Defects in ER-endosome contacts impact lysosome function in hereditary spastic paraplegia

Atlastin-1 regulates endosomal tubulation and lysosomal proteolysis in human cortical neurons

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.

Three-dimensional architecture of ESCRT-III flat spirals on the membrane

The endosomal sorting complexes required for transport (ESCRTs) are responsible for membrane remodeling in many cellular processes, such as multivesicular body biogenesis, viral budding, and cytokinetic abscission. ESCRT-III, the most abundant ESCRT subunit, assembles into flat spirals as the primed state, essential to initiate membrane invagination. However, the three-dimensional architecture of ESCRT-III flat spirals remained vague for decades due to highly curved filaments with a small diameter and a single preferred orientation on the membrane. Here, we unveiled that yeast Snf7, a component of ESCRT-III, forms flat spirals on the lipid monolayers using cryogenic electron microscopy. We developed a geometry-constrained Euler angle-assigned reconstruction strategy and obtained moderate-resolution structures of Snf7 flat spirals with varying curvatures. Our analyses showed that Snf7 subunits recline on the membrane with N-terminal motifs α0 as anchors, adopt an open state with fused α2/3 helices, and bend α2/3 gradually from the outer to inner parts of flat spirals. In all, we provide the orientation and conformations of ESCRT-III flat spirals on the membrane and unveil the underlying assembly mechanism, which will serve as the initial step in understanding how ESCRTs drive membrane abscission.

Unraveling the link between neuropathy target esterase NTE/SWS, lysosomal storage diseases, inflammation, abnormal fatty acid metabolism, and leaky brain barrier

Mutations in Drosophila Swiss cheese (SWS) gene or its vertebrate orthologue neuropathy target esterase (NTE) lead to progressive neuronal degeneration in flies and humans. Despite its enzymatic function as a phospholipase is well established, the molecular mechanism responsible for maintaining nervous system integrity remains unclear. In this study, we found that NTE/SWS is present in surface glia that forms the blood-brain barrier (BBB) and that NTE/SWS is important to maintain its structure and permeability. Importantly, BBB glia-specific expression of Drosophila NTE/SWS or human NTE in the sws mutant background fully rescues surface glial organization and partially restores BBB integrity, suggesting a conserved function of NTE/SWS. Interestingly, sws mutant glia showed abnormal organization of plasma membrane domains and tight junction rafts accompanied by the accumulation of lipid droplets, lysosomes, and multilamellar bodies. Since the observed cellular phenotypes closely resemble the characteristics described in a group of metabolic disorders known as lysosomal storage diseases (LSDs), our data established a novel connection between NTE/SWS and these conditions. We found that mutants with defective BBB exhibit elevated levels of fatty acids, which are precursors of eicosanoids and are involved in the inflammatory response. Also, as a consequence of a permeable BBB, several innate immunity factors are upregulated in an age-dependent manner, while BBB glia-specific expression of NTE/SWS normalizes inflammatory response. Treatment with anti-inflammatory agents prevents the abnormal architecture of the BBB, suggesting that inflammation contributes to the maintenance of a healthy brain barrier. Considering the link between a malfunctioning BBB and various neurodegenerative diseases, gaining a deeper understanding of the molecular mechanisms causing inflammation due to a defective BBB could help to promote the use of anti-inflammatory therapies for age-related neurodegeneration.

A chemical inhibitor of IST1-CHMP1B interaction impairs endosomal recycling and induces noncanonical LC3 lipidation

The endosomal sorting complex required for transport (ESCRT) machinery constitutes multisubunit protein complexes that play an essential role in membrane remodeling and trafficking. ESCRTs regulate a wide array of cellular processes, including cytokinetic abscission, cargo sorting into multivesicular bodies (MVBs), membrane repair, and autophagy. Given the versatile functionality of ESCRTs, and the intricate organizational structure of the ESCRT machinery, the targeted modulation of distinct ESCRT complexes is considerably challenging. This study presents a pseudonatural product targeting IST1-CHMP1B within the ESCRT-III complexes. The compound specifically disrupts the interaction between IST1 and CHMP1B, thereby inhibiting the formation of IST1-CHMP1B copolymers essential for normal-topology membrane scission events. While the compound has no impact on cytokinesis, MVB sorting, or biogenesis of extracellular vesicles, it rapidly inhibits transferrin receptor recycling in cells, resulting in the accumulation of transferrin in stalled sorting endosomes. Stalled endosomes become decorated by lipidated LC3, suggesting a link between noncanonical LC3 lipidation and inhibition of the IST1-CHMP1B complex.

ER-export and ARFRP1/AP-1-dependent delivery of SARS-CoV-2 Envelope to lysosomes controls late stages of viral replication

The β-coronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the global COVID-19 pandemic. Coronaviral Envelope (E) proteins are pentameric viroporins that play essential roles in assembly, release, and pathogenesis. We developed a nondisruptive tagging strategy for SARS-CoV-2 E and find that, at steady state, it localizes to the Golgi and to lysosomes. We identify sequences in E, conserved across Coronaviridae, responsible for endoplasmic reticulum-to-Golgi export, and relate this activity to interaction with COP-II via SEC24. Using proximity biotinylation, we identify an ADP ribosylation factor 1/adaptor protein-1 (ARFRP1/AP-1)-dependent pathway allowing Golgi-to-lysosome trafficking of E. We identify sequences in E that bind AP-1, are conserved across β-coronaviruses, and allow E to be trafficked from Golgi to lysosomes. We show that E acts to deacidify lysosomes and, by developing a trans-complementation assay for SARS-CoV-2 structural proteins, that lysosomal delivery of E and its viroporin activity is necessary for efficient viral replication and release.

Axonal organelle buildup from loss of AP-4 complex function causes exacerbation of amyloid plaque pathology and gliosis in Alzheimer's disease mouse model

Lysosomes and related precursor organelles robustly build up in swollen axons that surround amyloid plaques and disrupted axonal lysosome transport has been implicated in worsening Alzheimer's pathology. Our prior studies have revealed that loss of Adaptor protein-4 (AP-4) complex function, linked primarily to Spastic Paraplegia (HSP), leads to a similar build of lysosomes in structures we term "AP-4 dystrophies". Surprisingly, these AP-4 dystrophies were also characterized by enrichment of components of APP processing machinery, β-site cleaving enzyme 1 (BACE1) and Presenilin 2. Our studies examining whether the abnormal axonal lysosome build up resulting from AP-4 loss could lead to amyloidogenesis revealed that the loss of AP-4 complex function in an Alzheimer's disease model resulted in a strong increase in size and abundance of amyloid plaques in the hippocampus and corpus callosum as well as increased microglial association with the plaques. Interestingly, we found a further increase in enrichment of the secretase, BACE1, in the axonal swellings of the plaques of Alzheimer model mice lacking AP-4 complex compared to those having normal AP-4 complex function, suggestive of increased amyloidogenic processing under this condition. Additionally, the exacerbation of plaque pathology was region-specific as it did not increase in the cortex. The burden of the AP-4 linked axonal dystrophies/AP-4 dystrophies was higher in the corpus callosum and hippocampus compared to the cortex, establishing the critical role of AP-4 -dependent axonal lysosome transport and maturation in regulating amyloidogenic amyloid precursor protein processing.

