Optimizing AAV2/6 microglial targeting identified enhanced efficiency in the photoreceptor degenerative environment

Optic Nerve Crush Does Not Induce Retinal Ganglion Cell Loss in the Contralateral Eye

Purpose

Optic nerve crush (ONC) is a model for studying optic nerve trauma. Unilateral ONC induces massive retinal ganglion cell (RGC) degeneration in the affected eye, leading to vision loss within a month. A common assumption has been that the non-injured contralateral eye is unaffected due to the minimal retino-retinal projections of the RGCs at the chiasm. Yet, recently, microglia, the brain-resident macrophages, have shown a responsive phenotype in the contralateral eye after ONC. Whether RGC loss accompanies this phenotype is still controversial.

Methods

Using the available RGCode algorithm and developing our own RGC-Quant deep-learning-based tool, we quantify RGC's total number and density across the entire retina after ONC.

Results

We confirm a short-term microglia response in the contralateral eye after ONC, but this did not affect the microglia number. Furthermore, we cannot confirm the previously reported RGC loss between naïve and contralateral retinas 5 weeks after ONC induction across the commonly used Cx3cr1creERT2 and C57BL6/J mouse models. Neither sex nor the direct comparison of the RGC markers Brn3a and RBPMS, with Brn3a co-labeling, on average, 89% of the RBPMS+-cells, explained this discrepancy, suggesting that the early microglia-responsive phenotype does not have immediate consequences on the RGC number.

Conclusions

Our results corroborate that unilateral optic nerve injury elicits a microglial response in the uninjured contralateral eye but without RGC loss. Therefore, the contralateral eye should be treated separately and not as an ONC control.

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.

Oligodendrocytes, the Forgotten Target of Gene Therapy

If the billions of oligodendrocytes (OLs) populating the central nervous system (CNS) of patients could express their feelings, they would undoubtedly tell gene therapists about their frustration with the other neural cell populations, neurons, microglia, or astrocytes, which have been the favorite targets of gene transfer experiments. This review questions why OLs have been left out of most gene therapy attempts. The first explanation is that the pathogenic role of OLs is still discussed in most CNS diseases. Another reason is that the so-called ubiquitous CAG, CBA, CBh, or CMV promoters-widely used in gene therapy studies-are unable or poorly able to activate the transcription of episomal transgene copies brought by adeno-associated virus (AAV) vectors in OLs. Accordingly, transgene expression in OLs has either not been found or not been evaluated in most gene therapy studies in rodents or non-human primates. The aims of the current review are to give OLs their rightful place among the neural cells that future gene therapy could target and to encourage researchers to test the effect of OL transduction in various CNS diseases.

A comprehensive review of AAV-mediated strategies targeting microglia for therapeutic intervention of neurodegenerative diseases

Neurodegenerative diseases pose a significant health burden globally, with limited treatment options available. Among the various cell types involved in the pathogenesis of these disorders, microglia, the resident immune cells of the central nervous system, play a pivotal role. Dysregulated microglial activation contributes to neuroinflammation and neuronal damage, making them an attractive target for therapeutic intervention. Adeno-associated virus (AAV) vectors have emerged as powerful tools for delivering therapeutic genes to specific cell types in the central nervous system with remarkable precision and safety. In the current review, we discuss the strategies employed to achieve selective transduction of microglia, including the use of cell-specific promoters, engineered capsids, and microRNA (miRNA) strategies. Additionally, we address the challenges and future directions in the development of AAV-based therapies targeting microglia. Overall, AAV-mediated targeting of microglia holds promise as a novel therapeutic approach for neurodegenerative diseases, offering the potential to modify disease progression and improve patient outcomes.

Microglia: The Drunken Gardeners of Early Adversity

Early life adversity (ELA) is a heterogeneous group of negative childhood experiences that can lead to abnormal brain development and more severe psychiatric, neurological, and medical conditions in adulthood. According to the immune hypothesis, ELA leads to an abnormal immune response characterized by high levels of inflammatory cytokines. This abnormal immune response contributes to more severe negative health outcomes and a refractory response to treatment in individuals with a history of ELA. Here, we examine this hypothesis in the context of recent rodent studies that focus on the impact of ELA on microglia, the resident immune cells in the brain. We review recent progress in our ability to mechanistically link molecular alterations in microglial function during a critical period of development with changes in synaptic connectivity, cognition, and stress reactivity later in life. We also examine recent research showing that ELA induces long-term alterations in microglial inflammatory response to "secondary hits" such as traumatic brain injury, substance use, and exposure to additional stress in adulthood. We conclude with a discussion on future directions and unresolved questions regarding the signals that modify microglial function and the clinical significance of rodent studies for humans.

Microglia targeting by adeno-associated viral vectors

Microglia play a crucial role in maintaining homeostasis of the central nervous system and they are actively involved in shaping the brain's inflammatory response to stress. Among the multitude of involved molecules, purinergic receptors and enzymes are of special importance due to their ability to regulate microglia activation. By investigating the mechanisms underlying microglial responses and dysregulation, researchers can develop more precise interventions to modulate microglial behavior and alleviate neuroinflammatory processes. Studying gene function selectively in microglia, however, remains technically challenging. This review article provides an overview of adeno-associated virus (AAV)-based microglia targeting approaches, discussing potential prospects for refining these approaches to improve both specificity and effectiveness and encouraging future investigations aimed at connecting the potential of AAV-mediated microglial targeting for therapeutic benefit in neurological disorders.

Use of adeno-associated viruses for transgenic modulation of microglia structure and function: A review of technical considerations and challenges

Microglia play a central role in the etiology of many neuropathologies. Transgenic tools are a powerful experiment approach to gain reliable and specific control over microglia function. Adeno-associated virus (AAVs) vectors are already an indispensable tool in neuroscience research. Despite ubiquitous use of AAVs and substantial interest in the role of microglia in the study of central nervous system (CNS) function and disease, transduction of microglia using AAVs is seldom reported. This review explores the challenges and advancements made in using AAVs for expressing transgenes in microglia. First, we will examine the functional anatomy of the AAV capsid, which will serve as a basis for subsequent discussions of studies exploring the relationship between capsid mutations and microglia transduction efficacy. After outlining the functional anatomy of AAVs, we will consider the experimental evidence demonstrating AAV-mediated transduction of microglia and microglia-like cell lines followed by an examination of the most promising experimental approaches identified in the literature. Finally, technical limitations will be considered in future applications of AAV experimental approaches.

Directed evolution of adeno-associated virus for efficient gene delivery to microglia

As the resident immune cells in the central nervous system (CNS), microglia orchestrate immune responses and dynamically sculpt neural circuits in the CNS. Microglial dysfunction and mutations of microglia-specific genes have been implicated in many diseases of the CNS. Developing effective and safe vehicles for transgene delivery into microglia will facilitate the studies of microglia biology and microglia-associated disease mechanisms. Here, we report the discovery of adeno-associated virus (AAV) variants that mediate efficient in vitro and in vivo microglial transduction via directed evolution of the AAV capsid protein. These AAV-cMG and AAV-MG variants are capable of delivering various genetic payloads into microglia with high efficiency, and enable sufficient transgene expression to support fluorescent labeling, Ca²⁺ and neurotransmitter imaging and genome editing in microglia in vivo. Furthermore, single-cell RNA sequencing shows that the AAV-MG variants mediate in vivo transgene delivery without inducing microglia immune activation. These AAV variants should facilitate the use of various genetically encoded sensors and effectors in the study of microglia-related biology.