Class III PI3K Vps34 Controls Thyroid Hormone Production by Regulating Thyroglobulin Iodination, Lysosomal Proteolysis, and Tissue Homeostasis.

Background: The production of thyroid hormones [triiodothyronine (T3), thyroxine (T4)] depends on the organization of the thyroid in follicles, which are lined by a monolayer of thyrocytes with strict apicobasal polarity. This polarization supports vectorial transport of thyroglobulin (Tg) for storage into, and recapture from, the colloid. It also allows selective addressing of channels, transporters, ion pumps, and enzymes to their appropriate basolateral [Na+/I- symporter (NIS), SLC26A7, and Na+/K+-ATPase] or apical membrane domain (anoctamin, SLC26A4, DUOX2, DUOXA2, and thyroperoxidase). How these actors of T3/T4 synthesis reach their final destination remains poorly understood. The PI 3-kinase isoform Vps34/PIK3C3 is now recognized as a main component in the general control of vesicular trafficking and of cell homeostasis through the regulation of endosomal trafficking and autophagy. We recently reported that conditional Vps34 inactivation in proximal tubular cells in the kidney prevents normal addressing of apical membrane proteins and causes abortive macroautophagy. Methods:Vps34 was inactivated using a Pax8-driven Cre recombinase system. The impact of Vps34 inactivation in thyrocytes was analyzed by histological, immunolocalization, and messenger RNA expression profiling. Thyroid hormone synthesis was assayed by 125I injection and plasma analysis. Results:Vps34 conditional knockout (Vps34cKO) mice were born at the expected Mendelian ratio and showed normal growth until postnatal day 14 (P14), then stopped growing and died at ∼1 month of age. We therefore analyzed thyroid Vps34cKO at P14. We found that loss of Vps34 in thyrocytes causes (i) disorganization of thyroid parenchyma, with abnormal thyrocyte and follicular shape and reduced PAS+ colloidal spaces; (ii) severe noncompensated hypothyroidism with extremely low T4 levels (0.75 ± 0.62 µg/dL) and huge thyrotropin plasma levels (19,300 ± 10,500 mU/L); (iii) impaired 125I organification at comparable uptake and frequent occurrence of follicles with luminal Tg but nondetectable T4-bearing Tg; (iv) intense signal in thyrocytes for the lysosomal membrane marker, LAMP-1, as well as Tg and the autophagy marker, p62, indicating defective lysosomal proteolysis; and (v) presence of macrophages in the colloidal space. Conclusions: We conclude that Vps34 is crucial for thyroid hormonogenesis, at least by controlling epithelial organization, Tg iodination as well as proteolytic T3/T4 excision in lysosomes.

Protection of Cystinotic Mice by Kidney-Specific Megalin Ablation Supports an Endocytosis-Based Mechanism for Nephropathic Cystinosis Progression.

BACKGROUND: Deletions or inactivating mutations of the cystinosin gene CTNS lead to cystine accumulation and crystals at acidic pH in patients with nephropathic cystinosis, a rare lysosomal storage disease and the main cause of hereditary renal Fanconi syndrome. Early use of oral cysteamine to prevent cystine accumulation slows progression of nephropathic cystinosis but it is a demanding treatment and not a cure. The source of cystine accumulating in kidney proximal tubular cells and cystine's role in disease progression are unknown. METHODS: To investigate whether receptor-mediated endocytosis by the megalin/LRP2 pathway of ultrafiltrated, disulfide-rich plasma proteins could be a source of cystine in proximal tubular cells, we used a mouse model of cystinosis in which conditional excision of floxed megalin/LRP2 alleles in proximal tubular cells of cystinotic mice was achieved by a Cre-LoxP strategy using Wnt4-CRE. We evaluated mice aged 6-9 months for kidney cystine levels and crystals; histopathology, with emphasis on swan-neck lesions and proximal-tubular-cell apoptosis and proliferation (turnover); and proximal-tubular-cell expression of the major apical transporters sodium-phosphate cotransporter 2A (NaPi-IIa) and sodium-glucose cotransporter-2 (SGLT-2). RESULTS: Wnt4-CRE-driven megalin/LRP2 ablation in cystinotic mice efficiently blocked kidney cystine accumulation, thereby preventing lysosomal deformations and crystal deposition in proximal tubular cells. Swan-neck lesions were largely prevented and proximal-tubular-cell turnover was normalized. Apical expression of the two cotransporters was also preserved. CONCLUSIONS: These observations support a key role of the megalin/LRP2 pathway in the progression of nephropathic cystinosis and provide a proof of concept for the pathway as a therapeutic target.

