Targeted AAV Delivery Strategies for Adipose Tissue: From the Discovery of the Novel BAT Target to In Vivo Validation
In addressing the global challenge of obesity and metabolic syndrome, the research focus has increasingly shifted from merely suppressing appetite to increasing total energy expenditure. Among potential therapeutic targets, brown adipose tissue (BAT) remains highly promising due to its unique capacity to dissipate energy efficiently through adaptive thermogenesis.
Recently, research into the thermogenic mechanisms of adipose tissue has achieved a breakthrough. In a study published in Nature Communications in March 2026, researchers showed that SLIT3 fragments coordinate neurovascular expansion required for brown adipose tissue thermogenesis. [1] The study demonstrated that adipocyte progenitors within BAT secrete the SLIT3 protein, which is subsequently cleaved by the enzyme bone morphogenetic protein-1 (BMP1) into two distinct functional fragments: SLIT3-N and SLIT3-C. Acting as divergent signaling cues, these fragments independently promote angiogenesis and sympathetic innervation, respectively; notably, SLIT3-C was shown to regulate neural development by specifically binding to the PLXNA1 receptor. This SLIT3-mediated mechanism of neurovascular expansion provides the indispensable oxygen and activation signals required for BAT to generate heat in response to cold stimulation. This discovery not only highlights the central role of adipocyte progenitors in modulating tissue innervation but also establishes the SLIT3 pathway as a compelling novel target for future obesity interventions.
However, advancing emerging metabolic targets like SLIT3 from mechanistic discovery to in vivo functional validation presents a core challenge for researchers: How can we safely, efficiently, and specifically deliver gene-editing tools or therapeutic nucleic acids into targeted adipose tissues?
Overcoming Gene Delivery Challenges in Heterogeneous Adipose Tissues with AAV Vectors
Adipose tissue is traditionally classified into two distinct types, namely white adipose tissue (WAT) and brown adipose tissue (BAT), which possess different developmental origins, morphology, and functions. WAT is composed of white adipocytes, which have a single unilocular lipid droplet in the cytoplasm, accounting for the vast majority of adipose tissue. BAT is composed of brown adipocytes, which feature multiple small lipid droplets in the cytoplasm and abundant uncoupling protein-1 (UCP1) in the mitochondria. Adipose tissue consists of adipocytes and a stromal vascular fraction (or cells); the stromal vascular fraction includes preadipocytes, fibroblasts, mesenchymal stem cells (MSCs), endothelial and smooth muscle cells, macrophages, and immune cells.
Figure 1. Schematic diagram of adipose tissue cell population composition(https://ambrosecelltherapy.com/why-adipose/)
Due to the complex cellular composition of adipose tissue and its widespread anatomical distribution, targeted gene delivery has long been hindered by off-target effects and low transduction efficiency. In this context, Adeno-Associated Virus (AAV)—a viral vector known for its high safety profile, low immunogenicity, and ability to achieve long-term expression in non-dividing cells—is emerging as a game-changer for adipose-targeted delivery. Whether validating novel targets like SLIT3 in animal models or developing innovative gene therapies for metabolic diseases, researchers can achieve highly efficient and precise transgene expression within specific fat depots by strategically optimizing AAV serotypes, utilizing adipose-specific promoters, and selecting appropriate administration routes.
Optimizing Transduction Efficiency: AAV Serotype Selection for Adipose Tissue (AAV8 and AAV9)
The most commonly used AAV serotypes for infecting adipose tissue are AAV8 and AAV9.
In the article In Vivo Adeno-Associated Viral Vector–Mediated Genetic Engineering of White and Brown Adipose Tissue in Adult Mice [2], the authors compared the efficiency of multiple serotypes in transducing adipose tissue and discovered that AAV serotypes 8 and 9 are particularly adept at transducing WAT and BAT, thereby enabling the induction of functional changes in adipocytes. This approach provides a valuable tool for investigating the metabolic and molecular mechanisms of adipose tissue dysfunction, as well as for developing novel therapies for obesity and type 2 diabetes. Intravenous injection of 5 x 10^12 vg of AAV8 or AAV9-CAG-GFP vectors into normal mice can transfect systemic WAT and BAT tissues, although the transfection efficiency varies among different adipose tissues. Local administration can more effectively transfect specific adipose tissues. For example, direct injection into brown adipose tissue (iBAT) can efficiently transfect brown adipocytes.
Figure 2. Transduction efficiency of different serotypes in adipose tissue [2]
Because AAV8 and AAV9 exhibit a broad spectrum of affinity for tissues such as the liver and muscle, it is possible to avoid expression in non-targeted cells or organs through in situ administration and the use of adipose-specific promoters.
Enhancing Target Specificity: The Role of Adipose-Specific Promoters (mAP2.2, Adiponectin, UCP1)
mAP2.2 (2.2-kb fragment of murine adiponectin promoter) adipose-specific promoter. Kreuz and colleagues [2] found that by utilizing a 2.2-kb murine adiponectin promoter fragment (mAP2.2), AAV8-mAP2.2-EGFP transduced subcutaneous and visceral fat depots while simultaneously significantly restricting off-target transduction. When tested in adult male C57BL/6 mice, the injection of AAV8-mAP2.2-mediated perilipin A resulted in strong expression of mediated perilipin A in subcutaneous and visceral fat depots, accompanied by significant alterations in metabolic parameters (serum free fatty acids, blood glucose, and respiratory exchange ratio).
