Modulation of Sirt1-mTORC1 Pathway in Microglia Attenuates Retinal Ganglion Cell Loss After Optic Nerve Injury

Rethinking the role of microglia in obesity

Microglia are the macrophages of the central nervous system (CNS), implying their role in maintaining brain homeostasis. To achieve this, these cells are sensitive to a plethora of endogenous and exogenous signals, such as neuronal activity, cellular debris, hormones, and pathological patterns, among many others. More recent research suggests that microglia are highly responsive to nutrients and dietary variations. In this context, numerous studies have demonstrated their significant role in the development of obesity under calorie surfeit. Because many reviews already exist on this topic, we have chosen to present the state of our reflections on various concepts put forth in the literature, bringing a new perspective whenever possible. Our literature review focuses on studies conducted in the arcuate nucleus of the hypothalamus, a key structure in the control of food intake. Specifically, we present the recent data available on the modifications of microglial energy metabolism following the consumption of an obesogenic diet and their consequences on hypothalamic neuron activity. We also highlight the studies unraveling the mechanisms underlying obesity-related sexual dimorphism. The review concludes with a list of questions that remain to be addressed in the field to achieve a comprehensive understanding of the role of microglia in the regulation of body energy metabolism. This article is part of the Special Issue on "Microglia".

Neutrophil-inflicted vasculature damage suppresses immune-mediated optic nerve regeneration

In adult mammals, injured retinal ganglion cells (RGCs) fail to spontaneously regrow severed axons, resulting in permanent visual deficits. Robust axon growth, however, is observed after intra-ocular injection of particulate β-glucan isolated from yeast. Blood-borne myeloid cells rapidly respond to β-glucan, releasing numerous pro-regenerative factors. Unfortunately, the pro-regenerative effects are undermined by retinal damage inflicted by an overactive immune system. Here, we demonstrate that protection of the inflamed vasculature promotes immune-mediated RGC regeneration. In the absence of microglia, leakiness of the blood-retina barrier increases, pro-inflammatory neutrophils are elevated, and RGC regeneration is reduced. Functional ablation of the complement receptor 3 (CD11b/integrin-αM), but not the complement components C1q-/- or C3-/-, reduces ocular inflammation, protects the blood-retina barrier, and enhances RGC regeneration. Selective targeting of neutrophils with anti-Ly6G does not increase axogenic neutrophils but protects the blood-retina barrier and enhances RGC regeneration. Together, these findings reveal that protection of the inflamed vasculature promotes neuronal regeneration.

Cell-free fat extract promotes axon regeneration and retinal ganglion cells survival in traumatic optic neuropathy

Injuries to axons within the central nervous system (CNS) pose a substantial clinical challenge due to their limited regenerative capacity. This study investigates the therapeutic potential of Cell-free fat extract (CEFFE) in CNS injury. CEFFE was injected intravitreally after the optic nerve was crushed. Two weeks post-injury, quantification of regenerated axons and survival rates of retinal ganglion cells (RGCs) were performed. Subsequently, comprehensive gene ontology (GO) an-notation elucidated the cellular origins and functional attributes of CEFFE components. Molecular mechanisms underlying CEFFE's therapeutic effects were explored through Western blotting (WB). Additionally, levels of inflammatory factors within CEFFE were determined using enzyme-linked immunosorbent assay (ELISA), and histological staining of microglia was conducted to assess its impact on neuroinflammation. CEFFE demonstrated a significant capacity to promote axon re-generation and enhance RGCs survival. GO annotation revealed the involvement of 146 proteins within CEFFE in axonogenesis and neurogenesis. WB analysis unveiled the multifaceted pathways through which CEFFE exerts its therapeutic effects. Elevated levels of inflammatory factors were detected through ELISA, and CEFFE exhibited a modulatory effect on microglial activation in the retinal tissue following optic nerve crush (ONC). The present study highlights the therapeutic promise of CEFFE in the management of CNS injuries, exemplified by its ability to foster axon regeneration and improve RGCs survival.

Astragaloside IV against Alzheimer's disease via microglia-mediated neuroinflammation using network pharmacology and experimental validation

Alzheimer's disease (AD) is one of the most prevalent neurodegenerative diseases in the world. The effective therapeutic methods and drugs are still not clear. Astragaloside IV (AS-IV), a triterpenoid saponin isolated from the root of Huangqi, has a beneficial effect in the treatment of AD. However, whether AS-IV alters microglia in the inflammation of AD is still ambiguous. In our study, 99 common targets were collected between AS-IV and AD. BCL2 apoptosis regulator (Bcl-2), pro-apoptotic BCL-2 protein BAX, epidermal growth factor receptor (EGFR), and receptor tyrosine phosphatase type C (PTPRC) were screened for inflammation and microglia in the above targets by network pharmacology. Interleukin-1β (IL-1β) and EGFR both interact with signal transducer and activator of transcription 3 (STAT3) by a protein interaction network, and IL-1β had a higher affinity for AS-IV based on molecular docking. Enrichment revealed targets involved in the regulation of neuronal cell bodies, growth factor receptor binding, EGFR tyrosine kinase inhibitor resistance., etc. Besides, AS-IV alleviated the reduced cell proliferation in amyloid-beta (Aβ)-treated microglial BV2 cells. AS-IV affected BV2 cell morphological changes and decreased cluster of differentiation 11b (CD11b) gene, IL-1β, and EGFR mRNA levels increment during lipopolysaccharide (LPS) injury in BV2 cell activation. Therefore, AS-IV may regulate microglial activation and inflammation via EGFR-dependent pathways in AD. EGFR and IL-1β are vital targets that may relate to each other to coregulate downstream molecular functions in the cure of AD. Our study provides a candidate drug and disease target for the treatment of neurodegenerative diseases in the clinic.

