We have previously demonstrated that DHA metabolism to N-docosahexaenoylethanolamine (synaptamide) is a significant endogenous mechanism for promoting neurogenesis, neuritogenesis and synaptogensis in a cAMP dependent manner. We further demonstrated that orphan G-protein coupled receptor 110 (GPR110, ADGRF1) is the synaptamide target receptor, triggering cAMP production with low nM potency. We generated GPR110 knock-out (KO) mice, and confirmed that synaptamide-induced bioactivity is GPR110-dependent. GPR110 KO mice exhibited reduced synaptic protein expression and memory deficits, providing in vivo evidence that GPR110 is a functional synaptamide receptor for promoting brain development and function. The gpr110 expression profile indicated that this gene is primarily a developmental gene at least in the brain. During this review period, we continued our investigation on the role of GPR110/cAMP signaling in neurodevelopment and neuroprotection, together with biochemical and structural characterization of newly deorphanized GPR110. Our previously observation indicated that intravitreal administration of synaptamide immediately after optic nerve crush (ONC) enables the robust regeneration of the lesioned optic nerve, leading to partial functional recovery of vision in adult mice. We continued to test the effect of synaptamide on axon regeneration in vivo and found that gpr110 expression in retinal ganglion cells is significantly elevated after injury. We also found that synatamide is no longer capable of inducing axon regrowth or recovering visual function in GPR110 KO mice, confirming that the observed synaptamide effect is mediated through GPR110 signaling. For development of potential therapeutic agents for optic nerve injury, we screened a few stable analogues of synaptamide in ONC model. We identified an analogue that exhibits significantly enhanced activity for axon regeneration and functional recovery. Our findings suggest that this stable analogue of synaptamide may have potential therapeutic utility in optic nerve injury, offering a new possibility for clinical translation. We have previously demonstrated that DHA-derived synaptamide is a potent suppressor of neuroinflammation in an LPS-induced model, by enhancing cAMP/PKA signaling and inhibiting NF-kappaB activation. During this period we established that anti-inflammatory effect of synaptamide is also GPR110-dependent. The anti-inflammatory effects of synaptamide was not seen in microglia cells obtained from GPR110 KO mice. In vivo, synaptamide was no longer able to inhibit proinflammatory cytokine production increased by LPS in GPR110 KO mice. We demonstrated that GPR110 as a functional synaptamide receptor mediates potent anti-inflammatory effects by inhibiting the activation of innate immune cells in the brain and periphery. Our study provides the essential role of GPR110/cAMP signaling in the anti-inflammatory action of synaptamide, suggesting a novel therapeutic target for ameliorating the inflammation-associated diseases. We extended the investigation of synaptamide-induced GPR110 signaling to EtOH-enhanced neuroinflammation as well as sepsis. We found that ethanol exacerbates the LPS-induced inflammatory responses including neuroinflammation and septic shock. Synaptamide effectively blocked the EtOH-enhanced LPS injury in vivo and in vitro models, and reduced mortality caused by septic shock. These findings reveal the adverse impact of ethanol on inflammatory responses and may lead to a new strategy to prevent the ethanol-induced disturbance of the innate immune system as well as brain dysfunction. To understand the molecular basis of the GPR110 activation, we have recently probed the conformational changes of GPR110 by in-cell chemical cross-linking and quantitative mass spectrometry, which revealed that synaptamide induced conformational changes of GPR110 in the GAIN domain of the extracellular region and in the intracellular regions involving the G protein binding. For detailed molecular mechanism of GPR110 activation, we constructed the 3-D structure of GPR110 using computer modeling and performed structure-activity relationship studies using site-directed mutagenesis. We found that there is a specific synaptamide binding pocket located at the interface between the GAIN subdomains of GPR110. N512, Q511 and Y513 in the pocket interact with the polar ethanolamine group, carboxy group and the hydrophobic DHA chain of synaptamide, respectively. Mutation of these residues impairs or abolishes the synaptamide binding as well as synaptamide-induced cAMP production. Our data reveal a previously unknown ligand-induced and intact GAIN domain-mediated mechanism that regulate aGPCR signal transduction pathways. Given that synaptamide is the first small-molecule ligand identified for aGPCRs, our data can provide the basis for uncovering potential synaptamide-like ligands for this emerging class of GPCRs. Furthermore, our discovery of the critical role of ligand-GAIN domain interaction may facilitate the development of novel GAIN domain-targeting agents as potential drugs for aGPCRs-related human diseases. Provision of synaptamide as a neurogenic, synaptogenic and anti-inflammatory mediator may be important for developing human infants. As synaptamide is an endogenous metabolite of docosahexaenoic acid (DHA), synaptamide production likely responds to its precursor DHA level as altered by dietary omega-3 intake. Since breast milk is the primary source of DHA in most neonatal infants, we evaluated synaptamide concentrations in relation to DHA in expressed breast milk from lactating mothers in a preliminary clinical study. We found a significant correlation between DHA and synaptamide levels in the breast milk of lactating mothers (r=0.624, p<0.001).
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