About two and a half years ago, I injured my left middle finger while rock climbing with some friends. Although other athletes might sneer at the insignificance of an injured finger, to a rock climber, to whom hand strength is everything, an injury to one of these little extremities is dreaded, as it almost certainly entails an extended hiatus from climbing. In the event of injury, relatively enjoyable—although taxing—climbing based training is replaced with dull cross training and tedious therapy, a process which can last several months.
Unfortunately for me, due to both a series of misdiagnoses and a lack of willingness to stay off of the climbing wall, I was unable to use my left hand to its fullest extent on the climbing wall for almost two years, greatly limiting my capacity for both competitive and outdoor rock climbing. Needless to say, being injured for so long was incredibly frustrating, pushing me to seek closure and obtain a certain diagnosis of my finger.
Almost immediately after I originally hurt myself, I knew I had sustained a Proximal Interphalangeal (PIP) Joint injury on my left middle finger (the PIP is the joint about halfway between the knuckle and the fingertip). More than I year later, I discovered conclusively—through means of MRI—that I had damaged the left collateral ligament of the PIP joint, one of two ligaments on either side of the joint that holds it in place. Luckily for me, the ligament had almost completely healed—albeit with a noticeable amount of scar tissue. However, another finding on this MRI was a good deal more troubling: along with damage to a collateral ligament, I had sustained a small amount of damage to the articular (interjoint) cartilage of the PIP joint.
Luckily, the damage, however troubling, proved too small to be of any significance. My ongoing pain, as it turned out, was more psychological than physical. Simply being injured and experiencing pain, inflammation, and general discomfort for so long had created a sort of mental expectation of finger pain, and by spending time retraining my brain, I was eventually able to return to climbing at a level that was challenging for me.
But what if my cartilage damage had been more severe? This question was on my mind for months after the MRI, causing me to research and eventually write this. For those of you who are unaware of the significance of articular cartilage damage, for the majority of the history of modern orthopedics, articular cartilage injuries were considered almost untreatable. This is because, unlike most areas in the body, the cartilage between joints receives no blood supply, obtaining information only through diffusible factors within the surrounding matrix (other nearby cells and tissue) and loading of the joint. Thus, in the event of injury, healthy chondrocytes (cartilage cells) typically have no way to detect their necessity to fill a newly created defect. Furthermore, chondrocytes themselves have little capacity to increase their metabolic rate to manufacture and regenerate chondrocyte tissue, rendering them incapable of healing an injury even if they could detect its presence. And as cartilage tissue is instrumental in ensuring the health of joints by mitigating friction during movement and preventing bone on bone contact, those who lose a significant amount of articular cartilage loose joint mobility and experience pain and inflammation in the afflicted joint.
The degeneration of articular cartilage, better known as Osteoarthritis, not only affects many of the elderly but also has the potential to end one’s climbing career. As climbing puts a virtually incomparable amount of stress on the finger joints, demanding full mobility of the phalanges, any significant loss of cartilage, which certainly leads to inflammation and a decrease in mobility, could prevent any high-level climbing performance. And as this cartilage does not naturally heal, this debilitation could very well be permanent. So what can be done, both for the afflicted climber and the Osteoarthritic? The answer might be stem cell treatment.
First, a brief introduction to stem cells: although all your cells share the same DNA, groups of these cells undergo a process called differentiation, specializing into certain types of cells, (muscle cells, neurons, etc.) expressing different genes, manufacturing different proteins, and behaving differently. For example, fat cells, which are optimized for storage, contain a relatively small amount of mitochondria, whereas muscle cells, which spend high levels of energy contracting and expanding, contain relatively large amounts of the energy-producing organelle. These two cells types share the same genetic information, but express certain sequences of DNA with varying frequencies in order to specialize themselves for their delegated task.
