Sexually Dimorphic Differentiation of a C. elegans Hub Neuron Is Cell Autonomously Controlled by a Conserved Transcription Factor

Abstract

SUMMARY Functional and anatomical sexual dimorphisms in the brain are either the result of cells that are generated only in one sex, or a manifestation of sex-specific differentiation of neurons present in both sexes. The PHC neurons of the nematode C. elegans differentiate in a strikingly sex-specific manner. While in hermaphrodites the PHC neurons display a canonical pattern of synaptic connectivity similar to that of other sensory neurons, PHC differentiates into a densely connected hub sensory/interneuron in males, integrating a large number of male-specific synaptic inputs and conveying them to both male-specific and sex-shared circuitry. We show that the differentiation into such a hub neuron involves the sex-specific scaling of several components of the synaptic vesicle machinery, including the vesicular glutamate transporter eat-4/VGLUT, induction of neuropeptide expression, changes in axonal projection morphology and a switch in neuronal function. We demonstrate that these molecular and anatomical remodeling events are controlled cell-autonomously by the phylogenetically conserved Doublesex homolog dmd-3, which is both required and sufficient for sex-specific PHC differentiation. Cellular specificity of dmd-3 action is ensured by its collaboration with non-sex specific terminal selector-type transcription factors whereas sex-specificity of dmd-3 action is ensured by the hermaphrodite-specific, transcriptional master regulator of hermaphroditic cell identity, tra-1, which represses transcription of dmd-3 in hermaphrodite PHC. Taken together, our studies provide mechanistic insights into how neurons are specified in a sexually dimorphic manner.

Timing mechanism of sexually dimorphic nervous system differentiation

“…We found that like the male-specific, neuronal expression of lin-29a, male-specific SMD expression of mab-3 is precisely timed to commence at the L4 stage (Fig.6C). Similarly, the gfp-tagged dmd-3 locus is expressed in a sex-shared neuron class, the PHC neurons (Fig.6D), where dmd-3 acts to control the sexually dimorphic remodeling of PHC (Mason et al, 2008;Serrano-Saiz et al, 2017a). dmd-3::gfp expression in PHC also commences at the L4 stage (Fig.6D), at the time when PHC neurons remodel in a male-specific manner (Serrano-Saiz et al, 2017a).…”

“…To assess whether lin-29a acts cell-autonomously in AIM to control its neurotransmitter switch, we expressed lin-29a under an AIM-specific promoter (Serrano-Saiz et al, 2017a). This driver is not only active in both sexes, but it is already active in embryos and early larval stages, hence allowing us to assess three key issues: (1) whether LIN-29A acts cell-autonomously, (2) whether LIN-29A can induce the neurotransmitter switch prematurely in males and (3) whether LIN-29A can ectopically induce the AIM neurotransmitter switch in neurons of hermaphrodites.…”

“…The second type of sexual dimorphisms in the C. elegans nervous system are observed within embryonically born, postmitotic neurons that are present in both sexes but adopt sex-specific features during sexual maturation in later larval stages (Portman, 2017). These features include sexually dimorphic synaptic wiring patterns (Jarrell et al, 2012) and sex-specific expression of signaling molecules, such as neurotransmitters, neuropeptides and chemoreceptors (Hilbert and Kim, 2017;Pereira et al, 2015;Ryan et al, 2014;Serrano-Saiz et al, 2017a;Serrano-Saiz et al, 2017b). Most if not all of these sexually dimorphic features of sex-shared neurons also come into existence at the fourth larval stage.…”

Molecular mechanisms underlying sexual differentiation of the nervous system

“…Work by Hobert’s group on the sex-shared PHC neurons provides new molecular insights into repurposing of shared neurons [17••]. PHC neurons appear to be typical sensory neurons in hermaphrodites, with connections to a limited number of shared neurons, that are required for response to harsh touch.…”

fruitlessfunctions downstream ofdoublesexto promote sexual dimorphism of the gonad stem cell niche

“…The Doublesex/Mab-3 Related Transcription Factors (DMRTs) act downstream of sex determination and play an evolutionarily conserved role to establish and maintain sexual dimorphism in the gonad [1]. Meanwhile, sexual dimorphism in other tissues such as the brain is controlled, to varying degrees in different animals, through autonomous control by the sex determination and non-autonomous signaling from the gonads [2,3]. In many invertebrate species, another sex-determination gene fruitless (fru), which encodes several BTB-Zinc finger transcription factors, plays a central role in controlling sexual orientation and courtship behaviors [4].…”