Intestinal brush border assembly driven by protocadherin-based intermicrovillar adhesion.

Transporting epithelial cells build apical microvilli to increase membrane surface area and enhance absorptive capacity. The intestinal brush border provides an elaborate example with tightly packed microvilli that function in nutrient absorption and host defense. Although the brush border is essential for physiological homeostasis, its assembly is poorly understood. We found that brush border assembly is driven by the formation of Ca(2+)-dependent adhesion links between adjacent microvilli. Intermicrovillar links are composed of protocadherin-24 and mucin-like protocadherin, which target to microvillar tips and interact to form a trans-heterophilic complex. The cytoplasmic domains of microvillar protocadherins interact with the scaffolding protein, harmonin, and myosin-7b, which promote localization to microvillar tips. Finally, a mouse model of Usher syndrome lacking harmonin exhibits microvillar protocadherin mislocalization and severe defects in brush border morphology. These data reveal an adhesion-based mechanism for brush border assembly and illuminate the basis of intestinal pathology in patients with Usher syndrome. PAPERFLICK:

PACSIN2-dependent apical endocytosis regulates the morphology of epithelial microvilli.

Apical microvilli are critical for the homeostasis of transporting epithelia, yet mechanisms that control the assembly and morphology of these protrusions remain poorly understood. Previous studies in intestinal epithelial cell lines suggested a role for the F-BAR domain protein PACSIN2 in normal microvillar assembly. Here we report the phenotype of PACSIN2 KO mice and provide evidence that through its role in promoting apical endocytosis, this molecule plays a role in controlling microvillar morphology. PACSIN2 KO enterocytes exhibit reduced numbers of microvilli and defects in the microvillar ultrastructure, with membranes lifting away from rootlets of core bundles. Dynamin2, a PACSIN2 binding partner, and other endocytic factors were also lost from their normal localization near microvillar rootlets. To determine whether loss of endocytic machinery could explain defects in microvillar morphology, we examined the impact of PACSIN2 KD and endocytosis inhibition on live intestinal epithelial cells. These assays revealed that when endocytic vesicle scission fails, tubules are pulled into the cytoplasm and this, in turn, leads to a membrane-lifting phenomenon reminiscent of that observed at PACSIN2 KO brush borders. These findings lead to a new model where inward forces generated by endocytic machinery on the plasma membrane control the membrane wrapping of cell surface protrusions.

IRTKS (BAIAP2L1) Elongates Epithelial Microvilli Using EPS8-Dependent and Independent Mechanisms.

Transporting epithelial cells like those that line the gut build large arrays of actin-supported protrusions called microvilli, which extend from the apical surface into luminal spaces to increase functional surface area. Although critical for maintaining physiological homeostasis, mechanisms controlling the formation of microvilli remain poorly understood. Here, we report that the inverse-bin-amphiphysin-Rvs (I-BAR)-domain-containing protein insulin receptor tyrosine kinase substrate (IRTKS) (also known as BAIAP2L1) promotes the growth of epithelial microvilli. Super-resolution microscopy and live imaging of differentiating epithelial cells revealed that IRTKS localizes to the distal tips of actively growing microvilli via a mechanism that requires its N-terminal I-BAR domain. At microvillar tips, IRTKS promotes elongation through a mechanism involving its C-terminal actin-binding WH2 domain. IRTKS can also drive microvillar elongation using its SH3 domain to recruit the bundling protein EPS8 to microvillar tips. These results provide new insight on mechanisms that control microvillar growth during the differentiation of transporting epithelial cells and help explain why IRTKS is targeted by enteric pathogens that disrupt microvillar structure during infection of the intestinal epithelium.

MyTH4-FERM myosins in the assembly and maintenance of actin-based protrusions.

Unconventional myosins are actin-based molecular motors that serve a multitude of roles within the cell. One group of myosin motors, the MyTH4-FERM myosins, play an integral part in building and maintaining finger-like protrusions, which allow cells to interact with their external environment. Suggested to act primarily as transporters, these motor proteins enrich adhesion molecules, actin-regulatory proteins and other factors at the tips of filopodia, microvilli, and stereocilia. Below we review data from biophysical, biochemical, and cell biological studies, which implicate these myosins as central players in the assembly, maintenance and function of actin-based protrusions.

Impact of cordon-bleu expression on actin cytoskeleton architecture and dynamics.

Cordon-bleu (COBL) is a multifunctional WASP-Homology 2 (WH2) domain-containing protein implicated in a wide variety of cellular functions ranging from dendritic arborization in neurons to the assembly of microvilli on the surface of transporting epithelial cells. In vitro biochemical studies suggest that COBL is capable of nucleating and severing actin filaments, among other activities. How the multiple activities of COBL observed in vitro contribute to its function in cells remains unclear. Here, we used live imaging to evaluate the impact of COBL expression on the actin cytoskeleton in cultured cells. We found that COBL induces the formation of dynamic linear actin structures throughout the cytosol. We also found that stabilizing these dynamic structures with the parallel actin-bundling protein espin slows down their turnover and enables the robust formation of self-supported protrusions on the dorsal cell surface. Super-resolution imaging revealed a global remodeling of the actin cytoskeleton in cells expressing these two factors. Taken together, these results provide insight as to how COBL contributes to the assembly of actin-based structures such as epithelial microvilli. © 2016 Wiley Periodicals, Inc.

