Identification and mechanism of G protein-biased ligands for chemokine receptor CCR1.

Biased signaling of G protein-coupled receptors describes an ability of different ligands that preferentially activate an alternative downstream signaling pathway. In this work, we identified and characterized different N-terminal truncations of endogenous chemokine CCL15 as balanced or biased agonists targeting CCR1, and presented three cryogenic-electron microscopy structures of the CCR1-Gi complex in the ligand-free form or bound to different CCL15 truncations with a resolution of 2.6-2.9 Å, illustrating the structural basis of natural biased signaling that initiates an inflammation response. Complemented with pharmacological and computational studies, these structures revealed it was the conformational change of Tyr291 (Y2917.43) in CCR1 that triggered its polar network rearrangement in the orthosteric binding pocket and allosterically regulated the activation of ß-arrestin signaling. Our structure of CCL15-bound CCR1 also exhibited a critical site for ligand binding distinct from many other chemokine-receptor complexes, providing new insights into the mode of chemokine recognition.

HDAC2 attenuates airway inflammation by suppressing IL-17A production in HDM-challenged mice.

Histone deacetylase (HDAC)2 is expressed in airway epithelium and plays a pivotal role in inflammatory cells. However, the role of HDAC2 in allergic airway inflammation remains poorly understood. In the present study, we determined the role of HDAC2 in airway inflammation using in vivo models of house dust mite (HDM)-induced allergic inflammation and in vitro cultures of human bronchial epithelial (HBE) cells exposed to HDM, IL-17A, or both. We observed that HDM-challenged Hdac2+/- mice exhibited substantially enhanced infiltration of inflammatory cells. Higher levels of T helper 2 cytokines and IL-17A expression were found in lung tissues of HDM-challenged Hdac2+/- mice. Interestingly, IL-17A deletion or anti-IL-17A treatment reversed the enhanced airway inflammation induced by HDAC2 impairment. In vitro, HDM and IL-17A synergistically decreased HDAC2 expression in HBE cells. HDAC2 gene silencing further enhanced HDM- and/or IL-17A-induced inflammatory cytokines in HBE cells. HDAC2 overexpresion or blocking IL-17A gene expression restored the enhanced inflammatory cytokines. Collectively, these results support a protective role of HDAC2 in HDM-induced airway inflammation by suppressing IL-17A production and might suggest that activation of HDAC2 and/or inhibition of IL-17A production could prevent the development of allergic airway inflammation.

ß-Arrestin1 regulates the morphology and dynamics of microglia in zebrafish in vivo.

Microglia are the primary immune cells in the central nervous system. Microglia typically exist in a 'resting' state in the healthy brain, with ramified processes dynamically exploring the surrounding microenvironment. They become 'activated' under pathological conditions with marked changes in morphology. However, the regulation of their morphology dynamics remains poorly understood. Here, using in vivo time-lapse imaging and three-dimensional morphology analysis of microglia in intact zebrafish larvae, we found that ß-arrestin1, a multifunctional protein involved in various signal transductions, cell-autonomously regulated the microglial morphology. Knockdown of ß-arrestin1 increased the volume size and process number of microglia but reduced the deformation speed in the resting state. Meanwhile, ß-arrestin1 down-regulation led to a high frequency of phagocytic behaviour of microglia. These defects were partially rescued by over-expressing human ß-arrestin1 in microglia. Our study indicated that microglial dynamics in the resting state can be regulated cell-autonomously by ß-arrestin1 signalling.

CCL11/CCR3-dependent eosinophilia alleviates malignant pleural effusions and improves prognosis.

Malignant pleural effusion (MPE) is a common occurrence in advanced cancer and is often linked with a poor prognosis. Eosinophils were reported to involve in the development of MPE. However, the role of eosinophils in MPE remains unclear. To investigate this, we conducted studies using both human samples and mouse models. Increased eosinophil counts were observed in patients with MPE, indicating that the higher the number of eosinophils is, the lower the LENT score is. In our animal models, eosinophils were found to migrate to pleural cavity actively upon exposure to tumor cells. Intriguingly, we discovered that a deficiency in eosinophils exacerbated MPE, possibly due to their anti-tumor effects generated by modifying the microenvironment of MPE. Furthermore, our experiments explored the role of the C-C motif chemokine ligand 11 (CCL11) and its receptor C-C motif chemokine receptor 3 (CCR3) in MPE pathology. As a conclusion, our study underscores the protective potential of eosinophils against the development of MPE, and that an increase in eosinophils through adoptive transfer of eosinophils or increasing their numbers improved MPE.

