Modeling of axonal endoplasmic reticulum network by spastic paraplegia proteins.
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ABSTRACT:
SUBMITTER: Yalcın B
PROVIDER: S-EPMC5576921 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:BACKGROUND:Cortical motor neurons, also known as upper motor neurons, are large projection neurons whose axons convey signals to lower motor neurons to control the muscle movements. Degeneration of cortical motor neuron axons is implicated in several debilitating disorders, including hereditary spastic paraplegia (HSP) and amyotrophic lateral sclerosis (ALS). Since the discovery of the first HSP gene, SPAST that encodes spastin, over 70 distinct genetic loci associated with HSP have been identified. How the mutations of these functionally diverse genes result in axonal degeneration and why certain axons are affected in HSP remains largely unknown. The development of induced pluripotent stem cell (iPSC) technology has provided researchers an excellent resource to generate patient-specific human neurons to model human neuropathologic processes including axonal defects. METHODS:In this article, we will frst review the pathology and pathways affected in the common forms of HSP subtypes by searching the PubMed database. We will then summurize the findings and insights gained from studies using iPSC-based models, and discuss the challenges and future directions. RESULTS:HSPs, a heterogeneous group of genetic neurodegenerative disorders, are characterized by lower extremity weakness and spasticity that result from retrograde axonal degeneration of cortical motor neurons. Recently, iPSCs have been generated from several common forms of HSP including SPG4, SPG3A, and SPG11 patients. Neurons derived from HSP iPSCs exhibit disease-relevant axonal defects, such as impaired neurite outgrowth, increased axonal swellings, and reduced axonal transport. CONCLUSION:These patient-derived neurons offer unique tools to study the pathogenic mechanisms and explore the treatments for rescuing axonal defects in HSP, as well as other diseases involving axonopathy.
| S-EPMC5147749 | biostudies-literature
Project description:ObjectiveTo establish the phenotypic spectrum of KIF5A mutations and to investigate whether KIF5A mutations cause axonal neuropathy associated with hereditary spastic paraplegia (HSP) or typical Charcot-Marie-Tooth disease type 2 (CMT2).MethodsKIF5A sequencing of the motor-domain coding exons was performed in 186 patients with the clinical diagnosis of HSP and in 215 patients with typical CMT2. Another 66 patients with HSP or CMT2 with pyramidal signs were sequenced for all exons of KIF5A by targeted resequencing. One additional patient was genetically diagnosed by whole-exome sequencing.ResultsFive KIF5A mutations were identified in 6 unrelated patients: R204W and D232N were novel mutations; R204Q, R280C, and R280H have been previously reported. Three patients had CMT2 as the predominant and presenting phenotype; 2 of them also had pyramidal signs. The other 3 patients presented with HSP but also had significant axonal neuropathy or other additional features.ConclusionThis is currently the largest study investigating KIF5A mutations. By combining next-generation sequencing and conventional sequencing, we confirm that KIF5A mutations can cause variable phenotypes ranging from HSP to CMT2. The identification of mutations in CMT2 broadens the phenotypic spectrum and underlines the importance of KIF5A mutations, which involve degeneration of both the central and peripheral nervous systems and should be tested in HSP and CMT2.
| S-EPMC4141994 | biostudies-literature
Project description:The endoplasmic reticulum (ER) in plant cells forms a highly dynamic network of complex geometry. ER network morphology and dynamics are influenced by a number of biophysical processes, including filament/tubule tension, viscous forces, Brownian diffusion, and interactions with many other organelles and cytoskeletal elements. Previous studies have indicated that ER networks can be thought of as constrained minimal-length networks acted on by a variety of forces that perturb and/or remodel the network. Here, we study two specific biophysical processes involved in remodeling. One is the dynamic relaxation process involving a combination of tubule tension and viscous forces. The other is the rapid creation of cross-connection tubules by direct or indirect interactions with cytoskeletal elements. These processes are able to remodel the ER network: the first reduces network length and complexity whereas the second increases both. Using live cell imaging of ER network dynamics in tobacco leaf epidermal cells, we examine these processes on ER network dynamics. Away from regions of cytoplasmic streaming, we suggest that the dynamic network structure is a balance between the two processes, and we build an integrative model of the two processes for network remodeling. This model produces quantitatively similar ER networks to those observed in experiments. We use the model to explore the effect of parameter variation on statistical properties of the ER network.