Spastin regulates anaphase chromosome separation distance and microtubule-containing nuclear tunnels

Nuclear envelope reassembly during the final stages of each mitosis depends on disassembling spindle microtubules without disrupting chromosome separation. This process involves the transient recruitment of the ESCRT-III complex and spastin, a microtubule-severing AAA (ATPases associated with diverse cellular activities) mechanoenzyme, to late-anaphase chromosomes. However, dissecting mechanisms underlying these rapid processes, which can be completed within minutes, has been difficult. Here, we combine fast-acting chemical inhibitors with live-cell imaging and find that spindle microtubules, along with spastin activity, regulate the number and lifetimes of spastin foci at anaphase chromosomes. Unexpectedly, spastin inhibition impedes chromosome separation, but does not alter the anaphase localization dynamics of CHMP4B, an ESCRT-III protein, or increase γ-H2AX foci, a DNA damage marker. We show spastin inhibition increases the frequency of lamin-lined nuclear microtunnels that can include microtubules penetrating the nucleus. Our findings suggest failure to sever spindle microtubules impedes chromosome separation, yet reforming nuclear envelopes can topologically accommodate persistent microtubules ensuring nuclear DNA is not damaged or exposed to cytoplasm.

An Automated Imaging-Based Screen for Genetic Modulators of ER Organisation in Cultured Human Cells

Hereditary spastic paraplegias (HSPs) are a heterogeneous group of mono-genetic inherited neurological disorders, whose primary manifestation is the disruption of the pyramidal system, observed as a progressive impaired gait and leg spasticity in patients. Despite the large list of genes linked to this group, which exceeds 80 loci, the number of cellular functions which the gene products engage is relatively limited, among which endoplasmic reticulum (ER) morphogenesis appears central. Mutations in genes encoding ER-shaping proteins are the most common cause of HSP, highlighting the importance of correct ER organisation for long motor neuron survival. However, a major bottleneck in the study of ER morphology is the current lack of quantitative methods, with most studies to date reporting, instead, on qualitative changes. Here, we describe and apply a quantitative image-based screen to identify genetic modifiers of ER organisation using a mammalian cell culture system. An analysis reveals significant quantitative changes in tubular ER and dense sheet ER organisation caused by the siRNA-mediated knockdown of HSP-causing genes ATL1 and RTN2. This screen constitutes the first attempt to examine ER distribution in cells in an automated and high-content manner and to detect genes which impact ER organisation.

Stay in touch with the endoplasmic reticulum

The endoplasmic reticulum (ER), which is composed of a continuous network of tubules and sheets, forms the most widely distributed membrane system in eukaryotic cells. As a result, it engages a variety of organelles by establishing membrane contact sites (MCSs). These contacts regulate organelle positioning and remodeling, including fusion and fission, facilitate precise lipid exchange, and couple vital signaling events. Here, we systematically review recent advances and converging themes on ER-involved organellar contact. The molecular basis, cellular influence, and potential physiological functions for ER/nuclear envelope contacts with mitochondria, Golgi, endosomes, lysosomes, lipid droplets, autophagosomes, and plasma membrane are summarized.

IST1 regulates select recycling pathways

ESCRTs (Endosomal Sorting Complex Required for Transports) are a modular set of protein complexes with membrane remodeling activities that include the formation and release of intraluminal vesicles (ILVs) to generate multivesicular endosomes. While most of the 12 ESCRT-III proteins are known to play roles in ILV formation, IST1 has been associated with a wider range of endosomal remodeling events. Here, we extend previous studies of IST1 function in endosomal trafficking and confirm that IST1, along with its binding partner CHMP1B, contributes to scission of early endosomal carriers. Functionally, depleting IST1 impaired delivery of transferrin receptor from early/sorting endosomes to the endocytic recycling compartment and instead increased its rapid recycling to the plasma membrane via peripheral endosomes enriched in the clathrin adaptor AP-1. IST1 is also important for export of mannose 6-phosphate receptor from early/sorting endosomes. Examination of IST1 binding partners on endosomes revealed that IST1 interacts with the MIT domain-containing sorting nexin SNX15, a protein previously reported to regulate endosomal recycling. Our kinetic and spatial analyses establish that SNX15 and IST1 occupy a clathrin-containing subdomain on the endosomal perimeter distinct from those previously implicated in cargo retrieval or degradation. Using live-cell microscopy, we see that SNX15 and CHMP1B alternately recruit IST1 to this subdomain or the base of endosomal tubules. These findings indicate that IST1 contributes to a subset of recycling pathways from the early/sorting endosome.

Copper-independent lysosomal localisation of the Wilson disease protein ATP7B

In hepatocytes, the Wilson disease protein ATP7B resides on the trans-Golgi network (TGN) and traffics to peripheral lysosomes to export excess intracellular copper through lysosomal exocytosis. We found that in basal copper or even upon copper chelation, a significant amount of ATP7B persists in the endolysosomal compartment of hepatocytes but not in non-hepatic cells. These ATP7B-harbouring lysosomes lie in close proximity of ~10 nm to the TGN. ATP7B constitutively distributes itself between the sub-domain of the TGN with a lower pH and the TGN-proximal lysosomal compartments. The presence of ATP7B on TGN-lysosome colocalising sites upon Golgi disruption suggested a possible exchange of ATP7B directly between the TGN and its proximal lysosomes. Manipulating lysosomal positioning significantly alters the localisation of ATP7B in the cell. Contrary to previous understanding, we found that upon copper chelation in a copper-replete hepatocyte, ATP7B is not retrieved back to TGN from peripheral lysosomes; rather, ATP7B recycles to these TGN-proximal lysosomes to initiate the next cycle of copper transport. We report a hitherto unknown copper-independent lysosomal localisation of ATP7B and the importance of TGN-proximal lysosomes but not TGN as the terminal acceptor organelle of ATP7B in its retrograde pathway.

Inhibition of spastin impairs motor function recovery after spinal cord injury

Promoting axonal regeneration is an effective strategy for recovery from traumatic spinal cord injury (SCI). Spastin, a microtubule-severing protein, modulates axonal outgrowth and branch formation by regulating microtubule dynamics. However, the exact role of spastin during recovery from SCI remains unknown. Therefore, we utilized a hemisection injury model of the mouse spinal cord and explored the effect of spastin using a spastin inhibitor, spastazoline. Results showed that spastazoline significantly suppressed the microtubule-severing activity of spastin in COS-7 cells and inhibited the promoting effect of spastin on neurite outgrowth in primarily cultured hippocampal neurons. The protein expression level of spastin was significantly upregulated in the injured spinal cord. Injured mice showed impaired motor functions, which included increased toe-off angle and foot fault steps and decreased stride length and Basso mouse scale score. Notably, these motor function impairments were aggravated by the application of spastazoline. Inhibition of spastin exacerbated neurogenesis impairment, as demonstrated by neuronal nuclei antigen staining, the inflammatory response, as shown by Iba-1 and GFAP staining, and axonal regeneration impairment, as shown by 5-hydroxytryptamine staining. Furthermore, mass spectrometry analysis revealed that the inhibition of spastin resulted in numerous dysregulated differentially expressed proteins that were closely associated with vesicle organization and transport. Taken together, our data suggest that spastin is critical for recovery from SCI and may be a potential target for the treatment of SCI.

