Hnrnpk, a protein differentially expressed in immature rat ovarian development, is required for normal primordial follicle assembly and development

Causal and Candidate Gene Variants in a Large Cohort of Women With Primary Ovarian Insufficiency

CONTEXT

A genetic etiology likely accounts for the majority of unexplained primary ovarian insufficiency (POI).

OBJECTIVE

We hypothesized that heterozygous rare variants and variants in enhanced categories are associated with POI.

DESIGN

The study was an observational study.

SETTING

Subjects were recruited at academic institutions.

PATIENTS

Subjects from Boston (n = 98), the National Institutes of Health and Washington University (n = 98), Pittsburgh (n = 20), Italy (n = 43), and France (n = 32) were diagnosed with POI (amenorrhea with an elevated follicle-stimulating hormone level). Controls were recruited for health in old age or were from the 1000 Genomes Project (total n = 233).

INTERVENTION

We performed whole exome sequencing (WES), and data were analyzed using a rare variant scoring method and a Bayes factor-based framework for identifying genes harboring pathogenic variants. We performed functional studies on identified genes that were not previously implicated in POI in a D. melanogaster model.

MAIN OUTCOME

Genes with rare pathogenic variants and gene sets with increased burden of deleterious variants were identified.

RESULTS

Candidate heterozygous variants were identified in known genes and genes with functional evidence. Gene sets with increased burden of deleterious alleles included the categories transcription and translation, DNA damage and repair, meiosis and cell division. Variants were found in novel genes from the enhanced categories. Functional evidence supported 7 new risk genes for POI (USP36, VCP, WDR33, PIWIL3, NPM2, LLGL1, and BOD1L1).

CONCLUSIONS

Candidate causative variants were identified through WES in women with POI. Aggregating clinical data and genetic risk with a categorical approach may expand the genetic architecture of heterozygous rare gene variants causing risk for POI.

Establishment and depletion of the ovarian reserve: physiology and impact of environmental chemicals

The reproductive life span in women starts at puberty and ends at menopause, following the exhaustion of the follicle stockpile termed the ovarian reserve. Increasing data from experimental animal models and epidemiological studies indicate that exposure to a number of ubiquitously distributed reproductively toxic environmental chemicals (RTECs) can contribute to earlier menopause and even premature ovarian failure. However, the causative relationship between environmental chemical exposure and earlier menopause in women remains poorly understood. The present work, is an attempt to review the current evidence regarding the effects of RTECs on the main ovarian activities in mammals, focusing on how such compounds can affect the ovarian reserve at any stages of ovarian development. We found that in rodents, strong evidence exists that in utero, neonatal, prepubescent and even adult exposure to RTECs leads to impaired functioning of the ovary and a shortening of the reproductive lifespan. Regarding human, data from cross-sectional surveys suggest that human exposure to certain environmental chemicals can compromise a woman's reproductive health and in some cases, correlate with earlier menopause. In conclusion, evidences exist that exposure to RTECs can compromise a woman's reproductive health. However, human exposures may date back to the developmental stage, while the adverse effects are usually diagnosed decades later, thus making it difficult to determine the association between RTECs exposure and human reproductive health. Therefore, epidemiological surveys and more experimental investigation on humans, or alternatively primates, are needed to determine the direct and indirect effects caused by RTECs exposure on the ovary function, and to characterize their action mechanisms.

Starvation at birth impairs germ cell cyst breakdown and increases autophagy and apoptosis in mouse oocytes

The female reproductive lifespan is largely determined by the size of primordial follicle pool, which is established following germ cell cyst breakdown around birth. Almost two-third of oocytes are lost during germ cell cysts breakdown, following autophagic and apoptosis mechanisms. To investigate a possible relationship between germ cell cyst breakdown and nutrition supply, we established a starvation model in mouse pups at birth and evaluated the dynamics of cyst breakdown during nutrient deprivation. Our results showed that after 36 h of starvation between 1.5 and 3 d.p.p., indicators of metabolism both at systemic and ovarian level were significantly altered and the germ cell cyst breakdown markedly decreased. We also found that markers of oxidative stress, autophagy and apoptosis were increased and higher number of oocytes in cyst showing autophagic markers and of TUNEL-positive oocytes and somatic cells were present in the ovaries of starved pups. Moreover, the proliferation of pre-granulosa cells and the expression of the oocyte-specific transcription factor Nobox were decreased in such ovaries. Finally, we observed that the ovaries of the starved pups could recover a normal number of follicles after about 3 weeks from re-feeding. In conclusion, these data indicate that nutrient deficiency at birth can generate a number of adaptive metabolic and oxidative responses in the ovaries causing increased apoptosis both in the somatic cells and oocyte and autophagy mainly in these latter and leading to a delay of germ cell cyst breakdown and follicle assembly.

