- Email: [email protected]
Synaptic Transmission: Excitatory Autapses Find a Function?
Current Biology Vol 19 No 7 R296
hypersensitivity system, an in vivo reaction that involves poorly defined haptenated cell surface ‘antigens’, is not ideal for defining novel recognition receptors. The new data mark an evolution from the view that NK cells respond de novo to each insult. The sustained sensitization of NK cells as a result of cytokines or infection at the least constitutes a form of hazy, fairly short-term memory, wherein a previous encounter ensures that NK cells will, for a period of weeks or months, lash out vigorously when exposed to the same or a different insult. The clonal expansion data and contact hypersensitivity data indicate that more specific NK cell memories exist as well, but a detailed understanding of how this works, and how important it is, awaits data that clarify whether novel NK cell receptors, or specificities, exist. References 1. Medzhitov, R. (2007). Recognition of microorganisms and activation of the immune response. Nature 449, 819–826.
2. Sun, J.C., Beilke, J.N., and Lanier, L.L. (2009). Adaptive immune features of natural killer cells. Nature 457, 557–561. 3. Cooper, M.A., Elliott, J.M., Keyel, P.A., Yang, L., Carrero, J.A., and Yokoyama, W.M. (2009). Cytokine-induced memory-like natural killer cells. Proc. Natl. Acad. Sci. USA 106, 1915–1919. 4. Raulet, D.H. (2004). Interplay of natural killer cells and their receptors with the adaptive immune response. Nat. Immunol. 5, 996–1002. 5. Dorshkind, K., Pollack, S.B., Bosma, M.J., and Phillips, R.A. (1985). Natural killer (NK) cells are present in mice with severe combined immunodeficiency (scid). J. Immunol. 134, 3798–3801. 6. Lanier, L.L. (2008). Up on the tightrope: natural killer cell activation and inhibition. Nat. Immunol. 9, 495–502. 7. Murali-Krishna, K., Altman, J.D., Suresh, M., Sourdive, D., Zajac, A., Miller, J., Slansky, J., and Ahmed, R. (1998). Counting antigen-specific CD8 T cells: a reevaluation of bystander activation during viral infection. Immunity 8, 177–187. 8. Kaech, S.M., Wherry, E.J., and Ahmed, R. (2002). Effector and memory T-cell differentiation: implications for vaccine development. Nat. Rev. Immunol. 2, 251–262. 9. Dokun, A.O., Kim, S., Smith, H.R., Kang, H.S., Chu, D.T., and Yokoyama, W.M. (2001). Specific and nonspecific NK cell activation during virus infection. Nat. Immunol. 2, 951–956. 10. Cudkowicz, G., and Stimpfling, J.H. (1964). Induction of immunity and of unresponsiveness
Synaptic Transmission: Excitatory Autapses Find a Function? An autapse is a synapse between a neuron and itself, a peculiar structure with an unclear function. A new study suggests that excitatory autapses contribute to a positive-feedback loop that maintains persistent electrical activity in neurons. John M. Bekkers Although the brain is complicated, we are told in undergraduate biology classes that the basic circuit element is simple: the axon of one neuron forms synapses on the dendrites of another neuron, and information is conveyed from cell to cell like the baton in a relay race. The reality is, of course, more complex. The brain has evolved other avenues of communication that seem to lack the sequential, neuron-to-neuron discipline of synaptic transmission. For example, neurotransmitter might spill out of the synaptic cleft and activate receptors on nearby neurons, including the neuron that released the transmitter [1,2]. Crosstalk could also occur between neurons and non-neuronal cells, such as glia [3].
Finallly, neurotransmission might occur at autapses. An autapse is a self-synapse, a specialized structure in which a neuron forms a synaptic connection between its axon and its own dendrites [4,5]. Anatomical autapses are not uncommon in neural circuits, but their purpose has remained puzzling. Inhibitory autapses — those made by a neuron that releases an inhibitory neurotransmitter such as g-aminobutyric acid (GABA) — seem to make sense because they could provide a self-stabilizing influence (‘negative feedback’) [6]. But what about excitatory autapses? Why should a neuron want to destabilize itself by self-excitation in a positive-feedback loop? In a recent issue of Current Biology, Saada et al. [7] propose an answer to this question: that the
11.
