Spastin and microtubules: Functions in health and disease

Pluripotent Stem Cells as a Preclinical Cellular Model for Studying Hereditary Spastic Paraplegias

Hereditary spastic paraplegias (HSPs) comprise a family of degenerative diseases mostly hitting descending axons of corticospinal neurons. Depending on the gene and mutation involved, the disease could present as a pure form with limb spasticity, or a complex form associated with cerebellar and/or cortical signs such as ataxia, dysarthria, epilepsy, and intellectual disability. The progressive nature of HSPs invariably leads patients to require walking canes or wheelchairs over time. Despite several attempts to ameliorate the life quality of patients that have been tested, current therapeutical approaches are just symptomatic, as no cure is available. Progress in research in the last two decades has identified a vast number of genes involved in HSP etiology, using cellular and animal models generated on purpose. Although unanimously considered invaluable tools for basic research, those systems are rarely predictive for the establishment of a therapeutic approach. The advent of induced pluripotent stem (iPS) cells allowed instead the direct study of morphological and molecular properties of the patient's affected neurons generated upon in vitro differentiation. In this review, we revisited all the present literature recently published regarding the use of iPS cells to differentiate HSP patient-specific neurons. Most studies have defined patient-derived neurons as a reliable model to faithfully mimic HSP in vitro, discovering original findings through immunological and -omics approaches, and providing a platform to screen novel or repurposed drugs. Thereby, one of the biggest hopes of current HSP research regards the use of patient-derived iPS cells to expand basic knowledge on the disease, while simultaneously establishing new therapeutic treatments for both generalized and personalized approaches in daily medical practice.

14-3-3 protein augments the protein stability of phosphorylated spastin and promotes the recovery of spinal cord injury through its agonist intervention

Axon regeneration is abortive in the central nervous system following injury. Orchestrating microtubule dynamics has emerged as a promising approach to improve axonal regeneration. The microtubule severing enzyme spastin is essential for axonal development and regeneration through remodeling of microtubule arrangement. To date, however, little is known regarding the mechanisms underlying spastin action in neural regeneration after spinal cord injury. Here, we use glutathione transferase pulldown and immunoprecipitation assays to demonstrate that 14-3-3 interacts with spastin, both in vivo and in vitro, via spastin Ser233 phosphorylation. Moreover, we show that 14-3-3 protects spastin from degradation by inhibiting the ubiquitination pathway and upregulates the spastin-dependent severing ability. Furthermore, the 14-3-3 agonist Fusicoccin (FC-A) promotes neurite outgrowth and regeneration in vitro which needs spastin activation. Western blot and immunofluorescence results revealed that 14-3-3 protein is upregulated in the neuronal compartment after spinal cord injury in vivo. In addition, administration of FC-A not only promotes locomotor recovery, but also nerve regeneration following spinal cord injury in both contusion and lateral hemisection models; however, the application of spastin inhibitor spastazoline successfully reverses these phenomena. Taken together, these results indicate that 14-3-3 is a molecular switch that regulates spastin protein levels, and the small molecule 14-3-3 agonist FC-A effectively mediates the recovery of spinal cord injury in mice which requires spastin participation.

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.

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.

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.

Degradation of MinD oscillator complexes by Escherichia coli ClpXP

MinD is a cell division ATPase in Escherichia coli that oscillates from pole to pole and regulates the spatial position of the cell division machinery. Together with MinC and MinE, the Min system restricts assembly of the FtsZ-ring to midcell, oscillating between the opposite ends of the cell and preventing FtsZ-ring misassembly at the poles. Here, we show that the ATP-dependent bacterial proteasome complex ClpXP degrades MinD in reconstituted degradation reactions in vitro and in vivo through direct recognition of the MinD N-terminal region. MinD degradation is enhanced during stationary phase, suggesting that ClpXP regulates levels of MinD in cells that are not actively dividing. ClpXP is a major regulator of growth phase–dependent proteins, and these results suggest that MinD levels are also controlled during stationary phase. In vitro, MinC and MinD are known to coassemble into linear polymers; therefore, we monitored copolymers assembled in vitro after incubation with ClpXP and observed that ClpXP promotes rapid MinCD copolymer destabilization and direct MinD degradation by ClpXP. The N terminus of MinD, including residue Arg 3, which is near the ATP-binding site in sequence, is critical for degradation by ClpXP. Together, these results demonstrate that ClpXP degradation modifies conformational assemblies of MinD in vitro and depresses Min function in vivo during periods of reduced proliferation.

Anterograde Axonal Transport in Neuronal Homeostasis and Disease

Neurons are highly polarized cells with an elongated axon that extends far away from the cell body. To maintain their homeostasis, neurons rely extensively on axonal transport of membranous organelles and other molecular complexes. Axonal transport allows for spatio-temporal activation and modulation of numerous molecular cascades, thus playing a central role in the establishment of neuronal polarity, axonal growth and stabilization, and synapses formation. Anterograde and retrograde axonal transport are supported by various molecular motors, such as kinesins and dynein, and a complex microtubule network. In this review article, we will primarily discuss the molecular mechanisms underlying anterograde axonal transport and its role in neuronal development and maturation, including the establishment of functional synaptic connections. We will then provide an overview of the molecular and cellular perturbations that affect axonal transport and are often associated with axonal degeneration. Lastly, we will relate our current understanding of the role of axonal trafficking concerning anterograde trafficking of mRNA and its involvement in the maintenance of the axonal compartment and disease.

Lipid metabolic pathways converge in motor neuron degenerative diseases

Motor neuron diseases (MNDs) encompass an extensive and heterogeneous group of upper and/or lower motor neuron degenerative disorders, in which the particular clinical outcomes stem from the specific neuronal component involved in each condition. While mutations in a large number of molecules associated with lipid metabolism are known to be implicated in MNDs, there remains a lack of clarity regarding the key functional pathways involved, and their inter-relationships. This review highlights evidence that defines defects within two specific lipid (cholesterol/oxysterol and phosphatidylethanolamine) biosynthetic cascades as being centrally involved in MND, particularly hereditary spastic paraplegia. We also identify how other MND-associated molecules may impact these cascades, in particular through impaired organellar interfacing, to propose ‘subcellular lipidome imbalance’ as a likely common pathomolecular theme in MND. Further exploration of this mechanism has the potential to identify new therapeutic targets and management strategies for modulation of disease progression in hereditary spastic paraplegias and other MNDs.

