- Email: [email protected]
Abscisic Acid Antagonism in Balancing Growth and Stress
Developmental Cell
Previews Brassinosteroid/Abscisic Acid Antagonism in Balancing Growth and Stress Steven D. Clouse1,* 1Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.devcel.2016.07.005
In this issue of Developmental Cell, Gui et al. (2016) show that an abscisic acid-inducible remorin protein in rice directly interacts with critical brassinosteroid signaling components to attenuate the brassinosteroid response, thus illuminating one aspect of the brassinosteroid/abscisic acid antagonism. Plant hormones regulate numerous aspects of plant growth, development, and environmental response through a diverse array of signal transduction pathways that modulate the expression of thousands of different genes required for cell elongation, division, and differentiation. Brassinosteroids (BRs), belonging to one such class of plant hormones, have structural similarity to mammalian steroid hormones and, like their animal counterparts, have a profound influence on cellular dynamics and function. Null mutations in genes encoding BR biosynthetic enzymes or signal transduction components show severe phenotypes including dwarfism, disruptions in leaf, flower, and root development, and altered vascular structure, suggesting that BRs are essential for normal plant development (Belkhadir and Jaillais, 2015). In changing environmental conditions, plants must shift resources from growth and development to adaptation to stresses such as drought and temperature extremes. This process is accelerated by another plant hormone, abscisic acid (ABA), which responds to drought in part by regulating leaf stomatal aperture, although it also functions in embryo development and seed germination (Hauser et al., 2011). As part of the adaptive response to stress, ABA can inhibit vegetative growth and thus oppose the growth-promoting properties of BRs. Twenty years ago it was found that mutants unresponsive to the inhibitory effect of BRs on root elongation were hypersensitive to root length inhibition by ABA, suggesting an antagonistic interaction between these two hormones (Clouse et al., 1996). Numerous other examples of the BR/ABA antagonism have been reported since, but the precise molecular
mechanisms that are responsible for this physiological response could not be uncovered until both BR and ABA signal transduction pathways became well characterized (Saini et al., 2015). Gui et al. (2016) now describe one mechanism of BR/ABA interaction involving a remorin protein that is regulated by ABA and in turn regulates BR signal transduction. BRs are perceived by the extracellular domain of a leucine-rich repeat receptor kinase named BRASSINOSTEROID INSENSITIVE 1 (BRI1), which, in the presence of ligand, interacts with and reciprocally transphosphorylates a co-receptor from the SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) family of smaller leucine-rich repeat receptor kinases (Figure 1). The formation of an active complex between BRI1 and SERK family members such as SERK1 and SERK3 (also known as BRI1-ASSOCIATED RECEPTOR KINASE 1 or BAK1) is a critical step in BR signaling because it initiates a phospho-relay involving cytoplasmic kinases and phosphatases that ultimately results in the activation of specific transcription factors that regulate the expression of nearly 1,000 genes (Belkhadir and Jaillais, 2015). In both Arabidopsis thaliana and rice, BRI1 INHIBITIOR KINASE 1 (BKI1) is a membrane-bound negative regulator of BR signaling that binds to BRI1 in the absence of BR and prevents the interaction of BRI1 with SERK co-receptors. Upon BR binding to BRI1, BKI1 is phosphorylated on a tyrosine residue and released from the membrane, allowing the BRI1/SERK complex to form, which propagates BR signaling downstream (Jiang et al., 2015). Gui et al. (2016) have now identified a protein that has a function similar to BKI1, i.e., disruption of the BRI1/SERK complex
