Dynamics of Mitochondrial Inheritance in C. elegans Primordial Germ Cells

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PROJECT SUMMARY Mitochondria are dynamic double-membraned organelles that contain an independent genome (mtDNA). Each mitochondrial gene is essential; however, mitochondrial DNA has a substantially higher rate of mutation when compared to the nuclear genome. As a result, mtDNA mutation is one of the most common sources of genetic disease in humans. Nonetheless, relatively few mutations have been established in mitochondrial genomes across generations. Genetic evidence suggests that selection against mutant mtDNAs occurs in the female germline, however, the mechanisms by which mtDNA selection occurs remain poorly understood. The goal of the proposed research is to advance our understanding of mitochondrial DNA inheritance by investigating the molecular and cellular mechanisms of mitochondrial inheritance in C. elegans PGCs, which provide an outstanding model for examining mitochondrial inheritance at cellular resolution. During late embryogenesis, C. elegans PGCs undergo a drastic remodeling process, whereby much of their cell mass and content is discarded. PGC remodeling occurs when PGCs form organelle-filled lobe-like protrusions, which are cut off and digested by adjacent endodermal cells. In the process, most PGC mitochondrial mass is lost. We have observed that mitochondria initially localize into PGC lobes, but a subset subsequently migrates back into PGCs prior to lobe removal. These are presumably the mitochondria that are inherited. I hypothesize that PGC lobe formation and removal is a mechanism whereby the number and/or quality of mitochondria/mtDNAs are regulated to ensure that fit mitochondria are passed on to the next generation. I will approach this hypothesis in two ways. First, I will empirically determine which mitochondria are inherited by PGCs and test the hypothesis that mitochondrial fission is required for proper mitochondrial segregation in PGCs using live imaging. (AIM1). Second, I will determine if C. elegans PGC lobe formation/removal regulates mtDNA/mitochondrial quality during inheritance. I will test the two primary hypotheses of mtDNA selection, the mitochondrial bottleneck and purifying selection, by quantifying mtDNA prior to and following lobe removal in wild type and mitochondrial mutant strains respectively. Then I will test the hypothesis that mitochondrial functionality drives mitochondrial selection in PGC lobes using conserved markers of mitochondrial health (AIM2). Mitochondrial mutations have particularly severe effects on embryonic development and male/female infertility resulting from spermatogenesis defects, premature aging, and developmental arrest. I anticipate that my findings will contribute to a deeper understanding of these mechanisms, and thus, will be essential for developing treatments of human mitochondrial disease and reproductive disorders in the future.