Suppression by glutamate of proliferative activity through glutathione depletion mediated by the cystine/glutamate antiporter in mesenchymal C3H10T1/2 stem cells

Controlling Redox Status for Stem Cell Survival, Expansion, and Differentiation

Reactive oxygen species (ROS) have long been considered as pathological agents inducing apoptosis under adverse culture conditions. However, recent findings have challenged this dogma and physiological levels of ROS are now considered as secondary messengers, mediating numerous cellular functions in stem cells. Stem cells represent important tools for tissue engineering, drug screening, and disease modeling. However, the safe use of stem cells for clinical applications still requires culture improvements to obtain functional cells. With the examples of mesenchymal stem cells (MSCs) and pluripotent stem cells (PSCs), this review investigates the roles of ROS in the maintenance of self-renewal, proliferation, and differentiation of stem cells. In addition, this work highlights that the tight control of stem cell microenvironment, including cell organization, and metabolic and mechanical environments, may be an effective approach to regulate endogenous ROS generation. Taken together, this paper indicates the need for better quantification of ROS towards the accurate control of stem cell fate.

Poly(γ-Glutamic Acid) as an Exogenous Promoter of Chondrogenic Differentiation of Human Mesenchymal Stem/Stromal Cells

Cartilage damage and/or aging effects can cause constant pain, which limits the patient's quality of life. Although different strategies have been proposed to enhance the limited regenerative capacity of cartilage tissue, the full production of native and functional cartilaginous extracellular matrix (ECM) has not yet been achieved. Poly(γ-glutamic acid) (γ-PGA), a naturally occurring polyamino acid, biodegradable into glutamate residues, has been explored for tissue regeneration. In this work, γ-PGA's ability to support the production of cartilaginous ECM by human bone marrow mesenchymal stem/stromal cells (MSCs) and nasal chondrocytes (NCs) was investigated. MSC and NC pellets were cultured in basal medium (BM), chondrogenic medium (CM), and CM-γ-PGA-supplemented medium (CM+γ-PGA) over a period of 21 days. Pellet size/shape was monitored with time. At 14 and 21 days of culture, the presence of sulfated glycosaminoglycans (sGAGs), type II collagen (Col II), Sox-9, aggrecan, type XI collagen (Col XI), type X collagen (Col X), calcium deposits, and type I collagen (Col I) was analyzed. After excluding γ-PGA's cytotoxicity, earlier cell condensation, higher sGAG content, Col II, Sox-9 (day 14), aggrecan, and Col X (day 14) production was observed in γ-PGA-supplemented MSC cultures, with no signs of mineralization or Col I. These effects were not evident with NCs. However, Sox-9 (at day 14) and Col X (at days 14 and 21) were increased, decreased, or absent, respectively. Overall, γ-PGA improved chondrogenic differentiation of MSCs, increasing ECM production earlier in culture. It is proposed that γ-PGA incorporation in novel biomaterials has a beneficial impact on future approaches for cartilage regeneration.

[Universality of amino acid signaling between diverse plasma cells]

Both glutamic (Glu) and gamma-aminobutyric (GABA) acids are believed to play roles as neurotransmitters released from particular neurons into synaptic clefts in the mammalian central nervous system. Although GABA has been shown to act as an extracellular signal outside the brain, little attention has been paid to the possible expression of machineries required for neuronal glutamatergic signaling in cells other than central neurons. We first demonstrated the presence of Glu receptors in peripheral tissues such as the adrenal and pituitary glands three decades ago. In this review, I will outline our experimental findings accumulated since then on the physiological and pathological significance of neuronal amino acids as an extracellular signal for the maintenance of homeostasis in a variety of plasma cells. For example, Glu is released upon stimulation in a Ca2+-dependent manner for signal output in osteoblasts, where Glu is essential for the expression of the master regulator of osteoblastogenesis through a particular inotropic receptor subtype. In contrast, GABA plays a role in mechanisms underlying the suppression of cellular differentiation and maturation through a particular metabotropic receptor subtype in osteoblasts. Taken together, osteoblastic maturation proceeds as a delicate balancing between excitatory glutamatergic and inhibitory GABAergic signals, as seen in the brain. Re-evaluation of drugs currently used could be beneficial for the efficient discovery and development of innovative drugs useful for the prophylaxis and/or therapy of a variety of diseases relevant to the disturbance of glutamatergic and GABAergic signaling in diverse plasma cells.

