Pathophysiological insights into ALS with C9ORF72 expansions

The genotype-phenotype landscape of familial amyotrophic lateral sclerosis in Australia

Amyotrophic lateral sclerosis (ALS) is a clinically and genetically heterogeneous fatal neurodegenerative disease. Around 10% of ALS cases are hereditary. ALS gene discoveries have provided most of our understanding of disease pathogenesis. We aimed to describe the genetic landscape of ALS in Australia by assessing 1013 Australian ALS patients for known ALS mutations by direct sequencing, whole exome sequencing or repeat primed polymerase chain reaction. Age of disease onset and disease duration were used for genotype-phenotype correlations. We report 60.8% of Australian ALS families in this cohort harbour a known ALS mutation. Hexanucleotide repeat expansions in C9orf72 accounted for 40.6% of families and 2.9% of sporadic patients. We also report ALS families with mutations in SOD1 (13.7%), FUS (2.4%), TARDBP (1.9%), UBQLN2 (.9%), OPTN (.5%), TBK1 (.5%) and CCNF (.5%). We present genotype-phenotype correlations between these genes as well as between gene mutations. Notably, C9orf72 hexanucleotide repeat expansion positive patients experienced significantly later disease onset than ALS mutation patients. Among SOD1 families, p.I114T positive patients had significantly later onset and longer survival. Our report highlights a unique spectrum of ALS gene frequencies among patients from the Australian population, and further, provides correlations between specific ALS mutations with disease onset and/or duration.

Transcranial Magnetic Stimulation for the Assessment of Neurodegenerative Disease

Transcranial magnetic stimulation (TMS) is a noninvasive technique that has provided important information about cortical function across an array of neurodegenerative disorders, including Alzheimer's disease, frontotemporal dementia, Parkinson's disease, and related extrapyramidal disorders. Application of TMS techniques in neurodegenerative diseases has provided important pathophysiological insights, leading to the development of pathogenic and diagnostic biomarkers that could be used in the clinical setting and therapeutic trials. Abnormalities of TMS outcome measures heralding cortical hyperexcitability, as evidenced by a reduction of short-interval intracortical inhibition and increased in motor-evoked potential amplitude, have been consistently identified as early and intrinsic features of amyotrophic lateral sclerosis (ALS), preceding and correlating with the ensuing neurodegeneration. Cortical hyperexcitability appears to form the pathogenic basis of ALS, mediated by trans-synaptic glutamate-mediated excitotoxic mechanisms. As a consequence of these research findings, TMS has been developed as a potential diagnostic biomarker, capable of identifying upper motor neuronal pathology, at earlier stages of the disease process, and thereby aiding in ALS diagnosis. Of further relevance, marked TMS abnormalities have been reported in other neurodegenerative diseases, which have varied from findings in ALS. With time and greater utilization by clinicians, TMS outcome measures may prove to be of utility in future therapeutic trial settings across the neurodegenerative disease spectrum, including the monitoring of neuroprotective, stem-cell, and genetic-based strategies, thereby enabling assessment of biological effectiveness at early stages of drug development.

Increased prevalence of autoimmune disease within C9 and FTD/MND cohorts: Completing the picture

Objective:

To determine the prevalence of autoimmune disease in symptomatic C9ORF72 (C9) mutation carriers and frontotemporal dementia with motor neuron disease (FTD/MND) cohorts.

Methods:

In this case-control study, we reviewed the clinical histories of 66 patients with FTD/MND and 57 symptomatic C9 carriers (24 overlapping cases), a total of 99 charts, for history of autoimmune disease. The prevalence of autoimmune disease in C9 and FTD/MND cohorts was determined by χ² and Fisher exact comparisons between the combined C9 and FTD/MND group with normal control, Alzheimer disease, and progressive supranuclear palsy cohorts, as well as comparisons within C9 and FTD/MND cohorts.

Results:

Our combined C9 and FTD/MND cohort has a 12% prevalence of nonthyroid autoimmune disease. The prevalence of nonthyroid autoimmune disease in C9 and FTD/MND is similar to the rates in previously detailed progranulin and semantic variant primary progressive aphasia cohorts and elevated in comparison to previously collected normal control and typical Alzheimer disease cohorts, as well as a newly screened progressive supranuclear palsy cohort. Furthermore, the types of autoimmune disease in this combined C9 and FTD/MND cohort cluster within the same 3 categories previously described in progranulin and semantic variant primary progressive aphasia: inflammatory arthritides, cutaneous conditions, and gastrointestinal disorders.

