These papers represent exciting work describing a new genetic mutation associated with familial ALS. The results further highlight the importance for RNA processing in at least familial forms of motor neuron disease. Much work remains to determine the exact mechanisms by which FUS modulates motor neuron survival. It may be related to that of TDP-43. However, the lack of cytoplasmic aggregation of TDP-43, and rare ubiquitin inclusions in the patients with FUS mutations, suggest the mechanisms may be distinct. It is interesting that FUS protein did not accumulate in the cytoplasm of motor neurons in sporadic ALS patients, again suggestive that the pathogenic mechanisms of mutant FUS-induced motor neuron degeneration may be distinct from that in sporadic ALS.

View all comments by Robert Bowser

These studies raise interesting questions about whether one problem in ALS and perhaps other neurodegenerative diseases is that RNA trafficking proteins fail to properly deliver RNAs to dendritic spines. The paper by Kwiatkowski et al. reports evidence that wild-type FUS and TDP-43 may be involved in transporting RNA into dendrites, where it mediates local protein synthesis that can be stimulated by neural activity. The clumping of the mutant form described by both new papers could therefore perturb the transport of RNA. Local protein synthesis in dendrites plays a major role in the activity-dependent modulation of synaptic strength. Changes in synaptic activity have been recently reported in the mouse model of SOD1 mutation (van Zundert et al., 2008), so it will be worthwhile to examine this issue in the FUS mice that will certainly be developed by these investigators.

View all comments by Eric Frank

This is an extremely exiting story in the understanding of ALS pathogenesis. It actually it dates back to 1998—with the first description of mRNA processing errors in sporadic ALS (Lin et al., 1998), which, interestingly, was made not in the SOD1 mouse model. At the same time, the spinal muscular atrophy gene was discovered. SMA is not unlike a childhood ALS, though predominately lower motor neurons are affected in that disease. The SMA gene defect is involved in RNA metabolism. So for the next 10 years, the SMA field has investigated the pathobiology of the defective protein. At the time it made the link between sporadic ALS and the SMA story intriguing. But there was no clear genetic link (or cause for the changes in sporadic ALS).

Feed forward to 2008, when Chris Shaw and others found a true genetic defect in RNA metabolism-based protein TDP-43. (Of course more work needs to be done on that.) And now another gene by the Shaw group, and now verified by the group in Boston, does set a string of targets that all focus on RNA...  Read more

This is an extremely exiting story in the understanding of ALS pathogenesis. It actually it dates back to 1998—with the first description of mRNA processing errors in sporadic ALS (Lin et al., 1998), which, interestingly, was made not in the SOD1 mouse model. At the same time, the spinal muscular atrophy gene was discovered. SMA is not unlike a childhood ALS, though predominately lower motor neurons are affected in that disease. The SMA gene defect is involved in RNA metabolism. So for the next 10 years, the SMA field has investigated the pathobiology of the defective protein. At the time it made the link between sporadic ALS and the SMA story intriguing. But there was no clear genetic link (or cause for the changes in sporadic ALS).

Feed forward to 2008, when Chris Shaw and others found a true genetic defect in RNA metabolism-based protein TDP-43. (Of course more work needs to be done on that.) And now another gene by the Shaw group, and now verified by the group in Boston, does set a string of targets that all focus on RNA metabolism and (lower) motor neurons.

By the way, all these cases appear to predominately involve a lower motor neuron form of ALS. The hint from genetics does suggest more of a loss of function rather than gain, but cell biology will ultimately sort that out. We certainly await the generation of mouse or fly models, which are now well underway for TDP-43. However, this may be a particularly difficult target for specific, non-toxic drug therapy.

View all comments by Jeffrey D. Rothstein

These back-to-back papers on the identification of FUS (fused in sarcoma) gene as a new genetic component of ALS open a new era of research and direct our attention to mRNA biology with respect to disease. After the first identification of mRNA processing errors in ALS patients (Lin, Bristol et al., 1998), the discovery of TDP-43 (Neumann, Sampathu et al., 2006) and now the FUS gene clearly indicate the importance of mRNA management in neurodegenerative diseases. Defects in RNA transcription, splicing, and trafficking may be the reason for cell-type-specific degeneration of motor neurons in ALS. Motor neurons both in the cortex and spinal cord are very large excitatory neurons that extend long axons to their targets and require high levels of energy and protein integrity for survival and function. Defects in transcriptional mechanisms may result in splicing defects, which could give rise to formation of non-functional proteins that would deplete the pool of required proteins for cellular function, and these non-functional proteins may form aggregates that are toxic to neurons. In...  Read more

