Retromer is a multimeric protein complex that mediates endosometo-trans-Golgi network (TGN) and endosome-to-plasma membrane trafficking of integral membrane proteins. Dysfunction of this complex has been linked to Alzheimer's disease and Parkinson's disease. The recruitment of retromer to endosomes is regulated by Rab7 (also known as RAB7A) to coordinate endosome-to-TGN trafficking of cargo receptor complexes. Rab7 is also required for the degradation of internalized integral membrane proteins, such as the epidermal growth factor receptor (EGFR). We found that Rab7 is palmitoylated and that this modification is not required for membrane anchoring. Palmitoylated Rab7 colocalizes efficiently with and has a higher propensity to interact with retromer than nonpalmitoylatable Rab7. Rescue of Rab7 knockout cells by expressing wild-type Rab7 restores efficient endosome-to-TGN trafficking, while rescue with nonpalmitoylatable Rab7 does not. Interestingly, Rab7 palmitoylation does not appear to be required for the degradation of EGFR or for its interaction with its effector, Rab-interacting lysosomal protein (RILP). Overall, our results indicate that Rab7 palmitoylation is required for the spatiotemporal recruitment of retromer and efficient endosome-to-TGN trafficking of the lysosomal sorting receptors.

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.

The small GTPase Rab7 is the main regulator of membrane trafficking at late endosomes. This small GTPase regulates endosome-to-trans Golgi Network trafficking of sorting receptors, membrane fusion of late endosomes to lysosomes, and autophagosomes to lysosomes during autophagy. Rab7, like all Rab GTPases, binds downstream effectors coordinating several divergent pathways. How cells regulate these interactions and downstream functions is not well understood. Recent evidence suggests that Rab7 function can be modulated by the combination of several post-translational modifications that facilitate interactions with one effector while preventing binding to another one. In this review, we discuss recent data on how phosphorylation, palmitoylation and ubiquitination modulate the ability of this small GTPase to orchestrate membrane trafficking at the late endosomes.

There is growing evidence that molecular chaperones/heat shock proteins are involved in the pathogenesis of a number of human diseases, known as chaperonopathies. A better molecular understanding of the pathogenetic mechanisms is essential for addressing new strategies in diagnostics, therapeutics and clinical management of chaperonopathies, including those in which Hsp10 is involved. This chaperonin has been studied for a long time as a member of the mitochondrial protein-folding machine. However, although in normal cells Hsp10 is mainly localized in the mitochondrial matrix, it has also been found during and after stress in other subcellular compartments, such as cytosol, vesicles and secretory granules, alone or in combination with other proteins. In these extramitochondrial locales, Hsp10 plays an active role in cell signalling. For example, cancer cells often show altered levels of Hsp10, compared to normal cells. Hsp10 may also be found in the extracellular space and in the bloodstream, with a possible immunomodulatory activity. This minireview focuses on some studies to date on the involvement of Hsp10 in human disease pathogenesis.