SpiCee: A Genetic Tool for Subcellular and Cell-Specific Calcium Manipulation

The ion channel Anoctamin 10/TMEM16K coordinates organ morphogenesis across scales in the urochordate notochord

During embryonic development, tissues and organs are gradually shaped into their functional morphologies through a series of spatiotemporally tightly orchestrated cell behaviors. A highly conserved organ shape across metazoans is the epithelial tube. Tube morphogenesis is a complex multistep process of carefully choreographed cell behaviors such as convergent extension, cell elongation, and lumen formation. The identity of the signaling molecules that coordinate these intricate morphogenetic steps remains elusive. The notochord is an essential tubular organ present in the embryonic midline region of all members of the chordate phylum. Here, using genome editing, pharmacology and quantitative imaging in the early chordate Ciona intestinalis we show that Ano10/Tmem16k, a member of the evolutionarily ancient family of transmembrane proteins called Anoctamin/TMEM16 is essential for convergent extension, lumen expansion, and connection during notochord morphogenesis. We find that Ano10/Tmem16k works in concert with the plasma membrane (PM) localized Na+/Ca2+ exchanger (NCX) and the endoplasmic reticulum (ER) residing SERCA, RyR, and IP3R proteins to establish developmental stage specific Ca2+ signaling molecular modules that regulate notochord morphogenesis and Ca2+ dynamics. In addition, we find that the highly conserved Ca2+ sensors calmodulin (CaM) and Ca2+/calmodulin-dependent protein kinase (CaMK) show an Ano10/Tmem16k-dependent subcellular localization. Their pharmacological inhibition leads to convergent extension, tubulogenesis defects, and deranged Ca2+ dynamics, suggesting that Ano10/Tmem16k is involved in both the "encoding" and "decoding" of developmental Ca2+ signals. Furthermore, Ano10/Tmem16k mediates cytoskeletal reorganization during notochord morphogenesis, likely by altering the localization of 2 important cytoskeletal regulators, the small GTPase Ras homolog family member A (RhoA) and the actin binding protein Cofilin. Finally, we use electrophysiological recordings and a scramblase assay in tissue culture to demonstrate that Ano10/Tmem16k likely acts as an ion channel but not as a phospholipid scramblase. Our results establish Ano10/Tmem16k as a novel player in the prevertebrate molecular toolkit that controls organ morphogenesis across scales.

High-affinity tuning of single fluorescent protein-type indicators by flexible linker length optimization in topology mutant

Genetically encoded Ca2+ indicators (GECIs) are versatile for live imaging of cellular activities. Besides the brightness and dynamic range of signal change of GECIs, Ca2+ affinity is another critical parameter for successful Ca2+ imaging, as the concentration range of Ca2+ dynamics differs from low nanomolar to sub-millimolar depending on the celltype and organism. However, ultrahigh-affinity GECIs, particularly the single fluorescent protein (1FP)-type, are lacking. Here, we report a simple strategy that increases Ca2+ affinity through the linker length optimization in topology mutants of existing 1FP-type GECIs. The resulting ultrahigh-affinity GECIs, CaMPARI-nano, BGECO-nano, and RCaMP-nano (Kd = 17-25 nM), enable unique biological applications, including the detection of low nanomolar Ca2+ dynamics, highlighting active signaling cells, and multi-functional imaging with other second messengers. The linker length optimization in topology mutants could be applied to other 1FP-type indicators of glutamate and potassium, rendering it a widely applicable technique for modulating indicator affinity.

Starvation-resistant cavefish reveal conserved mechanisms of starvation-induced hepatic lipotoxicity

Starvation causes the accumulation of lipid droplets in the liver, a somewhat counterintuitive phenomenon that is nevertheless conserved from flies to humans. Much like fatty liver resulting from overfeeding, hepatic lipid accumulation (steatosis) during undernourishment can lead to lipotoxicity and atrophy of the liver. Here, we found that although surface populations of Astyanax mexicanus undergo this evolutionarily conserved response to starvation, the starvation-resistant cavefish larvae of the same species do not display an accumulation of lipid droplets upon starvation. Moreover, cavefish are resistant to liver atrophy during starvation, providing a unique system to explore strategies for liver protection. Using comparative transcriptomics between zebrafish, surface fish, and cavefish, we identified the fatty acid transporter slc27a2a/fatp2 to be correlated with the development of fatty liver. Pharmacological inhibition of slc27a2a in zebrafish rescues steatosis and atrophy of the liver upon starvation. Furthermore, down-regulation of FATP2 in Drosophila larvae inhibits the development of starvation-induced steatosis, suggesting the evolutionarily conserved importance of the gene in regulating fatty liver upon nutrition deprivation. Overall, our study identifies a conserved, druggable target to protect the liver from atrophy during starvation.

