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Ant-icipating Change: An Epigenetic Switch in Reprogramming the Social Lives of Ants
Molecular Cell
Previews Ant-icipating Change: An Epigenetic Switch in Reprogramming the Social Lives of Ants Marilyn G. Pray-Grant1 and Patrick A. Grant1,* 1Department of Biochemistry and Molecular Genetics, University of Virginia School of Medicine, Charlottesville, VA 22908, USA *Correspondence: [email protected] https://doi.org/10.1016/j.molcel.2019.12.006
Glastad et al. (2019) describe a role for the neuronal CoREST corepressor and changes in juvenile hormone (JH) and ecdysone signaling during the reprogramming of social behavioral phenotypes in ants that are reflective of a natural mechanism differentiating ‘‘Major’’ and ‘‘Minor’’ worker ants.
Dimorphic ant species, including Camponotus floridanus, have two distinctly sized classes of workers, called ‘‘Major’’ and ‘‘Minor’’ castes, but a single common genome. A prominent manifestation of these castes, as in many eusocial insect species, is the division of distinct colony roles and social behaviors among worker groups (Ho¨lldobler and Wilson, 1990). Various genomic approaches have provided insights into the molecular mechanisms underlying natural caste differentiation in ants (Favreau et al., 2018). Notably, the larger Major workers of C. floridanus, which are tasked with defending the colony, can be reprogrammed to have a Minor-like foraging behavior following injection of the histone deacetylase (HDAC) inhibitor trichostatin A (TSA) into the brain (Simola et al., 2013). However, the intrinsic epigenetic mechanisms at play in the reprogramming of Majors and Minors have remained largely elusive. Here, Glastad et al. (2019) uncover an epigenetic switch underlying caste fate and demonstrate an important function for the CoREST corepressor in the induced reprogramming of transcriptional networks in Major worker ants that lead to a Minor-like foraging behavior. In this study, the injection of Major ants with TSA at 0 and 5 days post eclosion, defined as the emergence of the adult animal from the pupal case, led to increased foraging as soon as 1 day after injection and was maintained throughout the 10 days of study. Notably, the strongest induction of foraging occurred on day 5 at levels similar to that of natural Minors, but it was ineffective at a later stage post eclosion. An analysis of differentially expressed genes (DEGs) identified by
RNA-seq performed on the whole brain at the day 5 peak of induced foraging revealed many significant gene ontology terms associated with neuronal function and additionally those associated with epigenetic regulation. The deacetylase HDAC1 and repressor CoREST were among the top 20 significant DEGs with TSA treatment. RNA-seq and ChIP-seq approaches provided data consistent with a role for upregulated CoREST in specifically repressing Minor genes in reprogrammed Major ants and support a role for CoREST in mediating natural caste behavioral reprogramming. CoREST is a highly conserved protein that functions as an integral component of chromatin corepressor complexes that contain the histone deacetylase HDAC1/2 and demethylase activities (Meier and Brehm, 2014). It is an important regulator of neurogenesis and differentia-
tion from flies to mammals (Andre´s et al., 1999; Abrajano et al., 2010; Meier et al., 2012). Consistent with these observations, ChIP-seq for the acetylation mark on histone H3K27 (H3K27ac), a modification associated with gene activation, revealed higher H3K27ac at upregulated genes at 1 h following TSA treatment. Among the top genes, CoREST and HDAC1 showed increased proximal H3K27ac, and the authors noted that CoREST and HDAC1 upregulation may be a response to HDAC inhibition. Collectively, their studies suggest epigenomic changes in neuronal function mediated by CoREST occur during a peak period of reprogramming and that a transient and early point of gene reprogramming occurs during this window. Among the potential targets of CoREST-mediated gene alterations were those involved in juvenile hormone (JH)
Figure 1. Model for the Role of CoREST in the Reprogramming of C. floridanus Major Social Behavior TSA treatment of Major ants induces upregulation of CoREST and a subsequent repression of JHe/JHeh, which leads to elevated levels of JH and a switch to Minor-like caste foraging behavior.
