Mammalian homolog of Drosophila retinal degeneration B rescues the mutant fly phenotype

Emerging perspectives on multidomain phosphatidylinositol transfer proteins

The phosphatidylinositol transfer protein domain (PITPd) is an evolutionarily conserved protein that is able to transfer phosphatidylinositol between membranes in vitro and in vivo. However some animal genomes also include genes that encode proteins where the PITPd is found in cis with a number of additional domains and recent large scale genome sequencing efforts indicate that this type of multidomain architecture is widespread in the animal kingdom. In Drosophila photoreceptors, the multidomain phosphatidylinositol transfer protein RDGB is required to regulate phosphoinositide turnover during G-protein activated phospholipase C signalling. Recent studies in flies and mammalian cell culture models have begun to elucidate functions for the non-PITPd of RDGB and its vertebrate orthologs. We review emerging evidence on the genomics, functional and cell biological perspectives of these multi-domain PITPd containing proteins.

Lipid transfer proteins and instructive regulation of lipid kinase activities: Implications for inositol lipid signaling and disease

Cellular membranes are critical platforms for intracellular signaling that involve complex interfaces between lipids and proteins, and a web of interactions between a multitude of lipid metabolic pathways. Membrane lipids impart structural and functional information in this regulatory circuit that encompass biophysical parameters such as membrane thickness and fluidity, as well as chaperoning the interactions of protein binding partners. Phosphatidylinositol and its phosphorylated derivatives, the phosphoinositides, play key roles in intracellular membrane signaling, and these involvements are translated into an impressively diverse set of biological outcomes. The phosphatidylinositol transfer proteins (PITPs) are key regulators of phosphoinositide signaling. Found in a diverse array of organisms from plants, yeast and apicomplexan parasites to mammals, PITPs were initially proposed to be simple transporters of lipids between intracellular membranes. It now appears increasingly unlikely that the soluble versions of these proteins perform such functions within the cell. Rather, these serve to facilitate the activity of intrinsically biologically insufficient inositol lipid kinases and, in so doing, promote diversification of the biological outcomes of phosphoinositide signaling. The central engine for execution of such functions is the lipid exchange cycle that is a fundamental property of PITPs. How PITPs execute lipid exchange remains very poorly understood. Molecular dynamics simulation approaches are now providing the first atomistic insights into how PITPs, and potentially other lipid-exchange/transfer proteins, operate.

Neuronal ER-plasma membrane junctions organized by Kv2-VAP pairing recruit Nir proteins and affect phosphoinositide homeostasis

The association of plasma membrane (PM)-localized voltage-gated potassium (Kv2) channels with endoplasmic reticulum (ER)-localized vesicle-associated membrane protein-associated proteins VAPA and VAPB defines ER-PM junctions in mammalian brain neurons. Here, we used proteomics to identify proteins associated with Kv2/VAP-containing ER-PM junctions. We found that the VAP-interacting membrane-associated phosphatidylinositol (PtdIns) transfer proteins PYK2 N-terminal domain-interacting receptor 2 (Nir2) and Nir3 specifically associate with Kv2.1 complexes. When coexpressed with Kv2.1 and VAPA in HEK293T cells, Nir2 colocalized with cell-surface-conducting and -nonconducting Kv2.1 isoforms. This was enhanced by muscarinic-mediated PtdIns(4,5)P₂ hydrolysis, leading to dynamic recruitment of Nir2 to Kv2.1 clusters. In cultured rat hippocampal neurons, exogenously expressed Nir2 did not strongly colocalize with Kv2.1, unless exogenous VAPA was also expressed, supporting the notion that VAPA mediates the spatial association of Kv2.1 and Nir2. Immunolabeling signals of endogenous Kv2.1, Nir2, and VAP puncta were spatially correlated in cultured neurons. Fluorescence-recovery-after-photobleaching experiments revealed that Kv2.1, VAPA, and Nir2 have comparable turnover rates at ER-PM junctions, suggesting that they form complexes at these sites. Exogenous Kv2.1 expression in HEK293T cells resulted in significant differences in the kinetics of PtdIns(4,5)P₂ recovery following repetitive muscarinic stimulation, with no apparent impact on resting PtdIns(4,5)P₂ or PtdIns(4)P levels. Finally, the brains of Kv2.1-knockout mice had altered composition of PtdIns lipids, suggesting a crucial role for native Kv2.1-containing ER-PM junctions in regulating PtdIns lipid metabolism in brain neurons. These results suggest that ER-PM junctions formed by Kv2 channel-VAP pairing regulate PtdIns lipid homeostasis via VAP-associated PtdIns transfer proteins.

