Proteomic Identification of Caldesmon as a Physiological Substrate of Proprotein Convertase 6 in Human Uterine Decidual Cells Essential for Pregnancy Establishment

Quantitative analysis of the serum proteome during early pregnancy in mares

Equine pregnancy is currently diagnosed by rectal palpation, ultrasonographic examination, or by measuring changes in hormones in the blood. In the present study, we identified proteins that are differentially expressed in the sera of early pregnant and non-pregnant mares in order to develop a novel method for diagnosing equine pregnancy. Serum samples were obtained from 18 adult mares, pregnancy at day 32 after ovulation (n = 9) and in diestrus (n = 9). Proteomic analysis of the samples was conducted using liquid chromatography-electrospray ionization-tandem mass spectrometry. We identified 467 proteins from a total of 3514 peptides. Thirty-two proteins (15 upregulated and 17 downregulated) were significantly differentially expressed between the two groups. The Gene Ontology enrichment analysis revealed that they are related to extracellular matrix assembly, blood coagulation, and hemostasis, and the prominent molecular functions were integrin binding, cell adhesion molecule binding, and glycine C-acetyltransferase activity. The pathway analysis of Kyoto Encyclopaedia of Genes and Genomes showed that the top three pathways identified were glycine, serine, and threonine metabolism; cysteine and methionine metabolism; and ether lipid metabolism. The selected five serum proteins were newly potential candidates for pregnancy diagnosis in mares.

Molecular mimicry between SARS-CoV-2 and the female reproductive system

Introduction

Oogenesis, the process of egg production by the ovary, involves a complex differentiation program leading to the production of functional oocytes. This process comprises a sequential pathway of steps that are finely regulated. The question related to SARS‐CoV‐2 infection and fertility has been evoked for several reasons, including the mechanism of molecular mimicry, which may contribute to female infertility by leading to the generation of deleterious autoantibodies, possibly contributing to the onset of an autoimmune disease in infected patients.

Objective

The immunological potential of the peptides shared between SARS‐CoV‐2 spike glycoprotein and oogenesis‐related proteins; Thus we planned a systematic study to improve our understanding of the possible effects of SARS‐CoV‐2 infection on female fertility using the angle of molecular mimicry as a starting point.

Methods

A library of 82 human proteins linked to oogenesis was assembled at random from UniProtKB database using oogenesis, uterine receptivity, decidualization, and placentation as a key words. For the analyses, an artificial polyprotein was built by joining the 82 a sequences of the oogenesis‐associated proteins. These were analyzed by searching the Immune Epitope DataBase for immunoreactive SARS‐CoV‐2 spike glycoprotein epitopes hosting the shared pentapeptides.

Results

SARS‐CoV‐2 spike glycoprotein was found to share 41 minimal immune determinants, that is, pentapeptides, with 27 human proteins that relate to oogenesis, uterine receptivity, decidualization, and placentation. All the shared pentapeptides that we identified, with the exception of four, are also present in SARS‐CoV‐2 spike glycoprotein–derived epitopes that have been experimentally validated as immunoreactive.

Caldesmon ablation in mice causes umbilical herniation and alters contractility of fetal urinary bladder smooth muscle

The actin-, myosin-, and calmodulin-binding protein caldesmon (CaD) is expressed in two splice isoforms: h-CaD, which is an integral part of the actomyosin domain of smooth muscle cells, and l-CaD, which is widely expressed and is involved in many cellular functions. Despite extensive research for many years, CaD's in vivo function has remained elusive. To explore the role of CaD in smooth muscle contraction in vivo, we generated a mutant allele that ablates both isoforms. Heterozygous animals were viable and had a normal life span, but homozygous mutants died perinatally, likely because of a persistent umbilical hernia. The herniation was associated with hypoplastic and dysmorphic abdominal wall muscles. We assessed mechanical parameters in isometrically mounted longitudinal strips of E18.5 urinary bladders and in ring preparations from abdominal aorta using wire myography. Ca²⁺ sensitivity was higher and relaxation rate was slower in Cald1^−/− compared with Cald1^+/+ skinned bladder strips. However, we observed no change in the content and phosphorylation of regulatory proteins of the contractile apparatus and myosin isoforms known to affect these contractile parameters. Intact fibers showed no difference in actin and myosin content, regardless of genotype, although KCl-induced force tended to be lower in homozygous and higher in heterozygous mutants than in WTs. Conversely, in skinned fibers, myosin content and maximal force were significantly lower in Cald1^−/− than in WTs. In KO abdominal aortas, resting and U46619 elicited force were lower than in WTs. Our results are consistent with the notion that CaD impacts smooth muscle function dually by (1) acting as a molecular brake on contraction and (2) maintaining the structural integrity of the contractile machinery. Most importantly, CaD is essential for resolution of the physiological umbilical hernia and ventral body wall closure.

