Ion channels in epilepsy

Abstract Neuronal excitability is determined by the flux of ions through ion channels. Many types of ion channels are expressed in the central nervous system, each responsible for its own aspect of neuronal excitability, from postsynaptic depolarization to action potential generation to neurotransmitter release. These mechanisms are tightly regulated to create a balance between excitation and inhibition. Disruption of this balance is thought to be key in many neurological disorders, including epilepsy syndromes. More and more ion channel mutations are being identified through genetic studies; however, their incidence is still small, suggesting the presence of undiscovered mutations or other causative mechanisms. Understanding wild-type channel function during epileptic activity may also provide vital insights into the remaining idiopathic epilepsies and provide targets for future antiepileptic drugs. Epilepsy is one of the most common neurological disorders affecting 2% of the world's population. It varies widely in type and severity of seizures and should not be considered as a single disorder. It is currently defined as 'a tendency to have unprovoked recurrent seizures'. Epilepsy can result from brain injury caused by head trauma, stroke or infection, but in 6 out of 10 people seizures have no known cause. Seizures are the result of excessive neuronal firing temporarily disrupting neuronal signalling. This aberrant brain activity is the result of a shift in the balance between excitation and inhibition created by ion channels. Signal transduction in neurons is controlled by electrical signals created by ion flux across the plasma membrane Excitatory ion channels iGluRs (ionotropic glutamate receptors) would be expected to play a major role in the hyperexcitable state in epileps

One target for amyotrophic lateral sclerosis therapy?

Repeat expansion mutations cause a range of developmental, neurodegenerative, and neuromuscular disorders. The repeat sequences generally comprise a 3– to 6–base pair repeat unit that expands above a critical threshold, leading to disease. Expanded repeats cause disease via a range of mechanisms, including loss of function of the repeat-containing protein and production of toxic repeat RNAs and proteins, making the disorders difficult to treat. In 2011, a hexanucleotide repeat expansion in the C9orf72 gene was identified as the most common cause of frontotemporal dementia and amyotrophic lateral sclerosis (termed c9FTD/ALS) (1, 2). On page 708 of this issue, Kramer et al. (3) report that targeting a single factor, Spt4, reduced production of C9orf72 repeat expansion–associated RNA and protein, and ameliorated neurodegeneration in model systems

C9orf72 amyotrophic lateral sclerosis and frontotemporal dementia: gain or loss of function?

The molecular mechanisms that underlie chromosome 9 open reading frame 72 (C9orf72)-associated amyotrophic lateral sclerosis and frontotemporal dementia are rapidly emerging. Two potential disease mechanisms have been postulated - gain or loss of function. We provide an overview of recent advances that support or oppose gain-of-function and loss-of-function mechanisms

C9orf72 repeat expansions cause neurodegeneration in Drosophila through arginine-rich proteins

An expanded GGGGCC repeat in C9orf72 is the most common genetic cause of frontotemporal dementia and amyotrophic lateral sclerosis. A fundamental question is whether toxicity is driven by the repeat RNA itself and/or by dipeptide repeat proteins generated by repeat-associated, non-ATG translation. To address this question we developed in vitro and in vivo models to dissect repeat RNA and dipeptide repeat protein toxicity. Expression of pure repeats in Drosophila caused adult-onset neurodegeneration attributable to poly-(glycine-arginine) proteins. Thus, expanded repeats promoted neurodegeneration through neurotoxic proteins. Expression of individual dipeptide repeat proteins with a non-GGGGCC RNA sequence showed both poly-(glycine-arginine) and poly-(proline-arginine) proteins caused neurodegeneration. These findings are consistent with a dual toxicity mechanism, whereby both arginine-rich proteins and repeat RNA contribute to C9orf72-mediated neurodegeneration

Frontotemporal dementia caused by CHMP2B mutation is characterised by neuronal lysosomal storage pathology

Mutations in the charged multivesicular body protein 2B (CHMP2B) cause frontotemporal dementia (FTD). We report that mice which express FTD-causative mutant CHMP2B at physiological levels develop a novel lysosomal storage pathology characterised by large neuronal autofluorescent aggregates. The aggregates are an early and progressive pathology that occur at 3 months of age and increase in both size and number over time. These autofluorescent aggregates are not observed in mice expressing wild-type CHMP2B, or in non-transgenic controls, indicating that they are a specific pathology caused by mutant CHMP2B. Ultrastructural analysis and immuno- gold labelling confirmed that they are derived from the endolysosomal system. Consistent with these findings, CHMP2B mutation patient brains contain morphologically similar autofluorescent aggregates. These aggregates occur significantly more frequently in human CHMP2B mutation brain than in neurodegenerative disease or age-matched control brains. These data suggest that lysosomal storage pathology is the major neuronal pathology in FTD caused by CHMP2B mutation. Recent evidence suggests that two other genes associated with FTD, GRN and TMEM106B are important for lysosomal function. Our identification of lysosomal storage pathology in FTD caused by CHMP2B mutation now provides evidence that endolysosomal dysfunction is a major degenerative pathway in FTD

