Osmotically relevant membrane signaling complex: Association between HB- EGF, β₁-integrin, and CD9 in mTAL

The integral membrane proteins cluster of differentiation-9 (CD9), β₁- integrin, and heparin-binding epidermal growth factor-like (HB-EGF) exist in association in many cell lines and are linked to intracellular signaling mechanisms. Two of the proteins (CD9 and β₁-integrin) are induced by hypertonicity, suggesting that their related signaling processes may be relevant to osmotic stress. The validity of this hypothesis rests upon coexpression and physical association between these molecules in nephron segments that are normally exposed to high and variable ambient osmolality. In this work, we show that CD9 and β₁-integrin are induced in rat kidney medulla after dehydration. Immunohistochemistry and immunoprecipitation studies show that CD9, HB-EGF, and β₁-integrin are coexpressed and physically associated in medullary thick ascending limbs (mTAL), nephron segments that are normally exposed to high and variable extracellular osmolality. Our findings are consistent with the existence of a cluster of integral membrane proteins in mTAL that may initiate or modulate osmotically relevant signaling pathways.