Endogenous spartin, mutated in hereditary spastic paraplegia, has a complex subcellular localization suggesting diverse roles in neurons

Methylation Heterogeneity and Gene Expression of SPG20 in Solid Tumors

Introduction. The downregulation of the Spastic Paraplegia-20 (SPG20) gene is correlated with a rare autosomal recessive disorder called Troyer Syndrome. Only in recent years has SPG20 been studied and partially characterized in cancer. SPG20 has been shown to be hypermethylated in colorectal cancer, gastric cancer, non-Hodgkin’s lymphoma and hepatocellular carcinoma. In this study, we analyze the methylation status and the gene expression of SPG20 in different tumors of various histological origins. Methods. We analyzed the data generated through Infinium Human Methylation 450 BeadChip arrays and RNA-seq approaches extrapolated from The Cancer Genome Atlas (TCGA) database. The statistics were performed with R 4.0.4. Results. We aimed to assess whether the hypermethylation of this target gene was a common characteristic among different tumors and if there was a correlation between the m-values and the gene expression in paired tumor versus solid tissue normal. Overall, our analysis highlighted that SPG20 open sea upstream the TSS is altogether hypermethylated, and the tumor tissues display a higher methylation heterogeneity compared to the solid tissue normal. The gene expression evidences a reproducible, higher gene expression in normal tissues. Conclusion. Our research, based on data mining from TCGA, evidences that colon and liver tumors display a consistent methylation heterogeneity compared to their normal counterparts. This parallels a downregulation of SPG20 gene expression in tumor samples and suggests a role for this multifunctional protein in the control of tumor progression.

13q Deletion Syndrome Involving RB1: Characterization of a New Minimal Critical Region for Psychomotor Delay

Retinoblastoma (RB) is an ocular tumor of the pediatric age caused by biallelic inactivation of the RB1 gene (13q14). About 10% of cases are due to gross-sized molecular deletions. The deletions can involve the surrounding genes delineating a contiguous gene syndrome characterized by RB, developmental anomalies, and peculiar facial dysmorphisms. Overlapping deletions previously found by traditional and/or molecular cytogenetic analysis allowed to define some critical regions for intellectual disability (ID) and multiple congenital anomalies, with key candidate genes. In the present study, using array-CGH, we characterized seven new patients with interstitial 13q deletion involving RB1. Among these cases, three patients with medium or large 13q deletions did not present psychomotor delay. This allowed defining a minimal critical region for ID that excludes the previously suggested candidate genes (HTR2A, NUFIP1, PCDH8, and PCDH17). The region contains 36 genes including NBEA, which emerged as the candidate gene associated with developmental delay. In addition, MAB21L1, DCLK1, EXOSC8, and SPART haploinsufficiency might contribute to the observed impaired neurodevelopmental phenotype. In conclusion, this study adds important novelties to the 13q deletion syndrome, although further studies are needed to better characterize the contribution of different genes and to understand how the haploinsufficiency of this region can determine ID.

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.

Dwarfism in Troyer syndrome: a family with SPG20 compound heterozygous mutations and a literature review

Troyer syndrome is an autosomal recessive disease characterized by spastic paralysis, dysarthria, distal amyotrophy, and short stature. Recently, two siblings (an older brother and a younger sister) were admitted to our hospital for the chief complaints of "short stature and intellectual disability." Through whole exome sequencing of the sister, who is the proband, it was found that her SPG20 gene had compound heterozygous mutations: c.364_365delAT (p.Met122Valfs^* 2) and c.892delA (p.Thr298Glnfs^* 30). Target testing revealed that the brother had the same genotype as the sister, and the former mutation originated from the father, while the latter mutation originated from the mother. In summary, this is the first report of Troyer syndrome in a family caused by SPG20 compound heterozygous mutations. A novel SPG20 mutation was found, namely c.892delA (p.Thr298Glnfs^* 30). In addition, we also summarize these Troyer syndrome patients' heights and their clinical characteristics, and provide a brief review of all known pathogenic mutations of SPG20.

Mitochondrial energy metabolism is required for lifespan extension by the spastic paraplegia-associated protein spartin

Hereditary spastic paraplegias, a group of neurodegenerative disorders, can be caused by loss-of-function mutations in the protein spartin. However, the physiological role of spartin remains largely elusive. Here we show that heterologous expression of human or Drosophila spartin extends chronological lifespan of yeast, reducing age-associated ROS production, apoptosis, and necrosis. We demonstrate that spartin localizes to the proximity of mitochondria and physically interacts with proteins related to mitochondrial and respiratory metabolism. Interestingly, Nde1, the mitochondrial external NADH dehydrogenase, and Pda1, the core enzyme of the pyruvate dehydrogenase complex, are required for spartin-mediated cytoprotection. Furthermore, spartin interacts with the glycolysis enhancer phospo-fructo-kinase-2,6 (Pfk26) and is sufficient to complement for PFK26-deficiency at least in early aging. We conclude that mitochondria-related energy metabolism is crucial for spartin's vital function during aging and uncover a network of specific interactors required for this function.

