Increased Gsα within blood cell membrane lipid microdomains in some depressive disorders: An exploratory study

Abstract

The stimulatory guanine nucleotide binding protein Gs couples many cellular receptors to adenylate cyclase, and the Gsα subunit activates all 9 isoforms of the adenylate cyclase catalytic unit to produce the enzyme product cyclicAMP or cAMP. In prefrontal cortex and cerebellum of unipolar depressive suicides, Rasenick and colleagues have found increased concentrations of Gsα in membrane lipid microdomains (Donati et al., 2008), where the ensconced Gsα is less likely to activate adenylate cyclase by receptor and postreceptor pathways (Allen et al., 2005, 2009).

We report that a group of 7 depressed patients (DP-1) had (1) reduced activation of platelet receptor-stimulated adenylate cyclase by both prostaglandins E2 and D2 compared to controls, and (2) reduced postreceptor stimulation of adenylate cyclase by aluminum fluoride ion in both platelets and mononuclear leukocytes when compared to both another group of depressed patients (DP-2, n = 17) and to controls (n = 21). Our observations in the blood cells of the group DP-1 support the findings of Donati et al. (2008), and they reflect the importance of this interaction between the activated Gsα subunit and membrane lipid microdomains in the pathophysiology and treatment of some major depressive disorders.

Introduction

Circulating blood platelets and mononuclear leukocytes have been used to study the receptor—Gs protein—adenylate cyclase (AC) enzyme catalytic unit complex in depressive disorders before and during treatment (Dwivedi and Pandey, 2008). The stimulatory guanine nucleotide binding protein Gs, which couples many cellular receptors to AC, activates all 9 isoforms of the AC catalytic unit, and Gs has a tissue distribution which is ubiquitous.

Gs is a heterotrimeric protein Gsα·β·γ, composed of the alpha subunit Gsα, and the Gs beta-gamma heterodimer, Gsβ·γ. The Gsα subunit is expressed by the complex imprinted locus GNAS on chromosome 20 (Kelsey, 2010; Mantovani, 2011), and there is a predominant maternal origin of GNAS transcripts in three seemingly unrelated tissues: renal proximal tubules, pituitary somatotrophs, and thyroid gland (Weinstein et al., 2000; Mantovani et al., 2002; Kelsey, 2010; Mantovani, 2011). The Gsα subunit is biallelically expressed in many other tissues, including leukocytes and platelets (Kelsey, 2010; Mantovani, 2011).

In the quiescent state, guanosine diphosphate (GDP) is bound to the Gsα subunit of Gs. Receptor activation of Gs leads to replacement of GDP with guanosine triphosphate (GTP) and the binding of GTP is associated with conformational changes in so called ‘switches’ or regions of the Gsα subunit structure near the guanine nucleotide binding site. The activated Gsα subunit, Gsα·GTP, dissociates from both the receptor and Gsβ·γ, and stimulates the catalytic unit of membrane bound AC that converts ATP into cyclicAMP (cAMP) until the bound GTP is dephosphorylated back to GDP by the GTPase at the guanine nucleotide binding site.

The Gs protein can also be activated in postreceptor fashion by GTP analogues such as guanosine-5′-3-O-(thio)triphosphate (GTPγS), or aluminum tetrafluoride ion (AlF4 ion, herein abbreviated as AlF; Sternweis and Gilman, 1982). In the case of GTPγS, the dissociation of Gsα-bound GDP is the rate-limiting step for GTPγS binding and activation (Ferguson et al., 1986).

The steric configuration of AlF ion closely resembles a phosphate group (Bigay et al., 1985). Activation of Gs by AlF is very rapid since the AlF attaches adjacent to the cleft-bound GDP of Gsα, forming Gsα–GDP–AlF–Mg⁺² which slows the release of GDP, and mimics a transitional state of GTP (Higashijima et al., 1987; Coleman et al., 1994). Thus, GTPγS and AlF activate Gs by different postreceptor mechanisms.

Dwivedi and Pandey (2008) noted that basal levels of cAMP in plasma, cerebrospinal fluid, and blood cells were not altered in mood disorders. In platelets, AC activity is regulated by both the stimulatory Gs protein and the inhibitory G protein Gi (Katada et al., 1984a). Several laboratories observed that cAMP formation was diminished in platelets from depressed patients following AC activation by prostaglandins through Gs. Using intact leukocytes, Mann et al. (1997) found that in depressed patients, cAMP production was reduced in the presence of either the beta-adrenergic agonist isoproterenol or prostaglandin E1. In mononuclear leukocytes, AC activity is regulated by Gs but not by the inhibitory guanine nucleotide protein Gi, although Gi is present in mononuclear leukocytes (Motulsky et al., 1986; Maisel et al., 1990). This suggests that diminished signaling through Gs rather than enhanced inhibitory signaling through Gi is responsible for the reductions in cAMP formation in leukocytes from depressed patients.

Activated Gsα can accumulate in membrane lipid microdomains, resulting in reductions in AC signaling (Allen et al., 2005; 2009; Head et al., 2006; Donati et al., 2008). Allen et al. (2009) has drawn attention to a study of platelet AC in 1,481 subjects with lifetime histories of alcohol and/or drug dependence, where subjects who also had lifetime histories of depression (n = 226) had reductions in the stimulation of platelet AC by both cesium fluoride and forskolin (Hines and Tabakoff, 2005). Allen et al. (2009) suggested that these attenuated signals may reflect an increased proportion of Gsα being localized to lipid rafts. Donati et al. (2008) found that there was a greater fraction of brain Gsα ensconced in lipid microdomains in the prefrontal cortex and cerebellum of unipolar depressed suicides compared to controls. Such lipid-rich regions are abundant in platelets (Dorahy et al., 1996; Jayachandran and Miller, 2003) and leukocytes (Szöőr et al., 2010).

Here we describe a subgroup of untreated depressed patients DP-1 who had evidence of reduced AC signaling in both leukocytes and platelets. We propose that our findings in blood cells from the depressed DP-1 support the observations of Donati et al. (2008) in unipolar depressed suicides.