Endocytic uptake of SARS-CoV-2: the critical roles of pH, Ca2+, and NAADP.

Abstract

Endocytic acidification occurs at the same time as Ca2+ is lost from endosomes (Figure 1). The fluid taken into cells by endocytosis has, of course, the normal concentration of free Ca2+ found in the extracellular fluid, namely ∼1mM. However, as seen in Figure 1, at the earliest time points allowing assessment of endosomal [Ca2+], the concentration had already dropped to ∼30 μM, which is still markedly higher than the level in the cytosol (∼0.1 μM). The endosomal [Ca2+] then declined further to ∼4 μM in late endosomes (Figure 1). Endosomal acidification and loss of Ca2+ are interlinked. Blockade of the proton pump by bafilomycin prevented release of Ca2+ from the endosomes, with endosomal [Ca2+] remaining high.4 If [Ca2+] in the early endosomes was kept low, by reducing the extracellular [Ca2+] to less than 300 μM, acidification did not occur.4 The mechanism underlying the linkage between Ca2+ and H+ movements is not fully understood, but the fact that the uptake of H+ into endosomes occurs simultaneously with release of Ca2+, and that the two processes are interdependent, means that it is difficult to determine whether inhibition of virus entry by blockade of the proton pump or by TPC inhibition1,7 is primarily due to the high pH or to the relatively high [Ca2+] in the endosomes, or to a mixture of both.