Spatacsin regulates directionality of lysosome trafficking by promoting the degradation of its partner AP5Z1

The endoplasmic reticulum (ER) forms contacts with the lysosomal compartment, regulating lysosome positioning and motility. The movements of lysosomes are controlled by the attachment of molecular motors to their surface. However, the molecular mechanisms by which ER controls lysosome dynamics are still elusive. Here, using mouse brain extracts and mouse embryonic fibroblasts, we demonstrate that spatacsin is an ER-resident protein regulating the formation of tubular lysosomes, which are highly dynamic. Screening for spatacsin partners required for tubular lysosome formation showed spatacsin to act by regulating protein degradation. We demonstrate that spatacsin promotes the degradation of its partner AP5Z1, which regulates the relative amount of spastizin and AP5Z1 at lysosomes. Spastizin and AP5Z1 contribute to regulate tubular lysosome formation, as well as their trafficking by interacting with anterograde and retrograde motor proteins, kinesin KIF13A and dynein/dynactin subunit p150Glued, respectively. Ultimately, investigations in polarized mouse cortical neurons in culture demonstrated that spatacsin-regulated degradation of AP5Z1 controls the directionality of lysosomes trafficking. Collectively, our results identify spatacsin as a protein regulating the directionality of lysosome trafficking.

Mind the gap: Methods to study membrane contact sites

Organelles are dynamic entities whose functions are essential for the optimum functioning of cells. It is now known that the juxtaposition of organellar membranes is essential for the exchange of metabolites and their communication. These functional apposition sites are termed membrane contact sites. Dynamic membrane contact sites between various sub-cellular structures such as mitochondria, endoplasmic reticulum, peroxisomes, Golgi apparatus, lysosomes, lipid droplets, plasma membrane, endosomes, etc. have been reported in various model systems. The burgeoning area of research on membrane contact sites has witnessed several manuscripts in recent years that identified the contact sites and components involved. Several methods have been developed to identify, measure and analyze the membrane contact sites. In this manuscript, we aim to discuss important methods developed to date that are used to study membrane contact sites.

IST1 regulates select endosomal recycling pathways

ESCRTs (Endosomal Sorting Complex Required for Transport) are a modular set of protein complexes with membrane remodeling activities that include the formation and release of intralumenal vesicles (ILVs) to generate multivesicular endosomes. While most of the 12 ESCRT-III proteins are known to play roles in ILV formation, IST1 has been associated with a wider range of endosomal remodeling events. Here, we extend previous studies of IST1 function in endosomal trafficking and confirm that IST1, along with its binding partner CHMP1B, contributes to scission of early endosomal carriers. Depleting IST1 impaired delivery of transferrin receptor from early/sorting endosomes to the endocytic recycling compartment and instead increased its rapid recycling to the plasma membrane via peripheral endosomes enriched in the clathrin adaptor AP-1. IST1 is also important for export of mannose 6-phosphate receptor from early/sorting endosomes. Examination of IST1 binding partners on endosomes revealed that IST1 interacts with the MIT domain-containing sorting nexin SNX15, a protein previously reported to regulate endosomal recycling. Our kinetic and spatial analyses establish that SNX15 and IST1 occupy a clathrin-containing subdomain on the endosomal perimeter distinct from those previously implicated in cargo retrieval or degradation. Using live-cell microscopy we see that SNX15 and CHMP1B alternately recruit IST1 to this subdomain or the base of endosomal tubules. These findings indicate that IST1 contributes to a subset of recycling pathways from the early/sorting endosome.

Microtubule remodelling as a driving force of axon guidance and pruning

The establishment of neuronal connectivity relies on the microtubule (MT) cytoskeleton, which provides mechanical support, roads for axonal transport and mediates signalling events. Fine-tuned spatiotemporal regulation of MT functions by tubulin post-translational modifications and MT-associated proteins is critical for the coarse wiring and subsequent refinement of neuronal connectivity. The defective regulation of these processes causes a wide range of neurodevelopmental disorders associated with connectivity defects. This review focuses on recent studies unravelling how MT composition, post-translational modifications and associated proteins influence MT functions in axon guidance and/or pruning to build functional neuronal circuits. We here summarise experimental evidence supporting the key role of this network as a driving force for growth cone steering and branch-specific axon elimination. We further provide a global overview of the MT-interactors that tune developing axon behaviours, with a special emphasis on their emerging versatility in the regulation of MT dynamics/structure. Recent studies establishing the key and highly selective role of the tubulin code in the regulation of MT functions in axon pathfinding are also reported. Finally, our review highlights the emerging molecular links between these MT regulation processes and guidance signals that wire the nervous system.

Endosome biogenesis is controlled by ER and the cytoskeleton at tripartite junctions

The plasma membrane (PM) and its associated cargo are internalized into small vesicles via endocytosis funneling cargo into endosomes. The endosomal system must efficiently deliver cargos, as well as recycle cargo receptors and membrane to maintain homeostasis. In animal cells, endosome trafficking, maturation, and cargo recycling rely on the actin and microtubule cytoskeleton. Microtubules and their associated motor proteins provide the roads on which endosomes move and fuse during cargo sorting and delivery. In addition, highly dynamic assemblies of actin adjust the shape of the endosomal membrane to promote cargo segregation into budding domains allowing for receptor recycling. Recent work has revealed that the endoplasmic reticulum (ER) frequently acts as an intermediary between endosomes and their cytoskeletal regulators via membrane contact sites (MCSs). This review will discuss the factors which form these tripartite junction between the ER, endosomes, and the cytoskeleton as well as their function.

RUFY1 binds Arl8b and mediates endosome-to-TGN CI-M6PR retrieval for cargo sorting to lysosomes

Arl8b, an Arf-like GTP-binding protein, regulates cargo trafficking and positioning of lysosomes. However, it is unknown whether Arl8b regulates lysosomal cargo sorting. Here, we report that Arl8b binds to the Rab4 and Rab14 interaction partner, RUN and FYVE domain-containing protein (RUFY) 1, a known regulator of cargo sorting from recycling endosomes. Arl8b determines RUFY1 endosomal localization through regulating its interaction with Rab14. RUFY1 depletion led to a delay in CI-M6PR retrieval from endosomes to the TGN, resulting in impaired delivery of newly synthesized hydrolases to lysosomes. We identified the dynein-dynactin complex as an RUFY1 interaction partner, and similar to a subset of activating dynein adaptors, the coiled-coil region of RUFY1 was required for interaction with dynein and the ability to mediate dynein-dependent organelle clustering. Our findings suggest that Arl8b and RUFY1 play a novel role on recycling endosomes, from where this machinery regulates endosomes to TGN retrieval of CI-M6PR and, consequently, lysosomal cargo sorting.

FBXL17/spastin axis as a novel therapeutic target of hereditary spastic paraplegia

Background

Spastin significantly influences microtubule regulation in neurons and is implicated in the pathogenesis of hereditary spastic paraplegia (HSP). However, post-translational regulation of the spastin protein remains nebulous. The association between E3 ubiquitin ligase and spastin provides a potential therapeutic strategy.