Figure 3: mAP2.2 enables specific expression in adipose tissue, avoiding leakage in the liver [3]
Adiponectin mature adipocyte-specific promoter. Gencer Sancar et al. [4] utilized the adiponectin mature adipocyte-specific promoter to achieve the specific expression of target proteins (such as perilipin-GFP and HSL-mCherry) within mature adipocytes. Immunoblotting validation demonstrated that the adiponectin promoter can be specifically expressed in adipose tissue, with an absence of expression in regions such as the liver and kidneys.
Figure 4. Immunoblotting validation of the specificity of the adiponectin promoter against non-target organs [4]
UCP1 brown adipose-specific promoter. Uncoupling protein 1 (UCP1) is a mitochondrial thermogenic gene critical for energy expenditure in the form of heat in brown adipose tissue; UCP1 is uniquely expressed during brown adipocyte differentiation and represents a candidate target for obesity treatment [4]. Winifred et al. [6] injected AAV8-UCP1-iCre virus into Atg5Flox/Flox mice to generate an Atg5 conditional knockout mouse model; the BAT-specific Atg5 knockout mice exhibited lower body temperatures than the control group under hyperthyroid conditions, verifying the importance of autophagy for T3-induced thermogenesis.
Strategic Administration Routes: Systemic vs. Local Delivery of AAV Vectors
Systemic delivery can distribute AAV across a wider array of fat depots, but it necessitates a higher dose than local delivery, and it may provoke a greater immune response against the capsid as well as produce off-target effects. Local delivery of AAV assists in mitigating off-target effects because the dose is lower and there is reduced leakage into the circulation. However, injecting multiple fat depots is time-consuming, invasive, and particularly challenging for visceral fat depots. Therefore, the injection method must be determined based on the specific targeted adipose tissue. This is precisely why selecting the optimal combination of AAV serotypes and tissue-specific promoters is critical. It empowers researchers to achieve high-specificity transgene expression through less invasive administration routes, thereby reducing experimental variability and accelerating the transition from preclinical models to clinical applications.
In the review Adipose Tissue: An Emerging Target for Adeno-associated Viral Vectors, Rhiannon Bates [7] summarized numerous applications of AAVs in adipose-related diseases, which included information such as serotype, injection method, and injection volume (Table 1). In summary, systemic delivery is predominantly achieved via tail vein administration and intraperitoneal administration, with the total injected virus volume being approximately 1E12 vg; the amount of virus utilized is even higher when specific promoters are employed. The virus injection volume for in situ administration ranges between 1E10 vg and 2E11 vg, but the majority of in situ administrations require surgical exposure of the adipose tissue.
Table 1: Applications of AAV in adipose-related diseases [7]
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Cyagen’s Featured Case Studies: AAV-Mediated Disease Modeling
| AAV Category | Virus Name | Product ID |
|---|---|---|
| Atherosclerosis | AAV8-ApoEHCR-hAAT-mPCSK9-D377Y | AAVRN-110001 |
| AAV8-TBG-mPCSK9 (D377Y)-3xFLAG-P2A-EGFP | AAVRN-110002 | |
| AAV8-ApoEHCR-hAAT-mPCSK9 (D377Y)-3xFLAG | AAVRN-110003 | |
| AAV8-TBG- hPCSK9-3xFLAG-WPRE | AAVRN-110004 | |
| AAV8-TBG-Kozak- hPCSK9 (D374Y)-3xFLAG-WPRE | AAVRN-110005 | |
| PD | AAV5-CAG-hSNCA -WPRE | AAVRN-110006 |
| AAV9-PM228-CAG-hSNCA (A53T)-WPRE | AAVRN-110007 | |
| AAV5-CAG-hSNCA (A53T)-WPRE | AAVRN-110008 | |
| AAV1/2-CAG-hSNCA (A53T)-WPRE | AAVRN-110009 | |
| AD | AAV9-CAG-MAPT (P301L)-WPRE | AAVRN-110010 |
| AAV9-CAG-MAPT (P301S)-WPRE | AAVRN-110011 | |
| AAVrh10-CAG-hAPP 3XFAD-WPRE | AAVRN-110012 | |
| AAVrh10-CAG-hPSEN1 (M146L &L286V)-WPRE | AAVRN-110013 |
Reference：
[1]. Serdan, T.D.A., Cervantes, H., Frank, B. et al. SLIT3 fragments orchestrate neurovascular expansion and thermogenesis in brown adipose tissue. Nat Commun 17, 2445 (2026). https://doi.org/10.1038/s41467-026-70310-9
[2]. Jimenez, V., et al., In vivo adeno-associated viral vector-mediated genetic engineering of white and brown adipose tissue in adult mice. Diabetes, 2013. 62(12): p. 4012-4022.
[3]. Uhrig-Schmidt, S., et al., Gene Delivery to Adipose Tissue Using Transcriptionally Targeted rAAV8 Vectors. Plos One, 2014. 9(12): p. e116288.
[4]. Sancar, G., et al., FGF1 and insulin control lipolysis by convergent pathways. Cell Metabolism, 2022. 34(1): p. 171-183.e6.
[5]. Yuanbo, Z. and J.P. Uhler, Identification of a G-quadruplex forming sequence in the promoter of UCP1. Acta Biochimica et Biophysica Sinica, 2018(7): p. 7.
[6]. Winifred, et al., Thyroid hormone (T3) stimulates brown adipose tissue activation via mitochondrial biogenesis and MTOR-mediated mitophagy. Autophagy, 2018.
[7] Bates, R., W. Huang and L. Cao, Adipose Tissue: An Emerging Target for Adeno-associated Viral Vectors. Molecular Therapy — Methods & Clinical Development, 2020. 19: p. 236-249.
