Quantitative and Functional Analyses of Spastin in the Nervous System: Implications for Hereditary Spastic Paraplegia

Department of Neurobiology and Anatomy, Drexel University College of Medicine, Philadelphia, Pennsylvania 19129, USA.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.34). 03/2008; 28(9):2147-57. DOI: 10.1523/JNEUROSCI.3159-07.2008
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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.

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Available from: Gerardo Morfini, Jun 27, 2014
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    • "We detected the full-length M1 and M1ΔEx4 isoforms specifically in human neuronal cells. Similar protein expression patterns have been described for rodent tissues (8,23). Taking into account that the sole cell type affected in HSP is neurons, one might speculate that specifically the M1 isoforms have a crucial role in neuron integrity and/or survival, and in particular a reduction in these M1 isoforms is relevant for disease manifestation. "
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    ABSTRACT: The hereditary spastic paraplegias (HSP) are a heterogeneous group of motorneuron diseases characterized by progressive spasticity and paresis of the lower limbs. Mutations in SPG4, encoding Spastin, are the most frequent cause of HSP. To understand how mutations in SPG4 affect human neurons, we generated human induced pluripotent stem cells (hiPSCs) from fibroblasts of two patients carrying a c.1684C>T nonsense mutation, and from two controls. These SPG4 and control hiPSCs were able to differentiate into neurons and glia at comparable efficiency.All known Spastin isoforms were reduced in SPG4 neuronal cells. The complexity of SPG4 neurites was decreased, which was paralleled by an imbalance of axonal transport with less retrograde movement. Prominent neurite swellings with disrupted microtubules were present in SPG4 neurons on an ultrastructural level. While some of these swellings contain acetylated and detyrosinated tubulin, these tubulin modifications were unchanged in total cell lysates of SPG4 neurons. Upregulation of another microtubule severing protein, p60 Katanin, may partially compensate microtubuli dynamics in SPG4 neurons.Overexpression of the M1 or M87 Spastin isoforms restored neurite length, branching, numbers of primary neurites and reduced swellings in SPG4 neuronal cells. We conclude that neurite complexity and maintenance in HSP patient-derived neurons are critically sensitive to Spastin gene dosage. Our data show that elevation of single Spastin isoform levels is sufficient to restore neurite complexity and reduce neurite swellings in patient cells. Furthermore, our human model offers an ideal platform for pharmacological screenings with the goal to restore physiological Spastin levels in SPG4 patients.
    Human Molecular Genetics 12/2013; 23(10). DOI:10.1093/hmg/ddt644 · 6.39 Impact Factor
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    • "The simplest explanation for the involvement of 3 HSP proteins, including M1 spastin, in membrane shaping at the ER is that abnormality of this process leads directly to the disease, and this would suggest that it is the M1 isoform that is pathogenic. In keeping with this, the M1 isoform has higher expression in spinal cord than in other tissues [6] [68]. However, since M1 and M87 spastin could theoretically participate in hexamers together, other interpretations are also possible; it is conceivable that some abnormalities of the ER morphogen complex might sequester M87 spastin from its normal sites of action, so affecting, for example, endosomal functions of spastin. "
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    ABSTRACT: In 1999, mutations in the gene encoding the microtubule severing AAA ATPase spastin were identified as a major cause of a genetic neurodegenerative condition termed hereditary spastic paraplegia (HSP). This finding stimulated intense study of the spastin protein and over the last decade, a combination of cell biological, in vivo, in vitro and structural studies have provided important mechanistic insights into the cellular functions of the protein, as well as elucidating cell biological pathways that might be involved in axonal maintenance and degeneration. Roles for spastin have emerged in shaping the endoplasmic reticulum and the abscission stage of cytokinesis, in which spastin appears to couple membrane modelling to microtubule regulation by severing.
    Biochimica et Biophysica Acta 08/2011; 1823(1):192-7. DOI:10.1016/j.bbamcr.2011.08.010 · 4.66 Impact Factor
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    • "This ex vivo model system allows for a direct, quantitative evaluation of different effectors on anterograde (conventional kinesin-dependent) and retrograde (cytoplasmic dynein-dependent) FAT rates by video microscopy (Brady et al., 1993). Isolated axoplasm has been used to examine effects of numerous pathogenic proteins, including huntingtin (Morfini et al., 2009b), spastin (Solowska et al., 2008), amyloid-β (Pigino et al., 2009), superoxide dismutase type 1 (Bosco et al., 2010), and MPTP/MPP+ (Morfini et al., 2007b). In each case, the results obtained in isolated axoplasms were confirmed in mammalian models. "
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    ABSTRACT: Alzheimer's disease (AD) and other tauopathies are characterized by fibrillar inclusions composed of the microtubule-associated protein, tau. Recently, we demonstrated that the N-terminus of tau (amino acids [aa] 2-18) in filamentous aggregates or N-terminal tau isoforms activate a signaling cascade involving protein phosphatase 1 and glycogen synthase kinase 3 that results in inhibition of anterograde fast axonal transport (FAT). We have termed the functional motif comprised of aa 2-18 in tau the phosphatase-activating domain (PAD). Here, we show that phosphorylation of tau at tyrosine 18, which is a fyn phosphorylation site within PAD, prevents inhibition of anterograde FAT induced by both filamentous tau and 6D tau. Moreover, Fyn-mediated phosphorylation of tyrosine 18 is reduced in disease-associated forms of tau (e.g., tau filaments). A novel PAD-specific monoclonal antibody revealed that exposure of PAD in tau occurs before and more frequently than tyrosine 18 phosphorylation in the evolution of tangle formation in AD. These results indicate that N-terminal phosphorylation may constitute a regulatory mechanism that controls tau-mediated inhibition of anterograde FAT in AD.
    Neurobiology of aging 07/2011; 33(4):826.e15-30. DOI:10.1016/j.neurobiolaging.2011.06.006 · 5.01 Impact Factor
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