Results

As evidenced by protein chip analysis, FBXL17 inversely correlated with SPAST-M1 at the protein level in vitro and, also in vivo during embryonic developmental stage. SPAST-M1 protein interacted with FBXL17 specifically via the BTB domain at the N-terminus of SPAST-M1. The SCF^FBXL17 E3 ubiquitin ligase complex degraded SPAST-M1 protein in the nuclear fraction in a proteasome-dependent manner. SPAST phosphorylation occurred only in the cytoplasmic fraction by CK2 and was involved in poly-ubiquitination. Inhibition of SCF^FBXL17 E3 ubiquitin ligase by small chemical and FBXL17 shRNA decreased proteasome-dependent degradation of SPAST-M1 and induced axonal extension. The SPAST Y52C mutant, harboring abnormality in BTB domain could not interact with FBXL17, thereby escaping protein regulation by the SCF^FBXL17 E3 ubiquitin ligase complex, resulting in loss of functionality with aberrant quantity. Although this mutant showed shortening of axonal outgrowth, low rate proliferation, and poor differentiation capacity in a 3D model, this phenotype was rescued by inhibiting SCF^FBXL17 E3 ubiquitin ligase.

Conclusions

We discovered that a novel pathway, FBXL17-SPAST was involved in pathogenicity of HSP by the loss of function and the quantitative regulation. This result suggested that targeting FBXL17 could provide new insight into HSP therapeutics.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-022-00851-1.

Recycling of cell surface membrane proteins from yeast endosomes is regulated by ubiquitinated Ist1

Upon internalization, many surface membrane proteins are recycled back to the plasma membrane. Although these endosomal trafficking pathways control surface protein activity, the precise regulatory features and division of labor between interconnected pathways are poorly defined. In yeast, we show recycling back to the surface occurs through distinct pathways. In addition to retrograde recycling pathways via the late Golgi, used by synaptobrevins and driven by cargo ubiquitination, we find nutrient transporter recycling bypasses the Golgi in a pathway driven by cargo deubiquitination. Nutrient transporters rapidly internalize to, and recycle from, endosomes marked by the ESCRT-III associated factor Ist1. This compartment serves as both “early” and “recycling” endosome. We show Ist1 is ubiquitinated and that this is required for proper endosomal recruitment and cargo recycling to the surface. Additionally, the essential ATPase Cdc48 and its adaptor Npl4 are required for recycling, potentially through regulation of ubiquitinated Ist1. This collectively suggests mechanistic features of recycling from endosomes to the plasma membrane are conserved.

TFG regulates secretory and endosomal sorting pathways in neurons to promote their activity and maintenance

Molecular pathways that intrinsically regulate neuronal maintenance are poorly understood, but rare pathogenic mutations that underlie neurodegenerative disease can offer important insights into the mechanisms that facilitate lifelong neuronal function. Here, we leverage a rat model to demonstrate directly that the TFG p.R106C variant implicated previously in complicated forms of hereditary spastic paraplegia (HSP) underlies progressive spastic paraparesis with accompanying ventriculomegaly and thinning of the corpus callosum, consistent with disease phenotypes identified in adolescent patients. Analyses of primary cortical neurons obtained from CRISPR-Cas9-edited animals reveal a kinetic delay in biosynthetic secretory protein transport from the endoplasmic reticulum (ER), in agreement with prior induced pluripotent stem cell-based studies. Moreover, we identify an unexpected role for TFG in the trafficking of Rab4A-positive recycling endosomes specifically within axons and dendrites. Impaired TFG function compromises the transport of at least a subset of endosomal cargoes, which we show results in down-regulated inhibitory receptor signaling that may contribute to excitation-inhibition imbalances. In contrast, the morphology and trafficking of other organelles, including mitochondria and lysosomes, are unaffected by the TFG p.R106C mutation. Our findings demonstrate a multifaceted role for TFG in secretory and endosomal protein sorting that is unique to cells of the central nervous system and highlight the importance of these pathways to maintenance of corticospinal tract motor neurons.

UPRmt and coordinated UPRER in type 2 diabetes

The mitochondrial unfolded protein response (UPR^mt) is a molecular mechanism that maintains mitochondrial proteostasis under stress and is closely related to various metabolic diseases, such as type 2 diabetes (T2D). Similarly, the unfolded protein response of the endoplasmic reticulum (UPR^ER) is responsible for maintaining proteomic stability in the endoplasmic reticulum (ER). Since the mitochondria and endoplasmic reticulum are the primary centers of energy metabolism and protein synthesis in cells, respectively, a synergistic mechanism must exist between UPR^mt and UPR^ER to cooperatively resist stresses such as hyperglycemia in T2D. Increasing evidence suggests that the protein kinase RNA (PKR)-like endoplasmic reticulum kinase (PERK) signaling pathway is likely an important node for coordinating UPR^mt and UPR^ER. The PERK pathway is activated in both UPR^mt and UPR^ER, and its downstream molecules perform important functions. In this review, we discuss the mechanisms of UPR^mt, UPR^ER and their crosstalk in T2D.

Life in lockdown: Orchestrating endoplasmic reticulum and lysosome homeostasis for quiescent cells

Cellular quiescence-reversible exit from the cell cycle-is an important feature of many cell types important for organismal health. Quiescent cells activate protective mechanisms that allow their persistence in the absence of growth and division for long periods of time. Aging and cellular dysfunction compromise the survival and re-activation of quiescent cells over time. Counteracting this decline are two interconnected organelles that lie at opposite ends of the secretory pathway: the endoplasmic reticulum and lysosomes. In this review, we highlight recent studies exploring the roles of these two organelles in quiescent cells from diverse contexts and speculate on potential other roles they may play, such as through organelle contact sites. Finally, we discuss emerging models of cellular quiescence, utilizing new cell culture systems and model organisms, that are suited to the mechanistic investigation of the functions of these organelles in quiescent cells.

Dysregulation of organelle membrane contact sites in neurological diseases

The defining evolutionary feature of eukaryotic cells is the emergence of membrane-bound organelles. Compartmentalization allows each organelle to maintain a spatially, physically, and chemically distinct environment, which greatly bolsters individual organelle function. However, the activities of each organelle must be balanced and are interdependent for cellular homeostasis. Therefore, properly regulated interactions between organelles, either physically or functionally, remain critical for overall cellular health and behavior. In particular, neuronal homeostasis depends heavily on the proper regulation of organelle function and cross talk, and deficits in these functions are frequently associated with diseases. In this review, we examine the emerging role of organelle contacts in neurological diseases and discuss how the disruption of contacts contributes to disease pathogenesis. Understanding the molecular mechanisms underlying the formation and regulation of organelle contacts will broaden our knowledge of their role in health and disease, laying the groundwork for the development of new therapies targeting interorganelle cross talk and function.

Coronin 1C restricts endosomal branched actin to organize ER contact and endosome fission

ER contact sites define the position of endosome bud fission during actin-dependent cargo sorting. Disrupting endosomal actin structures prevents retrograde cargo movement; however, how actin affects ER contact site formation and endosome fission is not known. Here we show that in contrast with the WASH complex, actin, its nucleator ARP2/3, and COR1C form a contained structure at the bud neck that defines the site of bud fission. We found that actin confinement is facilitated by type I coronins. Depletion of type I coronins allows actin to extend along the length of the bud in an ARP2/3-dependent manner. We demonstrate that extension of branched actin prevents ER recruitment and stalls buds before fission. Finally, our structure-function studies show that the COR1C’s coiled-coil domain is sufficient to restore actin confinement, ER recruitment, and endosome fission. Together, our data reveal how the dynamics of endosomal actin and activity of actin regulators organize ER-associated bud fission.

