Targeted evolution of adeno-associated virus capsids for systemic transgene delivery to microglia and tissue-resident macrophages

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.

Transgenic Targeting of Fcrls Creates a Highly Efficient Constitutively Active Microglia Cre Line with Differentiated Specificity

Microglia carry out important functions as the resident macrophages of the brain. To study their role in health and disease, the research community needs tools to genetically modify them with maximum completeness in a manner that distinguishes them from closely related cell types, such as monocytes. While currently available tamoxifen-inducible CreERT2 lines can achieve the differentiation from other cells, the field needs improved and publicly available constitutively active Cre lines, especially ones with favorable efficiency and specificity profiles for studies where high recombination efficiency is imperative and where tamoxifen administration is contraindicated. Here, we leverage the microglia-specific Fcrls gene to generate mice expressing Cre. Using genomic methods, we show correct positioning of the transgene and intact microglia homeostasis in Fcrls-2A-Cre mice. Crossing Fcrls-2A-Cre mice to four different reporters, we demonstrate highly efficient recombination in microglia across differentially sensitive loxP alleles in different genomic contexts, indicating robust applicability of the line. Further, we show that microglia recombine a loxP reporter during early embryonic development, supporting the use of the line for developmental studies. Finally, using immunofluorescence and flow cytometry, we reveal that most border-associated macrophages are also targeted whereas only few liver and spleen macrophages and virtually no white blood cell subsets exhibit Cre activity, distinguishing this line from another publicly available Cre line, Cx3cr1-CreM Fcrls-2A-Cre mice are immediately available (JAX #036591) and serve as a valuable addition to the community's microglia toolbox by providing highly efficient constitutive Cre activity with excellent specificity, particularly for studies where tamoxifen administration is undesirable.

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.

Glial cells as a promising therapeutic target of glaucoma: beyond the IOP

Glial cells, a type of non-neuronal cell found in the central nervous system (CNS), play a critical role in maintaining homeostasis and regulating CNS functions. Recent advancements in technology have paved the way for new therapeutic strategies in the fight against glaucoma. While intraocular pressure (IOP) is the most well-known modifiable risk factor, a significant number of glaucoma patients have normal IOP levels. Because glaucoma is a complex, multifactorial disease influenced by various factors that contribute to its onset and progression, it is imperative that we consider factors beyond IOP to effectively prevent or slow down the disease's advancement. In the realm of CNS neurodegenerative diseases, glial cells have emerged as key players due to their pivotal roles in initiating and hastening disease progression. The inhibition of dysregulated glial function holds the potential to protect neurons and restore brain function. Consequently, glial cells represent an enticing therapeutic candidate for glaucoma, even though the majority of glaucoma research has historically concentrated solely on retinal ganglion cells (RGCs). In addition to the neuroprotection of RGCs, the proper regulation of glial cell function can also facilitate structural and functional recovery in the retina. In this review, we offer an overview of recent advancements in understanding the non-cell-autonomous mechanisms underlying the pathogenesis of glaucoma. Furthermore, state-of-the-art technologies have opened up possibilities for regenerating the optic nerve, which was previously believed to be incapable of regeneration. We will also delve into the potential roles of glial cells in the regeneration of the optic nerve and the restoration of visual function.

Simple and Highly Specific Targeting of Resident Microglia with Adeno-Associated Virus

Microglia, as the immune cells of the central nervous system (CNS), play dynamic roles in both health and diseased conditions. The ability to genetically target microglia using viruses is crucial for understanding their functions and advancing microglia-based treatments. We here show that resident microglia can be simply and specifically targeted using adeno-associated virus (AAV) vectors containing a 466-bp DNA fragment from the human IBA1 (hIBA1) promoter. This targeting approach is applicable to both resting and reactive microglia. When combining the short hIBA1 promoter with the target sequence of miR124, up to 95% of transduced cells are identified as microglia. Such a simple and highly specific microglia-targeting strategy may be further optimized for research and therapeutics.