Hereditary spastic paraparesis: Disrupted intracellular transport associated with spastin mutation

Abstract

The commonest cause of hereditary spastic paraplegia (HSP) is mutation in the spastin gene. Both the normal function of spastin in the central nervous system and the mechanism by which mutation in spastin causes axonal degeneration are unknown. One hypothesis is that mutant spastin disrupts microtubule dynamics, causing an impairment of organelle transport on the microtubule network, which leads to degeneration in the distal parts of long axons. To study this neuronal and non-neuronal cells were transfected with either wild type or mutant spastin proteins. We demonstrated evidence of a transient interaction of wild-type spastin with microtubules, with resulting disassembly of microtubules, supporting a role for wild-type spastin as a microtubule-severing protein. Mutant spastin demonstrated an abnormal interaction with microtubules, colocalizing with but no longer severing microtubules. The abnormal interaction of mutant spastin with microtubules was demonstrated to be associated with an abnormal perinuclear clustering of mitochondria and peroxisomes, suggestive of an impairment of kinesin-mediated intracellular transport. Our findings indicate that an abnormal interaction of mutant spastin with microtubules, which disrupts organelle transport on the microtubule cytoskeleton, is likely to be the primary disease mechanism in HSP caused by missense mutations in the spastin gene.

Genetic and chemical modulation of spastin-dependent axon outgrowth in zebrafish embryos indicates a role for impaired microtubule dynamics in hereditary spastic paraplegia

“…Genetic and biochemical studies have demonstrated that SPAST mutations cause loss of spastin function, making haploinsufficiency a likely disease mechanism (Charvin et al, 2003;Evans et al, 2005;Patrono et al, 2002), although dominant-negative effects could contribute to disease pathogenesis in some cases (Errico et al, 2002;McDermott et al, 2003). To investigate the developmental requirements for spastin, we previously used morpholino antisense methods to knockdown spast expression in the zebrafish embryo and demonstrated a crucial requirement for spastin to promote axon outgrowth during embryonic development (Wood et al, 2006).…”

Disease-related phenotypes in a Drosophila model of hereditary spastic paraplegia are ameliorated by treatment with vinblastine

“…A wide variety of nonsense, missense, and frameshift mutations have been identified in SPG4 patients and produce clinically indistinguishable phenotypes, suggesting that the molecular mechanism of spastin mutations is haploinsufficiency (8,9). Nevertheless, cellular expression of pathogenic missense mutations potentially inactivating the ATPase domain of spastin has led to the hypothesis that truncated or missense mutant spastin may cause HSP through a dominant-negative mechanism (10,11).…”

In vivo assay of presynaptic microtubule cytoskeleton dynamics in Drosophila

“…Previous studies have made strong inferences about altered microtubule dynamics in different genetic mutants and disease models based wholly on very indirect assays of the local microtubule state within synapses (Zhang et al, 2001;Huot et al, 2001;McDermott et al, 2003;Lu et al, 2004;Sherwood et al, 2004;Trotta et al, 2004;Zhang and Broadie, 2005;Orso et al, 2005). The goal of this study was to develop techniques to directly assay microtubule dynamics, locally at synapses, in living neurons in situ.…”

“…Microtubules are abundant throughout neurons and are known to be critical for the generation and maintenance of neuronal processes, including dendrites and axons, transport along these exceptionally elongated, polarized processes, and the development and maintenance of synaptic transmission in both pre-and postsynaptic compartments (Barth et al, 1997;Terada and Hirokawa, 2000;Kneussel 2005;Marques 2005). Not surprisingly, therefore, disrupted neuronal microtubule dynamics has been suggested to be the underlying dysfunction of several human neurological diseases, including Fragile X Syndrome (FXS) and Hereditary Spastic Paraplegia (HSP) (Zhang et al, 2001;Huot et al, 2001;McDermott et al, 2003;Lu et al, 2004;Sherwood et al, 2004;Trotta et al, 2004;Zhang and Broadie, 2005;Orso et al, 2005). To study these disease states, we have developed FXS and HSP models in the Drosophila genetic system (Zhang et al, 2001;Trotta et al, 2004;Zhang and Broadie, 2005).…”

“…To study these disease states, we have developed FXS and HSP models in the Drosophila genetic system (Zhang et al, 2001;Trotta et al, 2004;Zhang and Broadie, 2005). However, our studies, like all others (Huot et al, 2001;McDermott et al, 2003;Lu et al, 2004;Sherwood et al, 2004;Orso et al, 2005), have been hampered by the available, very indirect assays of microtubule states in neuronal synapses. Published studies are restricted to inferences based on immunocytochemistry of tubulin isoforms and microtubule associated proteins in static images of fixed tissue.…”