The p21 gene has long been known for its role in cell cycle arrest and apoptosis. In case of DNA damage it signals to the p53 gene to initiate apoptosis of the cell, averting the possibility of tumorgenesis. Very-recent research indicates something else – expression of P21 is part of what keeps us from growing new limbs or other body parts like salamanders or newts. This blog post reviews the new research in the context of what has long been known about P21.
Some background on P21
P21 is a cell cycle regulator, specifically a CDK (cyclin-dependent kinase) inhibitor(ref). It has long been known to impede stem cell differentiation and proper embryonic development. For example, the 1996 publication Targeted in vivo expression of the cyclin-dependent kinase inhibitor p21 halts hepatocyte cell-cycle progression, postnatal liver development and regeneration states “The CDK inhibitor p21 (WAF-1/CIP-1/SDI-1) has been implicated in DNA damage-induced p53-mediated G1 arrest, as well as in physiological processes, such as cell differentiation and senescence, that do not involve p53 function. — These results provide the first in vivo evidence that appropriate p21 levels are critical in normal development and further implicate p21 in the control of multiple cell-cycle phases.”
P21 has been studied for its role in fibrosis and other lung diseases. A 2004 publication is entitled Induction of CDK inhibitor p21 gene as a new therapeutic strategy against pulmonary fibrosis. The 2008 publication P21 regulates TGF-beta1-induced pulmonary responses via a TNF-alpha-signaling pathway relates “Transforming growth factor (TGF)-beta(1) is an essential regulatory cytokine that has been implicated in the pathogenesis of diverse facets of the injury and repair responses in the lung. The types of responses that it elicits can be appreciated in studies from our laboratory that demonstrated that the transgenic (Tg) overexpression of TGF-beta(1) in the murine lung causes epithelial apoptosis followed by fibrosis, inflammation, and parenchymal destruction. Because a cyclin-dependent kinase inhibitor, p21, is a key regulator of apoptosis, we hypothesized that p21 plays an important role in the pathogenesis of TGF-beta(1)-induced tissue responses. — Collectively, our studies demonstrate that p21 regulates TGF-beta(1)-induced apoptosis, inflammation, fibrosis, and alveolar remodeling by interacting with TNF-alpha-signaling pathways.”
Expression of P21 is a barrier to stem cell differentiation. The 2000 publication Hematopoietic Stem Cell Quiescence Maintained by p21cip1/waf1 states “Therefore, p21 is the molecular switch governing the entry of stem cells into the cell cycle, and in its absence, increased cell cycling leads to stem cell exhaustion. Under conditions of stress, restricted cell cycling is crucial to prevent premature stem cell depletion and hematopoietic death.” In the absence of P21, hematopoietic stem cells would not remain quiescent in their niches but would instead prematurely differentiate when stress occurs exhausting the pools of those cells and interrupting the normal functioning of the stem cell supply chain leading to premature death. The 2009 paper Accelerating stem cell proliferation by down-regulation of cell cycle regulator p21 offers a consistent message. “Inhibition of the cell cycle regulator p21 results in significant acceleration of mesenchymal stem cell proliferation without promoting spontaneous cellular differentiation.”
P21 is also implicated in active disease processes especially in certain cancers. For example, the 2009 paper Cell-cycle restriction limits DNA damage and maintains self-renewal of leukemia stem cells argues that p21 gives cancer cells the chance to repair their DNA and keep living. “Here we demonstrate that expression of the cell-cycle inhibitor p21 is indispensable for maintaining self-renewal of leukemia stem cells. Expression of leukemia-associated oncogenes in mouse haematopoietic stem cells (HSCs) induces DNA damage and activates a p21-dependent cellular response, which leads to reversible cell-cycle arrest and DNA repair. Activated p21 is critical in preventing excess DNA-damage accumulation and functional exhaustion of leukemic stem cells. These data unravel the oncogenic potential of p21 and suggest that inhibition of DNA repair mechanisms might function as potent strategy for the eradication of the slowly proliferating leukemia stem cells.”
There is a lot more that can be said about P21 but the above is sufficient for this post.
