Trans-Modulation of the Somatostatin Type 2A Receptor Trafficking by Insulin-Regulated Aminopeptidase Decreases Limbic Seizures

Abstract

Within the hippocampus, the major somatostatin (SRIF) receptor subtype, the sst2A receptor, is localized at postsynaptic sites of the principal neurons where it modulates neuronal activity. Following agonist exposure, this receptor rapidly internalizes and recycles slowly through the trans-Golgi network. In epilepsy, a high and chronic release of somatostatin occurs, which provokes, in both rat and human tissue, a decrease in the density of this inhibitory receptor at the cell surface. The insulin-regulated aminopeptidase (IRAP) is involved in vesicular trafficking and shares common regional distribution with the sst2A receptor. In addition, IRAP ligands display anticonvulsive properties. We therefore sought to assess by in vitro and in vivo experiments in hippocampal rat tissue whether IRAP ligands could regulate the trafficking of the sst2A receptor and, consequently, modulate limbic seizures. Using pharmacological and cell biological approaches, we demonstrate that IRAP ligands accelerate the recycling of the sst2A receptor that has internalized in neurons in vitro or in vivo. Most importantly, because IRAP ligands increase the density of this inhibitory receptor at the plasma membrane, they also potentiate the neuropeptide SRIF inhibitory effects on seizure activity. Our results further demonstrate that IRAP is a therapeutic target for the treatment of limbic seizures and possibly for other neurological conditions in which downregulation of G-protein-coupled receptors occurs.

International Union of Basic and Clinical Pharmacology. CV. Somatostatin Receptors: Structure, Function, Ligands, and New Nomenclature

“…Assuming that SST 2 signals in intracellular organelles, this could explain, at least in part, the long-lasting actions of SRIF analogs such as octreotide. Of note, although ECE-1 might be present in the hippocampus ( Barnes et al, 1997 ), the kinetics of SST 2 recycling were the same after activation by SRIF-14 or octreotide in hippocampal neurons ( De Bundel et al, 2015 ), suggesting that the ECE-1 role on SST 2 recycling is dependent upon cell types. Filamin A (FLNA), a scaffolding protein involved in intracellular trafficking of several transmembrane proteins ( Onoprishvili et al, 2003 ; Noam et al, 2014 ), has been shown to interact with the SST 2 in melanoma and pancreatic cell lines ( Najib et al, 2012 ).…”

“…Collectively, these studies highlight that SRIF released under physiologic or pathophysiological conditions regulates localization and trafficking of SST 2 consistent with results obtained in cell lines ( Csaba and Dournaud, 2001 ; Tulipano and Schulz, 2007 ; Jacobs and Schulz, 2008 ; Treppiedi et al, 2017 ). In hippocampal neuronal cells, SST 2 trafficking was analyzed in detail at different times after acute intracerebral octreotide injections or in primary neuronal culture exposed to SRIF ligands ( Csaba et al, 2001 , 2007 ; Lelouvier et al, 2008 ; De Bundel et al, 2015 ). These experiments demonstrated for the first time that GPCR cargoes recycle through the trans-Golgi network (TGN) after endocytosis.…”

“…Modulating recycling of a particular receptor can indeed impact its physiologic fate and therefore offer a potential therapeutic value. Using pharmacological and cell biologic approaches, it was demonstrated that leucyl-cysteinyl aminopeptidase (LNPEP; formerly known as insulin-regulated aminopeptidase) ligands accelerate recycling of internalized SST 2 in neurons in vitro or in vivo ( De Bundel et al, 2015 ). LNPEP, which shares common regional and subcellular distribution with internalized SST 2 , was shown to be involved in vesicular trafficking ( Wright and Harding, 2011 ).…”

“…LNPEP, which shares common regional and subcellular distribution with internalized SST 2 , was shown to be involved in vesicular trafficking ( Wright and Harding, 2011 ). Importantly, because LNPEP ligands increase the density of SST 2 at the plasma membrane, they also potentiate SRIF-inhibitory effects on seizure activity ( De Bundel et al, 2015 ). LNPEP therefore represents a therapeutic target for treatment of limbic seizures and possibly for other neurologic conditions in which downregulation of GPCRs occurs.…”

Is There an Interplay Between the Functional Domains of IRAP?

“…In the basal state, newly synthesized IRAP is targeted to the trans-Golgi network (TGN), evidenced by colocalization with a TGN-specific marker (TGN38) ( De Bundel et al, 2015 ). It is subsequently transported for storage to specialized endosomal vesicles in the perinuclear region of the cell, with slow recycling of these endosomes to the plasma membrane ( Figure 2 ; Shi et al, 2008 ; Jordens et al, 2010 ).…”