Mammalian germ cells are determined after PGC colonization of the nascent gonad

Abstract

Mammalian primordial germ cells (PGCs) are induced in the embryonic epiblast, before migrating to the nascent gonads. In fish, frogs, and birds, the germline segregates even earlier, through the action of maternally inherited germ plasm. Across vertebrates, migrating PGCs retain a broad developmental potential, regardless of whether they were induced or maternally segregated. In mammals, this potential is indicated by expression of pluripotency factors, and the ability to generate teratomas and pluripotent cell lines. How the germline loses this developmental potential remains unknown. Our genome-wide analyses of embryonic human and mouse germlines reveal a conserved transcriptional program, initiated in PGCs after gonadal colonization, that differentiates germ cells from their germline precursors and from somatic lineages. Through genetic studies in mice and pigs, we demonstrate that one such gonad-induced factor, the RNA-binding protein DAZL, is necessary in vivo to restrict the developmental potential of the germline; DAZL’s absence prolongs expression of aNanogpluripotency reporter, facilitates derivation of pluripotent cell lines, and causes spontaneous gonadal teratomas. Based on these observations in humans, mice, and pigs, we propose that germ cells are determined after gonadal colonization in mammals. We suggest that germ cell determination was induced late in embryogenesis—after organogenesis has begun—in the common ancestor of all vertebrates, as in modern mammals, where this transition is induced by somatic cells of the gonad. We suggest that failure of this process of germ cell determination likely accounts for the origin of human testis cancer.

Testes of DAZL null sheep lack spermatogonia and maintain normal somatic cells

“…Likewise, DAZL mutations in humans were associated with an increased testicular teratoma risk 48 . In contrast to pigs, humans and 129 mice, no teratomas were observed in several other Dazl -/mouse strains 17 55 . This is a considerable limitation that has so far not been resolved in livestock.…”

“…This leads the aberrant germ cells down an apoptotic pathway, producing adult mice of both sexes that entirely lack germ cells 14,15 . In livestock, DAZL has been targeted in pigs, confirming complete ablation of the germline 16,17 .…”

“…These data indicate that the somatic cells of the testis may produce a low-level background germ cell transcript signature or that there may be some rare germ cells remaining at the neonatal stage. The notion that tumorigenic germ cells may remain in DAZL -/gonads is supported by the observation that DAZL -/female pigs frequently form teratomas, which are absent from DAZL +/+ wild-type controls 17 . The rate of teratoma formation was also significantly elevated in DAZL -/-129S mice 17 , which naturally show incidences of spontaneous testicular teratomas between 1-3% 47 .…”

“…The notion that tumorigenic germ cells may remain in DAZL -/gonads is supported by the observation that DAZL -/female pigs frequently form teratomas, which are absent from DAZL +/+ wild-type controls 17 . The rate of teratoma formation was also significantly elevated in DAZL -/-129S mice 17 , which naturally show incidences of spontaneous testicular teratomas between 1-3% 47 . Likewise, DAZL mutations in humans were associated with an increased testicular teratoma risk 48 .…”

“…This was confirmed by in situ hybridisation colocalising DAZL, NANOS2 and DDX4 mRNA to germ cells in mice 10,45,46 . Dazl-deficient germ cells retain gene expression characteristic of migrating PGCs, suggesting that DAZL is involved in committing pluripotent PGCs to the germ cell fate 13,14,17 . Thus, we also surveyed expression of pluripotency-related genes LIN28, SALL4 and NANOG in DAZL -/germ cells.…”