Alka Sharma

Role of Mitogen Activated Protein Kinase and Maturation Promoting Factor During the Achievement of Meiotic Competency in Mammalian Oocytes: MAPK3/1 AND ACHIEVEMENT OF OOCYTE MEIOTIC COMPETENCY

The oocyte quality remains as one of the major problems associated with poor in vitro fertilization (IVF) rate and assisted reproductive technology (ART) failure worldwide. The oocyte quality is dependent on its meiotic maturation that begins inside the follicular microenvironment and gets completed at the time of ovulation in most of the mammalian species. Follicular oocytes are arrested at diplotene stage of first meiotic prophase. The resumption of meiosis from diplotene arrest, progression through metaphase‐I (M‐I) and further arrest at metaphase‐II (M‐II) are important physiological requirements for the achievement of meiotic competency in mammalian oocytes. The achievement of meiotic competency is dependent upon cyclic stabilization/destabilization of maturation promoting factor (MPF). The mitogen‐activated protein kinase3/1 (MAPK3/1) modulates stabilization/destabilization of MPF in oocyte by interacting either with signal molecules, transcription and post‐transcription factors in cumulus cells or cytostatic factors (CSFs) in oocyte. MPF regulates meiotic cell cycle progression from diplotene arrest to M‐II arrest and directly impacts oocyte quality. The MAPK3/1 activity is not reported during spontaneous meiotic resumption but its activity in cumulus cells is required for gonadotropin‐induced oocyte meiotic resumption. Although high MAPK3/1 activity is required for the maintenance of M‐II arrest in several mammalian species, its cross‐talk with MPF remains to be elucidated. Further studies are required to find out the MAPK3/1 activity and its impact on MPF destabilization/stabilization during achievement of meiotic competency, an important period that decides oocyte quality and directly impacts ARTs outcome in several mammalian species including human. J. Cell. Biochem. 119: 123–129, 2018.

Germ cell depletion from mammalian ovary: possible involvement of apoptosis and autophagy

Mammalian ovary contains millions of germ cells during embryonic life but only few of them are culminated into oocytes that achieve meiotic competency just prior to ovulation. The majority of germ cells are depleted from ovary through several pathways. Follicular atresia is one of the major events that eliminate germ cells from ovary by engaging apoptotic as well as non-apoptotic pathways of programmed cell death. Apoptosis is characterized by several morphological changes that include cell shrinkage, nuclear condensation, membrane blebbing and cytoplasmic fragmentation by both mitochondria- as well as death receptor-mediated pathways in encircling granulosa cells and oocyte. Although necroapoptosis have been implicated in germ cell depletion, autophagy seems to play an active role in the life and death decisions of ovarian follicles. Autophagy is morphologically characterized by intracellular reorganization of membranes and increased number of autophagic vesicles that engulf bulk cytoplasm as well as organelles. Autophagy begins with the encapsulation of cytoplasmic constituents in a membrane sac known as autophagosomes. The autophagic vesicles are then destroyed by the lysosomal enzymes such as hydrolases that results in follicular atresia. It seems that apoptosis as well as autophagy could play active roles in germ cells depletion from ovary. Hence, it is important to prevent these two pathways in order to retain the germ cells in ovary of several mammalian species that are either threatened or at the verge of extinction. The involvement of apoptosis and autophagy in germ cell depletion from mammalian ovary is reviewed and possible pathways have been proposed.

Necrosis and necroptosis in germ cell depletion from mammalian ovary: CHAUDHARY et al.

The maximum number of germ cells is present during the fetal life in mammals. Follicular atresia results in rapid depletion of germ cells from the cohort of the ovary. At the time of puberty, only a few hundred (<1%) germ cells are either culminated into oocytes or further get eliminated during the reproductive life. Although apoptosis plays a major role, necrosis as well as necroptosis, might also be involved in germ cell elimination from the mammalian ovary. Both necrosis and necroptosis show similar morphological features and are characterized by an increase in cell volume, cell membrane permeabilization, and rupture that lead to cellular demise. Necroptosis is initiated by tumor necrosis factor and operated through receptor interacting protein kinase as well as mixed lineage kinase domain‐like protein. The acetylcholinesterase, cytokines, starvation, and oxidative stress play important roles in necroptosis‐mediated granulosa cell death. The granulosa cell necroptosis directly or indirectly induces susceptibility toward necroptotic or apoptotic cell death in oocytes. Indeed, prevention of necrosis and necroptosis pathways using their specific inhibitors could enhance growth/differentiation factor‐9 expression, improve survivability as well as the meiotic competency of oocytes, and prevent decline of reproductive potential in several mammalian species and early onset of menopause in women. This study updates the information and focuses on the possible involvement of necrosis and necroptosis in germ cell depletion from the mammalian ovary.

