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

Cul-4 inhibition rescues spastin levels and reduces defects in hereditary spastic paraplegia models

Hereditary spastic paraplegias (HSPs) are degenerative motor neuron diseases characterized by progressive spasticity and weakness in the lower limbs. The most common form of HSP is due to SPG4 gene haploinsufficiency. SPG4 encodes the microtubule severing enzyme spastin. Although, there is no cure for SPG4-HSP, strategies to induce a spastin recovery are emerging as promising therapeutic approaches. Spastin protein levels are regulated by poly-ubiquitination and proteasomal-mediated degradation, in a neddylation-dependent manner. However, the molecular players involved in this regulation are unknown. Here, we show that the Cullin-4-RING E3 ubiquitin ligase complex (CRL4) regulates spastin stability. Inhibition of CRL4 increases spastin levels by preventing its poly-ubiquitination and subsequent degradation in spastin-proficient and in patient derived SPG4 haploinsufficient cells. To evaluate the role of CRL4 complex in spastin regulation in vivo, we developed a Drosophila melanogaster model of SPG4 haploinsufficiency which show alterations of synapse morphology and locomotor activity, recapitulating phenotypical defects observed in patients. Downregulation of the CRL4 complex, highly conserved in Drosophila, rescues spastin levels and the phenotypical defects observed in flies. As a proof of concept of possible pharmacological treatments, we demonstrate a recovery of spastin levels and amelioration of the SPG4-HSP-associated defects both in the fly model and in patient-derived cells by chemical inactivation of the CRL4 complex with NSC1892. Taken together, these findings show that CRL4 contributes to spastin stability regulation and that it is possible to induce spastin recovery and rescue of SPG4-HSP defects by blocking the CRL4-mediated spastin degradation.

Spastin accumulation and motor neuron defects caused by a novel SPAST splice site mutation

BACKGROUND

Hereditary spastic paraplegia (HSP) is a rare genetically heterogeneous neurodegenerative disorder. The most common type of HSP is caused by pathogenic variants in the SPAST gene. Various hypotheses regarding the pathogenic mechanisms of HSP-SPAST have been proposed. However, a single hypothesis may not be sufficient to explain HSP-SPAST.

OBJECTIVE

To determine the causative gene of autosomal dominant HSP-SPAST in a pure pedigree and to study its underlying pathogenic mechanism.

METHODS

A four-generation Chinese family was investigated. Genetic testing was performed for the causative gene, and a splice site variant was identified. In vivo and in vitro experiments were conducted separately. Western blotting and immunofluorescence were performed after transient transfection of cells with the wild-type (WT) or mutated plasmid. The developmental expression pattern of zebrafish spasts was assessed via whole-mount in situ hybridization. The designed guide RNA (gRNA) and an antisense oligo spast-MO were microinjected into Tg(hb9:GFP) zebrafish embryos, spinal cord motor neurons were observed, and a swimming behavioral analysis was conducted.

RESULTS

A novel heterozygous intron variant, c.1004 + 5G > A, was identified in a pure HSP-SPAST pedigree and shown to cosegregate with the disease phenotypes. This intron splice site variant skipped exon 6, causing a frameshift mutation that resulted in a premature termination codon. In vitro, the truncated protein was evenly distributed throughout the cytoplasm, formed filamentous accumulations around the nucleus, and colocalized with microtubules. Truncated proteins diffusing in the cytoplasm appeared denser. No abnormal microtubule structures were observed, and the expression levels of α-tubulin remained unchanged. In vivo, zebrafish larvae with this mutation displayed axon pathfinding defects, impaired outgrowth, and axon loss. Furthermore, spast-MO larvae exhibited unusual behavioral preferences and increased acceleration.

CONCLUSION

The adverse effects of premature stop codon mutations in SPAST result in insufficient levels of functional protein, and the potential toxicity arising from the intracellular accumulation of spastin serves as a contributing factor to HSP-SPAST.

A novel variant (p.A524P) in Spastin is responsible for a Chinese family with hereditary spastic paraplegia

BACKGROUND

Hereditary spastic paraplegia (HSP) represents a group of monogenic neurodegenerative disorders characterized by high clinical and genetic heterogeneity. HSP is characterized by slowly progressing hypertonia of both lower extremities, spastic gait, and myasthenia. The most prevalent autosomal dominant form of HSP, known as spastic paraplegia 4 (SPG4), is attributed to variants in the spastin (SPAST) gene.

METHODS AND RESULTS

Here, a Chinese family presenting with spasticity in both legs and a shuffling gait participated in our investigation. Whole exome sequencing of the proband was utilized to identify the genetic lesion in the family. Through data filtering, Sanger sequencing validation, and co-separation analysis, a novel variant (NM_014946.3: c.1669G > C:p.A557P) of SPAST was identified as the genetic lesion of this family. Furthermore, bioinformatic analysis revealed that this variant was deleterious and located in a highly evolutionarily conserved site.

CONCLUSION

Our study confirmed the diagnosis of SPG4 in this family, contributing to genetic counseling for families affected by SPG4. Additionally, our study broadened the spectrum of SPAST variants and highlighted the importance of ATPases associated with various cellular activity domains of SPAST.

AP2A2 mutation and defective endocytosis in a Malian family with hereditary spastic paraplegia

Hereditary spastic paraplegia (HSP) comprises a large group of neurogenetic disorders characterized by progressive lower extremity spasticity. Neurological evaluation and genetic testing were completed in a Malian family with early-onset HSP. Three children with unaffected consanguineous parents presented with symptoms consistent with childhood-onset complicated HSP. Neurological evaluation found lower limb weakness, spasticity, dysarthria, seizures, and intellectual disability. Brain MRI showed corpus callosum thinning with cortical and spinal cord atrophy, and an EEG detected slow background in the index patient. Whole exome sequencing identified a homozygous missense variant in the adaptor protein (AP) complex 2 alpha-2 subunit (AP2A2) gene. Western blot analysis showed reduced levels of AP2A2 in patient-iPSC derived neuronal cells. Endocytosis of transferrin receptor (TfR) was decreased in patient-derived neurons. In addition, we observed increased axon initial segment length in patient-derived neurons. Xenopus tropicalis tadpoles with ap2a2 knockout showed cerebral edema and progressive seizures. Immunoprecipitation of the mutant human AP-2-appendage alpha-C construct showed defective binding to accessory proteins. We report AP2A2 as a novel genetic entity associated with HSP and provide functional data in patient-derived neuron cells and a frog model. These findings expand our understanding of the mechanism of HSP and improve the genetic diagnosis of this condition.

