Lactate as a determinant of neuronal excitability, neuroenergetics and beyond

Lactate: Beyond a mere fuel in the epileptic brain

Epilepsy, a prevalent neurological disorder characterized by spontaneous recurrent seizures, significantly impacts physiological and cognitive functions. Emerging evidence suggests a crucial role for metabolic factors, particularly lactate, in epilepsy. We discuss the applicability of the astrocyte-neuron lactate shuttle (ANLS) model during acute seizure events and examine lactate's metabolic adaptation in epilepsy progression. Additionally, the roles of lactate metabolism in microglia and oligodendrocytes are considered, aiming to supplement our understanding of neuro-glial metabolic interactions as extensions of the ANLS model. Additionally, lactate modulates neuronal excitability via its interaction with hydroxycarboxylic acid receptor 1 (HCAR1), alongside additional mechanisms involving acid-sensing ion channels (ASICs) and ATP-sensitive potassium (KATP) channels, which contribute as secondary modulatory pathways. In conclusion, we propose that lactate functions as more than a mere fuel source in the epileptic brain, offering potential insights into new therapeutic targets for seizure control.

Lactic acid contributes to the emergence of depression-like behaviors triggered by blue light exposure during sleep

The threat posed by light pollution to human health is increasing remarkably. As demand for high-efficiency and bright lighting increases, so does the blue light content from artificial sources. Although animal studies suggested blue light induced depression-like behaviors, human evidence remained limited, and the mechanisms by which blue light affects depression remained elusive. This study aimed to investigate the association between blue light exposure and depression in humans, and explored the underlying mechanisms that driving depression-like behaviors induced by blue light. Our population findings showed that the high-blue-light exposure at night was positively associated with depressive symptoms. Lactic acid was relevant to depression with Mendelian randomization analysis. Moreover, animal studies demonstrated that exposure to blue light during sleep (BLS) induced depression-like behaviors in the animals. Metabolomics and colorimetric analyses revealed elevated levels of lactic acid in the cerebrospinal fluid and lateral habenula (LHb) of rats with depression-like behaviors induced by BLS. The administration of a lactate inhibitor (Oxamate) alleviated these behaviors, along with changes in neuronal excitability, synaptic function, and morphology in the LHb. Overall, our study suggests that excessive exposure to high blue light-content artificial light at night links to increased depressive symptoms, revealing possible molecular mechanisms and prevention strategies, which are crucial for addressing environmentally related mental health issues.

Lactylation modification in cardio-cerebral diseases: A state-of-the-art review

Cardio-cerebral diseases (CCDs), encompassing conditions such as coronary heart disease, myocardial infarction, stroke, Alzheimer's disease, et al., represent a significant threat to human health and well-being. These diseases are often characterized by metabolic abnormalities and remodeling in the process of pathology. Glycolysis and hypoxia-induced lactate accumulation play critical roles in cellular energy dynamics and metabolic imbalances in CCDs. Lactylation, a post-translational modification driven by excessive lactate accumulation, occurs in both histone and non-histone proteins. It has been implicated in regulating protein function across various pathological processes in CCDs, including inflammation, angiogenesis, lipid metabolism dysregulation, and fibrosis. Targeting key proteins involved in lactylation, as well as the enzymes regulating this modification, holds promise as a therapeutic strategy to modulate disease progression by addressing these pathological mechanisms. This review provides a holistic picture of the types of lactylation and the associated modifying enzymes, highlights the roles of lactylation in different pathological processes, and synthesizes the latest clinical evidence and preclinical studies in a comprehensive view. We aim to emphasize the potential of lactylation as an innovative therapeutic target for preventing and treating CCD-related conditions.

Astrocyte aquaporin mediates a tonic water efflux maintaining brain homeostasis

Brain water homeostasis not only provides a physical protection, but also determines the diffusion of chemical molecules key for information processing and metabolic stability. As a major type of glia in brain parenchyma, astrocytes are the dominant cell type expressing aquaporin water channel. How astrocyte aquaporin contributes to brain water homeostasis in basal physiology remains to be understood. We report that astrocyte aquaporin 4 (AQP4) mediates a tonic water efflux in basal conditions. Acute inhibition of astrocyte AQP4 leads to intracellular water accumulation as optically resolved by fluorescence-translated imaging in acute brain slices, and in vivo by fiber photometry in mobile mice. We then show that aquaporin-mediated constant water efflux maintains astrocyte volume and osmotic equilibrium, astrocyte and neuron Ca2+ signaling, and extracellular space remodeling during optogenetically induced cortical spreading depression. Using diffusion-weighted magnetic resonance imaging (DW-MRI), we observed that in vivo inhibition of AQP4 water efflux heterogeneously disturbs brain water homeostasis in a region-dependent manner. Our data suggest that astrocyte aquaporin, though bidirectional in nature, mediates a tonic water outflow to sustain cellular and environmental equilibrium in brain parenchyma.