[Physiological properties and functions of microglia].

Microglia, the resident immune effective cells of the central nervous system, play crucial roles in mediating immune-related process. It becomes activated quickly in response to even minor pathological insults and participates in series of immune responses. Under physiological conditions, most microglia stay in a typical resting state, with ramified processes continuously extending and retracting from surrounding neural tissues, suggesting an important function of resting microglia. Recent studies indicate that resting microglia can regulate many physiological processes, including neural development, neural circuit formation, neuronal activity and plasticity, and animal grooming behavior. Here, we review the properties of resting microglia and further discuss how microglia participate in the above-mentioned functional regulation under physiological conditions.

Circadian regulation of developmental synaptogenesis via the hypocretinergic system.

The circadian clock orchestrates a wide variety of physiological and behavioral processes, enabling animals to adapt to daily environmental changes, particularly the day-night cycle. However, the circadian clock's role in the developmental processes remains unclear. Here, we employ the in vivo long-term time-lapse imaging of retinotectal synapses in the optic tectum of larval zebrafish and reveal that synaptogenesis, a fundamental developmental process for neural circuit formation, exhibits circadian rhythm. This rhythmicity arises primarily from the synapse formation rather than elimination and requires the hypocretinergic neural system. Disruption of this synaptogenic rhythm, by impairing either the circadian clock or the hypocretinergic system, affects the arrangement of the retinotectal synapses on axon arbors and the refinement of the postsynaptic tectal neuron's receptive field. Thus, our findings demonstrate that the developmental synaptogenesis is under hypocretin-dependent circadian regulation, suggesting an important role of the circadian clock in neural development.

Roles of Ca2+ activity in injury-induced migration of microglia in zebrafish in vivo.

Microglia are the resident immune cells in the brain. It is well known that brain injury can activate the microglia and induce its directional migration towards the injury sites for exerting immune functions. While extracellular ATP released from the injury site mediates the directionality of activated microglia's migration, what endows activated microglia with migration capability remains largely unexplored. In the present study, we used the larval zebrafish as an in vivo model to visualize the dynamics of both morphology and Ca2+ activity of microglia during its migration evoked by local brain injury. We found that, in response to local injury, activated microglia exhibited an immediate Ca2+ transient and later sustained Ca2+ bursts during its migration towards the local injury site. Furthermore, suppression of Ca2+ activities significantly retarded microglial cell migration. Thus, our study suggests that intracellular Ca2+ activity is required for activated microglia's migration.

Construction of Attenuated Strains for Red-Spotted Grouper Nervous Necrosis Virus (RGNNV) via Reverse Genetic System.

The nervous necrosis virus (NNV) mainly attacks the central nervous system of fish to cause viral nervous necrosis, which is an acute and serious prevalent disease in fish. Among different genotypes of NNV, red-spotted grouper nervous necrosis virus (RGNNV) is the most widely reported, with the highest number of susceptible species. To better understand the pathogenicity of RGNNV, we first developed a reverse genetic system for recombinant RGNNV rescue using B7GG and striped snakehead (SSN-1) cells. Furthermore, we constructed attenuated RGNNV strains rRGNNV-B2-M1 and rRGNNV-B2-M2 with the loss of B2 protein expression, which grew slower and induced less Mx1 expression than that of wild-type RGNNV. Moreover, rRGNNV-B2-M1 and rRGNNV-B2-M2 were less virulent than the wild-type RGNNV. Our study provides a potential tool for further research on the viral protein function, virulence pathogenesis, and vaccine development of RGNNV, which is also a template for the rescue of other fish viruses.

Molecular insights into ligand recognition and activation of chemokine receptors CCR2 and CCR3.

Chemokine receptors are a family of G-protein-coupled receptors with key roles in leukocyte migration and inflammatory responses. Here, we present cryo-electron microscopy structures of two human CC chemokine receptor-G-protein complexes: CCR2 bound to its endogenous ligand CCL2, and CCR3 in the apo state. The structure of the CCL2-CCR2-G-protein complex reveals that CCL2 inserts deeply into the extracellular half of the transmembrane domain, and forms substantial interactions with the receptor through the most N-terminal glutamine. Extensive hydrophobic and polar interactions are present between both two chemokine receptors and the Gα-protein, contributing to the constitutive activity of these receptors. Notably, complemented with functional experiments, the interactions around intracellular loop 2 of the receptors are found to be conserved and play a more critical role in G-protein activation than those around intracellular loop 3. Together, our findings provide structural insights into chemokine recognition and receptor activation, shedding lights on drug design targeting chemokine receptors.