Identification of novel dipeptidyl peptidase 9 substrates by two-dimensional differential in-gel electrophoresis

Dipeptidyl peptidase 9 (DPP9) is a member of the S9B/DPPIV (DPP4) serine protease family, which cleaves N-terminal dipeptides at an Xaa-Pro consensus motif. Cytoplasmic DPP9 has roles in epidermal growth factor signalling and in antigen processing, whilst the role of the recently discovered nuclear form of DPP9 is unknown. Mice lacking DPP9 proteolytic activity die as neonates. We applied a modified 2D differential in-gel electrophoresis approach to identify novel DPP9 substrates, using mouse embryonic fibroblasts lacking endogenous DPP9 activity. A total of 111 potential new DPP9 substrates were identified, with nine proteins/peptides confirmed as DPP9 substrates by MALDI-TOF or immunoblotting. Moreover, we also identified the dipeptide Val-Ala as a consensus site for DPP9 cleavage that was not recognized by DPP8, suggesting different in vivo roles for these closely related enzymes. The relative kinetics for the cleavage of these nine candidate substrates by DPP9, DPP8 and DPP4 were determined. This is the first identification of DPP9 substrates from cells lacking endogenous DPP9 activity. These data greatly expand the potential roles of DPP9 and suggest different in vivo roles for DPP9 and DPP8.

Label-free proteomics uncovers energy metabolism and focal adhesion regulations responsive for endometrium receptivity

The menstrual cycle of the female uterus leads to periodic changes of the endometrium. These changes are important for developing the endometrial receptivity and for achieving competency of embryo implantation. However, the molecular events underlying the endometrial receptivity process remain poorly understood. Here we applied an LC-MS-based label-free quantitative proteomic approach to compare the endometrial tissues in the midsecretory (receptive) phase with the endometrial tissues in the proliferative phase from age-matched woman (n = 6/group). The proteomes of endometrial tissues were extracted using an SDS-based detergent, digested by the filter-aided sample preparation procedures, and subsequently analyzed by nano-LC-MS/MS (Orbitrap XL) with a 4 h gradient. Reliable protein expression profiles were reproducibly obtained from the endometrial tissues in the receptive and proliferative phases. A total of 2138 protein groups were quantified under highly stringent criteria with a false discovery rate of <1% for peptide and protein groups. Among these proteins, 317 proteins had differences in expression that were statistically significant between the receptive and proliferative phases. Direct protein-protein interaction network analyses of these significantly changed proteins showed that the up-regulation of creatine kinase B-type (CKB) in the receptive phase may be related to endometrium receptivity. The interaction network also showed that proteins related to cell-cell adhesion were down-regulated. Moreover, the results from KEGG pathway analyses are consistent with the protein-protein interaction results. The proteins, including alpha-actinin (ACTN), extracellular matrix proteins, integrin alpha-V, and so on, that are involved in the focal adhesion pathway were down-regulated in the receptive phase compared with the proliferative phase, which may facilitate the implantation of the fertilized ovum. Selected proteins were validated by Western blot analysis and indirect immunofluorescence, including the up-regulation of CKB and down-regulation ACTN in the receptive phase. In summary, our proteomic analysis study shows potential for predicting the endometrial remodeling from the proliferative to the receptivity phase in women, and these results also reveal the key biological mechanisms (such as energy metabolism and focal adhesion) underlying human endometrial receptivity.