The role of astrocytes from synaptic to non-synaptic plasticity

Information storage and transfer in the brain require a high computational power. Neuronal network display various local or global mechanisms to allow information storage and transfer in the brain. From synaptic to intrinsic plasticity, the rules of input-output function modulation have been well characterized in neurons. In the past years, astrocytes have been suggested to increase the computational power of the brain and we are only just starting to uncover their role in information processing. Astrocytes maintain a close bidirectional communication with neurons to modify neuronal network excitability, transmission, axonal conduction, and plasticity through various mechanisms including the release of gliotransmitters or local ion homeostasis. Astrocytes have been significantly studied in the context of long-term or short-term synaptic plasticity, but this is not the only mechanism involved in memory formation. Plasticity of intrinsic neuronal excitability also participates in memory storage through regulation of voltage-gated ion channels or axonal morphological changes. Yet, the contribution of astrocytes to these other forms of non-synaptic plasticity remains to be investigated. In this review, we summarized the recent advances on the role of astrocytes in different forms of plasticity and discuss new directions and ideas to be explored regarding astrocytes-neuronal communication and regulation of plasticity.

A neurometabolic mechanism involving dmPFC/dACC lactate in physical effort-based decision-making

Motivation levels vary across individuals, yet the underlying mechanisms driving these differences remain elusive. The dorsomedial prefrontal cortex/dorsal anterior cingulate cortex (dmPFC/dACC) and the anterior insula (aIns) play crucial roles in effort-based decision-making. Here, we investigate the influence of lactate, a key metabolite involved in energy metabolism and signaling, on decisions involving both physical and mental effort, as well as its effects on neural activation. Using proton magnetic resonance spectroscopy and functional MRI in 63 participants, we find that higher lactate levels in the dmPFC/dACC are associated with reduced motivation for physical effort, a relationship mediated by neural activity within this region. Additionally, plasma and dmPFC/dACC lactate levels correlate, suggesting a systemic influence on brain metabolism. Supported by path analysis, our results highlight lactate's role as a modulator of dmPFC/dACC activity, hinting at a neurometabolic mechanism that integrates both peripheral and central metabolic states with brain function in effort-based decision-making.

Ketosis regulates K+ ion channels, strengthening brain-wide signaling disrupted by age

Aging is associated with impaired signaling between brain regions when measured using resting-state fMRI. This age-related destabilization and desynchronization of brain networks reverses itself when the brain switches from metabolizing glucose to ketones. Here, we probe the mechanistic basis for these effects. First, we confirmed their robustness across measurement modalities using two datasets acquired from resting-state EEG (Lifespan: standard diet, 20-80 years, N = 201; Metabolic: individually weight-dosed and calorically-matched glucose and ketone ester challenge,μ a g e = 26.9 ± 11.2 years , N = 36). Then, using a multiscale conductance-based neural mass model, we identified the unique set of mechanistic parameters consistent with our clinical data. Together, our results implicate potassium (K+) gradient dysregulation as a mechanism for age-related neural desynchronization and its reversal with ketosis, the latter finding of which is consistent with direct measurement of ion channels. As such, the approach facilitates the connection between macroscopic brain activity and cellular-level mechanisms.

Acute Administration of Lactate Exerts Antidepressant-like Effect Through cAMP-dependent Protein Synthesis

Lactate acts as an important metabolic substrate and signalling molecule modulating neural activities in the brain, and recent preclinical and clinical studies have revealed its antidepressant effect after acute or chronic peripheral administration. However, the neural mechanism underlying the antidepressant effect of lactate, in particular when lactate is acutely administered remains largely unknown. In the current study, we focused on forced swimming test (FST) to elucidate the neural mechanisms through which acute intracerebroventricular (ICV) infusion of lactate exerts antidepressant-like effect. A total of 238 male Sprague Dawley rats were used as experimental subjects. Results showed lactate produced antidepressant-like effect, as indicated by reduced immobility, in a dose- and time-dependent manner. Moreover, the antidepressant-like effect of lactate was dependent of new protein synthesis but not new gene expression, lactate's metabolic effect or hydroxy-carboxylic acid receptor 1 (HCAR1) activation. Furthermore, lactate rapidly promoted dephosphorylation of eukaryotic elongation factor 2 (eEF2) and increased brain-derived neurotrophic factor (BDNF) protein synthesis in the hippocampus in a cyclic adenosine monophosphate (cAMP)-dependent manner. Finally, inhibition of cAMP production blocked the antidepressant-like effect of lactate. These findings suggest that acute administration of lactate exerts antidepressant-like effect through cAMP-dependent protein synthesis.