Induction of the unfolded protein response drives enhanced metabolism and chemoresistance in glioma cells

The unfolded protein response (UPR) is an endoplasmic reticulum (ER)-based cytoprotective mechanism acting to prevent pathologies accompanying protein aggregation. It is frequently active in tumors, but relatively unstudied in gliomas. We hypothesized that UPR stress effects on glioma cells might protect tumors from additional exogenous stress (ie, chemotherapeutics), postulating that protection was concurrent with altered tumor cell metabolism. Using human brain tumor cell lines, xenograft tumors, human samples and gene expression databases, we determined molecular features of glioma cell UPR induction/activation, and here report a detailed analysis of UPR transcriptional/translational/metabolic responses. Immunohistochemistry, Western and Northern blots identified elevated levels of UPR transcription factors and downstream ER chaperone targets in gliomas. Microarray profiling revealed distinct regulation of stress responses between xenograft tumors and parent cell lines, with gene ontology and network analyses linking gene expression to cell survival and metabolic processes. Human glioma samples were examined for levels of the ER chaperone GRP94 by immunohistochemistry and for other UPR components by Western blotting. Gene and protein expression data from patient gliomas correlated poor patient prognoses with increased expression of ER chaperones, UPR target genes, and metabolic enzymes (glycolysis and lipogenesis). NMR-based metabolomic studies revealed increased metabolic outputs in glucose uptake with elevated glycolytic activity as well as increased phospholipid turnover. Elevated levels of amino acids, antioxidants, and cholesterol were also evident upon UPR stress; in particular, recurrent tumors had overall higher lipid outputs and elevated specific UPR arms. Clonogenicity studies following temozolomide treatment of stressed or unstressed cells demonstrated UPR-induced chemoresistance. Our data characterize the UPR in glioma cells and human tumors, and link the UPR to chemoresistance possibly via enhanced metabolism. Given the role of the UPR in the balance between cell survival and apoptosis, targeting the UPR and/or controlling metabolic activity may prove beneficial for malignant glioma therapeutics.

[Regulatory mechanisms of skeletal tissues by amino acid signaling]

In this review, we would outline the possible signaling system for three types of amino acids including glutamate (Glu), γ-aminobutyric acid (GABA) and D-serine (D-Ser) to play a role as an extracellular signal mediator in mechanisms underlying maintenance of the cellular homeostasis in skeletal tissues. Although Glu and GABA has been thought to be an excitatory/inhibitory amino acid neurotransmitter in the mammalian central nervous system, our molecular biological analyses give rise to a novel function for Glu and GABA as an autocrine and/or paracrine factor in three types of distinct cell types including osteoblasts, osteoclasts and chondrocytes in bone tissues. Moreover, D-Ser plays a pivotal role in osteoclastogenesis through a mechanism related to the incorporation of serine enantiomers in osteoclasts after the synthesis and subsequent release from adjacent osteoblasts. Accordingly, bone formation and maintenance seems to be under control by amino acid signaling in skeletal tissues as seen with neurotransmission in the brain.