Oxidative Stress in Caenorhabditis elegans: Protective Effects of Spartin

Troyer syndrome is caused by a mutation in the SPG20 gene, which results in complete loss of expression of the protein spartin. We generated a genetic model of Troyer syndrome in worms to explore the locomotor consequences of a null mutation of the Caenorhabditis elegans SPG20 orthologue, F57B10.9, also known as spg-20. Spg-20 mutants showed decreased length, crawling speed, and thrashing frequency, and had a shorter lifespan than wild-type animals. These results suggest an age-dependent decline in motor function in mutant animals. The drug paraquat was used to induce oxidative stress for 4 days in the animals. We measured survival rate and examined locomotion by measuring crawling speed and thrashing frequency. After 4 days of paraquat exposure, 77% of wild-type animals survived, but only 38% of spg-20 mutant animals survived. Conversely, animals overexpressing spg-20 had a survival rate of 95%. We also tested lifespan after a 1 hour exposure to sodium azide. After a 24 hour recovery period, 87% of wild type animals survived, 57% of spg-20 mutant animals survived, and 82% of animals overexpressing spg-20 survived. In the behavioral assays, spg-20 mutant animals showed a significant decrease in both crawling speed and thrashing frequency compared with wild-type animals. Importantly, the locomotor phenotype for both crawling and thrashing was rescued in animals overexpressing spg-20. The animals overexpressing spg-20 had crawling speeds and thrashing frequencies similar to those of wild-type animals. These data suggest that the protein F57B10.9/SPG-20 might have a protective role against oxidative stress.

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.

Hereditary spastic paraplegias: one disease for many genes, and still counting

Hereditary spastic paraplegias (HSPs) are genetically heterogeneous Mendelian disorders characterized by spastic gait with stiffness and weakness in the legs and an associated plethora of neurological or extraneurological signs in "complicated" forms. Major advances have been made during the past two decades in our understanding of their molecular bases with the identification of a large number of gene loci and the cloning of a set of them. The combined genetic and clinical information obtained has permitted a new, molecularly-driven classification and an improved diagnosis of these conditions. This represents a prerequisite for better counseling in families and more appropriate therapeutic options. However, further heterogeneity is expected and new insight into the possible mechanisms anticipated.

Spg20-/- mice reveal multimodal functions for Troyer syndrome protein spartin in lipid droplet maintenance, cytokinesis and BMP signaling

Hereditary spastic paraplegias (HSPs; SPG1-48) are inherited neurological disorders characterized by lower extremity spasticity and weakness. Loss-of-function mutations in the SPG20 gene encoding spartin cause autosomal recessive Troyer syndrome (SPG20), which has additional features of short stature, cognitive deficits and distal amyotrophy. To identify cellular impairments underlying Troyer syndrome, we generated Spg20-/- mice, which exhibit progressive gait defects. Although gross central nervous system pathology appeared largely normal, cerebral cortical neurons cultured from neonatal Spg20-/- mice exhibited increased axon branching, a phenotype suppressed by reintroducing spartin and which required its interaction with the endosomal sorting complex required for transport (ESCRT)-III protein IST1. Analysis of the bone morphogenetic protein (BMP) signaling pathway in Spg20-/- embryonic fibroblasts indicated that Smad1/5 phosphorylation is modestly elevated, possibly due to alterations in BMP receptor trafficking. Cytokinesis was impaired in embryonic fibroblasts cultured from Spg20-/- mice, and binucleated chondrocytes were prominent in epiphyseal growth plates of bones in Spg20-/- mice, perhaps explaining the short stature of patients. Finally, adipose tissue from Spg20-/- female mice exhibited increased lipid droplet (LD) numbers and alterations in perilipin levels, supporting a role for spartin in LD maintenance. Taken together, our results support multimodal functions for spartin that provide important insights into HSP pathogenesis.

