Angiotensin-converting enzyme 2 (ACE2) plays an important role as a member of the renin-angiotensin-aldosterone system (RAAS) in regulating the conversion of angiotensin II (Ang II) into angiotensin (1-7) (Ang [1-7]). But at the same time, while expressed on the surface of human cells, ACE2 is the entry receptor for SARS-CoV-2. Expression of this receptor has been described in several types of cells, including hematopoietic stem cells (HSCs) and endothelial progenitor cells (EPCs), which raises a concern that the virus may infect and damage the stem cell compartment. We demonstrate for the first time that ACE2 and the entry-facilitating transmembrane protease TMPRSS2 are expressed on very small CD133 + CD34 + Lin − CD45 − cells in human umbilical cord blood (UCB), which can be specified into functional HSCs and EPCs. The existence of these cells known as very small embryonic-like stem cells (VSELs) has been confirmed by several laboratories, and some of them may correspond to putative postnatal hemangioblasts. Moreover, we demonstrate for the first time that, in human VSELs and HSCs, the interaction of the ACE2 receptor with the SARS-CoV-2 spike protein activates the Nlrp3 inflammasome, which if hyperactivated may lead to cell death by pyroptosis. Based on this finding, there is a possibility that human VSELs residing in adult tissues could be damaged by SARS-CoV-2, with remote effects on tissue/organ regeneration. We also report that ACE2 is expressed on the surface of murine bone marrow-derived VSELs and HSCs, although it is known that murine cells are not infected by SARS-CoV-2. Finally, human and murine VSELs express several RAAS genes, which sheds new light on the role of these genes in the specification of early-development stem cells.

Background . The SARS-CoV-2 pandemic, with its high mortality, has become an urgent clinical problem. It is well established that SARS-CoV-2 enters human cells after binding to the angiotensin-converting enzyme 2 (ACE2) receptor and utilizes a spike protein (S) for attachment and entry into the cells, leading to their lysis or damage. This infection damages several organs, including lungs, heart, blood vessels, kidneys, and intestines and may lead to a fatal complication known as a "cytokine storm", which is the result of uncontrolled hyperactivation of the innate immunity-initiated response and the release of several pro-inflammatory cytokines. An important potential aspect of SARS-CoV-2 infection is damage to the stem cell compartment, which may lead to severe complications from the infection. The ACE2 receptor has been described as being expressed on the surfaces of certain types of stem cells, including specified hematopoietic stem cells (HSCs) and endothelial progenitor cells (EPCs). These cells could be potentially damaged and lysed after virus entry or could undergo pyroptosis due to hyperactivation of Nlrp3 inflammasomes. Supporting this possibility, we recently reported that all Nlrp3 inflammasome components are expressed in HSCs (Adamiak, M et al. Nlrp3 Inflammasome Signaling Regulates the Homing and Engraftment of Hematopoietic Stem Cells (HSPCs) by Enhancing Incorporation of CXCR4 Receptor into Membrane Lipid Rafts. Stem Cell Rev and Rep (2020).https://doi.org/10.1007/s12015-020-10005-w). Hypothesis. We hypothesized that one of the triggers of a cytokine storm could be interaction of the ACE2 receptor with the SARS-CoV-2 spike protein, leading to hyperactivation of Nlrp3 inflammsomes in target cells including population of stem cells.Materials and Methods. Experiments were performed on human stem cells at different levels of specification, including very small CD34+CD133+lin-CD45- cells, which may become specified into HSCs and EPCs, as well as human CD34+Lin-CD45+ HSCs and CD34+ CD133+ KDR+ CD31+ EPCs. These cells were phenotyped for expression of ACE2 and the SARS-CoV-2 entry-facilitating transmembrane protease TMPRSS2 at the mRNA level and by FACS at the protein level. Next, we exposed these cells to the NCP-CoV (2019-nCoV) spike protein (S1+S2 ECD, expressed with a His-tag; Sino Biological) at a concentration of 10 nM. After 16 h of incubation, the cells were lysed, and total RNA was isolated for qRT-PCR analysis of Nlrp3 and essential Nlrp3 inflammasome components, including ASC, caspase 1, IL1b, and IL18. In some experiments, UCB-derived HSCs were plated into 96-well plates and stimulated with NCP-CoV (2019-nCoV) spike protein, as described above, alone or together with angiotensin 1-7 or the anti-inflammatory heme oxygenase 1 (HO-1) activator CoPP. Results. First, we observed that the ACE2 receptor and SARS-CoV-2 entry-facilitating transmembrane protease TMPRSS2 are expressed by all types of stem cells evaluated in our studies. Moreover, we detected activation of Nlrp3 inflammasomes in response to viral spike protein. This activation was inhibited by exposure of the stimulated cells to angiotensin 1-7 or CoPP. Conclusions. We envision that, in addition to directly infecting target cells, virus can hyperactivate the Nlrp3 inflammasome in stem cells, which may trigger their pyroptosis. Therefore, since we still do not have an effective SARS-CoV-2 vaccine in hand, the results presented in our current work suggest that inhibition of the Nlrp3 inflammasome by the small-molecule inhibitor MCC950 or application of Nlrp3 inflammasome inhibitors, such as Ang (1-7) or heme oxygenase 1 activators, could find potential clinical application to prevent the onset of a cytokine storm and cell pyroptosis. Disclosures No relevant conflicts of interest to declare.

The effect of a sucrose diet and repeated one-day starvation on oxidative status in the ovary and uterus is still unknown. Our analysis focused on carbohydrate-lipid metabolism parameters and the changes in red blood cells, ovary and uterus superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) activities and malonylodialdehyde (MDA) concentration in rats fed with a diet containing 16% of sucrose and subjected to systematic one-day starvation when using such a diet. It was found that a diet with 16% sucrose contributed to the increase of antioxidant enzyme activity in the blood (GPx and CAT) and uterus (SOD), without changes in MDA concentrations, which indicates an increase in reactive oxygen species (ROS) concentration in these tissues, being balanced by an increase in antioxidant enzyme activity. The introduction of a regular one-day starvation period into the diet intensified oxidative stress and led to a redox imbalance in the reproductive tissues of female rats. This was manifested by higher GPx activity, lower CAT activity and higher MDA concentration in the uterus and lower GPx and CAT activities and lower MDA concentration in the ovaries. The observed changes may be the cause of fertility disorders and possible problems with fertilised egg cell implantation into the uterine tissue.

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