Expression of the prion protein gene (PRNP) and cellular prion protein (PrPc) in cattle and sheep fetuses and maternal tissues during pregnancy

Physiology of Cellular Prion Proteins in Reproduction

Cellular prion protein (PrPC) encoded at Prnp gene is well-known to form a misfolded isoform, termed scrapie PrP (PrPSC) that cause transmissible degenerative diseases in central nervous system. The physiological role of PrPC has been proposed by many studies, showing that PrPC interacts with various intracellular, membrane, and extracellular molecules including mitochondrial inner membrane as a scaffold. PrPC is expressed in most cell types including reproductive organs. Numerous studies using PrPC knockout rodent models found no obvious phenotypic changes, in particular the clear phenotypes in development and reproduction have not demonstrated in these knockout models. However, various roles of PrPC have been evaluated at the cellular levels. In this review, we summarized the known roles of PrPC in various cell types and tissues with a special emphasis on those involved in reproduction.

A Potential biomarker for the early diagnosis of OSCC: saliva and serum PrPC

Background: Oral squamous cell carcinoma (OSCC) is frequently diagnosed at an advanced stage, and the high mortality of patients is mainly due to the delay of diagnosis. Cellular prion protein (PrPC) contributes to the occurrence and development of many malignant tumors. However, little has been known about the clinical and diagnostic value of PrPC in OSCC. This study investigated the levels of PrPC in the saliva and serum of patients with OSCC, OPMD and control group and their diagnostic value. Methods: The Cancer Genome Atlas (TCGA), Gene Expression Omnibus (GEO) and Clinical Proteome Tumor Analysis Consortium (CPTAC) databases were analyzed to evaluate the expression of human prion protein gene (PRNP) mRNA and PrPC in OSCC. Enzyme-linked Immunosorbent Assay (ELISA) was utilized to detect the expression of PrPC in saliva and serum samples of OSCC, OPMD and control groups. Furthermore, diagnostic value and clinical significance of PrPC in OSCC was identified. Protein-protein interaction (PPI) network was constructed by STRING. GO and KEGG analysis were performed by ClusterProfiler. Results: The levels of PRNP mRNA and PrPC in OSCC were significantly higher than those in the control group from databases (P<0.05). Besides, salivary and serum PrPC of OSCC patients showed increased levels compared with OPMD and control groups (P<0.05). The expression of salivary and serum PrPC of OSCC was correlated with the degree of differentiation (P<0.05), and the expression of PrPC from CPTAC was related to tumor stage of OSCC (P<0.05). The areas under the diagnostic curves (AUCs) of salivary and serum PrPC were 0.807 and 0.671, respectively. GO and KEGG analysis revealed that PrPC might be related to cell adhesion, cell differentiation, signal transduction and apoptosis, and participate in the pathways of focal adhesion, PI3K-Akt signaling pathway and ECM- receptor interaction in OSCC. Conclusion: PrPC in saliva and serum may be a potential biomarker for early diagnosis of OSCC.

Harnessing the Physiological Functions of Cellular Prion Protein in the Kidneys: Applications for Treating Renal Diseases

A cellular prion protein (PrP^C) is a ubiquitous cell surface glycoprotein, and its physiological functions have been receiving increased attention. Endogenous PrP^C is present in various kidney tissues and undergoes glomerular filtration. In prion diseases, abnormal prion proteins are found to accumulate in renal tissues and filtered into urine. Urinary prion protein could serve as a diagnostic biomarker. PrP^C plays a role in cellular signaling pathways, reno-protective effects, and kidney iron uptake. PrP^C signaling affects mitochondrial function via the ERK pathway and is affected by the regulatory influence of microRNAs, small molecules, and signaling proteins. Targeting PrP^C in acute and chronic kidney disease could help improve iron homeostasis, ameliorate damage from ischemia/reperfusion injury, and enhance the efficacy of mesenchymal stem/stromal cell or extracellular vesicle-based therapeutic strategies. PrP^C may also be under the influence of BMP/Smad signaling and affect the progression of TGF-β-related renal fibrosis. PrP^C conveys TNF-α resistance in some renal cancers, and therefore, the coadministration of anti-PrP^C antibodies improves chemotherapy. PrP^C can be used to design antibody-drug conjugates, aptamer-drug conjugates, and customized tissue inhibitors of metalloproteinases to suppress cancer. With preclinical studies demonstrating promising results, further research on PrP^C in the kidney may lead to innovative PrP^C-based therapeutic strategies for renal disease.

