Substance P and norepinephrine modulate murine chondrocyte proliferation and apoptosis

Substance P (SP) and norepinephrine (NE) containing sensory and sympathetic nerve fibers innervate bone and fracture callus. They are involved in controlling vascularization and matrix differentiation during skeletal growth. Both types of nerve fibers are known to modulate growth and metabolic activity of osteoblasts and osteoclasts. The aim of this study was to understand the roles of SP and NE in chondrocyte metabolism and their impact on chondrocyte proliferation, apoptosis, and cell adhesion.

Primary costal chondrocytes were isolated from newborn mice. Micromass and monolayer cell culture regimens were used to analyze the effects of SP and NE on matrix formation, as determined by quantitative polymerase chain reaction and immunohistochemistry. The effects of SP and NE on proliferation, adhesion, and apoptosis of chondrocytes were determined by enzyme-linked immunosorbent assay, bromodeoxyuridine, TUNEL, and morphometric analyses.

SP, neurokinin type 1 (NK-1) receptor, α-adrenergic receptor (α-AR), and β-AR were abundantly expressed in primary costal chondrocytes. Stimulation with SP or NE did not affect extracellular matrix formation with respect to types I, II, and IX collagen and aggrecan in micromass pellets. SP dose-dependently increased the rate of proliferation of chondrocytes via the NK-1 receptor, whereas NE decreased the apoptosis rate of chondrocytes by stimulating β-AR. Both neurotransmitters induced the formation of focal adhesion contacts.

Transmitters of sympathetic and sensory nerve fibers modulate the metabolic activity of chondrocytes. Endogenous SP, NK-1 receptor, and adrenoceptor expression in chondrocytes implicates as-yet-unknown, presumably trophic, functions of neurotransmitter for skeletal growth and might be of interest for use in cartilage regenerative medicine.