Starvation resistant cavefish reveal conserved mechanisms of starvation-induced hepatic lipotoxicity

Starvation causes the accumulation of lipid droplets in the liver, a somewhat counterintuitive phenomenon that is nevertheless conserved from flies to humans. Much like fatty liver resulting from overfeeding, hepatic lipid accumulation (steatosis) during undernourishment can lead to lipotoxicity and atrophy of the liver. Here, we found that while surface populations of Astyanax mexicanus undergo this evolutionarily conserved response to starvation, the starvation-resistant cavefish larvae of the same species do not display an accumulation of lipid droplets upon starvation. Moreover, cavefish are resistant to liver atrophy during starvation, providing a unique system to explore strategies for liver protection. Using comparative transcriptomics between zebrafish, surface fish, and cavefish, we identified the fatty acid transporter slc27a2a/fatp2 to be correlated with the development of fatty liver. Pharmacological inhibition of slc27a2a in zebrafish rescues steatosis and atrophy of the liver upon starvation. Further, down-regulation of FATP2 in drosophila larvae inhibits the development of starvation-induced steatosis, suggesting the evolutionary conserved importance of the gene in regulating fatty liver upon nutrition deprivation. Overall, our study identifies a conserved, druggable target to protect the liver from atrophy during starvation.

Oligodendrocyte calcium signaling promotes actin-dependent myelin sheath extension

Myelin is essential for rapid nerve signaling and is increasingly found to play important roles in learning and in diverse diseases of the CNS. Morphological parameters of myelin such as sheath length are thought to precisely tune conduction velocity, but the mechanisms controlling sheath morphology are poorly understood. Local calcium signaling has been observed in nascent myelin sheaths and can be modulated by neuronal activity. However, the role of calcium signaling in sheath formation remains incompletely understood. Here, we use genetic tools to attenuate oligodendrocyte calcium signaling during myelination in the developing mouse CNS. Surprisingly, genetic calcium attenuation does not grossly affect the number of myelinated axons or myelin thickness. Instead, calcium attenuation causes myelination defects resulting in shorter, dysmorphic sheaths. Mechanistically, calcium attenuation reduces actin filaments in oligodendrocytes, and an intact actin cytoskeleton is necessary and sufficient to achieve accurate myelin morphology. Together, our work reveals a cellular mechanism required for accurate CNS myelin formation and may provide mechanistic insight into how oligodendrocytes respond to neuronal activity to sculpt and refine myelin sheaths.

CXCL12 targets the primary cilium cAMP/cGMP ratio to regulate cell polarity during migration

Directed cell migration requires sustained cell polarisation. In migrating cortical interneurons, nuclear movements are directed towards the centrosome that organises the primary cilium signalling hub. Primary cilium-elicited signalling, and how it affects migration, remain however ill characterised. Here, we show that altering cAMP/cGMP levels in the primary cilium by buffering cAMP, cGMP or by locally increasing cAMP, influences the polarity and directionality of migrating interneurons, whereas buffering cAMP or cGMP in the apposed centrosome compartment alters their motility. Remarkably, we identify CXCL12 as a trigger that targets the ciliary cAMP/cGMP ratio to promote sustained polarity and directed migration. We thereby uncover cAMP/cGMP levels in the primary cilium as a major target of extrinsic cues and as the steering wheel of neuronal migration.

Subcellular second messenger networks drive distinct repellent-induced axon behaviors

Second messengers, including cAMP, cGMP and Ca2+ are often placed in an integrating position to combine the extracellular cues that orient growing axons in the developing brain. This view suggests that axon repellents share the same set of cellular messenger signals and that axon attractants evoke opposite cAMP, cGMP and Ca2+ changes. Investigating the confinement of these second messengers in cellular nanodomains, we instead demonstrate that two repellent cues, ephrin-A5 and Slit1, induce spatially segregated signals. These guidance molecules activate subcellular-specific second messenger crosstalk, each signaling network controlling distinct axonal morphology changes in vitro and pathfinding decisions in vivo.

