Summary The synthesis of Type I collagen, the main component of the bone matrix, precedes the expression of Runx2, the earliest determinant of osteoblast differentiation. We hypothesized that the osteoblast's energetic needs might explain this apparent paradox. We show here that glucose, the main nutrient of osteoblasts, is transported in these cells through Glut1 whose expression precedes that of Runx2. Glucose uptake favors osteoblast differentiation by suppressing the AMPK-dependent proteasomal degradation of Runx2 and promotes bone formation by inhibiting another function of AMPK. While Runx2 cannot induce osteoblast differentiation when glucose uptake is compromised, raising blood glucose levels restores collagen synthesis in Runx2-null osteoblasts and initiates bone formation in Runx2-deficient embryos. Moreover, Runx2 favors Glut1 expression, and this feed-forward regulation between Runx2 and Glut1 determines the onset of osteoblast differentiation during development and the extent of bone formation throughout life. These results reveal an unexpected intricacy between bone and glucose metabolism.

Global gene deletion studies in mice and humans have established the pivotal role of runt related transcription factor-2 (Runx2) in both intramembranous and endochondral ossification processes during skeletogenesis. In this study, we for the first time generated mice carrying a conditional Runx2 allele with exon 4, which encodes the Runt domain, flanked by loxP sites. These mice were crossed with a1(I)-collagen-Cre or a1(II)-collagen-Cre transgenic mice to obtain osteoblast-specific or chondrocyte-specific Runx2 deficient mice, respectively. As seen in Runx2 À/À mice, perinatal lethality was observed in a1(II)-Cre;Runx2 flox/flox mice, but this was not the case in animals in which a1(I)-collagen-Cre was used to delete Runx2. When using double-staining with Alizarin red for mineralized matrix and Alcian blue for cartilaginous matrix, we observed previously that mineralization was totally absent at embryonic day 15.5 (E15.5) throughout the body in Runx2 À/À mice, but was found in areas undergoing intramembranous ossification such as skull and clavicles in a1(II)-Cre;Runx2 flox/flox mice. In newborn a1(II)-Cre; Runx2 flox/flox mice, mineralization impairment was restricted to skeletal areas undergoing endochondral ossification including long bones and vertebrae. In contrast, no apparent skeletal abnormalities were seen in mutant embryo, newborn, and 3-week-old to 6-week old-mice in which Runx2 had been deleted with the a1(I)-collagen-Cre driver. These results suggest that Runx2 is absolutely required for endochondral ossification during embryonic and postnatal skeletogenesis, but that disrupting its expression in already committed osteoblasts as achieved here with the a1(I)-collagen-Cre driver does not affect overtly intramembranous and endochondral ossification. The Runx2 floxed allele established here is undoubtedly useful for investigating the role of Runx2 in particular cells.

In the originally published author list, we inadvertently misspelled Takashi Iezaki's surname. The author list has been corrected online.

The hypothesis that L-glutamate (Glu) is an excitatory amino acid neurotransmitter in the mammalian central nervous system is now gaining more support after the successful cloning of a number of genes coding for the signaling machinery required for this neurocrine at synapses in the brain. These include Glu receptors (signal detection), Glu transporters (signal termination) and vesicular Glu transporters (signal output through exocytotic release). Relatively little attention has been paid to the functional expression of these molecules required for Glu signaling in peripheral neuronal and non-neuronal tissues; however, recent molecular biological analyses show a novel function for Glu as an extracellular signal mediator in the autocrine and/or paracrine system. Emerging evidence suggests that Glu could play a dual role in mechanisms underlying the maintenance of cellular homeostasis -as an excitatory neurotransmitter in the central neurocrine system and an extracellular signal mediator in peripheral autocrine and/or paracrine tissues. In this review, the possible Glu signaling methods are outlined in specific peripheral tissues including bone, testis, pancreas, and the adrenal, pituitary and pineal glands.Keywords: autocrine; glutamate; glutamate receptor; glutamate transporter; neurotransmitter; paracrine; vesicular glutamate transporter; peripheral tissues. Glutamate signaling moleculesGlutamate receptors L-Glutamate (Glu) is accepted as an excitatory amino acid neurotransmitter in the mammalian central nervous system (CNS). Receptors for Glu (GluRs) are categorized into two major classes, metabotropic (mGluRs) and ionotropic (iGluRs) receptors, according to their differential intracellular signal transduction mechanisms and molecular homologies (Fig. 1) [1-3]. mGluRs are further divided into three distinct subtypes containing seven transmembrane domains, including group I (mGluR1 and mGluR5), group II (mGluR2 and mGluR3) and group III (mGluR4, mGluR6, mGluR7 and mGluR8), in line with each receptor's exogenous agonists and intracellular second messengers [4,5]. The group I subtype stimulates formation of inositol 1,4,5-triphosphate and diacylglycerol, while both group II and III subtypes induce reduction of intracellular cyclic AMP (cAMP). On the basis of sequence homology and agonist preference, the latter iGluRs are classified into N-methyl-D-aspartate (NMDA), DL-a-amino-3-hydroxy-5-methylisoxasole-4-propionate (AMPA), and kainate (KA) receptors, which are associated with ion channels permeable to particular cations [6,7].NMDA receptor channels. These channels are highly permeable to Ca 2+ , with sensitivity to blockade by Mg 2+ in a voltage-dependent manner [8,9]. Functional NMDA receptor channels are comprised of heteromeric assemblies between the essential NR1 subunit and one of four different NR2 (A-D) subunits, in addition to one of two different NR3 (A-B) subunits. Expression of the NR2 subunit alone does not lead to composition of functional ion channels in any expression system, while coexpression of...

Background: Clock genes are expressed in different peripheral organs. Results: Rhythmic expression was lost with Ihh in the growth plate from mice defective of BMAL1 with a small body size. Conclusion: Endochondral ossification is under the control by clock genes in chondrocytes. Significance: Peripheral clocks are a target for treating cartilaginous diseases relevant to abnormal postnatal chondrogenesis.