Biology subjects

Simmonds, R. E.

Publications and source records attributed to Simmonds, R. E..

4 recordsLinked to original sources

bioRxiv2026

Modelling the formation of skin ulcers observed in Buruli ulcer disease

Buruli ulcer (BU) is a neglected tropical skin disease caused by infection with Mycobacterium ulcerans. The mode of its transmission into the skin has not been fully clarified, but it is believed to involve mechanical transfer, perhaps by insect bites. M. ulcerans secretes a toxin called mycolactone which causes necrosis of the skin and formation of large skin lesions. Understanding the development of these skin lesions over time is crucial to understand the pathogenesis of BU. We formulate a mathematical model to describe the space-time dynamics of M. ulcerans bacteria, mycolactone and skin cell densities as well as fibrin deposition. We investigate the relative contributions to ulcer formation of direct cytotoxicity of mycolactone and ischaemia following the loss of coagulation control. Our results are consistent with existing biological evidence that higher mycolactone concentrations accelerate cell death. Furthermore, the model indicates that the rate of wound expansion is primarily controlled by the diffusion and net birth rate of bacteria, which sets the overall scaling for the speed of wound expansion, along with two dimensionless constants that define the shape of the trajectory of wound edge position against time, namely the ratio of the mycolactone diffusion coefficient to the bacterial diffusion coefficient, and the ratio of the mycolactone degradation rate to the bacterial net birth rate. When mycolactone diffuses significantly faster than bacteria spread or when mycolactone degrades more slowly than bacteria reproduce, the initial development of the wound is faster. In contrast, the long-term wound expansion speed appears to be relatively unchanged across a wide range of the dimensionless parameters.

biophysics↗

bioRxiv2025

A mycobacterial Sec61 inhibitor disrupts lysosome function by blocking Vacuolar-ATPase biosynthesis.

Mycolactone is the virulence toxin of Mycobacterium ulcerans, causative agent of Buruli ulcer. Mycolactone inhibits the Sec61-dependent co-translational translocation of signal peptide-bearing secreted and membrane proteins into the endoplasmic reticulum. Sec61 inhibition leads to accumulation of mislocalised proteins in the cytosol and initially triggers an integrated stress response-dependent activation of autophagy that contributes to cell survival. Here we show that sustained exposure of cells to mycolactone causes a block in late-stage autophagy and induces nuclear translocation of the lysosomal stress marker TFEB. This coincides with loss of ATP6AP1 and ATP6AP2, Sec61-substrates required for assembly of the Vacuolar-ATPase, leading to reduced lysosomal biogenesis and acidification. This compromises the cells capacity to withstand the proteostatic stress caused by Sec61 inhibition and impairs the ability of phagocytes to combat infection with M. ulcerans. Loss of lysosomal function could contribute to both the tissue necrosis and immunosuppression seen in Buruli ulcer. Furthermore, since Sec61 inhibition is being pursued as a therapeutic target in several diseases, potential drugs should be screened against this activity to avoid unwanted side-effects.

cell biology↗

bioRxiv2023

Mycolactone causes catastrophic Sec61-dependent loss of the endothelial glycocalyx and basement membrane: a new indirect mechanism driving tissue necrosis in Mycobacterium ulcerans infection

The drivers of tissue necrosis in Mycobacterium ulcerans infection (Buruli ulcer disease) have historically been ascribed solely to the directly cytotoxic action of the diffusible exotoxin, mycolactone. However, its role in the clinically-evident vascular component of disease aetiology remains poorly explained. We have now dissected mycolactones effects on primary vascular endothelial cells in vitro and in vivo. We show that mycolactone-induced changes in endothelial morphology, adhesion, migration, and permeability are dependent on its action at the Sec61 translocon. Unbiased quantitative proteomics identified a profound effect on proteoglycans, driven by rapid loss of type II transmembrane proteins of the Golgi, including enzymes required for glycosaminoglycan (GAG) synthesis, combined with a reduction in the core proteins themselves. Loss of the glycocalyx is likely to be of particular mechanistic importance, since knockdown of galactosyltransferase II (beta-1,3-galactotransferase 6; B3GALT6), the GAG linker-building enzyme, phenocopied the permeability and phenotypic changes induced by mycolactone. Additionally, mycolactone depleted many secreted basement membrane components and microvascular basement membranes were disrupted in vivo. Remarkably, exogenous addition of laminin-511 reduced endothelial cell rounding, restored cell attachment and reversed the defective migration caused by mycolactone. Hence supplementing mycolactone-depleted extracellular matrix may be a future therapeutic avenue, to improve wound healing rates.

microbiology↗

bioRxiv2021

Mycolactone enhances the Ca2+ leakage from endoplasmic reticulum by trapping Sec61 translocons in a Ca2+ permeable state

The Mycobacterium ulcerans exotoxin, mycolactone, is an inhibitor of co-translational translocation via the Sec61 complex. Mycolactone has previously been shown to bind to, and alter the structure of, the major translocon subunit Sec61, and change its interaction with ribosome nascent chain complexes. In addition to its function in protein translocation into the ER, Sec61 also plays a key role in cellular Ca2+ homeostasis, acting as a leak channel between the endoplasmic reticulum (ER) and cytosol. Here, we have analysed the effect of mycolactone on cytosolic and ER Ca2+ levels using compartment-specific sensors. We also used molecular docking analysis to explore potential interaction sites for mycolactone on translocons in various states. These results show that mycolactone enhances the leak of Ca2+ ions via the Sec61 translocon, resulting in a slow but substantial depletion of ER Ca2+. This leak was dependent on mycolactone binding to Sec61 because resistance mutations in this protein completely ablated the increase. Molecular docking supports the existence of a mycolactone-binding transient inhibited state preceding translocation and suggests mycolactone may also bind Sec61 in its idle state. We propose that delayed ribosomal release after translation termination and/or translocon "breathing" during rapid transitions between the idle and intermediate-inhibited states allow for transient Ca2+ leak, and mycolactones stabilisation of the latter underpins the phenotype observed.

biochemistry↗