Caldesmon: new insights for diagnosing endometriosis

Considerable effort has been invested in searching for less invasive methods of diagnosing endometriosis. Previous studies have indicated altered levels of the CALD1 gene (encoding the protein caldesmon) in endometriosis. The aims of our study were to investigate whether average CALD1 expression and caldesmon protein levels are differentially altered in the endometrium and endometriotic lesions and to evaluate the performance of the CALD1 gene and caldesmon protein as potential biomarkers for endometriosis. Paired biopsies of endometrial tissue (eutopic endometrium) and endometriotic lesions (ectopic endometrium) were obtained from patients with endometriosis to evaluate CALD1 gene expression and caldesmon protein levels by real-time PCR and Western blot analysis, respectively. In addition, immunostaining for caldesmon to determine cellular localization was also performed. Endometrium from women without endometriosis was used as a control. Increased CALD1 expression and caldesmon levels were detected in the endometriotic lesions. The electrophoretic profile of caldesmon by Western blot analysis was clearly different between the control group (endometrium of women without endometriosis) and the group of women with endometriosis (eutopic endometrium and endometriotic lesions). Caldesmon expression as determined by immunostaining showed no variation among the cell types in endometriotic lesions and eutopic endometrium. Stromal cells marked positively in eutopic endometrium from control patients and in the endometriotic lesions. The presence of caldesmon in the endometrium of patients with and without endometriosis permitted diagnoses with 95% sensitivity (specificity 100%) and 100% sensitivity (specificity 100%) for the disease and for minimal to mild endometriosis in the proliferative phase of the menstrual cycle, respectively. In the secretory phase, minimal to mild endometriosis was detected with 90% sensitivity and 93.3% specificity. Caldesmon is a possible predictor of endometrial dysregulation in patients with endometriosis. A potential limitation of our study is the fact that other endometrial diseases were not excluded, and therefore prospective studies are needed to confirm the potential of caldesmon as a biomarker exclusively for endometriosis.

Proprotein convertase 5/6 is critical for embryo implantation in women: regulating receptivity by cleaving EBP50, modulating ezrin binding, and membrane-cytoskeletal interactions

Establishment of endometrial receptivity is vital for successful embryo implantation; its failure causes infertility. Epithelial receptivity acquisition involves dramatic structural changes in the plasma membrane and cytoskeleton. Proprotein convertase 5/6 (PC6), a serine protease of the proprotein convertase (PC) family, is up-regulated in the human endometrium specifically at the time of epithelial receptivity and stromal cell decidualization. PC6 is the only PC member tightly regulated in this manner. The current study addressed the importance and mechanisms of PC6 action in regulating receptivity in women. PC6 was dysregulated in the endometrial epithelium during the window of implantation in infertile women of three demographically different cohorts. Its critical role in receptivity was evidenced by a significant reduction in mouse blastocyst attachment of endometrial epithelial cells after PC6 knockdown by small interfering RNA. Using a proteomic approach, we discovered that PC6 cleaved the key scaffolding protein, ezrin-radixin-moesin binding phosphoprotein 50 (EBP50), thereby profoundly affecting its interaction with binding protein ezrin (a key protein bridging actin filaments and plasma membrane), EBP50/ezrin cellular localization, and cytoskeleton-membrane connections. We further validated this novel PC6 regulation of receptivity in human endometrium in vivo in fertile vs. infertile patients. These results strongly indicate that PC6 plays a key role in regulating fundamental cellular remodeling processes, such as plasma membrane transformation and membrane-cytoskeletal interface reorganization. PC6 cleavage of a crucial scaffolding protein EBP50, thereby profoundly regulating membrane-cytoskeletal reorganization, greatly extends the current knowledge of PC biology and provides substantial new mechanistic insight into the fields of reproduction, basic cellular biology, and PC biochemistry.

