Pharmacological depletion of microglia alleviates neuronal and vascular damage in the diabetic CX3CR1-WT retina but not in CX3CR1-KO or hCX3CR1I249/M280-expressing retina

Progerin mRNA Is Associated with Smoking and Signs of Increased Microvascular Damage in Patients with Diabetic Macular Edema

The premature aging disease Hutchinson-Gilford Syndrome (HGPS) is caused by defined mutations in the LMNA gene, resulting in the activation of a cryptic splice donor site, which leads to a defective truncated prelamin A protein called progerin. Notably, progerin expression has also been detected in non-mutated healthy individuals, and therefore, its involvement in the physiological aging process has been widely discussed. Since diabetes mellitus is associated with premature aging and increased cardiovascular mortality, we aimed to investigate the role of progerin expression in patients with diabetic retinopathy (DR). mRNA expression of progerin was analyzed in blood samples from 140 patients with DR who received anti-vascular endothelial growth factor (VEGF) therapy. Progerin mRNA levels were significantly lower in female compared to male patients (n = 42 vs. n = 98; 0.67 ± 0.19 vs. 0.89 ± 0.51, p = 0.006) and higher in patients with non-proliferative (NP)DR (n = 87 vs. n = 53; 0.9 ± 0.51 vs. 0.71 ± 0.29, p = 0.013) compared to those with proliferative (P)DR. Additionally, a positive correlation was found between progerin mRNA expression and the number of intravitreal anti-VEGF applications (n = 139, r = 0.21, p = 0.015), central macula thickness (CMT), (n = 137, r = 0.18, p = 0.036) and nicotine consumption (n = 105, r = 0.235, p = 0.002). The nuclear localization and significant upregulation of progerin mRNA and protein levels in dermal fibroblasts from HGPS donors emphasize its role in cellular aging mechanisms. Progerin mRNA levels were higher in patients with NPDR. CMT, number of intravitreal anti-VEGF therapy treatments, and cigarette consumption were positively related to progerin mRNA, suggesting an association with disease progression and premature aging.

Microglial regulation of the retinal vasculature in health and during the pathology associated with diabetes

The high metabolic demand of retinal neurons requires a precisely regulated vascular system that can deliver rapid changes in blood flow in response to neural need. In the retina, this is achieved via the action of a coordinated group of cells that form the neurovascular unit. While cells such as pericytes, Müller cells, and astrocytes have long been linked to neurovascular coupling, more recently the resident microglial population have also been implicated. In the healthy retina, microglia make extensive contact with blood vessels, as well as neuronal synapses, and are important in vascular patterning during development. Work in the brain and retina has recently indicated that microglia can directly regulate the local vasculature. In the retina, the fractalkine-Cx3cr1 signalling axis has been shown to induce local capillary constriction within the superficial vascular plexus via a mechanism involving components of the renin-angiotensin system. Furthermore, aberrant microglial induced vasoconstriction may be at the centre of early vascular reactivity changes observed in those with diabetes. This review summarizes the recent emerging evidence that microglia play multiple roles in retinal homeostasis especially in regulating the vasculature. We highlight what is known about the role of microglia under normal circumstances, and then build on this to discuss how microglia contribute to early vascular compromise during diabetes. Further understanding of the mechanisms of microglial-vascular regulation may allow alternate treatment strategies to be devised to reduce vascular pathology in diseases such as diabetic retinopathy.

CX3CR1-Fractalkine Dysregulation Affects Retinal GFAP Expression, Inflammatory Gene Induction, and LPS Response in a Mouse Model of Hypoxic Retinopathy

