Hereditary spastic paraplegia proteins REEP1, spastin, and atlastin-1 coordinate microtubule interactions with the tubular ER network

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

Hereditary spastic paraplegia (HSP) is a genetically heterogeneous disease caused by mutations in many genes, including those encoding spastin, strumpellin, or REEP1. Allison et al. show that similar lysosomal phenotypes are associated with mutations in different classes of HSP proteins and suggest that defective ER–endosome contacts and endosome tubule fission may be a common cause of axon degeneration in the disease.

Contacts between endosomes and the endoplasmic reticulum (ER) promote endosomal tubule fission, but the mechanisms involved and consequences of tubule fission failure are incompletely understood. We found that interaction between the microtubule-severing enzyme spastin and the ESCRT protein IST1 at ER–endosome contacts drives endosomal tubule fission. Failure of fission caused defective sorting of mannose 6-phosphate receptor, with consequently disrupted lysosomal enzyme trafficking and abnormal lysosomal morphology, including in mouse primary neurons and human stem cell–derived neurons. Consistent with a role for ER-mediated endosomal tubule fission in lysosome function, similar lysosomal abnormalities were seen in cellular models lacking the WASH complex component strumpellin or the ER morphogen REEP1. Mutations in spastin, strumpellin, or REEP1 cause hereditary spastic paraplegia (HSP), a disease characterized by axonal degeneration. Our results implicate failure of the ER–endosome contact process in axonopathy and suggest that coupling of ER-mediated endosomal tubule fission to lysosome function links different classes of HSP proteins, previously considered functionally distinct, into a unifying pathway for axonal degeneration.

Reconstitution of the tubular endoplasmic reticulum network with purified components

Organelles display characteristic morphologies that are intimately tied to their cellular function, but how organelles are shaped is poorly understood. The endoplasmic reticulum (ER) is particularly intriguing, as it is comprised of morphologically distinct domains, including a dynamic network of interconnected membrane tubules. Several membrane proteins have been implicated in network formation^1–5, but how exactly they mediate network formation and whether they are all required is unclear. Here, we have reconstituted a dynamic tubular membrane network with purified ER proteins. Proteoliposomes containing the membrane-fusing GTPase Sey1p^6,7 and the curvature-stabilizing protein Yop1p^8,9 from Saccharomyces cerevisiae form a tubular network upon GTP addition. The tubules rapidly fragment when GTP hydrolysis of Sey1p is inhibited, indicating that network maintenance requires continuous membrane fusion and that Yop1p favors the generation of highly curved membrane structures. Sey1p also forms networks with other curvature-stabilizing proteins, including reticulon⁸ and REEP¹⁰ proteins from different species. Atlastin, the vertebrate ortholog of Sey1p^6,11, forms a GTP-hydrolysis dependent network on its own, serving as both a fusion and curvature-stabilizing protein. Our results show that organelle shape can be generated by a surprisingly small set of proteins and represents an energy-dependent steady state between formation and disassembly.

Hereditary spastic paraplegia due to a novel mutation of the REEP1 gene: Case report and literature review

RATIONALE

Hereditary spastic paraplegia (HSP) is a heterogeneous group of diseases little known in clinical practice due to its low prevalence, slow progression, and difficult diagnosis. This results in an underestimation of HSP leading to belated diagnosis and management. In depth diagnosis is based on clinical presentation and identification of genomic mutations. We describe the clinical presentation and pathogeny of HSP through a report of a case due to a novel mutation of the REEP1 gene (SPG31).

PATIENT CONCERNS

A 64-year-old woman presented gait disturbances due to spasticity of the lower limbs progressing since her third decade. Previous investigations failed to find any cause.

INTERVENTIONS

DNA analysis was performed to search for HSP causing mutations.

DIAGNOSES

A novel heterozygote mutation (c.595 + 1G>A) of the REEP1 gene, within the splice site of intron 6, was discovered. This nucleotide change causes exon 6 skipping leading to frame shift and a truncated transcript identified by complementary DNA sequencing of reverse transcription polymerase chain reaction products.

OUTCOMES

REEP1 is a known protein predominantly located in the upper motor neurons. Mutation of REEP1 primary affects the longest axons explaining predominance of pyramidal syndrome on lower limbs.

LESSONS

Slow progressive pyramidal syndrome of the lower limbs should elicit a diagnosis of HSP. We describe a novel mutation of the REEP1 gene causing HSP. Pathogeny is based on resulting abnormal REEP1 protein which is involved in the development of longest axons constituting the corticospinal tracts.

Endoplasmic Reticulum-Localized Transmembrane Protein Dpy19L1 Is Required for Neurite Outgrowth

The endoplasmic reticulum (ER), including the nuclear envelope, is a continuous and intricate membrane-bound organelle responsible for various cellular functions. In neurons, the ER network is found in cell bodies, axons, and dendrites. Recent studies indicate the involvement of the ER network in neuronal development, such as neuronal migration and axonal outgrowth. However, the regulation of neural development by ER-localized proteins is not fully understood. We previously reported that the multi-transmembrane protein Dpy19L1 is required for neuronal migration in the developing mouse cerebral cortex. A Dpy19L family member, Dpy19L2, which is a causative gene for human Globozoospermia, is suggested to act as an anchor of the acrosome to the nuclear envelope. In this study, we found that the patterns of exogenous Dpy19L1 were partially coincident with the ER, including the nuclear envelope in COS-7 cells at the level of the light microscope. The reticular distribution of Dpy19L1 was disrupted by microtubule depolymerization that induces retraction of the ER. Furthermore, Dpy19L1 showed a similar distribution pattern with a ER marker protein in embryonic mouse cortical neurons. Finally, we showed that Dpy19L1 knockdown mediated by siRNA resulted in decreased neurite outgrowth in cultured neurons. These results indicate that transmembrane protein Dpy19L1 is localized to the ER membrane and regulates neurite extension during development.

The topology, structure and PE interaction of LITAF underpin a Charcot-Marie-Tooth disease type 1C

BACKGROUND

Mutations in Lipopolysaccharide-induced tumour necrosis factor-α factor (LITAF) cause the autosomal dominant inherited peripheral neuropathy, Charcot-Marie-Tooth disease type 1C (CMT1C). LITAF encodes a 17 kDa protein containing an N-terminal proline-rich region followed by an evolutionarily-conserved C-terminal 'LITAF domain', which contains all reported CMT1C-associated pathogenic mutations.

RESULTS

Here, we report the first structural characterisation of LITAF using biochemical, cell biological, biophysical and NMR spectroscopic approaches. Our structural model demonstrates that LITAF is a monotopic zinc-binding membrane protein that embeds into intracellular membranes via a predicted hydrophobic, in-plane, helical anchor located within the LITAF domain. We show that specific residues within the LITAF domain interact with phosphoethanolamine (PE) head groups, and that the introduction of the V144M CMT1C-associated pathogenic mutation leads to protein aggregation in the presence of PE.

CONCLUSIONS

In addition to the structural characterisation of LITAF, these data lead us to propose that an aberrant LITAF-PE interaction on the surface of intracellular membranes contributes to the molecular pathogenesis that underlies this currently incurable disease.