The interface between phosphatidylinositol transfer protein function and phosphoinositide signaling in higher eukaryotes

Phosphoinositides are key regulators of a large number of diverse cellular processes that include membrane trafficking, plasma membrane receptor signaling, cell proliferation, and transcription. How a small number of chemically distinct phosphoinositide signals are functionally amplified to exert specific control over such a diverse set of biological outcomes remains incompletely understood. To this end, a novel mechanism is now taking shape, and it involves phosphatidylinositol (PtdIns) transfer proteins (PITPs). The concept that PITPs exert instructive regulation of PtdIns 4-OH kinase activities and thereby channel phosphoinositide production to specific biological outcomes, identifies PITPs as central factors in the diversification of phosphoinositide signaling. There are two evolutionarily distinct families of PITPs: the Sec14-like and the StAR-related lipid transfer domain (START)-like families. Of these two families, the START-like PITPs are the least understood. Herein, we review recent insights into the biochemical, cellular, and physiological function of both PITP families with greater emphasis on the START-like PITPs, and we discuss the underlying mechanisms through which these proteins regulate phosphoinositide signaling and how these actions translate to human health and disease.

Topological organisation of the phosphatidylinositol 4,5-bisphosphate-phospholipase C resynthesis cycle: PITPs bridge the ER-PM gap

Phospholipase C (PLC) is a receptor-regulated enzyme that hydrolyses phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) at the plasma membrane (PM) triggering three biochemical consequences, the generation of soluble inositol 1,4,5-trisphosphate (IP₃), membrane-associated diacylglycerol (DG) and the consumption of PM PI(4,5)P₂ Each of these three signals triggers multiple molecular processes impacting key cellular properties. The activation of PLC also triggers a sequence of biochemical reactions, collectively referred to as the PI(4,5)P₂ cycle that culminates in the resynthesis of this lipid. The biochemical intermediates of this cycle and the enzymes that mediate these reactions are topologically distributed across two membrane compartments, the PM and the endoplasmic reticulum (ER). At the PM, the DG formed during PLC activation is rapidly converted into phosphatidic acid (PA) that needs to be transported to the ER where the machinery for its conversion into PI is localised. Conversely, PI from the ER needs to be rapidly transferred to the PM where it can be phosphorylated by lipid kinases to regenerate PI(4,5)P₂ Thus, two lipid transport steps between membrane compartments through the cytosol are required for the replenishment of PI(4,5)P₂ at the PM. Here, we review the topological constraints in the PI(4,5)P₂ cycle and current understanding how these constraints are overcome during PLC signalling. In particular, we discuss the role of lipid transfer proteins in this process. Recent findings on the biochemical properties of a membrane-associated lipid transfer protein of the PITP family, PITPNM proteins (alternative name RdgBα/Nir proteins) that localise to membrane contact sites are discussed. Studies in both Drosophila and mammalian cells converge to provide a resolution to the conundrum of reciprocal transfer of PA and PI during PLC signalling.