118 Developmental Cell 38, July 25, 2016 ª 2016 Elsevier Inc.
formation and downregulation of BR signaling, but with a twist: this rice protein, termed OsREM4.1, is encoded by a gene whose transcript levels are regulated by ABA. Specifically, OsREM4.1 levels are increased by elevated ABA levels through a bZIP transcription factor (OsbZIP23) in the ABA signal transduction pathway. In Arabidopsis, it is known that binding of ABA to its soluble receptor results in inactivation of a PP2C phosphatase, which in turn allows activation of a SnRK2 kinase that phosphorylates downstream transcription factors and ion channels required for the ABA response (Hauser et al., 2011). Recently, it was also shown in rice that OsbZIP23 is phosphorylated by a SnRK2 kinase in response to ABA (Zong et al., 2016). OsREM4.1 belongs to the plant-specific remorin family of proteins whose members are membrane-associated and often involved in plant-microbe interactions or hormone response. After establishing that OsREM4.1 transcription was specifically upregulated by ABA, Gui et al. (2016) then searched for OsREM4.1-interacting proteins by immunoprecipitation followed by liquid chromatography/tandem mass spectrometry. A particularly interesting candidate uncovered was OsSERK1, the co-receptor of BRI1 in rice. Numerous independent lines of analysis confirmed the OsREM4.1/OsSERK1 interaction, and examination of the phenotype of transgenic lines expressing either high or low levels of OsREM4.1 showed that OsREM4.1 was a negative regulator of BR signaling. Moreover, OsREM4.1 binds to the activation loop of the OsSERK1 kinase domain, thereby inhibiting the formation of the OsSERK1/OsBRI1 complex and preventing transphosphorylation of BRI1 and SERK1, thus negatively
Developmental Cell
Previews
Figure 1. Intersection of BR and ABA Signal Transduction Pathways The BRI1 receptor kinase binds brassinolide (BL), the most active naturally occurring BR, which leads to phosphorylation and release of BKI1 to allow the BRI1/SERK1 (and/or BAK1) complex to form and transphosphorylate both partners. Activated BRI1 phosphorylates BR SIGNALING KINASE 1 (BSK1), initiating a phosphorylation/dephosphorylation cascade that ultimately yields the unphosphorylated, active form of the BES1/BZR1 transcription factors that move to the nucleus to regulate BR-responsive gene expression. Under stress conditions, ABA increases rapidly and binds to the soluble PYRABACTIN RESISTANT/ REGULATORY COMPONENT OF ABA RECEPTOR (PYR) protein, which inactivates PROTEIN PHOSPHATASE 2C (PP2C), a negative regulator of ABA signaling. This allows SNF1-RELATED KINASE 2 (SnRK2) to phosphorylate a variety of transcription factors, including OsbZIP23, which activates transcription of the OsREM4.1 gene. The REM4.1 protein then binds to SERK1 to inactivate BR signaling, as described in the text. It is currently unknown whether BKI1 and REM4.1 act independently or are coordinated in some manner. For simplicity, the nucleus is not shown.
regulating BR signaling. Interestingly, increased BR levels caused OsBRI1 to directly phosphorylate OsREM4.1 and release its inhibition of the OsSERK1/ OsBRI1 active complex in a manner reminiscent of the BKI1 protein. When viewed together, the accumulated evidence of Gui et al. (2016) suggests a
central role for OSREM4.1 in mediating the ABA/BR antagonism in rice. Under conditions of stress, ABA levels are rapidly elevated, increasing OsREM4.1 levels to facilitate disruption of the critical OsBRI1/ OsSERK1 complex and downregulation of BR signaling, which would favor shifting plant resources from BR-promoted growth
to ABA-regulated stress responses. When stress decreases, ABA levels fall and BRpromoted phosphorylation of OsREM4.1 by OsBRI1 increases, releasing OsREM4.1 from the complex and thus allowing transphosphorylation of OsBRI1/OsSERK1 to reactivate the growth-promoting BR signal transduction pathway (Figure 1). Developmental Cell 38, July 25, 2016 119
Developmental Cell
Previews A recent study demonstrated direct interaction between downstream elements of the BR and ABA signaling pathways in Arabidopsis, providing another mechanism of antagonistic crosstalk in the regulation of root growth. BRASSINAZOLE RESISTANT 1 (BZR1), one of the primary transcription factors regulating BR-responsive genes (Figure 1), was found to directly bind to the promoter of ABSCISIC ACID INSENSTIVE 5 (ABI5), an important transcription factor in ABA signaling, leading to suppressed ABI5 expression and reduced ABA response (Yang et al., 2016). Several other examples of direct interaction between BR and ABA signaling components in a variety of physiological processes have also been reported (Hu and Yu, 2014; Shang et al., 2016). The work of Gui et al. in this issue provides
an addition to this growing list of mechanisms integrating distinct hormone signal transduction pathways that balance complex developmental programs leading to plant growth with those regulating adaptation to changing environments. ACKNOWLEDGMENTS This article was written while serving in a paid position at the National Science Foundation. Any opinions or conclusions expressed in this article are those of the author and do not necessarily reflect the views of the National Science Foundation.
