Hong-Sheng Li - US grants

DESCRIPTION (provided by applicant): The long-term goal of the proposed research is to understand how calcium, a universal messenger, regulates gene expression and cell activities. Calcium ions are pivotal in the regulation of a variety of cellular processes. Abnormal calcium homeostasis has been implicated in aging and in numerous human diseases, such as Alzheimer's disease. The enduring regulatory effects of calcium depend on changes in gene expression. The mechanisms by which cells decode the information carried by calcium signals and convert them into distinct alterations in gene expression is poorly understood. A prevailing model is that Ca2+ entry through different ion channels activates distinct transcription factors. Through the mediation of a series of protein kinases or phosphatases, Ca2+/calmodulin stimulates a range of transcription factors. Recent bioinformatic analyses have suggested the existence of transcription factors that are activated directly by Ca2+/calmodulin, which could respond to Ca2+ signals more quickly. A group of candidates, the calmodulin-binding transcription activators (CAMTAs) were recently identified in plants. Their homologs in animals have yet to be studied. We have isolated two Drosophila mutant alleles of dCAMTA, the fly CAMTA gene, and detected a strong and consistent phenotype in these mutants that implies an impaired deactivation of the fly light-stimulated Ca2+ channel TRP. Therefore we plan to use the fly photoreceptor cells as an assay system to study this new group of Ca2+-regulated transcription factors. A combination of molecular and cell biological, genetic, electrophysiological and calcium imaging approaches will be used to: 1. Test the hypothesis that dCAMTA is activated through Ca2+/calmodulin-binding in vivo. 2: Test the hypothesis that dCAMTA exists in the cytoplasm and translocates into the nuclei upon activation. 3: Test the hypothesis that dCAMTA proteins form homomultimers. 4. Test the hypothesis that dCAMTA depends on TRP channels for activation in the photoreceptor cells. 5. Test the hypothesis that the loss of dCAMTA function leads to increased Ca2+ level in the photoreceptor cells. 6. Determine the target genes of dCAMTA.

DESCRIPTION (provided by applicant): The long-term goal of the proposed research is to reveal the physiological functions and in vivo mechanisms of G protein-coupled receptor (GPCR) endocytosis and postendocytic trafficking. GPCRs are the largest family of membrane receptors that receive sensory stimuli and mediate responses to neurotransmitters, neuropeptides, hormones, cytokines and growth factors. Activity-dependent endocytosis of GPCR reduces receptor numbers on the cell surface and is an important feedback regulation on the receptor signaling. In addition to being sorted into lysosome for degradation, endocytosed GPCRs are more frequently recycled back to the plasma membrane. The process of endocytosis and recycling is required for many receptors to dissociate from the binding ligand so that they can receive new stimuli. Failure of this process has been implicated in drug tolerances such as that to morphine. Although a large body of works has elicited various mechanisms of receptor endocytosis in cultured cells, the studies on GPCR endocytosis in intact organisms are still limited. More importantly, it is unclear how the endocytosed receptors are recycled back to the cell surface. In addition, the specific physiological functions of endocytosis and recycling have yet to identify for each GPCR. The major light receptor Rh1 rhodopsin in Drosophila eye is a model molecule for genetic characterization of GPCR signaling and regulation. Recently we identified a null mutant of a gene that encodes a CUB- and LDLa-domain protein (CULD), and found that a large amount of endocytosed Rh1 protein was retained in the cell body of the mutant photoreceptor. Our preliminary studies suggest that this is due to a failure of Rh1 recycling. We propose to take advantage of this culd mutant and several additional new mutant flies to study the mechanisms and the regulations of Rh1 endocytosis and recycling, and to characterize their impacts on the visual sensory function. Using a combination of molecular genetic, biochemical and electrophysiological approaches, we will 1. Confirm that the CULD protein is required for the recycling of Rh1 in photoreceptor 2. Test the hypothesis that CULD interacts with Arr1 for the localization of Rh1 in the rhabdomere 3. Test the hypothesis that loss of CULD impairs the development of light sensitivity in photoreceptors 4. Test the hypothesis that LAP is involved in the Rh1 endocytosis 5. Test the hypothesis that the deglycosylation of Rh1 restricts its endocytosis 6 Screen for additional molecules involved in the recycling of Rh1. PUBLIC HEALTH RELEVANCE: G protein-coupled receptor (GPCR) proteins on the cell membrane mediate >80% of transmembrane signaling activities, and are the major targets for pharmaceutical drug designs. In this proposal we plan to use Drosophila rhodopsin Rh1, a light-stimulated GPCR, as a model to genetically characterize the mechanisms underlying the receptor endocytosis and recycling. These processes regulate the intensity of GPCR signaling, and have been implicated in clinical disorders such as retinal degenerations and opioid tolerance.

