Synaptic dysfunction, memory deficits and hippocampal atrophy due to ablation of mitochondrial fission in adult forebrain neurons

MSK1 is required for the experience- and ampakine-dependent enhancement of spatial reference memory and reversal learning and for the induction of Arc and BDNF

There is considerable interest in the development of nootropics, pharmacological agents that can improve cognition across a range of both cognitive modalities and cognitive disabilities. One class of cognitive enhancers, the ampakines, has attracted particular attention by virtue of improving cognition associated with animal models of neurodevelopmental, neurodegenerative, and psychiatric conditions, as well as in age-related cognitive impairment. Ampakines elevate CNS levels of BDNF, and it is through this elevation that their beneficial actions are believed to occur. However, what transduces the elevation of BDNF into long-lasting cognitive enhancement is not known. We have previously shown that MSK1, by virtue of its ability to regulate gene transcription, converts the elevation of BDNF associated with environmental enrichment into molecular, synaptic, cognitive and genomic adaptations that underlie enrichment-induced enhanced synaptic plasticity and learning and memory, a property that MSK1 retains across the lifespan. To establish whether MSK1 similarly converts ampakine-induced elevations of BDNF into cognitive enhancement we tested an ampakine (CX929) in male WT mice and in male mice in which the kinase activity of MSK1 was inactivated. We found that MSK1 is required for the ampakine-dependent improvement in spatial reference memory and cognitive flexibility, and for the elevations of BDNF and the plasticity-related protein Arc associated with ampakines and experience. These observations implicate MSK1 as a key enabler of the beneficial effects of ampakines on cognitive function, and furthermore identify MSK1 as a hub for BDNF-elevating nootropic strategies.

Synaptic mitochondria: A crucial factor in the aged hippocampus

Aging is a multifaceted biological process characterized by progressive molecular and cellular damage accumulation. The brain hippocampus undergoes functional deterioration with age, caused by cellular deficits, decreased synaptic communication, and neuronal death, ultimately leading to memory impairment. One of the factors contributing to this dysfunction is the loss of mitochondrial function. In neurons, mitochondria are categorized into synaptic and non-synaptic pools based on their location. Synaptic mitochondria, situated at the synapses, play a crucial role in maintaining neuronal function and synaptic plasticity, whereas non-synaptic mitochondria are distributed throughout other neuronal compartments, supporting overall cellular metabolism and energy supply. The proper function of synaptic mitochondria is essential for synaptic transmission as they provide the energy required and regulate calcium homeostasis at the communication sites between neurons. Maintaining the structure and functionality of synaptic mitochondria involves intricate processes, including mitochondrial dynamics such as fission, fusion, transport, and quality control mechanisms. These processes ensure that mitochondria remain functional, replace damaged organelles, and sustain cellular homeostasis at synapses. Notably, deficiencies in these mechanisms have been increasingly associated with aging and the onset of age-related neurodegenerative diseases. Synaptic mitochondria from the hippocampus are particularly vulnerable to age-related changes, including alterations in morphology and a decline in functionality, which significantly contribute to decreased synaptic activity during aging. This review comprehensively explores the critical roles that mitochondrial dynamics and quality control mechanisms play in preserving synaptic activity and neuronal function. It emphasizes the emerging evidence linking the deterioration of synaptic mitochondria to the aging process and the development of neurodegenerative diseases, highlighting the importance of these organelles from hippocampal neurons as potential therapeutic targets for mitigating cognitive decline and synaptic degeneration associated with aging. The novelty of this review lies in its focus on the unique vulnerability of hippocampal synaptic mitochondria to aging, underscoring their importance in maintaining brain function across the lifespan.

Mitochondrial Dynamics and mRNA Translation: A Local Synaptic Tale

Mitochondria are dynamic organelles that can adjust and respond to different stimuli within a cell. This plastic ability allows them to effectively coordinate several cellular functions in cells and becomes particularly relevant in highly complex cells such as neurons. An imbalance in mitochondrial dynamics can disrupt mitochondrial function, leading to abnormal cellular function and ultimately to a range of diseases, including neurodegenerative disorders. Regulation of mRNA transport and local translation inside neurons is crucial for maintaining the proteome of distal mitochondria, which is vital for energy production and synaptic function. A significant portion of the axonal transcriptome is dedicated to mRNAs for mitochondrial proteins, emphasizing the importance of local translation in sustaining mitochondrial function in areas far from the cell body. In neurons, local translation and the regulation of mRNAs encoding mitochondrial-shaping proteins could be essential for synaptic plasticity and neuronal health. The dynamics of these mRNAs, including their transport and local translation, may influence the morphology and function of mitochondria, thereby affecting the overall energy status and responsiveness of synapses. Comprehending the mitochondria-related mRNA regulation and local translation, as well as its influence on mitochondrial morphology near the synapses will help to better understand neuronal physiology and neurological diseases where mitochondrial dysfunction and impaired synaptic plasticity play a central role.

Resveratrol: A Multifaceted Guardian against Anxiety and Stress Disorders-An Overview of Experimental Evidence

The medicinal properties of resveratrol have garnered increasing attention from researchers. Extensive data have been accumulated on its use in treating cardiovascular diseases, immune system disorders, cancer, neurological diseases, and behavioral disorders. The protective mechanisms of resveratrol, particularly in anxiety-related stress disorders, have been well documented. However, less attention has been given to the side effects of resveratrol. This review explores not only the mechanisms underlying the anxiolytic effects of resveratrol but also the mechanisms that may lead to increased anxiety following resveratrol treatment. Understanding these mechanisms is crucial for enhancing the efficacy of resveratrol in managing anxiety disorders associated with stress and PTSD.

Patient mutations in DRP1 perturb synaptic maturation of cortical neurons

With the advent of exome sequencing, a growing number of children are being identified with de novo loss of function mutations in the dynamin 1 like (DNM1L) gene encoding the large GTPase essential for mitochondrial fission, dynamin-related protein 1 (DRP1); these mutations result in severe neurodevelopmental phenotypes, such as developmental delay, optic atrophy, and epileptic encephalopathies. Though it is established that mitochondrial fission is an essential precursor to the rapidly changing metabolic needs of the developing cortex, it is not understood how identified mutations in different domains of DRP1 uniquely disrupt cortical development and synaptic maturation. We leveraged the power of induced pluripotent stem cells (iPSCs) harboring DRP1 mutations in either the GTPase or stalk domains to model early stages of cortical development in vitro. High-resolution time-lapse imaging of axonal transport in mutant DRP1 cortical neurons reveals mutation-specific changes in mitochondrial motility of severely hyperfused mitochondrial structures. Transcriptional profiling of mutant DRP1 cortical neurons during maturation also implicates mutation dependent alterations in synaptic development and calcium regulation gene expression. Disruptions in calcium dynamics were confirmed using live functional recordings of 100 DIV (days in vitro) mutant DRP1 cortical neurons. These findings and deficits in pre- and post-synaptic marker colocalization using super resolution microscopy, strongly suggest that altered mitochondrial morphology of DRP1 mutant neurons leads to pathogenic dysregulation of synaptic development and activity.

Delineating the contribution of ageing and physical activity to changes in mitochondrial characteristics across the lifespan

Ageing is associated with widespread physiological changes prominent within all tissues, including skeletal muscle and the brain, which lead to a decline in physical function. To tackle the growing health and economic burdens associated with an ageing population, the concept of healthy ageing has become a major research priority. Changes in skeletal muscle mitochondrial characteristics have been suggested to make an important contribution to the reductions in skeletal muscle function with age, and age-related changes in mitochondrial content, respiratory function, morphology, and mitochondrial DNA have previously been reported. However, not all studies report changes in mitochondrial characteristics with ageing, and there is increasing evidence to suggest that physical activity (or inactivity) throughout life is a confounding factor when interpreting age-associated changes. Given that physical activity is a potent stimulus for inducing beneficial adaptations to mitochondrial characteristics, delineating the influence of physical activity on the changes in skeletal muscle that occur with age is complicated. This review aims to summarise our current understanding and knowledge gaps regarding age-related changes to mitochondrial characteristics within skeletal muscle, as well as to provide some novel insights into brain mitochondria, and to propose avenues of future research and targeted interventions. Furthermore, where possible, we incorporate discussions of the modifying effects of physical activity, exercise, and training status, to purported age-related changes in mitochondrial characteristics.