A novel SPAST gene mutation identified in a Chinese family with hereditary spastic paraplegia

Background

Hereditary spastic paraplegia is a heterogeneous group of clinically and genetically neurodegenerative diseases characterized by progressive gait disorder. Hereditary spastic paraplegia can be inherited in various ways, and all modes of inheritance are associated with multiple genes or loci. At present, more than 76 disease-causing loci have been identified in hereditary spastic paraplegia patients. Here, we report a novel mutation in SPAST gene associated with hereditary spastic paraplegia in a Chinese family, further enriching the hereditary spastic paraplegia spectrum.

Methods

Whole genomic DNA was extracted from peripheral blood of the 15 subjects from a Chinese family using DNA Isolation Kit. The Whole Exome Sequencing of the proband was analyzed and the result was identified in the rest individuals. RaptorX prediction tool and Protein Variation Effect Analyzer were used to predict the effects of the mutation on protein tertiary structure and function.

Results

Spastic paraplegia has been inherited across at least four generations in this family, during which only four HSP patients were alive. The results obtained by analyzing the Whole Exome Sequencing of the proband exhibited a novel disease-associated in-frame deletion in the SPAST gene, and this mutation also existed in the rest three HSP patients in this family. This in-frame deletion consists of three nucleotides deletion (c.1710_1712delGAA) within the exon 16, resulting in lysine deficiency at the position 570 of the protein (p.K570del). This novel mutation was also predicted to result in the synthesis of misfolded SPAST protein and have the deleterious effect on the function of SPAST protein.

Conclusion

In this case, we reported a novel mutation in the known SPAST gene that segregated with HSP disease, which can be inherited in each generation. Simultaneously, this novel discovery significantly enriches the mutation spectrum, which provides an opportunity for further investigation of genetic pathogenesis of HSP.

Mutant spastin proteins promote deficits in axonal transport through an isoform-specific mechanism involving casein kinase 2 activation

Mutations of various genes cause hereditary spastic paraplegia (HSP), a neurological disease involving dying-back degeneration of upper motor neurons. From these, mutations in the SPAST gene encoding the microtubule-severing protein spastin account for most HSP cases. Cumulative genetic and experimental evidence suggests that alterations in various intracellular trafficking events, including fast axonal transport (FAT), may contribute to HSP pathogenesis. However, the mechanisms linking SPAST mutations to such deficits remain largely unknown. Experiments presented here using isolated squid axoplasm reveal inhibition of FAT as a common toxic effect elicited by spastin proteins with different HSP mutations, independent of microtubule-binding or severing activity. Mutant spastin proteins produce this toxic effect only when presented as the tissue-specific M1 isoform, not when presented as the ubiquitously-expressed shorter M87 isoform. Biochemical and pharmacological experiments further indicate that the toxic effects of mutant M1 spastins on FAT involve casein kinase 2 (CK2) activation. In mammalian cells, expression of mutant M1 spastins, but not their mutant M87 counterparts, promotes abnormalities in the distribution of intracellular organelles that are correctable by pharmacological CK2 inhibition. Collectively, these results demonstrate isoform-specific toxic effects of mutant M1 spastin on FAT, and identify CK2 as a critical mediator of these effects.

The role of ESCRT during development and functioning of the nervous system

The endosomal sorting complex required for transport (ESCRT) is made of subcomplexes (ESCRT 0-III), crucial to membrane remodelling at endosomes, nuclear envelope and cell surface. ESCRT-III shapes membranes and in most cases cooperates with the ATPase VPS4 to mediate fission of membrane necks from the inside. The first ESCRT complexes mainly serve to catalyse the formation of ESCRT-III but can be bypassed by accessory proteins like the Alg-2 interacting protein-X (ALIX). In the nervous system, ALIX/ESCRT controls the survival of embryonic neural progenitors and later on the outgrowth and pruning of axons and dendrites, all necessary steps to establish a functional brain. In the adult brain, ESCRTs allow the endosomal turn over of synaptic vesicle proteins while stable ESCRT complexes might serve as scaffolds for the postsynaptic parts. The necessity of ESCRT for the harmonious function of the brain has its pathological counterpart, the mutations in CHMP2B of ESCRT-III giving rise to several neurodegenerative diseases.

α-Synuclein is a Novel Microtubule Dynamase

α-Synuclein is a presynaptic protein associated to Parkinson’s disease, which is unstructured when free in the cytoplasm and adopts α helical conformation when bound to vesicles. After decades of intense studies, α-Synuclein physiology is still difficult to clear up due to its interaction with multiple partners and its involvement in a pletora of neuronal functions. Here, we looked at the remarkably neglected interplay between α-Synuclein and microtubules, which potentially impacts on synaptic functionality. In order to identify the mechanisms underlying these actions, we investigated the interaction between purified α-Synuclein and tubulin. We demonstrated that α-Synuclein binds to microtubules and tubulin α₂β₂ tetramer; the latter interaction inducing the formation of helical segment(s) in the α-Synuclein polypeptide. This structural change seems to enable α-Synuclein to promote microtubule nucleation and to enhance microtubule growth rate and catastrophe frequency, both in vitro and in cell. We also showed that Parkinson’s disease-linked α-Synuclein variants do not undergo tubulin-induced folding and cause tubulin aggregation rather than polymerization. Our data enable us to propose α-Synuclein as a novel, foldable, microtubule-dynamase, which influences microtubule organisation through its binding to tubulin and its regulating effects on microtubule nucleation and dynamics.

Impairments in dendrite morphogenesis as etiology for neurodevelopmental disorders and implications for therapeutic treatments

Dendrite morphology is pivotal for neural circuitry functioning. While the causative relationship between small-scale dendrite morphological abnormalities (shape, density of dendritic spines) and neurodevelopmental disorders is well established, such relationship remains elusive for larger-scale dendrite morphological impairments (size, shape, branching pattern of dendritic trees). Here, we summarize published data on dendrite morphological irregularities in human patients and animal models for neurodevelopmental disorders, with focus on autism and schizophrenia. We next discuss high-risk genes for these disorders and their role in dendrite morphogenesis. We finally overview recent developments in therapeutic attempts and we discuss how they relate to dendrite morphology. We find that both autism and schizophrenia are accompanied by dendritic arbor morphological irregularities, and that majority of their high-risk genes regulate dendrite morphogenesis. Thus, we present a compelling argument that, along with smaller-scale morphological impairments in dendrites (spines and synapse), irregularities in larger-scale dendrite morphology (arbor shape, size) may be an important part of neurodevelopmental disorders' etiology. We suggest that this should not be ignored when developing future therapeutic treatments.