Mutations in REEP6 Cause Autosomal-Recessive Retinitis Pigmentosa

Retinitis pigmentosa (RP) is the most frequent form of inherited retinal dystrophy. RP is genetically heterogeneous and the genes identified to date encode proteins involved in a wide range of functional pathways, including photoreceptor development, phototransduction, the retinoid cycle, cilia, and outer segment development. Here we report the identification of biallelic mutations in Receptor Expression Enhancer Protein 6 (REEP6) in seven individuals with autosomal-recessive RP from five unrelated families. REEP6 is a member of the REEP/Yop1 family of proteins that influence the structure of the endoplasmic reticulum but is relatively unstudied. The six variants identified include three frameshift variants, two missense variants, and a genomic rearrangement that disrupts exon 1. Human 3D organoid optic cups were used to investigate REEP6 expression and confirmed the expression of a retina-specific isoform REEP6.1, which is specifically affected by one of the frameshift mutations. Expression of the two missense variants (c.383C>T [p.Pro128Leu] and c.404T>C [p.Leu135Pro]) and the REEP6.1 frameshift mutant in cultured cells suggest that these changes destabilize the protein. Furthermore, CRISPR-Cas9-mediated gene editing was used to produce Reep6 knock-in mice with the p.Leu135Pro RP-associated variant identified in one RP-affected individual. The homozygous knock-in mice mimic the clinical phenotypes of RP, including progressive photoreceptor degeneration and dysfunction of the rod photoreceptors. Therefore, our study implicates REEP6 in retinal homeostasis and highlights a pathway previously uncharacterized in retinal dystrophy.

NOGO-A/RTN4A and NOGO-B/RTN4B are simultaneously expressed in epithelial, fibroblast and neuronal cells and maintain ER morphology

Reticulons (RTNs) are a large family of membrane associated proteins with various functions. NOGO-A/RTN4A has a well-known function in limiting neurite outgrowth and restricting the plasticity of the mammalian central nervous system. On the other hand, Reticulon 4 proteins were shown to be involved in forming and maintaining endoplasmic reticulum (ER) tubules. Using comparative transcriptome analysis and qPCR, we show here that NOGO-B/RTN4B and NOGO-A/RTN4A are simultaneously expressed in cultured epithelial, fibroblast and neuronal cells. Electron tomography combined with immunolabelling reveal that both isoforms localize preferably to curved membranes on ER tubules and sheet edges. Morphological analysis of cells with manipulated levels of NOGO-B/RTN4B revealed that it is required for maintenance of normal ER shape; over-expression changes the sheet/tubule balance strongly towards tubules and causes the deformation of the cell shape while depletion of the protein induces formation of large peripheral ER sheets.

Modeling Axonal Defects in Hereditary Spastic Paraplegia with Human Pluripotent Stem Cells

BACKGROUND

Cortical motor neurons, also known as upper motor neurons, are large projection neurons whose axons convey signals to lower motor neurons to control the muscle movements. Degeneration of cortical motor neuron axons is implicated in several debilitating disorders, including hereditary spastic paraplegia (HSP) and amyotrophic lateral sclerosis (ALS). Since the discovery of the first HSP gene, SPAST that encodes spastin, over 70 distinct genetic loci associated with HSP have been identified. How the mutations of these functionally diverse genes result in axonal degeneration and why certain axons are affected in HSP remains largely unknown. The development of induced pluripotent stem cell (iPSC) technology has provided researchers an excellent resource to generate patient-specific human neurons to model human neuropathologic processes including axonal defects.

METHODS

In this article, we will frst review the pathology and pathways affected in the common forms of HSP subtypes by searching the PubMed database. We will then summurize the findings and insights gained from studies using iPSC-based models, and discuss the challenges and future directions.

RESULTS

HSPs, a heterogeneous group of genetic neurodegenerative disorders, are characterized by lower extremity weakness and spasticity that result from retrograde axonal degeneration of cortical motor neurons. Recently, iPSCs have been generated from several common forms of HSP including SPG4, SPG3A, and SPG11 patients. Neurons derived from HSP iPSCs exhibit disease-relevant axonal defects, such as impaired neurite outgrowth, increased axonal swellings, and reduced axonal transport.

CONCLUSION

These patient-derived neurons offer unique tools to study the pathogenic mechanisms and explore the treatments for rescuing axonal defects in HSP, as well as other diseases involving axonopathy.

Spastin, atlastin, and ER relocalization are involved in axon but not dendrite regeneration

A Drosophila model system is used to show that the hereditary spastic paraplegia proteins spastin and atlastin help axons but not dendrites regenerate. The endoplasmic reticulum concentrates at tips of regenerating axons but not dendrites, and this depends on spastin and atlastin.

Mutations in >50 genes, including spastin and atlastin, lead to hereditary spastic paraplegia (HSP). We previously demonstrated that reduction of spastin leads to a deficit in axon regeneration in a Drosophila model. Axon regeneration was similarly impaired in neurons when HSP proteins atlastin, seipin, and spichthyin were reduced. Impaired regeneration was dependent on genetic background and was observed when partial reduction of HSP proteins was combined with expression of dominant-negative microtubule regulators, suggesting that HSP proteins work with microtubules to promote regeneration. Microtubule rearrangements triggered by axon injury were, however, normal in all genotypes. We examined other markers to identify additional changes associated with regeneration. Whereas mitochondria, endosomes, and ribosomes did not exhibit dramatic repatterning during regeneration, the endoplasmic reticulum (ER) was frequently concentrated near the tip of the growing axon. In atlastin RNAi and spastin mutant animals, ER accumulation near single growing axon tips was impaired. ER tip concentration was observed only during axon regeneration and not during dendrite regeneration. In addition, dendrite regeneration was unaffected by reduction of spastin or atlastin. We propose that the HSP proteins spastin and atlastin promote axon regeneration by coordinating concentration of the ER and microtubules at the growing axon tip.

Dysfunctional tubular endoplasmic reticulum constitutes a pathological feature of Alzheimer's disease

Pathological features in Alzheimer's brains include mitochondrial dysfunction and dystrophic neurites (DNs) in areas surrounding amyloid plaques. Using a mouse model that overexpresses reticulon 3 (RTN3) and spontaneously develops age-dependent hippocampal DNs, here we report that DNs contain both RTN3 and REEPs, topologically similar proteins that can shape tubular endoplasmic reticulum (ER). Importantly, ultrastructural examinations of such DNs revealed gradual accumulation of tubular ER in axonal termini, and such abnormal tubular ER inclusion is found in areas surrounding amyloid plaques in biopsy samples from Alzheimer's disease (AD) brains. Functionally, abnormally clustered tubular ER induces enhanced mitochondrial fission in the early stages of DN formation and eventual mitochondrial degeneration at later stages. Furthermore, such DNs are abrogated when RTN3 is ablated in aging and AD mouse models. Hence, abnormally clustered tubular ER can be pathogenic in brain regions: disrupting mitochondrial integrity, inducing DNs formation and impairing cognitive function in AD and aging brains.

