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
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 α2A-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 α2A-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 α2A-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.
Section snippets
Patient selection, inclusion/exclusion criteria, and antidepressant treatment
Outpatients with MDD, aged over 18, were recruited, diagnosed and treated in Hospital Santa Creu i Sant Pau (Álvarez et al., 1997), Autonomous University of Barcelona. The study protocols (clinical assessment, treatment, and biochemical assays) were approved by Hospital Santa Creu i Sant Pau ethical committee, and all procedures were conducted in accordance with the Declaration of Helsinki (World Medical Association, 2008). The main inclusion criteria were: a diagnosis of MDD according to
Platelet GRK2, GRK3 and GRK5 proteins in patients with MDD: basal content and effect of antidepressant treatment
In platelet membranes of drug-free depressed patients (N = 19; 15 women, 4 men), compared with matched controls, the basal immunodensity of GRK2 was reduced (36%, p < 0.001) (Fig. 1a,b,c). This abnormality was not related to a gender specific effect (mean GRK2 reduction in male patients: 26%, p < 0.05). The severity of depression (HRSD score) correlated inversely with platelet GRK2 content (Spearman's ρ = − 0.51, N = 19, p = 0.03). Treatment (6 weeks) with SSRI or SNRI antidepressants induced a significant
Discussion
The results confirm and extend previous findings on platelet GRK2 downregulation in patients with MDD (basal reduction correlating with severity of depression) and its normalization after effective antidepressant treatment (García-Sevilla et al., 2004). In the current study, platelet GRK2 and its close homolog GRK3 (but not GRK5) were reduced in untreated depressed patients; the severity of symptoms correlated inversely with GRK2 (r = − 0.51); and antidepressant treatments normalized GRK2 content.
Role of the funding source
Funding for this study was provided by Instituto de Salud Carlos III and Ministerio de Ciencia e Innovación (MICINN/FEDER, Madrid, Spain). A clinical research grant was provided by Lilly Laboratories (Madrid, Spain). These Public and Private Entities had no further role in the study design, collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the paper for publication.
Contributors
Authors JAG-S, EA and JJM conceived the study, designed the experimental protocols, and coordinated the project. Authors DP and VP recruited, diagnosed and treated the participant patients. Authors MA-B, RD-A, and AR-M carried out the biochemical assays, data collection and statistical analyses. Author JAG-S wrote the paper with inputs from all the co-authors. All the authors discussed the results and approved the final manuscript.
Conflict of interest
The authors state that they have no conflict of interest.
Acknowledgments
The authors gratefully acknowledge the patients and healthy controls who participated in this study. This study was supported by grants PI04/0196 (Instituto de Salud Carlos III), RETICS-RD06/0001/0003 (MICINN/FEDER) and SAF2008-01311 (MICINN/FEDER) to JAG-S; PI04/2601 (Instituto de Salud Carlos III) and CIBERSAM (MICINN/FEDER) to EA; PI04/0190 (Instituto de Salud Carlos III) and CIBERSAM (MICINN/FEDER) to JJM. MA-B and AR-M were supported by predoctoral fellowships (FPI) from MICINN/FEDER. The
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