Proteomic Analysis of Fetal Ovaries Reveals That Primordial Follicle Formation and Transition Are Differentially Regulated

Primordial follicle formation represents a critical phase of the initiation of embryonic reproductive organ development, while the primordial follicle transition into primary follicle determines whether oestrus or ovulation will occur in female animals. To identify molecular mechanism of new proteins which are involved in ovarian development, we employed 2D-DIGE to compare the protein expression profiles of primordial follicles and primary follicles of fetal ovaries in pigs. Fetal ovaries were collected at distinct time-points of the gestation cycle (g55 and g90). The identified proteins at the g55 time-point are mainly involved in the development of anatomical structures [reticulocalbin-1 (RCN1), reticulocalbin-3 (RCN3)], cell differentiation (actin), and stress response [heterogeneous nuclear ribonucleoprotein K (HNRNPK)]. Meanwhile, at the g90 stage, the isolated proteins with altered expression levels were mainly associated with cell proliferation [major vault protein (MVP)] and stress response [heat shock-related 70 kDa protein 2 (HSPA2)]. In conclusion, our work revealed that primordial follicle formation is regulated by RCN1, RCN3, actin, and HNRNPK, while the primordial follicle transformation to primary follicle is regulated by MVP and HSPA2. Therefore, our results provide further information for the prospective understanding of the molecular mechanism(s) involved in the regulation of the ovarian follicle development.

Expression and localization of heterogeneous nuclear ribonucleoprotein K in mouse ovaries and preimplantation embryos

Heterogeneous nuclear ribonucleoprotein K (hnRNP K), an evolutionarily conserved protein, is involved in several important cellular processes that are relevant to cell proliferation, differentiation, apoptosis, and cancer development. However, details of hnRNP K expression during mammalian oogenesis and preimplantation embryo development are lacking. The present study investigates the expression and cellular localization of K protein in the mouse ovaries and preimplantation embryos using immunostaining. We demonstrate, for the first time, that hnRNP K is abundantly expressed in the nuclei of mouse oocytes in primordial, primary and secondary follicles. In germ vesicle (GV)-stage oocytes, hnRNP K accumulates in the germinal vesicle in a spot distribution manner. After germinal vesicle breakdown, speckled hnRNP K is diffusely distributed in the cytoplasm. However, after fertilization, the K protein relocates into the female and male pronucleus and persists in the blastomere nuclei. Localization of K protein in the human ovary and ovarian granulosa cell tumor (GCT) was also investigated. Overall, this study provides important morphological evidence to better understand the possible roles of hnRNP K in mammalian oogenesis and early embryo development.

hnRNP K Is a Haploinsufficient Tumor Suppressor that Regulates Proliferation and Differentiation Programs in Hematologic Malignancies

hnRNP K regulates cellular programs, and changes in its expression and mutational status have been implicated in neoplastic malignancies. To directly examine its role in tumorigenesis, we generated a mouse model harboring an Hnrnpk knockout allele (Hnrnpk(+/-)). Hnrnpk haploinsufficiency resulted in reduced survival, increased tumor formation, genomic instability, and the development of transplantable hematopoietic neoplasms with myeloproliferation. Reduced hnRNP K expression attenuated p21 activation, downregulated C/EBP levels, and activated STAT3 signaling. Additionally, analysis of samples from primary acute myeloid leukemia patients harboring a partial deletion of chromosome 9 revealed a significant decrease in HNRNPK expression. Together, these data implicate hnRNP K in the development of hematological disorders and suggest hnRNP K acts as a tumor suppressor.

Transcriptional regulation of heterogeneous nuclear ribonucleoprotein K gene expression

Heterogeneous nuclear ribonucleoprotein K (hnRNP K) is importantly involved in the regulation of development, DNA damage response, and several human diseases. The molecular mechanisms that control the expression of hnRNP K are largely unknown. In the present study, we investigated the detailed mechanism of the transcriptional regulation of human hnRNP K gene. Two activating and one repressive elements located in the proximal segment of the transcriptional initiation site were identified in hnRNP K gene. A 19 bp-region was responsible for the inhibitory activities of the repressor element. Twenty proteins were identified by DNA-affinity purification and mass spectrometry analyses as binding partners of the primary activating element in the hnRNP K promoter. Chromatin immunoprecipitation and EMSA analysis confirmed the binding of Sp1 with hnRNP K promoter. Sp1 enhanced the promoter activity, increased the expression of hnRNP K, and reduced the mRNA level of angiotensinogen, a gene known to be negatively regulated by hnRNP K. In summary, the current study characterized the promoter elements that regulate the transcription of human hnRNP K gene, identified 20 proteins that bind to the primary activating element of hnRNP K promoter, and demonstrated a functional effect of Sp1 on hnRNP K transcription.