PC6 levels in uterine lavage are closely associated with uterine receptivity and significantly lower in a subgroup of women with unexplained infertility

BACKGROUND Embryo implantation requires a healthy embryo and a receptive uterus. Uterine incompetence contributes significantly to implantation failure and infertility. To date, there are no reliable biochemical methods that can determine whether the uterus is receptive. Proprotein convertase 5/6 (PC6) is tightly regulated in the uterus and critical for receptivity and implantation; its secretory nature predicts PC6 to be secreted into the uterine cavity. The present study examines whether PC6 is detectable in uterine lavage and whether there is any correlation between secreted PC6 levels and uterine receptivity. METHODS Western blotting determined the presence of PC6 protein in uterine lavage. A sensitive and high-throughput activity assay was established and validated. This assay was applied to 103 lavages collected from different phases of the menstrual cycle from women with proven fertility or unexplained infertility. RESULTS Uterine lavage contained PC6 protein with levels paralleling enzymatic activity. PC6 levels were significantly higher in the receptive than in the non-receptive phase in fertile women, and the putative receptive phase levels in a subgroup of women with unexplained infertility were significantly lower than in the fertile counterparts. CONCLUSIONS PC6 levels in uterine lavage are significantly elevated in the luteal phase of fertile women and markedly reduced in a subgroup of women with unexplained infertility. Uterine fluid is a valuable source of material to evaluate uterine function. Detection of PC6 in uterine fluid may lead to the development of a rapid and relatively non-surgical assessment of uterine receptivity.

A proteomic protocol to identify physiological substrates of pro-protein convertases

Proprotein convertases (PCs) convert pro-proteins into their bioactive forms through limited proteolytic cleavage, thereby regulating the temporal and spatial activation of a large number of functionally important proteins. This "converting" process is involved in a wide range of essential physiological and pathological processes, making PCs valuable therapeutic targets. One of the challenges in the field of PC research has been to identify the physiological substrates of a particular PC in a specific tissue or cellular process. Proteomics provides an unprecedented opportunity to identify novel PC substrates in a physiological context. Here we provide a detailed practical procedure utilizing two-dimensional fluorescent differential gel electrophoresis (2D-DiGE) and tandem mass spectrometry techniques, in combination with other standard molecular and biochemical methods, to identify and subsequently validate novel PC6 substrates in a critical uterine event called decidualization. This method is applicable to the study of any PC members and their relevant cellular processes.

Proteomic approach identifies alterations in cytoskeletal remodelling proteins during decidualization of human endometrial stromal cells

Decidualization is a tissue remodelling process within the uterus in preparation for embryo implantation and pregnancy. In this study we isolated primary human endometrial stromal cells and stimulated decidualization with cAMP. We then used 2D- differential in-gel electrophoresis (DIGE) to identify proteins induced by decidualization. Eighty-eight out of 2714 spots were differentially regulated, 18 of which were assigned clear identities by mass spectrometry. Many of these are proteins known to be associated with cell structure and cytoskeletal remodelling. We validated five of these proteins by Western blot and immunohistochemistry on human endometrial tissue. The validated proteins are caldesmon 1, src substrate contactin 8, tropomyosin alpha-4 chain, protein disulfide isomerase 1A, and LIM and SH3 domain protein. With the exception of caldesmon 1, none of the identified proteins have previously been associated with decidualization. This study provides insight into our understanding of decidualization, which is important for successful embryo implantation and establishment of pregnancy.

Posttranslational activation of bone morphogenetic protein 2 is mediated by proprotein convertase 6 during decidualization for pregnancy establishment

Bone morphogenetic proteins (BMPs) require major posttranslational modifications to become biologically active. One such key modification is endoproteolytic cleavage of the initially synthesized nonactive precursor protein to release the mature ligand. Here we show in a physiological context of uterine stromal decidualization that BMP2 cleavage is mediated by proprotein convertase 5/6 (PC6). Decidualization is a uterine remodeling event critical for embryo implantation. Deletion or knockdown of either BMP2 or PC6 inhibits decidualization causing implantation failure and female infertility. In this study we provide biochemical and physiological evidence that PC6 proteolytically activates BMP2. We used freshly isolated primary human endometrial stromal cells and demonstrated that PC6 was the sole member of the PC family significantly up-regulated during decidualization. The precursor form of BMP2 was reduced, whereas its active form was increased during decidualization. Inhibition of PC6 activity inhibited decidualization, and this was accompanied by a total blockade of BMP2 activation. Addition of recombinant active BMP2 partially rescued the decidualization arrest caused by PC6 inhibition. PC6 processed BMP2 at the KREKR(282) downward arrow cleavage site, and mutating this site prevented the cleavage. This study thus demonstrates for the first time that the proteolytic activation and thus bioavailability of BMP2 is controlled by PC6.