Unexpected requirement for ELMO1 in clearance of apoptotic germ cells in vivo.

Apoptosis and the subsequent clearance of dying cells occurs throughout development and adult life in many tissues. Failure to promptly clear apoptotic cells has been linked to many diseases. ELMO1 is an evolutionarily conserved cytoplasmic engulfment protein that functions downstream of the phosphatidylserine receptor BAI1, and, along with DOCK1 and the GTPase RAC1, promotes internalization of the dying cells. Here we report the generation of ELMO1-deficient mice, which we found to be unexpectedly viable and grossly normal. However, they had a striking testicular pathology, with disrupted seminiferous epithelium, multinucleated giant cells, uncleared apoptotic germ cells and decreased sperm output. Subsequent in vitro and in vivo analyses revealed a crucial role for ELMO1 in the phagocytic clearance of apoptotic germ cells by Sertoli cells lining the seminiferous epithelium. The engulfment receptor BAI1 and RAC1 (upstream and downstream of ELMO1, respectively) were also important for Sertoli-cell-mediated engulfment. Collectively, these findings uncover a selective requirement for ELMO1 in Sertoli-cell-mediated removal of apoptotic germ cells and make a compelling case for a relationship between engulfment and tissue homeostasis in vivo.

Nucleotides released by apoptotic cells act as a find-me signal to promote phagocytic clearance.

Phagocytic removal of apoptotic cells occurs efficiently in vivo such that even in tissues with significant apoptosis, very few apoptotic cells are detectable. This is thought to be due to the release of 'find-me' signals by apoptotic cells that recruit motile phagocytes such as monocytes, macrophages and dendritic cells, leading to the prompt clearance of the dying cells. However, the identity and in vivo relevance of such find-me signals are not well understood. Here, through several lines of evidence, we identify extracellular nucleotides as a critical apoptotic cell find-me signal. We demonstrate the caspase-dependent release of ATP and UTP (in equimolar quantities) during the early stages of apoptosis by primary thymocytes and cell lines. Purified nucleotides at these concentrations were sufficient to induce monocyte recruitment comparable to that of apoptotic cell supernatants. Enzymatic removal of ATP and UTP (by apyrase or the expression of ectopic CD39) abrogated the ability of apoptotic cell supernatants to recruit monocytes in vitro and in vivo. We then identified the ATP/UTP receptor P2Y(2) as a critical sensor of nucleotides released by apoptotic cells using RNA interference-mediated depletion studies in monocytes, and macrophages from P2Y(2)-null mice. The relevance of nucleotides in apoptotic cell clearance in vivo was revealed by two approaches. First, in a murine air-pouch model, apoptotic cell supernatants induced a threefold greater recruitment of monocytes and macrophages than supernatants from healthy cells did; this recruitment was abolished by depletion of nucleotides and was significantly decreased in P2Y(2)(-/-) (also known as P2ry2(-/-)) mice. Second, clearance of apoptotic thymocytes was significantly impaired by either depletion of nucleotides or interference with P2Y receptor function (by pharmacological inhibition or in P2Y(2)(-/-) mice). These results identify nucleotides as a critical find-me cue released by apoptotic cells to promote P2Y(2)-dependent recruitment of phagocytes, and provide evidence for a clear relationship between a find-me signal and efficient corpse clearance in vivo.

The role of regulatory proteins and S-nitrosylation of endothelial nitric oxide synthase in the human clitoris: implications for female sexual function.

INTRODUCTION: During female sexual arousal, clitoral blood flow is controlled by endothelial nitric oxide synthase (eNOS) and its product, nitric oxide (NO). The mechanisms regulating eNOS activity and NO bioavailability in the clitoris are largely unknown. AIM: To identify proteins involved in regulation of eNOS activity within the clitoris and to evaluate the effects of S-nitrosoglutathione reductase (GSNO-R) and eNOS nitrosylation/denitrosylation on clitoral blood flow. METHODS: Immunohistochemistry for eNOS, caveolin-1 (Cav1), heat shock protein-90 (Hsp90), phosphodiesterase type 5 (PDE5), GSNO-R, and soluble guanylate cyclase (sGC) was performed on human and murine clitoral tissue. Western blot analysis was performed for eNOS, phosphorylated eNOS (phospho-eNOS, Ser1177), Cav1, Hsp90, sGC, PDE5, phosphoinositide 3-kinase (PI3K), Akt (protein kinase B), and GSNO-R on protein from human clitoral tissue. A biotin switch assay was used to analyze the S-nitrosylation of eNOS, nNOS, and GSNO-R. Clitoral blood flow was measured in wild-type and GSNO-R(-/-) mice at baseline and during cavernous nerve electrical stimulation (CNES). MAIN OUTCOME MEASURES: Localization of eNOS regulatory proteins and clitoral blood flow. RESULTS: eNOS and GSNO-R co-localized to the vascular endothelium and sinusoids of human clitoral tissue. Immunohistochemistry also localized Cav1 and Hsp90 to the endothelium and PDE5 and sGC to the trabecular smooth muscle. Expression of S-nitrosylated (SNO)-eNOS and SNO-GSNO-R was detected by biotin switch assays. Wild-type control mice exhibited increased clitoral blood flow with CNES whereas GSNO-R(-/-) animals failed to show an increase in blood flow. CONCLUSIONS: Several key eNOS regulatory proteins are present in the clitoral tissue in a cellular specific pattern. S-nitrosylation of eNOS may also represent a key regulatory mechanism governing eNOS activation/deactivation since mice deficient in GSNO-R failed to increase clitoral blood flow. Additional studies are necessary to define the role of S-nitrosylation in the genital vascular response and its subsequent impact on female sexual function.

Hypoxia-inducible factor-1alpha is constitutively expressed in murine Leydig cells and regulates 3beta-hydroxysteroid dehydrogenase type 1 promoter activity.

Hypoxia-inducible factor-1alpha (HIF-1alpha) is a transcription factor that plays an essential role in oxygen homeostasis. HIF-1alpha is constitutively made in cells; however, it is ubiquitinated and degraded under normoxic conditions. Hypoxia prevents the ubiquitination of HIF-1alpha, resulting in stabilization of the protein and activation of target genes. Because of its vascular arrangement and the high metabolic demand of spermatogenesis, the testis has been described previously as functioning on the brink of hypoxia; thus, we have hypothesized that HIF-1alpha is constitutively expressed and stabilized in the testis, where it could play a role in testicular homeostasis. Western blot analysis using nuclear proteins from liver, kidney, and testis revealed the presence of HIF-1alpha only in the testis. Immunohistochemistry confirmed this result and revealed that HIF-1alpha was specifically located in interstitial Leydig cells. Electromobility shift assays employing nuclear extracts from the TM3 Leydig cell line revealed that these cells express HIF-1alpha that is capable of binding DNA under normoxic conditions. Furthermore, we found that protein levels can be increased further when the TM3 cells are cultured under hypoxic conditions. Finally, transient transfections of TM3 Leydig cells revealed that the promoter of the mouse 3beta-hydroxysteroid dehydrogenase type 1 (Hsd3b1) gene, which encodes a key enzyme in testosterone production, is a potential target of HIF-1alpha. In conclusion, HIF-1alpha is constitutively present in the Leydig cells of the murine testis, where it potentially regulates Hsd3b1 transcription, and thus male reproductive function.