Genetic analysis of novel loci in the kal-1 pathway

Abstract

The development of the nervous system is a complex process through which many cues and instructions need to be followed to form a functional unit. KAL1 regulates neuronal branching and cell migration through mechanisms still unidentified. When mutated, KAL 1 causes Kallmann syndrome (KS); a genetic disease with neuronal targeting and migration defects that manifest in the inability to smell and infertility. Misexpressing the homolog of KAL1 in C. elegans causes a highly penetrant axonal branching phenotype. In a forward genetic screen for modifiers of this phenotype we identified sax-7, dig-1, and mutants that affected genes involved in heparan sulfate (HS) biosynthesis and modification, namely the xylosyltransferase sqv-6, the HS-6-O-sulfotransferase hst-6, and the HS-3-O-sulfotransferase hst-3.2. In this work we show that sax-7, which is the C. elegans ortholog of the L1 cell adhesion molecule (L1CAM), is a branching factor that can act at the axon or at the dendrite by two independent pathways. At the dendrite level, it acts through mnr-1 and dma-1; while at the axonal level it acts through kal-1 and egl-15/FGFR. We further show that at the axonal level, kal-1, sax-7/L1CAM and egl-15/FGFR act in the same genetic pathway and form a complex, suggesting that SAX-7/L1CAM and EGL-15/FGFR may be acting as co-receptors for KAL-1. Characterization of the mutants affecting the HS biosynthesis and modification showed that HS with distinct modification patterns of different cellular origin are required cell non-autonomously for kal-1 function in vivo. Genetic analysis of mutants of the proteins that contain the HS chains, the heparan sulfate proteoglycan (HSPG), revealed that these act redundantly to mediate the kal-1 dependent branching. Our results support a complex cooperative interaction model in which HS form a three-dimensional scaffold necessary for the function of kal-1. Altogether, all the genes identified in this work represent candidate disease-causing genes for KS.