Unravelling the genetics of inherited retinal dystrophies: Past, present and future

The identification of the genes underlying monogenic diseases has been of interest to clinicians and scientists for many years. Using inherited retinal dystrophies as an example of monogenic disease we describe the history of molecular genetic techniques that have been pivotal in the discovery of disease causing genes. The methods that were developed in the 1970's and 80's are still in use today but have been refined and improved. These techniques enabled the concept of the Human Genome Project to be envisaged and ultimately realised. When the successful conclusion of the project was announced in 2003 many new tools and, as importantly, many collaborations had been developed that facilitated a rapid identification of disease genes. In the post-human genome project era advances in computing power and the clever use of the properties of DNA replication has allowed the development of next-generation sequencing technologies. These methods have revolutionised the identification of disease genes because for the first time there is no need to define the position of the gene in the genome. The use of next generation sequencing in a diagnostic setting has allowed many more patients with an inherited retinal dystrophy to obtain a molecular diagnosis for their disease. The identification of novel genes that have a role in the development or maintenance of retinal function is opening up avenues of research which will lead to the development of new pharmacological and gene therapy approaches. Neither of which can be used unless the defective gene and protein is known. The continued development of sequencing technologies also holds great promise for the advent of truly personalised medicine.

The Drosophila photoreceptor as a model system for studying signalling at membrane contact sites

Several recent studies have demonstrated the existence of membrane contact sites (MCS) between intracellular organelles in eukaryotic cells. Recent exciting studies have also demonstrated the existence of biomolecular interactions at these contact sites in mediating changes in the membrane composition of the cellular compartments. However, the role of such contact sites in regulating organelle function and physiological processes remains less clear. In this review we discuss the existence of a contact site between the plasma membrane (PM) and the endoplasmic reticulum (ER) inDrosophilaphotoreceptors. Further, we discuss the role of specific proteins present at this location in regulating phospholipid turnover and its impact in regulating a physiological process, namely phototransduction.

Homeostatic regulation of the PI(4,5)P2-Ca(2+) signaling system at ER-PM junctions

The phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2)-Ca(2+) signaling system is important for cell activation in response to various extracellular stimuli. This signaling system is initiated by receptor-induced hydrolysis of PI(4,5)P2 in the plasma membrane (PM) to generate the soluble second messenger inositol 1,4,5-trisphosphate (IP3). IP3 subsequently triggers the release of Ca(2+) from the endoplasmic reticulum (ER) store to the cytosol to activate Ca(2+)-mediated responses, such as secretion and proliferation. The consumed PM PI(4,5)P2 and ER Ca(2+) must be quickly restored to sustain signaling responses, and to maintain the homeostasis of PI(4,5)P2 and Ca(2+). Since phosphatidylinositol (PI), the precursor lipid for PM PI(4,5)P2, is synthesized in the ER membrane, and a Ca(2+) influx across the PM is required to refill the ER Ca(2+) store, efficient communications between the ER and the PM are critical for the homeostatic regulation of the PI(4,5)P2-Ca(2+) signaling system. This review describes the major findings that established the framework of the PI(4,5)P2-Ca(2+) signaling system, and recent discoveries on feedback control mechanisms at ER-PM junctions that sustain the PI(4,5)P2-Ca(2+) signaling system. Particular emphasis is placed on the characterization of ER-PM junctions where efficient communications between the ER and the PM occur, and the activation mechanisms of proteins that dynamically localize to ER-PM junctions to provide the feedback control during PI(4,5)P2-Ca(2+) signaling, including the ER Ca(2+) sensor STIM1, the extended synaptotagmin E-Syt1, and the PI transfer protein Nir2. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

RDGBα, a PtdIns-PtdOH transfer protein, regulates G-protein-coupled PtdIns(4,5)P2 signalling during Drosophila phototransduction

