ABSTRACT Clathrin‐mediated endocytosis is a process by which eukaryotic cells internalise a wide range of molecules such as signalling factors, nutrients and membrane proteins, via the formation of a plasma‐membrane‐derived vesicle. This complex mechanism involves a coordinated assembly of the scaffold protein clathrin and more than 50 other endocytic proteins at endocytic sites. Clathrin polymerises into sturdy supramolecular lattices, yet its assembly is highly dynamic and controlled by the DnaJ/Hsc70 uncoating machinery. Although clathrin turnover at endocytic sites was proposed early on to allow membrane deformation to occur, more recent studies have started hinting at possible regulatory functions. In this review, we summarise and discuss the key findings that have advanced our understanding of the role played by clathrin dynamics, hoping to shed light on the complex regulation of the endocytic process.
ABSTRACT Lysosomes are a major organelle involved in intracellular degradation, membrane traffic and metabolic signaling, but how their biochemical composition changes during normal aging remains poorly defined. Puszynska and colleagues now combine lysosome purification with metabolomics and lipidomics across tissues, species and ages to reveal a conserved signature of lysosomal aging characterized by accumulation of glycerophosphodiesters (GPDs) and cystine and remodeling of bis(monoacylglycero)phosphate (BMP). Strikingly, these changes are largely obscured in whole‐tissue analyses and resemble metabolic abnormalities associated with lysosomal storage disorders, including CLN3 disease and cystinosis. The signature develops progressively with age, can predict chronological age, and is attenuated by caloric restriction in peripheral tissues but not in the brain. Within the brain, age‐dependent metabolite accumulation is particularly prominent in microglia and is also detected in neurons, linking lysosomal aging to cell populations central to neurodegenerative disease. These findings define an organelle‐level metabolic clock, suggesting that one might be only as young as one's lysosomes. Establishing whether GPDs, cystine and BMP remodeling merely mark lysosomal age or actively contribute to declining lysosomal degradation and membrane traffic may uncover a metabolic connection between physiological aging, lysosomal storage disorders and vulnerability to late‐onset neurodegeneration.
ABSTRACT Recent work by Hirata and colleagues identifies site‐specific O ‐GlcNAc modification of the cargo‐selecting COPII subunit Sec24D as a dynamic regulatory switch that promotes collagen export from the endoplasmic reticulum (ER), providing new insight into how COPII accommodates cargoes far larger than canonical transport vesicles. Two glycosites in the poorly characterized N‐terminal intrinsically disordered region (IDR) of Sec24D, T9 and S13, display strikingly different dynamics: S13 is constitutively glycosylated, whereas T9 is rapidly modified following a synchronized burst of collagen transport and returns to baseline within minutes. Both sites are required for collagen I trafficking and rescue of Sec24D‐deficient bulldog zebrafish, while T9 is also required for efficient transport of the smaller cargo E‐cadherin, suggesting that a single intrinsically disordered region can encode cargo‐selective information. Crosslinking proteomics further reveals distinct glycosylation‐dependent changes in the Sec24D interactome and identifies myoferlin (MYOF) as a partner proposed to promote fusion of ER exit sites (ERES) with ER‐Golgi intermediate compartment (ERGIC) membranes. In this commentary, we discuss how these findings recast the COPII inner coat as a signal‐integrating platform, intersect with TANGO1‐dependent ERGIC recruitment and competing models of large collagen carriers, and raise key mechanistic questions concerning signal transmission across the ER membrane, glycan stoichiometry, and how the recruitment or tethering function proposed for MYOF is coupled to membrane fusion.
Signal peptides direct secretory and membrane proteins to the endoplasmic reticulum (ER), but proteins lacking classical signal peptides can occasionally engage the ER translocation machinery. TDP-43 is a nuclear RNA-binding protein implicated in amyotrophic lateral sclerosis and frontotemporal lobar degeneration. TDP-43 pathology has been linked to extracellular TDP-43 species, association with the ER luminal oxidoreductase PDI, and ER stress-related phenotypes, yet whether TDP-43 fragments can directly access the ER lumen remains unclear. Here, we used budding yeast to examine signal peptide-independent ER entry of TDP-43. C-terminal fragments of TDP-43 acquired N-glycans in ste24Δ cells, whereas full-length TDP-43 showed little detectable ER entry. Endo H digestion confirmed N-glycosylation of the fragments, and a protease protection assay supported ER luminal localization of TDP-43(Δ2-89). ER entry was reduced by sec61-41 and sec66Δ, indicating engagement of a Sec61/Sec66-dependent translocation pathway. Deletion analyses identified opposing sequence elements: residues 320-343 facilitated ER entry, whereas an N-terminal region upstream of the nuclear localization signal suppressed it. TDP-43(Δ2-89) also accessed the secretory pathway, and selected disease-associated variants caused Ire1-dependent growth vulnerability without increasing detectable ER translocation. These findings reveal a Ste24-suppressed route for noncanonical ER entry of TDP-43 fragments.
Tauopathies are a class of neurodegenerative diseases characterized by the accumulation of hyperphosphorylated, misfolded and aggregated Tau proteins and by dysfunctions in the autophagy-lysosome system. Whether the latter are a cause or a consequence of the former is unclear. The answer may come from a recent study by Mirfakhar et al. Using human iPSC-derived neurons harboring the MAPT p.R406W mutation in Tau, they were able to show that pathogenic Tau is able to broadly impair lysosomal function ahead of Tau accumulation. They also show that the degradative function of lysosomes, but not their motility, can be restored through pharmacological activation of autophagy, leading to reduced Tau levels. This work opens new therapeutic opportunities to eliminate early-on pathological misfolded Tau proteins before they can engage in a vicious cycle of aggregation, amplification and propagation.