Diabetic retinopathy (DR) causes vision loss due to sustained inflammation and vascular damage. The vascular damage is evident by fibrinogen leakage, angiogenesis, and hypoxia. Neuronal regulation of microglia via the CX3CL1 (Fractalkine or FKN)-CX3CR1 pathway plays a significant role in retinal pathology. Defects in FKN or CX3CR1 exacerbate inflammation, vascular damage, and vision impairment. However, the contribution of hypoxic astrocytes to the pathological process of DR is unclear. A hypoxic model (7 days of systemic 7.5% O2) was utilized to induce retinal damage in adult mice in the absence of systemic inflammatory signals. This model induced vascular and microglial responses similar to 10 weeks of STZ-induced hyperglycemia. The goal of this study is to characterize retinal damage in WT and mice with defects in the FKN-CX3CR1 signaling axis and hence assess the impact of the microglial inflammatory responses to hypoxic retinopathy. Tissues were analyzed by immunostaining, RNA sequencing, and cytokine quantification. We found that CX3CR1 deficiency in hypoxic animals induced reactive astrogliosis and that Müller glial responses to hypoxia and systemic inflammation were dependent on FKN signaling. Exacerbated microglial reactivity to hypoxic conditions significantly altered the expression of HIF transcripts. Microglial dysregulation was found to reduce the anti-inflammatory response to hypoxic conditions, downregulate hypoxia-responsive gene expression, and restrained LPS-induced inflammatory responses. We found that microglia dysregulation alters the hypoxic response by inhibiting the upregulation of HIF2α/3α, increasing CD31 immunoreactivity, and altering the expression of ECM-associated transcripts such as type I, III, and XVIII collagens to hypoxic conditions.

CX3CL1 Regulation of Gliosis in Neuroinflammatory and Neuroprotective Processes

Among the different chemokines, C-X3-C motif chemokine ligand 1 or CX3CL1, also named fractalkine, is one of the most interesting due to its characteristics, including its unique structure, not shared by any other chemokine, and its ability to function both in a membrane-bound form and in a soluble form, among others. However, undoubtedly, its most relevant characteristic from the neuroscientific point of view is its role as a messenger used by neurons to communicate with microglia. The study of the interaction between both cell types and the key role that CX3CL1 seems to play has facilitated the identification of CX3CL1 as a crucial modulator of microglial activation and a promising target in the fight against neuroinflammation. As a result, numerous studies have contributed to elucidate the involvement of CX3CL1 and its specific receptor CCX3CR1 in the progression of different neuroinflammatory and neurodegenerative processes, with Alzheimer's and Parkinson's diseases being the most studied ones. However, the different animal and cellular models used to reproduce the pathological conditions to be analyzed, as well as the difficulties inherent to studies performed on human samples, have hindered the collection of compatible results in many cases. In this review, we summarize some of the most relevant data describing the alterations found for the CX3CL1/CX3CR1 signaling axis in different neurodegenerative conditions in which neuroinflammation is known to play a relevant role.

The Role of Fractalkine in Diabetic Retinopathy: Pathophysiology and Clinical Implications

Diabetic retinopathy (DR) is a complication of diabetes, characterized by progressive microvascular dysfunction that can result in vision loss. Chronic hyperglycemia drives oxidative stress, endothelial dysfunction, and inflammation, leading to retinal damage and complications such as neovascularization. Current treatments, including anti-VEGF agents, have limitations, necessitating the exploration of alternative therapeutic strategies. Fractalkine (CX3CL1), a chemokine with dual roles as a membrane-bound adhesion molecule and a soluble chemoattractant, has emerged as a potential therapeutic target. Its receptor, CX3CR1, is expressed on immune cells and mediates processes such as immune cell recruitment and microglial activation through intracellular signaling pathways. In DR, soluble fractalkine plays critical roles in retinal inflammation, angiogenesis, and neuroprotection, balancing tissue damage and repair. In DR, elevated fractalkine levels are associated with retinal inflammation and endothelial dysfunction. Experimental studies suggest that fractalkine deficiency exacerbates the severity of diabetic retinopathy (DR), whereas exogenous fractalkine appears to reduce inflammation, oxidative stress, and neuronal damage. However, its role in pathological angiogenesis within DR remains unclear and warrants further investigation. Preclinical evidence indicates that fractalkine may hold therapeutic potential, particularly in mitigating tissue injury and inflammation associated with early-stage DR.

