Osteoclastogenesis is negatively regulated by D‐serine produced by osteoblasts

Abstract

We have shown the functional expression by chondrocytes of serine racemase (SR) which is responsible for the synthesis of D-serine (Ser) from L-Ser in cartilage. In this study, we evaluated the possible functional expression of SR by bone-forming osteoblasts and bone-resorbing osteoclasts. Expression of SR mRNA was seen in osteoblasts localized at the cancellous bone surface in neonatal rat tibial sections and in cultured rat calvarial osteoblasts endowed to release D-Ser into extracellular medium, but not in cultured osteoclasts differentiated from murine bone marrow progenitor cells. Sustained exposure to D-Ser failed to significantly affect alkaline phosphatase activity and Ca(2+) accumulation in cultured osteoblasts, but significantly inhibited differentiation and maturation in a concentration-dependent manner at a concentration range of 0.1-1 mM without affecting cellular survival in cultured osteoclasts. By contrast, L-Ser promoted osteoclastic differentiation in a manner sensitive to the inhibition by D-Ser. Matured osteoclasts expressed mRNA for the amino acid transporter B(0,+) (ATB(0,+) ) and the system alanine, serine, and cysteine amino acid transporter-2 (ASCT2), which are individually capable of similarly incorporating extracellular L- and D-Ser. Knockdown of these transporters by siRNA prevented both the promotion by L-Ser and the inhibition by D-Ser of osteoclastic differentiation in pre-osteoclastic RAW264.7 cells. These results suggest that D-Ser may play a pivotal role in osteoclastogenesis through a mechanism related to the incorporation mediated by both ATB(0,+) and ASCT2 of serine enantiomers in osteoclasts after the synthesis and subsequent release from adjacent osteoblasts.

D-Serine and D-Cycloserine Reduce Compulsive Alcohol Intake in Rats

“…Importantly, D-serine within the NAcore but not DLS selectively reduced aversion-resistant alcohol drinking, identifying the NAcore as a critical region where D-serine regulates aversion-resistant consumption. Finally, we provide in vitro evidence that D-serine and DCS inhibited NAcore HA-NMDARs in alcohol-drinking rats, similar to other studies where D-serine inhibited HA-NMDARs (Chatterton et al, 2002;Takarada et al, 2009Takarada et al, , 2012. D-serine and DCS had no effect on electrically evoked EPSCs, indicating that they did not affect AMPAR EPSCs in general.…”

“…D-serine is an agonist at most NMDAR subunits, but can also inhibit HA-NMDARs (Chatterton et al, 2002;Takarada et al, 2009Takarada et al, , 2012. We recently showed that inhibiting NAcore HA-NMDARs selectively decreases aversion-resistant alcohol drinking, with no effect on intake not explicitly paired with aversive consequences (Seif et al, 2013).…”

“…D-Serine Reduced NMDAR Function at − 70 mV Resting Potential In vitro HA-NMDARs in the NAcore of alcohol-drinking rats promote aversion-resistant alcohol intake (Seif et al, 2013). As D-serine can inhibit HA-NMDARs (Chatterton et al, 2002;Takarada et al, 2009Takarada et al, , 2012, we determined whether D-serine inhibited NAcore HA-NMDARs in vitro in alcoholdrinking rats. In particular, we examined EPSCs generated at − 70 mV by ChR2 stimulation of mPFC-NAcore inputs without the AMPAR blocker DNQX, as in Seif et al (2013).…”

“…Although D-serine is a canonical positive modulator of NMDARs (Cull-Candy and Leszkiewicz, 2004;Millan, 2005), it can also inhibit HA-NMDARs (Chatterton et al, 2002;Takarada et al, 2009Takarada et al, , 2012. Here, we examined whether D-serine or DCS systemically, or D-serine within the NAcore, could suppress aversion-resistant alcohol intake.…”

Do specific NMDA receptor subunits act as gateways for addictive behaviors?

“…One additional finding from our studies is that d ‐serine, either systemically or within the NAc, selectively reduces compulsion‐like drinking in a manner similar to the NMDAR blocker AP‐5 (Seif et al ). Although d ‐serine is canonically considered an NMDAR activator at the glycine site (see following section), it can also inhibit non‐canonical NMDARs containing the GluN3 subunit (Chatterton et al ; Takarada et al , ), although at higher concentrations than those that promote NMDAR activity (Takarada et al , ). In this regard, aversion‐resistant alcohol drinking is inhibited by 50–100 µg but not 25 µg d ‐serine within the NAc core (Seif et al ), while 4 µg d ‐serine within the NAc enhances NMDAR function and promotes memory formation (Curcio et al ).…”