Eosinophil-derived chemokine (hCCL15/23, mCCL6) interacts with CCR1 to promote eosinophilic airway inflammation.

Eosinophils are terminally differentiated cells derived from hematopoietic stem cells (HSCs) in the bone marrow. Several studies have confirmed the effective roles of eosinophils in asthmatic airway pathogenesis. However, their regulatory functions have not been well elucidated. Here, increased C-C chemokine ligand 6 (CCL6) in asthmatic mice and the human orthologs CCL15 and CCL23 that are highly expressed in asthma patients are described, which are mainly derived from eosinophils. Using Ccl6 knockout mice, further studies revealed CCL6-dependent allergic airway inflammation and committed eosinophilia in the bone marrow following ovalbumin (OVA) challenge and identified a CCL6-CCR1 regulatory axis in hematopoietic stem cells (HSCs). Eosinophil differentiation and airway inflammation were remarkably decreased by the specific CCR1 antagonist BX471. Thus, the study identifies that the CCL6-CCR1 axis is involved in the crosstalk between eosinophils and HSCs during the development of allergic airway inflammation, which also reveals a potential therapeutic strategy for targeting G protein-coupled receptors (GPCRs) for future clinical treatment of asthma.

Eosinophilic inflammation promotes CCL6-dependent metastatic tumor growth.

Compelling evidence suggests that inflammatory components contribute to cancer development. However, eosinophils, involved in several inflammatory diseases, were not fully explored in cancer metastasis. We show that airway inflammatory eosinophilia and colonic inflammation with eosinophil infiltration are both associated with increased metastasis in mice. Eosinophilia is responsible for increased bone metastasis in eosinophil-enriched Cd3δ-Il-5 transgenic (Il-5 Tg) mice. We also observe increased eosinophils in the malignant pleural effusion of cancer patients with pleural metastasis. Mechanistically, eosinophils promote tumor cell migration and metastasis formation through secreting C-C motif chemokine ligand 6 (CCL6). Genetic knockout of Ccl6 in Il-5 Tg mice remarkably attenuates bone metastasis. Moreover, inhibition of C-C chemokine receptor 1 (CCR1, the receptor of CCL6) in tumor cells reduces tumor cell migration and metastasis. Thus, our study identifies a CCL6-dependent prometastatic activity of eosinophils, which can be inhibited by targeting CCR1 and represent an approach to preventing metastatic disease.

Therapeutic Effects of the Bcl-2 Inhibitor on Bleomycin-induced Pulmonary Fibrosis in Mice.

Idiopathic pulmonary fibrosis (IPF) is a distressing lung disorder with poor prognosis and high mortality rates. Limited therapeutic options for IPF is a major clinical challenge. Well-known for its anti-apoptotic properties, B-cell lymphoma 2 (Bcl-2) plays a critical role in the pathology of malignancies and inflammatory diseases, including IPF. In this study, we aimed to investigate the therapeutic effect of a Bcl-2 homology domain 3 mimetic inhibitor, ABT-199, on bleomycin (BLM)-induced pulmonary fibrosis in mice, and explore possible underlying mechanism. The lung inflammation and fibrosis model was established by intratracheal instillation of a single dose of BLM. We observed elevated Bcl-2 in the alveolar macrophages and fibroblasts derived from BLM-instilled mice from day 7. Further, we obtained in vivo evidence that early therapeutic treatment with Bcl-2 inhibitor ABT-199 from day 3, and late treatment from day 10, both alleviated airway inflammation and lung fibrosis induced by BLM. Our data suggest that ABT-199 might be an effective antifibrotic agent that interferes with profibrogenic cells, which may be a promising therapy in the treatment of clinical IPF patients.

Piezo1-Mediated Ca2+ Activities Regulate Brain Vascular Pathfinding during Development.