Novel Genetic, Clinical, and Pathomechanistic Insights into TFG-Associated Hereditary Spastic Paraplegia

Hereditary spastic paraplegias (HSPs) are genetically and clinically heterogeneous axonopathies primarily affecting upper motor neurons and, in complex forms, additional neurons. Here, we report two families with distinct recessive mutations in TFG, previously suggested to cause HSP based on findings in a single small family with complex HSP. The first carried a homozygous c.317G>A (p.R106H) variant and presented with pure HSP. The second carried the same homozygous c.316C>T (p.R106C) variant previously reported and displayed a similarly complex phenotype including optic atrophy. Haplotyping and bisulfate sequencing revealed evidence for a c.316C>T founder allele, as well as for a c.316_317 mutation hotspot. Expression of mutant TFG proteins in cultured neurons revealed mitochondrial fragmentation, the extent of which correlated with clinical severity. Our findings confirm the causal nature of bi-allelic TFG mutations for HSP, broaden the clinical and mutational spectra, and suggest mitochondrial impairment to represent a pathomechanistic link to other neurodegenerative conditions.

ER-shaping proteins are required for ER and mitochondrial network organization in motor neurons

Hereditary spastic paraplegias (HSPs) are a group of neurodegenerative disorders characterized by degeneration of the longest motor neurons in the corticospinal tract, leading to muscle weakness and spasticity of the lower limbs. Pathogenic variants in genes encoding proteins that shape the endoplasmic-reticulum (ER) network are a leading cause of HSP, however, the mechanisms by which loss of ER-shaping proteins underpin degeneration of selective neurons in HSP remain poorly understood. To begin to address this, we have generated a novel in vivo model of HSP in Drosophila melanogaster by targeted knockdown of the ER-shaping protein Arl6IP1 Variants in the human homolog of this gene have recently been linked to HSP subtype SPG61. Arl6IP1 RNAi flies display progressive locomotor deficits without a marked reduction in lifespan, recapitulating key features of HSP in human patients. Loss of Arl6IP1 leads to fragmentation of the smooth ER and disrupted mitochondrial network organization within the distal ends of long motor neurons. Furthermore, genetically increasing mitochondrial fission, by overexpression of dynamin-related protein 1 (Drp1), restores mitochondrial network organization and rescues locomotor deficits in two independent Drosophila models of HSP. Taken together, these results propose a role for ER-shaping proteins in mitochondrial network organization in vivo and suggest that impaired mitochondrial organization may be a common mechanism underpinning some forms of HSP.

The endoplasmic reticulum: structure, function and response to cellular signaling

The endoplasmic reticulum (ER) is a large, dynamic structure that serves many roles in the cell including calcium storage, protein synthesis and lipid metabolism. The diverse functions of the ER are performed by distinct domains; consisting of tubules, sheets and the nuclear envelope. Several proteins that contribute to the overall architecture and dynamics of the ER have been identified, but many questions remain as to how the ER changes shape in response to cellular cues, cell type, cell cycle state and during development of the organism. Here we discuss what is known about the dynamics of the ER, what questions remain, and how coordinated responses add to the layers of regulation in this dynamic organelle.

Rare disease models provide insight into inherited forms of neurodegeneration

Hereditary spastic paraplegias (HSPs) are a group of inherited neurodegenerative conditions characterised by retrograde degeneration of the longest motor neurons in the corticospinal tract, resulting in muscle weakness and spasticity of the lower limbs. To date more than 70 genetic loci have been associated with HSP, however the majority of cases are caused by mutations that encode proteins responsible for generating and maintaining tubular endoplasmic reticulum (ER) structure. These ER-shaping proteins are vital for the long-term survival of axons, however the mechanisms by which mutations in these proteins give rise to HSP remain poorly understood. To begin to address this we have characterized in vivo loss of function models of two very rare forms of HSP caused by loss of the ER-shaping proteins ARL6IP1 (SPG61) and RTN2 (SPG12). These models display progressive locomotor defects, disrupted organisation of the tubular ER and length-dependant defects in the axonal mitochondrial network. Here we compare our findings with those associated with more common forms HSP including: Spastin, Atlastin-1 and REEP 1 which together account for over half of all cases of autosomal dominant HSP. Furthermore, we discuss recent observations in other HSP models which are directly implicated in mitochondrial function and localization. Overall, we highlight the common features of our rare models of HSP and other models of disease which could indicate shared mechanisms underpinning neurodegeneration in these disorders.

Lipid droplets, lipophagy, and beyond

Lipids are essential components for life. Their various structural and physical properties influence diverse cellular processes and, thereby, human health. Lipids are not genetically encoded but are synthesized and modified by complex metabolic pathways, supplying energy, membranes, signaling molecules, and hormones to affect growth, physiology, and response to environmental insults. Lipid homeostasis is crucial, such that excess fatty acids (FAs) can be harmful to cells. To prevent such lipotoxicity, cells convert excess FAs into neutral lipids for storage in organelles called lipid droplets (LDs). These organelles do not simply manage lipid storage and metabolism but also are involved in protein quality management, pathogenesis, immune responses, and, potentially, neurodegeneration. In recent years, a major trend in LD biology has centered around the physiology of lipid mobilization via lipophagy of fat stored within LDs. This review summarizes key findings in LD biology and lipophagy, offering novel insights into this rapidly growing field. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Carnitine palmitoyltransferase 1C: From cognition to cancer

Carnitine palmitoyltransferase 1 (CPT1) C was the last member of the CPT1 family of genes to be discovered. CPT1A and CPT1B were identified as the gate-keeper enzymes for the entry of long-chain fatty acids (as carnitine esters) into mitochondria and their further oxidation, and they show differences in their kinetics and tissue expression. Although CPT1C exhibits high sequence similarity to CPT1A and CPT1B, it is specifically expressed in neurons (a cell-type that does not use fatty acids as fuel to any major extent), it is localized in the endoplasmic reticulum of cells, and it has minimal CPT1 catalytic activity with l-carnitine and acyl-CoA esters. The lack of an easily measurable biological activity has hampered attempts to elucidate the cellular and physiological role of CPT1C but has not diminished the interest of the biomedical research community in this CPT1 isoform. The observations that CPT1C binds malonyl-CoA and long-chain acyl-CoA suggest that it is a sensor of lipid metabolism in neurons, where it appears to impact ceramide and triacylglycerol (TAG) metabolism. CPT1C global knock-out mice show a wide range of brain disorders, including impaired cognition and spatial learning, motor deficits, and a deregulation in food intake and energy homeostasis. The first disease-causing CPT1C mutation was recently described in humans, with Cpt1c being identified as the gene causing hereditary spastic paraplegia. The putative role of CPT1C in the regulation of complex-lipid metabolism is supported by the observation that it is highly expressed in certain virulent tumor cells, conferring them resistance to glucose- and oxygen-deprivation. Therefore, CPT1C may be a promising target in the treatment of cancer. Here we review the molecular, biochemical, and structural properties of CPT1C and discuss its potential roles in brain function, and cancer.

ER network homeostasis is critical for plant endosome streaming and endocytosis

Eukaryotic cells internalize cargo at the plasma membrane via endocytosis, a vital process that is accomplished through a complex network of endosomal organelles. In mammalian cells, the ER is in close association with endosomes and regulates their fission. Nonetheless, the physiological role of such interaction on endocytosis is yet unexplored. Here, we probed the existence of ER–endosome association in plant cells and assayed its physiological role in endocytosis. Through live-cell imaging and electron microscopy studies, we established that endosomes are extensively associated with the plant ER, supporting conservation of interaction between heterotypic organelles in evolutionarily distant kingdoms. Furthermore, by analyzing ER–endosome dynamics in genetic backgrounds with defects in ER structure and movement, we also established that the ER network integrity is necessary for homeostasis of the distribution and streaming of various endosome populations as well as for efficient endocytosis. These results support a novel model that endocytosis homeostasis depends on a spatiotemporal control of the endosome dynamics dictated by the ER membrane network.