CD44 integrates signaling in normal stem cell, cancer stem cell and (pre)metastatic niches

The stem cell niche provides a regulatory microenvironment for cells as diverse as totipotent embryonic stem cells to cancer stem cells (CSCs) which exhibit stem cell-like characteristics and have the capability of regenerating the bulk of tumor cells while maintaining self-renewal potential. The transmembrane glycoprotein CD44 is a common component of the stem cell niche and exists as a standard isoform (CD44s) and a range of variant isoforms (CD44v) generated though alternative splicing. CD44 modulates signal transduction through post-translational modifications as well as interactions with hyaluronan, extracellular matrix molecules and growth factors and their cognate receptor tyrosine kinases. While the function of CD44 in hematopoietic stem cells has been studied in considerable detail, our knowledge of CD44 function in tissue-derived stem cell niches remains limited. Here we review CD44s and CD44v in both hematopoietic and tissue-derived stem cell niches, focusing on their roles in regulating stem cell behavior including self-renewal and differentiation in addition to cell-matrix interactions and signal transduction during cell migration and tumor progression. Determining the role of CD44 and CD44v in normal stem cell, CSC and (pre)metastatic niches and elucidating their unique functions could provide tools and therapeutic strategies for treating diseases as diverse as fibrosis during injury repair to cancer progression.

[Signaling factors in a variety of cells derived from mesenchymal stem cells]

L-Glutamate (Glu) and γ-aminobutyric acid (GABA) has been thought to be an excitatory/inhibitory amino acid neurotransmitter in the mammalian central nervous system (CNS). Limited information is available in the literature with regard to an extracellular transmitter role of Glu and GABA in peripheral neuronal and non-neuronal tissues, whereas recent molecular biological analyses including ours give rise to a novel function for Glu and GABA as an autocrine and/or paracrine factor in a variety of cells derived from mesenchymal stem cells, in addition to other peripheral tissues including pancreas, adrenal, and pituitary glands. Emerging evidence suggests that Glu and GABA could play a dual role in mechanisms underlying maintenance of cellular homeostasis as a neurotransmitter in the CNS and as an extracellular signal mediator in peripheral autocrine and/or paracrine tissues. In this review, therefore, we summarized the possible signaling by Glu and GABA as an extracellular signal mediator in mechanisms underlying maintenance of cellular homeostasis in mesenchymal stem cells, osteoblasts and chondrocytes.

Glutamate signaling in healthy and diseased bone

Bone relies on multiple extracellular signaling systems to maintain homeostasis of its normal structure and functions. The amino acid glutamate is a fundamental extracellular messenger molecule in many tissues, and is used in bone for both neural and non-neural signaling. This review focuses on the non-neural interactions, and examines the evolutionarily ancient glutamate signaling system in the context of its application to normal bone functioning and discusses recent findings on the role of glutamate signaling as they pertain to maintaining healthy bone structure. The underlying mechanisms of glutamate signaling and the many roles glutamate plays in modulating bone physiology are featured, including those involved in osteoclast and osteoblast differentiation and mature cell functions. Moreover, the relevance of glutamate signaling systems in diseases that affect bone, such as cancer and rheumatoid arthritis, is discussed, and will highlight how the glutamate system may be exploited as a viable therapeutic target. We will identify novel areas of research where knowledge of glutamate communication mechanisms may aid in our understanding of the complex nature of bone homeostasis. By uncovering the contributions of glutamate in maintaining healthy bone, the reader will discover how this complex molecular signaling system may advance our capacity to treat bone pathologies.

Yokukansan, a kampo medicine, protects against glutamate cytotoxicity due to oxidative stress in PC12 cells

AIM OF THE STUDY

Yokukansan is a traditional Japanese medicine consisted of seven medicinal herbs and has been used for treatment of neurosis, insomnia, and behavioral and psychological symptoms of dementia in Japan. The aim of the present study is to clarify the active compounds responsible for the protective effect of yokukansan against glutamate-induced cytotoxicity in PC12 cells.

MATERIALS AND METHODS

PC12 cells which is a tool for selective evaluation of test substances against oxidative stress was used in the present study. The cell survival rates or glutathione (GSH) levels were evaluated by a MTT reduction assay or GSH assay based on the GSH reductase enzymatic recycling method, respectively.

