Defective endosomal sorting and trafficking are increasingly recognised as key drivers of neurodegeneration, including hereditary spastic paraplegia (HSP) and other motor neuron disorders. Early endosomal tubule fission (ETF) is essential for sorting cargoes for recycling and retrograde transport, yet the mechanisms coordinating this process are incompletely defined. Here, we identify the endoplasmic reticulum (ER)-resident protein protrudin-previously shown to promote axonal regeneration after injury-as a key regulator of ETF. Using CRISPR interference in human cells, we show that loss of protrudin causes marked accumulation of elongated endosomal tubules, caused by defective fission. Protrudin-mediated ETF required its ability to interact with ER-localised VAP proteins, endosomal phosphoinositides, and the kinesin motor KIF5, indicating a function at ER-endosome contact sites. The endosomal tubulation phenotype depended on dynamic microtubules and dynein and was phenocopied by KIF5 depletion, suggesting that protrudin coordinates opposing microtubule motor forces to drive fission. Beyond this direct role, protrudin connects multiple ETF machineries implicated in lipid transfer, actin regulation, and ER shaping, positioning it as a central scaffold for ETF. Importantly, depletion of protrudin or the HSP-associated kinesin KIF5A produced similar endosomal tubulation defects in human cortical neurons, underscoring the neurophysiological and disease relevance of this pathway. These findings identify protrudin as a key molecular link between ER-endosome communication, neuronal membrane trafficking, and axonal maintenance-processes whose disruption underlies neurodegenerative disease.

Mutation of the ATL1 gene is one of the most common causes of hereditary spastic paraplegia (HSP), a group of genetic neurodegenerative conditions characterised by distal axonal degeneration of the corticospinal tract axons. Atlastin-1, the protein encoded by ATL1 , is one of three mammalian atlastins, which are homologous dynamin-like GTPases that control endoplasmic reticulum (ER) morphology by fusing tubules to form the three-way junctions that characterise ER networks. However, it is not clear whether atlastin-1 is required for correct ER morphology in human neurons and if so what the functional consequences of lack of atlastin-1 are. Using CRISPR-inhibition we generated human cortical neurons lacking atlastin-1. We demonstrate that ER morphology was altered in these neurons, with a reduced number of three-way junctions. Neurons lacking atlastin-1 had longer endosomal tubules, suggestive of defective tubule fission. This was accompanied by reduced lysosomal proteolytic capacity. As well as demonstrating that atlastin-1 is required for correct ER morphology in human neurons, our results indicate that lack of a classical ER-shaping protein such as atlastin-1 may cause altered endosomal tubulation and lysosomal proteolytic dysfunction. Furthermore, they strengthen the idea that defective lysosome function contributes to the pathogenesis of a broad group of HSPs, including those where the primary localisation of the protein involved is not at the endolysosomal system.

Fission of transport tubules from early endosomes is required for endosomal sorting, but mechanisms of endosomal tubule fission (ETF) are incompletely understood. We show protrudin acts at ER-endosome contacts to promote ETF and endosome-to-Golgi traffic. Protrudin-mediated ETF required its ability to interact with ER-localised VAP proteins, endosomal phosphoinositides and KIF5. These properties also regulated the distance between protrudin and endosomal tubules. The defective ETF phenotype of increased endosomal tubulation in cells lacking protrudin was phenocopied by depletion of KIF5, but not FYCO1, a motor protein adaptor implicated in protrudin-dependent late endosome motility. It also required intact microtubules and dynein, consistent with a model where protrudin facilitates a tug-of-war between KIF5 and dynein to fission tubules. In addition to its direct role, protrudin links many other machineries involved in ETF, thus our findings elucidate how ETF is co-ordinated. These machineries are enriched for proteins implicated in hereditary motor neuron disorders, and protrudin or KIF5 depletion caused defective ETF in human neurons. Summary Protrudin binds ER-localised VAPs and endosomal phosphoinositides to form ER-endosome contacts that promote endosomal tubule fission and endosome-to-Golgi traffic. Protrudin recruits KIF5 to provide a FYCO1-independent force to fission endosomal tubules in neurons and non-polarised cells. ### Competing Interest Statement Evan Reid provided consultancy services (unrelated to protrudin) for SwanBio Therapeutics Ltd from 2020-2023

