The Role of Adipocytes and Adipocyte‐Like Cells in the Severity of COVID‐19 Infections

Coronavirus disease‐2019 (COVID‐19), caused by the highly pathogenic severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2), demonstrates high morbidity and mortality caused by development of a severe acute respiratory syndrome connected with extensive pulmonary fibrosis. In this Perspective, we argue that adipocytes and adipocyte‐like cells, such as pulmonary lipofibroblasts, may play an important role in the pathogenic response to SARS‐CoV‐2. Expression of angiotensin‐converting enzyme 2 (the functional receptor for SARS‐CoV) is upregulated in adipocytes of patients with obesity and diabetes, which turns adipose tissue into a potential target and viral reservoir. This may explain why obesity and diabetes are potential comorbidities for COVID‐19 infections. Similar to the recently established adipocyte‐myofibroblast transition, pulmonary lipofibroblasts located in the alveolar interstitium and closely related to classical adipocytes demonstrate the ability to transdifferentiate into myofibroblasts that play an integral part of pulmonary fibrosis. This may significantly increase the severity of the local response to SARS‐CoV‐2 in the lung. To reduce the severity and mortality associated with COVID‐19, we propose to probe for the clinical response to thiazolidinediones, peroxisome proliferator activated receptor γ agonists that are well‐known antidiabetic drugs. Thiazolidinediones are able to stabilize lipofibroblasts in their “inactive” state, preventing the transition to myofibroblasts and thereby reducing the development of pulmonary fibrosis and stimulating its resolution.     

The puzzle of TRPV4 channelopathies

Hereditary channelopathies, that is, mutations in channel genes that alter channel function and are causal for the pathogenesis of the disease, have been described for several members of the transient receptor potential channel family. Mutations in the TRPV4 gene, encoding a polymodal Ca²⁺ permeable channel, are causative for several human diseases, which affect the skeletal system and the peripheral nervous system, with highly variable phenotypes. In this review, we describe the phenotypes of TRPV4 channelopathies and overlapping symptoms. Putative mechanisms to explain the puzzle, and how mutations in the same region of the channel cause different diseases, are discussed and experimental approaches to tackle this surprising problem are suggested.     

Low Genetic Diversity in Wide-Spread Eurasian Liver Fluke Opisthorchis felineus Suggests Special Demographic History of This Trematode Species

Opisthorchis felineus or Siberian liver fluke is a trematode parasite (Opisthorchiidae) that infects the hepato-biliary system of humans and other mammals. Despite its public health significance, this wide-spread Eurasian species is one of the most poorly studied human liver flukes and nothing is known about its population genetic structure and demographic history. In this paper, we attempt to fill this gap for the first time and to explore the genetic diversity in O. felineus populations from Eastern Europe (Ukraine, European part of Russia), Northern Asia (Siberia) and Central Asia (Northern Kazakhstan). Analysis of marker DNA fragments from O. felineus mitochondrial cytochrome c oxidase subunit 1 and 3 (cox1, cox3) and nuclear rDNA internal transcribed spacer 1 (ITS1) sequences revealed that genetic diversity is very low across the large geographic range of this species. Microevolutionary processes in populations of trematodes may well be influenced by their peculiar biology. Nevertheless, we suggest that lack of population genetics structure observed in O. felineus can be primarily explained by the Pleistocene glacial events and subsequent sudden population growth from a very limited group of founders. Rapid range expansion of O. felineus through Asian and European territories after severe bottleneck points to a high dispersal potential of this trematode species.     

TRPM8-independent menthol-induced Ca2+ release from endoplasmic reticulum and Golgi

Menthol, a secondary alcohol produced by the peppermint herb, Mentha piperita, is widely used in the food and pharmaceutical industries as a cooling/soothing compound and odorant. It induces Ca2+ influx in a subset of sensory neurons from dorsal root and trigeminal ganglia, due to activation of TRPM8, a Ca2+-permeable, cold-activated member of the TRP superfamily of cation channels. Menthol also induces Ca2+ release from intracellular stores in several TRPM8-expressing cell types, which has led to the suggestion that TRPM8 can function as an intracellular Ca2+-release channel. Here we show that menthol induces Ca2+ release from intracellular stores in four widely used cell lines (HEK293, lymph node carcinoma of the prostate (LNCaP), Chinese hamster ovary (CHO), and COS), and provide several lines of evidence indicating that this release pathway is TRPM8-independent: 1) menthol-induced Ca2+ release was potentiated at higher temperatures, which contrasts to the cold activation of TRPM8; 2) overexpression of TRPM8 did not enhance the menthol-induced Ca2+) release; 3) menthol-induced Ca2+ release was mimicked by geraniol and linalool, which are structurally related to menthol, but not by the more potent TRPM8 agonists icilin or eucalyptol; and 4) TRPM8 expression in HEK293 cells was undetectable at the protein and mRNA levels. Moreover, using a novel TRPM8-specific antibody we demonstrate that both heterologously expressed TRPM8 (in HEK293 cells) and endogenous TRPM8 (in LNCaP cells) are mainly localized in the plasma membrane, which contrast to previous localization studies using commercial anti-TRPM8 antibodies. Finally, aequorin-based measurements demonstrate that the TRPM8-independent menthol-induced Ca2+ release originates from both endoplasmic reticulum and Golgi compartments.     