These back-to-back papers on the identification of FUS (fused in sarcoma) gene as a new genetic component of ALS open a new era of research and direct our attention to mRNA biology with respect to disease. After the first identification of mRNA processing errors in ALS patients (Lin, Bristol et al., 1998), the discovery of TDP-43 (Neumann, Sampathu et al., 2006) and now the FUS gene clearly indicate the importance of mRNA management in neurodegenerative diseases. Defects in RNA transcription, splicing, and trafficking may be the reason for cell-type-specific degeneration of motor neurons in ALS. Motor neurons both in the cortex and spinal cord are very large excitatory neurons that extend long axons to their targets and require high levels of energy and protein integrity for survival and function. Defects in transcriptional mechanisms may result in splicing defects, which could give rise to formation of non-functional proteins that would deplete the pool of required proteins for cellular function, and these non-functional proteins may form aggregates that are toxic to neurons. In addition, defects in the trafficking of mRNA may lead to depletion of key proteins that are in high demand locally for motor neuron function. But if FUS has a general function in mRNA transcription, splicing, and trafficking, why do mutations in this gene cause ALS and not other neurodegenerative diseases? What makes motor neurons more vulnerable in the presence of defective FUS? It could be true that in motor neurons FUS controls the transcription of a distinct set of mRNA that is expressed in a cell-type-specific manner in motor neurons, or that FUS controls the production of a key protein that is highly required in motor neurons when compared to other cell-types, and thus motor neurons may become vulnerable first. FUS seems to be the tip of the iceberg. Finding effectors, binding partners including mRNA, may lead to the identification of key components of both familial and sporadic ALS. More work is on the way!

References:

Kneussel M. Dynamic regulation of GABA(A) receptors at synaptic sites. Brain Res Brain Res Rev. 2002 Jun ;39(1):74-83. Abstract

Lin CL, Bristol LA, Jin L, Dykes-Hoberg M, Crawford T, Clawson L, Rothstein JD. Aberrant RNA processing in a neurodegenerative disease: the cause for absent EAAT2, a glutamate transporter, in amyotrophic lateral sclerosis. Neuron. 1998 Mar;20(3):589-602. Abstract

Neumann M, Sampathu DM, Kwong LK, Truax AC, Micsenyi MC, Chou TT, Bruce J, Schuck T, Grossman M, Clark CM, McCluskey LF, Miller BL, Masliah E, Mackenzie IR, Feldman H, Feiden W, Kretzschmar HA, Trojanowski JQ, Lee VM. Ubiquitinated TDP-43 in frontotemporal lobar degeneration and amyotrophic lateral sclerosis. Science. 2006 Oct 6;314(5796):130-3. Abstract

Vance C, Rogelj B, Hortobágyi T, De Vos KJ, Nishimura AL, Sreedharan J, Hu X, Smith B, Ruddy D, Wright P, Ganesalingam J, Williams KL, Tripathi V, Al-Saraj S, Al-Chalabi A, Leigh PN, Blair IP, Nicholson G, de Belleroche J, Gallo JM, Miller CC, Shaw CE. Mutations in FUS, an RNA processing protein, cause familial amyotrophic lateral sclerosis type 6. Science. 2009 Feb 27;323(5918):1208-11. Abstract

View all comments by P. Hande Ozdinler

We would like to comment on the interesting results of the recent study by Morfini et al. (1). Kinesin-1, a major microtubule motor that transports cargo in the plus-end direction of microtubules, is a heterotetramer consisting of two microtubule-binding, motor polypeptides (the heavy chains; KHCs) and two cargo-binding polypeptides (the light chains; KLCs). Being a soluble, cytoplasmic protein, kinesin-1 needs to bind the cargo in order to transport it. Therefore, recruitment of kinesin-1 to the cargo vesicle, and its release from it, are important regulatory steps of axonal transport. About 10 years ago, Scott Brady’s laboratory identified the first mechanism leading to the release of kinesin-1 from vesicles. According to this model, kinesin-1 is released through the action of the chaperone HSC70, and is nucleotide-dependent and NEM-sensitive (2). One year later, work from Larry Goldstein’s laboratory suggested that the premature release of kinesin-1 from cargo vesicles in neurons could impair fast axonal transport and lead to neuronal pathology and disease (3). Although the...  Read more

