Mitochondrial dysfunction in microglia: a novel perspective of depression pathogenesis

Abstract

Major depressive disorder is a prevalent psychiatric disease that is closely linked to chronic low-grade neuroinflammation, which is primarily driven by microglia, the primary resident immune cells in the brain. Accumulating studies started to reveal that mitochondria, the powerhouses of the cell, play an important role in microglia-mediated neuroinflammation and neurotoxicity in depression. Stress-induced mitochondrial dysfunction triggers microglial activation through distinct mechanisms including energy metabolism patterns transition, electron transport chain impairment, reactive oxygen species accumulation, mitochondrial DNA damage, imbalanced fission and fusion, autophagy, and mitophagy deficiency, which cause the onset and progression of depression. In this review, we described microglial activation in animal models and clinically depressed patients, summarized current understanding of mitochondrial dysfunction-mediated microglial activation, and emphasized the possible mechanisms by which microglial mitochondrial dysfunction participates in the pathogenesis of depression. Understanding the roles and underlying mechanisms of mitochondrial dysfunction in neuroinflammation and neurotoxicity in depression may shed light on developing novel therapeutic strategies to fight against this devastating mental illness.

Graphical Abstract

Introduction

Major depression disorder (MDD) is a debilitating mental disorder affecting over 300 million people worldwide, leading to significant social dysfunction and high suicide rates [1, 2]. Current clinical interventions, including antidepressants and psychotherapy, often show limited efficacy and delayed responses, highlighting the urgent need for a deeper understanding of its pathophysiology [2]. While multiple hypotheses such as monoamine deficiency and hypothalamic–pituitary–adrenal (HPA) axis dysregulation have been proposed, none fully explains the heterogeneous nature of depression [3, 4]. Recently, microglia-driven neuroinflammation and neuronal damage have emerged as key pathological factors in the onset and progression of depression [5]. Microglia, the resident macrophages of the central nervous system (CNS), account for about 10% to 15% of all glial cells and serve as the central innate immune cells to build the first line of defense in the CNS [6]. Uncontrolled activation of microglia induces pro-inflammatory responses, excessive synaptic pruning, and neurotoxic factor production and release that seriously affect neurological structure and functions in depression [5]. However, the mechanisms of microglial activation are far away from fully elucidated. Inspiringly, mounting studies have revealed the involvement of mitochondria in the regulation of microglial activation in physio-pathological conditions [7].

Mitochondria, known as the “powerhouse of the cell,” are maternally inherited organelles that play central roles in energy metabolism, calcium homeostasis, substance synthesis, and oxidative stress [7, 8]. Among all these vital metabolic processes, the main function of mitochondria is to generate adenosine 5′-triphosphate (ATP), the primary energy-carrying molecule used in all living cells, by oxidative phosphorylation (OXPHOS), performed by the mitochondrial respiratory chain [9]. Being cells with high energy consumption, microglia require mitochondria to maintain their energy metabolism [7]. Importantly, a strong correlation between microglial activation and mitochondrial dysfunction has been observed in depression in both basic research and clinical studies [5, 10–13]. Following investigations have demonstrated that mitochondrial dysfunctions trigger microglial activation via multiple mechanisms including metabolic reprogramming, oxidative stress, mitochondrial deoxyribonucleic acid (mtDNA) abnormalities, mitochondrial quality-control disorder, cellular calcium homeostasis disruption, and mitophagy deficiency in depression [14–19]. This review aims to explore the emerging roles of microglial mitochondrial dysfunction in the pathogenesis of depression, integrating evidence from molecular, cellular, animal, and human population studies to provide a comprehensive perspective on this novel pathogenic axis.

The origin and physio-pathological roles of microglia

The origin of microglia

Microglia were once believed to originate from neuroectoderm, similar to other CNS-resident glial cells such as astrocytes and oligodendrocytes [6]. Currently, it is firmly established that microglia are actually macrophages belonging to the immune system. Microglia primarily derive from c-Kit^lowCD41^low progenitors in the yolk sac at the embryonic stage, and are with the potential for primitive erythropoiesis [20–22]. Fate-mapping experiments also suggest negligible contribution of marrow-derived monocytes to microglia, that is confirmed by transgenic animals lacking hematopoietic stem cells (HSCs) [23, 24]. Microglia invade the CNS at very early stage of development as microglia are observed to disperse throughout the developing cortex in the human forebrain during the first month of gestation [25]. Thus, microglia are a specialized group of prenatally established, hematopoietic stem cell-independent macrophages that persist throughout life [26].

The physiological functions of microglia

Growing evidence has implicated essential roles of microglia in brain development, homeostasis maintenance, and neurological diseases [27]. Serving as the primary innate immune cells in the brain, microglia are mainly in the resting state (M0 state) under physiological conditions [27–30]. Microglial numbers and spatial distribution are tightly regulated, forming a tile-like network to cover the entire neuropil [6, 31]. Microglia are composed of small, dense cell bodies and numerous short, irregular protrusions that directly contact neurons, astrocytes, and blood vessels, allowing microglia to dynamically surveil surrounding environment and rapidly respond to injury and infection [32–35]. Being phagocytes within the CNS, microglia recognize, engulf, and digest large extracellular particles or synaptic structures through receptor-mediated phagocytosis, which is vital for neural development and homeostasis maintenance of the brain after birth [36, 37]. Microglia also express receptors for multiple neurotransmitters and neuropeptides to detect and eliminate damaged neurons, and release trophic factors such as insulin-like growth factor-1 (IGF-1), basic fibroblast growth factor (bFGF), nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF) to maintain neuronal development, survival, and functions [38, 39]. Besides, microglia crosstalk with oligodendrocyte precursor cells (OPCs) and oligodendrocytes to modulate myelinogenesis, and act as cellular chaperones for neovascularization [40, 41].

