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Genetic Models of Chronic Neuronal Degeneration Causing Hereditary Spastic Paraplegia

Final Report Summary - SPASTICMODELS (Genetic Models of Chronic Neuronal Degeneration Causing Hereditary Spastic Paraplegia)

Neurodegenerative disorders, a heterogeneous group of chronic progressive diseases, are among the most puzzling and devastating diseases in medicine. Indeed, they are characterised by onset in adult life, distinct clinical phenotypes, and specific degeneration of subsets of neurons and axons. Hereditary spastic paraplegia (HSP) is a disorder that results in progressive weakness and spasticity of the lower limbs affecting approximately 1 in 10 000 individuals. Heterogeneity characterises HSP in both clinical and genetic aspects. Electrophysiological and pathological findings point to the corticospinal tracts, dorsal columns and the spinocerebellar fibres as the structures primarily affected by HSP.

Two main pathogenetic hypotheses for the neurodegeneration seen in HSP have recently emerged, suggesting that impaired mitochondrial function and / or defective subcellular transportation mechanics play a crucial role. In fact, HSP-causing mutations have been found in gene products involved in mitochondrial function, such as paraplegin and HSP60, and axonal trafficking, such as kinesin and, possibly, spastin and spartin.

HSP has been classified traditionally as 'pure' or 'complicated', depending on whether spastic paraplegia is the only symptom or whether it is found in association with other neurological abnormalities, such as optic neuropathy, retinopathy, extrapyramidal symptoms, dementia, ataxia, mental retardation and deafness.

Neuropathological analyses of tissues from a small number of individuals with pure HSP have shown axonal degeneration involving the more distal portions of the longest motor and sensory axons of the central nervous system (i.e. the crossed and uncrossed corticospinal tracts, the fasciculus gracilis and the spinocerebellar tracts). A specific pattern of degeneration is seen in HSP, during which the cell bodies remain largely intact while the degeneration is principally limited to the cell axon and may be a 'dying back' axonopathy, beginning distally and proceeding towards the cell body.

Spastin interacts with microtubules and seems to modulate microtubule dynamics, an essential attainment for maintenance of long axons. Although this major role of spastin in the cytoplasm, one of the project partners recently showed evidence that spastin is also a nuclear protein. Nuclear targeting of spastin is regulated through two mechanisms, the usage of alternative translational start sites and active nuclear export to the cytoplasm.

The specificity of the axonal damage in HSP apparently contrasts with the wide distribution and range of functions carried out by the few gene products known to cause the clinical phenotypes. However, we think to envisage two possible pathogenetic mechanisms. The mitochondrial way seems to affect neuronal metabolism, in particular at the axon extremity, by reducing the available energy. On the other hand, the cellular trafficking impairment would limit the turnover of fresh molecules and organelles to and from the periphery of neurons. Both mechanisms would result in a dying-back effect, typical of this late progressive neurodegeneration.

A possible unifying alternative mechanism can be hypothesised: impaired mitochondrial function generates huge aberrant mitochondria, which engulf the peripheral cellular trafficking.

In the proposal, we planned to answer to this and other open questions on this specific neurodegeneration by developing a formidable set of cellular and animal models of HSP. The workplan included the development of seven novel animal models beside the further characterisation of the paraplegin null mutant already generated, the functional characterisation of the HSP mitochondrial dysfunction and the impaired mitochondrial protein quality control, and the relationship between defective trafficking dynamics and axonal degeneration.

More specifically, we have obtained two mouse models (one null and one carrier of a missense mutation) for the paraplegin interactor Afg3l2, the conditional mutant for the prohibitin Phb2 and the HSP60 null mutant. Conditional knockout model for Afg3l1 and spartin and spastin null models are being generated. This large set of murine models has been also used to generate double mutants to study the possible genetic interactions among the different genes and to identify redundant or independent pathways. A fine characterisation of the mouse phenotypes have been performed for all the available models, following a common procedure including behavioural, neuropathological and biochemical studies.

Mutant mouse tissues and derived cell lines have been examined for mitochondrial protein quality control, protein folding and degradation efficiency. In addition, the characterisation of the different m-AAA complexes in human and mice and the identification of binding partners of the human m-AAA protease have been performed.

Furthermore, the role of spastin and spartin has been investigated to address the transportation hypothesis and their role in vesicular dynamics. This is accomplished by identifying interacting partners by yeast two-hybrid studies. The different mouse models will provide an invaluable resource for the investigation of the transportation systems in HSP and range of primary cultures are now being used to examine the axonal transport by neuronal tracers.

Participants have direct connections to patients associations and charity organisations that have great impact to non-specialised general audience in Europe. In particular, the Tom-Wahlig-Stiftung (please see online) and the Italian Telethon Foundation (please see online) will be updated with the new results obtained within the consortium and translated to information comprehensible to a large audience.

We have prepared a set of information (one or several board members in charge) on hereditary spastic paraplegia and related neurodegenerative diseases, the relevance of research on this topic and the expertise of the Sixth Framework Programme (FP6) funded project. Every consortium member can use this basic material to inform local press and other channels on accomplishments of the project.