Theberge-Julien, G. | bioRxiv Science

bioRxiv2025

Manganese homeostasis modulates glucan and chitin unmasking in the opportunistic yeast Candida albicans

Candida albicans is a commensal fungus and also the most prevalent human fungal pathogen. The ability of this opportunistic yeast to acquire and maintain homeostatic levels of manganese (Mn), particularly in the metal-limited host environment, is an important determinant of its fitness. Recent studies have underscored the importance of Mn acquisition through members of Smf transporters, in C. albicans virulence and its ability to withstand various stresses. In the present study, we undertook transcriptional profiling in the mutant of the Mn transporter Smf12 under restricted Mn availability to identify processes that are directly affected in defective Mn uptake. Our analysis revealed that smf12 displayed a transcriptional pattern suggestive of a cell wall defect, with many transcripts associated with cell wall biogenesis being differentially regulated. smf12 together with smf11, a mutant of the closest homolog of Smf12, exhibited hypersensitivity to cell wall stressors and an altered cell wall ultrastructure. The smf mutants also exhibited unmasking of both {beta}-glucan and chitin, which unexpectedly resulted in a decreased rate of phagocytosis by macrophages, suggesting impaired recognition or internalization--an observation that challenges the prevailing paradigm. Furthermore, we showed that Mn-mediated unmasking of {beta}-glucan required modulation of glucanase activity and was not mediated through the calcineurin pathway. This study uncovers a novel role for Mn in maintaining cell wall integrity and modulating the exposure of fungal antigenic determinants, further emphasizing the critical role of this metal in supporting the opportunistic nature of C. albicans.

microbiology↗

bioRxiv2020

Expression, subcellular localization, and phosphorylation of MK5 in adult cardiac ventricular fibroblasts

MAP kinase-activated protein kinase-5 (MK5) plays an important role in cardiac fibroblast function. Although p38 MAPK and atypical MAPKs and ERK3 and ERK4 have been identified as activators of MK5, the kinases that activate MK5 remain controversial. Here we examined the expression, subcellular distribution, and regulation of MK5 in cardiac ventricular myofibroblasts and myocytes. The copy numbers for MK5 and ERK4 mRNA were comparable in myocytes and myofibroblasts, whereas that of ERK3 was much higher in myofibroblasts. Interestingly, MK5 and ERK3 immunoreactivity was detected in myofibroblasts but not myocytes whereas ERK4 immunoreactivity was detected in myocytes: treating in myocytes with a proteasome inhibitor or hypertrophic agonists failed to rescue MK5 immunoreactivity. In myofibroblasts, MK5 and ERK3 immunoreactivity was predominantly nuclear and cytosolic, respectively. In serum-starved cardiac myofibroblasts, phosphothreonine-182 MK5 (pT182-MK5) immunoreactivity was predominantly nuclear but increased in intensity and relocated to the cytoplasm in response to serum, sorbitol, angiotensin II, TGF{beta}, or H2O2 and this was prevented by inhibition of p38/{beta}. Phos-tag SDS-PAGE revealed multiple slower migrating bands of MK5 immunoreactivity, indicating phosphorylation of MK5 at multiple sites. Phos-tag PAGE also revealed MK5 phosphorylation was increased with fibroblast activation and in hearts exposed to a chronic increase in afterload. MK5 and ERK3 co-immunoprecipitated and proximity ligation assays revealed ERK3 and MK5 in close proximity in myofibroblast cytoplasmic compartment. Furthermore, p38/{beta} inhibition decreased the abundance of MK5 immunoreactivity in ERK3 immunoprecipitates. Finally, deleting MK5 did not reduce the abundance of ERK3 immunoreactivity. These observations suggest that p38 and/or p38{beta} are the primary mediators of T182-MK5 phosphorylation and hence MK5 activation in cardiac myofibroblasts.

biochemistry↗