Effects of DeSUMOylated Spastin on AMPA Receptor Surface Delivery and Synaptic Function Are Enhanced by Phosphorylating at Ser210

Surface trafficking of AMPA receptors (AMPARs) is one of the important mechanisms mediating synaptic plasticity which is essential for cognitive functions such as learning and memory. Spastin, as a novel binding partner for the AMPAR, has been reported to regulate AMPAR surface expression and synaptic function. Additionally, Spastin undergoes two posttranslational modifications, phosphorylation and SUMOylation, both of which are crucial for synaptic function. However, gaps exist in our knowledge of how Spastin phosphorylation cross-talks with its SUMOylation in the regulation of AMPAR surface expression and synaptic function. Here, we reported that deSUMOylation of Spastin at Lys427 increased the surface level of AMPAR GluA2 subunit, the amplitude and frequency of miniature excitatory synaptic currents (mEPSC), and facilitated the morphological maturation of dendritic spines in cultured hippocampal neurons. Further studies demonstrated that Spastin phosphorylation at Ser210 further increased the enhancement of GluA2 surface expression and synaptic function by deSUMOylated Spastin, while dephosphorylation had the opposite effect. Simultaneously, deSUMOylation at Lys427 significantly increased the promoting effect of Spastin phosphorylation on synaptic function. In conclusion, our study suggests that cooperative interactions between phosphorylated and deSUMOylated Spastin are novel pathways to enhance synaptic function.

Novel de novo SPAST mutation in a Han Chinese SPG4 patient: a case report

Spastic paraplegia type 4 (SPG4), the predominant form of Autosomal Dominant Hereditary spastic paraplegia (AD-HSP), is characterized by variants in the SPAST gene. This study reports a unique case of a late-onset SPG4 in a Han Chinese male, manifesting primarily as gait disturbances from lower extremity spasticity. Uncovered through whole-genome sequencing, a previously undocumented frameshift variant, c.1545dupA in exon 14 of the SPAST gene, was identified. Notably, this variant was absent in asymptomatic parents with confirmed paternity and maternity status, suggesting a de novo variant occurrence. This discovery emphasizes the potential of de novo variants to exhibit a late-onset pure pattern, extending the SPG4 variant spectrum, and consideration of such variants should be given in HSP patients with a negative family history.

The various forms of hereditary motor neuron disorders and their historical descriptions

Motor neuron disorders comprise a clinically and pathologically heterogeneous group of neurologic diseases characterized by progressive degeneration of motor neurons (including both sporadic and hereditary diseases), affecting the upper motor neurons, lower motor neurons, or both. Hereditary motor neuron disorders themselves represent a vast and heterogeneous group, with numerous clinical and genetic overlaps that can be a source of error. This narrative review aims at providing an overview of the main types of inherited motor neuron disorders by recounting the stages in their historical descriptions. For practical purposes, this review of the literature sets out their various clinical characteristics and updates the list of all the genes involved in the various forms of inherited motor neuron disorders, including spinal muscular atrophy, familial amyotrophic lateral sclerosis, hereditary spastic paraplegia, distal hereditary motor neuropathies/neuronopathies, Kennedy's disease, riboflavin transporter deficiencies, VCPopathy and the neurogenic scapuloperoneal syndrome.

Clinical and genetic characteristics in a Chinese cohort of complex spastic paraplegia type 4

Spastic paraplegia type 4 (SPG4), caused by SPAST mutations, is the most predominant subtype of hereditary spastic paraplegia. Most documented SPG4 patients present as pure form, with the complex form rarely reported. We described the clinical and genetic features of 20 patients with complex phenotypes of SPG4 and further explored the genotype-phenotype correlations. We collected detailed clinical data of all SPG4 patients and assessed their phenotypes. SPAST gene mutations were identified by Multiplex ligation-dependent probe amplification in combination with whole exome sequencing. We further performed statistical analysis in genotype and phenotype among patients with various manifestations and different variants. Out of 90 SPG4 patients, 20 patients (male:female = 16:4) with additional neurologic deficits, namely complex form, were included in our study. The bimodal distribution of age of onset at 0-10 and 21-40 years old is concluded. On cranial MRI, obvious white matter lesions can be observed in five patients. We identified 9 novel and 8 reported SPAST mutations, of which 11 mutations were located in AAA (ATPase associated with various cellular activities) domain. The AAA cassette of spastin is the hottest mutated region among complex SPG4. All patients with cognitive impairment (CI) are males (n = 9/9). Additionally, 80% patients with ataxia are due to frameshift mutations (n = 4/5). Overall, our study summarized and analyzed the genetic and phenotypic characteristics of complex SPG4, making up over 1/5 of in-house SPG4 cohort, among which CI and ataxia are the most common features. Further studies are expected to explore the underlying mechanisms.

A novel SPAST gene splicing variant (c.1617-2A>C) in a heterozygous carrier with hereditary spastic paraplegia

Hereditary spastic paraplegia (HSP) is a group of genetically heterogenous neurodegenerative disorders characterized by progressive spasticity and weakness of lower limbs. We report a novel splicing variant (c.1617-2A>C) of the SPAST gene in a heterozygous carrier from an Italian family with autosomal dominant HSP. The case study describes a pure form of spastic paraparesis with the cardinal clinical features of SPG4. The novel variant affects a canonical splice site and is likely to disrupt RNA splicing. We conclude that the c.1617-2A>C substitution is a null variant, which could be classified as pathogenic; its penetrance should be further investigated.

SPAST Intragenic CNVs Lead to Hereditary Spastic Paraplegia via a Haploinsufficiency Mechanism

The most common form of hereditary spastic paraplegia (HSP), SPG4 is caused by single nucleotide variants and microrearrangements in the SPAST gene. The high percentage of multi-exonic deletions or duplications observed in SPG4 patients is predisposed by the presence of a high frequency of Alu sequences in the gene sequence. In the present study, we analyzed DNA and RNA samples collected from patients with different microrearrangements in SPAST to map gene breakpoints and evaluate the mutation mechanism. The study group consisted of 69 individuals, including 50 SPG4 patients and 19 healthy relatives from 18 families. Affected family members from 17 families carried varying ranges of microrearrangements in the SPAST gene, while one individual had a single nucleotide variant in the 5'UTR of SPAST. To detect the breakpoints of the SPAST gene, long-range PCR followed by sequencing was performed. The breakpoint sequence was detected for five different intragenic SPAST deletions and one duplication, revealing Alu-mediated microhomology at breakpoint junctions resulting from non-allelic homologous recombination in these patients. Furthermore, SPAST gene expression analysis was performed using patient RNA samples extracted from whole blood. Quantitative real-time PCR tests performed in 14 patients suggest no expression of transcripts with microrearrangements in 5 of them. The obtained data indicate that nonsense-mediated decay degradation is not the only mechanism of hereditary spastic paraplegia in patients with SPAST microrearrangements.

A novel truncated variant in SPAST results in spastin accumulation and defects in microtubule dynamics

OBJECTIVE

Haploinsufficiency is widely accepted as the pathogenic mechanism of hereditary spastic paraplegias type 4 (SPG4). However, there are some cases that cannot be explained by reduced function of the spastin protein encoded by SPAST. The aim of this study was to identify the causative variant of SPG4 in a large Chinese family and explore its pathological mechanism.

MATERIALS AND METHODS

A five-generation family with 49 members including nine affected (4 males and 5 females) and 40 unaffected individuals in Mongolian nationality was recruited. Whole exome sequencing was employed to investigate the genetic etiology. Western blotting and immunofluorescence were used to analyze the effects of the mutant proteins in vitro.