12.
13.
14.
to parental marrow grafts in adult F1 hybrid mice. Nature 204, 450–453. Glas, R., Franksson, L., Une, C., Eloranta, M.L., Ohlen, C., Orn, A., and Karre, K. (2000). Recruitment and activation of natural killer (NK) cells in vivo determined by the target cell phenotype: An adaptive component of NK cell-mediated responses. J. Exp. Med. 191, 129–138. Gidlund, M., Orn, A., Wigzell, H., Senik, A., and Gresser, I. (1978). Enhanced NK cell activity in mice injected with interferon and interferon inducers. Nature 273, 759. Carson, W.E., Giri, J.G., Lindemann, M.J., Linett, M.L., Ahdieh, M., Paxton, R., Anderson, D., Eisenmann, J., Grabstein, K., and Caligiuri, M.A. (1994). Interleukin (IL) 15 is a novel cytokine that activates human natural killer cells via components of the IL-2 receptor. J. Exp. Med. 180, 1395–1403. O’Leary, J.G., Goodarzi, M., Drayton, D.L., and von Andrian, U.H. (2006). T cell- and B cell-independent adaptive immunity mediated by natural killer cells. Nat. Immunol. 7, 507–516.
Department of Molecular and Cell Biology and Cancer Research Laboratory, 489 Life Sciences Addition, University of California at Berkeley, Berkeley, CA 94720, USA. E-mail: [email protected] DOI: 10.1016/j.cub.2009.02.023
sustained activation of certain Aplysia neurons during feeding behavior may be aided by excitatory autapses. In this context, then, an excitatory autapse makes perfect sense: autaptic self-excitation tips the neuron into a hyperexcitable state that requires the persistent firing of action potentials. Saada et al. [7] studied the buccal ganglia of Aplysia, focusing on B31/B32 neurons, motor neurons that drive muscles involved in grasping food during a repetitive feeding behavior in these animals [8]. When briefly stimulated in intact ganglia, B31/B32 neurons exhibit a prolonged depolarization and firing of action potentials (Figure 1), producing sustained muscle contraction. The authors wished to understand the mechanism of the persistent activity in B31/B32 cells. These cells are embedded in a surprisingly complex circuit (Figure 1). They receive both fast and slow excitatory synaptic inputs mediated by release of acetylcholine from presynaptic neurons, the B63 cells. In addition, they are electrically connected to B63 cells via gap junctions. These connections make it difficult to establish with certainty the
Dispatch R297
B31/B32 cell starts firing
B31/B32 autapse is active
Repolarization EPSPs from B63 cell 10 mV 1 sec
Stimulate
B63
Increasing depolarization
B31/B32
Muscle
existence of a postulated excitatory autapse in a B31/B32 neuron (red in Figure 1). If an action potential occurs in the B31/B32 cell it could pass through the gap junction into the B63 cell, exciting it to cause normal synaptic transmitter release. Such a response would look just like an autapse: an action potential in the B31/B32 neuron produces a postsynaptic potential in the same cell. One would be fooled into thinking an autapse is present when it is not. Saada et al. [7] elegantly resolved this ambiguity by simultaneously recording from the soma and axon of a B31/B32 neuron. By stimulating the axon directly, they were able to show that a slow, muscarinic synaptic (or autaptic) response could be recorded in the B31/B32 neuron, even when the B63 synapse was clearly silent. Hence, they tentatively concluded that B31/B32 cells do possess autapses, and they release acetylcholine onto themselves to produce a slow, muscarinic excitation that aids persistent activity. Next, Saada et al. [7] turned to a simplified culture system in which they were able to demonstrate the presence of autaptic connections in an isolated B31/B32 neuron, without the complications of the intact ganglion. This is a risky strategy. Mammalian neurons that do not form autapses in vivo commonly establish profuse autaptic connections when they are grown in cell culture, presumably as an adaptation to the lack of axonal guidance cues [4,9]. Thus, the presence of autapses in vitro by no means confirms their presence in vivo. Nevertheless, the authors found that isolated B31/B32 neurons in culture did form autaptic connections with properties similar to those seen in the intact ganglion. Importantly, they also showed that some other types of buccal ganglion neurons that did not form autapses in situ also did not form them in culture. In other words, unlike mammalian neurons, Aplysia neurons seem to retain their synaptic phenotype when placed in cell culture. This encourages confidence in the validity of the test. Finally, Saada et al. [7] used fluorescence microscopy to show the presence of putative boutons on the axon of B31/B32 neurons close to the cell soma. They suggest that these boutons, which other evidence
Current Biology
Figure 1. Electrical activity in a B31/B32 neuron in an intact ganglion (top), and the postulated circuit that gives rise to this behavior (bottom). Following brief electrical stimulation to either the B63 or B31/B32 neuron, excitatory postsynaptic potentials (EPSPs), due to the release of acetylcholine from the B63 cell, are observed in the B31/B32 cell. These EPSPs accelerate and merge, causing increasing depolarization. Eventually the B31/B32 cell starts firing action potentials, which activates the slow, muscarinic B31/B32 autapse (red connection, bottom panel). Synaptic release from the B63 cell can also occur due to electrical coupling between the B31/B32 cell and the B63 axon (zigzag line). Repolarization is produced by inhibitory inputs that are not shown in this figure.