Stem cells, however, have not yet been differentiated, and thus can specialize into a multitude of different cells. Embryonic Stem Cells, obtained from human embryos, can differentiate into virtually any human cell imaginable, and are thus referred to as pluripotent. However, because of the controversial ethics surrounding the acquisition and use of Embryonic Stem Cells, adult Mesenchymal Stem Cells (MSCs), which are harvested primarily autologously (from the same person they will be used to treat), obtained from bone marrow and adipose (fat) tissue, are more commonly used in medical treatment today.
MSCs have a much more limited potential for differentiation; however, this actually makes them better specifically for the treatment of cartilage. As the differentiation potential of MSCs is far less varied than that of Embryonic Stem Cells, there is less risk associated with the use of MSCs to treat Osteoarthritis, as they are able to differentiate into cartilage, but are less likely than Embryonic cells to differentiate into cell types that may exacerbate the injury. However, the associated risk of this happening with either stem cell type is rather low, as transplanted stem cells would receive signals from the Extracellular Matrix (EM) surrounding damaged cartilage tissue, giving them information about the various cell types present in the area and making them far less likely to differentiate into a cell type not found in said area (a stem cell placed in an environment occupied by cartilage and bone tissue would be unlikely to differentiate into a muscle cell).
So, how can stem cells help with cartilage injuries? In short, as stem cells can differentiate into articular cartilage, they have the potential to allow the regrowth of this tissue, effectively offering a cure to Osteoarthritis.
So what’s the hold-up, why aren’t stem cells being used in all orthopedic practices? Well, the application of MSCs in treatment has some challenges, the primary challenge being the difficulty surrounding the ability to control stem cell differentiation so that it results in the correct tissue and is integrated properly and permanently into the surrounding tissue matrix. In order for treatment to be effective, the MSCs used must be coerced into differentiating in a specific type of joint cartilage that best minimizes joint friction, and must be prevented from experiencing any further phenotypic (physical) changes.
One difficulty is that this desired tissue, called hyaline cartilage, is only one of several similar types of cartilage and cartilage-like tissue into which MSCs can differentiate, given the information present in the extracellular matrix surrounding the joint area. For example, many current stem cell injection treatments, which directly inject autologously obtained MSCs into an afflicted joint area, rather than resulting in a successful differentiation and integration of hyaline cartilage, instead result in the presence of lower quality fibrocartilage, which is significantly worse of minimizing the friction associated with joint movement, therefore doing far less to mitigate the symptoms associated with Osteoarthritis.
Furthermore, even the amount of hyaline cartilage that is sometimes produced from these injections does not always integrate properly into the surrounding tissue matrix, usually rendering the treatment ineffective in the long-term. Furthermore, stem cells that, at first, correctly differentiate and integrate often experience chondrocyte hypertrophy, a process associated with the pathology of Osteoarthritis which enlarges cartilage cells and produces detectable changes in their gene expression. This change in gene expression is most associated with a notable increase in the production of collagen type X, a protein which facilitates ossification (bone formation) from chondrocytes, effectively rendering the obtained hyaline cartilage ineffective in the long-term reduction of symptoms associated with osteoarthritis, as the normally friction reducing articular chondrocytes begin to work to form new bone tissue rather than maintaining their original function of producing collagen type II (a similar protein found in healthy articular cartilage) to minimize the friction between bones.
The limitation of the process of chondrocyte hypertrophy is the primary challenge associated with MSC based therapy that has yet to be solved. Many factors influence the differentiation and subsequent behavior of stem cells, but in short, “an improved microenvironment with timely correlated signals from biomaterials, growth factors, proteases, adjacent cartilage and subchondral bone may be key to a third generation of techniques to regenerate hyaline cartilage” (Richter). Basically, in order to ensure the correct differentiation and subsequent sustainability of MSCs into hyaline cartilage, we must be able to control the messages, the biological signals and growth factors these cells receive, both from adjacent tissue and the rest of the body.
Possible solutions to this seemingly daunting problem include the use of extracellular vesicles, epigenetic techniques, and gene editing techniques, each of which I will discuss briefly (although I encourage you to do more reading on these subjects if you are interested, as they all have potential to be applied in many other areas of bioengineering).