Microglia activation and polarization

When the CNS microenvironment homeostasis is disrupted by viral and bacterial infection, trauma pathology, or stress and bad environmental factors such as interference, resting state microglia are rapidly activated. Activated microglia undergo proliferation, retraction, conversion, and rapid transition from the branching form to amoebic form, secrete immuno-modulatory factors, and phagocytose pathogens, myelin debris, damaged and dead neurons and other cells [6, 39, 42]. According to their different secreted factors and functional states, activated microglia are roughly classified as the classically activated pro-inflammatory (M1) phenotype and the alternatively activated anti-inflammatory (M2) phenotype [39]. Classically activated microglia release pro-inflammatory factors such as interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), and chemical mediators to attract and activate other immune cells. They trigger signaling pathway molecules such as nuclear factor kappa B (NF-κB), nucleotide-binding oligomerization domain (NOD)-like receptor pyrin containing 3 (NLRP3), and epigenetic modifying enzymes such as histone deacetylases (HDACs), which cause inflammatory response and maintain neuroinflammation [43, 44]. Activated microglia exhibit swelled cell body and increased protrusions, while enhancing their phagocytic and antigen presentation functions [45]. Unlike the M1 type, alternatively activated microglia inhibit neuroinflammation, enhance cell proliferation, trigger wound healing, phagocytose debris, and restore homeostasis by secreting anti-inflammatory factors such as interleukin-10 (IL-10), transforming growth factor β (TGF-β), arginase-1 (Arg-1), growth factors, neurotrophic factors, and colony-stimulating factor 1 (CSF1) [46, 47].

Notably, in 2017, Keren-Shaul et al. reported a new and rare subset of microglia, disease-associated microglia (DAM) conserved in human patients and rodent models of Alzheimer’s disease (AD) [48, 49]. They found that DAM express typical microglial markers like ionized calcium-binding adapter molecule 1 (Iba1), cystatin C (Cst3), and hexosaminidase subunit beta (Hexb), together with downregulation of “homeostatic” microglial genes, including purinergic receptor P2Y12 (P2ry12), P2ry13, C-X3-C motif chemokine receptor 1 (Cx3cr1), CD33, and transmembrane protein 119 (Tmem119) [49, 50]. These AD-related DAM also excessively express several known AD risk genes such as apolipoprotein E (Apoe), cathepsin D (Ctsd), lipoprotein lipase (Lpl), transmembrane immune signaling adaptor TYROBP (Tyrobp), and triggering receptor expressed on myeloid cells 2 (Trem2) that are involved in lysosomal, phagocytic, and lipid metabolism pathways [48]. Moreover, AD-related DAMs are observed in disease-affected brain regions (e.g., cortex and hippocampus), and are co-localized with β-amyloid (Aβ) plaques [48]. Afterwards, DAM have been identified in many other neurodegenerative diseases including amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS), and Parkinson’s disease (PD) [51–53]. These findings raise the probability that the DAM phenotype is a common signature of microglia response to CNS pathology, irrespective of the disease etiology. Overall, although classical concept divides activated microglia into 2 distinct phenotypes, recent studies demonstrate a much more complicated polarization of microglia, highlighting the importance of investigating the dynamic transition of microglial phenotypes in neurological diseases and their different stages [49].

The role of microglia in depression

Microglia activation and polarization in depression

Plenty of brain-imaging studies including positron emission tomography (PET) scans have revealed significant microglial activation in the prefrontal cortex (PFC) and anterior cingulate cortex (ACC) of patients with MDD, which is positively correlated with the severity of the depressive episode [5, 54, 55]. Postmortem examinations further show increased density of microglia that are in a state of prolonged activation, ascertained as elevated expression levels of Iba1, CD45, and monocyte chemoattractant protein-1, in both PFC and ACC white matters [55–57]. Similarly, sustained microglial activation is also observed in various brain regions of diverse animal models of depression, including lipopolysaccharide (LPS)-induced acute depression, chronic unpredictable mild stress (CUMS), chronic social defeat stress (CSDS), and chronic restraint stress (CRS) (Fig. 1) [58–63].

Fig. 1.

Microglial dysfunctions trigger neuroinflammation and neuronal damage in depression. Microglial dysfunction is a key pathological change in depression. Under pathological conditions, microglia continuously activate in response to stress-related information from the molecular patterns (e.g., DAMPs) in the local brain environment, leading to the hyper-production of inflammatory cytokines/chemokines, accumulation of ROS, reduced release of trophic factors, and excessive synaptic pruning, which triggers neuroinflammation, induces neuronal/axonal damage, disrupts the integrity of the BBB in the brain, and consequently causes depression

Interestingly, a recent study demonstrates that microglia in the gray matter of depression patients show a distinct DAM transcriptomic profile [64]. Unlike AD-related DAM that exhibit strong pro-inflammatory and phagocytic properties, depression DAM (DepDAM) reveal an immune-suppressed state, with lower levels of inflammation-related genes including the marker of proliferation Ki-67 (MKI67), secreted phosphoprotein 1 (SPP1), complement C1q chain A/B/C (C1QA/B/C), and phagocytosis-related ones such as Fc gamma receptor 1 A/C (FCGR1A/C), Fc gamma receptor 3 A (FCGR3A), CD14, and CD163 [64]. This finding has been supported by other groups, demonstrating increased expression of microglial homeostatic signature proteins P2RY12, TMEM119, CX3CR1, and C–C motif chemokine receptor 5 (CCR5), and negligible changes in the expression of pro-inflammatory factors such as major histocompatibility complex class II DR alpha (HLA-DRA), IL-1β, IL-6, and TNF-α in the brains of depressed patients [65, 66]. These studies implicate that, although neuroinflammation has been recognized as a common pathological feature of many neurological diseases, the dynamic changes of microglial phenotypes in response to the physio-pathological conditions of depression display strong heterogeneity compared with those of other diseases (e.g., AD).