RESULTS

A novel frameshift variant NM_014946.4: c.483_484delinsC (p.Val162Leufs*2) was identified in SPAST from a pedigree with SPG4. The variant segregated with the disease in the family and thus determined as the disease-causing variant. The c.483_484delinsC variant produced two truncated mutants (mutant M1 and M87 isoforms). They accumulated to a higher level and presented increased stability than their wild-type counterparts and may lost the microtubule severing activity.

CONCLUSION

SPAST mutations leading to premature stop codons do not always act through haploinsufficiency. The potential toxicity to the corticospinal tract caused by the intracellular accumulation of truncated spastin should be considered as the pathological mechanism of SPG4.

Cohort analysis of novel SPAST variants in SPG4 patients and implementation of in vitro and in vivo studies to identify the pathogenic mechanism caused by splicing mutations

Introduction

Pure hereditary spastic paraplegia (SPG) type 4 (SPG4) is caused by mutations of SPAST gene. This study aimed to analyze SPAST variants in SPG4 patients to highlight the occurrence of splicing mutations and combine functional studies to assess the relevance of these variants in the molecular mechanisms of the disease.

Methods

We performed an NGS panel in 105 patients, in silico analysis for splicing mutations, and in vitro minigene assay.

Results and discussion

The NGS panel was applied to screen 105 patients carrying a clinical phenotype corresponding to upper motor neuron syndrome (UMNS), selectively affecting motor control of lower limbs. Pathogenic mutations in SPAST were identified in 12 patients (11.42%), 5 missense, 3 frameshift, and 4 splicing variants. Then, we focused on the patients carrying splicing variants using a combined approach of in silico and in vitro analysis through minigene assay and RNA, if available. For two splicing variants (i.e., c.1245+1G>A and c.1414-2A>T), functional assays confirm the types of molecular alterations suggested by the in silico analysis (loss of exon 9 and exon 12). In contrast, the splicing variant c.1005-1delG differed from what was predicted (skipping exon 7), and the functional study indicates the loss of frame and formation of a premature stop codon. The present study evidenced the high splice variants in SPG4 patients and indicated the relevance of functional assays added to in silico analysis to decipher the pathogenic mechanism.

Long-read sequencing revealing intragenic deletions in exome-negative spastic paraplegias

Hereditary spastic paraplegias (HSPs) are a heterogeneous group of neurodegenerative disorders characterized by progressive spasticity and weakness in the lower extremities. To date, a total of 88 types of SPG are known. To diagnose HSP, multiple technologies, including microarray, direct sequencing, multiplex ligation-dependent probe amplification, and short-read next-generation sequencing, are often chosen based on the frequency of HSP subtypes. Exome sequencing (ES) is commonly used. We used ES to analyze ten cases of HSP from eight families. We identified pathogenic variants in three cases (from three different families); however, we were unable to determine the cause of the other seven cases using ES. We therefore applied long-read sequencing to the seven undetermined HSP cases (from five families). We detected intragenic deletions within the SPAST gene in four families, and a deletion within PSEN1 in the remaining family. The size of the deletion ranged from 4.7 to 12.5 kb and involved 1-7 exons. All deletions were entirely included in one long read. We retrospectively performed an ES-based copy number variation analysis focusing on pathogenic deletions, but were not able to accurately detect these deletions. This study demonstrated the efficiency of long-read sequencing in detecting intragenic pathogenic deletions in ES-negative HSP patients.

Alu Retrotransposition Event in SPAST Gene as a Novel Cause of Hereditary Spastic Paraplegia

OBJECTIVES

To diagnose the molecular cause of hereditary spastic paraplegia (HSP) observed in a four-generation family with autosomal dominant inheritance.

METHODS

Multiplex ligation-dependent probe amplification (MLPA), whole-exome sequencing (WES), and RNA sequencing (RNA-seq) of peripheral blood leukocytes were performed. Reverse transcription polymerase chain reaction (RT-PCR) and Sanger sequencing were used to characterize target regions of SPAST.

RESULTS

A 121-bp AluYb9 insertion with a 30-bp poly-A tail flanked by 15-bp direct repeats on both sides was identified in the edge of intron 16 in SPAST that segregated with the disease phenotype.

CONCLUSIONS

We identified an intronic AluYb9 insertion inducing splicing alteration in SPAST causing pure HSP phenotype that was not detected by routine WES analysis. Our findings suggest RNA-seq is a recommended implementation for undiagnosed cases by first-line diagnostic approaches. © 2023 International Parkinson and Movement Disorder Society.

Identification of c.1495C > T mutation in SPAST gene in a family of Han Chinese with hereditary spastic paraplegia

BACKGROUND

Hereditary spastic paraplegia 4 (SPG4) caused by spastin (SPAST) gene mutations accounts for 40-45% of hereditary spastic paraplegia (HSP) cases. To search for more genetic evidences for the pathogenesis of HSP, the SPAST genotype and clinical phenotype of a Chinese Han SPG4 family were analysed in this study.

METHODS

The clinical data of the proband and his family members were collected. Whole genomic DNA was extracted from peripheral blood, and the gene detection and pathogenicity analysis of mutations were conducted using whole-exome sequencing technology. Suspected pathogenic mutations were identified. Verification within this family was conducted by Sanger sequencing.

RESULTS

Eight (4 males and 4 females) of 20 members in 4 generations had SPG4. All patients presented with the high feet arches (pes cavus), the abnormal gait, the active tendon reflexes of the upper limbs, the hyperreflexia of the lower limbs, and the positive ankle clonus and Babinski's signs bilaterally. In the proband, we found a heterozygous mutation c.1495C > T in SPAST gene, which was associated with the autosomal dominant SPG4. Both the daughters and granddaughters of the proband in this family were verified to carry this mutation. The clinical characteristics of the SPG4 patients in this family are in line with the simple type of HSP. Heterozygous c.1495C > T is a pathogenic mutation in this family.

CONCLUSION

In this study, we identified a c.1495C > T mutation in the SPAST gene in a Han Chinese family, enriching the mutation spectrum of SPG4.

Spastin locally amplifies microtubule dynamics to pattern the axon for presynaptic cargo delivery

Neurons rely on long-range trafficking of synaptic components to form and maintain the complex neural networks that encode the human experience. With a single neuron capable of forming thousands of distinct en passant synapses along its axon, spatially precise delivery of the necessary synaptic components is paramount. How these synapses are patterned, and how efficient delivery of synaptic components is regulated, remains largely unknown. Here, we reveal a novel role for the microtubule severing enzyme spastin in locally enhancing microtubule polymerization to influence presynaptic cargo pausing and retention along the axon. In human neurons derived from induced pluripotent stem cells (iPSCs), we identify sites stably enriched for presynaptic components, termed 'protosynapses', which are distributed along the axon prior to the robust assembly of mature presynapses apposed by postsynaptic contacts. These sites are capable of cycling synaptic vesicles, are enriched with spastin, and are hotspots for new microtubule growth and synaptic vesicle precursor (SVP) pausing/retention. Disruption of neuronal spastin, either by CRISPRi-mediated depletion or transient overexpression, interrupts the localized enrichment of dynamic microtubule plus ends and diminishes SVP accumulation. Using an innovative human heterologous synapse model, where microfluidically isolated human axons recognize and form presynaptic connections with neuroligin-expressing non-neuronal cells, we reveal that neurons deficient for spastin do not achieve the same level of presynaptic component accumulation as control neurons. We propose a model where spastin acts locally as an amplifier of microtubule polymerization to pattern specific regions of the axon for synaptogenesis and guide synaptic cargo delivery.

