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.

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 α1(I)‐collagen‐Cre or α1(II)‐collagen‐Cre transgenic mice to obtain osteoblast‐specific or chondrocyte‐specific Runx2 deficient mice, respectively. As seen in Runx2^−/− mice, perinatal lethality was observed in α1(II)‐Cre;Runx2^flox/flox mice, but this was not the case in animals in which α1(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^−/− mice, but was found in areas undergoing intramembranous ossification such as skull and clavicles in α1(II)‐Cre;Runx2^flox/flox mice. In newborn α1(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 α1(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 α1(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. © 2013 American Society for Bone and Mineral Research.

Circadian rhythm disturbances are well established in neurological diseases. However, how these disruptions cause homeostatic imbalances remains poorly understood. Brain and muscle aryl hydrocarbon receptor nuclear translocator-like protein 1 (Bmal1) is a major circadian clock transcriptional activator, and Bmal1 deficiency in maleBmal1_nestin^−/−mice induced marked astroglial activation without affecting the number of astrocytes in the brain and spinal cord. Bmal1 deletion caused blood–brain barrier (BBB) hyperpermeability with an age-dependent loss of pericyte coverage of blood vessels in the brain. Using Nestin-green fluorescent protein (GFP) transgenic mice, we determined that pericytes are Nestin-GFP⁺in the adult brain. Bmal1 deletion caused Nestin-GFP⁺pericyte dysfunction, including the downregulation of platelet-derived growth factor receptor β (PDGFRβ), a protein necessary for maintaining BBB integrity. Knockdown of Bmal1 downregulated PDGFRβ transcription in the brain pericyte cell line. Thus, the circadian clock component Bmal1 maintains BBB integrity via regulating pericytes.SIGNIFICANCE STATEMENTCircadian rhythm disturbances may play a role in neurodegenerative disorders, such as Alzheimer's disease. Our results revealed that one of the circadian clock components maintains the integrity of the blood–brain barrier (BBB) by regulating vascular-embedded pericytes. These cells were recently identified as a vital component for the control of BBB permeability and cerebral blood flow. Our present study demonstrates the involvement of circadian clock component Bmal1 in BBB homeostasis and highlights the role of Bmal1 dysfunction in multiple neurological diseases.

The circadian clock controls many behavioral and physiological processes beyond daily rhythms. Circadian dysfunction increases the risk of cancer, obesity, and cardiovascular and metabolic diseases. Although clinical studies have shown that bone resorption is controlled by circadian rhythm, as indicated by diurnal variations in bone resorption, the molecular mechanism of circadian clock–dependent bone resorption remains unknown. To clarify the role of circadian rhythm in bone resorption, aryl hydrocarbon receptor nuclear translocator-like (Bmal1), a prototype circadian gene, was knocked out specifically in osteoclasts. Osteoclast-specific Bmal1-knockout mice showed a high bone mass phenotype due to reduced osteoclast differentiation. A cell-based assay revealed that BMAL1 upregulated nuclear factor of activated T cells, cytoplasmic, calcineurin-dependent 1 (Nfatc1) transcription through its binding to an E-box element located on the Nfatc1 promoter in cooperation with circadian locomotor output cycles kaput (CLOCK), a heterodimer partner of BMAL1. Moreover, steroid receptor coactivator (SRC) family members were shown to interact with and upregulate BMAL1:CLOCK transcriptional activity. Collectively, these data suggest that bone resorption is controlled by osteoclastic BMAL1 through interactions with the SRC family and binding to the Nfatc1 promoter. © 2016 American Society for Bone and Mineral Research.