RESULTS

Glutamate (1-17.5mM) induced cell death of PC12 cells in a concentration- dependent manner. Yokukansan (125-500μg/ml) inhibited the glutamate-induced PC12 cell death. When the effects of extracts of the seven constituent herbs in yokukansan on the cell death were examined, Uncaria thorn was found to have the highest potency in the protection. To clarify the active compounds in Uncaria thorn, the effects of seven alkaloids (rhynchophylline, isorhynchophylline, corynoxeine, isocorynoxeine, hirsutine, hirsuteine, and geissoschizine methyl ether) on the cell death were further examined. The protective effects were found in hirsutine, hirsuteine, and geissoschizine methyl ether, which also ameliorated the glutamate-induced decrease in GSH levels.

CONCLUSION

These results suggest that yokukansan protects against PC12 cell death induced by glutamate-mediated oxidative stress, i.e., reduction of intracellular GSH level, and the effect may be mainly attributed to a synergistic effect of the hirsutine, hirsuteine, and geissoschizine methyl ether in Uncaria thorn.

Glutamate preferentially suppresses osteoblastogenesis than adipogenesis through the cystine/glutamate antiporter in mesenchymal stem cells

We have shown that glutamate (Glu) signaling machineries, such as receptors (GluR) and transporters, are functionally expressed by mesenchymal stem cells, in addition to by their progeny cells such as osteoblasts and chondrocytes. Sustained exposure to Glu induced significant decreases in alkaline phosphatase (ALP) staining and osteoblastic marker gene expression in the mesenchymal C3H10T1/2 stem cells infected with runt-related transcription factor-2 (Runx2) adenovirus, without markedly affecting Oil Red O staining for adipocytes in cells cultured with adipogenic inducers. In cells with Runx2 adenovirus, the cystine/Glu antiporter substrate cystine significantly prevented the decreases by Glu in both ALP staining and osteoblastic marker gene expression, with GluR agonists being ineffective. In cells with Runx2 adenovirus, Glu significantly decreased [14C]cystine uptake, intracellular glutathione (GSH) level, Runx2 recruitment to osteocalcin promoter and nuclear Runx2 protein level, respectively. Cystine again significantly prevented the decreases by Glu in both GSH levels and Runx2 recruitment. In mouse bone marrow stromal cells, Glu and a GSH depleter significantly decreased ALP staining without affecting Oil Red O staining. Knockdown of the cystine/Glu antiporter led to markedly decreased ALP staining and GSH levels, with concomitant prevention of the decrease by Glu, in cells with Runx2 adenovirus. These results suggest that Glu may play a role as a negative regulator at an early differentiation stage into osteoblasts than adipocytes through a mechanism relevant to nuclear translocation of Runx2 after regulation of intracellular GSH levels by the cystine/Glu antiporter expressed in mesenchymal stem cells.

Extracellular glutamate alters mature osteoclast and osteoblast functions

Glutamatergic intercellular communication is involved in many aspects of metabolic homeostasis in normal bone. In bone metastasis, the balance between bone formation and degradation is disrupted. Although the responsible mechanisms are not clear, we have previously identified that cancer cell lines used in bone tumour models secrete glutamate, suggesting that tumour-derived glutamate may disrupt sensitive signalling systems in bone. This study examines the role of glutamate in mature osteoclastic bone resorption, osteoblast differentiation, and bone nodule formation. Glutamate was found to have no effect on the survival or activity of mature osteoclasts, although glutamate transporter inhibition and receptor blockade increased the number of bone resorption pits. Furthermore, transporter inhibition increased the area of resorbed bone while significantly decreasing the number of osteoclasts. Alkaline phosphatase activity and extracellular matrix mineralization were used as measurements of osteoblast differentiation. Glutamate significantly increased osteoblast differentiation and mineralization, but transport inhibitors had no effect. These studies support earlier findings suggesting that glutamate may be more important for osteoclastogenesis than for osteoclast proliferation or functions. Since glutamate is capable of changing the differentiation and activities of both osteoclast and osteoblast cell types in bone, it is reasonable to postulate that tumour-derived glutamate may impact bone homeostasis in bone metastasis.

