In vitro rejuvenation of brain mitochondria by the inhibition of actin polymerization

Beyond fission and fusion-Diving into the mysteries of mitochondrial shape

Mitochondrial shape and network formation have been primarily associated with the well-established processes of fission and fusion. However, recent research has unveiled an intricate and multifaceted landscape of mitochondrial morphology that extends far beyond the conventional fission-fusion paradigm. These less-explored dimensions harbor numerous unresolved mysteries. This review navigates through diverse processes influencing mitochondrial shape and network formation, highlighting the intriguing complexities and gaps in our understanding of mitochondrial architecture. The exploration encompasses various scales, from biophysical principles governing membrane dynamics to molecular machineries shaping mitochondria, presenting a roadmap for future research in this evolving field.

Actin Polymerization Defects Induce Mitochondrial Dysfunction in Cellular Models of Nemaline Myopathies

Nemaline myopathy (NM) is one of the most common forms of congenital myopathy and it is identified by the presence of "nemaline bodies" (rods) in muscle fibers by histopathological examination. The most common forms of NM are caused by mutations in the Actin Alpha 1 (ACTA1) and Nebulin (NEB) genes. Clinical features include hypotonia and muscle weakness. Unfortunately, there is no curative treatment and the pathogenetic mechanisms remain unclear. In this manuscript, we examined the pathophysiological alterations in NM using dermal fibroblasts derived from patients with mutations in ACTA1 and NEB genes. Patients' fibroblasts were stained with rhodamine-phalloidin to analyze the polymerization of actin filaments by fluorescence microscopy. We found that patients' fibroblasts showed incorrect actin filament polymerization compared to control fibroblasts. Actin filament polymerization defects were associated with mitochondrial dysfunction. Furthermore, we identified two mitochondrial-boosting compounds, linoleic acid (LA) and L-carnitine (LCAR), that improved the formation of actin filaments in mutant fibroblasts and corrected mitochondrial bioenergetics. Our results indicate that cellular models can be useful to study the pathophysiological mechanisms involved in NM and to find new potential therapies. Furthermore, targeting mitochondrial dysfunction with LA and LCAR can revert the pathological alterations in NM cellular models.

The actin binding protein profilin 1 is critical for mitochondria function

Profilin 1 (PFN1) is an actin binding protein that is vital for the polymerization of monomeric actin into filaments. Here we screened knockout cells for novel functions of PFN1 and discovered that mitophagy, a type of selective autophagy that removes defective or damaged mitochondria from the cell, was significantly upregulated in the absence of PFN1. Despite successful autophagosome formation and fusion with the lysosome, and activation of additional mitochondrial quality control pathways, PFN1 knockout cells still accumulate damaged, dysfunctional mitochondria. Subsequent imaging and functional assays showed that loss of PFN1 significantly affects mitochondria morphology, dynamics, and respiration. Further experiments revealed that PFN1 is located to the mitochondria matrix and is likely regulating mitochondria function from within rather than through polymerizing actin at the mitochondria surface. Finally, PFN1 mutants associated with amyotrophic lateral sclerosis (ALS) fail to rescue PFN1 knockout mitochondrial phenotypes and form aggregates within mitochondria, further perturbing them. Together, these results suggest a novel function for PFN1 in regulating mitochondria and identify a potential pathogenic mechanism of ALS-linked PFN1 variants.

The multiple links between actin and mitochondria

Actin plays many well-known roles in cells, and understanding any specific role is often confounded by the overlap of multiple actin-based structures in space and time. Here, we review our rapidly expanding understanding of actin in mitochondrial biology, where actin plays multiple distinct roles, exemplifying the versatility of actin and its functions in cell biology. One well-studied role of actin in mitochondrial biology is its role in mitochondrial fission, where actin polymerization from the endoplasmic reticulum through the formin INF2 has been shown to stimulate two distinct steps. However, roles for actin during other types of mitochondrial fission, dependent on the Arp2/3 complex, have also been described. In addition, actin performs functions independent of mitochondrial fission. During mitochondrial dysfunction, two distinct phases of Arp2/3 complex-mediated actin polymerization can be triggered. First, within 5 min of dysfunction, rapid actin assembly around mitochondria serves to suppress mitochondrial shape changes and to stimulate glycolysis. At a later time point, at more than 1 h post-dysfunction, a second round of actin polymerization prepares mitochondria for mitophagy. Finally, actin can both stimulate and inhibit mitochondrial motility depending on the context. These motility effects can either be through the polymerization of actin itself or through myosin-based processes, with myosin 19 being an important mitochondrially attached myosin. Overall, distinct actin structures assemble in response to diverse stimuli to affect specific changes to mitochondria.

