Cortical hyperexcitability in mouse models and patients with amyotrophic lateral sclerosis is linked to noradrenaline deficiency.

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease, characterized by the death of upper (UMN) and lower motor neurons (LMN) in the motor cortex, brainstem, and spinal cord. Despite decades of research, ALS remains incurable, challenging to diagnose, and of extremely rapid progression. A unifying feature of sporadic and familial forms of ALS is cortical hyperexcitability, which precedes symptom onset, negatively correlates with survival, and is sufficient to trigger neurodegeneration in rodents. Using electrocorticography in the Sod1G86R and FusΔNLS/+ ALS mouse models and standard electroencephalography recordings in patients with sporadic ALS, we demonstrate a deficit in theta-gamma phase-amplitude coupling (PAC) in ALS. In mice, PAC deficits started before symptom onset, and in patients, PAC deficits correlated with the rate of disease progression. Using mass spectrometry analyses of CNS neuropeptides, we identified a presymptomatic reduction of noradrenaline (NA) in the motor cortex of ALS mouse models, further validated by in vivo two-photon imaging in behaving SOD1G93A and FusΔNLS/+ mice, that revealed pronounced reduction of locomotion-associated NA release. NA deficits were also detected in postmortem tissues from patients with ALS, along with transcriptomic alterations of noradrenergic signaling pathways. Pharmacological ablation of noradrenergic neurons with DSP-4 reduced theta-gamma PAC in wild-type mice and administration of a synthetic precursor of NA augmented theta-gamma PAC in ALS mice. Our findings suggest theta-gamma PAC as means to assess and monitor cortical dysfunction in ALS and warrant further investigation of the NA system as a potential therapeutic target.

Evidence that corticofugal propagation of ALS pathology is not mediated by prion-like mechanism.

Amyotrophic lateral sclerosis (ALS) arises from the combined degeneration of motor neurons (MN) and corticospinal neurons (CSN). Recent clinical and pathological studies suggest that ALS might start in the motor cortex and spread along the corticofugal axonal projections (including the CSN), either via altered cortical excitability and activity or via prion-like propagation of misfolded proteins. Using mouse genetics, we recently provided the first experimental arguments in favour of the corticofugal hypothesis, but the mechanism of propagation remained an open question. To gain insight into this matter, we tested here the possibility that the toxicity of the corticofugal projection neurons (CFuPN) to their targets could be mediated by their cell autonomous-expression of an ALS causing transgene and possible diffusion of toxic misfolded proteins to their spinal targets. We generated a Crym-CreERT2 mouse line to ablate the SOD1G37R transgene selectively in CFuPN. This was sufficient to fully rescue the CSN and to limit spasticity, but had no effect on the burden of misfolded SOD1 protein in the spinal cord, MN survival, disease onset and progression. The data thus indicate that in ALS corticofugal propagation is likely not mediated by prion-like mechanisms, but could possibly rather rely on cortical hyperexcitability.

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

OBJECTIVE: Recent studies carried out on amyotrophic lateral sclerosis patients suggest that the disease might initiate in the motor cortex and spread to its targets along the corticofugal tracts. In this study, we aimed to test the corticofugal hypothesis of amyotrophic lateral sclerosis experimentally. METHODS: Sod1G86R and Fezf2 knockout mouse lines were crossed to generate a model that expresses a mutant of the murine Sod1 gene ubiquitously, a condition sufficient to induce progressive motor symptoms and premature death, but genetically lacks corticospinal neurons and other subcerebral projection neurons, one of the main populations of corticofugal neurons. Disease onset and survival were recorded, and weight and motor behavior were followed longitudinally. Hyper-reflexia and spasticity were monitored using electromyographic recordings. Neurodegeneration and gliosis were assessed by histological techniques. RESULTS: Absence of subcerebral projection neurons delayed disease onset, reduced weight loss and motor impairment, and increased survival without modifying disease duration. Absence of corticospinal neurons also limited presymptomatic hyper-reflexia, a typical component of the upper motoneuron syndrome. INTERPRETATION: Major corticofugal tracts are crucial to the onset and progression of amyotrophic lateral sclerosis. In the context of the disease, subcerebral projection neurons might carry detrimental signals to their downstream targets. In its entirety, this study provides the first experimental arguments in favor of the corticofugal hypothesis of amyotrophic lateral sclerosis. ANN NEUROL 2020;88:688-702.

Cortical Circuit Dysfunction as a Potential Driver of Amyotrophic Lateral Sclerosis.

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease that affects selected cortical and spinal neuronal populations, leading to progressive paralysis and death. A growing body of evidences suggests that the disease may originate in the cerebral cortex and propagate in a corticofugal manner. In particular, transcranial magnetic stimulation studies revealed that ALS patients present with early cortical hyperexcitability arising from a combination of increased excitability and decreased inhibition. Here, we discuss the possibility that initial cortical circuit dysfunction might act as the main driver of ALS onset and progression, and review recent functional, imaging and transcriptomic studies conducted on ALS patients, along with electrophysiological, pathological and transcriptomic studies on animal and cellular models of the disease, in order to evaluate the potential cellular and molecular origins of cortical hyperexcitability in ALS.

A possible link between KCNQ2- and STXBP1-related encephalopathies: STXBP1 reduces the inhibitory impact of syntaxin-1A on M current.

OBJECTIVE: Kv7 channels mediate the voltage-gated M-type potassium current. Reduction of M current due to KCNQ2 mutations causes early onset epileptic encephalopathies (EOEEs). Mutations in STXBP1 encoding the syntaxin binding protein 1 can produce a phenotype similar to that of KCNQ2 mutations, suggesting a possible link between STXBP1 and Kv7 channels. These channels are known to be modulated by syntaxin-1A (Syn-1A) that binds to the C-terminal domain of the Kv7.2 subunit and strongly inhibits M current. Here, we investigated whether STXBP1could prevent this inhibitory effect of Syn-1A and analyzed the consequences of two mutations in STXBP1 associated with EOEEs. METHODS: Electrophysiologic analysis of M currents mediated by homomeric Kv7.2 or heteromeric Kv7.2/Kv7.3 channels in Chinese hamster ovary (CHO) cells coexpressing Syn-1A and/or STXBP1 or mutants STXBP1 p.W28* and p.P480L. Expression and interaction of these different proteins have been investigated using biochemical and co-immunoprecipitation experiments. RESULTS: Syn-1A decreased M currents mediated by Kv7.2 or Kv7.2/Kv7.3 channels. STXBP1 had no direct effects on M current but dampened the inhibition produced by Syn-1A by abrogating Syn-1A binding to Kv7 channels. The mutation p.W28*, but not p.P480L, failed to rescue M current from Syn-1A inhibition. Biochemical analysis showed that unlike the mutation p.W28*, the mutation p.P480L did not affect STXBP1 expression and reduced the interaction of Syn-1A with Kv7 channels. SIGNIFICANCE: These data indicate that there is a functional link between STXBP1 and Kv7 channels via Syn-1A, which may be important for regulating M-channel activity and neuronal excitability. They suggest also that a defect in Kv7 channel activity or regulation could be one of the consequences of some STXBP1 mutations associated with EOEEs. Furthermore, our data reveal that STXBP1 mutations associated with the Ohtahara syndrome do not necessarily result in protein haploinsufficiency.