Recessive REEP1 mutation is associated with congenital axonal neuropathy and diaphragmatic palsy

OBJECTIVE

To identify the underlying genetic cause of a congenital neuropathy in a 5-year-old boy as part of a cohort of 32 patients from 23 families with genetically unresolved neuropathies.

METHODS

We used autozygosity mapping coupled with next-generation sequencing to investigate a consanguineous family from Lebanon with 1 affected and 2 healthy children. Variants were investigated for segregation in the family by Sanger sequencing. A splice site mutation was further evaluated on the messenger RNA level by quantitative reverse transcription PCR. Subsequently, a larger cohort was specifically screened for receptor expression-enhancing protein 1 (REEP1) gene mutations.

RESULTS

We detected a homozygous splice donor mutation in REEP1 (c.303+1-7GTAATAT>AC, p.F62Kfs23*; NM_022912) that cosegregated with the phenotype in the family, leading to complete skipping of exon 4 and a premature stop codon. The phenotype of the patient is similar to spinal muscular atrophy with respiratory distress type 1 (SMARD1) with additional distal arthrogryposis and involvement of the upper motor neuron manifested by pronounced hyperreflexia.

CONCLUSION

To date, only dominant REEP1 mutations have been reported to be associated with a slowly progressive hereditary spastic paraplegia. The findings from our patient expand the phenotypical spectrum and the mode of inheritance of REEP1-associated disorders. Recessive mutations in REEP1 should be considered in the molecular genetic workup of patients with a neuromuscular disorder resembling SMARD1, especially if additional signs of upper motor neuron involvement and distal arthrogryposis are present.

Hereditary spastic paraplegia-linked REEP1 modulates endoplasmic reticulum/mitochondria contacts

OBJECTIVE

Mutations in receptor expression enhancing protein 1 (REEP1) are associated with hereditary spastic paraplegias (HSPs). Although axonal degeneration is thought to be a predominant feature in HSP, the role of REEP1 mutations in degeneration is largely unknown. Previous studies have implicated a role for REEP1 in the endoplasmic reticulum (ER), whereas others localized REEP1 with mitochondria. We sought to resolve the cellular localization of REEP1 and further elucidate the pathobiology underlying REEP1 mutations in patients.

METHODS

A combination of cellular imaging and biochemical approaches was used to refine the cellular localization of REEP1. Next, Reep1 mutations associated with HSP were functionally tested in neuritic growth and degeneration assays using mouse cortical culture. Finally, a novel assay was developed and used with wild-type and mutant Reep1s to measure the interactions between the ER and mitochondria.

RESULTS

We found that REEP1 is present at the ER-mitochondria interface, and it contains subdomains for mitochondrial as well as ER localization. Knockdown of Reep1 and expression of pathological Reep1 mutations resulted in neuritic growth defects and degeneration. Finally, using our novel split-RLuc8 assay, we show that REEP1 facilitates ER-mitochondria interactions, a function diminished by disease-associated mutations.

INTERPRETATION

Our data potentially reconcile the current conflicting reports regarding REEP1 being either an ER or a mitochondrial protein. Furthermore, our results connect, for the first time, the disrupted ER-mitochondria interactions to a failure in maintaining health of long axons in HSPs. Finally, the split-RLuc8 assay offers a new tool to identify potential drugs for multiple neurodegenerative diseases with ER-mitochondria interaction defects.

Connecting the cytoskeleton to the endoplasmic reticulum and Golgi

A tendency in cell biology is to divide and conquer. For example, decades of painstaking work have led to an understanding of endoplasmic reticulum (ER) and Golgi structure, dynamics, and transport. In parallel, cytoskeletal researchers have revealed a fantastic diversity of structure and cellular function in both actin and microtubules. Increasingly, these areas overlap, necessitating an understanding of both organelle and cytoskeletal biology. This review addresses connections between the actin/microtubule cytoskeletons and organelles in animal cells, focusing on three key areas: ER structure and function; ER-to-Golgi transport; and Golgi structure and function. Making these connections has been challenging for several reasons: the small sizes and dynamic characteristics of some components; the fact that organelle-specific cytoskeletal elements can easily be obscured by more abundant cytoskeletal structures; and the difficulties in imaging membranes and cytoskeleton simultaneously, especially at the ultrastructural level. One major concept is that the cytoskeleton is frequently used to generate force for membrane movement, with two potential consequences: translocation of the organelle, or deformation of the organelle membrane. While initially discussing issues common to metazoan cells in general, we subsequently highlight specific features of neurons, since these highly polarized cells present unique challenges for organellar distribution and dynamics.

The endogenous caspase-8 inhibitor c-FLIPL regulates ER morphology and crosstalk with mitochondria

Components of the death receptor-mediated pathways like caspase-8 have been identified in complexes at intracellular membranes to spatially restrict the processing of local targets. In this study, we report that the long isoform of the cellular FLICE-inhibitory protein (c-FLIP(L)), a well-known inhibitor of the extrinsic cell death initiator caspase-8, localizes at the endoplasmic reticulum (ER) and mitochondria-associated membranes (MAMs). ER morphology was disrupted and ER Ca(2+)-release as well as ER-mitochondria tethering was decreased in c-FLIP(-/-) mouse embryonic fibroblasts (MEFs). Mechanistically, c-FLIP ablation resulted in enhanced basal caspase-8 activation and in caspase-mediated processing of the ER-shaping protein reticulon-4 (RTN4) that was corrected by re-introduction of c-FLIP(L) and caspase inhibition, resulting in the recovery of a normal ER morphology and ER-mitochondria juxtaposition. Thus, the caspase-8 inhibitor c-FLIP(L) emerges as a component of the MAMs signaling platforms, where caspases appear to regulate ER morphology and ER-mitochondria crosstalk by impinging on ER-shaping proteins like the RTN4.

Hereditary spastic paraplegia SPG4: what is known and not known about the disease

Mutations in more than 70 distinct loci and more than 50 mutated gene products have been identified in patients with hereditary spastic paraplegias, a diverse group of neurological disorders characterized predominantly, but not exclusively, by progressive lower limb spasticity and weakness resulting from distal degeneration of corticospinal tract axons. Mutations in the SPAST (previously known as SPG4) gene that encodes the microtubule-severing protein called spastin, are the most common cause of the disease. The aetiology of the disease is poorly understood, but partial loss of microtubule-severing activity resulting from inactivating mutations in one SPAST allele is the most postulated explanation. Microtubule severing is important for regulating various aspects of the microtubule array, including microtubule number, length, and mobility. In addition, higher numbers of dynamic plus-ends of microtubules, resulting from microtubule-severing events, may play a role in endosomal tubulation and fission. Even so, there is growing evidence that decreased severing of microtubules does not fully explain HSP-SPG4. The presence of two translation initiation codons in SPAST allows synthesis of two spastin isoforms: a full-length isoform called M1 and a slightly shorter isoform called M87. M87 is more abundant in both neuronal and non-neuronal tissues. Studies on rodents suggest that M1 is only readily detected in adult spinal cord, which is where nerve degeneration mainly occurs in humans with HSP-SPG4. M1, due to its hydrophobic N-terminal domain not shared by M87, may insert into endoplasmic reticulum membrane, and together with reticulons, atlastin and REEP1, may play a role in the morphogenesis of this organelle. Some mutated spastins may act in dominant-negative fashion to lower microtubule-severing activity, but others have detrimental effects on neurons without further lowering microtubule severing. The observed adverse effects on microtubule dynamics, axonal transport, endoplasmic reticulum, and endosomal trafficking are likely caused not only by diminished severing of microtubules, but also by neurotoxicity of mutant spastin proteins, chiefly M1. Some large deletions in SPAST might also affect the function of adjacent genes, further complicating the aetiology of the disease.

