XBP1 Activation Reduces Severity of Polycystic Kidney Disease due to a Nontruncating Polycystin-1 Mutation in Mice

Significance Statement XBP1 activation in neonatal and adult doxycycline-inducible murine models of ADPKD due to a hypomorphic polycystin-1 missense mutation orthologous to human PC1R2220W delays cyst formation. Activating XBP1s, a pro-chaperone inducer of the endoplasmic reticulum stress response, can improve steady-state expression, ciliary trafficking, and cleavage of the mutant protein, providing initial in vivo proof of concept that modulating levels of poorly functioning hypomorphic PC1 alleles can slow progression of kidney cyst formation in ADPKD. Background Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in Pkd1 and Pkd2. They encode the polytopic integral membrane proteins polycystin-1 (PC1) and polycystin-2 (PC2), respectively, which are expressed on primary cilia. Formation of kidney cysts in ADPKD starts when a somatic second hit mechanism inactivates the wild-type Pkd allele. Approximately one quarter of families with ADPDK due to Pkd1 have germline nonsynonymous amino acid substitution (missense) mutations. A subset of these mutations is hypomorphic, retaining some residual PC1 function. Previous studies have shown that the highly conserved Ire1 α -XBP1 pathway of the unfolded protein response can modulate levels of functional PC1 in the presence of mutations in genes required for post-translational maturation of integral membrane proteins. We examine how activity of the endoplasmic reticulum chaperone-inducing transcription factor XBP1 affects ADPKD in a murine model with missense Pkd1 . Methods We engineered a Pkd1 REJ domain missense murine model, Pkd1R2216W , on the basis of the orthologous human hypomorphic allele Pkd1R2220W , and examined the effects of transgenic activation of XBP1 on ADPKD progression. Results Expression of active XBP1 in cultured cells bearing PC1 R2216W mutations increased levels and ciliary trafficking of PC1 R2216W . Mice homozygous for Pkd1R2216W or heterozygous for Pkd1R2216W in trans with a conditional Pkd1fl allele exhibit severe ADPKD following inactivation in neonates or adults. Transgenic expression of spliced XBP1 in tubule segments destined to form cysts reduced cell proliferation and improved Pkd progression, according to structural and functional parameters. Conclusions Modulating ER chaperone function through XBP1 activity improved Pkd in a murine model of PC1, suggesting therapeutic targeting of hypomorphic mutations.