A by-product of glutathione production in cancer cells may cause disruption in bone metabolic processes

Bone is a frequent site for metastasis of breast and prostate cancers, often resulting in pathologic changes in bone metabolism and severe pain. The mechanisms involved are not well understood, but tumour cells may release factors that interfere with bone homeostasis. Several observations have led us to hypothesize that the functional disruptions in bone metastasis are the result of a biological process common to many cell types. The high metabolic activity characteristic of cancer cells often upregulates oxidative stress protection mechanisms such as the antioxidant molecule glutathione. In maintaining redox balance, this normal metabolic response may result in unintended pathologic effects in certain sensitive organ sites. Malignant glioma cells kill surrounding neurons in the brain specifically by secreting the amino acid glutamate, an obligatory waste product of glutathione synthesis. We suggest that glutamate release is a plausible mechanism that may account for the pathologic changes in bone metastasis, since bone, like brain, is also highly sensitive to glutamatergic disruption. This report reviews the available evidence to draw a mechanistic connection between tumour cell oxidative stress and the pathology seen in patients with bone metastasis.

Developmentally regulated ceramide synthase 6 increases mitochondrial Ca2+ loading capacity and promotes apoptosis

Ceramides, which are membrane sphingolipids and key mediators of cell-stress responses, are generated by a family of (dihydro) ceramide synthases (Lass1-6/CerS1-6). Here, we report that brain development features significant increases in sphingomyelin, sphingosine, and most ceramide species. In contrast, C(16:0)-ceramide was gradually reduced and CerS6 was down-regulated in mitochondria, thereby implicating CerS6 as a primary ceramide synthase generating C(16:0)-ceramide. Investigations into the role of CerS6 in mitochondria revealed that ceramide synthase down-regulation is associated with dramatically decreased mitochondrial Ca(2+)-loading capacity, which could be rescued by addition of ceramide. Selective CerS6 complexing with the inner membrane component of the mitochondrial permeability transition pore was detected by immunoprecipitation. This suggests that CerS6-generated ceramide could prevent mitochondrial permeability transition pore opening, leading to increased Ca(2+) accumulation in the mitochondrial matrix. We examined the effect of high CerS6 expression on cell survival in primary oligodendrocyte (OL) precursor cells, which undergo apoptotic cell death during early postnatal brain development. Exposure of OLs to glutamate resulted in apoptosis that was prevented by inhibitors of de novo ceramide biosynthesis, myriocin and fumonisin B1. Knockdown of CerS6 with siRNA reduced glutamate-triggered OL apoptosis, whereas knockdown of CerS5 had no effect: the pro-apoptotic role of CerS6 was not stimulus-specific. Knockdown of CerS6 with siRNA improved cell survival in response to nerve growth factor-induced OL apoptosis. Also, blocking mitochondrial Ca(2+) uptake or decreasing Ca(2+)-dependent protease calpain activity with specific inhibitors prevented OL apoptosis. Finally, knocking down CerS6 decreased calpain activation. Thus, our data suggest a novel role for CerS6 in the regulation of both mitochondrial Ca(2+) homeostasis and calpain, which appears to be important in OL apoptosis during brain development.

Role of mechanical stress-induced glutamate signaling-associated molecules in cytodifferentiation of periodontal ligament cells

