Improving Mitochondrial Function in the Aging and Diseased Brain

Abstract

Dysfunctional mitochondria are a hallmark of the ageing brain and are implicated in neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). As such, healthy neuron function is dependent on the maintenance of mitochondrial processes such as mitochondrial redox state, membrane potential, network, superoxide production and cellular bioenergetics. Indeed, there is evidence that natural compounds are capable of improving these mitochondrial processes prior to the onset of disease- or agerelated neuronal mitochondrial dysfunction. Thus, this thesis explores therapeutic ways to improve mitochondrial function in the diseased and ageing brain. An in vitro model and a mimetic that induces disease- and age-related neuronal mitochondrial dysfunction was established. The human neuroblastoma cell line, SH-SY5Y, was chosen as an in vitro model and was differentiated using retinoic acid (RA) to produce a mature, and cholinergic phenotype. This RA-differentiation of SH-SY5Y also induced changes at the mitochondrial level, which recapitulated several reported aspects of mitochondrial function in rodent primary neurons. Hence, RA-differentiated SH-SY5Y was an appropriate in vitro model to use. Next, understanding new AD pathology provides targets for therapeutic intervention. It has been reported that glycation of Aβ1-42, a peptide involved in AD and pathological ageing with a non-reversible posttranslational modification, is more neurotoxic relative to unglycated Aβ1-42. However, it is unknown whether a specific type or site of glycation on Aβ1-42 contributes to neurotoxicity and if mitochondrial dysfunction is involved. Thus, synthesis of type- and site-specific glycation of Aβ1-42 was undertaken, which added Nε- (carboxyethyl)lysine (CEL), a common glycation found in AD patients, to the two lysine residues on Aβ1- 42. The results show that single, but not double, CEL-glycated Aβ1-42 are neurotoxic in RA-differentiated SH-SY5Y. This neurotoxicity was independent of mitochondrial dysfunction. The double CEL-glycated Aβ1- 42 may have induced mitophagy (i.e. selective mitochondrial degradation), which protected against neurotoxicity. To investigate mitophagy further, a plant-derived gut-metabolite, urolithin A (UA), known to induce mitophagy in myoblasts and cross the blood brain barrier, was tested on RA-differentiated SH-SY5Y. Under non-stressed conditions, UA dose- and time-dependently improved mitochondrial processes such as respiration, mitochondrial network and mitophagy in RA-differentiated SH-SY5Y. Further investigations are necessary to understand the effect of UA on attenuating Aβ1-42- and rotenone-mediated mitochondrial dysfunction associated with AD and PD respectively. Therefore, given the safety of UA in Phase I trials, UA may be repurposed in the clinic to improve mitochondrial function in the diseased and ageing brain.