No abstract

Cartilaginous structures are at the core of embryo growth and shaping before the bone forms. Here we report a novel principle of vertebrate cartilage growth that is based on introducing transversally-oriented clones into pre-existing cartilage. This mechanism of growth uncouples the lateral expansion of curved cartilaginous sheets from the control of cartilage thickness, a process which might be the evolutionary mechanism underlying adaptations of facial shape. In rod-shaped cartilage structures (Meckel, ribs and skeletal elements in developing limbs), the transverse integration of clonal columns determines the well-defined diameter and resulting rod-like morphology. We were able to alter cartilage shape by experimentally manipulating clonal geometries. Using in silico modeling, we discovered that anisotropic proliferation might explain cartilage bending and groove formation at the macro-scale.DOI: http://dx.doi.org/10.7554/eLife.25902.001

Articular cartilage has little regenerative capacity. Recently, genetic lineage tracing experiments have revealed chondrocyte progenitors at the articular surface. We further characterized these progenitors by using genetic approaches. Histone H2B-green fluorescent protein retention revealed that superficial cells divide more slowly than underlying articular chondrocytes. Clonal genetic tracing combined with immunohistochemistry revealed that superficial cells renew their number by symmetric division, express mesenchymal stem cell markers, and generate chondrocytes both asymmetric and symmetric differentiation. Quantitative analysis of cellular kinetics, in combination with phosphotungstic acid-enhanced micro-computed tomography, showed that superficial cells generate chondrocytes and contribute to the growth and reshaping of articular cartilage. Furthermore, we found that cartilage renewal occurs as the progeny of superficial cells fully replace fetal chondrocytes during early postnatal life. Thus, superficial cells are self-renewing progenitors that are capable of maintaining their own population and fulfilling criteria of unipotent adult stem cells. Furthermore, the progeny of these cells reconstitute adult articular cartilage entirely substituting fetal chondrocytes.-Li, L., Newton, P. T., Bouderlique, T., Sejnohova, M., Zikmund, T., Kozhemyakina, E., Xie, M., Krivanek, J., Kaiser, J., Qian, H., Dyachuk, V., Lassar, A. B., Warman, M. L., Barenius, B., Adameyko, I., Chagin, A. S. Superficial cells are self-renewing chondrocyte progenitors, which form the articular cartilage in juvenile mice.

To determine the long-term role of ER␤ in the regulation of longitudinal bone growth, appendicular and axial skeletal growth was followed and compared in female ER␤ ؊/؊ , ER␣ ؊/؊ , and ER␣ ؊/؊ ␤ ؊/؊ mice. Our results show that ER␤ inhibits appendicular and axial skeletal growth and has the capacity to induce fusion of the growth plates.

Facial shaping results from oriented divisions and crowd movements of ectomesenchymal cells during morphogenetic events.

Although glucocorticoids are known to induce apoptosis in chondrocytes, the mechanisms for this effect and the potential antiapoptotic role of IGF-I are unknown. To address this, we studied the effects of dexamethasone (Dexa) on apoptosis in the HCS-2/8 chondrocytic cell line. Dexa (25 microm) increased apoptosis (cell death ELISA) by 39% and 45% after 48 and 72 h, respectively (P< 0.01 and P < 0.05, respectively). IGF-I (100 ng/ml) decreased Dexa-induced apoptosis to levels similar to control cells. Apoptosis was associated with cleavage of poly-ADP-ribose polymerase (PARP) and alpha-fodrin and activation of caspases-8, -9, and -3 (Western), an effect that was counteracted when chondrocytes were cocultured with Dexa + IGF-I. Inhibitors for caspases-8, -9, and -3 (50 microm each) equally suppressed Dexa-induced apoptosis (P < 0.01). Time-response experiments showed that caspase-8 was activated earlier (at 12 h) than caspase-9 (at 36 h). We studied the phosphatidylinositol 3'-kinase (PI3K) pathway to further investigate the mechanisms of Dexa-induced apoptosis. Dexa decreased Akt phosphorylation by 93% (P < 0.001) without affecting total Akt and increased the p85alpha subunit 4-fold. The Akt inhibitor SH-6 (10 microm) increased apoptosis by 54% (P < 0.001). When combining Dexa with SH-6, apoptosis was not further increased, showing that Dexa-induced apoptosis is mediated through inhibition of the PI3K pathway. Addition of IGF-I to SH-6- or Dexa + SH-6-treated cells decreased apoptosis by 21.2% (P < 0.001) and 20.6% (P < 0.001), respectively. We conclude that Dexa-induced apoptosis is caspase dependent with an early activation of caspase-8. IGF-I can rescue chondrocytes from Dexa-induced apoptosis partially through the activation of other pathways than the PI3K signaling pathway. Based on our in vitro data, we speculate that in vivo treatment with glucocorticoids may diminish longitudinal growth by increasing apoptosis of proliferative growth plate chondrocytes.

ObjectivesIt has been suggested that the lysosomal recycling process called macro-autophagy plays a role in osteoarthritis development. We thus decided to genetically ablate the autophagy-indispensable Atg5 gene specifically in chondrocytes and analyse the development of osteoarthritis upon aging and in a post-traumatic model.MethodsMice lacking the Atg5 gene in their chondrocytes (Atg5cKO) were generated by crossing Atg5-floxed mice with transgenic mice that expressed cre recombinase driven by the collagen type 2 promoter. Animals were analysed at the age of 2, 6 and 12 months for age-related osteoarthritis or underwent mini-open partial medial meniscectomy at 2 months of age and were analysed 1 or 2 months after surgery. We evaluated osteoarthritis using the Osteoarthritis Research Society International (OARSI) scoring on safranin-O-stained samples. Cell death was evaluated by terminal deoxy-nucleotidyl-transferase-mediated deoxy-UTP nick end labelling (TUNEL) and by immunostaining of cleaved caspases.ResultsWe observed the development of osteoarthritis in Atg5cKO mice with aging including fibrillation and loss of proteoglycans, which was particularly severe in males. The ablation of Atg5 was associated with an increased cell death as assessed by TUNEL, cleaved caspase 3 and cleaved caspase 9. Surprisingly, no difference in the development of post-traumatic osteoarthritis was observed between Atg5cKO and control mice.ConclusionsAutophagy protects from age-related osteoarthritis by facilitating chondrocyte survival.

Parathyroid hormone (PTH)-related protein (PTHrP), regulated by Indian hedgehog and acting through the PTH/PTHrP receptor (PPR), is crucial for normal cartilage development. These observations suggest a possible role of PPR signaling in the postnatal growth plate; however, the role of PPR signaling in postnatal chondrocytes is unknown. In this study, we have generated tamoxifen-inducible and cartilage-specific PPR KO mice to evaluate the physiological role of PPR signaling in postnatal chondrocytes. We found that inactivation of the PPR in chondrocytes postnatally leads to accelerated differentiation of chondrocytes, followed by disappearance of the growth plate. We also observed an increase of TUNEL-positive cells and activities of caspase-3 and caspase-9 in the growth plate, along with a decrease in phosphorylation of Bad at Ser155 in postnatal PPR KO mice. Administration of a lowphosphate diet, which prevents apoptosis of chondrocytes, prevented the disappearance of the growth plate. Taken together, these observations suggest that the major consequences of PPR activation are similar in both the fetal and postnatal growth plates. Moreover, chondrocyte apoptosis through the activation of a mitochondrial pathway may be involved in the process of premature disappearance of the growth plate by postnatal inactivation of the PPR in chondrocytes.