Reduced platelet G protein-coupled receptor kinase 2 in major depressive disorder: Antidepressant treatment-induced upregulation of GRK2 protein discriminates between responder and non-responder patients

Abstract

The homologous regulation of neurotransmitter receptors by G protein-coupled receptor kinases (GRKs) is important in the pathogenesis and treatment of major depressive disorder (MDD). Previous studies have reported that the basal status of GRK2 is different in brains (upregulation) and platelets (downregulation) of subjects with MDD. The principal aim of this study was to re-examine the status of platelet membrane GRK2 protein in patients with MDD, along with GRK3 (a close kinase homolog) and GRK5 (a kinase with different properties), before and after treatment with serotonin-selective reuptake inhibitor (SSRI) or serotonin noradrenaline reuptake inhibitor (SNRI) antidepressants. The main findings indicated that platelet GRK2 and p-Ser670 GRK2 were reduced (36–41%) in unmedicated MDD subjects, and that GRK2 content correlated inversely with the severity of depression (r = − 0.51). Effective antidepressant treatments normalized platelet GRK2, and, notably, GRK2 upregulation discriminated between responder and non-responder patients. Other findings revealed a modest reduction of platelet GRK3 (23%) and no alteration of platelet GRK5 content. In untreated subjects with MDD, lymphocyte GRK2 and GRK5 mRNAs were unaltered but antidepressant treatment-induced upregulation of GRK2 mRNA expression. The reduced content of platelet GRK2 protein is a relevant target in MDD. Although this peripheral GRK2 defect does not mirror the canonical regulation of brain GRK2 in depressed suicides, it could well represent a disease state marker as well as a surrogate of response to effective antidepressant treatment.

Introduction

Major depressive disorder (MDD) is a multifactorial and recurrent disease, and a leading cause of disability and mortality associated with suicide. The mechanisms of pathogenesis of MDD are largely unknown, and consequently there is a lack of reliable biomarkers for characterizing the disease. However, numerous investigations have established that MDD is related to dysfunctions of brain monoamine systems (reviewed in Heninger et al., 1996, Charney, 1998, Ressler & Nemeroff, 1999, Belmaker & Agam, 2008). In fact, antidepressant drugs (monoamine reuptake inhibitors, monoamine oxidase inhibitors and monoamine receptor antagonists) mediate their effects through enhancing the availability of noradrenaline and/or serotonin in the brain (Nutt, 2002), which is followed by long-lasting neuroplastic changes that hopefully are more closely associated with the therapeutic response (Krishnan & Nestler, 2008, Lucassen et al., 2010, Yanpallewar et al., 2010). In this context, biochemical and functional studies revealed the presence of supersensitive inhibitory α_2A-adrenoceptors in brains of depressed suicides (Callado et al., 1998, García-Sevilla et al., 1999, Fu et al., 2001, González-Maeso et al., 2002, Ordway et al., 2003) as well as in blood platelets of depressed patients (García-Sevilla et al., 1981, García-Sevilla et al., 1986, García-Sevilla et al., 1990, García-Sevilla et al., 2004, Piletz et al., 1991, Gurguis et al., 1999), which suggested that MDD might be associated with reduced noradrenergic function. Neurotransmitter receptor regulation is also important in the treatment of depression because most antidepressants induce indirect effects on these receptors (Esteban et al., 1999, Nutt, 2002). The increased functionality of platelet α_2A-adrenoceptors in depressed patients was downregulated by chronic antidepressant treatment (García-Sevilla et al., 1986, García-Sevilla et al., 1990, Gurguis et al., 1999).

The homologous regulation of neurotransmitter receptors (e.g. in MDD: basal receptor state and antidepressant effects) is induced by a family of G protein-coupled receptor kinases (GRKs) that phosphorylate the agonist (endogenous ligand)-activated G protein-coupled receptor (GPCR) leading to desensitization (Premont and Gainetdinov, 2007). Seven GRK members have been identified and, with the exception of GRK1/7 (retinal rods/cones), the other GRKs are ubiquitously expressed (except GRK4) and GRK2 is present at higher levels in most tissues. The canonical role of GRKs and non-visual β-arrestins1/2 is to terminate signaling of agonist-occupied GPCRs. In addition, GRKs/arrestins can simultaneously enable alternate receptor pathways such as the stimulation of extracellular signal-regulated kinases (ERK1/2) (Lefkowitz, 1998, Pierce et al., 2001, Dromey & Pfleger, 2008).

Few studies have dealt with the involvement of brain GRKs in MDD and the mechanism of action of antidepressants (García-Sevilla et al., 1999, Miralles et al., 2002, Grange-Midroit et al., 2003). Basal α_2A-adrenoceptor/Gαi/GRK2 complex was increased in brains of depressed subjects (homologous receptor regulation) (García-Sevilla et al., 1999). Moreover, antidepressant treatment was associated with reduced brain GRK2 content (García-Sevilla et al., 1999, Grange-Midroit et al., 2003). In marked contrast, GRK2 was reduced in blood cells (platelets and leukocytes) of patients with MDD (García-Sevilla et al., 2004, Matuzany-Ruban et al., 2010), indicating that peripheral GRK2 is not a surrogate for brain GRK2 in depression.

In this study, the regulation of platelet GRK2 was re-examined, along with GRK3 (close kinase homolog) and GRK5 (a kinase with different properties), in patients with MDD before and after treatment with serotonin-selective reuptake inhibitors (SSRIs: fluoxetine or escitalopram) and serotonin noradrenaline reuptake inhibitors (SNRIs: duloxetine or venlafaxine). Moreover, the aim was also directed to assess if the modulation of platelet GRK2 could discriminate between responder and non-responders to SSRIs and SNRIs.