Early alterations of RNA metabolism and splicing from adult corticospinal neurons in an ALS mouse model

Abstract

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease clinically defined as the combined degeneration of corticospinal and corticobulbar neurons (CSN), and bulbar and spinal motor neurons (MN). A growing body of evidence points to the motor cortex, where CSN are located, as the potential initiation site of ALS. However, little is known about the spatiotemporal dynamics of CSN degeneration and the molecular pathways involved. Here, we show in the Sod1 G86R mouse model of ALS that CSN loss precedes MN degeneration and that CSN and MN degenerations are somatotopically related, highlighting the relevance of CSN to ALS onset and progression. To gain insights into the molecular mechanisms that selectively trigger CSN degeneration, we purified CSN from the motor and somatosensory cortex of adult mice and analysed their transcriptome from presymptomatic ages to disease end-stage. Significant RNA metabolism and splicing alterations, novel in the context of Sod1 mutation, were identified, including mis-splicing events that largely trigger genes involved in neuronal functions. Together, the data indicate that CSN dysfunction and degeneration upon mutant Sod1 expression involve alterations of RNA metabolism and splicing, emphasizing shared mechanisms across various ALS-related genes.3 ALS is an incurable and fatal neurodegenerative disease that mostly starts in adulthood with a rapidly progressing paralysis and death within only 2 to 5 years of diagnosis 1,2 . In clinics, ALS is defined as the combined degeneration of both corticospinal and corticobulbar neurons (CSN, or upper motor neurons) whose cell bodies are located in the cerebral cortex and that extend axons to the medulla and spinal cord, and of spinal and bulbar motor neurons (MN, or lower motor neurons) whose cell bodies are located in the medulla and spinal cord, and that connect to the skeletal muscles. If the cellular and molecular mechanisms behind MN degeneration are relatively well known, the numerous related clinical trials have unfortunately failed to translate into improved treatment of ALS 1-4 , a discrepancy that contributed to the emergence of the CSN and the overall cerebral cortex as alternative therapeutic targets.A growing body of evidence points to the motor cortex as the likely initiation site of ALS 5,6 . A first series of arguments arose from the recent genetic, pathological and clinical evolution links established between ALS and FrontoTemporal Dementia (FTD), a neurodegenerative disease that affects solely the cerebral cortex and results in behavioural and cognitive deficits 7 . A second series of arguments emerged following extensive examination of the TDP-43 aggregates, a pathological hallmark of the disease in post-mortem brains from ALS patients, identifying the motor cortex as the most affected region of the brain, and leading to the corticofugal hypothesis 8-10 which proposes a sequential pattern of pathology spreading from the motor cortex to its projection sites. Finally, transcranial magnetic stimulation studi...

Absence of Subcerebral Projection Neurons Is Beneficial in a Mouse Model of Amyotrophic Lateral Sclerosis

“…The absence of H‐reflex recording in WT animals does not rule out the existence of a small‐amplitude H‐reflex in these animals, below the detection threshold, as already observed 30 . In contrast, the presence of an H‐reflex in subgroups of presymptomatic Sod1 animals suggests that hyper‐reflexia precedes the appearance of motor symptoms in these animals and is correlated with the presymptomatic degeneration of the CSN 11 . Absence of an H‐reflex in KO animals suggests that developmental absence of CSN and other SubCerPN could have been compensated by other supraspinal controls or by spinal network rearrangements, or both.…”

“…Although major differences exist between primates and rodents regarding the route of the corticospinal tract and the connectivity of CSN onto alpha motoneurons, many mouse models of ALS recapitulate CSN or SubCerPN degeneration (reviewed by Brunet and colleagues 8 ). Likewise, we recently showed that Sod1 G86R mice display presymptomatic CSN degeneration and a somatotopic relationship between CSN and spinal motoneuron degeneration, 11 as reported in ALS patients 12 …”

“…Further support for the appropriateness of rodents to study the cortical contributions to ALS is provided by their ability to recapitulate CSN or SubCerPN degeneration 8 . In the Sod1 G86R mouse model, CSN precedes MN degeneration and NMJ denervation 11 . Thus, rodent models and Sod1 G86R mice in particular seem suitable to assess the contribution of SubCerPN to ALS.…”

“…Given the reported alterations in excitability of cortical neuron populations in different mouse models of the disease, 8,42 comparison between silencing and genetic ablation of adult corticofugal neurons would be particularly informative to gain a better understanding of the mechanism by which these neurons might contribute to disease onset and progression. Finally, deep molecular analysis of CSN dysfunction in ALS 11 might in the future provide a better understanding of the role of the cerebral cortex and its outputs to ALS and potentially unravel new therapeutic targets.…”

Exciting Complexity: The Role of Motor Circuit Elements in ALS Pathophysiology

“…Inhibitory interneurons in cortex and spinal cord also express glutamate receptors, some of which even highly Ca 2+ -permeable AMPA receptors lacking the GluA2 subunit (Akgul and McBain, 2016), thus potentially also putting interneurons at risk in ALS. Transcriptomic and proteomic datasets of these distinct populations have only recently been gathered, leaving many aspects of neuronal activity regulations still unanswered (D'Erchia et al, 2017;Maniatis et al, 2019;Marques et al, 2019). (iv) Homeostatic and compensatory mechanisms or the failure thereof have not been addressed yet.…”