has shown are unlikely to contact other neurons in the ganglion, may be the sites of autaptic release of acetylcholine. Strictly speaking, these data are insufficient to formally define an autapse, which is a structural specialization that is indistinguishable, at the ultrastructural level, from a synapse [10,11]. The possibility remains that the boutons on B31/B32 axons simply release acetylcholine into the neuropil, giving rise to a slow, autocrine response. If so, this would not be a true autapse. Electron microscopy would be necessary to resolve this uncertainty. The paper [7] also provokes other questions. For example, it is unclear how much of the sustained activity is due to the autapse and how much to other circuit elements, notably the strong synaptic input from B63 cells.
Indeed, Saada et al. [7] suggest, somewhat paradoxically, that the autapse might be more important for turning off the sustained activity than maintaining it. This and other questions could be resolved by finding a way to selectively block the autaptic input in the intact ganglion and measuring the effect of its absence. This paper [7] is significant because it provides the first evidence of a function for excitatory autapses (or autapse-like structures) in an intact neural circuit. It is also satisfying that these autapses are present in exactly the kind of circuit where one would expect to find them: a positive-feedback loop in which electrical activity must be sustained for several seconds in order to effect a behavior [8]. These
Current Biology Vol 19 No 7 R298
findings suggest that excitatory autapses, like their inhibitory cousins, may well have evolved to fulfil a legitimate role in all kinds of nervous systems. References 1. Bacci, A., Huguenard, J.R., and Prince, D.A. (2004). Long-lasting self-inhibition of neocortical interneurons mediated by endocannabinoids. Nature 431, 312–316. 2. Rusakov, D.A., Kullmann, D.M., and Stewart, M.G. (1999). Hippocampal synapses: do they talk to their neighbours? Trends Neurosci. 22, 382–388. 3. Auld, D.S., and Robitaille, R. (2003). Glial cells and neurotransmission: An inclusive view of synaptic function. Neuron 40, 389–400. 4. Ikeda, K., and Bekkers, J.M. (2006). Autapses. Curr. Biol. 16, R308.
5. Bekkers, J.M. (2003). Synaptic transmission: Functional autapses in the cortex. Curr. Biol. 13, R433–R435. 6. Bacci, A., Huguenard, J.R., and Prince, D.A. (2003). Functional autaptic neurotransmission in fast-spiking interneurons: A novel form of feedback inhibition in the neocortex. J. Neurosci. 23, 859–866. 7. Saada, R., Miller, N., Hurwitz, I., and Susswein, A.J. (2009). Autaptic muscarinic excitation underlies a plateau potential and persistent activity in a neuron of known behavioral function. Curr. Biol. 19, 479–484. 8. Hurwitz, I., Cropper, E.C., Vilim, F.S., Alexeeva, V., Susswein, A.J., Kupfermann, I., and Weiss, K.R. (2000). Serotonergic and peptidergic modulation of the buccal mass protractor muscle (I2) in Aplysia. J. Neurophysiol. 84, 2810–2820. 9. Craig, A.M., and Boudin, H. (2001). Molecular heterogeneity of central synapses: afferent and target regulation. Nat. Neurosci. 4, 569–578.
