Paracrine-mediated apoptosis in reproductive tract development

In mammalian development, the signaling pathways that couple extracellular death signals with the apoptotic machinery are still poorly understood. We chose to examine Müllerian duct regression in the developing reproductive tract as a possible model of apoptosis during morphogenesis. The TGFbeta-like hormone, Müllerian inhibiting substance (MIS), initiates regression of the Müllerian duct or female reproductive tract anlagen; this event is essential for proper male sexual differentiation and occurs between embryonic days (E) 14 and 17 in the rat. Here, we show that apoptosis occurs during Müllerian duct regression in male embryos beginning at E15. Female Müllerian ducts exposed to MIS also exhibited prominent apoptosis within 13 h, which was blocked by a caspase inhibitor. In both males and females the MIS type-II receptor is expressed exclusively in the mesenchymal cell layer surrounding the duct, whereas apoptotic cells localize to the epithelium. In addition, tissue recombination experiments provide evidence that MIS does not act directly on the epithelium to induce apoptosis. Based on these data, we suggest that MIS triggers cell death by altering mesenchymal-epithelial interactions.     

Involvement of a matrix metalloproteinase in MIS-induced cell death during urogenital development

Programmed cell death of the Müllerian duct eliminates the primitive female reproductive tract during normal male sexual differentiation. Müllerian inhibiting substance (MIS or AMH) triggers regression by propagating a BMP-like signaling pathway in the Müllerian mesenchyme that culminates in apoptosis of the Müllerian duct epithelium. Presently, the paracrine signal(s) used in this developmental event are undefined. We have identified a member of the matrix metalloproteinase gene family, Mmp2, as one of the first candidate target genes downstream of the MIS cascade to function as a paracrine death factor in Müllerian duct regression. Consistent with a role in regression, Mmp2 expression was significantly elevated in male but not female Müllerian duct mesenchyme. Furthermore, this sexually dimorphic expression of Mmp2 was extinguished in mice lacking the MIS ligand, suggesting strongly that Mmp2 expression is regulated by MIS signaling. Using rat organ genital ridge organ cultures, we found that inhibition of MMP2 activity prevented MIS-induced regression, whereas activation of MMP2 promoted ligand-independent Müllerian duct regression. Finally, MMP2 antisense experiments resulted in partial blockage of Müllerian duct regression. Based on our findings, we propose that similar to other developmental programs where selective elimination or remodeling of tissues occurs, localized induction of extracellular proteinases is critical for normal male urogenital development.     

The serine/threonine transmembrane receptor ALK2 mediates Müllerian inhibiting substance signaling

Müllerian inhibiting substance (MIS or anti-Müllerian hormone) is a member of the transforming growth factor-beta family and plays a pivotal role in proper male sexual differentiation. Members of this family signal by the assembly of two related serine/threonine kinase receptors, referred to as type I or type II receptors, and downstream cytoplasmic Smad effector proteins. Although the MIS type II receptor (MISRII) has been identified, the identity of the type I receptor is unclear. Here we report that MIS activates a bone morphogenetic protein-like signaling pathway, which is solely dependent on the presence of the MISRII and bioactive MIS ligand. Among the multiple type I candidates tested, only ALK2 resulted in significant enhancement of the MIS signaling response. Furthermore, dominant-negative and antisense strategies showed that ALK2 is essential for MIS-induced signaling in two independent assays, the cellular Tlx-2 reporter gene assay and the Müllerian duct regression organ culture assay. In contrast, ALK6, the other candidate MIS type I receptor, was not required. Expression analyses revealed that ALK2 is present in all MIS target tissues including the mesenchyme surrounding the epithelial Müllerian duct. Collectively, we conclude that MIS employs a bone morphogenetic protein-like signaling pathway and uses ALK2 as its type I receptor. The use of this ubiquitously expressed type I receptor underscores the role of the MIS ligand and the MIS type II receptor in establishing the specificity of the MIS signaling cascade.     

Sexually dimorphic expression of a teleost homologue of Müllerian inhibiting substance during gonadal sex differentiation in Japanese flounder, Paralichthys olivaceus

Müllerian inhibiting substance (MIS), also known as anti-Müllerian hormone, is a glycoprotein belonging to transforming growth factor beta superfamily. In mammals, MIS is responsible for regression of Müllerian ducts, anlagen of the female reproductive ducts, in the male fetus. However, the role of MIS in gonadal sex differentiation of teleost fishes, which do not have the Müllerian ducts, has yet to be clarified. To address the role of MIS on gonadal sex differentiation in fishes, we isolated a MIS cDNA from the Japanese flounder testis and examined the expression pattern of MIS mRNA in gonads of both sexes during sex differentiation period. In this study, we present the first demonstration of sexually dimorphic expression of MIS mRNA during sex differentiation in teleost fishes, similarly to amniote vertebrates which possess the Müllerian ducts.