Systemic compensatory response to neonatal estradiol exposure does not prevent depletion of the oocyte pool in the rat

The formation of ovarian follicles is a finely tuned process that takes place within a narrow time-window in rodents. Multiple factors and pathways have been proposed to contribute to the mechanisms triggering this process but the role of endocrine factors, especially estrogens, remains elusive. It is currently hypothesized that removal from the maternal hormonal environment permits follicle formation at birth. However, experimentally-induced maintenance of high 17β-estradiol (E2) levels leads to subtle, distinct, immediate effects on follicle formation and oocyte survival depending on the species and dose. In this study, we examined the immediate effects of neonatal E2 exposure from post-natal day (PND) 0 to PND2 on the whole organism and on ovarian follicle formation in rats. Measurements of plasma E2, estrone and their sulfate conjugates after E2 exposure showed that neonatal female rats rapidly acquire the capability to metabolize and clear excessive E2 levels. Concomitant modifications to the mRNA content of genes encoding selected E2 metabolism enzymes in the liver and the ovary in response to E2 exposure indicate that E2 may modify the neonatal maturation of these organs. In the liver, E2 treatment was associated with lower acquisition of the capability to metabolize E2. In the ovary, E2 depleted the oocyte pool in a dose dependent manner by PND3. In 10 µg/day E2-treated ovaries, apoptotic oocytes were observed in newly formed follicles in addition to areas of ovarian cord remodeling. At PND6, follicles without any visible oocyte were present and multi-oocyte follicles were not observed. Our study reveals a major species-difference. Indeed, neonatal exposure to E2 depletes the oocyte pool in the rat ovary, whereas in the mouse it is well known to increase oocyte survival.

Heterogeneous nuclear ribonucleoprotein K supports vesicular stomatitis virus replication by regulating cell survival and cellular gene expression

The heterogeneous nuclear ribonucleoprotein K (hnRNP K) is a member of the family of hnRNPs and was recently shown in a genome-wide small interfering RNA (siRNA) screen to support vesicular stomatitis virus (VSV) growth. To decipher the role of hnRNP K in VSV infection, we conducted studies which suggest that the protein is required for VSV spreading. Virus binding to cells, entry, and nucleocapsid uncoating steps were not adversely affected in the absence of hnRNP K, whereas viral genome transcription and replication were reduced slightly. These results indicate that hnRNP K is likely involved in virus assembly and/or release from infected cells. Further studies showed that hnRNP K suppresses apoptosis of virus-infected cells, resulting in increased cell survival during VSV infection. The increased survival of the infected cells was found to be due to the suppression of proapoptotic proteins such as Bcl-XS and Bik in a cell-type-dependent manner. Additionally, depletion of hnRNP K resulted in not only significantly increased levels of T-cell-restricted intracellular antigen 1 (TIA1) but also switching of the expression of the two isoforms of the protein (TIA1a and TIA1b), both of which inhibited VSV replication. hnRNP K was also found to support expression of several cellular proteins known to be required for VSV infection. Overall, our studies demonstrate hnRNP K to be a multifunctional protein that supports VSV infection via its role(s) in suppressing apoptosis of infected cells, inhibiting the expression of antiviral proteins, and maintaining the expression of proteins required for the virus.

RNAi-Mediated Gene Silencing in a Gonad Organ Culture to Study Sex Determination Mechanisms in Sea Turtle

The autosomal Sry-related gene, Sox9, encodes a transcription factor, which performs an important role in testis differentiation in mammals. In several reptiles, Sox9 is differentially expressed in gonads, showing a significant upregulation during the thermo-sensitive period (TSP) at the male-promoting temperature, consistent with the idea that SOX9 plays a central role in the male pathway. However, in spite of numerous studies, it remains unclear how SOX9 functions during this event. In the present work, we developed an RNAi-based method for silencing Sox9 in an in vitro gonad culture system for the sea turtle, Lepidochelys olivacea. Gonads were dissected as soon as the embryos entered the TSP and were maintained in organ culture. Transfection of siRNA resulted in the decrease of both Sox9 mRNA and protein. Furthermore, we found coordinated expression patterns for Sox9 and the anti-Müllerian hormone gene, Amh, suggesting that SOX9 could directly or indirectly regulate Amh expression, as it occurs in mammals. These results demonstrate an in vitro method to knockdown endogenous genes in gonads from a sea turtle, which represents a novel approach to investigate the roles of important genes involved in sex determination or differentiation pathways in species with temperature-dependent sex determination.

Follicular assembly: mechanisms of action

The differentiation of primordial germ cells (PGCs) into functional oocytes is important for the continuation of species. In mammals, PGCs begin to differentiate into oocytes during embryonic development. Oocytes develop in clusters called germ line cysts. During fetal or neonatal development, germ cell cysts break apart into single oocytes that become surrounded by pregranulosa cells to form primordial follicles. During the process of cyst breakdown, a subset of cells in each cyst undergoes cell death with only one-third of the initial number of oocytes surviving to form primordial follicles. The mechanisms that control cyst breakdown, oocyte survival, and follicle assembly are currently under investigation. This review describes the mechanisms that have been implicated in the control of primordial follicle formation, which include programmed cell death regulation, growth factor and other signaling pathways, regulation by transcription factors and hormones, meiotic progression, and changes in cell adhesion. Elucidation of mechanisms leading to formation of the primordial follicle pool will help research efforts in ovarian biology and improve treatments of female infertility, premature ovarian failure, and reproductive cancers.