Estrogen-Dependent Proteolytic Cleavage of Semaphorin 4D and Plexin-B1 Enhances Semaphorin 4D-Induced Apoptosis during Postnatal Vaginal Remodeling in Pubescent Mice

Takuji Ito,Masayasu Miyajima,Takashi Ueyama,Kazunori Yukawa,Tetsuji Tanaka,Takahiko Kawasaki,Takayuki Negishi,Atsushi Kumanogoh,Hitoshi Kikutani,Tao Bai,Hyota Takamatsu,Kenji Yoshida,Hiroyasu Nakano

doi:10.1371/journal.pone.0097909

Abstract

Around the fifth week after birth, the vaginal cavity in female mouse pups opens to the overlaying skin. This postnatal tissue remodeling of the genital tract occurs during puberty, and it largely depends upon hormonally induced apoptosis that mainly occurs in the epithelium at the lower part of the mouse vaginal cavity. Previously, we showed that most BALB/c mice lacking the class IV Semaphorin (Sema4D) develop imperforate vagina and hydrometrocolpos; therefore, we reasoned that the absence of Sema4D-induced apoptosis in vaginal epithelial cells may cause the imperforate vagina. Sema4D signals via the Plexin-B1 receptor; nevertheless detailed mechanisms mediating this hormonally triggered apoptosis are not fully documented. To investigate the estrogen-dependent control of Sema4D signaling during the apoptosis responsible for mouse vaginal opening, we examined structural and functional modulation of Sema4D, Plexin-B1, and signaling molecules by analyzing both wild-type and Sema4D−/− mice with or without ovariectomy. Both the release of soluble Sema4D and the conversion of Plexin-B1 by proteolytic processing in vaginal tissue peaked 5 weeks after birth of wild-type BALB/c mice at the time of vaginal opening. Estrogen supplementation of ovariectomized wild-type mice revealed that both the release of soluble Sema4D and the conversion of Plexin-B1 into an active form were estrogen-dependent and concordant with apoptosis. Estrogen supplementation of ovariectomized Sema4D−/− mice did not induce massive vaginal apoptosis in 5-week-old mice; therefore, Sema4D may be an essential apoptosis-inducing ligand that acts downstream of estrogen action in vaginal epithelium during this postnatal tissue remodeling. Analysis of ovariectomized mice also indicated that Sema4D contributed to estrogen-dependent dephosphorylation of Akt and ERK at the time of vaginal opening. Based on our results, we propose that apoptosis in vaginal epithelium during postnatal vaginal opening is induced by enhanced Sema4D signaling that is caused by estrogen-dependent structural changes of Sema4D and Plexin-B1.

Highlights

In mice, the blind ending of the vaginal cavity in each female pup opens to the skin around 5 weeks after birth when sex hormone level rises in the internal environment; vaginal opening is one of very few postnatal tissue remodeling events in mice [1]
Using Sema4D2/2 BALB/c mice, we documented three novel, crucial observations regarding the estrogen-dependent apoptosis that occurs during postnatal vaginal opening in mice
We found that 1) Semaphorin 4D (Sema4D), which is classified as a class 4 semaphorin, played an indispensable role as a downstream effector of estrogen action during apoptosis of vaginal epithelial cells as the vagina opens; 2) estrogen-dependent Sema4D processing and estrogen-dependent Plexin-B1 reorganization increased Sema4D signal transduction efficiency during vaginal opening; and 3)

Summary

Introduction

The blind ending of the vaginal cavity in each female pup opens to the skin around 5 weeks after birth when sex hormone level rises in the internal environment; vaginal opening is one of very few postnatal tissue remodeling events in mice [1]. The study of transgenic mice that overexpress the human antiapoptotic protein Bcl-2 in the vaginal mucosa clearly shows that this postnatal tissue remodeling process depends heavily on massive mucosal apoptosis; these cell deaths occur in a very limited time window and only at the lower distal end of mouse vaginal cavity in the vicinity of skin [1]. The augmented Plexin-B1 GAP activities in neurons 1) downregulate activities of Ras family members and 2) induce dephosphorylation of Akt and ERK and activation of glycogen synthase kinase (GSK)-3b; these events reduce integrin-mediated cell adhesion to the extracellular matrix, and induce morphological remodeling of growth cones and dendrites in cultured neurons [14,15,16]. The proteolytically processed, active form of Plexin-B1 transmits a more intense Sema4D-dependent intracellular signal than does the unprocessed precursor [20]

Methods

Results

Conclusion