Abstract: BACKGROUND: Our previous experimental results have shown that hyaluronic acid hydrogel can act as a vehicle for bone marrow mesenchymal stem cell delivery to improve the cardiac function of rats with myocardial infarction. 
OBJECTIVE: To explore the molecular mechanism of bone marrow mesenchymal stem cells and hyaluronic acid hydrogel in promoting damaged heart repair. 
METHODS: Bone marrow mesenchymal stem cells from male Sprague-Dawley rats were isolated and cultured, and then hyaluronic acid-encapsulated bone marrow mesenchymal stem cells were cultured in vitro in a three-dimensional manner. A model of myocardial infarction was made by ligating the left anterior descending artery of female Sprague-Dawley rats. After 1 week, the model rats were screened by ultrasonic testing and then eligible ones were randomly divided into four groups: PBS group (n=12), hyaluronic acid group (n=12), bone marrow mesenchymal stem cell group (n=15), and hyaluronic acid-encapsulated bone marrow mesenchymal stem cell group (n=15). At 1 week after ligation, the model rats underwent the secondary thoracotomy followed by corresponding injections into the infarcted region and its marginal zone. The expression levels of matrix metalloproteinase-2, vascular endothelial growth factor, thymosin β4 and c-Kit were examined at post-injection day 1, week 1 and week 2 by western blot assay. At post-injection week 2, immunofluorescence staining was used to detect the differentiation of transplanted cells.
RESULTS AND CONCLUSION: (1) The expression levels of matrix metalloproteinase-2 and vascular endothelial growth factor protein in the infarct zone in the bone marrow mesenchymal stem cell group were significantly up-regulated at week 1 compared with the other three groups (P < 0.05). At week 2, the hyaluronic acid group had a lower expression of matrix metalloproteinase-2 and vascular endothelial growth factor protein than the other three groups (P < 0.05). However, the expression of matrix metalloproteinase-2 and vascular endothelial growth factor protein in the hyaluronic acid+bone marrow mesenchymal stem cell group was not significantly different compared with the bone marrow mesenchymal stem cell group. This was primarily attributable to a prolonged paracrine effect via the controlled release of the hyaluronic acid hydrogel. This prolonged paracrine effect offsets the inhibitory effect induced by hyaluronic acid hydrogel at 2 weeks. (2) Compared with the PBS group, thymosin β4 and c-Kit expression levels in the hyaluronic acid group, bone marrow mesenchymal stem cell group and bone marrow mesenchymal stem cell+hyaluronic acid group were significantly increased (P < 0.05). (3) No differentiation of transplanted cells into cardiomyocytes or blood vessels was detected 2 weeks after transplantation. (4) It is indicated that transplanted bone marrow mesenchymal stem cells promote myocardial repair through the paracrine effect, and hyaluronic acid hydrogel prolongs the paracrine effect of transplanted bone marrow mesenchymal stem cells.

Key words: hyaluronic acid, bone marrow mesenchymal stem cell, myocardial infarction, cell transplantation, cardiac function, stem cell