Hereditary spastic paraplegias proteome: common pathways and pathogenetic mechanisms

INTRODUCTION

Hereditary spastic paraplegias (HSPs) are a group of inherited neurodegenerative disorders characterized by progressive spasticity and weakness of the lower limbs. These conditions are caused by lesions in the neuronal pyramidal tract and exhibit clinical and genetic variability. Ongoing research focuses on understanding the underlying mechanisms of HSP onset, which ultimately lead to neuronal degeneration. Key molecular mechanisms involved include axonal transport, cytoskeleton dynamics, myelination abnormalities, membrane trafficking, organelle morphogenesis, ER homeostasis, mitochondrial dysfunction, and autophagy deregulation.

AREAS COVERED

This review aims to provide an overview of the shared pathogenetic mechanisms in various forms of HSPs. By examining disease-causing gene products and their associated functional pathways, this understanding could lead to the discovery of new therapeutic targets and the development of treatments to modify the progression of the disease.

EXPERT OPINION

Investigating gene functionality is crucial for identifying shared pathogenetic pathways underlying different HSP subtypes. Categorizing protein function and identifying pathways aids in finding biomarkers, predicting early onset, and guiding treatment for a better quality of life. Targeting shared mechanisms enables efficient and cost-effective therapies. Prospects involve identifying new disease-causing genes, refining molecular processes, and implementing findings in diagnosis, key for advancing HSP understanding and developing effective treatments.

Spastin is required for human immunodeficiency virus-1 efficient replication through cooperation with the endosomal sorting complex required for transport (ESCRT) protein

Human immunodeficiency virus-1 (HIV-1) encodes simply 15 proteins and thus depends on multiple host cellular factors for virus reproduction. Spastin, a microtubule severing protein, is an identified HIV-1 dependency factor, but the mechanism regulating HIV-1 is unclear. Here, the study showed that knockdown of spastin inhibited the production of the intracellular HIV-1 Gag protein and new virions through enhancing Gag lysosomal degradation. Further investigation showed that increased sodium tolerance 1 (IST1), the subunit of endosomal sorting complex required for transport (ESCRT), could interact with the MIT domain of spastin to regulate the intracellular Gag production. In summary, spastin is required for HIV-1 replication, while spastin-IST1 interaction facilitates virus production by regulating HIV-1 Gag intracellular trafficking and degradation. Spastin may serve as new target for HIV-1 prophylactic and therapy.

A novel missense mutation in SPAST causes hereditary spastic paraplegia in male members of a family: A case report

Hereditary spastic paraplegia (HSP) comprises a group of hereditary and neurodegenerative diseases that are characterized by axonal degeneration or demyelination of bilateral corticospinal tracts in the spinal cord; affected patients exhibit progressive spasticity and weakness in the lower limbs. The most common manifestation of HSP is spastic paraplegia type 4 (SPG4), which is caused by mutations in the spastin (SPAST) gene. The present study reports the clinical characteristics of affected individuals and sequencing analysis of a mutation that caused SPG4 in a family. All affected family members exhibited spasticity and weakness of the lower limbs and, notably, only male members of the family were affected. Whole‑exome sequencing revealed that all affected individuals had a novel c.1785C>A (p. Ser595Arg) missense mutation in SPAST. Bioinformatics analysis revealed changes in both secondary and tertiary structures of the mutated protein. The novel missense mutation in SPAST supported the diagnosis of SPG4 in this family and expands the spectrum of pathogenic mutations that cause SPG4. Analysis of SPAST sequences revealed that most pathogenic mutations occurred in the AAA domain of the protein, which may have a close relationship with SPG4 pathogenesis.

Severe Pressure Ulcers in Two Patients With Adrenoleukodystrophy

Adrenoleukodystrophy (ALD) is a rare X-linked disease that affects the metabolism of very long-chain fatty acids (VLCFAs), leading to cognitive deterioration, progressive spastic paraplegia, sensory disturbance, adrenocortical insufficiency, and bladder and bowel abnormalities. Although the symptoms of ALD correspond to the risk of developing pressure ulcers, a pressure ulcer has never been listed as a complication of ALD. We present two cases of ALD with severe pressure ulcers in the pelvic region and feet. The first case was a 27-year-old male patient with adolescent cerebral-type ALD who had pressure ulcers with bone exposure on the sacral and bilateral greater trochanter region. The second case was a 64-year-old male patient with adrenomyeloneuropathy (AMN) phenotype who had pressure ulcers on the sacral region and both feet. Both patients had VLCFA accumulation and a likely pathogenic variant in the ABCD1 gene, the causative gene of ALD. These cases indicate that ALD patients with immobility and incontinence have a higher risk of developing severe pressure ulcers, which requires the proactive identification of ALD patients and early multidisciplinary intervention for patients and their families to prevent the development of pressure ulcers.

Recent Development of Advanced Fluorescent Molecular Probes for Organelle-Targeted Cell Imaging

Fluorescent molecular probes are very powerful tools that have been generally applied in cell imaging in the research fields of biology, pathology, pharmacology, biochemistry, and medical science. In the last couple of decades, numerous molecular probes endowed with high specificity to particular organelles have been designed to illustrate intracellular images in more detail at the subcellular level. Nowadays, the development of cell biology has enabled the investigation process to go deeply into cells, even at the molecular level. Therefore, probes that can sketch a particular organelle's location while responding to certain parameters to evaluate intracellular bioprocesses are under urgent demand. It is significant to understand the basic ideas of organelle properties, as well as the vital substances related to each unique organelle, for the design of probes with high specificity and efficiency. In this review, we summarize representative multifunctional fluorescent molecular probes developed in the last decade. We focus on probes that can specially target nuclei, mitochondria, endoplasmic reticulums, and lysosomes. In each section, we first briefly introduce the significance and properties of different organelles. We then discuss how probes are designed to make them highly organelle-specific. Finally, we also consider how probes are constructed to endow them with additional functions to recognize particular physical/chemical signals of targeted organelles. Moreover, a perspective on the challenges in future applications of highly specific molecular probes in cell imaging is also proposed. We hope that this review can provide researchers with additional conceptual information about developing probes for cell imaging, assisting scientists interested in molecular biology, cell biology, and biochemistry to accelerate their scientific studies.

Clinical and genetic spectrum of hereditary spastic paraplegia in Chinese children

AIM

To explore the clinical and genetic spectrum of hereditary spastic paraplegia (HSP) in Chinese children.

METHOD

This retrospective study was conducted between January 2014 and October 2021 in children clinically diagnosed with either pure HSP (pHSP) or complex HSP (cHSP).

