Parkinson disease remains a debilitating neurodegenerative disorder, despite the discovery of multiple causative genes that account for familial forms. Prominent among these are PRKN/Parkin and PINK1, whose protein products participate in mitochondrial turnover, or mitophagy. But our poor understanding of the basic biological mechanisms driven by those genes in neurons limits our ability to target them therapeutically. Here, we summarize our recent findings enabled by a new platform to track individual mitochondria in neurons. Our analysis delineates the steps of PINK1- and PRKN-dependent mitochondrial turnover, including the unexplored fates of mitochondria after fusion with lysosomes. These studies reveal unexpected mechanisms of mitochondrial quality control, which may contribute to the reliance of neurons on PINK1 under conditions of stress.

Copper (Cu) in trace amounts is essential for biological organisms. However, dysregulation of the redox-active metal has been implicated in different neurological disorders such as Wilson's, Menkes', Alzheimer's, and Parkinson's diseases. Since many households use Cu tubing in the plumbing system, and corrosion causes the metal to leach into the drinking water, there may be adverse effects on the central nervous system connected with low-level chronic exposure. The present study demonstrates that treatment with a biologically relevant concentration of Cu for 3 months significantly increases activation of the redox-modulated transcription factor AP-1 in mouse brains. This was independent of an upstream kinase indicated in AP-1 activation. Another redox-active transcription factor, NF-κB, was not significantly modified by the Cu exposure. These results indicate that the effect of Cu on AP-1 is unique and may involve direct modulation of DNA binding.

Male mice were fed 40 ppm melatonin for 2 months prior to sacrifice at age 26 months, and compared with both 26 and 4 month-old untreated controls. The nuclear translocation of NF-κB increased with age in both brain and spleen and this was reversed by melatonin only in brain. Another transcription factor, AP-1 was increased with age in the spleen and not in brain and this could be blocked by melatonin treatment. The fraction of the active relative to the inactive form of several enabling kinases was compared. The proportion of activated ERK was elevated with age in brain and spleen but this change was unresponsive to melatonin. A similar age-related increase in glial fibrillary acidic protein (GFAP) was also refractory to melatonin treatment. The cerebral melatonin M1 receptor decreased with age in brain but increased in spleen. The potentially beneficial nature of melatonin for the preservation of brain function with aging was suggested by the finding that an age-related decline in cortical synaptophysin levels was prevented by dietary melatonin.

Mutations in the mitochondrial protein CHCHD2 cause autosomal dominant Parkinson's disease characterized by the preferential loss of substantia nigra dopamine (DA) neurons. Therefore, understanding the function of CHCHD2 in neurons may provide vital insights into how mitochondrial dysfunction contributes to neurodegeneration in PD. To investigate the normal requirement and function of CHCHD2 in neurons, we first examined CHCHD2 levels and showed that DA neurons have higher CHCHD2 levels than other neuron types, both in vivo and in co-culture. We then generated mice with either a targeted deletion of CHCHD2 in DA neurons or a deletion in the brain or total body. All three models were viable, and loss of CHCHD2 in the brain did not cause degeneration of DA neurons. Mice lacking CHCHD2 in DA neurons did display sex-specific changes to locomotor activity, but we did not observe differences in assays of muscle strength, exercise endurance or motor coordination. Furthermore, mitochondria derived from mice lacking CHCHD2 did not display abnormalities in OXPHOS function. Lastly, resilience to CHCHD2 deletion could not be explained by functional complementation by its paralog CHCHD10, as deletion of both CHCHD10 and CHCHD2 did not cause degeneration of DA neurons in the midbrain. These findings support the hypothesis that pathogenic CHCHD2 mutations cause PD through a toxic gain-of-function, rather than loss-of-function mechanism.

Altered mitochondrial quality control and dynamics may contribute to neurodegenerative diseases, including Parkinson's disease, but we understand little about these processes in neurons. We combined time-lapse microscopy and correlative light and electron microscopy to track individual mitochondria in neurons lacking the fission-promoting protein dynamin-related protein 1 (Drp1) and delineate the kinetics of PINK1-dependent pathways of mitochondrial quality control. Depolarized mitochondria recruit Parkin to the outer mitochondrial membrane, triggering autophagosome formation, rapid lysosomal fusion, and Parkin redistribution. Unexpectedly, these mitolysosomes are dynamic and persist for hours. Some are engulfed by healthy mitochondria, and others are deacidified before bursting. In other cases, Parkin is directly recruited to the matrix of polarized mitochondria. Loss of PINK1 blocks Parkin recruitment, causes LC3 accumulation within mitochondria, and exacerbates Drp1KO toxicity to dopamine neurons. These results define a distinct neuronal mitochondrial life cycle, revealing potential mechanisms of mitochondrial recycling and signaling relevant to neurodegeneration.

