The cortical actin cytoskeleton and desmosomes act together in keratin filament network maintenance

Abstract

Keratin intermediate filaments are hallmark features of epithelial tissue differentiation forming complex cytoplasmic networks that are connected to subplasmalemmal cortical keratin filaments and anchored at desmosomal junctions. The mechanisms determining keratin filament network morphogenesis and maintenance are poorly understood. We previously generated a homozygous knock-in mouse line expressing YFP-tagged keratin 8, which functionally substitutes for the wild-type keratin 8. 3D time-lapse fluorescence recording of developing pre-implantation embryos allowed monitoring the de novo formation of the keratin filament network, which implicated desmosomes and the actin-rich cell cortex in nucleation and transport of nascent keratin particles. To further explore the relevance of the cell cortex for keratin filament network maintenance, we now studied Krt8-YFP-producing late blastocysts that had developed cytoplasmic and cortical keratin networks. We treated them with drugs to modulate the actomyosin cytoskeleton and analyzed desmosome-deficient blastocysts. We find that overall keratin network organization is barely affected in either scenario. Detailed analyses, however, reveal distinct changes in cytoplasmic keratin filament abundance, cortical anchorage and keratin filament turnover. We conclude that mature keratin networks withstand drastic changes in cellular organization and that maintenance of their spatial organization is secured in a “belt and braces” fashion by multiple mechanisms, notably desmosomal anchorage and attachment to the actin-rich cell cortex.