Infectious Prions in the Pregnancy Microenvironment of Chronic Wasting Disease-Infected Reeves' Muntjac Deer

Ample evidence exists for the presence of infectious agents at the maternal-fetal interface, often with grave outcomes to the developing fetus (i.e., Zika virus, brucella, cytomegalovirus, and toxoplasma). While less studied, pregnancy-related transmissible spongiform encephalopathies (TSEs) have been implicated in several species, including humans. Our previous work has shown that prions can be transferred from mother to offspring, resulting in the development of clinical TSE disease in offspring born to muntjac dams infected with chronic wasting disease (CWD) (1). We further demonstrated protein misfolding cyclic amplification (PMCA)-competent prions within the female reproductive tract and in fetal tissues harvested from CWD experimentally and naturally exposed cervids (1, 2). To assess whether the PMCA-competent prions residing at the maternal-fetal interface were infectious and to determine if the real-time quaking-induced conversion (RT-QuIC) methodology may enhance our ability to detect amyloid fibrils within the pregnancy microenvironment, we employed a mouse bioassay and RT-QuIC. In this study, we have demonstrated RT-QuIC seeding activity in uterus, placentome, ovary, and amniotic fluid but not in allantoic fluids harvested from CWD-infected Reeves' muntjac dams showing clinical signs of infection (clinically CWD-infected) and in some placentomes from pre-clinically CWD-infected dams. Prion infectivity was confirmed within the uterus, amniotic fluid, and the placentome, the semipermeable interface that sustains the developing fetus, of CWD-infected dams. This is the first report of prion infectivity within the cervid pregnancy microenvironment, revealing a source of fetal CWD exposure prior to the birthing process, maternal grooming, or encounters with contaminated environments.IMPORTANCE The facile dissemination of chronic wasting disease within captive and free-range cervid populations has led to questions regarding the transmission dynamics of this disease. Direct contact with infected animals and indirect contact with infectious prions in bodily fluids and contaminated environments are suspected to explain the majority of this transmission. A third mode of transmission, from mother to offspring, may be underappreciated. The presence of pregnancy-related prion infectivity within the uterus, amniotic fluid, and the placental structure reveals that the developing fetus is exposed to a source of prions long before exposure to the infectious agent during and after the birthing process or via contact with contaminated environments. These findings have impact on our current concept of CWD disease transmission.

Prenatal transmission of scrapie in sheep and goats: A case study for veterinary public health

Unsettled knowledge as to whether scrapie transmits prenatally in sheep and goats and transmits by semen and preimplantation embryos has a potential to compromise measures for controlling, preventing and eliminating the disease. The remedy may be analysis according to a systematic review, allowing comprehensive and accessible treatment of evidence and reasoning, clarifying the issue and specifying the uncertainties. Systematic reviews have clearly formulated questions, can identify relevant studies and appraise their quality and can summarise evidence and reasoning with an explicit methodology. The present venture lays a foundation for a possible systematic review and applies three lines of evidence and reasoning to two questions. The first question is whether scrapie transmits prenatally in sheep and goats. It leads to the second question, which concerns the sanitary safety of artificial breeding technologies, and is whether scrapie transmits in sheep and goats by means of semen and washed or unwashed in vivo derived embryos. The three lines of evidence derive from epidemiological, field and clinical studies, experimentation, and causal reasoning, where inferences are made from the body of scientific knowledge and an understanding of animal structure and function. Evidence from epidemiological studies allow a conclusion that scrapie transmits prenatally and that semen and embryos are presumptive hazards for the transmission of scrapie. Evidence from experimentation confirms that semen and washed or unwashed in vivo derived embryos are hazards for the transmission of scrapie. Evidence from causal reasoning, including experience from other prion diseases, shows that mechanisms exist for prenatal transmission and transmission by semen and embryos in both sheep and goats.

