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Phosphorylation of Mitogen-Activated Protein Kinase 8 (MAPK8) Is Associated With Germ Cell Apoptosis and Redistribution of the Bcl2-Modifying Factor (BMF)

CHRISTINE M. HILL

MATTHEW D. ANWAY

From the Division of Reproductive Biology, Department of Biochemistry and Molecular Biology, Johns Hopkins University Bloomberg School of Public Health, Baltimore, Maryland.

Correspondence to: Dr Barry R. Zirkin, Division of Reproductive Biology, Department of Biochemistry and Molecular Biology, Johns Hopkins University Bloomberg School of Public Health, Baltimore, MD 21205 (e-mail: brzirkin{at}jhsph.edu).

Successful spermatogenesis requires that germ cells remain in physical contact with Sertoli cells until spermiation. Previous studies have shown that the Bcl2-modifying factor (BMF) is a proapoptotic protein found in many epithelial cells which, when phosphorylated by the active form of mitogen-activated protein kinase 8 (p-MAPK8), initiates apoptosis in response to loss of adhesion of the cells to their basal lamina. Based on this, we hypothesized that p-MAPK8 and BMF may play important roles in the apoptotic death of testicular germ cells in response to their detachment from Sertoli cells. Immunohistochemical analysis of the normal rat testis revealed p-MAPK8 expression in spermatocytes and elongated spermatids but not in round spermatids. This localization was opposite to that of BMF, which is expressed in round spermatids but not in spermatocytes or elongated spermatids. When freshly isolated germ cells were cultured in the absence of Sertoli cells, a condition in which there was widespread germ cell apoptosis, an increase in p-MAPK8 relative to overall MAPK8 protein, was seen by Western blot analysis. Additionally, immunocytochemical analysis showed an increase in immunoreactive p-MAPK8 in round spermatids and spermatocytes in association with BMF expression. From these correlative data, we propose that the activation of MAPK8 and redistribution of BMF may be integrally involved in the mechanism by which specific germ cells undergo programmed cell death in response to their detachment from Sertoli cells.

     Key words: Spermatogenesis, Sertoli cells, cell adhesion