Natural selection also makes extensive use of replacement in organ maintenance in long-lived organisms such as humans. Replacement involves proliferation of progenitor or stem cells. In humans, there is a great amount of tissue-specific cell turnover. For example, new skin cells, gastrointestinal epithelial cells, and hematopoietic cells are produced continually. At the other extreme, new skeletal muscle, cardiomyocytes, and neurons are rarely formed, although in each case a limited capacity for renewal results from embedded stem cells in adult tissue. Although both progenitor and stem cells may be involved in organ homeostasis, a distinction can be made in tissues that have high cell turnover rates, such as tissue composed of epithelial cells. In the lung, progenitor cells maintain homeostasis, with an overall proliferation rate of less than 1% per day.139,140
Clara cell secretory protein expressing stem cells are responsible for resistance to pollutants and regenerate specialized epithelial cell types in the bronchioles. Such cells function to repair more serious injuries that result in local depletion of epithelial progenitor cells.141
Moreover, some tissues, such as pancreatic islet cells, are believed to lack stem cells and solely use progenitor cells for maintenance.142
Conversely, natural selection of organisms that lack replacement mechanisms are largely postmitotic as adults and are short-lived, as is the case of C. elegans.138
Because organs are not needed after the period of successful reproduction, mechanisms to maintain organ function past this time are not likely to have been selected. This likely explains the small number of cardiac stem cells in the adult mammalian heart and the low level of cardiomyocyte turnover during the human lifespan.143
Enhanced regeneration appears to be the mechanism used by natural selection to maintain viability in the few examples in the animal kingdom in which the soma is not disposable. Asexual species of planarians and hydra lack germ cells, but instead have pluripotent somatic cells to replace aging, postmitotic tissue. Hydra eliminate aging cells by sloughing off differentiated cells and are considered essentially immortal.144
Asexual planaria detach their tails and regenerate two halves of their body, a process that requires their pluripotent somatic stem cells, neoblasts, to not only differentiate, but to recreate the positional information inherent in their body plan.145
Although the planarian's method of maintaining healthspan is not practical for humans, the idea that it may be possible to significantly enhance tissue regeneration has significant merit.
AHE seeks replacement strategies that include enhanced rejuvenation/regeneration, stem cell–based tissue/organ repair, tissue/organ replacement, and the development of durable artificial organs.
Stimulation of endogenous tissue maintenance pathways to remove poorly functioning cells and replace them with new cells derived from progenitor or stem cells is the least disruptive strategy for tissue rejuvenation. Successful development of this strategy necessitates avoidance of dysfunction cellular immortalization (cancer) and replicative senescence (Hayflick limit).
Enhanced regeneration is likely to be highly tissue specific. Blastema formation, a hallmark of limb regeneration in newts and axolotls, has been observed in punctured ears of MRL mice146
as well as other strains.147
Thus, mammals may possess far more extensive regenerative capacity than is usually believed, although it appears that critical organs such as the heart and brain possess very limited regenerative capacity.
Until recently, the idea that the heart had significant regenerative potential or the capacity to generate new cardiomyocytes was controversial. Recent evidence suggests the human heart does have limited regenerative capacity that might be exploited for AHE. Hsieh et al. have provided evidence for the existence of at least a small population of adult cardiomyocyte stem cells that replace damaged cardiomyocytes after injury.148
Furthermore, human cardiomyocytes are not a static postmitotic population. Bergmann et al. used carbon-14 (originating from 1950s atmospheric nuclear bomb tests) levels in DNA of human cardiomyocytes to demonstrate cell proliferation ranging from 1% in 25 year olds to 0.45% in 75 year olds.143
This implies that almost 50% of cardiomyocytes may be renewed during a human lifetime.
The first promising step in the development of pharmaceutical rejuvenation techniques to increase new cardiomyocyte formation has been recently described. Stimulation of mononucleated cardiomyocytes (most cardiomyocytes are binucleated) by neuregulin 1149
causes up to 6.5% of mononucleated cells to undergo DNA synthesis. Sufficient cardiomyocyte replacement prevented cardiac dilatation in a murine congestive heart failure model (following ischemia reperfusion induced by ligated left anterior descending artery [LAD]). Cell cycle reentry of postmitotic cells may be possible in skeletal muscle as well. A small synthetic molecule, reversine, stimulates skeletal muscle to dedifferentiate into a mesenchymal-like stem cell capable of forming chondrocytes and adipocytes.150
Development of new drugs that enhance preexisting regenerative capacity are an important goal of AHE, although serious hurdles persist. At a minimum, tissue specific/localized delivery may be required to contain the potentially deleterious activities of such compounds.
Stem cells and artificial organs
Stem cells provide a potential means to replace or augment old or malfunctioning tissue. In principle, physiological reserve and maintenance of homeostasis could result. However, four basic problems need to be solved for optimal use of stem cells: (1) Means to introduce the cells into a target tissue, (2) means to insure correct cell differentiation, (3) means to remove damaged or dysfunctional cells, and (4) means to insure integration into the target tissue. Replacement of damaged or diseased tissue is already being attempted with a variety of adult stem cells. Sources include hematopoietic, bone-marrow derived mesenchymal cells, and adipose cells.
