Mesenchymal Stem Cells Derived from Induced Pluripotent Stem Cells are Epigenetically Rejuvenated


Earlier this year, Miltalipov and his research group published a paper in Nature that compared the genetic integrity of embryonic stem cells made from embryos, to induced pluripotent stem cells and embryonic stem cells made from cloned embryos.  All three sets of stem cells seemed to have comparable numbers of mutations, but the induced pluripotent stem cells had “epigenetic changes” that were not found in either stem cell line from cloned or non-cloned embryos.

Genetic characteristics have to do with the sequence of the DNA molecules that make up the genome of an organism.  Epigenetic characteristics have nothing to do with the sequence of DNA, but instead are the result of small chemicals that are attached to the DNA molecule.  These small chemical tags affect gene expression patterns.  Every cell has a specific epigenetic signature.

During development, the cells that will form our eggs and sperm in our bodies, the “primordial germ cells,” begin their lives in the outer layer of the embryo.  During the third week of life, these primordial germ cells or PGCs move like amoebas and wander into the yolk sac wall and collect near the exit of a sac called the “allantois.”  The PGCs are outside the embryo at this time or extraembryonal.  Incidentallyyolk sac is a terrible name for this structure, since it does not produce yolk proteins.  Therefore other textbooks have renamed it the “primary umbilical vesicle,” which is a bit of a mouthful, but it probably better than “yolk sac.”

 

1 - Primordial germ cells 2 - Allantois 3 - Rectum 4 - Ectoderm 5 - Foregut 6 - Primordial Heart 7 - Secondary yolk sac 8 - Endoderm 9 - Mesoderm 10 - Amniotic cavity
1 – Primordial germ cells
2 – Allantois
3 – Rectum
4 – Ectoderm
5 – Foregut
6 – Primordial Heart
7 – Secondary yolk sac
8 – Endoderm
9 – Mesoderm
10 – Amniotic cavity

The embryo around this time undergoes a bending process as a result of its growth and the head bends toward the tail (known as the cranio-caudal curvature) and then the sides of the embryo fold downwards and eventually fuse (lateral folding).  This bending of the embryo allows the PGCs to wander back into the embryo again between the fourth and sixth week.  The PGCs move along the yolk sac wall to the vitelline and into the wall of the rectum.  After crossing the dorsal mesentery (which holds the developing intestines in place) they colonize the gonadal or genital ridge (which is the developing gonad). During their journey, and while in the gonadal ridge, the PGCs divide many times.

1 - Rectum 2 - Vitelline 3 - Allantois 4 - Nephrogenic cord (pink) 5 - Gonadal ridge (green) 6 - Primordial germ cells (red dots) 7 - Heart prominence
1 – Rectum
2 – Vitelline
3 – Allantois
4 – Nephrogenic cord (pink)
5 – Gonadal ridge (green)
6 – Primordial germ cells (red dots)
7 – Heart prominence

When the PGCs move into the developing gonad, the chemical tags on their DNA are completely removed (rather famous paper – Lee, et al., Development 129, 1807–1817 (2002).  This epigenetic erasure proceeds in order for the PGCs to develop into gametes and then received a gamete-specific set of epigenetic modifications.  These epigenetic modifications also extend to the proteins that package the DNA into chromosomes – proteins called histones.  Specific modifications of histone proteins and DNA lead to gamete-specific expression of genes.  Once fertilization occurs, and the embryological program is initiated, tissue-specific epigenetic modifications are conveyed onto the DNA and histones of particular cell populations.

This is a long-winded explanation, but because many cancer cells have abnormal epigenetic modifications, these epigenetic abnormalities in induced pluripotent stem cells (iPSCs) have been taken with some degree of seriousness.  Although, there is little evidence to date that links the cancer-causing capabilities of iPSCs with specific epigenetic modifications, although it certainly affects the ability of these cells to differentiate into various cell types.

A paper has just come from the laboratory of Wolfgang Wagner from the Aachen University Medical School, in Aachen, Germany that derived iPSCs from mesenchymal stem cells from human bone marrow, and then in a cool one-step procedure, differentiated these cells into mesenchymal stem cells (MSCs).  These  iPS-MSCs looked the same, and acted the same in cell culture as the parent MSCs, and had the same gene expression profiles as primary MSCs.  However, all age-related and tissue-specific epigenetic patterns had been erased by the reprogramming process.  This means that all the tissue-specific, senescence-associated, and age-related epigenetic patterns were erased during reprogramming.  Another feature of these iPS-MSCs is that they lacked but the ability to down-regulate the immune response, which is a major feature of MSCs.

