Files
Abstract
Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are devastating and fatal degenerative diseases. ALS is the most common motor neuron disease and FTD represents an estimated 10%-20% of all dementia cases. The discovery in 2011 that the C9orf72 gene mutation is the major genetic cause of both ALS and FTD provided a molecular link for these two diseases. Both loss of function and gain of function hypotheses have been proposed for the disease mechanisms. Loss of function mechanisms focus on loss of the C9orf72 protein, whose function was largely unknown back then. Gain of function mechanisms focus on toxicity from C9orf72 repeat RNA or from dipeptide repeat proteins produced by repeat-associated on-ATG translation.
This thesis work has examined the normal molecular and cellular functions of C9orf72 as well as its associated protein Smcr8, and how partial depletion of C9orf72 affects the cellular and behavioral functions under the gain of toxicity background. I have examined the roles of the C9orf72/Smcr8 complex in autophagy and axonal transport. C9orf72/Smcr8 complex associates with the ULK1/ATG13/FIP200 complex. Loss of C9orf72 or Smcr8 disrupts autophagy induction and autophagy flux. Smcr8 deficient mice exhibit motor behavioral deficits, axonal swellings and autophagosome-lysosomal accumulations. Combining the Smcr8 deficient mouse model with a C9orf72 deficient mouse model, I discovered that C9orf72 deficiency exacerbates the lysosomal and axonal deficits of Smcr8 deficient mice. Time-lapse imaging revealed roles of C9orf72 and Smcr8 in axonal transport of autophagosomes. These studies have uncovered potential functions of C9orf72 in maintaining neuronal functions, which may be masked by its interactor Smcr8.
To understand how loss of function of C9orf72 affects neurodegeneration in the disease conditions, I crossed the C9orf72 deficient mouse model with a C9ALS/FTD gain of function mouse model. C9orf72 haploinsufficiency exacerbated the motor, social and cognitive deficits of the gain of function mouse model. Mechanistic studies revealed more severe axonal swellings at the neuromuscular junctions and increased dipeptide repeat accumulations due to partial loss of C9orf72 function in the gain of function background. These experiments have filled the gap between the loss- and gain-of-function and provided the field with a broader picture of the disease mechanisms and potential therapeutic targets.
This thesis work has examined the normal molecular and cellular functions of C9orf72 as well as its associated protein Smcr8, and how partial depletion of C9orf72 affects the cellular and behavioral functions under the gain of toxicity background. I have examined the roles of the C9orf72/Smcr8 complex in autophagy and axonal transport. C9orf72/Smcr8 complex associates with the ULK1/ATG13/FIP200 complex. Loss of C9orf72 or Smcr8 disrupts autophagy induction and autophagy flux. Smcr8 deficient mice exhibit motor behavioral deficits, axonal swellings and autophagosome-lysosomal accumulations. Combining the Smcr8 deficient mouse model with a C9orf72 deficient mouse model, I discovered that C9orf72 deficiency exacerbates the lysosomal and axonal deficits of Smcr8 deficient mice. Time-lapse imaging revealed roles of C9orf72 and Smcr8 in axonal transport of autophagosomes. These studies have uncovered potential functions of C9orf72 in maintaining neuronal functions, which may be masked by its interactor Smcr8.
To understand how loss of function of C9orf72 affects neurodegeneration in the disease conditions, I crossed the C9orf72 deficient mouse model with a C9ALS/FTD gain of function mouse model. C9orf72 haploinsufficiency exacerbated the motor, social and cognitive deficits of the gain of function mouse model. Mechanistic studies revealed more severe axonal swellings at the neuromuscular junctions and increased dipeptide repeat accumulations due to partial loss of C9orf72 function in the gain of function background. These experiments have filled the gap between the loss- and gain-of-function and provided the field with a broader picture of the disease mechanisms and potential therapeutic targets.