HSP Flashcards
What are the four characteristics of HSP?
- Heterogenous group of NDD
- Clinically progressive spasticity and weakness of lower limbs
- Pathologically characterised by the retrograde axonal degeneration of the cortical spinal tracts and the posterior colums
- Hypothesis of axonal transport
Where did the axonal tranposrt hypothesis come from
In recent years genetic studies have identified crucial genes for the maintenance of axonal homeostasis in HSP. If disrupted, these genes lead to the disruption and dsyregulation of axonal transport of macromolecules, organelles and other cargoes, which predominantly effect distal neuronal regions.
Where does retrograde axonal degeneration occur?
In the longest nerve fibres of the cortical spinal tracts and posterior columns
What are the 4 key clinical diagnostic findings?
Progressive lower limb spasticity, pyramidal weakness, hyper-reflexia and extensor plantar responses
What is the prevelance of HSP in europe
Although few epidemiological studies have been performed it is predicted that between 3-4 in every 100,000 have HSP
How many loci have been mapped, and how many genes
41 loci, 17 genes
What is the most prevelant form of inheritence
Autosomal Dominant in 70% of cases
What gene is disupted in 40-50% of cases
SPG4 encoding spastin, onset caries between childhood to late adult life - complete loss of mobility occurs 20 years after onset, thanks to its prevelance it is the most widely studied form
What gene is disrupted in 10% of cases
SPG3 encoding atlastin - typically presenting with a pure yet late onset with a slow progression of symptoms
What was the original argument for the pathogenesis of HSP
Originally argued, along the findings of Casari et al. 1998, that it was owed to mitochondrial deficits
What did Casari et al. 1998 find?
In SPG7 mutant families they recognised clinical characteristics identical to that of mitochondrial dsyfunction in muscle biopsies of those with HSP and mito disease
How “mastered” the theory of mitochondrial disorder in the pathogenesis of HSP?
Ferreirinha et al. 2001 - findings - theory was made in 2004
What did Ferreirinha et al. 2001 discover
Following their study of SPG7 knockout mice they formulated their theory. They generated a successful mouse model which fully embodied the cellular characteristics of HSP. They demonstrated that long before physical phenotypes occured their were EM studies demonstrating axons to be filled with abnormal mitochondria, subsequently swollen axons containing accumulate organelles and neurofilaments were observed, suggestig mito dsyfunction may lead to neurodegen through impairing axonal transport. However, their motor assessment of the mice could be quarrelled, and ultimately SPG7 mutations (paraplegin) only account for ~30 families of HSP.
Who took the mito theory further with the development of the axonal transport theory
Crosby et al. 2012
What must the hypothesis of HSP pathogenesis take into account?
The pathophysiological mechanism proposed must explain why the longest neurons are effected
Why is the axonal transport theory so attractive?
It supports the selectivity of the disease pathogenesis, in that longer axons are more susceptible due to failures in axonal trafficking and transport, and are more dependent on mitochondrial function for their great energy demands.
Additionally the causitive mutations are all implicated in axonal transport and mito function.
Finally it has been supported by both in vivo and in vitro studies.
On what basis can the mito theory be argued against.
- Motor studies didnt show much
- ~30 families only
- EM studies were performed at large time intervals, potentially overlooking the intermediate stages involving other mechanisms
What are the genes implicated in trafficking dsyfunction?