Vps34/PI3KC3 deletion in kidney proximal tubules impairs apical trafficking and blocks autophagic flux, causing a Fanconi-like syndrome and renal insufficiency.

Kidney proximal tubular cells (PTCs) are highly specialized for ultrafiltrate reabsorption and serve as paradigm of apical epithelial differentiation. Vps34/PI3-kinase type III (PI3KC3) regulates endosomal dynamics, macroautophagy and lysosomal function. However, its in vivo role in PTCs has not been evaluated. Conditional deletion of Vps34/PI3KC3 in PTCs by Pax8-Cre resulted in early (P7) PTC dysfunction, manifested by Fanconi-like syndrome, followed by kidney failure (P14) and death. By confocal microscopy, Vps34∆/∆ PTCs showed preserved apico-basal specification (brush border, NHERF-1 versus Na+/K+-ATPase, ankyrin-G) but basal redistribution of late-endosomes/lysosomes (LAMP-1) and mis-localization to lysosomes of apical recycling endocytic receptors (megalin, cubilin) and apical non-recycling solute carriers (NaPi-IIa, SGLT-2). Defective endocytosis was confirmed by Texas-red-ovalbumin tracing and reduced albumin content. Disruption of Rab-11 and perinuclear galectin-3 compartments suggested mechanistic clues for defective receptor recycling and apical biosynthetic trafficking. p62-dependent autophagy was triggered yet abortive (p62 co-localization with LC3 but not LAMP-1) and PTCs became vacuolated. Impaired lysosomal positioning and blocked autophagy are known causes of cell stress. Thus, early trafficking defects show that Vps34 is a key in vivo component of molecular machineries governing apical vesicular trafficking, thus absorptive function in PTCs. Functional defects underline the essential role of Vps34 for PTC homeostasis and kidney survival.

Cellular uptake of proMMP-2:TIMP-2 complexes by the endocytic receptor megalin/LRP-2.

Matrix metalloproteinases (MMPs) are regulated at multiple transcriptional and post-transcriptional levels, among which receptor-mediated endocytic clearance. We previously showed that low-density lipoprotein receptor-related protein-1 (LRP-1) mediates the clearance of a complex between the zymogen form of MMP-2 (proMMP-2) and tissue inhibitor of metalloproteinases, TIMP-2, in HT1080 human fibrosarcoma cells. Here we show that, in BN16 rat yolk sac cells, proMMP-2:TIMP-2 complex is endocytosed through a distinct LRP member, megalin/LRP-2. Addition of receptor-associated protein (RAP), a natural LRP antagonist, caused accumulation of endogenous proMMP-2 and TIMP-2 in conditioned media. Incubation with RAP also inhibited membrane binding and cellular uptake of exogenous iodinated proMMP-2:TIMP-2. Moreover, antibodies against megalin/LRP-2, but not against LRP-1, inhibited binding of proMMP-2:TIMP-2 to BN16 cell surface. BIAcore analysis confirmed direct interaction between the complex and megalin/LRP-2. Conditional renal invalidation of megalin/LRP-2 in mice resulted in accumulation of proMMP-2 and TIMP-2 in their urine, highlighting the physiological relevance of the binding. We conclude that megalin/LRP-2 can efficiently mediate cell-surface binding and endocytosis of proMMP-2:TIMP-2 complex. Therefore megalin/LRP-2 can be considered as a new actor in regulation of MMP-2 activity, an enzyme crucially involved in many pathological processes.

Time course of pathogenic and adaptation mechanisms in cystinotic mouse kidneys.

