Uromodulin: old friend with new roles in health and disease.

The most abundant urinary protein, Tamm-Horsfall protein, later renamed uromodulin, is expressed exclusively by the thick ascending limb cells of the kidney and released into urine from the apical cell membrane. Uromodulin is believed to protect against urinary tract infections and stones, but its other physiologic functions have remained obscure until recently. Renewed interest in uromodulin has been brought about by the identification of uromodulin mutations as causes of a discrete group of diseases that are distinct from nephronophthisis. The three overlapping clinical uromodulin-associated kidney diseases (UAKD) are medullary cystic disease type 2, familial juvenile hyperuricemic nephropathy and glomerulocystic kidney disease. Previously thought of as "adult diseases", it is now recognized that they may also present in childhood and even in infancy. Common characteristics of all three diseases are autosomal dominant inheritance, unremarkable urine sediment and slow progression to end-stage renal disease (ESRD). They are frequently associated with hyperuricemia and gout. These diseases appear to result from failure of the mutant uromodulin to be incorporated into the apical cilium, thereby placing UAKD in the category of "ciliopathies". In addition to causing specific UAKD, certain uromodulin gene polymorphisms have been linked to ESRD in general, suggesting that uromodulin plays a modulatory role in kidney disease progression.