A neuron-glia lipid metabolic cycle couples daily sleep to mitochondrial homeostasis

Sleep is thought to be restorative to brain energy homeostasis, but it is not clear how this is achieved. We show here that Drosophila glia exhibit a daily cycle of glial mitochondrial oxidation and lipid accumulation that is dependent on prior wake and requires the Drosophila APOE orthologs NLaz and GLaz, which mediate neuron-glia lipid transfer. In turn, a full night of sleep is required for glial lipid clearance, mitochondrial oxidative recovery and maximal neuronal mitophagy. Knockdown of neuronal NLaz causes oxidative stress to accumulate in neurons, and the neuronal mitochondrial integrity protein, Drp1, is required for daily glial lipid accumulation. These data suggest that neurons avoid accumulation of oxidative mitochondrial damage during wake by using mitophagy and passing damage to glia in the form of lipids. We propose that a mitochondrial lipid metabolic cycle between neurons and glia reflects a fundamental function of sleep relevant for brain energy homeostasis.

Mitochondrial Dysfunction: A Key Player in Brain Aging and Diseases

Mitochondria are thought to have become incorporated within the eukaryotic cell approximately 2 billion years ago and play a role in a variety of cellular processes, such as energy production, calcium buffering and homeostasis, steroid synthesis, cell growth, and apoptosis, as well as inflammation and ROS production. Considering that mitochondria are involved in a multitude of cellular processes, mitochondrial dysfunction has been shown to play a role within several age-related diseases, including cancers, diabetes (type 2), and neurodegenerative diseases, although the underlying mechanisms are not entirely understood. The significant increase in lifespan and increased incidence of age-related diseases over recent decades has confirmed the necessity to understand the mechanisms by which mitochondrial dysfunction impacts the process of aging and age-related diseases. In this review, we will offer a brief overview of mitochondria, along with structure and function of this important organelle. We will then discuss the cause and consequence of mitochondrial dysfunction in the aging process, with a particular focus on its role in inflammation, cognitive decline, and neurodegenerative diseases, such as Huntington's disease, Parkinson's disease, and Alzheimer's disease. We will offer insight into therapies and interventions currently used to preserve or restore mitochondrial functioning during aging and neurodegeneration.

Cell-Type-Specific Mitochondrial Quality Control in the Brain: A Plausible Mechanism of Neurodegeneration

Neurodegeneration is an age-dependent progressive phenomenon with no defined cause. Aging is the main risk factor for neurodegenerative diseases. During aging, activated microglia undergo phenotypic alterations that can lead to neuroinflammation, which is a well-accepted event in the pathogenesis of neurodegenerative diseases. Several common mechanisms are shared by genetically or pathologically distinct neurodegenerative diseases, such as excitotoxicity, mitochondrial deficits and oxidative stress, protein misfolding and translational dysfunction, autophagy and microglia activation. Progressive loss of the neuronal population due to increased oxidative stress leads to neurodegenerative diseases, mostly due to the accumulation of dysfunctional mitochondria. Mitochondrial dysfunction and excessive neuroinflammatory responses are both sufficient to induce pathology in age-dependent neurodegeneration. Therefore, mitochondrial quality control is a key determinant for the health and survival of neuronal cells in the brain. Research has been primarily focused to demonstrate the significance of neuronal mitochondrial health, despite the important contributions of non-neuronal cells that constitute a significant portion of the brain volume. Moreover, mitochondrial morphology and function are distinctly diverse in different tissues; however, little is known about their molecular diversity among cell types. Mitochondrial dynamics and quality in different cell types markedly decide the fate of overall brain health; therefore, it is not justifiable to overlook non-neuronal cells and their significant and active contribution in facilitating overall neuronal health. In this review article, we aim to discuss the mitochondrial quality control of different cell types in the brain and how important and remarkable the diversity and highly synchronized connecting property of non-neuronal cells are in keeping the neurons healthy to control neurodegeneration.

Enhanced presynaptic mitochondrial energy production is required for memory formation

Some of the prominent features of long-term memory formation include protein synthesis, gene expression, enhanced neurotransmitter release, increased excitability, and formation of new synapses. As these processes are critically dependent on mitochondrial function, we hypothesized that increased mitochondrial respiration and dynamics would play a prominent role in memory formation. To address this possibility, we measured mitochondrial oxygen consumption (OCR) in hippocampal tissue punches from trained and untrained animals. Our results show that context fear training significantly increased basal, ATP synthesis-linked, and maximal OCR in the Shaffer collateral-CA1 synaptic region, but not in the CA1 cell body layer. These changes were recapitulated in synaptosomes isolated from the hippocampi of fear-trained animals. As dynamin-related protein 1 (Drp1) plays an important role in mitochondrial fission, we examined its role in the increased mitochondrial respiration observed after fear training. Drp1 inhibitors decreased the training-associated enhancement of OCR and impaired contextual fear memory, but did not alter the number of synaptosomes containing mitochondria. Taken together, our results show context fear training increases presynaptic mitochondria respiration, and that Drp-1 mediated enhanced energy production in CA1 pre-synaptic terminals is necessary for context fear memory that does not result from an increase in the number of synaptosomes containing mitochondria or an increase in mitochondrial mass within the synaptic layer.

Calcineurin inhibition protects against dopamine toxicity and attenuates behavioral decline in a Parkinson's disease model

BACKGROUND

Parkinson's disease (PD), a highly prevalent neuro-motor disorder is caused due to progressive loss of dopaminergic (DAergic) neurons at substantia nigra region of brain. This leads to depleted dopamine (DA) content at striatum, thus affecting the fine tuning of basal ganglia. In patients, this imbalance is manifested by akinesia, catalepsy and tremor. PD associated behavioral dysfunctions are frequently mitigated by l-DOPA (LD) therapy, a precursor for DA synthesis. Due to progressive neurodegeneration, LD eventually loses applicability in PD. Although DA is cytotoxic, it is unclear whether LD therapy can accelerate PD progression or not. LD itself does not lead to neurodegeneration in vivo, but previous reports demonstrate that LD treatment mediated excess DA can potentiate neurotoxicity when PD associated genetic or epigenetic aberrations are involved. So, minimizing DA toxicity during the therapy is an absolute necessity to halt or slowdown PD progression. The two major contributing factors associated with DA toxicity are: degradation by Monoamine oxidase and DAquinone (DAQ) formation.

RESULTS

Here, we report that apoptotic mitochondrial fragmentation via Calcineurin (CaN)-DRP1 axis is a common downstream event for both these initial cues, inhibiting which can protect cells from DA toxicity comprehensively. No protective effect is observed, in terms of cell survival when only PxIxIT domain of CaN is obstructed, demonstrating the importance to block DRP1-CaN axis specifically. Further, evaluation of the impact of DA exposure on PD progression in a mice model reveal that LD mediated behavioral recovery diminishes with time, mostly because of continued DAergic cell death and dendritic spine loss at striatum. CaN inhibition, alone or in combination with LD, offer long term behavioral protection. This protective effect is mediated specifically by hindering CaN-DRP1 axis, whereas inhibiting interaction between CaN and other substrates, including proteins involved in neuro-inflammation, remained ineffective when LD is co-administered.

CONCLUSIONS

In this study, we conclude that DA toxicity can be circumvented by CaN inhibition and it can mitigate PD related behavioral aberrations by protecting neuronal architecture at striatum. We propose that CaN inhibitors might extend the therapeutic efficacy of LD treatment.

MSK1 is required for the beneficial synaptic and cognitive effects of enriched experience across the lifespan

Positive experiences, such as social interaction, cognitive training and physical exercise, have been shown to ameliorate some of the harms to cognition associated with ageing. Animal models of positive interventions, commonly known as environmental enrichment, strongly influence neuronal morphology and synaptic function and enhance cognitive performance. While the profound structural and functional benefits of enrichment have been appreciated for decades, little is known as to how the environment influences neurons to respond and adapt to these positive sensory experiences. We show that adult and aged male wild-type mice that underwent a 10-week environmental enrichment protocol demonstrated improved performance in a variety of behavioural tasks, including those testing spatial working and spatial reference memory, and an enhancement in hippocampal LTP. Aged animals in particular benefitted from enrichment, performing spatial memory tasks at levels similar to healthy adult mice. Many of these benefits, including in gene expression, were absent in mice with a mutation in an enzyme, MSK1, which is activated by BDNF, a growth factor implicated in rodent and human cognition. We conclude that enrichment is beneficial across the lifespan and that MSK1 is required for the full extent of these experience-induced improvements of cognitive abilities, synaptic plasticity and gene expression.

