6 studies reviewed covering 5 HSP genes
Over the past three months, six studies of how HSP happens in five different HSP genotypes have been published. This is an unprecedented volume of publications on the mechanisms and pathways from genetic mutation to HSP symptoms.
Knowledge of causal mechanisms, dynamics and pathways at the cell and molecular level is fundamental to identifying potential therapies that treat the disease at or close to its cause. Let’s hope that this increased research focus on HSP mechanisms continues.
Mitochondrial structural impairment explained
Person with both SPG7 HSP and ataxia mutations
A protein, OPA1, is identified as playing a crucial role in mitochondrial fusion, and when impaired, causes severe fragmentation of the mitochondrial network.
A coincidental sporadic spinocerebellar ataxia gene mutation combined with an SPG7 HSP mutation has resulted in a new complex phenotype, with both mutations impairing mitochondrial structure, dynamics and function.
Mitochondrial dynamics and quality control are crucial for neuronal survival and their perturbation is a major cause of neurodegeneration. m-AAA complex is an ATP-dependent metalloprotease located in the inner mitochondrial membrane and involved in protein quality control. Mutations in the m-AAA subunits AFG3L2 and paraplegin are associated with autosomal dominant spinocerebellar ataxia (SCA28) and autosomal recessive hereditary spastic paraplegia (SPG7), respectively.
We report a novel m-AAA-associated phenotype characterized by early-onset optic atrophy with spastic ataxia and L-dopa-responsive parkinsonism. The proband carried a de novo AFG3L2 heterozygous mutation (p.R468C) along with a heterozygous maternally inherited intragenic deletion of SPG7. Functional analysis in yeast demonstrated the pathogenic role of AFG3L2 p.R468C mutation shedding light on its pathogenic mechanism. Analysis of patient’s fibroblasts showed an abnormal processing pattern of OPA1, a dynamin-related protein essential for mitochondrial fusion and responsible for most cases of hereditary optic atrophy. Consistently, assessment of mitochondrial morphology revealed a severe fragmentation of the mitochondrial network, not observed in SCA28 and SPG7 patients’ cells.
This case suggests that coincidental mutations in both components of the mitochondrial m-AAA protease may result in a complex phenotype and reveals a crucial role for OPA1 processing in the pathogenesis of neurodegenerative disease caused by m-AAA defects.
SOURCE: Hum Mutat. 2018 Dec;39(12):2060-2071. doi: 10.1002/humu.23658. Epub 2018 Oct 10. PMID: 30252181
Concurrent AFG3L2 and SPG7 mutations associated with syndromic parkinsonism and optic atrophy with aberrant OPA1 processing and mitochondrial network fragmentation.
1 Unit of Medical Genetics and Neurogenetics, Fondazione IRCCS Istituto Neurologico Carlo Besta, Milan, Italy.
2 Unit of Developmental Neurology, Fondazione IRCCS Istituto Neurologico Carlo Besta, Milan, Italy.
3 Dipartimento di Fisiopatologia Medico-Chirurgica e dei Trapianti, Università degli Studi di Milano, Milan, Italy.
4 Dipartimento di Scienze Farmacologiche e Biomolecolari (DiSFeB), Centro di Eccellenza sulle Malattie Neurodegenerative, Università degli Studi di Milano, Milan, Italy.
5 Unit of Neurology 1, Fondazione IRCCS Istituto Neurologico Carlo Besta, Milan, Italy.
M1 and M87 forms of spastin
M1 strongly impairs axonal transport
Mutations of the SPG4 (SPAST) gene encoding for spastin protein are the main causes of hereditary spastic paraplegia. Spastin binds to microtubules and severs them through the enzymatic activity of its AAA domain. Several missense mutations located in this domain lead to stable, nonsevering spastins that decorate a subset of microtubules, suggesting a possible negative gain-of-function mechanism for these mutants.
Of the two main isoforms of spastin, only mutations of the long isoform, M1, are supposed to be involved in the onset of the pathology, leaving the role of the ubiquitously expressed shorter one, M87, not fully investigated and understood. Here, we show that two isoforms of spastin harboring the same missense mutation bind and bundle different subsets of microtubules in HeLa cells, and likely stabilize them by increasing the level of acetylated tubulin.
