Pelota Regulates Epidermal Differentiation by Modulating BMP and PI3K/AKT Signaling Pathways.

The depletion of evolutionarily conserved pelota protein causes impaired differentiation of embryonic and spermatogonial stem cells. In this study, we show that temporal deletion of pelota protein before epidermal barrier acquisition leads to neonatal lethality due to perturbations in permeability barrier formation. Further analysis indicated that this phenotype is a result of failed processing of profilaggrin into filaggrin monomers, which promotes the formation of a protective epidermal layer. Molecular analyses showed that pelota protein negatively regulates the activities of bone morphogenetic protein and phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) signaling pathways in the epidermis. To address whether elevated activities of bone morphogenetic protein and PI3K/AKT signaling pathways were the cause for the perturbed epidermal barrier in Pelo-deficient mice, we made use of organotypic cultures of skin explants from control and mutant embryos at embryonic day 15.5. Inhibition of PI3K/AKT signaling did not significantly affect the bone morphogenetic protein activity. However, inhibition of bone morphogenetic protein signaling caused a significant attenuation of PI3K/AKT activity in mutant skin and, more interestingly, the restoration of profilaggrin processing and normal epidermal barrier function. Therefore, increased activity of the PI3K/AKT signaling pathway in Pelo-deficient skin might conflict with the dephosphorylation of profilaggrin and thereby affect its proper processing into filaggrin monomers and ultimately the epidermal differentiation.

Pelota mediates gonocyte maturation and maintenance of spermatogonial stem cells in mouse testes.

Pelota (Pelo) is an evolutionarily conserved gene, and its deficiency in Drosophila affects both male and female fertility. In mice, genetic ablation of Pelo leads to embryonic lethality at the early implantation stage as a result of the impaired development of extra-embryonic endoderm (ExEn). To define the consequences of Pelo deletion on male germ cells, we temporally induced deletion of the gene at both embryonic and postnatal stages. Deletion of Pelo in adult mice resulted in a complete loss of whole-germ cell lineages after 45 days of deletion. The absence of newly emerging spermatogenic cycles in mutants confirmed that spermatogonial stem cells (SSCs) were unable to maintain spermatogenesis in the absence of PELO protein. However, germ cells beyond the undifferentiated SSC stage were capable of completing spermatogenesis and producing spermatozoa, even in the absence of PELO. Following the deletion of Pelo during embryonic development, we found that although PELO is dispensable for maintaining gonocytes, it is necessary for the transition of gonocytes to SSCs. Immunohistological and protein analyses revealed the attenuation of FOXO1 transcriptional activity, which induces the expression of many SSC self-renewal genes. The decreased transcriptional activity of FOXO1 in mutant testes was due to enhanced activity of the PI3K/AKT signaling pathway, which led to phosphorylation and cytoplasmic sequestration of FOXO1. These results suggest that PELO negatively regulates the PI3K/AKT pathway and that the enhanced activity of PI3K/AKT and subsequent FOXO1 inhibition are responsible for the impaired development of SSCs in mutant testes.

Pelota regulates the development of extraembryonic endoderm through activation of bone morphogenetic protein (BMP) signaling.

Pelota (Pelo) is ubiquitously expressed, and its genetic deletion in mice leads to embryonic lethality at an early post-implantation stage. In the present study, we conditionally deleted Pelo and showed that PELO deficiency did not markedly affect the self-renewal of embryonic stem cells (ESCs) or their capacity to differentiate in teratoma assays. However, their differentiation into extraembryonic endoderm (ExEn) in embryoid bodies (EBs) was severely compromised. Conversely, forced expression of Pelo in ESCs resulted in spontaneous differentiation toward the ExEn lineage. Failure of Pelo-deficient ESCs to differentiate into ExEn was accompanied by the retained expression of pluripotency-related genes and alterations in expression of components of the bone morphogenetic protein (BMP) signaling pathway. Further experiments have also revealed that attenuated activity of BMP signaling is responsible for the impaired development of ExEn. The recovery of ExEn and down-regulation of pluripotent genes in BMP4-treated Pelo-null EBs indicate that the failure of mutant cells to down-regulate pluripotency-related genes in EBs is not a result of autonomous defect, but rather to failed signals from surrounding ExEn lineage that induce the differentiation program. In vivo studies showed the presence of ExEn in Pelo-null embryos at E6.5, yet embryonic lethality at E7.5, suggesting that PELO is not required for the induction of ExEn development, but rather for ExEn maintenance or for terminal differentiation toward functional visceral endoderm which provides the embryos with growth factors required for further development. Moreover, Pelo-null fibroblasts failed to reprogram toward induced pluripotent stem cells (iPSCs) due to inactivation of BMP signaling and impaired mesenchymal-to-epithelial transition. Thus, our results indicate that PELO plays an important role in the establishment of pluripotency and differentiation of ESCs into ExEn lineage through activation of BMP signaling.