DESCRIPTION (provided by applicant): The long-term goal of our research is to understand how cells maintain intracellular Ca2+ homeostasis in diverse physiological conditions. As a versatile signaling molecule, Ca2+ regulates the proliferation, differentiation, function, aging, and apoptosis of virtually all types of cells. Abnormal calcium homeostasis may cause damage to the cell, and has been implicated in aging and in numerous human diseases, such as Alzheimer's disease and spinocerebellar ataxia. To avoid undesirable effects of Ca2+, all pathways of Ca2+ entry are tightly controlled in the cell. A group of ubiquitously expressed Ca2+ influx channels, termed receptor-operated Ca2+/cation channels (ROCs), are stimulated by G protein-coupled receptors (GPCRs) through Gq/11 proteins and phospholipase C (PLC). Since most GPCRs and ROCs have much higher protein levels in excitable cells including neurons, to maintain the intracellular Ca2+ homeostasis, these cells need to fortify the regulatory machinery of GPCR/ROC by expressing more regulatory molecules. We have previously demonstrated that dCAMTA, a transcription factor responding to the Ca2+ sensor calmodulin, is indispensable for rapid deactivation of the light-stimulated GPCR rhodopsin in the Drosophila eye. dCAMTA belongs to a new family of transcription factors named calmodulin-binding transcription activators (CAMTAs). Interestingly, both human CAMTAs, CAMTA1 and CAMTA2, are highly expressed in the brain. We predict that the calmodulin/CAMTA-stimulated gene expression may fortify the control machinery of GPCR/ROC-mediated Ca2+ entry in neurons, in a long-term feedback manner. To test this hypothesis, we propose to use dCAMTA as model and to take advantage of the fly phototransduction cascade, a typical GPCR/PLC cascade that has been successfully used for the identification of the first ROC channel TRP. In this proposal, we will 1. Test the hypothesis that the dCAMTA target gene dFbxl4 is indispensable for rapid deactivation of rhodopsin; 2. Test the hypothesis that dFbxl4 interacts with the myosin III NINAC for rhodopsin deactivation; 3. Test the hypothesis that dCAMTA promotes expression of calmodulin to facilitate the deactivation of rhodopsin; 4. Test the hypothesis that loss of dCAMTA leads to Ca2+-dependent, vacuolar photoreceptor degeneration in older flies; 5. identify the nuclear localization sequences of dCAMTA and human CAMTA1; 6. Test the hypothesis that Fbxl4 and/or cam1 are target genes of human CAMTA1. PUBLIC HEALTH RELEVANCE: Defects in calcium homeostasis have been implicated in a variety of human disorders including several neurodegeneration diseases (Alzheimer's disease and spinocerebellar ataxia) and several forms of immunodeficiency The long- term goal of this research is to fully understand how the intracellular Ca2+ homeostasis is maintained in both physiological and pathological conditions and to use this knowledge to facilitate treatment and perhaps prevention of these human diseases. This proposal will study how a new group of transcription factors regulate the Ca2+ homeostasis in a feedback manner.