REFERENCES Belkhadir, Y., and Jaillais, Y. (2015). New Phytol. 206, 522–540. Clouse, S.D., Langford, M., and McMorris, T.C. (1996). Plant Physiol. 111, 671–678.
Gui, J., Zheng, S., Liu, C., Shen, J., Li, J., and Li, L. (2016). Dev. Cell 38, this issue, 201–213. Hauser, F., Waadt, R., and Schroeder, J.I. (2011). Curr. Biol. 21, R346–R355. Hu, Y., and Yu, D. (2014). Plant Cell 26, 4394–4408. Jiang, J., Wang, T., Wu, Z., Wang, J., Zhang, C., Wang, H., Wang, Z.X., and Wang, X. (2015). Mol. Plant 8, 1675–1678. Saini, S., Sharma, I., and Pati, P.K. (2015). Front. Plant Sci. 6, 1–17. Shang, Y., Dai, C., Lee, M.M., Kwak, J.M., and Nam, K.H. (2016). Mol. Plant 9, 447–460. Yang, X., Bai, Y., Shang, J., Xin, R., and Tang, W. (2016). Plant Cell Environ., in press. Published online May 5, 2016. http://dx.doi.org/10.1111/ pce.12763. Zong, W., Tang, N., Yang, J., Peng, L., Ma, S., Xu, Y., Li, G., and Xiong, L. (2016). Plant Physiol., in press. Published online June 20, 2016. http://dx. doi.org/10.1104/pp.16.00469.
A Death Trap for Microglia Xu-fei Du1,* and Jiu-lin Du1,* 1Institute of Neuroscience, State Key Laboratory of Neuroscience, Center for Excellence in Brain Science and Intelligence Technology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, 320 Yue-Yang Road, Shanghai 200031, China *Correspondence: [email protected] (X.-f.D.), [email protected] (J.-l.D.) http://dx.doi.org/10.1016/j.devcel.2016.07.004
Microglia, immune cells of the brain, originate from erythromyeloid precursors, far from the central nervous system. Xu et al. (2016) in this issue of Developmental Cell and Casano et al. (2016) recently in Cell Reports show that apoptotic neurons act as bait to ‘‘trap’’ microglia into colonizing the developing brain. Microglia are blood-born resident macrophages in the central nervous system (CNS) and have been known to play crucial roles in regulating immune-related processes in the brain (Ransohoff and Cardona, 2010; Shemer et al., 2015). In the past decade, accumulating evidence has revolutionized our understanding of this unique cell population : microglia act not only as immune system guardians of the CNS but also as versatile sculptors for the normal development and functions of the brain (Frost and Schafer, 2016; Salter and Beggs, 2014; Wu et al., 2015). What makes microglia different from other cell types in the CNS is that they continuously patrol in the neural parenchyma and interact with all other cell types, like little
‘‘spirits’’ of the brain. Notably, microglia are not born in the CNS, but derive from erythromyeloid precursors (EMPs) present in the yolk sac and immigrate into the CNS during embryogenesis. How these cells colonize the CNS is a question that has long been fascinating biologists. This long-lasting puzzle had remained unresolved, because non-invasive longterm imaging of the dynamics of microglial precursors at a high spatiotemporal resolution in live animals is required. In light of these barriers, the transparent nature of the zebrafish brain at early developmental stages, together with advanced imaging techniques, has made it an ideal model to study microglial dynamics (Li et al., 2012; Peri and Nu¨sslein-Volhard, 2008).
120 Developmental Cell 38, July 25, 2016 ª 2016 Elsevier Inc.
By using in vivo imaging, Xu et al. (2016) in this issue of Developmental Cell and Casano et al. (2016) in a recent issue of Cell Reports explored how microglial precursors immigrate into the brain during development. Because microglia in nature are mononuclear phagocytes that inhabit the CNS and execute many of their functions through phagocytosis and clearance, it is interesting to hypothesize that the driving force for microglial colonization of the brain is the need for developmentally apoptotic cells to be cleared from the organ. In these two elegant studies, the authors provide evidence supporting this hypothesis through a series of well-designed experiments (Figure 1).
