A role of the LIN-12/Notch signaling pathway in diversifying the non-striated egg-laying muscles in C. elegans

• An RNAi screen identified novel factors critical for sex muscle development.
• LIN-12/Notch signaling first specifies the fate of the sex muscle precursors, the sex myoblasts (SMs).
• LIN-12/Notch signaling then specifies the type II vulval muscles (VM2) required for egg-laying.
• The LAG-2/DSL ligand likely functions in the anchor cell (AC) to induce VM2 specification.

The proper formation and function of an organ is dependent on the specification and integration of multiple cell types and tissues. An example of this is the Caenorhabditis elegans hermaphrodite egg-laying system, which requires coordination between the vulva, uterus, neurons, and musculature. While the genetic constituents of the first three components have been well studied, little is known about the molecular mechanisms underlying the specification of the egg-laying musculature. The egg-laying muscles are non-striated in nature and consist of sixteen cells, four each of type I and type II vulval muscles and uterine muscles. These 16 non-striated muscles exhibit distinct morphology, location, synaptic connectivity and function. Using an RNAi screen targeting the putative transcription factors in the C. elegans genome, we identified a number of novel factors important for the diversification of these different types of egg-laying muscles. In particular, we found that RNAi knockdown of lag-1, which encodes the sole C. elegans ortholog of the transcription factor CSL (CBF1, Suppressor of Hairless, LAG-1), an effector of the LIN-12/Notch pathway, led to the production of extra type I vulval muscles. Similar phenotypes were also observed in animals with down-regulation of the Notch receptor LIN-12 and its DSL (Delta, Serrate, LAG-2) ligand LAG-2. The extra type I vulval muscles in animals with reduced LIN-12/Notch signaling resulted from a cell fate transformation of type II vulval muscles to type I vulval muscles. We showed that LIN-12/Notch was activated in the undifferentiated type II vulval muscle cells by LAG-2/DSL that is likely produced by the anchor cell (AC). Our findings provide additional evidence highlighting the roles of LIN-12/Notch signaling in coordinating the formation of various components of the functional C. elegans egg-laying system. We also identify multiple new factors that play critical roles in the proper specification of the different types of egg-laying muscles.