When neurons degenerate…
Be it tasting your favorite wine, admiring a Van Gogh painting, listening to music, or smelling the perfume of your beloved, it is all mediated by the nervous system. The nervous system controls the functioning of all organs of our body. The motor system is essential for voluntary movements, and the sense of touch is conveyed by the sensory nervous system. As a consequence, nervous system dysfunction, as is the case in neurological disorders, can jeopardize one or several body functions.
In particular, neurodegenerative disorders can have devastating effects on the quality of life, and are often fatal. These disorders typically strike adult or adolescent people, are progressive in nature, and are incurable. Despite the high incidence of neurodegeneration and its increasing medical significance, our present understanding of the molecular pathogenesis of these diseases is still quite incomplete.
In our group, we seek to unravel the molecular pathogenesis of the motor neurodegenerative disorder ALS, and of Charcot-Marie-Tooth disease, a peripheral motor and sensory neuropathy. To this purpose, we combine the power of Drosophila and mouse genetics with novel disruptive technologies such as deep sequencing and high-end imaging.
Amyotrophic Lateral Sclerosis
Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disorder, characterized by the selective loss of both upper and lower motor neurons. This results in progressive muscle atrophy, weakness and fasciculation, as well as spasticity. In the United States, ALS is commonly known as Lou Gehrig’s disease, named after the famous baseball player in the 1920s and 1930s, who succumbed to the disease at age 38. ALS inevitably results in death, usually due to respiratory insufficiency, within 3 to 5 years after diagnosis. Apart from Lou Gehrig, other famous ALS victims include the Russian composer Dmitri Shostakovich, the Chinese military and political leader Mao Zedong, and the jazz composer and bassist Charles Mingus. Probably the most famous victim of this devastating disease is Stephen Hawking, English theoretical physicist and cosmologist. Unlike most ALS patients, Hawking suffers from a markedly mild form of the disease – he has been living with ALS for more than 40 years. Nevertheless, he cannot walk, talk, breathe easily, swallow and has difficulty in holding up his head. Still, one has to admire his attitude: “I have been lucky, that my condition has progressed more slowly than is often the case. But it shows that one need not lose hope.”
The vast majority (90%) of ALS patients suffers from a sporadic form of the disease, without an apparent family history, but 10% of cases have familial ALS (FALS). Causative mutations in the SOD1 gene have been identified in 20% of FALS cases, but despite the availability of mutant SOD1 mouse models since 1994, the molecular pathogenesis of ALS has remained enigmatic. The current hypothesis is that the molecular pathogenesis of ALS is "complex", in that simultaneous dysfunction of multiple cellular and molecular pathways may ultimately converge into motor neuron degeneration. These pathways may include oxidative stress, protein aggregation, mitochondrial dysfunction, axonal transport defects, excitotoxicity, ER stress and insufficient growth factor signaling. However, ALS is still an incurable disease, and the only drug with proven efficacy is riluzole (Rilutek), which prolongs life by approximately 3 months, without affecting disease progression. For these reasons, there is an urgent need to gain new and better insights into the molecular pathogenesis of ALS, and to identify therapeutic targets.
Recently, dominant mutations in FUS, TDP-43 and SETX have been identified as a genetic cause of FALS. These genes encode nuclear proteins that are thought to be involved in multiple steps of gene expression, including regulation of transcription, pre-mRNA splicing, mRNA nuclear export, mRNA subcellular localization, and possibly microRNA biogenesis. It is still not resolved whether mutations in these genes cause ALS through a toxic gain of function mechanism, or a loss of function mechanism, or a combination of both. Nevertheless, the striking functional similarity between these three ALS-associated proteins strongly suggests that defects in RNA biogenesis may contribute to ALS pathogenesis.
Charcot-Marie-Tooth disease (CMT) – also known as hereditary motor and sensory neuropathy (HMSN) – is the most common inherited neuromuscular disorder, characterized by length-dependent degeneration of peripheral motor and sensory nerves. This results in distal muscle wasting and weakness, sensory loss, reduced tendon reflexes and foot deformities. CMT is named after the French neurologists Jean-Martin Charcot and Pierre Marie, and the British neurologist Howard Henry Tooth, who were the first to describe the disease in 1886. Traditionally, CMT is divided into 2 major clinical entities: demyelinating forms (CMT1), in which nerve conduction velocities (NCVs) are severely reduced, and axonal forms (CMT2), in which NCVs are normal or slightly reduced. More recently, a third class has been added, dominant intermediate CMT (DI-CMT), which is characterized by intermediate NCVs and histological evidence of both axonal degeneration and demyelinating features. DI-CMT type C (DI-CMTC) is caused by dominant mutations in the gene encoding tyrosyl-tRNA synthetase (YARS), an enzyme that catalyzes the aminoacylation of tRNATyr with tyrosine. So far, three DI-CMTC associated mutations have been identified: two missense mutations (G41R and E196K) and one 12-bp in-frame deletion that results in the deletion of 4 amino acids in the YARS protein (153-156delVKQV). Interestingly, besides YARS, dominant mutations in two other tRNA synthetases, glycyl-tRNA synthetase (GARS) and alanyl-tRNA synthetase (AARS) cause axonal forms of CMT. The molecular mechanisms through which mutations in YARS, GARS and AARS lead to peripheral motor and sensory neuropathy are currently unknown, and no effective therapies are available.
A Drosophila model for DI-CMTC
We have generated a Drosophila model for DI-CMTC, as expression of the three mutant – but not wild type – YARS proteins in Drosophila recapitulated several hallmarks of the human disease, including a progressive deficit in motor performance, electrophysiological evidence of neuronal dysfunction and morphological signs of axonal degeneration. Not only ubiquitous, but also neuron-specific expression of mutant YARS, induces these phenotypes, indicating that the mutant enzyme has cell-autonomous effects in neurons. Since DI-CMTC is characterized by both demyelination and axonal degeneration, this finding suggest that axonal degeneration in DI-CMTC patients is not just secondary to demyelination. Finally, biochemical and genetic complementation experiments revealed that loss of enzymatic activity is not a common feature of DI-CMTC associated mutations, indicating that the DI-CMTC phenotype is not due to haploinsufficiency of aminoacylation activity (Storkebaum et al. 2009, Proc. Natl. Acad. Sci. USA 106: 11782-11787). This is consistent with the finding that approximately half of the CMT2D-associated mutations in GARS have no effect on aminoacylation activity, whereas the other half displays severe reduction or loss of aminoacylation activity. Obviously, this does not exclude the possibility that in neurons, defects in local protein translation may occur, since mislocalization of the mutant YARS proteins may result in defects in local protein synthesis.
Keywords: neurodegeneration, ALS, CMT, tRNA synthetase, RNA biogenesis