The inhibitor of apoptosis proteins (IAPs) constitute a family of highly conserved apoptosis suppressor proteins that was originally identified in baculoviruses. XIAP, HIAP1, and HIAP2 show extensive homology to baculovirus IAP's. Uren et al.(1996) reported that XIAP shares 43% protein sequence identity with HIAP1 and HIAP2. Deveraux et al.(1997) showed that human XIAP directly inhibits caspase-3 and caspase-7. Endogenous XIAP is cleaved into two fragments during apoptosis induced by the Fas. The two fragments produced comprise the baculoviral inhibitory repeat (BIR) 1 and 2 domains (BIR1-2) and the BIR3 and RING
(BIR3-Ring) domains of XIAP. Overexpression of the BIR1-2 fragment inhibits Fas-induced apoptosis, albeit at significantly reduced efficiency compared with full-length XIAP. Cleavage of XIAP may be one mechanism by which cell death programs circumvent the anti-apoptotic barrier posed by XIAP. The ectopic expression of the BIR3-Ring fragment results in nearly complete protection from Bax-induced apoptosis. Use of purified recombinant proteins revealed that BIR3-Ring is a specific inhibitor of caspase-9 whereas BIR1-2 is specific for caspases 3 and 7. Therefore XIAP possesses two different caspase inhibitory activities which can be attributed to distinct domains within XIAP. These data may provide an explanation for why IAPs have evolved with multiple BIR domains (Deveraux et al., 1997).
As the caspases are the principal effectors of apoptosis, these findings suggested how IAPs might inhibit cell death, providing evidence for a mechanism of action for these mammalian cell-death suppressors. Farahani et al. (1997) isolated MIAP3, the mouse XIAP homolog, which had 94% identical homology to human XIAP.
NCBI Summary:
This gene encodes a protein that belongs to a family of apoptotic suppressor proteins. Members of this family share a conserved motif termed, baculovirus IAP repeat, which is necessary for their anti-apoptotic function. This protein functions through binding to tumor necrosis factor receptor-associated factors TRAF1 and TRAF2 and inhibits apoptosis induced by menadione, a potent inducer of free radicals, and interleukin 1-beta converting enzyme. This protein also inhibits at least two members of the caspase family of cell-death proteases, caspase-3 and caspase-7. Mutations in this gene are the cause of X-linked lymphoproliferative syndrome. Alternate splicing results in multiple transcript variants. Pseudogenes of this gene are found on chromosomes 2 and 11.[provided by RefSeq, Feb 2011]
General function
Cell death/survival, Anti-apoptotic, Metabolism
Comment
Uren et al. (1996) determined that expression of XIAP in mammalian cells significantly reduced ICE (interleukin 1beta converting enzyme)-mediated apoptosis.
Interplay between Caspase 9 and X-linked Inhibitor of Apoptosis Protein (XIAP) in the oocyte elimination during fetal mouse development. Liu X et al. (2019) Mammalian female fertility is limited by the number and quality of oocytes in the ovarian reserve. The number of oocytes is finite since all germ cells cease proliferation to become oocytes in fetal life. Moreover, 70-80% of the initial oocyte population is eliminated during fetal and neonatal development, restricting the ovarian reserve. Why so many oocytes are lost during normal development remains an enigma. In Meiotic Prophase I (MPI), oocytes go through homologous chromosome synapsis and recombination, dependent on formation and subsequent repair of DNA double strand breaks (DSBs). The oocytes that have failed in DSB repair or synapsis get eliminated mainly in neonatal ovaries. However, a large oocyte population is eliminated before birth, and the cause or mechanism of this early oocyte loss is not well understood. In the current paper, we show that the oocyte loss in fetal ovaries was prevented by a deficiency of Caspase 9 (CASP9), which is the hub of the mitochondrial apoptotic pathway. Furthermore, CASP9 and its downstream effector Caspase 3 were counteracted by endogenous X-linked Inhibitor of Apoptosis (XIAP) to regulate the oocyte population; while XIAP overexpression mimicked CASP9 deficiency, XIAP deficiency accelerated oocyte loss. In the CASP9 deficiency, more oocytes were accumulated at the pachytene stage with multiple γH2AFX foci and high LINE1 expression levels, but with normal levels of synapsis and overall DSB repair. We conclude that the oocytes with LINE1 overexpression were preferentially eliminated by CASP9-dependent apoptosis in balance with XIAP during fetal ovarian development. When such oocytes were retained, however, they get eliminated by a CASP9-independent mechanism during neonatal development. Thus, the oocyte is equipped with multiple surveillance mechanisms during MPI progression to safe-guard the quality of oocytes in the ovarian reserve.////////////////// Identification and expression of X-linked inhibitor of apoptosis protein during follicular development in goat ovary. Han P et al. (2017) X-linked inhibitor of apoptosis protein (XIAP), an endogenous of inhibitor of caspases, plays crucial roles in regulating ovarian granulosa cell apoptosis during follicular atresia. The aim of the present study was to determine the presence and localization of XIAP in the goat ovary and its expression level during follicular development. The full length cDNA of XIAP from goat ovary cells was cloned using reverse transcription PCR. A total of 497 amino acid residues were encoded by open reading frame and had high identity with homologous sequences from other mammals. XIAP was widely expressed in adult goat tissues as determined by real-time PCR and it demonstrated higher expression in propagative organs. High level of XIAP was detected in large healthy follicles and corpus luteum in comparison with that in small antral follicles, which was in accordance with the immunohistochemistry results and atretic follicles had very low expression. XIAP was localized in both granulosa and theca cells in antral follicles but not in primordial follicles. Furthermore, luteinizing hormone stimulated the proliferation of mRNA encoding XIAP in granulosa cells in vitro. The present study demonstrated that XIAP was expressed in a follicular-stage-dependent manner in goat ovaries.//////////////////
HIAP-2 and XIAP expression in both granulosa and theca cells increase with follicular maturation, reaching maximal levels at the antral stage of development. XIAP and HIAP-2 decrease markedly in atretic follicles at the small to medium sized antral stage of development, suggesting follicular atresia may be associated with decreased granulosa cell IAP protein content. Atresia was also associated with a change in the intracellular distribution of IAPs in granulosa cells; preantral and early antral follicles indicated extensive apoptosis associated with minimal IAP protein content (Li et al.,1998) IAPs may be involved in the suppression of granulosa cell apoptosis by gonadotropins in small to medium-sized antral follicles and play an important role in determining the fate of the cells, and thus also the eventual follicular destiny (atresia vs. ovulation) (Li et al.,1998).
