Frontotemporal dementia and amyotrophic lateral sclerosis proteins in neurite health and dysfunction

A number of proteins have been identified which are pathologically and/or genetically associated with both frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). A hexanucleotide repeat expansion in the non-coding region of the C9ORF72 gene is the largest genetic factor associated with FTD and ALS. Additionally, pathological inclusions of the TDP-43 protein are found in brains of approximately 50% of FTD cases and 90% of ALS cases. However, the normal function of these two proteins and how this relates to the degeneration of neurons in disease are not yet well understood. Accumulating evidence suggests that both proteins may have involvement with the neuronal cytoskeleton. The cytoskeleton is of particular interest in FTD/ALS due to significant axon pathology and loss observed in post mortem cases. It is known that the cytoskeleton is often a key effector to changes in axon integrity in many neurodegenerative diseases. This thesis examined the normal functions of TDP-43 and C9ORF72, and their links with the cytoskeleton through use of mouse primary cell culture, histological analysis of intact mouse brain, and viral-mediated expression of proteins of interest in a retina model in the mouse. TDP-43 is a predominantly nuclear protein, with much known about its nuclear roles in DNA and RNA binding, and regulation of transcription and translation. Evidence suggests that TDP-43 is also important for neurite outgrowth, remodelling and can regulate many components of the neuronal cytoskeleton. The effects of overexpression of TDP-43 in primary cortical neurons were examined and demonstrated distinct alterations to actin-associated cellular processes including neurite branching and growth cone morphology, as well as down-regulation of actin-binding proteins in the proteome of these cells. To examine the effect of pathogenic alterations to TDP-43 in vivo AAV2 virus was used to mediate the expression of wildtype human TDP-43 and human TDP-43 with a mutation to the nuclear localisation signal (NLS) in mouse retinal ganglion cells, through intraocular injection of the virus. These changes induced axon pathology in the optic nerve, with indications of perturbed axonal transport due to presence of organelle accumulation. Several mechanisms for how the non-coding repeat expansion in C9ORF72 may lead to disease have been described, however little is known about the normal function and the expression pattern of the C9ORF72 protein. This thesis characterized the expression pattern and cellular localisation of the three reported mouse isoforms of C9ORF72 in cell culture and in vivo, and demonstrated that C9ORF72 was present in synaptosomes and within actin-rich structures of neurons. In summary, these results indicate that both C9ORF72 and TDP-43 may have links to the neuronal cytoskeleton, and in particular the actin cytoskeleton. Modulation of the neuronal cytoskeleton is a compelling target for providing therapeutic protection to vulnerable cellular components, such as the axon. Studies such as those described in this thesis may provide insight for whether TDP-43- and C9ORF72-related FTD/ALS are candidates for these types of interventions.