Thus, this paper by the Wagner lab shows that MSCs derived from iPSCs are rejuvenated by the reprogramming process.  Also, the donor-specific epigenetic features are maintained, which was also discovered by Shao and others last year.  This suggests that epigenetic abnormalities are not an inherent property of the derivation of iPSCs, and that this feature is not an intractable characteristic of iPSCs derivation and may not prevent these cells from being successfully and safely used in the clinic.  However, this might be a cell type-specific phenomenon.  Also, the loss of the immune system regulatory capabilities of these iPS-MSCs is troubling and this requires further work.

iPS-MSCs

Blood Vessel-Making Stem Cells From Fat


Blood vessel obstruction deprives tissues of life-giving oxygen and leads to the death of cells. If enough cells within a tissue die, the organ in which whose tissues reside could experience organ failure.

To quote the Sound of Music, “How does one solve a problem like blood vessel obstruction?” The obvious answer is to make new blood vessels to replace the blocked ones. Scientists have identified growth factors that are important in blood vessel formation during development. Therefore, injecting these growth factors should lead to the formation of new blood vessels, right? Unfortunately, such a strategy does not work very well (see Collison and Donnelly, Eur J Vasc Endovasc Surg 2004 28:9-23). Therefore, vascular specialists have focused on the ability of stem cells make new blood vessels, and this approach has yielded some very definite successes.

During development, the same stem cell gives rise to blood vessels and blood cells. This stem cell, the hemangioblast is found in a structure known as the yolk sac (even though it never functions as a yolk sac). In the yolk sac, during the third week of development, little specs form called “blood islands. These blood islands are small clusters of hemangioblasts with the cells at the center of the cluster forming blood cells and the cells at the periphery of the blood island forming blood vessels.

In adults, blood cell-making stem cells are found in the bone marrow. Blood vessel-making stem cells are endothelial progenitor cells or EPCs can be rather easily isolated from peripheral blood, however they are thought to originate from bone marrow. EPCs are not a homogeneous group of cells. There are different types with different surface molecules found in different locations.

Recently another cell from circulating blood called an “endothelial colony forming cell” or ECFC has been discovered, and this cell can attach to uncoated plastic surfaces in a growth medium. These cells can be grown to high numbers, even though it takes a rather long time to expand them. Once the ECFC culture system is further perfected, ECFCs will be excellent candidates for therapeutic trials (Reinisch et al., Blood 2009 113: 6716-25).

Fat tissue is also a reservoir of EPCs and mesenchymal stem cells. Fat-based mesenchymal stem cells help induce blood vessel formation and stimulate fat-based EPCs form blood vessels. Because of this remarkable “one-two punch” in fat, with cells that stimulate blood vessel formation and cells that actually form blood vessels, fat is a source of blood vessel-forming cells that can be used for therapeutic purposes.

Stem cells from fat.
Stem cells from fat.

Several pre-clinical experiments and presently ongoing clinical trials have examined the ability of fat-based stems to treat patients with conditions that result from insufficient circulation to various tissues. In rodents, experimental obstruction of the blood vessels in the hindlimb create a condition called “hindlimb ischemia.” In a rodent model of hindlimb ischemia, human fat-based stem cell applications not only improve the use of the limb and decrease limb damage, but also induce the formation of new blood vessels that definitely come from the applied stem cells (Miranville, et al., Circulation 2004 110: 349-55; Planat-Bernard, et al., Circulation 2004 109: 656-63 & Moon et al., Cell Physiol Biochem 2006 17: 279-90). Several clinical trials have been conducted with bone marrow-based EPCs for limb-based ischemia in humans, and these trials have been largely successful(see Szoke and Brinchmann, Stem Cells Translational Medicine 2012: 658-67 for a list of these trials). Adding mesenchymal stem cells from fat might improve the results of these trials.

In the heart, obstructed blood vessels can cause intense chest pain, a condition known as “angina pectoris.” EPCs have been used in clinical trials to treat patients with angina pectoris, and these trials have all been successful and have all used EPCs from bone marrow. These experiments, despite their success, have used bone marrow-based cells that were not fractionated and EPCs are less than 1% of the total number of cells. Also, the vast majority of cells introduced into heart migrate into the lungs, spleen and other organs. Also, those cells that remain tend to die off. A way to improve the survival of these implanted cells might be to combine them with mesenchymal stem cells from fat with EPCs from fat. Presently, the MyStromalCell trial is underway to test the efficacy of fat-based stem cells on the heart.

Fat provides an incredible treasure-trove of healing cells that have been demonstrated in animal experiments to relieve tissue ischemia and generate new blood vessels (for a summary of pre-clinical experiments in laboratory animals, see Qayyum AA, et al., Regen Med. 2012 May;7(3):421-8). Clinical trials with these cells are also underway. We have almost certainly only begun to tap to potential of these exciting cells that can be extracted so easily for our bodies.