Insights into the regulation of 5-HT2A serotonin receptors by scaffolding proteins and kinases

Abstract

5-HT(2A) serotonin receptors are essential molecular targets for the actions of LSD-like hallucinogens and atypical antipsychotic drugs. 5-HT(2A) serotonin receptors also mediate a variety of physiological processes in peripheral and central nervous systems including platelet aggregation, smooth muscle contraction, and the modulation of mood and perception. Scaffolding proteins have emerged as important regulators of 5-HT(2A) receptors and our recent studies suggest multiple scaffolds exist for 5-HT(2A) receptors including PSD95, arrestin, and caveolin. In addition, a novel interaction has emerged between p90 ribosomal S6 kinase and 5-HT(2A) receptors which attenuates receptor signaling. This article reviews our recent studies and emphasizes the role of scaffolding proteins and kinases in the regulation of 5-HT(2A) trafficking, targeting and signaling.

FIGURE 1. Neuronal scaffolding mechanisms for sorting and targeting of 5-HT_2A receptors

Scaffolding proteins including PSD95, β-arrestins and caveolins interact with 5-HT_2A receptors and influence the subcellular targeting and signaling of the receptor. Subsequent to expression in neurons and sorting within the endoplasmic reticulum and golgi, 5-HT_2A receptors traffic within vesicles to various neuronal subdomains including endosomal membranes, microtubules and apical dendrites. 5-HT_2A receptors interact with microtubule associated protein 1A (MAP1A) which may scaffold the receptor to the microtubule cytoskeleton and facilitate trafficking to apical dendrites. 5-HT_2A receptors contain a canonical PDZ binding motif at their extreme carboxy-terminus which interacts with PSD95. The 5-HT2A-PDZ-PSD95 interaction is essential for the polarized sorting and targeting of the receptor to dendrites. In addition to dendritic targeting, many 5-HT_2A receptors are found in intracellular endomembranes where they associate with β-arrestins, which may scaffold the receptor in intracellular compartments. Caveolins also interact with 5-HT_2A receptors which may target the receptor to cholesterol-enriched membrane microdomains of the plasma membrane to enable efficient receptor-mediated signaling. The kinase RSK2 interacts with and phosphorylates 5-HT_2A receptors which attenuates both basal and serotonin-induced signaling. These interactions between the receptor and scaffolding proteins or kinases provide multiple and overlapping mechanisms for sorting 5-HT_2A receptors and regulating serotonergic signaling.

FIGURE 2. Scaffolding proteins and serotonin-induced endocytosis of 5-HT_2A receptors

Agonist-induced internalization of 5-HT_2A receptors may involve two distinct endocytosis pathways in neurons and depend upon interactions with caveolins and/or β-arrestins. Upon serotonin binding, the receptors undergo conformational shifts that promote G protein activation and the receptors are internalized. Scaffolding interactions of the receptor with caveolins or arrestins may recruit the receptors with the appropriate endocytosis protein machinery to facilitate dynamin-dependent endocytsis through either a caveolae/caveolin or clathrin-dependent mechanism. Depending on the endocytosis pathway and scaffolding interaction, activated 5-HT_2A receptors traffic into caveolin containing caveosomes or clathrin-coated vesicles. The receptors are trafficked further into sorting endosomes where they may recycle back to the plasma membrane or be targeted for degradation in the lysosomal pathway. A scaffolding interaction between PSD95 and 5-HT_2A receptors can stabilize the receptors at the plasma membrane, possibly in post-synaptic sites, and also prevents agonist-induced internalization. Therefore, depending on the specific scaffold interaction, 5-HT_2A internalization can be facilitated (caveolin or β-arrestins) or prevented (PSD95).