Probing the constitutive activity among dopamine D1 and D5 receptors and their mutants

Chlorpromazine, an Inverse Agonist of D1R-Like, Differentially Targets Voltage-Gated Calcium Channel (CaV) Subtypes in mPFC Neurons

The dopamine receptor type 1 (D1R) and the dopamine receptor type 5 (D5R), which are often grouped as D1R-like due to their sequence and signaling similarities, exhibit high levels of constitutive activity. The molecular basis for this agonist-independent activation has been well characterized through biochemical and mutagenesis in vitro studies. In this regard, it was reported that many antipsychotic drugs act as inverse agonists of D1R-like constitutive activity. On the other hand, D1R is highly expressed in the medial prefrontal cortex (mPFC), a brain area with important functions such as working memory. Here, we studied the impact of D1R-like constitutive activity and chlorpromazine (CPZ), an antipsychotic drug and D1R-like inverse agonist, on various neuronal Ca_V conductances, and we explored its effect on calcium-dependent neuronal functions in the mouse medial mPFC. Using ex vivo brain slices containing the mPFC and transfected HEK293T cells, we found that CPZ reduces Ca_V2.2 currents by occluding D1R-like constitutive activity, in agreement with a mechanism previously reported by our lab, whereas CPZ directly inhibits Ca_V1 currents in a D1R-like activity independent manner. In contrast, CPZ and D1R constitutive activity did not affect Ca_V2.1, Ca_V2.3, or Ca_V3 currents. Finally, we found that CPZ reduces excitatory postsynaptic responses in mPFC neurons. Our results contribute to understanding CPZ molecular targets in neurons and describe a novel physiological consequence of CPZ non-canonical action as a D1R-like inverse agonist in the mouse brain.

Constitutive activity of dopamine receptor type 1 (D1R) increases CaV2.2 currents in PFC neurons

Alterations in dopamine receptor type 1 (D1R) density are associated with cognitive deficits of aging and schizophrenia. In the prefrontal cortex (PFC), D1R plays a critical role in the regulation of working memory, which is impaired in these cognitive deficit states, but the cellular events triggered by changes in D1R expression remain unknown. A previous report demonstrated that interaction between voltage-gated calcium channel type 2.2 (Ca_V2.2) and D1R stimulates Ca_V2.2 postsynaptic surface location in medial PFC pyramidal neurons. Here, we show that in addition to the occurrence of the physical receptor-channel interaction, constitutive D1R activity mediates up-regulation of functional Ca_V2.2 surface density. We performed patch-clamp experiments on transfected HEK293T cells and wild-type C57BL/6 mouse brain slices, as well as imaging experiments and cAMP measurements. We found that D1R coexpression led to ∼60% increase in Ca_V2.2 currents in HEK293T cells. This effect was occluded by preincubation with a D1/D5R inverse agonist, chlorpromazine, and by replacing D1R with a D1R mutant lacking constitutive activity. Moreover, D1R-induced increase in Ca_V2.2 currents required basally active Gs protein, as well as D1R-Ca_V2.2 interaction. In mice, intraperitoneal administration of chlorpromazine reduced native Ca_V currents’ sensitivity to ω-conotoxin-GVIA and their size by ∼49% in layer V/VI pyramidal neurons from medial PFC, indicating a selective effect on Ca_V2.2. Additionally, we found that reducing D1/D5R constitutive activity correlates with a decrease in the agonist-induced D1/D5R inhibitory effect on native Ca_V currents. Our results could be interpreted as a stimulatory effect of D1R constitutive activity on the number of Ca_V2.2 channels available for dopamine-mediated modulation. Our results contribute to the understanding of the physiological role of D1R constitutive activity and may explain the noncanonical postsynaptic distribution of functional Ca_V2.2 in PFC neurons.

Constitutive activity of a G protein-coupled receptor, DRD1, contributes to human cerebral organoid formation

The intricate balance of neural stem cell (NSC) amplification and neurogenesis is central to nervous system development. Dopamine D1 receptor (DRD1) is a typical G protein‐coupled receptor (GPCR) mainly expressed in neurogenic area, with high constitutive activity. The receptor appears in the embryonic period before the formation of mature synaptic contacts, which indicates that dopamine receptor and its constitutive activity play crucial roles in the embryonic brain development. Here, we found that DRD1 was enriched in human NSCs. Inhibition of the receptor activity by its inverse agonists promoted human NSCs proliferation and impeded its differentiation. These results were also mimicked by genetic knockdown of DRD1, which also blocked the effects of inverse agonists, suggesting a receptor‐dependent manner. More interestingly, knock‐in A229T mutant with reduced DRD1 constitutive activity by CRISPR‐Cas9 genome editing technology resulted into increased endogenous human NSCs proliferation. These results were well reproduced in human cerebral organoids, and inhibition of the DRD1 constitutive activity by its inverse agonists induced the expansion and folding of human cerebral organoids. The anatomic analysis uncovered that decreasing the constitutive activity of DRD1 by its inverse agonists promoted the NSCs proliferation and maintenance that led to hindered cortical neurogenesis. Further mechanistic studies revealed that the PKC‐CBP pathway was involved in the regulation by DRD1. Thus, our findings indicate that the constitutive activity of DRD1 and possibly other GPCRs plays an important role in the development of human nervous system.

