The process of cellular differentiation, from progenitor to fully mature and differentiated cell can be understood as a sequence of activation of one or several gene expression programs (Sokpor et al.,
9 2017). A key aspect in the activation or repression of genetic programs is the process of chromatin remodeling, by which chromatin can be opened into the accessible euchromatin or closed in the form of heterochromatin, respectively enabling or preventing the binding of TFs necessary for the initiation of transcription (Coskun et al., 2012; Hirabayashi and Gotoh, 2010; Juliandi et al., 2010;
Narayanan and Tuoc, 2014a; Ronan et al., 2013; Sokpor et al., 2017; Watson and Tsai, 2017). A crucial chromatin regulator is the ATP-dependent SWItch/Sucrose Non-Fermentable (mSWI/SNF), better known as BRG1/BRM-associated factor (BAF), a multi-subunit chromatin remodeling complex, henceforth referred to as BAF complex (Ho and Crabtree, 2010; Wang et al., 1996).
BAF complex from the biochemical side
Biochemically, the BAF complex is a conglomerate of 15 or more different protein subunits, comprising core and peripheral elements (Ho and Crabtree, 2010; Kadoch and Crabtree, 2015;
Lessard et al., 2007; Wu et al., 2007). The core subunits of the BAF complex include the ubiquitous BAF47 as well as two scaffolding subunits, BAF170 and/or BAF155, providing structural support for the assembly of the entire BAF complex (Mashtalir et al., 2018; Narayanan et al., 2015; Phelan et al., 1999; Sokpor et al., 2017). In addition, the BAF complex core also comprises one of two ATPase subunits, brahma related gene 1 (BRG1) or brahma (BRM) (Ho and Crabtree, 2010; Lessard et al., 2007; Wang et al., 1996). Peripheral subunits (also referred to as variant subunits) bind to the core subunits, and exhibit far greater diversity (Ho and Crabtree, 2010; Mashtalir et al., 2018). As a result of this diversity, it is believed that hundreds of possible permutations exist in assembling discrete BAF complexes (Narayanan and Tuoc, 2014a; Ronan et al., 2013; Sokpor et al., 2017). Because distinct subunits possess various protein domains (including but not limited to, DNA binding domains as well as bromo- and chromodomains, etc.), the properties of the BAF complex as a whole is a reflection of its particular subunit composition (Ho and Crabtree, 2010; Sokpor et al., 2017; Yoo and Crabtree, 2009). Accordingly, BAF complex interacts with TFs, coactivators, corepressors and histone modifiers in a manner that depends on its assembly (Ho and Crabtree, 2010; Narayanan et al., 2015;
Narayanan and Tuoc, 2014a; Wu, 2012). This structural and functional diversity is presumed to endow the BAF complex with the ability to regulate gene expression profiles in a cell lineage-specific manner (Ho and Crabtree, 2010; Sokpor et al., 2017). For example, in embryonic stems cells (ESCs), proliferation and pluripotency are associated with the presence of a BAF complex comprising BRG1, BAF60a/b, BAF155 and BAF250a but not BRM, BAF60c, BAF170 and BAF250b (Ho et al., 2009; Kaeser et al., 2008; Kidder et al., 2009; Sokpor et al., 2017). In another example, neuronal progenitors express BAF45A and BAF53A, which is replaced by BAF45B and BAF53B as progenitors leave mitosis and differentiate into neurons (Ho and Crabtree, 2010; Olave et al., 2002; Yoo et al., 2009).
Interestingly, the BAF53B subunit is a key regulator of activity-dependent dendritic morphogenesis
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in neurons (Wu et al., 2007). From a mechanistic point of view, it is plausible that all possible combinations of the BAF complex share a similar mode of action, namely the utilization of energy derived from the hydrolysis of ATP by its core subunits BRG1 or BRM to orchestrate structural changes within the chromatin by direct alteration of histones and nucleosomes (Cairns, 2007;
Hargreaves and Crabtree, 2011; Yoo and Crabtree, 2009).
The role of BAF complex in regulation of developmental processes
As described above, the diverse subunit composition of the BAF complex enables it to play various roles in a cell lineage dependent manner, with wide ranging implications for embryonic development and tissue formation (Lessard et al., 2007; Matsumoto et al., 2006; Nguyen et al., 2016; Tuoc et al., 2013a). Unsurprisingly, the BAF complex has also been implicated in numerous pathologies, such as developmental disorders and several forms of cancer (Alfert et al., 2019; Kadoch et al., 2017; Sokpor et al., 2017).