Circadian Biology: An Unexpected Invitee to New Time Zones Adaptation to changing light conditions is a hallmark of the circadian clock. A new study points to the critical role played by a transcriptional repressor previously implicated in cell differentiation, highlighting unappreciated links between the clock and the control of development and tumorigenesis. Satoru Masubuchi and Paolo Sassone-Corsi* Flying from Europe to California imposes on travelers an almost full reversal of the light–dark regime, a drastic event that forces the circadian clock to reset its gears in order to adjust to the new cycle. Resetting occurs through alterations in the molecular machinery that constitutes the clock [1,2], a system that basically operates in each cell of our body [3]. Several studies have documented a correlation between repeated jet lag and a number of pathological conditions, including metabolic disorders, insomnia, cognitive deficits and cancer [4–7]. Experiments reported in a recent issue of Current Biology by Duffield et al. [8] may provide a clue as to the molecular mechanisms behind these observations. The molecular organization of the circadian clock is based on interlocking positive and negative transcriptional– translational feedback loops, in which a heterodimer comprising the basic helix-loop-helix (bHLH) transcription factors CLOCK and BMAL induces the expression of its own repressors — the PER and CRY proteins [3]. Duffield and colleages [8] introduce a new element
in this equation — Id2 (inhibitor of DNA-binding gene 2), a transcriptional repressor that controls bHLH activators that, like CLOCK and BMAL1, regulate genes with E-box promoter elements. The first studies of the Id gene showed that it encoded an inhibitor of the muscle-differentiationinducing transcription factor MyoD [9], and a group of similar genes was subsequently identified [10]. All Id proteins have a HLH domain but lack a DNA-binding domain, thereby acting as transcriptional repressors by simply sequestering the activators by heterodimerization [10]. For years, studies have focused on the function of Id proteins in early development, in the context of specific differentiation programs or in cancer [10]. Little was known, however, regarding their possible physiological role in adult life, so the reported interplay with the clock system constitutes a surprising and revealing twist. A first hint of the involvement of Id genes in circadian regulation was their oscillatory expression, observed in the mammalian master clock — the suprachiasmatic nucleus (SCN) — as well as in peripheral tissues (e.g., heart and liver) and in serum-stimulated fibroblasts [11–13]. Conceptually, the
10. Lu¨bke, J., Markram, H., Frotscher, M., and Sakmann, B. (1996). Frequency and dendritic distribution of autapses established by Layer 5 pyramidal neurons in the developing rat neocortex: Comparison with synaptic innervation of adjacent neurons of the same class. J. Neurosci. 16, 3209–3218. 11. Tama´s, G., Buhl, E.H., and Somogyi, P. (1997). Massive autaptic self-innervation of GABAergic neurons in cat visual cortex. J. Neurosci. 17, 6352–6364.
John Curtin School of Medical Research, The Australian National University, Canberra, ACT 0200, Australia. E-mail: [email protected]
DOI: 10.1016/j.cub.2009.02.010
rhythmic synthesis of transcriptional repressors could result in the cyclic transcription of genes controlled by activators that are targets of the repressors [14]. The data presented by Duffield and colleagues [8] seem to fall into this regulatory scenario. Indeed, Id2 has a repressive effect on CLOCK–BMAL1-mediated transactivation of a clock promoter, suggesting that Id2 could physiologically participate in regulating the clock machinery. Although direct molecular interaction between Id2 and components of the circadian clock was not formally demonstrated, the results suggest an unexpected example of a regulatory cross-talk, in which the mechanism of the clock would be controlled by a non-canonical clock transcription factor. How is Id2 involved in the circadian rhythm? The authors exposed Id2-null mice to a time-zone change in the light–dark cycle, a procedure that mimics jet lag. Wild-type mice normally require four or five days to fully adjust to a ten-hour shift (humans need virtually an equivalent adjusting period), whereas the mutant mice took only one or two days to recover from the ‘jet lag’ (Figure 1). Thus, it would seem that Id2 operates as a modulator of the adaptation to light, one of the key features of the circadian clock. Intriguingly, a very similar jet lag phenotype has been observed in mPer1 mutant mice [15], which show an enhanced phase delay response to continuous treatments of light (e.g., 8 hours and 12 hours) but do not exhibit a different phase response curve
hypersensitivity system, an in vivo reaction that involves poorly defined haptenated cell surface ‘antigens’, is not ideal for defining novel recognition receptors. The new data mark an evolution from the view that NK cells respond de novo to each insult. The sustained sensitization of NK cells as a result of cytokines or infection at the least constitutes a form of hazy, fairly short-term memory, wherein a previous encounter ensures that NK cells will, for a period of weeks or months, lash out vigorously when exposed to the same or a different insult. The clonal expansion data and contact hypersensitivity data indicate that more specific NK cell memories exist as well, but a detailed understanding of how this works, and how important it is, awaits data that clarify whether novel NK cell receptors, or specificities, exist. References 1. Medzhitov, R. (2007). Recognition of microorganisms and activation of the immune response. Nature 449, 819–826.
