FKBP immunophilins and Alzheimer's disease: a chaperoned affair.

The FK506-binding protein (FKBP) family of immunophilins consists of proteins with a variety of protein-protein interaction domains and versatile cellular functions. Analysis of the functions of immunophilins has been the focus of studies in recent years and has led to the identification of various molecular pathways in which FKBPs play an active role. All FKBPs contain a domain with prolyl cis/trans isomerase (PPIase) activity. Binding of the immunosuppressant molecule FK506 to this domain inhibits their PPIase activity while mediating immune suppression through inhibition of calcineurin. The larger members, FKBP51 and FKBP52, interact with Hsp90 and exhibit chaperone activity that is shown to regulate steroid hormone signalling. From these studies it is clear that FKBP proteins are expressed ubiquitously but show relatively high levels of expression in the nervous system. Consistent with this expression, FKBPs have been implicated with both neuroprotection and neurodegeneration. This review will focus on recent studies involving FKBP immunophilins in Alzheimer's-disease-related pathways.