Microglia drive neuroinflammation in depression

The most obvious pathological roles of microglia in neurological diseases are to mediate immune responses and trigger neuroinflammation (Fig. 1) [67–69]. Clinical studies have noted elevated levels of multiple cytokines in the peripheral blood and cerebrospinal fluid (CSF) of depressed patients [67]. Significantly increased levels of pro-inflammatory cytokines are also observed in the brains of depression mice, accompanied by microglial activation and hyperactivities of pro-inflammatory pathways such as NLRP3 and toll-like receptor 4 (TLR4)-NF-κB [58–63, 66, 70]. Excessive pro-inflammatory cytokines secreted by microglia destabilize NMDA receptors and cause excitotoxicity, which impairs neuronal function and leads to neuronal loss, thereby affecting mood regulation [71, 72]. Activated microglia also transfer miRNAs to astrocytes through exosomes, promoting pro-inflammatory astrocytic phenotype (A1) transition and aggravating neuroinflammation [70]. Depletion of microglia, blockage of pro-inflammatory exosome release, and inhibition of microglial activation have been shown to alleviate neuroinflammation and depression-like behaviors, indicating microglia-driven neuroinflammation as a fundamental pathological process and a pivotal therapeutic target for depression [63, 70, 73–75].

Microglia influence neuroplasticity in depression

Neuroplasticity refers to changes in connections between neurons, which are involved in processes such as learning, memory, and mood regulation [76]. Emerging research indicates that neuroplasticity is often impaired in depressed patients, particularly in key regions such as the hippocampus [76, 77]. Deng et al. highlighted that microglia are closely associated with synaptic plasticity in neuropsychiatric disorders like depression [78]. Under stress, microglia increase their phagocytic activity. They engulf and eliminate excitatory synapses (via CX3CR1 and complement pathways), leading to spine loss in the hippocampus and prefrontal cortex. This structural change directly underlies the cognitive and affective symptoms of depression [13, 79]. Meanwhile, activated microglia release proinflammatory factors, such as TNF-α and IL-1β, that impair long-term potentiation (LTP) and facilitate long-term depression (LTD), effectively damaging the cellular basis of learning and resilience [79, 80]. However, till today, the mechanisms by which microglia mediate depression remain not clearly understood, representing a promising direction for future research.

The neuroprotective roles of microglia in depression

Microglia activation in depression is largely portrayed as detrimental. However, the portrayal of activated microglia in depression as purely a pathological flaw overlooks their fundamental role as evolutionary sentinels. Microglia’s pro-inflammatory activation represents a physiological trade-off where acute, beneficial defenses against pathogens or injury become maladaptive under chronic stress. While initially serving as a calculated sacrifice of well-being for survival and necessary synaptic pruning, this response turns pathological when it becomes sustained, leading to excessive synaptic loss and neurotoxicity. Besides, a group of microglia also transit into anti-inflammatory phenotypes in response to CNS injury, thereby clearing damaged cells and metabolic waste, secreting anti-inflammatory cytokines like IL-10, and promoting neural regeneration and repair [81–86]. Multiple studies have demonstrated that microglia depletion by PLX3397 aggravates chronic unpredictable stress-induced depressive behaviors, suggesting a nonnegligible anti-depressant role of microglia [87, 88]. Thus, microglia activation cannot be viewed as an evolutionary flaw, but as a maladaptive consequence of an ancient immune system constrained by its design for acute crises, now chronically activated by modern psychosocial stressors, highlighting the need for therapies that restore microglial homeostasis and enhance anti-inflammatory capacity rather than simply suppress their functions.

Microglial mitochondria dysfunctions in depression

The high energy consumption of microglia in maintaining immune function is regulated by mitochondrial energy metabolism [7]. Growing evidence has demonstrated important roles of mitochondrial metabolism in microglia activation (Fig. 2) [89]. More importantly, the involvement of microglial mitochondrial dysfunction in depression has been extensively investigated in vivo and in vitro [10, 90]. In this section, we discuss different aspects of mitochondrial dysfunctions in microglia and their roles in depression (Fig. 3).

Fig. 2.

Mitochondria modulate microglial activities through distinct mechanisms. Mitochondria are widely involved in the regulation of microglial activities via multiple mechanisms, including the shift of energy metabolic patterns between OXPHOS and aerobic glycolysis, ROS-driven oxidative stress, mitochondrial DNA damage and release, autophagy/mitophagy processes, calcium homeostasis and transportation, TSPO- and HK-dependent control of metabolism and phagocytosis, fission/fusion balance in mitochondrial quality control, and the activities of AMPK-mTOR signaling

Fig. 3.

Mitochondrial dysfunction-driven microglial activation induces pathological changes in depression. Under stress stimulations, mitochondrial functions are disrupted, leading to metabolic reprogramming from OXPHOS to glycolysis, ROS-driven oxidative stress, mtDNA damage and release, fission-fusion imbalance, mitophagy impairment, calcium homeostasis alteration, and TSPO and AMPK-mTOR signaling deregulation, which ultimately triggers microglial activation. Activated microglia further release pro-inflammatory and neurotoxic factors, and engulf health synaptic structures to induce neural damage and function abnormalities, resulting in the onset and progression of depression

Microglia metabolic reprogramming in depression

Metabolic reprogramming in microglia activation

Microglia dynamically adjust their energy metabolism between OXPHOS and glycolysis depending on physiological or pathological demands. Under resting conditions, microglia primarily rely on mitochondrial OXPHOS for efficient ATP production. Upon exposure to inflammatory stimuli or stress, microglia undergo metabolic reprogramming, shifting their primary energy producing route from OXPHOS to aerobic glycolysis, a phenomenon also known as the Warburg effect [91]. While OXPHOS generates ATP more efficiently and does not produce lactic acid, it involves more reaction steps and produces reactive oxygen species (ROS) as a byproduct [92, 93]. Glycolysis only predominates microglial energy production when cells require rapid energy, such as during inflammation or under hypoxia [92, 94].

