The mammalian brain consists of many inhibitory, excitatory, and modulatory cells interconnected into circuits by trillions of synapses1. One of the central challenges of neuroscience is to decode the role of distinct cell types in the organization and function of brain circuits and behavior. Manipulating genetically defined cells within the brain requires methods to introduce and express transgenes. Viral-based gene delivery systems are by far the most effective and simple method for gene delivery into the central nervous system2. Viral delivery systems are based on replicating viruses (adenoviruses, adeno-associated viruses (AAVs), lentiviruses, and retroviruses) that have the ability to deliver genetic information into a host cell2,3.
AAV-based vectors have now become one of the most widely used tools for the delivery of desired transgenes to cells within the brain, both for purposes of basic neuroscience research and to develop gene therapy for neurological diseases. When compared against other viruses, replication-defective AAVs possess many features that make them ideal vectors for these purposes. Most notably, AAV vectors efficiently transduce nondividing (terminally differentiated) cells such as neurons and glial cells, resulting in high levels of transgene expression in vivo2. The vectors can be easily produced at a high functional titer suitable for in vivo use3,4,5. Importantly, adeno-associated virus-mediated gene delivery in vivo does not produce histopathological alterations and vector-related toxicity6. Unlike adenoviral vectors, in vivo administration of AAV vectors in animal models usually does not elicit host immune responses against transduced cells, enabling stable transgene expression within the brain parenchyma for extended periods of time2,7,8.
Another reason for the popularity of AAV vectors is the broad array of AAV serotypes with unique tissue and cell-type tropisms9,10,11,12,13,14. Distinct capsid proteins expressed by different AAV serotypes result in the use of different cell surface receptors for cell entry and, thus, specific tropisms10,14.
AAV tropism is determined not only by capsid proteins but by many other factors14. It has been shown that AAV serotypes 1, 2, 6, 7, 8, and 9 transduced both neurons and astrocytes in primary culture15,16, but exhibited strong neuronal tropism following intraparenchymal brain injection17,18. The method used for AAV vector preparation can also influence nervous cell tropism, even for the same serotype. For example, CsCl-purified AAV8 possessed strong astroglial tropism following intraparenchymal brain injection, while iodixanol-purified AAV8, injected under identical conditions, transduced only neurons19. AAV tropism may also be affected by the injected dose and volume14. For example, high titer rAAV2/1 efficiently transduced both cortical excitatory and inhibitory neurons, but the use of lower titers exposed a strong preference for transduction of cortical inhibitory neurons20.
Thus, it is not possible to achieve robust cell-type specificity based solely on the capsid serotype. Cell-type specific promoters can be used to overcome the broad natural tropism of the AAV capsid. For example, human synapsin I is used for targeting neurons21, the CaMKII promoter can drive transgene expression in glutamatergic excitatory neurons with high specificity20, the ppHcrt promoter targets hypocretin (HCRT)-expressing neurons in the lateral hypothalamus22, the PRSx8 promoter targets noradrenergic and adrenergic neurons that express dopamine beta-hydroxylase23, and the GFAP promoter can drive astrocyte-specific expression24. However, some cell-specific promoters have weak transcriptional activity and cannot drive sufficient levels of transgene expression25. Furthermore, the short promoters that fit in AAV viral vectors often do not retain cell-type specificity1,26. For example, it has been shown that a CaMKII construct also transduced inhibitory neurons12.
Besides cell-type specificity (tropism), another significant feature of AAVs is transduction efficiency. The various AAV serotypes have different diffusional properties. AAV2 and four viral vectors diffuse less readily through the brain parenchyma and, therefore, mediate transduction over a smaller area17,27. The most widespread neuronal transduction is observed with AAV serotypes 1, 9, and rh.1011,17,18,19,28.
The most popular method to drive the expression of a desired transgene in a particular brain area is to inject the AAV vector directly into the brain region of interest (parenchyma)3. Following intra-parenchymal injection, even AAV serotypes with more effective diffusion through the brain transduce typically only a local area around the injection site 12. Moreover, intraparenchymal injection is an invasive procedure and leads to tissue damage adjacent to the region of interest. Thus, this method of virus injection is unsuitable for some experimental tasks. For example, extensive labeling of cells is highly desirable in experiments aimed at studying cortical neuron functions in freely moving animals, including with the use of one- or two-photon microscopy29,30,31,32.
Here, we describe a new adeno-associated virus injection technique that uses subarachnoid virus infusion to provide widespread transduction of neocortical neurons in adult mice and preserve brain tissue for subsequent optical or electrophysiological recordings of neuronal activity. This method not only ensured widespread transduction of neurons in superficial neocortical layers but resulted in expression of the transgene in a large population of layer five pyramidal neurons with high specificity even when using a strong non-selective promoter such as CAG.