Neuroprotective effect of cellular prion protein (PrPC) is related with activation of alpha7 nicotinic acetylcholine receptor (α7nAchR)-mediated autophagy flux

Activation of the alpha7 nicotinic acetylcholine receptor (α7nAchR) is regulated by prion protein (PrPC) expression and has a neuroprotective effect by modulating autophagic flux. In this study, we hypothesized that PrPC may regulate α7nAchR activation and that may prevent prion-related neurodegenerative diseases by regulating autophagic flux. PrP(106-126) treatment decreased α7nAchR expression and activation of autophagic flux. In addition, the α7nAchR activator PNU-282987 enhanced autophagic flux and protected neuron cells against PrP(106-126)-induced apoptosis. However, activation of autophagy and the protective effects of PNU-282987 were inhibited in PrPC knockout hippocampal neuron cells. In addition, PrPC knockout hippocampal neuron cells showed decreased α7nAchR expression levels. Adenoviral overexpression of PrPC in PrPC knockout hippocampal neuron cells resulted in activation of autophagic flux and inhibition of prion peptide-mediated cell death via α7nAchR activation. This is the first report demonstrating that activation of α7nAchR-mediated autophagic flux is regulated by PrPC, and that activation of α7nAchR regulated by PrPC expression may play a pivotal role in protection of neuron cells against prion peptide-induced neuron cell death by autophagy. These results suggest that α7nAchR-mediated autophagic flux may be involved in the pathogenesis of prion-related diseases and may be a therapeutic target for prion-related neurodegenerative diseases.

The prion protein family: a view from the placenta

Based on its developmental pattern of expression, early studies suggested the implication of the mammalian Prion protein PrP, a glycosylphosphatidylinositol-anchored ubiquitously expressed and evolutionary conserved glycoprotein encoded by the Prnp gene, in early embryogenesis. However, gene invalidation in several species did not result in obvious developmental abnormalities and it was only recently that it was associated in mice with intra-uterine growth retardation and placental dysfunction. A proposed explanation for this lack of easily detectable developmental-related phenotype is the existence in the genome of one or more gene (s) able to compensate for the absence of PrP. Indeed, two other members of the Prnp gene family have been recently described, Doppel and Shadoo, and the consequences of their invalidation alongside that of PrP tested in mice. No embryonic defect was observed in mice depleted for Doppel and PrP. Interestingly, the co-invalidation of PrP and Shadoo in two independent studies led to apparently conflicting observations, with no apparent consequences in one report and the observation of a developmental defect of the ectoplacental cone that leads to early embryonic lethality in the other. This short review aims at summarizing these recent, apparently conflicting data highlighting the related biological questions and associated implications in terms of animal and human health.

Prion (PrPC) expression in ovine uteroplacental tissues increases after estrogen treatment of ovariectomized ewes and during early pregnancy