Under inflammatory conditions, the stabilization of hypoxia-inducible factor-1α (HIF-1α) plays a central role in driving OXPHOS to glycolytic reprogramming. HIF-1α upregulates glycolytic enzymes including hexokinase 2 (HK2), 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 (PFKFB3), and lactate dehydrogenase A (LDHA), while simultaneously inducing pyruvate dehydrogenase kinase 1, which phosphorylates and inactivates pyruvate dehydrogenase. This blocks pyruvate entry into the TCA cycle and diverts it toward lactate production [95, 96]. Concurrently, succinate accumulates via the GABA shunt or glutaminolysis, inhibiting prolyl hydroxylases and further stabilizing HIF-1α in a feed-forward loop that sustains glycolysis [96]. Additional signaling pathways such as NF-κB and mTOR also promote glycolytic flux, supplying biosynthetic precursors like nucleotides and lipids required for cytokine production [97].

The metabolic shift from OXPHOS to glycolysis in activated microglia is a functional adaptation that supports rapid immune responses, distinct from cancer cells that use the Warburg effect primarily for proliferation [7, 98]. Glycolysis provides immediate energy for phagocytosis, chemotaxis, and cytokine release, bypassing the slower kinetics of mitochondrial ATP synthesis [97, 99]. Moreover, glycolysis feeds into the pentose phosphate pathway (PPP) to generate ribose-5-phosphate for RNA/DNA synthesis, and provide acetyl-CoA and glycerol for membrane and lipid production [99]. The Warburg effect also enhances microglial survival in the high-ROS environment characteristic of neuroinflammation [99].

Mitochondria actively regulate glycolysis. HK2, a key rate-limiting glycolytic enzyme that converts glucose to glucose-6-phosphate (G-6-P) [100], translocates to mitochondria via an N-terminal motif, conferring cytoprotective effects and improving glucose utilization efficiency [101]. However, LPS stimulation increases mitochondrial recruitment of HK2 in microglia, promoting glycolysis while impairing phagocytic capacity [102]. Interestingly, cytoplasmic HK2 enhances phagocytosis independently of its metabolic activity, suggesting additional immune signaling functions. These findings highlight that targeting the interaction between mitochondria and glycolytic enzymes such as HK2 may offer a novel immunotherapeutic strategy to modulate microglial functions [102].

Microglial metabolic reprogramming in depression

Altered glucose metabolism and enhanced glycolysis in microglia are key pathological changes in depression [18, 19]. Increased glucose levels, upregulated expression of key glycolytic enzymes, and enhanced hexokinase (HK) activity have been reported in CUMS-exposed mouse brains [18]. Metabolic reprogramming of microglia provides more energy and promotes the synthesis of pro-inflammatory cytokines such as IL-1β and TNF-α [18, 19]. Moreover, increased glycolysis also activates the NLRP3 inflammasome via lactate accumulation, further promoting neuroinflammation [103]. The treatment of glycolysis inhibitor 2-deoxy-d-glucose (2-DG) significantly inhibits ROS and proinflammatory cytokines production in corticosterone-exposed microglia in vitro, and alleviates CUMS-induced neuroinflammation and depression-like behaviors, confirming the pathological roles of altered metabolic patterns in depression [18, 19]. Thus, abnormal glucose metabolism in microglia, particularly excessive glycolysis, forms a feedback loop that triggers and aggravates excessive microglial activation, contributing to depression.

Mitochondria-driven oxidative stress drives microglial activation in depression

Microglia activation and mitochondria-driven oxidative stress

Mitochondria in microglia also play a key role in regulating oxidative stress. Activated microglia produce high levels of ROS during inflammation. In M1 microglia, ROS generated by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX1), mitochondrial oxidative phosphorylation, and iron‑dependent Fenton reaction act as key signaling molecules, enhancing the core functions of microglia including phagocytosis and inflammatory cytokine production via [104]. Excessive ROS produced by mitochondria may cause cellular damage. High concentrations of NO (generated by inducible nitric oxide synthase) react with mitochondrial ROS to form peroxynitrite, leading to irreversible damage to the electron transport chain [105]. Thus, microglia maintain redox balance by regulating mitochondrial antioxidant mechanisms to avoid excessive oxidative damage. For example, LPS stimulation increases the activity of key enzymes in the PPP, promoting NADPH production to maintain redox balance [106]. Additionally, overexpression of mitochondrial mortalin is able to inhibit LPS-induced abnormal metabolic transitions and inflammatory responses.

Earlier in vitro research has demonstrated that inflammation leads to increased intracellular succinate accumulation in microglia. This excess succinate drives a shift from standard forward electron flow along the electron transport chain to a process called reverse electron transport (RET) via mitochondrial complex I [107]. RET depends on a substantial proton gradient across the inner mitochondrial membrane (a high proton motive force) [108] and functionally re-tasks mitochondria. Instead of primarily generating ATP, they produce substantial superoxide, which can be converted to hydrogen peroxide and other ROS that are collectively termed mitochondrial ROS (mtROS) [107]. Limiting the activity of mitochondrial complex II (also known as succinate dehydrogenase) with reversible inhibitors such as itaconate or malonate is able to reduce RET-driven mtROS generation and help establish an anti-inflammatory state in cultured microglia [109, 110]. Comparable protective effects are observed in vivo that inhibiting complex II or complex I activity mitigates RET-mediated mtROS injury during cardiac reperfusion following a heart attack [111, 112].

