Deciphering the Mechanisms Underlying Metabolic Dysregulation in ALS

Abstract

Amyotrophic lateral sclerosis (ALS) is a fatal, progressive neurodegenerative disorder that causes selective degeneration of upper and lower motor neurons with TDP-43 inclusions as a major pathological hallmark. ALS patients exhibit systemic metabolic deficits such as hypermetabolism, weight loss, dyslipidemia, insulin resistance, and altered glucose metabolism. Although evidence supports metabolic changes in ALS patients, metabolic alterations at a cellular level remain poorly understood. We used a Drosophila model of ALS based on TDP-43 (WT or ALS associated mutant variant, G298S) overexpression in motor neurons that recapitulates hallmark features of motor neuron disease to study metabolic dysregulation in ALS. In Chapter 2.2/Appendix D, we report alterations in multiple metabolic pathways including upregulation of Tricarboxylic acid (TCA) cycle metabolites and defects in neurotransmitter levels. Using genetic and dietary interventions, we show that modulating TCA cycle flux mitigates TDP-43-dependent locomotor defects. In addition, dopamine levels are significantly reduced in the context of TDP-43G298S, and we find that treatment with pramipexole, a dopamine agonist, improves locomotor function in Drosophila models of TDP-43 proteinopathy. Next, in Chapter 2.3, we show that overexpression of the rate limiting enzyme of glycolysis, Phosphofructokinase (PFK), mitigates TDP-43 induced locomotor deficits, lethality, and synaptic deficits consistent with upregulation of glycolysis being neuroprotective in ALS. In contrast, PFK mutations (R252S, G282A) that are thought to alter its enzymatic activity, abolish PFK’s neuroprotective role. This study observed PFK forming puncta in motor neuron cell bodies in response to TDP-43 proteinopathy, and PFKR252S, G282A formed significantly more cytoplasmic puncta compared to PFKWT, indicating an ineffective attempt to compensate for the lack of enzymatic activity, which is exacerbated by the presence of TDP-43 proteinopathy. Furthermore, alterations were observed in the localization of endogenous PFK in Drosophila neuromuscular junctions and human iPSC derived motor neurons in the context of disease-associated TDP-43 mutations. Together, these findings emphasize the potential for therapeutic approaches based on restoring metabolic balance in ALS and similar neurodegenerative conditions, by highlighting the role of cellular metabolism.