A Pilot Study on Blood Components in COVID-19 Affected Subjects: A Correlation to UPR Signalling and ER-Stress

Abstract

Abstract: The endoplasmic reticulum (ER) is the site for protein synthesis, its folding and secretion. An intricate set of signalling pathways, called UPR pathways, have been evolved by ER in mammalian cells, to allow the cell to respond the presence of misfolded proteins within the ER. Breaching of these signalling systems by disease oriented accumulation of unfolded proteins may develop cellular stress. The aim of this study is to explore whether COVID-19 infection is responsible for developing this kind of endoplasmic reticulum related stress (ER-stress). ER-stress was evaluated by checking the expression of ER-stress markers e.g. PERK (adapting) and TRAF2 (alarming). ER-stress was correlated to several blood parameters viz. IgG, pro- and anti-inflammatory cytokines, leukocytes, lymphocytes, RBC, haemoglobin and PaO₂/FiO₂ ratio (ratio of arterial oxygen partial pressure to fractional inspired oxygen) in COVID-19 affected subjects. COVID-19 infection was found to be a state of protein homeostasis (proteostasis) collapse. Changes in IgG levels showed very poor immune response by the infected subjects. At the initial phase of the disease, pro-inflammatory cytokine levels were high and anti-inflammatory cytokines levels were low; though they were partly compromised at later phase of the disease. Total leukocyte concentration increased over the period of time; while percentage of lymphocytes were dropped. No significant changes were observed in cases of RBC counts and haemoglobin (Hb) levels. Both RBC and Hb were maintained at their normal range. In mildly stressed group, PaO₂/FiO₂ ratio (oxygenation status) was in the higher side of normal range; whereas in other two groups the ratio was in respiratory distress syndrome mode. Virus could induce mild to severe ER-stress, which could be the cause of cellular death and systemic dysfunction introducing fatal consequences.

Graphical abstract: Schematic representation of SARS-CoV-2 infection and related consequences.

Keywords: COVID-19; ER-stress; IgG response; PERK; Protein misfolding; TRAF2; Unfolded protein response.

Fig. 1

ER endoplasmic reticulum, UPR unfolded protein response, Grp78 78-kda glucose-regulated protein, PERK protein kinase R-like ER kinase, Ire1 inositol-requiring enzyme1, ATF4 activating transcription factor 4, TRAF2 TNF receptor-associated factor 2, NF-kB nuclear factor-κB, P³⁸MARK P³⁸ mitogen-activated protein kinase, JNK c-Jun N-terminal kinase. Figure shows signal transduction events associated with ER stress. Chaperone Grp78 binds the N-termini of Ire1 and PERK, and prevents their activation. Unfolded proteins in the ER cause Grp78 to release Ire1, and PERK. Upon Grp78 release, Ire1 and PERK oligomerize in ER membranes. Oligomerized Ire1 binds TRAF2, signalling downstream kinases that activate NF-kB, P³⁸MAPK, and c-Jun (JNK), causing expression of genes associated with host defense (alarm). Activation of defense genes induces expression of genes involved in restoring protein folding or degradation of unfolded proteins. Oligomerization of PERK activates its intrinsic kinase activity and translates only selected mRNA(s) like ATF4. ATF4 induces expression of genes involved in restoring ER homeostasis