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 the dopamine (D5 ) receptor, highly expressed in CA1 hippocampal neurons, selectively reduces CaV 3.2 and CaV 3.3 currents

BACKGROUND AND PURPOSE

Ca_V 3.1-3 currents differentially contribute to neuronal firing patterns. Ca_V 3 are regulated by G protein-coupled receptors (GPCRs) activity, but information about Ca_V 3 as targets of the constitutive activity of GPCRs is scarce. We investigate the impact of D₅ recpetor constitutive activity, a GPCR with high levels of basal activity, on Ca_V 3 functionality. D₅ recpetor and Ca_V 3 are expressed in the hippocampus and have been independently linked to pathophysiological states associated with epilepsy.

EXPERIMENTAL APPROACH

Our study models were HEK293T cells heterologously expressing D₁ or D₅ receptor and Ca_V 3.1-3, and mouse brain slices containing the hippocampus. We used chlorpromazine (D₁ /D₅ inverse agonist) and a D₅ receptor mutant lacking constitutive activity as experimental tools. We measured Ca_V 3 currents and excitability parameters using the patch-clamp technique. We completed our study with computational modelling and imaging technique.

KEY RESULTS

We found a higher sensitivity to TTA-P2 (Ca_V 3 blocker) in CA1 pyramidal neurons obtained from chlorpromazine-treated animals compared with vehicle-treated animals. We found that Ca_V 3.2 and Ca_V 3.3-but not Ca_V 3.1-are targets of D₅ receptor constitutive activity in HEK293T cells. Finally, we found an increased firing rate in CA1 pyramidal neurons from chlorpromazine-treated animals in comparison with vehicle-treated animals. Similar changes in firing rate were observed on a neuronal model with controlled Ca_V 3 currents levels.

CONCLUSIONS AND IMPLICATIONS

Native hippocampal Ca_V 3 and recombinant Ca_V 3.2-3 are sensitive to D₅ receptor constitutive activity. Manipulation of D₅ receptor constitutive activity could be a valuable strategy to control neuronal excitability, especially in exacerbated conditions such as epilepsy.

ACE2 internalization induced by a SARS-CoV-2 recombinant protein is modulated by angiotensin II type 1 and bradykinin 2 receptors

AIMS

Angiotensin-converting enzyme 2 (ACE2) is a key regulator of the renin-angiotensin system (RAS) recently identified as the membrane receptor for the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Here we aim to study whether two receptors from RAS, the angiotensin receptor type 1 (AT1R) and the bradykinin 2 receptor (B2R) modulate ACE2 internalization induced by a recombinant receptor binding domain (RBD) of SARS-CoV-2 spike protein. Also, we investigated the impact of ACE2 coexpression on AT1R and B2R functionality.

MATERIALS AND METHODS

To study ACE2 internalization, we assessed the distribution of green fluorescent protein (GFP) signal in HEK293T cells coexpressing GFP-tagged ACE2 and AT1R, or B2R, or AT1R plus B2R in presence of RBD alone or in combination with AT1R or B2R ligands. To estimate ACE2 internalization, we classified GFP signal distribution as plasma membrane uniform GFP (PMU-GFP), plasma membrane clustered GFP (PMC-GFP) or internalized GFP and calculated its relative frequency. Additionally, we investigated the effect of ACE2 coexpression on AT1R and B2R inhibitory action on voltage-gated calcium channels (CaV2.2) currents by patch-clamp technique.

KEY FINDINGS

RBD induced ACE2-GFP internalization in a time-dependent manner. RBD-induced ACE2-GFP internalization was increased by angiotensin II and reduced by telmisartan in cells coexpressing AT1R. RBD-induced ACE2-GFP internalization was strongly inhibited by B2R co-expression. This effect was mildly modified by bradykinin and rescued by angiotensin II in presence of AT1R. ACE2 coexpression impacted on B2R- and AT1R-mediated inhibition of CaV2.2 currents.

SIGNIFICANCE

Our work contributes to understand the role of RAS modulators in the susceptibility to SARS-CoV-2 infection and severity of COVID-19.

Metabolic insights from a GHSR-A203E mutant mouse model

Objective

Binding of ghrelin to its receptor, growth hormone secretagogue receptor (GHSR), stimulates GH release, induces eating, and increases blood glucose. These processes may also be influenced by constitutive (ghrelin-independent) GHSR activity, as suggested by findings in short people with naturally occurring GHSR-A204E mutations and reduced food intake and blood glucose in rodents administered GHSR inverse agonists, both of which impair constitutive GHSR activity. In this study, we aimed to more fully determine the physiologic relevance of constitutive GHSR activity.

Methods

We generated mice with a GHSR mutation that replaces alanine at position 203 with glutamate (GHSR-A203E), which corresponds to the previously described human GHSR-A204E mutation, and used them to conduct ex vivo neuronal electrophysiology and in vivo metabolic assessments. We also measured signaling within COS-7 and HEK293T cells transfected with wild-type GHSR (GHSR-WT) or GHSR-A203E constructs.