Molecular Cell 77, January 16, 2020 ª 2019 Elsevier Inc. 205
Molecular Cell
Previews and ecdysone signaling. In fact, the JH protein levels are downregulated in Majors, while the JHe and JHeh enzymes that degrade JH are upregulated. This suggests that Major- and Minor-specific behaviors may be regulated by a balance between JH and ecdysone signaling as in other social hymenoptera (Glastad et al., 2019). Notably, the authors found that TSA leads to behavioral reprogramming of Majors by inducing a Minor-like program of gene expression in the brain (Figure 1). Given the role of CoREST in neuronal differentiation and gene expression (Andre´s et al., 1999), the authors tested the requirement for CoREST in reprogramming. CoREST knockdown blocked the TSA behavioral effects and many of the associated changes in gene expression, including that of the JHe and JHeh genes. CoREST and ttk, a homolog of the REST protein that recruits CoREST to repressed genes, were found to both be more highly expressed in natural Minor late pupal brains, an effect lost at later developmental stages. This might imply an early-stage ‘‘window’’ of Minor caste specification that accompanies a gene expression pattern and behavior in adulthood, despite the later caste normalization of CoREST levels. Similarly, in posteclosion Minors, a relatively transient period of epigenetic plasticity was linked to TSA-mediated reprogramming and a long-lasting foraging behavioral pheno-
206 Molecular Cell 77, January 16, 2020
type. This leads to the question as to what determines the opening and closing of such windows? In addition, while reprogramming can be induced using TSA, the question remains to what natural cue(s) may regulate this epigenetic switch? A clue may come from the fact that most social insects share fluid mouth-to-mouth with other individuals in their colony, in a behavior called trophallaxis. This allows these species to pass around food and to chemically communicate, both between adults and between adults and larvae (LeBoeuf et al., 2016). Trophallactic fluid contains juvenile hormones and other molecules that have been suggested to regulate colony development. LeBoeuf and colleagues also obtained evidence using transcriptomic and proteomic studies that some components of trophallactic fluid can be modulated by social environment and may influence larval development. Given the central role CoREST appears to play in programming social behavior, it will be interesting to determine the mechanisms by which natural influences may target this key regulator and which pathways lie upstream of CoREST.
decisions. Proc. Natl. Acad. Sci. USA 107, 16685–16690. Andre´s, M.E., Burger, C., Peral-Rubio, M.J., Battaglioli, E., Anderson, M.E., Grimes, J., Dallman, J., Ballas, N., and Mandel, G. (1999). CoREST: a functional corepressor required for regulation of neural-specific gene expression. Proc. Natl. Acad. Sci. USA 96, 9873–9878. Favreau, E., Martı´nez-Ruiz, C., Rodrigues Santiago, L., Hammond, R.L., and Wurm, Y. (2018). Genes and genomic processes underpinning the social lives of ants. Curr. Opin. Insect Sci. 25, 83–90. Glastad, K.M., Graham, R.J., Ju, L., Roessler, J., Brady, C.M., and Berger, S.L. (2019). Epigenetic Regulator CoREST Controls Social Behavior in Ants. Mol. Cell 77, S1097-2765(19)30790-7. Ho¨lldobler, B., and Wilson, E.O. (1990). The Ants (Cambridge, MA: Belknap Press of Harvard Univ Press). LeBoeuf, A.C., Waridel, P., Brent, C.S., Gonc¸alves, A.N., Menin, L., Ortiz, D., Riba-Grognuz, O., Koto, A., Soares, Z.G., Privman, E., et al. (2016). Oral transfer of chemical cues, growth proteins and hormones in social insects. eLife 5, e20375. Meier, K., and Brehm, A. (2014). Chromatin regulation: how complex does it get? Epigenetics 9, 1485–1495.
REFERENCES
Meier, K., Mathieu, E.-L., Finkernagel, F., Reuter, L.M., Scharfe, M., Doehlemann, G., Jarek, M., and Brehm, A. (2012). LINT, a novel dL(3)mbt-containing complex, represses malignant brain tumour signature genes. PLoS Genet. 8, e1002676.
Abrajano, J.J., Qureshi, I.A., Gokhan, S., Molero, A.E., Zheng, D., Bergman, A., and Mehler, M.F. (2010). Corepressor for element-1-silencing transcription factor preferentially mediates gene networks underlying neural stem cell fate
Simola, D.F., Ye, C., Mutti, N.S., Dolezal, K., Bonasio, R., Liebig, J., Reinberg, D., and Berger, S.L. (2013). A chromatin link to caste identity in the carpenter ant Camponotus floridanus. Genome Res. 23, 486–496.