ANKS4B Is Essential for Intermicrovillar Adhesion Complex Formation.

Transporting and sensory epithelial cells shape apical specializations using protocadherin-based adhesion. In the enterocyte brush border, protocadherin function requires a complex of cytoplasmic binding partners, although the composition of this complex and logic governing its assembly remain poorly understood. We found that ankyrin repeat and sterile α motif domain containing 4B (ANKS4B) localizes to the tips of adherent brush border microvilli and is essential for intermicrovillar adhesion. ANKS4B interacts with USH1C and MYO7B, which link protocadherins to the actin cytoskeleton. ANKS4B and USH1C also bind to the MYO7B cargo-binding tail at distinct sites. However, a tripartite complex only forms if ANKS4B and MYO7B are first activated by USH1C. This study uncovers an essential role for ANKS4B in brush border assembly, reveals a hierarchy in the molecular interactions that drive intermicrovillar adhesion, and informs our understanding of diseases caused by mutations in USH1C and ankyrin repeat proteins, such as Usher syndrome.

Cortactin promotes exosome secretion by controlling branched actin dynamics.

Exosomes are extracellular vesicles that influence cellular behavior and enhance cancer aggressiveness by carrying bioactive molecules. The mechanisms that regulate exosome secretion are poorly understood. Here, we show that the actin cytoskeletal regulatory protein cortactin promotes exosome secretion. Knockdown or overexpression of cortactin in cancer cells leads to a respective decrease or increase in exosome secretion, without altering exosome cargo content. Live-cell imaging revealed that cortactin controls both trafficking and plasma membrane docking of multivesicular late endosomes (MVEs). Regulation of exosome secretion by cortactin requires binding to the branched actin nucleating Arp2/3 complex and to actin filaments. Furthermore, cortactin, Rab27a, and coronin 1b coordinately control stability of cortical actin MVE docking sites and exosome secretion. Functionally, the addition of purified exosomes to cortactin-knockdown cells rescued defects of those cells in serum-independent growth and invasion. These data suggest a model in which cortactin promotes exosome secretion by stabilizing cortical actin-rich MVE docking sites.

Cordon bleu promotes the assembly of brush border microvilli.

Microvilli are actin-based protrusions found on the surface of diverse cell types, where they amplify membrane area and mediate interactions with the external environment. In the intestinal tract, these protrusions play central roles in nutrient absorption and host defense and are therefore essential for maintaining homeostasis. However, the mechanisms controlling microvillar assembly remain poorly understood. Here we report that the multifunctional actin regulator cordon bleu (COBL) promotes the growth of brush border (BB) microvilli. COBL localizes to the base of BB microvilli via a mechanism that requires its proline-rich N-terminus. Knockdown and overexpression studies show that COBL is needed for BB assembly and sufficient to induce microvillar growth using a mechanism that requires functional WH2 domains. We also find that COBL acts downstream of the F-BAR protein syndapin-2, which drives COBL targeting to the apical domain. These results provide insight into a mechanism that regulates microvillar growth during epithelial differentiation and have significant implications for understanding the maintenance of intestinal homeostasis.

Dynamics of brush border remodeling induced by enteropathogenic E. coli.

Enteropathogenic Escherichia coli (EPEC) induces dramatic remodeling of enterocyte brush borders, a process that includes microvillar effacement and actin pedestal formation. Although the Arp2/3 complex is involved in formation of a branched actin network within pedestals, the fate of parallel actin bundles in microvilli during infection remains unclear. Here, we find that in polarized intestinal epithelial cells, EPEC stimulates long-range microvillar dynamics, pulling protrusions toward sites of bacterial attachment in a process mediated by the adhesion molecule protocadherin-24. Additionally, retraction of the EPEC bundle forming pilus stimulates directed elongation of nearby microvilli. These processes lead to coalescence of microvilli and incorporation of the underlying parallel actin bundles into pedestals. Furthermore, stabilization of microvillar actin bundles delays pedestal formation. Together, these results suggest a model where EPEC takes advantage of pre-existing actin filaments in microvillar core bundles to facilitate pedestal formation.

Bves and NDRG4 regulate directional epicardial cell migration through autocrine extracellular matrix deposition.

Directional cell movement is universally required for tissue morphogenesis. Although it is known that cell/matrix interactions are essential for directional movement in heart development, the mechanisms governing these interactions require elucidation. Here we demonstrate that a novel protein/protein interaction between blood vessel epicardial substance (Bves) and N-myc downstream regulated gene 4 (NDRG4) is critical for regulation of epicardial cell directional movement, as disruption of this interaction randomizes migratory patterns. Our studies show that Bves/NDRG4 interaction is required for trafficking of internalized fibronectin through the "autocrine extracellular matrix (ECM) deposition" fibronectin recycling pathway. Of importance, we demonstrate that Bves/NDRG4-mediated fibronectin recycling is indeed essential for epicardial cell directional movement, thus linking these two cell processes. Finally, total internal reflectance fluorescence microscopy shows that Bves/NDRG4 interaction is required for fusion of recycling endosomes with the basal cell surface, providing a molecular mechanism of motility substrate delivery that regulates cell directional movement. This is the first evidence of a molecular function for Bves and NDRG4 proteins within broader subcellular trafficking paradigms. These data identify novel regulators of a critical vesicle-docking step required for autocrine ECM deposition and explain how Bves facilitates cell-microenvironment interactions in the regulation of epicardial cell-directed movement.