Results

In COS-7 cells, GHSR-A203E resulted in lower baseline IP₃ accumulation than GHSR-WT; ghrelin-induced IP₃ accumulation was observed in both constructs. In HEK293T cells co-transfected with voltage-gated Ca_V2.2 calcium channel complex, GHSR-A203E had no effect on basal Ca_V2.2 current density while GHSR-WT did; both GHSR-A203E and GHSR-WT inhibited Ca_V2.2 current in the presence of ghrelin. In cultured hypothalamic neurons from GHSR-A203E and GHSR-deficient mice, native calcium currents were greater than those in neurons from wild-type mice; ghrelin inhibited calcium currents in cultured hypothalamic neurons from both GHSR-A203E and wild-type mice. In brain slices, resting membrane potentials of arcuate NPY neurons from GHSR-A203E mice were hyperpolarized compared to those from wild-type mice; the same percentage of arcuate NPY neurons from GHSR-A203E and wild-type mice depolarized upon ghrelin exposure. The GHSR-A203E mutation did not significantly affect body weight, body length, or femur length in the first ∼6 months of life, yet these parameters were lower in GHSR-A203E mice after 1 year of age. During a 7-d 60% caloric restriction regimen, GHSR-A203E mice lacked the usual marked rise in plasma GH and demonstrated an exaggerated drop in blood glucose. Administered ghrelin also exhibited reduced orexigenic and GH secretagogue efficacies in GHSR-A203E mice.

Conclusions

Our data suggest that the A203E mutation ablates constitutive GHSR activity and that constitutive GHSR activity contributes to the native depolarizing conductance of GHSR-expressing arcuate NPY neurons. Although the A203E mutation does not block ghrelin-evoked signaling as assessed using in vitro and ex vivo models, GHSR-A203E mice lack the usual acute food intake response to administered ghrelin in vivo. The GHSR-A203E mutation also blunts GH release, and in aged mice leads to reduced body length and femur length, which are consistent with the short stature of human carriers of the GHSR-A204E mutation.

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We generated mice with a GHSR mutation replacing Ala at position 203 with Glu.

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The A203E mutation ablates constitutive GHSR activity & hyperpolarizes NPY neurons.

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GHSR-A203E mice lack the usual orexigenic response to administered ghrelin.

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The GHSR-A203E mutation blunts GH release and causes reduced body length.

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This finding is consistent with short stature in human carriers of the GHSR-A204E mutation.

Dopamine Receptor Type 2 and Ghrelin Receptor Coexpression Alters CaV2.2 Modulation by G Protein Signaling Cascades

Voltage-gated calcium channels type 2.2 (Ca_V2.2) are activated by action potentials at presynaptic terminals, and their calcium current induces neurotransmitter release. In this context, regulating Ca_V2.2 is critical, and one of the most important mechanisms for doing so is through its G protein-coupled receptor (GPCR) activity. Two such GPCRs are the ghrelin (GHSR) and the dopamine type 2 (D2R) receptors. We previously demonstrated that constitutive GHSR activity reduces Ca_V2.2 forward trafficking and that ghrelin-induced GHSR activity inhibits Ca_V2.2 currents. On the other hand, dopamine-induced D2R activity also inhibits Ca_V2.2 currents. It has been recently shown that D2R and GHSR form heteromers in hypothalamic neurons. This interaction profoundly changes the signaling cascades activated by dopamine and is necessary for dopamine-dependent anorexia. Here we explored how D2R-GHSR coexpression in HEK293T cells modulates the effect that each GPCR has on Ca_V2.2. We found that D2R-GHSR coexpression reduces the inhibition of Ca_V2.2 currents by agonist-induced D2R activation and added a new source of basal Ca_V2.2 current inhibition to the one produced by GHSR solely expression. We investigated the signaling cascades implicated and found that constitutive GHSR activity, G_q protein, and Gβγ subunit play a critical role in these altered effects. Moreover, we found that the effect of D2R agonist on native calcium currents in hypothalamic neurons is reduced when both D2R and GHSR are overexpressed. In summary, our results allow us to propose a novel mechanism for controlling Ca_V2.2 currents involving the coexpression of two physiologically relevant GPCRs.

Free fatty acid receptor 3 is a key target of short chain fatty acid. What is the impact on the sympathetic nervous system?

Nervous system (NS) activity participates in metabolic homeostasis by detecting peripheral signal molecules derived from food intake and energy balance. High quality diets are thought to include fiber-rich foods like whole grain rice, breads, cereals, and grains. Several studies have associated high consumption of fiber-enriched diets with a reduced risk of diabetes, obesity, and gastrointestinal disorders. In the lower intestine, anaerobic fermentation of soluble fibers by microbiota produces short chain fatty acids (SCFAs), key energy molecules that have a recent identified leading role in the intestinal gluconeogenesis, promoting beneficial effects on glucose tolerance and insulin resistance. SCFAs are also signaling molecules that bind to specific G-protein coupled receptors (GPCRs) named Free Fatty Acid Receptor 3 (FFA3, GPR41) and 2 (FFA2, GPR43). However, how SCFAs impact NS activity through their GPCRs is poorly understood. Recently, studies have demonstrated the presence of FFA2 and FFA3 in the sympathetic NS of rat, mouse and human. Two studies have showed that FFA3 activation by SCFAs increases firing and norepinephrine (NE) release from sympathetic neurons. However, the recent study from the Ikeda Laboratory revealed that activation of FFA3 by SCFAs impairs N-type calcium channel (NTCC) activity, which contradicts the idea of FFA3 activation leading to increased action potential evoked NE release. Here we will discuss the scope of the latter study and the putative physiological role of SCFAs and FFAs in the sympathetic NS.