Single cell tracing of Pomc neurons reveals recruitment of 'Ghost' subtypes with atypical identity in a mouse model of obesity.

The hypothalamus contains a remarkable diversity of neurons that orchestrate behavioural and metabolic outputs in a highly plastic manner. Neuronal diversity is key to enabling hypothalamic functions and, according to the neuroscience dogma, it is predetermined during embryonic life. Here, by combining lineage tracing of hypothalamic pro-opiomelanocortin (Pomc) neurons with single-cell profiling approaches in adult male mice, we uncovered subpopulations of 'Ghost' neurons endowed with atypical molecular and functional identity. Compared to 'classical' Pomc neurons, Ghost neurons exhibit negligible Pomc expression and are 'invisible' to available neuroanatomical approaches and promoter-based reporter mice for studying Pomc biology. Ghost neuron numbers augment in diet-induced obese mice, independent of neurogenesis or cell death, but weight loss can reverse this shift. Our work challenges the notion of fixed, developmentally programmed neuronal identities in the mature hypothalamus and highlight the ability of specialised neurons to reversibly adapt their functional identity to adult-onset obesogenic stimuli.

Phenotyping xylem connections in grafted plants using X-ray micro-computed tomography.

Plants are able to naturally graft or inosculate their trunks, branches and roots together, this mechanism is used by humans to graft together different genotypes for a range of purposes. Grafts are considered successful if functional vascular connections between the two genotypes occur. Various techniques can evaluate xylem connections across the graft interface. However, these methods are generally unable to assess the heterogeneity and three-dimensional (3D) structure of xylem vessel connections. Here we present the use of X-ray micro-computed tomography to characterize the 3D morphology of grafts of grapevine. We show that xylem vessels form between the two plants of natural root and human-made stem grafts. The main novelty of this methodology is that we were able to visualize the 3D network of functional xylem vessels connecting the scion and rootstock in human-made stem grafts thanks to the addition of a contrast agent to the roots and improved image analysis pipelines. In addition, we reveal the presence of extensive diagonal xylem connections between the main axial xylem vessels in 2-year old grapevine stems. In conclusion, we present a method that has the potential to provide new insights into the structure and function of xylem vessels in large tissue samples.

Autophagy protein 5 controls flow-dependent endothelial functions.

Dysregulated autophagy is associated with cardiovascular and metabolic diseases, where impaired flow-mediated endothelial cell responses promote cardiovascular risk. The mechanism by which the autophagy machinery regulates endothelial functions is complex. We applied multi-omics approaches and in vitro and in vivo functional assays to decipher the diverse roles of autophagy in endothelial cells. We demonstrate that autophagy regulates VEGF-dependent VEGFR signaling and VEGFR-mediated and flow-mediated eNOS activation. Endothelial ATG5 deficiency in vivo results in selective loss of flow-induced vasodilation in mesenteric arteries and kidneys and increased cerebral and renal vascular resistance in vivo. We found a crucial pathophysiological role for autophagy in endothelial cells in flow-mediated outward arterial remodeling, prevention of neointima formation following wire injury, and recovery after myocardial infarction. Together, these findings unravel a fundamental role of autophagy in endothelial function, linking cell proteostasis to mechanosensing.

Scavenger Receptor Cysteine-Rich domains of Lysyl Oxidase-Like2 regulate endothelial ECM and angiogenesis through non-catalytic scaffolding mechanisms.

Lysyl oxidases are major actors of microenvironment and extracellular matrix (ECM) remodeling. These cross-linking enzymes are thus involved in many aspects of physiopathology, including tumor progression, fibrosis and cardiovascular diseases. We have already shown that Lysyl Oxidase-Like 2 (LOXL2) regulates collagen IV deposition by endothelial cells and angiogenesis. We here provide evidence that LOXL2 also affects deposition of other ECM components, including fibronectin, thus altering structural and mechanical properties of the matrix generated by endothelial cells. LOXL2 interacts intracellularly and directly with collagen IV and fibronectin before incorporation into ECM fibrillar structures upon exocytosis, as demonstrated by TIRF time-lapse microscopy. Furthermore, surface plasmon resonance experiments using recombinant scavenger receptor cysteine-rich (SRCR) domains truncated for the catalytic domain demonstrated their direct binding to collagen IV. We thus used directed mutagenesis to investigate the role of LOXL2 catalytic domain. Neither enzyme activity nor catalytic domain were necessary for collagen IV deposition and angiogenesis, whereas the SRCR domains were effective for these processes. Finally, surface coating with recombinant SRCR domains restored deposition of collagen IV by LOXL2-depleted cells. We thus propose that LOXL2 SRCR domains orchestrate scaffolding of the vascular basement membrane and angiogenesis through interactions with collagen IV and fibronectin, independently of the enzymatic cross-linking activity.