However, only mutated M1 has the ability to interact with wild-type M1, and decorates a subset of perinuclear microtubules associated with the endoplasmic reticulum that display higher resistance to microtubule depolymerization and increased intracellular ionic strength, compared with those decorated by mutated M87.
We further show that only mutated M1 decorates microtubules of proximal axons and dendrites, and strongly impairs axonal transport in cortical neurons through a mechanism likely independent of the microtubule-severing activity of this protein.
SOURCE: Dis Model Mech. 2018 Sep 10;11(9). pii: dmm033704. doi: 10.1242/dmm.033704. PMID: 30213879
Functional differences of short and long isoforms of spastin harboring missense mutation.
1 Structure and Activity of Normal and Pathological Biomolecules, INSERM U1204, Université Paris Saclay, Université d’Evry, 91000 Evry, France.
2 INSERM UMR-S 1193, Faculty of Pharmacy, Univirsité Paris-Sud, Université Paris-Saclay, 92296 Châtenay-Malabry, France.
3 Structure and Activity of Normal and Pathological Biomolecules, INSERM U1204, Université Paris Saclay, Université d’Evry, 91000 Evry, France [email protected]
Spastin impairment mechanisms
Two separate mechanisms specific to the two Spastin isoforms
Spastin has 2 forms in cells, called M1 and M87. M87 is the most prevalent form in neuronal axons where depletion results in defective neuron development. Depletion of M1 spastin operates differently, with the combined effect of both likely being motor/mobility impairment of the zebrafish studied.
Functional analyses of genes responsible for neurodegenerative disorders have unveiled crucial links between neurodegenerative processes and key developmental signalling pathways. Mutations in SPG4-encoding spastin cause hereditary spastic paraplegia (HSP). Spastin is involved in diverse cellular processes that couple microtubule severing to membrane remodelling.
Two main spastin isoforms are synthesised from alternative translational start sites (M1 and M87). However, their specific roles in neuronal development and homeostasis remain largely unknown.
To selectively unravel their neuronal function, we blocked spastin synthesis from each initiation codon during zebrafish development and performed rescue analyses. The knockdown of each isoform led to different motor neuron and locomotion defects, which were not rescued by the selective expression of the other isoform. Notably, both morphant neuronal phenotypes were observed in a CRISPR/Cas9 spastin mutant. We next showed that M1 spastin, together with HSP proteins atlastin 1 and NIPA1, drives motor axon targeting by repressing BMP signalling, whereas M87 spastin acts downstream of neuropilin 1 to control motor neuron migration.
Our data therefore suggest that defective BMP and neuropilin 1 signalling may contribute to the motor phenotype in a vertebrate model of spastin depletion.
SOURCE: Development. 2018 Sep 12;145(17). pii: dev162701. doi: 10.1242/dev.162701. PMID: 30082270
BMP- and neuropilin 1-mediated motor axon navigation relies on spastin alternative translation.
1 Sorbonne Universités, UPMC Université Paris 06, INSERM, CNRS, Neuroscience Paris Seine – Institut de Biologie Paris-Seine (NPS-IBPS), 75005 Paris, France.
2 Sorbonne Universités, UPMC Université Paris 06, CNRS, INSERM, Biologie du Développement Paris Seine – Institut de Biologie Paris-Seine (LBD-IBPS), 75005 Paris, France.
3 Cambridge Institute for Medical Research, University of Cambridge, Cambridge CB2 OXY, UK.
4 Medical Research Council Centre for Developmental Neurobiology, King’s College London, London SE1 1UL, UK.
SPG11 and SPG 15 HSPs mechanisms expanded
Shared, but also different roles for the two genes involved, Spatacsin and Spastizin
ZFYVE26/Spastizin and SPG11/Spatacsin encode 2 large proteins that are mutated in hereditary autosomal recessive spastic paraplegia / paraparesis (HSP) type 15 (AR-SPG15) and type 11 (AR-SPG11), respectively. We previously have reported that AR-SPG15-related ZFYVE26 mutations lead to autophagy defects with accumulation of immature autophagosomes. ZFYVE26 and SPG11 were found to be part of a complex including the AP5 (adaptor related protein complex 5) and to have a critical role in autophagic lysosomal reformation with identification of autophagic and lysosomal defects in cells with both AR-SPG15- and AR-SPG11-related mutations.