Functional importance of two conserved residues in intracellular loop 1 and transmembrane region 2 of Family A GPCRs: insights from ligand binding and signal transduction responses of D1 and D5 dopaminergic receptor mutants

For many G protein-coupled receptors (GPCRs), the role of the first intracellular loop (IL1) and its connections with adjacent transmembrane (TM) regions have not been investigated. Notably, these regions harbor several polar residues such as Ser and Thr. To begin uncovering how these polar residues may contribute to the structural basis for GPCR functionality, we have designed human D1-class receptor mutants (hD1-ST1 and hD5-ST1) whereby all Ser and Thr of IL1 and IL1/TM2 juncture have been replaced by Ala and Val, respectively. Both ST1 mutants exhibited a loss of dopamine affinity but similar binding properties for inverse agonists compared to their parent receptors. As well, these mutations diminished receptor activation for both subtypes, as indicated by an ablated constitutive activity and a pronounced decrease in dopamine potency. Interestingly, both mutants exhibited enhanced dopamine-mediated maximal stimulation (Emax) of adenylyl cyclase that was at least two-fold higher than wild-type. Point mutations for hD1R revealed that the loss in dopamine affinity and potency was attributed to Thr59, while the enhanced Emax of adenylyl cyclase was directly influenced by Ser65. These two residues are conserved among many Family A GPCRs and have recurring molecular interactions among crystallized structures. As such, their functional roles for IL1 and its transition into TM2 reported herein may also be applicable to other GPCRs. Our work thus potentially highlights a structural role of Thr59 and Ser65 in the formation of critical intramolecular interactions for ligand binding and signal transduction of D1-class dopaminergic receptors.

Constitutive activities and inverse agonism in dopamine receptors

The concept of activation in the absence of agonists has been demonstrated for many GPCRs and is now solidified as one of the principal aspects of GPCR signaling. In this chapter, we review how dopamine receptors demonstrate this ability. Although difficult to prove in vivo due to the presence of endogenous dopamine and lack of subtype-selective inverse agonists and "pure" antagonists (neutral ligands), in vitro assays such as measuring intracellular cAMP, [(35)S]GTPγS binding, and [(3)H]thymidine incorporation have uncovered the constitutive activation of D1- and D2-class receptors. Nevertheless, because of limited and inconsistent findings, the existence of constitutive activity for D2-class receptors is currently not well established. Mutagenesis studies have shown that basal signaling, notably by D1-class receptors, is governed by the collective contributions of transmembrane domains and extracellular/intracellular loops, such as the third extracellular loop, the third intracellular loop, and C-terminal tail. Furthermore, constitutive activities of D1-class receptors are subjected to regulation by kinases. Among the dopamine receptor family, the D5 receptor subtype exhibits a higher basal signaling and bears resemblance to constitutively active mutant forms of GPCRs. The presence of its constitutive activity in vivo and its pathophysiological relevance, with a brief mention of other subtypes, are also discussed.

Functional analysis of human D1 and D5 dopaminergic G protein-coupled receptors: lessons from mutagenesis of a conserved serine residue in the cytosolic end of transmembrane region 6

In mammals, dopamine G protein-coupled receptors (GPCR) are segregated into two categories: D1-like (D1R and D5R) and D2-like (D2R(short), D2R(long), D3R, and D4R) subtypes. D1R and D5R are primarily coupled to stimulatory heterotrimeric GTP-binding proteins (Gs/olf) leading to activation of adenylyl cyclase and production of intracellular cAMP. D1R and D5R share high level of amino acid identity in transmembrane (TM) regions. Yet these two GPCR subtypes display distinct ligand binding and G protein coupling properties. In fact, our studies suggest that functional properties reported for constitutively active mutants of GPCRs (e.g., increased basal activity, higher agonist affinity and intrinsic activity) are also observed in cells expressing wild type D5R when compared with wild type D1R. Herein, we describe an experimental method based on mutagenesis and transfection of human embryonic kidney 293 (HEK293) cells to explore the molecular mechanisms regulating ligand affinity, agonist-independent and dependent activity of D1R and D5R. We will demonstrate how to mutate one conserved residue in the cytosolic end of TM6 of D1R (Ser263) and D5R (Ser287) by modifying two or three nucleotides in the cDNA of human D1-like receptors. Genetically modified D1R and D5R cDNAs are prepared using a polymerase chain reaction method, propagated in E. coli, purified and mutations confirmed by DNA sequencing. Receptor expression constructs are transfected into HEK293 cells cultured in vitro at 37°C in 5% CO(2) environment and used in radioligand binding and whole cAMP assays. In this study, we will test the effect of S263A/G/D and S287A/G/D mutations on ligand binding and DA-dependent activation of D1R and D5R.

