Matrix hyaluronan-activated CD44 signaling promotes keratinocyte activities and improves abnormal epidermal functions

Abstract

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-Nγ, and phosphoinositide-specific phospholipases (phospholipases Cε and Cγ1) 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.

Figure 1

A proposed model for HA/CD44-mediated signaling in keratinocytes. The binding of HA to the extracellular domain of CD44 promotes activation of RhoGTPases [eg, RhoA (A) and Rac1 (B)], which then generate specific signaling cascades described as follows. A: RhoA-specific signaling pathways. HA-CD44 interaction (left, 1) activates RhoA-ROK signaling, promotes the membrane localization of Gab-1, and activates certain isoforms of PI3K, leading to AKT activation and keratinocyte cell growth and survival. The binding of HA to CD44 induces activation of ROK, which, in turn, phosphorylates myosin light chain phosphatase and LIM kinase, thereby generating actomyosin contractility and actin assembly-disassembly required for keratinocyte migration (left, 2). HA-CD44–mediated ROK signaling also phosphorylates PLCε (left, 3) which, in turn, produces IP₃ production and IP₃ receptor–triggered intracellular Ca²⁺ mobilization, resulting in CaMKII activation. CaMKII then phosphorylates the cytoskeletal protein, filamin, leading to cytoskeleton reorganization and keratinocyte migration. B: Rac1-specific signaling pathways. HA-CD44 interaction activates Rac1 signaling and PKN activity (right, 1). Activated PKN then phosphorylates certain cellular proteins, including PLC-γ1, which, in turn, produces IP₃ production and IP₃ receptor–triggered intracellular Ca²⁺ mobilization, resulting in CaMKII activation. CaMKII then phosphorylates the cytoskeletal protein, filamin, leading to cytoskeleton function and keratinocyte differentiation. The binding of HA to CD44 also promotes PKN-mediated phosphorylation of cytoskeletal protein, cortactin, thereby generating actin filament reorganization and keratinocyte cell-cell adhesion (right, 2).

Figure 2

A proposed model for selective effects of HA_S versus HA_L on CD44-mediated epidermal functions during skin aging. A: HA_S-mediated RhoA-ROK signaling in aged skin: In epidermis, HA_S binding to CD44 promotes RhoA-ROK signaling, resulting in proliferation and migration, leading to an increase in epidermal thickness and acceleration of wound healing in aged skin. ROK inhibitor, Y27632, effectively reduces HA_S-mediated RhoA-ROK signaling and a variety of keratinocyte functions (eg, proliferation, migration, epidermal thickness, and skin wound healing) in aged mouse skin. B: HA_L-mediated Rac1-PKN signaling in aged skin: In epidermis, HA_L binding to CD44 promotes Rac1-PKN signaling, resulting in cell-cell adhesion, differentiation, and lipid synthesis, leading to restoration of permeability barrier functions in aged skin. PKN inhibitor, Ro31-8220, effectively reduces HA_L-mediated Rac1-PKN signaling and a variety of keratinocyte functions (eg, cell-cell adhesion, differentiation, lipid synthesis, and permeability barrier function) in aged mouse skin. Both skin aging and CD44 deficiency in knockout (k/o) mice cause down-regulation of HA_S-mediated RhoA-ROK signaling and HA_L-induced Rac1-PKN activation events, resulting in abnormal epidermal structure and function.