Disease-associated mutations in Drp1 have fundamentally different effects on the mitochondrial fission machinery

Patient mutations have been identified throughout dynamin-related protein 1 (Drp1), the key protein mediator of mitochondrial fission. These changes generally impact young children and often result in severe neurological defects and, in some instances, death. Until now, the underlying functional defect leading to patient phenotypes has been largely speculative. We therefore analyzed six disease-associated mutations throughout the GTPase and middle domains (MD) of Drp1. The MD plays a role in Drp1 oligomerization, and three mutations in this region were predictably impaired in self-assembly. However, another mutant in this region (F370C) retained oligomerization capability on pre-curved membranes despite being assembly-limited in solution. Instead, this mutation impaired membrane remodeling of liposomes, which highlights the importance of Drp1 in generating local membrane curvature before fission. Two GTPase domain mutations were also observed in different patients. The G32A mutation was impaired in GTP hydrolysis both in solution and in the presence of lipid but remains capable of self-assembly on these lipid templates. The G223V mutation also exhibited decreased GTPase activity and was able to assemble on pre-curved lipid templates; however, this change impaired membrane remodeling of unilamellar liposomes similar to F370C. This demonstrates that the Drp1 GTPase domain also contributes to self-assembly interactions that drive membrane curvature. Overall, the functional defects caused by mutations in Drp1 are highly variable even for mutations that reside within the same functional domain. This study provides a framework for characterizing additional Drp1 mutations to provide a comprehensive understanding of functional sites within this essential protein.

Mitochondria as central hubs in synaptic modulation

Mitochondria are present in the pre- and post-synaptic regions, providing the energy required for the activity of these very specialized neuronal compartments. Biogenesis of synaptic mitochondria takes place in the cell body, and these organelles are then transported to the synapse by motor proteins that carry their cargo along microtubule tracks. The transport of mitochondria along neurites is a highly regulated process, being modulated by the pattern of neuronal activity and by extracellular cues that interact with surface receptors. These signals act by controlling the distribution of mitochondria and by regulating their activity. Therefore, mitochondria activity at the synapse allows the integration of different signals and the organelles are important players in the response to synaptic stimulation. Herein we review the available evidence regarding the regulation of mitochondrial dynamics by neuronal activity and by neuromodulators, and how these changes in the activity of mitochondria affect synaptic communication.

Neurological Phenotypes in Mouse Models of Mitochondrial Disease and Relevance to Human Neuropathology

Mitochondrial diseases represent the most common inherited neurometabolic disorders, for which no effective therapy currently exists for most patients. The unmet clinical need requires a more comprehensive understanding of the disease mechanisms and the development of reliable and robust in vivo models that accurately recapitulate human disease. This review aims to summarise and discuss various mouse models harbouring transgenic impairments in genes that regulate mitochondrial function, specifically their neurological phenotype and neuropathological features. Ataxia secondary to cerebellar impairment is one of the most prevalent neurological features of mouse models of mitochondrial dysfunction, consistent with the observation that progressive cerebellar ataxia is a common neurological manifestation in patients with mitochondrial disease. The loss of Purkinje neurons is a shared neuropathological finding in human post-mortem tissues and numerous mouse models. However, none of the existing mouse models recapitulate other devastating neurological phenotypes, such as refractory focal seizures and stroke-like episodes seen in patients. Additionally, we discuss the roles of reactive astrogliosis and microglial reactivity, which may be driving the neuropathology in some of the mouse models of mitochondrial dysfunction, as well as mechanisms through which cellular death may occur, beyond apoptosis, in neurons undergoing mitochondrial bioenergy crisis.

SETD5 haploinsufficiency affects mitochondrial compartment in neural cells

BACKGROUND

Neurodevelopmental disorders (NDDs) are heterogeneous conditions due to alterations of a variety of molecular mechanisms and cell dysfunctions. SETD5 haploinsufficiency leads to NDDs due to chromatin defects. Epigenetic basis of NDDs has been reported in an increasing number of cases while mitochondrial dysfunctions are more common within NDD patients than in the general population.

METHODS

We investigated in vitro neural stem cells as well as the brain of the Setd5 haploinsufficiency mouse model interrogating its transcriptome, analyzing mitochondrial structure, biochemical composition, and dynamics, as well as mitochondrial functionality.

RESULTS

Mitochondrial impairment is facilitated by transcriptional aberrations originated by the decrease of the SETD5 enzyme. Low levels of SETD5 resulted in fragmented mitochondria, reduced mitochondrial membrane potential, and ATP production both in neural precursors and neurons. Mitochondria were also mislocalized in mutant neurons, with reduced organelles within neurites and synapses.

LIMITATIONS

We found several defects in the mitochondrial compartment; however, we can only speculate about their position in the hierarchy of the pathological mechanisms at the basis of the disease.

CONCLUSIONS

Our study explores the interplay between chromatin regulation and mitochondria functions as a possible important aspect of SETD5-associated NDD pathophysiology. Our data, if confirmed in patient context, suggest that the mitochondrial activity and dynamics may represent new therapeutic targets for disorders associated with the loss of SETD5.

Therapeutics for mitochondrial dysfunction-linked diseases in Down syndrome

Genome-wide deregulation contributes to mitochondrial dysfunction and impairment in oxidative phosphorylation (OXPHOS) mechanism resulting in oxidative stress, increased production of reactive oxygen species (ROS) and cell death in individuals with Down syndrome (DS). The cells, which require more energy, such as muscles, brain and heart are greatly affected. Impairment in mitochondrial network has a direct link with patho-mechanism at cellular and systemic levels at the backdrop of generalized metabolic perturbations in individuals with DS. Myriads of clinico-phenotypic features, including intellectual disability, early aging and neurodegeneration, and Alzheimer disease (AD)-related dementia are inevitable in DS-population where mitochondrial dysfunctions play the central role. Collectively, the mitochondrial abnormalities and altered energy metabolism perturbs several signaling pathways, particularly related to neurogenesis, which are directly associated with cognitive development and early onset of AD in individuals with DS. Therefore, therapeutic challenges for amelioration of the mitochondrial defects were perceived to improve the quality of life of the DS population. A number of pharmacologically active natural compounds such as polyphenols, antioxidants and flavonoids have shown convincing outcome for reversal of the dysfunctional mitochondrial network and oxidative metabolism, and improvement in intellectual skill in mouse models of DS and humans with DS.

Emerging roles of brain metabolism in cognitive impairment and neuropsychiatric disorders