ATP6AP2 variant impairs CNS development and neuronal survival to cause fulminant neurodegeneration.

Vacuolar H+-ATPase-dependent (V-ATPase-dependent) functions are critical for neural proteostasis and are involved in neurodegeneration and brain tumorigenesis. We identified a patient with fulminant neurodegeneration of the developing brain carrying a de novo splice site variant in ATP6AP2 encoding an accessory protein of the V-ATPase. Functional studies of induced pluripotent stem cell-derived (iPSC-derived) neurons from this patient revealed reduced spontaneous activity and severe deficiency in lysosomal acidification and protein degradation leading to neuronal cell death. These deficiencies could be rescued by expression of full-length ATP6AP2. Conditional deletion of Atp6ap2 in developing mouse brain impaired V-ATPase-dependent functions, causing impaired neural stem cell self-renewal, premature neuronal differentiation, and apoptosis resulting in degeneration of nearly the entire cortex. In vitro studies revealed that ATP6AP2 deficiency decreases V-ATPase membrane assembly and increases endosomal-lysosomal fusion. We conclude that ATP6AP2 is a key mediator of V-ATPase-dependent signaling and protein degradation in the developing human central nervous system.

Nerves Control Redox Levels in Mature Tissues Through Schwann Cells and Hedgehog Signaling.

AIMS: Recent advances in redox biology have emphasized the role of hydrogen peroxide (H2O2) in the modulation of signaling pathways and revealed that H2O2 plays a role in cellular remodeling in adults. Thus, an understanding of the mechanisms that control H2O2 levels in mature tissue would be of great interest. RESULTS: We used a denervation strategy to demonstrate that sensory neurons are responsible for controlling H2O2 levels under normal conditions and after being lesioned. Moreover, we demonstrate that severed nerves respond to appendage amputation via the induction of Hedgehog signaling and that this signaling is responsible for H2O2 production in the wounded epidermis. Finally, we show that H2O2 and nerve growth are regulated via reciprocal action in adults. INNOVATION AND CONCLUSION: These data support a new paradigm for the regulation of tissue homeostasis: H2O2 attracts nerves and nerves control H2O2 levels in a positive feedback loop. This finding suggests that the peripheral nerve redox environment could be a target for manipulating cell plasticity in adults.

Astroglial calcium signaling displays short-term plasticity and adjusts synaptic efficacy.

Astrocytes are dynamic signaling brain elements able to sense neuronal inputs and to respond by complex calcium signals, which are thought to represent their excitability. Such signaling has been proposed to modulate, or not, neuronal activities ranging from basal synaptic transmission to epileptiform discharges. However, whether calcium signaling in astrocytes exhibits activity-dependent changes and acutely modulates short-term synaptic plasticity is currently unclear. We here show, using dual recordings of astroglial calcium signals and synaptic transmission, that calcium signaling in astrocytes displays, concomitantly to excitatory synapses, short-term plasticity in response to prolonged repetitive and tetanic stimulations of Schaffer collaterals. We also found that acute inhibition of calcium signaling in astrocytes by intracellular calcium chelation rapidly potentiates excitatory synaptic transmission and short-term plasticity of Shaffer collateral CA1 synapses, i.e., paired-pulse facilitation and responses to tetanic and prolonged repetitive stimulation. These data reveal that calcium signaling of astrocytes is plastic and down-regulates basal transmission and short-term plasticity of hippocampal CA1 glutamatergic synapses.

Sustained production of ROS triggers compensatory proliferation and is required for regeneration to proceed.

A major issue in regenerative medicine is the role of injury in promoting cell plasticity. Here we explore the function of reactive oxygen species (ROS) induced through lesions in adult zebrafish. We show that ROS production, following adult fin amputation, is tightly regulated in time and space for at least 24 hours, whereas ROS production remains transient (2 hours) in mere wound healing. In regenerative tissue, ROS signaling triggers two distinct parallel pathways: one pathway is responsible for apoptosis, and the other pathway is responsible for JNK activation. Both events are involved in the compensatory proliferation of stump epidermal cells and are necessary for the progression of regeneration. Both events impact the Wnt, SDF1 and IGF pathways, while apoptosis only impacts progenitor marker expression. These results implicate oxidative stress in regeneration and provide new insights into the differences between healing and regeneration.

Olfactory deficits cause anxiety-like behaviors in mice.

