How the serotonin story is being rewritten by new gene-based discoveries principally related to SLC6A4, the serotonin transporter gene, which functions to influence all cellular serotonin systems

Abstract

Discovered and crystallized over sixty years ago, serotonin's important functions in the brain and body were identified over the ensuing years by neurochemical, physiological and pharmacological investigations. This 2008 M. Rapport Memorial Serotonin Review focuses on some of the most recent discoveries involving serotonin that are based on genetic methodologies. These include examples of the consequences that result from direct serotonergic gene manipulation (gene deletion or overexpression) in mice and other species; an evaluation of some phenotypes related to functional human serotonergic gene variants, particularly in SLC6A4, the serotonin transporter gene; and finally, a consideration of the pharmacogenomics of serotonergic drugs with respect to both their therapeutic actions and side effects. The serotonin transporter (SERT) has been the most comprehensively studied of the serotonin system molecular components, and will be the primary focus of this review. We provide in-depth examples of gene-based discoveries primarily related to SLC6A4 that have clarified serotonin's many important homeostatic functions in humans, non-human primates, mice and other species.

Introduction

Serotonin and the serotonin transporter modulate many brain and body processes involved in health and disease (Fig. 1). Early vasoactive response assays and pharmacological studies by M. Rapport, V. Erspamer, I.H. Page and others first identified serotonin as an endogenous substance that stimulated smooth muscle responses in the periphery in studies that began over sixty years ago (Erspamer, 1954, Page, 1976, Rapport, 1997, Rapport et al., 1948). However, gene variants that change the serotonergic system so as to alter vasoactive and other peripheral and central serotonin functions have only recently been discovered (Chen et al., 2001, Linder et al., 2008, Murphy and Lesch, 2008). Many earlier books, reviews and research papers have explicated the remarkable roles of the 14-plus serotonin receptors and the single transporter in the functions that serotonin plays as a neurotransmitter and neuromodulator in neurochemical, pharmacological, physiological and behavioral processes (Baumgarten and Gothert, 2000, Fox et al., 2007a, Hahn and Blakely, 2002, Murphy et al., 2004a, Murphy and Lesch, 2008, Roth, 2006, Vanhoutte et al., 1990).

This Rapport Memorial Review focuses on recent developments based upon gene manipulations in non-human species and gene variant studies in humans that have challenged and changed some of our more classical views and interpretations of the pharmacology and physiology of serotonin's actions. In doing so, it highlights important examples of how gene-based discoveries have been re-writing the history of the central and peripheral serotonergic systems. The emphasis is on examples from experiments focused on one major component of the serotonergic systems, the serotonin transporter and its gene (SLC6A4 in humans; Slc6a4 in mice, otherwise known in some published papers as 5-HTT or SERT).

SERT functions in all serotonergic systems via transport-mediated regulation of the availability of serotonin to its homo- and hetero-receptors in brain, blood and peripheral organs. Changes in SERT, as documented below, alter the expression and/or function of most, if not all, serotonin receptors as well as the synthesis, clearance and metabolism of serotonin. These changes have important clinical implications as SERT inhibitors. These include the drug classes of serotonin reuptake inhibitors (SRIs), the selective serotonin reuptake inhibitors (SSRIs) and also combined serotonin-norepinephrine reuptake inhibitors (SNRIs), that are among the world's most widely prescribed drugs for the treatment of neuropsychiatric and many other disorders and are also the target of serotonergic recreational drugs such as MDMA (Ecstasy) as well as cocaine. The review perspectives and additional supporting data presented here are from two sources that have provided the most up-to-date recent history of the contributions attributable to genetic alterations in serotonergic system genes, principally involving SLC6A4. The two examples that are examined are primarily from our laboratory and from colleagues in this field: (1) First, there are human gene variants in SLC6A4, many of which have now been documented to produce changes in SERT expression, function and/or regulation. These changes have been shown to be associated with alterations in brain SERT binding site densities, brain-imaged responses to emotional stimuli, personality traits and multiple disorders involving the central nervous system (CNS) and cardiac, pulmonary and other systems (reviews: Murphy et al., 2004a, Murphy and Lesch, 2008). Additionally, some of these SLC6A4 variants have been found to be associated with therapeutic responses as well as side effects of drugs affecting SERT and the serotonergic systems, especially SRIs and related SERT-altering medications. As human SLC6A4 variants are associated with prominent psychological and behavioral phenotypes in anxiety-related emotional domains, special attention is directed towards studies that have documented these findings in humans, and to relevant anxiety-related behavioral phenotypes in mice. This review will note recent findings in one anxiety disorder, obsessive–compulsive disorder (OCD) and related OCD spectrum disorders, as prime examples of conditions with exceptional serotonergic interest since specific functional SLC6A4 variants have been found to be associated with these disorders in multiple studies (Delorme et al., 2005, Hu et al., 2006, Kilic et al., 2003, Ozaki et al., 2003, Sutcliffe et al., 2005, Wendland et al., 2008a, Wendland et al., 2008b). Of special interest, SRIs are the only drug group found in replicated studies to be therapeutically useful in OCD, an anxiety disorder that does not respond to other anxiolytic or antidepressant agents such as tricyclic antidepressants (Greist et al., 1995, Pigott et al., 1990). (2) Secondly, genetically engineered SERT-deficient mice (Bengel et al., 1998, Murphy and Lesch, 2008) have elucidated the roles of changes in extracellular fluid (ECF) and intracellular (whole tissue) serotonin levels with regard to serotonin receptor activation and adaptation, serotonin clearance and synthesis, plus pharmacological, physiological and behavioral phenotypes that emerge in Slc6a4 knockout (−/−) mice, produced by homologous recombination in ES cells, and in single-allele-deficient, heterozygous (+/−) mice (Ansorge et al., 2004, Bengel et al., 1998, Fox et al., 2007a, Li, 2006), or in other mice produced by transgenic techniques that are either SERT-deficient (Thakker et al., 2005, Zhao et al., 2006) or have SERT overexpression (Jennings et al., 2006).