Integrin CD11b Deficiency Aggravates Retinal Microglial Activation and RGCs Degeneration After Acute Optic Nerve Injury

Ablation of the Integrin CD11b Mac-1 Limits Deleterious Responses to Traumatic Spinal Cord Injury and Improves Functional Recovery in Mice

Spinal cord injury (SCI) triggers microglial/monocytes activation with distinct pro-inflammatory or inflammation-resolving phenotypes, which potentiate tissue damage or facilitate functional repair, respectively. The major integrin Mac-1 (CD11b/CD18), a heterodimer consisting of CD11b and CD18 chains, is expressed in multiple immune cells of the myeloid lineage. Here, we examined the effects of CD11b gene ablation in neuroinflammation and functional outcomes after SCI. qPCR analysis of C57BL/6 female mice showed upregulation of CD11b mRNA starting from 1 d after injury, which persisted up to 28 d. CD11b knockout (KO) mice and their wildtype littermates were subjected to moderate SCI. At 1 d post-injury, qPCR showed increased expression of genes involved with inflammation-resolving processes in CD11b KO mice. Flow cytometry analysis of CD45intLy6C-CX3CR1+ microglia, CD45hiLy6C+Ly6G- monocytes, and CD45hiLy6C+Ly6G+ neutrophils revealed significantly reduced cell counts as well as reactive oxygen species (ROS) production in CD11b KO mice at d3 post-injury. Further examination with NanoString and RNA-seq showed upregulation of pro-inflammatory genes, but downregulation of the ROS pathway. Importantly, CD11b KO mice exhibited significantly improved locomotor function, reduced cutaneous mechanical/thermal hypersensitivity, and limited tissue damage at 8 weeks post-injury. Collectively, our data suggest an important role for CD11b in regulating tissue inflammation and functional outcome following SCI.

The integrin CD11b inhibits MSU-induced NLRP3 inflammasome activation in macrophages and protects mice against MSU-induced joint inflammation

OBJECTIVE

In gout, monosodium urate crystals are taken up by macrophages, triggering the activation of the NLRP3 inflammasome and the maturation of IL-1β. This study aimed to investigate the role of integrin CD11b in inflammasome activation in macrophages stimulated by MSU.

METHODS

BMDM from WT and CD11b KO mice were stimulated in vitro with MSU crystals. Cellular supernatants were collected to assess the expression of the inflammatory cytokines by enzyme-linked immunosorbent assay and western blot methods. The role of integrin CD11b in MSU-induced gouty arthritis in vivo was investigated by intra-articular injection of MSU crystals. Real-time extracellular acidification rate and oxygen consumption rate of BMDMs were measured by Seahorse Extracellular Flux Analyzer.

RESULTS

We demonstrate that CD11b-deficient mice developed exacerbated gouty arthritis with increased recruitment of leukocytes in the joint and higher IL-1β levels in the sera. In macrophages, genetic deletion of CD11b induced a shift of macrophage metabolism from oxidative phosphorylation to glycolysis, thus decreasing the overall generation of intracellular ATP. Upon MSU stimulation, CD11b-deficient macrophages showed an exacerbated secretion of IL-1β. Treating wild-type macrophages with a CD11b agonist, LA1, inhibited MSU-induced release of IL-1β in vitro and attenuated the severity of experimental gouty arthritis. Importantly, LA1, was also effective in human cells as it inhibited MSU-induced release of IL-1β by peripheral blood mononuclear cells from healthy donors.

CONCLUSION

Our data identified the CD11b integrin as a principal cell membrane receptor that modulates NLRP3 inflammasome activation by MSU crystal in macrophages, which could be a potential therapeutic target to treat gouty arthritis in human patients.

Effect of astrocyte GPER on the optic nerve inflammatory response following optic nerve injury in mice

Activated astrocytes are a primary source of inflammatory factors following traumatic optic neuropathy (TON). Accumulation of inflammatory factors in this context leads to increased axonal damage and loss of retinal ganglion cells (RGCs). Therefore, in the present study, we explored the role of the astrocyte G protein-coupled estrogen receptor (GPER) in regulating inflammatory factors following optic nerve crush (ONC), and analyzed its potential regulatory mechanisms. Overall, our results showed that GPER was abundantly expressed in the optic nerve, and co-localized with glial fibrillary acidic proteins (GFAP). Exogenous administration of G-1 led to a significant reduction in astrocyte activation and expression of inflammation-related factors (including IL-1β, TNF-α, NFκB, and p-NFκB). Additionally, it dramatically increased the survival of RGCs. In contrast, astrocytes were activated to a greater extent by exogenous G15 administration; however, RGCs survival was significantly reduced. In vitro, GPER activation significantly reduced astrocyte activation and the release of inflammation-related factors. In conclusion, activation of astrocyte GPER significantly reduced ONC inflammation levels, and should be explored as a potential target pathway for protecting the optic nerve and RGCs after TON.

