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A schematic view of the interplay between autophagy, abnormal mitochondria and cell death in cystinosis. Abnormal induction of autophagy, typically mitophagy, forces cells into a starvation mode leading to cell death; and renders cystine-laden lysosomes sensitive to lysosomal membrane permeabilization (Lmp) making it readily enter the apoptosis pathway. A potential block in autophagic flux, after autophagosome-lysosome fusion, remains to be elucidated. preferential severe kidney damage in nephropathic cystinosis may be due to the tissue-and organ-specific injury effect of autophagy.
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Cystinosis, which is characterized by lysosomal accumulation of cystine in many tissues, was the first known storage disorder caused by defective metabolite export from the lysosome. The molecular and cellular mechanisms underlying nephropathic cystinosis, the most severe form, which exhibits generalized proximal tubular dysfunction and progressive...
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... conclusion, as illustrated in Figure 1, we speculate that there is a multifac- eted impact of autophagy in nephropathic cystinosis as follows: (1) the mechanism linking autophagy to lysosomal cystine or apoptosis in cystinotic cells could poten- tially be related to lysosomal membrane permeabilization (LMP), proposed as an early step in apoptosis in cystinosis. We hypothesize that abnormal induction of autophagy besides providing more cargo to be digested in the lysosomes, leads to increased fusion of autophagosomes with cystine-laden lysosomes, rendering them more sensitive to membrane desta- bilization, and thus making them readily enter the apoptotic pathway; (2) the sec- ond most important question is the link between abnormal mitochondria and mitophagy in cystinosis. ...
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Nephropathic cystinosis is an inherited lysosomal transport disorder caused by mutations in the CTNS gene that encodes for a lysosomal membrane transporter, cystinosin. Dysfunction in this protein leads to cystine accumulation in the cells of different organs. The accumulation of cystine in the kidneys becomes apparent with renal tubular Fanconi sy...
Citations
... Previous studies have explored the downstream nephropathic cystinosis phenotype beyond the lysosomal cystine accumulation to provide more druggable targets for this disease [9][10][11]46], including mitochondrial function [47][48][49][50]. Mitochondrial respiration defects can lead to an increased oxidative stress, increased GSH levels, and a decrease in cell energy production, which has been linked to the development and exacerbation of other metabolic diseases as well, such as diabetes, obesity, cardiovascular diseases, and nephropathic cystinosis. ...
Nephropathic cystinosis is a rare monogenetic kidney disease caused by mutations in the lysosomal transporter cystinosin (encoded by CTNS) that, to date, has no cure. The hallmark of this disease is lysosomal accumulation of cystine and decline in proximal tubular function leading to kidney failure early in life. In this project, we developed a novel gene repair strategy using CRISPR/Cas9 Homology-Independent Targeted Integration (HITI) to restore CTNS. A novel, non-viral peptide-mediated approach was used to deliver the Cas9-guideRNA ribonucleoprotein (RNP) complex and repair templates to conditionally immortalized proximal tubule epithelial cell (ciPTEC) lines. The repair constructs contained either mCherry (1.7 kb), the CTNS Superexon (1.7 Kb) or both (3.2 Kb). The results demonstrated that the smaller mCherry construct achieved a higher repair efficiency (63%) compared to the CTNS-mCherry construct (16%). Clonal expansion of repaired cells showed restoration of lysosomal cystine levels in 70-75% of the clones, which was accompanied by improved mitochondrial bioenergetics. In conclusion, CRISPR/Cas9 HITI can be used to precisely insert repair templates into the genome, resulting in a functional cystinosin restoration, and a reversal of the cystinotic disease phenotype.
... In order to treat growth retardation, growth hormone replacement therapy is administered (Hohenfellner et al. 2019b). (Sansanwal and Sarwal 2010). Patients with intermediate cystinosis now have a better chance of surviving into adulthood attributable to therapies involving kidney transplantation, cystine lowering therapy, and other medications, as long as they receive prompt and efficient care. ...
... An effect of sex on cystine content (higher levels in female versus male kidneys) has also been observed, while no evidence for such an effect has been detected in humans (16). Discrepancies in diverse forms of autophagy between human-and mousederived samples have also been reported (11,17,18). More generally, mouse models have failed in many cases to be predictive due to evolutionary differences with humans (19). ...
Recessive mutations in the CTNS gene encoding the lysosomal transporter cystinosin cause cystinosis, a lysosomal storage disease leading to kidney failure and multisystem manifestations. A Ctns knock-out mouse model recapitulates features of cystinosis, but the delayed onset of kidney manifestations, phenotype variability, and strain effects limit its use for mechanistic and drug development studies. To provide a better model for cystinosis, we generated a Ctns knock-out rat model using CRISPR/Cas9 technology. The Ctns-/- rats display progressive cystine accumulation and crystal formation in multiple tissues including kidney, liver and thyroid. They show an early onset and progressive loss of urinary solutes, indicating generalized proximal tubule dysfunction, with development of typical swan-neck lesions, tubulointerstitial fibrosis and kidney failure, and decreased survival. The Ctns-/- rats also present crystals in the cornea, and bone and liver defects, like in patients. Mechanistically, the loss of cystinosin induces a phenotype switch associating abnormal proliferation and dedifferentiation, loss of apical receptors and transporters, and defective lysosomal activity and autophagy in the cells. Primary cultures of proximal tubule cells derived from the Ctns-/- rat kidneys confirmed the key changes caused by cystine overload, including reduced endocytic uptake, increased proliferation and defective lysosomal dynamics and autophagy. The novel Ctns-/- rat model and derived proximal tubule cell system provide invaluable tools to investigate the pathogenesis of cystinosis and to accelerate drug discovery.