Alterations in mitochondrial fission may contribute to the pathophysiology of several neurodegenerative diseases, including Alzheimer's disease (AD). However, we understand very little about the normal functions of fission or how fission disruption may interact with AD-associated proteins to modulate pathogenesis. Here we show that loss of the central mitochondrial fission protein dynamin-related protein 1 (Drp1) in CA1 and other forebrain neurons markedly worsens the learning and memory of mice expressing mutant human amyloid precursor protein (hAPP) in neurons. In cultured neurons, Drp1KO and hAPP converge to produce mitochondrial Ca²⁺ (mitoCa²⁺) overload, despite decreasing mitochondria-associated ER membranes (MAMs) and cytosolic Ca²⁺. This mitoCa²⁺ overload occurs independently of ATP levels. These findings reveal a potential mechanism by which mitochondrial fission protects against hAPP-driven pathology.

Although COVID-19 causes cardiac dysfunction in up to 25% of patients, its pathogenesis remains unclear. Exposure of human iPSC-derived heart cells to SARS-CoV-2 revealed productive infection and robust transcriptomic and morphological signatures of damage, particularly in cardiomyocytes. Transcriptomic disruption of structural proteins corroborated adverse morphologic features, which included a distinct pattern of myofibrillar fragmentation and numerous iPSC-cardiomyocytes lacking nuclear DNA. Human autopsy specimens from COVID-19 patients displayed similar sarcomeric disruption, as well as cardiomyocytes without DNA staining. These striking cytopathic features provide new insights into SARS-CoV-2 induced cardiac damage, offer a platform for discovery of potential therapeutics, and raise serious concerns about the long-term consequences of COVID-19.

Neurons require large amounts of energy, but whether they can perform glycolysis or require glycolysis to maintain energy remains unclear. Using metabolomics, we show that human neurons do metabolize glucose through glycolysis and can rely on glycolysis to supply tricarboxylic acid (TCA) cycle metabolites. To investigate the requirement for glycolysis, we generated mice with postnatal deletion of either the dominant neuronal glucose transporter (GLUT3cKO) or the neuronal-enriched pyruvate kinase isoform (PKM1cKO) in CA1 and other hippocampal neurons. GLUT3cKO and PKM1cKO mice show age-dependent learning and memory deficits. Hyperpolarized magnetic resonance spectroscopic (MRS) imaging shows that female PKM1cKO mice have increased pyruvate-to-lactate conversion, whereas female GLUT3cKO mice have decreased conversion, body weight, and brain volume. GLUT3KO neurons also have decreased cytosolic glucose and ATP at nerve terminals, with spatial genomics and metabolomics revealing compensatory changes in mitochondrial bioenergetics and galactose metabolism. Therefore, neurons metabolize glucose through glycolysis in vivo and require glycolysis for normal function.

Obesity-driven cardiac lipid accumulation can progress to lipotoxic cardiomyopathy. Soluble epoxide hydrolase (sEH) is the major enzyme that metabolizes epoxyeicosatrienoic acids (EETs), which have biological activity of regulating lipid metabolism. The current study explores the unknown role of sEH deficiency in lipotoxic cardiomyopathy and its underlying mechanism. Wild-type and Ephx2 knock out (sEH KO) C57BL/6 J mice were fed with high-fat diet (HFD) for 24 weeks to induce lipotoxic cardiomyopathy animal models. Palmitic acid (PA) was utilized to induce lipotoxicity to cardiomyocytes for in vitro study. We found sEH KO, independent of plasma lipid and blood pressures, significantly attenuated HFD-induced myocardial lipid accumulation and cardiac dysfunction in vivo. HFD-induced lipotoxic cardiomyopathy and dysfunction of adenosine 5'-monophosphate-activated protein kinase-mammalian target of rapamycin complex (AMPK-mTORC) signaling mediated lipid autophagy in heart were restored by sEH KO. In primary neonatal mouse cardiomyocytes, both sEH KO and sEH substrate EETs plus sEH inhibitor AUDA treatments attenuated PA-induced lipid accumulation. These effects were blocked by inhibition of AMPK or autophagy. The outcomes were supported by the results that sEH KO and EETs plus AUDA rescued HFD- and PA-induced impairment of autophagy upstream signaling of AMPK-mTORC, respectively. These findings revealed that sEH deficiency played an important role in attenuating myocardial lipid accumulation and provided new insights into treating lipotoxic cardiomyopathy. Regulation of autophagy via AMPK-mTORC signaling pathway is one of the underlying mechanisms.

Although coronavirus disease 2019 (COVID-19) causes cardiac dysfunction in up to 25% of patients, its pathogenesis remains unclear. Exposure of human induced pluripotent stem cell (iPSC)-derived heart cells to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) revealed productive infection and robust transcriptomic and morphological signatures of damage, particularly in cardiomyocytes. Transcriptomic disruption of structural genes corroborates adverse morphologic features, which included a distinct pattern of myofibrillar fragmentation and nuclear disruption. Human autopsy specimens from patients with COVID-19 reflected similar alterations, particularly sarcomeric fragmentation. These notable cytopathic features in cardiomyocytes provide insights into SARS-CoV-2-induced cardiac damage, offer a platform for discovery of potential therapeutics, and raise concerns about the long-term consequences of COVID-19 in asymptomatic and severe cases.