Selective vulnerability of the primary motor cortex in amyotrophic lateral sclerosis

The ‘selective vulnerability’ of cells and systems to disease pathomechanisms is a defining feature of amyotrophic lateral sclerosis (ALS), where the relative clinical and pathological involvement of cortical motor neurons varies between patients. Mislocalisation and aggregation of hyperphosphorylated TDP-43 (pTDP-43) is the major neuropathological finding in the majority of ALS cases (ALS-TDP), however the relationship between TDP-43 aggregation and neurodegeneration remains unclear. Here, we sought to reconcile the selective vulnerability of specific cell types within the human primary motor cortex to the pathological hallmarks of ALS, and define commonalities and differences of neurodegenerative phenotypes across the genetic spectrum of the disease. We analysed human post-mortem tissue from a large cohort of genetically defined ALS/FTD (n = 78), and mean age-matched control cases (n = 15) using a combination of quantitative immunohistochemical techniques and multi-level generalized linear modelling. The utility of novel methods such as CLARITY, imaging mass cytometry, combined RNA-ISH/IHC and multiplexed immunofluorescence is also explored. Additionally, two monogenic forms of the disease which exhibit striking UMN/LMN predominance, ALS-FUS and ALS-OPTN, are discussed and compared in the context of a novel BAC transgenic mouse and a unique human index case. We show that there is considerable variance in neuropathology within the ALS primary motor cortex both within and across the genetic spectrum of disease, including in pTDP-43 aggregation, microglial activation, and inhibitory interneuron number. We also assess the extent of this pathology across the corticospinal neuraxis, and begin to examine the interactive relationship of pathologies using multiplexed immunofluorescence. Using RNA-ISH, we also show that while FUS is significantly more highly expressed in the primary motor cortex, TARDBP demonstrates a higher retention of transcripts localised to the nucleus. We also begin to assess the expression of transcription factors associated with projection neuron diversity including CTIP2, SATB2 and TBR1 in the primary motor cortex. Broadly, this thesis refines the concept of selective vulnerability within the ALS motor cortex across genetic subtypes, whilst highlighting the distinction between cellular vulnerability to proteinopathy and vulnerability to degeneration itself. These findings have implications for our understanding of disease pathogenesis and the development of genotype-specific therapies.