Cellular pathways of hereditary spastic paraplegia

Human voluntary movement is controlled by the pyramidal motor system, a long CNS pathway comprising corticospinal and lower motor neurons. Hereditary spastic paraplegias (HSPs) are a large, genetically diverse group of inherited neurologic disorders characterized by a length-dependent distal axonopathy of the corticospinal tracts, resulting in lower limb spasticity and weakness. A range of studies are converging on alterations in the shaping of organelles, particularly the endoplasmic reticulum, as well as intracellular membrane trafficking and distribution as primary defects underlying the HSPs, with clear relevance for other long axonopathies affecting peripheral nerves and lower motor neurons.

Overlapping molecular pathological themes link Charcot-Marie-Tooth neuropathies and hereditary spastic paraplegias

In this review we focus on Charcot-Marie-Tooth (CMT) neuropathies and hereditary spastic paraplegias (HSPs). Although these diseases differ in whether they primarily affect the peripheral or central nervous system, both are genetically determined, progressive, long axonopathies that affect motor and sensory pathways. This commonality suggests that there might be similarities in the molecular pathology underlying these conditions, and here we compare the molecular genetics and cellular pathology of the two groups.

Spartin recruits PKC-ζ via the PKC-ζ-interacting proteins ZIP1 and ZIP3 to lipid droplets

Protein kinase C-ζ interacting proteins (ZIP1-3) recruit the enzymatic activity of the atypical protein kinase C isoforms PKC-λ/ι or PKC-ζ to target proteins. In this study, we searched for binding partners of ZIP3 in the CNS and identified spartin, a multifunctional protein that is mutated in spastic paraplegia type 20. In transfected cells, spartin was present on the surface of lipid droplets (LD), whereas ZIP proteins appeared in intracellular speckles. In the presence of spartin, ZIP1 and ZIP3 were translocated to spartin-positive LD. This translocation was mediated by amino acids 196-393 of spartin that interacted with an N-terminal region of ZIP proteins. Furthermore, ZIP proteins interacted simultaneously with spartin and PKC-ζ, resulting in an enrichment of PKC-ζ on spartin/ZIP-labelled LD. Without spartin, neither ZIP proteins nor PKC-ζ were detected on LD. Interestingly, the presence of the spartin/ZIP/PKC-ζ complex increased LD size. This effect was most pronounced upon incorporation of the ZIP3 isoform into the trimer. Finally, we co-localized spartin, ZIP proteins and PKC-ζ in axon terminals of neurons in the mammalian retina. In summary, we describe spartin as new binding partner of the ZIP/PKC-ζ dimer that recruits PKC-ζ to LD and show that the expressed ZIP isoform regulates LD size.

SPG20 protein spartin associates with cardiolipin via its plant-related senescence domain and regulates mitochondrial Ca2+ homeostasis

Hereditary spastic paraplegias (HSPs) are a group of neurological disorders characterized clinically by spasticity of lower limbs and pathologically by degeneration of the corticospinal tract. Troyer syndrome is an autosomal recessive HSP caused by a frameshift mutation in the spartin (SPG20) gene. Previously, we established that this mutation results in a lack of expression of the truncated mutant spartin protein. Spartin is involved in many cellular processes and associates with several intracellular organelles, including mitochondria. Spartin contains a conserved plant-related senescence domain at its C-terminus. However, neither the function of this domain nor the roles of spartin in mitochondrial physiology are currently known. In this study, we determined that the plant-related senescence domain of spartin interacts with cardiolipin but not with two other major mitochondrial phospholipids, phosphatidylcholine and phosphatidylethanolamine. We also found that knockdown of spartin by small interfering RNA in a human neuroblastoma cell line resulted in depolarization of the mitochondrial membrane. In addition, depletion of spartin resulted in a significant decrease in both mitochondrial calcium uptake and mitochondrial membrane potential in cells treated with thapsigargin. Our results suggest that impairment of mitochondrial calcium uptake might contribute to the neurodegeneration of long corticospinal axons and the pathophysiology of Troyer syndrome.

SPG20, a novel biomarker for early detection of colorectal cancer, encodes a regulator of cytokinesis

Colorectal cancer is a common disease with high mortality. Suitable biomarkers for detection of tumors at an early curable stage would significantly improve patient survival. Here, we show that the SPG20 (spastic paraplegia-20) promoter, encoding the multifunctional Spartin protein, is hypermethylated in 89% of colorectal carcinomas, 78% of adenomas and only 1% of normal mucosa samples. SPG20 methylation was also present in a pilot series of stool samples and corresponding tumors from colorectal cancer patients. SPG20 promoter hypermethylation resulted in loss of mRNA expression in various cancer types and subsequent depletion of Spartin. We further showed that Spartin downregulation in cancer cells resulted in cytokinesis arrest, which was reversed when SPG20 methylation was inhibited. The present study identifies SPG20 promoter hypermethylation as a biomarker suitable for non-invasive detection of colorectal cancer, and a possible mechanism for cytokinesis arrest in colorectal tumorigenesis.

