Expression of CD44 and Its Spliced Variants: Innate and Inducible Roles in Nervous Tissue Cells and Their Environment.

CD44, a structurally diverse cell-surface glycoprotein, plays a multifaceted and indispensable role in neural tissue across both physiological and pathological conditions. It orchestrates complex cell-extracellular matrix interactions and intracellular signaling through its variant isoforms and post-translational modifications and is broadly expressed in neural stem/progenitor cells, microglia, astrocytes, and selected neuronal populations. The interactions of CD44 with ligands such as hyaluronan and osteopontin regulate critical cellular functions, including migration, differentiation, inflammation, and synaptic plasticity. In microglia and macrophages, CD44 mediates immune signaling and phagocytic activity, and it is dynamically upregulated in neuroinflammatory diseases, particularly through pathways involving Toll-like receptor 4. CD44 expression in astrocytes is abundant during central nervous system development and in diseases, contributing to glial differentiation, reactive astrogliosis, and scar formation. Though its expression is less prominent in mature neurons, CD44 supports neural plasticity, circuit organization, and injury-induced repair mechanisms. Additionally, its expression at nervous system barriers, such as the blood-brain barrier, underscores its role in regulating vascular permeability during inflammation and ischemia. Collectively, CD44 emerges as a critical integrator of neural cell function and intercellular communication. Although the roles of CD44 in glial cells appear to be similar to those explored in other tissues, the expression of this molecule and its variants on neurons reveals peculiar functions. Elucidating the cell-type-specific roles and regulation of CD44 variants may offer novel therapeutic strategies for diverse neurological disorders.