... However, in some studies, exogenous expression of the dynein subunit, e.g., DYNC1LI2 (dynein, cytoplasmic 1 light intermediate chain 2)-a key cytoskeletal motor protein involved intracellular transport of cargo, organelle trafficking, mitotic spindle assembly, and positioning-rescues the localization of the chaperone-mediated autophagy (CMA) receptor LAMP2A, CMA activity, and the cellular homeostasis in cystinosis-affected PT cells [97]. Regardless of the mechanisms involved, the concept that defects in endolysosomes and autophagy pathways might contribute to cystinosis pathogenesis is in line with recent studies that indicate an accumulation of autophagosomes engulfing damaged and/or dysfunction mitochondria, and increased formation of aggregate-prone SQSTM1/p62 inclusions in both kidney biopsies [98,99] and patient cells [100]. ...
Epithelial cells that form the kidney proximal tubule (PT) rely on an intertwined ecosystem of vesicular membrane trafficking pathways to ensure the reabsorption of essential nutrients—a key requisite for homeostasis. The endolysosome stands at the crossroads of this sophisticated network, internalizing molecules through endocytosis, sorting receptors and nutrient transporters, maintaining cellular quality control via autophagy, and toggling the balance between PT differentiation and cell proliferation. Dysregulation of such endolysosome-guided trafficking pathways might thus lead to a generalized dysfunction of PT cells, often causing chronic kidney disease and life-threatening complications. In this review, we highlight the biological functions of endolysosome-residing proteins from the perspectives of understanding—and potentially reversing—the pathophysiology of rare inherited diseases affecting the kidney PT. Using cystinosis as a paradigm of endolysosome disease causing PT dysfunction, we discuss how the endolysosome governs the homeostasis of specialized epithelial cells. This review also provides a critical analysis of the molecular mechanisms through which defects in autophagy pathways can contribute to PT dysfunction, and proposes potential interventions for affected tissues. These insights might ultimately accelerate the discovery and development of new therapeutics, not only for cystinosis, but also for other currently intractable endolysosome-related diseases, eventually transforming our ability to regulate homeostasis and health.
... This usually leads to the abnormal induction of mitochondrial autophagy (mitophagy), particularly in cystinotic PTECs and fibroblasts, with the enhanced expression of the microtubule-associated protein 1 light chain 3 (LC3-II/LC3-I) and beclin-1 [53]. This process was further associated with reduced mitochondrial ATP generation, cell starvation, increased generation of ROS, autophagic flux blockade, and enhanced apoptosis [53,54]. Moreover, mitochondrial dysfunction is usually associated with ER stress, which is a common pathogenic process that is linked to altered autophagy in various lysosomal storage disorders, including cystinosis [55,56]. ...
The activation of several inflammatory pathways has recently been documented in patients and different cellular and animal models of nephropathic cystinosis. Upregulated inflamma-tory signals interact with many pathogenic aspects of the disease, such as enhanced oxidative stress, abnormal autophagy, inflammatory cell recruitment, enhanced cell death, and tissue fibrosis. Cys-teamine, the only approved specific therapy for cystinosis, ameliorates many but not all pathogenic aspects of the disease. In the current review, we summarize the inflammatory mechanisms involved in cystinosis and their potential impact on the disease pathogenesis and progression. We further elaborate on the crosstalk between inflammation, autophagy, and apoptosis, and discuss the potential of experimental drugs for suppressing the inflammatory signals in cystinosis.
... Despite this, no visible histological differences were detected in the muscles. The decreased locomotor activity might be due to mitochondrial dysfunction which could not be detected by routine histology studies [35][36][37]. Hence, a comprehensive characterization of mitochondria in cystinosis zebrafish needs further evaluation. ...
Cystinosis is a rare, incurable, autosomal recessive disease caused by mutations in the CTNS gene. This gene encodes the lysosomal cystine transporter cystinosin, leading to lysosomal cystine accumulation in all cells of the body, with kidneys being the first affected organs. The current treatment with cysteamine decreases cystine accumulation, but does not reverse the proximal tubular dysfunction, glomerular injury or loss of renal function. In our previous study, we have developed a zebrafish model of cystinosis through a nonsense mutation in the CTNS gene and have shown that zebrafish larvae recapitulate the kidney phenotype described in humans. In the current study, we characterized the adult cystinosis zebrafish model and evaluated the long-term effects of the disease on kidney and extra renal organs through biochemical, histological, fertility and locomotor activity studies. We found that the adult cystinosis zebrafish presents cystine accumulation in various organs, altered kidney morphology, impaired skin pigmentation, decreased fertility, altered locomotor activity and ocular anomalies. Overall, our data indicate that the adult cystinosis zebrafish model reproduces several human phenotypes of cystinosis and may be useful for studying pathophysiology and long-term effects of novel therapies.