We would like to comment on the interesting results of the recent study by Morfini et al. (1). Kinesin-1, a major microtubule motor that transports cargo in the plus-end direction of microtubules, is a heterotetramer consisting of two microtubule-binding, motor polypeptides (the heavy chains; KHCs) and two cargo-binding polypeptides (the light chains; KLCs). Being a soluble, cytoplasmic protein, kinesin-1 needs to bind the cargo in order to transport it. Therefore, recruitment of kinesin-1 to the cargo vesicle, and its release from it, are important regulatory steps of axonal transport. About 10 years ago, Scott Brady’s laboratory identified the first mechanism leading to the release of kinesin-1 from vesicles. According to this model, kinesin-1 is released through the action of the chaperone HSC70, and is nucleotide-dependent and NEM-sensitive (2). One year later, work from Larry Goldstein’s laboratory suggested that the premature release of kinesin-1 from cargo vesicles in neurons could impair fast axonal transport and lead to neuronal pathology and disease (3). Although the mechanisms for the release of kinesin-1 from its vesicular cargos were incompletely understood at that time, the general idea that a premature release of the motor from its cargo could be at the core of the pathology in neurodegenerative diseases turned out to be correct, and generated an increased interest for research in this direction. Thus, work from the Brady and Busciglio laboratories identified at least two pathways for release of kinesin-1 from vesicles and halt of transport, which are likely to be factors leading to the axonal pathology and synaptic failure in Alzheimer’s disease (AD) (4-6).

Both pathways lead to phosphorylation of the KLCs, followed by detachment of kinesin-1 from the cargo, and impairment of vesicle transport. They are initiated by the addition of soluble Aβ oligomers, or expression of FAD-linked presenilin 1 variants, which trigger aberrant activation of casein kinase 2 or glycogen synthase 3β, which phosphorylate the KLCs. Why the phosphorylated kinesin-1 is released from vesicles is still not fully understood.

Along with AD, kinesin-1 is a target for abnormal phosphorylation in other neurodegenerative diseases, such as spinal and bulbar muscular atrophy (SBMA) and Huntington’s disease, as revealed by the studies from the Brady laboratory, including the work featured here (1, 7). However, in this case, the phosphorylation targets the KHCs, and the activated kinase that performs the phosphorylation is the cJun-N-terminal kinease (JNK). The phosphorylation of the KHCs leads to inhibition of binding of kinesin-1 to microtubules. As a result, the kinesin-1-cargo complex is released from the microtubules, and the transport is halted. These studies showed that the abnormal activation of JNK is triggered by the pathogenic, polyglutaminated, mutant proteins characteristic for polyglutamine (polyQ) expansion diseases: polyQ-androgen receptor in SBMA) (7), and polyQ-huntingtin in Huntingon’s disease(1). As the study by Morfini et al. (1) showed, polyQ-huntingtin activates JNK3, a neuron-specific JNK, that in turn phosphorylates KHC at a serine residue critical for the microtubule-binding function of kinesin-1. While in this case JNK3 is aberrantly activated by a disease factor, it is likely that, under normal conditions, the JNK-3 pathway contributes to the regulation of axonal transport.

Interestingly, in the squid axon system used in these studies, polyQ-huntingtin inhibits, not only the anterograde (kinesin-driven), but also the retrograde (cytoplasmic dynein-driven) fast axonal transport (1). It is not clear whether this inhibition of transport in both directions is due to the fact that kinesin-1 and cytoplasmic dynein interact and coordinate each other’s function (8), or is caused by a direct effect on the dynein machinery. Other studies showed that huntingtin regulates dynein-mediated vesicle transport, and can interact with both dynein and its accessory complex, dynactin (9, 10); however, the assays used by Morfini et al. (1) did not detect an interaction of huntingtin with dynein.

Certainly, other mechanisms, besides the release of the kinesin motor from the cargo or the microtubules, could contribute to the pathogenic processes in these neurodegenerative diseases. Other potentially damaging pathways that target the intracellular transport by affecting the cytoskeleton or the supply of ATP (by disrupting mitochondrial function) have been described (reviewed in (11)). Also, the activation of the kinases is likely to lead to the abnormal phosphorylation of other protein targets as well, with detrimental consequences for the function of neurons via mechanisms that may not involve abnormal axonal transport. For now, a picture emerges where the release of kinesin-1 from either cargo or microtubules, followed by impairment of axonal transport, becomes an important component of the pathogenic process in many neurodegenerative diseases. Therefore, it is the time to think of possibilities to correct the deficiencies, or to find means to enhance the disease-inflicted axonal transport.

References:
1. Morfini GA, You YM, Pollema SL, Kaminska A, Liu K, Yoshioka K, Björkblom B, Coffey ET, Bagnato C, Han D, Huang CF, Banker G, Pigino G, Brady ST. Pathogenic huntingtin inhibits fast axonal transport by activating JNK3 and phosphorylating kinesin. Nat Neurosci. 2009 Jul;12(7):864-71. Abstract

2. Tsai MY, Morfini G, Szebenyi G, Brady ST. Release of kinesin from vesicles by hsc70 and regulation of fast axonal transport. Mol Biol Cell. 2000 Jun;11(6):2161-73. Abstract

3. Kamal A, Almenar-Queralt A, LeBlanc JF, Roberts EA, Goldstein LS. Kinesin-mediated axonal transport of a membrane compartment containing beta-secretase and presenilin-1 requires APP. Nature. 2001 Dec 6;414(6864):643-8. Abstract