Clinical and genetic heterogeneity in hereditary spastic paraplegias: from SPG1 to SPG72 and still counting

Hereditary spastic paraplegias (HSPs) are genetically determined neurodegenerative disorders characterized by progressive weakness and spasticity of lower limbs, and are among the most clinically and genetically heterogeneous human diseases. All modes of inheritance have been described, and the recent technological revolution in molecular genetics has led to the identification of 76 different spastic gait disease-loci with 59 corresponding spastic paraplegia genes. Autosomal recessive HSP are usually associated with diverse additional features (referred to as complicated forms), contrary to autosomal dominant HSP, which are mostly pure. However, the identification of additional mutations and families has considerably enlarged the clinical spectra, and has revealed a huge clinical variability for almost all HSP; complicated forms have also been described for primary pure HSP subtypes, adding further complexity to the genotype-phenotype correlations. In addition, the introduction of next generation sequencing in clinical practice has revealed a genetic and phenotypic overlap with other neurodegenerative disorders (amyotrophic lateral sclerosis, neuropathies, cerebellar ataxias, etc.) and neurodevelopmental disorders, including intellectual disability. This review aims to describe the most recent advances in the field and to provide genotype-phenotype correlations that could help clinical diagnoses of this heterogeneous group of disorders.

ER morphology: sculpting with XendoU

Endoplasmic reticulum (ER) sheet membranes are covered with ribosomes and RNAs that are involved in protein synthesis. A new study reveals that a calcium-activated endoribonuclease of the EndoU protein family promotes the formation of tubular ER networks, contributing to dynamic shaping of the ER in cells.

Strumpellin and Spartin, Hereditary Spastic Paraplegia Proteins, are Binding Partners

Hereditary spastic paraplegia (HSP) is one of the most heterogeneous neurodegenerative diseases with more than 50 identified genes causing a relatively stereotypical phenotypic presentation. Recent studies of HSP pathogenesis have suggested the existence of shared biochemical pathways that are crucial for axonal maintenance and degeneration. We explored possible interactions of several proteins associated with this condition. Here we report interactions of endogenous and overexpressed strumpellin with another HSP-associated protein, spartin. This biochemical interaction does not appear to be a part of the Wiskott–Aldrich syndrome protein and Scar homologue (WASH) complex because spartin is not co-immunoprecipitated with WASH1 protein. The spartin–strumpellin association does not require the presence of the microtubule interacting and trafficking domain of spartin. Over-expression of mutant forms of strumpellin with the introduced HSP-causing mutations does not alter the colocalization of these two proteins. Knockdown of strumpellin in cultured cortical rat neurons interferes with development of neuronal branching and results in reduced expression of endogenous spartin. Proteosomal inhibition stabilized the levels of spartin and WASH1 proteins, supporting increased spartin degradation in the absence of strumpellin.

Mutation in CPT1C Associated With Pure Autosomal Dominant Spastic Paraplegia

IMPORTANCE

The family of genes implicated in hereditary spastic paraplegias (HSPs) is quickly expanding, mostly owing to the widespread availability of next-generation DNA sequencing methods. Nevertheless, a genetic diagnosis remains unavailable for many patients.

OBJECTIVE

To identify the genetic cause for a novel form of pure autosomal dominant HSP.

DESIGN, SETTING, AND PARTICIPANTS

We examined and followed up with a family presenting to a tertiary referral center for evaluation of HSP for a decade until August 2014. Whole-exome sequencing was performed in 4 patients from the same family and was integrated with linkage analysis. Sanger sequencing was used to confirm the presence of the candidate variant in the remaining affected and unaffected members of the family and screen the additional patients with HSP. Five affected and 6 unaffected participants from a 3-generation family with pure adult-onset autosomal dominant HSP of unknown genetic origin were included. Additionally, 163 unrelated participants with pure HSP of unknown genetic cause were screened.

MAIN OUTCOME AND MEASURE

Mutation in the neuronal isoform of carnitine palmitoyl-transferase (CPT1C) gene.

RESULTS

We identified the nucleotide substitution c.109C>T in exon 3 of CPT1C, which determined the base substitution of an evolutionarily conserved Cys residue for an Arg in the gene product. This variant strictly cosegregated with the disease phenotype and was absent in online single-nucleotide polymorphism databases and in 712 additional exomes of control participants. We showed that CPT1C, which localizes to the endoplasmic reticulum, is expressed in motor neurons and interacts with atlastin-1, an endoplasmic reticulum protein encoded by the ATL1 gene known to be mutated in pure HSPs. The mutation, as indicated by nuclear magnetic resonance spectroscopy studies, alters the protein conformation and reduces the mean (SD) number (213.0 [46.99] vs 81.9 [14.2]; P < .01) and size (0.29 [0.01] vs 0.26 [0.01]; P < .05) of lipid droplets on overexpression in cells. We also observed a reduction of mean (SD) lipid droplets in primary cortical neurons isolated from Cpt1c-/- mice as compared with wild-type mice (1.0 [0.12] vs 0.44 [0.05]; P < .001), suggesting a dominant negative mechanism for the mutation.

CONCLUSIONS AND RELEVANCE

This study expands the genetics of autosomal dominant HSP and is the first, to our knowledge, to link mutation in CPT1C with a human disease. The association of the CPT1C mutation with changes in lipid droplet biogenesis supports a role for altered lipid-mediated signal transduction in HSP pathogenesis.

Spastin binds to lipid droplets and affects lipid metabolism

Mutations in SPAST, encoding spastin, are the most common cause of autosomal dominant hereditary spastic paraplegia (HSP). HSP is characterized by weakness and spasticity of the lower limbs, owing to progressive retrograde degeneration of the long corticospinal axons. Spastin is a conserved microtubule (MT)-severing protein, involved in processes requiring rearrangement of the cytoskeleton in concert to membrane remodeling, such as neurite branching, axonal growth, midbody abscission, and endosome tubulation. Two isoforms of spastin are synthesized from alternative initiation codons (M1 and M87). We now show that spastin-M1 can sort from the endoplasmic reticulum (ER) to pre- and mature lipid droplets (LDs). A hydrophobic motif comprised of amino acids 57 through 86 of spastin was sufficient to direct a reporter protein to LDs, while mutation of arginine 65 to glycine abolished LD targeting. Increased levels of spastin-M1 expression reduced the number but increased the size of LDs. Expression of a mutant unable to bind and sever MTs caused clustering of LDs. Consistent with these findings, ubiquitous overexpression of Dspastin in Drosophila led to bigger and less numerous LDs in the fat bodies and increased triacylglycerol levels. In contrast, Dspastin overexpression increased LD number when expressed specifically in skeletal muscles or nerves. Downregulation of Dspastin and expression of a dominant-negative variant decreased LD number in Drosophila nerves, skeletal muscle and fat bodies, and reduced triacylglycerol levels in the larvae. Moreover, we found reduced amount of fat stores in intestinal cells of worms in which the spas-1 homologue was either depleted by RNA interference or deleted. Taken together, our data uncovers an evolutionarily conserved role of spastin as a positive regulator of LD metabolism and open up the possibility that dysfunction of LDs in axons may contribute to the pathogenesis of HSP.

