In normal cells, most Hsp90 resides in an uncomplexed state. In the presence of abnormal client proteins, Hsp90 associates with co-chaperones. When complexed with chaperones like Hsp70 and Hsp40, the degradation of client proteins is favored. When complexed to another subset of chaperones, including p23, protein folding is favored. When bound by HSP90i, Hsp90 adopts the conformation that drives protein degradation. In a similar fashion, the acetylation state of Hsp90 modulates Hsp90 function; Hsp90 acetylation disrupts the assembly of Hsp90 with chaperones involved in protein folding, resulting in the degradation of client proteins. Acetylated Hsp90 also binds to HSP90i with a higher affinity, thus enhancing HSP90i-induced protein degradation.
PGRN levels affect many biological functions. Elevated PGRN levels stimulate proliferation, survival and motility in a variety of cells. Consequently, PGRN acts as a tumorigenic agent leading to several different types of cancers. Genetic and functional analysis has shown that decreased levels of PGRN due to null mutations within the PGRN gene are a major cause of FTLD-U with TDP-43-positive inclusions. The broad range of age at onset of FTLD-U patients with PGRN mutations indicates that environmental and/or other genetic factors are also likely to play a role in disease pathogenesis.