Jun Zhang, Hongxu Qi, Hongjun Wang, Ping Hu, Lailiang Ou, Shuhua Guo, Jing Li, Yongzhe Che, Yaoting Yu, Deling Kong (2006) Artificial Organs 30 (12), 898–905.
Nitric oxide (NO) is an important factor in the regulation of proper blood vessel regulation. Its production also has a strong positive correlation with the patency of blood vessels used in bypass surgery. In this paper it was hypothesized that grafts engineered with endothelial nitric oxide synthase (eNOS)-modified MSCs would produce an NO level comparable to that of native blood vessels, and greater than that of unmodified MSCs. In order to make the small vascular grafts, MSCs were extracted from the bone marrow of rats, expanded in culture, and modified with eNOS. The cultures were then seeded on a tubular poly (propylene carbonate) (PPC) graft which was produced by electrospinning. Grafts of both eNOS-modified and unmodified MSCs were developed, with a rat artery of similar size as a control.
A number of the techniques that we will be using or already have used in our lab classes were performed for this paper. RT-PCR was used to quantify the amount of eNOS expressed in the grafts. Proteins in the grafts were separated and identified using western blotting. Fluorescence microscopy was used to examine the immunohistochemistry of the cultures. The results of the study showed that NO production level of the non-eNOS modified MSC grafts was similar to the baseline, whereas the eNOS-modified MSC grafts produced NO levels comparable to that of the control vessels.
This paper is interesting because it helps show the importance of understanding the regulatory pathways of different tissues and organs when designing artificial organs. They also use a number of techniques that we will be using in lab. Heart and vascular disease/problems are on the list of top killers in the world1. I think it is important to investigate both means of preventing these diseases and means of repair/regeneration (in this case, replacement of artificial blood vessels) upon occurrence of the disease. In future studies, it would be interesting to establish what other regulatory pathways may have been compromised in previous designs of vascular grafts less than 5mm. Additional studies should also be done showing the effects of different mechanical forces on these eNOS-modified MSC grafts, to ensure that they would be able to withstand forces that occur naturally within the vascular system. It would also be good to measure the patency of these grafts compared to grafts made with other polymers in vivo to determine which material has the best long-term outcome under “normal” conditions of the body.
1http://www.who.int/research/en/
6 comments:
I'm guessing one of the limiting factors of fully functional blood vessels less than 5mm in diameter is the ability to provide sufficient vascularization. In the paper that I posted, functionality of the implant was dependent on vascularization, sourced from systemic circulation. I wonder if similar results would occur if the polymer scaffold was attached to a vascular pedicle and what type of results would occur if the same setup incorporated the eNOS modification?
Some other studies do work with grafts made of NO-emitting polyurethane, which seems to be promising. I think this would be much easier to get around the regulatory hurdles. However one of the questions with these grafts is whether or not they would be able to withstand in vivo shear stress.
The main problem with the blood vessels less than 5mm in diameter is that they often suffer from thrombosis (clotting) and intimal hyperplasia (when there is an increased number of cells on the inner lining of the blood vessel- can make the vessel get too big and eventually burst). Some studies have shown that an early interface between the blood and the artificial vessel wall are needed to avoid this. Maybe it's possible to coat the graft with a protein that attracts endothelial cells more quickly to form an interface.
The experiment shows that the eNOS- modified blood vessel is one step closer to the natural blood vessel. However,I wonder this level of NO keeps constant over a long time period. Do the modified cells have a mechanism to regulate the level of NO produced?
Besides the concerns about mechanical properties of the vascular and regulatory hurdles, I am wondering if the use of poly (propylene carbonate) (PPC) will have any long term effects once being implanted. What about the use of biodegradable scaffold?
They didn't mention whether or not the modified cells continue to release NO over time. However they have done studies on NO-eluting polyurethane grafts that do continue to release NO. It happens in two bursts- one fast one that occurs over 48 hours, and then it continues for a length of at least 60 days (which is how long they continued the study). The study showed that the vascular graft actually continued high performance (with minimal platelet aggregation and SMC proliferation and an increase in EC growth and proliferation) during both bursts of time. I would assume that the genetically modified cells would also be able to regulate the amount of NO produced though. There are a number of pathways that are involved in NO production- both flow dependent and independent. The flow independent pathways rely on the positive and negative feedback cycles that are naturally existent within the cells. Thus there should be a reasonable, non-toxic amount of NO produced.
I haven't seen much literature using biodegradable scaffolds, although I'm sure they are being researched. It would probably be similar to the materials used in stents right now which I believe are also not biodegradable. I think that many of them have to be replaced after a few years or at least need to have their location surgically adjusted due to movement over time. The biodegradable scaffold would be a good idea but a lot of research would have to be conducted to determine when the cells would be able to create an environment/structure stable enough for them to support themselves on for a lifetime.
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