The SWI/SNF Chromatin Remodeling Complex Selectively Affects Multiple Aspects of Serotonergic Neuron Differentiation

Abstract

Regulatory programs that control the specification of serotonergic neurons have been investigated by genetic mutant screens in the nematode Caenorhabditis elegans. Loss of a previously uncloned gene, ham-3, affects migration and serotonin antibody staining of the hermaphrodite-specific neuron (HSN) pair. We characterize these defects here in more detail, showing that the defects in serotonin antibody staining are paralleled by a loss of the transcription of all genes involved in serotonin synthesis and transport. This loss is specific to the HSN class as other serotonergic neurons appear to differentiate normally in ham-3 null mutants. Besides failing to migrate appropriately, the HSNs also display axon pathfinding defects in ham-3 mutants. However, the HSNs are still generated and express a subset of their terminal differentiation features in ham-3 null mutants, demonstrating that ham-3 is a specific regulator of select features of the HSNs. We show that ham-3 codes for the C. elegans ortholog of human BAF60, Drosophila Bap60, and yeast Swp73/Rsc6, which are subunits of the yeast SWI/SNF and vertebrate BAF chromatin remodeling complex. We show that the effect of ham-3 on serotonergic fate can be explained by ham-3 regulating the expression of the Spalt/SALL-type Zn finger transcription factor sem-4, a previously identified regulator of serotonin expression in HSNs and of the ham-2 Zn transcription factor, a previously identified regulator of HSN migration and axon outgrowth. Our findings provide the first evidence for the involvement of the BAF complex in the acquisition of terminal neuronal identity and constitute genetic proof by germline knockout that a BAF complex component can have cell-type-specific roles during development. N EURONS that express the neurotransmitter serotonin fulfill a number of critical functions in all nervous systems examined to date. In vertebrates, serotonergic neurons modulate anxiety, cognitive processes, mood, body temperature, sleep, sexual behavior, appetite, and metabolism, and their dysfunction has been connected to a variety of human disorders (Muller and Jacobs 2010). In the hermaphroditic Caenorhabditis elegans nervous system, serotonin also controls a number of distinct behaviors (Schafer 2005;Chase and Koelle 2007). Serotonin is utilized as a neurotransmitter under normal conditions by seven neuron types, the sensory neuron ADF; the interneurons AIM and RIH; the motor neurons hermaphroditespecific neurons (HSNs), the ventral cord motor neurons VC4 and VC5; and the neurosecretory NSM cells (Schafer 2005;Chase and Koelle 2007). Under stress conditions, serotonin is used by an additional neuron type, ASG (Pocock and Hobert 2010). One of the serotonergic neurons, the hermaphrodite-specific motor neuron, HSN, utilizes serotonin to signal to vulval muscles to control egg-laying behavior (Schafer 2005). Unlike most other neurons in the C. elegans nervous system, the two bilaterally symmetric HSNs undergo long-range migration. After terminating migration, they extend th...

SWI/SNF chromatin remodeling regulates alcohol response behaviors in Caenorhabditis elegans and is associated with alcohol dependence in humans

“…Thirteen C. elegans SWI/SNF genes have been characterized to date, and they include homologs of all major human SWI/SNF subunits (26)(27)(28)(29)(30)(31). Mammalian SWI/SNF complexes have been biochemically purified from different tissues and found to contain unique combinations of subunits depending upon cell type and developmental stage (32).…”

“…The SWI/SNF complex is important for embryonic and larval development (26,(28)(29)(30)(31); therefore SWI/SNF may be required during development and/or in adulthood for AFT. To distinguish between these possibilities, we took advantage of a temperaturesensitive allele of swsn-1 (28).…”

Functional Interplay of Two Paralogs Encoding SWI/SNF Chromatin-Remodeling Accessory Subunits DuringCaenorhabditis elegansDevelopment

“…Traditionally, SWI/SNF complexes have been classified into two subclasses, named BAF/BAP or PBAF/PBAP, depending on their signature subunits ( Figure S1A). The human accessory subunits BAF60a/SMARCD1, BAF60b/SMARCD2, and BAF60c/SMARCD3 and their worm homologs HAM-3 and SWSN-2.2 derive from the same evolutionary ancestor and are expected to belong to both subclasses of complexes (Shibata et al 2012;Weinberg et al 2013) ( Figure S1 and Figure S2). The three human BAF60 proteins, which present 60% of similarity in their amino acid sequences, are mutually exclusive in a given SWI/SNF complex displaying distinct expression patterns and functions in humans (Oh et al 2008;Puri and Mercola 2012;Jordan et al 2013;Watanabe et al 2014 ham-3 and swsn-2.2 are paralog genes with 67% similarity at the amino acid sequence level ( Figure S3).…”

“…Others and we have found these three modes of interaction between ham-3 and swsn-2.2. While swsn-2.2 has a unique function in early embryonic cell divisions, only ham-3 affects the development of the hermaphrodite specific neurons (HSNs) (Desai et al 1988;Weinberg et al 2013). Since the HSNs stimulate the vulva muscle cells to extrude embryos, the high penetrance of the Egl phenotype in animals where ham-3 was inactivated can be explained by this neuronal defect.…”

“…ham-3 and swsn-2.2 exhibit RNA interference (RNAi) phenotypes in vulva development and fertility, present a synthetic genetic interaction with lin-35/Rb (Cui et al 2004;Ceron et al 2007), and are implicated in somatic gonad development (Large and Mathies 2014). In addition to these common functions, ham-3 has been described to be involved in neuronal specification and in the transcriptional regulation of specific microRNAs (Hayes et al 2011;Weinberg et al 2013). Differently from ham-3, loss of swsn-2.2 produces Emb (embryonic lethality) and Psa (phasmid socket absent; specific cells acquiring hypodermal fate instead of the neuronal fate) phenotypes at high penetrance (Sawa et al 2000;Large and Mathies 2014).…”