Store-operated calcium entry is reduced in spastin-linked hereditary spastic paraplegia

Pathogenic variants in SPAST, the gene coding for spastin, are the single most common cause of hereditary spastic paraplegia, a progressive motor neuron disease. Spastin regulates key cellular functions, including microtubule-severing and endoplasmic reticulum-morphogenesis. However, it remains unclear how alterations in these cellular functions due to SPAST pathogenic variants result in motor neuron dysfunction. Since spastin influences both microtubule network and endoplasmic reticulum structure, we hypothesized that spastin is necessary for the regulation of Ca2+ homeostasis via store-operated calcium entry. Here, we show that the lack of spastin enlarges the endoplasmic reticulum and reduces store-operated calcium entry. In addition, elevated levels of different spastin variants induced clustering of STIM1 within the endoplasmic reticulum, altered the transport of STIM1 to the plasma membrane and reduced store-operated calcium entry, which could be rescued by exogenous expression of STIM1. Importantly, store-operated calcium entry was strongly reduced in induced pluripotent stem cell-derived neurons from hereditary spastic paraplegia patients with pathogenic variants in SPAST resulting in spastin haploinsufficiency. These neurons developed axonal swellings in response to lack of spastin. We were able to rescue both store-operated calcium entry and axonal swellings in SPAST patient neurons by restoring spastin levels, using CRISPR/Cas9 to correct the pathogenic variants in SPAST. These findings demonstrate that proper amounts of spastin are a key regulatory component for store-operated calcium entry mediated Ca2+ homeostasis and suggest store-operated calcium entry as a disease relevant mechanism of spastin-linked motor neuron disease.

Friction-driven membrane scission by the human ESCRT-III proteins CHMP1B and IST1

The endosomal sorting complexes required for transport (ESCRT) system is an ancient and ubiquitous membrane scission machinery that catalyzes the budding and scission of membranes. ESCRT-mediated scission events, exemplified by those involved in the budding of HIV-1, are usually directed away from the cytosol ("reverse topology"), but they can also be directed toward the cytosol ("normal topology"). The ESCRT-III subunits CHMP1B and IST1 can coat and constrict positively curved membrane tubes, suggesting that these subunits could catalyze normal topology membrane severing. CHMP1B and IST1 bind and recruit the microtubule-severing AAA+ ATPase spastin, a close relative of VPS4, suggesting that spastin could have a VPS4-like role in normal-topology membrane scission. Here, we reconstituted the process in vitro using membrane nanotubes pulled from giant unilamellar vesicles using an optical trap in order to determine whether CHMP1B and IST1 are capable of membrane severing on their own or in concert with VPS4 or spastin. CHMP1B and IST1 copolymerize on membrane nanotubes, forming stable scaffolds that constrict the tubes, but do not, on their own, lead to scission. However, CHMP1B-IST1 scaffolded tubes were severed when an additional extensional force was applied, consistent with a friction-driven scission mechanism. We found that spastin colocalized with CHMP1B-enriched sites but did not disassemble the CHMP1B-IST1 coat from the membrane. VPS4 resolubilized CHMP1B and IST1 without leading to scission. These observations show that the CHMP1B-IST1 ESCRT-III combination is capable of severing membranes by a friction-driven mechanism that is independent of VPS4 and spastin.

The Role of Spastin in Axon Biology

Neurons are highly polarized cells with elaborate shapes that allow them to perform their function. In neurons, microtubule organization—length, density, and dynamics—are essential for the establishment of polarity, growth, and transport. A mounting body of evidence shows that modulation of the microtubule cytoskeleton by microtubule-associated proteins fine tunes key aspects of neuronal cell biology. In this respect, microtubule severing enzymes—spastin, katanin and fidgetin—a group of microtubule-associated proteins that bind to and generate internal breaks in the microtubule lattice, are emerging as key modulators of the microtubule cytoskeleton in different model systems. In this review, we provide an integrative view on the latest research demonstrating the key role of spastin in neurons, specifically in the context of axonal cell biology. We focus on the function of spastin in the regulation of microtubule organization, and axonal transport, that underlie its importance in the intricate control of axon growth, branching and regeneration.

New phenotype of RTN2-related spectrum: Complicated form of spastic paraplegia-12

Objective

Spastic paraplegia‐12 (SPG12) is a subtype of hereditary spastic paraplegia caused by Reticulon‐2 (RTN2) mutations. We described the clinical and genetic features of three SPG12 patients, functionally explored the potential pathogenic mechanism of RTN2 mutations, and reviewed RTN2‐related cases worldwide.

Methods

The three patients were 31, 36, and 50 years old, respectively, with chronic progressive lower limb spasticity and walking difficulty. Physical examination showed elevated muscle tone, hyperreflexia and Babinski signs in the lower limbs. Patients 1 and 3 additionally had visual, urinary, and/or coordination dysfunctions. Patient 2 also had epileptic seizures. RTN2 mutations were identified by whole‐exome sequencing, followed by Sanger sequencing, segregation analysis, and phenotypic reevaluation. Functional examination of identified mutations was further explored.

Results

Three variants in RTN2 were identified in Patient 1 (c.103C>T, p.R35X), Patient 2 (c.230G>A, p.G77D), and Patient 3 (c.337C>A, p.P113T) with SPG, respectively. Western blotting revealed the p.R35X with smaller molecular weight than WT and other two missense mutants. Immunostaining showed the wild type colocalized with endoplasmic reticulum (ER) in vitro. p.R35X mutant diffusely distributes in the cytoplasm, losing colocalization with ER. p.G77D and p.P113T co‐localized with ER, which was abnormally aggregated in clumps.

Interpretation

In this study, we identified three cases with complicated SPG12 due to three novel RTN2 mutations, respectively, presenting various phenotypes: classic SPG symptoms with (1) visual abnormalities and sphincter disturbances or (2) seizures. The phenotypic heterogeneity might arise from the abnormal subcellular localization of mutant Reticulon‐2 and improper ER morphogenesis, revealing the RTN2‐related spectrum is still expanding.

Current methods to analyze lysosome morphology, positioning, motility and function

Since the discovery of lysosomes more than 70 years ago, much has been learned about the functions of these organelles. Lysosomes were regarded as exclusively degradative organelles, but more recent research has shown that they play essential roles in several other cellular functions, such as nutrient sensing, intracellular signalling and metabolism. Methodological advances played a key part in generating our current knowledge about the biology of this multifaceted organelle. In this review, we cover current methods used to analyze lysosome morphology, positioning, motility and function. We highlight the principles behind these methods, the methodological strategies and their advantages and limitations. To extract accurate information and avoid misinterpretations, we discuss the best strategies to identify lysosomes and assess their characteristics and functions. With this review, we aim to stimulate an increase in the quantity and quality of research on lysosomes and further ground-breaking discoveries on an organelle that continues to surprise and excite cell biologists.