Anxiety disorders are characterized by persistent fear in the absence of immediate threat and represent the most common psychiatric diseases, with an estimated 28% lifetime prevalence worldwide (Kessler et al., 2010). While symptoms of anxiety are typically evoked by sensory stimuli, it is unknown whether sensory deficits contribute to the development of anxiety disorders. Here we examine the effect of defined genetic mutations that compromise the function of the olfactory system on the development of anxiety-like behaviors in mice. We show that the functional inactivation of the main olfactory epithelium, but not the vomeronasal organ, causes elevated levels of anxiety. Anxiety-like behaviors are also observed in mice with a monoclonal nose, that are able to detect and discriminate odors but in which the patterns of odor-evoked neural activity are perturbed. In these mice, plasma corticosterone levels are elevated, suggesting that olfactory deficits can lead to chronic stress. These results demonstrate a central role for olfactory sensory cues in modulating anxiety in mice.

Protection against myocardial infarction and no-reflow through preservation of vascular integrity by angiopoietin-like 4.

BACKGROUND: Increased permeability, predominantly controlled by endothelial junction stability, is an early event in the deterioration of vascular integrity in ischemic disorders. Hemorrhage, edema, and inflammation are the main features of reperfusion injuries, as observed in acute myocardial infarction (AMI). Thus, preservation of vascular integrity is fundamental in ischemic heart disease. Angiopoietins are pivotal modulators of cell-cell junctions and vascular integrity. We hypothesized that hypoxic induction of angiopoietin-like protein 4 (ANGPTL4) might modulate vascular damage, infarct size, and no-reflow during AMI. METHODS AND RESULTS: We showed that vascular permeability, hemorrhage, edema, inflammation, and infarct severity were increased in angptl4-deficient mice. We determined that decrease in vascular endothelial growth factor receptor 2 (VEGFR2) and VE-cadherin expression and increase in Src kinase phosphorylation downstream of VEGFR2 were accentuated after ischemia-reperfusion in the coronary microcirculation of angptl4-deficient mice. Both events led to altered VEGFR2/VE-cadherin complexes and to disrupted adherens junctions in the endothelial cells of angptl4-deficient mice that correlated with increased no-reflow. In vivo injection of recombinant human ANGPTL4 protected VEGF-driven dissociation of the VEGFR2/VE-cadherin complex, reduced myocardial infarct size, and the extent of no-reflow in mice and rabbits. CONCLUSIONS: These data showed that ANGPTL4 might constitute a relevant target for therapeutic vasculoprotection aimed at counteracting the effects of VEGF, thus being crucial for preventing no-reflow and conferring secondary cardioprotection during AMI.

Lysyl oxidase-like protein-2 regulates sprouting angiogenesis and type IV collagen assembly in the endothelial basement membrane.

Sprouting angiogenesis is associated with extensive extracellular matrix (ECM) remodeling. The molecular mechanisms involved in building the vascular microenvironment and its impact on capillary formation remain elusive. We therefore performed a proteomic analysis of ECM from endothelial cells maintained in hypoxia, a major stimulator of angiogenesis. Here, we report the characterization of lysyl oxidase-like protein-2 (LOXL2) as a hypoxia-target expressed in neovessels and accumulated in the endothelial ECM. LOXL2 belongs to the lysyl oxidase family of secreted enzymes involved in ECM crosslinking. Knockdown experiments in Tg(fli1:egfp)y1 zebrafish embryos resulted in lack of intersegmental vessel circulation and demonstrated LOXL2 involvement in proper capillary formation. Further investigation in vitro by loss and gain of function experiments confirmed that LOXL2 was required for tubulogenesis in 3D fibrin gels and demonstrated that this enzyme was required for collagen IV assembly in the ECM. In addition, LOXL2 depletion down-regulated cell migration and proliferation. These data suggest a major role for LOXL2 in the organization of endothelial basal lamina and in the downstream mechanotransductive signaling. Altogether, our study provides the first evidence for the role of LOXL2 in regulating angiogenesis through collagen IV scaffolding.

Alteration of developmental and pathological retinal angiogenesis in angptl4-deficient mice.

Proper vessel maturation, remodeling of endothelial junctions, and recruitment of perivascular cells is crucial for establishing and maintaining vessel functions. In proliferative retinopathies, hypoxia-induced angiogenesis is associated with disruption of the vascular barrier, edema, and vision loss. Therefore, identifying factors that regulate vascular maturation is critical to target pathological angiogenesis. Given the conflicting role of angiopoietin-like-4 (ANGPTL4) reported in the current literature using gain of function systems both in vitro and in vivo, the goal of this study was to characterize angiogenesis, focusing on perinatal retinal vascularization and pathological circumstances in angpl4-deficient mice. We report altered organization of endothelial junctions and pericyte coverage, both leading to impaired angiogenesis and increased vascular leakage that were eventually caught up, suggesting a delay in vessel maturation. In a model of oxygen-induced retinopathy, pathological neovascularization, which results from tissue hypoxia, was also strongly inhibited in angptl4-deficient mice. This study therefore shows that ANGPTL4 tunes endothelial cell junction organization and pericyte coverage and controls vascular permeability and angiogenesis, both during development and in pathological conditions.