2. Sun, J.C., Beilke, J.N., and Lanier, L.L. (2009). Adaptive immune features of natural killer cells. Nature 457, 557–561. 3. Cooper, M.A., Elliott, J.M., Keyel, P.A., Yang, L., Carrero, J.A., and Yokoyama, W.M. (2009). Cytokine-induced memory-like natural killer cells. Proc. Natl. Acad. Sci. USA 106, 1915–1919. 4. Raulet, D.H. (2004). Interplay of natural killer cells and their receptors with the adaptive immune response. Nat. Immunol. 5, 996–1002. 5. Dorshkind, K., Pollack, S.B., Bosma, M.J., and Phillips, R.A. (1985). Natural killer (NK) cells are present in mice with severe combined immunodeficiency (scid). J. Immunol. 134, 3798–3801. 6. Lanier, L.L. (2008). Up on the tightrope: natural killer cell activation and inhibition. Nat. Immunol. 9, 495–502. 7. Murali-Krishna, K., Altman, J.D., Suresh, M., Sourdive, D., Zajac, A., Miller, J., Slansky, J., and Ahmed, R. (1998). Counting antigen-specific CD8 T cells: a reevaluation of bystander activation during viral infection. Immunity 8, 177–187. 8. Kaech, S.M., Wherry, E.J., and Ahmed, R. (2002). Effector and memory T-cell differentiation: implications for vaccine development. Nat. Rev. Immunol. 2, 251–262. 9. Dokun, A.O., Kim, S., Smith, H.R., Kang, H.S., Chu, D.T., and Yokoyama, W.M. (2001). Specific and nonspecific NK cell activation during virus infection. Nat. Immunol. 2, 951–956. 10. Cudkowicz, G., and Stimpfling, J.H. (1964). Induction of immunity and of unresponsiveness
Synaptic Transmission: Excitatory Autapses Find a Function? An autapse is a synapse between a neuron and itself, a peculiar structure with an unclear function. A new study suggests that excitatory autapses contribute to a positive-feedback loop that maintains persistent electrical activity in neurons. John M. Bekkers Although the brain is complicated, we are told in undergraduate biology classes that the basic circuit element is simple: the axon of one neuron forms synapses on the dendrites of another neuron, and information is conveyed from cell to cell like the baton in a relay race. The reality is, of course, more complex. The brain has evolved other avenues of communication that seem to lack the sequential, neuron-to-neuron discipline of synaptic transmission. For example, neurotransmitter might spill out of the synaptic cleft and activate receptors on nearby neurons, including the neuron that released the transmitter [1,2]. Crosstalk could also occur between neurons and non-neuronal cells, such as glia [3].
Finallly, neurotransmission might occur at autapses. An autapse is a self-synapse, a specialized structure in which a neuron forms a synaptic connection between its axon and its own dendrites [4,5]. Anatomical autapses are not uncommon in neural circuits, but their purpose has remained puzzling. Inhibitory autapses — those made by a neuron that releases an inhibitory neurotransmitter such as g-aminobutyric acid (GABA) — seem to make sense because they could provide a self-stabilizing influence (‘negative feedback’) [6]. But what about excitatory autapses? Why should a neuron want to destabilize itself by self-excitation in a positive-feedback loop? In a recent issue of Current Biology, Saada et al. [7] propose an answer to this question: that the
11.
12.
13.