In this study, we analyzed the effects of tensile mechanical stress on the gene expression profile of in vitro-maintained human periodontal ligament (PDL) cells. A DNA chip analysis identified 17 up-regulated genes in human PDL cells under the mechanical stress, including HOMER1 (homer homolog 1) and GRIN3A (glutamate receptor ionotropic N-methyl-d-aspartate 3A), which are related to glutamate signaling. RT-PCR and real-time PCR analyses revealed that human PDL cells constitutively expressed glutamate signaling-associated genes and that mechanical stress increased the expression of these mRNAs, leading to release of glutamate from human PDL cells and intracellular glutamate signal transduction. Interestingly, exogenous glutamate increased the mRNAs of cytodifferentiation and mineralization-related genes as well as the ALP (alkaline phosphatase) activities during the cytodifferentiation of the PDL cells. On the other hand, the glutamate signaling inhibitors riluzole and (+)-MK801 maleate suppressed the alkaline phosphatase activities and mineralized nodule formation during the cytodifferentiation and mineralization. Riluzole inhibited the mechanical stress-induced glutamate signaling-associated gene expressions in human PDL cells. Moreover, in situ hybridization analyses showed up-regulation of glutamate signaling-associated gene expressions at tension sites in the PDL under orthodontic tooth movement in a mouse model. The present data demonstrate that the glutamate signaling induced by mechanical stress positively regulates the cytodifferentiation and mineralization of PDL cells.

Glutamate signaling in chondrocytes and the potential involvement of NMDA receptors in cell proliferation and inflammatory gene expression

OBJECTIVE

Increased levels of glutamate, the main excitatory neurotransmitter, are found in the synovial fluid of osteoarthritis (OA) patients. Our aim was to study glutamate signaling in chondrocytes, focusing on the composition, pharmacology, and functional role of N-methyl-d-aspartate (NMDA) glutamate receptors.

METHODS

We used the human chondrocyte cell line SW1353 and, in parallel, primary rat articular chondrocytes. Glutamate release and uptake were measured by fluorimetric and radiometric methods, respectively. Gene expression was analyzed by quantitative polymerase chain reaction. NMDA receptor pharmacology was studied in binding experiments with [3H]MK-801, a specific NMDA receptor antagonist. RNA interference was used to knock-down the expression of NR1, a subunit of NMDA receptors.

RESULTS

Glutamate release, sodium- and calcium-dependent glutamate uptake, and the expression of a glutamate transporter were observed in chondrocytes. NR2D was the most abundant NMDA receptor subunit in these cells. Consistent with this observation, the binding affinity of [3H]MK-801 was much lower in chondrocytes than in rat brain membranes (mean K(d) values of 700 and 2.6 nM, respectively). NR1 knock-down, as well as NMDA receptor blockade with MK-801, reduced chondrocyte proliferation. Interleukin (IL)-1beta significantly altered glutamate release and uptake (about 90% increase and 50% decrease, respectively, in SW1353 cells). Moreover, IL-1beta induced the gene expression of cytokines and enzymes involved in cartilage degradation, and MK-801 significantly inhibited this response.

CONCLUSIONS

Our findings suggest that chondrocytes express a self-sufficient machinery for glutamate signaling, including a peripheral NMDA receptor with unique properties. This receptor may have a role in the inflammatory process associated with cartilage degradation, thus emerging as a potential pharmacological target in OA.

Functional expression of beta2 adrenergic receptors responsible for protection against oxidative stress through promotion of glutathione synthesis after Nrf2 upregulation in undifferentiated mesenchymal C3H10T1/2 stem cells