Hereditary spastic paraplegia (HSP) is a genetically heterogeneous neurological disease characterized by weakness and spasticity of the lower limbs, caused by progressive retrograde degeneration of the corticospinal axons, the longest in the central nervous system. The most commonly mutated gene in autosomal dominant forms of HSP, SPAST, encodes for spastin, a microtubule-severing protein. Spastin has been implicated in several processes involving remodeling of membrane structures. We now show that the longest spastin form, spastin-M1, harbors a lipid droplet targeting sequence, which allows targeting of the protein to the surface of lipid droplets, the organelles where cells store neutral lipids. Furthermore, we demonstrate that depletion of the homologous spastin proteins in both flies and worms affects lipid droplet number and triacylglycerol content. Our study adds to recent discoveries that implicate other HSP proteins in lipid droplet and lipid metabolism, and strongly suggests that lipid droplet dysfunction in neurons should be investigated to understand pathogenesis of HSP.

Loss of Clcc1 results in ER stress, misfolded protein accumulation, and neurodegeneration

Folding of transmembrane and secretory proteins occurs in the lumen of the endoplasmic reticulum (ER) before transportation to the cell surface and is monitored by the unfolded protein response (UPR) signaling pathway. The accumulation of unfolded proteins in the ER activates the UPR that restores ER homeostasis by regulating gene expression that leads to an increase in the protein-folding capacity of the ER and a decrease in the ER protein-folding load. However, prolonged UPR activity has been associated with cell death in multiple pathological conditions, including neurodegeneration. Here, we report a spontaneous recessive mouse mutation that causes progressive cerebellar granule cell death and peripheral motor axon degeneration. By positional cloning, we identify the mutation in this strain as a retrotransposon insertion in the Clcc1 gene, which encodes a putative chloride channel localized to the ER. Furthermore, we demonstrate that the C3H/HeSnJ inbred strain has late onset cerebellar degeneration due to this mutation. Interestingly, acute knockdown of Clcc1 expression in cultured cells increases sensitivity to ER stress. In agreement, GRP78, the major HSP70 family chaperone in the ER, is upregulated in Clcc1-deficient granule cells in vivo, and ubiquitinated proteins accumulate in these neurons before their degeneration. These data suggest that disruption of chloride homeostasis in the ER disrupts the protein-folding capacity of the ER, leading to eventual neuron death.

Cis and trans interactions between atlastin molecules during membrane fusion

Atlastin (ATL), a membrane-anchored GTPase that mediates homotypic fusion of endoplasmic reticulum (ER) membranes, is required for formation of the tubular network of the peripheral ER. How exactly ATL mediates membrane fusion is only poorly understood. Here we show that fusion is preceded by the transient tethering of ATL-containing vesicles caused by the dimerization of ATL molecules in opposing membranes. Tethering requires GTP hydrolysis, not just GTP binding, because the two ATL molecules are pulled together most strongly in the transition state of GTP hydrolysis. Most tethering events are futile, so that multiple rounds of GTP hydrolysis are required for successful fusion. Supported lipid bilayer experiments show that ATL molecules sitting on the same (cis) membrane can also undergo nucleotide-dependent dimerization. These results suggest that GTP hydrolysis is required to dissociate cis dimers, generating a pool of ATL monomers that can dimerize with molecules on a different (trans) membrane. In addition, tethering and fusion require the cooperation of multiple ATL molecules in each membrane. We propose a comprehensive model for ATL-mediated fusion that takes into account futile tethering and competition between cis and trans interactions.

ER network formation and membrane fusion by atlastin1/SPG3A disease variants

Atlastin catalyzes GTP-dependent membrane fusion to form the ER network. Mutations in atlastin1 cause the disease hereditary spastic paraplegia (HSP), implying that defects in ER membrane fusion cause HSP. Surprisingly, several disease variants are functional in assays for ER network formation and membrane fusion, warranting rethinking of HSP causation by atlastin1 mutations.

At least 38 distinct missense mutations in the neuronal atlastin1/SPG3A GTPase are implicated in an autosomal dominant form of hereditary spastic paraplegia (HSP), a motor-neurological disorder manifested by lower limb weakness and spasticity and length-dependent axonopathy of corticospinal motor neurons. Because the atlastin GTPase is sufficient to catalyze membrane fusion and required to form the ER network, at least in nonneuronal cells, it is logically assumed that defects in ER membrane morphogenesis due to impaired fusion activity are the primary drivers of SPG3A-associated HSP. Here we analyzed a subset of established atlastin1/SPG3A disease variants using cell-based assays for atlastin-mediated ER network formation and biochemical assays for atlastin-catalyzed GTP hydrolysis, dimer formation, and membrane fusion. As anticipated, some variants exhibited clear deficits. Surprisingly however, at least two disease variants, one of which represents that most frequently identified in SPG3A HSP patients, displayed wild-type levels of activity in all assays. The same variants were also capable of co-redistributing ER-localized REEP1, a recently identified function of atlastins that requires its catalytic activity. Taken together, these findings indicate that a deficit in the membrane fusion activity of atlastin1 may be a key contributor, but is not required, for HSP causation.

Delving into the complexity of hereditary spastic paraplegias: how unexpected phenotypes and inheritance modes are revolutionizing their nosology

Hereditary spastic paraplegias (HSP) are rare neurodegenerative diseases sharing the degeneration of the corticospinal tracts as the main pathological characteristic. They are considered one of the most heterogeneous neurological disorders. All modes of inheritance have been described for the 84 different loci and 67 known causative genes implicated up to now. Recent advances in molecular genetics have revealed clinico-genetic heterogeneity of these disorders including their clinical and genetic overlap with other diseases of the nervous system. The systematic analysis of a large set of genes, including exome sequencing, is unmasking unusual phenotypes or inheritance modes associated with mutations in HSP genes and related genes involved in various neurological diseases. A new nosology may emerge after integration and understanding of these new data to replace the current classification. Collectively, functions of the known genes implicate the disturbance of intracellular membrane dynamics and trafficking as the consequence of alterations of cytoskeletal dynamics, lipid metabolism and organelle structures, which represent in fact a relatively small number of cellular processes that could help to find common curative approaches, which are still lacking.

A conserved amphipathic helix is required for membrane tubule formation by Yop1p

The integral membrane proteins of the DP1 (deleted in polyposis) and reticulon families are responsible for maintaining the high membrane curvature required for both smooth endoplasmic reticulum (ER) tubules and the edges of ER sheets, and mutations in these proteins lead to motor neuron diseases, such as hereditary spastic paraplegia. Reticulon/DP1 proteins contain reticulon homology domains (RHDs) that have unusually long hydrophobic segments and are proposed to adopt intramembrane helical hairpins that stabilize membrane curvature. We have characterized the secondary structure and dynamics of the DP1 family protein produced from the YOP1 gene (Yop1p) and identified a C-terminal conserved amphipathic helix (APH) that, on its own, interacts strongly with negatively charged membranes and is necessary for membrane tubule formation. Analyses of DP1 and reticulon family members indicate that most, if not all, contain C-terminal sequences capable of forming APHs. Together, these results indicate that APHs play a previously unrecognized role in RHD membrane curvature stabilization.