PI4P/PS countertransport by ORP10 at ER-endosome membrane contact sites regulates endosome fission

Membrane contact sites (MCSs) serve as a zone for nonvesicular lipid transport by oxysterol-binding protein (OSBP)-related proteins (ORPs). ORPs mediate lipid countertransport, in which two distinct lipids are transported counterdirectionally. How such lipid countertransport controls specific biological functions, however, remains elusive. We report that lipid countertransport by ORP10 at ER–endosome MCSs regulates retrograde membrane trafficking. ORP10, together with ORP9 and VAP, formed ER–endosome MCSs in a phosphatidylinositol 4-phosphate (PI4P)-dependent manner. ORP10 exhibited a lipid exchange activity toward its ligands, PI4P and phosphatidylserine (PS), between liposomes in vitro, and between the ER and endosomes in situ. Cell biological analysis demonstrated that ORP10 supplies a pool of PS from the ER, in exchange for PI4P, to endosomes where the PS-binding protein EHD1 is recruited to facilitate endosome fission. Our study highlights a novel lipid exchange at ER–endosome MCSs as a nonenzymatic PI4P-to-PS conversion mechanism that organizes membrane remodeling during retrograde membrane trafficking.

Regulation of Endosomal Sorting and Maturation by ER-Endosome Contact Sites

Endosomes are a heterogeneous population of intracellular organelles responsible for sorting, recycling, or transporting internalized materials for degradation. Endosomal sorting and maturation are controlled by a complex interplay of regulators, with RAB GTPases and phosphoinositides playing key roles. In this decade, another layer of regulation surfaced with the role played by membrane contact sites between the endoplasmic reticulum (ER) and endosomes. Specific regulators of ER-endosome contact sites or proteins localized at these sites are emerging as modulators of this complex endosomal ballet. In particular, lipid transfer or recruitment of various complexes and enzymes at ER-endosome contact sites play an active role in endosome sorting, scission, and maturation. In this short review, we focus on studies describing ER-endosome contact sites in these three endosomal processes.

ER Morphology in the Pathogenesis of Hereditary Spastic Paraplegia

The endoplasmic reticulum (ER) is the most abundant and widespread organelle in cells. Its peculiar membrane architecture, formed by an intricate network of tubules and cisternae, is critical to its multifaceted function. Regulation of ER morphology is coordinated by a few ER-specific membrane proteins and is thought to be particularly important in neurons, where organized ER membranes are found even in the most distant neurite terminals. Mutation of ER-shaping proteins has been implicated in the neurodegenerative disease hereditary spastic paraplegia (HSP). In this review we discuss the involvement of these proteins in the pathogenesis of HSP, focusing on the experimental evidence linking their molecular function to disease onset. Although the precise biochemical activity of some ER-related HSP proteins has been elucidated, the pathological mechanism underlying ER-linked HSP is still undetermined and needs to be further investigated.

Molecular and cellular mechanisms of spastin in neural development and disease (Review)

Spastin is a microtubule (MT)‑severing enzyme identified from mutations of hereditary spastic paraplegia in 1999 and extensive studies indicate its vital role in various cellular activities. In the past two decades, efforts have been made to understand the underlying molecular mechanisms of how spastin is linked to neural development and disease. Recent studies on spastin have unraveled the mechanistic processes of its MT‑severing activity and revealed that spastin acts as an MT amplifier to mediate its remodeling, thus providing valuable insight into the molecular roles of spastin under physiological conditions. In addition, recent research has revealed multiple novel molecular mechanisms of spastin in cellular biological pathways, including endoplasmic reticulum shaping, calcium trafficking, fatty acid trafficking, as well as endosomal fission and trafficking. These processes are closely involved in axonal and dendritic development and maintenance. The current review presents recent biological advances regarding the molecular mechanisms of spastin at the cellular level and provides insight into how it affects neural development and disease.

Phagosome maturation in macrophages: Eat, digest, adapt, and repeat

Phagocytosis is a dynamic process that requires an intricate interplay between phagocytic receptors, membrane lipids, and numerous signalling proteins and their effectors, to coordinate the engulfment of a bound particle. These particles are diverse in their physico-chemical properties such as size and shape and include bacteria, fungi, apoptotic cells, living tumour cells, and abiotic particles. Once engulfed, these particles are enclosed within a phagosome, which undergoes a striking transformation referred to as phagosome maturation, which will ultimately lead to the processing and degradation of the enclosed particulate. In this review, we focus on recent advancements in phagosome maturation in macrophages, highlighting new discoveries and emerging themes. Such advancements include identification of new GTPases and their effectors and the intricate spatio-temporal dynamics of phosphoinositides in governing phagosome maturation. We then explore phagosome fission and recycling, the emerging role of membrane contact sites, and delve into mechanisms of phagosome resolution to recycle and reform lysosomes. We further illustrate how phagosome maturation is context-dependent, subject to the type of particle, phagocytic receptors, the phagocytes and their state of activation during phagocytosis. Lastly, we discuss how phagosomes serve as signalling platforms to help phagocytes adapt to their environmental conditions. Overall, this review aims to cover recent findings, identify emerging themes, and highlight current challenges and directions to improve our understanding of phagosome maturation in macrophages.

Intertwined and Finely Balanced: Endoplasmic Reticulum Morphology, Dynamics, Function, and Diseases

The endoplasmic reticulum (ER) is an organelle that is responsible for many essential subcellular processes. Interconnected narrow tubules at the periphery and thicker sheet-like regions in the perinuclear region are linked to the nuclear envelope. It is becoming apparent that the complex morphology and dynamics of the ER are linked to its function. Mutations in the proteins involved in regulating ER structure and movement are implicated in many diseases including neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and amyotrophic lateral sclerosis (ALS). The ER is also hijacked by pathogens to promote their replication. Bacteria such as Legionella pneumophila and Chlamydia trachomatis, as well as the Zika virus, bind to ER morphology and dynamics-regulating proteins to exploit the functions of the ER to their advantage. This review covers our understanding of ER morphology, including the functional subdomains and membrane contact sites that the organelle forms. We also focus on ER dynamics and the current efforts to quantify ER motion and discuss the diseases related to ER morphology and dynamics.

Advances in the study of organelle interactions and their role in neurodegenerative diseases enabled by super-resolution microscopy

From the first illustrations of neuronal morphology by Ramón y Cajal to the recent three-dimensional reconstruction of synaptic connections, the development of modern neuroscience has greatly benefited from breakthroughs in imaging technology. This also applies specifically to the study of neurodegenerative diseases. Much of the research into these diseases relies on the direct visualisation of intracellular structures and their dynamics in degenerating neural cells, which cannot be fully resolved by diffraction-limited microscopes. Progress in the field has therefore been closely linked to the development of super-resolution imaging methods. Their application has greatly advanced our understanding of disease mechanisms, ranging from the structural progression of protein aggregates to defects in organelle morphology. Recent super-resolution studies have specifically implicated the disruption of inter-organelle interactions in multiple neurodegenerative diseases. In this article, we describe some of the key super-resolution techniques that have contributed to this field. We then discuss work to visualise changes in the structure and dynamics of organelles and associated dysfunctions. Finally, we consider what future developments in imaging technology may further our knowledge of these processes.

