Interferes with fast axonal transport
A tissue-specific form of mutant Spastin activates a toxic compound that promotes abnormalities in the distribution of cell organelles by disrupting axonal transport. This was able to be corrected using drugs that inhibited the toxic compound.
Mutations of various genes cause hereditary spastic paraplegia (HSP), a neurological disease involving dying-back degeneration of upper motor neurons. From these, mutations in the SPAST gene encoding the microtubule-severing protein spastin account for most HSP cases.
Cumulative genetic and experimental evidence suggests that alterations in various intracellular trafficking events, including fast axonal transport (FAT), may contribute to HSP pathogenesis. However, the mechanisms linking SPAST mutations to such deficits remain largely unknown.
Experiments presented here using isolated squid axoplasm reveal inhibition of FAT as a common toxic effect elicited by spastin proteins with different HSP mutations, independent of microtubule-binding or severing activity. Mutant spastin proteins produce this toxic effect only when presented as the tissue-specific M1 isoform, not when presented as the ubiquitously-expressed shorter M87 isoform.
Biochemical and pharmacological experiments further indicate that the toxic effects of mutant M1 spastins on FAT involve casein kinase 2 (CK2) activation. In mammalian cells, expression of mutant M1 spastins, but not their mutant M87 counterparts, promotes abnormalities in the distribution of intracellular organelles that are correctable by pharmacological CK2 inhibition.
Collectively, these results demonstrate isoform-specific toxic effects of mutant M1 spastin on FAT, and identify CK2 as a critical mediator of these effects.
SOURCE: Hum Mol Genet. 2017 Apr 7. doi: 10.1093/hmg/ddx125. [Epub ahead of print] © The Author 2017. Published by Oxford University Press. All rights reserved. PMID: 28398512
Mutant spastin proteins promote deficits in axonal transport through an isoform-specific mechanism involving casein kinase 2 activation.
Leo L1, Weissmann C2, Burns M2, Kang M2,3, Song Y3,4, Qiang L1, Brady ST2,3, Baas PW1, Morfini G2,3.
1 Department of Neurobiology and Anatomy, Drexel University College of Medicine, Philadelphia, PA, USA.
2 Department of Anatomy and Cell Biology, University of Illinois at Chicago, Chicago, IL, USA.
3 Marine Biological Laboratory, Woods Hole, MA, USA.
4 Department of Genetics, School of Medicine, Yale University, New Haven, CT, USA.
Different mechanism of Spastin proposed
Toxicity and accumulation might be the cause of damage
Rather than impaired microtubule severing ability, the authors of this study suggest that greater toxicity and accumulation of a particular form of Spastin may cause the damage to corticospinal tracts in at least some forms of SPAST (SPG4) HSP.
The SPAST gene, which produces two isoforms (M1 and M87) of the microtubule-severing protein called spastin, is the chief gene mutated in hereditary spastic paraplegia. Haploinsufficiency is a popular explanation for the disease, in part because most of the over 200 pathogenic mutations of the gene are truncating, and expected to produce only vanishingly small amounts of shortened proteins.
Here we studied two such mutations, N184X and S245X, and our results suggest another possibility. We found that the truncated M1 proteins can accumulate to notably higher levels than their truncated M87 or wild-type counterparts.
Reminiscent of our earlier studies on a pathogenic mutation that generates full-length M1 and M87 proteins, truncated M1 was notably more detrimental to neurite outgrowth than truncated M87, and this was true for both N184X and S245X. The greater toxicity and tendency to accumulate suggest that truncated M1 could over time damage the corticospinal tracts of human patients.
Curiously, the N184X mutation triggers the re-initiation of translation at a third start codon in SPAST, resulting in synthesis of a novel M187 spastin isoform that is able to sever microtubules. Thus, microtubule severing may not be as reduced as previously assumed in the case of that mutation.
SOURCE: Mol Biol Cell. 2017 May 11. pii: mbc.E17-01-0047. doi: 10.1091/mbc.E17-01-0047. [Epub ahead of print] © 2017 by The American Society for Cell Biology. PMID: 28495799
Studies on truncating mutations of SPAST associated with Hereditary Spastic Paraplegia indicate greater accumulation and toxicity of the M1 isoform of spastin.
Solowska JM1, Rao AN1, Baas PW2.
1 Department of Neurobiology and Anatomy, Drexel University College of Medicine, 2900 Queen Lane, Philadelphia, PA 19129.
2 Department of Neurobiology and Anatomy, Drexel University College of Medicine, 2900 Queen Lane, Philadelphia, PA 19129 [email protected].