. Axonal transport deficits and degeneration can evolve independently in mouse models of amyotrophic lateral sclerosis. Proc Natl Acad Sci U S A. 2012 Mar 13;109(11):4296-301. PubMed.

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  1. The mechanisms that lead to synaptic failure, neuronal degeneration, and—finally—neuronal death in most neurodegenerative diseases are still poorly characterized. Abnormal axonal transport has been frequently brought into discussion as a possible cause of the neuronal pathology, especially because the task of transporting organelles, proteins, and mRNAs to their remote sites of function is much more difficult in neurons compared to other cell types, where transport distances are usually short. Certainly, axonal transport appears impeded in many animal models of neurodegenerative diseases, and it is conceivable to assume that the same would also hold true in disease-afflicted human neurons. However, whether the abnormal axonal transport is a cause, a contributing factor, or simply a consequence of the neuronal pathology is still an open question (see, e.g., [1]).

    In their recent paper discussed here, Marinkovic et al. address this question with regard to amyotrophic lateral sclerosis (ALS), using transgenic mouse models of familial ALS (FALS) carrying mutated, human SOD1 genes. By analyzing the transport of fluorescently tagged mitochondria and endocytic vesicles in explanted nerves, the authors set out to determine whether any detected abnormalities in transport precede, or are subsequent to, axon degeneration. Briefly, when performing this analysis in several mouse models of FALS, they find no consistent temporal correlation between the onset of axonal transport deficiencies and the first signs of neuronal degeneration. In some cases, motor neurons with severely compromised transport appear to show normal morphology and function, while in others, transport of mitochondria and endocytic vesicles seems unperturbed in neurons with degenerated axons. Marinkovic et al. conclude that deficits in axonal transport—at least of mitochondria and some endocytic vesicles—and the neuronal degeneration in ALS are independent processes. In other words, ALS-specific neurodegeneration is likely caused by factors other than axonal transport deficiencies. Considering the body of previously published data—strongly suggesting that the transport of mitochondria deteriorates prior to neurodegeneration, and likely causes the synaptic alterations in motor neurons—their results are somewhat surprising.

    The publication of this paper coincides with the publication last month of another paper analyzing transport of mitochondria in the context of a FALS-specific mutation in SOD1 (2). The mutation, G93A, is one of the mutations analyzed by Marinkovic et al. Like Marinkovic et al., Magrané et al. (2) find that transport of mitochondria is reduced in motor neurons. However, unlike Marinkovic et al., Magrané et al. find that the motor neurons expressing mutant SOD1 show severe synaptic alterations. Based mostly on correlative studies, the latter group concludes that "impaired mitochondrial dynamics may contribute to the selective degeneration of motor neurons in SOD1-FALS" (2). There are other, smaller discrepancies between the two studies, for example, the extent by which both anterograde and retrograde axonal transport of mitochondria are affected, or the effect of overexpressing of wild-type SOD1 on axonal transport (Marinkovic et al. find that overexpression of wild-type SOD1 causes significant reduction of mitochondrial transport, while Magrané et al. find no effect). These could be due to the different experimental systems used by the two groups: transgenic mice versus transgenic rats, nerve explants versus neuronal cultures, and—most importantly—adult neurons in situ versus embryonic neurons. Yet, the major discrepancy between the two studies is in the conclusions they reach; this is certainly due to the fact that Marinkovic et al. perform their analysis not only on one, but on several, mouse models of ALS. It would be interesting to see what results Magrané et al. would have obtained in the context of the other SOD1 mutations investigated by Marinkovic et al.

    We think that any conclusion on the role of an abnormal axonal transport on neuronal pathology in ALS—as in any other neurodegenerative disease—should not be drawn solely based on transport of mitochondria or other slowly moving cargo. Mitochondria do not represent the typical cargo for fast axonal transport. They use several molecular motors, and switch quite frequently the direction of movement. Most importantly, mitochondria are stationary most of the time. When they move, they do so by translocating mostly over short distances. Long-distance transport of mitochondria is quite infrequent. If needed in a particular location along the axon, they are brought mostly from vicinal locations. Of course, their importance as providers of energy and relief from oxidative stress makes them particularly important. However, the deficiency in the transport of a cargo that normally is robustly transported over long distances would probably be more detrimental to the health of the neuron than a slightly reduced transport rate of mitochondria.

    On the other hand, even a subtle slowing down of axonal transport could become detrimental, if it triggers a secondary response that itself leads to neurodegeneration. We have recently proposed such a mechanism, where a slowed down axonal transport could trigger a stress response—aimed at correcting axonal transport—that itself leads to the neuronal pathology typical for Alzheimer's disease (3). Certainly, many more studies are needed before one can conclude that a deficient axonal transport causes—or does not cause—the neuronal pathologies characteristic for the various neurodegenerative diseases. One of the difficulties may come from not knowing exactly what is the normal range of axonal transport, and how much variability in transport can be tolerated by neurons.

    References:

    . Is abnormal axonal transport a cause, a contributing factor or a consequence of the neuronal pathology in Alzheimer's disease?. Future Neurol. 2009 Nov 1;4(6):761-773. PubMed.

    . Mitochondrial dynamics and bioenergetic dysfunction is associated with synaptic alterations in mutant SOD1 motor neurons. J Neurosci. 2012 Jan 4;32(1):229-42. PubMed.

    . A persistent stress response to impeded axonal transport leads to accumulation of amyloid-β in the endoplasmic reticulum, and is a probable cause of sporadic Alzheimer's disease. Neurodegener Dis. 2012;10(1-4):60-3. PubMed.

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