Abstract The long-term goal of the proposed research is to reveal functions and mechanisms of neuron-glia interaction in visual systems. As previous studies on vision have mostly been focused on the visual transduction cascades and the neuronal circuits, our knowledge about the role of glial cells in visual signaling is still very limited. Intriguingly, glial cells express receptors for neurotransmitters of visual interneurons in both mammalian and fly eyes. It is unknown whether glial cells directly communicate with those neurons for regulation of visual signaling. In our preliminary studies on Drosophila vision, we found that the visual epithelial glia may concentrate a glutamate-gated chloride channel GluCl in 'gnarl', a special glial membrane structure that typically interposes between a glutamatergic interneuron amacrine cell and its postsynaptic partner T1 cell. When the glutamate release from amacrine cell was blocked, the speed of photoreceptor repolarization at the end of light response reduced significantly. Importantly, this visual defect was phenocopied by knocking down the GluCl expression specifically in the epithelial glia, and was also observed in an ADAM protein mutant of impaired gnarl structure. Based on these observations, we propose the existence of a photoreceptor-amacrine cell-epithelial glia-photoreceptor feedback loop, which functions to reinforce the speed of photoreceptor repolarization and is thus important for the temporal resolution of vision. Using a combination of molecular and cell biological, genetic, histological, and electrophysiological approaches, we propose to further investigate this novel function of visual epithelial glia. Specifically, we will 1. Test the hypothesis that laminar epithelial glia receive neuronal input through the glutamate-gated chloride channel GluCl; 2. Test the hypothesis that an ADAM protein MMD is required to localize GluCl in the gnarl structure for the neuron-glia signaling; 3. Identify the mechanism of epithelial glia-mediated photoreceptor regulation. These studies are not only important to visual biology, but will also contribute to our understanding of glial function in general.

ABSTRACT The long-term goal of the proposed research is to reveal how electrically non-excitable glial cells support synaptic transmission under the control of neurons using visual system as a model. As previous studies on vision mostly have been focused on the visual transduction cascades and the neuronal circuits, our knowledge about the role of glial cells in vision is still very limited. It is well established that visual glial cells are known for their functions in neuron protection, structural maintenance and control of environmental K+ and neurotransmitter levels. But neurobiological research in the last decade has found that, in addition to supportive roles, the central neuropil glia astrocytes can respond to various neurotransmitters, thus they may actively modulate neuronal synaptic transmissions. Neurotransmitter receptors are also detected in visual glia including Müller cells. However, it remains to be elucidated on how glia support signal transmission in visual system, and how this glial function is regulated conversely by neurons. We are proposing to use the Drosophila visual system as a novel, genetic model to study this function of perisynaptic glia, which allows the observation of neuronal activities in live animals after signaling molecules are genetically manipulated in either glia or neuron. In the first visual neuropil region (lamina) of fly, photoreceptor axons release histamine upon light stimulation to hyperpolarize projective large monopolar cells (LMC). All neuronal processes are wrapped laterally by three epithelial glia cells (EG) in each laminar cartridge. We have previous found that EG concentrate a glutamate-gated chloride channel GluCl in special membrane processes abutting terminals of T1 interneuron. In our preliminary study, loss of GluCl diminished the Ca2+ response of LMC to light change, and impaired fly locomotion vision in dim conditions. Both dark-vision and electroretinogram defects of the GluCl mutant were phenocopied by downregulation of a glutamate transporter EAAT1 in T1, suggesting the involvement of T1 neuron and EAAT1 in the stimulation of GluCl. In addition, a cation channel NA in T1 appeared to function upstream of GluCl as well. Based on these observations, we propose to test a voltage-dependent, non-vesicular mechanism of neuron-glia communication, in which T1 neuron releases glutamate through EAAT1 to open a GluCl-gated Cl- pool in EG, and thereby facilitate the inhibition of LMC by photoreceptors. This model may represent a general mechanism for interneuron to modulate synaptic weight through glia in both fly and mammals. Using a combination of molecular and cell biological, genetic, histological, electrophysiological and in vivo imaging approaches, we will 1. Test the hypothesis that a GluCl-gated glial Cl- pool is essential for the inhibitory visual; 2. Test the hypothesis that T1 neuron releases glutamate through EAAT1 to open glial GluCl channel; 3. Test the hypothesis that depolarization of T1 is required for EAAT1 to release glutamate.