Mitochondrial respiration in microglia is essential for response to demyelinating injury but not proliferation

Microglia modulate integrity of myelin and proliferation of oligodendrocyte precursor cells in murine model of bone cancer pain

Cancer pain, a frequent complication in patients with cancer, adversely affects quality of life and survival rates. Microglia promote nociceptive information transmission by modulating myelin integrity during pain perception. However, the specific mechanisms by which microglia regulate myelin in the context of cancer pain remain poorly understood. In this study, we developed a bone cancer pain model to examine the interactions among microglia, myelin, and oligodendrocyte precursor cells and their roles in cancer pain. Our study found that mice with bone cancer pain had oligodendrocyte differentiation defects and myelin loss, and that promoting myelination did not relieve pain. In addition, we observed that reactive microglia and inflammatory cytokines increased and microglia phagocytosed myelin in mice with bone cancer pain. Inhibition of microglia not only alleviated pain behaviors in mice with bone cancer but also mitigated myelin phagocytosis and the proliferation of oligodendrocyte precursor cells. Our study suggests that microglia-mediated myelin loss and oligodendrocyte precursor cell proliferation may be one of the pathological mechanisms underlying pain in mice with bone cancer.

The foam cell-derived exosomes exacerbate ischemic white matter injury via transmitting metabolic defects to microglia

Atherosclerosis (AS) has been shown to be an independent risk factor for vascular cognitive impairment (VCI), but the mechanisms remain unclear. Here, we found that AS circulating exosomes exacerbated ischemic white matter injury and VCI. Exosomes originating from macrophage-derived foam cells targeted microglia. Mechanistically, foam cell-derived exosomes transmitted redox imbalance, mitochondrial dysfunction, and metabolic defects to microglia via the miR-101-3p-Nrf2-Slc2a1 axis. Anti-miR-101-3p or activation of Nrf2, both genetically and pharmacologically, could antagonize AS exosomes and ameliorate VCI. In conclusion, our findings reveal a distant connection between peripheral macrophages and brain microglia, which provides new insights and potential targets of AS-induced VCI.

Neuroinflammation and immunometabolism in neurodegenerative diseases

PURPOSE OF REVIEW

Immunometabolism is an emerging field of research investigating the ability of immune cells to modulate their metabolic activity for optimal function. While this has been extensively examined in peripheral immune cells like macrophages, only recently have these studies been extended to assess the immunometabolic activity of microglia, the innate immune cells of the brain.

RECENT FINDINGS

Microglia are highly metabolically flexible and can utilize different nutrients for their diverse functions. Like other immune cells, they undergo metabolic reprogramming on immune stimulation and in inflammatory, neurodegenerative conditions such as Alzheimer's disease (AD). In recent years, researchers have looked at the intricate mechanisms that modulate microglial activity and have uncovered key links between altered metabolism, neuroinflammation, and the involvement of disease-associated risk genes.

SUMMARY

This review highlights the recent studies that have significantly contributed to our understanding of the metabolic dysregulation observed in activated microglia in conditions such as AD, unveiling novel targets for therapeutic intervention.

Intranasal delivery of AEP inhibitor-loaded neuron-targeted liposome ameliorates radiation-induced brain injury

Acute exposure to high-dose radiation during head and neck tumors radiotherapy can result in radiation-induced brain injury (RIBI), characterized by neurocognitive deficits, dementia, and epilepsy. Asparagine endopeptidase (AEP), a cysteine proteinase, is effective in preventing neurodegenerative diseases and RIBI. However, the limited permeability of selective AEP inhibitor (AEPI) delivery to the brain reduces its effectiveness in preventing RIBI. This study constructed a nose-to-brain delivery platform for AEPI by encapsulating it in liposomes that are surface modified with rabies virus glycoprotein (RVG29), creating RVG29-AEPI liposomes. These RVG29-AEPI liposomes demonstrated efficient cellular uptake and blood-brain barrier penetration in vitro and in vivo. RVG29-AEPI liposomes effectively shielded DNA from radiation-induced damage and resulted in more effective reactive oxygen species removal than liposomes in primary neurons and microglial cells. Notably, the treatment with RVG29-AEPI liposomes (10 mg/kg AEPI) was highly systemically safe and significantly reduced brain injury. Behavioral tests demonstrated that RVG29-AEPI liposomes-treated mice had less radiation-induced brain damage and motor dysfunction. Moreover, it significantly prevented neuronal injury and microglia cell activation under photon and modern proton irradiation. These findings demonstrate the potential of nose-to-brain medication delivery of RVG29-AEPI liposomes for effective radioprotection, indicating a viable technique with enormous potential for clinical translation.

