Nuclear localization of the actin regulatory protein Palladin in sertoli cells

In the testis, F-actin structures are involved in spermatid nuclear remodeling and cytoplasm reduction, maintenance of the blood-testis barrier, support of the spermatogonial stem cell niche, and release of spermatids into the tubular lumen. To gain a better understanding of actin regulation in Sertoli-germ cell interactions, we investigated the expression of the Palladin (Palld) gene, which encodes a widely expressed phosphoprotein that localizes to actin-rich cytoplasmic structures, including focal adhesions, cell-cell junctions, podosomes, and stress fibers, and serves as a molecular scaffold to bundle actin fibers. In germ cells, PALLD was concentrated along the tubulin- and F-actin-containing cytoplasmic manchette that forms adjacent to the elongating spermatid nucleus during spermiogenesis. To our surprise, PALLD relocated from the cytoplasm to the nucleus of Sertoli cells in the juvenile testis, coincident with the onset of puberty, and this localization was maintained in the adult. We provide evidence that the 140 kDa isoform of PALLD predominates in Sertoli cells, and that it is apparently cleaved, with the C-terminus localizing to the nucleus while the N-terminus remains cytoplasmic. We investigated the nuclear localization of the C-terminus of PALLD and found that it is regulated by a putative nuclear export signal. These results provide the foundation for future work employing Sertoli cell- and spermatid-specific Palld-knockout mice to study diverse roles of PALLD as both a nuclear-actin regulatory protein and as a potential regulator of manchette formation during spermatogenesis.