Improved detection of RNA foci in C9orf72 amyotrophic lateral sclerosis post-mortem tissue using BaseScope™ shows a lack of association with cognitive dysfunction

Acknowledgements The authors would like to thank: (i) the Medical Research Council Edinburgh Brain Bank (ethics approval from East of Scotland Research Ethics Service, 16/ES/0084)) for supplying all post-mortem brain material and the Scottish Motor Neurone Disease Register (SMNDR)/Care Audit Research and Evaluation for Motor Neurone Disease (CARE-MND) consortium for all clinical and demographic data (ethical approval from Scotland A Research Ethics Committee 10/MRE00/78 and 15/SS/0216); (ii) The Scottish MND Clinical Specialist team for obtaining consent from patients with MND for inclusion in these studies; (iii) MND Scotland and the Sylvia Aitken Charitable Trust for funding CS to help to establish the MND Tissue Bank. The authors would also like to thank Advanced Cell Diagnostics for gifting the C9orf72 probe (BaseScopeTM Probe - BA-GGGGCCn-3zz-st, Cat Code: 704181), prior to it being made commercially available. Funding A.R.M. is a Lady Edith Wolfson Clinical Fellow and is jointly funded by the Medical Research Council and the Motor Neurone Disease Association (MR/R001162/1). He also acknowledges support from the Rowling Scholars scheme, administered by the Anne Rowling Regenerative Neurology Clinic, University of Edinburgh, Edinburgh, UK. The Chandran laboratory is supported by the Euan MacDonald Centre and the UK Dementia Research Institute, which receives its funding from UK DRI Ltd, funded by the UK Medical Research Council, Alzheimer’s Society and Alzheimer’s Research UK. J.M.G. is funded by a starter grant for clinical lecturers from the Academy of Medical Sciences (AMS: 210JMG 3102 R45620) and C.S. is supported by a Medical Research Council grant (MR/L016400/1).Peer reviewe

C9orf72 frontotemporal lobar degeneration is characterised by frequent neuronal sense and antisense RNA foci.

An expanded GGGGCC repeat in a non-coding region of the C9orf72 gene is a common cause of frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis. Non-coding repeat expansions may cause disease by reducing the expression level of the gene they reside in, by producing toxic aggregates of repeat RNA termed RNA foci, or by producing toxic proteins generated by repeat-associated non-ATG translation. We present the first definitive report of C9orf72 repeat sense and antisense RNA foci using a series of C9FTLD cases, and neurodegenerative disease and normal controls. A sensitive and specific fluorescence in situ hybridisation protocol was combined with protein immunostaining to show that both sense and antisense foci were frequent, specific to C9FTLD, and present in neurons of the frontal cortex, hippocampus and cerebellum. High-resolution imaging also allowed accurate analyses of foci number and subcellular localisation. RNA foci were most abundant in the frontal cortex, where 51 % of neurons contained foci. RNA foci also occurred in astrocytes, microglia and oligodendrocytes but to a lesser degree than in neurons. RNA foci were observed in both TDP-43- and p62-inclusion bearing neurons, but not at a greater frequency than expected by chance. RNA foci abundance in the frontal cortex showed a significant inverse correlation with age at onset of disease. These data establish that sense and antisense C9orf72 repeat RNA foci are a consistent and specific feature of C9FTLD, providing new insight into the pathogenesis of C9FTLD

C9orf72 poly GA RAN-translated protein plays a key role in amyotrophic lateral sclerosis via aggregation and toxicity

An intronic GGGGCC (G4C2) hexanucleotide repeat expansion inC9orf72 is the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia (C9ALS/FTD). Repeat-associated non-AUG (RAN) translation of G4C2 RNA can result in five different dipeptide repeat proteins (DPR: poly GA, poly GP, poly GR, poly PA, and poly PR), which aggregate into neuronal cytoplasmic and nuclear inclusions in affected patients, however their contribution to disease pathogenesis remains controversial. We show that among the DPR proteins, expression of poly GA in a cell culture model activates programmed cell death and TDP-43 cleavage in a dose-dependent manner. Dual expression of poly GA together with other DPRs revealed that poly GP and poly PA are sequestered by poly GA, whereas poly GR and poly PR are rarely co-localised with poly GA. Dual expression of poly GA and poly PA ameliorated poly GA toxicity by inhibiting poly GA aggregation both in vitro and in vivo in the chick embryonic spinal cord. Expression of alternative codon-derived DPRs in chick embryonic spinal cord confirmed in vitro data, revealing that each of the dipeptides caused toxicity, with poly GA being the most toxic. Further, in vivo expression of G4C2 repeats of varying length caused apoptotic cell death, but failed to generate DPRs. Together, these data demonstrate that C9-related toxicity can be mediated by either RNA or DPRs. Moreover, our findings provide evidence that poly GA is a key mediator of cytotoxicity and that cross-talk between DPR proteins likely modifies their pathogenic status in C9ALS/FTD