Previews Brassinosteroid/Abscisic Acid Antagonism in Balancing Growth and Stress Steven D. Clouse1,* 1Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.devcel.2016.07.005
In this issue of Developmental Cell, Gui et al. (2016) show that an abscisic acid-inducible remorin protein in rice directly interacts with critical brassinosteroid signaling components to attenuate the brassinosteroid response, thus illuminating one aspect of the brassinosteroid/abscisic acid antagonism. Plant hormones regulate numerous aspects of plant growth, development, and environmental response through a diverse array of signal transduction pathways that modulate the expression of thousands of different genes required for cell elongation, division, and differentiation. Brassinosteroids (BRs), belonging to one such class of plant hormones, have structural similarity to mammalian steroid hormones and, like their animal counterparts, have a profound influence on cellular dynamics and function. Null mutations in genes encoding BR biosynthetic enzymes or signal transduction components show severe phenotypes including dwarfism, disruptions in leaf, flower, and root development, and altered vascular structure, suggesting that BRs are essential for normal plant development (Belkhadir and Jaillais, 2015). In changing environmental conditions, plants must shift resources from growth and development to adaptation to stresses such as drought and temperature extremes. This process is accelerated by another plant hormone, abscisic acid (ABA), which responds to drought in part by regulating leaf stomatal aperture, although it also functions in embryo development and seed germination (Hauser et al., 2011). As part of the adaptive response to stress, ABA can inhibit vegetative growth and thus oppose the growth-promoting properties of BRs. Twenty years ago it was found that mutants unresponsive to the inhibitory effect of BRs on root elongation were hypersensitive to root length inhibition by ABA, suggesting an antagonistic interaction between these two hormones (Clouse et al., 1996). Numerous other examples of the BR/ABA antagonism have been reported since, but the precise molecular
mechanisms that are responsible for this physiological response could not be uncovered until both BR and ABA signal transduction pathways became well characterized (Saini et al., 2015). Gui et al. (2016) now describe one mechanism of BR/ABA interaction involving a remorin protein that is regulated by ABA and in turn regulates BR signal transduction. BRs are perceived by the extracellular domain of a leucine-rich repeat receptor kinase named BRASSINOSTEROID INSENSITIVE 1 (BRI1), which, in the presence of ligand, interacts with and reciprocally transphosphorylates a co-receptor from the SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) family of smaller leucine-rich repeat receptor kinases (Figure 1). The formation of an active complex between BRI1 and SERK family members such as SERK1 and SERK3 (also known as BRI1-ASSOCIATED RECEPTOR KINASE 1 or BAK1) is a critical step in BR signaling because it initiates a phospho-relay involving cytoplasmic kinases and phosphatases that ultimately results in the activation of specific transcription factors that regulate the expression of nearly 1,000 genes (Belkhadir and Jaillais, 2015). In both Arabidopsis thaliana and rice, BRI1 INHIBITIOR KINASE 1 (BKI1) is a membrane-bound negative regulator of BR signaling that binds to BRI1 in the absence of BR and prevents the interaction of BRI1 with SERK co-receptors. Upon BR binding to BRI1, BKI1 is phosphorylated on a tyrosine residue and released from the membrane, allowing the BRI1/SERK complex to form, which propagates BR signaling downstream (Jiang et al., 2015). Gui et al. (2016) have now identified a protein that has a function similar to BKI1, i.e., disruption of the BRI1/SERK complex