Previews Ant-icipating Change: An Epigenetic Switch in Reprogramming the Social Lives of Ants Marilyn G. Pray-Grant1 and Patrick A. Grant1,* 1Department of Biochemistry and Molecular Genetics, University of Virginia School of Medicine, Charlottesville, VA 22908, USA *Correspondence: [email protected] https://doi.org/10.1016/j.molcel.2019.12.006
Glastad et al. (2019) describe a role for the neuronal CoREST corepressor and changes in juvenile hormone (JH) and ecdysone signaling during the reprogramming of social behavioral phenotypes in ants that are reflective of a natural mechanism differentiating ‘‘Major’’ and ‘‘Minor’’ worker ants.
Dimorphic ant species, including Camponotus floridanus, have two distinctly sized classes of workers, called ‘‘Major’’ and ‘‘Minor’’ castes, but a single common genome. A prominent manifestation of these castes, as in many eusocial insect species, is the division of distinct colony roles and social behaviors among worker groups (Ho¨lldobler and Wilson, 1990). Various genomic approaches have provided insights into the molecular mechanisms underlying natural caste differentiation in ants (Favreau et al., 2018). Notably, the larger Major workers of C. floridanus, which are tasked with defending the colony, can be reprogrammed to have a Minor-like foraging behavior following injection of the histone deacetylase (HDAC) inhibitor trichostatin A (TSA) into the brain (Simola et al., 2013). However, the intrinsic epigenetic mechanisms at play in the reprogramming of Majors and Minors have remained largely elusive. Here, Glastad et al. (2019) uncover an epigenetic switch underlying caste fate and demonstrate an important function for the CoREST corepressor in the induced reprogramming of transcriptional networks in Major worker ants that lead to a Minor-like foraging behavior. In this study, the injection of Major ants with TSA at 0 and 5 days post eclosion, defined as the emergence of the adult animal from the pupal case, led to increased foraging as soon as 1 day after injection and was maintained throughout the 10 days of study. Notably, the strongest induction of foraging occurred on day 5 at levels similar to that of natural Minors, but it was ineffective at a later stage post eclosion. An analysis of differentially expressed genes (DEGs) identified by
RNA-seq performed on the whole brain at the day 5 peak of induced foraging revealed many significant gene ontology terms associated with neuronal function and additionally those associated with epigenetic regulation. The deacetylase HDAC1 and repressor CoREST were among the top 20 significant DEGs with TSA treatment. RNA-seq and ChIP-seq approaches provided data consistent with a role for upregulated CoREST in specifically repressing Minor genes in reprogrammed Major ants and support a role for CoREST in mediating natural caste behavioral reprogramming. CoREST is a highly conserved protein that functions as an integral component of chromatin corepressor complexes that contain the histone deacetylase HDAC1/2 and demethylase activities (Meier and Brehm, 2014). It is an important regulator of neurogenesis and differentia-
tion from flies to mammals (Andre´s et al., 1999; Abrajano et al., 2010; Meier et al., 2012). Consistent with these observations, ChIP-seq for the acetylation mark on histone H3K27 (H3K27ac), a modification associated with gene activation, revealed higher H3K27ac at upregulated genes at 1 h following TSA treatment. Among the top genes, CoREST and HDAC1 showed increased proximal H3K27ac, and the authors noted that CoREST and HDAC1 upregulation may be a response to HDAC inhibition. Collectively, their studies suggest epigenomic changes in neuronal function mediated by CoREST occur during a peak period of reprogramming and that a transient and early point of gene reprogramming occurs during this window. Among the potential targets of CoREST-mediated gene alterations were those involved in juvenile hormone (JH)
Figure 1. Model for the Role of CoREST in the Reprogramming of C. floridanus Major Social Behavior TSA treatment of Major ants induces upregulation of CoREST and a subsequent repression of JHe/JHeh, which leads to elevated levels of JH and a switch to Minor-like caste foraging behavior.
