Mislocalisation of TDP‐43 to the cytoplasm causes cortical hyperexcitability and reduced excitatory neurotransmission in the motor cortex

• Published in: Journal of neurochemistry 2021-05, Vol.157 (4), p.1300-1315
• Main Authors: Dyer, Marcus S., Reale, Laura A., Lewis, Katherine E., Walker, Adam K., Dickson, Tracey C., Woodhouse, Adele, Blizzard, Catherine A.
• Format: Article
• Language: English
• Subjects: ALS; Amyotrophic lateral sclerosis; Animal models; cortex; Cortex (motor); Cytoplasm; Deoxyribonucleic acid; DNA; Electrophysiology; Excitability; Forebrain; glutamate; Glutamate receptors; Glutamic acid receptors (ionotropic); hyperexcitability; Immunohistochemistry; Localization; mislocalisation; mouse model; Neurodegenerative diseases; Neurons; Neurotransmission; Pathophysiology; Phenotypes; Proteins; RNA-binding protein; TDP‐43; Toxicity
• ISSN: 0022-3042; 1471-4159
• DOI: 10.1111/jnc.15214

Amyotrophic lateral sclerosis (ALS) is a chronic neurodegenerative disease pathologically characterised by mislocalisation of the RNA‐binding protein TAR‐DNA‐binding protein 43 (TDP‐43) from the nucleus to the cytoplasm. Changes to neuronal excitability and synapse dysfunction in the motor cortex are early pathological changes occurring in people with ALS and mouse models of disease. To investigate the effect of mislocalised TDP‐43 on the function of motor cortex neurons we utilised mouse models that express either human wild‐type (TDP‐43WT) or nuclear localisation sequence‐deficient TDP‐43 (TDP‐43ΔNLS) on an inducible promoter that enriches expression to forebrain neurons. Pathophysiology was investigated through immunohistochemistry and whole‐cell patch‐clamp electrophysiology. Thirty days expression of TDP‐43ΔNLS in adult mice did not cause any changes in the number of CTIP2‐positive neurons in the motor cortex. However, at this time‐point, the expression of TDP‐43ΔNLS drives intrinsic hyperexcitability in layer V excitatory neurons of the motor cortex. This hyperexcitability occurs concomitantly with a decrease in excitatory synaptic input to these cells and fluctuations in both directions of ionotropic glutamate receptors. This pathophysiology is not present with TDP‐43WT expression, demonstrating that the localisation of TDP‐43 to the cytoplasm is crucial for the altered excitability phenotype. This study has important implications for the mechanisms of toxicity of one of the most notorious proteins linked to ALS, TDP‐43. We provide the first evidence that TDP‐43 mislocalisation causes aberrant synaptic function and a hyperexcitability phenotype in the motor cortex, linking some of the earliest dysfunctions to arise in people with ALS to mislocalisation of TDP‐43. Cytoplasmic accumulation of TDP‐43 is the most common pathology in ALS. ALS patients and ALS models show cortical hyperexcitability prior to symptom onset and during disease. Here, we reveal that TDP‐43 mislocalised to the cytoplasm in the forebrain of TDP‐43NLS mice causes hyperexcitability of layer V pyramidal neurons in the motor cortex. We also detected a decrease in excitatory synaptic input to these cells and alterations in glutamate receptor subunit expression, without cell loss. These are important discoveries for the ALS field as we demonstrate that the most common pathology in ALS, TDP‐43 mislocalisation, causes the earliest physiological change, cortical hyperexcitability.