Scrapie in sheep is spread laterally by placental transmission of an infectious misfolded form (PrPSc) of a normal prion protein (PrPC) used as a template in PrPSc formation. We hypothesized that PrPC would be expressed in uterine and placental tissues and estradiol-17β (E2) would affect uterine PrPC expression. PrPC expression was evaluated in the uterus of long-term ovariectomized (OVX) ewes treated with an E2 implant for 2-24 h and in uteroplacental tissues from day 20 to day 30 of pregnancy. Expression of PrPC mRNA and PrPC protein increased in the uterus after E2 treatment of OVX ewes. In the maternal placenta, expression of PrPC mRNA and PrPC protein were unchanged, but in the fetal membranes (FM) PrPC mRNA and PrPC protein expression increased from day 20 to day 28. In the nonpregnant uterus, PrPC protein was immunolocalized at apical borders of the surface epithelium, in outer smooth muscle layers of large blood vessels, and in scattered stromal cells of the deep intercaruncular areas of the uterus. In the maternal placenta, PrPC protein was immunolocalized in the cytoplasm of flattened luminal epithelial cells apposed to the FM, whereas in the FM PrPC protein was in trophoblast cells and was also in several tissues of the developing embryo during early pregnancy. These data linking estrogen stimulation to increases in PrPC expression in uteroplacental tissues suggest that PrPC has a specific function during the estrous cycle and early pregnancy. Future studies should determine whether or not estrogen influences PrPC expression in other tissues, such as the nervous system and brain.

Developmental expression of the cellular prion protein (PrP(C) ) in bovine embryos

The mammalian cellular prion protein (PrP(C) ) is a highly conserved glycoprotein that may undergo conversion into a conformationally altered isoform (scrapie prion protein or PrP(Sc) ), widely believed to be the pathogenic agent of transmissible spongiform encephalopathies (TSEs). Although much is known about PrP(Sc) conversion and its role in TSEs, the normal function of PrP(C) has not been elucidated. In adult mammals, PrP(C) is most abundant in the central nervous tissue, with intermediate levels in the intestine and heart, and lower levels in the pancreas and liver. PrP(C) is expressed during neurogenesis throughout development, and it has recently been proposed that PrP(C) participates in neural cell differentiation during embryogenesis. In order to establish the developmental timing and to address the cell-specific expression of PrP(C) during mammalian development, we examined PrP(C) expression in bovine gametes and embryos through gestation Day 39. Our data revealed differential levels of Prnp mRNA at Days 4 and 18 in pre-attachment embryos. PrP(C) was detected in the developing central and peripheral nervous systems in Day-27, 32-, and -39 embryos. PrP(C) was particularly expressed in differentiated neural cells located in the marginal regions of the central nervous system, but was absent from mitotically active, periventricular areas. Moreover, a PrP(C) cell-specific pattern of expression was detected in non-nervous tissues, including liver and mesonephros, during these stages. The potential participation of PrP(C) in neural cell differentiation is supported by its specific expression in differentiated states of neurogenesis.

Mapping the prion protein distribution in marsupials: insights from comparing opossum with mouse CNS

The cellular form of the prion protein (PrP^C) is a sialoglycoprotein widely expressed in the central nervous system (CNS) of mammalian species during neurodevelopment and in adulthood. The location of the protein in the CNS may play a role in the susceptibility of a species to fatal prion diseases, which are also known as the transmissible spongiform encephalopathies (TSEs). To date, little is known about PrP^C distribution in marsupial mammals, for which no naturally occurring prion diseases have been reported. To extend our understanding of varying PrP^C expression profiles in different mammals we carried out a detailed expression analysis of PrP^C distribution along the neurodevelopment of the metatherian South American short-tailed opossum (Monodelphis domestica). We detected lower levels of PrP^C in white matter fiber bundles of opossum CNS compared to mouse CNS. This result is consistent with a possible role for PrP^C in the distinct neurodevelopment and neurocircuitry found in marsupials compared to other mammalian species.