Given the heart's limited regenerative capacity, a variety of attempts have been made to treat heart failure resulting from myocardial infarction using injection of either hematopoietic or bone marrow-derived mesenchymal autologous stem cells. Improvement of up to 10% of left ventricular ejection fraction has been observed. It appears that these adult stem cells mainly increase blood supply to the heart by differentiating into endothelial cells. The problem of delivery and retention of stem calls in the area of injury can be addressed by bispecific antibodies, one binding to the stem cell and the other binding an injury antigen such as myosin light chain.151
Unfortunately, this approach does not solve the cell differentiation problem, i.e., provide viable replacement cardiomyocytes.
The recent development of techniques to reprogram somatic cells to become pluripotent (induced pluripotent stem [iPS] cells) may be the breakthrough needed to generate sufficient numbers of specialized autologous cells for repair and replacement of damaged tissue. Takahashi152,153
and colleagues found that ectopic expression of four transcription factors, Oct4, Sox2, Klf-4, and c-Myc, could reprogram somatic mouse and human tissue into cells that resemble embryonic stem (ES) cells. The original methodology used retroviral and lentiviral gene therapy techniques that are currently problematic for human therapy. Presently there is no gene therapy treatment approved as a therapeutic in the United States. However, recent work suggests a combination of small molecule drugs and cell-transducible reprogramming proteins will increase the safety of iPS production.154–156
Techniques to differentiate iPS cells into many cell types, including cardiomyocytes, are under active development,157
and clinical trials are on the immediate horizon.
Despite substantial progress, outstanding problems remain. Among these are: How best to remove dead or damaged cells without scar formation, how best to remove aged cells, and how best to promote cellular integration without disrupting tissue function. The latter problems are likely to be more difficult than the former, because the human body has mechanisms for removal of dead cells that can be co-opted.
Haphazard cellular integration may be problematic because appropriate structural and electrical connections must be established in the heart. Unfortunately delivery of skeletal muscle cells, originally thought to be a ready source of large numbers of autologous repair cells, failed to revive damaged hearts due to associated serious arrhythmias.158
Transplanted cells can apparently sense potential problems with tissue integration; as a result, cardiomyocytes directly injected into rodent hearts have a strong tendency to die before successful integration.159
iPS cells and ES cells are also being used to create whole organs ex vivo
for transplantation. Although this goal remains in the realm of science fiction, various hybrid organs are being developed using a combination of differentiated cells derived from ES or iPS cells, bioengineered microenvironments, scaffolds, and encapsulation materials. For example, substantial progress has been made toward an artificial liver.160
A serious drawback for AHE is that organ replacement requires transplantation surgery. Although iPS cells solve the immune rejection problem associated with current techniques, incomplete reenervation and the risks of surgery, especially on the elderly, make such a strategy impractical. Delivery of new tissue by catheterization may reduce this risk, although organ size limits this approach. Perhaps cells and tissues can be programmed for self-assembly in vivo.
For example, heart valve engineers are designing possible in vivo
assembly strategies in conjunction with catheter-based delivery.161
Bioengineered mechanical organs
Although AHE seeks to optimize biologically based tissues and organs (“carbon-organic-based solutions”), it is necessary to mention that competing biodevice technology (“silicon-inorganic based solutions”) may ultimately prove superior. For example, the implementation of an implantable artificial heart has been a holy grail of bioengineering for quite some time.162
Successful development of left ventricle assist devices (LVAD) represents an important milestone with survival of some patients for months to years. Current problems include strokes, unwieldy power connections, anticoagulation, and increased risk of potentially fatal infections. Interestingly, implantation of LVADs actually restored native heart function in some cases where a transplantable heart was not identified (reviewed by Mountis and Starling163
Problems of integration of artificial biomaterials with biological systems are likely to be solved by hybrid approaches. We hypothesized that repair is best implemented by replacing the highest-level module above the identified malfunction (). This approach requires the least understanding of the underlying system components. Thus, stem cell–based and biomechanical organ transplantation may be predicted to advance more rapidly than technologies impinging at lower levels (e.g., pharmaceuticals).
Potential problems with regenerating the brain
The brain poses a special problem for AHE replacement strategies. Whole brain replacement is obviously nonsensical (although head transplants have been patented!; e.g., US Patent 4,666,425), and even replacement of individual neurons by regenerative therapies may be problematic for maintenance of memory and identity, depending upon the role some individual neurons play in distributed memory networks.164
The creation and maintenance of memory remains a topic of active research, and the degree to which loss of specific neurons or addition of new neurons will disrupt brain memory can only be crudely estimated from brain injury patients.
Although the brain possesses limited homeostatic regenerative capacity, neurogenesis occurs in the hippocampal dentate gyrus and the subventricular zone of the lateral ventricles. The function of this regenerative capacity in adults is not completely understood, but may play a role in spatial memory formation (reviewed in Lee and Son165
). Neurogenesis can be stimulated by a variety of compounds in mammals, including dietary supplements such as curcumin.166
However, it is especially interesting that conventional aerobic exercise is among the most effective means to stimulate neurogenesis.167
Although it is unclear to what extent these cells contribute to maintenance of brain homeostasis, exercise may provide a low-tech means to achieve increased maintenance of brain function. Certainly the cardiopulmonary benefits of aerobic exercise on brain function cannot be ignored.