Objective-Bone

A point mutation, Gly380Arg, in the transmembrane domain of fibroblast growth factor receptor 3 (FGFR3) leads to achondroplasia, the most common form of genetic dwarfism in humans. This substitution was suggested to enhance mutant receptor dimerization, leading to constitutive, ligand-independent activation. We found that dimerization and activation of the G380R mutant receptor are predominantly ligand dependent. However, using both transient and stable transfections, we found significant overexpression only of the mutant receptor protein. Metabolic pulse-chase experiments, cell surface labeling, and kinetics of uptake of radiolabeled ligand demonstrated a selective delay in the down-regulation of the mutant receptor. Moreover, this receptor was now resistant to ligand-mediated internalization, even at saturating ligand concentrations. Finally, transgenic mice expressing the human G380R mutant receptor under the mouse receptor transcriptional control demonstrated a markedly expanded area of FGFR3 immunoreactivity within their epiphyseal growth plates, compatible with an in vivo defect in receptor down-regulation. We propose that the achondroplasia mutation G380R uncouples ligand-mediated receptor activation from down-regulation at a site where the levels and kinetics of FGFR3 signals are crucial for chondrocyte maturation and bone formation.Fibroblast growth factor (FGF) receptors (FGFR) constitute a family of four genes that encode multiple receptor isoforms, all of which have intrinsic tyrosine kinase activity (8,12). Upon binding of a ligand, receptor dimerization is induced, leading to auto-and transphosphorylation followed by receptor internalization and down-regulation. These lead to the controlled activation of specific signal transduction pathways and the expression of FGF target genes, critically required during embryogenesis, tissue repair, and angiogenesis (1).Multiple mutations in FGFR 1, 2, and 3 (FGFR1, FGFR2, and FGFR3, respectively) give rise to a variety of inherited skeletal malformations (40). Mutations in FGFR3 are responsible for disorders predominantly of the long bones, including achondroplasia, the most common form of human genetic dwarfism (27,29). Over 97% of cases of achondroplasia result from either a G-to-A transition or a G-to-C transversion, changing the codon for Gly380 (GGG) to Arg (AGG or CGG) in the transmembrane domain of FGFR3. An Asn540Lys mutation in the proximal tyrosine kinase domain of FGFR3 is found in the milder disorder of hypochondroplasia (2), while substitution to a cysteine of residue 248, 249, 370, or 371 in the extracellular domain or a Lys650Glu mutation in the kinase activation loop gives rise to the most severe and neonatal lethal thanatophoric dysplasia (TD) types, I and II, respectively (28, 33). All of these skeletal malformations represent autosomal dominant disorders characterized by disproportionately short limbs and relative macrocephaly (23).The cellular basis underlying the clinical features of achondroplasia is a defect in chondrocyte function durin...

The shell of turtles is a shield which needs to be stiff at high loads but should provide sufficient flexibility for respiration and locomotion at smaller loads. We show that this seemingly contradictory requirement is met by a self‐locking material, whereby stiff bony elements are connected by a much softer suture with a complex three‐dimensional shape.

SUMMARYThe histological diversity of the skeletal tissues of fishes is impressive compared with that of other vertebrate groups, yet our understanding of the functional consequences of this diversity is limited. In particular, although it has been known since the mid1800s that a large number of fish species possess acellular bones, the mechanical advantages and consequences of this structural characteristic -and therefore the nature of the evolution of this feature -remain unclear. Although several studies have examined the material properties of fish bone, these have used a variety of techniques and there have been no direct contrasts of acellular and cellular bone. We report on a comparison of the structural and mechanical properties of the ribs and opercula between two freshwater fish -the common carp Cyprinus carpio (a fish with cellular bone) and the tilapia Oreochromis aureus (a fish with acellular bone). We used light microscopy to show that the bones in both fish species exhibit poor blood supply and possess discrete tissue zones, with visible layering suggesting differences in the underlying collagen architecture. We performed identical micromechanical testing protocols on samples of the two bone types to determine the mechanical properties of the bone material of opercula and ribs. Our data support the consensus of literature values, indicating that Youngʼs moduli of cellular and acellular bones are in the same range, and lower than Youngʼs moduli of the bones of mammals and birds. Despite these similarities in mechanical properties between the bone tissues of the fish species tested here, cellular bone had significantly lower mineral content than acellular bone; furthermore, the percentage ash content and bone mineral density values (derived from micro-CT scans) show that the bone of these fishes is less mineralized than amniote bone. Although we cannot generalize from our data to the numerous remaining teleost species, the results presented here suggest that while cellular and acellular fish bone may perform similarly from a mechanical standpoint, there are previously unappreciated differences in the structure and composition of these bone types.

This study determined the role of MMP9/gelatinase B during the migration onset of Neural Crest Cells (NCC) in avian embryos. NCC are neuroepithelial progenitors that convert into mesenchyme and migrate along defined paths throughout the embryo. To engage in migration, NCC loose cell contacts, detach from the neural tube and invade the surrounding environment. Multiple signals and transcription factors that regulate these events have been identified. Nevertheless, little is known regarding effectors that act downstream to execute the actual NCC migration. Matrix metalloproteinases (MMPs) compose a large family of enzymes whose principal substrates are basement membranes, adhesion proteins and the extracellular matrix (ECM) components. A major subgroup of MMPs, the gelatinases (MMP9 and 2) are central to many adult physiological and pathological processes, such as tumor metastasis and angiogenesis, in which cell-cell and cell-matrix contacts are degraded to allow migration. As NCC undergo similar processes during development, we hypothesized that MMP9 may also promote the migration of NCC. MMP9 was found to be expressed in delaminating and migrating NCC of both cranial and trunk axial levels. Blocking MMP9 resulted in a dramatic inhibition of NCC delamination and migration, without perturbing specification or survival. This inhibition occurred at regions containing both premigratory and migrating cells, indicative for the central role of MMP9 in executing the detachment of NCC from the neural tube as well as their migration. Conversely, excess MMP9 enhanced mesenchymalization and delamination of NCC and accelerated progenitors to undergo precocious migration. Examination of the mechanistic activity of MMP9 revealed its capability to degrade the adhesion molecule N-cadherin as well as the basement-membrane protein laminin within or around NCC, respectively. Altogether, our study reveals MMP9 as a novel effector which is required for NCC delamination and migration.

Various human skeletal disorders are thought to be caused by mutations in fibroblast growth factor receptor 3 (FGFR3). These result in chronic FGFR3 hyperactivation and inhibition of bone growth. One such disorder, thanatophoric dysplasia, the most common form of sporadic, lethal dwarfism, is associated frequently with cysteine substitutions (G370C, S371C, and Y373C) in the extracellular juxtamembrane region of the receptor.

The primary cilium is an immotile, solitary, and microtubule-based structure that projects from cell surfaces into the extracellular environment. The primary cilium functions as a dual sensor, as mechanosensors and chemosensors. The primary cilia coordinate several essential cell signaling pathways that are mainly involved in cell division and differentiation. A primary cilium malfunction can result in several human diseases. Mechanical loading is sense by mechanosensitive cells in nearly all tissues and organs. With this sensation, the mechanical signal is further transduced into biochemical signals involving pathways such as Akt, PKA, FAK, ERK, and MAPK. In this review, we focus on the fundamental functional and structural features of primary cilia in chondrocytes and chondrogenic cells.

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