Molecular Cell 77, January 16, 2020 ª 2019 Elsevier Inc. 205
Molecular Cell
Previews and ecdysone signaling. In fact, the JH protein levels are downregulated in Majors, while the JHe and JHeh enzymes that degrade JH are upregulated. This suggests that Major- and Minor-specific behaviors may be regulated by a balance between JH and ecdysone signaling as in other social hymenoptera (Glastad et al., 2019). Notably, the authors found that TSA leads to behavioral reprogramming of Majors by inducing a Minor-like program of gene expression in the brain (Figure 1). Given the role of CoREST in neuronal differentiation and gene expression (Andre´s et al., 1999), the authors tested the requirement for CoREST in reprogramming. CoREST knockdown blocked the TSA behavioral effects and many of the associated changes in gene expression, including that of the JHe and JHeh genes. CoREST and ttk, a homolog of the REST protein that recruits CoREST to repressed genes, were found to both be more highly expressed in natural Minor late pupal brains, an effect lost at later developmental stages. This might imply an early-stage ‘‘window’’ of Minor caste specification that accompanies a gene expression pattern and behavior in adulthood, despite the later caste normalization of CoREST levels. Similarly, in posteclosion Minors, a relatively transient period of epigenetic plasticity was linked to TSA-mediated reprogramming and a long-lasting foraging behavioral pheno-
206 Molecular Cell 77, January 16, 2020
type. This leads to the question as to what determines the opening and closing of such windows? In addition, while reprogramming can be induced using TSA, the question remains to what natural cue(s) may regulate this epigenetic switch? A clue may come from the fact that most social insects share fluid mouth-to-mouth with other individuals in their colony, in a behavior called trophallaxis. This allows these species to pass around food and to chemically communicate, both between adults and between adults and larvae (LeBoeuf et al., 2016). Trophallactic fluid contains juvenile hormones and other molecules that have been suggested to regulate colony development. LeBoeuf and colleagues also obtained evidence using transcriptomic and proteomic studies that some components of trophallactic fluid can be modulated by social environment and may influence larval development. Given the central role CoREST appears to play in programming social behavior, it will be interesting to determine the mechanisms by which natural influences may target this key regulator and which pathways lie upstream of CoREST.
decisions. Proc. Natl. Acad. Sci. USA 107, 16685–16690. Andre´s, M.E., Burger, C., Peral-Rubio, M.J., Battaglioli, E., Anderson, M.E., Grimes, J., Dallman, J., Ballas, N., and Mandel, G. (1999). CoREST: a functional corepressor required for regulation of neural-specific gene expression. Proc. Natl. Acad. Sci. USA 96, 9873–9878. Favreau, E., Martı´nez-Ruiz, C., Rodrigues Santiago, L., Hammond, R.L., and Wurm, Y. (2018). Genes and genomic processes underpinning the social lives of ants. Curr. Opin. Insect Sci. 25, 83–90. Glastad, K.M., Graham, R.J., Ju, L., Roessler, J., Brady, C.M., and Berger, S.L. (2019). Epigenetic Regulator CoREST Controls Social Behavior in Ants. Mol. Cell 77, S1097-2765(19)30790-7. Ho¨lldobler, B., and Wilson, E.O. (1990). The Ants (Cambridge, MA: Belknap Press of Harvard Univ Press). LeBoeuf, A.C., Waridel, P., Brent, C.S., Gonc¸alves, A.N., Menin, L., Ortiz, D., Riba-Grognuz, O., Koto, A., Soares, Z.G., Privman, E., et al. (2016). Oral transfer of chemical cues, growth proteins and hormones in social insects. eLife 5, e20375. Meier, K., and Brehm, A. (2014). Chromatin regulation: how complex does it get? Epigenetics 9, 1485–1495.
REFERENCES
Meier, K., Mathieu, E.-L., Finkernagel, F., Reuter, L.M., Scharfe, M., Doehlemann, G., Jarek, M., and Brehm, A. (2012). LINT, a novel dL(3)mbt-containing complex, represses malignant brain tumour signature genes. PLoS Genet. 8, e1002676.
Abrajano, J.J., Qureshi, I.A., Gokhan, S., Molero, A.E., Zheng, D., Bergman, A., and Mehler, M.F. (2010). Corepressor for element-1-silencing transcription factor preferentially mediates gene networks underlying neural stem cell fate
Simola, D.F., Ye, C., Mutti, N.S., Dolezal, K., Bonasio, R., Liebig, J., Reinberg, D., and Berger, S.L. (2013). A chromatin link to caste identity in the carpenter ant Camponotus floridanus. Genome Res. 23, 486–496.