Here we discuss the role of diverse environmental manipulations affecting cognition with special regard to psychiatric conditions. We present evidence supporting a direct causal correlation between the valence of the environmental stimulation and some psychopathological traits and how the environment influences brain structure and function with special regard to oxidative stress and mitochondrial activity. Increasing experimental evidence supports a role for mitochondrial dysfunctions in neuropsychiatric disorders. Brain mitochondria are considered crucial mediators of allostasis, that is the capability to adapt to stress via a complex interaction between the autonomic, metabolic, and immune systems to maintain cellular homeostasis. In this process, mitochondria act as highly dynamic integrators by sensing and transducing stressors into adaptation mechanisms via metabolic stress mediators, such as glucocorticoids and catecholamines. Alterations in cellular homeostasis induced by chronic stress are thought to predispose to disease by triggering the so-called "mitochondrial allostatic load". This process is characterized by functional and structural changes of the mitochondria, ultimately leading to oxidative stress, inflammation, mitochondrial DNA damage and apoptosis. In this review we discuss the role of diverse environmental manipulations to affect cognition with special regard to psychiatric conditions. How the environment influences brain structure and function, and the interactions between rearing conditions, oxidative stress and mitochondrial activity are fundamental questions that are still poorly understood. As will be discussed, increasing experimental evidence supports a role for mitochondrial dysfunctions in neuropsychiatric disorders. Brain mitochondria are considered crucial mediators of allostasis, that is the capability to adapt to stress via a complex interaction between the autonomic, metabolic, and immune systems to maintain cellular homeostasis. In this process, mitochondria act as highly dynamic integrators by sensing and transducing stressors into adaptation mechanisms via metabolic stress mediators, such as glucocorticoids and catecholamines. Alterations in cellular homeostasis induced by chronic stress are thought to predispose to disease by triggering the so-called "mitochondrial allostatic load". This process is characterized by functional and structural changes of the mitochondria, ultimately leading to oxidative stress, inflammation, mitochondrial DNA damage and apoptosis. The brain requires considerable mitochondrial reserve not only to sustain basal neuronal needs but a also to provide increasing energy demands during stress. Consistently with these high energetic requirements, it is reasonable to hypothesise that the brain is particularly vulnerable to mitochondrial defects. Thus, even subtle metabolic alterations might have a substantial impact on cognitive functions. Over the last decade, several experimental evidence supported the hypothesis that a suboptimal mitochondrial function, which could be of genetic origin or acquired following adverse life events, is a key vulnerability factor for stress-related psychopathologies. Chronic psychological stress is a major promoter of anxiety as well as of oxidative damage, as shown in several studies. Recent evidence from mouse models harbouring mutations in mitochondrial genes demonstrated the role of mitochondria in modulating the response to acute psychological stress. However, it has yet to be determined whether mitochondrial dysfunctions are the cause or the consequence of anxiety. In this review, we discuss how adverse psychosocial environments can impact mitochondrial bioenergetics at the molecular level and we gather evidence from several studies linking energy metabolism and stress resilience/vulnerability. Moreover, we review recent findings supporting that metabolic dysfunction can underlie deficits in complex social behaviours. As will be discussed, aberrations in mitochondrial functionality have been found in the nucleus accumbens of highly anxious mice and mediate low social competitiveness. In addition, alterations in sociability can be reversed by enhancing mitochondrial functions. Recent evidence also demonstrated that a specific mutation in mitochondrial DNA, previously linked to autism spectrum disorder, produces autistic endophenotypes in mice by altering respiration chain and reactive oxygen species (ROS) production. Finally, we discuss a "Negative Enrichment" model that can explain some of the psychopathological conditions relevant to humans. Evidence of a direct causal correlation of valence of environmental stimulation and psychopathological traits will be presented, and possible molecular mechanisms that focus on oxidative stress. Collectively, the findings described here have been achieved with a wide set of behavioural and cognitive tasks with translational validity. Thus, they will be useful for future work aimed to elucidate the fine metabolic alterations in psychopathologies and devise novel approaches targeting mitochondria to alleviate these conditions.

Error-prone protein synthesis recapitulates early symptoms of Alzheimer disease in aging mice

Age-related neurodegenerative diseases (NDDs) are associated with the aggregation and propagation of specific pathogenic protein species (e.g., Aβ, α-synuclein). However, whether disruption of synaptic homeostasis results from protein misfolding per se rather than accumulation of a specific rogue protein is an unexplored question. Here, we show that error-prone translation, with its frequent outcome of random protein misfolding, is sufficient to recapitulate many early features of NDDs, including perturbed Ca²⁺ signaling, neuronal hyperexcitability, and mitochondrial dysfunction. Mice expressing the ribosomal ambiguity mutation Rps9 D95N exhibited disrupted synaptic homeostasis resulting in behavioral changes reminiscent of early Alzheimer disease (AD), such as learning and memory deficits, maladaptive emotional responses, epileptiform discharges, suppressed circadian rhythmicity, and sleep fragmentation, accompanied by hippocampal NPY expression and cerebral glucose hypometabolism. Collectively, our findings suggest that random protein misfolding may contribute to the pathogenesis of age-related NDDs, providing an alternative framework for understanding the initiation of AD.

CREB serine 133 is necessary for spatial cognitive flexibility and long-term potentiation

The transcription factor cAMP response element binding protein (CREB) is widely regarded as orchestrating the genomic response that underpins a range of physiological functions in the central nervous system, including learning and memory. Of the means by which CREB can be regulated, emphasis has been placed on the phosphorylation of a key serine residue, S133, in the CREB protein, which is required for CREB-mediated transcriptional activation in response to a variety of activity-dependent stimuli. Understanding the role of CREB S133 has been complicated by molecular genetic techniques relying on over-expression of either dominant negative or activating transgenes that may distort the physiological role of endogenous CREB. A more elegant recent approach targeting S133 in the endogenous CREB gene has yielded a mouse with constitutive replacement of this residue with alanine (S133A), but has generated results (no behavioural phenotype and no effect on gene transcription) at odds with contemporary views as to the role of CREB S133, and which may reflect compensatory changes associated with the constitutive mutation. To avoid this potential complication, we generated a post-natal and forebrain-specific CREB S133A mutant in which the expression of the mutation was under the control of CaMKIIα promoter. Using male and female mice we show that CREB S133 is necessary for spatial cognitive flexibility, the regulation of basal synaptic transmission, and for the expression of long-term potentiation (LTP) in hippocampal area CA1. These data point to the importance of CREB S133 in neuronal function, synaptic plasticity and cognition in the mammalian brain.

Mitochondrial behavior when things go wrong in the axon

Axonal homeostasis is maintained by processes that include cytoskeletal regulation, cargo transport, synaptic activity, ionic balance, and energy supply. Several of these processes involve mitochondria to varying degrees. As a transportable powerplant, the mitochondria deliver ATP and Ca2+-buffering capabilities and require fusion/fission to maintain proper functioning. Taking into consideration the long distances that need to be covered by mitochondria in the axons, their transport, distribution, fusion/fission, and health are of cardinal importance. However, axonal homeostasis is disrupted in several disorders of the nervous system, or by traumatic brain injury (TBI), where the external insult is translated into physical forces that damage nervous tissue including axons. The degree of damage varies and can disconnect the axon into two segments and/or generate axonal swellings in addition to cytoskeletal changes, membrane leakage, and changes in ionic composition. Cytoskeletal changes and increased intra-axonal Ca2+ levels are the main factors that challenge mitochondrial homeostasis. On the other hand, a proper function and distribution of mitochondria can determine the recovery or regeneration of the axonal physiological state. Here, we discuss the current knowledge regarding mitochondrial transport, fusion/fission, and Ca2+ regulation under axonal physiological or pathological conditions.

Mitochondrial heterogeneity and homeostasis through the lens of a neuron

Mitochondria are vital organelles with distinct morphological features and functional properties. The dynamic network of mitochondria undergoes structural and functional adaptations in response to cell-type-specific metabolic demands. Even within the same cell, mitochondria can display wide diversity and separate into functionally distinct subpopulations. Mitochondrial heterogeneity supports unique subcellular functions and is crucial to polarized cells, such as neurons. The spatiotemporal metabolic burden within the complex shape of a neuron requires precisely localized mitochondria. By travelling great lengths throughout neurons and experiencing bouts of immobility, mitochondria meet distant local fuel demands. Understanding mitochondrial heterogeneity and homeostasis mechanisms in neurons provides a framework to probe their significance to many other cell types. Here, we put forth an outline of the multifaceted role of mitochondria in regulating neuronal physiology and cellular functions more broadly.

Mitochondrial function and dynamics in neural stem cells and neurogenesis: Implications for neurodegenerative diseases

Mitochondria have been largely described as the powerhouse of the cell and recent findings demonstrate that this organelle is fundamental for neurogenesis. The mechanisms underlying neural stem cells (NSCs) maintenance and differentiation are highly regulated by both intrinsic and extrinsic factors. Mitochondrial-mediated switch from glycolysis to oxidative phosphorylation, accompanied by mitochondrial remodeling and dynamics are vital to NSCs fate. Deregulation of mitochondrial proteins, mitochondrial DNA, function, fission/fusion and metabolism underly several neurodegenerative diseases; data show that these impairments are already present in early developmental stages and NSC fate decisions. However, little is known about mitochondrial role in neurogenesis. In this Review, we describe the recent evidence covering mitochondrial role in neurogenesis, its impact in selected neurodegenerative diseases, for which aging is the major risk factor, and the recent advances in stem cell-based therapies that may alleviate neurodegenerative disorders-related neuronal deregulation through improvement of mitochondrial function and dynamics.

