How Stem Cells Make New Skin Cells Throughout Life

Beneath the upper epidermal layers of our skin lies a layer of stem cells and their progeny (human epidermal progenitor cells) that continually make new skin throughout our lifetime. How these stem cells manage to form skin and not some other structure is still poorly understood, but a new study from the University of San Diego School of Medicine in the laboratory of George L. Sen has pulled back the curtain on this vital process.

Sen and his colleagues have examined a component of the machinery of the cell known as the “exosome.” The term exosome is confusing because it refers to two different entities. Exosomes are vesicles secreted by cells that are loaded with proteins and RNA molecules that the cell wants to dump (Kooijmans, et al., Int J Nanomedicine. 2012; 7: 1525–41). Exosomes are used by cells to export materials to other others cells. Cells also use exosomes to regulate processes, since by ridding themselves of proteins and RNAs that direct particular processes, effectively shuts those processes down. However, exosome also refers to a complex of proteins that are involved in 3′–5′ exonucleolytic degradation. This exosome consists of ~11 proteins that degrade RNAs and regulate processes.

In skin-based stem cells, the exosome (RNA degradation machinery) functions in skin stem cells and provides one of the main mechanisms by which stem cells stay stem cells and skin cells stay skin cells. Exosomes and their targets may help point the way to new drugs or therapies for not just skin diseases, but other disorders in which stem and progenitor cell populations are affected.

Stem cells can divide throughout their lifetime, and their progeny can differentiate to become any required cell type. The progeny of stem cells, progenitor cells, have more limited developmental capabilities, and are only able to divide only a fixed number of times and form a few distinct cell types. When it comes to skin, progenitor and stem cells deep in the epidermis constantly produce new skin cells called keratinocytes that gradually rise to the surface where they will mature, die, and be sloughed off.

Exosomes degrade and recycle different RNA molecules, such as messenger RNAs that wear out or that contain errors. Such errors would cause the production of junk protein, and this would be deleterious to the cell.

According to Sen: “In short, the exosome functions as a surveillance system in cells to regulate the normal turnover of RNAs as well as to destroy RNAs with errors in them.” Sen and his colleagues discovered that in the epidermis the exosome functions to target and destroy mRNAs that encode for transcription factors that induce differentiation. One of the targets of the exosome in epidermal progenitor cells is a transcription factor called GRHL3. GRHL3 promotes the expression of genes necessary for skin cell differentiation. Routine destruction of GRHL3 keeps epidermal progenitor cells undifferentiated. When the epidermal progenitor cells receive signals to differentiate, the progenitor cells down-regulate the expression of certain subunits of the exosome, and this leads to higher levels of GRHL3 protein. The increase in GRHL3 levels promotes the differentiation of the progenitor cells to skin cells.

“Without a functioning exosome in progenitor cells,” said Sen, “the progenitor cells prematurely differentiate due to increased levels of GRHL3 resulting in loss of epidermal tissue over time.” Sen also noted that these findings could have particular relevance if future research determines that mutations in exosome genes are linked to skin disorders or other diseases.

“Recently there was a study showing that recessive mutations in a subunit of the exosome complex can lead to pontocerebellar hypoplasia, a rare neurological disorder characterized by impaired development or atrophy of parts of the brain,” said Sen. “This may potentially be due to loss of progenitor cells. Once mutations in exosome complex genes are identified in either skin diseases or other diseases like pontocerebellar hypoplasia, it may be possible to design drugs targeting these defects.”

Plant Virus Speeds Bone Growth from Stem Cells

Sometimes great scientific discoveries are the result of serendipity, The German organic chemist August Kekulé supposedly came upon the structure of the chemical benzene because of a day-dream in which he saw a snake biting its own tail. This story illustrates that even brilliant scientists conceive of some of their greatest ideas in ways that are accidental.

Today’s discovery is a stem cell version of August Kekulé’s day-dream. Researchers at the University of South Carolina have been interested in growing bone from stem cells. In order to get the cells to form bone, Qian Wang, Robert L. Sumwalt Professor of Chemistry at the University of South Carolina, grew mesenchymal stem cells (MSCs) from bone marrow on plastic culture dishes. However, he wanted to test the ability of different surfaces to influence bone formation by MSCs. In order to compare each surface he tested with something that seemed rather innocuous, Wang decided to use plant viruses.

Plant viruses such as turnip yellow mosaic virus are completely harmless to animal cells and they can be isolated in gram quantities from cabbages, which makes them very inexpensive to work with. Wang decided to coat his culture dishes with these plant viruses in order to have something that was a sort of ground-zero surface that did not do anything to the cells. Except there was one problem: The plant virus-coated surfaces helped the stem cells make bone faster than anything else Wang’s group examined.

This gave Wang an idea. When bones break in our bodies, stem cells make new bone at the site of the break in order to help the bones reform and heal themselves. The process can take some time and people with broken bones are usually incapacitated for some time. Wang wondered, “what if we could make that healing process go faster?”