Microglial oxidative stress and depression

Oxidative stress is a central feature of mitochondrial metabolic abnormalities in microglia during depression [75]. In CSDS-induced depression and anxiety model, excessive ROS accumulation has been observed in microglia [16]. Metabolic disorders induce ROS production in microglia, which further damages mitochondrial DNA and membrane structures, impairing mitochondrial energy metabolism and signal transduction [67, 75]. It consequently activates pro-inflammatory signaling pathways such as NF-κB and mitogen-activated protein kinase (MAPK), which are closely linked to the pathological progression of depression. CSDS-induced depression-like behaviors are either inhibited by N-acetylcysteine (NAC)-based ROS elimination or CSF1 receptor (CSF1R) inhibitor PLX5622-induced microglia/macrophages depletion, confirming the roles of microglial oxidative stress in depression [75].

Microglial mitochondrial DNA abnormalities in depression

Mitochondrial DNA abnormalities in microglial activation

Microglia perceive cellular damage and stress by recognizing damage-associated molecular patterns (DAMPs) through pattern recognition receptors (PRRs) [113]. Recent studies have suggested mtDNA as an important category of DAMPs [114]. mtDNA is a 16,569 base-pair multicopy circular genome containing 37 genes [113]. mtDNA plays essential roles in energy production and cell metabolism [113]. Under pathological conditions, damaged mitochondria release a great amount of mtDNA primarily through the mitochondrial permeability transition pores such as BCL2 antagonist/killer 1 (BAK) and BCL2-associated X (BAX) [115]. The cell-free mtDNA is more stable and resistant to nuclease digestion than genomic DNA in the bloodstream and CSF [113, 114].

Accumulating findings indicate that mitochondrial DNA is extruded into extracellular spaces as a consequence of impaired mitochondrial function. This pool, termed circulatory cell-free mtDNA (ccf-mtDNA), substantially contributes to stress-induced inflammatory processes [116]. Ccf-mtDNA, being a DAMP, acts as a key factor to trigger inflammatory responses of microglia through multiple mechanisms [117]. Liberated mtDNA is able to engage multiple pro-inflammatory signaling cascades, notably the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) axis. Upon binding cytosolic mtDNA, cGAS-STING activation initiates innate immune signaling and promotes interferon regulatory factor 3 (IRF3)-driven transcription, culminating in type I interferon (IFN-I) secretion [118]. Besides, the binding of mtDNA to cGAS-STING also activates tank-binding kinase 1 (TBK1) and NLRP3 inflammasome. Consequently, IFN and NF-κB signaling pathways are activated, resulting in microglial activation and production of pro-inflammatory molecules [119].

Furthermore, mitochondrial DNA can be sensed by endosomal Toll-like receptor 9 (TLR9), a specific member of TLR family that is mainly located on the surface of the endosome, to trigger myeloid differentiation primary response 88 (MyD88)-mediated activation of MAPK and NF-κB signaling [119, 120]. mtDNA ligation of TLR9 upregulates NF-κB signaling, thereby enhancing the production of IFN-I and TNF-α [121]. It leads to the activation of more surrounding microglia to spread neuroinflammation and forms a vicious cycle, ultimately resulting in irreversible neuronal death and loss of glial function [114].

Mitochondrial DNA abnormalities in depression

Both clinical investigations and preclinical models have shown that heightened levels of circulating mtDNA exacerbate pathological immune activation and neuroinflammation in individuals with MDD [122, 123]. As a result, recent progress in ccf-mtDNA quantification has generated considerable interest in evaluating its utility as a biomarker for MDD and stress-induced neuroinflammation [123–125]. Contrary to the situation in peripheral blood, the copy number of mtDNA is significantly reduced in the hippocampi of CRS-exposed rats [126]. Moreover, increased release of mtDNA has been observed in LPS-treated BV2 microglial cells, that are utilized to mimic neuroinflammation of adolescent depression [127]. Studies on hypertensive mice with lead (Pb) exposure and mice with neuropathic pain have further revealed that mtDNA leakage activates cGAS/STING signaling in microglia, causing neuroinflammation and depression-like behaviors [128, 129]. These findings clearly suggest a great deal of contribution of mtDNA-driven microglial activation to depression, although comprehensive validations are urgently required with more classical animal models of depression.

Microglial mitochondrial fusion-fission imbalance in depression

Altered mitochondrial dynamics in microglia activation

Mitochondria constitute dynamic organelles that continuously remodel their architecture through cycles of fusion and fission in accordance with environmental cues, while possessing robust quality-control systems to govern mitochondrial turnover and the removal of compromised organelles [130]. Microglia mitochondrial dynamics, particularly fusion-fission balance, influence cellular function. Mitochondrial fission helps cells to adapt to changing cellular demands, while fusion maintains mitochondrial functional integrity [131].

The integrity of a healthy mitochondrial network, essential for cellular homeostasis and viability, is governed by the dynamic behavior and renewal of mitochondria, collectively referred to as the mitochondrial quality-control apparatus [130]. Basically, fusion promotes complementation between healthy mitochondria, while fission is essential for mitochondrial transport to synapses, mitochondrial quality control, damaged organelle segregation, and brain development [132]. Mitochondrial fission is triggered by the recruitment of dynamin-related protein 1 (Drp1) from the cytoplasm to the outer mitochondrial membrane, mediated by various adapter proteins [133, 134]. Studies have shown that Drp1-dependent mitochondrial fission plays essential roles in microglial activation without significantly interfering with metabolic reprogramming or repolarization [135]. Thus, dysregulation of mitochondrial dynamics is implicated in the pathogenesis of various neurological diseases [136–138]. For instance, mitochondrial dynamics are disrupted, leading to mitochondrial damage in activated microglia in AD mouse models [7]. These damaged mitochondria are released into the extracellular space, inducing innate immune responses by targeting nearby astrocytes, thereby releasing more dysfunctional mitochondria, generating positive feedback, and accelerating neuroinflammation. Therefore, abnormal mitochondrial division may serve as a promising therapeutic target for correcting hyper-activation of glial cells and inflammatory response in various neuropathological conditions [7].