Adrenaline is believed to play a dual role as a neurotransmitter in the central nervous system and an adrenomedullary hormone in the peripheral tissues. In contrast to accumulating evidence for the involvement in endochondral ossification, osteoblastogenesis, and osteoclastogenesis, little attention has been paid to the role of adrenergic signals in the mechanisms underlying proliferation and differentiation of mesenchymal stem cells with self-renewal capacity and multi-potentiality to differentiate into osteoblast, chondrocyte, adipocyte, and myocyte lineages. Expression of mRNA was seen for different adrenergic receptor (AdR) subtypes, including beta(2)AdR, in the mesenchymal stem cell line C3H10T1/2 cells and mouse bone marrow mesenchymal stem cells before differentiation. Exposure to adrenaline not only increased cAMP formation, phosphorylation of cAMP responsive element (CRE) binding protein (CREB) on serine133 and CRE reporter activity in a manner sensitive to propranolol, but also rendered C3H10T1/2 cells resistant to the cytotoxicity of hydrogen peroxide, but not of either 2,4-dinitirophenol or tunicamycin. Adrenaline induced a rapid but transient increase in mRNA expression of the antioxidative gene nuclear factor E2 p45-related factor-2 (Nrf2) along with an increase in the cystine/glutamate antiporter subunit xCT mRNA expression. Hydrogen peroxide was less cytotoxic in cells overexpressing Nrf2, moreover, while adrenaline significantly increased xCT promoter activity with an increase in endogenous glutathione levels. These results suggest that adrenaline may selectively protect mesenchymal C3H10T1/2 cells from oxidative stress through a mechanism related to the promoted biosynthesis of glutathione in association with transient Nrf2 expression after activation of beta(2)AdR.

Plasma membrane glutathione transporters and their roles in cell physiology and pathophysiology

Reduced glutathione (GSH) is critical for many cellular processes, and both its intracellular and extracellular concentrations are tightly regulated. Intracellular GSH levels are regulated by two main mechanisms: by adjusting the rates of synthesis and of export from cells. Some of the proteins responsible for GSH export from mammalian cells have recently been identified, and there is increasing evidence that these GSH exporters are multispecific and multifunctional, regulating a number of key biological processes. In particular, some of the multidrug resistance-associated proteins (Mrp/Abcc) appear to mediate GSH export and homeostasis. The Mrp proteins mediate not only GSH efflux, but they also export oxidized glutathione derivatives (e.g., glutathione disulfide (GSSG), S-nitrosoglutathione (GS-NO), and glutathione-metal complexes), as well as other glutathione S-conjugates. The ability to export both GSH and oxidized derivatives of GSH, endows these transporters with the capacity to directly regulate the cellular thiol-redox status, and therefore the ability to influence many key signaling and biochemical pathways. Among the many processes that are influenced by the GSH transporters are apoptosis, cell proliferation, and cell differentiation. This report summarizes the evidence that Mrps contribute to the regulation of cellular GSH levels and the thiol-redox state, and thus to the many biochemical processes that are influenced by this tripeptide.

Serine racemase suppresses chondrogenic differentiation in cartilage in a Sox9-dependent manner

Serine racemase (SR) is responsible for the biosynthesis of D-serine (D-Ser), an endogenous co-agonist for N-methyl-D-aspartate (NMDA) receptors, from L-serine (L-Ser) in the central nervous system. In the present study, we investigated the role of SR in the regulation of chondrogenic differentiation in cartilage. On in situ hybridization analysis of tibia from neonatal rats, SR mRNA was ubiquitously expressed in all cell layers of proliferating to hypertrophic chondrocytes. In the pre-chondrogenic cell line ATDC5 cells, mRNA expression was seen with SR irrespective of the cellular maturity, with no mRNA expression of the NR1 subunit essential for the heteromeric assembly of functional NMDA receptor channels. In ATDC5 cells stably overexpressing SR, significant inhibition was found with the maturation-dependent temporal increases in Alcian blue staining, alkaline phosphatase (ALP) activity and mRNA expression of type II and type X collagens. Stable overexpression of SR significantly impaired the sry-type HMG box 9 (Sox9) transcriptinal activity in ATDC5 cells, while Sox9 transcriptional activity was significantly inhibited in COS7 cells with co-introduction of SR and Sox9. However, no significant inhibition was seen with Sox9 transcriptional activity in COS7 cells co-introduced of either SR(K56G) defective of D-Ser formation ability or 3-phosphoglycerate dehydrogenase essential for D-Ser biosynthesis. The co-introduction of SR with Sox9 significantly decreased the Sox9 protein level with that of Sox9 mRNA being unchanged. These results suggest that SR may negatively regulate cellular differentiation through the inhibition of Sox9 transcriptional activity in chondrocytes.