Beta-Integrin Implicated In Slow Healing Of Aged Muscles


With age, the function and regenerative abilities of skeletal muscles decrease. Therefore, the elderly can find it difficult to recover from injury or surgery.

A new study from the laboratory of Chen-Ming Fan from Johns Hopkins University has shown that a protein called β1-integrin is crucial for muscle regeneration. β1-integrin seems to provide a promising target for therapeutic intervention to combat muscle aging or disease.

Muscle stem cells are the primary source of muscle regeneration after muscle injury from exercise, accidents, or surgery. These specialized adult stem cells lie dormant in the muscle tissue, and muscles even have them stored off to the side of the individual muscle fibers. Because of their location, these muscle stem cells are known as muscle “satellite cells.” After damage, these satellite cells awaken and proliferate, and go on to make new muscle fibers and restore muscle function. Some satellite cells return to dormancy, which allows the muscle to keep a reservoir of healing cells that can repair the muscle over and over again. Fan and her colleagues determined that proteins called integrins, and in particular, β1-integrin, are integral for maintaining the cycle of hibernation, activation, proliferation, and then return to hibernation, in muscle stem cells.

Integrins are cell surface proteins that provide tight connections between cells and the immediate external environment.

Integrin Dimer Structure: Globular domain structures of α and β subunits in a stable dimer. Ligand binding happens at the interface of the αI (left panel) or β-propeller (right panel) and the βI domain.
Integrin Dimer Structure: Globular domain structures of α and β subunits in a stable dimer. Ligand binding happens at the interface of the αI (left panel) or β-propeller (right panel) and the βI domain.

Without integrins, almost every stage of the regeneration is disrupted. Fan and her group predicted that defects in β1-integrin likely contribute to aging, which is associated with reduced muscle stem cell function and decreased quantities of muscle stem cells. This means that healing after injury or surgery is very slow, which can cause a long period of immobility and an accompanying loss of muscle mass. Inefficient muscular healing in the elderly is a significant clinical problem. Therapeutic approaches would be quite welcome by the aging population and their physicians. One way to improve muscle regeneration would be to stimulate muscle satellite cells in older individuals.

Fan and others determined that β1-integrin function is diminished in aged muscle stem cells. When they artificially activated integrins in aged mice, their regenerative abilities were restored to youthful levels. Improvement in regeneration, strength, and function were also seen when this treatment was applied to animals with muscular dystrophy, which underscores the potential importance of such an approach for the treatment of muscle disorders.

Muscle stem cells use β1-integrin to interact with many other proteins in the external environment of the muscle. Among this forest of proteins in the external environment of the muscle, Fan and her coworkers found one called fibronectin that might be the most relevant. They discovered that aged muscles contain substantially less fibronectin compared to young muscles. Like β1-integrin, eliminating fibronectin from young muscles makes them function as though they were old. However, restoring fibronectin to aged muscle tissue restores muscle regeneration to youthful levels. Fan’s group demonstrated a strong link between β1-integrin, fibronectin and muscle stem cell regeneration.

Taken together, the results show that aged muscle stem cells with compromised β1-integrin activity and aged muscles with insufficient amount of fibronectin both root causes of muscle aging. This makes β1-integrin and fibronectin very promising therapeutic targets.

This work appeared in the following journal: Michelle Rozo et al., “Targeting β1-integrin signaling enhances regeneration in aged and dystrophic muscle in mice,” Nature Medicine, 2016; DOI: 10.1038/nm.4116.

How Stem Cells Exit The Bloodstream


New research from a laboratory at North Carolina State University has changed our understanding of how therapeutic stem cells exit the bloodstream.  Understanding this new process, which has been given the name “angiopellosis” may not only increase our understanding of how intravenous stem cells home to their target tissues, but also how metastatic cancer cells invade new sites.

When white blood cells are summoned to a site of infection, they exit the bloodstream by means of a rather well understood process called “Leukocyte extravasation” or “diapedesis.”.

Leukocyte extravasation mostly occurs in post-capillary venules, where hemodynamic shear force are low.  This process is characterized by 4 steps: 1)  “chemoattraction;” 2)  “rolling adhesion;” 3) “tight adhesion;” and 4)  “(endothelial) transmigration.”. If any of these steps are inhibited, diapedesis does not occur.