ER - lysosome contacts at a pre-axonal region regulate axonal lysosome availability

Neuronal function relies on careful coordination of organelle organization and transport. Kinesin-1 mediates transport of the endoplasmic reticulum (ER) and lysosomes into the axon and it is increasingly recognized that contacts between the ER and lysosomes influence organelle organization. However, it is unclear how organelle organization, inter-organelle communication and transport are linked and how this contributes to local organelle availability in neurons. Here, we show that somatic ER tubules are required for proper lysosome transport into the axon. Somatic ER tubule disruption causes accumulation of enlarged and less motile lysosomes at the soma. ER tubules regulate lysosome size and axonal translocation by promoting lysosome homo-fission. ER tubule - lysosome contacts often occur at a somatic pre-axonal region, where the kinesin-1-binding ER-protein P180 binds microtubules to promote kinesin-1-powered lysosome fission and subsequent axonal translocation. We propose that ER tubule - lysosome contacts at a pre-axonal region finely orchestrate axonal lysosome availability for proper neuronal function.

Current Knowledge of Endolysosomal and Autophagy Defects in Hereditary Spastic Paraplegia

Hereditary spastic paraplegia (HSP) refers to a group of neurological disorders involving the degeneration of motor neurons. Due to their clinical and genetic heterogeneity, finding common effective therapeutics is difficult. Therefore, a better understanding of the common pathological mechanisms is necessary. The role of several HSP genes/proteins is linked to the endolysosomal and autophagic pathways, suggesting a functional convergence. Furthermore, impairment of these pathways is particularly interesting since it has been linked to other neurodegenerative diseases, which would suggest that the nervous system is particularly sensitive to the disruption of the endolysosomal and autophagic systems. In this review, we will summarize the involvement of HSP proteins in the endolysosomal and autophagic pathways in order to clarify their functioning and decipher some of the pathological mechanisms leading to HSP.

Lipid Droplets in the Pathogenesis of Hereditary Spastic Paraplegia

Hereditary spastic paraplegias (HSPs) are genetically heterogeneous conditions caused by the progressive dying back of the longest axons in the central nervous system, the corticospinal axons. A wealth of data in the last decade has unraveled disturbances of lipid droplet (LD) biogenesis, maturation, turnover and contact sites in cellular and animal models with perturbed expression and function of HSP proteins. As ubiquitous organelles that segregate neutral lipid into a phospholipid monolayer, LDs are at the cross-road of several processes including lipid metabolism and trafficking, energy homeostasis, and stress signaling cascades. However, their role in brain cells, especially in neurons remains enigmatic. Here, we review experimental findings linking LD abnormalities to defective function of proteins encoded by HSP genes, and discuss arising questions in the context of the pathogenesis of HSP.

Hereditary Spastic Paraplegia and Future Therapeutic Directions: Beneficial Effects of Small Compounds Acting on Cellular Stress

Hereditary spastic paraplegia (HSP) is a group of inherited neurodegenerative conditions that share a characteristic feature of degeneration of the longest axons within the corticospinal tract, which leads to progressive spasticity and weakness of the lower limbs. Mutations of over 70 genes produce defects in various biological pathways: axonal transport, lipid metabolism, endoplasmic reticulum (ER) shaping, mitochondrial function, and endosomal trafficking. HSPs suffer from an adequate therapeutic plan. Currently the treatments foreseen for patients affected by this pathology are physiotherapy, to maintain the outgoing tone, and muscle relaxant therapies for spasticity. Very few clinical studies have been conducted, and it's urgent to implement preclinical animal studies devoted to pharmacological test and screening, to expand the rose of compounds potentially attractive for clinical trials. Small animal models, such as Drosophila melanogaster and zebrafish, have been generated, analyzed, and used as preclinical model for screening of compounds and their effects. In this work, we briefly described the role of HSP-linked proteins in the organization of ER endomembrane system and in the regulation of ER homeostasis and stress as a common pathological mechanism for these HSP forms. We then focused our attention on the pharmacodynamic and pharmacokinetic features of some recently identified molecules with antioxidant property, such as salubrinal, guanabenz, N-acetyl cysteine, methylene blue, rapamycin, and naringenin, and on their potential use in future clinical studies. Expanding the models and the pharmacological screening for HSP disease is necessary to give an opportunity to patients and clinicians to test new molecules.

Interorganelle communication, aging, and neurodegeneration

Our cells are comprised of billions of proteins, lipids, and other small molecules packed into their respective subcellular organelles, with the daunting task of maintaining cellular homeostasis over a lifetime. However, it is becoming increasingly evident that organelles do not act as autonomous discrete units but rather as interconnected hubs that engage in extensive communication through membrane contacts. In the last few years, our understanding of how these contacts coordinate organelle function has redefined our view of the cell. This review aims to present novel findings on the cellular interorganelle communication network and how its dysfunction may contribute to aging and neurodegeneration. The consequences of disturbed interorganellar communication are intimately linked with age-related pathologies. Given that both aging and neurodegenerative diseases are characterized by the concomitant failure of multiple cellular pathways, coordination of organelle communication and function could represent an emerging regulatory mechanism critical for long-term cellular homeostasis. We anticipate that defining the relationships between interorganelle communication, aging, and neurodegeneration will open new avenues for therapeutics.

Hereditary Spastic Paraplegia: From Genes, Cells and Networks to Novel Pathways for Drug Discovery

Hereditary spastic paraplegia (HSP) is a diverse group of Mendelian genetic disorders affecting the upper motor neurons, specifically degeneration of their distal axons in the corticospinal tract. Currently, there are 80 genes or genomic loci (genomic regions for which the causative gene has not been identified) associated with HSP diagnosis. HSP is therefore genetically very heterogeneous. Finding treatments for the HSPs is a daunting task: a rare disease made rarer by so many causative genes and many potential mutations in those genes in individual patients. Personalized medicine through genetic correction may be possible, but impractical as a generalized treatment strategy. The ideal treatments would be small molecules that are effective for people with different causative mutations. This requires identification of disease-associated cell dysfunctions shared across genotypes despite the large number of HSP genes that suggest a wide diversity of molecular and cellular mechanisms. This review highlights the shared dysfunctional phenotypes in patient-derived cells from patients with different causative mutations and uses bioinformatic analyses of the HSP genes to identify novel cell functions as potential targets for future drug treatments for multiple genotypes.

Dysregulation of mitochondria-lysosome contacts by GBA1 dysfunction in dopaminergic neuronal models of Parkinson's disease

Mitochondria-lysosome contacts are recently identified sites for mediating crosstalk between both organelles, but their role in normal and diseased human neurons remains unknown. In this study, we demonstrate that mitochondria-lysosome contacts can dynamically form in the soma, axons, and dendrites of human neurons, allowing for their bidirectional crosstalk. Parkinson's disease patient derived neurons harboring mutant GBA1 exhibited prolonged mitochondria-lysosome contacts due to defective modulation of the untethering protein TBC1D15, which mediates Rab7 GTP hydrolysis for contact untethering. This dysregulation was due to decreased GBA1 (β-glucocerebrosidase (GCase)) lysosomal enzyme activity in patient derived neurons, and could be rescued by increasing enzyme activity with a GCase modulator. These defects resulted in disrupted mitochondrial distribution and function, and could be further rescued by TBC1D15 in Parkinson's patient derived GBA1-linked neurons. Together, our work demonstrates a potential role of mitochondria-lysosome contacts as an upstream regulator of mitochondrial function and dynamics in midbrain dopaminergic neurons in GBA1-linked Parkinson's disease.