Programming axonal mitochondrial maintenance and bioenergetics in neurodegeneration and regeneration

Mitochondria generate ATP essential for neuronal growth, function, and regeneration. Due to their polarized structures, neurons face exceptional challenges to deliver mitochondria to and maintain energy homeostasis throughout long axons and terminal branches where energy is in high demand. Chronic mitochondrial dysfunction accompanied by bioenergetic failure is a pathological hallmark of major neurodegenerative diseases. Brain injury triggers acute mitochondrial damage and a local energy crisis that accelerates neuron death. Thus, mitochondrial maintenance defects and axonal energy deficits emerge as central problems in neurodegenerative disorders and brain injury. Recent studies have started to uncover the intrinsic mechanisms that neurons adopt to maintain (or reprogram) axonal mitochondrial density and integrity, and their bioenergetic capacity, upon sensing energy stress. In this review, we discuss recent advances in how neurons maintain a healthy pool of axonal mitochondria, as well as potential therapeutic strategies that target bioenergetic restoration to power neuronal survival, function, and regeneration.

Phenotype of Mrps5-Associated Phylogenetic Polymorphisms Is Intimately Linked to Mitoribosomal Misreading

We have recently identified point mutation V336Y in mitoribosomal protein Mrps5 (uS5m) as a mitoribosomal ram (ribosomal ambiguity) mutation conferring error-prone mitochondrial protein synthesis. In vivo in transgenic knock-in animals, homologous mutation V338Y was associated with a discrete phenotype including impaired mitochondrial function, anxiety-related behavioral alterations, enhanced susceptibility to noise-induced hearing damage, and accelerated metabolic aging in muscle. To challenge the postulated link between Mrps5 V338Y-mediated misreading and the in vivo phenotype, we introduced mutation G315R into the mouse Mrps5 gene as Mrps5 G315R is homologous to the established bacterial ram mutation RpsE (uS5) G104R. However, in contrast to bacterial translation, the homologous G → R mutation in mitoribosomal Mrps5 did not affect the accuracy of mitochondrial protein synthesis. Importantly, in the absence of mitochondrial misreading, homozygous mutant MrpS5^G315R/G315R mice did not show a phenotype distinct from wild-type animals.

Alterations in hippocampal mitochondrial dynamics are associated with neurodegeneration and recognition memory decline in old male mice

Mitochondrial dynamics is a key process that modulates the ultrastructure, quality and function of mitochondria. It is disrupted in numerous major neurodegenerative disorders including Parkinson's, Alzheimer's and Huntington's disease. Mitochondrial dysfunction has been correlated with the loss of memory. Previous studies suggest the involvement of Vdac1 and Drp1 in outer mitochondrial membrane permeabilization and promotion of mitochondrial fragmentation through Drp1 phosphorylation at S616. However, alterations in mitochondrial dynamics with respect to aging, memory loss and neurodegeneration remain unexplored. Therefore, the present study focuses on the involvement of mitochondrial dynamics in neurodegeneration and recognition memory decline during aging. The recognition memory decline was validated by the novel object recognition test and measurement of hippocampal Arc protein level during aging. The ultrastructure analysis revealed a decline in mitochondrial length and area, while an increase in the number of fragmented, round and disrupted mitochondria in the hippocampus during aging. Disruption was also evident in mitochondrial cristae and membrane with advancing age. The change in mitochondrial morphology was corroborated by an increase in the expression of phospho-Drp1 (S616) and Cyt-c proteins but decline in Mfn2, LC3B, Vdac1, Bcl-XL and Bcl-2 proteins in the hippocampus during aging. Taken together, our findings reveal that an increase in the expression of phospho-Drp1 (S616) and decrease in Mfn2 and LC3B proteins in the hippocampus bring about a reduction in mitochondrial length and area, and rise in mitochondrial fragmentation leading to reduced neuronal cell density, increased neurodegeneration and recognition memory decline in old male mice. Diagram depicts the increase in hippocampal mitochondrial fragmentation during aging of mice. Increased mitochondrial fragmentation causes distorted mitochondrial function such as decrease in ATP/ADP transportation due to decrease in Vdac1 protein level and increase in oxidative damage. These alterations result in hippocampal neurodegeneration and consequently impairment in recognition memory during aging.

Pyk2 Regulates MAMs and Mitochondrial Dynamics in Hippocampal Neurons

Pyk2 is a non-receptor tyrosine kinase enriched in hippocampal neurons, which can be activated by calcium-dependent mechanisms. In neurons, Pyk2 is mostly localised in the cytosol and dendritic shafts but can translocate to spines and/or to the nucleus. Here, we explore the function of a new localisation of Pyk2 in mitochondria-associated membranes (MAMs), a subdomain of ER-mitochondria surface that acts as a signalling hub in calcium regulation. To test the role of Pyk2 in MAMs’ calcium transport, we used full Pyk2 knockout mice (Pyk2^−/−) for in vivo and in vitro studies. Here we report that Pyk2^−/− hippocampal neurons present increased ER-mitochondrial contacts along with defective calcium homeostasis. We also show how the absence of Pyk2 modulates mitochondrial dynamics and morphology. Taken all together, our results point out that Pyk2 could be highly relevant in the modulation of ER-mitochondria calcium efflux, affecting in turn mitochondrial function.

Endoplasmic reticulum-mitochondria signaling in neurons and neurodegenerative diseases

Recent advances have revealed common pathological changes in neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis with related frontotemporal dementia (ALS/FTD). Many of these changes can be linked to alterations in endoplasmic reticulum (ER)-mitochondria signaling, including dysregulation of Ca2+ signaling, autophagy, lipid metabolism, ATP production, axonal transport, ER stress responses and synaptic dysfunction. ER-mitochondria signaling involves specialized regions of ER, called mitochondria-associated membranes (MAMs). Owing to their role in neurodegenerative processes, MAMs have gained attention as they appear to be associated with all the major neurodegenerative diseases. Furthermore, their specific role within neuronal maintenance is being revealed as mutant genes linked to major neurodegenerative diseases have been associated with damage to these specialized contacts. Several studies have now demonstrated that these specialized contacts regulate neuronal health and synaptic transmission, and that MAMs are damaged in patients with neurodegenerative diseases. This Review will focus on the role of MAMs and ER-mitochondria signaling within neurons and how damage of the ER-mitochondria axis leads to a disruption of vital processes causing eventual neurodegeneration.

Energy matters: presynaptic metabolism and the maintenance of synaptic transmission

Synaptic activity imposes large energy demands that are met by local adenosine triphosphate (ATP) synthesis through glycolysis and mitochondrial oxidative phosphorylation. ATP drives action potentials, supports synapse assembly and remodelling, and fuels synaptic vesicle filling and recycling, thus sustaining synaptic transmission. Given their polarized morphological features - including long axons and extensive branching in their terminal regions - neurons face exceptional challenges in maintaining presynaptic energy homeostasis, particularly during intensive synaptic activity. Recent studies have started to uncover the mechanisms and signalling pathways involved in activity-dependent and energy-sensitive regulation of presynaptic energetics, or 'synaptoenergetics'. These conceptual advances have established the energetic regulation of synaptic efficacy and plasticity as an exciting research field that is relevant to a range of neurological disorders associated with bioenergetic failure and synaptic dysfunction.

Silencing of the ER and Integrative Stress Responses in the Liver of Mice with Error-Prone Translation

Translational errors frequently arise during protein synthesis, producing misfolded and dysfunctional proteins. Chronic stress resulting from translation errors may be particularly relevant in tissues that must synthesize and secrete large amounts of secretory proteins. Here, we studied the proteostasis networks in the liver of mice that express the Rps2-A226Y ribosomal ambiguity (ram) mutation to increase the translation error rate across all proteins. We found that Rps2-A226Y mice lack activation of the eIF2 kinase/ATF4 pathway, the main component of the integrated stress response (ISR), as well as the IRE1 and ATF6 pathways of the ER unfolded protein response (ER-UPR). Instead, we found downregulation of chronic ER stress responses, as indicated by reduced gene expression for lipogenic pathways and acute phase proteins, possibly via upregulation of Sirtuin-1. In parallel, we observed activation of alternative proteostasis responses, including the proteasome and the formation of stress granules. Together, our results point to a concerted response to error-prone translation to alleviate ER stress in favor of activating alternative proteostasis mechanisms, most likely to avoid cell damage and apoptotic pathways, which would result from persistent activation of the ER and integrated stress responses.