The fate of mitochondrial respiratory complexes in aging

While mitochondrial dysfunction is one of the canonical hallmarks of aging, it remains only vaguely defined. Its core feature embraces defects in energy-producing molecular machinery, the mitochondrial respiratory complexes (MRCs). The causes and consequences of these defects hold research attention. In this review, we assess the lifecycle of respiratory complexes, from biogenesis to degradation, and look closely at the mechanisms that could underpin their dysfunction in aged cells. We discuss how these processes could be altered by aging and expand on the fate of MRCs in age-associated pathologies. Given the complexity behind MRC maintenance and functionality, several traits could contribute to the phenomenon known as age-associated mitochondrial dysfunction. New advances will help us better understand the fate of this machinery in aging and age-related diseases.

Mitochondria complex III-generated superoxide is essential for IL-10 secretion in macrophages

Mitochondrial electron transport chain (ETC) function modulates macrophage biology; however, mechanisms underlying mitochondria ETC control of macrophage immune responses are not fully understood. Here, we report that mutant mice with mitochondria ETC complex III (CIII)-deficient macrophages exhibit increased susceptibility to influenza A virus (IAV) and LPS-induced endotoxic shock. Cultured bone marrow-derived macrophages (BMDMs) isolated from these mitochondria CIII-deficient mice released less IL-10 than controls following TLR3 or TLR4 stimulation. Unexpectedly, restoring mitochondrial respiration without generating superoxide using alternative oxidase (AOX) was not sufficient to reverse LPS-induced endotoxic shock susceptibility or restore IL-10 release. However, activation of protein kinase A (PKA) rescued IL-10 release in mitochondria CIII-deficient BMDMs following LPS stimulation. In addition, mitochondria CIII deficiency did not affect BMDM responses to interleukin-4 (IL-4) stimulation. Thus, our results highlight the essential role of mitochondria CIII-generated superoxide in the release of anti-inflammatory IL-10 in response to TLR stimulation.

Metabolic control of microglia in health and disease

Metabolic states within cells are tightly linked to functional outcomes and finely regulated by nutrient availability. A growing body of the literature supports the idea that various metabolites can influence cellular functions, such as cell differentiation, migration, and proliferation in different contexts, with ample evidence coming from the immune system. Additionally, certain functional programs can trigger significant metabolic changes within cells, which are crucial not only to meet high energy demands, but also to produce intermediate metabolites necessary to support specific tasks. Microglia, the resident innate immune cells of the central nervous system, are constantly active, surveying the brain parenchyma and providing support to neighboring cells in the brain. They exhibit high metabolic flexibility, capable of quickly undergoing metabolic reprogramming based on nutrient availability and functional requirements. In this chapter, we will discuss the major metabolic pathways within cells and provide examples of how relevant enzymes and metabolites can impact microglial function in physiologic and pathologic contexts.

Mitochondria are absent from microglial processes performing surveillance, chemotaxis, and phagocytic engulfment

Microglia continually surveil the brain allowing for rapid detection of tissue damage or infection. Microglial metabolism is linked to tissue homeostasis, yet how mitochondria are subcellularly partitioned in microglia and dynamically reorganize during surveillance, injury responses, and phagocytic engulfment in the intact brain are not known. Here, we performed intravital imaging of microglia mitochondria, revealing that microglial processes diverge, with some containing multiple mitochondria while others are completely void. Microglial processes that engage in minute-to-minute surveillance typically do not have mitochondria. Moreover, unlike process surveillance, mitochondrial motility does not change with animal anesthesia. Likewise, the processes that acutely chemoattract to a lesion site or initially engage with a neuron undergoing programmed cell death do not contain mitochondria. Rather, microglia mitochondria have a delayed arrival into the responding cell processes. Thus, there is subcellular heterogeneity of mitochondrial partitioning and asymmetry between mitochondrial localization and cell process motility or acute damage responses.

Microglia mitochondrial complex I deficiency during development induces glial dysfunction and early lethality

Primary mitochondrial diseases (PMDs) are associated with pediatric neurological disorders and are traditionally related to oxidative phosphorylation system (OXPHOS) defects in neurons. Interestingly, both PMD mouse models and patients with PMD show gliosis, and pharmacological depletion of microglia, the innate immune cells of the brain, ameliorates multiple symptoms in a mouse model. Given that microglia activation correlates with the expression of OXPHOS genes, we studied whether OXPHOS deficits in microglia may contribute to PMDs. We first observed that the metabolic rewiring associated with microglia stimulation in vitro (via IL-33 or TAU treatment) was partially changed by complex I (CI) inhibition (via rotenone treatment). In vivo, we generated a mouse model deficient for CI activity in microglia (MGcCI). MGcCI microglia showed metabolic rewiring and gradual transcriptional activation, which led to hypertrophy and dysfunction in juvenile (1-month-old) and adult (3-month-old) stages, respectively. MGcCI mice presented widespread reactive astrocytes, a decrease of synaptic markers accompanied by an increased number of parvalbumin neurons, a behavioral deficit characterized by prolonged periods of immobility, loss of weight and premature death that was partially rescued by pharmacologic depletion of microglia. Our data demonstrate that microglia development depends on mitochondrial CI and suggest a direct microglial contribution to PMDs.