Cellular endocytosis and gene delivery

Abstract

Endocytosis is the process by which cells take up macromolecules from the surrounding medium. The best-characterized process is the so-called clathrin-dependent endocytosis, although much is also currently known about clathrin-independent endocytic processes such as those involving caveolae and lipid rafts. An understanding of endocytosis and the cellular trafficking that occurs thereafter has a great deal of relevance to current molecular medicine. Gene therapy, which is presently being investigated for its therapeutic potential in treating immunodeficiency and metabolic diseases, cancer and heart disease, employs a variety of viral and nonviral vectors, which can be delivered to the target cells of the body and are subsequently endocytosed and dissembled. A variety of vectors can be used to deliver genes to organs in vivo or cells ex vivo. Various routes of vector delivery have been investigated. The mechanisms by which vectors such as adenoviruses, adeno-associated viruses, retroviruses and liposomes enter the cell are increasingly being investigated as the effort to increase the efficiency of gene therapy continues. This review focuses on mechanisms of endocytosis and how they relate to the internal trafficking of viral and nonviral vectors in gene therapy.

Figure 1

The endocytic pathway in gene therapy. Gene therapy vectors enter the cells via membrane structures that allow endocytosis to occur, such as clathrin-coated pits, caveolae and lipid rafts. Clathrin-coated pits are lined by clathrin, caveolae are lined by caveolin and lipid rafts are cell membrane domains that contain cholesterol, glycosphino-golipids and possibly the protein flotillin. Each structure invaginates to form a vesicle and, subsequently, an early endosome. The early endosome sorts the endocytosed material into material that will be recycled back to the plasma membrane, material that will be secreted and material that will be degraded by fusion with the lysosome in later steps. Some viruses and proteins can escape lysosomal degradation via a decrease in the pH of the endosome. These particles/molecules can then be trafficked into the nucleus, where they can act to silence or activate various genes. GSL, glycosphingolipids.