Mitochondrial fusion-fission imbalance in depression

Stress-induced fusion-fission imbalance is a common pathological change in depression [139, 140]. CUMS-exposed rats exhibit increased Drp1 expression and reduced the expression levels of fusion protein mitofusin 2 (MFN2) in the hippocampus, together with increased mitochondrial fragmentation and depression-like behaviors [141]. Following animal studies further demonstrate that either MFN2 overexpression or genetic knockdown of DRP1 knockdown/inhibition attenuate depression-like behaviors, supporting a causal relationship between mitochondrial fusion-fission imbalance and depression [142]. However, these studies are conducted either on brain tissues or cultured neurons, leaving the pathological roles of microglial mitochondria fusion-fission imbalance in depression an incomplete question to be answered.

Microglial mitochondrial calcium homeostasis in depression

Mitochondrial calcium homeostasis in microglial activation

Mitochondria are integral components of cellular calcium signaling. Increases in the concentration of calcium ion (Ca²⁺) in the mitochondrial matrix enhance ATP synthase and the TCA cycle activities [143, 144]. Calcium ion moves from the cytosol to the mitochondrial intermembrane space (IMS) through the 30- to 35-kDa voltage-dependent anion channel (VDAC) located on the outer mitochondrial membrane (OMM) [145]. Calcium ion is then transported across the inner mitochondrial membrane (IMM) into the mitochondrial matrix mainly through mitochondrial uniporter channel (mtCU), consisting of mitochondrial calcium uniporter (MCU), essential mitochondrial response element (EMRE), mitochondrial calcium uptake 1, 2, and 3 (MICU1/2/3), and mitochondrial calcium uniporter regulator 1 (MCUR1). Besides, calcium ion uptake across the IMM can also be mediated by the rapid mode of uptake (RaM), mitochondrial ryanodine receptor (mRyR), the mitochondrial sodium/calcium exchanger (NCLX), leucine zipper and EF-hand containing transmembrane protein 1 (LETM1), and other machineries [144–146].

The excitability of microglia, similar to all other types of glial cells, is mainly regulated by calcium homeostasis [147]. Under physiological conditions, intracellular calcium signals support the main functions of microglia including surveilling capacities [148]. In pathological contexts, the failure of calcium homeostatic mechanisms in response to DAMPs or pathogen-associated molecular patterns (PAMPs) increases matrix calcium ion levels, inducing defensive responses and activation of microglia [148, 149]. In a nitrogen-doped graphene quantum dots (N-GQDs)-induced inflammation model, disruption of calcium homeostasis causes pro-inflammatory responses and ferroptosis of microglia by altering the normal opening of calcium channels in plasma and endoplasmic reticulum (ER) membranes [150]. Moreover, Zhang et al. found that the mitochondrial-associated membrane (MAM), a platform that promotes calcium transport between ER and mitochondria, regulates ER stress response and mitochondrial homeostasis, therefore affecting the pro-inflammatory responses of microglia under stress [11]. These observations clearly indicate calcium homeostasis as a key switch for dynamic phenotype transition of microglia.

Microglial mitochondria calcium homeostasis alteration and depression

Intracellular calcium signaling is the substrate of microglial excitability. Spatial and temporal changes of the intracellular calcium signaling are observed in response to pathological stimuli in various neurological disorders including depression [147, 151]. In postpartum depression (PPD) mouse and cell models, Zhai et al. further demonstrate that mitochondrial calcium homeostasis is regulated by the interaction of transport proteins (TSPO) and translocase of outer mitochondrial membrane 40 (TOM40), in collaboration with the mitochondrial inner membrane multiprotein channel composed of mitochondrial calcium uniporter (MCU) [152]. Restoring calcium homeostasis by blocking calcium influx of mitochondria significantly inhibits microglia-driven neuroinflammation, alleviating depressive behaviors in PPD mice [152]. Furthermore, the formation of a tripartite complex involving the inositol 1,4,5-triphosphate receptor 3 (IP3R3), voltage-dependent anion channel 1 (VDAC1), and glucose-regulated protein 75 (GRP75) within MAMs enhances ER-mitochondria contact in microglia of CSDS mice, facilitating the transfer of calcium ions to the mitochondria. Consequently, the transmission of extracellular stress signals including ATP is promoted, leading to mitochondrial damage and NLRP3-driven neuroinflammation to mediate the stress-induced depression-like behaviors [11]. These observations suggest essential roles of calcium homeostasis disruption in depression.

Microglial mitophagy deficiency in depression

Autophagy and mitophagy deficiency in microglial activation

Autophagy is a major pathway for the removal of damaged organelles from cells, and mitophagy is the selective mitochondrial autophagy that is involved in the clearance of damaged mitochondria [153]. Although different mitophagy classifications have been proposed, canonical mitophagy can be generally grouped into PTEN-induced kinase 1 (PINK1)-parkin dependent and PINK1-parkin independent ones [154]. For the PINK1-parkin-dependent mitophagy, beclin1/phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)-mediated formation of phagophore coordinates with mitochondrial fission to isolate mitochondria into mitophagosomes, and mitophagosome then fuses with the lysosomes for digesting the encapsulated mitochondria [155]. PINK1-parkin-independent mitophagy involves receptors including BCL2 interacting protein 3 (BNIP3), BCL2 interacting protein 3 like (BNIP3L), and FUN14 domain containing 1 (FUNDC1) that bind to microtubule associated protein 1 light chain 3 (LC3) to generate mitophagosomes for lysosome-mediated degradation and recycling [155]. Moreover, noncanonical mitophagy has also been found to be associated with the formation, degradation, and recycling of mitochondria-derived vesicles (MDVs) in PINK1/Parkin- and lysosome-dependent pathways [155].