118 Developmental Cell 38, July 25, 2016 ª 2016 Elsevier Inc.
formation and downregulation of BR signaling, but with a twist: this rice protein, termed OsREM4.1, is encoded by a gene whose transcript levels are regulated by ABA. Specifically, OsREM4.1 levels are increased by elevated ABA levels through a bZIP transcription factor (OsbZIP23) in the ABA signal transduction pathway. In Arabidopsis, it is known that binding of ABA to its soluble receptor results in inactivation of a PP2C phosphatase, which in turn allows activation of a SnRK2 kinase that phosphorylates downstream transcription factors and ion channels required for the ABA response (Hauser et al., 2011). Recently, it was also shown in rice that OsbZIP23 is phosphorylated by a SnRK2 kinase in response to ABA (Zong et al., 2016). OsREM4.1 belongs to the plant-specific remorin family of proteins whose members are membrane-associated and often involved in plant-microbe interactions or hormone response. After establishing that OsREM4.1 transcription was specifically upregulated by ABA, Gui et al. (2016) then searched for OsREM4.1-interacting proteins by immunoprecipitation followed by liquid chromatography/tandem mass spectrometry. A particularly interesting candidate uncovered was OsSERK1, the co-receptor of BRI1 in rice. Numerous independent lines of analysis confirmed the OsREM4.1/OsSERK1 interaction, and examination of the phenotype of transgenic lines expressing either high or low levels of OsREM4.1 showed that OsREM4.1 was a negative regulator of BR signaling. Moreover, OsREM4.1 binds to the activation loop of the OsSERK1 kinase domain, thereby inhibiting the formation of the OsSERK1/OsBRI1 complex and preventing transphosphorylation of BRI1 and SERK1, thus negatively
Developmental Cell
Previews
Figure 1. Intersection of BR and ABA Signal Transduction Pathways The BRI1 receptor kinase binds brassinolide (BL), the most active naturally occurring BR, which leads to phosphorylation and release of BKI1 to allow the BRI1/SERK1 (and/or BAK1) complex to form and transphosphorylate both partners. Activated BRI1 phosphorylates BR SIGNALING KINASE 1 (BSK1), initiating a phosphorylation/dephosphorylation cascade that ultimately yields the unphosphorylated, active form of the BES1/BZR1 transcription factors that move to the nucleus to regulate BR-responsive gene expression. Under stress conditions, ABA increases rapidly and binds to the soluble PYRABACTIN RESISTANT/ REGULATORY COMPONENT OF ABA RECEPTOR (PYR) protein, which inactivates PROTEIN PHOSPHATASE 2C (PP2C), a negative regulator of ABA signaling. This allows SNF1-RELATED KINASE 2 (SnRK2) to phosphorylate a variety of transcription factors, including OsbZIP23, which activates transcription of the OsREM4.1 gene. The REM4.1 protein then binds to SERK1 to inactivate BR signaling, as described in the text. It is currently unknown whether BKI1 and REM4.1 act independently or are coordinated in some manner. For simplicity, the nucleus is not shown.
regulating BR signaling. Interestingly, increased BR levels caused OsBRI1 to directly phosphorylate OsREM4.1 and release its inhibition of the OsSERK1/ OsBRI1 active complex in a manner reminiscent of the BKI1 protein. When viewed together, the accumulated evidence of Gui et al. (2016) suggests a
central role for OSREM4.1 in mediating the ABA/BR antagonism in rice. Under conditions of stress, ABA levels are rapidly elevated, increasing OsREM4.1 levels to facilitate disruption of the critical OsBRI1/ OsSERK1 complex and downregulation of BR signaling, which would favor shifting plant resources from BR-promoted growth
to ABA-regulated stress responses. When stress decreases, ABA levels fall and BRpromoted phosphorylation of OsREM4.1 by OsBRI1 increases, releasing OsREM4.1 from the complex and thus allowing transphosphorylation of OsBRI1/OsSERK1 to reactivate the growth-promoting BR signal transduction pathway (Figure 1). Developmental Cell 38, July 25, 2016 119
Developmental Cell
Previews A recent study demonstrated direct interaction between downstream elements of the BR and ABA signaling pathways in Arabidopsis, providing another mechanism of antagonistic crosstalk in the regulation of root growth. BRASSINAZOLE RESISTANT 1 (BZR1), one of the primary transcription factors regulating BR-responsive genes (Figure 1), was found to directly bind to the promoter of ABSCISIC ACID INSENSTIVE 5 (ABI5), an important transcription factor in ABA signaling, leading to suppressed ABI5 expression and reduced ABA response (Yang et al., 2016). Several other examples of direct interaction between BR and ABA signaling components in a variety of physiological processes have also been reported (Hu and Yu, 2014; Shang et al., 2016). The work of Gui et al. in this issue provides
an addition to this growing list of mechanisms integrating distinct hormone signal transduction pathways that balance complex developmental programs leading to plant growth with those regulating adaptation to changing environments. ACKNOWLEDGMENTS This article was written while serving in a paid position at the National Science Foundation. Any opinions or conclusions expressed in this article are those of the author and do not necessarily reflect the views of the National Science Foundation.
