Caveolin-1 protects endothelial cells from extensive expansion of transcellular tunnel by stiffening the plasma membrane.

Large transcellular pores elicited by bacterial mono-ADP-ribosyltransferase (mART) exotoxins inhibiting the small RhoA GTPase compromise the endothelial barrier. Recent advances in biophysical modeling point toward membrane tension and bending rigidity as the minimal set of mechanical parameters determining the nucleation and maximal size of transendothelial cell macroaperture (TEM) tunnels induced by bacterial RhoA-targeting mART exotoxins. We report that cellular depletion of caveolin-1, the membrane-embedded building block of caveolae, and depletion of cavin-1, the master regulator of caveolae invaginations, increase the number of TEMs per cell. The enhanced occurrence of TEM nucleation events correlates with a reduction in cell height due to the increase in cell spreading and decrease in cell volume, which, together with the disruption of RhoA-driven F-actin meshwork, favor membrane apposition for TEM nucleation. Strikingly, caveolin-1 specifically controls the opening speed of TEMs, leading to their dramatic 5.4-fold larger widening. Consistent with the increase in TEM density and width in siCAV1 cells, we record a higher lethality in CAV1 KO mice subjected to a catalytically active mART exotoxin targeting RhoA during staphylococcal bloodstream infection. Combined theoretical modeling with independent biophysical measurements of plasma membrane bending rigidity points toward a specific contribution of caveolin-1 to membrane stiffening in addition to the role of cavin-1/caveolin-1-dependent caveolae in the control of membrane tension homeostasis.

A mechanosensitive caveolae-invadosome interplay drives matrix remodelling for cancer cell invasion.

Invadosomes and caveolae are mechanosensitive structures that are implicated in metastasis. Here, we describe a unique juxtaposition of caveola clusters and matrix degradative invadosomes at contact sites between the plasma membrane of cancer cells and constricting fibrils both in 2D and 3D type I collagen matrix environments. Preferential association between caveolae and straight segments of the fibrils, and between invadosomes and bent segments of the fibrils, was observed along with matrix remodelling. Caveola recruitment precedes and is required for invadosome formation and activity. Reciprocally, invadosome disruption results in the accumulation of fibril-associated caveolae. Moreover, caveolae and the collagen receptor ß1 integrin co-localize at contact sites with the fibrils, and integrins control caveola recruitment to fibrils. In turn, caveolae mediate the clearance of ß1 integrin and collagen uptake in an invadosome-dependent and collagen-cleavage-dependent mechanism. Our data reveal a reciprocal interplay between caveolae and invadosomes that coordinates adhesion to and proteolytic remodelling of confining fibrils to support tumour cell dissemination.

Transposable elements in normal and malignant hematopoiesis.

Transposable elements (TEs) are dispersed repetitive DNA sequences that can move within a genome. Even though hundreds of years of evolution have led to the accumulation of mutations that render most TEs unable to transpose, they still exert multiple important functions. They play a role in hematopoiesis, especially during periods of high cellular plasticity, such as development, regeneration and aging. This is because TEs can populate functional elements, such as enhancers. Furthermore, TE RNA can be sensed by innate immune sensors that play a role in inflammation and inflammaging. TEs also play an important role in different aspects of leukemia and lymphoma, leading to either beneficial or detrimental outcomes. Further studies into the function of TEs in healthy or diseased hematopoietic systems are necessary to manipulate them for therapeutic benefit.

Caveolae and Bin1 form ring-shaped platforms for T-tubule initiation.

Excitation-contraction coupling requires a highly specialized membrane structure, the triad, composed of a plasma membrane invagination, the T-tubule, surrounded by two sarcoplasmic reticulum terminal cisternae. Although the precise mechanisms governing T-tubule biogenesis and triad formation remain largely unknown, studies have shown that caveolae participate in T-tubule formation and mutations of several of their constituents induce muscle weakness and myopathies. Here, we demonstrate that, at the plasma membrane, Bin1 and caveolae composed of caveolin-3 assemble into ring-like structures from which emerge tubes enriched in the dihydropyridine receptor. Bin1 expression lead to the formation of both rings and tubes and we show that Bin1 forms scaffolds on which caveolae accumulate to form the initial T-tubule. Cav3 deficiency caused by either gene silencing or pathogenic mutations results in defective ring formation and perturbed Bin1-mediated tubulation that may explain defective T-tubule organization in mature muscles. Our results uncover new pathophysiological mechanisms that may prove relevant to myopathies caused by Cav3 or Bin1 dysfunction.

Clathrin plaques and associated actin anchor intermediate filaments in skeletal muscle.

Clathrin plaques are stable features of the plasma membrane observed in several cell types. They are abundant in muscle, where they localize at costameres that link the contractile apparatus to the sarcolemma and connect the sarcolemma to the basal lamina. Here, we show that clathrin plaques and surrounding branched actin filaments form microdomains that anchor a three-dimensional desmin intermediate filament (IF) web. Depletion of clathrin plaque and branched actin components causes accumulation of desmin tangles in the cytoplasm. We show that dynamin 2, whose mutations cause centronuclear myopathy (CNM), regulates both clathrin plaques and surrounding branched actin filaments, while CNM-causing mutations lead to desmin disorganization in a CNM mouse model and patient biopsies. Our results suggest a novel paradigm in cell biology, wherein clathrin plaques act as platforms capable of recruiting branched cortical actin, which in turn anchors IFs, both essential for striated muscle formation and function.