| S-EPMC5510808 | biostudies-other
Project description:BackgroundHereditary spastic paraplegia defines a group of genetically heterogeneous diseases characterized by weakness and spasticity of the lower limbs owing to retrograde degeneration of corticospinal axons. One autosomal recessive form of the disease is caused by mutation in the SPG7 gene. Paraplegin, the product of SPG7, is a component of the m-AAA protease, a high molecular weight complex that resides in the mitochondrial inner membrane, and performs crucial quality control and biogenesis functions in mitochondria.Principal findingsHere we show the existence in the mouse of a novel isoform of paraplegin, which we name paraplegin-2, encoded by alternative splicing of Spg7 through usage of an alternative first exon. Paraplegin-2 lacks the mitochondrial targeting sequence, and is identical to the mature mitochondrial protein. Remarkably, paraplegin-2 is targeted to the endoplasmic reticulum. We find that paraplegin-2 exposes the catalytic domains to the lumen of the endoplasmic reticulum. Moreover, endogenous paraplegin-2 accumulates in microsomal fractions prepared from mouse brain and retina. Finally, we show that the previously generated mouse model of Spg7-linked hereditary spastic paraplegia is an isoform-specific knock-out, in which mitochondrial paraplegin is specifically ablated, while expression of paraplegin-2 is retained.Conclusions/significanceThese data suggest a possible additional role of AAA proteases outside mitochondria and open the question of their implication in neurodegeneration.
| S-EPMC3341365 | biostudies-literature
Project description:Hereditary spastic paraplegias are a group of inherited motor neuron diseases characterized by progressive paraparesis and spasticity. Mutations in the spastic paraplegia gene SPG11, encoding spatacsin, cause an autosomal-recessive disease trait; however, the precise knowledge about the role of spatacsin in neurons is very limited. We for the first time analyzed the expression and function of spatacsin in human forebrain neurons derived from human pluripotent stem cells including lines from two SPG11 patients and two controls. SPG11 patients'-derived neurons exhibited downregulation of specific axonal-related genes, decreased neurite complexity and accumulation of membranous bodies within axonal processes. Altogether, these data point towards axonal pathologies in human neurons with SPG11 mutations. To further corroborate spatacsin function, we investigated human pluripotent stem cell-derived neurons and mouse cortical neurons. In these cells, spatacsin was located in axons and dendrites. It colocalized with cytoskeletal and synaptic vesicle (SV) markers and was present in synaptosomes. Knockdown of spatacsin in mouse cortical neurons evidenced that the loss of function of spatacsin leads to axonal instability by downregulation of acetylated tubulin. Finally, time-lapse assays performed in SPG11 patients'-derived neurons and spatacsin-silenced mouse neurons highlighted a reduction in the anterograde vesicle trafficking indicative of impaired axonal transport. By employing SPG11 patient-derived forebrain neurons and mouse cortical neurons, this study provides the first evidence that SPG11 is implicated in axonal maintenance and cargo trafficking. Understanding the cellular functions of spatacsin will allow deciphering mechanisms of motor cortex dysfunction in autosomal-recessive hereditary spastic paraplegia.
| S-EPMC4140466 | biostudies-literature
Project description:Hereditary spastic paraplegia (HSP) is one of the most heterogeneous neurodegenerative diseases with more than 50 identified genes causing a relatively stereotypical phenotypic presentation. Recent studies of HSP pathogenesis have suggested the existence of shared biochemical pathways that are crucial for axonal maintenance and degeneration. We explored possible interactions of several proteins associated with this condition. Here we report interactions of endogenous and overexpressed strumpellin with another HSP-associated protein, spartin. This biochemical interaction does not appear to be a part of the Wiskott-Aldrich syndrome protein and Scar homologue (WASH) complex because spartin is not co-immunoprecipitated with WASH1 protein. The spartin-strumpellin association does not require the presence of the microtubule interacting and trafficking domain of spartin. Over-expression of mutant forms of strumpellin with the introduced HSP-causing mutations does not alter the colocalization of these two proteins. Knockdown of strumpellin in cultured cortical rat neurons interferes with development of neuronal branching and results in reduced expression of endogenous spartin. Proteosomal inhibition stabilized the levels of spartin and WASH1 proteins, supporting increased spartin degradation in the absence of strumpellin.
| S-EPMC4426939 | biostudies-literature
Project description:Autophagy allows cells to adapt to changes in their environment by coordinating the degradation and recycling of cellular components and organelles to maintain homeostasis. Lysosomes are organelles critical for terminating autophagy via their fusion with mature autophagosomes to generate autolysosomes that degrade autophagic materials; therefore, maintenance of the lysosomal population is essential for autophagy-dependent cellular clearance. Here, we have demonstrated that the two most common autosomal recessive hereditary spastic paraplegia gene products, the SPG15 protein spastizin and the SPG11 protein spatacsin, are pivotal for autophagic lysosome reformation (ALR), a pathway that generates new lysosomes. Lysosomal targeting of spastizin required an intact FYVE domain, which binds phosphatidylinositol 3-phosphate. Loss of spastizin or spatacsin resulted in depletion of free lysosomes, which are competent to fuse with autophagosomes, and an accumulation of autolysosomes, reflecting a failure in ALR. Moreover, spastizin and spatacsin were essential components for the initiation of lysosomal tubulation. Together, these results link dysfunction of the autophagy/lysosomal biogenesis machinery to neurodegeneration.