Many membrane receptors activate phospholipase C (PLC) during signalling, triggering changes in the levels of several plasma membrane lipids including phosphatidylinositol (PtdIns), phosphatidic acid (PtdOH) and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]. It is widely believed that exchange of lipids between the plasma membrane and endoplasmic reticulum (ER) is required to restore lipid homeostasis during PLC signalling, yet the mechanism remains unresolved. RDGBα (hereafter RDGB) is a multi-domain protein with a PtdIns transfer protein (PITP) domain (RDGB-PITPd). We find that, in vitro, the RDGB-PITPd binds and transfers both PtdOH and PtdIns. In Drosophila photoreceptors, which experience high rates of PLC activity, RDGB function is essential for phototransduction. We show that binding of PtdIns to RDGB-PITPd is essential for normal phototransduction; however, this property is insufficient to explain the in vivo function because another Drosophila PITP (encoded by vib) that also binds PtdIns cannot rescue the phenotypes of RDGB deletion. In RDGB mutants, PtdIns(4,5)P2 resynthesis at the plasma membrane following PLC activation is delayed and PtdOH levels elevate. Thus RDGB couples the turnover of both PtdIns and PtdOH, key lipid intermediates during G-protein-coupled PtdIns(4,5)P2 turnover.

Potential role for phosphatidylinositol transfer protein (PITP) family in lipid transfer during phospholipase C signalling

The hallmark of mammalian phosphatidylinositol transfer proteins (PITPs) is to transfer phosphatidylinositol between membrane compartments. In the mammalian genome, there are three genes that code for soluble PITP proteins, PITPα, PITPβ and RdgBβ and two genes that code for membrane-associated multi-domain proteins (RdgBαI and II) containing a PITP domain. PITPα and PITPβ constitute Class I PITPs whilst the RdgB proteins constitute Class II proteins based on sequence analysis. The PITP domain of both Class I and II can sequester one molecule of phosphatidylinositol (PI) in its hydrophobic cavity. Therefore, in principle, PITPs are therefore ideally poised to couple phosphatidylinositol delivery to the PI kinases for substrate provision for phospholipases C during cell activation. Since phosphatidylinositol (4,5)bisphosphate plays critical roles in cells, particularly at the plasma membrane, where it is a substrate for both phospholipase C and phosphoinositide-3-kinases as well as required as an intact lipid to regulate ion channels and the actin cytoskeleton, homeostatic mechanisms to maintain phosphatidylinositol(4,5)bisphosphate levels are vital. To maintain phosphatidylinositol levels, phospholipase C activation inevitably leads to the resynthesis of PI at the endoplasmic reticulum where the enzymes are located. Phosphatidic acid generated at the plasma membrane during phospholipase C activation needs to move to the ER for conversion to PI and here we provide evidence that Class II PITPs are also able to bind and transport phosphatidic acid. Thus RdgB proteins could couple PA and PI transport bidirectionally during phospholipase C signalling.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Probing mechanisms that underlie human neurodegenerative diseases in Drosophila

The fruit fly, Drosophila melanogaster, is an excellent organism for the study of the genetic and molecular basis of metazoan development. Drosophila provides numerous tools and reagents to unravel the molecular and cellular functions of genes that cause human disease, and the past decade has witnessed a significant expansion of the study of neurodegenerative disease mechanisms in flies. Here we review the interplay between oxidative stress and neuronal toxicity. We cover some of the studies that show how proteasome degradation of protein aggregates, autophagy, mitophagy, and lysosomal function affect the quality control mechanisms required for neuronal survival. We discuss how forward genetic screens in flies have led to the isolation of a few loci that cause neurodegeneration, paving the way for large-scale systematic screens to identify such loci in flies as well as promoting gene discovery in humans.

The Diverse Biological Functions of Phosphatidylinositol Transfer Proteins in Eukaryotes

Phosphatidylinositol/phosphatidylcholine transfer proteins (PITPs) remain largely functionally uncharacterized, despite the fact that they are highly conserved and are found in all eukaryotic cells thus far examined by biochemical or sequence analysis approaches. The available data indicate a role for PITPs in regulating specific interfaces between lipid-signaling and cellular function. In this regard, a role for PITPs in controlling specific membrane trafficking events is emerging as a common functional theme. However, the mechanisms by which PITPs regulate lipid-signaling and membrane-trafficking functions remain unresolved. Specific PITP dysfunctions are now linked to neurodegenerative and intestinal malabsorption diseases in mammals, to stress response and developmental regulation in higher plants, and to previously uncharacterized pathways for regulating membrane trafficking in yeast and higher eukaryotes, making it clear that PITPs are integral parts of a highly conserved signal transduction strategy in eukaryotes. Herein, we review recent progress in deciphering the biological functions of PITPs, and discuss some of the open questions that remain.