Oligodendrocyte calcium signaling sculpts myelin sheath morphology

Myelin is essential for rapid nerve signaling and is increasingly found to play important roles in learning and in diverse diseases of the CNS. Morphological parameters of myelin such as sheath length and thickness are regulated by neuronal activity and can precisely tune conduction velocity, but the mechanisms controlling sheath morphology are poorly understood. Local calcium signaling has been observed in nascent myelin sheaths and can be modulated by neuronal activity. However, the role of calcium signaling in sheath formation and remodeling is unknown. Here, we used genetic tools to attenuate oligodendrocyte calcium signaling during active myelination in the developing mouse CNS. Surprisingly, we found that genetic calcium attenuation did not grossly affect the number of myelinated axons or myelin thickness. Instead, calcium attenuation caused striking myelination defects resulting in shorter, dysmorphic sheaths. Mechanistically, calcium attenuation reduced actin filaments in oligodendrocytes, and an intact actin cytoskeleton was necessary and sufficient to achieve accurate myelin morphology. Together, our work reveals a novel cellular mechanism required for accurate CNS myelin formation and provides mechanistic insight into how oligodendrocytes may respond to neuronal activity to sculpt myelin sheaths throughout the nervous system.

In vivo imaging of calcium dynamics in zebrafish hepatocytes

BACKGROUND AND AIMS

Hepatocytes were the first cell type for which oscillations of cytoplasmic calcium levels in response to hormones were described. Since then, investigation of calcium dynamics in liver explants and culture has greatly increased our understanding of calcium signaling. A bottleneck, however, exists in observing calcium dynamics in a noninvasive manner because of the optical inaccessibility of the mammalian liver. Here, we aimed to take advantage of the transparency of the zebrafish larvae to image hepatocyte calcium dynamics in vivo at cellular resolution.

APPROACH AND RESULTS

We developed a transgenic model expressing a calcium sensor, GCaMP6s, specifically in zebrafish hepatocytes. Using this, we provide a quantitative assessment of intracellular calcium dynamics during multiple contexts, including growth, feeding, ethanol-induced stress, and cell ablation. Specifically, we show that synchronized calcium oscillations are present in vivo , which are lost upon starvation. Starvation induces lipid accumulation in the liver. Feeding recommences calcium waves in the liver, but in a spatially restricted manner, as well as resolves starvation-induced hepatic steatosis. By using a genetically encoded scavenger for calcium, we show that dampening of calcium signaling accelerates the accumulation of starvation-related lipid droplets in the liver. Furthermore, ethanol treatment, as well as cell ablation, induces calcium flux, but with different dynamics. The former causes asynchronous calcium oscillations, whereas the latter leads to a single calcium spike.

CONCLUSIONS

We demonstrate the presence of oscillations, waves, and spikes in vivo . Calcium waves are present in response to nutrition and negatively regulate starvation-induced accumulation of lipid droplets.

The role of astrocytes in behaviors related to emotion and motivation

Astrocytes are present throughout the brain and intimately interact with neurons and blood vessels. Three decades of research have shown that astrocytes reciprocally communicate with neurons and other non-neuronal cells in the brain and dynamically regulate cell function. Astrocytes express numerous receptors for neurotransmitters, neuromodulators, and cytokines and receive information from neurons, other astrocytes, and other non-neuronal cells. Among those receptors, the main focus has been G-protein coupled receptors. Activation of G-protein coupled receptors leads to dramatic changes in intracellular signaling (Ca²⁺ and cAMP), which is considered a form of astrocyte activity. Methodological improvements in measurement and manipulation of astrocytes have advanced our understanding of the role of astrocytes in circuits and have begun to reveal unexpected functions of astrocytes in behavior. Recent studies have suggested that astrocytic activity regulates behavior flexibility, such as coping strategies for stress exposure, and plays an important role in behaviors related to emotion and motivation. Preclinical evidence suggests that impairment of astrocytic function contributes to psychiatric diseases, especially major depression. Here, we review recent progress on the role of astrocytes in behaviors related to emotion and motivation.

