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[...]reducing hSODG93A directly in the muscles of the SODG93A mouse model, as well as crossing lipoxygenase (LOX) SODG37R with the Cre coding sequence under the control of the muscle creatine kinase (MCK) promoter, or performing manipulations using follistatin did not affect the disease’s onset and survival (Miller et al., 2006). [...]application of muscle condition media (CM) from SODG93A-expressing muscles on healthy spinal cord neurons or embryonic stem cell-derived motor neurons in vitro resulted in no appreciable effect (Nagai et al., 2007). [...]eventually the connections between axons and muscles are established. [...]at least in our system, apparently the non-cell autonomous contributions of the muscle are insufficient to recapitulate all the toxic effects observed in ALS. Taking this into account, we speculate that there are undiscovered retrograde death pathways specifically in ALS-diseased MNs that cause the normal MNs to be more vulnerable to its toxic distal environment. [...]this basic mechanism, of cell bodies respond to distal stress in health and disease has to be deeply characterized in order to progress toward possible future treatment for ALS [Figure 1]. [...]aside from targeting Sema3A and NRP1, miR126-5p is thought to regulate other Semaphorin-related genes such as other type 3 Semaphorins, several Plexins, and further downstream signaling molecules such as c-jun n-terminal kinase (JNK) and phosphatase and tensin homologue. [...]miR126-5p can regulate a few ALS-related genes such as vascular endothelial growth factor A, SPAST, matrix metalloproteinases, AGRIN, and C9orf72 which are directly involved in ALS.

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease affecting both the upper and lower motor neurons (MNs), with no effective treatment currently available. Early pathological events in ALS include perturbations in axonal transport (AT), formation of toxic protein aggregates and Neuromuscular Junction (NMJ) disruption, which all lead to axonal degeneration and motor neuron death. Pridopidine is a small molecule that has been clinically developed for Huntington disease. Here we tested the efficacy of pridopidine for ALS using in vitro and in vivo models. Pridopidine beneficially modulates AT deficits and diminishes NMJ disruption, as well as motor neuron death in SOD1 G93A MNs and in neuromuscular co-cultures. Furthermore, we demonstrate that pridopidine activates the ERK pathway and mediates its beneficial effects through the sigma-1 receptor (S1R). Strikingly, in vivo evaluation of pridopidine in SOD1 G93A mice reveals a profound reduction in mutant SOD1 aggregation in the spinal cord, and attenuation of NMJ disruption, as well as subsequent muscle wasting. Taken together, we demonstrate for the first time that pridopidine improves several cellular and histological hallmark pathologies of ALS through the S1R.

Parkinson’s disease is characterized by loss of dopamine neurons in the substantia nigra 1 . Similar to other major neurodegenerative disorders, there are no disease-modifying treatments for Parkinson’s disease. While most treatment strategies aim to prevent neuronal loss or protect vulnerable neuronal circuits, a potential alternative is to replace lost neurons to reconstruct disrupted circuits 2 . Here we report an efficient one-step conversion of isolated mouse and human astrocytes to functional neurons by depleting the RNA-binding protein PTB (also known as PTBP1). Applying this approach to the mouse brain, we demonstrate progressive conversion of astrocytes to new neurons that innervate into and repopulate endogenous neural circuits. Astrocytes from different brain regions are converted to different neuronal subtypes. Using a chemically induced model of Parkinson’s disease in mouse, we show conversion of midbrain astrocytes to dopaminergic neurons, which provide axons to reconstruct the nigrostriatal circuit. Notably, re-innervation of striatum is accompanied by restoration of dopamine levels and rescue of motor deficits. A similar reversal of disease phenotype is also accomplished by converting astrocytes to neurons using antisense oligonucleotides to transiently suppress PTB. These findings identify a potentially powerful and clinically feasible approach to treating neurodegeneration by replacing lost neurons. Depletion of the RNA-binding protein PTB (also known as PTBP1) in astrocytes reprograms these cells to become functional neurons and, in a mouse model of Parkinson’s disease, reverses the disease phenotype.

Methods to enhance adult neurogenesis by reprogramming glial cells into neurons enable production of new neurons in the adult nervous system. Development of therapeutically viable approaches to induce new neurons is now required to bring this concept to clinical application. Here, we successfully generate new neurons in the cortex and dentate gyrus of the aged adult mouse brain by transiently suppressing polypyrimidine tract binding protein 1 using an antisense oligonucleotide delivered by a single injection into cerebral spinal fluid. Radial glial-like cells and other GFAP-expressing cells convert into new neurons that, over a 2-month period, acquire mature neuronal character in a process mimicking normal neuronal maturation. The new neurons functionally integrate into endogenous circuits and modify mouse behavior. Thus, generation of new neurons in the dentate gyrus of the aging brain can be achieved with a therapeutically feasible approach, thereby opening prospects for production of neurons to replace those lost to neurodegenerative disease. Maimon et al. demonstrate a therapeutically viable approach, single-dose injection of a DNA drug to suppress synthesis of PTB, to generate new neurons in the aged mouse hippocampus and enhance memory after their integration into endogenous circuits.

Huntington’s disease (HD) is caused by a CAG repeat expansion in the HTT gene, leading to altered gene expression. However, the mechanisms leading to disrupted RNA processing in HD remain unclear. Here we identify TDP-43 and the N6-methyladenosine (m6A) writer protein METTL3 to be upstream regulators of exon skipping in multiple HD systems. Disrupted nuclear localization of TDP-43 and cytoplasmic accumulation of phosphorylated TDP-43 occurs in HD mouse and human brains, with TDP-43 also co-localizing with HTT nuclear aggregate-like bodies distinct from mutant HTT inclusions. The binding of TDP-43 onto RNAs encoding HD-associated differentially expressed and aberrantly spliced genes is decreased. Finally, m6A RNA modification is reduced on RNAs abnormally expressed in the striatum of HD R6/2 mouse brain, including at clustered sites adjacent to TDP-43 binding sites. Our evidence supports TDP-43 loss of function coupled with altered m6A modification as a mechanism underlying alternative splicing in HD. Nguyen et al. identify TDP-43 and METTL3 as key regulators of disrupted RNA splicing in Huntington’s disease, offering insight into how TDP-43 mislocalization and aberrant m6A RNA modification and localization relate to disease pathogenesis.

