In our third and final post of this series we’ll discuss some of the difficulties in using mesenchymal stem cells (MSCs) for regenerative therapies for degenerative disc disease (DDD). To recap from the last post, here’s what we know so far about MSCs in the treatment of DDD: First, we know that MSCs can be safely and efficiently harvested and cultured to a quantity of cells suitable for implantation. We also know that these MSCs can be cultured in vitro (in a test tube) into cells similar to nucleus pulposus (NP) cells. We know that MSCs can be safely implanted into an intervertebral disc (IVD) of animals or humans. Finally, we know that implantation of MSCs can lead to improvement in the cellular appearance and MRI appearance of the IVD. In some very preliminary human studies this has translated into some clinical benefit. Sounds like we’ve got this all worked out right?
Unfortunately, we’re not quite there yet. It’s long been know that the problem with stem cells isn’t getting them to a location, it’s getting them to stay there and flourish. Stem cell scientists will frequently report that when they survey a site of implantation weeks after MSCs are implanted there are few cells left. We suspect that this is because once the cells are injected, if they don’t have the proper nutritional and structural support they will undergo apoptosis (cell suicide) and die. The next step in getting these regenerative therapies to work, then, is to ensure that the injected MSCs have what they need to survive and produce the extracellular matrix (ECM) necessary for IVD regeneration.
First, it’s clear from many years of in vitro stem cell work that MSCs prefer three-dimensional organization to thrive. Stem cell scientists often culture MSCs with beads of aliginate, a non-soluble gelatinous substance that acts a scaffold to promote proliferation. Recently more complex scaffolds have been developed. These scaffolds are composed of materials ranging from simple, naturally-derived collagen to exotic nano-designed proprietary materials (see figure 1). The materials not only have to be able to provide a hospitable locale for the MSCs, but also have to be structurally robust so that they can stand up to the mechanical stresses in the IVD. Ultimately, though, the goal of these various materials is the same: to provide structural support to anchor the MSCs so that they can differentiate into cells necessary for IVD regeneration. Future therapies will likely include some sort of scaffold material injected concurrently with the MSCs.
Figure 1. Various types of scaffolds used for MSC culture. (Source: Bertolo et al, 2012)
Another obstacle to in vivo (in the body) MSC differentiation and proliferation is that the environment within the IVD is quite harsh. Even in a young healthy IVD the blood supply that provides oxygen and nutrients to the disc is quite tenuous. Over time with repeated mechanical loading it is believed that this delicate blood supply (most of which comes through the bony endplates of the adjacent vertebral bodies) is damaged resulting in the death of IVD cells. Some groups have experimented with various substances (i.e. calcium channel blockers) to help increase blood flow to the disc. Others have looked at mechanically altering the adjacent endplates to allow more blood to flow to the IVD. We have quite a way to go before any of these therapies are ready for clinical use, however.
Ok let’s say that you’ve gotten the MSCs to stay in place on a nice scaffold. You’ve also figured out a way to augment blood flow and nutrition to the disc space. How do you actually get the MSCs to differentiate into the various cell types of the IVD? Native NP cells produce ECM that has a very specific ratio of proteoglycans (the water-carrying substance so important to healthy IVDs) and collagen (27:1 for those interested.) Unfortunately, using current culture methods MSCs often will by default differentiate into cells similar to chondrocytes, cells that form cartilage. This results in the formation of ECM that is thick and fibrous like the cartilage in a knee rather than the soft and pliable ECM of an NP. Researches are getting closer, however, to the proper culture techniques that promote the differentiation of MSCs into cells nearly identical to NP cells. Some of these techniques involve the implantation of a concoction of MSCs and specific growth factors that promote proper differentiation. Other techniques utilize special scaffolds that secrete growth factors to stimulate attached MSCs. These techniques have shown promise in animal studies. One last thing to consider: remember that the NP is only one component of the IVD and that the annulus fibrosus (AF) is equally important for the structure and function of the IVD. Despite this fact, almost no one is working on the differentiation of the MSCs into AF-like cells. We clearly have a long way to go before we can reliably direct implanted MSCs to differentiate into cells of each distinct component of the IVD.
Perhaps rather than trying to coax foreign cells to become NP cells we should be trying to find cells within the IVD that have the potential to form the various IVD cell types. Every tissue type in your body is composed of cells that originate from parent progenitor cells. These progenitor cells are originally quite pluripotent in that they are able to differentiate into a variety of related cell types. While most of these progenitor cells die off as we mature, a small number remain dormant within the tissue in case they’re needed for purposes of regeneration. It turns out that the IVD also has nests progenitor cells. Recently a group in Japan (Sakai et al, 2012) discovered a population of cells, so-called Tie+ cells, that can differentiate into mature NP cells. The fact that the survival of these cells was dependent on angiopoietin-1, a potent promoter of blood vessel formation, stresses the importance of adequate blood flow on the health of the cells within the IVD. These progenitor cells may be the key to unlocking the mystery of IVD regeneration.
Readers of this post will realize that many hurdles still must be overcome in the development of stem cell therapies for IVD regeneration. Regardless, this is a very exciting frontier in the treatment of DDD. A review of any current spine journal reveals a new in vivo or animal study of these therapies. Also, a search of clinicaltrials.gov lists several clinical trials enrolling patients to study some sort of implantable stem cell therapy for DDD. Clearly momentum is building and it won’t be long before stem cell therapies for the treatment of DDD become reality.
Thanks for reading!
J. Alex Thomas, M.D.
1. Sakai D, Nakamura Y, Nakai T, Mishima T, Kato S, Grad S, et al.: Exhaustion of nucleus pulposus progenitor cells with ageing and degeneration of the intervertebral disc. Nat Commun 3:1264, 2012.
2. Bertolo A, Mehr M, Aebli N, Baur M, Ferguson SJ, Stoyanov J V: Influence of different commercial scaffolds on the in vitro differentiation of human mesenchymal stem cells to nucleus pulposus-like cells. Eur Spine J 21 Suppl 6:S826–38, 2012.