Serum Autoantibodies in Chronic Prostate Inflammation in Prostate Cancer Patients

Background

Chronic inflammation is frequently observed on histological analysis of malignant and non-malignant prostate specimens. It is a suspected supporting factor for prostate diseases and their progression and a main cause of false positive PSA tests in cancer screening. We hypothesized that inflammation induces autoantibodies, which may be useful biomarkers. We aimed to identify and validate prostate inflammation associated serum autoantibodies in prostate cancer patients and evaluate the expression of corresponding autoantigens.

Methods

Radical prostatectomy specimens of prostate cancer patients (N = 70) were classified into high and low inflammation groups according to the amount of tissue infiltrating lymphocytes. The corresponding pre-surgery blood serum samples were scrutinized for autoantibodies using a low-density protein array. Selected autoantigens were identified in prostate tissue and their expression pattern analyzed by immunohistochemistry and qPCR. The identified autoantibody profile was cross-checked in an independent sample set (N = 63) using the Luminex-bead protein array technology.

Results

Protein array screening identified 165 autoantibodies differentially abundant in the serum of high compared to low inflammation patients. The expression pattern of three corresponding antigens were established in benign and cancer tissue by immunohistochemistry and qPCR: SPAST (Spastin), STX18 (Syntaxin 18) and SPOP (speckle-type POZ protein). Of these, SPAST was significantly increased in prostate tissue with high inflammation. All three autoantigens were differentially expressed in primary and/or castration resistant prostate tumors when analyzed in an inflammation-independent tissue microarray. Cross-validation of the inflammation autoantibody profile on an independent sample set using a Luminex-bead protein array, retrieved 51 of the significantly discriminating autoantibodies. Three autoantibodies were significantly upregulated in both screens, MUT, RAB11B and CSRP2 (p>0.05), two, SPOP and ZNF671, close to statistical significance (p = 0.051 and 0.076).

Conclusions

We provide evidence of an inflammation-specific autoantibody profile and confirm the expression of corresponding autoantigens in prostate tissue. This supports evaluation of autoantibodies as non-invasive markers for prostate inflammation.

Conserved pharmacological rescue of hereditary spastic paraplegia-related phenotypes across model organisms

Hereditary spastic paraplegias (HSPs) are a group of neurodegenerative diseases causing progressive gait dysfunction. Over 50 genes have now been associated with HSP. Despite the recent explosion in genetic knowledge, HSP remains without pharmacological treatment. Loss-of-function mutation of the SPAST gene, also known as SPG4, is the most common cause of HSP in patients. SPAST is conserved across animal species and regulates microtubule dynamics. Recent studies have shown that it also modulates endoplasmic reticulum (ER) stress. Here, utilizing null SPAST homologues in C. elegans, Drosophila and zebrafish, we tested FDA-approved compounds known to modulate ER stress in order to ameliorate locomotor phenotypes associated with HSP. We found that locomotor defects found in all of our spastin models could be partially rescued by phenazine, methylene blue, N-acetyl-cysteine, guanabenz and salubrinal. In addition, we show that established biomarkers of ER stress levels correlated with improved locomotor activity upon treatment across model organisms. Our results provide insights into biomarkers and novel therapeutic avenues for HSP.

Tau missorting and spastin-induced microtubule disruption in neurodegeneration: Alzheimer Disease and Hereditary Spastic Paraplegia

In Alzheimer Disease (AD), the mechanistic connection of the two major pathological hallmarks, namely deposition of Amyloid-beta (Aβ) in the form of extracellular plaques, and the pathological changes of the intracellular protein Tau (such as phosphorylation, missorting, aggregation), is not well understood. Genetic evidence from AD and Down Syndrome (Trisomy 21), and animal models thereof, suggests that aberrant production of Aβ is upstream of Tau aggregation, but also points to Tau as a critical effector in the pathological process. Yet, the cascade of events leading from increased levels of Aβ to Tau-dependent toxicity remains a matter of debate.

Using primary neurons exposed to oligomeric forms of Aβ, we have found that Tau becomes mislocalized (missorted) into the somatodendritic compartment. Missorting of Tau correlates with loss of microtubules and downstream consequences such as loss of mature spines, loss of synaptic activity, and mislocalization of mitochondria.

In this cascade, missorting of Tau induces mislocalization of TTLL6 (Tubulin-Tyrosine-Ligase-Like 6) into the dendrites. TTLL6 induces polyglutamylation of microtubules, which acts as a trigger for spastin mediated severing of dendritic microtubules. Loss of microtubules makes cells unable to maintain transport of mitochondria, which in turn results in synaptic dysfunction and loss of mature spines. These pathological changes are absent in TauKO derived primary neurons. Thus, Tau mediated mislocalization of TTLL6 and spastin activation reveals a pathological gain of function for Tau and spastin in this cellular model system of AD.

In contrast, in hereditary spastic paraplegia (HSP) caused by mutations of the gene encoding spastin (spg4 alias SPAST), spastin function in terms of microtubule severing is decreased at least for the gene product of the mutated allele, resulting in overstable microtubules in disease model systems. Whether total spastin severing activity or microtubule stability in human disease is also affected is not yet clear. No human disease has been associated so far with the long-chain polyglutamylation enzyme TTLL6, or the other TTLLs (1,5,11) possibly involved.

Here we review the findings supporting a role for Tau, spastin and TTLL6 in AD and other tauopathies, HSP and neurodegeneration, and summarize possible therapeutic approaches for AD and HSP.

Importance of gene dosage in controlling dendritic arbor formation during development

Proper dendrite morphology is crucial for normal nervous system functioning. While a number of genes have been implicated in dendrite morphogenesis in both invertebrates and mammals, it remains unclear how developing dendrites respond to changes in gene dosage and what type of patterns their responses may follow. To understand this, I review here evidence from the recent literature, focusing on the genetic studies performed in the Drosophila larval dendritic arborization class IV neuron, an excellent cell type to understand dendrite morphogenesis. I summarize how class IV arbors change morphology in response to developmental fluctuations in the expression levels of 47 genes, studied by means of genetic manipulations such as loss-of-function and gain-of-function, and for which sufficient information is available. I find that arbors can respond to changing gene dosage in several distinct ways, each characterized by a singular dose-response curve. Interestingly, in 72% of cases arbors are sensitive, and thus adjust their morphology, in response to both decreases and increases in the expression of a given gene, indicating that dendrite morphogenesis is a process particularly sensitive to gene dosage. By summarizing the parallels between Drosophila and mammals, I show that many Drosophila dendrite morphogenesis genes have orthologs in mammals, and that some of these are associated with mammalian dendrite outgrowth and human neurodevelopmental disorders. One notable disease-related molecule is kinase Dyrk1A, thought to be a causative factor in Down syndrome. Both increases and decreases in Dyrk1A gene dosage lead to impaired dendrite morphogenesis, which may contribute to Down syndrome pathoetiology.