Coordinated lipid transfer between the endoplasmic reticulum and the Golgi complex requires the VAP proteins and is essential for Golgi-mediated transport

Lipid transport between intracellular organelles is mediated by vesicular and nonvesicular transport mechanisms and is critical for maintaining the identities of different cellular membranes. Nonvesicular lipid transport between the endoplasmic reticulum (ER) and the Golgi complex has been proposed to affect the lipid composition of the Golgi membranes. Here, we show that the integral ER-membrane proteins VAP-A and VAP-B affect the structural and functional integrity of the Golgi complex. Depletion of VAPs by RNA interference reduces the levels of phosphatidylinositol-4-phosphate (PI4P), diacylglycerol, and sphingomyelin in the Golgi membranes, and it leads to substantial inhibition of Golgi-mediated transport events. These effects are coordinately mediated by the lipid-transfer/binding proteins Nir2, oxysterol-binding protein (OSBP), and ceramide-transfer protein (CERT), which interact with VAPs via their FFAT motif. The effect of VAPs on PI4P levels is mediated by the phosphatidylinositol/phosphatidylcholine transfer protein Nir2, which is required for Golgi targeting of OSBP and CERT and the subsequent production of diacylglycerol and sphingomyelin. We propose that Nir2, OSBP, and CERT function coordinately at the ER-Golgi membrane contact sites, thereby affecting the lipid composition of the Golgi membranes and consequently their structural and functional identities.

RdgB proteins: Functions in lipid homeostasis and signal transduction

The RdgBs are a group of evolutionarily conserved molecules that contain a phosphatidylinositol transfer protein (PITP) domain. However in contrast to classical PITPs (PITPalpha) with whom they share the conserved PITP domain, these proteins also contain several additional sequence elements whose functional significance remains unknown. The founding member of the family DrdgB alpha (Drosophila rdgB) appears to be essential for sensory transduction and maintenance of ultra structure in photoreceptors (retinal sensory neurons). Although proposed to support the maintenance of phosphatidylinositol 4, 5 bisphosphate [PI (4, 5) P(2)] levels during G-protein coupled phospholipase C activity in these cells, the biochemical mechanism of DrdgB alpha function remains unresolved. More recently, a mammalian RdgB protein has been implicated in the maintenance of diacylglycerol (DAG) levels and secretory function at Golgi membranes. In this review we discuss existing work on the function of RdgB proteins and set out future challenges in understanding this group of lipid transfer proteins.

Phosphatidylinositol transfer proteins and cellular nanoreactors for lipid signaling

Membrane lipids function as structural molecules, reservoirs for second messengers, membrane platforms that scaffold protein assembly and regulators of enzymes and ion channels. Such diverse lipid functions contribute substantially to cellular mechanisms for fine-tuning membrane-signaling events. Meaningful coordination of these events requires exquisite spatial and temporal control of lipid metabolism and organization, and reliable mechanisms for specifically coupling these parameters to dedicated physiological processes. Recent studies suggest such integration is linked to the action of phosphatidylinositol transfer proteins that operate at the interface of the metabolism, trafficking and organization of specific lipids.

Isolation and characterization of murine Cds (CDP-diacylglycerol synthase) 1 and 2

Phototransduction in Drosophila is a phosphoinositide-mediated signalling pathway. Phosphatidylinositol 4,5-bisphosphate (PIP2) plays a central role in this process, and its levels are tightly regulated. A photoreceptor-specific form of the enzyme CDP-diacylglycerol synthase (CDS), which catalyzes the formation of CDP-diacylglycerol from phosphatidic acid, is a key regulator of the amount of PIP2 available for signalling. cds mutants develop light-induced retinal degeneration. We report here the isolation and characterization of two murine genes encoding this enzyme, Cds1 and Cds2. The genes encode proteins that are 73% identical and 92% similar but exhibit very different expression patterns. Cds1 shows a very restricted expression pattern but is expressed in the inner segments of the photoreceptors whilst Cds2 shows a ubiquitous pattern of expression. Using fluorescent in situ hybridization we have mapped Cds1 and Cds2 to chromosomes 5E3 and 2G1 respectively. These are regions of synteny with the corresponding human gene localization (4q21 and 20p13). Transient transfection experiments with epitope tagged proteins have also demonstrated that both are associated with the endoplasmic reticulum.

