The growth plate's response to load is partially mediated by mechano-sensing via the chondrocytic primary cilium.

Mechanical load plays a significant role in bone and growth-plate development. Chondrocytes sense and respond to mechanical stimulation; however, the mechanisms by which those signals exert their effects are not fully understood. The primary cilium has been identified as a mechano-sensor in several cell types, including renal epithelial cells and endothelium, and accumulating evidence connects it to mechano-transduction in chondrocytes. In the growth plate, the primary cilium is involved in several regulatory pathways, such as the non-canonical Wnt and Indian Hedgehog. Moreover, it mediates cell shape, orientation, growth, and differentiation in the growth plate. In this work, we show that mechanical load enhances ciliogenesis in the growth plate. This leads to alterations in the expression and localization of key members of the Ihh-PTHrP loop resulting in decreased proliferation and an abnormal switch from proliferation to differentiation, together with abnormal chondrocyte morphology and organization. Moreover, we use the chondrogenic cell line ATDC5, a model for growth-plate chondrocytes, to understand the mechanisms mediating the participation of the primary cilium, and in particular KIF3A, in the cell's response to mechanical stimulation. We show that this key component of the cilium mediates gene expression in response to mechanical stimulation.

IL-1RI participates in normal growth plate development and bone modeling.

The proinflammatory cytokine interleukin-1 (IL-1) signals through IL-1 receptor type I (IL-1RI) and induces osteoclastogenesis and bone resorption mainly during pathological conditions. Little is known about the effect of excess or absence of IL-1 signaling on the physiological development of the growth plate and bone. In this study, we examine growth plate morphology, bone structure, and mechanical properties as well as osteoclast number in IL-1RI knockout mice to evaluate the role of IL-1RI in the normal development of the growth plate and bone. We show for the first time that IL-1RI knockout mice have narrower growth plates due to a smaller hypertrophic zone, suggesting a role for this cytokine in hypertrophic differentiation, together with higher proteoglycan content. The bones of theses mice exhibit higher trabecular and cortical mass, increased mineral density, and superior mechanical properties. In addition, IL-1RI knockout mice have significantly reduced osteoclast numbers in the chondro-osseous junction, trabecular bone, and cortical bone. These results suggest that IL-1RI is involved in normal growth plate development and ECM homeostasis and that it is significant in the physiological process of bone modeling.

Exposure to omega-3 fatty acids at early age accelerate bone growth and improve bone quality.

Omega-3 fatty acids (FAs) are essential nutritional components that must be obtained from foods. Increasing evidence validate that omega-3 FAs are beneficial for bone health, and several mechanisms have been suggested to mediate their effects on bone, including alterations in calcium absorption and urinary calcium loss, prostaglandin synthesis, lipid oxidation, osteoblast formation and inhibition of osteoclastogenesis. However, to date, there is scant information regarding the effect of omega-3 FAs on the developing skeleton during the rapid growth phase. In this study we aim to evaluate the effect of exposure to high levels of omega-3 FAs on bone development and quality during prenatal and early postnatal period. For this purpose, we used the fat-1 transgenic mice that have the ability to convert omega-6 to omega-3 fatty acids and the ATDC5 chondrogenic cell line as models. We show that exposure to high concentrations of omega-3 FAs at a young age accelerates bone growth through alterations of the growth plate, associated with increased chondrocyte proliferation and differentiation. We further propose that those effects are mediated by the receptors G-protein coupled receptor 120 (GPR120) and hepatic nuclear factor 4α, which are expressed by chondrocytes in culture. Additionally, using a combined study on the structural and mechanical bone parameters, we show that high omega-3 levels contribute to superior trabecular and cortical structure, as well as to stiffer bones and improved bone quality. Most interestingly, the fat-1 model allowed us to demonstrate the role of maternal high omega-3 concentration on bone growth during the gestation and postnatal period.

Interleukin-1ß promotes proliferation and inhibits differentiation of chondrocytes through a mechanism involving down-regulation of FGFR-3 and p21.

The proinflammatory cytokine IL-1ß is elevated in many childhood chronic inflammatory diseases as well as obesity and can be associated with growth retardation. Here we show that IL-1ß affects bone growth by directly disturbing the normal sequence of events in the growth plate, resulting in increased proliferation and widening of the proliferative zone, whereas the hypertrophic zone becomes disorganized, with impaired matrix structure and increased apoptosis and osteoclast activity. This was also evident in vitro: IL-1ß increased proliferation and caused a G1-to-S phase shift in the cell cycle in ATDC5 chondrocytes, accompanied by a reduction in fibroblast growth factor receptor-3 (FGFR-3) and its downstream gene, the cell-cycle inhibitor p21 and its family member p57, whereas the cell-cycle promoter E2F-2 was increased. The reduction in FGFR-3, p21, and p57 was followed by delayed cell differentiation, manifested by decreases in proteoglycan synthesis, mineralization, alkaline phosphatase activity, and the expression of Sox9, RunX2, collagen type II, collagen type X, and other matrix proteins. Taken together, we suggest that IL-1ß alters normal chondrogenesis and bone growth through a mechanism involving down-regulation of FGFR-3 and p21.

The role of matrix gla protein in ossification and recovery of the avian growth plate.

Extracellular matrix mineralization is an essential physiologic process in bone, teeth, and hypertrophic cartilage. Matrix Gla protein (MGP), an inhibitor of mineralization, is expressed by chondrocytes and vascular smooth muscle cells to inhibit calcification of those soft tissues. Tibial dyschondroplasia (TD), a skeletal abnormality apparent as a plug of non-vascularized, non-mineralized, white opaque cartilage in the tibial growth plate of avian species can serve as a good model for studying process and genes involved in matrix mineralization and calcification. In this work, we studied the involvement of MGP in the development of TD, as well as in the processes of spontaneous and induced recovery from this syndrome. First, we found that during normal bone development, MGP is expressed in specific time and locations, starting from wide-spread expression in the yet un-ossified diaphysis during embryonic development, to specific expression in hypertrophic chondrocytes adjacent to the chondro-osseous junction and the secondary ossification center just prior to calcification. In addition, we show that MGP is not expressed in the impaired TD lesion, however when the lesion begins to heal, it strongly express MGP prior to its calcification. Moreover, we show that when calcification is inhibited, a gap is formed between the expression zones of MGP and BMP2 and that this gap is closed during the healing process. To conclude, we suggest that MGP, directly or through interaction with BMP2, plays a role as ossification regulator that acts prior to ossification, rather then simple inhibitor.