β-Lactolin Reduces Age-Related Inflammation and Cognitive Decline

With the rapid increase in aging populations worldwide, there has been an increase in demand for preventive and therapeutic measures for age-related cognitive decline and dementia. Epidemiological studies show that consumption of dairy products reduces the risk for cognitive decline and dementia in the elderly. We have previously demonstrated in randomized trials that the consumption of β-lactolin, a whey-derived Gly-Thr-Trp-Tyr lactotetrapeptide, improves cognitive function in older adults. Orally administered β-lactolin is delivered to the brain and inhibits monoamine oxidase, resulting in alleviation of memory impairment. However, there is currently no evidence of the effects of long-term β-lactolin intake on aging. Here, we found that the discrimination index in the novel object recognition test for object recognition memory was reduced in mice aged 20 months compared with that in young mice, indicating that age-related cognitive decline was induced in the aged mice; in aged mice fed β-lactolin for 3 months, memory impairment was subsequently alleviated. In aged mice, impairment of light/dark activity cycles was found to be induced, which was subsequently alleviated by β-lactolin consumption. Additionally, the number of activated microglia in the hippocampus and cortex and the production of cytokines (tumor necrosis factor-α, macrophage inflammatory protein-1α, and macrophage chemoattractant protein-1) were increased in aged mice compared with those in young mice but were reduced in aged mice fed β-lactolin. The age-related hippocampal atrophy was improved in aged mice fed β-lactolin. Cytochrome c levels in the hippocampus and cortex were increased in aged mice compared with those in young mice but were also reduced by β-lactolin consumption. These results suggest that β-lactolin consumption prevents neural inflammation and alleviates aging-related cognitive decline.

Dexmedetomidine maintains blood-brain barrier integrity by inhibiting Drp1-related endothelial mitochondrial dysfunction in ischemic stroke

Stroke is the second leading cause of death and long-term disability worldwide, which lacks effective treatment. Perioperative stroke is associated with much higher rates of mortality and disability. The neuroprotective role of dexmedetomidine (Dex), a highly selective agonist of alpha2-adrenergic receptor, has been reported in a stroke rat model, and it was found that pretreatment of Dex before stroke could alleviate blood-brain barrier (BBB) breakdown. However, the underlying mechanisms are still unknown. As the brain endothelial cells are the main constituents of BBB and in high demand of energy, mitochondrial function of endothelial cells plays an important role in the maintenance of BBB. Given that dynamin-related protein 1 (Drp1) is a protein mediating mitochondrial fission, with mitochondrial fusion that balances mitochondrial morphology and ensures mitochondria function, the present study was designed to investigate the possible role of Drp1 in endothelial cells involved in the neuroprotective effects of Dex in ischemic stroke. Our results showed that preconditioning with Dex reduced infarction volume, alleviated brain water content and BBB damage, and improved neurological scores in middle cerebral artery occlusion rats. Meanwhile, Dex enhanced cell activity and decreased cell apoptosis in oxygen-glucose deprivation human brain microvascular endothelial cells in vitro. These protective effects of Dex were correlated with the mitochondrial morphology integrality of endothelial cells, mediated by increased phosphorylation of serine 637 in Drp1, and could be reversed by α2-adrenergic receptor antagonist Yohimbine and AMP-activated protein kinase inhibitor Compound C. These findings suggest new molecular pathways involved in the neuroprotective effects of Dex in ischemic stroke. As Dex is routinely used as a sedative drug clinically, our findings provide molecular evidence that it has perioperative neuroprotection from ischemic stroke.

Mitochondrial fission, integrity and completion of mitophagy require separable functions of Vps13D in Drosophila neurons

A healthy population of mitochondria, maintained by proper fission, fusion, and degradation, is critical for the long-term survival and function of neurons. Here, our discovery of mitophagy intermediates in fission-impaired Drosophila neurons brings new perspective into the relationship between mitochondrial fission and mitophagy. Neurons lacking either the ataxia disease gene Vps13D or the dynamin related protein Drp1 contain enlarged mitochondria that are engaged with autophagy machinery and also lack matrix components. Reporter assays combined with genetic studies imply that mitophagy both initiates and is completed in Drp1 impaired neurons, but fails to complete in Vps13D impaired neurons, which accumulate compromised mitochondria within stalled mito-phagophores. Our findings imply that in fission-defective neurons, mitophagy becomes induced, and that the lipid channel containing protein Vps13D has separable functions in mitochondrial fission and phagophore elongation.

Ca2+ handling at the mitochondria-ER contact sites in neurodegeneration

Mitochondria-endoplasmic reticulum (ER) contact sites (MERCS) are morpho-functional units, formed at the loci of close apposition of the ER-forming endomembrane and outer mitochondrial membrane (OMM). These sites contribute to fundamental cellular processes including lipid biosynthesis, autophagy, apoptosis, ER-stress and calcium (Ca²⁺) signalling. At MERCS, Ca²⁺ ions are transferred from the ER directly to mitochondria through a core protein complex composed of inositol-1,4,5 trisphosphate receptor (InsP₃R), voltage-gated anion channel 1 (VDAC1), mitochondrial calcium uniporter (MCU) and adaptor protein glucose-regulated protein 75 (Grp75); this complex is regulated by several associated proteins. Deregulation of ER-mitochondria Ca²⁺ transfer contributes to pathogenesis of neurodegenerative and other diseases. The efficacy of Ca²⁺ transfer between ER and mitochondria depends on the protein composition of MERCS, which controls ER-mitochondria interaction regulating, for example, the transversal distance between ER membrane and OMM and the extension of the longitudinal interface between ER and mitochondria. These parameters are altered in neurodegeneration. Here we overview the ER and mitochondrial Ca²⁺ homeostasis, the composition of ER-mitochondrial Ca²⁺ transfer machinery and alterations of the ER-mitochondria Ca²⁺ transfer in three major neurodegenerative diseases: motor neurone diseases, Parkinson disease and Alzheimer's disease.

Random errors in protein synthesis activate an age-dependent program of muscle atrophy in mice

Random errors in protein synthesis are prevalent and ubiquitous, yet their effect on organismal health has remained enigmatic for over five decades. Here, we studied whether mice carrying the ribosomal ambiguity (ram) mutation Rps2-A226Y, recently shown to increase the inborn error rate of mammalian translation, if at all viable, present any specific, possibly aging-related, phenotype. We introduced Rps2-A226Y using a Cre/loxP strategy. Resulting transgenic mice were mosaic and showed a muscle-related phenotype with reduced grip strength. Analysis of gene expression in skeletal muscle using RNA-Seq revealed transcriptomic changes occurring in an age-dependent manner, involving an interplay of PGC1α, FOXO3, mTOR, and glucocorticoids as key signaling pathways, and finally resulting in activation of a muscle atrophy program. Our results highlight the relevance of translation accuracy, and show how disturbances thereof may contribute to age-related pathologies.

By introducing a ribosomal ambiguity mutation into mice, Moore et al. establish an in-vivo model to investigate how age-related diseases are related to decreasing accuracy in protein synthesis. Their findings potentially offer new insights into the pathological changes observed in age-related diseases, such as muscle atrophy

Telencephalon-specific Alkbh1 conditional knockout mice display hippocampal atrophy and impaired learning

AlkB homolog 1 (ALKBH1) is responsible for the biogenesis of 5-formylcytidine (f⁵ C) on mitochondrial tRNA^Met and essential for mitochondrial protein synthesis. The brain, especially the hippocampus, is highly susceptible to mitochondrial dysfunction; hence, the maintenance of mitochondrial activity is strongly required to prevent disorders associated with hippocampal malfunction. To study the role of ALKBH1 in the hippocampus, we generated dorsal telencephalon-specific Alkbh1 conditional knockout (cKO) mice in inbred C57BL/6 background. These mice showed reduced activity of the respiratory chain complex, hippocampal atrophy, and CA1 pyramidal neuron abnormalities. Furthermore, performances in the fear-conditioning and Morris water maze tests in cKO mice indicated that the hippocampal abnormalities led to impaired hippocampus-dependent learning. These findings indicate critical roles of ALKBH1 in the hippocampus.

