Birth Dating of Cells by Retrovirus

BIRTH DATING OF cells is important to fully elucidate when a cell has been created, to determine what factors or conditions may have led to its conception. Birth dating has been applied within many cell constructs, though recent attention has focused on cells within the central nervous system. With the relatively recent discovery of neuro—genesis, a process of creating functionally integrated neurons from progenitor cells, a plethora of innovative techniques to track the rate of cell birth have been developed.

Among some of the most widely used techniques for birth dating of cells are analyses based on the incorporation of nucleotide analogs during cell division, expression of specific markers during the maturation process, and genetic marking with retroviruses. The most robust and reliable results for birth dating are generated from nucleotide analogs and genetic marking by retrovirus, whereas the expression of specific markers has elicited relatively poor results. Birth dating of cells by retrovirus has stimulated a great deal of interest because of the ability to visualize tissue directly, as opposed to the nucleotide analog methodology, which requires tissue fixation and DNA denaturing. Though the retroviral method is invasive, many researchers feel that the pros outweigh the cons of this newly developed birth dating technique.

Within the paradigm of neurogenesis, it is extremely important to understand when a functional neuron has been created. This information is critical, as it allows researchers to focus on aspects of the microenvironment at a specific time point that led to the production of new cells. This essentially enables researchers to “rewind time” to reliably chart cell birth and development in a living system. As a consequence, important facts about neurogenesis can be collected, such as the region of the brain in which this process is occurring, particular factors/neurotransmitters that are present in the microenvironment at the appropriate time, genes that may be upregulated [Page 30]or downregulated, and so on. Birth dating of cells by retrovirus can provide all of these details in a living system, which has afforded the most convincing evidence thus far that newborn neurons in the adult mammalian central nervous system are in fact functional and physiologically active. The ability of a retrovirus to integrate into normally functioning tissue offers a tremendous advantage compared with other techniques.

To fully comprehend the use of a retrovirus to date the birth of a cell, one must understand the properties of a retrovirus. Retroviruses are enveloped viruses with an RNA genome that replicate via a DNA intermediate. Retroviruses rely on the enzyme reverse transcriptase to reverse transcribe its genome from RNA into DNA, which can then be integrated into the host's genome using the enzyme integrase. The virus then replicates as part of the host cell's DNA. Furthermore, there are many different subfamilies of retroviruses that each have different properties. For example, the oncovirus subfamily of retroviruses depends on host cell proliferation for completion of the viral life cycle, whereas the lentivirus subfamily replicates without this process. It is important for researchers to understand the intricacies of the retroviral vector they choose, as some vectors rely on the breakdown of the host cell's nuclear membrane for integration, whereas some possess the appropriate nuclear import abilities so that integration into the host cell's genome can occur at all times.

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