Dysregulated autophagy and mitophagy serve as critical regulatory mechanisms underlying microglia-mediated neuroinflammation in depression and other neurological disorders via multiple signaling cascades including PI3K, AMP-activated protein kinase (AMPK), mammalian target of rapamycin (mTOR), and cytokine-mediated pathways [153, 156–158]. Defective mitophagy also leads to impaired mitochondria and mtDNA degradation, resulting in DAMPs accumulation and inflammatory responses of microglia [159]. These studies suggested complicated pathological processes under the control of mitophagy. More importantly, stimulation of mitophagy by mitophagy-inducing agents such as urolithin A (UA) and nicotinamide riboside (NR) restore mitochondrial function, therefore enhancing polarization of microglia from pro-inflammatory phenotype to anti-inflammatory one and inhibiting subsequent neuroinflammation [157, 160, 161].

Nowadays, many mitophagy regulators have been identified that modulate microglial activities. For instance, NOD-like receptor X1 (NLRX1), the only NOD family receptor protein located in mitochondria, plays a role in morphine-induced mitophagy in microglia [162, 163]. NLRX1 triggers incomplete mitophagy in microglia that leads to immune suppression and susceptibility to pathogenic attacks after morphine treatment, and NLRX1 silencing inhibits mitophagy activity and rescues lysosomal function, including the generation and acidification of microglia [163].

Microglial mitophagy impairment in depression

Mitophagy serves as a first protective means of cells as it facilitates cellular adaptation to stress by eliminating damaged and senescent mitochondria [164]. Conversely, a failure in this process allows defective mitochondria to accumulate, disrupting homeostasis and precipitating neuronal degeneration or death, which ultimately contributes to depression. In depression, microglial mitophagy is often inhibited, leading to the accumulation of dysfunctional mitochondria [17]. Abnormal mitophagy has been found in the brains of multiple depression model animals including chronic mild stress (CMS)-exposed rats [14]. In experiments where fluoxetine was administered to counteract corticosterone-induced injury, the antidepressant was shown to suppress cell death. This effect was achieved by enhancing the removal of compromised mitochondria, boosting mitophagic flux in astrocytes, and mitigating the buildup of mtROS [15]. Investigating this effect further, De Rudranil et al. [165] demonstrated that cold restraint stress, a rodent model for stress-related mucosal disease, induces significant mitochondrial dysfunction in the gut. This occurs via excessive mitochondrial fission alongside irregular mitophagy within the intestinal lining. Subsequently, mitochondrial damage induces ROS accumulation, and NLRP3 inflammasome activation, causing excessive production and release of inflammatory and neurotoxic molecules (e.g., TNF-α, IL-1β, etc.), which results in neuronal dysfunction and depressive behaviors [166, 167]. The study also highlighted that inadequate mitophagy activation during the later phases of stress causes the mitochondrial quality control system to fail, triggering a cascade of caspases.

Inspiringly, many chemicals and natural substances have been identified to activate mitophagy in depression. For instance, Quercetin, a natural flavonoid, significantly promotes microglial mitophagy to reduce NLRP3 inflammasome and pyroptosis-related proteins expression and activities, therefore exhibiting neuroprotective and anti-inflammatory effects in both in vitro and in vivo models of depression [17]. Similarly, AdipoRon, an orally active synthetic adiponectin agonist, and exendin-4, a glucagon-like peptide-1 receptor (GLP-1R) agonist, also improve mitochondrial function and mitophagy by decreasing the accumulation of mitochondrial ROS and intracellular ROS production, suggesting microglial mitophagy as a promising target for antidepressant therapies [166, 168]. Supporting these findings, research has indicated that both G11-5 and fluoxetine ameliorate depressive phenotypes in mice. These compounds were shown to reduce instances of escape failure in the learned helplessness (LH) paradigm and enhance social interactions in the social defeat stress (SDS) model, through mechanisms attributed to their preservation of mitophagy and their role in reinstating retrograde axonal transport and neurotransmitter release [169, 170].

Microglial mitochondria-driven TSPO and AMPK-mTOR pathways in depression

Importantly, mitochondria mediate microglial activities through different molecular signaling pathways. Among them, TSPO and AMPK-mTOR pathways belong to the most investigated and recognized ones for neurological diseases in general and depression in particular.

TSPO in microglial activation

TSPO, which is predominately localized to the outer mitochondrial membrane, is in relatively low expression in brain cells, and exhibits enhanced expression in response to microglial activation [171]. The roles of TSPO in microglia expand into multiple pathways. First, TSPO regulates the opening of mitochondrial permeability transition pore (mPTP) that controls the entry of molecules necessary for mitochondrial function [172]. For instance, TSPO transports cholesterol from cytosol to inner mitochondrial membranes for the cleavage of cholesterol into pregnenolone, affecting steroidogenesis in microglia [173]. Second, TSPO plays a crucial role in controlling the recruitment of mitochondrial HK2, therefore coordinating metabolic modes of microglia to control phagocytosis [102]. Third, TSPO acts as a central player of redox homeostasis maintenance via NADPH oxidases-dependent neurotoxic ROS production [174–176]. Fourth, TSPO manipulates inflammatory transcriptional pathways including MAPK and the NLRP3 inflammasome, resulting in the production and release of cytokines [174]. Hence, TSPO has been widely used as a key biomarker of microglial activation, making TSPO a prominent target for PET imaging of neurological disorders [177].

