Primary congenital hydrocephalus (CH) is a progressive, life-threatening condition that arises from abnormal buildup of cerebrospinal fluid (CSF) in the ventricles of the brain. If untreated, intracranial pressure due to CSF accumulation can result in brain damage and death. While the pathophysiology underlying CH is undefined, the most common cause is loss or impaired function of L1, a transmembrane immunoglobulin cell adhesion molecule with well-defined roles in neuronal migration, synaptic regulation, and axon guidance. It is not known how impaired L1 function causes CH, but the incomplete penetrance and variable expressivity of L1-associated CH within families is indicative of complex genetic interactions. Uncovering L1 functions and mechanisms of action are key to developing novel therapies and preventative measures for CH. The C. elegans L1 homologue, SAX-7, has well-characterized roles in nervous system development and maintenance. Recently, we identified a genetic interaction between SAX-7 and the Ras/MAPK signaling pathway, uncovering novel roles for SAX-7 in fluid regulation and vulval development.
sax-7 null animals with hyperactive Ras/LET-60(gf) display a synthetic phenotype of progressive fluid buildup that often results in mid-to-late larval lethality. Dying animals often show ruptures at the excretory pore, suggestive of increased internal pressure. Consistent with internal fluid build-up,
sax-7 let-60(gf) animals also show hypersensitivity to hypotonic solutions, their bodies tending to burst shortly after immersion as compared to
sax-7 and
let-60(gf) single mutants. Our conditional knockout experiments reveal a neuronal requirement for SAX-7 in this process. While
sax-7 null animals do not exhibit vulva abnormalities, loss of SAX-7 synergistically enhances the multiple vulva phenotype (Muv) exhibited by
let-60(gf) animals, revealing a previously uncharacterized role for SAX-7 in vulva development. Both the
sax-7 let-60(gf) fluid buildup and Muv phenotypes can be suppressed by knocking out KSR-1, a key protein scaffold that promotes MAPK signaling, consistent with elevated MAPK activity as underlying the
sax-7 let-60(gf) synergistic phenotypes. Interestingly, CH is sometimes presented in a cluster of diseases known as "Rasopathies," characterized by elevated Ras/MAPK signaling. Based on these findings, we hypothesize that SAX-7 and Ras/MAPK signaling act synergistically in fluid homeostasis and vulval development.