14.
to parental marrow grafts in adult F1 hybrid mice. Nature 204, 450–453. Glas, R., Franksson, L., Une, C., Eloranta, M.L., Ohlen, C., Orn, A., and Karre, K. (2000). Recruitment and activation of natural killer (NK) cells in vivo determined by the target cell phenotype: An adaptive component of NK cell-mediated responses. J. Exp. Med. 191, 129–138. Gidlund, M., Orn, A., Wigzell, H., Senik, A., and Gresser, I. (1978). Enhanced NK cell activity in mice injected with interferon and interferon inducers. Nature 273, 759. Carson, W.E., Giri, J.G., Lindemann, M.J., Linett, M.L., Ahdieh, M., Paxton, R., Anderson, D., Eisenmann, J., Grabstein, K., and Caligiuri, M.A. (1994). Interleukin (IL) 15 is a novel cytokine that activates human natural killer cells via components of the IL-2 receptor. J. Exp. Med. 180, 1395–1403. O’Leary, J.G., Goodarzi, M., Drayton, D.L., and von Andrian, U.H. (2006). T cell- and B cell-independent adaptive immunity mediated by natural killer cells. Nat. Immunol. 7, 507–516.
Department of Molecular and Cell Biology and Cancer Research Laboratory, 489 Life Sciences Addition, University of California at Berkeley, Berkeley, CA 94720, USA. E-mail: [email protected] DOI: 10.1016/j.cub.2009.02.023
sustained activation of certain Aplysia neurons during feeding behavior may be aided by excitatory autapses. In this context, then, an excitatory autapse makes perfect sense: autaptic self-excitation tips the neuron into a hyperexcitable state that requires the persistent firing of action potentials. Saada et al. [7] studied the buccal ganglia of Aplysia, focusing on B31/B32 neurons, motor neurons that drive muscles involved in grasping food during a repetitive feeding behavior in these animals [8]. When briefly stimulated in intact ganglia, B31/B32 neurons exhibit a prolonged depolarization and firing of action potentials (Figure 1), producing sustained muscle contraction. The authors wished to understand the mechanism of the persistent activity in B31/B32 cells. These cells are embedded in a surprisingly complex circuit (Figure 1). They receive both fast and slow excitatory synaptic inputs mediated by release of acetylcholine from presynaptic neurons, the B63 cells. In addition, they are electrically connected to B63 cells via gap junctions. These connections make it difficult to establish with certainty the
Dispatch R297
B31/B32 cell starts firing
B31/B32 autapse is active
Repolarization EPSPs from B63 cell 10 mV 1 sec
Stimulate
B63
Increasing depolarization
B31/B32
Muscle
existence of a postulated excitatory autapse in a B31/B32 neuron (red in Figure 1). If an action potential occurs in the B31/B32 cell it could pass through the gap junction into the B63 cell, exciting it to cause normal synaptic transmitter release. Such a response would look just like an autapse: an action potential in the B31/B32 neuron produces a postsynaptic potential in the same cell. One would be fooled into thinking an autapse is present when it is not. Saada et al. [7] elegantly resolved this ambiguity by simultaneously recording from the soma and axon of a B31/B32 neuron. By stimulating the axon directly, they were able to show that a slow, muscarinic synaptic (or autaptic) response could be recorded in the B31/B32 neuron, even when the B63 synapse was clearly silent. Hence, they tentatively concluded that B31/B32 cells do possess autapses, and they release acetylcholine onto themselves to produce a slow, muscarinic excitation that aids persistent activity. Next, Saada et al. [7] turned to a simplified culture system in which they were able to demonstrate the presence of autaptic connections in an isolated B31/B32 neuron, without the complications of the intact ganglion. This is a risky strategy. Mammalian neurons that do not form autapses in vivo commonly establish profuse autaptic connections when they are grown in cell culture, presumably as an adaptation to the lack of axonal guidance cues [4,9]. Thus, the presence of autapses in vitro by no means confirms their presence in vivo. Nevertheless, the authors found that isolated B31/B32 neurons in culture did form autaptic connections with properties similar to those seen in the intact ganglion. Importantly, they also showed that some other types of buccal ganglion neurons that did not form autapses in situ also did not form them in culture. In other words, unlike mammalian neurons, Aplysia neurons seem to retain their synaptic phenotype when placed in cell culture. This encourages confidence in the validity of the test. Finally, Saada et al. [7] used fluorescence microscopy to show the presence of putative boutons on the axon of B31/B32 neurons close to the cell soma. They suggest that these boutons, which other evidence
Current Biology
Figure 1. Electrical activity in a B31/B32 neuron in an intact ganglion (top), and the postulated circuit that gives rise to this behavior (bottom). Following brief electrical stimulation to either the B63 or B31/B32 neuron, excitatory postsynaptic potentials (EPSPs), due to the release of acetylcholine from the B63 cell, are observed in the B31/B32 cell. These EPSPs accelerate and merge, causing increasing depolarization. Eventually the B31/B32 cell starts firing action potentials, which activates the slow, muscarinic B31/B32 autapse (red connection, bottom panel). Synaptic release from the B63 cell can also occur due to electrical coupling between the B31/B32 cell and the B63 axon (zigzag line). Repolarization is produced by inhibitory inputs that are not shown in this figure.