... Autophagy is normally considered as a protective mechanism of cell survival under various conditions including nutrient deprivation and hypoxia. On the contrary, excessive stress causes detrimental autophagy that leads to cellular dysfunctions by destroying organelles, especially the mitochondria and endoplasmic reticulum, and induces cell death [33,34]. The present study found that trehalose treatment significantly induced the activation of TFEB-mediated autophagy and caused cell death of the epithelioid clone of kidney cells, which were markedly blocked by bafilomycin (Figs. 5 and 6). ...
Autophagy has been implicated in the pathogenesis of chronic kidney disease (CKD). Transcription factor EB (TFEB) is a master controller of autophagy. However, the pathophysiological roles of TFEB in modulating autophagy and tubulointerstitial injury in CKD are unknown. This study aimed to determine whether TFEB-mediated autophagy contributed to the tubulointerstitial injury in mice with CKD. After the mice were treated with an adenine diet (0.2 % adenine) for 8 weeks, the development of CKD was observed to be characterised by increased levels of plasma blood urea nitrogen (BUN), creatinine (Cre), tubulointerstitial inflammation and fibrosis. Immunohistochemical and Western blot analysis further revealed that TFEB and autophagy genes were significantly up-regulated in the kidney of the mice with adenine-induced CKD, and this increase was mostly found in the tubular epithelial cells. Interestingly, a similar expression pattern of TFEB-autophagy genes was observed in tubular epithelial cells in the kidney tissue of patients with immunoglobulin A (IgA) nephropathy. Moreover, a pathogenic role of TFEB in adenine-induced CKD was speculated because the pharmacological activation of TFEB by trehalose failed to protect mice from tubulointerstitial injuries. In the epithelioid clone of normal rat kidney cells (NRK-52E), the activation of TFEB by trehalose increased autophagy induction, cell death and inflammatory cytokine (Interleukin-6, IL-6) release. Collectively, these results suggested that the activation of TFEB-mediated autophagy might cause autophagic cell death and inflammation in tubular epithelial cells, contributing to renal fibrosis in adenine-induced CKD. This study provided novel insights into the pathogenic role of TFEB in CKD associated with a high purine diet.
... Cysteine is the oxidized dimer form of the amino acid cysteine produced after degradation of endocytosed protein. Increased concentrations of cysteine con ined in the lysosomes lead to precipitations of cysteine crystals disrupting cellular oxidative metabolism and glutathione status, leading to altered mitochondrial energy metabolism, autophagy, and apoptosis overtime [2]. ...
... Cysteine is the oxidized dimer form of the amino acid cysteine produced after degradation of endocytosed protein. Increased concentrations of cysteine con ined in the lysosomes lead to precipitations of cysteine crystals disrupting cellular oxidative metabolism and glutathione status, leading to altered mitochondrial energy metabolism, autophagy, and apoptosis overtime [2]. ...
... Its anterior location means the cornea directly interfaces with the immediate environment, where it is exposed to myriad environmental stresses and serves as a protective barrier by absorbing UV-B light. Impaired autophagic flux and mitochondrial quality control in the cornea has been linked to granular corneal dystrophy type 2, Fuchs' endothelial corneal dystrophy and non-nephropathic cystinosis [21][22][23]. Furthermore, impaired mitochondrial morphology and defective respiration has been observed in human corneal biopsies from diabetic patients [24]. ...
Photoreception is pivotal to our experience and perception of the natural world; hence the eye is of
prime importance for most vertebrate animals to sense light. Central to visual health is mitochondrial
homeostasis, and the selective autophagic turnover of mitochondria (mitophagy) is predicted to play
a key role here. Despite studies that link aberrant mitophagy to ocular dysfunction, little is known about
the prevalence of basal mitophagy, or its relationship to general autophagy, in the visual system. In this
study, we utilize the mito-QC mouse and a closely related general macroautophagy reporter model to
profile basal mitophagy and macroautophagy in the adult and developing eye. We report that ocular
macroautophagy is widespread, but surprisingly mitophagy does not always follow the same pattern of
occurrence. We observe low levels of mitophagy in the lens and ciliary body, in stark contrast to the high
levels of general MAP1LC3-dependent macroautophagy in these regions. We uncover a striking reversal
of this process in the adult retina, where mitophagy accounts for a larger degree of the macroautophagy
taking place, specifically in the photoreceptor neurons of the outer nuclear layer. We also show the
developmental regulation of autophagy in a variety of ocular tissues. In particular, mitophagy in the
adult mouse retina is reversed in localization during the latter stages of development. Our work thus
defines the landscape of mitochondrial homeostasis in the mammalian eye, and in doing so highlights
the selective nature of autophagy in vivo and the specificity of the reporters used.