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.

Retinal cell-targeted liposomal ginsenoside Rg3 attenuates retinal ischemia-reperfusion injury via alleviating oxidative stress and promoting microglia/macrophage M2 polarization

Retinal ischemia-reperfusion (RIR) injury remains a major challenge that is detrimental to retinal cell survival in a variety of ocular diseases. However, current clinical treatments focus on a single pathological mechanism, making them unable to provide comprehensive retinal protection. A variety of natural products including ginsenoside Rg3 (Rg3) exhibit potent antioxidant and anti-inflammatory activities. Unfortunately, the hydrophobicity of Rg3 and the presence of various intraocular barriers limit its effective application in clinical settings. Hyaluronic acid (HA)- specifically binds to cell surface receptors, CD44, which is widely expressed in retinal pigment epithelial cells and M1-type macrophage. Here, we developed HA-decorated liposomes loaded with Rg3, termed Rg3@HA-Lips, to protect against retinal damage caused by RIR injury. Treatment with Rg3@HA-Lips significantly inhibited the oxidative stress induced by RIR injury. In addition, Rg3@HA-Lips promoted the transition of M1-type macrophage to the M2 type, ultimately reversing the pro-inflammatory microenvironment. The mechanism of Rg3@HA-Lips was further investigated and found that they can regulateSIRT/FOXO3a, NF-κB and STAT3 signaling pathways. Together with as well demonstrated good safety profiles, this CD44-targeted platform loaded with a natural product alleviates RIR injury by modulating the retinal microenvironment and present a potential clinical treatment strategy.

Physcion prevents induction of optic nerve injury in rats via inhibition of the JAK2/STAT3 pathway

Optic nerve injury is a type of neurodegenerative disease. Physcion is an anthraquinone that exerts a protective role against various diseases. However, its function in regulating optic nerve injury remains largely unknown. An in vitro model of optic nerve injury was established in HAPI cells treated with IFN-β. Functional assays were used to detect HAPI cell viability and apoptosis. The levels of inflammation and the expression levels of oxidative stress-related genes were measured in HAPI cells. In addition, western blot analysis was used to detect the expression levels of Janus kinase 2 (JAK2)/STAT3-linked genes in HAPI cells. Treatment of the cells with physcion prevented cells against IFN-β-induced neuronal injury. Physcion restrained IFN-β-induced inflammatory response and oxidative stress in HAPI cells. In addition, it improved IFN-β-induced injury in HAPI cells by suppressing the JAK2/STAT3 pathway. In conclusion, the present study revealed that physcion improved optic nerve injury in vitro by inhibiting the JAK2/STAT3 pathway. Physcion may be a promising therapeutic target for the treatment of this disease.

Microglial CD11b Knockout Contributes to Axonal Debris Clearance and Axonal Degradation Attenuation via IGF-1 After Acute Optic Nerve Injury

Purpose

Microglial clearance of axonal debris is an essential response for management of traumatic optic neuropathy. Inadequate removal of axonal debris leads to increased inflammation and axonal degeneration after traumatic optic neuropathy. The present study investigated the role of CD11b (Itgam) in axonal debris clearance and axonal degeneration.

Methods

Western blot and immunofluorescence were used to detect CD11b expression in the mouse optic nerve crush (ONC) model. Bioinformatics analysis predicted the possible role of CD11b. Cholera toxin subunit B (CTB) and zymosan were used to assay phagocytosis by microglia in vivo and in vitro, respectively. CTB was also used to label functionally intact axons after ONC.

Results

CD11b is abundantly expressed after ONC and participates in phagocytosis. Microglia from Itgam-/- mice exhibited more significant phagocytosis of axonal debris than wild-type microglia. In vitro experiments confirmed that the CD11b gene defect in M2 microglia leads to increased insulin-like growth factor-1 secretion and thus promotes phagocytosis. Lastly, following ONC, Itgam-/- mice exhibited elevated expression of neurofilament heavy peptide and Tuj1, along with more intact CTB-labeled axons when compared with wild-type mice. Moreover, the inhibition of insulin-like growth factor-1 decreased CTB labeling in Itgam-/- mice after injury.