Ablation of dynamin-related protein 1 promotes diabetes-induced synaptic injury in the hippocampus

Dynamin-related protein 1 (Drp1)-mediated mitochondrial dysfunction is associated with synaptic injury in the diabetic brain. However, the dysfunctional mitochondria by Drp1 deletion in the diabetic brain are poorly understood. Here, we investigated the effects of neuron-specific Drp1 deletion on synaptic damage and mitophagy in the hippocampus of a high-fat diet (HFD)/streptozotocin (STZ)-induced diabetic mice. HFD/STZ-induced diabetic mice exhibited metabolic disturbances and synaptic damages. Floxed Drp1 mice were crossed with Ca²⁺/calmodulin-dependent protein kinase IIα (CaMKIIα)-Cre mice, to generate neuron-specific Drp1 knockout (Drp1cKO) mice, which showed marked mitochondrial swelling and dendritic spine loss in hippocampal neurons. In particular, diabetic Drp1cKO mice exhibited an increase in dendritic spine loss and higher levels of oxidative stress and neuroinflammation compared with diabetic wild-type (WT) mice. Diabetic WT mice generally displayed increased Drp1-induced small mitochondrial morphology in hippocampal neurons, but large mitochondria were prominently observed in diabetic Drp1cKO mice. The levels of microtubule-associated protein 1 light-chain 3 and lysosomal-associated membrane protein 1 proteins were significantly increased in the hippocampus of diabetic Drp1cKO mice compared with diabetic WT mice. The inhibition of Drp1 adversely promotes synaptic injury and neurodegeneration in the diabetic brain. The findings suggest that the exploratory mechanisms behind Drp1-mediated mitochondrial dysfunction could provide a possible therapeutic target for diabetic brain complications.

Improving Age-Related Cognitive Decline through Dietary Interventions Targeting Mitochondrial Dysfunction

Aging is inevitable and it is one of the major contributors to cognitive decline. However, the mechanisms underlying age-related cognitive decline are still the object of extensive research. At the biological level, it is unknown how the aging brain is subjected to progressive oxidative stress and neuroinflammation which determine, among others, mitochondrial dysfunction. The link between mitochondrial dysfunction and cognitive impairment is becoming ever more clear by the presence of significant neurological disturbances in human mitochondrial diseases. Possibly, the most important lifestyle factor determining mitochondrial functioning is nutrition. Therefore, with the present work, we review the latest findings disclosing a link between nutrition, mitochondrial functioning and cognition, and pave new ways to counteract cognitive decline in late adulthood through diet.

Genetic Neuropathy Due to Impairments in Mitochondrial Dynamics

Mitochondria are dynamic organelles capable of fusing, dividing, and moving about the cell. These properties are especially important in neurons, which in addition to high energy demand, have unique morphological properties with long axons. Notably, mitochondrial dysfunction causes a variety of neurological disorders including peripheral neuropathy, which is linked to impaired mitochondrial dynamics. Nonetheless, exactly why peripheral neurons are especially sensitive to impaired mitochondrial dynamics remains somewhat enigmatic. Although the prevailing view is that longer peripheral nerves are more sensitive to the loss of mitochondrial motility, this explanation is insufficient. Here, we review pathogenic variants in proteins mediating mitochondrial fusion, fission and transport that cause peripheral neuropathy. In addition to highlighting other dynamic processes that are impacted in peripheral neuropathies, we focus on impaired mitochondrial quality control as a potential unifying theme for why mitochondrial dysfunction and impairments in mitochondrial dynamics in particular cause peripheral neuropathy.

Dynamic properties of mitochondria during human corticogenesis

Mitochondria are signaling hubs responsible for the generation of energy through oxidative phosphorylation, the production of key metabolites that serve the bioenergetic and biosynthetic needs of the cell, calcium (Ca2+) buffering and the initiation/execution of apoptosis. The ability of mitochondria to coordinate this myriad of functions is achieved through the exquisite regulation of fundamental dynamic properties, including remodeling of the mitochondrial network via fission and fusion, motility and mitophagy. In this Review, we summarize the current understanding of the mechanisms by which these dynamic properties of the mitochondria support mitochondrial function, review their impact on human cortical development and highlight areas in need of further research.

CHIP protects against MPP+/MPTP-induced damage by regulating Drp1 in two models of Parkinson's disease

Mitochondrial dysfunction has been implicated in the pathogenesis of Parkinson's disease (PD). Carboxyl terminus of Hsp70-interacting protein (CHIP) is a key regulator of mitochondrial dynamics, and mutations in CHIP or deficits in its expression have been associated with various neurological diseases. This study explores the protective role of CHIP in cells and murine PD models. In SH-SY5Y cell line, overexpression of CHIP improved the cell viability and increased the ATP levels upon treatment with 1-methyl-4-phenylpyridinium (MPP⁺). To achieve CHIP overexpression in animal models, we intravenously injected mice with AAV/BBB, a new serotype of adeno-associated virus that features an enhanced capacity to cross the blood-brain barrier. We also generated gene knock-in mice that overexpressed CHIP in neural tissue. Our results demonstrated that CHIP overexpression in mice suppressed 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced damage, including movement impairments, motor coordination, and spontaneous locomotor activity, as well as loss of dopaminergic neurons. In vitro and in vivo experiments showed that overexpression of CHIP inhibited the pathological increase in Drp1 observed in the PD models, suggesting that CHIP regulates Drp1 degradation to attenuate MPP⁺/MPTP-induced injury. We conclude that CHIP plays a protective role in MPP⁺/MPTP-induced PD models. Our experiments further revealed that CHIP maintains the integrity of mitochondria.

Abnormalities of synaptic mitochondria in autism spectrum disorder and related neurodevelopmental disorders

Autism spectrum disorder (ASD) is a neurodevelopmental condition primarily characterized by an impairment of social interaction combined with the occurrence of repetitive behaviors. ASD starts in childhood and prevails across the lifespan. The variability of its clinical presentation renders early diagnosis difficult. Mutations in synaptic genes and alterations of mitochondrial functions are considered important underlying pathogenic factors, but it is obvious that we are far from a comprehensive understanding of ASD pathophysiology. At the synapse, mitochondria perform diverse functions, which are clearly not limited to their classical role as energy providers. Here, we review the current knowledge about mitochondria at the synapse and summarize the mitochondrial disturbances found in mouse models of ASD and other ASD-related neurodevelopmental disorders, like DiGeorge syndrome, Rett syndrome, Tuberous sclerosis complex, and Down syndrome.

Mitochondrial Dynamics in the Brain Are Associated With Feeding, Glucose Homeostasis, and Whole-Body Metabolism

The brain is responsible for maintaining whole-body energy homeostasis by changing energy input and availability. The hypothalamus and dorsal vagal complex (DVC) are the primary sites of metabolic control, able to sense both hormones and nutrients and adapt metabolism accordingly. The mitochondria respond to the level of nutrient availability by fusion or fission to maintain energy homeostasis; however, these processes can be disrupted by metabolic diseases including obesity and type II diabetes (T2D). Mitochondrial dynamics are crucial in the development and maintenance of obesity and T2D, playing a role in the control of glucose homeostasis and whole-body metabolism across neurons and glia in the hypothalamus and DVC.

Altered Outer Hair Cell Mitochondrial and Subsurface Cisternae Connectomics Are Candidate Mechanisms for Hearing Loss in Mice

Organelle crosstalk is vital for cellular functions. The propinquity of mitochondria, ER, and plasma membrane promote regulation of multiple functions, which include intracellular Ca2+ flux, and cellular biogenesis. Although the purposes of apposing mitochondria and ER have been described, an understanding of altered organelle connectomics related to disease states is emerging. Since inner ear outer hair cell (OHC) degeneration is a common trait of age-related hearing loss, the objective of this study was to investigate whether the structural and functional coupling of mitochondria with subsurface cisternae (SSC) was affected by aging. We applied functional and structural probes to equal numbers of male and female mice with a hearing phenotype akin to human aging. We discovered the polarization of cristae and crista junctions in mitochondria tethered to the SSC in OHCs. Aging was associated with SSC stress and decoupling of mitochondria with the SSC, mitochondrial fission/fusion imbalance, a remarkable reduction in mitochondrial and cytoplasmic Ca2+ levels, reduced K+-induced Ca2+ uptake, and marked plasticity of cristae membranes. A model of structure-based ATP production predicts profound energy stress in older OHCs. This report provides data suggesting that altered membrane organelle connectomics may result in progressive hearing loss.

