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Cardiovascular complications are major clinical hallmarks of acute and post-acute coronavirus disease 2019 (COVID-19). However, the mechanistic details of SARS-CoV-2 infectivity of endothelial cells remain largely unknown. Here, we demonstrate that the receptor binding domain (RBD) of the SARS-CoV-2 spike (S) protein shares a similarity with the proline-rich binding ena/VASP homology (EVH1) domain and identified the endoplasmic reticulum (ER) resident calreticulin (CALR) as an S-RBD interacting protein. Our biochemical analysis showed that CALR, via its proline-rich (P) domain, interacts with S-RBD and modulates proteostasis of the S protein. Treatment of cells with the proteasomal inhibitor bortezomib increased the expression of the S protein independent of CALR, whereas the lysosomal/autophagy inhibitor bafilomycin 1A, which interferes with the acidification of lysosome, selectively augmented the S protein levels in a CALR-dependent manner. More importantly, the shRNA-mediated knockdown of CALR increased SARS-CoV-2 infection and impaired calcium homeostasis of human endothelial cells. This study provides new insight into the infectivity of SARS-CoV-2, calcium hemostasis, and the role of CALR in the ER-lysosome-dependent proteolysis of the spike protein, which could be associated with cardiovascular complications in COVID-19 patients.

Highly neurotoxic A1-reactive astrocytes have been associated with several human neurodegenerative diseases. Complement protein C3 expression is strongly upregulated in A1 astrocytes, and this protein has been shown to be a specific biomarker of these astrocytes. Several cytokines released in neurodegenerative diseases have been shown to upregulate the production of amyloid β protein precursor (APP) and neurotoxic amyloid β (Aβ) peptides in reactive astrocytes. Also, aberrant Ca2+ signals have been proposed as a hallmark of astrocyte functional remodeling in Alzheimer’s disease mouse models. In this work, we induced the generation of A1-like reactive astrocytes after the co-treatment of U251 human astroglioma cells with a cocktail of the cytokines TNF-α, IL1-α and C1q. These A1-like astrocytes show increased production of APP and Aβ peptides compared to untreated U251 cells. Additionally, A1-like astrocytes show a (75 ± 10)% decrease in the Ca2+ stored in the endoplasmic reticulum (ER), (85 ± 10)% attenuation of Ca2+ entry after complete Ca2+ depletion of the ER, and three-fold upregulation of plasma membrane calcium pump expression, with respect to non-treated Control astrocytes. These altered intracellular Ca2+ dynamics allow A1-like astrocytes to efficiently counterbalance the enhanced release of Ca2+ from the ER, preventing a rise in the resting cytosolic Ca2+ concentration.

Endoplasmic reticulum (ER) stress has been demonstrated to play important roles in the pathogenesis of various cardiovascular diseases. The ER stress pathway is therefore a promising therapeutic target in cardiovascular disease. Although Panax notoginseng saponins (PNS) are one of the patent medicines that are traditionally used to treat cardiovascular disorders, their effects on ER stress in cardiac myocytes remain unexploited so far. This study investigates the effects of PNS on ER stress and its associated cell apoptosis along with the related mechanism in cardiac myocytes. PNS compounds were identified via high-performance liquid chromatograph (HPLC) assay. PNS-pretreated H9c2 cells, HL-1 cells, and primary cultured neonatal rat cardiomyocytes were stimulated with thapsigargin (TG) to induce ER stress response and apoptosis. ER stress response was tested by immunofluorescence or immunoblot of the ER protein chaperones—calnexin, binding immunoglobulin protein (BiP) and the C/EBP homologous protein (CHOP). Cell viability was tested by methyl thiazolyl tetrazolium (MTT) assay. Cell apoptosis was detected by immunoblot of Cleaved caspase-3 and flow cytometry analysis of Annexin V/propidium iodide (PI) staining. Cytosolic, mitochondrial, and ER calcium dynamics were investigated by calcium imaging. Moreover, a ryanodine receptor type-2 (RyR2) overexpression stable cell line was generated to verify the mechanism of RyR2 involved in PNS in the inhibition of ER stress and cell apoptosis. We demonstrate here that PNS protected cardiac myocytes from ER stress response and associated cell death in a concentration-dependent manner. Importantly, PNS reduced the elevation of cytosolic calcium, mitochondria calcium, as well as ER calcium in response to either TG or histamine treatment. PNS protection in ER stress was regulated by RyR2 expression. In summary, PNS protection against TG-induced ER stress response and its associated cell apoptosis in cardiac myocytes is calcium dependent. Through the regulation of ER calcium release mediated by RyR2, a novel mechanism for PNS in the prevention of cardiovascular diseases is thereby identified.

(1) Background: Members of the TRPC3/TRPC6/TRPC7 subfamily of canonical transient receptor potential (TRP) channels share an amino acid similarity of more than 80% and can form heteromeric channel complexes. They are directly gated by diacylglycerols in a protein kinase C-independent manner. To assess TRPC3 channel functions without concomitant protein kinase C activation, direct activators are highly desirable. (2) Methods: By screening 2000 bioactive compounds in a Ca2+ influx assay, we identified artemisinin as a TRPC3 activator. Validation and characterization of the hit was performed by applying fluorometric Ca2+ influx assays and electrophysiological patch-clamp experiments in heterologously or endogenously TRPC3-expressing cells. (3) Results: Artemisinin elicited Ca2+ entry through TRPC3 or heteromeric TRPC3:TRPC6 channels, but did not or only weakly activated TRPC6 and TRPC7. Electrophysiological recordings confirmed the reversible and repeatable TRPC3 activation by artemisinin that was inhibited by established TRPC3 channel blockers. Rectification properties and reversal potentials were similar to those observed after stimulation with a diacylglycerol mimic, indicating that artemisinin induces a similar active state as the physiological activator. In rat pheochromocytoma PC12 cells that endogenously express TRPC3, artemisinin induced a Ca2+ influx and TRPC3-like currents. (4) Conclusions: Our findings identify artemisinin as a new biologically active entity to activate recombinant or native TRPC3-bearing channel complexes in a membrane-confined fashion.