Conclusions:

The association between selective autoimmune disease and neurodegenerative disorders unified by the underlying pathology frontotemporal lobar degeneration with TDP-43–positive inclusions (FTLD-TDP) extends to C9 and FTD/MND cohorts, providing further evidence that select autoimmune inflammation may be intrinsically linked to FTLD-TDP pathophysiology.

Epidemiology and molecular mechanism of frontotemporal lobar degeneration/amyotrophic lateral sclerosis with repeat expansion mutation in C9orf72

GGGGCC hexanucleotide repeat expansions in C9orf72 were identified in 2011 as the genetic cause of frontotemporal lobar degeneration (FTLD)/amyotrophic lateral sclerosis (ALS) linked to chromosome 9. Since then, a number of studies have been conducted to delineate the molecular epidemiology of the repeat expansions and the molecular pathophysiology of the disease. The frequency of the repeat expansions considerably varied among countries. The frequency of the repeat expansions was high in European populations and populations of European descent and a substantial proportion of sporadic FTLD or ALS patients also have the mutations in these populations. On the other hand, the frequency was extremely low in Asia or Oceania except for limited regions including Kii Peninsula of Japan. A founder effect seems to strongly influence the regional differences in the frequency, but there is no definitive evidence that supports the notion that the repeat expansions arose in a single founder or multiple founders. As a disease-causing mechanism, several molecular mechanisms have been proposed, including conformational changes of DNA (G-quadruplex formation and hypermethylation) or RNA (G-quadruplex formation) molecules, altered transcriptional levels of C9orf72, sequestration of RNA-binding proteins, bidirectional transcription, formation of RNA foci, and neurotoxicity of dipeptide repeat proteins generated by repeat-associated non-ATG-initiated translation. Further investigations on the molecular mechanisms of neurodegeneration are expected to lead to the development of therapeutic interventions for this disease as well as for other diseases associated with non-coding repeat expansions.

Amyotrophic lateral sclerosis and motor neuron syndromes in Asia

While the past 2 decades have witnessed an increasing understanding of amyotrophic lateral sclerosis (ALS) arising from East Asia, particularly Japan, South Korea, Taiwan and China, knowledge of ALS throughout the whole of Asia remains limited. Asia represents >50% of the world population, making it host to the largest patient cohort of ALS. Furthermore, Asia represents a diverse population in terms of ethnic, social and cultural backgrounds. In this review, an overview is presented that covers what is currently known of ALS in Asia from basic epidemiology and genetic influences, through to disease characteristics including atypical phenotypes which manifest a predilection for Asians. With the recent establishment of the Pan-Asian Consortium for Treatment and Research in ALS to facilitate collaborations between clinicians and researchers across the region, it is anticipated that Asia and the Pacific will contribute to unravelling the uncertainties in ALS.

Loss-of-function mutations in the C9ORF72 mouse ortholog cause fatal autoimmune disease

C9ORF72 mutations are found in a significant fraction of patients suffering from amyotrophic lateral sclerosis and frontotemporal dementia, yet the function of the C9ORF72 gene product remains poorly understood. We show that mice harboring loss-of-function mutations in the ortholog of C9ORF72 develop splenomegaly, neutrophilia, thrombocytopenia, increased expression of inflammatory cytokines, and severe autoimmunity, ultimately leading to a high mortality rate. Transplantation of mutant mouse bone marrow into wild-type recipients was sufficient to recapitulate the phenotypes observed in the mutant animals, including autoimmunity and premature mortality. Reciprocally, transplantation of wild-type mouse bone marrow into mutant mice improved their phenotype. We conclude that C9ORF72 serves an important function within the hematopoietic system to restrict inflammation and the development of autoimmunity.

