Meiotic onset is reliant on spatial distribution but independent of germ cell number in the mouse ovary.

Mouse ovarian germ cells enter meiosis in a wave that propagates from anterior to posterior, but little is known about contribution of germ cells to initiation or propagation of meiosis. In a Ror2 mutant with diminished germ cell number and migration, we find that overall timing of meiotic initiation is delayed at the population level. We use chemotherapeutic depletion to exclude a profoundly reduced number of germ cells as a cause for meiotic delay. We rule out sex reversal or failure to specify somatic support cells as contributors to the meiotic phenotype. Instead, we find that anomalies in the distribution of germ cells as well as gonad shape in mutants contribute to aberrant initiation of meiosis. Our analysis supports a model of meiotic initiation via diffusible signal(s), excludes a role for germ cells in commencing the meiotic wave and furnishes the first phenotypic demonstration of the wave of meiotic entry. Finally, our studies underscore the importance of considering germ cell migration defects while studying meiosis to discern secondary effects resulting from positioning versus primary meiotic entry phenotypes.

Primordial germ cells and gastrointestinal stromal tumors respond distinctly to a cKit overactivating allele.

KitL, via its receptor cKit, supports primordial germ cell (PGC) growth, survival, migration and reprogramming to pluripotent embryonic germ cells (EGCs). However, the signaling downstream of KitL and its regulation in PGCs remain unclear. A constitutively activating mutation, cKit(V558Δ), causes gain-of-function phenotypes in mast cells and intestines, and gastrointestinal stromal tumors (GISTs) when heterozygous. Unexpectedly, we find that PGC growth is not significantly affected in cKit(V558Δ) heterozygotes, whereas in homozygotes, increased apoptosis and inefficient migration lead to the depletion of PGCs. Through genetic studies, we reveal that this oncogenic cKit allele exhibits loss-of-function behavior in PGCs distinct from that in GIST development. Examination of downstream signaling in GISTs from cKit(V558Δ/+) mice confirmed hyperphosphorylation of AKT and ERK, but both remain unperturbed in cKit(V558Δ/+) PGCs and EGCs. In contrast, we find reduced activation of ERK1/2 and JNK1 in cKit(V558Δ) homozygous PGCs and EGCs. Inhibiting JNK, though not ERK1/2, increased apoptosis of wild-type PGCs, but did not further affect the already elevated apoptosis of cKit(V558Δ)(/V558Δ) PGCs. These results demonstrate a cell-context-dependent response to the cKit(V558Δ) mutation. We propose that AKT overload protection and JNK-mediated survival comprise PGC-specific mechanisms for regulating cKit signaling.

Ror2 enhances polarity and directional migration of primordial germ cells.

The trafficking of primordial germ cells (PGCs) across multiple embryonic structures to the nascent gonads ensures the transmission of genetic information to the next generation through the gametes, yet our understanding of the mechanisms underlying PGC migration remains incomplete. Here we identify a role for the receptor tyrosine kinase-like protein Ror2 in PGC development. In a Ror2 mouse mutant we isolated in a genetic screen, PGC migration and survival are dysregulated, resulting in a diminished number of PGCs in the embryonic gonad. A similar phenotype in Wnt5a mutants suggests that Wnt5a acts as a ligand to Ror2 in PGCs, although we do not find evidence that WNT5A functions as a PGC chemoattractant. We show that cultured PGCs undergo polarization, elongation, and reorientation in response to the chemotactic factor SCF (secreted KitL), whereas Ror2 PGCs are deficient in these SCF-induced responses. In the embryo, migratory PGCs exhibit a similar elongated geometry, whereas their counterparts in Ror2 mutants are round. The protein distribution of ROR2 within PGCs is asymmetric, both in vitro and in vivo; however, this asymmetry is lost in Ror2 mutants. Together these results indicate that Ror2 acts autonomously to permit the polarized response of PGCs to KitL. We propose a model by which Wnt5a potentiates PGC chemotaxis toward secreted KitL by redistribution of Ror2 within the cell.