Microglia in retinal angiogenesis and diabetic retinopathy

Diabetic retinopathy has a high probability of causing visual impairment or blindness throughout the disease progression and is characterized by the growth of new blood vessels in the retina at an advanced, proliferative stage. Microglia are a resident immune population in the central nervous system, known to play a crucial role in regulating retinal angiogenesis in both physiological and pathological conditions, including diabetic retinopathy. Physiologically, they are located close to blood vessels and are essential for forming new blood vessels (neovascularization). In diabetic retinopathy, microglia become widely activated, showing a distinct polarization phenotype that leads to their accumulation around neovascular tufts. These activated microglia induce pathogenic angiogenesis through the secretion of various angiogenic factors and by regulating the status of endothelial cells. Interestingly, some subtypes of microglia simultaneously promote the regression of neovascularization tufts and normal angiogenesis in neovascularization lesions. Modulating the state of microglial activation to ameliorate neovascularization thus appears as a promising potential therapeutic approach for managing diabetic retinopathy.

Astrogliosis in the GFAP-CreERT2:Rosa26iDTR Mouse Model Does Not Exacerbate Retinal Microglia Activation or Müller Cell Gliosis under Hypoxic Conditions

Diabetic retinopathy (DR) affects over 140 million people globally. The mechanisms that lead to blindness are still enigmatic but there is evidence that sustained inflammation and hypoxia contribute to vascular damage. Despite efforts to understand the role of inflammation and microglia in DR's pathology, the contribution of astrocytes to hypoxic responses is less clear. To investigate the role of astrocytes in hypoxia-induced retinopathy, we utilized a 7-day systemic hypoxia model using the GFAP-CreERT2:Rosa26iDTR transgenic mouse line. This allows for the induction of inflammatory reactive astrogliosis following tamoxifen and diphtheria toxin administration. We hypothesize that DTx-induced astrogliosis is neuroprotective during hypoxia-induced retinopathy. Glial, neuronal, and vascular responses were quantified using immunostaining, with antibodies against GFAP, vimentin, IBA-1, NeuN, fibrinogen, and CD31. Cytokine responses were measured in both the brain and serum. We report that while both DTx and hypoxia induced a phenotype of reduced microglia morphological activation, DTx, but not hypoxia, induced an increase in the Müller glia marker vimentin. We did not observe that the combination of DTx and hypoxic treatments exacerbated the signs of reactive glial cells, nor did we observe a significant change in the expression immunomodulatory mediators IL-1β, IL2, IL-4, IL-5, IL-6, IL-10, IL-18, CCL17, TGF-β1, GM-CSF, TNF-α, and IFN-γ. Overall, our results suggest that, in this hypoxia model, reactive astrogliosis does not alter the inflammatory responses or cause vascular damage in the retina.

Macrophage activation contributes to diabetic retinopathy

Diabetic retinopathy (DR) is recognized as a neurovascular complication of diabetes, and emerging evidence underscores the pivotal role of inflammation in its pathophysiology. Macrophage activation is increasingly acknowledged as a key contributor to the onset and progression of DR. Different populations of macrophages originating from distinct sources contribute to DR-associated inflammation. Retinal macrophages can be broadly categorized into two main groups based on their origin: intrinsic macrophages situated within the retina and vitreoretinal interface and macrophages derived from infiltrating monocytes. The former comprises microglia (MG), perivascular macrophages, and macrophage-like hyalocytes. Retinal MG, as the principal population of tissue-resident population of mononuclear phagocytes, exhibits high heterogeneity and plasticity while serving as a crucial connector between retinal capillaries and synapses. This makes MG actively involved in the pathological processes across various stages of DR. Activated hyalocytes also contribute to the pathological progression of advanced DR. Additionally, recruited monocytes, displaying rapid turnover in circulation, augment the population of retinal macrophages during DR pathogenesis, exerting pathogenic or protective effect based on different subtypes. In this review, we examine novel perspectives on macrophage biology based on recent studies elucidating the diversity of macrophage identity and function, as well as the mechanisms influencing macrophage behavior. These insights may pave the way for innovative therapeutic strategies in the management of DR.

