The ability of the heart to adapt to increased stress is dependent on modification of its extracellular matrix (ECM) architecture that is established during postnatal development as cardiomyocytes differentiate, a process that is poorly understood. We hypothesized that the small leucine-rich proteoglycan (SLRP) lumican (LUM), which binds collagen and facilitates collagen assembly in other tissues, may play a critical role in establishing the postnatal murine myocardial ECM. Although previous studies suggest LUM deficient mice (lum−/−) exhibit skin anomalies consistent with Ehlers-Danlos syndrome, lum−/− hearts have not been evaluated. These studies show LUM was immunolocalized to non-cardiomyocytes of the cardiac ventricles and its expression increased throughout development. Lumican deficiency resulted in significant (50%) perinatal death and further examination of the lum−/− neonatal hearts revealed an increase in myocardial tissue without a significant increase in cell proliferation. However cardiomyocytes from surviving postnatal day 0 (P0), 1 month (1 mo) and adult (4 mo) lum−/− hearts were significantly larger than their wild type (WT) littermates. Immunohistochemistry revealed that the increased cardiomyocyte size in the lum−/− hearts correlated with alteration of the cardiomyocyte pericellular ECM components collagenα1(I) and the class I SLRP decorin (DCN). Western blot analysis demonstrated that the ratio of glycosaminoglycan (GAG) decorated DCN to core DCN was reduced in P0 and 1 mo lum−/− hearts. There was also a reduction in the β and γ forms of collagenα1(I) in lum−/− hearts. While the total insoluble collagen content was significantly reduced, the fibril size was increased in lum−/− hearts, indicating LUM may play a role in collagen fiber stability and lateral fibril assembly. These results suggest that LUM controls cardiomyocyte growth by regulating the pericellular ECM and also indicates that LUM may coordinate multiple factors of collagen assembly in the murine heart. Further investigation into the role of LUM may yield novel therapeutic targets and/or biomarkers for patients with cardiovascular disease.

Background There are many patients that exhibit connective tissue related cardiac malformations but do not have mutations in collagen genes. The Small Leucine Rich Proteoglycans (SLRP) fibromodulin (FMOD) and lumican (LUM) bind collagen and regulate fibril assembly in other biological contexts. Results FMOD deficient mice and double deficient FMOD;LUM mice exhibited anomalies in regions where cardiac valve tissue interdigitates with adjacent muscle for support. Ectopic connective and/or myocardial tissue(s) was associated with the more severe cardiac valve anomalies in FMOD;LUM deficient mice. At postnatal day 0 (P0) there was an increase in the mesenchymal cell number in the regions where valve cusps anchor in FMOD;LUM deficient mice compared to WT. The cardiac valve anomalies correlated with the highest levels of FMOD expression in the heart and also where myotendinous junctions (MTJ) components biglycan, collagen type I alpha 1, and collagen type VI, are also localized. Conclusion The postnatal assembly of the collagen-rich ECM in regions where cardiac valves anchor, that we have designated ‘myotendinous-like junctions’ (MTLJ) requires the SLRPs FMOD and LUM. Moreover, FMOD and LUM may facilitate mesenchymal cell differentiation in late stages of cardiac valve development.