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Stress-inducible phosphoprotein 1 (STI1) is part of the chaperone machinery, but it also functions as an extracellular ligand for the prion protein. However, the physiological relevance of these STI1 activities in vivo is unknown. Here, we show that in the absence of embryonic STI1, several Hsp90 client proteins are decreased by 50%, although Hsp90 levels are unaffected. Mutant STI1 mice showed increased caspase-3 activation and 50% impairment in cellular proliferation. Moreover, placental disruption and lack of cellular viability were linked to embryonic death by El0.5 in STIl-mutant mice. Rescue of embryonic lethality in these mutants, by transgenic expression of the STII gene, supported a unique role for STI1 during embryonic development. The response of STI1 haploinsufficient mice to cellular stress seemed compromised, and mutant mice showed increased vulnerability to ischemic insult. At the cellular level, ischemia increased the secretion of STI1 from wild-type astrocytes by 3-fold, whereas STI1 haploinsufficient mice secreted half as much STI1. Interesting, extracellular STI1 prevented ischemia-mediated neuronal death in a prion proteindependent way. Our study reveals essential roles for intracellular and extracellular STI1 in cellular resilience. [ABSTRACT FROM AUTHOR]

Protein misfolding is a central feature of most neurodegenerative diseases. Molecular chaperones can modulate the toxicity associated with protein misfolding, but it remains elusive which molecular chaperones and co‐chaperones interact with specific misfolded proteins. TDP‐43 misfolding and inclusion formation are a hallmark of amyotrophic lateral sclerosis (ALS) and other neurodegenerative diseases. Using yeast and mammalian neuronal cells we find that Hsp90 and its co‐chaperone Sti1 have the capacity to alter TDP‐43 misfolding, inclusion formation, aggregation, and cellular toxicity. Our data also demonstrate that impaired Hsp90 function sensitizes cells to TDP‐43 toxicity and that Sti1 specifically interacts with and strongly modulates TDP‐43 toxicity in a dose‐dependent manner. Our study thus uncovers a previously unrecognized tie between Hsp90, Sti1, TDP‐43 misfolding, and cellular toxicity. [ABSTRACT FROM AUTHOR]