In spite of these similarities between the 2 proteins, here we report that ZFYVE26 and SPG11 are differently involved in autophagy and endocytosis. We found that both ZFYVE26 and SPG11 interact with RAB5A and RAB11, 2 proteins regulating endosome trafficking and maturation, but only ZFYVE26 mutations affected RAB protein interactions and activation. ZFYVE26 mutations lead to defects in the fusion between autophagosomes and endosomes, while SPG11 mutations do not affect this step and lead to a milder autophagy defect.
We thus demonstrate that ZFYVE26 and SPG11 affect the same cellular physiological processes, albeit at different levels: both proteins have a role in autophagic lysosome reformation, but only ZFYVE26 acts at the intersection between endocytosis and autophagy, thus representing a key player in these 2 processes. Indeed expression of the constitutively active form of RAB5A in cells with AR-SPG15-related mutations partially rescues the autophagy defect.
Finally the model we propose demonstrates that autophagy and the endolysosomal pathway are central processes in the pathogenesis of these complicated forms of hereditary spastic paraparesis.
Abbreviations: ALR, autophagic lysosome reformation; AP5, adaptor related protein complex 5; AR, autosomal-recessive; HSP, hereditary spastic paraplegia/paraparesis; ATG14, autophagy related 14; BafA, bafilomycin A1; BECN1, beclin 1; EBSS, Earle balanced salt solution; EEA1, early endosome antigen 1; EGF, epidermal growth factor; EGFR, epidermal growth factor receptor; GDP, guanosine diphosphate; GFP, green fluorescent protein; GTP, guanosine triphosphate; HSP, hereditary spastic paraplegias; LBPA, lysobisphosphatidic acid; MAP1LC3B/LC3B, microtubule associated protein 1 light chain 3 beta; MVBs, multivesicular bodies; PIK3C3, phosphatidylinositol 3-kinase, catalytic subunit type 3; PIK3R4, phosphoinositide-3-kinase regulatory subunit 4; PtdIns3P, phosphatidylinositol-3-phosphate; RFP, red fluorescent protein; RUBCN, RUN and cysteine rich domain containing beclin 1 interacting protein; shRNA, short hairpin RNA; SQSTM1/p62, sequestosome 1; TCC: thin corpus callosum; TF, transferrin; UVRAG, UV radiation resistance associated.
SOURCE: Autophagy. 2019 Jan;15(1):34-57. doi: 10.1080/15548627.2018.1507438. Epub 2018 Sep 13. PMID: 30081747
ZFYVE26/SPASTIZIN and SPG11/SPATACSIN mutations in hereditary spastic paraplegia types AR-SPG15 and AR-SPG11 have different effects on autophagy and endocytosis.
1 a Scientific Institute, IRCCS E. Medea, Laboratory of Molecular Biology , Bosisio Parini , Lecco , Italy.
2 b Department of Molecular Medicine , IRCCS Fondazione Stella Maris , Calambrone, Pisa , Italy.
3 c Department of Biomedical and Neuromotor Sciences , University of Bologna; IRCCS Institute of Neurological Sciences , Bologna , Italy.
4 d Dept. of Neurosciences and Institute of Experimental Neurology (INSpe) , San Raffaele Scientific Institute , Milan , Italy.
5 e Department of Clinical and Experimental Medicine , University of Messina , Messina , Italy.
6 f Unit of Clinical Pharmacology, CNR Institute of Neuroscience, Department of Biomedical and Clinical Sciences , University Hospital “Luigi Sacco”, Università di Milano , Milan , Italy.
SPG28 HSP mechanism proposed
Lipids and lipid dynamics studied in mice
Deleterious mutations in the serine hydrolase DDHD domain containing 1 (DDHD1) cause the SPG28 subtype of the neurological disease hereditary spastic paraplegia (HSP), which is characterized by axonal neuropathy and gait impairments. DDHD1 has been shown to display PLA1-type phospholipase activity with a preference for phosphatidic acid. However, the endogenous lipid pathways regulated by DDHD1 in vivo remain poorly understood.
