The actin nucleator Cobl organises the terminal web of enterocytes

Abstract

Brush borders of intestinal epithelial cells are mandatory for nutrient uptake. Yet, which actin nucleators are crucial for forming the F-actin bundles supporting microvilli and the actin filaments of the terminal web, in which microvilli are rooted, is unknown. We show that mice lacking the actin nucleator cobl surprisingly did not display reduced microvilli densities or changes in microvillar f-actin bundles or microvilli diameter but particularly in the duodenum displayed increased microvillar length. Interestingly, Cobl-deficient mice furthermore showed a significant widening of the terminal web. Quantitative analyses of high-resolution cryo-scanning electron microscopy (eM) of deep-etched duodenum samples revealed that Cobl is specifically important for the formation of fine filaments in the central terminal web that connect the apical structure of the terminal web underlying the plasma membrane, the microvilli rootlets and the basal structure of the terminal web with each other. Thus, the actin nucleator cobl is critically involved in generating one of the cellular structures of the brush borderdecorated apical cortex of enterocytes representing the absorptive intestinal surface. Surface extensions of epithelial cells (enterocytes) by microvilli are mandatory for nutrient uptake in the gut. New-borns with reduced length of microvilli, as it e.g. occurs in the inherited microvillar inclusion disease, thus usually die during their early infancy 1,2. Yet, despite this importance, it is still unknown which actin nucleators are crucial for forming the F-actin bundles supporting microvilli and the actin filaments of the terminal web, in which microvilli are rooted 3-5. Microvilli are uniform membrane protrusions that are structurally supported by a dense bundle of unbranched actin filaments, that are organised in a very dense, almost hexagonal packing by actin-crosslinking proteins and that show very uniform length in brush borders. This length differs along the intestinal tract with about 1300-1400 nm in the small intestine and only 800 nm in the colon 6. The minus ends of microvillar F-actin bundles reach into the cells as rootlets and are anchored-presumably by plastin 1 (fimbrin) interlinking them with the keratin network 7-in the terminal web, which spans the entire apical cortex of enterocytes 8,9. Although a variety of F-actin-binding proteins including e.g. myosins, tropomyosin, villin, spectrin, ezrin and espin have been identified in biochemical isolations of microvilli and attached terminal web structures 3 , loss-of-function data identifying the actin nucleator(s) that give rise to the F-actin structures in microvilli and/or in the terminal web is lacking. Despite significant scientific efforts during several decades, also microvilli formation and length control in intestinal brush borders still are poorly understood, as i) intestine tissues are not well accessible for molecular manipulations, ii) enterocytes in the murine gut only have a lifespan of a few days and iii) once intestinal microvilli are ...

Functional interdependence of the actin nucleator Cobl and Cobl-like in dendritic arbor development

“…Further studies of Cobl knock-out (KO) mice unveiled an importance of Cobl for a specialized set of filaments interconnecting structural elements in the F-actin-rich terminal web of microvilli-decorated epithelial cells in the small intestine. However, there were no indications of an additional Cobl-like involvement ( Beer et al, 2020 ). Also in proteomic analyses of myoblasts, which upon IGFN1 deficiency show altered G-to-F-actin ratios, only Cobl was identified but not Cobl-like ( Cracknell et al, 2020 ).…”

Functional interdependence of the actin nucleator Cobl and Cobl-like in dendritic arbor development

“…Further studies of Cobl KO mice unveiled an importance of Cobl for a specialized set of filaments interconnecting structural elements in the F-actin-rich terminal web of microvilli-decorated epithelial cells in the small intestine. However, there were no indications of an additional Cobl-like involvement (Beer et al, 2020). Also in proteomic analyses of myoblasts, which upon IGFN1 deficiency show altered G-to-F-actin ratios, only Cobl was identified but not Cobl-like (Cracknell et al, 2020).…”

Role of Lipids in Morphogenesis of T-Cell Microvilli

“…In the shaft, the membrane is tightly linked to actin bundles via actin- and membrane-binding proteins [e.g., myosins and ERM proteins; see Figure 1 ( 12 )]. At the base, at least in epithelial cells, actin bundle terminates in the network of intermediate filaments known as ‘terminal web’ ( 13 , 14 ). The size of microvilli is regulated by the growth of actin fibres at the tip ( 15 ).…”