The Atlastin C-terminal tail is an amphipathic helix that perturbs the bilayer structure during endoplasmic reticulum homotypic fusion

Fusion of tubular membranes is required to form three-way junctions found in reticular subdomains of the endoplasmic reticulum. The large GTPase Atlastin has recently been shown to drive endoplasmic reticulum membrane fusion and three-way junction formation. The mechanism of Atlastin-mediated membrane fusion is distinct from SNARE-mediated membrane fusion, and many details remain unclear. In particular, the role of the amphipathic C-terminal tail of Atlastin is still unknown. We found that a peptide corresponding to the Atlastin C-terminal tail binds to membranes as a parallel α helix, induces bilayer thinning, and increases acyl chain disorder. The function of the C-terminal tail is conserved in human Atlastin. Mutations in the C-terminal tail decrease fusion activity in vitro, but not GTPase activity, and impair Atlastin function in vivo. In the context of unstable lipid bilayers, the requirement for the C-terminal tail is abrogated. These data suggest that the C-terminal tail of Atlastin locally destabilizes bilayers to facilitate membrane fusion.

Lunapark stabilizes nascent three-way junctions in the endoplasmic reticulum

The endoplasmic reticulum (ER) consists of a polygonal network of sheets and tubules interconnected by three-way junctions. This network undergoes continual remodeling through competing processes: the branching and fusion of tubules forms new three-way junctions and new polygons, and junction sliding and ring closure leads to polygon loss. However, little is known about the machinery required to generate and maintain junctions. We previously reported that yeast Lnp1 localizes to ER junctions, and that loss of Lnp1 leads to a collapsed, densely reticulated ER network. In mammalian cells, only approximately half the junctions contain Lnp1. Here we use live cell imaging to show that mammalian Lnp1 (mLnp1) affects ER junction mobility and hence network dynamics. Three-way junctions with mLnp1 are less mobile than junctions without mLnp1. Newly formed junctions that acquire mLnp1 remain stable within the ER network, whereas nascent junctions that fail to acquire mLnp1 undergo rapid ring closure. These findings imply that mLnp1 plays a key role in stabilizing nascent three-way ER junctions.

ER - the key to the highway

The endoplasmic reticulum (ER) is the key organelle at the start of the secretory pathway and the list of its functions is continually growing. The ER organization as a tubular/cisternal network at the cortex of plant cells has recently been shown to be governed by the membrane tubulation proteins of the reticulon family working alongside plant atlastin homologues, members of the RHD3 group of proteins. Such a network has intimate connections with other organelles such as peroxisomes via peroxules, chloroplasts, Golgi bodies and at the cell cortex to the plasma membrane with cytoskeleton at so called 'anchor/contact sites'. The ER network is by no means static displaying a range of different movements and acting as a subcellular highway supports the motility of organelles such as peroxisomes, mitochondria and Golgi bodies plus the transport of macromolecules such as viral movement proteins, nucleocapsid proteins and RNA. Here we highlight recent and exciting discoveries on the maintenance of the ER structure and its role on movement and biology of other organelles.

Spastin-interacting protein NA14/SSNA1 functions in cytokinesis and axon development

Hereditary spastic paraplegias (HSPs) are a genetically diverse group of inherited neurological disorders (SPG1-72) with the cardinal feature of prominent lower-extremity spasticity due to a length-dependent axonopathy of corticospinal motor neurons. The most frequent form of autosomal dominant HSP results from mutations of the SPG4 gene product spastin. This is an ATPase associated with diverse cellular activities (AAA) protein that binds to and severs microtubules. While spastin participates in crucial cellular processes such as cytokinesis, endosomal tubulation, and axon development, its role in HSP pathogenesis remains unclear. Spastin interacts in cells with the NA14 protein, a major target for auto-antibodies in Sjögren's syndrome (nuclear autoantigen 1; SSNA1). Our analysis of endogenous spastin and NA14 proteins in HeLa cells and rat cortical neurons in primary culture revealed a clear distribution of both proteins to centrosomes, with NA14 localizing specifically to centrioles. Stable NA14 knockdown in cell lines dramatically affected cell division, in particular cytokinesis. Furthermore, overexpression of NA14 in neurons significantly increased axon outgrowth and branching, while also enhancing neuronal differentiation. We postulate that NA14 may act as an adaptor protein regulating spastin localization to centrosomes, temporally and spatially regulating the microtubule-severing activity of spastin that is particularly critical during the cell cycle and neuronal development.

Membrane-shaping disorders: a common pathway in axon degeneration

Neurons with long projections are particularly liable to damage, which is reflected by a large group of hereditary neurodegenerative disorders that primarily affect these neurons. In the group of hereditary spastic paraplegias motor axons of the central nervous system degenerate, while distal pure motor neuropathies, Charcot-Marie-Tooth disorders and the group of hereditary sensory and autonomic neuropathies are characterized by degeneration of peripheral nerve fibres. Because the underlying pathologies share many parallels, the disorders are also referred to as axonopathies. A large number of genes has been associated with axonopathies and one of the emerging subgroups encodes membrane-shaping proteins with a central reticulon homology domain. Association of these proteins with lipid bilayers induces positive membrane curvature and influences the architecture of cellular organelles. Membrane-shaping proteins closely cooperate and directly interact with each other, but their structural features and localization to distinct subdomains of organelles suggests mutually exclusive roles. In some individuals a mutation in a shaping protein can result in upper motor neuron dysfunction, whereas in other patients it can lead to a degeneration of peripheral neurons. This suggests that membrane-shaping disorders might be considered as a continuous disease-spectrum of the axon.

Cell biology. Metabolic control of cell death

Beyond their contribution to basic metabolism, the major cellular organelles, in particular mitochondria, can determine whether cells respond to stress in an adaptive or suicidal manner. Thus, mitochondria can continuously adapt their shape to changing bioenergetic demands as they are subjected to quality control by autophagy, or they can undergo a lethal permeabilization process that initiates apoptosis. Along similar lines, multiple proteins involved in metabolic circuitries, including oxidative phosphorylation and transport of metabolites across membranes, may participate in the regulated or catastrophic dismantling of organelles. Many factors that were initially characterized as cell death regulators are now known to physically or functionally interact with metabolic enzymes. Thus, several metabolic cues regulate the propensity of cells to activate self-destructive programs, in part by acting on nutrient sensors. This suggests the existence of "metabolic checkpoints" that dictate cell fate in response to metabolic fluctuations. Here, we discuss recent insights into the intersection between metabolism and cell death regulation that have major implications for the comprehension and manipulation of unwarranted cell loss.