RESULTS

We investigated 45 children (32 males, 13 females; mean age [SD] at symptom onset 4 years [7 months]). clinically diagnosed with HSP and identified genetic causes in 35 patients. Most patients with autosomal dominant HSP had pHSP (16/18), whereas most patients with autosomal recessive HSP tended to have cHSP (14/16). SPG11 was the most common autosomal recessive subtype, followed by FA2H/SPG35, whereas SPAST/SPG4 was the most frequent cause of autosomal dominant HSP. Two patients with CPT1C mutations presented with a complex phenotype. Meanwhile, 10 patients were found to have likely pathogenic variants/variants of uncertain clinical significance in six genes related to HSP.

INTERPRETATION

SPG11 and SPG4 were the most frequent subtypes in Chinese children with autosomal recessive HSP and autosomal dominant HSP. However, the prevalence of SPG4 was much lower than that in adults, which might be explained by the late onset of the disease. On the other hand, FA2H/SPG35 was common in our cohort, while it contributed to only a small proportion of adult cases, which might be explained by its rapid progression and early death in some patients. We also expanded the genetic and clinical spectra of SPG73.

Phosphorylation mutation impairs the promoting effect of spastin on neurite outgrowth without affecting its microtubule severing ability

Spastin, a microtubule-severing enzyme, is known to be important for neurite outgrowth. However, the role of spastin post-translational modification, particularly its phosphorylation regulation in neuronal outgrowth, remains unclear. This study aimed to investigate the effects of eliminating spastin phosphorylation on the neurite outgrowth of rat hippocampal neurons. To accomplish this, we constructed a spastin mutant with eleven potential phosphorylation sites mutated to alanine. The phosphorylation levels of the wildtype spastin (WT) and the mutant (11A) were then detected using Phos-tag SDS-PAGE. The spastin constructs were transfected into COS7 cells for the observation of microtubule severing, and into rat hippocampal neurons for the detection of neuronal outgrowth. The results showed that compared to the spastin WT, the phosphorylation levels were significantly reduced in the spastin 11A mutant. The spastin mutant 11A impaired its ability to promote neurite length, branching, and complexity in hippocampal neurons, but did not affect its ability to sever microtubules in COS7 cells. In conclusion, the data suggest that mutations at multiple phosphorylation sites of spastin do not impair its microtubule cleavage ability in COS7 cells, but reduce its ability to promote neurite outgrowth in rat hippocampal neurons.

Copy-number analysis by base-level normalization: An intuitive visualization tool for evaluating copy number variations

Next-generation sequencing (NGS) facilitates comprehensive molecular analyses that help with diagnosing unsolved disorders. In addition to detecting single-nucleotide variations and small insertions/deletions, bioinformatics tools can identify copy number variations (CNVs) in NGS data, which improves the diagnostic yield. However, due to the possibility of false positives, subsequent confirmation tests are generally performed. Here, we introduce Copy-number Analysis by BAse-level NormAlization (CABANA), a visualization tool that allows users to intuitively identify candidate CNVs using the normalized single-base-level read depth calculated from NGS data. To demonstrate how CABANA works, NGS data were obtained from 474 patients with neuromuscular disorders. CNVs were screened using a conventional bioinformatics tool, ExomeDepth, and then we normalized and visualized those data at the single-base level using CABANA, followed by manual inspection by geneticists to filter out false positives and determine candidate CNVs. In doing so, we identified 31 candidate CNVs (7%) in 474 patients and subsequently confirmed all of them to be true using multiplex ligation-dependent probe amplification. The performance of CABANA was deemed acceptable by comparing its diagnostic yield with previous data about neuromuscular disorders. Despite some limitations, we expect CABANA to help researchers accurately identify CNVs and reduce the need for subsequent confirmation testing.

Neurogenetic motor disorders

Advances in the field of neurogenetics have practical applications in rapid diagnosis on blood and body fluids to extract DNA, obviating the need for invasive investigations. The ability to obtain a presymptomatic diagnosis through genetic screening and biomarkers can be a guide to life-saving disease-modifying therapy or enzyme replacement therapy to compensate for the deficient disease-causing enzyme. The benefits of a comprehensive neurogenetic evaluation extend to family members in whom identification of the causal gene defect ensures carrier detection and at-risk counseling for future generations. This chapter explores the many facets of the neurogenetic evaluation in adult and pediatric motor disorders as a primer for later chapters in this volume and a roadmap for the future applications of genetics in neurology.

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

Background

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

Results

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

Conclusions

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

Supplementary Information

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

Early-Onset and Severe Complex Hereditary Spastic Paraplegia Caused by De Novo Variants in SPAST

BACKGROUND

Familial hereditary spastic paraplegia (HSP)-SPAST (SPG4) typically presents with a pure HSP phenotype.

OBJECTIVE

The aim of this study was to delineate the genotypic and phenotypic spectrum of children with de novo HSP-SPAST.

METHODS

This study used a systematic cross-sectional analysis of clinical and molecular features.

RESULTS

We report the clinical and molecular spectrum of 40 patients with heterozygous pathogenic de novo variants in SPAST (age range: 2.2-27.7 years). We identified 19 unique variants (16/40 carried the same recurrent variant, p.Arg499His). Symptom onset was in early childhood (median: 11.0 months, interquartile range: 6.0 months) with significant motor and speech delay, followed by progressive ascending spasticity, dystonia, neurogenic bladder dysfunction, gastrointestinal dysmotility, and epilepsy. The mean Spastic Paraplegia Rating Scale score was 32.8 ± 9.7 (standard deviation).

CONCLUSIONS

These results confirm that de novo variants in SPAST lead to a severe and complex form of HSP that differs from classic familial pure HSP-SPAST. Clinicians should be aware of this syndrome in the differential diagnosis for cerebral palsy. © 2022 International Parkinson and Movement Disorder Society.

The enhanced association between mutant CHMP2B and spastin is a novel pathological link between frontotemporal dementia and hereditary spastic paraplegias

Chromosome 3-linked frontotemporal dementia (FTD3) is caused by a gain-of-function mutation in CHMP2B, resulting in the production of a truncated toxic protein, CHMP2BIntron5. Loss-of-function mutations in spastin are the most common genetic cause of hereditary spastic paraplegias (HSP). How these proteins might interact with each other to drive pathology remains to be explored. Here we found that spastin binds with greater affinity to CHMP2BIntron5 than to CHMP2BWT and colocalizes with CHMP2BIntron5 in p62-positive aggregates. In cultured cells expressing CHMP2BIntron5, spastin level in the cytoplasmic soluble fraction is decreased while insoluble spastin level is increased. These pathological features of spastin are validated in brain neurons of a mouse model of FTD3. Moreover, genetic knockdown of spastin enhances CHMP2BIntron5 toxicity in a Drosophila model of FTD3, indicating the functional significance of their association. Thus, our study reveals that the enhanced association between mutant CHMP2B and spastin represents a novel potential pathological link between FTD3 and HSP.

A novel truncating variant of SPAST associated with hereditary spastic paraplegia indicates a haploinsufficiency pathogenic mechanism

Introduction

Hereditary spastic paraplegias (HSPs) are genetic neurodegenerative diseases. The most common form of pure HSP that is inherited in an autosomal dominant manner is spastic paraplegia type 4 (SPG4), which is caused by mutations in the SPAST gene. Different theories have been proposed as the mechanism underlying SPAST-HSP for different types of genetic mutations, including gain- and loss-of-function mechanisms. To better understand the mutation mechanisms, we performed genetic analysis and investigated a truncating SPAST variant that segregated with disease in one family.