Distinct neurological disorders with C9orf72 mutations: genetics, pathogenesis, and therapy

The G4C2 repeat expansion within C9orf72 has been recently identified as the most common genetic cause of frontotemporal dementia and amyotrophic lateral sclerosis. This mutation has also been detected in a variety of other neurological diseases with distinct clinical manifestations. The exact mechanisms of how this mutation leads to the wide spectrum of clinical syndromes remain unknown. A series of molecular changes together with some potential modifiers may play a key role. Nucleolar stress, nucleocytoplasmic transport defect, oxidative damage, inhibited stress granules assembly, activated endoplasmic reticulum stress, and inhibited proteasome activity are mechanisms that contribute to the pathogenesis of these diseases. Additional mutations, epigenetic modifiers, and repeat size are potential modifiers that modulate specific phenotypes on the basis of the molecular changes. Here, we summarize distinct C9orf72-related neurological disorders and their corresponding neuropathological changes. Then, we elucidate the existing molecular knowledge and the potential modifiers. Finally, we detail the main target of treatment aiming at controlling expanded RNA transcripts.

ACAID as a potential therapeutic approach to modulate inflammation in neurodegenerative diseases

The progressive loss of neurons and inflammation characterizes neurodegenerative diseases. Although the etiology, progression and outcome of different neurodegenerative diseases are varied, they share chronic inflammation maintained largely by central nervous system (CNS)-derived antigens recognized by T cells. Inflammation can be beneficial by recruiting immune cells to kill pathogens or to clear cell debris resulting from the primary insult. However, chronic inflammation exacerbates and perpetuates tissue damage. An increasing number of therapies that attempt to modulate neuroinflammation have been developed. However, so far none has succeeded in decreasing the secondary damage associated with chronic inflammation. A potential strategy to modulate the immune system is related to the induction of tolerance to CNS antigens. In this line, it is our hypothesis that this could be accomplished by using anterior chamber associated immune deviation (ACAID) as a strategy. Thus, we review current knowledge regarding some neurodegenerative diseases and the associated immune response that causes inflammation. In addition, we discuss further our hypothesis of the possible usefulness of ACAID as a therapeutic strategy to ameliorate damage to the CNS.

Can cannabinoids be a potential therapeutic tool in amyotrophic lateral sclerosis?

Amyotrophic lateral sclerosis (ALS) is the most common degenerative disease of the motor neuron system. Over the last years, a growing interest was aimed to discovery new innovative and safer therapeutic approaches in the ALS treatment. In this context, the bioactive compounds of Cannabis sativa have shown antioxidant, anti-inflammatory and neuroprotective effects in preclinical models of central nervous system disease. However, most of the studies proving the ability of cannabinoids in delay disease progression and prolong survival in ALS were performed in animal model, whereas the few clinical trials that investigated cannabinoids-based medicines were focused only on the alleviation of ALS-related symptoms, not on the control of disease progression. The aim of this report was to provide a short but important overview of evidences that are useful to better characterize the efficacy as well as the molecular pathways modulated by cannabinoids.

Jump from pre-mutation to pathologic expansion in C9orf72

An expanded G4C2 repeat in C9orf72 represents the most common known genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). However, the lower limit for pathological expansions is unknown (the suggested cutoff is 30 repeats). It has been proposed that the expansion might have occurred only once in human history and subsequently spread throughout the population. However, our present findings support a hypothesis of multiple origins for the expansion. We report a British-Canadian family in whom a ∼70-repeat allele from the father (unaffected by ALS or FTLD at age 89 years) expanded during parent-offspring transmission and started the first generation affected by ALS (four children carry an ∼1,750-repeat allele). Epigenetic and RNA-expression analyses further discriminated the offspring's large expansions (which were methylated and associated with reduced C9orf72 expression) from the ∼70-repeat allele (which was unmethylated and associated with upregulation of C9orf72). Moreover, RNA foci were only detected in fibroblasts from offspring with large expansions, but not in the father, who has the ∼70-repeat allele. All family members with expansions were found to have an ancient known risk haplotype, although it was inherited on a unique 5-Mb genetic backbone. We conclude that small expansions (e.g., 70 repeats) might be considered "pre-mutations" to reflect their propensity to expand in the next generation. Follow-up studies might help explain the high frequency of ALS- or FTLD-affected individuals with an expansion but without a familial history (e.g., 21% among Finnish ALS subjects).

