Microglia targeting by adeno-associated viral vectors

Abstract

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.

Keywords: adeno-associated virus (AAV); gene targeting; microglia; neuroinflammation; purinergic signaling.

Figure 1

AAV-based microglia targeting approaches. Overview of AAV vectors that have successfully been used to transduce microglia. (A) AAV6 and AAV8 were shown to efficiently transduce primary microglia in vitro when employing the ubiquitous CMV promoter (18), while the AAV6 capsid mutant AAV TM6 in combination with the F4/80 or CD68 promoter showed specificity for microglia upon incubation with primary microglia, astrocytes and mixed neuroglia (19). (B) The AAV6 capsid mutants AAV TM6 and AAV6Δ4 showed successful microglia transduction in vivo upon intravitreal and subretinal injection when employing the CD68 promoter (21). AAV8 in combination with the ubiquitous CAG promoter was shown to transduce microglia upon intravitreal injection (20). (C) AAV5, AAV TM6, AAV8, AAV9 and the AAV9 peptide-displaying variants MG1.1 and MG1.2 successfully transduced microglia upon intraparenchymal brain injection while assessing transgene (mScarlet) expression by inducible vectors in cre-positive mice (24). AAV9-mediated microglia-specific transgene expression upon intraparenchymal brain injection was further enhanced by employing the Iba1 promoter and targeting elements for miR-9 and miR-129-23 in four copies each (16). (D) The AAV9 peptide-displaying variants ALAVPFR, ALAVPFK, HGTAASH, and YAFGGEG were recently shown to transduce microglia and macrophages with high efficiently even upon intravenous injection in mice. Gene expression was accomplished by employing the CD11b promoter (25). This figure was generated with Biorender.com.