Objectives and methods

We described a pure HSP pedigree with family members across four generations. We performed genetic analysis and investigated a novel frameshift pathogenic variant (c.862_863dupAC, p. H289Lfs*27) in this family. We performed reverse transcription-polymerase chain reaction (RT-PCR), Sanger sequencing, and quantitative RT-PCR using total RNA from an Epstein-Barr virus-induced lymphoblastoid cell line produced from the proband. We also performed Western blotting on cell lysates to investigate if the protein expression of spastin is affected by this variant.

Results

This variant (c.862_863dupAC, p. H289Lfs*27) co-segregated with pure HSP in this family and is not registered in any public database. Measurement of SPAST transcripts in lymphoblasts from the proband demonstrated a reduction of SPAST transcript levels through likely nonsense-mediated mRNA decay. Immunoblot analyses demonstrated a reduction of spastin protein expression levels in lymphoblasts.

Conclusion

We report an SPG4 family with a novel heterozygous frameshift variant p.H289Lfs*27 in SPAST. Our study implies haploinsufficiency as the pathogenic mechanism for this variant and expands the known mutation spectrum of SPAST.

Autologous iPSC-Derived Human Neuromuscular Junction to Model the Pathophysiology of Hereditary Spastic Paraplegia

Hereditary spastic paraplegia (HSP) is a heterogeneous group of genetic neurodegenerative disorders, characterized by progressive lower limb spasticity and weakness resulting from retrograde axonal degeneration of motor neurons (MNs). Here, we generated in vitro human neuromuscular junctions (NMJs) from five HSP patient-specific induced pluripotent stem cell (hiPSC) lines, by means of microfluidic strategy, to model disease-relevant neuropathologic processes. The strength of our NMJ model lies in the generation of lower MNs and myotubes from autologous hiPSC origin, maintaining the genetic background of the HSP patient donors in both cell types and in the cellular organization due to the microfluidic devices. Three patients characterized by a mutation in the SPG3a gene, encoding the ATLASTIN GTPase 1 protein, and two patients with a mutation in the SPG4 gene, encoding the SPASTIN protein, were included in this study. Differentiation of the HSP-derived lines gave rise to lower MNs that could recapitulate pathological hallmarks, such as axonal swellings with accumulation of Acetyl-α-TUBULIN and reduction of SPASTIN levels. Furthermore, NMJs from HSP-derived lines were lower in number and in contact point complexity, denoting an impaired NMJ profile, also confirmed by some alterations in genes encoding for proteins associated with microtubules and responsible for axonal transport. Considering the complexity of HSP, these patient-derived neuronal and skeletal muscle cell co-cultures offer unique tools to study the pathologic mechanisms and explore novel treatment options for rescuing axonal defects and diverse cellular processes, including membrane trafficking, intracellular motility and protein degradation in HSP.

Evolution of factors shaping the endoplasmic reticulum

Endomembrane system compartments are significant elements in virtually all eukaryotic cells, supporting functions including protein synthesis, post-translational modifications and protein/lipid targeting. In terms of membrane area the endoplasmic reticulum (ER) is the largest intracellular organelle, but the origins of proteins defining the organelle and the nature of lineage-specific modifications remain poorly studied. To understand the evolution of factors mediating ER morphology and function we report a comparative genomics analysis of experimentally characterized ER-associated proteins involved in maintaining ER structure. We find that reticulons, REEPs, atlastins, Ufe1p, Use1p, Dsl1p, TBC1D20, Yip3p and VAPs are highly conserved, suggesting an origin at least as early as the last eukaryotic common ancestor (LECA), although many of these proteins possess additional non-ER functions in modern eukaryotes. Secondary losses are common in individual species and in certain lineages, for example lunapark is missing from the Stramenopiles and the Alveolata. Lineage-specific innovations include protrudin, Caspr1, Arl6IP1, p180, NogoR, kinectin and CLIMP-63, which are restricted to the Opisthokonta. Hence, much of the machinery required to build and maintain the ER predates the LECA, but alternative strategies for the maintenance and elaboration of ER shape and function are present in modern eukaryotes. Moreover, experimental investigations for ER maintenance factors in diverse eukaryotes are expected to uncover novel mechanisms.

Protein disulfide isomerase A6 promotes the repair of injured nerve through interactions with spastin

The maintenance of appropriate endoplasmic reticulum (ER) homeostasis is critical to effective spinal cord injury (SCI) repair. In previous reports, protein disulfide isomerase A6 (PDIA6) demonstrated to serve as a reversible functional modulator of ER stress responses, while spastin can coordinate ER organization through the modulation of the dynamic microtubule network surrounding this organelle. While both PDIA6 and spastin are thus important regulators of the ER, whether they interact with one another for SCI repair still needs to be determined. Here a proteomics analysis identified PDIA6 as being related to SCI repair, and protein interaction mass spectrometry further confirmed the ability of PDIA6 and spastin to interact with one another. Pull-down and co-immunoprecipitation assays were further performed to validate and characterize the interactions between these two proteins. The RNAi-based knockdown of PDIA6 in COS-7 cells inhibited the activity of spastin-dependent microtubule severing. PDIA6 was also found to promote injured neuron repair, while spastin knockdown reversed this reparative activity. Together, these results thus confirm that PDIA6 and spastin function together as critical mediators of nerve repair, highlighting their potential value as validated targets for efforts to promote SCI repair.

The Role of Spastin in Axon Biology

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

Genetic, structural and clinical analysis of spastic paraplegia 4

INTRODUCTION

Spastic paraplegia type 4 (SPG4), resulting from heterozygous mutations in the SPAST gene, is the most common form among the heterogeneous group of hereditary spastic paraplegias (HSPs). We aimed to study genetic and clinical characteristics of SPG4 across Canada.

METHODS

The SPAST gene was analyzed in a total of 696 HSP patients from 431 families by either HSP-gene panel sequencing or whole exome sequencing (WES). We used Multiplex ligation-dependent probe amplification to analyze copy number variations (CNVs), and performed in silico structural analysis of selected mutations. Clinical characteristics of patients were assessed, and long-term follow-up was done to study genotype-phenotype correlations.

RESULTS

We identified 157 SPG4 patients from 65 families who carried 41 different SPAST mutations, six of which are novel and six are CNVs. We report novel aspects of mutations occurring in Arg499, a case with homozygous mutation, a family with probable compound heterozygous mutations, three patients with de novo mutations, three cases with pathogenic synonymous mutation, co-occurrence of SPG4 and clinically isolated syndrome, and novel or rarely reported signs and symptoms seen in SPG4 patients.

CONCLUSION

Our study demonstrates that SPG4 is a heterogeneous type of HSP, with diverse genetic features and clinical manifestations. In rare cases, biallelic inheritance, de novo mutation, pathogenic synonymous mutations and CNVs should be considered.

