Priming Cocktail for Cardiac Stem Cell Grafts


Approximately 700,000 Americans suffer a heart attack every year and stem cells have the potential to heal the damage wrought by a heart attack. Stem cells therapy has tried to take stem cells cultured in the laboratory and apply them to damaged tissues.

In the case of the heart, transplanted stem cells do not always integrate into the heart tissue. In the words of Jeffrey Spees, Associate Professor of Medicine at the University of Vermont, “many grafts simply didn’t take. The cells would stick or would die.”

To solve this problem, Spees and his colleagues examined ways to increase the efficiency of stem cell engraftment. In his experiments, Spees and others used mesenchymal stem cells from bone marrow. Mesenchymal stem cells are also called stromal cells because they help compose the spider web-like filigree within the bone marrow known as “stroma.” Even though the stroma does not make blood cells, it supports the hematopoietic stem cells that do make all blood cells.  Here is a picture of bone marrow stroma to give you an idea of what it looks like:

Immunohistochemistry-Paraffin: Bone marrow stromal cell antigen 1 Antibody [NBP2-14363] Staining of human smooth muscle shows moderate cytoplasmic positivity in smooth muscle cells.
Immunohistochemistry-Paraffin: Bone marrow stromal cell antigen 1 Antibody [NBP2-14363] Staining of human smooth muscle shows moderate cytoplasmic positivity in smooth muscle cells.
Stromal cells are known to secrete a host of molecules that protect injured tissue, promote tissue repair, and support the growth and proliferation of stem cells.

Spees suspected that some of the molecules made by bone marrow stromal cells could enhance the engraftment of stem cells patches in the heart. To test this idea, Spees and others isolated proteins from the culture medium of bone marrow stem cells grown in the laboratory and tested their ability to improve the survival and tissue integration of stem cell patches in the heart.

Spees tenacity paid off when he and his team discovered that a protein called “Connective tissue growth factor” or CTGF plus the hormone insulin were in the culture medium of these stem cells. Furthermore, when this culture medium was injected into the heart prior to treating them with stem cells, the stem cell patches engrafted at a higher rate.

“We broke the record for engraftment,” said Spees. Spees and his co-workers called their culture medium from the bone marrow stem cells “Cell-Kro.” Cell-Kro significantly increases cell adhesion, proliferation, survival, and migration.

Spees is convinced that the presence of CTGF and insulin in Cell-Kro have something to do with its ability to enhance stem cell engraftment. “Both CTGF and insulin are protective,” said Spees. “Together they have a synergistic effect.”

Spees is continuing to examine Cell-Kro in rats, but he wants to take his work into human trials next. His goal is to use cardiac stem cells (CSCs) from humans, which already have a documented ability to heal the heart after a heart attack. See here, here, and here.

“There are about 650,000 bypass surgeries annually,” said Spees. “These patients could have cells harvested at their first surgery and banked for future application. If they return for another procedure, they could then receive a graft of their own cardiac progenitor cells, primed in Cell-Kro, and potentially re-build part of their injured heart.”

Encapsulation of Cardiac Stem Cells and Their Effect on the Heart


Earlier I blogged about an experiment that encapsulated mesenchymal stem cells into alginate hydrogels and implanted them into the hearts of rodents after a heart attack. The encapsulated mesenchymal stem cells showed much better retention in the heart and survival and elicited better healing and recovery of cardiac function than their non-encapsulated counterparts.

This idea seems to be catching on because another paper reports doing the same thing with cardiac stem cells extracted from heart biopsies. Audrey Mayfield and colleagues in the laboratory of Darryl Davis at the University of Ottawa Heart Institute and in collaboration with Duncan Steward and his colleagues from the Ottawa Hospital Research Institute used cardiac stem cells extracted from human patients that were encased in agarose hydrogels to treat mice that had suffered heart attacks. These experiments were reported in the journal Biomaterials (2013).

Cardiac stem cells (CSCs) were extracted from human patients who were already undergoing open heart procedures. Small biopsies were taken from the “atrial appendages” and cultured in cardiac explants medium for seven days.

atrial appendage

Migrating cells in the culture were harvested and encased in low melt agarose supplemented with human fibrinogen. To form a proper hydrogel, the cells/agarose mixture was added drop-wise to dimethylpolysiloxane (say that fast five times) and filtered. Filtration guaranteed that only small spheres (100 microns) were left. All the larger spheres were not used.

Those CSCs that were not encased in hydrogels were used for gene profiling studies. These studies showed that cultured CSCs expressed a series of cell adhesion molecules known as “integrins.” Integrins are 2-part proteins that are embedded in the cell membrane and consist of an “alpha” and “beta” subunit. Integrin subunits, however, come in many forms, and there are multiple alpha subunits and multiple beta subunits.

integrin-actin2

This mixing and matching of integrin subunits allows integrins to bind many different types of substrates. Consequently it is possible to know what kinds of molecules these cells will stick to based on the types of integrins they express. The gene prolifing experiments showed that CSC expressed integrin alpha-5 and the beta 1 and 3 subunits, which shows that CSC can adhere to fibronectin and fibrinogen.

fibronectin

fibrinogen-cleave

When encapsulated CSCs were supplemented with fibrinogen and fibronectin, CSCs showed better survival than their unencapsulated counterparts, and grew just as fast ans unencapsulated CSCs. Other experiments showed that the encapsulated CSCs made just as many healing molecules as the unencapsulated CSCs, and were able to attract circulating angiogenic (blood vessel making) cells. Also, the culture medium of the encapsulated cells was also just as potent as culture medium from suspended CSCs.

With these laboratory successes, encapsulated CSCs were used to treat non-obese diabetic mice with dysfunctional immune systems that had suffered a heart attack. The CSCs were injected into the heart, and some mice received encapsulated CSCs, other non-encapsulated CSCs, and others only buffer.

The encapsulated CSCs showed better retention in the heart; 2.5 times as many encapsulated CSCs were retained in the heart in comparison to the non-encapsulated CSCs. Also, the ejection fraction of the hearts that received the encapsulated CSCs increased from about 35% to almost 50%. Those hearts that had received the non-encapsulated CSCs showed an ejection fraction that increased from around 33% to about 39-40%. Those mice that had received buffer only showed deterioration of heart function (ejection fraction decreased from 36% to 28%). Also, the heart scar was much smaller in the hearts that had received encapsulated CSCs. Less than 10% of the heart tissue was scarred in those mice that received encapsulated CSCs, but 16% of the heart was scarred in the mice that received free CSCs. Those mice that received buffer had 20% of their hearts scarred.

Finally, did encapsulated CSCs engraft into the heart muscle? CSCs have been shown to differentiate into heart-specific tissues such as heart muscle, blood vessels, and heart connective tissue. Encapsulation might prevent CSCs from differentiating into heart-specific cell types and connecting to other heart tissues and integrating into the existing tissues. However, at this point, w have a problem with this paper. The text states that “encapsulated CSCs provided a two-fold increase in the number of engrafted human CSCs as compared transplant of non-encapsulated CSCs.” The problem is that the bar graft shown in the paper shows that the non-encapsulated CSCs have twice the engraftment of the capsulated CSCs. I think the reviewers might have missed this one. Nevertheless, the other data seem to show that encapsulation did not affect engraftment of the CSCs.

The conclusion of this paper is that “CSC capsulation provides an easy, fast and non-toxic way to treat the cells prior to injection through a clinically acceptable process.”

Hopefully large-animal tests will come next. If these are successful, then maybe human trials should be on the menu.