Roots to start research in amyotrophic lateral sclerosis: molecular pathways and novel therapeutics for future

Amyotrophic lateral sclerosis (ALS) is a devastating neurological disease that rapidly progresses from mild motor symptoms to severe motor paralysis and premature death. There is currently no cure for this devastating disease; most ALS patients die of respiratory failure generally within 3–5 years from the onset of signs and symptoms. Approximately 90% of ALS cases are sporadic in nature, with no clear associated risk factors. It is reported that ALS is a complex and multifaceted neurodegenerative disease. Less is known about the key factors involved in the sporadic form of the disease. The intricate pathogenic mechanisms that target motor neurons in ALS includes oxidative stress, glutamate excitotoxicity, mitochondrial damage, protein aggregation, glia and neuroinflammation pathology, defective axonal transport, and aberrant RNA metabolism. Despite aggressive research, no therapy has been yet proven to completely reverse the core symptoms of the disease. Riluzole is the only drug approved by the Food and Drug Administration and recommended by the National Institute for Clinical Excellence so far proven to be successful against ALS and may prevent progression and extend life for a few months or so. This article provides a novel understanding in key findings of pathogenesis and interventions currently under investigation to slow disease progression in ALS.

Frontotemporal dementia: a bridge between dementia and neuromuscular disease

The concept that frontotemporal dementia (FTD) is a purely cortical dementia has largely been refuted by the recognition of its close association with motor neuron disease, and the identification of transactive response DNA-binding protein 43 (TDP-43) as a major pathological substrate underlying both diseases. Genetic findings have transformed this field and revealed connections between disorders that were previous thought clinically unrelated. The discovery that the C9ORF72 locus is responsible for the majority of hereditary FTD, amyotrophic lateral sclerosis (ALS), and FTD-ALS cases and the understanding that repeat-containing RNA plays a crucial role in pathogenesis of both disorders has paved the way for the development of potential biomarkers and therapeutic targets for these devastating diseases. In this review, we summarize the historical aspects leading up to our current understanding of the genetic, clinical, and neuropathological overlap between FTD and ALS, and include brief discussions on chronic traumatic encephalopathy (CTE), given its association with TDP-43 pathology, its associated increased dementia risk, and reports of ALS in CTE patients. In addition, we describe other genetic associations between dementia and neuromuscular disease, such as inclusion body myositis with Paget's disease and FTD.

Quantifying disease progression in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) exhibits characteristic variability of onset and rate of disease progression, with inherent clinical heterogeneity making disease quantitation difficult. Recent advances in understanding pathogenic mechanisms linked to the development of ALS impose an increasing need to develop strategies to predict and more objectively measure disease progression. This review explores phenotypic and genetic determinants of disease progression in ALS, and examines established and evolving biomarkers that may contribute to robust measurement in longitudinal clinical studies. With targeted neuroprotective strategies on the horizon, developing efficiencies in clinical trial design may facilitate timely entry of novel treatments into the clinic.

Increased expression of microRNA-29a in ALS mice: functional analysis of its inhibition

Endoplasmic reticulum (ER) stress has been implicated in a number of neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). MicroRNAs are small ribonucleic acids which can modulate protein expression by binding to the 3'UTR of target mRNAs. We recently identified increased miR-29a expression in response to ER stress in neurons, with members of the miR-29 family implicated in cancer and neurodegeneration. We found high expression of miR-29a in the mouse brain and spinal cord by quantitative PCR analysis and increased expression of miR-29a in the spinal cord of SOD1(G93A) transgenic mice, a mouse model of familial ALS. In situ hybridisation experiments revealed increased miR-29a expression in the lumbar spinal cord of SOD1(G93A) transgenic mice from postnatal day 70 onward when compared to wild-type mice. miR-29a knockdown was achieved in the CNS in vivo after a single intracerebroventricular injection of a miR-29a-specific antagomir. While analysis of disease progression and motor function could not identify a significant alteration in ALS disease manifestations, a trend towards increased lifespan was observed in male SOD1(G93A) mice. These findings demonstrate that miR-29a may act as a marker for disease progression in SOD1(G93A) mice, and provide first proof-of-concept for a therapeutic modulation of miR-29a function in ALS.

Intrinsic membrane hyperexcitability of amyotrophic lateral sclerosis patient-derived motor neurons

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease of the motor nervous system. We show using multielectrode array and patch-clamp recordings that hyperexcitability detected by clinical neurophysiological studies of ALS patients is recapitulated in induced pluripotent stem cell-derived motor neurons from ALS patients harboring superoxide dismutase 1 (SOD1), C9orf72, and fused-in-sarcoma mutations. Motor neurons produced from a genetically corrected but otherwise isogenic SOD1(+/+) stem cell line do not display the hyperexcitability phenotype. SOD1(A4V/+) ALS patient-derived motor neurons have reduced delayed-rectifier potassium current amplitudes relative to control-derived motor neurons, a deficit that may underlie their hyperexcitability. The Kv7 channel activator retigabine both blocks the hyperexcitability and improves motor neuron survival in vitro when tested in SOD1 mutant ALS cases. Therefore, electrophysiological characterization of human stem cell-derived neurons can reveal disease-related mechanisms and identify therapeutic candidates.