Clinical-Genetic Features Influencing Disability in Spastic Paraplegia Type 4

Background and Objectives

Hereditary spastic paraplegias (HSPs) are a group of inherited rare neurologic disorders characterized by length-dependent degeneration of the corticospinal tracts and dorsal columns, whose prominent clinical feature is represented by spastic gait. Spastic paraplegia type 4 (SPG4, SPAST-HSP) is the most common form. We present both clinical and molecular findings of a large cohort of patients, with the aim of (1) defining the clinical spectrum of SPAST-HSP in Italy; (2) describing their molecular features; and (3) assessing genotype-phenotype correlations to identify features associated with worse disability.

Methods

A cross-sectional retrospective study with molecular and clinical data collected in an anonymized database was performed.

Results

A total of 723 Italian patients with SPAST-HSP (58% men) from 316 families, with a median age at onset of 35 years, were included. Penetrance was 97.8%, with men showing higher Spastic Paraplegia Rating Scale (SPRS) scores (19.67 ± 12.58 vs 16.15 ± 12.61, p = 0.009). In 26.6% of patients with SPAST-HSP, we observed a complicated phenotype, mainly including intellectual disability (8%), polyneuropathy (6.7%), and cognitive decline (6.5%). Late-onset cases seemed to progress more rapidly, and patients with a longer disease course displayed a more severe neurologic disability, with higher SPATAX (3.61 ± 1.46 vs 2.71 ± 1.20, p < 0.001) and SPRS scores (22.63 ± 11.81 vs 12.40 ± 8.83, p < 0.001). Overall, 186 different variants in the SPAST gene were recorded, of which 48 were novel. Patients with SPAST-HSP harboring missense variants displayed intellectual disability (14.5% vs 4.4%, p < 0.001) more frequently, whereas patients with truncating variants presented more commonly cognitive decline (9.7% vs 2.6%, p = 0.001), cerebral atrophy (11.2% vs 3.4%, p = 0.003), lower limb spasticity (61.5% vs 44.5%), urinary symptoms (50.0% vs 31.3%, p < 0.001), and sensorimotor polyneuropathy (11.1% vs 1.1%, p < 0.001). Increasing disease duration (DD) and abnormal motor evoked potentials (MEPs) were also associated with increased likelihood of worse disability (SPATAX score>3).

Discussion

The SPAST-HSP phenotypic spectrum in Italian patients confirms a predominantly pure form of HSP with mild-to-moderate disability in 75% of cases, and slight prevalence of men, who appeared more severely affected. Early-onset cases with intellectual disability were more frequent among patients carrying missense SPAST variants, whereas patients with truncating variants showed a more complicated disease. Both longer DD and altered MEPs are associated with worse disability.

Phosphorylation of Spastin Promotes the Surface Delivery and Synaptic Function of AMPA Receptors

Synaptic plasticity is essential for cognitive functions such as learning and memory. One of the mechanisms involved in synaptic plasticity is the dynamic delivery of AMPA receptors (AMPARs) in and out of synapses. Mutations of SPAST, which encodes SPASTIN, a microtubule-severing protein, are considered the most common cause of hereditary spastic paraparesis (HSP). In some cases, patients with HSP also manifest cognitive impairment. In addition, mice with Spastin depletion exhibit working and associative memory deficits and reduced AMPAR levels. However, the exact effect and molecular mechanism of Spastin on AMPARs trafficking has remained unclear. Here, we report that Spastin interacts with AMPAR, and phosphorylation of Spastin enhances its interaction with AMPAR subunit GluA2. Further study shows that phosphorylation of Spastin can increase AMPAR GluA2 surface expression and the amplitude and frequency of miniature excitatory synaptic currents (mEPSC) in cultured hippocampal neurons. Moreover, phosphorylation of Spastin at Ser210 is crucial for GluA2 surface expression. Phosphorylation of Spastin K353A, which obliterates microtubule-severing activity, also promotes AMPAR GluA2 subunit trafficking to the surface and increases the amplitude and frequency of mEPSCs in cultured neurons. Taken together, our data demonstrate that Spastin phosphorylation promotes the surface delivery of the AMPAR GluA2 subunit independent of microtubule dynamics.

Dictyostelium spastin is involved in nuclear envelope dynamics during semi-closed mitosis

Dictyostelium amoebae perform a semi-closed mitosis, in which the nuclear envelope is fenestrated at the insertion sites of the mitotic centrosomes and around the central spindle during karyokinesis. During late telophase the centrosome relocates to the cytoplasmic side of the nucleus, the central spindle disassembles and the nuclear fenestrae become closed. Our data indicate that Dictyostelium spastin (DdSpastin) is a microtubule-binding and severing type I membrane protein that plays a role in this process. Its mitotic localization is in agreement with a requirement for the removal of microtubules that would hinder closure of the fenestrae. Furthermore, DdSpastin interacts with the HeH/ LEM-family protein Src1 in BioID analyses as well as the inner nuclear membrane protein Sun1, and shows subcellular co-localizations with Src1, Sun1, the ESCRT component CHMP7 and the IST1-like protein filactin, suggesting that the principal pathway of mitotic nuclear envelope remodeling is conserved between animals and Dictyostelium amoebae.

Tract-specific damage at spinal cord level in pure hereditary spastic paraplegia type 4: a diffusion tensor imaging study

BACKGROUND

SPG4 is a subtype of hereditary spastic paraplegia (HSP), an upper motor neuron disorder characterized by axonal degeneration of the corticospinal tracts and the fasciculus gracilis. The few neuroimaging studies that have focused on the spinal cord in HSP are based mainly on the analysis of structural characteristics.

METHODS

We assessed diffusion-related characteristics of the spinal cord using diffusion tensor imaging (DTI), as well as structural and shape-related properties in 12 SPG4 patients and 14 controls. We used linear mixed effects models up to T3 in order to analyze the global effects of 'group' and 'clinical data' on structural and diffusion data. For DTI, we carried out a region of interest (ROI) analysis in native space for the whole spinal cord, the anterior and lateral funiculi, and the dorsal columns. We also performed a voxelwise analysis of the spinal cord to study local diffusion-related changes.

RESULTS

A reduced cross-sectional area was observed in the cervical region of SPG4 patients, with significant anteroposterior flattening. DTI analyses revealed significantly decreased fractional anisotropy (FA) and increased radial diffusivity at all the cervical and thoracic levels, particularly in the lateral funiculi and dorsal columns. The FA changes in SPG4 patients were significantly related to disease severity, measured as the Spastic Paraplegia Rating Scale score.

CONCLUSIONS

Our results in SPG4 indicate tract-specific axonal damage at the level of the cervical and thoracic spinal cord. This finding is correlated with the degree of motor disability.

