Mitophagy mechanisms in neuronal physiology and pathology during ageing

Mifepristone and rapamycin have non-additive benefits for life span in mated female Drosophila

The drugs mifepristone and rapamycin were compared for their relative ability to increase the life span of mated female Drosophila melanogaster. Titration of rapamycin indicated an optimal concentration of approximately 50 μM, which increased median life span here by average +81%. Meta-analysis of previous mifepristone titrations indicated an optimal concentration of approximately 466 μM, which increased median life span here by average +114%. Combining mifepristone with various concentrations of rapamycin did not produce further increases in life span, and instead reduced life span relative to either drug alone. Assay of maximum midgut diameter indicated that rapamycin was equally efficacious as mifepristone in reducing mating-induced midgut hypertrophy. The mito-QC mitophagy reporter is a previously described green fluorescent protein (GFP)-mCherry fusion protein targeted to the outer mitochondrial membrane. Inhibition of GFP fluorescence by the acidic environment of the autophagolysosome yields an increased red/green fluorescence ratio indicative of increased mitophagy. Creation of a multi-copy mito-QC reporter strain facilitated assay in live adult flies, as well as in dissected midgut tissue. Mifepristone was equally efficacious as rapamycin in activating the mito-QC mitophagy reporter in the adult female fat-body and midgut. The data suggest that mifepristone and rapamycin act through a common pathway to increase mated female Drosophila life span, and implicate increased mitophagy and decreased midgut hypertrophy in that pathway.

Longitudinal autophagy profiling of the mammalian brain reveals sustained mitophagy throughout healthy aging

Mitophagy neutralizes mitochondrial damage, thereby preventing cellular dysfunction and apoptosis. Defects in mitophagy have been strongly implicated in age-related neurodegenerative disorders such as Parkinson's and Alzheimer's disease. While mitophagy decreases throughout the lifespan of short-lived model organisms, it remains unknown whether such a decline occurs in the aging mammalian brain-a question of fundamental importance for understanding cell type- and region-specific susceptibility to neurodegeneration. Here, we define the longitudinal dynamics of basal mitophagy and macroautophagy across neuronal and non-neuronal cell types within the intact aging mouse brain in vivo. Quantitative profiling of reporter mouse cohorts from young to geriatric ages reveals cell- and tissue-specific alterations in mitophagy and macroautophagy between distinct subregions and cell populations, including dopaminergic neurons, cerebellar Purkinje cells, astrocytes, microglia and interneurons. We also find that healthy aging is hallmarked by the dynamic accumulation of differentially acidified lysosomes in several neural cell subsets. Our findings argue against any widespread age-related decline in mitophagic activity, instead demonstrating dynamic fluctuations in mitophagy across the aging trajectory, with strong implications for ongoing theragnostic development.

Mitochondrial dysfunction at the crossroad of cardiovascular diseases and cancer

A large body of evidence indicates the existence of a complex pathophysiological relationship between cardiovascular diseases and cancer. Mitochondria are crucial organelles whose optimal activity is determined by quality control systems, which regulate critical cellular events, ranging from intermediary metabolism and calcium signaling to mitochondrial dynamics, cell death and mitophagy. Emerging data indicate that impaired mitochondrial quality control drives myocardial dysfunction occurring in several heart diseases, including cardiac hypertrophy, myocardial infarction, ischaemia/reperfusion damage and metabolic cardiomyopathies. On the other hand, diverse human cancers also dysregulate mitochondrial quality control to promote their initiation and progression, suggesting that modulating mitochondrial homeostasis may represent a promising therapeutic strategy both in cardiology and oncology. In this review, first we briefly introduce the physiological mechanisms underlying the mitochondrial quality control system, and then summarize the current understanding about the impact of dysregulated mitochondrial functions in cardiovascular diseases and cancer. We also discuss key mitochondrial mechanisms underlying the increased risk of cardiovascular complications secondary to the main current anticancer strategies, highlighting the potential of strategies aimed at alleviating mitochondrial impairment-related cardiac dysfunction and tumorigenesis. It is hoped that this summary can provide novel insights into precision medicine approaches to reduce cardiovascular and cancer morbidities and mortalities.

RA, Flanagan-Steet H, Edison AS, Lyons MJ, Steet R. Functional assessment of homozygous ALDH18A1 variants reveals alterations in amino acid and antioxidant metabolism