Exploiting Connections for Viral Replication

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of the COVID-19 (coronavirus disease 2019) pandemic, is a positive strand RNA (+RNA) virus. Like other +RNA viruses, SARS-CoV-2 is dependent on host cell metabolic machinery to survive and replicate, remodeling cellular membranes to generate sites of viral replication. Viral RNA-containing double-membrane vesicles (DMVs) are a striking feature of +RNA viral replication and are abundant in SARS-CoV-2-infected cells. Their generation involves rewiring of host lipid metabolism, including lipid biosynthetic pathways. Viruses can also redirect lipids from host cell organelles; lipid exchange at membrane contact sites, where the membranes of adjacent organelles are in close apposition, has been implicated in the replication of several +RNA viruses. Here we review current understanding of DMV biogenesis. With a focus on the exploitation of contact site machinery by +RNA viruses to generate replication organelles, we discuss evidence that similar mechanisms support SARS-CoV-2 replication, protecting its RNA from the host cell immune response.

Axonal Organelles as Molecular Platforms for Axon Growth and Regeneration after Injury

Investigating the molecular mechanisms governing developmental axon growth has been a useful approach for identifying new strategies for boosting axon regeneration after injury, with the goal of treating debilitating conditions such as spinal cord injury and vision loss. The picture emerging is that various axonal organelles are important centers for organizing the molecular mechanisms and machinery required for growth cone development and axon extension, and these have recently been targeted to stimulate robust regeneration in the injured adult central nervous system (CNS). This review summarizes recent literature highlighting a central role for organelles such as recycling endosomes, the endoplasmic reticulum, mitochondria, lysosomes, autophagosomes and the proteasome in developmental axon growth, and describes how these organelles can be targeted to promote axon regeneration after injury to the adult CNS. This review also examines the connections between these organelles in developing and regenerating axons, and finally discusses the molecular mechanisms within the axon that are required for successful axon growth.

Brain Damage and Gene Expression Across Hereditary Spastic Paraplegia Subtypes

BACKGROUND

Spinal cord has been considered the main target of damage in hereditary spastic paraplegias (HSPs), but mounting evidence indicates that the brain is also affected. Despite this, little is known about the brain signature of HSPs, in particular regarding stratification for specific genetic subtypes.

OBJECTIVE

We aimed to characterize cerebral and cerebellar damage in five HSP subtypes (9 SPG3A, 27 SPG4, 10 SPG7, 9 SPG8, and 29 SPG11) and to uncover the clinical and gene expression correlates.

METHODS

We obtained high-resolution brain T1 and diffusion tensor image (DTI) datasets in this cross-sectional case-control study (n = 84). The MRICloud, FreeSurfer, and CERES-SUIT pipelines were employed to assess cerebral gray (GM) and white matter (WM) as well as the cerebellum.

RESULTS

Brain abnormalities were found in all but one HSP group (SPG3A), but the patterns were gene-specific: basal ganglia, thalamic, and posterior WM involvement in SPG4; diffuse WM and cerebellar involvement in SPG7; cortical thinning at the motor cortices and pallidal atrophy in SPG8; and widespread GM, WM, and deep cerebellar nuclei damage in SPG11. Abnormal regions in SPG4 and SPG8 matched those with higher SPAST and WASHC5 expression, whereas in SPG7 and SPG11 this concordance was only noticed in the cerebellum.

CONCLUSIONS

Brain damage is a conspicuous feature of HSPs (even for pure subtypes), but the pattern of abnormalities is genotype-specific. Correlation between brain structural damage and gene expression maps is different for autosomal dominant and recessive HSPs, pointing to distinct pathophysiological mechanisms underlying brain damage in these subgroups of the disease. © 2021 International Parkinson and Movement Disorder Society.

Lysosome Function and Dysfunction in Hereditary Spastic Paraplegias

Hereditary Spastic Paraplegias (HSPs) are a genetically diverse group of inherited neurological diseases with over 80 associated gene loci. Over the last decade, research into mechanisms underlying HSPs has led to an emerging interest in lysosome dysfunction. In this review, we highlight the different classes of HSPs that have been linked to lysosome defects: (1) a subset of complex HSPs where mutations in lysosomal genes are causally linked to the diseases, (2) other complex HSPs where mutation in genes encoding membrane trafficking adaptors lead to lysosomal defects, and (3) a subset of HSPs where mutations affect genes encoding proteins whose function is primarily linked to a different cellular component or organelle such as microtubule severing and Endoplasmic Reticulum-shaping, while also altering to lysosomes. Interestingly, aberrant axonal lysosomes, associated with the latter two subsets of HSPs, are a key feature observed in other neurodegenerative diseases such as Alzheimer's disease. We discuss how altered lysosome function and trafficking may be a critical contributor to HSP pathology and highlight the need for examining these features in the cortico-spinal motor neurons of HSP mutant models.

Janus-faced spatacsin (SPG11): involvement in neurodevelopment and multisystem neurodegeneration

Hereditary spastic paraplegia (HSP) is a heterogeneous group of rare motor neuron disorders characterized by progressive weakness and spasticity of the lower limbs. HSP type 11 (SPG11-HSP) is linked to pathogenic variants in the SPG11 gene and it represents the most frequent form of complex autosomal recessive HSP. The majority of SPG11-HSP patients exhibit additional neurological symptoms such as cognitive decline, thin corpus callosum, and peripheral neuropathy. Yet, the mechanisms of SPG11-linked spectrum diseases are largely unknown. Recent findings indicate that spatacsin, the 280 kDa protein encoded by SPG11, may impact the autophagy-lysosomal machinery. In this update, we summarize the current knowledge of SPG11-HSP. In addition to clinical symptoms and differential diagnosis, our work aims to link the different clinical manifestations with the respective structural abnormalities and cellular in vitro phenotypes. Moreover, we describe the impact of localization and function of spatacsin in different neuronal systems. Ultimately, we propose a model in which spatacsin bridges between neurodevelopmental and neurodegenerative phenotypes of SPG11-linked disorders.

Complex Interactions Between Membrane-Bound Organelles, Biomolecular Condensates and the Cytoskeleton

Membrane-bound and membraneless organelles/biomolecular condensates ensure compartmentalization into functionally distinct units enabling proper organization of cellular processes. Membrane-bound organelles form dynamic contacts with each other to enable the exchange of molecules and to regulate organelle division and positioning in coordination with the cytoskeleton. Crosstalk between the cytoskeleton and dynamic membrane-bound organelles has more recently also been found to regulate cytoskeletal organization. Interestingly, recent work has revealed that, in addition, the cytoskeleton and membrane-bound organelles interact with cytoplasmic biomolecular condensates. The extent and relevance of these complex interactions are just beginning to emerge but may be important for cytoskeletal organization and organelle transport and remodeling. In this review, we highlight these emerging functions and emphasize the complex interplay of the cytoskeleton with these organelles. The crosstalk between membrane-bound organelles, biomolecular condensates and the cytoskeleton in highly polarized cells such as neurons could play essential roles in neuronal development, function and maintenance.