Chromosomal localization, genomic organization and evolution of the genes encoding human phosphatidylinositol transfer protein membrane-associated (PITPNM) 1, 2 and 3

Eukaryotic proteins containing a phosphatidylinositol transfer (PITP) domain can be divided into two groups, one consisting of small soluble 35-kDa proteins and the other those that are membrane-associated and show sequence similarities to the Drosophila retinal degeneration B (rdgB) protein. The rdgB protein consists of four domains, an amino terminal PITP domain, a Ca2+-binding domain, a transmembrane domain and a carboxyl terminal domain that interacts with the protein tyrosine kinase PYK2. Three mammalian phosphatidylinositol transfer protein membrane-associated genes (PITPNM1, 2 and 3) with homology to Drosophila rdgB have previously been described and shown to be expressed in the mammalian retina. These findings and the demonstration that the rdgB gene plays a critical role in the invertebrate phototransduction pathway have led to the mammalian genes being considered as candidate genes for human eye diseases. In order to facilitate the analysis of these genes we have used radiation hybrid mapping and fluorescence in situ hybridization to localize the PITPNM2 and 3 genes to human chromosomes 12p24 and 17p13 respectively and hybrid mapping to confirm the localization of PITPNM1 to chromosome 11q13. We have also determined the genomic organization of both the soluble and membrane-associated Drosophila and human PITP domain-containing genes. Phylogenetic analysis indicates that the two groups arose by gene duplication that occurred very early in animal evolution.

The role of the Nir/rdgB protein family in membrane trafficking and cytoskeleton remodeling

The Nir/rdgB family of proteins has been identified in a variety of eukaryotic organisms, ranging from worms to mammals. The Drosophila retinal degeneration B (rdgB), a protein that is required for photoreceptor cell viability and light response, was the first to be identified. It consists an amino-terminal phosphatidylinositol (PI)-transfer domain and was proposed to play an essential role in photoreceptor membrane renewal and biogenesis. The other Nir/rdgB family members are functionally and structurally related to the Drosophila homolog and are implicated in regulation of lipid trafficking, metabolism, and signaling. Recent advances have revealed that Nir/rdgB proteins are also involved in regulation of cytoskeletal elements. Thus, these family members exert a broad spectrum of cellular functions and are involved in multiple cellular processes. The physiological functions of these closely related proteins are described in this review.

The TRP calcium channel and retinal degeneration

The Drosophila light activated channel TRP is the founding member of a large and diverse family of channel proteins that is conserved throughout evolution. These channels are Ca2+ permeable and have been implicated as important component of cellular Ca2+ homeostasis in neuronal and non-neuronal cells. The power of the molecular genetics of Drosophila has yielded several mutants in which constitutive activity of TRP leads to a rapid retinal degeneration in the dark. Metabolic stress activates rapidly and reversibly the TRP channels in the dark in a constitutive manner by a still unknown mechanism. The link of TRP gating to the metabolic state of the cell is shared also by mammalian homologues of TRP and makes cells expressing TRP extremely vulnerable to metabolic stress, a mechanism that may underlie retinal degeneration and neuronal cell death.