Conclusions

CD11b limits microglial phagocytosis of axonal debris in traumatic optic neuropathy, as demonstrated by increased phagocytosis with CD11b knockout. The inhibition of CD11b activity may be a novel approach to promote central nerve repair.

ITGB4 deficiency in airway epithelia enhances HDM-induced airway inflammation through hyperactivation of TLR4 signaling pathway

Airway epithelial cells (AECs) are the first cell barrier of the respiratory system against external stimuli that play a critical role in the development of asthma. It is known that AECs play a key role in asthma susceptibility and severity. ITGB4 is a downregulated adhesion molecule in the airway epithelia of asthma patients, which was involved in the exaggerated lung inflammation after allergy stimulation. Toll-like receptor 4 (TLR4) in AECs has also been shown to play a crucial role in the development of lung inflammation in asthma patients. However, the specific intrinsic regulatory mechanism of TLR4 in AECs are still obscure. In this article, we demonstrated that ITGB4 deficiency in AECs enhances HDM-induced airway inflammation through hyperactivation of the TLR4 signaling pathway, which is mediated by inhibition of FYN phosphorylation. Moreover, TLR4-antagonist treatment or blockade of FYN can inhibit or exaggerate lung inflammation in HDM-stressed ITGB4-deficient mice, separately. Together, these results demonstrated that ITGB4 deficiency in AECs enhances HDM-induced lung inflammatory response through the ITGB4-FYN-TLR4 axis, which may provide new therapeutic approaches for the management of lung inflammation in asthma.

Establishment of a minimally invasive distal traumatic optic neuropathy model in mice to investigate cascade reactions of retinal glial cells

Traumatic optic neuropathy (TON) is a complication of craniocerebral, orbital and facial injuries, leading to irreversible vision loss. At present, there is no reliable, widely used animal model, although it has been confirmed that TON can cause the loss of retinal ganglion cells (RGC). However, the cascade reaction of retinal glial cells underlying TON is unclear. Therefore, the establishment of an animal model to explore the pathological mechanism of TON would be of great interest to the scientific community. In this study, we propose a novel mouse model utilizing a 3D stereotaxic apparatus combined with a 27G needle to evaluate damage to the optic nerve by micro-CT, anatomy, SD-OCT and F-VEP. Immunofluorescence, western blotting, qPCR experiments were conducted to investigate the loss of RGCs and activation or inactivation of microglia, astrocytes and Müller glial cells in the retina from the first week to the fourth week after modeling. The results showed that this minimally invasive method caused damage to the distal optic nerve and loss of RGC after optic nerve injury. Microglia cells were found to be activated from the first week to the third week; however, they were inactivated at the fourth week; astrocytes were activated at the second week of injury, while Müller glial cells were gradually inactivated following injury. In conclusion, this method can be used as a novel animal model of distal TON, that results in a series of cascade reactions of retinal glial cells, which will provide a basis for future studies aimed at exploring the mechanism of TON and the search for effective treatment methods.

Anti-inflammatory effect of glucagon-like Peptide-1 receptor agonist on the neurosensory retina in an acute optic nerve injury rat model

PURPOSE

To explore the possibility of using glucagon-like peptide-1 receptor agonist (GLP-1RA) as a new treatment for neuroinflammation, by analyzing retinal pathological changes in an optic nerve crush rat model.

METHODS

Eight-week-old male Sprague-Dawley rats were divided into lixisenatide (LIX, n = 10), traumatic control (T-CON, n = 10), and normal control (n = 5) groups. The optic nerves of left eyes in the LIX and T-CON groups were crushed in a standardized manner. The LIX group was treated with subcutaneous injections of lixisenatide (200 μg/kg/day) for 5 days. One week after initiating treatment, quantitative polymerase chain reaction, Western blot, and immunohistochemistry analyses were performed on the retinal tissues of each group to identify inflammatory markers.

RESULTS

The LIX group showed significantly lower mRNA levels of interleukin 1 beta (IL-1β), tumor necrosis factor-alpha (TNF-α), thioredoxin interacting protein (TXNIP), and glial fibrillary acidic protein (GFAP) than the T-CON group. Also, the LIX group exhibited decreased TXNIP and GFAP expression compared with the T-CON group, and similar expression to the normal control group, according to Western blot analysis. Significantly increased immunohistochemistry staining of Brn3a and decreased TUNEL staining were seen in the LIX group compared with the T-CON group, indicating that lixisenatide contributes to retinal ganglion cell survival in cases of acute optic nerve injury.