Multimodal MRI-based study in patients with SPG4 mutations

Mutations in the SPG4 gene (SPG4-HSP) are the most frequent cause of hereditary spastic paraplegia, but the extent of the neurodegeneration related to the disease is not yet known. Therefore, our objective is to identify regions of the central nervous system damaged in patients with SPG4-HSP using a multi-modal neuroimaging approach. In addition, we aimed to identify possible clinical correlates of such damage. Eleven patients (mean age 46.0 ± 15.0 years, 8 men) with molecular confirmation of hereditary spastic paraplegia, and 23 matched healthy controls (mean age 51.4 ± 14.1years, 17 men) underwent MRI scans in a 3T scanner. We used 3D T1 images to perform volumetric measurements of the brain and spinal cord. We then performed tract-based spatial statistics and tractography analyses of diffusion tensor images to assess microstructural integrity of white matter tracts. Disease severity was quantified with the Spastic Paraplegia Rating Scale. Correlations were then carried out between MRI metrics and clinical data. Volumetric analyses did not identify macroscopic abnormalities in the brain of hereditary spastic paraplegia patients. In contrast, we found extensive fractional anisotropy reduction in the corticospinal tracts, cingulate gyri and splenium of the corpus callosum. Spinal cord morphometry identified atrophy without flattening in the group of patients with hereditary spastic paraplegia. Fractional anisotropy of the corpus callosum and pyramidal tracts did correlate with disease severity. Hereditary spastic paraplegia is characterized by relative sparing of the cortical mantle and remarkable damage to the distal portions of the corticospinal tracts, extending into the spinal cord.

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

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

Identification and characterization of a neuron-specific isoform of protrudin

Protrudin is a membrane protein that regulates polarized vesicular transport. Now, we have identified a novel isoform of protrudin (protrudin-L) that contains an additional seven amino acids between the FFAT motif and the coiled-coil domain compared with the conventional isoform (protrudin-S) as a result of alternative splicing of a microexon (exon L). Protrudin-L mRNA was found to be mostly restricted to the central nervous system in mice, whereas protrudin-S mRNA was detected in all tissues examined. With the use of a splicing reporter minigene that produces two distinct fluorescent proteins in a manner dependent on the splicing pattern of protrudin transcripts, we found that most neurons express protrudin-L, whereas astrocytes express both protrudin isoforms and oligodendrocytes express only protrudin-S. Protrudin-L associated to a greater extent with vesicle-associated membrane protein-associated protein (VAP) than protrudin-S. Expression of protrudin-L in hippocampal neurons of protrudin-deficient mice also promoted neurite outgrowth more efficiently than protrudin-S. Our results suggest that protrudin-L is a neuron-specific protrudin isoform that promotes axonal elongation and contributes to the establishment of neuronal polarity.

Microtubules: greater than the sum of the parts

The post-genomic era has produced a variety of new investigation technologies, techniques and approaches that may offer exciting insights into many long-standing questions of scientific research. The microtubule cytoskeleton is a highly conserved system that shows a high degree of internal complexity, is known to be integral to many cell systems and functions on a fundamental level. After decades of study, much is still unknown about microtubules in vivo from the control of dynamics in living cells to their responses to environmental changes and responses to other cellular processes. In the present article, we examine some outstanding questions in the microtubule field and propose a combination of emerging interdisciplinary approaches, i.e. high-throughput functional genomics techniques, quantitative and super-resolution microscopy, and in silico modelling, that could shed light on the systemic regulation of microtubules in cells by networks of regulatory factors. We propose that such an integrative approach is key to elucidate the function of the microtubule cytoskeleton as a complete responsive integral biological system.

Striatal dopaminergic functioning in patients with sporadic and hereditary spastic paraplegias with parkinsonism

Sporadic spastic paraplegia (SSP) and hereditary spastic paraplegia (HSP) belong to a clinical and genetically heterogeneous group of disorders characterized by progressive spasticity and weakness in the lower extremities. The symptoms are associated with pyramidal tract dysfunction and degeneration of the corticospinal tracts. Parkinsonism is uncommon in SSP/HSP patients. However, both disorders are associated with damage to the nigrostriatal dopaminergic system. In the present study, the clinical features of patients with SSP/HSP were investigated, and nigrostriatal dopaminergic binding potential was assessed using dopamine transporter (DAT) single-photon emission computer tomography (SPECT). Nine patients with spastic paraplegia participated in the present study. The subjects underwent DAT SPECT using the agent [2-[[2-[[[3-(4-chlorophenyl)-8-methyl-8-azabicyclo[3,2,1]oct-2-yl]methyl](2-mercaptoethyl)amino]ethyl]amino]ethanethiolato (3-)-N2,N20,S2,S20]oxo-[IR-(exo-exo)])-[(99)mTc]technetium ([(99)mTc]TRODAT-1). The [(99)mTc]TRODAT-1 SPECT images of five patients appeared normal, whereas the images of four patients revealed reduced striatal ligand uptake. Among the four patients with reduced uptake, two had parkinsonism, and one exhibited periodic limb movements and restless leg syndrome. Our DAT SPECT imaging study shows that reduced DAT density may be observed in patients with parkinsonism. The results of the present study offer an explanation for the spectrum of spastic paraplegia symptoms and the progression of the disorder.

Detection of microtubules in vivo using antibody-immobilized nanoneedles

We present here an alternative, force-based measurement method for the detection of intracellular cytoskeletal proteins in the live cell. High aspect ratio nanoneedles of 200 nm in diameter were functionalized with anti-tubulin antibodies and inserted, using an atomic force microscope (AFM), into live NIH3T3 cells, without affecting cell viability. Force curves were recorded during insertion and evacuation of nanoneedles from the cells, and used to analyse intracellular interactions of the nanoneedles with the microtubule cytoskeleton during evacuation from the cell. Disruption of microtubules led to a correlated time-dependent decrease in the measured intracellular binding forces, pointing to the high-sensitivity and high-specificity of this detection method. This analytical technique allows for real-time evaluation of the microtubule network in the live cell, without the need to use potentially harmful molecular markers as do conventional detection methods, and may prove beneficial in the diagnosis and investigation of cytoskeleton-associated diseases.

