Gone Without the WNT: A Requirement for WNT5A in Germ Cell Migration and Testis Development

Abstract

The article by Chawengsaksophak et al. in this issue of Biology of Reproduction identifies the WNT5A signaling molecule as an important component that guides PGCs to their destination.

In most mammals, sex is determined genetically by inheritance of fathers' Y chromosome by males, but not females, at the time of conception. Despite this fundamental commitment, the sexual fate remains temporarily concealed, and the gonads develop in both sexes as bipotential primordia well into midgestation. Testis differentiation is triggered by the expression of the Sry gene from the Y chromosome that activates its downstream target Sox9 and tips the balance of the preexisting milieu of regulatory molecules toward the male fate. In the absence of Sry/Sox9, the bipotential gonad halts the testis-determining preparations and instead up-regulates the genes of the ovarian developmental program. In contrast to the testis that relies on a single Sry master switch to ignite its cellular differentiation, it is believed that ovarian determination is more of a collective effort. The two converging pathways that figure most prominently in the ovarian differentiation are regulated by the canonical Rspo1-Wnt4/β-catenin signaling and the FOXL2 transcription factor (reviewed in [1–3]).

Differentiation of the gonadal somatic precursor cells into Sertoli or granulosa cells instructs the rest of the gonadal inhabitants to take on the appropriate sexual identity. The primordial germ cells (PGCs) that independently give rise to equally important constituents of gonads, sperm, and eggs originate far apart from their permanent home (developing somatic primordia) and travel across the embryo to become reproductively competent. Not surprisingly, the cumbersome nature of this migratory process mandates that the voyage of the PGC is under precise regulation to ensure that they navigate a complex path through the continuously changing environment of the developing tissues. The torturous travel of PGCs is thought to be guided by a precisely formulated blend of attractive and repulsive signals. The PGC migratory process is tightly correlated with the dynamic expression of regulatory cytokines at the genital ridges and in the surrounding mesenchyme. This cytokine expression pattern dictates both the intermediate and the final targets for PGCs' migration. A separate important signaling system controls proliferation and survival of the travelers (reviewed in [4]).

The article by Chawengsaksophak et al. in this issue of Biology of Reproduction identifies the WNT5A signaling molecule as an important component that guides PGCs to their destination [5]. It is generally acknowledged that the canonical Wnt pathway (known to play a central role in ovarian determination) is not active in the developing testis (reviewed in [6]). Indeed, WNT5A acts in development mostly through engaging the noncanonical pathways via the orphan tyrosine kinase receptor, ROR2. The noncanonical branches of Wnt signaling that include the cell polarity and calcium-dependent signaling pathways remain mechanistically less understood and explored, in part because of the technical difficulty of identifying them in vivo. By definition, canonical WNT signaling converges on nuclear β-catenin complex, and reporter mouse strains (TOPGAL or Axin2LacZ) allow for a fairly straightforward detection of the in vivo activity. Such definitive assays are not available for noncanonical branches of WNT signaling, and thus their involvement often comes to light only because of serendipitous circumstances (reviewed in [7, 8]).

However, this time the hint for the WNT5A link came from a study that intentionally compared embryos with genetically induced deficiencies in the components of the noncanonical Wnt signaling pathway. Several of these mutants (including WNT5A null embryos) exhibited similar phenotypes that included abnormal gubernacular development and transabdominal testicular descent [9]. The publication in this issue of Biology of Reproduction extends these previous findings and strengthens the recently established connection between WNT5A and Robinow syndrome [10], a rare genetic disorder also associated with mutations in ROR2. Robinow syndrome patients have a broad spectrum of phenotypes, including shortened limbs (dwarfism), craniofacial dysmorphology (“fetal face”), and genital hypoplasia. Interestingly, mouse loss-of-function phenotypes of WNT5A and ROR2 are quite similar, and both grossly resemble those observed in Robinow syndrome (reviewed in [11]). Chawengsaksophak et al. report here that WNT5A exhibits an unusual dorso-ventral (strongest in the epithelium along the ventral side) XY-specific interstitial expression shortly after testis determination. The authors compellingly demonstrate that the proliferation defect in WNT5A null mutant is responsible for testicular hypoplasia and conclude that WNT5A is required for normal proliferation of the somatic component of the testis (Fig. 1). The presentation of the male Robinow syndrome patients with general hypoplasia of the reproductive organs and frequent cryptorchidism [12] closely matches the phenotypes described for the WNT5A-null XY embryos. Given that the level of expression for a Insl3 (a gene that is a usual suspect in cases of gubernacular abnormalities) did not change, the authors concluded that WNT5A acts either independently or downstream of INSL3 during this process [5].

FIG. 1.

WNT5A controls several steps (red arrows) in gonadal development. Initially, WNT5A is required for the PGCs (yellow) migration into bipotential gonad. Upon sex determination, WNT5A controls proliferation of the somatic component of the testis and testicular descent. An unusual WNT5A expression pattern in the interstitial compartment of the testis is shown.

WNT5A is known to regulate a wide range of developmental processes that rely on cellular migration (reviewed in [13]). In this respect, the authors uncovered a significant reduction in germ cell numbers in both sexes as a result of abnormal migration of PGCs into gonadal primordia. The work by Chawengsaksophak et al. convincingly shows that WNT5A controls the migration of the PGCs into the gonadal anlagen (Fig. 1): instead of colonizing gonads, most of the germ cells in WNT5A null mutants either prematurely terminate their migration or wander off course to never complete the journey. Given that the germ cell phenotype precedes sex determination (and hence male-specific increase in gonadal WNT5A), guiding PGCs to their destination has to rely on a bipotential gonad or some other source of WNT5A common to both genetic sexes. Subsequent work will have to identify the origin of WNT5A expression that controls germ cell migration process. It will be also important to know whether germ cells engage ROR2 to respond to somatic WNT5A signal.