During development, endothelial tip cells (ETCs) located at the leading edge of growing vascular plexus guide angiogenic sprouts to target vessels, and thus, ETC pathfinding is fundamental for vascular pattern formation in organs, including the brain. However, mechanisms of ETC pathfinding remain largely unknown. Here, we report that Piezo1-mediated Ca2+ activities at primary branches of ETCs regulate branch dynamics to accomplish ETC pathfinding during zebrafish brain vascular development. ETC branches display spontaneous local Ca2+ transients, and high- and low-frequency Ca2+ transients cause branch retraction through calpain and branch extension through nitric oxide synthase, respectively. These Ca2+ transients are mainly mediated by Ca2+-permeable Piezo1 channels, which can be activated by mechanical force, and mutating piezo1 largely impairs ETC pathfinding and brain vascular patterning. These findings reveal that Piezo1 and downstream Ca2+ signaling act as molecular bases for ETC pathfinding and highlight a novel function of Piezo1 and Ca2+ in vascular development.

Prevalence and risk factors of mental distress in China during the outbreak of COVID-19: A national cross-sectional survey.

OBJECTIVE: As a result of the pandemic of COVID-19, the public have been experiencing psychological distress. However, the prevalence of psychological distress during the COVID-19 pandemic remains unknown. Our objective was to evaluate the prevalence of psychological distress during COVID-19 outbreak and their risk factors, especially their internal paths and causality. METHODS: A nationwide cross-sectional survey of the prevalence of mental disorders was conducted. We used Hospital Anxiety and Depression Scale (HADS) to estimate the prevalence of anxiety and depression. The internal paths and the causality of the psychological health were analyzed using a structural equation modeling (SEM) approach. RESULTS: A total of 24,789 respondents completed the survey. We found that the overall prevalence of anxiety, depression, combination of anxiety, and depression were 51.6% (95% CI: 51.0-52.2), 47.5% (95% CI: 46.9-48.1), and 24.5% (95% CI: 24.0-25.0), respectively. The risk of psychological disorders in men was higher than that in women. The status of psychological health was different across different age groups, education levels, occupations, and income levels. The SEM analysis revealed that inadequate material supplies, low income, low education, lack of knowledge or confidence of the epidemic, and lack of exercise are major risk factors for psychological distress. CONCLUSIONS: The evidence from this survey poses serious challenges related to the high prevalence of psychological distress, but also offers strategies to deal with the mental health problems caused by the COVID-19 pandemic.

Apoptosis of Endothelial Cells Contributes to Brain Vessel Pruning of Zebrafish During Development.

During development, immature blood vessel networks remodel to form a simplified and efficient vasculature to meet the demand for oxygen and nutrients, and this remodeling process is mainly achieved via the pruning of existing vessels. It has already known that the migration of vascular endothelial cells (ECs) is one of the mechanisms underlying vessel pruning. However, the role of EC apoptosis in vessel pruning remains under debate, especially in the brain. Here, we reported that EC apoptosis makes a significant contribution to vessel pruning in the brain of larval zebrafish. Using in vivo long-term time-lapse confocal imaging of the brain vasculature in zebrafish larvae, we found that EC apoptosis was always accompanied with brain vessel pruning and about 15% of vessel pruning events were resulted from EC apoptosis. In comparison with brain vessels undergoing EC migration-associated pruning, EC apoptosis-accompanied pruned vessels were longer and showed higher probability that the nuclei of neighboring vessels' ECs occupied their both ends. Furthermore, we found that microglia were responsible for the clearance of apoptotic ECs accompanying vessel pruning, though microglia themselves were dispensable for the occurrence of vessel pruning. Thus, our study demonstrates that EC apoptosis contributes to vessel pruning in the brain during development in a microglial cell-independent manner.

A transgenic zebrafish model for in vivo long-term imaging of retinotectal synaptogenesis.

The retinotectal synapse in larval zebrafish, combined with live time-lapse imaging, provides an advantageous model for study of the development and remodelling of central synapses in vivo. In previous studies, these synapses were labelled by transient expression of fluorescence-tagged synaptic proteins, which resulted in the dramatic variation of labelling patterns in each larva. Here, using GAL4-Upstream Activating Sequence (GAL4-UAS) methodology, we generated stable transgenic lines, which express EGFP-tagged synaptophysin (a presynaptic protein) in retinal ganglion cells (RGCs), to reliably label the pre-synaptic site of retinotectal synapses. This tool avoids the variable labelling of RGCs that occurs in transient transgenic larvae. We obtained several stable transgenic lines that differ consistently in the number of labelled RGCs. Using stable lines that consistently had a single labelled RGC, we could trace synaptogenic dynamics on an individual RGC axonal arbor across different developmental stages. In the stable lines that consistently had multiple labelled RGCs, we could simultaneously monitor both pre- and post-synaptic compartments by combining transient labelling of post-synaptic sites on individual tectal neurons. These tools allowed us to investigate molecular events underlying synaptogenesis and found that the microRNA-132 (miR-132) is required for developmental synaptogenesis. Thus, these transgenic zebrafish stable lines provide appropriate tools for studying central synaptogenesis and underlying molecular mechanisms in intact vertebrate brain.

