A Physiological Approach to the Study of Pseudopod Extension in the Amoeboid Sperm of the Nematode Caenorhabditis elegans
Fertilization is the process in which both the spermatozoon and the oocyte must undergo a myriad of physiological changes resulting in successful fusion to produce progeny. Spermatozoa are highly motile cells that must reach the oocyte in the environment where fertilization takes place and in this regard the study of acquisition of motility in sperm cells is important to understand sperm-egg interactions. In the case of the nematode Caenorhabditis elegans, fertilization takes place in the hermaphrodite reproductive tract where sessile spermatids must extend a pseudopod to become motile. The process of pseudopod extension initiates with rearrangement of the plasma membrane and culminates with the extension of a pseudopod by an MSP-based cytoskeleton. Thus, successful fertilization depends on the initiation of a signaling pathway that acts on proteins present on the plasma membrane and has as an ultimate target the MSP filament. A genetic model of pseudopod extension suggests the interaction of proteins from the SPE group in a multicomponent complex that regulates the timing for pseudopod extension. This complex of proteins resembles membrane microdomains, regions of the plasma membrane enriched in cholesterol and sphingolipids, where membrane and cytosolic proteins from a common signaling pathway interact. In the present work, the presence and involvement of membrane microdomains in C. elegans spermatids is tested. This hypothesis is approached by the use of biochemical assays and microscopy techniques that give useful information on the physiological process of motility acquisition. The results suggest that male-derived spermatids are subject to physiological changes that prevent pseudopod extension prior to ejaculation and that membrane microdomains are present in these cells and involved in successful fertilization of the oocyte through the extension of a pseudopod. Using an integrative approach, these results revealed that membrane dynamics are responsible for the signaling cascade that triggers acquisition of motility, complementing the genetic model of pseudopod extension.