Here we use a combination of untargeted and targeted metabolomics to compare the lipid content of brain tissue from DDHD1+/+ and DDHD1-/- mice, revealing that DDHD1 inactivation causes a substantial decrease in the level of polyunsaturated lysophosphatidylinositol (LPI) lipids and a corresponding increase in the level of phosphatidylinositol (PI) lipids. Levels of other phospholipids were mostly unchanged, with the exception of decreases in the levels of select polyunsaturated lysophosphatidylserine (LPS) and lysophosphatidylcholine lipids and a striking remodeling of PI phosphates (e.g., PIP and PIP2) in DDHD1-/- brain tissue. Biochemical assays confirmed that DDHD1 hydrolyzes PI/PS to LPI/LPS with sn-1 selectivity and accounts for a substantial fraction of the PI/PS lipase activity in mouse brain tissue.
These data indicate that DDHD1 is a principal regulator of bioactive LPI and other lysophospholipids, as well as PI phosphates, in the mammalian nervous system, pointing to a potential role for these lipid pathways in HSP.
SOURCE: Biochemistry. 2018 Oct 2;57(39):5759-5767. doi: 10.1021/acs.biochem.8b00810. Epub 2018 Sep 17. PMID: 30221923
The Spastic Paraplegia-Associated Phospholipase DDHD1 Is a Primary Brain Phosphatidylinositol Lipase.
1 Department of Chemistry, The Skaggs Institute for Chemical Biology , The Scripps Research Institute , La Jolla , California 92037, United States.
Mechanism of SPG57 HSP clarified
TFG gene mutation role in cell transport
Secretion from the endoplasmic reticulum and the process of axon development and axons growing together are both impaired by pathogenic mutations in the TFG gene.
Length-dependent axonopathy of the corticospinal tract causes lower limb spasticity and is characteristic of several neurological disorders, including hereditary spastic paraplegia (HSP) and amyotrophic lateral sclerosis.
Mutations in Trk-fused gene (TFG) have been implicated in both diseases, but the pathomechanisms by which these alterations cause neuropathy remain unclear. Here, we biochemically and genetically define the impact of a mutation within the TFG coiled-coil domain, which underlies early-onset forms of HSP.
We find that the TFG (p.R106C) mutation alters compaction of TFG ring complexes, which play a critical role in the export of cargoes from the endoplasmic reticulum (ER). Using CRISPR-mediated genome editing, we engineered human stem cells that express the mutant form of TFG at endogenous levels and identified specific defects in secretion from the ER and axon fasciculation following neuronal differentiation.
Together, our data highlight a key role for TFG-mediated protein transport in the pathogenesis of HSP.
SOURCE: Cell Rep. 2018 Aug 28;24(9):2248-2260. doi: 10.1016/j.celrep.2018.07.081. PMID: 30157421
Pathogenic TFG Mutations Underlying Hereditary Spastic Paraplegia Impair Secretory Protein Trafficking and Axon Fasciculation.
Slosarek EL1, Schuh AL1, Pustova I1, Johnson A1, Bird J1, Johnson M2, Frankel EB1, Bhattacharya N2, Hanna MG1, Burke JE3, Ruhl DA4, Quinney K1, Block S1, Peotter JL1, Chapman ER4, Sheets MD1, Butcher SE3, Stagg SM2, Audhya A5.
1 Department of Biomolecular Chemistry, University of Wisconsin-Madison School of Medicine and Public Health, 440 Henry Mall, Madison, WI 53706, USA.
2 Department of Chemistry and Biochemistry, Institute of Molecular Biophysics, Florida State University, 91 Chieftan Way, Tallahassee, FL 32306, USA.
3 Department of Biochemistry, University of Wisconsin-Madison, Madison, WI 53706, USA.
4 Howard Hughes Medical Institute and Department of Neuroscience, University of Wisconsin-Madison, Madison, WI 53705, USA.
5 Department of Biomolecular Chemistry, University of Wisconsin-Madison School of Medicine and Public Health, 440 Henry Mall, Madison, WI 53706, USA. Electronic address: [email protected]