Gi/o-Protein Coupled Receptors in the Aging Brain

Abstract

Cells translate extracellular signals to regulate processes such as differentiation, metabolism and proliferation, via transmembranar receptors. G protein-coupled receptors (GPCRs) belong to the largest family of transmembrane receptors, with over 800 members in the human species. Given the variety of key physiological functions regulated by GPCRs, these are main targets of existing drugs. During normal aging, alterations in the expression and activity of GPCRs have been observed. The central nervous system (CNS) is particularly affected by these alterations, which results in decreased brain functions, impaired neuroregeneration, and increased vulnerability to neuropathologies, such as Alzheimer's and Parkinson diseases. GPCRs signal via heterotrimeric G proteins, such as G_o, the most abundant heterotrimeric G protein in CNS. We here review age-induced effects of GPCR signaling via the G_i/o subfamily at the CNS. During the aging process, a reduction in protein density is observed for almost half of the G_i/o-coupled GPCRs, particularly in age-vulnerable regions such as the frontal cortex, hippocampus, substantia nigra and striatum. G_i/o levels also tend to decrease with aging, particularly in regions such as the frontal cortex. Alterations in the expression and activity of GPCRs and coupled G proteins result from altered proteostasis, peroxidation of membranar lipids and age-associated neuronal degeneration and death, and have impact on aging hallmarks and age-related neuropathologies. Further, due to oligomerization of GPCRs at the membrane and their cooperative signaling, down-regulation of a specific G_i/o-coupled GPCR may affect signaling and drug targeting of other types/subtypes of GPCRs with which it dimerizes. G_i/o-coupled GPCRs receptorsomes are thus the focus of more effective therapeutic drugs aiming to prevent or revert the decline in brain functions and increased risk of neuropathologies at advanced ages.

Keywords: G protein-coupled receptors GPCRs; Gi/o heterotrimeric G proteins; aging; basal ganglia; frontal cortex; hippocampus; receptor density and binding potential.

FIGURE 1

Relative abundance of G_i/o-coupled GPCRs. (A) GPCRs’ main functions. There are about 800 human GPCRs, and around half have sensory functions that mediate olfaction, taste, light perception and pheromone signaling. Another big slice of the pie belongs to non-sensory GPCRs that mediate signaling from other type of ligands, mainly from hormones and neurotransmitters (NTs). (B) Relative abundance of G_i/o-coupled GPCRs in 13 human tissues/regions was taken from the Human Protein Atlas (in Supplementary Table S1), quantitatively transformed (value 1 was attributed to ‘low’, 2.5 for ‘medium’ and 3.5 for ‘high’ abundance), and the relative % of tissue distribution calculated by taking the sum of all abundance values, in all the 13 tissues, as 100%.

FIGURE 2

GPCR and G-proteins activation models. (A) Classical GPCR/G protein activation model. When inactive, heterotrimeric G proteins exist as a complex comprising the α subunit (bound to GDP) and the βγ subunit. (1a) The binding of a ligand (as a neurotransmitter) to a GPCR results in a conformational change that allows (1b) the binding of the receptor to the G protein. (2) This in turn causes a second conformational change on the α-subunit that results in the exchange of its GDP by GTP, and the separation of the α and βγ subunits. (3) At this point the G protein is active, and both the α and βγ subunits interact with downstream effectors to modulate different signaling pathways. (4) The G protein activation is terminated by the hydrolysis of the GTP molecule into GDP, a reaction that can be accelerated by the binding of Regulators of G-protein Signaling (RGS) to the α subunit. (5) The GTP hydrolysis results in the re-formation of the trimeric complex, bringing the G protein back to its inactive state. (B) Pre-coupled model(s). New mechanisms of GPCRs’ activation have emerged from various studies, particularly on G_i/o-coupled GPCRs. In these, inactive GPCRs monomers (left) or homo/hetero dimers (right) exist in an inactive G protein-binding state. In some cases, effector proteins (as adenylyl cyclase, AC) can even be part of the inactive pre-coupled receptorsome complex (not shown). (1) When the GPCR is activated by a ligand (as dopamine, serotonin, etc.) (2) its coupled G protein becomes active by exchanging GDP by GTP and either dissociates from the GPCR and its βγ subunit to initiate the signaling response (left) or remain associated to the complex and activate downstream effector proteins at the membrane (right). (3) In either case, the GTPase activity (accelerated, e.g., by binding to RGS proteins) terminates the activation cycle for that G protein, and the GPCR-inactive G protein complex reassembles/is reinstalled. Note that, in both classical and pre-coupled models, when an active G_α subunit dissociates from the GPCR, another cytosolic inactive heterotrimeric G protein can bind to the activated GPCR to get activated, in an amplification mechanism. GIRK, G-protein activated Inwardly Rectifying K⁺ channel; PLC, phospholipase C; PDE 6, phosphodiesterase 6.

FIGURE 3

Main mechanisms of age-induced GPCRs decline. With aging, there is a general decrease in G_i/o-coupled GPCRs protein levels throughout the brain. Though many elements are involved, there are three main factors: (1) alterations in the plasma membrane structure and fluidity caused by events like lipid peroxidation; these lead to the instability of membrane proteins, including GPCRS, and result either in their altered function or decreased levels and densities at, e.g., signaling nanodomains; (2) hindered proteostasis: decreased protein synthesis due to alterations in transcription factors or increased mRNA instability, and accumulation of aberrant GPCRs and other signaling proteins due to less efficient proteasome or autophagy (Aphg) quality control systems, may lead to a decrease in the levels of functional GPCRs in the aged brain; (3) the increased incidence of neurodegenerative diseases and physical trauma with aging, together with exposure to toxins, all lead to neuronal death. Moreover, the alterations in protein synthesis and membrane structure can eventually contribute to the onset or progression of neurodegenerative pathologies, with a subsequent further decrease in some GPCRs’ levels.

FIGURE 4

Major alterations in G_o/i-coupled GPCRs’ pathways in the aged brain, affecting four main areas. In the frontal lobe, particularly at the pre-frontal cortex (1), the protein densities and/or affinity of α2 adrenoceptors, acetylcholine M2 receptors (M2), serotonin 5-HT1A receptors, opioid receptors (including the kappa receptor, κ), somatostatin receptors (SSTRs), cannabinoid receptor CB1, the metabotropic GABA_B receptor, and of the Gα proteins G_αo/i, are found decreased. The same occurs in the (2) striatum, excluding the serotonin 5-HT1A receptors, the GABA metabotropic receptors and Gαo/i, and including dopamine D2 receptors and the oxytocin (OT) receptors. The (3) hippocampus also presents decreases in the same receptors as the frontal cortex, with the exclusion of the adrenoceptors and the GABA_B receptor. Contrarily, mGluR2 and 3 glutamate receptors are up-regulated in these three brain areas. In the (4) substantia nigra, dopamine D2, opioid κ and cannabinoid CB1 receptors are also depleted with age, while angiotensin AT1 receptor levels are increased.