Microglia mediated neuroinflammation in neurodegenerative diseases: A review on the cell signaling pathways involved in microglial activation

Microglia, the immune sentinels of the central nervous system (CNS), have emerged to be the central players in many neurological and neurodegenerative diseases. Recent studies on large genome databases and omics studies in fact provide support to the idea that microglial cells could be the drivers of these diseases. Microglial cells have the capacity to undergo morphological and phenotypic transformations depending on its microenvironment. From the homeostatic ramified state, they can shift their phenotypes between the two extremes, known as the proinflammatory M1 and anti-inflammatory M2 phenotype, with intermediate transitional states, characterized by different transcriptional signature and release of inflammatory mediators. The temporal regulation of the release of the inflammatory factors are critical for damage control and steering the microglia back towards homeostatic conditions. A dysregulation in these can lead to excessive tissue damage and neuronal death. Therefore, targeting the cell signaling pathways that are the underpinnings of microglial modulations are considered to be an important avenue for treatment of various neurodegenerative diseases. In this review we have discussed various signaling pathways that trigger microglial activation from its ramified state and highlight the mechanisms of microglia-mediated neuroinflammation that are associated with various neurodegenerative diseases. Most of the cellular factors that drive microglia towards a proinflammatory phenotype are components of the immune system signaling pathways and cell proliferation, along with certain ion channels. The anti-inflammatory phenotype is mainly elicited by purinoceptors, metabolic receptors and other receptors that primarily suppress the production proinflammatory mediators.

Microglial SIRT1 activation attenuates synapse loss in retinal inner plexiform layer via mTORC1 inhibition

BACKGROUND

Optic nerve injury (ONI) is a key cause of irreversible blindness and triggers retinal ganglion cells (RGCs) change and synapse loss. Microglia is the resistant immune cell in brain and retina and has been demonstrated to be highly related with neuron and synapse injury. However, the function of Sirtuin 1 (SIRT1), a neuroprotective molecule, in mediating microglial activation, retinal synapse loss and subsequent retinal ganglion cells death in optic nerve injury model as well as the regulatory mechanism remain unclear.

METHOD

To this end, optic nerve crush (ONC) model was conducted to mimic optic nerve injury. Resveratrol and EX527, highly specific activator and inhibitor of SIRT1, respectively, were used to explore the function of SIRT1 in vivo and vitro. Cx3Cr1-CreERT2/RaptorF/F mice were used to delete Raptor for inhibiting mammalian target of rapamycin complex 1 (mTORC1) activity in microglia. HEK293 and BV2 cells were transfected with plasmids to explore the regulatory mechanism of SIRT1.

RESULTS

We discovered that microglial activation and synapse loss in retinal inner plexiform layer (IPL) occurred after optic nerve crush, with later-development retinal ganglion cells death. SIRT1 activation induced by resveratrol inhibited microglial activation and attenuated synapse loss and retinal ganglion cells injury. After injury, microglial phagocytosed synapse and SIRT1 inhibited this process to protect synapse and retinal ganglion cells. Moreover, SIRT1 exhibited neuron protective effects via activating tuberous sclerosis complex 2 (TSC2) through deacetylation, and enhancing the inhibition effect of tuberous sclerosis complex 2 on mammalian target of rapamycin complex 1 activity.

CONCLUSION

Our research provides novel insights into microglial SIRT1 in optic nerve injury and suggests a potential strategy for neuroprotective treatment of optic nerve injury disease.

Oxidative Stress: A Suitable Therapeutic Target for Optic Nerve Diseases?

Optic nerve disorders encompass a wide spectrum of conditions characterized by the loss of retinal ganglion cells (RGCs) and subsequent degeneration of the optic nerve. The etiology of these disorders can vary significantly, but emerging research highlights the crucial role of oxidative stress, an imbalance in the redox status characterized by an excess of reactive oxygen species (ROS), in driving cell death through apoptosis, autophagy, and inflammation. This review provides an overview of ROS-related processes underlying four extensively studied optic nerve diseases: glaucoma, Leber's hereditary optic neuropathy (LHON), anterior ischemic optic neuropathy (AION), and optic neuritis (ON). Furthermore, we present preclinical findings on antioxidants, with the objective of evaluating the potential therapeutic benefits of targeting oxidative stress in the treatment of optic neuropathies.