Astrocyte-derived lactate in stress disorders

Stress disorders are psychiatric disorders arising following stressful or traumatic events. They could deleteriously affect an individual's health because they often co-occur with mental illnesses. Considerable attention has been focused on neurons when considering the neurobiology of stress disorders. However, like other mental health conditions, recent studies have highlighted the importance of astrocytes in the pathophysiology of stress-related disorders. In addition to their structural and homeostatic support role, astrocytes actively serve several functions in regulating synaptic transmission and plasticity, protecting neurons from toxic compounds, and providing metabolic support for neurons. The astrocyte-neuron lactate shuttle model sets forth the importance of astrocytes in providing lactate for the metabolic supply of neurons under intense activity. Lactate also plays a role as a signaling molecule and has been recently studied regarding its antidepressant activity. This review discusses the involvement of astrocytes and brain energy metabolism in stress and further reflects on the importance of lactate as an energy supply in the brain and its emerging antidepressant role in stress-related disorders.

Intracellular Lactate Dynamics Reveal the Metabolic Diversity of Drosophila Glutamatergic Neurons

Lactate, an intermediary between glycolysis and mitochondrial oxidative phosphorylation, reflects the metabolic state of neurons. Here, we utilized a genetically-encoded lactate FRET biosensor to uncover subpopulations of distinct metabolic states among Drosophila glutamatergic neurons. Neurons within specific subpopulations exhibited correlated lactate flux patterns that stemmed from inherent cellular properties rather than neuronal interconnectivity. Further, individual neurons exhibited consistent patterns of lactate flux over time such that stimulus-evoked changes in lactate were correlated with pre-treatment fluctuations. Leveraging these temporal autocorrelations, deep-learning models accurately predicted post-stimulus responses from pre-stimulus fluctuations. These findings point to the existence of distinct neuronal subpopulations, each characterized by unique lactate dynamics, and raise the possibility that neurons with correlated metabolic activities might synchronize across different neural circuits. Such synchronization, rooted in neuronal metabolic states, could influence information processing in the brain.

IGF1R deficiency in vascular smooth muscle cells impairs myogenic autoregulation and cognition in mice

Introduction

Cerebrovascular pathologies contribute to cognitive decline during aging, leading to vascular cognitive impairment and dementia (VCID). Levels of circulating insulin-like growth factor 1 (IGF-1), a vasoprotective hormone, decrease during aging. Decreased circulating IGF-1 in animal models leads to the development of VCID-like symptoms, but the cellular mechanisms underlying IGF-1-deficiency associated pathologies in the aged cerebrovasculature remain poorly understood. Here, we test the hypothesis that vascular smooth muscle cells (VSMCs) play an integral part in mediating the vasoprotective effects of IGF-1.

Methods

We used a hypertension-based model of cerebrovascular dysfunction in mice with VSMC-specific IGF-1 receptor (Igf1r) deficiency and evaluated the development of cerebrovascular pathologies and cognitive dysfunction.

Results

VSMC-specific Igf1r deficiency led to impaired cerebral myogenic autoregulation, independent of blood pressure changes, which was also associated with impaired spatial learning and memory function as measured by radial arm water maze and impaired motor learning measured by rotarod. In contrast, VSMC-specific IGF-1 receptor knockdown did not lead to cerebral microvascular rarefaction.

Discussion

These studies suggest that VSMCs are key targets for IGF-1 in the context of cerebrovascular health, playing a role in vessel stability alongside other cells in the neurovascular unit, and that VSMC dysfunction in aging likely contributes to VCID.

BDNF and Lactate as Modulators of Hippocampal CA3 Network Physiology

Growing evidence supports the notion that brain-derived neurotrophic factor (BDNF) and lactate are potent modulators of mammalian brain function. The modulatory actions of those biomolecules influence a wide range of neuronal responses, from the shaping of neuronal excitability to the induction and expression of structural and synaptic plasticity. The biological actions of BDNF and lactate are mediated by their cognate receptors and specific transporters located in the neuronal membrane. Canonical functions of BDNF occur via the tropomyosin-related kinase B receptor (TrkB), whereas lactate acts via monocarboxylate transporters or the hydroxycarboxylic acid receptor 1 (HCAR1). Both receptors are highly expressed in the central nervous system, and some of their physiological actions are particularly well characterized in the hippocampus, a brain structure involved in the neurophysiology of learning and memory. The multifarious neuronal circuitry between the axons of the dentate gyrus granule cells, mossy fibers (MF), and pyramidal neurons of area CA3 is of great interest given its role in specific mnemonic processes and involvement in a growing number of brain disorders. Whereas the modulation exerted by BDNF via TrkB has been extensively studied, the influence of lactate via HCAR1 on the properties of the MF-CA3 circuit is an emerging field. In this review, we discuss the role of both systems in the modulation of brain physiology, with emphasis on the hippocampal CA3 network. We complement this review with original data that suggest cross-modulation is exerted by these two independent neuromodulatory systems.