| S-EPMC4348974 | biostudies-literature
Project description:Hereditary spastic paraplegias (HSPs; SPG1-45) are inherited neurological disorders characterized by lower extremity spastic weakness. More than half of HSP cases result from autosomal dominant mutations in atlastin-1 (also known as SPG3A), receptor expression enhancing protein 1 (REEP1; SPG31), or spastin (SPG4). The atlastin-1 GTPase interacts with spastin, a microtubule-severing ATPase, as well as with the DP1/Yop1p and reticulon families of ER-shaping proteins, and SPG3A caused by atlastin-1 mutations has been linked pathogenically to abnormal ER morphology. Here we investigated SPG31 by analyzing the distribution, interactions, and functions of REEP1. We determined that REEP1 is structurally related to the DP1/Yop1p family of ER-shaping proteins and localizes to the ER in cultured rat cerebral cortical neurons, where it colocalizes with spastin and atlastin-1. Upon overexpression in COS7 cells, REEP1 formed protein complexes with atlastin-1 and spastin within the tubular ER, and these interactions required hydrophobic hairpin domains in each of these proteins. REEP proteins were required for ER network formation in vitro, and REEP1 also bound microtubules and promoted ER alignment along the microtubule cytoskeleton in COS7 cells. A SPG31 mutant REEP1 lacking the C-terminal cytoplasmic region did not interact with microtubules and disrupted the ER network. These data indicate that the HSP proteins atlastin-1, spastin, and REEP1 interact within the tubularER membrane in corticospinal neurons to coordinate ER shaping and microtubule dynamics. Thus, defects in tubular ER shaping and network interactions with the microtubule cytoskeleton seem to be the predominant pathogenic mechanism of HSP.
| S-EPMC2846052 | biostudies-literature
Project description:The physical continuity of axons over long cellular distances poses challenges for their maintenance. One organelle that faces this challenge is endoplasmic reticulum (ER); unlike other intracellular organelles, this forms a physically continuous network throughout the cell, with a single membrane and a single lumen. In axons, ER is mainly smooth, forming a tubular network with occasional sheets or cisternae and low amounts of rough ER. It has many potential roles: lipid biosynthesis, glucose homeostasis, a Ca2+ store, protein export, and contacting and regulating other organelles. This tubular network structure is determined by ER-shaping proteins, mutations in some of which are causative for neurodegenerative disorders such as hereditary spastic paraplegia (HSP). While axonal ER shares many features with the tubular ER network in other contexts, these features must be adapted to the long and narrow dimensions of axons. ER appears to be physically continuous throughout axons, over distances that are enormous on a subcellular scale. It is therefore a potential channel for long-distance or regional communication within neurons, independent of action potentials or physical transport of cargos, but involving its physiological roles such as Ca2+ or organelle homeostasis. Despite its apparent stability, axonal ER is highly dynamic, showing features like anterograde and retrograde transport, potentially reflecting continuous fusion and breakage of the network. Here we discuss the transport processes that must contribute to this dynamic behavior of ER. We also discuss the model that these processes underpin a homeostatic process that ensures both enough ER to maintain continuity of the network and repair breaks in it, but not too much ER that might disrupt local cellular physiology. Finally, we discuss how failure of ER organization in axons could lead to axon degenerative diseases, and how a requirement for ER continuity could make distal axons most susceptible to degeneration in conditions that disrupt ER continuity.
| S-EPMC7025499 | biostudies-literature
Project description:Hereditary spastic paraplegias are inherited neurological disorders characterized by progressive lower-limb spasticity and weakness. Although more than 50 genetic loci are known [spastic gait (SPG)1 to -57], over half of hereditary spastic paraplegia cases are caused by pathogenic mutations in four genes encoding proteins that function in tubular endoplasmic reticulum (ER) network formation: atlastin-1 (SPG3A), spastin (SPG4), reticulon 2 (SPG12), and receptor expression-enhancing protein 1 (SPG31). Here, we show that the SPG33 protein protrudin contains hydrophobic, intramembrane hairpin domains, interacts with tubular ER proteins, and functions in ER morphogenesis by regulating the sheet-to-tubule balance and possibly the density of tubule interconnections. Protrudin also interacts with KIF5 and harbors a Rab-binding domain, a noncanonical FYVE (Fab-1, YGL023, Vps27, and EEA1) domain, and a two phenylalanines in an acidic tract (FFAT) domain and, thus, may also function in the distribution of ER tubules via ER contacts with the plasma membrane or other organelles.
| S-EPMC3773775 | biostudies-literature