Vaccinia-Related Kinase 1 Is Required for the Maintenance of Undifferentiated Spermatogonia in Mouse Male Germ Cells

Abstract

Vaccinia-related kinase 1 (VRK1) is a crucial protein kinase for mitotic regulation. VRK1 is known to play a role in germ cell development, and its deficiency results in sterility. Here we describe that VRK1 is essential for the maintenance of spermatogonial stem cells. To determine whether VRK1 plays a role in these cells, we assessed the population size of undifferentiated spermatogonia. Flow cytometry analyses showed that the number of undifferentiated spermatogonia was markedly reduced in VRK1-deficient testes. VRK1 was highly expressed in spermatogonial populations, and approximately 66% of undifferentiated spermatogonia that were sorted as an Ep-CAM+/c-kit−/alpha-6-integrin+ population showed a positive signal for VRK1. Undifferentiated stem cells expressing Plzf and Oct4 but not c-kit also expressed VRK1, suggesting that VRK1 is an intrinsic factor for the maintenance of spermatogonial stem cells. Microarray analyses of the global testicular transcriptome and quantitative RT-PCR of VRK1-deficient testes revealed significantly reduced expression levels of undifferentiated spermatogonial marker genes in early postnatal mice. Together, these results suggest that VRK1 is required for the proliferation and differentiation of undifferentiated spermatogonia, which are essential for spermatogenic cell maintenance.

The kinase VRK1 is required for normal meiotic progression in mammalian oogenesis

“…Our results show that a reduction in VRK1 causes sterility in both male and female mice. Male mice homozygous for the Vrk1 gene trap allele, while fertile at first, display a progressive loss of spermatogenesis that results in early onset sterility, similar to previous reports (Choi et al, 2010;Wiebe et al, 2010). Female homozygous mice are completely sterile and display multiple defects during oocyte development, including delays in meiotic progression and lagging chromosomes during metaphase II.…”

“…Additional evidence implicating a role for VRK1 in germ cell development has also been demonstrated in the germline stem cells of Caenorhabditis elegans where a reduction in proliferation, but not an increase in apoptosis, causes the sterility defect observed in vrk-1 mutants (Gorjanacz et al, 2007;Waters et al, 2010). In mammals, VRK1 has been shown to affect germ cell proliferation in males, leading to sterility (Choi et al, 2010;Wiebe et al, 2010). Female mice lacking VRK1 function are reported to be sterile as well, but the basis for this phenotype is unknown.…”

VRK1 interacts with p53 forming a basal complex that is activated by UV‐induced DNA damage

“…In addition, VRK1 kinase activity is enhanced in response to DNA damage and is an early participant necessary for the formation of 53BP1 foci in response to DNA double-strand breaks induced by ionizing radiation, in both resting and dividing cells [31]. In murine gene-trap models, VRK1 deficiency results in sterility due to either meiotic defects in females or by affecting maintenance of spermatogonial stem cells prior to meiosis in males [40][41][42][43].…”

Sensitivity of the kinase activity of human vaccinia-related kinase proteins to toxic metals

“…The elimination of VRK1 in gene-trap mice has revealed that reduced levels of this kinase play an important role in meiosis, resulting in male and female sterility [47][48][49]. Also, the loss in mice of coilin, a phosphorylation target of VRK1 [50], is manifested by a sterility phenotype [51].…”