Hsp90 has been shown to mediate interactions between PLN, SERCA, and HAX-1. By recruiting Hsp90 to the SR Ca2+ uptake complex, the function of IRE-1, another Hsp90 client protein, was impaired
[56][57][58]. Furthermore, the function of PLN and ryanodine receptor can be regulated by Ca
2+/calmodulin-dependent protein kinase II (CaMKII) phosphorylation
[59][60]. This kinase is also stabilized by Hsp90
[22]. CaMKII is relevant in intra-nuclear phosphorylation of transcription factors including HSF-1, CREB, and SRF. It also may activate NF-kB signaling leading to inflammatory response
[61]. Given the role of CaMKII and SR Ca
2+ cycling in the development of heart diseases, it is intriguing to examine if Hsp90 can be targeted to correct these dysfunctions. When Ca
2+ levels increase in the cytosol, two important Ca
2+-dependent proteins can be activated, calcineurin and calmodulin. Upon Ca
2+ binding, these enzymes dimerize to form a functional phosphatase
[62]. Hsp90 is found to stabilize both calcineurin and calmodulin and inhibition of Hsp90 leads to decreased nuclear factor of activated T-cells (NFAT) signaling
[63][64]. Once the calcineurin/calmodulin phosphatase (CaM) is active, it dephosphorylates NFAT which is translocated to the nucleus as a transcription factor. Here, NFAT can activate genes controlled by MEF2 and GATA which are implicated in cardiac hypertrophy
[65]. NFAT has been shown to be relevant in pathological cardiac hypertrophy and may also cross-talk with MAPK to accentuate pathological effects
[66][67].