The nucleotide cycle of spastin correlates with its microtubule-binding properties

Spastin is an AAA (ATPase associated with diverse cellular activities) protein with microtubule (MT)-severing activity. The spastin-encoding gene was identified as the most often mutated gene in the human neurodegenerative disease hereditary spastic paraplegia. Although the structure of the AAA domain of spastin has been determined, the mechanism by which spastin severs MTs remains elusive. Here, we studied the MT-binding and nucleotide-binding properties of spastin, as well as their interplay. The results suggest that ATP-bound spastin interacts strongly and cooperatively with MTs; this interaction stimulates ATP hydrolysis by spastin. After ATP hydrolysis, spastin dissociates from MTs, and then exchanges ADP for ATP in solution for the next round of work. In particular, spastin in the ternary complex of MT-spastin-ATP is the most cooperative state during the working cycle, and is probably the force-generating state that is responsible for MT severing. The results presented in this study establish the nucleotide cycle of spastin in correlation with its MT-binding properties, and provide a biochemical framework for further studies of the working mechanism of spastin.

Autoantibodies to Non-myelin Antigens as Contributors to the Pathogenesis of Multiple Sclerosis

For years, investigators have sought to prove that myelin antigens are the primary targets of autoimmunity in multiple sclerosis (MS). Recent experiments have begun to challenge this assumption, particularly when studying the neurodegenerative phase of MS. T-lymphocyte responses to myelin antigens have been extensively studied, and are likely early contributors to the pathogenesis of MS. Antibodies to myelin antigens have a much more inconstant association with the pathogenesis of MS. Recent studies indicate that antibodies to non-myelin antigens such as neurofilaments, neurofascin, RNA binding proteins and potassium channels may contribute to the pathogenesis of MS. The purpose of this review is to analyze recent studies that examine the role that autoantibodies to non-myelin antigens might play in the pathogenesis of MS.

Hereditary Spastic Paraplegia with a Novel SPAST Mutation Misdiagnosed with Subacute Combined Degeneration

Autosomal dominant hereditary spastic paraplegia (AD-HSP) is due to mutations in the "spastin" gene (SPAST gene) encoding the AAA protein. The main clinical features of "pure" HSP are progressive lower-limb spasticity with corticospinal tracts and dorsal column degeneration without peripheral neuropathy. Here we report the case of HSP with novel SPAST gene mutation that misdiagnosed with subacute combined degeneration initially. A 58-year-old man with gait disturbance came to our hospital. He was unable to regulate his steps by himself. The impaired gait began 3 years after he had undergone subtotal gastrectomy and chemotherapy for 6 months. Thereafter, he started feeling tingling sensations in the hands and feet and acquired gait difficulties. He denied having a family history of abnormal gait or developmental problem. We diagnosed him with subacute combined degeneration on the evidence of history of gastrectomy, lower normal limit of vitamin B12 (363 pg/ml), apparent absence of vibration sensations and paresthesia in the feet. He was intramuscularly administered cyanocobalamin regularly. However, there was no improvement in his condition. We reconsidered his symptoms and signs, decided to examine the SPAST gene, which is the most common mutation in HSP. The SPAST gene, c.870+1delG, heterozygote, splicing mutation is detected from the gene sample. There was no previous information of this polymorphism or mutation at this locus. We examined his two children, and the same mutation was founded in his son. We report a patient of novel SPAST gene mutation with AD-HSP which is misdiagnosed with SCD.

A patient-derived stem cell model of hereditary spastic paraplegia with SPAST mutations

Hereditary spastic paraplegia (HSP) leads to progressive gait disturbances with lower limb muscle weakness and spasticity. Mutations in SPAST are a major cause of adult-onset, autosomal-dominant HSP. Spastin, the protein encoded by SPAST, is a microtubule-severing protein that is enriched in the distal axon of corticospinal motor neurons, which degenerate in HSP patients. Animal and cell models have identified functions of spastin and mutated spastin but these models lack the gene dosage, mutation variability and genetic background that characterize patients with the disease. In this study, this genetic variability is encompassed by comparing neural progenitor cells derived from biopsies of the olfactory mucosa from healthy controls with similar cells from HSP patients with SPAST mutations, in order to identify cell functions altered in HSP. Patient-derived cells were similar to control-derived cells in proliferation and multiple metabolic functions but had major dysregulation of gene expression, with 57% of all mRNA transcripts affected, including many associated with microtubule dynamics. Compared to control cells, patient-derived cells had 50% spastin, 50% acetylated α-tubulin and 150% stathmin, a microtubule-destabilizing enzyme. Patient-derived cells were smaller than control cells. They had altered intracellular distributions of peroxisomes and mitochondria and they had slower moving peroxisomes. These results suggest that patient-derived cells might compensate for reduced spastin, but their increased stathmin expression reduced stabilized microtubules and altered organelle trafficking. Sub-nanomolar concentrations of the microtubule-binding drugs, paclitaxel and vinblastine, increased acetylated α-tubulin levels in patient cells to control levels, indicating the utility of this cell model for screening other candidate compounds for drug therapies.

Pathogenic mechanisms of neurodegeneration based on the phenotypic expression of progressive forms of immune-mediated neurologic disease

Considering there are no treatments for progressive forms of multiple sclerosis (MS), a comprehensive understanding of the role of neurodegeneration in the pathogenesis of MS should lead to novel therapeutic strategies to treat it. Many studies have implicated viral triggers as a cause of MS, yet no single virus has been exclusively shown to cause MS. Given this, human and animal viral models of MS are used to study its pathogenesis. One example is human T-lymphotropic virus type 1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Importantly, HAM/TSP is similar clinically, pathologically, and immunologically to progressive MS. Interestingly, both MS and HAM/TSP patients were found to make antibodies to heterogeneous nuclear ribonucleoprotein (hnRNP) A1, an RNA-binding protein overexpressed in neurons. Anti-hnRNP A1 antibodies reduced neuronal firing and caused neurodegeneration in neuronal cell lines, suggesting the autoantibodies are pathogenic. Further, microarray analyses of neurons exposed to anti-hnRNP A1 antibodies revealed novel pathways of neurodegeneration related to alterations of RNA levels of the spinal paraplegia genes (SPGs). Mutations in SPGs cause hereditary spastic paraparesis, genetic disorders clinically indistinguishable from progressive MS and HAM/TSP. Thus, there is a strong association between involvement of SPGs in neurodegeneration and the clinical phenotype of progressive MS and HAM/TSP patients, who commonly develop spastic paraparesis. Taken together, these data begin to clarify mechanisms of neurodegeneration related to the clinical presentation of patients with chronic immune-mediated neurological disease of the central nervous system, which will give insights into the design of novel therapies to treat these neurological diseases.