Extracellular vesicles (EVs), in simplest terms, are packages secreted by cells into the Extracellular Matrix (EM) that surrounds them. EVs can contain a variety of biological signals, growth factors, etc., and allow cells to communicate with their neighbors, as neighboring cells receive the messages contained in EVs. So how can this help with stem cell treatment? Basically, by deriving EVs from genetically engineered MSCs and injecting them articularly (into an afflicted joint), the messages contained within the EVs promote chondrogenesis (cartilage regrowth) within the articular tissue. In fact, recent studies have shown that EVs derived from MSCs can promote hyaline cartilage regeneration within rats, a promising finding which suggests EVs have the potential to be the future of stem cell treatment. However, the finer points of the mechanisms behind how the EVs actually promote chondrogenesis are still not fully understood, which—in part, along with the relative youth of EV based therapy—is why such techniques, by and large, are not currently used to treat humans.
Epigenetics, or the modification of gene expression without altering the fundamental genetic code—in this case, the DNA contained in cells—is another promising alternative in the future of stem cell-based therapy. In brief summary, specific groups of miRNAs (micro-ribonucleic acids), which regulate MSC differentiation into chondrocytes and other joint-related cell types seem to show signs of therapeutic potential. By understanding more about the function of miRNAs in the regulation of articular tissue and the differentiation of MSCs, it is feasible that miRNAs could be used to regulate the differentiation of MSCs into desired cell types, as well as regulate their gene expression post-differentiation to ensure their continued functionality as chondrocytes and minimize hypertrophy.
Finally, gene editing technologies, which fundamentally change the DNA of target cells—in this case, MSCs—could enhance the therapeutic potential of stem cell treatment. Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) technology, which allows targeting and editing of DNA at precise sequences, could enhance MSC applications by regulating various growth factors associated with chondrogenic development. Basically, CRISPR uses special RNA sequences called guide RNAs (gRNAs) which can be engineered to lead the system to a designated sequence of DNA. Once there, CRISPR, by using either Cas9 or Cpf1 enzymes, which are capable of making physical changes to the DNA, can either shut a targeted gene off, stopping its expression, or promote the expression of a certain gene. Moreover, the CRISPR-Cpf1 system is capable of integrating sequences of specially engineered DNA into a given genomic sequence, furthering the editing possibilities. And while CRISPR could be a powerful tool for controlling MSC differentiation and gene expression, it raises a multitude of ethical and safety concerns regarding the use of genomic editing techniques on humans. Specifically, many are concerned with the risk of tumorigenicity (the formation of tumors) as a result of genetic modification—although, with CRISPR, this risk is quite low—as well as the ethics associated with genomic modification: some see it as “playing god,” while others may be worried about future developments of such powerful technologies, as future genomic editing techniques, by potentially allowing almost limitless control over the human genome, could be used for eugenics if they fell into the wrong hands. Despite these concerns, which are somewhat valid, CRISPR technology has been used in several preclinical studies and will likely remain an important tool in the future of bioengineering.
So, back to the original question: can stem cells cure my finger injury? Well, probably not right now. Current stem cell-based therapy, at best, can result in a temporary reduction in symptoms associated with Osteoarthritis and some cartilage renewal. Unfortunately, recent trials of MSC based therapy have not provided a satisfactory amount of evidence to support the capability of such therapy to restore hyaline cartilage and reduce Osteoarthritis symptoms in the long-term. However, given the numerous techniques in contention to improve current stem cell therapy, it is likely that MSC treatment will soon be an option for chondrogenesis, effectively offering a cure for Osteoarthritis and allowing an athlete to return to the acute stress of the finger joints associated with rock climbing.
References: https://docs.google.com/document/d/1xiTopPfZ-G8kz5Jlc1yd67PVTas4UkTVJslMoFn5Ceo/edit?usp=sharing