The widening spectrum of C9ORF72-related disease; genotype/phenotype correlations and potential modifiers of clinical phenotype

The GGGGCC (G4C2) repeat expansion in C9ORF72 is the most common cause of familial amyotrophic lateral sclerosis (ALS), frontotemporal lobar dementia (FTLD) and ALS-FTLD, as well as contributing to sporadic forms of these diseases. Screening of large cohorts of ALS and FTLD cohorts has identified that C9ORF72-ALS is represented throughout the clinical spectrum of ALS phenotypes, though in comparison with other genetic subtypes, C9ORF72 carriers have a higher incidence of bulbar onset disease. In contrast, C9ORF72-FTLD is predominantly associated with behavioural variant FTD, which often presents with psychosis, most commonly in the form of hallucinations and delusions. However, C9ORF72 expansions are not restricted to these clinical phenotypes. There is a higher than expected incidence of parkinsonism in ALS patients with C9ORF72 expansions, and the G4C2 repeat has also been reported in other motor phenotypes, such as primary lateral sclerosis, progressive muscular atrophy, corticobasal syndrome and Huntington-like disorders. In addition, the expansion has been identified in non-motor phenotypes including Alzheimer's disease and Lewy body dementia. It is not currently understood what is the basis of the clinical variation seen with the G4C2 repeat expansion. One potential explanation is repeat length. Sizing of the expansion by Southern blotting has established that there is somatic heterogeneity, with different expansion lengths in different tissues, even within the brain. To date, no correlation with expansion size and clinical phenotype has been established in ALS, whilst in FTLD only repeat size in the cerebellum was found to correlate with disease duration. Somatic heterogeneity suggests there is a degree of instability within the repeat and evidence of anticipation has been reported with reducing age of onset in subsequent generations. This variability/instability in expansion length, along with its interactions with environmental and genetic modifiers, such as TMEM106B, may be the basis of the differing clinical phenotypes arising from the mutation.

Early dipeptide repeat pathology in a frontotemporal dementia kindred with C9ORF72 mutation and intellectual disability

Familial cases of frontotemporal dementia (FTD) provide an opportunity to study the pathophysiology of this clinically diverse condition. The C9ORF72 mutation is the most common cause of familial FTD, recent pathological descriptions challenge existing TDP-43 based hypotheses of sporadic FTD pathogenesis. Non-ATG dependent translation of the hexanucleotide expansion into aggregating dipeptide repeat (DPR) proteins may represent a novel pathomechanism. We report detection of the DPR aggregates very early in C9ORF72 FTD development and also describe childhood intellectual disability as a clinical feature preceding dementia. The index case presented with psychiatric symptoms, progressing into typical FTD. Autopsy revealed extensive neuronal DPR aggregates but only minimal TDP-43 pathology. Her intellectually disabled elder son, also carrying the C9ORF72 mutation, died aged 26 years and at autopsy only DPR aggregates without TDP-43 were found. A second son also has intellectual disability, his C9ORF72 status is unknown, but chromosomal microarray revealed no other cause of disability. These cases both extend the existing phenotype of C9ORF72 mutation and highlight the potential significance of DPR translation early in disease development.

The C9ORF72 expansion mutation: gene structure, phenotypic and diagnostic issues

The discovery of the C9ORF72 hexanucleotide repeat expansion in 2011 and the immediate realisation of a remarkably high prevalence in both familial and sporadic frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS) triggered an explosion of interest in studies aiming to define the associated clinical and investigation phenotypes and attempts to develop technologies to measure more accurately the size of the repeat region. This article reviews progress in these areas over the subsequent 2 years, focussing on issues directly relevant to the practising physician. First, we summarise findings from studies regarding the global prevalence of the expansion, not only in FTLD and ALS cases, but also in other neurological diseases and its concurrence with other genetic mutations associated with FTLD and ALS. Second, we discuss the variability in normal repeat number in cases and controls and the theories regarding the relevance of intermediate and pathological repeat number for disease risk and clinical phenotype. Third, we discuss the usefulness of various features within the FTLD and ALS clinical phenotype in aiding differentiation between cases with and without the C9ORF72 expansion. Fourth, we review clinical investigations used to identify cases with the expansion, including neuroimaging and cerebrospinal fluid markers, and describe the mechanisms and limitations of the various diagnostic laboratory techniques used to quantify repeat number in cases and controls. Finally, we discuss the issues surrounding accurate clinical and technological diagnosis of patients with FTLD and/or ALS associated with the C9ORF72 expansion, and outline areas for future research that might aid better diagnosis and genetic counselling of patients with seemingly sporadic or familial FTLD or ALS and their relatives.

