Mitochondrial abnormalities have been identified as a central mechanism in multiple neurodegenerative diseases and, therefore, the mitochondria have been explored as a therapeutic target. This review will focus on the evidence for mitochondrial abnormalities in the two most common neurodegenerative diseases, Parkinson's disease and Alzheimer's disease. In addition, we discuss the main strategies which have been explored in these diseases to target the mitochondria for therapeutic purposes, focusing on mitochondrially targeted antioxidants, peptides, modulators of mitochondrial dynamics and phenotypic screening outcomes.

Alzheimer's disease (AD) is the leading cause of dementia worldwide. Mitochondrial abnormalities have been identified in many cell types in AD, with deficits preceding the development of the classical pathological aggregations. Ursodeoxycholic acid (UDCA), a treatment for primary biliary cirrhosis, improves mitochondrial function in fibroblasts derived from Parkinson's disease patients as well as several animal models of AD and Parkinson's disease. In this paper, we investigated both mitochondrial function and morphology in fibroblasts from patients with both sporadic and familial AD. We show that both sporadic AD (sAD) and PSEN1 fibroblasts share the same impairment of mitochondrial membrane potential and alterations in mitochondrial morphology. Mitochondrial respiration, however, was decreased in sAD fibroblasts and increased in PSEN1 fibroblasts. Morphological changes seen in AD fibroblasts include reduced mitochondrial number and increased mitochondrial clustering around the cell nucleus as well as an increased number of long mitochondria. We show here for the first time in AD patient tissue that treatment with UDCA increases mitochondrial membrane potential and respiration as well as reducing the amount of long mitochondria in AD fibroblasts. In addition, we show reductions in dynamin-related protein 1 (Drp1) level, particularly the amount localized to mitochondria in both sAD and familial patient fibroblasts. Drp1 protein amount and localization were increased after UDCA treatment. The restorative effects of UDCA are abolished when Drp1 is knocked down. This paper highlights the potential use of UDCA as a treatment for neurodegenerative disease.

Alzheimer’s disease (AD) is diagnosed using neuropsychological testing, supported by amyloid and tau biomarkers and neuroimaging abnormalities. The cause of neuropsychological changes is not clear since they do not correlate with biomarkers. This study investigated if changes in cellular metabolism in AD correlate with neuropsychological changes. Fibroblasts were taken from 10 AD patients and 10 controls. Metabolic assessment included measuring total cellular ATP, extracellular lactate, mitochondrial membrane potential (MMP), mitochondrial respiration and glycolytic function. All participants were assessed with neuropsychological testing and brain structural MRI. AD patients had significantly lower scores in delayed and immediate recall, semantic memory, phonemic fluency and Mini Mental State Examination (MMSE). AD patients also had significantly smaller left hippocampal, left parietal, right parietal and anterior medial prefrontal cortical grey matter volumes. Fibroblast MMP, mitochondrial spare respiratory capacity (MSRC), glycolytic reserve, and extracellular lactate were found to be lower in AD patients. MSRC/MMP correlated significantly with semantic memory, immediate and delayed episodic recall. Correlations between MSRC and delayed episodic recall remained significant after controlling for age, education and brain reserve. Grey matter volumes did not correlate with MRSC/MMP. AD fibroblast metabolic assessment may represent an emergent disease biomarker of AD.

Alzheimer’s disease (AD) is the most common cause of dementia worldwide and is characterised pathologically by the accumulation of amyloid beta and tau protein aggregates. Currently, there are no approved disease modifying therapies for clearance of either of these proteins from the brain of people with AD. As well as abnormalities in protein aggregation, other pathological changes are seen in this condition. The function of mitochondria in both the nervous system and rest of the body is altered early in this disease, and both amyloid and tau have detrimental effects on mitochondrial function. In this review article, we describe how the function and structure of mitochondria change in AD. This review summarises current imaging techniques that use surrogate markers of mitochondrial function in both research and clinical practice, but also how mitochondrial functions such as ATP production, calcium homeostasis, mitophagy and reactive oxygen species production are affected in AD mitochondria. The evidence reviewed suggests that the measurement of mitochondrial function may be developed into a future biomarker for early AD. Further work with larger cohorts of patients is needed before mitochondrial functional biomarkers are ready for clinical use.

Dysfunctional mitochondria characterise Parkinson’s Disease (PD). Uncovering etiological molecules, which harm the homeostasis of mitochondria in response to pathological cues, is therefore pivotal to inform early diagnosis and therapy in the condition, especially in its idiopathic forms. This study proposes the 18 kDa Translocator Protein (TSPO) to be one of those. Both in vitro and in vivo data show that neurotoxins, which phenotypically mimic PD, increase TSPO to enhance cellular redox-stress, susceptibility to dopamine-induced cell death, and repression of ubiquitin-dependent mitophagy. TSPO amplifies the extracellular signal-regulated protein kinase 1 and 2 (ERK1/2) signalling, forming positive feedback, which represses the transcription factor EB (TFEB) and the controlled production of lysosomes. Finally, genetic variances in the transcriptome confirm that TSPO is required to alter the autophagy–lysosomal pathway during neurotoxicity.

Background In clinical settings, AD is defined by characteristic deficits in neuropsychological testing supported by amyloid and tau biomarkers and neuroimaging abnormalities. The biological cause of neuropsychological changes is not established. Tau deposition correlates with, but does not fully account for all observed neuropsychological impairments. We have shown mitochondrial spare respiratory capacity (MRSC) is lowered in AD patient fibroblasts. This study investigates if fibroblast mitochondrial functional abnormalities correlate with neuropsychological/neuroimaging changes in AD. Method 10 AD patient and 10 control fibroblast samples were taken via skin biopsy. Adenosine Triphosphate (ATP) and extracellular lactate were measured using luminescent and fluorescent protocols respectively. Mitochondrial membrane potential (MMP) was measured using tetramethylrhodamine dye. Mitochondrial respiration and glycolytic function were measured using a Seahorse XF Analyzer. In‐depth Neuropsychological profiling, and brain structural MRIs were undertaken on all participants. Correlations were performed between MMP, MRSC and neuropsychological/MRI AD markers. Result Reductions in delayed (p<0.0001) and immediate recall (p<0.0001), semantic fluency (p<0.0001), phonemic fluency (p=0.0033) and MMSE (p=0.0009) scores were seen in AD patients. After controlling for age, education and brain reserve; left hippocampal (p=0.001), left parietal (p=0.002), right parietal (p=0.001) and anterior medial prefrontal cortical (p=0.017) gray matter volumes were reduced in AD patients. Fibroblast metabolic markers showed a reduction in MMP (p=0.001), MRSC (p<0.0001), glycolytic reserve(p=0.05), and extracellular lactate (p<0.05) in AD patients. MRSC and MMP correlated significantly with immediate recall ([MRSC, p=0.0041],[MMP, p=0.0115]), delayed recall ([MRSC, p=0.0013],[MMP, p=0.0138]) and semantic memory ([MRSC, p=0.0039],[MMP, p=0.009]) tests. The correlations between MRSC and neuropsychological measures remained after controlling for age, education and brain reserve. No correlations were seen between grey matter volumes and fibroblast metabolism. Conclusion This study highlights how in‐depth metabolic analysis of sporadic AD fibroblasts identifies functional abnormalities that correlate with neuropsychological features distinctive to AD. This work may also explain how some of the fundamental biological processes that are affected in Alzheimer’s disease may contribute to the neuropsychological profiles that define the condition.