SARS-CoV-2 remains an endemic virus, that can potentially infect, hospitalize, and even kill people. The mortality rate from the SARS-CoV-2 virus infection appeared to drop in early 2023, however, the repercussions on afflicted people continue to worry the scientific community. These effects are no longer limited to the respiratory system, but have spread to other systems of the human body, generating a variety of symptoms such as excessive weariness, brain fog, irregular menstrual cycle, fever, and impaired mobility. Such persistence or development of new symptoms in individuals infected with SARS-CoV-2 is usually referred to as post-COVID conditions. A critical molecular investigation of the SARS-CoV-2 infected patients stands to be a promising approach to gain insights into the molecular changes taking place in the patients with a history of SARS-CoV-2 infection.
The consequences of viral exploitation on the host transcriptome were investigated, and changes in the transcriptome of SARS-CoV-2 infected patients were studied. This revealed differences in the expression of several HICs and lipid metabolism-related genes in patients. Pathologically significant annotated groups of altered genes that may cause pathology in patients were found to be enriched using GSEA. Annotations - in terms of biological processes, cellular components, and molecular functions that highlighted ion channels transport and activity; cholesterol transport and binding; insulin response, gap junction assembly, and activity; lipid transportation and activation were obtained through this study.
Further, PPINs depicted the interactions between the HICs and lipid metabolism-related proteins and led to identification of pathways. Interesting findings in the form of overlapping pathways depicted the deregulation of pathways such as insulin secretion, calcium signaling, gap junction, cholesterol metabolism, inflammatory mediator regulation of TRP channels, long-term depression, gap junction, renin secretion, and apelin signaling, type II diabetes mellitus, taste transduction, GABAergic synapse, serotonergic synapse, cAMP signaling pathway, RAS signaling pathway, cortisol synthesis and secretion, pancreatic secretion, thyroid hormone synthesis, glucagon signaling pathway and other pathologically critical pathway in SARS-CoV-2 infection. Moreover, these findings correlated with our previous study (Munjal et al., 2022).
Furthermore, a pathway map consisting of the pathways including long-term depression, cholesterol metabolism, and type II diabetes mellitus was generated. In addition, calcium signaling, inflammatory mediator regulation of TRP channels, insulin secretion, and taste transduction were also represented in the pathway map. Here, we highlight the pathophysiology of following pathways:
• Long-term depression
Persistence or development of new symptoms in patients infected with SARS-CoV-2 infection referred to as post-COVID affects various organ systems including the central and peripheral nervous system (Zawilska and Kuczynska, 2022). Several patients also experience sleep disorders and depression with other nervous breakdowns (Zawilska and Kuczynska, 2022), (Olgun Yildizeli et al., 2023). HICs including the InsP3 receptor family, and phospholipase A2, among others (Supplementary Table S2) were identified to be deregulated in patients. HICs play a critical role by connecting the inferior olivary neurons in the medulla oblongata through gap junctions (Leznik and Llinas, 2005) (also identified to be deregulated as a pathway), maintaining their membrane potential (Llinas and Yarom, 1986). A decrease in the diffusion of AMPAR, a ligand-gated ion channel, at the synapses of the neurons could be a result of a rise in the intracellular calcium, which is again a central element in the synaptic plasticity (Henley and Wilkinson, 2013), (Anggono and Huganir, 2012). Our study has identified these ion channels namely CACNA1A, and ITPRs which are responsible for maintaining the intracellular calcium levels, to be deregulated.
• Cholesterol metabolism
Alterations in HICs are known to contribute in the deregulation of cholesterol metabolism (Cure and Cumhur Cure, 2021), (Maxfield and Wustner, 2002), (Craig et al., 2023). Cholesterol-enriched lipid rafts serve as a platform for entry of SARS-CoV-2 by endocytosis (Palacios-Rapalo et al., 2021). It is known to recruit receptors like ACE2, and transmembrane serine protease 2 for their interaction with viral spike-protein (Palacios-Rapalo et al., 2021), describing cholesterol levels are crucial for viral penetration (Kowalska et al., 2022). Lower levels of high-density lipoprotein cholesterol (HDL-C) are associated with increased vulnerability to SARS-CoV-2 infection along with other infections, whereas elevated HDL-C levels are linked to reduced susceptibility to these conditions (Kocar et al., 2021, Kowalska et al., 2022).
Voltage-dependent anion channels (VDACs), are located on the mitochondrial membrane and facilitate the transfer of metabolites and ions in the mitochondria (Varughese et al., 2021), (Campbell and Chan, 2007). Its alteration results in the alteration of cholesterol synthesis and transport (Varughese et al., 2021). VDACs, apolipoproteins, among other molecules (Supplementary Table S2) were found to be deregulated in the patients.
The pathway map highlights the alteration in pathophysiologically critical pathways in the context of SARS-CoV-2 infection and the role of deregulated HICs and lipid metabolism-related proteins in it. Some of these pathways were also identified in our previous study (Munjal et al., 2022) highlighting the importance of network-based approaches.
4 out of 376 HICs, that interact with lipid metabolism and SARS-CoV-2 proteins, were identified to interact with 17 drug molecules using DGIdb with a known mode of action.
Gap Junction Protein Alpha 1 (GJA1) was identified to interact with Caveolin-1 (CAV1), a lipid metabolism-related protein, and seven SARS-CoV-2 proteins (M, nsp4, nsp6, ORF14, ORF3b, ORF7a, ORF7b). CAV1, which is an integral membrane protein, is critically involved in regulating the blood-brain barrier (Huang et al., 2018). Several studies report the upregulation of CAV1 in the forebrain of SARS-CoV-2-infected patients (Green et al., 2022), (Premkumar and Sajitha Lulu, 2023). Upregulation of CAV1 further results in increased expression of vascular cell adhesion molecule-1 (VCAM-1) and CD3+ T cell infiltration of the hippocampus, a region responsible for memory and short-term learning. This cascading upregulation of proteins was observed in SARS-CoV-2 infection, which contributed to neuroinflammatory symptoms including deficiency in learning and memory (Trevino et al., 2023). Knockout studies on mice with CAV1 deficiency showed protection against neuroinflammatory symptoms during SARS-CoV-2 infection (Gioiosa et al., 2008), (Trushina et al., 2006). CAV1 which interacted with GJA1, was also found to be upregulated in SARS-CoV-2-infected patients. Langlois and colleagues reported the direct binding and interdependence between GJA1 and CAV1 (Langlois et al., 2008). Moreover, potential drug molecules like Carbenoxolone, Octanol, and Flufenamic acid were found to interact with GJA1 (Fig. 7). Most likely, GJAI could be one of the potential target molecules for drug repurposing.
Likewise, we identified drugs interacting with other three significantly deregulated HICs namely, Inositol 1,4,5-Trisphosphate Receptor Type 2 (ITPR2), Inositol 1,4,5-Trisphosphate Receptor Type 2 (ITPR3), and Potassium Voltage-Gated Channel Subfamily B Member 2 (KCNB2) (Supplementary Table S6 and S7) identified to be interacting with lipid metabolism-related and SARS-CoV-2 proteins.
Similarly, drugs that interact with HICs with known functions could be studied further to understand their physiological and therapeutic roles in SARS-CoV-2.