Expression regulated by
FSH, LH
Comment
Gonadotropin treatment increased Hiap-2 and Xiap protein content and suppressed apoptosis in granulosa
cells, resulting in the development of follicles to the antral and preovulatory stages. In addition, gonadotropin withdrawal induced apoptotic DNA fragmentation in granulosa cells in early antral and antral follicles, which is accompanied by a marked decrease in Hiap-2 and Xiap expression (Li et al.,1998).
Ovarian localization
Oocyte, Granulosa, Theca
Comment
Germline Quality Control: eEF2K Stands Guard to Eliminate Defective Oocytes. Chu HP 2014 et al.
The control of germline quality is critical to reproductive success and survival of a species; however, the mechanisms underlying this process remain unknown. Here, we demonstrate that elongation factor 2 kinase (eEF2K), an evolutionarily conserved regulator of protein synthesis, functions to maintain germline quality and eliminate defective oocytes. We show that disruption of eEF2K in mice reduces ovarian apoptosis and results in the accumulation of aberrant follicles and defective oocytes at advanced reproductive age. Furthermore, the loss of eEF2K in Caenorhabditis elegans results in a reduction of germ cell death and significant decline in oocyte quality and embryonic viability. Examination of the mechanisms by which eEF2K regulates apoptosis shows that eEF2K senses oxidative stress and quickly downregulates short-lived antiapoptotic proteins, XIAP and c-FLIPL by inhibiting global protein synthesis. These results suggest that eEF2K-mediated inhibition of protein synthesis renders cells susceptible to apoptosis and functions to eliminate suboptimal germ cells.
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X-linked inhibitor of apoptosis protein (XIAP) and active caspase-3 expression patterns in antral follicles in the sheep ovary. Phillipps HR et al. X-linked inhibitor of apoptosis protein (XIAP) interacts with caspases to inhibit their activity, thereby providing a potential mechanism for regulation of granulosa apoptosis occurring during follicular atresia. The aims of the study were to determine the presence and localization of XIAP mRNA and protein content in the sheep ovary and compare these expression patterns with active caspase-3 protein in the same antral follicles. Romney ewe estrous cycles (N=25) were synchronized with 2-3 Estrumate injections and ovarian tissue collected during the luteal and follicular phases of the cycle. The presence of XIAP mRNA was confirmed by RT-PCR using laser capture microdissected ovarian cell samples. XIAP mRNA was subsequently localized by in situ hybridization histochemistry and XIAP and active caspase-3 protein visualized by immunohistochemistry. In antral follicles extensive XIAP protein localization was evident in both granulosa and thecal cells. In contrast, mRNA expression was widespread in granulosa cells and only detected in thecal tissue from a small proportion of antral follicles. Active caspase-3 and XIAP comparative expression analysis showed positive XIAP mRNA expression in all late luteal phase (day 14) follicles, despite varying levels of active caspase-3 protein. A proportion of follicular phase (days 15 and 16) follicles, however, showed an inverse expression relationship at the protein and mRNA levels in both granulosa and thecal tissue, as did XIAP protein in day 14 follicles. These results suggest high XIAP may prevent activation of caspase-3, thereby regulating follicular atresia in antral follicles and could potentially be utilized as a marker of follicular health.
Changes in Expression and Localization of X-linked Inhibitor of Apoptosis Protein (XIAP) in Follicular Granulosa Cells During Atresia in Porcine Ovaries. Cheng Y et al. Follicular selection predominantly depends on granulosa cell apoptosis in porcine ovaries, but the molecular mechanisms regulating the induction of apoptosis in granulosa cells during follicular selection remain incompletely understood. To determine the role of X-linked inhibitor of apoptosis protein (XIAP), which suppresses caspase-3, -7 and -9 activities and acts as an endogenous inhibitor of apoptotic cell death, in the regulation of granulosa cell apoptosis during follicular atresia, we examined the changes in the expression level and localization of XIAP mRNA and protein in granulosa cells during follicular atresia using reverse transcription-polymerase chain reaction (RT-PCR), in situ hybridization, Western blotting and immunohistochemistry, respectively. High levels of XIAP mRNA and protein were noted in the granulosa cells of healthy follicles, and decreased levels were noted during follicular atresia. In situ hybridization and immunohistochemistry demonstrated that XIAP mRNA and protein were strongly expressed in the granulosa cells of healthy follicles, but negative/trace stainings were noted in those of atretic follicles. The present findings strongly indicate that XIAP is a candidate molecule which acts as an anti-apoptotic/pro-survival factor by inhibiting intracellular apoptosis signaling and is involved in the regulation of apoptosis in porcine granulosa cells.
Follicle stages
Antral, Preovulatory
Comment
Xiap protein suppressed apoptosis in granulosa cells after gonadotropin treatment, resulting in development of follicles to the antral and preovulatory stages (Li et al.,1998).