White blood cells or leukocytes phagocytose or gobble up foreign particles, produce antibodies, secrete inflammatory response triggers (histamine and heparin), and neutralize histamine.  In general, leukocytes defend an organism and protect it from disease by promoting or inhibiting inflammatory responses. Leukocytes do most of their specific functions in tissues and they use the blood as a transport medium to reach the tissues of the body.

extravasation1319669430297

Below is a brief summary of each of the four steps involved in leukocyte extravasation:

1) Chemoattraction

Upon recognition of and activation by pathogenic organisms, resident macrophages in the affected tissue release small signaling proteins called “cytokines” such as IL-1, TNFα and chemokines (small molecules that induce cell migration). IL-1, TNFα and other blood-based  molecules induce the endothelial cells that line blood vessels near the site of infection to express cellular adhesion molecules, including selectins.  Circulating leukocytes are localized to the site of injury or infection as a result of secreted chemokines.

2) Rolling adhesion

Sugar residues on they surfaces of circulating leukocytes bind to these selectin molecules on the inner wall of the blood vessels.  This interaction, however, is relatively modest in its binding strength.  The sugar-selectin interaction causes the leukocytes to roll along the inner surface of the vessel wall as transient bounds are constantly broken and reformed between selectins and cell-bound sugars.

The carbohydrate binding partner for P-selectin, P-selectin glycoprotein ligand-1 (PSGL-1), is an expressed by different types of leukocytes. The binding of PSGL-1 on the leukocyte to P-selectin on the endothelial cell allows for the leukocyte to roll along the endothelial surface. This interaction can be fine-tuned by the different ways that sugars are attached to PSGL-1.   These different forms of PSGL-1 that have distinct patterns of sugar attachment have unique affinities for different selectins.  This gives different leukocytes varying abilities to migrate to distinct specific sites within the body.

3) Tight adhesion

The chemokines released by macrophages activate the rolling leukocytes and induce them to synthesize surface integrin molecules.   Integrin molecules create high-affinity associations between cells and bind tightly to complementary receptors expressed on endothelial cells. This immobilized the leukocytes, despite the shear forces of the ongoing blood flow.

4) Transmigration

The internal cytoskeleton of the leukocytes are reorganizes that the leukocytes spread out over the endothelial cells. In this form, leukocytes extend pseudopodia and pass through gaps between endothelial cells.  This migratory step requires the expression of PECAM proteins on both the surface  of the leukocytes and the endothelial cells.  PECAM interaction effectively pulls the cell through the endothelium. Once through the endothelium, the leukocyte must penetrate the underlying basement membrane.  The mechanism by which the leukocytes does this remains a source of some dispute.  Once in the interstitial fluid, leukocytes migrate along a gradient of attractant molecules towards the site of injury or infection.

When stem cells are administered intravenously, they too Havre a similar ability to leave the bloodstream, but the means by which they do so was poorly understood.

Ke Cheng and colleagues examined zebrafish and used genetically engineered fish whose blood vessels glowed a fluorescent color.  Next, these fish were injected with leukocytes, and stem cells from rats, humans, and dogs that had been labeled with a red fluorescent protein.  These cells were followed by means of time-lapsed, three-dimensional light sheet microscopic imaging.  This technology allowed Cheng and others to view the stem cells as they left the blood vessels.

As predicted, the leukocytes exited the bloodstream by means of leukocytes extravasation.  The stem cells, however, were actively expelled from the blood vessels by the endothelial cells.  The endothelial cells membranes moved around the stem cells, surrounded them, moved them through the endothelial cells and then extruded them on the opposite side of the blood vessel.  This is a very different process than diapedesis in which the leukocyte is the active participant.  In the case of the stem cells, the endothelial cells are the active participants and the stem cells passively exit the bloodstream.  Cheng and company called this process angiopellosis.

Other differences between angiopellosis and diapedesis involved the time of the process.  Diapedesis can occur rather quickly whereas angiopellosis takes hours.  During diapedesis, one cell moves at a time, but during angiopellosis, several cells are moved at a time.

How effective of a method is this to leave the bloodstream?  If cancer cells used angiopellosis to facilitate metastasis, cent we inhibit it?

Further work should answer these important questions.  This work was published in the journal Stem Cells, 2016; DOI:10.1002/stem.2451.

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.

Stem Cell Behavior in Three-Dimensional Matrices


Scientists from Case Western Reserve in Cleveland, Ohio have used hydrogels (jello-like materials) to make three-dimensional structures that direct stem cell behavior.

Physical and biochemical signals guide stem cell behavior and directs them to differentiate and make tissues like muscle, blood vessels, or bone. The exact recipes to produce each particular tissue remains unknown, but the Case Western Reserve team has provided a way to discover these recipes.

