Investigating effect of telomerase on differentiation of human embryonic stem cells and expansion of the progenitors

Ageing is a natural process that all living organisms have to bear. When humans get older, the incidence for certain conditions and diseases becomes much higher, such as infection, cancer, Alzheimer s and Parkinson s diseases. This is mainly due to special cells that normally protect human, by replacing non-functional cells or responding to stress conditions (e.g. infection), cease to function properly. These cells called adult stem cells. The secret behind this lies in part with a structure that caps both ends of every chromosome in our cells. These structures are telomeres , consisting primarily of an array of thousands of DNA TTAGGG repeats. The main function of telomeres is to keep the chromosomes in good order so that all the genetic information can be faithfully passed from mother cells to daughter cells.

The telomere DNA can only be synthesized by a special enzyme complex, called telomerase . In normal human development, telomerase is restricted to be active in germ cells and early embryos (and cells derived from early embryos e.g. embryonic stem cells) but absent in most of the cells in the body. In the adult stem cells, telomerase is active but at very low levels. Hence, in the majority of cells, including stem cells, telomeres are shortened each time that cells divide and eventually, telomeres get too short to protect chromosome ends and cells will stop dividing or they even die. However, in cancer cells, telomerase expression is higher thus enabling the cancer cells to divide indefinitely. Therefore, telomerase is like a double-edged sword: its absence may account for adult stem cell ageing but on the other hand, its expression may lead to cancer.

It remains unclear what function telomerase plays in human development and cancer. If telomerase is highly active in adult stem cells, does it prevent stem cell ageing or does it produce cancer? Also, do stem cells with active telomerase can develop into functional cell types (e.g. heart cells, liver cells and brain cells)? In this project, we will investigate these issues using cell and molecular biology techniques in a specific cell model system. Answers to these questions will provide important information about telomerase function in development, ageing and cancer. The information will also help scientists and medical doctors to use stem cells for the treatment of age-related diseases.

There is increasing evidence that telomere and telomerase play an important role in stem cell ageing, cell replicative senescence, maintaining genome stability and human cancer. Understanding telomere and telomerase function in stem cell proliferation and differentiation is important not only for better understanding of basic developmental and cancer biology but also for using stem cells in biomedical and medical applications. Although some studies had been carried out in this area over the past several years, the results are conflicting and confusing, especially in the effect of telomerase expression on stem cell differentiation. This may be attributed, at least in part, to the gene expression systems used in those studies. Therefore, there is a need to investigate the impact of telomerase expression on stem cells differentiation and proliferation in a controllable manner.

The ultimate aim of this research is to understand role of telomerase in stem cell proliferation and differentiation so that we can apply it adequately to promote stem cell application in biomedical and medical fields. To achieve this, we will generate human embryonic stem cell (hESC) lines in which telomerase can be expressed at specific stages of differentiation as required. In this way, it will be possible to study the effect of telomerase at alternative stages during differentiation. We will use protocols already established in our laboratory to differentiate hESCs to neural cells and functional hepatocytes. During the differentiation, we will induce or knockdown telomerase expression at specific developmental stages and investigate the effect on proliferation and differentiation using molecular and cellular biological techniques. In addition, we will also initiate the study to explore possible molecular mechanisms underlying the effect. Our objectives are to address the following questions:
1. Does deficient telomerase expression affect self-renewal of hESCs?
2. Does ectopic expression of telomerase affect hESCs differentiation and at what developmental stage?
3. Are effects of hTERT on proliferation and differentiation of stem cells mediated by telomere-related mechanism or extra-telomeric function?

Our findings will provide new insights for understanding telomerase function in stem cell proliferation and differentiation, which will have important implication in better understanding of stem cell biology, ageing and cancer formation as well as stem cell applications in biomedical and medical fields.