Melatonin protects against NMDA-induced retinal ganglion cell injury by regulating the microglia-TNFα-RGC p38 MAPK pathway

Glaucoma, one of the most common ocular neurodegenerative diseases worldwide, is characterized by retinal ganglion cell (RGC) loss. There is a large body of literature that describes the neuroprotective role of melatonin against neurodegenerative diseases by regulating neuroinflammation, although the exact mechanism through which melatonin acts on RGC is still uncertain. This study assessed the protective effects of melatonin using a NMDA-induced RGC injury model, and studied the possible mechanisms involved in this process. Melatonin promoted RGC survival, improved retinal function, and inhibited the apoptosis and necrosis of retinal cells. To understand the mechanism of the neuroprotective effects of melatonin on RGC, microglia and inflammation-related pathways were assessed after melatonin administration and microglia ablation. Melatonin promoted RGC survival by suppressing microglia-derived proinflammatory cytokines, in particular TNFα, which in turn inhibited the activation of p38 MAPK pathway. Inhibiting TNFα or manipulating p38 MAPK pathway protected damaged RGC. Our results suggest that melatonin protects against NMDA-induced RGC injury by inhibiting the microglial TNFα-RGC p38 MAPK pathway. It should be considered a candidate neuroprotective therapy against retinal neurodegenerative diseases.

Rapamycin suppresses neuroinflammation and protects retinal ganglion cell loss after optic nerve crush

Pyroptosis, an inflammasome-mediated mode of death, plays an important role in glaucoma. It has been shown that regulating the mTOR pathway can inhibit pyroptosis. Unfortunately, whether rapamycin (RAPA), a specific inhibitor of the mTOR pathway, can inhibit optic nerve crush (ONC)-induced pyroptosis to protect retinal ganglion cells (RGCs) has not been investigated. Our research aimed to confirm the effect of intravitreal injection of RAPA on RGCs. Furthermore, we used the ONC model to explore the underlying mechanisms. First, we observed that intravitreal injection of RAPA alleviated RGC damage induced by various types of injury. We then used the ONC model to further explore the potential mechanism of RAPA. Mechanistically, RAPA not only reduced the activation of glial cells in the retina but also inhibited retinal pyroptosis-induced expression of inflammatory factors such as nucleotide-binding oligomeric domain-like receptor 3 (NLRP3), apoptosis-associated speckle-like protein containing a CARD (ASC), N-terminal of gasdermin-D (GSDMD-N), IL-18 and IL-1β. Moreover, RAPA exerted protective effects on RGC axons, possibly by inhibiting glial activation and regulating the mTOR/ROCK pathway. Therefore, this study demonstrates a novel mechanism by which RAPA protects against glaucoma and provides further evidence for its application in preclinical studies.

Neuroinflammation, Microglia and Implications for Retinal Ganglion Cell Survival and Axon Regeneration in Traumatic Optic Neuropathy

Traumatic optic neuropathy (TON) refers to a pathological condition caused by a direct or indirect insult to the optic nerves, which often leads to a partial or permanent vision deficit due to the massive loss of retinal ganglion cells (RGCs) and their axonal fibers. Retinal microglia are immune-competent cells residing in the retina. In rodent models of optic nerve crush (ONC) injury, resident retinal microglia gradually become activated, form end-to-end alignments in the vicinity of degenerating RGC axons, and actively internalized them. Some activated microglia adopt an amoeboid morphology that engulf dying RGCs after ONC. In the injured optic nerve, the activated microglia contribute to the myelin debris clearance at the lesion site. However, phagocytic capacity of resident retinal microglia is extremely poor and therefore the clearance of cellular and myelin debris is largely ineffective. The presence of growth-inhibitory myelin debris and glial scar formed by reactive astrocytes inhibit the regeneration of RGC axons, which accounts for the poor visual function recovery in patients with TON. In this Review, we summarize the current understanding of resident retinal microglia in RGC survival and axon regeneration after ONC. Resident retinal microglia play a key role in facilitating Wallerian degeneration and the subsequent axon regeneration after ONC. However, they are also responsible for producing pro-inflammatory cytokines, chemokines, and reactive oxygen species that possess neurotoxic effects on RGCs. Intraocular inflammation triggers a massive influx of blood-borne myeloid cells which produce oncomodulin to promote RGC survival and axon regeneration. However, intraocular inflammation induces chronic neuroinflammation which exacerbates secondary tissue damages and limits visual function recovery after ONC. Activated retinal microglia is required for the proliferation of oligodendrocyte precursor cells (OPCs); however, sustained activation of retinal microglia suppress the differentiation of OPCs into mature oligodendrocytes for remyelination after injury. Collectively, controlled activation of retinal microglia and infiltrating myeloid cells facilitate axon regeneration and nerve repair. Recent advance in single-cell RNA-sequencing and identification of microglia-specific markers could improve our understanding on microglial biology and to facilitate the development of novel therapeutic strategies aiming to switch resident retinal microglia’s phenotype to foster neuroprotection.