Eosinophil-derived CCL-6 impairs hematopoietic stem cell homeostasis.

Eosinophils (Eos) have been long considered as end-stage effector cells in the hierarchical hematopoietic system. Numerous lines of evidence have suggested that Eos are multifunctional leukocytes with respect to the initiation, propagation and regulation of various inflammatory or immune reactions, especially in allergic diseases. Recent studies have shown that Eos are also required for maintenance of bone marrow plasma cells and differentiation of B cells. However, it remains unclear whether Eos contributes to regulation of hematopoietic stem cell (HSC) homeostasis. Here, we demonstrate that Eos disrupt HSC homeostasis by impairing HSC quiescence and reconstitution ability in wild-type mice following ovalbumin (OVA) challenge and even by causing bone marrow HSC failure and exhaustion in Cd3δ-Il-5 transgenic mice. The impaired maintenance and function of HSCs were associated with Eos-induced redox imbalance (increased oxidative phosphorylation and decreased anti-oxidants levels). More importantly, using mass spectrometry, we determined that CCL-6 is expressed at a high level under eosinophilia. We demonstrate that CCL-6 is Eos-derived and responsible for the impaired HSC homeostasis. Interestingly, blockage of CCL-6 with a specific neutralizing antibody, restored the reconstitution ability of HSCs while exacerbating eosinophilia airway inflammation in OVA-challenged mice. Thus, our study reveals an unexpected function of Eos/CCL-6 in HSC homeostasis.

Neurons secrete miR-132-containing exosomes to regulate brain vascular integrity.

Vascular integrity helps maintain brain microenvironment homeostasis, which is critical for the normal development and function of the central nervous system. It is known that neural cells can regulate brain vascular integrity. However, due to the high complexity of neurovascular interactions involved, understanding of the neural regulation of brain vascular integrity is still rudimentary. Using intact zebrafish larvae and cultured rodent brain cells, we find that neurons transfer miR-132, a highly conserved and neuron-enriched microRNA, via secreting exosomes to endothelial cells (ECs) to maintain brain vascular integrity. Following translocation to ECs through exosome internalization, miR-132 regulates the expression of vascular endothelial cadherin (VE-cadherin), an important adherens junction protein, by directly targeting eukaryotic elongation factor 2 kinase (eef2k). Disruption of neuronal miR-132 expression or exosome secretion, or overexpression of vascular eef2k impairs VE-cadherin expression and brain vascular integrity. Our study indicates that miR-132 acts as an intercellular signal mediating neural regulation of the brain vascular integrity and suggests that the neuronal exosome is a novel avenue for neurovascular communication.

A Death Trap for Microglia.

Microglia, immune cells of the brain, originate from erythromyeloid precursors, far from the central nervous system. Xu et al. (2016) in this issue of Developmental Cell and Casano et al. (2016) recently in Cell Reports show that apoptotic neurons act as bait to "trap" microglia into colonizing the developing brain.

Resting microglia respond to and regulate neuronal activity in vivo.

Microglia are the primary immune cells in the brain. Under pathological conditions, they become activated and participate in scavenging, inflammation and tissue repair in response to brain injury. While the function and underlying mechanism of activated microglia have been intensively studied in the past decades, physiological functions of resting microglia remain largely underestimated. In our recent work, by simultaneously monitoring both the motility of resting microglial processes and the activity of surrounding neurons in intact zebrafish optic tectum, we examined the interaction between resting microglia and neurons. Local increase in neuronal activity attracts resting microglial processes and drives them to contact neurons with high levels of activity. This process is mediated by neuronal release of "find-me" signals such as ATP via pannexin-1 hemichannels and requires small Rho GTPase Rac in microglia. Reciprocally, the microglia-neuron contact reduces both the spontaneous and visually evoked activities of contacted neurons. We here summarize and explain the key results in the context of our previous work.