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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, Pkd1 R2216W , on the basis of the orthologous human hypomorphic allele Pkd1 R2220W , 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 Pkd1 R2216W or heterozygous for Pkd1 R2216Win trans with a conditional Pkd1 fl 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., (Copyright © 2022 by the American Society of Nephrology.)

Background: SEC63 encodes a resident protein in the endoplasmic reticulum membrane that, when mutated, causes human autosomal dominant polycystic liver disease. Selective inactivation of Sec63 in all distal nephron segments in embryonic mouse kidney results in polycystin-1-mediated polycystic kidney disease (PKD). It also activates the Ire1 α -Xbp1 branch of the unfolded protein response, producing Xbp1s, the active transcription factor promoting expression of specific genes to alleviate endoplasmic reticulum stress. Simultaneous inactivation of Xbp1 and Sec63 worsens PKD in this model., Methods: We explored the renal effects of postnatal inactivation of Sec63 alone or with concomitant inactivation of Xbp1 or Ire1α , specifically in the collecting ducts of neonatal mice., Results: The later onset of inactivation of Sec63 restricted to the collecting duct does not result in overt activation of the Ire1 α -Xbp1 pathway or cause polycystin-1-dependent PKD. Inactivating Sec63 along with either Xbp1 or Ire1α in this model causes interstitial inflammation and associated fibrosis with decline in kidney function over several months. Re-expression of XBP1s in vivo completely rescues the chronic kidney injury observed after inactivation of Sec63 with either Xbp1 or Ire1α ., Conclusions: In the absence of Sec63 , basal levels of Xbp1s activity in collecting ducts is both necessary and sufficient to maintain proteostasis (protein homeostasis) and protect against inflammation, myofibroblast activation, and kidney functional decline. The Sec63-Xbp1 double knockout mouse offers a novel genetic model of chronic tubulointerstitial kidney injury, using collecting duct proteostasis defects as a platform for discovery of signals that may underlie CKD of disparate etiologies., (Copyright © 2019 by the American Society of Nephrology.)

Polycystic kidney disease (PKD) can arise from either developmental or postdevelopmental processes. Recessive PKD, caused by mutations in PKHD1, is a developmental defect, whereas dominant PKD, caused by mutations in PKD1 or PKD2, occurs by a cellular recessive mechanism in mature kidneys. Oriented cell division is a feature of planar cell polarity that describes the orientation of the mitotic axes of dividing cells during development with respect to the luminal vector of the elongating nephron. In polycystic mutant mice, the loss of oriented cell division may also contribute to the pathogenesis of PKD. Here, we examined the role of oriented cell division in mouse models based on mutations in Pkd1, Pkd2, and Pkhd1. Precystic tubules after kidney-selective inactivation of either Pkd1 or Pkd2 did not lose oriented division before cystic dilation but lost oriented division after tubular dilation began. In contrast, Pkhd1(del4/del4) mice lost oriented cell division but did not develop kidney cysts. Increased intercalation of cells into the plane of the tubular epithelium maintained the normal tubular morphology in Pkhd1(del4/del4) mice, which had more cells present in transverse tubular profiles. In conclusion, loss of oriented cell division is a feature of Pkhd1 mutation and cyst formation, but it is neither sufficient to produce kidney cysts nor required to initiate cyst formation after mutation in Pkd1 or Pkd2.

The cloning of the PKD1 and PKD2 genes has led to promising new insight into the mechanisms that are responsible for cyst development in patients with autosomal dominant polycystic kidney disease. Although the dominant pattern of inheritance would argue for haploinsufficiency, a gain of function, or a dominant negative mechanism, there is good evidence that autosomal dominant polycystic kidney disease behaves like a recessive disease on a cellular level (two-hit mechanism of cystogenesis). For testing of whether other pathomechanisms in addition to the two-hit hypothesis can explain cyst formation, two transgenic rat lines that contain a truncated human polycystin-2 cDNA were generated. The protein product lacks almost the entire COOH-terminus and mimics mutations that frequently are found in patients. The transgene-encoded mRNA could be detected in multiple tissues of both transgenic lines, with the highest expression in the kidney. Both lines present with renal cysts that originate predominantly from the proximal tubule; in the tubular epithelial cells, the epitope-tagged mutant protein was detected in the brush border and in primary cilia. Further evidence of the involvement of primary cilia stems from the finding of retinal degeneration in the transgenic rats and from the fact that stably transfected LLC-PK(1) cells that inducibly produced the truncated polycystin-2 protein elaborated shorter cilia. Other experimental approaches, such as a knock-in strategy, will be necessary to validate these results, but this is the first preliminary evidence that cyst formation is due not only to somatic mutations.

The great advantage of the tetracycline-inducible system lies in its ability to address a large variety of biological questions in a time-dependent and tissue-specific manner. This study describes a transgenic mouse line, rTA(LAP)-1, which produces the reverse tetracycline transactivator under control of the liver activator protein (LAP) promoter. Two reporter lines with luciferase and LacZ reporter genes were used to demonstrate predominant expression in the kidney and liver when doxycycline was added to the drinking water. In the kidney, transgene expression was found primarily in cortical proximal tubules. No luciferase and beta-galactosidase activity was detected in mice without doxycycline in the drinking water, which attests to the tight control of this system. One of the advantages of the tet system lies in its reversibility, and indeed, a virtually complete remission of transgene activity in both the kidney and liver was observed when doxycycline was withdrawn. Also examined was transactivator activity during development by exposing the mothers producing the reverse transactivator to doxycycline before mating. Transgene activity was detected in newborn kidneys and liver, indicating that sufficient amounts of doxycycline had crossed the placental barrier. During nephron development, the LAP promoter appeared to be only active in the more mature proximal tubules. Finally, the rTA(LAP)-1 line was used to inducibly express the human PKD2 cDNA in proximal tubules of transgenic mice, but no cystic changes were detected, even after 6 mo of induction.