Figure 2. Expression of nicotinic acetylcholine receptor (nAChR) subtypes is dependent upon differentiation, free acetylcholine (ACh) and free Ca2+ concentration gradients.
The nAChR superfamily of ligand-gated ionotropic receptors are differentially expressed during the course of keratinocyte differentiation. Corneocyte cells comprising the top layer of the epidermis (stratum corneum) express primarily α7 nAChR. Transitional (prickle) keratinocytes located in the middle layer of the epidermis express primarily α3(β2/β4)α5 nAChR. Immature (basal) keratinocytes located within the base layer of the epidermis express primarily α3β2 or nAChRα3β4 nAChR. All keratinocyte cells appear to express α9 nAChR. Activation of each nAChR subtype is associated with a different cellular response, including apoptosis, cornification, proliferation, migration, or differentiation. Atypical expression of specific nAChR subtypes throughout the epidermis has been correlated with disease. The gradients on the right represent the state of differentiation and the concentrations of ACh and free calcium, with the greatest degree of differentiation and highest ACh and free calcium concentrations occurring at the interface with the external environment (e.g. stratum corneum).