An essential role for katanin p80 and microtubule severing in male gamete production

Katanin is an evolutionarily conserved microtubule-severing complex implicated in multiple aspects of microtubule dynamics. Katanin consists of a p60 severing enzyme and a p80 regulatory subunit. The p80 subunit is thought to regulate complex targeting and severing activity, but its precise role remains elusive. In lower-order species, the katanin complex has been shown to modulate mitotic and female meiotic spindle dynamics and flagella development. The in vivo function of katanin p80 in mammals is unknown. Here we show that katanin p80 is essential for male fertility. Specifically, through an analysis of a mouse loss-of-function allele (the Taily line), we demonstrate that katanin p80, most likely in association with p60, has an essential role in male meiotic spindle assembly and dissolution and the removal of midbody microtubules and, thus, cytokinesis. Katanin p80 also controls the formation, function, and dissolution of a microtubule structure intimately involved in defining sperm head shaping and sperm tail formation, the manchette, and plays a role in the formation of axoneme microtubules. Perturbed katanin p80 function, as evidenced in the Taily mouse, results in male sterility characterized by decreased sperm production, sperm with abnormal head shape, and a virtual absence of progressive motility. Collectively these data demonstrate that katanin p80 serves an essential and evolutionarily conserved role in several aspects of male germ cell development.

Microtubules are critical components of cells, acting as a “scaffold” for the movement of organelles and proteins within the cytoplasm. The control of microtubule length, number, and movement is essential for many cellular processes, including division, architecture, and migration. We have defined the role of the microtubule severing protein katanin p80 in male germ cell development. Male mice carrying a point mutation in the p80 gene are sterile as a consequence of low numbers of sperm, abnormal sperm morphology, and poor motility (ability to “swim”). We show that this mutation is associated with defects in microtubule structures involved in the division of immature sperm cells, in structures that shape the sperm head, and in the sperm tail, which is essential for sperm movement in the female reproductive tract. This study is the first to show that katanin p80, via its effects on microtubule dynamics within the testis, is required for male fertility.

Molecular and genetic analysis of the Drosophila model of fragile X syndrome

The Drosophila genome contains most genes known to be involved in heritable disease. The extraordinary genetic malleability of Drosophila, coupled to sophisticated imaging, electrophysiology, and behavioral paradigms, has paved the way for insightful mechanistic studies on the causes of developmental and neurological disease as well as many possible interventions. Here, we focus on one of the most advanced examples of Drosophila genetic disease modeling, the Drosophila model of Fragile X Syndrome, which for the past decade has provided key advances into the molecular, cellular, and behavioral defects underlying this devastating disorder. We discuss the multitude of RNAs and proteins that interact with the disease-causing FMR1 gene product, whose function is conserved from Drosophila to human. In turn, we consider FMR1 mechanistic relationships in non-neuronal tissues (germ cells and embryos), peripheral motor and sensory circuits, and central brain circuits involved in circadian clock activity and learning/memory.

Protrudin serves as an adaptor molecule that connects KIF5 and its cargoes in vesicular transport during process formation

Neurons are highly polarized cells with long neurites. Vesicular transport is required for neurite extension. We recently identified protrudin as a key regulator of vesicular transport during neurite extension. Expression of protrudin in nonneuronal cells thus induces formation of neurite-like membrane protrusions. We adopted a proteomics approach to identify proteins that associate with protrudin. Among the protrudin-associated proteins, including many with a function related to intracellular trafficking, we focused on KIF5, a motor protein that mediates anterograde vesicular transport in neurons. A coimmunoprecipitation assay confirmed that endogenous protrudin and KIF5 interact in mouse brain. Overexpression of KIF5 induced the formation of membrane protrusions in HeLa cells, reminiscent of the effect of protrudin overexpression. Forced expression of both protrudin and KIF5 promoted protrusion extension in a synergistic manner, whereas depletion of either protein attenuated protrusion formation. Protrudin facilitated the interaction of KIF5 with Rab11, VAP-A and -B, Surf4, and RTN3, suggesting that protrudin serves as an adaptor protein and that the protrudin-KIF5 complex contributes to the transport of these proteins in neurons. Given that mutation of protrudin or KIF5 is a cause of human hereditary spastic paraplegia, the protrudin-KIF5 axis appears to be integral to neuronal function.

Detection of novel mutations and review of published data suggests that hereditary spastic paraplegia caused by spastin (SPAST) mutations is found more often in males

BACKGROUND

Hereditary spastic paraplegia (HSP) is characterised in its pure form by slowly progressive spastic paraparesis. Around 40% of autosomal dominant (AD) cases are caused by mutations in SPAST, encoding spastin.

PATIENTS AND METHODS

The clinical and investigation details of all patients with a SPAST mutation identified through our centre were reviewed. All published reports of SPAST mutations where the sex of patients was given were subsequently analysed in order to determine whether there is evidence of one sex being preferentially affected.

RESULTS

In total 22 probable pathogenic changes were detected, including 11 novel ones. One patient carried two adjacent missense mutations. The pathogenicity of a further novel missense mutation is uncertain. Most patients had a pure phenotype, although mild peripheral neuropathy was sometimes present. The total number of patients with SPAST mutations was 27, as three of the previously known mutations were present in more than one person. The excess of males over females in our population (17:10) prompted us to review all published studies where the sex of the patients was given (n=31). A significant excess of males was identified (ratio 1.29, p=0.0007).

CONCLUSIONS

Our results are consistent with data suggesting that SPAST mutations mostly cause a pure HSP phenotype. The excess of males in our sample and in published reports suggests that penetrance or severity may be sex-dependent, and merits further investigation as it may have important implications for counselling.

SPG4 gene promoter regulation via Elk1 transcription factor

The most common cause of autosomal dominant hereditary spastic paraplegia, that is characterized with axonal degeneration in corticospinal tracts and posterior columns, is known to be caused by mutations in the SPG4 gene which encodes spastin, a microtubule severing ATPase belonging to AAA family. Spastin promotes the formation of microtubule networks that are essential for axon growth and branching which are important for neuronal plasticity. Mutations observed in SPG4 gene of hereditary spastic paraplegia patients have been shown to cause reduced spastin levels. In addition to mutations, transcriptional regulation of spastin gene expression may also affect spastin level. ETS (E Twenty Six-specific)-domain transcription factor, Elk1, has been shown to be important for synaptic plasticity and interact with microtubules. In this study, we aimed to identify the critical promoter regions of SPG4 gene and effects of Elk on SPG4 gene expression. We identified 700 bp TATA-less promoter including a critical CpG island as an optimal promoter, and deletion of the CpG island gradually decreased the SPG4 promoter activity. In addition, we identified the binding sites of Elk1 on the SPG4 promoter by EMSA. Over-expression of Elk1 showed that it repressed the SPG4 promoter and also decreased spastin protein level in SHSY-5Y cells.