Fractalkine isoforms differentially regulate microglia-mediated inflammation and enhance visual function in the diabetic retina

Diabetic retinopathy (DR) affects about 200 million people worldwide, causing leakage of blood components into retinal tissues, leading to activation of microglia, the resident phagocytes of the retina, promoting neuronal and vascular damage. The microglial receptor, CX3CR1, binds to fractalkine (FKN), an anti-inflammatory chemokine that is expressed on neuronal membranes (mFKN), and undergoes constitutive cleavage to release a soluble domain (sFKN). Deficiencies in CX3CR1 or FKN showed increased microglial activation, inflammation, vascular damage, and neuronal loss in experimental mouse models. To understand the mechanism that regulates microglia function, recombinant adeno-associated viral vectors (rAAV) expressing mFKN or sFKN were delivered to intact retinas prior to diabetes. High-resolution confocal imaging and mRNA-seq were used to analyze microglia morphology and markers of expression, neuronal and vascular health, and inflammatory mediators. We confirmed that prophylactic intra-vitreal administration of rAAV expressing sFKN (rAAV-sFKN), but not mFKN (rAAV-mFKN), in FKNKO retinas provided vasculo- and neuro-protection, reduced microgliosis, mitigated inflammation, improved overall optic nerve health by regulating microglia-mediated inflammation, and prevented fibrin(ogen) leakage at 4 weeks and 10 weeks of diabetes induction. Moreover, administration of sFKN improved visual acuity. Our results elucidated a novel intervention via sFKN gene therapy that provides an alternative pathway to implement translational and therapeutic approaches, preventing diabetes-associated blindness.

The glucocorticoid receptor as a master regulator of the Müller cell response to diabetic conditions in mice

Diabetic retinopathy (DR) is considered a primarily microvascular complication of diabetes. Müller glia cells are at the centre of the retinal neurovascular unit and play a critical role in DR. We therefore investigated Müller cell-specific signalling pathways that are altered in DR to identify novel targets for gene therapy. Using a multi-omics approach on purified Müller cells from diabetic db/db mice, we found the mRNA and protein expression of the glucocorticoid receptor (GR) to be significantly decreased, while its target gene cluster was down-regulated. Further, oPOSSUM TF analysis and ATAC- sequencing identified the GR as a master regulator of Müller cell response to diabetic conditions. Cortisol not only increased GR phosphorylation. It also induced changes in the expression of known GR target genes in retinal explants. Finally, retinal functionality was improved by AAV-mediated overexpression of GR in Müller cells. Our study demonstrates an important role of the glial GR in DR and implies that therapeutic approaches targeting this signalling pathway should be aimed at increasing GR expression rather than the addition of more ligand.

Microglia depletion/repopulation does not affect light-induced retinal degeneration in mice

Reactive microglia are a hallmark of age-related retinal degenerative diseases including age-related macular degeneration (AMD). These cells are capable of secreting neurotoxic substances that may aggravate inflammation that leads to loss of photoreceptors and impaired vision. Despite their role in driving detrimental inflammation, microglia also play supporting roles in the retina as they are a crucial cellular component of the regulatory innate immune system. In this study, we used the colony stimulating factor 1 receptor (CSF1R)-antagonist PLX3397 to investigate the effects of microglia depletion and repopulation in a mouse model of acute retinal degeneration that mimics some aspects of dry AMD. Our main goal was to investigate whether microglia depletion and repopulation affects the outcome of light-induced retinal degeneration. We found that microglia depletion effectively decreased the expression of several key pro-inflammatory factors but was unable to influence the extent of retinal degeneration as determined by optical coherence tomography (OCT) and histology. Interestingly, we found prominent cell debris accumulation in the outer retina under conditions of microglia depletion, presumably due to the lack of efficient phagocytosis that could not be compensated by the retinal pigment epithelium. Moreover, our in vivo experiments showed that renewal of retinal microglia by repopulation did also not prevent rapid microglia activation or preserve photoreceptor death under conditions of light damage. We conclude that microglia ablation strongly reduces the expression of pro-inflammatory factors but cannot prevent photoreceptor loss in the light-damage paradigm of retinal degeneration.