006). PC12 cells regulate inducible cyclic AMP (cAMP) element repressor expression to differentially control cAMP response element-dependent transcription in response to nerve growth factor and cAMP. J. Neurochem. 99, 1517–1530. Published online October 24, 2006. 10.1111/j.1471-4159.2006.04196.x. Chwang, W.B., O’Riordan, K.J., Levenson, J.M., and Sweatt, J.D. (2006). ERK/MAPK regulates hippocampal histone phosphorylation following contextual fear conditioning. Learn. Mem. 13, 322–328. Colvis, C.M., Pollock, J.D., Goodman, R.H., Impey, S., Dunn, J., Mandel, G., Champagne, F.A., Mayford, M., Korzus, E., Kumar, A., et al.

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