Regulatory network of microRNAs in RAW 264.7 macrophage cells

microRNAs (miRNA) play important roles in regulating immunity. Here we utilize the systems biology approach to predict the regulatory network of miRNAs among the most down-regulated genes by the lipopolysaccharides (LPS) treatment in the macrophage RAW267.4 cell line. We combine the proteome and transcriptome data sets to define 200 target genes that are significantly down-regulated by the LPS treatment. We perform the profiling of over 300 miRNAs with the RNA-Seq method. Using the complementary binding rule between the seed sequences of profiled miRNAs and the 3'UTRs of target genes, we predict genes involved in mobility, metabolism, and oxidative phosphorylation as the top targets of miRNA negative regulation.

Hereditary spastic paraplegias: membrane traffic and the motor pathway

Voluntary movement is a fundamental way in which animals respond to, and interact with, their environment. In mammals, the main CNS pathway controlling voluntary movement is the corticospinal tract, which encompasses connections between the cerebral motor cortex and the spinal cord. Hereditary spastic paraplegias (HSPs) are a group of genetic disorders that lead to a length-dependent, distal axonopathy of fibres of the corticospinal tract, causing lower limb spasticity and weakness. Recent work aimed at elucidating the molecular cell biology underlying the HSPs has revealed the importance of basic cellular processes — especially membrane trafficking and organelle morphogenesis and distribution— in axonal maintenance and degeneration.

Activin signaling functions upstream of Gbb to regulate synaptic growth at the Drosophila neuromuscular junction

Activins are members of the TGF-ss superfamily of secreted growth factors that control a diverse array of processes in vertebrates including endocrine function, cell proliferation, differentiation, immune response and wound repair. In Drosophila, the Activin ligand Dawdle (Daw) has been shown to regulate several aspects of neuronal development such as embryonic axonal pathfinding, neuroblast proliferation in the larval brain and growth cone motility in the visual system. Here we identify a novel role for Activin signaling in regulating synaptic growth at the larval neuromuscular junction (NMJ). Mutants for Daw, the Activin type I receptor Baboon (Babo), and the signal transducer dSmad2, display reduced NMJ size suggesting that Daw utilizes a canonical Activin signal-transduction pathway in this context. Additionally, loss of Daw/Babo activity affects microtubule stability, axonal transport and distribution of Futsch, the Drosophila microtubule associated protein 1B (MAP1B) homolog. We find that Babo signaling is required postsynaptically in the muscle, in contrast to the well-characterized retrograde BMP/Gbb signal that is required for synaptic growth and function in presynaptic cells. Finally, we show that the Daw/Babo pathway acts upstream of gbb, and is involved in maintenance of muscle gbb expression, suggesting that Activins regulate NMJ growth by modulating BMP activity through transcriptional regulation of ligand expression.

ESCRT & Co

Components of the ESCRT (endosomal sorting complex required for transport) machinery mediate endosomal sorting of ubiquitinated membrane proteins. They are key regulators of biological processes important for cell growth and survival, such as growth-factor-mediated signalling and cytokinesis. In addition, enveloped viruses, such as HIV-1, hijack and utilize the ESCRTs for budding during virus release and infection. Obviously, the ESCRT-facilitated pathways require tight regulation, which is partly mediated by a group of interacting proteins, for which our knowledge is growing. In this review we discuss the different ESCRT-modulating proteins and how they influence ESCRT-dependent processes, for example, by acting as positive or negative regulators or by providing temporal and spatial control. A number of the interactors influence the classical ESCRT-mediated process of endosomal cargo sorting, for example, by modulating the interaction between ubiquitinated cargo and the ESCRTs. Certain accessory proteins have been implicated in regulating the activity or steady-state expression levels of the ESCRT components, whereas other interactors control the cellular localization of the ESCRTs, for example, by inducing shuttling between cytosol and nucleus or endosomes. In conclusion, the discovery of novel interactors has and will extend our knowledge of the biological roles of ESCRTs.

Functional conservation of human Spastin in a Drosophila model of autosomal dominant-hereditary spastic paraplegia

Mutations in spastin are the most frequent cause of the neurodegenerative disease autosomal dominant-hereditary spastic paraplegia (AD-HSP). Drosophila melanogaster lacking spastin exhibit striking behavioral similarities to human patients suffering from AD-HSP, suggesting conservation of Spastin function between the species. Consistent with this, we show that exogenous expression of wild-type Drosophila or human spastin rescues behavioral and cellular defects in spastin null flies equivalently. This enabled us to generate genetically representative models of AD-HSP, which arises from dominant mutations in spastin rather than a complete loss of the gene. Flies co-expressing one copy of wild-type human spastin and one encoding the K388R catalytic domain mutation in the fly spastin null background, exhibit aberrant distal synapse morphology and microtubule distribution, similar to but less severe than spastin nulls. R388 or a separate nonsense mutation act dominantly and are furthermore sufficient to confer partial rescue, supporting in vitro evidence for additional, non-catalytic Spastin functions. Using this model, we tested the observation from human pedigrees that S44L and P45Q are trans-acting modifiers of mutations affecting the Spastin catalytic domain. As in humans, both L44 and Q45 are largely silent when heterozygous, but exacerbate mutant phenotypes when expressed in trans with R388. These transgenic 'AD-HSP' flies therefore provide a powerful and tractable model to enhance our understanding of the cellular and behavioral consequences of human spastin mutations and test hypotheses directly relevant to the human disease.

Regulation of microtubule dynamic instability

Proper regulation of MT (microtubule) dynamics is essential for various vital processes, including the segregation of chromosomes, directional cell migration and differentiation. MT assembly and disassembly is modulated by a complex network of intracellular factors that co-operate or antagonize each other, are highly regulated in space and time and are thus attuned to the cell cycle and differentiation processes. While we only begin to appreciate how the concerted action of MT stabilizers and destabilizers shapes different MT patterns, a clear picture of how individual factors affect the MT structure is emerging. In this paper, we review the current knowledge about proteins that modulate MT dynamic instability.

Spinal muscular atrophy: why do low levels of survival motor neuron protein make motor neurons sick?

