Imaging Skin through the Lenses of Phase Separation: Liquid-like Membraneless Organelles Dictate Skin Barrier Formation

Felipe Garcia Quiroz, Ph.D., Postdoctoral Researcher, Laboratory of Mammalian Cell Biology and Development, The Rockefeller University

The skin forms an essential barrier for retaining body fluids and resisting environmental insults. Throughout life, epidermal stem cells constantly rejuvenate this barrier as they move upward and stratify into layers. Despite the importance of the skin barrier, its formation remains poorly understood. Intriguingly, a well-known yet loosely defined membraneless structure within skin cells, keratohyalin granules (KGs), is impaired in skin barrier disorders (e.g. eczema) that are also frequently associated with mutations in skin repeat proteins, namely Filaggrin (Flg). Building on my previous work that identified Flg as a putative phase transition protein (Nature Materials, 2015), in this talk I will present a novel approach to illuminate the process of skin barrier formation through the engineering of fluorescent proteins that sense phase behavior in mouse and human skin. Using these advanced tools, together with mouse genetics, live imaging and atomic force microscopy, we demonstrate that Flg drives a dramatic event of liquid-liquid phase separation in skin to assemble a surprisingly extensive network of KGs that are at the crux of skin barrier formation by virtue of their stimuli-responsiveness and unique liquid-like properties. Importantly, knockdown of Flg in mouse skin as well as mutations in human Flg both impair skin barrier formation and abolish or severely impact phase separation. Besides providing unprecedented and therapeutically-relevant insights into the process of skin barrier formation, this work has broad implications to the emerging field of cellular mechanisms driven by phase separation, and beg the exploration of the underlying principles for the engineering of stimuli-responsive materials and skins with novel functionality.