Clock-Controlled Mitochondrial Dynamics Correlates with Cyclic Pregnenolone Synthesis

Neurosteroids are steroids synthetized in the nervous system, with the first step of steroidogenesis taking place within mitochondria with the synthesis of pregnenolone. They exert important brain-specific functions by playing a role in neurotransmission, learning and memory processes, and neuroprotection. Here, we show for the first time that mitochondrial neurosteroidogenesis follows a circadian rhythm and correlates with the rhythmic changes in mitochondrial morphology. We used synchronized human A172 glioma cells, which are steroidogenic cells with a functional core molecular clock, to show that pregnenolone levels and translocator protein (TSPO) are controlled by the clock, probably via circadian regulation of mitochondrial fusion/fission. Key findings were recapitulated in mouse brains. We also showed that genetic or pharmacological abrogation of fusion/fission activity, as well as disturbing the core molecular clock, abolished circadian rhythms of pregnenolone and TSPO. Our findings provide new insights into the crosstalk between mitochondrial function (here, neurosteroidogenesis) and circadian cycles.

Mechanisms and roles of mitochondrial localisation and dynamics in neuronal function

Neurons are highly polarised, complex and incredibly energy intensive cells, and their demand for ATP during neuronal transmission is primarily met by oxidative phosphorylation by mitochondria. Thus, maintaining the health and efficient function of mitochondria is vital for neuronal integrity, viability and synaptic activity. Mitochondria do not exist in isolation, but constantly undergo cycles of fusion and fission, and are actively transported around the neuron to sites of high energy demand. Intriguingly, axonal and dendritic mitochondria exhibit different morphologies. In axons mitochondria are small and sparse whereas in dendrites they are larger and more densely packed. The transport mechanisms and mitochondrial dynamics that underlie these differences, and their functional implications, have been the focus of concerted investigation. Moreover, it is now clear that deficiencies in mitochondrial dynamics can be a primary factor in many neurodegenerative diseases. Here, we review the role that mitochondrial dynamics play in neuronal function, how these processes support synaptic transmission and how mitochondrial dysfunction is implicated in neurodegenerative disease.

Hippocampal mitochondrial dysfunction and psychiatric-relevant behavioral deficits in spinocerebellar ataxia 1 mouse model

Spinocerebellar ataxia 1 (SCA1) is a devastating neurodegenerative disease associated with cerebellar degeneration and motor deficits. However, many patients also exhibit neuropsychiatric impairments such as depression and apathy; nevertheless, the existence of a causal link between the psychiatric symptoms and SCA1 neuropathology remains controversial. This study aimed to explore behavioral deficits in a knock-in mouse SCA1 (SCA1^154Q/2Q) model and to identify the underlying neuropathology. We found that the SCA1 mice exhibit previously undescribed behavioral impairments such as increased anxiety- and depressive-like behavior and reduced prepulse inhibition and cognitive flexibility. Surprisingly, non-motor deficits characterize the early SCA1 stage in mice better than does ataxia. Moreover, the SCA1 mice exhibit significant hippocampal atrophy with decreased plasticity-related markers and markedly impaired neurogenesis. Interestingly, the hippocampal atrophy commences earlier than the cerebellar degeneration and directly reflects the individual severity of some of the behavioral deficits. Finally, mitochondrial respirometry suggests profound mitochondrial dysfunction in the hippocampus, but not in the cerebellum of the young SCA1 mice. These findings imply the essential role of hippocampal impairments, associated with profound mitochondrial dysfunction, in SCA1 behavioral deficits. Moreover, they underline the view of SCA1 as a complex neurodegenerative disease and suggest new avenues in the search for novel SCA1 therapies.

Neuronal Mitochondrial Dysfunction Activates the Integrated Stress Response to Induce Fibroblast Growth Factor 21

Stress adaptation is essential for neuronal health. While the fundamental role of mitochondria in neuronal development has been demonstrated, it is still not clear how adult neurons respond to alterations in mitochondrial function and how neurons sense, signal, and respond to dysfunction of mitochondria and their interacting organelles. Here, we show that neuron-specific, inducible in vivo ablation of the mitochondrial fission protein Drp1 causes ER stress, resulting in activation of the integrated stress response to culminate in neuronal expression of the cytokine Fgf21. Neuron-derived Fgf21 induction occurs also in murine models of tauopathy and prion disease, highlighting the potential of this cytokine as an early biomarker for latent neurodegenerative conditions.

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Neuronal Drp1 ablation is sensed by branches of the integrated stress response (ISR)

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Activation of the ISR induces catabolic cytokine Fgf21 in the brain

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Brain Fgf21 induced in neurodegeneration models may be a potential biomarker

Tourette Syndrome Risk Genes Regulate Mitochondrial Dynamics, Structure, and Function

Gilles de la Tourette syndrome (GTS) is a neurodevelopmental disorder characterized by motor and vocal tics with an estimated prevalence of 1% in children and adolescents. GTS has high rates of inheritance with many rare mutations identified. Apart from the role of the neurexin trans-synaptic connexus (NTSC) little has been confirmed regarding the molecular basis of GTS. The NTSC pathway regulates neuronal circuitry development, synaptic connectivity and neurotransmission. In this study we integrate GTS mutations into mitochondrial pathways that also regulate neuronal circuitry development, synaptic connectivity and neurotransmission. Many deleterious mutations in GTS occur in genes with complementary and consecutive roles in mitochondrial dynamics, structure and function (MDSF) pathways. These genes include those involved in mitochondrial transport (NDE1, DISC1, OPA1), mitochondrial fusion (OPA1), fission (ADCY2, DGKB, AMPK/PKA, RCAN1, PKC), mitochondrial metabolic and bio-energetic optimization (IMMP2L, MPV17, MRPL3, MRPL44). This study is the first to develop and describe an integrated mitochondrial pathway in the pathogenesis of GTS. The evidence from this study and our earlier modeling of GTS molecular pathways provides compounding support for a GTS deficit in mitochondrial supply affecting neurotransmission.

A Mutation in Hnrnph1 That Decreases Methamphetamine-Induced Reinforcement, Reward, and Dopamine Release and Increases Synaptosomal hnRNP H and Mitochondrial Proteins

Individual variation in the addiction liability of amphetamines has a heritable genetic component. We previously identified Hnrnph1 (heterogeneous nuclear ribonucleoprotein H1) as a quantitative trait gene underlying decreased methamphetamine-induced locomotor activity in mice. Here, we showed that mice (both females and males) with a heterozygous mutation in the first coding exon of Hnrnph1 (H1^+/-) showed reduced methamphetamine reinforcement and intake and dose-dependent changes in methamphetamine reward as measured via conditioned place preference. Furthermore, H1^+/- mice showed a robust decrease in methamphetamine-induced dopamine release in the NAc with no change in baseline extracellular dopamine, striatal whole-tissue dopamine, dopamine transporter protein, dopamine uptake, or striatal methamphetamine and amphetamine metabolite levels. Immunohistochemical and immunoblot staining of midbrain dopaminergic neurons and their forebrain projections for TH did not reveal any major changes in staining intensity, cell number, or forebrain puncta counts. Surprisingly, there was a twofold increase in hnRNP H protein in the striatal synaptosome of H1^+/- mice with no change in whole-tissue levels. To gain insight into the mechanisms linking increased synaptic hnRNP H with decreased methamphetamine-induced dopamine release and behaviors, synaptosomal proteomic analysis identified an increased baseline abundance of several mitochondrial complex I and V proteins that rapidly decreased at 30 min after methamphetamine administration in H1^+/- mice. In contrast, the much lower level of basal synaptosomal mitochondrial proteins in WT mice showed a rapid increase. We conclude that H1^+/- decreases methamphetamine-induced dopamine release, reward, and reinforcement and induces dynamic changes in basal and methamphetamine-induced synaptic mitochondrial function.SIGNIFICANCE STATEMENT Methamphetamine dependence is a significant public health concern with no FDA-approved treatment. We discovered a role for the RNA binding protein hnRNP H in methamphetamine reward and reinforcement. Hnrnph1 mutation also blunted methamphetamine-induced dopamine release in the NAc, a key neurochemical event contributing to methamphetamine addiction liability. Finally, Hnrnph1 mutants showed a marked increase in basal level of synaptosomal hnRNP H and mitochondrial proteins that decreased in response to methamphetamine, whereas WT mice showed a methamphetamine-induced increase in synaptosomal mitochondrial proteins. Thus, we identified a potential role for hnRNP H in basal and dynamic mitochondrial function that informs methamphetamine-induced cellular adaptations associated with reduced addiction liability.