Figure 1: The Hematopoietic Hierarchy. Long Term Hematopoietic Stem Cells (LT-HSCs), also known as real stem cells of hematopoietic system, are at the top of the hierarchy. They give rise to Short Term (ST)-HSC and then more differentiated blood cell types (Erythrocytes, platelets, granulocytes, macrophages, T cells, NK cells, and B cells) through common myeloid progenitors (CMP) and Common lymphoid progenitors (CLP). HSC are responsible for the replacement of blood cell lost by production of billions of new white blood cells, red blood cells and platelets each day.
Hierarchy of hematopoietic stem cells
The blood system, also known as hematopoietic system, has enormous regenerative capacity to maintain functional mature blood cells that arise from highly proliferative but short-lived progenitor cells. Those progenitors in turn are generated from very rare blood stem cells called hematopoietic stem cells (HSCs) (Fig. 1). HSCs are one of the most studied stem cells in our body which has greatly shaped our thinking on the features of adult stem cells. These stem cells are kept at the bone marrow in close proximity to bone cells and other supporting cells forming the specialized home known as niche (Fig. 2). In several aspects, a niche resembles a cellular stronghold that a queen lives in a safe and protected environment.
The interaction of stem cells with the niche is crucial as this prevents exhaustion of stem cells from uncontrolled cellular divisions and proliferation. While active progenitors account for the generation of mature blood cells, hematopoietic stem cells function as a reserved cell population. Interestingly, we observe the importance of the balance between those two cell populations in the aging process. Although the number of HSCs increases in aged animals, there is a decline in self-renewal of HSCs.
Other protective mechanisms include the low proliferation rates of HSCs in a relatively quiescent state, residing in a low oxygen environment , a relatively low degree of metabolism and preferential use of glycolysis as energy source, and additional protection mechanism against oxidative stress.
Low in oxygen but a good place to be!
Stem cells as the cell bank of the body are protected against internal and external insults by a number of mechanisms. Stem cell niche not only provides an environment that they can survive but also poses the lesser degree of internal and external insults. Those possible stresses on cells include, but not limited to, UV exposure, radiation, toxic chemicals, and free oxygen species that cause various damages in the cell including mutations in DNA (Fig. 3). Cells respond to those external and internal issues by various ways such as senescence (loss of stem cell activity), cell death or DNA repair. For example, blood stem cells mainly house in the bone marrow next to osteoblastic lining (blood-forming cells) and endothelial cells where they form the hypoxic (low oxygen tension) endosteal region. This hypoxic niche of HSCs provides lower levels of oxygen so that there are lower levels of free oxygen radicals that mainly arise from electrons leaking from mitochondria during oxidative phosphorylation. In addition, it has been shown that HSC express higher levels of hypoxia inducible factor-1α, a master regulator at low oxygen tension with hundreds of downstream targets regulating various aspects of metabolism including defense against oxidative stress and survival at low oxygen environment. It has also been shown that hypoxia increases self-renewal abilities of HSCs, thus keeps them healthy and functional for longer periods.
Protection from detrimental effects of reactive oxygen species (ROS)
Excess amounts of reactive oxygen species are detrimental to cells. ROS are found to cause hematopoietic stem cell defects as shown in mouse lacking FoxO and Atm genes. In those mutant mice, the hematopoietic defects could be rescued by the use of an antioxidant N-acetyl-cysteine. Anti-oxidants are one of the scavengers that diminish unwanted effects of reactive oxygen species. A number of fruits and vegetables such as beans, blueberry, strawberry, and apple are known with their high content of anti-oxidants. It is amazing to observe anti-oxidants being placed into our sustenance just as much as in some special genes (such as SOD2 and Hypoxia Inducible factor-2α) that provide additional protection for cells. Amazingly, stem cells show high levels of ROS scavenger genes.
Low metabolism provide protection for stem cells
Recent studies demonstrate that hematopoietic stem cells have lower rates metabolism as measured by lower oxygen consumption, lower ATP content and higher lactate production (an end product of cytoplasmic glycolysis) . This means that stem cells produce and consume lesser energy (ATP) compared to more differentiated cells and the by-products of the energy production are kept lower. As higher energy demand brings higher rates of internal insults like production of ROS which is associated with aging and cellular damages, HSCs are granted with another protective mechanism by preferential use of glycolysis (anaerobic) instead of oxidative phosphorylation (aerobic).
Hematopoietic stem cells are quiescent
Another defense mechanism is the quiescence of stem cells which is associated with slow cell-cycle progression. Quiescence of stem cells means that they are kept at a resting, inactive state thus sustaining a self-renewing HSC compartment for life. Because when cell divides, they have to undergo thousands of chemical reactions including making a copy of the three billion letter long DNA, which puts cells at risk to get mutations. Thus, they don’t undergo division unless there is a stimulus. In addition, it has been found that HSCs divide only once every 145 days on average.
There are a number of studies indicating that there are signals in the niche that keeps HSCs in a quiescent state. Tie2/Ang-1 signaling, for instance, has been demonstrated to contribute to the maintenance of HSCs by inducing quiescence. While Ang-1 is expressed in the mesenchymal/stromal cells of niche, its receptor Tie2 is expressed at HSCs. In addition, it has been shown that Ang-1 can inhibit HSC division in culture and promote quiescence of HSCs in the bone marrow .
It is also reported that the cell adhesion molecules that allow physical interaction between stem cells and their niche components may participate in regulation of stem cell quiescence through a process called contact dependent inhibition of proliferation. For instance, it has been found that cell adhesion molecules such as N-cadherin, β1-integrin, and osteopontin might be involved in the regulation of cell cycle status of HSCs .
One advantage of quiescence of HSC comes from the lower susceptibility of slowly proliferating cells to radiation than other cells due to the expression of cell cycle inhibitors like p21 and anti-apoptotic (controlled cell death) machinery like ATM in HSCs. In addition, studies in p21 (a cell cycle inhibitor gene) knockout mice suggest that maintaining cell cycle quiescence is directly linked to self-renewal of HSCs .
Toxics are exported from hematopoietic stem cells
There are other issues concerning external insults against toxics and unwanted chemicals. An HSC population described as side population has been equipped with a number of transporters such as ATP Binding Cassette (ABC) transporters, P-glycoprotein (P-gp/ABCB1) and Breast Cancer Resistance Protein (BCRP/ABCG2) on their membrane providing high efflux ability . They play an important role in the excretion of drugs and endogenous compounds. Those transporters work actively when there is an entrance or excess of such chemicals thus keeping damage minimal.
HSCs are placed in such an environment that even minimum damages by internal and external insults are prevented by different defense mechanisms including residing HSCs in the hypoxic niche, expression of ROS scavenger genes, preferential use of glycolytic metabolism, quiescence nature of HSCs, and removal of toxins by ABC transporters. It is very wise to home such an important cell in a place where it can prosper with a carefully balanced rate of cell division and metabolism. Hypoxic niche seems key to the protection of hematopoietic stem cells by supporting self-renewal and preservation of hematopoietic functions both at the same time. The presence of these protective systems that are graciously placed in our cells with perfect measurements provides an elusive mechanism to ensure healthy life-long reservoir of HSCs.
Ali Fethi Toprak is a PhD candidate at Southwestern Medical Center, Texas University.
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