The MT2 receptor stimulates axonogenesis and enhances synaptic transmission by activating Akt signaling

Abstract

The MT2 receptor is a principal type of G protein-coupled receptor that mainly mediates the effects of melatonin. Deficits of melatonin/MT2 signaling have been found in many neurological disorders, including Alzheimer's disease, the most common cause of dementia in the elderly, suggesting that preservation of the MT2 receptor may be beneficial to these neurological disorders. However, direct evidence linking the MT2 receptor to cognition-related synaptic plasticity remains to be established. Here, we report that the MT2 receptor, but not the MT1 receptor, is essential for axonogenesis both in vitro and in vivo. We find that axon formation is retarded in MT2 receptor knockout mice, MT2-shRNA electroporated brain slices or primary neurons treated with an MT2 receptor selective antagonist. Activation of the MT2 receptor promotes axonogenesis that is associated with an enhancement in excitatory synaptic transmission in central neurons. The signaling components downstream of the MT2 receptor consist of the Akt/GSK-3β/CRMP-2 cascade. The MT2 receptor C-terminal motif binds to Akt directly. Either inhibition of the MT2 receptor or disruption of MT2 receptor-Akt binding reduces axonogenesis and synaptic transmission. Our data suggest that the MT2 receptor activates Akt/GSK-3β/CRMP-2 signaling and is necessary and sufficient to mediate functional axonogenesis and synaptic formation in central neurons. Synaptic circuits are established at the sites of axon-dendritic, axon-somatic or axon-axonal contact, in which functional axonogenesis is a critical step. 1 Axonogenesis can be regulated by many intracellular signals that involve cytoskeletal rearrangements, 2 local protein degradation, 3 as well as diffusional barriers. 4 Additionally, several extracellular neurotrophic factors and hormones have also been shown to have a role in axon guidance and synaptic formation in central neurons. 5,6 To date, the role of melatonin and its receptors in axonogenesis remains unclear. Most of the biological functions of melatonin are mediated by its two receptors, MT1 and MT2 receptors, both of them belong to the G protein-coupled receptor (GPCR) subfamily and are widely expressed throughout the central nervous system (CNS). 7 Activation of the MT2 receptor in response to melatonin is critical for controlling circadian rhythms 7 and regulation of slow wave sleep. 8,9 Early studies have shown that activation of the MT2 receptor in the retina reduces the release of dopamine, while dopamine inhibits growth cone motility and neurite outgrowth during embryonic development, 10,11 suggesting the involvement of the MT2 receptor in functional axonogenesis. In mutant mice with deficient expression of the MT2 gene, the induction of long-term potentiation (LTP) of excitatory synaptic transmission is impaired, and this impairment is closely related to deficits in learning. 12 In the hippocampus, the MT2 receptor inhibits GABA A receptor-mediated current, 13 which is implicated in the synaptic transmission. In Alzheimer's disease, expression of the MT...

The role of melatonin in the neurodevelopmental etiology of schizophrenia: A study in human olfactory neuronal precursors

“…In support of this, Liu et al. found that melatonin induces axogenesis through MT2 receptors in embryonic hippocampal neurons from the rat . However, we cannot discard other intracellular/nuclear binding sites in the melatonin signaling involved in axonal formation.…”

“…Recently, it was demonstrated that melatonin induces the dendritogenesis process in the hilar zone of the hippocampus through CaM translocation to the soluble fraction and CaM‐kinase II stimulation, indicating that it plays a key role in neuronal development and differentiation in the adult brain . Also, in cultured embryonic hippocampal neurons, melatonin stimulates axonal formation through MT2 receptors and activation of MT1 receptor has been implicated in neurite formation at earlier stages …”

Targeting the HDAC2/HNF-4A/miR-101b/AMPK Pathway Rescues Tauopathy and Dendritic Abnormalities in Alzheimer’s Disease

“…41,42 The hippocampi of embryos were dissected and trypsinized by TrypLE Express (Invitrogen) for 15 min at 37 C. Dissociated neurons were plated onto poly(D-lysine)-coated (Sigma) coverslips in a 12-well plate or tissue culture dish covered with a glass bottom (WPI) at a higher density (400-600 neurons/mm 2 ) for western blotting and a lower density (100-200 neurons/mm 2 ) for immunofluorescence. Neurons usually attached to the substrate after they grew in the plating medium (DMEM/F12 with 10% fetal bovine serum [FBS] and 100 U/mL penicillin/streptomycin) for 2-4 hr.…”

“…Protein concentration was quantitated using the BCA Protein Assay Reagent (Pierce). 42,46 Equal amounts of protein were separated by SDS-PAGE (10% gel) and transferred to nitrocellulose membranes. The membranes were blocked with 5% non-fat milk dissolved in PBS (50 mM Tris-HCl, pH 7.6, 150 mM NaCl) for 30 min to 1 hr and were probed with primary antibodies overnight at 4 C. Then the blots were incubated with goat anti-mouse or anti-rabbit conjugated to IRdye 800 (1:15,000; Rockland Immunochemicals) for 1 hr at 25 C. The protein bands were visualized and quantified by the Odyssey infrared imaging system (LI-COR).…”

Enhancement of high glucose‐induced PINK1 expression by melatonin stimulates neuronal cell survival: Involvement of MT2/Akt/NF‐κB pathway

“…Furthermore, our investigation revealed that Akt/NF‐κB signaling activated by MT 2 is important for PINK1‐mediated neuronal cell survival under high glucose conditions. The C‐terminal motif of activated MT 2 directly binds to Akt, which leads to regulation of neuronal cell‐related physiology including hypothalamus‐liver communication, axonogenesis, and synaptic transmission . In addition, it has been reported that Akt phosphorylation by melatonin induces neuroprotective effect .…”