Wang explains: “With a broken femur, a leg, you can be really incapacitated for a long time. In cases like that, they sometimes inject a protein-based drug, BMP-2 [bone morphogen protein-2], which is very effective is speeding up the healing process. Unfortunately,, it’s very expensive and can also have some side effects.” One of those side effects is an increased risk of tumors.

By coating glass slides with turnip yellow mosaic viruses (TYMVs) or tobacco mosaic viruses (TMVs), Wang and his colleagues found that the stem cells did not take nearly as long to form bone in culture.

Since making that discovery four years ago, Wang has been trying to determine what is it about the viruses that cause the MSCs to make bone so effectively. It turns out that the viruses form a specific topography on the glass slides and this topography forms a kind of “easy chair” for the MSCs. The next question was, “Could this easy chair be made into more of a Sleep Comfort Bed for the MSCs?” Could they improve it?

Wang and his group set about chemically modifying the surfaces of the viruses and binding things to them in order to stabilize the interaction of the viruses with the MSCs and the slide.

The results were astounding. With the right concoction of plant viruses coating the glass slides, the right molecules bound to the plant viruses, and the right culture medium recipe, Wang’s group found that they could induce MSCs to form bone in two days. The cells are also make many proteins that are specific to bone formation.

According to Wang: “What we’ve seen could prove very useful, particularly when it comes to external implants in bones. With those, you have to add a foreign material, and knowing that a coating might increase the bone growth process is clearly beneficial. But more importantly, we fell we’re making progress in a more general sense in bone engineering. We’re really showing the direct correlation between nanotopography and cellular response. If our results can be further developed, in the future you could use titanium to replace the bone and you might be able to use different kinds of nanoscale patterning on the titanium surface to create all kinds of different cellular responses.”

In many ways this work is just the beginning of what will almost certainly become a remarkable advance in bone engineering.

Platelet-Lysate Bioactive Scafold for Tissue Engineered Cartilage

Cartilage replacement at joints represents a tremendous challenge for regenerative medicine. While growing cartilage in culture is possible, scaling this technology up to generate enough high-quality articular cartilage (the kind of cartilage found at joints), is still a distinct challenge. To date, stem cell treatments can heal small breaches in cartilage, but reconstructing large lesions is still not possible. In general, cartilage at joints has very poor healing properties, and therefore, is a major challenge in orthopedics.

A major improvement in therapeutics is the use of a technique called “autologous chondrocyte implantation” or ACI. ACI involves the delivery of healthy cartilage-making cells (chondrocytes) from the patient’s own body after these cells have been grown and expanded in culture. In order to coax these cartilage-making cells to make cartilage, special scaffolds are used that provide a three-dimensional matrix upon which the chrondrocytes grow and form cartilage. These 3-D scaffolds are essential to keep the chondrocytes differentiated and making cartilage.

One of the most promising types of scaffolds for making cartilage are “bioactive 3D scaffolds.” These types of scaffolds can deliver growth factors and other molecules to the chrondrocytes and boost their growth and cartilage production.

In a recent publication, Andrei Moroz and colleagues in the Extracellular Matrix Laboratory at the Botucatu Institute of Biosciences, São Paulo State University, Brazil, have used mesenchymal stem cells (MSCs) from rabbit bone marrow and differentiated them into chondrocytes. This allowed them to use stem cells from bone marrow instead of harvesting cartilage from the joints, which can be very painful and deleterious to the joint. The main innovation in this paper was the use of a platelet-lysate-based 3D bioactive scaffold to support the chondrogenic differentiation and maintenance of MSCs.

MSCs from adult rabbit bone marrow were isolated, characterized, and grown in 60 microliters of platelet lysate from rabbit blood. Platelets are very small cells from circulating blood that assist in the formation of clots that staunch bleeding after a blood vessel in damaged. Platelets are easy to isolate from circulating blood and the rabbit platelet-lysate clot scaffold was maintained is a standard tissue culture medium (Dulbecco’s Modified Eagle Medium Nutrient Mixture F-12) that was supplemented with other molecules known to induce cartilage formation in MSCs. After three weeks in culture, the MSCs were examined in detail. Not only were they nice and round, but they were filled with cartilage-specific molecules, and clumped together like chondrocytes.

According to this research group, they are on to something with this platelet-lysate bioactive scaffold. It provided a suitable system for culturing MSCs and allowed them to make lots of cartilage. The scaffold also was easy to make, and maintained the MSCs in a cartilage-making state without causing cell death or stressing the cells. Therefore, it might provide an alternative to autologous chondrocyte implantation. The next steps in this research will be to use this engineered cartilage to repair damaged joints to see if the cartilage made by cells embedded in platelet-lysate 3D bioactive scaffolds can act as functional cartilage.

For the article see Andrei Moroz, et al., Platelet lysate 3D scaffold supports mesenchymal stem cell chondrogenesis: An improved approach in cartilage tissue engineering.  Platelets. 2012.