The cadherin family of cell adhesion molecules are calcium-dependent membrane glycoproteins that are grouped into nine types based on structural and functional differences (204). Type I, or classical cadherins (E-, P-, N-, B-, R-, EP-cadherin), are the only cadherins that have been conclusively shown to promote cell adhesion. Type I cad-herins contain five extracellular domains of approximately 110 amino acids each, a single transmembrane domain, and two cytoplasmic domains. The extracellular domains contain Asp-X-Gln-Asp-Gln-Asp and Asp-X-Asp amino acid motifs, which are responsible for calcium binding. The amino-terminal extracellular domain contains the cell adhesion recognition sequence His-Ala-Val (HAV), which mediates calcium-dependent homotypic and heterotypic interactions between cadherins. The cytoplasmic domains of type I cadherins interact with a family of intracellular proteins termed catenins (105). The catenins in turn bind to microfilaments and serve to connect the cadherins to the cytoskeleton. The interaction of catenins with the cyto-plasmic domain of cadherins is thought to play an important role in cadherin adhesive function.

Cadherin interactions regulate the formation of adher-ens junctions and tight junctions between cells. In addition to mediating cell-cell contact, cadherins also regulate cell morphology and differentiation. E-cadherin, which has been classically associated with the formation, differentiation, and polarization of developing epithelia, has recently been shown to regulate erythropoietin-mediated differentiation of bone marrow mononuclear cells (205). Neural cadherin, or N-cadherin, promotes neurite outgrowth, which can be blocked by HAV-containing peptides (206). N-cadherin also regulates the adhesion of O-2A-lineage glial progenitor cells in culture (207).

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