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Tissues are not solely composed of cells. The extracellular matrix is important for the cell well-being and cell-cell communication. The glycosaminoglycan hyaluronan (HYA) is a widely distributed extracellular matrix (ECM) component. The molecule has prominent physicochemical properties, foremost viscoelastic and osmotic, but participates in many biological processes such as cell migration, proliferation, tissue turnover, wound healing and angiogenesis. HYA is synthesised by either of three different hyaluronan-synthesising enzymes, HAS1-3, and its main ligand is the transmembrane receptor CD44. In the heart and vessels the matrix components are of great importance for endurance and elasticity which are prerequisites for a normal function. The aims of the study were to describe the distribution of HYA and its receptor CD44 in normal cardiovascular tissue and to investigate the ECM composition in myocardial hypertrophy.

Normal conditions were studied in a rat model. These studies showed that the tunica adventitia in almost all vessels stained strongly for HYA. The expression in the tunica intima and media on the venous side, differed between the vessels and was almost absent on the arterial side. In the adult animals only minute amounts of CD44 were detected. The expression of both HYA and CD44 was increased in newborn rats.

In the heart HYA was unevenly distributed in the interstitium. Strong HYA-staining was seen in the valves and in the adventitia of intramyocardial vessels. Almost no CD44-staining was observed. Notably, there was no obvious difference between newborn and adult animals.

In an experimental rat model of pressure-induced cardiac hypertrophy the mRNA-levels of HAS1, HAS2, CD44, basic Fibroblast Growth Factor (FGF-2) and Fibroblast Growth Factor Receptor-1 (FGFR-1) were elevated on day 1 after aortic banding. HAS2, CD44 and FGFR-1 were at basal levels on day 42. The HYA-concentration was significally elevated on day 1. HYA was detected in the interstitium by histochemistry and CD44 was detected mainly in and around the intramyocardial vessels.

The HYA-staining was increased in myectomi specimens from patients with HCM compared to controls. HYA was detected in the interstitium, in fibrous septas and in the adventitia of intramyocardial vessels. No CD44 was detected in HCM or in control specimens.

Our results indicate that HYA and CD44 play an active role in the maturing vessel tree and that the ECM content of HYA is increased in experimental myocardial hypertrophy and human hypertrophic cardiomyopathy.

Background. Myocardial hypertrophy is a risk factor for cardiovascular morbidity and mortality. Independent of underlying disease, the cardiac muscle strives in different ways to compensate for an increased workload. This remodelling of the heart includes changes in the extracellular matrix which will affect systolic and diastolic cardiac function. Furthermore, signal transduction, molecular diffusion and microcirculation will be affected in the hypertrophic process. One important extracellular component is the glycosaminoglycan hyaluronan. It has been shown to play a major role in other conditions that feature cellular growth and proliferation, such as wound healing and malignancies. The aim of this thesis was to investigate hyaluronan and its role in both an experimental rat model of cardiac hypertrophy as well as in cultured mouse cardiomyocytes and fibroblasts.

Methods. Cardiac hypertrophy was induced in rats by aortic ligation. Hyaluronan concentration was measured and expression of genes coding for hyaluronan synthases were quantified after 1, 6 and 42 days after operation, in cardiac tissue from the left ventricular wall. Localization of hyaluronan and its receptor CD44 was studied histochemically. Hyaluronan synthesis was correlated to gene transcription using microarray gene expression analysis. Cultures of cardiomyocytes and fibroblasts were stimulated with growth factors. Hyaluronan concentration was measured and expression of genes coding for hyaluronan synthases were detected. Hyaluronan size was measured and crosstalk between cardiomyocytes and fibroblasts was investigated.

Results. Increased concentration of hyaluronan in hypertrophied cardiac tissue was observed together with an up-regulation of two hyaluronan synthase genes. Hyaluronan was detected in the myocardium and in the adventitia of cardiac arteries whereas CD44 staining was mainly found in and around the adventitia. Hyaluronan synthesis correlated to the expression of genes, regulated by transcription factors known to initiate cardiac hypertrophy. Stimulation of cardiomyocytes by PDGF-BB induced synthesis of hyaluronan. Cardiomyocytes also secreted a factor into culture media that after transfer to fibroblasts initiated an increased synthesis of hyaluronan. When stimulated with hyaluronan of different sizes, a change in cardiomyocyte gene expression was observed. Different growth factors induced production of different sizes of hyaluronan in fibroblasts. The main synthase detected was hyaluronan synthase-2. Cardiomyocytes were also shown to secrete microvesicles containing both DNA and RNA. Isolated microvesicles incubated with fibroblasts were observed by confocal microscopy to be internalized into fibroblasts. Altered gene expression was observed in microvesicle stimulated fibroblasts.

Conclusion. This study shows that increased hyaluronan synthesis in cardiac tissue during hypertrophic development is a part of the extracellular matrix remodelling. Cell cultures revealed the ability of cardiomyocytes to both synthesize hyaluronan and to convey signals to fibroblasts, causing them to increase hyaluronan synthesis. Cardiomyocytes are likely to express receptors for hyaluronan, which mediate intracellular signalling causing the observed altered gene expression in cardiomyocytes stimulated with hyaluronan. This demonstrates the extensive involvement of hyaluronan in cardiac hypertrophy.