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Review
. 2016 Sep 13:10:204.
doi: 10.3389/fncel.2016.00204. eCollection 2016.

A Case for Microtubule Vulnerability in Amyotrophic Lateral Sclerosis: Altered Dynamics During Disease

Affiliations
Review

A Case for Microtubule Vulnerability in Amyotrophic Lateral Sclerosis: Altered Dynamics During Disease

Jayden A Clark et al. Front Cell Neurosci. .

Abstract

Amyotrophic lateral sclerosis (ALS) is an aggressive multifactorial disease converging on a common pathology: the degeneration of motor neurons (MNs), their axons and neuromuscular synapses. This vulnerability and dysfunction of MNs highlights the dependency of these large cells on their intracellular machinery. Neuronal microtubules (MTs) are intracellular structures that facilitate a myriad of vital neuronal functions, including activity dependent axonal transport. In ALS, it is becoming increasingly apparent that MTs are likely to be a critical component of this disease. Not only are disruptions in this intracellular machinery present in the vast majority of seemingly sporadic cases, recent research has revealed that mutation to a microtubule protein, the tubulin isoform TUBA4A, is sufficient to cause a familial, albeit rare, form of disease. In both sporadic and familial disease, studies have provided evidence that microtubule mediated deficits in axonal transport are the tipping point for MN survivability. Axonal transport deficits would lead to abnormal mitochondrial recycling, decreased vesicle and mRNA transport and limited signaling of key survival factors from the neurons peripheral synapses, causing the characteristic peripheral "die back". This disruption to microtubule dependant transport in ALS has been shown to result from alterations in the phenomenon of microtubule dynamic instability: the rapid growth and shrinkage of microtubule polymers. This is accomplished primarily due to aberrant alterations to microtubule associated proteins (MAPs) that regulate microtubule stability. Indeed, the current literature would argue that microtubule stability, particularly alterations in their dynamics, may be the initial driving force behind many familial and sporadic insults in ALS. Pharmacological stabilization of the microtubule network offers an attractive therapeutic strategy in ALS; indeed it has shown promise in many neurological disorders, ALS included. However, the pathophysiological involvement of MTs and their functions is still poorly understood in ALS. Future investigations will hopefully uncover further therapeutic targets that may aid in combating this awful disease.

Keywords: amyotrophic lateral sclerosis; axon transport; dynamics; microtubules.

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Figures

Figure 1
Figure 1
Modifications to neuronal microtubules (MT) in amyotrophic lateral sclerosis (ALS). (A) Normal microtubule +TIP dynamics and kinesin/dynein transport, mRNA granule transport and chemical modifications. (B) Mutant superoxide dismutase 1 (SOD1) expression leading to microtubule hyperdynamics, increased +TIP protein density, decreased transport, increased acetylation, phosphorylation of microtubule associated proteins (MAPs) and accumulation of microtubule protein containing aggregates. A global decrease in Histone Deacetylase (HDAC) activity is also present. (C) Mutant TDP-43 (TARDBP) expression causes dysfunction in mRNA granule transport. Decreased local translation of MAP mRNA is also implicated in TDP-43 mutants. (D) Mutant TUBA4A expression alters microtubule dynamics and network stability, with unknown impact on +TIP proteins, transport or chemical modifications. Select mutations are incorporated into intracellular aggregates. (E) Energy depletion and calcium dysregulation generates increased microtubule depolymerization, tubulin guanosine triphosphate (GTP) cap hydrolysis, and increased MAP phosphorylation. (F) Neuronal oxidative stress leads to tubulin glutathionylation, increased microtubule depolymerization, decreased axonal transport and alterations to MAPs, with unknown impact on classical chemical modifications or +TIP proteins.

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