REFERENCES Belkhadir, Y., and Jaillais, Y. (2015). New Phytol. 206, 522–540. Clouse, S.D., Langford, M., and McMorris, T.C. (1996). Plant Physiol. 111, 671–678.
Gui, J., Zheng, S., Liu, C., Shen, J., Li, J., and Li, L. (2016). Dev. Cell 38, this issue, 201–213. Hauser, F., Waadt, R., and Schroeder, J.I. (2011). Curr. Biol. 21, R346–R355. Hu, Y., and Yu, D. (2014). Plant Cell 26, 4394–4408. Jiang, J., Wang, T., Wu, Z., Wang, J., Zhang, C., Wang, H., Wang, Z.X., and Wang, X. (2015). Mol. Plant 8, 1675–1678. Saini, S., Sharma, I., and Pati, P.K. (2015). Front. Plant Sci. 6, 1–17. Shang, Y., Dai, C., Lee, M.M., Kwak, J.M., and Nam, K.H. (2016). Mol. Plant 9, 447–460. Yang, X., Bai, Y., Shang, J., Xin, R., and Tang, W. (2016). Plant Cell Environ., in press. Published online May 5, 2016. http://dx.doi.org/10.1111/ pce.12763. Zong, W., Tang, N., Yang, J., Peng, L., Ma, S., Xu, Y., Li, G., and Xiong, L. (2016). Plant Physiol., in press. Published online June 20, 2016. http://dx. doi.org/10.1104/pp.16.00469.
A Death Trap for Microglia Xu-fei Du1,* and Jiu-lin Du1,* 1Institute of Neuroscience, State Key Laboratory of Neuroscience, Center for Excellence in Brain Science and Intelligence Technology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, 320 Yue-Yang Road, Shanghai 200031, China *Correspondence: [email protected] (X.-f.D.), [email protected] (J.-l.D.) http://dx.doi.org/10.1016/j.devcel.2016.07.004
Microglia, immune cells of the brain, originate from erythromyeloid precursors, far from the central nervous system. Xu et al. (2016) in this issue of Developmental Cell and Casano et al. (2016) recently in Cell Reports show that apoptotic neurons act as bait to ‘‘trap’’ microglia into colonizing the developing brain. Microglia are blood-born resident macrophages in the central nervous system (CNS) and have been known to play crucial roles in regulating immune-related processes in the brain (Ransohoff and Cardona, 2010; Shemer et al., 2015). In the past decade, accumulating evidence has revolutionized our understanding of this unique cell population : microglia act not only as immune system guardians of the CNS but also as versatile sculptors for the normal development and functions of the brain (Frost and Schafer, 2016; Salter and Beggs, 2014; Wu et al., 2015). What makes microglia different from other cell types in the CNS is that they continuously patrol in the neural parenchyma and interact with all other cell types, like little
‘‘spirits’’ of the brain. Notably, microglia are not born in the CNS, but derive from erythromyeloid precursors (EMPs) present in the yolk sac and immigrate into the CNS during embryogenesis. How these cells colonize the CNS is a question that has long been fascinating biologists. This long-lasting puzzle had remained unresolved, because non-invasive longterm imaging of the dynamics of microglial precursors at a high spatiotemporal resolution in live animals is required. In light of these barriers, the transparent nature of the zebrafish brain at early developmental stages, together with advanced imaging techniques, has made it an ideal model to study microglial dynamics (Li et al., 2012; Peri and Nu¨sslein-Volhard, 2008).
120 Developmental Cell 38, July 25, 2016 ª 2016 Elsevier Inc.
By using in vivo imaging, Xu et al. (2016) in this issue of Developmental Cell and Casano et al. (2016) in a recent issue of Cell Reports explored how microglial precursors immigrate into the brain during development. Because microglia in nature are mononuclear phagocytes that inhabit the CNS and execute many of their functions through phagocytosis and clearance, it is interesting to hypothesize that the driving force for microglial colonization of the brain is the need for developmentally apoptotic cells to be cleared from the organ. In these two elegant studies, the authors provide evidence supporting this hypothesis through a series of well-designed experiments (Figure 1).