Where genotype is not predictive of phenotype: towards an understanding of the molecular basis of reduced penetrance in human inherited disease

Some individuals with a particular disease-causing mutation or genotype fail to express most if not all features of the disease in question, a phenomenon that is known as 'reduced (or incomplete) penetrance'. Reduced penetrance is not uncommon; indeed, there are many known examples of 'disease-causing mutations' that fail to cause disease in at least a proportion of the individuals who carry them. Reduced penetrance may therefore explain not only why genetic diseases are occasionally transmitted through unaffected parents, but also why healthy individuals can harbour quite large numbers of potentially disadvantageous variants in their genomes without suffering any obvious ill effects. Reduced penetrance can be a function of the specific mutation(s) involved or of allele dosage. It may also result from differential allelic expression, copy number variation or the modulating influence of additional genetic variants in cis or in trans. The penetrance of some pathogenic genotypes is known to be age- and/or sex-dependent. Variable penetrance may also reflect the action of unlinked modifier genes, epigenetic changes or environmental factors. At least in some cases, complete penetrance appears to require the presence of one or more genetic variants at other loci. In this review, we summarize the evidence for reduced penetrance being a widespread phenomenon in human genetics and explore some of the molecular mechanisms that may help to explain this enigmatic characteristic of human inherited disease.

Repeat expansion in C9ORF72 is not a major cause of amyotrophic lateral sclerosis among Iranian patients

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron disease in populations of European descent. It was recently found that a hexanucleotide repeat expansion in C9ORF72 is its most common cause in these populations. The contribution of C9ORF72 to ALS is notably lower in the Far East, but its role in other populations is unknown. Results of C9ORF72 screening in 78 unrelated Iranian ALS patients are reported here. The repeat expansion was observed in only 1 (5.9%) of the familial and 1 (1.6%) of the sporadic cases. These figures are to be compared, respectively, with 30% and 6.9% among patients of European ethnicity. Screenings of C9ORF72 in other Middle East countries will reveal whether the low contribution of C9ORF72 to ALS is a feature of the entire region. During the screenings, it was noted that in a single family, 3 individuals affected with ALS, Parkinson's disease, or frontotemporal dementia all carried the repeat expansion. The finding suggests the mutation does rarely contribute to the etiology of Parkinson's disease.

Microtubule defects & Neurodegeneration

One of the major challenges facing the long term survival of neurons is their requirement to maintain efficient axonal transport over long distances. In humans as large, long-lived vertebrates, the machinery maintaining neuronal transport must remain efficient despite the slow accumulation of cell damage during aging. Mutations in genes encoding proteins which function in the transport system feature prominently in neurologic disorders. Genes known to cause such disorders and showing traditional Mendelian inheritance have been more readily identified. It has been more difficult, however, to isolate factors underlying the complex genetics contributing to the more common idiopathic forms of neurodegenerative disease. At the heart of neuronal transport is the rail network or scaffolding provided by neuron specific microtubules (MTs). The importance of MT dynamics and stability is underscored by the critical role tau protein plays in MT-associated stabilization versus the dysfunction seen in Alzheimer's disease, frontotemporal dementia and other tauopathies. Another example of the requirement for tight regulation of MT dynamics is the need to maintain balanced levels of post-translational modification of key MT building-blocks such as α-tubulin. Tubulins require extensive polyglutamylation at their carboxyl-terminus as part of a novel post-translational modification mechanism to signal MT growth versus destabilization. Dramatically, knock-out of a gene encoding a deglutamylation family member causes an extremely rapid cell death of Purkinje cells in the ataxic mouse model, pcd. This review will examine a range of neurodegenerative conditions where current molecular understanding points to defects in the stability of MTs and axonal transport to emphasize the central role of MTs in neuron survival.