Loss of spastin function results in disease-specific axonal defects in human pluripotent stem cell-based models of hereditary spastic paraplegia

Human neuronal models of hereditary spastic paraplegias (HSP) that recapitulate disease-specific axonal pathology hold the key to understanding why certain axons degenerate in patients and to developing therapies. SPG4, the most common form of HSP, is caused by autosomal dominant mutations in the SPAST gene, which encodes the microtubule-severing ATPase spastin. Here, we have generated a human neuronal model of SPG4 by establishing induced pluripotent stem cells (iPSCs) from an SPG4 patient and differentiating these cells into telencephalic glutamatergic neurons. The SPG4 neurons displayed a significant increase in axonal swellings, which stained strongly for mitochondria and tau, indicating the accumulation of axonal transport cargoes. In addition, mitochondrial transport was decreased in SPG4 neurons, revealing that these patient iPSC-derived neurons recapitulate disease-specific axonal phenotypes. Interestingly, spastin protein levels were significantly decreased in SPG4 neurons, supporting a haploinsufficiency mechanism. Furthermore, cortical neurons derived from spastin-knockdown human embryonic stem cells (hESCs) exhibited similar axonal swellings, confirming that the axonal defects can be caused by loss of spastin function. These spastin-knockdown hESCs serve as an additional model for studying HSP. Finally, levels of stabilized acetylated-tubulin were significantly increased in SPG4 neurons. Vinblastine, a microtubule-destabilizing drug, rescued this axonal swelling phenotype in neurons derived from both SPG4 iPSCs and spastin-knockdown hESCs. Thus, this study demonstrates the successful establishment of human pluripotent stem cell-based neuronal models of SPG4, which will be valuable for dissecting the pathogenic cellular mechanisms and screening compounds to rescue the axonal degeneration in HSP.

Valosin-containing protein-interacting membrane protein (VIMP) links the endoplasmic reticulum with microtubules in concert with cytoskeleton-linking membrane protein (CLIMP)-63

The distribution and morphology of the endoplasmic reticulum (ER) in mammalian cells depend on both dynamic and static interactions of ER membrane proteins with microtubules (MTs). Cytoskeleton-linking membrane protein (CLIMP)-63 is exclusively localized in sheet-like ER membranes, typical structures of the rough ER, and plays a pivotal role in the static interaction with MTs. Our previous study showed that the 42-kDa ER-residing form of syntaxin 5 (Syn5L) regulates ER structure through the interactions with both CLIMP-63 and MTs. Here, we extend our previous study and show that the valosin-containing protein/p97-interacting membrane protein (VIMP)/SelS is also a member of the family of proteins that shape the ER by interacting with MTs. Depletion of VIMP causes the spreading of the ER to the cell periphery and affects an MT-dependent process on the ER. Although VIMP can interact with CLIMP-63 and Syn5L, it does not interact with MT-binding ER proteins (such as Reep1) that shape the tubular smooth ER, suggesting that different sets of MT-binding ER proteins are used to organize different ER subdomains.

Hereditary spastic paraplegia: clinical-genetic characteristics and evolving molecular mechanisms

Hereditary spastic paraplegia (HSP) is a group of clinically and genetically heterogeneous neurological disorders characterized by pathophysiologic hallmark of length-dependent distal axonal degeneration of the corticospinal tracts. The prominent features of this pathological condition are progressive spasticity and weakness of the lower limbs. To date, 72 spastic gait disease-loci and 55 spastic paraplegia genes (SPGs) have been identified. All modes of inheritance (autosomal dominant, autosomal recessive, and X-linked) have been described. Recently, a late onset spastic gait disorder with maternal trait of inheritance has been reported, as well as mutations in genes not yet classified as spastic gait disease. Several cellular processes are involved in its pathogenesis, such as membrane and axonal transport, endoplasmic reticulum membrane modeling and shaping, mitochondrial function, DNA repair, autophagy, and abnormalities in lipid metabolism and myelination processes. Moreover, recent evidences have been found about the impairment of endosome membrane trafficking in vesicle formation and about the involvement of oxidative stress and mtDNA polymorphisms in the onset of the disease. Interactome networks have been postulated by bioinformatics and biological analyses of spastic paraplegia genes, which would contribute to the development of new therapeutic approaches.

Cold temperature improves mobility and survival in Drosophila models of autosomal-dominant hereditary spastic paraplegia (AD-HSP)

Autosomal-dominant hereditary spastic paraplegia (AD-HSP) is a crippling neurodegenerative disease for which effective treatment or cure remains unknown. Victims experience progressive mobility loss due to degeneration of the longest axons in the spinal cord. Over half of AD-HSP cases arise from loss-of-function mutations in spastin, which encodes a microtubule-severing AAA ATPase. In Drosophila models of AD-HSP, larvae lacking Spastin exhibit abnormal motor neuron morphology and function, and most die as pupae. Adult survivors display impaired mobility, reminiscent of the human disease. Here, we show that rearing pupae or adults at reduced temperature (18°C), compared with the standard temperature of 24°C, improves the survival and mobility of adult spastin mutants but leaves wild-type flies unaffected. Flies expressing human spastin with pathogenic mutations are similarly rescued. Additionally, larval cooling partially rescues the larval synaptic phenotype. Cooling thus alleviates known spastin phenotypes for each developmental stage at which it is administered and, notably, is effective even in mature adults. We find further that cold treatment rescues larval synaptic defects in flies with mutations in Flower (a protein with no known relation to Spastin) and mobility defects in flies lacking Kat60-L1, another microtubule-severing protein enriched in the CNS. Together, these data support the hypothesis that the beneficial effects of cold extend beyond specific alleviation of Spastin dysfunction, to at least a subset of cellular and behavioral neuronal defects. Mild hypothermia, a common neuroprotective technique in clinical treatment of acute anoxia, might thus hold additional promise as a therapeutic approach for AD-HSP and, potentially, for other neurodegenerative diseases.

Pharmacologic rescue of axon growth defects in a human iPSC model of hereditary spastic paraplegia SPG3A

Hereditary spastic paraplegias are a large, diverse group of neurological disorders (SPG1-71) with the unifying feature of prominent lower extremity spasticity, owing to a length-dependent axonopathy of corticospinal motor neurons. The most common early-onset form of pure, autosomal dominant hereditary spastic paraplegia is caused by mutation in the ATL1 gene encoding the atlastin-1 GTPase, which mediates homotypic fusion of ER tubules to form the polygonal ER network. We have identified a p.Pro342Ser mutation in a young girl with pure SPG3A. This residue is in a critical hinge region of atlastin-1 between its GTPase and assembly domains, and it is conserved in all known eukaryotic atlastin orthologs. We produced induced pluripotent stem cells from skin fibroblasts and differentiated these into forebrain neurons to generate a human neuronal model for SPG3A. Axons of these SPG3A neurons showed impaired growth, recapitulating axonal defects in atlastin-1-depleted rat cortical neurons and impaired root hair growth in loss-of-function mutants of the ATL1 ortholog rhd3 in the plant Arabidopsis. Both the microtubule cytoskeleton and tubular ER are important for mitochondrial distribution and function within cells, and SPG3A neurons showed alterations in mitochondrial motility. Even so, it is not clear whether this change is involved in disease pathogenesis. The SPG3A axon growth defects could be rescued with microtubule-binding agents, emphasizing the importance of tubular ER interactions with the microtubule cytoskeleton in hereditary spastic paraplegia pathogenesis. The prominent alterations in axon growth in SPG3A neurons may represent a particularly attractive target for suppression in screens for novel pharmacologic agents.