"The effect of hyaluronic acid size and concentration on branching morphogenesis and tubules differentiation in developing kidney culture systems: Potential applications to engineering of renal tissues"
by Eran Rosines, Heidi J. Schmidt and Sanjay K. Nigam. Biomaterials 28(2007): 4806-4817.
The research presented in this article is very insightful and will lead to many future applications in medicinal research involving the kidneys. Studies prior to this one have shown that hyaluronic acid (HA), a glycosaminoglycan, plays a vital role in mammalian development. It has also been shown to be important for tissue engineering purposes as it is antigenic and immunocompatible. Use as an injectible gel for viscosupplementation of osteoarthritic joints and as a dermatological treatment for wrinkles represent only a couple of its many potential uses. The purpose of the article was to investigate the importance of HA during renal organogenesis. Many branching morphogenesis and mesenchymal-to-epithelial transformation (MET) regulatory mechanisms have already been identified for renal organogenesis, however, it is poorly understood how branching morphogenesis is stopped and collecting duct and nephron differentiation is started. It is, however, known that in the absence of HA murine embryos exhibit growth retardation and death within ten days on embryonic development suggesting an essential role for HA. The authors hope that the results presented in this paper will shed some light on this matter.
HA is synthesized by three HA synthase genes, has-1, has-2 and has-3 with has-2 accounting for at least 97% of HA synthesized during murine embryonic development. All genes synthesize HA in different molecular weights (MW). HA is known to exist at different MWs and concentrations during kidney differentiation. The MW of HA is controlled in vivo by degradative hyaluronidases including that encoded by hyal-2.
In order to demonstrate the potential importance of HA several kidney culture methods were utilized. These include qPCR to detect the expression of has-2 and hyal-2 during in vivo kidney development; several tests adding hyaluronidase and HA at various MWs and concentrations to in vitro metanephric and isolated uretic bud (UB) culture systems; morphometric analysis to quantify the effects on branching morphogenesis and; quantitative PCR of functional renal differentiation markers to measure UB/metanephric mesenchym (MM) differentiation.
The results of this paper show that HA has the ability to simultaneously modulate UB branching, promote MET, and promote differentiation of both MM and the UB depending on the concentration and MW of HA. The results also suggest that endogenous HA is required for branching morphogenesis as the presence of hyaluronidase inhibited branching morphogenesis in both UB and whole kidney cultures. The in vitro tests involving the use of various concentrations and MWs of HA revealed that HA stimulates branching morphogenesis at low concentrations and MW but inhibits branching at high concentrations and high MW. qPCR results showed that has-2 and hyal-2, which regulate the size of HA molecules, are highly expressed during kidney development. This, in combination with the other results presented in this paper, suggests that specific sizes and concentrations of HA may act to independently regulate UB branching and promote tubular maturation and thus may be responsible for ending branching morphogenesis and initiating nephron differentiation. Quantitative PCR of functional renal differentiation markers to measure UB/MM differentiation demonstrated that HA of a variety of MWs strongly promotes mesenchymal epithelialization and nephron differentiation in a concentration-dependent manner.
So what does all this mean??
These results suggest that HA may act as a growth factor sink for the UB but the viscous high concentration and high MW may in turn act as a physical barrier to the growth required for UB branching morphogenesis which could have tissue engineering and developmental relevance. Furthermore, it appears that HA acts as a switch molecule controlling when branching morphogenesis is stopped and when collecting duct and nephron differentiation is started; something little has been known about prior to this research. But, in my opinion, the greatest contribution of this research is the insight into the potential use of HA in renal repair, including promoting the formation new nephrons in adult kidneys with renal disease and the use of HA receptors or HA analogs as a new set of drug targets or therapies to promote renal tubule regeneration. HA may also promote tubule regeneration in injured or cryopreserved kidneys, which is very useful for kidney transplantation and can be used for creating a 3D scaffold for in vitro kidney engineering from developmental tissues.
The results presented in this paper appear to greatly support the authors claims of future applications of HA in kidney engineering. All of the figures were quite informative as they were very visual using color images to show the effects of HA and hyal-2 on branching. The data from the color images was also displayed in bar graphs providing two ways to visualize the results. All graphs and figures were well presented and easy to understand with informative figure legends. Overall, I found this paper moderately easy to understand. I really liked how the results section was split up to distinguish between all the tests. The discussion was kind of wordy at times and I did have to go back over some parts to fully understand but the gist of the research presented came through clearly.
