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Chemokines play critical roles in numerous physiologic and pathologic processes through their action on seven-transmembrane (TM) receptors. The N-terminal domain of chemokines, which is a key determinant of signaling via its binding within a pocket formed by receptors' TM helices, can be the target of proteolytic processing. An illustrative case of this regulatory mechanism is the natural processing of CXCL12 that generates chemokine variants lacking the first two N-terminal residues. Whereas such truncated variants behave as antagonists of CXCR4, the canonical G protein-coupled receptor of CXCL12, they are agonists of the atypical chemokine receptor 3 (ACKR3/CXCR7), suggesting the implication of different structural determinants in the complexes formed between CXCL12 and its two receptors. Recent analyses have suggested that the CXCL12 N-terminus first engages the TM helices of ACKR3 followed by the receptor N-terminus wrapping around the chemokine core. Here we investigated the first stage of ACKR3-CXCL12 interactions by comparing the activity of substituted or N-terminally truncated variants of CXCL12 toward CXCR4 and ACKR3. We showed that modification of the first two N-terminal residues of the chemokine (K1R or P2G) does not alter the ability of CXCL12 to activate ACKR3. Our results also identified the K1R variant as a G protein-biased agonist of CXCR4. Comparative molecular dynamics simulations of the complexes formed by ACKR3 either with CXCL12 or with the P2G variant identified interactions between the N-terminal 2-4 residues of CXCL12 and a pocket formed by receptor's TM helices 2, 6, and 7 as critical determinants for ACKR3 activation.

G protein-coupled receptors (GPCRs) are classically characterized as cell-surface receptors transmitting extracellular signals into cells. Here we show that central components of a GPCR signaling system comprised of the melatonin type 1 receptor (MT1), its associated G protein, and β-arrestins are on and within neuronal mitochondria. We discovered that the ligand melatonin is exclusively synthesized in the mitochondrial matrix and released by the organelle activating the mitochondrial MT1 signal-transduction pathway inhibiting stress-mediated cytochrome c release and caspase activation. These findings coupled with our observation that mitochondrial MT1 overexpression reduces ischemic brain injury in mice delineate a mitochondrial GPCR mechanism contributing to the neuroprotective action of melatonin. We propose a new term, "automitocrine," analogous to "autocrine" when a similar phenomenon occurs at the cellular level, to describe this unexpected intracellular organelle ligand-receptor pathway that opens a new research avenue investigating mitochondrial GPCR biology.

Melatonin (5‐methoxy‐N‐acetyltryptamine) binds with high affinity and specificity to membrane receptors. Several receptor subtypes exist in different species, of which the mammalian MT1 and MT2 receptors are the best‐characterized. They are members of the G protein‐coupled receptor superfamily, preferentially coupling to Gi/o proteins but also to other G proteins in a cell‐context‐depending manner. In this review, experts on melatonin receptors will summarize the current state of the field. We briefly report on the discovery and classification of melatonin receptors, then focus on the molecular structure of human MT1 and MT2 receptors and highlight the importance of molecular simulations to identify new ligands and to understand the structural dynamics of these receptors. We then describe the state‐of‐the‐art of the intracellular signaling pathways activated by melatonin receptors and their complexes. Brief statements on the molecular toolbox available for melatonin receptor studies and future perspectives will round‐up this review. [ABSTRACT FROM AUTHOR]

As the COVID‐19 pandemic grows, several therapeutic candidates are being tested or undergoing clinical trials. Although prophylactic vaccination against SARS‐CoV‐2 infection has been shown to be effective, no definitive treatment exists to date in the event of infection. The rapid spread of infection by SARS‐CoV‐2 and its variants fully warrants the continued evaluation of drug treatments for COVID‐19, especially in the context of repurposing of already available and safe drugs. Here, we explored the therapeutic potential of melatonin and melatonergic compounds in attenuating COVID‐19 pathogenesis in mice expressing human ACE2 receptor (K18‐hACE2), strongly susceptible to SARS‐CoV‐2 infection. Daily administration of melatonin, agomelatine, or ramelteon delays the occurrence of severe clinical outcome with improvement of survival, especially with high melatonin dose. Although no changes in most lung inflammatory cytokines are observed, treatment with melatonergic compounds limits the exacerbated local lung production of type I and type III interferons, which is likely associated with the observed improved symptoms in treated mice. The promising results from this preclinical study should encourage studies examining the benefits of repurposing melatonergic drugs to treat COVID‐19 and related diseases in humans. [ABSTRACT FROM AUTHOR]

