Actin-related protein2/3 complex regulates tight junctions and terminal differentiation to promote epidermal barrier formation

Abstract

The epidermis provides an essential seal from the external environment and retains fluids within the body. To form an effective barrier, cells in the epidermis must form tight junctions and terminally differentiate into cornified envelopes. Here, we demonstrate that the branched actin nucleator, the actin-related protein (Arp)2/3 complex, is unexpectedly required for both these activities. Loss of the ArpC3 subunit of the Arp2/3 complex resulted in minimal changes in the morphogenesis and architecture of this stratified squamous epithelium, but resulted in profound defects in its physiology. Mutant embryos did not develop an effective barrier to the external environment and died within hours of birth. We discovered two underlying causes for these effects. First, ArpC3 was essential for robust assembly and function of tight junctions, specialized cell-cell adhesions that restrict water loss in the epidermis. Second, there were defects in differentiation of the epidermis and the production of cornified envelopes, structures essential for barrier activity. Underlying this defect, we found that YAP was inappropriately active not only in the ArpC3 mutant tissue, but also in cultured cells. Inhibition of YAP activity rescued the differentiation and barrier defects caused by loss of ArpC3. These results demonstrate previously unappreciated roles for the Arp2/3 complex and highlight the functions of branched actin networks in a complex tissue.

Loss of the Arp2/3 complex component ARPC1B causes platelet abnormalities and predisposes to inflammatory disease

“…These include cell secretion53, phagocytosis5455, autophagy56, migration5758, haptotaxis59, focal adhesions60 and intracellular tight junctions required for epidermal barrier formation61, vesicle trafficking and transcytosis in the small intestine62. With regards to immunity and inflammation, Arp2/3 function has been reported to be critical for the formation of immune cell synapses6364 and T-regulatory cell function, which is aberrant in WAS patients leading to a high susceptibility to develop Th2-mediated food allergies65.…”

Akt1-associated actomyosin remodelling is required for nuclear lamina dispersal and nuclear shrinkage in epidermal terminal differentiation

“…Myosin II activity is required for nuclear repositioning in cell migration (Gomes, Jani, and Gundersen 2005), constriction of the cytokinetic actomyosin contractile ring (Powell 2005;De Lozanne and Spudich 1987;Mabuchi 1977) and contraction of an actin network around the nucleus at nuclear envelope breakdown (Booth et al 2019). Arp2/3, required for branched actin filament nucleation, has previously been identified as required for epidermal differentiation (Lechler 2014;Zhou et al 2013), regulation of nuclear actin (Oma and Harata 2011) and correct formation of the actomyosin contractile ring in cell division (Chan et al 2019). Myosin IIA and Arp2/3 have also been identified as responsible for force generation in the secretion of vesicles in alveolar type II (ATII) cells (Miklavc et al 2012) and myosin IIA and actin are required for exocytosis in murine exocrine cells (Ebrahim et al 2019).…”

Architecture of tight junctions and principles of molecular composition

“…A number of actin polymerizing proteins have been localized to tight junctions, including Arp2/3 [105], N-WASP [106], cortactin [107] and VASP [108]. In addition, many regulators of actin organization have been implicated in tight junction biology, including members of the RhoA, Rac and Cdc42 family of small GTPase; their roles at the junction have been the subject of excellent recent reviews [109, 110] as has trafficking of tight junction components [111], so these proteins will not be explicitly covered here.…”