SPG20 protein spartin is recruited to midbodies by ESCRT-III protein Ist1 and participates in cytokinesis

Hereditary spastic paraplegias (HSPs, SPG1-46) are inherited neurological disorders characterized by lower extremity spastic weakness. Loss-of-function SPG20 gene mutations cause an autosomal recessive HSP known as Troyer syndrome. The SPG20 protein spartin localizes to lipid droplets and endosomes, and it interacts with tail interacting protein 47 (TIP47) as well as the ubiquitin E3 ligases atrophin-1-interacting protein (AIP)4 and AIP5. Spartin harbors a domain contained within microtubule-interacting and trafficking molecules (MIT) at its N-terminus, and most proteins with MIT domains interact with specific ESCRT-III proteins. Using yeast two-hybrid and in vitro surface plasmon resonance assays, we demonstrate that the spartin MIT domain binds with micromolar affinity to the endosomal sorting complex required for transport (ESCRT)-III protein increased sodium tolerance (Ist)1 but not to ESCRT-III proteins charged multivesicular body proteins 1-7. Spartin colocalizes with Ist1 at the midbody, and depletion of Ist1 in cells by small interfering RNA significantly decreases the number of cells where spartin is present at midbodies. Depletion of spartin does not affect Ist1 localization to midbodies but markedly impairs cytokinesis. A structure-based amino acid substitution in the spartin MIT domain (F24D) blocks the spartin-Ist1 interaction. Spartin F24D does not localize to the midbody and acts in a dominant-negative manner to impair cytokinesis. These data suggest that Ist1 interaction is important for spartin recruitment to the midbody and that spartin participates in cytokinesis.

Developmental and degenerative features in a complicated spastic paraplegia

Objective

We sought to explore the genetic and molecular causes of Troyer syndrome, one of several complicated hereditary spastic paraplegias (HSPs). Troyer syndrome had been thought to be restricted to the Amish; however, we identified 2 Omani families with HSP, short stature, dysarthria and developmental delay—core features of Troyer syndrome—and a novel mutation in the SPG20 gene, which is also mutated in the Amish. In addition, we analyzed SPG20 expression throughout development to infer how disruption of this gene might generate the constellation of developmental and degenerative Troyer syndrome phenotypes.

Methods

Clinical characterization of 2 non-Amish families with Troyer syndrome was followed by linkage and sequencing analysis. Quantitative polymerase chain reaction and in situ hybridization analysis of SPG20 expression were carried out in embryonic and adult human and mouse tissue.

Results

Two Omani families carrying a novel SPG20 mutation displayed clinical features remarkably similar to the Amish patients with Troyer syndrome. SPG20 mRNA is expressed broadly but at low relative levels in the adult brain; however, it is robustly and specifically expressed in the limbs, face, and brain during early morphogenesis.

Interpretation

Null mutations in SPG20 cause Troyer syndrome, a specific clinical entity with developmental and degenerative features. Maximal expression of SPG20 in the limb buds and forebrain during embryogenesis may explain the developmental origin of the skeletal and cognitive defects observed in this disorder. ANN NEUROL 2010;67:516–525

Identification of novel spartin-interactors shows spartin is a multifunctional protein

Hereditary spastic paraplegia describes a group of neurodegenerative diseases characterized by lower limb progressive weakness and spasticity. Troyer syndrome is an autosomal recessive form of hereditary spastic paraplegia caused by a frameshift mutation (1110delA) in the SPG20 gene encoding spartin protein, the cellular function of which remains unknown. Knowledge about spartin-interactors is also very limited. In this study, we apply a broad spectrum of proteomics techniques to identify novel spartin-binding proteins. We used a Tandem Affinity Purification technique followed by HPLC-mass spectrometry to characterize potential spartin-binding partners. Selected putative interactions were confirmed by co-immunoprecipitation experiments. We identified 94 potential spartin-binding proteins which were grouped into functional categories. We performed co-immunoprecipitation experiments to confirm that spartin interacts with GRP78, GRP75 and nucleolin proteins. Additionally, our mass spectrometry results confirmed previously published information about spartin interaction with ubiquitin and the E3 ubiquitin-protein ligases, AIP4/Itch and AIP5/WWP1. Our studies suggest that spartin is a multifunctional protein and for the first time we suggest a role for spartin in protein folding and turnover both in mitochondria and endoplasmic reticulum. We also show for the first time interaction between spartin and a nucleolar protein, nucleolin.