Mono- and bi-allelic variants in ALDH18A1 cause a spectrum of human disorders associated with cutaneous and neurological findings that overlap with both cutis laxa and spastic paraplegia. ALDH18A1 encodes the bifunctional enzyme pyrroline-5-carboxylate synthetase (P5CS) that plays a role in the de novo biosynthesis of proline and ornithine. Here we characterize a previously unreported homozygous ALDH18A1 variant (p.Thr331Pro) in four affected probands from two unrelated families, and demonstrate broad-based alterations in amino acid and antioxidant metabolism. These four patients exhibit variable developmental delay, neurological deficits and loose skin. Functional characterization of the p.Thr331Pro variant demonstrated a lack of any impact on the steady-state level of the P5CS monomer or mitochondrial localization of the enzyme, but reduced incorporation of the monomer into P5CS oligomers. Using an unlabeled NMR-based metabolomics approach in patient fibroblasts and ALDH18A1-null human embryonic kidney cells expressing the variant P5CS, we identified reduced abundance of glutamate and several metabolites derived from glutamate, including proline and glutathione. Biosynthesis of the polyamine putrescine, derived from ornithine, was also decreased in patient fibroblasts, highlighting the functional consequence on another metabolic pathway involved in antioxidant responses in the cell. RNA sequencing of patient fibroblasts revealed transcript abundance changes in several metabolic and extracellular matrix-related genes, adding further insight into pathogenic processes associated with impaired P5CS function. Together these findings shed new light on amino acid and antioxidant pathways associated with ALDH18A1-related disorders, and underscore the value of metabolomic and transcriptomic profiling to discover new pathways that impact disease pathogenesis.

Mitophagy: A promising therapeutic target for neuroprotection during ageing and age-related diseases

Mitochondria and mitochondria-mediated signalling pathways are known to control synaptic signalling, as well as long-lasting changes in neuronal structure and function. Mitochondrial impairment is linked to synaptic dysfunction in normal ageing and age-associated neurodegenerative ailments, including Parkinson's disease (PD) and Alzheimer's disease (AD). Both proteolysis and mitophagy perform a major role in neuroprotection, by maintaining a healthy mitochondrial population during ageing. Mitophagy, a highly evolutionarily conserved cellular process, helps in the clearance of damaged mitochondria and thereby maintains the mitochondrial and metabolic balance, energy supply, neuronal survival and neuronal health. Besides the maintenance of brain homeostasis, hippocampal mitophagy also helps in synapse formation, axonal development, dopamine release and long-term depression. In contrast, defective mitophagy contributes to ageing and age-related neurodegeneration by promoting the accumulation of damaged mitochondria leading to cellular dysfunction. Exercise, stress management, maintaining healthy mitochondrial dynamics and administering natural or synthetic pharmacological compounds are some of the strategies used for neuroprotection during ageing and age-related neurological diseases. The current review discusses the impact of defective mitophagy in ageing and age-associated neurodegenerative conditions, the underlying molecular pathways and potential therapies based on recently elucidated mitophagy-inducing strategies.

Effect of the Ketone Body, D-β-Hydroxybutyrate, on Sirtuin2-Mediated Regulation of Mitochondrial Quality Control and the Autophagy-Lysosomal Pathway

Mitochondrial activity and quality control are essential for neuronal homeostasis as neurons rely on glucose oxidative metabolism. The ketone body, D-β-hydroxybutyrate (D-BHB), is metabolized to acetyl-CoA in brain mitochondria and used as an energy fuel alternative to glucose. We have previously reported that D-BHB sustains ATP production and stimulates the autophagic flux under glucose deprivation in neurons; however, the effects of D-BHB on mitochondrial turnover under physiological conditions are still unknown. Sirtuins (SIRTs) are NAD⁺-activated protein deacetylases involved in the regulation of mitochondrial biogenesis and mitophagy through the activation of transcription factors FOXO1, FOXO3a, TFEB and PGC1α coactivator. Here, we aimed to investigate the effect of D-BHB on mitochondrial turnover in cultured neurons and the mechanisms involved. Results show that D-BHB increased mitochondrial membrane potential and regulated the NAD⁺/NADH ratio. D-BHB enhanced FOXO1, FOXO3a and PGC1α nuclear levels in an SIRT2-dependent manner and stimulated autophagy, mitophagy and mitochondrial biogenesis. These effects increased neuronal resistance to energy stress. D-BHB also stimulated the autophagic-lysosomal pathway through AMPK activation and TFEB-mediated lysosomal biogenesis. Upregulation of SIRT2, FOXOs, PGC1α and TFEB was confirmed in the brain of ketogenic diet (KD)-treated mice. Altogether, the results identify SIRT2, for the first time, as a target of D-BHB in neurons, which is involved in the regulation of autophagy/mitophagy and mitochondrial quality control.

Biophysical Reviews- 2021, the year that was

The current issue (volume 13 issue 6, 2021) is a Special Issue jointly dedicated to scientific content presented at the 20th triennial IUPAB Congress that was held in conjunction with both the 45th Annual Meeting of the Brazilian Biophysical Society (Sociedade Brasileira de Biofísica - SBBf) and the 50th Annual Meeting of the Brazilian Society for Biochemistry and Molecular Biology (Sociedade Brasileira de Bioquímica e Biologia Molecular - SBBq). In addition to describing the scientific and nonscientific content arising from the meeting this sub-editorial also provides a look back at some of the high points for Biophysical Reviews in the year 2021 before going on to describe a number of matters of interest to readers of the journal in relation to the coming year of 2022.