KIF5A, SPG4, SPG3, NIPA1, ZFVE26, SG20
What is encoded by KIF5A
kinesin heavy chain –> Reduced microtubule affinity and gliding velocity
What is encoded by SPG4
SPASTIN –> Swelling and mito present
What is encoded by SPG3
ATLASTIN –> golgi localised, interaction with spastin (implicated in either signalling or recruiting) and membrane distribution
What is encoded by NIPA1
Non-imprinted in Prader/Willi/Angelman syndrome region protein 1 –> Mg2+ transport alters endosomal trafficking
What is encoded by SPG11
Spasticsin –> thought to be implicated in trafficking
What is encoded by ZFVE26
Spastizin –> localised in the ER and with endosomes, indicating a role in trafficking
What is encoded by SPG20
Spartin –> EGFR trafficking and endocytosis
What motor governs retrograde transport
dynein, a member of the AAA (atpase associated with diverse activities)
What motor governs anteretrograde transport
kinesin, deletion of khc and khl in drosophila led to accumulation of mitochondria and synaptic vesicles
What did Saxton et al. show in 1991
That deletion of khc and klc lead to the accumulation of mito and synpatic vesicles in axons
What is the role of the heavy and light chains of kinesin
Heavy - microtubule binding and ATPase function - effected by mutations
Light - cargo selectivity
What provides the most direct evidence to support axonal transport hypothesis?
KIF5A mutations, they are associated with an early onset, pure form of HSP, with mutant studies demonstrating reduced cargo flux owed to lowered microtubule affinity and lowered gliding velocity
Who provided findings for Spastins function in microtubule maintenance?
Errico 2002, Salinas 2007, Yu 2008
What did Errico 2002 show (4 findings)
- By expressing wild-type or ATPase-defective spastin in several cell types, they showed that spastin interacts dynamically with microtubules.
- Spastin association with the microtubule cytoskeleton is mediated by the N-terminal region of the protein, and is regulated through the ATPase activity of the AAA domain.
- Expression of all the missense mutations into the AAA domain, which were previously identified in patients, leads to constitutive binding to microtubules in transfected cells and induces the disappearance of the aster and the formation of thick perinuclear bundles, suggesting a role of spastin in microtubule dynamics.
- Consistently, wild-type spastin promotes microtubule disassembly in transfected cells. These data suggest that spastin may be involved in microtubule dynamics similarly to the highly homologous microtubule-severing protein, katanin.
What did Salinas 2007 show
Deletion of AAA domain demonstrated abolished AAA severing activity, however it MTs could still bundle, which could explain why in axonal swelling neurofilaments of MT are found
What did Yu 2008 show
Overexpression - increased neuronal branching
Depletion - lead to shorter neurons
In Rat hippocampal cells
–> This was whilst comparing it to the function of katanin - a v. homologous protein
Who provided evidence for spastins role in axonal trafficking
tarade 2006, Kasher 2009
What did tarade 2006 show
In a mouse model with a deletion mutation in SPAST that leads to a premature stop codon (mimicking a human pathogenic mutant), axonal degeneration and accumulation of mitochondria in abnormal swellings close to the axonal growth cone were described.
What did Kasher 2009 show
Supported these findings with analysis of patient neurons in vitro with culture iPSCs (neuron derived) and post mortem analysis
Who argued for a gain of function mutation
Solowska 2008
What did Solowska 2008 show
Pathogenic form is formed from the first ATG (instead of 2nd), expressed in squid and this inhibited fast axonal transportwhilst the shorter peptide of the second ATG did not present the same deleterious effect - following a study demonstrating that exon 2 translated peptide was more prominent in spinal cord (only 20-25% of total spastin in the spinal cord)
What are the domains of spastin
N-terminal MIT MTBD and AAA
What domain do mutations effect?
AAA domain through missense or truncation
What did atlastin previously be thought to be involved in
Neurite outgrowth and membrane trafficking, on the basis of its similarity to proteins of the dynamin family of large GTPases
what is atlastin now implicated in
Functioning in ER and golgi morphogenesis
What did evans et al. 2006 show
The interaction of spastin NTD with the CTD of atlastin, they additionally identified a clinical mutation ins1688A which prevents this interaction, potentially highlighting a common pathogenic mechanism?
What are the implications of atlastins function in ER and golgi morphogenesis
Might intefere with membrane distribution and polarity of the corticospinal neurons
What is the function of paraplegin
It is an AAA family protein, that is localised to the mitochodnrila IMM and is implied in mito protein quality control