CONCLUSIONS

Neuroinflammation was significantly reduced in lixisenatide-treated retinas compared with untreated retinas in our acute optic nerve injury rat model. The neuroprotective effect of lixisenatide indicates that it can serve a new treatment option against clinically intractable traumatic optic neuropathy.

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

Purpose

Optic nerve injury (ONI) causes neuroinflammation and neurodegeneration leading to visual deficits. The response of microglia has emerged as an impactful component of etiology in neurodegeneration. This study aimed to investigate the effect of SIRT1-mTORC1 signaling pathway in microglia regulation after ONI.

Methods

Cx3Cr1-Cre^ERT2/Raptor^F/F and Cx3Cr1-Cre^ERT2/Sirt1^F/F mice were used to delete Raptor and Sirt1 in microglia, respectively. Optic nerve crush (ONC) model was established to mimic ONI. PLX5622, a highly specific inhibitor of the colony-stimulating factor 1 receptor (CSF1R), is used to eliminate microglia in optic nerve. Ionized calcium binding adaptor molecule 1 (Iba1) immunostaining was used to detect microglial activation. Retinal ganglion cells (RGCs) were quantified by Nissl staining and retinal whole-mount immunostaining with RNA-binding protein with multiple splicing (RBPMS). Axonal damage was valued by transmission electron microscopy (TEM).

Results

Microglial activation emerged on day 3 post ONC and was earlier than RGCs loss which occurred at day 5 after injury. Depleting microglia with PLX5622 could attenuate the loss of RGCs and axon damage after ONC. Gain- and loss-of-function studies revealed that SIRT1 determined the activation of microglia in optic nerve. In addition, microglia-specific deletion of Raptor resulted in decreased microglial activation. Interestingly, activating mTORC1 with CCT007093 could reverse the function of SIRT1 in regulating the process of microglial activation mediated RGCs loss.

Conclusion

Our study reveals a potential novel mechanism of SIRT1-mTORC1 pathway in microglia regulation, and indicates a therapeutic potential for the protection of RGCs in ONI.

Retinal Ganglion Cell Axon Regeneration Requires Complement and Myeloid Cell Activity within the Optic Nerve

Axon regenerative failure in the mature CNS contributes to functional deficits following many traumatic injuries, ischemic injuries, and neurodegenerative diseases. The complement cascade of the innate immune system responds to pathogen threat through inflammatory cell activation, pathogen opsonization, and pathogen lysis, and complement is also involved in CNS development, neuroplasticity, injury, and disease. Here, we investigated the involvement of the classical complement cascade and microglia/monocytes in CNS repair using the mouse optic nerve injury (ONI) model, in which axons arising from retinal ganglion cells (RGCs) are disrupted. We report that central complement C3 protein and mRNA, classical complement C1q protein and mRNA, and microglia/monocyte phagocytic complement receptor CR3 all increase in response to ONI, especially within the optic nerve itself. Importantly, genetic deletion of C1q, C3, or CR3 attenuates RGC axon regeneration induced by several distinct methods, with minimal effects on RGC survival. Local injections of C1q function-blocking antibody revealed that complement acts primarily within the optic nerve, not retina, to support regeneration. Moreover, C1q opsonizes and CR3+ microglia/monocytes phagocytose growth-inhibitory myelin debris after ONI, a likely mechanism through which complement and myeloid cells support axon regeneration. Collectively, these results indicate that local optic nerve complement-myeloid phagocytic signaling is required for CNS axon regrowth, emphasizing the axonal compartment and highlighting a beneficial neuroimmune role for complement and microglia/monocytes in CNS repair.SIGNIFICANCE STATEMENT Despite the importance of achieving axon regeneration after CNS injury and the inevitability of inflammation after such injury, the contributions of complement and microglia to CNS axon regeneration are largely unknown. Whereas inflammation is commonly thought to exacerbate the effects of CNS injury, we find that complement proteins C1q and C3 and microglia/monocyte phagocytic complement receptor CR3 are each required for retinal ganglion cell axon regeneration through the injured mouse optic nerve. Also, whereas studies of optic nerve regeneration generally focus on the retina, we show that the regeneration-relevant role of complement and microglia/monocytes likely involves myelin phagocytosis within the optic nerve. Thus, our results point to the importance of the innate immune response for CNS repair.

