Mutations in CLN3 are a cause of juvenile neuronal ceroid lipofuscinosis (JNCL), also known as Batten disease. Clinical manifestations include cognitive regression, progressive loss of vision and motor function, epileptic seizures and a significantly reduced lifespan. CLN3 localizes to endosomes and lysosomes, and has been implicated in intracellular trafficking and autophagy. However, the precise molecular function of CLN3 remains to be elucidated. Previous studies showed an interaction between CLN3 and Rab7A, a small GTPase that regulates several functions at late endosomes. We confirmed this interaction in live cells and found that CLN3 is required for the efficient endosome-to-TGN trafficking of the lysosomal sorting receptors because it regulates the Rab7A interaction with retromer. In cells lacking CLN3 or expressing CLN3 harbouring a disease-causing mutation, the lysosomal sorting receptors were degraded. We also demonstrated that CLN3 is required for the Rab7A-PLEKHM1 interaction, which is required for fusion of autophagosomes to lysosomes. Overall, our data provide a molecular explanation behind phenotypes observed in JNCL and give an indication of the pathogenic mechanism behind Batten disease. This article has an associated First Person interview with the first author of the paper.

CLN5 is a soluble endolysosomal protein whose function is poorly understood. Mutations in this protein cause a rare neurodegenerative disease, Neuronal Ceroid Lipofuscinosis. We previously found that depletion of CLN5 leads to dysfunctional retromer, resulting in the degradation of the lysosomal sorting receptor, sortilin. However, how a soluble lysosomal protein can modulate the function of a cytosolic protein, retromer, is not known. In this work, we show that deletion of CLN5 not only results in retromer dysfunction, but also in impaired endolysosome fusion events. This results in delayed degradation of endocytic proteins and in defective autophagy. CLN5 modulates these various pathways by regulating downstream interactions between CLN3, an endolysosomal integral membrane protein whose mutations also result in Neuronal Ceroid Lipofuscinosis, RAB7A, and a subset of RAB7A effectors. Our data supports a model where CLN3 and CLN5 function as an endolysosomal complex regulating various functions.

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CLN5 is a soluble endolysosomal protein that regulates the itinerary of the lysosomal sorting receptor sortilin. Mutations in this protein cause neuronal ceroid lipofuscinosis, a rare neurodegenerative disorder, and have also been associated with Alzheimer’s disease, suggesting functional defects in a common pathway. We previously found that depletion of CLN5 leads to dysfunctional retromer, resulting in the degradation of the lysosomal sorting receptor, sortilin. However, how a soluble lysosomal protein can modulate the function of a cytosolic protein is not known. In this work, we show that deletion of CLN5 not only results in retromer dysfunction, but also in impaired endolysosome fusion events. This results in delayed degradation of endocytic proteins and in defective autophagy. CLN5 modulates these various pathways by regulating downstream interactions between CLN3, an integral membrane protein, Rab7A and a subset of Rab7A effectors. Mutations in CLN3 are also a cause of neuronal ceroid lipofuscinosis. Our data supports a model where CLN3 and CLN5 function as an endolysosome complex regulating several endosomal functions.Summary StatementWe have previously demonstrated that CLN3 is required for efficient endosome-to-trans Golgi Network (TGN) trafficking of sortilin by regulating retromer function. In this work, we show that CLN5, which interacts with CLN3, regulates retromer function by modulating key interactions between CLN3, Rab7A, retromer, and sortilin. Therefore, CLN3 and CLN5 serve as endosomal switch regulating the itinerary of the lysosomal sorting receptors.

Wolfram syndrome (WFS) is a rare autosomal recessive disease with non-autoimmune childhood onset insulin dependent diabetes and optic atrophy. WFS type 2 (WFS2) differs from WFS type 1 (WFS1) with upper intestinal ulcers, bleeding tendency and the lack ofdiabetes insipidus. Li-fespan is short due to related comorbidities. Only a few familieshave been reported with this syndrome with the CISD2 mutation. Here we report two siblings with a clinical diagnosis of WFS2, previously misdiagnosed with type 1 diabetes mellitus and diabetic retinopathy-related blindness. We report possible additional clinical and laboratory findings that have not been pre-viously reported, such as asymptomatic hypoparathyroidism, osteomalacia, growth hormone (GH) deficiency and hepatomegaly. Even though not a requirement for the diagnosis of WFS2 currently, our case series confirm hypogonadotropic hypogonadism to be also a feature of this syndrome, as reported before.

Mutations in CLN3 are a cause of juvenile NCL (JNCL), also known as Batten Disease.Clinical manifestations includes cognitive regression, progressive loss of vision and motor function, epileptic seizures, and a significantly reduced lifespan. CLN3 localizes to endosomes and lysosomes, and has been implicated in intracellular trafficking and autophagy. However, the precise molecular function of CLN3 remains to be elucidated. We show that CLN3 interacts with Rab7A, a small GTPase that regulates several functions at late endosomes. We found that CLN3 is required for the efficient endosome-to-TGN trafficking of the lysosomal sorting receptors by regulating the Rab7A/retromer interaction. In cells lacking CLN3 or expressing CLN3 harbouring a disease-causing mutation, the lysosomal sorting receptors were degraded.We also demonstrated that CLN3 is required for the Rab7A/PLEKHM1 interaction, which is required for autophagosome/lysosome fusion. Overall, our data provides a molecular explanation behind phenotypes observed in JNCL.

The assembly of membrane-less organelles such as stress granules (SGs) is emerging as central in helping cells rapidly respond and adapt to stress. Following stress sensing, the resulting global translational shutoff leads to the condensation of stalled mRNAs and proteins into SGs. By reorganising cytoplasmic contents, SGs can modulate RNA translation, biochemical reactions and signalling cascades to promote survival until the stress is resolved. While mechanisms for SG disassembly are not widely understood, the resolution of SGs is important for maintaining cell viability and protein homeostasis. Mutations that lead to persistent of aberrant SGs are increasingly associated with neuropathology and a hallmark of several neurodegenerative diseases. Mutations in CLN3 are causative of juvenile neuronal ceroid lipofuscinosis (JNCL), a rare neurodegenerative disease affecting children. CLN3 encodes a transmembrane lysosomal protein implicated in autophagy, endosomal trafficking, metabolism, and response to oxidative stress. Using a HeLa KO model, we now show that CLN3KO is associated with an altered metabolic profile, reduced global translation, and altered stress signalling. We further demonstrate that loss of CLN3 results in perturbations in SG dynamics, resulting in assembly and disassembly defects, and altered expression of the key SG nucleating factor G3BP1. With a growing interest in SG-modulating drugs for the treatment of neurodegenerative diseases, novel insights into the molecular basis of CLN3 Batten disease may reveal avenues for disease-modifying treatments for this debilitating childhood disease.

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