Many neurogenetic disorders are caused by the mutation of ubiquitously expressed genes. One such disorder, spinal muscular atrophy, is caused by loss or mutation of the survival motor neuron1 gene (SMN1), leading to reduced SMN protein levels and a selective dysfunction of motor neurons. SMN, together with partner proteins, functions in the assembly of small nuclear ribonucleoproteins (snRNPs), which are important for pre-mRNA splicing. It has also been suggested that SMN might function in the assembly of other ribonucleoprotein complexes. Two hypotheses have been proposed to explain the molecular dysfunction that gives rise to spinal muscular atrophy (SMA) and its specificity to a particular group of neurons. The first hypothesis states that the loss of SMN's well-known function in snRNP assembly causes an alteration in the splicing of a specific gene (or genes). The second hypothesis proposes that SMN is crucial for the transport of mRNA in neurons and that disruption of this function results in SMA.

ClpXP protease degrades the cytoskeletal protein, FtsZ, and modulates FtsZ polymer dynamics

FtsZ is the major cytoskeletal protein in bacteria and a tubulin homologue. It polymerizes and forms a ring where constriction occurs to divide the cell. We found that FtsZ is degraded by E. coli ClpXP, an ATP-dependent protease. In vitro, ClpXP degrades both FtsZ protomers and polymers; however, polymerized FtsZ is degraded more rapidly than the monomer. Deletion analysis shows that the N-terminal domain of ClpX is important for polymer recognition and that the FtsZ C terminus contains a ClpX recognition signal. In vivo, FtsZ is turned over slower in a clpX deletion mutant compared with a WT strain. Overexpression of ClpXP results in increased FtsZ degradation and filamentation of cells. These results suggest that ClpXP may participate in cell division by modulating the equilibrium between free and polymeric FtsZ via degradation of FtsZ filaments and protomers.

Promotion of neurite extension by protrudin requires its interaction with vesicle-associated membrane protein-associated protein

Protrudin is a protein that contains a Rab11-binding domain and a FYVE (lipid-binding) domain and that functions to promote neurite formation through interaction with the GDP-bound form of Rab11. Protrudin also contains a short sequence motif designated FFAT (two phenylalanines in an acidic tract), which in other proteins has been shown to mediate binding to vesicle-associated membrane protein-associated protein (VAP). We now show that protrudin associates and colocalizes with VAP-A, an isoform of VAP expressed in the endoplasmic reticulum. Both the interaction between protrudin and VAP-A as well as the induction of process formation by protrudin were markedly inhibited by mutation of the FFAT motif. Furthermore, depletion of VAP-A by RNA interference resulted in mislocalization of protrudin as well as in inhibition of neurite outgrowth induced by nerve growth factor in rat pheochromocytoma PC12 cells. These defects resulting from depletion of endogenous rat VAP-A in PC12 cells were corrected by forced expression of (RNA interference-resistant) human VAP-A but not by VAP-A mutants that have lost the ability to interact with protrudin. These results suggest that VAP-A is an important regulator both of the subcellular localization of protrudin and of its ability to stimulate neurite outgrowth.

A cryptic promoter in the first exon of the SPG4 gene directs the synthesis of the 60-kDa spastin isoform

BACKGROUND

Mutations in SPG4 cause the most common form of autosomal dominant hereditary spastic paraplegia, a neurodegenerative disease characterized by weakness and spasticity of the lower limbs due to degeneration of the corticospinal tract. SPG4 encodes spastin, a microtubule-severing ATPase belonging to the AAA family. Two isoforms of spastin, 68 and 60 kDa, respectively, are variably abundant in tissues, show different subcellular localizations and interact with distinct molecules. The isoforms arise through alternative initiation of translation from two AUG codons in exon 1; however, it is unclear how regulation of their expression may be achieved.

RESULTS

We present data that rule out the hypothesis that a cap-independent mechanism may be involved in the translation of the 60-kDa spastin isoform. Instead, we provide evidence for a complex transcriptional regulation of SPG4 that involves both a TATA-less ubiquitous promoter and a cryptic promoter in exon 1. The cryptic promoter covers the 5'-UTR and overlaps with the coding region of the gene. By using promoter-less constructs in various experimental settings, we found that the cryptic promoter is active in HeLa, HEK293 and motoneuronal NSC34 cells but not in SH-SY-5Y neuroblastoma cells. We showed that the cryptic promoter directs the synthesis of a SPG4 transcript that contains a shorter 5'-UTR and translates the 60-kDa spastin isoform selectively. Two polymorphisms (S44L and P45Q), leading to an early onset severe form of hereditary spastic paraplegia when present in heterozygosity with a mutant allele, fall a few nucleotides downstream of the novel transcriptional start site, opening up the possibility that they may exert their modifier effect at the transcriptional level. We provide evidence that at least one of them decreases the activity of the cryptic promoter in luciferase assays.

CONCLUSION

We identified a cryptic promoter in exon 1 of the SPG4 gene that selectively drives the expression of the 60-kDa spastin isoform in a tissue-regulated manner. These data may have implications for the understanding of the biology of spastin and the pathogenic basis of hereditary spastic paraplegia.

Quantitative and functional analyses of spastin in the nervous system: implications for hereditary spastic paraplegia

Spastin and P60-katanin are two distinct microtubule-severing proteins. Autosomal dominant mutations in the SPG4 locus corresponding to spastin are the most common cause of hereditary spastic paraplegia (HSP), a neurodegenerative disease that afflicts the adult corticospinal tracts. Here we sought to evaluate whether SPG4-based HSP is best understood as a "loss-of-function" disease. Using various rat tissues, we found that P60-katanin levels are much higher than spastin levels during development. In the adult, P60-katanin levels plunge dramatically but spastin levels decline only slightly. Quantitative data of spastin expression in specific regions of the nervous system failed to reveal any obvious explanation for the selective sensitivity of adult corticospinal tracts to loss of spastin activity. An alternative explanation relates to the fact that the mammalian spastin gene has two start codons, resulting in a 616 amino acid protein called M1 and a slightly shorter protein called M85. We found that M1 is almost absent from developing neurons and most adult neurons but comprises 20-25% of the spastin in the adult spinal cord, the location of the axons that degenerate during HSP. Experimental expression in cultured neurons of a short dysfunctional M1 polypeptide (but not a short dysfunctional M85 peptide) is deleterious to normal axonal growth. In squid axoplasm, the M1 peptide dramatically inhibits fast axonal transport, whereas the M85 peptide does not. These results are consistent with a "gain-of-function" mechanism underlying HSP wherein spastin mutations produce a cytotoxic protein in the case of M1 but not M85.