Coenzyme Q10 supplementation improves the motor function of middle-aged mice by restoring the neuronal activity of the motor cortex

Physiological aging causes motor function decline and anatomical and biochemical changes in the motor cortex. We confirmed that middle-aged mice at 15-18 months old show motor function decline, which can be restored to the young adult level by supplementing with mitochondrial electron transporter coenzyme Q₁₀ (CoQ₁₀) as a water-soluble nanoformula by drinking water for 1 week. CoQ₁₀ supplementation concurrently improved brain mitochondrial respiration but not muscle strength. Notably, we identified an age-related decline in field excitatory postsynaptic potential (fEPSP) amplitude in the pathway from layers II/III to V of the primary motor area of middle-aged mice, which was restored to the young adult level by supplementing with CoQ₁₀ for 1 week but not by administering CoQ₁₀ acutely to brain slices. Interestingly, CoQ₁₀ with high-frequency stimulation induced NMDA receptor-dependent long-term potentiation (LTP) in layer V of the primary motor cortex of middle-aged mice. Importantly, the fEPSP amplitude showed a larger input‒output relationship after CoQ₁₀-dependent LTP expression. These data suggest that CoQ₁₀ restores the motor function of middle-aged mice by improving brain mitochondrial function and the basal fEPSP level of the motor cortex, potentially by enhancing synaptic plasticity efficacy. Thus, CoQ₁₀ supplementation may ameliorate the age-related decline in motor function in humans.

Regulation of Mitochondrial Function by the Actin Cytoskeleton

The regulatory role of actin cytoskeleton on mitochondrial function is a growing research field, but the underlying molecular mechanisms remain poorly understood. Specific actin-binding proteins (ABPs), such as Gelsolin, have also been shown to participate in the pathophysiology of mitochondrial OXPHOS disorders through yet to be defined mechanisms. In this mini-review, we will summarize the experimental evidence supporting the fundamental roles of actin cytoskeleton and ABPs on mitochondrial trafficking, dynamics, biogenesis, metabolism and apoptosis, with a particular focus on Gelsolin involvement in mitochondrial disorders. The functional interplay between the actin cytoskeleton, ABPs and mitochondrial membranes for the regulation of cellular homeostasis thus emerges as a new exciting field for future research and therapeutic approaches.

Hormetic effect of low doses of rapamycin triggers anti-aging cascades in WRL-68 cells by modulating an mTOR-mitochondria cross-talk

BACKGROUND

Rapamycin is hormetic in nature-it demonstrates contrasting effects at high and low doses. It is toxic at moderate/high doses, while it can restrain aging and extend lifespan at low doses. However, it is not fully understood how rapamycin governs cellular aging. On the other hand, aging is putatively correlated to mitochondrial dysregulation. Although previous studies have suggested that hormetic (low) doses of rapamycin can cause partial/incomplete inhibition of mTOR, the actual modus operandi of how such partial mTOR inhibition might modulate the mTOR-mitochondria cross-talk remained to be deciphered in the context of cellular aging. The present study was designed to understand the hormetic effects of rapamycin on cellular factors that govern aging-associated changes in mitochondrial facets, such as functional and metabolic homeostases, sustenance of membrane potential, biogenesis, mitophagy, and oxidative injury to mitochondrial macromolecules.

METHODS AND RESULTS

WRL-68 cells treated (24 h) with variable doses of rapamycin were studied for estimating their viability, apoptosis, senescence, mitochondrial density and Δψm. Expression levels of key functional proteins were estimated by immunofluorescence/immunoblots. Oxidative damage to mtDNA/mtRNA/proteins was measured in mitochondrial lysates. We demonstrated that hormetic doses (0.1 and 1 nM) of rapamycin can alleviate aging-associated mitochondrial dyshomeostasis in WRL-68 cells, such as oxidative injury to mitochondrial nucleic acids and proteins, as well as disequilibrium of mitochondrial density, membrane potential, biogenesis, mitophagy and overall metabolism.

