Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are overlapping neurodegenerative diseases that are increasingly understood to have long prodromal periods.Investigation of these early stages promises to yield valuable biomarkers of disease and will be key to understanding mechanisms underlying the genesis of ALS-FTD. Here, we use in vivo magnetic resonance imaging (MRI), histology and computed tomography to identify structural and cellular readouts of early stage disease in the TDP-43 Q331K knock-in mouse model of ALS-FTD. Adult mutant mice demonstrated parenchymal volume reductions affecting the frontal lobe and entorhinal cortex in a manner reminiscent of ALS-FTD. Subcortical, cerebellar and brain stem regions were also affected in line with observations in presymptomatic carriers of mutations in C9orf72, the commonest genetic cause of both ALS and FTD. Volume loss, as measured by MRI, was also observed in the dentate gyrus (DG) of the hippocampus, along with ventricular enlargement. Guided by these imaging findings, detailed post-mortem brain tissue analysis revealed reduced parvalbumin-positive (PV+) interneurons as a potential cellular correlate of MRI changes in mutant mice. By contrast, microglia were in a disease activated state even in the absence of brain volume loss. A reduction in immature neurons was found in the DG, indicative of impaired adult neurogenesis, while a paucity of PV+ interneurons in juvenile mutant mice (P14) suggests that TDP-43 Q331K disrupts neurodevelopment. Computerised tomography imaging also showed altered skull morphology in mutants, further suggesting a role for TDP-43 Q331K in development. Finally, analysis of human post-mortem prefrontal cortices confirmed a paucity of PV+ interneurons in the prefrontal cortex in cases with both sporadic ALS and ALS linked to C9orf72 mutations. This study suggests an important role for PV+ interneurons in regional brain vulnerability associated with ALS-FTD, and identifies novel MRI and histological biomarkers that will be of value in assessing the efficacy of putative therapeutics in TDP-43 Q331K knock-in mice.

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