Mg2+-dependent gating and strong inward rectification of the cation channel TRPV6

TRPV6 (CaT1/ECaC2), a highly Ca(2+)-selective member of the TRP superfamily of cation channels, becomes permeable to monovalent cations in the absence of extracellular divalent cations. The monovalent currents display characteristic voltage-dependent gating and almost absolute inward rectification. Here, we show that these two features are dependent on the voltage-dependent block/unblock of the channel by intracellular Mg(2+). Mg(2+) blocks the channel by binding to a site within the transmembrane electrical field where it interacts with permeant cations. The block is relieved at positive potentials, indicating that under these conditions Mg(2+) is able to permeate the selectivity filter of the channel. Although sizeable outward monovalent currents were recorded in the absence of intracellular Mg(2+), outward conductance is still approximately 10 times lower than inward conductance under symmetric, divalent-free ionic conditions. This Mg(2+)-independent rectification was preserved in inside-out patches and not altered by high intracellular concentrations of spermine, indicating that TRPV6 displays intrinsic rectification. Neutralization of a single aspartate residue within the putative pore loop abolished the Mg(2+) sensitivity of the channel, yielding voltage-independent, moderately inwardly rectifying monovalent currents in the presence of intracellular Mg(2+). The effects of intracellular Mg(2+) on TRPV6 are partially reminiscent of the gating mechanism of inwardly rectifying K(+) channels and may represent a novel regulatory mechanism for TRPV6 function in vivo.     

TRAIL receptor and CD95 signal to mitochondria via FADD,caspase-8/10, Bid,and Bax but differentially regulate events downstream from truncated Bid

The death receptor ligand TRAIL arouses much interest for clinical application. We found that TRAIL receptor could induce cytochrome c (Cyt c) release from mitochondria in cells that failed to respond to CD95. Therefore, we examined whether these two closely related death receptors use different intermediates to convey the apoptotic signal to mitochondria. Dominant negative FADD, FLIP(L), or a Bid mutant lacking cleavage sites for caspase-8/10 completely inhibited Cyt c release in response to either receptor. Depletion of Bid from TRAIL- or CD95-activated cytosols blocked their capacity to mediate Cyt c release from mitochondria in vitro, whereas Bax depletion reduced it. We conclude that FADD, caspase-8/10, and caspase-cleaved Bid are required for TRAIL receptor and CD95 signaling to mitochondria, whereas Bax is a common accessory. In vitro, caspase-8 treatment of cytosol from CD95-resistant cells permitted generation of truncated Bid and its association with mitochondria. However, this cytosol impaired the ability of truncated Bid to liberate Cyt c from exogenous mitochondria. We conclude that the TRAIL receptor can bypass or neutralize the activity of cytosolic factor that blocks truncated Bid function. This may benefit the capacity of TRAIL to break apoptosis resistance in tumor cells.     

The pore of TRP channels: trivial or neglected?

Ultimate proof that a protein contributes to the pore of an ion channel is demonstrating that pore properties can be altered by mutations to the putative pore-forming region. To date this has only been achieved for a few TRP proteins, and only within the TRPV subfamily. The location and structure of the pore region and selectivity filter of most TRP proteins, including all members of the TRPM and TRPC subfamilies, is currently unknown. In this review we give a short overview of the limited current knowledge about TRP channel pores and argue that further study is needed, not only for a better understanding of the mechanisms underlying cation permeation, but also to establish that all members of the TRP superfamily indeed function as bona fide ion channels.     

O-mannosylation precedes and potentially controls the N-glycosylation of a yeast cell wall glycoprotein

Secretory proteins in yeast are N- and O-glycosylated while they enter the endoplasmic reticulum. N-glycosylation is initiated by the oligosaccharyl transferase complex and O-mannosylation is initiated by distinct O-mannosyltransferase complexes of the protein mannosyl transferase Pmt1/Pmt2 and Pmt4 families. Using covalently linked cell-wall protein 5 (Ccw5) as a model, we show that the Pmt4 and Pmt1/Pmt2 mannosyltransferases glycosylate different domains of the Ccw5 protein, thereby mannosylating several consecutive serine and threonine residues. In addition, it is shown that O-mannosylation by Pmt4 prevents N-glycosylation by blocking the hydroxy amino acid of the single N-glycosylation site present in Ccw5. These data prove that the O- and N-glycosylation machineries compete for Ccw5; therefore O-mannosylation by Pmt4 precedes N-glycosylation.