Role of C-type natriuretic peptide signalling in maintaining cartilage and bone function.

C-type natriuretic peptide (CNP) has been demonstrated in human and mouse models to play critical roles in cartilage homeostasis and endochondral bone formation. Indeed, targeted inactivation of the genes encoding CNP results in severe dwarfism and skeletal defects with a reduction in growth plate chondrocytes. Conversely, cartilage-specific overexpression of CNP was observed to rescue the phenotype of CNP deficient mice and significantly enhanced bone growth caused by growth plate expansion. In vitro studies reported that exogenous CNP influenced chondrocyte differentiation, proliferation and matrix synthesis with the response dependent on CNP concentration. The chondroprotective effects were shown to be mediated by natriuretic peptide receptor (Npr)2 and enhanced synthesis of cyclic guanosine-3',5'-monophosphate (cGMP) production. Recent studies also showed certain homeostatic effects of CNP are mediated by the clearance inactivation receptor, Npr3, highlighting several mechanisms in maintaining tissue homeostasis. However, the CNP signalling systems are complex and influenced by multiple factors that will lead to altered signalling and tissue dysfunction. This review will discuss the differential role of CNP signalling in regulating cartilage and bone homeostasis and how the pathways are influenced by age, inflammation or sex. Evidence indicates that enhanced CNP signalling may prevent growth retardation and protect cartilage in patients with inflammatory joint disease.