Journey of oocyte from metaphase-I to metaphase-II stage in mammals

In mammals, journey from metaphase‐I (M‐I) to metaphase‐II (M‐II) is important since oocyte extrude first polar body (PB‐I) and gets converted into haploid gamete. The molecular and cellular changes associated with meiotic cell cycle progression from M‐I to M‐II stage and extrusion of PB‐I remain ill understood. Several factors drive oocyte meiosis from M‐I to M‐II stage. The mitogen‐activated protein kinase3/1 (MAPK3/1), signal molecules and Rho family GTPases act through various pathways to drive cell cycle progression from M‐I to M‐II stage. The down regulation of MOS/MEK/MAPK3/1 pathway results in the activation of anaphase‐promoting complex/cyclosome (APC/C). The active APC/C destabilizes maturation promoting factor (MPF) and induces meiotic resumption. Several signal molecules such as, c‐Jun N‐terminal kinase (JNK2), SENP3, mitotic kinesin‐like protein 2 (MKlp2), regulator of G‐protein signaling (RGS2), Epsin2, polo‐like kinase 1 (Plk1) are directly or indirectly involved in chromosomal segregation. Rho family GTPase is another enzyme that along with cell division cycle (Cdc42) to form actomyosin contractile ring required for chromosomal segregation. In the presence of origin recognition complex (ORC4), eccentrically localized haploid set of chromosomes trigger cortex differentiation and determine the division site for polar body formation. The actomyosin contractile activity at the site of division plane helps to form cytokinetic furrow that results in the formation and extrusion of PB‐I. Indeed, oocyte journey from M‐I to M‐II stage is coordinated by several factors and pathways that enable oocyte to extrude PB‐I. Quality of oocyte directly impact fertilization rate, early embryonic development, and reproductive outcome in mammals.

Role of granulosa cell mitogen-activated protein kinase 3/1 in gonadotropin-mediated meiotic resumption from diplotene arrest of mammalian oocytes

Abstract In mammals, preovulatory oocytes are encircled by several layers of granulosa cells (GCs) in follicular microenvironment. These follicular oocytes are arrested at diplotene arrest due to high level of cyclic nucleotides from encircling GCs. Pituitary gonadotropin acts at the level of encircling GCs and increases adenosine 3′,5′-cyclic monophosphate (cAMP) and guanosine 3′,5′-cyclic monophosphate (cGMP) and activates mitogen-activated protein kinase 3/1 (MAPK3/1) signaling pathway. The MAPK3/1 disrupts the gap junctions between encircling GCs and oocyte. The disruption of gap junctions interrupts the transfer of cyclic nucleotides to the oocyte that results a drop in intraoocyte cAMP level. A transient decrease in oocyte cAMP level triggers maturation promoting factor (MPF) destabilization. The destabilized MPF finally triggers meiotic resumption from diplotene arrest in follicular oocyte. Thus, MAPK3/1 from GCs origin plays important role in gonadotropin-mediated meiotic resumption from diplotene arrest in follicular oocyte of mammals.

Impact of stress on female reproductive health disorders: Possible beneficial effects of shatavari (Asparagus racemosus)

Stress is deeply rooted in the society and women are frequently exposed to psychological, physical and physiological stressors. Psychological stress disturbs reproductive health by inducing generation of reactive oxygen species (ROS) and thereby oxidative stress (OS). The increased OS may affect physiology of ovary, oocyte quality and cause female reproductive health disorders. To overcome stress-mediated reproductive health disorders in women, shatavari (Asparagus racemosus) is frequently recommended in Ayurvedic system of medicine. Although shatavari is one of the major health tonics and most popular rasayana drugs to treat reproductive ailments of women, underlying mechanism of shatavari action at the level of ovary remains poorly understood. Based on the existing studies, we propose that shatavari may improve female reproductive health complications including hormonal imbalance, polycystic ovarian syndrome (PCOS), follicular growth and development, oocyte quality and infertility possibly by reducing OS level and increasing antioxidants level in the body. Further studies are required to elucidate the mechanism of shatavari actions at the level of ovary and oocyte that directly impacts the reproductive health of women.