Expression of the integrin subunit α9 in the murine embryo

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Abstract
The α9 integrin subunit is expressed in adult skeletal muscle, visceral smooth muscle, hepatocytes, squamous epithelium, and airway epithelium. The in vivo function of this protein is unknown. Thus far, only a single α9‐containing integrin has been identified (α9β1) and only a single ligand (tenascin) has been found for this integrin. In order to gain insight into the potential function of α9 integrin(s), we examined the spatiotemporal distribution of the α9 subunit and tenascin during murine embryogenesis. In all tissues where α9 was expressed, its appearance was associated with other evidence of cell differentiation. In developing airway, visceral, and vascular smooth muscles, the onset of α9 expression either coincided with or immediately followed the expression of α‐SM actin. Expression of α9 in epithelia was restricted to the choroid plexus and the basal cell layer of squamous epithelia where its appearance coincided with the development of stratification. α9 immunostaining was first detected in developing skeletal musculature when skeletal myotubes formed. Tenascin expression was detected in many, but not all tissues found to express α9. For example, the hair germs of maturing hair follicles exhibited high levels of α9 staining, but no tenascin immunoreactivity was detected either within the hair germ themselves or in the adjacent dermis. In some tissues where tenascin expression colocalized with α9, expression patterns were not synchronous. Although α9 expression was associated with the onset of tissue differentiation, its expression was not limited to terminally differentiated cells. In fact, in the skin, α9 expression appeared restricted to cells known to retain the capacity to proliferate, i.e., basal cells and hair germs. Thus, α9 integrin(s) are not likely to contribute to the early steps in organ formation, but probably play a role in the maturation and/or maintenance of a variety of differentiated tissues. The expression of α9 without its only known ligand, tenascin, suggests the existence of additional ligands.