Activation of GABA(B) receptors inhibits protein kinase B/glycogen synthase kinase 3 signaling

Accumulated evidence has suggested that potentiation of cortical GABAergic inhibitory neurotransmission may be a key mechanism in the treatment of schizophrenia. However, the downstream molecular mechanisms related to GABA potentiation remain unexplored. Recent studies have suggested that dopamine D2 receptor antagonists, which are used in the clinical treatment of schizophrenia, modulate protein kinase B (Akt)/glycogen synthase kinase (GSK)-3 signaling. Here we report that activation of GABA_B receptors significantly inhibits Akt/GSK-3 signaling in a β-arrestin-dependent pathway. Agonist stimulation of GABA_B receptors enhances the phosphorylation of Akt (Thr-308) and enhances the phosphorylation of GSK-3α (Ser-21)/β (Ser-9) in both HEK-293T cells expressing GABA_B receptors and rat hippocampal slices. Furthermore, knocking down the expression of β-arrestin2 using siRNA abolishes the GABA_B receptor-mediated modulation of GSK-3 signaling. Our data may help to identify potentially novel targets through which GABA_B receptor agents may exert therapeutic effects in the treatment of schizophrenia.

TTX-resistant NMDA receptor-mediated membrane potential oscillations in neonatal mouse Hb9 interneurons

Conditional neuronal membrane potential oscillations have been identified as a potential mechanism to help support or generate rhythmogenesis in neural circuits. A genetically identified population of ventromedial interneurons, called Hb9, in the mouse spinal cord has been shown to generate TTX-resistant membrane potential oscillations in the presence of NMDA, serotonin and dopamine, but these oscillatory properties are not well characterized. Hb9 interneurons are rhythmically active during fictive locomotor-like behavior. In this study, we report that exogenous N-Methyl-D-Aspartic acid (NMDA) application is sufficient to produce membrane potential oscillations in Hb9 interneurons. In contrast, exogenous serotonin and dopamine application, alone or in combination, are not sufficient. The properties of NMDA-induced oscillations vary among the Hb9 interneuron population; their frequency and amplitude increase with increasing NMDA concentration. NMDA does not modulate the T-type calcium current (I_Ca(T)), which is thought to be important in generating locomotor-like activity, in Hb9 neurons. These results suggest that NMDA receptor activation is sufficient for the generation of TTX-resistant NMDA-induced membrane potential oscillations in Hb9 interneurons.

The third intracellular loop of D1 and D5 dopaminergic receptors dictates their subtype-specific PKC-induced sensitization and desensitization in a receptor conformation-dependent manner

We previously showed that phorbol-12-myristate-13-acetate (PMA) mediates a robust PKC-dependent sensitization and desensitization of the highly homologous human Gs protein and adenylyl cyclase (AC)-linked D1 (hD1R) and D5 (hD5R) dopaminergic receptors, respectively. Here, we demonstrate using forskolin-mediated AC stimulation that PMA-mediated hD1R sensitization and hD5R desensitization is not associated with changes in AC activity. We next employed a series of chimeric hD1R and hD5R to delineate the underlying structural determinants dictating the subtype-specific regulation of human D1-like receptors by PMA. We first used chimeric receptors in which the whole terminal region (TR) spanning from the extracellular face of transmembrane domain 6 to the end of cytoplasmic tail (CT) or CT alone were exchanged between hD1R and hD5R. CT and TR swaps lead to chimeric hD1R and hD5R retaining PMA-induced sensitization and desensitization of wild type parent receptors. In striking contrast, hD1R sensitization and hD5R desensitization mediated by PMA are correspondingly switched to PMA-induced receptor desensitization and sensitization following the IL3 swap between hD1R and hD5R. Cell treatment with the PKC blocker, Gö6983, inhibits PMA-induced regulation of these chimeric receptors in a similar fashion to wild type receptors. Further studies with chimeras constructed by exchanging IL3 and TR show that PMA-induced regulation of these chimeras remains fully switched relative to their respective wild type parent receptor. Interestingly, results obtained with the exchange of IL3 and TR also reveal that the D1-like subtype-specific regulation by PMA, while fully dictated by IL3, can be modulated in a receptor conformation-dependent manner. Overall, our results strongly suggest that IL3 is the critical determinant underlying the subtype-specific regulation of human D1-like receptor responsiveness by PKC.