Iatrogenic prion diseases in humans: an update

Although Creutzfeldt-Jakob disease (CJD) was first identified in 1920, prevention of transmission raised particular concern all over the world when a new variant of the disease was first described in 1996. There is good evidence of iatrogenic transmission of this new variant among human beings through blood, blood components, tissues and growth hormone. Furthermore, four cases of iatrogenic transmission of CJD through fertility treatment with human pituitary-derived gonadotrophins have been reported. It is important to distinguish the categories of infectivity and categories of risk, which require consideration not only of the level of infectivity of a given tissue or fluid, but also the amount of tissue/fluid to which a person is exposed, the duration of exposure and the route by which infection is transmitted. The potential presence and infectivity of prion proteins in human urinary gonadotrophin preparations is a matter of debate. Differences in the sensitivity of bioassay methods are of paramount importance when considering the infectivity of a tissue. Some new methods might detect small amounts of agent in some tissues currently thought to be free of infectivity. No cases of human prion disease due to the use of urinary gonadotrophins have been recognized to date. However, the detection of prions in the urine of experimental animals and in some urine-based preparations, and the young age of fertility drug recipients, require the application of the precautionary principle to urinary preparations.

Effects of nutrition and genotype on prion protein (PrPC) gene expression in the fetal and maternal sheep placenta

For placental transmission of scrapie to occur, the normal cellular prion protein (PrPC) must be converted to an abnormal infectious form known as PrPSc. PrPC genotype influences susceptibility to contracting scrapie, but we still do not understand whether genotype or expression levels of PrPC are important in transmission of scrapie. Some evidence exists that nutrition affects expression levels of PrPC. Thus, we evaluated the effects of genotype and nutrition on PrPC mRNA and protein expression in adolescent ewes fed at control (100% of National Research Council [NRC] requirements) or restricted (60% of NRC) levels of diet intake during two periods of pregnancy (days 50-90 and days 90-130)]. Gravid uteri (n=50) from singleton pregnancies were collected at day 130, and placentomes were either separated into caruncular (CAR; maternal) or cotyledonary (COT; fetal) placenta and snap-frozen for PrPC mRNA expression or perfusion fixed for PrPC protein expression. PrPC genotypes were determined (codons 136 and 171) using SNP assay. There were no genotype effects on PrPC mRNA expression in CAR or on PrPC protein expression in either CAR or COT, but PrPC mRNA expression in COT was greater (P<0.02) when codon 136 was homozygous for alanine. Some PrPC protein-positive cells were found in the epithelium of CAR, but most were found in trophoblast binucleate and mononucleate cells of COT. In CAR, from days 90 to 130, PrPC protein abundance was greater (P=0.003) in diet-restricted ewes than in control ewes, but was less uniformly distributed (P<0.007). Additionally, in COT, from days 90 to 130, PrPC protein was less uniformly distributed (P<0.01) in diet-restricted ewes. The localized increase in PrPC protein expression, found in ewes diet-restricted late in pregnancy, may suggest a protective role for PrPC in placental biology. Further study is needed to evaluate whether nutrition, PrPC genotype, and PrPC expression levels influence placental transmission of scrapie.

Susceptibility of cell substrates to PrPSc infection and safety control measures related to biological and biotherapeutical products

Concerns over the potential for infectious prion proteins to contaminate human biologics and biotherapeutics have been raised from time to time. Transmission of the pathogenic form of prion protein (PrP(Sc)) through veterinary vaccines has been observed, yet no human case through the use of vaccine products has been reported. However, iatrogenic transmissions of PrP(Sc) in humans through blood components, tissues and growth hormone have been reported. These findings underscore the importance of reliable detection or diagnostic methods to prevent the transmission of prion diseases, given that the number of asymptomatic infected individuals remains unknown, the perceived incubation time for human prion diseases could be decades, and no cure of the diseases has been found yet. A variety of biochemical and molecular methods can selectively concentrate PrP(Sc) to facilitate its detection in tissues and cells. Furthermore, some methods routinely used in the manufacturing process of biological products have been found to be effective in reducing PrP(Sc) from the products. Questions remain unanswered as to the validation criteria of these methods, the minimal infectious dose of the PrP(Sc) required to cause infection and the susceptibility of cells used in gene therapy or the manufacturing process of biological products to PrP(Sc) infections. Here, we discuss some of these challenging issues.