Alessandro Luciani

Elevated concentrations of albumin in the urine, albuminuria, are a hallmark of diabetic kidney disease and are associated with an increased risk for end-stage renal disease and cardiovascular events. To gain insight into the pathophysiological mechanisms underlying albuminuria, we conducted meta-analyses of genome-wide association studies and independent replication in up to 5,825 individuals of European ancestry with diabetes and up to 46,061 without diabetes, followed by functional studies. Known associations of variants in CUBN, encoding cubilin, with the urinary albumin-to-creatinine ratio (UACR) were confirmed in the overall sample (P = 2.4 × 10(-10)). Gene-by-diabetes interactions were detected and confirmed for variants in HS6ST1 and near RAB38/CTSC. Single nucleotide polymorphisms at these loci demonstrated a genetic effect on UACR in individuals with but not without diabetes. The change in the average UACR per minor allele was 21% for HS6ST1 (P = 6.3 × 10(-7)) and 13% for RAB38/CTSC (P = 5.8 × 10(-7)). Experiments using streptozotocin-induced diabetic Rab38 knockout and control rats showed higher urinary albumin concentrations and reduced amounts of megalin and cubilin at the proximal tubule cell surface in Rab38 knockout versus control rats. Relative expression of RAB38 was higher in tubuli of patients with diabetic kidney disease compared with control subjects. The loci identified here confirm known pathways and highlight novel pathways influencing albuminuria.

The endolysosomal system sustains the reabsorptive activity of specialized epithelial cells. Lysosomal storage diseases such as nephropathic cystinosis cause a major dysfunction of epithelial cells lining the kidney tubule, resulting in massive losses of vital solutes in the urine. The mechanisms linking lysosomal defects and epithelial dysfunction remain unknown, preventing the development of disease-modifying therapies. Here we demonstrate, by combining genetic and pharmacologic approaches, that lysosomal dysfunction in cystinosis results in defective autophagy-mediated clearance of damaged mitochondria. This promotes the generation of oxidative stress that stimulates Gα12/Src-mediated phosphorylation of tight junction ZO-1 and triggers a signaling cascade involving ZO-1-associated Y-box factor ZONAB, which leads to cell proliferation and transport defects. Correction of the primary lysosomal defect, neutralization of mitochondrial oxidative stress, and blockage of tight junction-associated ZONAB signaling rescue the epithelial function. We suggest a link between defective lysosome-autophagy degradation pathways and epithelial dysfunction, providing new therapeutic perspectives for lysosomal storage disorders.

BACKGROUND: An unresolved question in coeliac disease is to understand how some toxic gliadin peptides, in particular p31-43, can initiate an innate response and lead to tissue transglutaminase (TG2) upregulation in coeliac intestine and gliadin sensitive epithelial cell lines. Aim We addressed whether the epithelial uptake of p31-43 induces an intracellular pro-oxidative envoronment favouring TG2 activation and leading to the innate immune response. METHODS: The time course of intracellular delivery to lysosomes of p31-43, palpha-2 or palpha-9 gliadin peptides was analysed in T84 and Caco-2 epithelial cells. The effects of peptide challenge on oxidative stress, TG2 and peroxisome proliferator-activated receptor (PPAR)gamma ubiquitination and p42/44-mitogen activated protein (MAP) kinase or tyrosine phosphorylation were investigated in cell lines and cultured coeliac disease biopsies with/without anti-oxidant treatment or TG2 gene silencing by immunoprecipitation, western blot, confocal microscopy and Fluorenscence Transfer Resonance Energy (FRET) analysis. RESULTS: After 24 h of challenge p31-43, but not palpha-2 or palpha-9, is still retained within LAMP1-positive perinuclear vesicles and leads to increased levels of reactive oxygen species (ROS) that inhibit TG2 ubiquitination and lead to increases of TG2 protein levels and activation. TG2 induces cross-linking, ubiquitination and proteasome degradation of PPARgamma. Treatment with the antioxidant EUK-134 as well as TG2 gene silencing restored PPARgamma levels and reversed all monitored signs of innate activation, as indicated by the dramatic reduction of tyrosine and p42/p44 phosphorylation. CONCLUSION: p31-43 accumulation in lysosomes leads to epithelial activation via the ROS-TG2 axis. TG2 works as a rheostat of ubiquitination and proteasome degradation and drives inflammation via PPARgamma downregulation.