has shown are unlikely to contact other neurons in the ganglion, may be the sites of autaptic release of acetylcholine. Strictly speaking, these data are insufficient to formally define an autapse, which is a structural specialization that is indistinguishable, at the ultrastructural level, from a synapse [10,11]. The possibility remains that the boutons on B31/B32 axons simply release acetylcholine into the neuropil, giving rise to a slow, autocrine response. If so, this would not be a true autapse. Electron microscopy would be necessary to resolve this uncertainty. The paper [7] also provokes other questions. For example, it is unclear how much of the sustained activity is due to the autapse and how much to other circuit elements, notably the strong synaptic input from B63 cells.
Indeed, Saada et al. [7] suggest, somewhat paradoxically, that the autapse might be more important for turning off the sustained activity than maintaining it. This and other questions could be resolved by finding a way to selectively block the autaptic input in the intact ganglion and measuring the effect of its absence. This paper [7] is significant because it provides the first evidence of a function for excitatory autapses (or autapse-like structures) in an intact neural circuit. It is also satisfying that these autapses are present in exactly the kind of circuit where one would expect to find them: a positive-feedback loop in which electrical activity must be sustained for several seconds in order to effect a behavior [8]. These
Current Biology Vol 19 No 7 R298
findings suggest that excitatory autapses, like their inhibitory cousins, may well have evolved to fulfil a legitimate role in all kinds of nervous systems. References 1. Bacci, A., Huguenard, J.R., and Prince, D.A. (2004). Long-lasting self-inhibition of neocortical interneurons mediated by endocannabinoids. Nature 431, 312–316. 2. Rusakov, D.A., Kullmann, D.M., and Stewart, M.G. (1999). Hippocampal synapses: do they talk to their neighbours? Trends Neurosci. 22, 382–388. 3. Auld, D.S., and Robitaille, R. (2003). Glial cells and neurotransmission: An inclusive view of synaptic function. Neuron 40, 389–400. 4. Ikeda, K., and Bekkers, J.M. (2006). Autapses. Curr. Biol. 16, R308.
5. Bekkers, J.M. (2003). Synaptic transmission: Functional autapses in the cortex. Curr. Biol. 13, R433–R435. 6. Bacci, A., Huguenard, J.R., and Prince, D.A. (2003). Functional autaptic neurotransmission in fast-spiking interneurons: A novel form of feedback inhibition in the neocortex. J. Neurosci. 23, 859–866. 7. Saada, R., Miller, N., Hurwitz, I., and Susswein, A.J. (2009). Autaptic muscarinic excitation underlies a plateau potential and persistent activity in a neuron of known behavioral function. Curr. Biol. 19, 479–484. 8. Hurwitz, I., Cropper, E.C., Vilim, F.S., Alexeeva, V., Susswein, A.J., Kupfermann, I., and Weiss, K.R. (2000). Serotonergic and peptidergic modulation of the buccal mass protractor muscle (I2) in Aplysia. J. Neurophysiol. 84, 2810–2820. 9. Craig, A.M., and Boudin, H. (2001). Molecular heterogeneity of central synapses: afferent and target regulation. Nat. Neurosci. 4, 569–578.
