Evidence for a novel glutamate-mediated signaling pathway in keratinocytes

Phenotypic alterations in keratinocyte behavior are essential for maintaining epidermal integrity during growth and wound repair and rely on co-ordinated cell signaling events. Numerous growth factors and cytokines have been shown to be instrumental in guiding such changes in keratinocyte activity; here we provide evidence which proposes a novel epidermal signaling pathway mediated by the excitatory amino acid glutamate. Glutamate is the major excitatory neurotransmitter at synaptic junctions within the central nervous system; however, we have identified expression in vivo of several regulatory molecules associated with glutamate signaling in keratinocytes. In resting rat skin epidermis, different classes of glutamate receptors, transporters, and a recently described clustering protein were shown to display distinct distribution patterns, supportive of a multifunctional cellular communication pathway. Immunoreactive N-methyl-D-aspartate-type, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-type, and metabotropic-type glutamate receptors were colocalized with the specific glutamate transporter EAAC1 in basal layer keratinocytes, and GLT-1, a related transporter, was expressed suprabasally. In full-thickness rat skin wounds, marked modifications in the distribution of N-methyl-D-aspartate receptors and EAAC1 were observed during re-epithelialization, and alterations in N-methyl-D-aspartate receptor expression accompanied embryonic epidermal development, implicating glutamate signaling in these important biologic events. Furthermore, we provide evidence that these receptors are functional in vitro. These data provide strong evidence to support a role for glutamate in the control of epidermal renewal, and therefore suggest potentially novel therapeutic targets for the treatment of skin disease and enhancement of wound healing.