Cystinosis, a main cause of Fanconi syndrome, is reproduced in congenic C57BL/6 cystinosin knockout (KO) mice. To identify the sequence of pathogenic and adaptation mechanisms of nephropathic cystinosis, we defined the onset of Fanconi syndrome in KO mice between 3 and 6 months of age and analyzed the correlation with structural and functional changes in proximal tubular cells (PTCs), with focus on endocytosis of ultrafiltrated disulfide-rich proteins as a key source of cystine. Despite considerable variation between mice at the same age, typical event sequences were delineated. At the cellular level, amorphous lysosomal inclusions preceded cystine crystals and eventual atrophy without crystals. At the nephron level, lesions started at the glomerulotubular junction and then extended distally. In situ hybridization and immunofluorescence revealed progressive loss of expression of megalin, cubilin, sodium-glucose cotransporter 2, and type IIa sodium-dependent phosphate cotransporter, suggesting apical dedifferentiation accounting for Fanconi syndrome before atrophy. Injection of labeled proteins revealed that defective endocytosis in S1 PTCs led to partial compensatory uptake by S3 PTCs, suggesting displacement of endocytic load and injury by disulfide-rich cargo. Increased PTC apoptosis allowed luminal shedding of cystine crystals and was partially compensated for by tubular proliferation. We conclude that lysosomal storage triggered by soluble cystine accumulation induces apical PTC dedifferentiation, which causes transfer of the harmful load of disulfide-rich proteins to more distal cells, possibly explaining longitudinal progression of swan-neck lesions. Furthermore, our results suggest that subsequent adaptation mechanisms include lysosomal clearance of free and crystalline cystine into urine and ongoing tissue repair.

Class III phosphoinositide 3-kinase/VPS34 and dynamin are critical for apical endocytic recycling.

Recycling is a limiting step for receptor-mediated endocytosis. We first report three in vitro or in vivo evidences that class III PI3K/VPS34 is the key PI3K isoform regulating apical recycling. A substractive approach, comparing in Opossum Kidney (OK) cells a pan-class I/II/III PI3K inhibitor (LY294002) with a class I/II PI3K inhibitor (ZSTK474), suggested that class III PI3K/VPS34 inhibition induced selective apical endosome swelling and sequestration of the endocytic receptor, megalin/LRP-2, causing surface down-regulation. GFP-(FYVE)x2 overexpression to sequester PI(3)P caused undistinguishable apical endosome swelling. In mouse kidney proximal tubular cells, conditional Vps34 inactivation also led to vacuolation and intracellular megalin redistribution. We next report that removal of LY294002 from LY294002-treated OK cells induced a spectacular burst of recycling tubules and restoration of megalin surface pool. Acute triggering of recycling tubules revealed recruitment of dynamin-GFP and dependence of dynamin-GTPase, guidance directionality by microtubules, and suggested that a microfilamentous net constrained endosomal swelling. We conclude that (i) besides its role in endosome fusion, PI3K-III is essential for endosome fission/recycling; and (ii) besides its role in endocytic entry, dynamin also supports tubulation of recycling endosomes. The unleashing of recycling upon acute reversal of PI3K inhibition may help study its dynamics and associated machineries.

Clustering of (auto)immune diseases with early-onset and complicated inflammatory bowel disease.

Studies in adult inflammatory bowel disease (IBD) patients have highlighted associations with genetic and serologic markers and suggest an association with disease location, behaviour and natural history. Data on patients with Crohn's disease (CD, n=80), ulcerative colitis (UC, n=15) and indeterminate colitis (n=4) were collected. All individuals were analysed for CARD15 R702W, G908R and L1007fs for toll-like receptor 4 (TLR4) Asp299Gly and for anti-Saccharomyces cerevisiae antibodies (ASCA) and atypical perinuclear antineutrophil cytoplasmatic antibodies (pANCA). After a mean of 10.7 years of follow up, the disease behaviour changed in 45% of CD patients, in contrast to disease location, where only 12.5% had a change (p<0.001). The younger the age at diagnosis, the more patients presented with colonic disease (p=0.021). Also, more TLR4 Asp299 Gly variants were found when the age at onset was younger (p=0.018). A large number of concomitant diseases were observed. There was no difference in the prevalence of TLR4 variants nor ASCA or pANCA between the patients with or without concomitant diseases. Patients who progressed more often needed surgery as compared to patients who remained free of stenosing or fistulising disease (27/32 or 84% versus 3/35 or 8.6%, respectively, p<0.0001) and more often had concomitant immune-mediated diseases and a trend for more seroreactivity towards ASCA.