Mutations in the gene encoding the microtubule severing ATPase spastin are the most frequent cause of hereditary spastic paraplegia, a genetic condition characterised by length-dependent axonal degeneration. Here, we show that HeLa cells lacking spastin and embryonic fibroblasts from a spastin knock-in mouse model become highly polarised and develop cellular protrusions. In HeLa cells, this phenotype was rescued by wild-type spastin, but not by forms unable to sever microtubules or interact with endosomal ESCRT-III proteins. Cells lacking the spastin-interacting ESCRT-III-associated proteins IST1 or CHMP1B also developed protrusions. The protrusion phenotype required protrudin, a RAB-interacting protein that interacts with spastin and localises to ER–endosome contact sites, where it promotes KIF5-dependent endosomal motility to protrusions. Consistent with this, the protrusion phenotype in cells lacking spastin also required KIF5. Lack or mutation of spastin resulted in functional consequences for receptor traffic of a pathway implicated in HSP, as Bone Morphogenetic Protein receptor distribution became polarised. Our results, therefore, identify a novel role for ESCRT-III proteins and spastin in regulating polarised membrane traffic.

The spindle checkpoint acts as a mitotic surveillance system, monitoring interactions between kinetochores and spindle microtubules and ensuring high-fidelity chromosome segregation [1-3]. The checkpoint is activated by unattached kinetochores, and Mps1 kinase phosphorylates KNL1 on conserved MELT motifs to generate a binding site for the Bub3-Bub1 complex [4-7]. This leads to dynamic kinetochore recruitment of Mad proteins [8, 9], a conformational change in Mad2 [10-12], and formation of the mitotic checkpoint complex (MCC: Cdc20-Mad3-Mad2 [13-15]). MCC formation inhibits the anaphase-promoting complex/cyclosome (Cdc20-APC/C), thereby preventing the proteolytic destruction of securin and cyclin and delaying anaphase onset. What happens at kinetochores after Mps1-dependent Bub3-Bub1 recruitment remains mechanistically unclear, and it is not known whether kinetochore proteins other than KNL1 have significant roles to play in checkpoint signaling and MCC generation. Here, we take a reductionist approach, avoiding the complexities of kinetochores, and demonstrate that co-recruitment of KNL1(SPc7) and Mps1(Mph1) is sufficient to generate a robust checkpoint signal and prolonged mitotic arrest. We demonstrate that a Mad1-Bub1 complex is formed during synthetic checkpoint signaling. Analysis of bub3 Delta mutants demonstrates that Bub3 acts to suppress premature checkpoint signaling. This synthetic system will enable detailed, mechanistic dissection of MCC generation and checkpoint silencing. After analyzing several mutants that affect localization of checkpoint complexes, we conclude that spindle checkpoint arrest can be independent of their kinetochore, spindle pole, and nuclear envelope localization.

A novel method was developed for a parallel, rapid and precise tracking of dopamine and its metabolites ( homovanillic acid, 3-methoxytyramine and 3,4-dihydroxyphenylacetic acid) in the brain. The method consists of a sample collection using microdialysis from specific brain region, a lyophilization step to concentrate analytes from the microdialysates, and a quantification step exploiting high performance liquid chromatography combined with electrospray ionization tandem mass spectrometry (HPLC-ESI-MS/MS). This was employed in an experimental study investigating the effect of a prenatal exposure to methamphetamine on development of mesolimbic dopaminergic system. It was determined that abusing methamphetamine during pregnancy induces permanent changes in the nucleus accumbens of adult rats. Namely, the basal levels of dopamine and its metabolites are higher and a response to acute dose of methamphetamine is stronger in prenatally exposed rats compared to control rats. Furthermore, the developed methodology can be optimized for use in different brain regions and for analysis of diverse chemical substances. This highly applicable technique could thus be a source of valuable information about brain physiology as well as pathophysiology on the molecular level.