Transcriptional dysregulation of 5-HT1A autoreceptors in mental illness

Abstract

The serotonin-1A (5-HT1A) receptor is among the most abundant and widely distributed 5-HT receptors in the brain, but is also expressed on serotonin neurons as an autoreceptor where it plays a critical role in regulating the activity of the entire serotonin system. Over-expression of the 5-HT1A autoreceptor has been implicated in reducing serotonergic neurotransmission, and is associated with major depression and suicide. Extensive characterization of the transcriptional regulation of the 5-HT1A gene (HTR1A) using cell culture systems has revealed a GC-rich "housekeeping" promoter that non-selectively drives its expression; this is flanked by a series of upstream repressor elements for REST, Freud-1/CC2D1A and Freud-2/CC2D1B factors that not only restrict its expression to neurons, but may also regulate the level of expression of 5-HT1A receptors in various subsets of neurons, including serotonergic neurons. A separate set of allele-specific factors, including Deaf1, Hes1 and Hes5 repress at the HTR1A C(-1019)G (rs6295) polymorphism in serotonergic neurons in culture, as well as in vivo. Pet1, an obligatory enhancer for serotonergic differentiation, has been identified as a potent activator of 5-HT1A autoreceptor expression. Taken together, these results highlight an integrated regulation of 5-HT1A autoreceptors that differs in several aspects from regulation of post-synaptic 5-HT1A receptors, and could be selectively targeted to enhance serotonergic neurotransmission.

Figure 1

Components of 5-HT neurotransmission. Shown are the major components involved in the synthesis, vesicular packaging, reuptake, and degradation of serotonin in the brain, and the major receptors that mediate pre- and post-synaptic regulation of 5-HT neurotransmission. TRP, tryptophan; 5-HTP, 5-hydroxy-TRP; 5-HIAA, 5-hydroxy indole acetic acid; TPH2, tryptophan hydroxylase-2; AADC, aromatic amino-acid decarboxylase; VMAT2, vesicular monoamine transporter-2; MAO-B, monoamine oxidase B; 5-HTT, 5-HT transporter.

Figure 2

5-HT1A autoreceptor-mediated negative feedback on 5-HT neurons. A model of a serotonergic neuron (5-HT neuron) projecting to a target neuron (postsynaptic neuron) is shown, with 5-HT1A receptors depicted as yellow ovals, and other 5-HT receptors as other colored ovals. Acutely, SSRI's rapidly enter the brain and block 5-HT transporters (green diamonds), located at the raphe serotonin nerve terminal, but also at the cell body and dendrites in the raphe nuclei. This block of reuptake leads to accumulation of extracellular 5-HT (red dots) at both sites once released by depolarization. Activation of 5-HT1A autoreceptors located at the cell body and dendrites leads to inhibition of neuronal firing rate (red action potential), thus compensating for increase in 5-HT induced by SSRI treatment, resulting in little change in 5-HT neurotransmission initially.

Figure 3

Neuronal signaling of the 5-HT1A autoreceptor. The major signaling pathways of the 5-HT1A receptor in neurons are shown. The 5-HT1A receptor is shown figuratively as a heptahelical G-protein coupled receptor that couples via inhibitory G proteins (Gi/Go) to inhibit adenylyl cyclase (AC) and reduce cAMP levels; to open G-protein inward rectifying potassium channels (K⁺) to reduce membrane potential (Vm); and to inhibit voltage-gated calcium channels (Ca⁺⁺) and reduce intracellular free calcium concentration ([Ca⁺⁺]_i).

Figure 4

Identified promoter elements of the human 5-HT1A receptor gene (HTR1A). The location of identified DNA elements on the 5-HT1A 5' regions flanking the start of translation (bold arrow) are shown figuratively. Identified activators (arrows) or repressors (bars) of transcription are also shown. Within the minimal promoter (box) there are GC-rich Sp1 and MAZ elements (+); NFkB response element (NRE); and glucocorticoid receptors (GR/MR) that inhibit transcription by blocking Sp1. Further upstream in a repressor/enhancer regions, a negative glucocorticoid response element (nGRE) also mediates direct GR/MR-induced repression. Hes and Deaf1 proteins repress the 5-HT1A promoter at the C(-1019) allele, while in serotonin neurons Pet-1 exerts strong enhancer activity. A strong repressor region that silences expression in 5-HT1A-negative non-neuronal cells, but also represses in 5-HT1A-positive neuronal cells is located upstream that includes elements for REST (RE-1), Freud-1 and Freud-2 (DRE).

Figure 5

Proposed actions of 5-HT1A C(-1019)G genotype in normal vs. depressed subjects. In normal subjects containing homozygous HTR1A C/C (-1019) genotype, 5-HT neurons fire at their normal rate (red action potentials), releasing 5-HT (red dots) at the synapse for activation of receptors (ovals), and to a lesser degree at the raphe cell body to restrain neuronal activity by negative feedback through the 5-HT1A autoreceptor (yellow ovals). In the G/G genotype and in depressed subjects, 5-HT1A autoreceptor expression is increased which would reduce neuronal firing and 5-HT release, while post-synaptic 5-HT1A receptors are reduced in certain regions, which would decrease response to 5-HT release. Normal subjects with the G/G genotype have a lesser increase of 5-HT1A autoreceptors, and are able to compensate for the effect of the G/G genotype on transcription.