CONCLUSIONS

We established that low doses of rapamycin can hormetically amend the mTOR-mitochondria cross-talk, and can consequently promote anti-aging outcome in cells.

Neuroglobin effectively halts vision loss in Harlequin mice at an advanced stage of optic nerve degeneration

Mitochondrial diseases are among the most prevalent groups of inherited neurological disorders, affecting up to 1 in 5000 adults. Despite the progress achieved on the identification of gene mutations causing mitochondrial pathologies, they cannot be cured so far. Harlequin mice, a relevant model of mitochondrial pathology due to apoptosis inducing factor depletion, suffer from progressive disappearance of retinal ganglion cells leading to optic neuropathy. In our previous work, we showed that administering adeno-associated virus encompassing the coding sequences for neuroglobin, (a neuroprotective molecule belonging to the globin family) or apoptosis-inducing factor, before neurodegeneration onset, prevented retinal ganglion cell loss and preserved visual function. One of the challenges to develop an effective treatment for optic neuropathies is to consider that by the time patients become aware of their handicap, a large amount of nerve fibers has already disappeared. Gene therapy was performed in Harlequin mice aged between 4 and 5 months with either a neuroglobin or an apoptosis-inducing factor vector to determine whether the increased abundance of either one of these proteins in retinas could preserve visual function at this advanced stage of the disease. We demonstrated that gene therapy, by preserving the connectivity of transduced retinal ganglion cells and optic nerve bioenergetics, results in the enhancement of visual cortex activity, ultimately rescuing visual impairment. This study demonstrates that: (a) An increased abundance of neuroglobin functionally overcomes apoptosis-inducing factor absence in Harlequin mouse retinas at a late stage of neuronal degeneration; (b) The beneficial effect for visual function could be mediated by neuroglobin localization to the mitochondria, thus contributing to the maintenance of the organelle homeostasis.

Mitochondrial dynamics, positioning and function mediated by cytoskeletal interactions

The ability of a mitochondrion to undergo fission and fusion, and to be transported and localized within a cell are central not just to proper functioning of mitochondria, but also to that of the cell. The cytoskeletal filaments, namely microtubules, F-actin and intermediate filaments, have emerged as prime movers in these dynamic mitochondrial shape and position transitions. In this review, we explore the complex relationship between the cytoskeleton and the mitochondrion, by delving into: (i) how the cytoskeleton helps shape mitochondria via fission and fusion events, (ii) how the cytoskeleton facilitates the translocation and anchoring of mitochondria with the activity of motor proteins, and (iii) how these changes in form and position of mitochondria translate into functioning of the cell.

Measuring Respiration in Isolated Murine Brain Mitochondria: Implications for Mechanistic Stroke Studies

Measuring mitochondrial respiration in brain tissue is very critical in understanding the physiology and pathology of the central nervous system. Particularly, measurement of respiration in isolated mitochondria provides the advantage over the whole cells or tissues as the changes in respiratory function are intrinsic to mitochondrial structures rather than the cellular signaling that regulates mitochondria. Moreover, a high-throughput technique for measuring mitochondrial respiration minimizes the experimental time and the sample-to-sample variation. Here, we provide a detailed protocol for measuring respiration in isolated brain non-synaptosomal mitochondria using Agilent Seahorse XFe24 Analyzer. We optimized the protocol for the amount of mitochondria and concentrations of ADP, oligomycin, and trifluoromethoxy carbonylcyanide phenylhydrazone (FCCP) for measuring respiratory parameters for complex I-mediated respiration. In addition, we measured complex II-mediated respiratory parameters. We observed that 10 µg of mitochondrial protein per well, ADP concentrations ranging between 2.5 and 10 mmol/L along with 5 µmol/L of oligomycin, and 5 µmol/L of FCCP are ideal for measuring the complex I-mediated respiration in isolated mouse brain mitochondria. Furthermore, we determined that 2.5 µg of mitochondrial protein per well is ideal for measuring complex II-mediated respiration. Notably, we provide a discussion of logical analysis of data and how the assay could be utilized to design mechanistic studies for experimental stroke. In conclusion, we provide detailed experimental design for measurement of various respiratory parameters in isolated brain mitochondria utilizing a novel high-throughput technique along with interpretation and analysis of data.