Over the last few years, several studies have investigated the involvement of BAF complex in neuronal development in more detail (Sokpor et al., 2017). The most telling illustration of its importance comes from phenotype analysis of conditional mutant mice in which BAF complex expression is entirely lost (Narayanan et al., 2015). This can be achieved by simultaneous deletion of BAF155 and BAF170, which causes the dissociation of the entire BAF complex, followed by ubiquitination and subsequent degradation of its constituent subunits, effectively obliterating any BAF-complex dependent remodeling of chromatin (Narayanan et al., 2015). When the loss of BAF complex is triggered in the telencephalon at E8.5, shortly before the onset of neurogenesis, the telencephalon entirely fails to develop (Narayanan et al., 2015). When activated at E10.5 in cortex, the deletion of BAF complex results in hypotrophic and underdeveloped cortical structures (Narayanan et al., 2015). These results highlight the importance of BAF complex integrity for proper brain development.
The role of BAF complex has also been studied with respect to cellular proliferation and differentiation during neurogenesis (Sokpor et al., 2017). Several reports indicate that NSCs (including NECs and RGCs) express a BAF complex variation whose composition is characteristic of their type, the NSC/neuronal progenitor - npBAF complex (Chen et al., 2012; Ho and Crabtree, 2010;
Lei et al., 2015; Li et al., 2010; Meng et al., 2018; Oh et al., 2008). Among the subunits composing this cell-type specific BAF complex, BRG1, BAF45A/D, BAF53A and BAF55A are known to be indispensable for NSCs proliferation (Lessard et al., 2007; Matsumoto et al., 2006; Staahl et al., 2013). Furthermore, the composition of the BAF complex in RGCs determines whether these cells engage in direct or indirect modes of neurogenesis, that is, by directly generating neurons or by producing intermediate progenitors (Tuoc et al., 2013b). Two of the core subunits have been
11 implicated in this phenomenon, BAF170 and BAF155 (Tuoc et al., 2013b). More precisely, direct neurogenesis is favoured when BAF170 is incorporated into the BAF complex, whereas the dominance of BAF155 promotes indirect neurogenesis (Tuoc et al., 2013b). In addition, the BAF complex has been implicated in various other neurodevelopmental processes, such as neuronal migration, dendritic morphogenesis, neuronal subtype determination and even adult neurogenesis (Lessard et al., 2007; Ninkovic et al., 2013; Olave et al., 2002; Petrik et al., 2015; Tuoc et al., 2017;
Wiegreffe et al., 2015; Woodworth et al., 2016; Wu et al., 2007).
Overall, these wide ranging effects of BAF complex on neural development clearly identify it as a potential molecular switch capable of orienting progenitors born at different places and time towards a common fate.
BAF complex in regulation of gliogenic switch and astrocyte differentiation
Although BAF complex is known to have a profound effect on neurogenesis, only few studies have investigated its involvement in astrogliogenesis. What studies exist have relied on deletion of a single BAF complex subunit, have investigated the effect of BAF complex outside of the forebrain, or during adult gliogenesis rather than in embryonic development. As a result the interpretation of these studies in the context of embryonic forebrain astrogliogenesis is haphazard at best.
For example, Matsumoto and colleagues have investigated the effects of Nestin-Cre driven deletion of BRG1 on embryonic astrogliogenesis in the cortex and spinal cord (Matsumoto et al., 2006). The loss of BRG1 caused an apparent loss of astrogliogenesis. This was possibly due to a precocious exhaustion of the progenitor pool. Another study examined the role of BRG1 deletion in adult neurogenesis. Surprisingly the study found that deletion of BRG1 in adult NSCs abolishes production of neurons and directs differentiation towards astroglia (Ninkovic et al., 2013). These contradictory findings raise the possibly that BRG1 plays different roles in embryonic versus adult astrogliogenesis.
Finally, a previous study from our laboratory (Tuoc et al., 2017) investigated the role of BAF170 in adult neurogenesis and found that deletion of this subunit from adult NSCs in dentate gyrus (DG) leads to the depletion of their pool, and promotes terminal differentiation towards astrocytes.
Although these studies are insufficient to establish what role the BAF complex as a whole plays in embryonic astrogliogenesis, some hints can be found elsewhere in the literature. For example, it has been suggested that BAF complex interacts with the nuclear corepressor complex (N-CoR) through some of its subunits (Underhill et al., 2000). N-CoR was shown to repress astrogliogenesis: indeed, NSCs deprived of N-CoR are unable to proliferate in vitro, and their differentiation to GFAP-expressing astroglial cells is enhanced (Hermanson et al., 2002). Furthermore, another chromatin remodeler, the polycomb repressive complex (PcR) has been shown to repress neurogenesis, and thus indirectly enabling astrogliogenesis (Hirabayashi et al., 2009). Interestingly, it is known that BAF
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complex can evict PcR from its binding site on chromatin, through which it could potentially antagonize its repression of neurogenesis and putative enhancement of astrogliogenesis (Hirabayashi et al., 2009; Kadoch et al., 2017).
Overall, the existing literature on the involvement of BAF complex on astrogliogenesis is scarce and insufficient to come to a satisfying conclusion. However, the importance of BAF complex on neurogenesis, and the evidence described here led us to anticipate that BAF complex potentially also exerts a powerful regulatory effect on astrogliogenesis.