Matrix Hyaluronan-Activated CD44 Signaling Promotes Keratinocyte Activities and Improves Abnormal Epidermal Functions

Hyaluronan (HA), a major component of the extracellular matrix, is enriched in skin tissues, particularly the epidermis. HA binds to a ubiquitous, abundant, and functionally important family of cell surface receptors, CD44. This article reviews the current evidence for HA/CD44-mediated activation of RhoGTPase signaling and calcium mobilization, leading to the regulation of keratinocyte activities and various epidermal functions. It further discusses the role of HA-mediated CD44 interactions with unique downstream effectors, such as RhoGTPases (RhoA and Rac1), Rho-kinase, protein kinase-N gamma, and phosphoinositide-specific phospholipases (phospholipases C epsilon and C gamma 1) in coordinating certain intracellular signaling pathways, such as calcium mobilization, phosphatidylinositol 3-kinase-AKT activation, cortactin-actin binding, and actin-associated cytoskeleton reorganization; generating the onset of important keratinocyte activities, such as cell adhesion, proliferation, migration, and differentiation; and performing epidermal functions. Topical application of selective HA fragments (large versus small HA) to the skin of wild-type mice (but not CD44 knockout mice) improves keratinocyte-associated epidermal functions and accelerates permeability barrier recovery and skin wound healing. Consequently, specific HA fragment (large versus small HA)-mediated signaling events (through the CD44 receptor) are required for keratinocyte activities, which offer new HA-based therapeutic options for patients experiencing epidermal dysfunction and skin damage as well as aging-related skin diseases, such as epidermal thinning (atrophy), permeability barrier dysfunction, and chronic nonhealing wounds.