Ligand-independent activity of the ghrelin receptor modulates AMPA receptor trafficking and supports memory formation.

The biological signals of hunger, satiety, and memory are interconnected. The role of the hormone ghrelin in regulating feeding and memory makes ghrelin receptors attractive targets for associated disorders. We investigated the effects of the high ligand-independent activity of the ghrelin receptor GHS-R1a on the physiology of excitatory synapses in the hippocampus. Blocking this activity produced a decrease in the synaptic content of AMPA receptors in hippocampal neurons and a reduction in GluA1 phosphorylation at Ser845 Reducing the ligand-independent activity of GHS-R1a increased the surface diffusion of AMPA receptors and impaired AMPA receptor-dependent synaptic delivery induced by chemical long-term potentiation. Accordingly, we found that blocking this GHS-R1a activity impaired spatial and recognition memory in mice. These observations support a role for the ligand-independent activity of GHS-R1a in regulating AMPA receptor trafficking under basal conditions and in the context of synaptic plasticity that underlies learning.

Ghrelin triggers the synaptic incorporation of AMPA receptors in the hippocampus.

Ghrelin is a peptide mainly produced by the stomach and released into circulation, affecting energy balance and growth hormone release. These effects are guided largely by the expression of the ghrelin receptor growth hormone secretagogue type 1a (GHS-R1a) in the hypothalamus and pituitary. However, GHS-R1a is expressed in other brain regions, including the hippocampus, where its activation enhances memory retention. Herein we explore the molecular mechanism underlying the action of ghrelin on hippocampal-dependent memory. Our data show that GHS-R1a is localized in the vicinity of hippocampal excitatory synapses, and that its activation increases delivery of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic-type receptors (AMPARs) to synapses, producing functional modifications at excitatory synapses. Moreover, GHS-R1a activation enhances two different paradigms of long-term potentiation in the hippocampus, activates the phosphatidylinositol 3-kinase, and increases GluA1 AMPAR subunit and stargazin phosphorylation. We propose that GHS-R1a activation in the hippocampus enhances excitatory synaptic transmission and synaptic plasticity by regulating AMPAR trafficking. Our study provides insights into mechanisms that may mediate the cognition-enhancing effect of ghrelin, and suggests a possible link between the regulation of energy metabolism and learning.

Regulation of synapse composition by protein acetylation: the role of acetylated cortactin.

Protein acetylation affects synaptic plasticity and memory, but its effects on synapse composition have not been addressed. We found that protein acetylation promotes the dendritic clustering of the excitatory postsynaptic scaffold protein PSD95 in hippocampal neurons, without affecting the total levels of this protein. Cortactin, an F-actin-binding protein enriched in dendritic spines, is a substrate for acetylation and has a role in spine morphogenesis. Recent studies showed that cortactin acetylation changes its ability to bind F-actin and regulates cellular motility, but the function of cortactin acetylation in neuronal cells is so far unknown. We tested whether acetylation of cortactin influences its morphogenic function by overexpressing wild-type cortactin, or the mimetic mutants for acetylated or deacetylated cortactin, in hippocampal neurons, and found that cortactin acetylation has an impact on PSD95 clustering, independent from its function as actin dynamics regulator. Moreover, acetylated cortactin can rescue the reduction in PSD95 clustering mediated by knockdown of cortactin. We also found that acetylation of cortactin is correlated with decreased cortactin interaction with p140Cap and Shank1, and with lower cortactin phosphorylation at tyrosine 421. The neurotrophin BDNF promoted the acetylation of cortactin in hippocampal neurons, suggesting that BDNF may regulate excitatory synapses and PSD95 dendritic clustering at least in part by changing the acetylation level of cortactin. Our findings unravel an unsuspected role for cortactin acetylation in the regulation of PSD95 dendritic clustering, which may work in concert with cortactin's role in spine development.

Cleavage of the vesicular glutamate transporters under excitotoxic conditions.

Glutamate is loaded into synaptic vesicles by vesicular glutamate transporters (VGLUTs), and alterations in the transporters expression directly regulate neurotransmitter release. We investigated changes in VGLUT1 and VGLUT2 protein levels after ischemic and excitotoxic insults. The results show that VGLUT2 is cleaved by calpains after excitotoxic stimulation of hippocampal neurons with glutamate, whereas VGLUT1 is downregulated to a lower extent. VGLUT2 was also cleaved by calpains after oxygen/glucose deprivation (OGD), and downregulated after middle cerebral artery occlusion (MCAO) and intrahippocampal injection of kainate. In contrast, VGLUT1 was not affected after OGD. Incubation of isolated synaptic vesicles with recombinant calpain also induced VGLUT2 cleavage, with a little effect observed for VGLUT1. N-terminal sequencing analysis showed that calpain cleaves VGLUT2 in the C-terminus, at Asn(534) and Lys(542). The truncated GFP-VGLUT2 forms were found to a great extent in non-synaptic regions along neurites, when compared to GFP-VGLUT2. These findings show that excitotoxic and ischemic insults downregulate VGLUT2, which is likely to affect glutamatergic transmission and cell death, especially in the neonatal period when the transporter is expressed at higher levels.