Circadian Biology: An Unexpected Invitee to New Time Zones Adaptation to changing light conditions is a hallmark of the circadian clock. A new study points to the critical role played by a transcriptional repressor previously implicated in cell differentiation, highlighting unappreciated links between the clock and the control of development and tumorigenesis. Satoru Masubuchi and Paolo Sassone-Corsi* Flying from Europe to California imposes on travelers an almost full reversal of the light–dark regime, a drastic event that forces the circadian clock to reset its gears in order to adjust to the new cycle. Resetting occurs through alterations in the molecular machinery that constitutes the clock [1,2], a system that basically operates in each cell of our body [3]. Several studies have documented a correlation between repeated jet lag and a number of pathological conditions, including metabolic disorders, insomnia, cognitive deficits and cancer [4–7]. Experiments reported in a recent issue of Current Biology by Duffield et al. [8] may provide a clue as to the molecular mechanisms behind these observations. The molecular organization of the circadian clock is based on interlocking positive and negative transcriptional– translational feedback loops, in which a heterodimer comprising the basic helix-loop-helix (bHLH) transcription factors CLOCK and BMAL induces the expression of its own repressors — the PER and CRY proteins [3]. Duffield and colleages [8] introduce a new element
in this equation — Id2 (inhibitor of DNA-binding gene 2), a transcriptional repressor that controls bHLH activators that, like CLOCK and BMAL1, regulate genes with E-box promoter elements. The first studies of the Id gene showed that it encoded an inhibitor of the muscle-differentiationinducing transcription factor MyoD [9], and a group of similar genes was subsequently identified [10]. All Id proteins have a HLH domain but lack a DNA-binding domain, thereby acting as transcriptional repressors by simply sequestering the activators by heterodimerization [10]. For years, studies have focused on the function of Id proteins in early development, in the context of specific differentiation programs or in cancer [10]. Little was known, however, regarding their possible physiological role in adult life, so the reported interplay with the clock system constitutes a surprising and revealing twist. A first hint of the involvement of Id genes in circadian regulation was their oscillatory expression, observed in the mammalian master clock — the suprachiasmatic nucleus (SCN) — as well as in peripheral tissues (e.g., heart and liver) and in serum-stimulated fibroblasts [11–13]. Conceptually, the
10. Lu¨bke, J., Markram, H., Frotscher, M., and Sakmann, B. (1996). Frequency and dendritic distribution of autapses established by Layer 5 pyramidal neurons in the developing rat neocortex: Comparison with synaptic innervation of adjacent neurons of the same class. J. Neurosci. 16, 3209–3218. 11. Tama´s, G., Buhl, E.H., and Somogyi, P. (1997). Massive autaptic self-innervation of GABAergic neurons in cat visual cortex. J. Neurosci. 17, 6352–6364.
John Curtin School of Medical Research, The Australian National University, Canberra, ACT 0200, Australia. E-mail: [email protected]
DOI: 10.1016/j.cub.2009.02.010
rhythmic synthesis of transcriptional repressors could result in the cyclic transcription of genes controlled by activators that are targets of the repressors [14]. The data presented by Duffield and colleagues [8] seem to fall into this regulatory scenario. Indeed, Id2 has a repressive effect on CLOCK–BMAL1-mediated transactivation of a clock promoter, suggesting that Id2 could physiologically participate in regulating the clock machinery. Although direct molecular interaction between Id2 and components of the circadian clock was not formally demonstrated, the results suggest an unexpected example of a regulatory cross-talk, in which the mechanism of the clock would be controlled by a non-canonical clock transcription factor. How is Id2 involved in the circadian rhythm? The authors exposed Id2-null mice to a time-zone change in the light–dark cycle, a procedure that mimics jet lag. Wild-type mice normally require four or five days to fully adjust to a ten-hour shift (humans need virtually an equivalent adjusting period), whereas the mutant mice took only one or two days to recover from the ‘jet lag’ (Figure 1). Thus, it would seem that Id2 operates as a modulator of the adaptation to light, one of the key features of the circadian clock. Intriguingly, a very similar jet lag phenotype has been observed in mPer1 mutant mice [15], which show an enhanced phase delay response to continuous treatments of light (e.g., 8 hours and 12 hours) but do not exhibit a different phase response curve