Background

Obesity is a severe global health problem characterized by excess adipose tissue expansion and strongly associated with metabolic diseases such as diabetes, cardiovascular and hepatic lipid diseases, and some types of cancers (Haslam and James, 2005; Swinburn et al., 2011; Wabitsch et al., 2015). The growth of the adipose tissue can be attributed to the enlargement of the existing adipocytes (hypertrophy) or the formation of new adipocytes (hyperplasia) (Salans et al., 1973; Spalding et al., 2008; Jo et al., 2009). Unlike the protected role of adipocyte hyperplasia (Vishvanath and Gupta, 2019), hypertrophic adipocytes are more responsible for lipid homeostasis disorders and pathological consequences (Haczeyni et al., 2018). Moreover, besides its usage as a container for lipid storage, the adipose tissue also plays crucial roles in regulating metabolic and endocrine functions. Therefore, there is an urgent need to clarify the obesity pathogenic mechanism, develop safe anti-obesity therapeutic strategies, and further avoid the pandemic dimensions of obesity.

Genetic engineering of mice is a commonly used approach to clarify gene function in vivo. However, the production of new transgenic mice is time consuming and costly (Jimenez et al., 2013; Bates et al., 2020). Some undesired side effects can be observed in conventional genetically modified mice: the manipulated gene that works on the whole body may interfere with its primary function on a specific tissue. In vivo gene transfer to the target organ has become a faster, lower-cost, and more specific strategy. Using this strategy, the time points of gene delivery can be arranged based on experimental requirements. It can also eliminate the undesired effects on embryo development.

Adeno-associated virus (AAV) vectors, discovered in the 1960s, are considered one of the safest and most promising tools for in vivo delivery of gene therapies (Atchison et al., 1965; Wang et al., 2019). They are small (25–26 nm in diameter), non-enveloped viruses composed of an icosahedral capsid that contains a linear single-stranded DNA genome (approximately 4.7–4.9 Kb). In contrast to the adenovirus, retrovirus, and lentivirus, AAVs show a relatively safe profile with apathogenicity and low immunogenicity (Cao et al., 2011). They can transduce genes into dividing and non-dividing cells (Flotte et al., 1994), allowing long-term transgene expression in such tissues. Among the 13 AAV serotypes that have been identified (Pipe et al., 2019), at least three serotypes (AAV2/8/9) exhibit efficient gene transfer to adipose tissue of adult mice (O'Neill et al., 2014; Uhrig-Schmidt et al., 2014; Bates et al., 2020). Recently, we successfully applied this protocol on direct AAV8 injection into the subcutaneous adipose tissue. In this protocol, we demonstrate a detailed procedure for efficient gene delivery into adipocytes of adult mice.