P21 and organ renewal
Interesting news relating P21 to limb and appendage regeneration is reported in a March 2010 report Lack of p21 expression links cell cycle control and appendage regeneration in mice. “Animals capable of regenerating multiple tissue types, organs, and appendages after injury are common yet sporadic and include some sponge, hydra, planarian, and salamander (i.e., newt and axolotl) species, but notably such regenerative capacity is rare in mammals. The adult MRL mouse strain is a rare exception to the rule that mammals do not regenerate appendage tissue. Certain commonalities, such as blastema formation and basement membrane breakdown at the wound site, suggest that MRL mice may share other features with classical regenerators. As reported here, MRL fibroblast-like cells have a distinct cell-cycle (G2/M accumulation) phenotype and a heightened basal and wound site DNA damage/repair response that is also common to classical regenerators and mammalian embryonic stem cells.” MRL mice do not express P21 and are like salamanders in an important respect. When wounded “The super-healing mice form a “blastema”, a clump of immature cells that behave like stem cells, at the injury site. The blastema cells differentiate into the proper cells, leaving virtually no scar tissue or other trace of the injury. Such regenerative power is almost unknown in mammals. But it is common in amphibians such as the newt and axolotl, which can regrow entire limbs(ref).” MRL mice are known to be able to at least partially regenerate digits and “have a far superior ability to regenerate cardiac tissue than do regular mice, and humans(ref).”
The researchers discovered that the wound-injury response of P21-knockout mice was like that of MRL mice. In response to a hole punched in a mouse’s ear, the MRL and P21- mice formed healing blastemas which closed up the holes like they were never there while ordinary P21+ mice formed scars and the holes remained open.
What about the absence of the protective role of P21 in P21- and MRL mice? Are they more cancer prone? “DNA damage is a hallmark of cancer, and regeneration in the healer mice and p21 knockout mice features an increase in DNA damage in the dividing cells at the blastema. — The link between regeneration and cancer notwithstanding, the researchers who originally created the p21 knockout mice haven’t found any evidence that the lack of a p21 gene leads to increased rates of cancer. — Along with the cell proliferation and DNA damage in the heightened regeneration, there’s an increased rate of apoptosis, which kills off cells too severely damaged to be repaired, — “The combined effects of an increase in highly regenerative cells and apoptosis may allow the cells of these organisms to divide rapidly without going out of control and becoming cancerous,” Heber-Katz (an author of the study) said. “In fact, it is similar to what is seen in mammalian embryos, where p21 also happens to be inactive after DNA damage. The down regulation of p21 promotes the induced pluripotent state in mammalian cells, highlighting a correlation between stem cells, tissue regeneration, and the cell cycle(ref).”
The line of research was conducted in the Wistar Institute in the laboratory of Dr. Ellen Heber-Katz. Dr. Heber Katz has been involved in highly related research since the 1990s. In fact they accidentally discovered the regenerative capability of MRL mice back in the mid 90s when they punched holes in the ears of some of these animals to identify them, a standard laboratory procedure. In normal mice the holes stay permanently open. In the MRL mice the holes healed closed without a trace. This set Dr. Katz and her group off on a chain of discoveries based on working with normal and super-healer mice. In 1998 the group reported on multiple regions in 5 chromosomes connected with wound healing. The latest discovery is that reported here(ref).The hope is that it may ultimately be possible to regenerate damaged or missing organs, possibly even fingers or limbs in human beings through temporary inactivation of P21. “If humans can be induced to heal like these healer mice, it would be possible to repair skin wounds without scarring, and to induce regrowth of cartilage. Internal healing could be improved, including that of damage from heart attacks, as a 2001 study in PNAS said(ref).” Spinal cord tissue injury is another potential area of application.
On a personal note, I have been concerned about loss of nerve connections in two fingers and spinal cord tissue damage. See my posts Nerve regeneration and Spinal cord injury pain – a personal story and a new paradigm. And I have been looking forward to the still far-off days when these parts of me can be regenerated. I believe the current research about P21 moves us a step further up the long ladder leading to practical regenerative medicine.