4. Morfini G, Szebenyi G, Elluru R, Ratner N, Brady ST. Glycogen synthase kinase 3 phosphorylates kinesin light chains and negatively regulates kinesin-based motility. EMBO J. 2002 Feb 1;21(3):281-93. Abstract

5. Pigino G, Morfini G, Atagi Y, Deshpande A, Yu C, Jungbauer L, Ladu M, Busciglio J, Brady S. Disruption of fast axonal transport is a pathogenic mechanism for intraneuronal amyloid beta. Proc Natl Acad Sci U S A. 2009 Apr 7;106(14):5907-12. Abstract

6. Pigino G, Morfini G, Pelsman A, Mattson MP, Brady ST, Busciglio J. Alzheimer's presenilin 1 mutations impair kinesin-based axonal transport. J Neurosci. 2003 Jun 1;23(11):4499-508. Abstract

7. Morfini G, Pigino G, Szebenyi G, You Y, Pollema S, Brady ST. JNK mediates pathogenic effects of polyglutamine-expanded androgen receptor on fast axonal transport. Nat Neurosci. 2006 Jul;9(7):907-16. Abstract

8. Ligon LA, Tokito M, Finklestein JM, Grossman FE, Holzbaur EL. A direct interaction between cytoplasmic dynein and kinesin I may coordinate motor activity. J Biol Chem. 2004 Apr 30;279(18):19201-8. Abstract

9. Caviston JP, Ross JL, Antony SM, Tokito M, Holzbaur EL. Huntingtin facilitates dynein/dynactin-mediated vesicle transport. Proc Natl Acad Sci U S A. 2007 Jun 12;104(24):10045-50. Abstract

10. Zala D, Colin E, Rangone H, Liot G, Humbert S, Saudou F. Phosphorylation of mutant huntingtin at S421 restores anterograde and retrograde transport in neurons. Hum Mol Genet. 2008 Dec 15;17(24):3837-46. Abstract

11. De Vos KJ, Grierson AJ, Ackerley S, Miller CC. Role of axonal transport in neurodegenerative diseases. Annu Rev Neurosci. 2008;31:151-73. Abstract

View all comments by Zoia Muresan
View all comments by Virgil Muresan

This important study establishes for the first time a genetic risk factor for sporadic ALS, thus providing a long-sought entry point into mechanisms and genetics of ALS. Genetic risk factors for neurodegenerative diseases will likely lead to novel insights into mechanisms of disease.

The impact of the chromogranin B mutations is slightly lower than that of the ApoE4 allele in Alzheimer disease. Because the chromogranin B mutations had an impact on onset time in familial ALS, and on risk of disease in sporadic ALS, the findings provide important support for the notion that sporadic and familial ALS are mechanistically related.

Chromogranin has been linked to mutant SOD1 by two previous studies. A frequently raised question is to what extent SOD1-based ALS mice are relevant in mimicking human disease. The association to chromogranin B in the SOD1 mouse model and now patient cohorts for both sporadic and familial ALS strongly supports the notion of converging cellular and molecular mechanisms of disease.

One important implication for scientists working on ALS is that the...  Read more

This important study establishes for the first time a genetic risk factor for sporadic ALS, thus providing a long-sought entry point into mechanisms and genetics of ALS. Genetic risk factors for neurodegenerative diseases will likely lead to novel insights into mechanisms of disease.

The impact of the chromogranin B mutations is slightly lower than that of the ApoE4 allele in Alzheimer disease. Because the chromogranin B mutations had an impact on onset time in familial ALS, and on risk of disease in sporadic ALS, the findings provide important support for the notion that sporadic and familial ALS are mechanistically related.

Chromogranin has been linked to mutant SOD1 by two previous studies. A frequently raised question is to what extent SOD1-based ALS mice are relevant in mimicking human disease. The association to chromogranin B in the SOD1 mouse model and now patient cohorts for both sporadic and familial ALS strongly supports the notion of converging cellular and molecular mechanisms of disease.

One important implication for scientists working on ALS is that the research mechanistically links SOD1 mutations to a set of mutations in chromogranin B. We now have potentially interacting mutations in two genes to study; this will likely lead to new disease models and hopefully to first elements of a molecular disease pathway. It will be interesting to determine whether there is a relationship between mutant SOD1 secretion and chromogranin B mutations. Should this be the case, it may suggest that the risk involves local secretion of misfolded proteins such as mutant SOD1. Of further interest are the implications for the role of ER stress pathways in the pathogenesis of ALS.

It will be important to determine whether other genes implicated in ALS (including the RNA metabolism genes TDP-43 and FUS) also synergize with chromogranin B mutations to promote disease. Whether chromogranin A variants link to ALS is definitely worth investigating as well.

View all comments by Pico Caroni