Mcl-1 is a key regulator of the ovarian reserve

Abstract

A majority of ovarian follicles are lost to natural death, but the disruption of factors involved in maintenance of the oocyte pool results in a further untimely follicular depletion known as premature ovarian failure. The anti-apoptotic B-cell lymphoma 2 (Bcl-2) family member myeloid cell leukemia-1 (MCL-1) has a pro-survival role in various cell types; however, its contribution to oocyte survival is unconfirmed. We present a phenotypic characterization of oocytes deficient in Mcl-1, and establish its role in maintenance of the primordial follicle (PMF) pool, growing oocyte survival and oocyte quality. Mcl-1 depletion resulted in the premature exhaustion of the ovarian reserve, characterized by early PMF loss because of activation of apoptosis. The increasingly diminished surviving cohort of growing oocytes displayed elevated markers of autophagy and mitochondrial dysfunction. Mcl-1-deficient ovulated oocytes demonstrated an increased susceptibility to cellular fragmentation with activation of the apoptotic cascade. Concomitant deletion of the pro-apoptotic Bcl-2 member Bcl-2-associated X protein (Bax) rescued the PMF phenotype and ovulated oocyte death, but did not prevent the mitochondrial dysfunction associated with Mcl-1 deficiency and could not rescue long-term breeding performance. We thus recognize MCL-1 as the essential survival factor required for conservation of the postnatal PMF pool, growing follicle survival and effective oocyte mitochondrial function.

Humanized Mcl-1 mice enable accurate pre-clinical evaluation of MCL-1 inhibitors destined for clinical use

“…There have been many reports on the importance of MCL-1 in several vital tissues (Dzhagalov et al, 2008;Omari et al, 2015;Opferman et al, 2005;Opferman et al, 2003;Peperzak et al, 2013;Thomas et al, 2013;Vikstrom et al, 2010;Wang et al, 2013) raising the question whether an MCL-1 inhibitor would find its therapeutic use in the clinic. Previously it was shown that S63845 exerts very little toxic effects in mice expressing mouse MCL-1 (Kotschy et al, 2016), but given the higher affinity of this drug for human MCL-1 we revisited this question by treating huMcl-1 (and wild-type controls) mice for 5 consecutive days with either vehicle or 12.5 mg/kg S63845 and testing for acute impact and recovery 3 or 17 days post treatment, respectively.…”

“…MCL-1 is widely expressed (Kozopas et al, 1993), and essential for embryonic development with homozygous loss of Mcl-1 in mice resulting in failure to implant at the blastocyst stage (Rinkenberger et al, 2000). Furthermore, conditional gene knockout studies have shown that MCL-1 plays a vital role in the survival of cardiomyocytes (Thomas et al, 2013;Wang et al, 2013), hematopoietic stem cells (Opferman et al, 2005), developing and mature lymphocytes (Dzhagalov et al, 2008;Opferman et al, 2003;Peperzak et al, 2013;Vikstrom et al, 2010) and in the maintenance of oocytes in the ovarian reserve (Omari et al, 2015). In spite of the many important roles MCL-1 plays, very little toxicity was observed when healthy mice were treated in vivo with the MCL-1 inhibitor S63845 at a dose that ablates mouse lymphoma cells (Kotschy et al, 2016).…”

The Role of Mitochondria in Oocyte and Early Embryo Health

“…MCL1 is abundantly expressed in human and murine oocytes, and its disruption causes premature ovarian insufficiency. Mcl1 deficient oocytes that survive folliculogenesis and are ovulated exhibit a decrease in mitochondrial membrane potential increased ROS production and decreased metabolic substrates malate and fumarate; however no change in citrate or total ATP level was observed [134]. While cell death of Mcl1 deficient oocytes can be prevented by Bax inactivation, mitochondrial phenotypes and meiotic abnormalities cannot be reverted by Bax deficiency.…”

Humanized Mcl-1 mice enable accurate preclinical evaluation of MCL-1 inhibitors destined for clinical use

“…S63845 exerts no enduring toxicity in the huMcl-1 mice at the MTD There have been many reports on the importance of MCL-1 in several vital tissues [21][22][23][24][25][26][27][28][29] raising the question of whether an MCL-1 inhibitor would find its therapeutic use in the clinic. Previously and unexpectedly, it was shown that S63845 exerts very little toxic effects at the MTD in mice expressing mouse MCL-1.…”

“…20 Conditional gene knockout studies showed that MCL-1 plays a vital role in the survival of cardiomyocytes, 21,22 hematopoietic stem cells, 23 developing and mature lymphocytes, [24][25][26][27][28] and in the maintenance of oocytes in the ovarian reserve. 29 Despite these reports, little toxicity was observed in mice treated with doses of the MCL-1 inhibitor S63845 that ablates mouse lymphoma cells. 16 One caveat of these results is that S63845 has an approximately sixfold higher affinity for the human compared with the mouse MCL-1 protein and it is therefore possible that the mouse models used were not sensitive enough to reveal potential on-target toxicities.…”