AMPK-mTOR pathway in microglial activation

Several signaling pathways influence microglial mitochondrial metabolism, including mTOR and AMPK pathways [178]. AMPK-mTOR pathway is an energy-sensing mechanism that regulates mitochondrial metabolism. Under nutrient-rich conditions, mTOR activation promotes anabolic processes in microglia, increasing mitochondrial biogenesis, quantity, and OXPHOS. In energy-deficient states, mTOR is inhibited, reducing anabolic activity to avoid excessive energy consumption. AMPK activation, conversely, enhances mitochondrial function by promoting biogenesis, increasing mitochondrial number, and stimulating mitophagy.

Numerous studies further identified various downstream factors of AMPK-mTOR pathway in the regulation of microglial activities including NLRP3, NF-κB, and peroxisome proliferative activated receptor-γ (PPARγ) co-activator-1α (PGC-1α) [179]. For example, AMPK inhibits pro-inflammatory responses of microglia via repressing NF-κB and NLRP3 signaling in AD rodent models [179]. Studies have also shown that PGC-1α drives microglia toward an anti-inflammatory phenotype by repressing NF-κB activity while coactivating STAT6 and STAT3, and mice lacking PGC-1α suffer from pronounced neuronal injury following cerebral ischemia [180, 181]. Thus, dysregulation of AMPK-mTOR and downstream pathways impairs mitochondrial biogenesis and function, therefore modulating microglial activation and inflammatory responses.

Microglial TSPO and AMPK-mTOR pathway in depression

The pathological roles of TSPO and AMPK-mTOR pathways in microglial activation have been revealed in different depression models. Clinical PET studies have demonstrated a significant increase of TSPO bindings in the brains of patients with depression, suggesting promising diagnostic potential of TSPO in depression [151]. In vivo and in vitro studies further indicate significant increase of TSPO expression and TSPO-TOM40 binding, causing microglial activation and neuroinflammation in PPD models [152]. The treatment of TSPO ligand ZBD-2 shows great anti-inflammatory and anti-depressive effects on PPD mice, indicating TSPO as a great therapeutic target of depression [151]. Besides, Xu et al. found that mTOR pathway mediates glucose metabolic reprogramming of microglia in the cerebral striatum, therefore inducing depressive behaviors in single prolonged stress (SPS) rats [182]. Moreover, inhibited AMPK phosphorylation and enhanced mTOR phosphorylation are observed in high-fat diet–induced obesity mice, which drives neuroinflammation and depression-like behaviors [183]. Thus, mTOR signaling has been proposed as a potential target for depression treatment. For instance, the inhibition of mTOR signaling by rapamycin significantly promotes autophagy, represses microglial activation, alleviates neuroinflammation, and ameliorates depression-like behaviors [183]. Similarly, costunolide, the major active ingredient of the traditional Chinese medicinal herb Saussurea lappa, displays significant antidepressant effects through inhibiting mTOR signaling-induced microglial activation in CRS-exposed depression mice [184].

Notably, apart from the aspects discussed above, mitochondrial dysfunction also disrupts serotonin synthesis and release by disrupting tryptophan metabolism [185], suppresses neurotransmitter release by altering synaptic plasticity [135], and reduces neurogenesis by stimulating oxidative stress-mediated cell damage [103, 186] in other brain cells in depression and related disorders. Thus, mitochondria-related pathological changes weave a complicated network, in which microglial dysfunctions act as a key node that participates in the onset and progression of depression.

Conclusions and future directions

In conclusion, a wealth of information strongly suggests a great impact of mitochondria, the “power house” of cells, on microglia under physio-pathological conditions. Mitochondrial dysfunctions including mitochondrial energy metabolism alteration, oxidative stress, mtDNA damage, quality control impairment, and calcium homeostasis disruption, have emerged as a driving force of microglial abnormal activation in depression, contributing significantly to neuroinflammation and synaptic deficits. However, studies focusing on mitochondria-mediated microglial regulation in depression are limited, which severely restricts our understanding in the pathogenesis of depression. For instance, although one study has reported DepDAM, a group of microglia in immune-repressing state, in the brains of patients with depression and depression animal models, the roles of mitochondria in this heterogeneity of microglia in depression remain vague. In view of the association between mitochondrial activities and DAM transition in other neurological diseases [187, 188], mitochondrial dysfunction may also modulate DAM transition in depression, which needs to be clarified in future studies.

Current studies have implicated targeting microglial mitochondria as a promising therapeutic strategy for depression, especially for treatment-resistant cases. However, there remain many challenges such as poor BBB penetrability and insufficient microglial targeting capacity. Thus, comprehensive investigations are urgently needed to elucidate the precise molecular pathways linking mitochondrial dysfunction to microglial activation in depression, to develop novel brain-targeted delivery systems (e.g., biomimetic nanoparticles) and mitochondria-specific therapeutics (e.g., antioxidants, mitophagy inducers), and to conduct clinical trials with patient stratification based on inflammatory biomarkers to validate these novel therapeutic strategies of depression. Inspiringly, numerous pioneer investigations have demonstrated promising antidepressant effects of natural and synthetic products (e.g., Quercetin, AdipoRon, exendin-4) in animal and pre-clinical studies, shedding light on the rapid development of novel mitochondria-targeting therapies for depression.

Authors’ contributions

Conceptualization, X.X.; literature search and summary, Y.J., Shuyu.Z., F.M., Z.Y., Shiyi.Z., Y.W. and X.X.; manuscript preparation, Y.J., Shuyu.Z., F.M., Y.W., Z.Y., Shiyi.Z., Q.W., Y.W., and X.X.; funding acquisition, Q.W. and X.X.; supervision, X.X. All authors critically reviewed the manuscript and approved the final draft.

Funding

This work was supported in part by research grants from the National Natural Science Foundation of China (No. 82271477 to X.X.), Shanghai Oriental Talents Program (No. QNKJ2025035 to X.X.), Key Disciplines of Shanghai Health System (No. 2024ZDXK0036 to Q.W.).

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

The authors approved the final manuscript.

Competing interests

The authors declare no competing interests.