Ultimately, scientists would like to manipulate stem cells in order to repair or replace damaged tissues. They would also like to engineer new tissues and organs.

Eben Alsberg. associate professor of biomedical engineering and orthopedic surgery at Case Western Reserve, who was also the senior author on this research said, “If we can control the spatial preservation of signals, we have be able to have more control over cell behavior and enhance the rate and quality of tissue formation. Many tissues form during development and healing processes at least in part due to gradients of signals: gradients of growth factors, gradients of physical triggers.”

Alsberg and his colleagues have tested their system on mesenchymal stem cells, and in doing so have turned them into bone or cartilage cells. Regulating the presentation of certain signals in three-dimensional space may be a key to engineering complex tissues; such tissues as bone and cartilage. For example, if we want to convert cartilage-making cells into bone-making cells or visa-verse, several different signals are required to induce the stem cells to change into different cell types in order to form the tissues you need.

To test their ideas, Alsberg and coworkers two different growth factors directed the stem cells to differentiate into either bone or cartilage.  One of these growth factors, transforming growth factor-beta (TGF-beta) promotes cartilage formation while a different growth factor, bone morphogen protein-2 (BMP-2).  Alsberg and his crew placed mesenchymal stem cells into an alginate hydrogel with varying concentrations of these growth factors.  Alginate comes from seaweed and when you hit it with ultraviolet light, it crosslinks to form a jello-like material called a hydrogel.   To create gradients of these growth factors, Alsberg developed a very inventive method in which they loaded a syringes with these growth factors and hooked them to a computer controlled pump that released lots of BMP-2 and a little TGF-1beta and tapered the levels of BMP-2 and then gradually increased the levels of TGF-1beta (see panel A below).  

 Fabrication of microparticle-based gradient alginate hydrogels. (A) Photograph of gradient making system. (B) Flow rates of two syringes to pump a linear gradient for a 5 cm length × 2 mm diameter alginate hydrogel. After linear gradient pumping for 3 min, an additional 50 μL of alginate solution, which is the volume from the Y point to the beginning of quartz tube, was further pumped into a spiral mixer for 1 min. (C) Photomicrographs of microparticles in cross-sections of gradient alginate hydrogel segments. Segments 1-10 represent sequential segments of the gel. (D) Quantification of microparticles in each segment of gradient alginate hydrogels.
Fabrication of microparticle-based gradient alginate hydrogels. (A) Photograph of gradient making system. (B) Flow rates of two syringes to pump a linear gradient for a 5 cm length × 2 mm diameter alginate hydrogel. After linear gradient pumping for 3 min, an additional 50 μL of alginate solution, which is the volume from the Y point to the beginning of quartz tube, was further pumped into a spiral mixer for 1 min. (C) Photomicrographs of microparticles in cross-sections of gradient alginate hydrogel segments. Segments 1-10 represent sequential segments of the gel. (D) Quantification of microparticles in each segment of gradient alginate hydrogels.

The result has an alginate hydrogen with mesenchymal stem cell embedded in it that had a high concentration of BMP-2 at one end and a high concentration of TGF-1beta at the other end.  Alsberg also modified the hydrogel by attached RGD peptides to it so that the stem cells would bind the hydrogel.  The peptide RGD (arginine-glycine-aspartic acid) binds to the integrin receptors, which happen to be one of the main cell adhesion protein on the surfaces of these cells.  This modification increases the exposure of the mesenchymal stem cells to the growth factors.  After culturing mesenchymal stem cells in the hydrogel, they discovered that the majority of the cells were in the areas of the hydrogel that had the highest concentration of RDG peptides.  

In another other experiment Alsberg and others varied the crosslinks in the hydrogel.  They used hydrogels with few crosslinks that were more flexible and hydrogels that have quite a few crosslinks and were stiffer.  The stem cells clearly preferred the more flexible hydrogels.  Alsberg thinks that the more flexible hydrogels might show better diffusion of the growth factors and better waste removal.  

“This is exciting,” gushed Alsberg.  “We can look at this work as a proof of principle.  Using this approach, you can use any growth factor or any adhesion ligand that influences cell behavior and study the role of gradient presentation.  We can also examine multiple different parameters in one system to investigate the role of these gradients in combination on cell behavior.”  

This technology might also be a platform for testing different recipes that would direct stem cells to become fat, cartilage, bone, or other tissues.  Also, since this hydrogel is also biodegradable, stem cells grown in the hydrogel could be implanted into patients.  Since the cells would be in the process of forming the desired tissue, their implantation might restore function and promote healing.  Clearly Alsberg is on to something.