Looking for Guidance - Models and Methods to Study Axonal Navigation

The molecular mechanisms of neural circuit formation have been of interest to Santiago Ramón y Cajal and thousands of neuroscientists sharing his passion for neural circuits ever since. Cajal was a brilliant observer and taught us about the connections and the morphology of neurons in the adult and developing nervous system. Clearly, we will not learn about molecular mechanisms by just looking at brain sections or cells in culture. Technically, we had to come a long way to today's possibilities that allow us to perturb target gene expression and watch the consequences of our manipulations on navigating axons in situ. In this review, we summarize landmark steps towards modern live-imaging approaches used to study the molecular basis of axon guidance.

A perspective on astrocyte regulation of neural circuit function and animal behavior

Studies over the past two decades have demonstrated that astrocytes are tightly associated with neurons and play pivotal roles in neural circuit development, operation, and adaptation in health and disease. Nevertheless, precisely how astrocytes integrate diverse neuronal signals, modulate neural circuit structure and function at multiple temporal and spatial scales, and influence animal behavior or disease through aberrant excitation and molecular output remains unclear. This Perspective discusses how new and state-of-the-art approaches, including fluorescence indicators, opto- and chemogenetic actuators, genetic targeting tools, quantitative behavioral assays, and computational methods, might help resolve these longstanding questions. It also addresses complicating factors in interpreting astrocytes' role in neural circuit regulation and animal behavior, such as their heterogeneity, metabolism, and inter-glial communication. Research on these questions should provide a deeper mechanistic understanding of astrocyte-neuron assemblies' role in neural circuit function, complex behaviors, and disease.

From Synapses to Circuits, Astrocytes Regulate Behavior

Astrocytes are non-neuronal cells that regulate synapses, neuronal circuits, and behavior. Astrocytes ensheath neuronal synapses to form the tripartite synapse where astrocytes influence synapse formation, function, and plasticity. Beyond the synapse, recent research has revealed that astrocyte influences on the nervous system extend to the modulation of neuronal circuitry and behavior. Here we review recent findings on the active role of astrocytes in behavioral modulation with a focus on in vivo studies, primarily in mice. Using tools to acutely manipulate astrocytes, such as optogenetics or chemogenetics, studies reviewed here have demonstrated a causal role for astrocytes in sleep, memory, sensorimotor behaviors, feeding, fear, anxiety, and cognitive processes like attention and behavioral flexibility. Current tools and future directions for astrocyte-specific manipulation, including methods for probing astrocyte heterogeneity, are discussed. Understanding the contribution of astrocytes to neuronal circuit activity and organismal behavior will be critical toward understanding how nervous system function gives rise to behavior.

T2R bitter taste receptors regulate apoptosis and may be associated with survival in head and neck squamous cell carcinoma

Better management of head and neck squamous cell carcinomas (HNSCCs) requires a clearer understanding of tumor biology and disease risk. Bitter taste receptors (T2Rs) have been studied in several cancers, including thyroid, salivary, and GI, but their role in HNSCC has not been explored. We found that HNSCC patient samples and cell lines expressed functional T2Rs on both the cell and nuclear membranes. Bitter compounds, including bacterial metabolites, activated T2R‐mediated nuclear Ca²⁺ responses leading to mitochondrial depolarization, caspase activation, and ultimately apoptosis. Buffering nuclear Ca²⁺ elevation blocked caspase activation. Furthermore, increased expression of T2Rs in HNSCCs from The Cancer Genome Atlas is associated with improved overall survival. This work suggests that T2Rs are potential biomarkers to predict outcomes and guide treatment selection, may be leveraged as therapeutic targets to stimulate tumor apoptosis, and may mediate tumor‐microbiome crosstalk in HNSCC.

Approaches to Manipulate Ephrin-A:EphA Forward Signaling Pathway

Erythropoietin-producing hepatocellular carcinoma A (EphA) receptors and their ephrin-A ligands are key players of developmental events shaping the mature organism. Their expression is mostly restricted to stem cell niches in adults but is reactivated in pathological conditions including lesions in the heart, lung, or nervous system. They are also often misregulated in tumors. A wide range of molecular tools enabling the manipulation of the ephrin-A:EphA system are available, ranging from small molecules to peptides and genetically-encoded strategies. Their mechanism is either direct, targeting EphA receptors, or indirect through the modification of intracellular downstream pathways. Approaches enabling manipulation of ephrin-A:EphA forward signaling for the dissection of its signaling cascade, the investigation of its physiological roles or the development of therapeutic strategies are summarized here.