Direct evidence for axonal transport defects in a novel mouse model of mutant spastin‐induced hereditary spastic paraplegia (HSP) and human HSP patients

Abstract

Mutations in spastin are the most common cause of hereditary spastic paraplegia (HSP) but the mechanisms by which mutant spastin induces disease are not clear. Spastin functions to regulate microtubule organisation, and because of the essential role of microtubules in axonal transport, this has led to the suggestion that defects in axonal transport may underlie at least part of the disease process in HSP. However, as yet there is no direct evidence to support this notion. Here we analysed axonal transport in a novel mouse model of spastin‐induced HSP that involves a pathogenic splice site mutation, which leads to a loss of spastin protein. A mutation located within the same splice site has been previously described in HSP. Spastin mice develop gait abnormalities that correlate with phenotypes seen in HSP patients and also axonal swellings containing cytoskeletal proteins, mitochondria and the amyloid precursor protein (APP). Pathological analyses of human HSP cases caused by spastin mutations revealed the presence of similar axonal swellings. To determine whether mutant spastin influenced axonal transport we quantified transport of two cargoes, mitochondria and APP‐containing membrane bound organelles, in neurons from mutant spastin and control mice, using time‐lapse microscopy. We found that mutant spastin perturbs anterograde transport of both cargoes. In neurons with axonal swellings we found that the mitochondrial axonal transport defects were exacerbated; distal to axonal swellings both anterograde and retrograde transport were severely reduced. These results strongly support a direct role for defective axonal transport in the pathogenesis of HSP because of spastin mutation.

Inactivation of the hereditary spastic paraplegia-associated Hspd1 gene encoding the Hsp60 chaperone results in early embryonic lethality in mice

“…That the development of spastic paraplegia symptoms is caused by degeneration of primarily the spinal cord motorneurons is supported by post-mortem findings in patients (Deluca et al 2004) and the evidence of dying-back axonopathy in paraplegin-deficient (Ferreirinha et al 2004) and spastin-deficient mouse models (Kasher et al 2009;Tarrade et al 2006).…”

“…Future studies are needed to investigate whether or not the heterozygous Hspd1 +/− mice will develop symptoms of hereditary spastic paraplegia by age. Even though mice are both smaller and have a shorter life-span than humans, they have proven to be able to develop mitochondrial dysfunction and late-onset degeneration of the longest motor neurons in the spinal cord due to genetic inactivation of hereditary spastic paraplegia genes (Ferreirinha et al 2004;Kasher et al 2009;Tarrade et al 2006). Besides, if heterozygous Hspd1 +/− mice exhibit a discreet underlying mitochondrial stress phenotype before the onset of more pronounced disease symptoms, they will have the potential to serve as a modifying genetic background for mouse models of various diseases in which mitochondrial dysfunction is a potential common denominator (e.g., Parkinson's disease, amyotrophic lateral sclerosis, Alzheimer's disease, Huntington's disease, and multiple sclerosis; Dutta et al 2006;Kwong et al 2006).…”

Loss of Miro1-directed mitochondrial movement results in a novel murine model for neuron disease

“…It is not known, however, whether mitochondrial motility defects are a primary cause or a secondary consequence of MND progression. In addition, it has been difficult to isolate the primary effect of mitochondrial motility defects in MNDs because most mutations that impair mitochondrial motility in neurons also affect transport of other organelles and vesicles (1,(8)(9)(10)(11).In mammals, the movement of neuronal mitochondria between the cell body and the synapse is controlled by adaptors called trafficking kinesin proteins (Trak1 and Trak2) and molecular motors (kinesin heavy chain and dynein), which transport the organelle in the anterograde or retrograde direction along axonal microtubule tracks (7,(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24). Mitochondrial Rho (Miro) GTPase proteins are critical for transport because they are the only known surface receptors that attach mitochondria to these adaptors and motors (12-15, 18, 25, 26).…”

“…It is not known, however, whether mitochondrial motility defects are a primary cause or a secondary consequence of MND progression. In addition, it has been difficult to isolate the primary effect of mitochondrial motility defects in MNDs because most mutations that impair mitochondrial motility in neurons also affect transport of other organelles and vesicles (1,(8)(9)(10)(11).…”

Rare disease models provide insight into inherited forms of neurodegeneration

“…A mouse model of SPG4, caused by a loss of function mutation in spastin, resulted in disrupted anterograde (i.e. towards the distal ends of neurons) axonal transport of mitochondria 21 . Separately, olfactory mucosal cells obtained from a patient with Spastin mutations and differentiated to an adherent cell culture were found to exhibit a reduction in the presence of mitochondria at the cell periphery 22 .…”