Modeling gain-of-function and loss-of-function components of SPAST-based hereditary spastic paraplegia using transgenic mice

Hereditary spastic paraplegia (HSP) is a disease in which dieback degeneration of corticospinal tracts, accompanied by axonal swellings, leads to gait deficiencies. SPG4-HSP, the most common form of the disease, results from mutations of human spastin gene (SPAST), which is the gene that encodes spastin, a microtubule-severing protein. The lack of a vertebrate model that recapitulates both the etiology and symptoms of SPG4-HSP has stymied the development of effective therapies for the disease. hSPAST-C448Y mice, which express human mutant spastin at the ROSA26 locus, display corticospinal dieback and gait deficiencies but not axonal swellings. On the other hand, mouse spastin gene (Spast)-knockout (KO) mice display axonal swellings but not corticospinal dieback or gait deficiencies. One possibility is that reduced spastin function, resulting in axonal swellings, is not the cause of the disease but exacerbates the toxic effects of the mutant protein. To explore this idea, Spast-KO and hSPAST-C448Y mice were crossbred, and the offspring were compared with the parental lines via histological and behavioral analyses. The crossbred animals displayed axonal swellings as well as earlier onset, worsened gait deficiencies and corticospinal dieback compared with the hSPAST-C448Y mouse. These results, together with observations on changes in histone deacetylases 6 and tubulin modifications in the axon, indicate that each of these three transgenic mouse lines is valuable for investigating a different component of the disease pathology. Moreover, the crossbred mice are the best vertebrate model to date for testing potential therapies for SPG4-HSP.

A Novel SPAST Mutation Results in Spastin Accumulation and Defects in Microtubule Dynamics

Background

Haploinsufficiency is widely accepted as the pathogenic mechanism of spastic paraplegia type 4 (SPG4). However, there are some cases that cannot be explained by reduced function of the spastin protein encoded by SPAST.

Objectives

To identify the causative gene of autosomal dominant hereditary spastic paraplegia in three large Chinese families and explore the pathological mechanism of a spastin variant.

Methods

Three large Chinese hereditary spastic paraplegia families with a total of 247 individuals (67 patients) were investigated, of whom 59 members were recruited to the study. Genetic testing was performed to identify the causative gene. Western blotting and immunofluorescence were used to analyze the effects of the mutant proteins in vitro.

Results

In the three hereditary spastic paraplegia families, of whom three index cases were misdiagnosed as other types of neurological diseases, a novel c.985dupA (p.Met329Asnfs*3) variant in SPAST was identified and was shown to cosegregate with the phenotype in the three families. The c.985dupA mutation produced two truncated mutants (mutant M1 and M87 isoforms) that accumulated to a higher level than their wild‐type counterparts. Furthermore, the mutant M1 isoform heavily decorated the microtubules and rendered them resistant to depolymerization. In contrast, the mutant M87 isoform was diffusely localized in both the nucleus and the cytoplasm, could not decorate microtubules, and was not able to promote microtubule disassembly.

Conclusions

SPAST mutations leading to premature stop codons do not always act through haploinsufficiency. The truncated spastin may damage the corticospinal tracts through an isoform‐specific toxic effect.

Different fusion tags affect the activity of ubiquitin overexpression on spastin protein stability

Spastin is one of the proteins which lead to hereditary spastic paraplegia (HSP), whose dysfunction towards microtubule severing and membrane transporting is critically important. The present study is to elucidate the mechanisms of the protein stability regulation of spastin. The ubiquitin encoding plasmids were transfected into COS-7 cells with different fusion tags including Green Fluorescent Protein (GFP), mCherry and Flag. The expression level of spastin was detected, microtubule severing activity and neurite outgrowth were quantified. The data showed that ubiquitin overexpression significantly induced the decreased expression of spastin, suppressed the activity of microtubule severing in COS-7 cells and inhibited the promoting effect on neurite outgrowth in cultured hippocampal neurons. Furthermore, when modulating the overexpression experiments of ubiquitin, it was found that relatively small tag like Flag, but not large tags such as GFP or mCherry fused with ubiquitin, retained the activity on spastin stability. The present study investigated the effects of small/large tags addition to ubiquitin and the novel mechanisms of post-transcriptional modifications of spastin on regulating neurite outgrowth, in the attempt to experimentally elucidate the mechanisms that control the level or stability of spastin in hereditary spastic paraplegia.

A p.Arg499His mutation in SPAST is associated with infantile-onset complicated spastic paraplegia: a case report and review of the literature

Background

Spastic paraplegia type 4 (SPG4) is caused by mutations in the SPAST gene, is the most common form of autosomal-dominant pure hereditary spastic paraplegias (HSP), and is rarely associated with a complicated form that includes ataxia, epilepsy, and cognitive decline. To date, the genotype-phenotype correlation has not been substantially established for SPAST mutations.

Case presentation

We present a Japanese patient with infantile-onset HSP and a complex form with coexisting ataxia and epilepsy. The sequencing of SPAST revealed a de novo c.1496G > A (p.R499H) mutation. A review of the literature revealed 16 additional patients with p.R499H mutations in SPAST associated with an early-onset complicated form of HSP. We found that the complicated phenotype of patients with p.Arg499His mutations could be mainly divided into three subgroups: (1) infantile-onset ascending hereditary spastic paralysis, (2) HSP with severe dystonia, and (3) HSP with cognitive impairment. Moreover, the c.1496G > A mutation in SPAST may occur as a de novo variant at noticeably high rates.

Conclusion

We reviewed the clinical features of the patients reported in the literature with the p.Arg499His mutation in SPAST and described the case of a Japanese patient with this mutation presenting a new complicated form. Accumulating evidence suggests a possible association between infantile-onset complicated HSP and the p.Arg499His mutation in SPAST. The findings of this study may expand the clinical spectrum of the p.Arg499His mutation in SPAST and provide an opportunity to further study the genotype-phenotype correlation of SPG4.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12883-021-02478-0.

SNX14 deficiency-induced defective axonal mitochondrial transport in Purkinje cells underlies cerebellar ataxia and can be reversed by valproate

Loss-of-function mutations in sorting nexin 14 (SNX14) cause autosomal recessive spinocerebellar ataxia 20, which is a form of early-onset cerebellar ataxia that lacks molecular mechanisms and mouse models. We generated Snx14-deficient mouse models and observed severe motor deficits and cell-autonomous Purkinje cell degeneration. SNX14 deficiency disrupted microtubule organization and mitochondrial transport in axons by destabilizing the microtubule-severing enzyme spastin, which is implicated in dominant hereditary spastic paraplegia with cerebellar ataxia, and compromised axonal integrity and mitochondrial function. Axonal transport disruption and mitochondrial dysfunction further led to degeneration of high-energy-demanding Purkinje cells, which resulted in the pathogenesis of cerebellar ataxia. The antiepileptic drug valproate ameliorated motor deficits and cerebellar degeneration in Snx14-deficient mice via the restoration of mitochondrial transport and function in Purkinje cells. Our study revealed an unprecedented role for SNX14-dependent axonal transport in cerebellar ataxia, demonstrated the convergence of SNX14 and spastin in mitochondrial dysfunction, and suggested valproate as a potential therapeutic agent.

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

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

Therapeutic Strategies for Mutant SPAST-Based Hereditary Spastic Paraplegia

Mutations of the SPAST gene that encodes the microtubule-severing enzyme called spastin are the chief cause of Hereditary Spastic Paraplegia. Growing evidence indicates that pathogenic mutations functionally compromise the spastin protein and endow it with toxic gain-of-function properties. With each of these two factors potentially relevant to disease etiology, the present article discusses possible therapeutic strategies that may ameliorate symptoms in patients suffering from SPAST-based Hereditary Spastic Paraplegia, which is usually termed SPG4-HSP.