The liver is a vital organ that metabolizes nutrients and chemicals, but is also under constant exposure to cytotoxic and carcinogenic insults. While the adult liver is capable of regrowing after injury, its regenerative power is severely hampered by age1. To date, the only therapeutic option for patients with end-stage chronic liver diseases or massive acute liver damage is liver transplantation, which poses many challenges of its own2. To better understand the molecular pathophysiology of the liver by interrogating the functions of genes of interest, genetic manipulations have been developed for in vivo application, including RNAi-mediated knockdown and targeted deletion by a Cre recombinase in the presence of loxP sites3. Cre-expression cassette and shRNA construct can be delivered by viral vehicles.
Adeno-associated virus serotype 8 (AAV8) is a robust vector for gene delivery to selective tissue types (e.g., liver, skeletal muscle, heart, brain, and pancreas) with high efficiency and low inflammatory response4,5. To target internal body organs, AAV8 is commonly introduced by tail vein injection, in which viral particles first travel through the lung before reaching the systemic circulation. In contrast, portal vein injection allows them to reach the liver first before circulating through the lung, a potential source of sequestering and dilution. However, portal vein injection is technically challenging and often complicated by operation-induced bleeding and a high mortality rate. To achieve an efficient gene knockdown (KD) in the liver while avoiding the issue of high peri/postoperative mortality, we tested a method of intrasplenic injection of AAV8 carrying an engineered shRNA targeting nucleostemin (NS) (AAV8-shNS1) and compared its KD efficiency to that of portal vein-injected AAV8-shNS1.
NS is a stem/progenitor cell-enriched protein discovered first in neural stem cells and later in several other types of stem cells and cancers6,7. The biological importance of NS is shown by the early embryonic lethal phenotype of germline NS-knockout (NSKO) mice in vivo3,8 and by NSKD-induced perturbation of self-renewal in vitro9,10. In adult animals, high levels of NS expression are found in the testis and several tissues undergoing regeneration, including the dedifferentiating newt-pigmented epithelial cells after lentectomy and muscle cells after limb amputation11, as well as regenerating mammalian tissues, such as mouse cardiomyocytes after cardiac injury12 and hepatocytes after liver injury (e.g., CCl4) or surgical resection (e.g., partial hepatectomy)13,14. Furthermore, NS has been shown to play important roles in the development of mammary, liver, and oral tumors15,16,17. Mechanistically, NS has been shown to promote self-renewal by protecting the replicating genome from DNA damage18,19. However, due to the early embryonic lethal phenotype of germline NSKO, an experimental method to temporally introduce NSKD after the completion of tissue development is needed to further determine its biological activities in adult organs.
In this article, we use NS as a case in point to illustrate the establishment and testing of an in vivo liver gene KD method that is efficient and has a low procedure-induced mortality.