Introduction: Metabolic dysfunction is now recognized as a pivotal component of Alzheimer's disease (AD), the most common dementia worldwide. However, the precise molecular mechanisms linking metabolic dysfunction to AD remain elusive. Objective: Here, we investigated the direct impact of soluble oligomeric amyloid beta (Aβ) peptides, the main molecular hallmark of AD, on the leptin system, a major component of central energy metabolism regulation. Methods: We developed a new time-resolved fluorescence resonance energy transfer-based Aβ binding assay for the leptin receptor (LepR) and studied the effect of Aβ on LepR function in several in vitro assays. The in vivo effect of Aβ on LepR function was studied in an Aβ-specific AD mouse model and in pro-opiomelanocortin (POMC) neurons of the hypothalamic arcuate nucleus. Results: We revealed specific and high-affinity (Ki = 0.1 nM) binding of Aβ to LepR. Pharmacological characterization of this interaction showed that Aβ binds allosterically to the extracellular domain of LepR and negatively affects receptor function. Negative allosteric modulation of LepR by Aβ was detected at the level of signaling pathways (STAT-3, AKT, and ERK) in vitroand in vivo. Importantly, the leptin-induced response of POMC neurons, key players in the regulation of metabolic function, was completely abolished in the presence of Aβ. Conclusion: Our data indicate that Aβ is a negative allosteric modulator of LepR, resulting in impaired leptin action, and qualify LepR as a new and direct target of Aβ oligomers. Preventing the interaction of Aβ with LepR might improve both the metabolic and cognitive dysfunctions in AD condition. [ABSTRACT FROM AUTHOR]

G protein‐coupled receptors (GPCRs) transmit extracellular signals into cells by activating G protein‐ and β‐arrestin‐dependent pathways. Extracellular signal‐regulated kinases (ERKs) play a central role in integrating these different linear inputs coming from a variety of GPCRs to regulate cellular functions. Here, we investigated human melatonin MT1 and MT2 receptors signaling through the ERK1/2 cascade by employing different biochemical techniques together with pharmacological inhibitors and siRNA molecules. We show that ERK1/2 activation by both receptors is exclusively G protein‐dependent, without any participation of β‐arrestin1/2 in HEK293 cells. ERK1/2 activation by MT1 is only mediated though Gi/o proteins, while MT2 is dependent on the cooperative activation of Gi/o and Gq/11 proteins. In the absence of Gq/11 proteins, however, MT2‐induced ERK1/2 activation switches to a β‐arrestin1/2‐dependent mode. The signaling cascade downstream of G proteins is the same for both receptors and involves activation of the PI3K/PKCζ/c‐Raf/MEK/ERK cascade. The differential G protein dependency of MT1‐ and MT2‐mediated ERK activation was confirmed at the level of EGR1 and FOS gene expression, two ERK1/2 target genes. Gi/o/Gq/11 cooperativity was also observed in Neuroscreen‐1 cells expressing endogenous MT2, whereas in the mouse retina, where MT2 is engaged into MT1/MT2 heterodimers, ERK1/2 signaling is exclusively Gi/o‐dependent. Collectively, our data reveal differential signaling modes of MT1 and MT2 in terms of ERK1/2 activation, with an unexpected Gi/o/Gq/11 cooperativity exclusively for MT2. The plasticity of ERK activation by MT2 is highlighted by the switch to a β‐arrestin1/2‐dependent mode in the absence of Gq/11 proteins and by the switch to a Gi/o mode when engaged into MT1/MT2 heterodimers, revealing a new mechanism underlying tissue‐specific responses to melatonin. [ABSTRACT FROM AUTHOR]