Endogenous spartin (SPG20) is recruited to endosomes and lipid droplets and interacts with the ubiquitin E3 ligases AIP4 and AIP5

The HSPs (hereditary spastic paraplegias) are genetic conditions in which there is distal degeneration of the longest axons of the corticospinal tract, resulting in spastic paralysis of the legs. The gene encoding spartin is mutated in Troyer syndrome, an HSP in which paralysis is accompanied by additional clinical features. There has been controversy over the subcellular distribution of spartin. We show here that, at steady state, endogenous spartin exists in a cytosolic pool that can be recruited to endosomes and to lipid droplets. Cytosolic endogenous spartin is mono-ubiquitinated and we demonstrate that it interacts via a PPXY motif with the ubiquitin E3 ligases AIP4 [atrophin-interacting protein 4; WWP2 (WW domain-containing E3 ubiquitin protein ligase 2] and AIP5 (WWP1). Surprisingly, the PPXY motif, AIP4 and AIP5 are not required for spartin's ubiquitination, and so we propose that spartin acts as an adaptor for these proteins. Our results suggest that spartin is involved in diverse cellular functions, which may be of relevance to the complex phenotype seen in Troyer syndrome.

A role for ubiquitin ligases and Spartin/SPG20 in lipid droplet turnover

HECT (homologous to the E6AP C terminus) ubiquitin ligases have diverse functions in eukaryotic cells. In screens for proteins that bind to the HECT ubiquitin ligase WWP1, we identified Spartin, which is also known as SPG20. This protein is truncated in a neurological disease, Troyer syndrome. In this study, we show that SPG20 associates with the surface of lipid droplets (LDs) and can regulate their size and number. SPG20 binds to another LD protein, TIP47, and both proteins compete with an additional LD protein, adipophilin/adipocyte differentiation-related protein, for occupancy of LDs. The mutant SPG20 present in Troyer syndrome does not possess these activities. Depletion of SPG20 using RNA interference increases the number and size of LDs when cells are fed with oleic acid. Binding of WWP1 to SPG20 and the consequent ubiquitin transfer remove SPG20 from LDs and reduce the levels of coexpressed SPG20. These experiments suggest functions for ubiquitin ligases and SPG20 in the regulation of LD turnover and potential pathological mechanisms in Troyer syndrome.

Lack of spartin protein in Troyer syndrome: a loss-of-function disease mechanism?

BACKGROUND

Hereditary spastic paraplegias (SPG1-SPG33) are characterized by progressive spastic weakness of the lower limbs. A nucleotide deletion (1110delA) in the (SPG20; OMIM 275900) spartin gene is the origin of autosomal recessive Troyer syndrome. This mutation is predicted to cause premature termination of the spartin protein. However, it remains unknown whether this truncated spartin protein is absent or is present and partially functional in patients.

OBJECTIVE

To determine whether the truncated spartin protein is present or absent in cells derived from patients with Troyer syndrome.

DESIGN

Case report.

SETTING

Academic research.

PATIENTS

We describe a new family with Troyer syndrome due to the 1110delA mutation.

MAIN OUTCOME MEASURES

We cultured primary fibroblasts and generated lymphoblasts from affected individuals, carriers, and control subjects and subjected these cells to immunoblot analyses.

RESULTS

Spartin protein is undetectable in several cell lines derived from patients with Troyer syndrome.

CONCLUSIONS

Our data suggest that Troyer syndrome results from complete loss of spartin protein rather than from the predicted partly functional fragment. This may reflect increased protein degradation or impaired translation.

Troyer syndrome protein spartin is mono-ubiquitinated and functions in EGF receptor trafficking

Troyer syndrome is an autosomal recessive hereditary spastic paraplegia caused by mutation in the spartin (SPG20) gene, which encodes a widely expressed protein of unknown function. This mutation results in premature protein truncation and thus might signify a loss-of-function disease mechanism. In this study, we have found that spartin is mono-ubiquitinated and functions in degradation of the epidermal growth factor receptor (EGFR). Upon EGF stimulation, spartin translocates from the cytoplasm to the plasma membrane and colocalizes with internalized EGF-Alexa. Knockdown of spartin by small interfering RNA decreases the rate of EGFR degradation and also affects EGFR internalization, recycling, or both. Furthermore, overexpression of spartin results in a prominent decrease in EGFR degradation. Taken together, our data suggest that spartin is involved in the intracellular trafficking of EGFR and that impaired endocytosis may underlie the pathogenesis of Troyer syndrome.