Parkinson disease is characterized by loss of dopamine neurons in the substantia nigra1. Similar to other major neurodegenerative disorders, no disease-modifying treatment exists. While most treatment strategies aim to prevent neuronal loss or protect vulnerable neuronal circuits, a potential alternative is to replace lost neurons to reconstruct disrupted circuits2. Herein we report an efficient single-step conversion of isolated mouse and human astrocytes into functional neurons by depleting the RNA binding protein PTB. Applying this approach to the mouse brain, we demonstrate progressive conversion of astrocytes into new neurons that can innervate into endogenous neural circuits. Astrocytes in different brain regions are found to convert into different neuronal subtypes. Using a chemically induced model of Parkinson’s disease, we show conversion of midbrain astrocytes into dopaminergic neurons whose axons reconstruct the nigro-striatal circuit. Significantly, re-innervation of striatum is accompanied by restoration of dopamine levels and rescue of motor deficits. Similar disease phenotype reversal is also accomplished by converting astrocytes to neurons using antisense oligonucleotides to transiently suppress PTB. These findings identify a potentially powerful and clinically feasible new approach to treating neurodegeneration by replacing lost neurons.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Huntington's disease (HD) is a neurodegenerative disorder caused by a CAG repeat expansion in the first exon of the gene encoding huntingtin. Prior reports have established a correlation between CAG expanded and altered gene expression. However, the mechanisms leading to disruption of RNA processing in HD remain unclear. Here, our analysis of the reported HTT protein interactome identifies interactions with known RNA-binding proteins (RBPs). Total, long-read sequencing and targeted RASL-seq of RNAs from cortex and striatum of the HD mouse model R6/2 reveals increased exon skipping which is confirmed in Q150 and Q175 knock-in mice and in HD human brain. We identify the RBP TDP-43 and the N6-methyladenosine (m6A) writer protein methyltransferase 3 (METTL3) to be upstream regulators of exon skipping in HD. Along with this novel mechanistic insight, we observe decreased nuclear localization of TDP-43 and cytoplasmic accumulation of phosphorylated TDP-43 in HD mice and human brain. In addition, TDP-43 co-localizes with HTT in human HD brain forming novel nuclear aggregate-like bodies distinct from mutant HTT inclusions or previously observed TDP-43 pathologies. Binding of TDP-43 onto RNAs encoding HD-associated differentially expressed and aberrantly spliced genes is decreased. Finally, m6A RNA modification is reduced on RNAs abnormally expressed in striatum from HD R6/2 mouse brain, including at clustered sites adjacent to TDP-43 binding sites. Our evidence supports TDP-43 loss of function coupled with altered m6A modification as a novel mechanism underlying alternative splicing/unannotated exon usage in HD and highlights the critical nature of TDP-43 function across multiple neurodegenerative diseases.

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease affecting both the upper and lower motor neurons (MNs), with no effective treatment currently available. Early pathological events in ALS include perturbations in axonal transport (AT), formation of toxic protein aggregates and Neuromuscular Junction (NMJ) disruption, which all lead to axonal degeneration and motor neuron death. Pridopidine is a small molecule that has been clinically developed for Huntington disease. Here we tested the efficacy of pridopidine for ALS using in vitro and in vivo models. Pridopidine beneficially modulates AT deficits and diminishes NMJ disruption, as well as motor neuron death in SOD1 MNs and in neuromuscular co-cultures. Furthermore, we demonstrate that pridopidine activates the ERK pathway and mediates its beneficial effects through the sigma-1 receptor (S1R). Strikingly, in vivo evaluation of pridopidine in SOD1 mice reveals a profound reduction in mutant SOD1 aggregation in the spinal cord, and attenuation of NMJ disruption, as well as subsequent muscle wasting. Taken together, we demonstrate for the first time that pridopidine improves several cellular and histological hallmark pathologies of ALS through the S1R.

Recent studies proposing induced glia-to-neuron conversion raised the potential for generating new neurons to replace those lost due to injury, aging or neurodegenerative diseases. Here, single-cell spatial transcriptomics [Multiplexed Error Robust Fluorescence In Situ Hybridization (MERFISH)] is used to construct a spatial cell atlas of the subventricular and dentate gyrus neurogenic niches of young and aged adult murine brain. RNAs that encode the RNA binding protein Polypyrimidine Tract-Binding Protein (PTBP1) in the aged murine brain are determined to be highest in glia that line previously active neurogenic niches. A glial cell population with ependymal character within an initially quiescent subventricular neurogenic niche in the aged murine brain is identified that upon transient suppression of PTBP1 reenters the cell cycle, replicates DNA, and converts into neurons through a canonical adult neurogenesis pathway. Glia-derived neurons migrate from this niche, with some neurons transiting to the striatum and acquiring a transcriptome characteristic of GABAergic inhibitory neurons. Similar PTBP1 expressing quiescent glia are identified in the corresponding neurogenic niche of aged human brain. Thus, transient reduction of PTBP1 holds potential for inducing the generation of new neurons in quiescent neurogenic niches of the aged nervous system, thereby offering promising therapeutic applications.Competing Interest StatementD.W.C. is a consultant for and C.F.B, P. J-N. and C.H. are employees of Ionis Pharmaceuticals.