Cytoarcheology : understanding cellular turnover in the human brain and heart

Abstract

The total number of cells within a human being increases dramatically during growth, yet certain organs are thought to have distinct differences within this regard. Just as cells may be added to an organ, cells may die also, and these dynamics of cellular addition and cellular death are known as cellular turnover. The mere presence of cellular turnover of important cell types, within many organs is largely unknown. Given the technical limitations of studying human cellular turnover, new techniques were devised in order to study these important and fundamental biological questions. These answers are key to gaining a better understanding of human brain and heart development, and also may shed insight into future brain and heart regenerative strategies following pathological states.

The conventional methods for studying cellular turnover in experimental animal models are limited or unfeasible when studying humans. A novel strategy was created in order to answer these questions in humans. Motivated by the established tool of radiocarbon (14C) dating to obtain the date of formation of archeological relies, an analogous strategy was sought to be able to retrospectively date human cells. The key was once again 14C, with a half-life of 5730 years. 14C levels have been very stable on earth over the past few thousand years until very recently. From the mid 1950s to 1963, massive above-ground atomic bombs were detonated during the height of the Cold War, which resulted in a dramatic rise in atmospheric 14C levels. This increase distributed evenly throughout the atmosphere and decreased exponentially after the nuclear Test Ban Treaty in 1963, due to equilibration within the biosphere. Atmospheric 14 C enters the biotope via photosynthesis, and then enters the human body via ingestion of plants or animals. The level of 14C within a cell's genomic DNA mirrors atmospheric levels at the time of DNA synthesis, so that it is possible to retrospectively date human cells. This strategy revealed that there was human adult cellular turnover within the blood, intestine, and whole cerebral cortex.

In the past, all neurons in the human brain, were thought to be generated before birth, yet this view has begun to change. Studies looking at cerebral cortex neuronal turnover in adult primate models reported mixed results, causing controversy. The presence of neuronal turnover may confer a certain degree of plasticity. However, it may also be that the lack of neuronal turnover within the cerebral cortex allows for the stalwart stability of information and memory storage needed over the many decades of life. The 14C strategy was used to assess adult human neuronal turnover by investigating various neocortical brain regions in all lobes of the cerebral cortex. Nuclei from post mortem human brains were isolated from different cortical regions. In order to isolate only those nuclei from neurons, an antibody that specifically bound to neuronal nuclei (NeuN), was used to separate these from the non-neuronal nuclei. Flow cytometry was used to separate and collect the different nuclei populations so that their DNA could be measured for 14 C analysis. The results were that the non-neuronal cells were capable of turnover in the adult, but that the neuronal cells were not generated during adulthood at detectable levels in any of the cortical regions studied.

The presence of cellular turnover is also controversial in the adult heart. Some studies have indicated that new cardiomyocytes are produced in murine adult models. Human studies have also demonstrated that adult fusion of cardiomyocytes occurs during certain circumstances. The retrospective birth dating of cardiomyocyte cells would be able to shed important light into this subject. Human left ventricular heart tissue DNA underwent 14 C analysis, and turnover was conclusively observed. Since roughly 20% of all cells within heart tissue are in fact cardiomyocytes, isolation of this specific population was necessary to determine the level of turnover. The protein, cardiac troponin-I, is mainly located in the cytosol of cardiomyocytes, but it was also found wit in the nuclei. By using a nuclear cardiomyocyte specific antibody (cardiac troponin-I), the nuclei from both cardiomyocyte and non-cardiomyocyte populations were isolated and collected using flow cytometry. 14 C analysis of cardiomyocyte nuclei DNA was performed, and demonstrated that turnover is occurring after early development.

The dynamics of cellular turnover in the adult human being need to be better understood, both from a basic biological, and from a regenerative perspective. By developing and applying the 14C method to retrospectively date human cells within the brain and heart, a deeper understanding of these dynamics becomes apparent. Neocortical neurons appear to be generated only in early development, whereas the human heart is able to replenish some of its cardiomyocytes after this period. This knowledge may offer some insight into future heart repair strategies following ischemia, which is a very common source of human morbidity. Knowledge regarding the adult cellular dynamics of vital tissues, such as the heart and brain, is important with the hope of better treating ourselves in the future.