Apical projections are integral functional units of epithelial cells. Microvilli and stereocilia are cylindrical apical projections that are formed of bundled actin. Microridges on the other hand, extend laterally, forming labyrinthine patterns on surfaces of various kinds of squamous epithelial cells. So far, the structural organization and functions of microridges have remained elusive. We have analyzed microridges on zebrafish epidermal cells using confocal and electron microscopy methods including electron tomography, to show that microridges are formed of F-actin networks and require the function of the Arp2/3 complex for their maintenance. During development, microridges begin as F-actin punctae showing signatures of branching and requiring an active Arp2/3 complex. Using inhibitors of actin polymerization and the Arp2/3 complex, we show that microridges organize the surface glycan layer. Our analyses have unraveled the F-actin organization supporting the most abundant and evolutionarily conserved apical projection, which functions in glycan organization.

Microvillus inclusion disease (MVID) is a life-threatening enteropathy characterised by malabsorption and incapacitating fluid loss due to chronic diarrhoea. Histological analysis has revealed that enterocytes in MVID patients exhibit reduction of microvilli, presence of microvillus inclusion bodies and intestinal villus atrophy, whereas genetic linkage analysis has identified mutations in myosin Vb gene as the main cause of MVID. In order to understand the cellular basis of MVID and the associated formation of inclusion bodies, an animal model that develops ex utero and is tractable genetically as well as by microscopy would be highly useful. Here we report that the intestine of the zebrafish goosepimples (gsp)/myosin Vb (myoVb) mutant shows severe reduction in intestinal folds - structures similar to mammalian villi. The loss of folds is further correlated with changes in the shape of enterocytes. In striking similarity with MVID patients, zebrafish gsp/myoVb mutant larvae exhibit microvillus atrophy, microvillus inclusions and accumulation of secretory material in enterocytes. We propose that the zebrafish gsp/myoVb mutant is a valuable model to study the pathophysiology of MVID. Furthermore, owing to the advantages of zebrafish in screening libraries of small molecules, the gsp mutant will be an ideal tool to identify compounds having therapeutic value against MVID.

Polarity is one of the fundamental properties displayed by living organisms. In metazoans, cell polarity governs developmental processes and plays an essential role during maintenance of forms of tissues as well as their functions. The mechanisms of establishment and maintenance of cell polarity have been investigated extensively in the last two decades. This has resulted in identification of "core cell polarity modules" that control anterior-posterior, front-rear and apical-basal polarity across various cell types. Here, we review how these polarity modules interact closely with the cytoskeleton during establishment and maintenance of cytoskeletal polarity. We further suggest that reciprocal interactions between cell polarity modules and the cytoskeleton consolidate the initial weaker polarity, arising from an external cue, into a committed polarized system.

Actin is a conserved cytoskeletal protein with essential functions. Here, we review the state-of-the-art reagents, tools and methods used to probe actin biology and functions in zebrafish embryo and larvae. We also discuss specific cell types and tissues where the study of actin in zebrafish has provided new insights into its functions.

Bullet points (Key findings):• myosin Vb is expressed in the zebrafish intestine.• goosepimples/myosin Vb function is essential for the morphogenesis, cell shape modulation, secretion of glycoproteins and prevention of the formation of microvillus inclusions in the zebrafish intestine.• The function of myosin Vb in the intestine is conserved between fish and humans.• The goosepimples mutant is a good animal model for microvillus inclusion disease in humans. Grant Sponsor: Wellcome Trust-DBT India alliance (MS; 500129-Z-09-Z), Department of Science and Technology, India (MS; SR/S2/RJN-06/2006), TIFR-DAE (MS; 12P-121). 2 Abstract Background: Microvillus inclusion disease (MVID) is a life threatening enteropathy characterised by perpetual diarrhoea. Recent analysis has revealed that enterocytes in MVID patients exhibit reduction of microvilli, presence of microvillus inclusion bodies and intestinal villus atrophy. Genetic linkage analysis has identified mutations in myosin Vb gene as the main cause of MVID. An animal model that develops ex-utero and is tractable genetically as well as by microscopy would be highly useful to study the cellular basis of the formation of inclusion bodies.Results: Here we report that the intestine of the zebrafish goosepimples (gsp)/myosin Vb (myoVb) mutant show severe reduction in the intestinal folds-structures similar to mammalian villi. The loss of folds is further correlated with changes in the shape of enterocytes. In a striking similarity with MVID patients, zebrafish gsp/myoVb mutant larvae exhibit microvillus atrophy, microvillus inclusions and accumulation of secretory material in enterocytes.Conclusion: We propose that the zebrafish gsp/myoVb mutant is a valuable model to study the pathophysiology of MVID. Owing to the advantages of zebrafish in screening libraries of small molecules, the gsp mutant will be an appropriate tool to identify compounds that would alleviate the formation of microvillus inclusions and have therapeutic value.