Formation of high molecular weight complexes of mutant Cu,Zn-superoxide dismutase in a mouse model for familial amyotrophic lateral sclerosis

Abstract

Deposition of aggregated protein into neurofilament-rich cytoplasmic inclusion bodies is a common cytopathological feature of neurodegenerative disease. How-or indeed whether-protein aggregation and inclusion body formation cause neurotoxicity are presently unknown. Here, we show that the capacity of superoxide dismutase (SOD) to aggregate into biochemically distinct, high molecular weight, insoluble protein complexes (IPCs) is a gain of function associated with mutations linked to autosomal dominant familial amyotrophic lateral sclerosis. SOD IPCs are detectable in spinal cord extracts from transgenic mice expressing mutant SOD several months before inclusion bodies and motor neuron pathology are apparent. Sequestration of mutant SOD into cytoplasmic inclusion bodies resembling aggresomes requires retrograde transport on microtubules. These data indicate that aggregation and inclusion body formation are mechanistically and temporally distinct processes.F amilial amyotrophic lateral sclerosis (FALS), a dominantly inherited form of ALS, is a progressive paralytic disorder resulting from the degeneration of motor neurons in the cortex, brainstem, and spinal cord (1, 2). Between 10% and 20% of FALS cases are because of missense mutations in the SOD1 gene, which encodes the cytoplasmic metalloenzyme Cu, Znsuperoxide dismutase (SOD) (3). Mice expressing human FALS-linked SOD1 transgenes develop an age-dependent ALSlike disorder characterized by profound degeneration of spinal motor neurons and by the presence in surviving motor neurons of neurofilament-rich cytoplasmic inclusions resembling pathological inclusion bodies in spinal motor neurons in human ALS and FALS. The highly penetrant dominant inheritance pattern of both the human and mouse diseases (4), together with the absence of motor neuron disease from ''knockout'' mice lacking endogenous murine SOD1 (5), strongly suggests that FALS pathology is because of a toxic gain-of-function in SOD. The biochemical nature of this toxic gain of function, however, and the mechanism by which SOD mutations cause the degeneration of motor neurons have remained elusive, largely because of the failure to identify novel properties of mutant SOD that are unambiguously linked to early cytopathological changes.One hypothesis argues that toxicity results from the tendency of mutant SOD to ''aggregate'' into cytoplasmic inclusion bodies (6) that are evident in motor neurons from SOD transgenic mice (7,8) and in cultured COS cells (9) or motor neurons (10) expressing mutant SOD cDNA. Cytoplasmic inclusion bodies are a hallmark of motor neuron degeneration in ALS and, indeed, of nearly all neurodegenerative diseases (11). SOD is itself a component of inclusion bodies in degenerating spinal cords from FALS patients (12, 13) and in end-stage mice expressing human FALS-linked SOD1 transgenes (6,7,12). How these inclusion bodies could cause neuronal degeneration-and indeed whether the inclusions are cytotoxic or even, perhaps, cytoprotective-is controversial. It has been suggested that ne...

Heterozygous missense mutations in BSCL2 are associated with distal hereditary motor neuropathy and Silver syndrome

“…But we determined that seipin is glycosylated and that both N88S and S90L affect the consensus sequence for N-glycosylation. GFP-labeled mutant seipin also results in aggregates, which may share some features in common with aggresomes, a well known characteristic of a steadily increasing number of neurodegenerative disorders [14][15][16] . The fact that mutations in the N-glycosylation sequence of seipin seem to be specifically required to cause dHMN-V and Silver syndrome, whereas null mutations result in BerardinelliSeip congenital lipodystrophy type 2, supports the hypothesis that a gain-of-function mechanism underlies the motor neuron degeneration in these conditions.…”

Are amyloid diseases caused by protein aggregates that mimic bacterial pore-forming toxins?

“…There is mounting evidence that the sequestration process may be an active one. Expression of any one of the four disease-associated proteins ; cystic fibrosis transmembrane conductance regulator (CFTR, associated with cystic fibrosis) ( Johnston et al 1998), SOD1 (amyotrophic lateral sclerosis) ( Johnston et al 2000), the androgen receptor (AR, X-linked spinobulbar muscular atrophy) , or Parkin ( Junn et al 2002) in cultured cells under conditions of oxidative stress or proteasome impairment leads to sequestration of the mutant protein within multicomponent proteinaceous inclusions known as aggresomes. Sequestration is actively mediated by microtubules ( Johnston et al 1998).…”

Efficient Induction of Nuclear Aggresomes by Specific Single Missense Mutations in the DNA-binding Domain of a Viral AP-1 Homolog

“…Cytoplasmic aggresomes form by retrograde dynein-dependent transport of misfolded protein aggregates along microtubules toward the microtubule-organizing center (2,6,7,9). The microtubule-associated histone deacetylase HDAC6 was shown to play an essential role in the transport of polyubiquitinated misfolded protein aggregates by its ability to interact with both ubiquitin and dynein motors (10,11). Other ubiquitin-binding proteins, including protein linking integrin-associated protein to cytoskeleton and ataxin-3, were also shown to regulate targeting of proteins to aggresomes.…”