Axonal Injuries Cast Long Shadows: Long Term Glial Activation in Injured and Contralateral Retinas after Unilateral Axotomy

Background: To analyze the course of microglial and macroglial activation in injured and contralateral retinas after unilateral optic nerve crush (ONC). Methods: The left optic nerve of adult pigmented C57Bl/6 female mice was intraorbitally crushed and injured, and contralateral retinas were analyzed from 1 to 45 days post-lesion (dpl) in cross-sections and flat mounts. As controls, intact retinas were studied. Iba1⁺ microglial cells (MCs), activated phagocytic CD68⁺MCs and M2 CD206⁺MCs were quantified. Macroglial cell changes were analyzed by GFAP and vimentin signal intensity. Results: After ONC, MC density increased significantly from 5 to 21 dpl in the inner layers of injured retinas, remaining within intact values in the contralateral ones. However, in both retinas there was a significant and long-lasting increase of CD68⁺MCs. Constitutive CD206⁺MCs were rare and mostly found in the ciliary body and around the optic-nerve head. While in the injured retinas their number increased in the retina and ciliary body, in the contralateral retinas decreased. Astrocytes and Müller cells transiently hypertrophied in the injured retinas and to a lesser extent in the contralateral ones. Conclusions: Unilateral ONC triggers a bilateral and persistent activation of MCs and an opposed response of M2 MCs between both retinas. Macroglial hypertrophy is transient.

Semaphorin3A increases M1-like microglia and retinal ganglion cell apoptosis after optic nerve injury

Background

The mechanisms leading to retinal ganglion cell (RGC) death after optic nerve injury have not been fully elucidated. Current evidence indicates that microglial activation and M1- and M2-like dynamics may be an important factor in RGC apoptosis after optic nerve crush (ONC). Semaphorin3A (Sema3A) is a classic axonal guidance protein,which has been found to have a role in neuroinflammation processes. In this study, we investigated the contribution of microglial-derived Sema3A to progressive RGC apoptosis through regulating paradigm of M1- and M2-like microglia after ONC.

Method

A mouse ONC model and a primary microglial-RGC co-culture system were used in the present study. The expression of M1- and M2-like microglial activation markers were assessed by real-time polymerase chain reaction (RT-qPCR). Histological and Western blot (WB) analyses were used to investigate the polarization patterns of microglia transitions and the levels of Sema3A. RGC apoptosis was investigated by TUNEL staining and caspase-3 detection.

Results

Levels of Sema3A in the mouse retina increased after ONC. Treatment of mice with the stimulating factor 1 receptor antagonist PLX3397 resulted in a decrease of retinal microglia. The levels of CD16/32 (M1) were up-regulated at days 3 and 7 post-ONC. However, CD206 (M2) declined on day 7 after ONC. Exposure to anti-Sema3A antibodies (anti-Sema3A) resulted in a decrease in the number of M1-like microglia, an increase in the number of M2-like microglia, and the amelioration of RGC apoptosis.

Conclusions

An increase in microglia-derived Sema3A in the retina after ONC partially leads to a continuous increase of M1-like microglia and plays an important role in RGC apoptosis. Inhibition of Sema3A activity may be a novel approach to the prevention of RGC apoptosis after optic nerve injury.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00603-7.

The critical role of the hippocampal NLRP3 inflammasome in social isolation-induced cognitive impairment in male mice

Early life stress exerts detrimental effects on cognitive function, but the mechanism by which this occurs is unknown. The NLRP3 inflammasome-mediated inflammatory response has emerged as a prominent contributor to cognitive impairment induced by chronic stress. In the present study, we showed that 8-week chronic social isolation (SI) led to cognitive impairment in mice, remarkably increasing expression of the hippocampal NLRP3 inflammasome. Furthermore, the 8-week SI procedure significantly increased the levels of hippocampal IL-1β and IL-18 without significant alteration of the level of serum IL-1β, suggesting a central mechanism for IL-1β-related CNS inflammation. Moreover, inflammatory microglial and expression of AMPAR were reduced in the hippocampus of SI mice. Minocycline is an antibiotic that limits microglia responses, and previous study also showed that minocycline could prevent stress-induced pro-inflammatory cytokine expression in the brain. Our experiment found that minocycline improved cognitive behavior in SI mice. Minocycline also prevented expression of the hippocampal NLRP3 inflammasome, indicating that microglia might be the primary contributor to SI-induced hippocampal NLRP3 inflammasome activation. Furthermore, alterations in SI mice were also restored by chronic treatment with the NLRP3 inhibitor MCC950. These results indicate that the microglia-derived NLRP3 inflammasome may be primarily involved in the inflammatory response to social isolation and that specific NLRP3 inflammasome inhibition using MCC950 may represent a promising therapeutic approach for early stress induced cognitive impairment.