Regulation of TRP channels via lipid second messengers

In Drosophila photoreceptors, the light-sensitive current is mediated downstream of phospholipase C by TRP (transient receptor potential) channels. Recent evidence suggests that Drosophila TRP channels are activated by diacylglycerol (DAG) or its metabolites (polyunsaturated fatty acids), possibly in combination with the reduction in phosphatidyl inositol 4,5 bisphosphate (PIP2). Consistent with this view, diacylglycerol kinase is identified as a key enzyme required for response termination. Signaling is critically dependent upon efficient PIP2 synthesis; mutants of this pathway in combination with genetically targeted PIP2 reporters provide unique insights into the kinetics and regulation of PIP2 turnover. Recent evidence indicates that a growing number of mammalian TRP homologues are also regulated by lipid messengers, including DAG, arachidonic acid, and PIP2.

Cloning and characterization of a novel variant (mM-rdgBbeta1) of mouse M-rdgBs, mammalian homologs of Drosophila retinal degeneration B gene proteins, and its mRNA localization in mouse brain in comparison with other M-rdgBs

We report the cloning, characterization and localization in the brain of a novel isoform termed mM-rdgBbeta1 (mouse type of mammalian retinal degeneration Bbeta1 protein) in comparison with the localization of three known mammalian homologs (M-rdgBbeta, M-rdgB1, M-rdgB2). mM-rdgBbeta1 cDNA contains a sequence of 119 bp as a form of insertion in the open reading frame of the known mM-rdgBbeta, and encodes a protein of 269 amino acids with a calculated molecular mass of 31.7 kDa, different from the molecular mass of 38.3 kDa of mM-rdgBbeta. It also contains a phosphatidylinositol transfer protein (PITP)-like domain similar to the known three homologs, as well as D-rdgB. The recombinant mM-rdgBbeta1 protein shows the specific binding activity to phosphatidylinositol but not to other phospholipids. This novel molecule is localized not only in the cytoplasm but also in the nucleus, different from the cytoplasmic localization of mM-rdgBbeta. In in situ hybridization analysis, the gene expression for mM-rdgBbeta1 in the brain, though weak, is rather confined to the embryonic stage, different from wider expression of mM-rdgBbeta in the gray matters of pre- and post-natal brains. Taken together, mM-rdgBbeta1 is suggested to play a role in the phosphoinositide-mediated signaling in the neural development.

Targeting of Nir2 to lipid droplets is regulated by a specific threonine residue within its PI-transfer domain

Nir2, like its Drosophila homolog retinal degeneration B (RdgB), contains an N-terminal phosphatidylinositol-transfer protein (PI-TP)-like domain. Previous studies have suggested that RdgB plays an important role in the fly phototransduction cascade and that its PI-transfer domain is critical for this function. In this domain, a specific mutation, T59E, induces a dominant retinal degeneration phenotype. Here we show that a similar mutation, T59E in the human Nir2 protein, targets Nir2 to spherical cytosolic structures identified as lipid droplets by the lipophilic dye Nile red. A truncated Nir2T59E mutant consisting of only the PI-transfer domain was also targeted to lipid droplets, whereas neither the wild-type Nir2 nor the Nir2T59A mutant was associated with lipid droplets under regular growth conditions. However, oleic-acid treatment caused translocation of wild-type Nir2, but not translocation of the T59A mutant, to lipid droplets. This treatment also induced partial targeting of endogenous Nir2, which is mainly associated with the Golgi apparatus, to lipid droplets. Targeting of Nir2 to lipid droplets was attributed to its enhanced threonine phosphorylation. These results suggest that a specific threonine within the PI-transfer domain of Nir2 provides a regulatory site for targeting to lipid droplets. In conjunction with the role of PI-TPs in lipid transport, this targeting may affect intracellular lipid trafficking and distribution and may provide the molecular basis underlying the dominant effect of the RdgB-T59E mutant on retinal degeneration.

Nir2, a human homolog of Drosophila melanogaster retinal degeneration B protein, is essential for cytokinesis

Cytokinesis, the final stage of eukaryotic cell division, ensures the production of two daughter cells. It requires fine coordination between the plasma membrane and cytoskeletal networks, and it is known to be regulated by several intracellular proteins, including the small GTPase Rho and its effectors. In this study we provide evidence that the protein Nir2 is essential for cytokinesis. Microinjection of anti-Nir2 antibodies into interphase cells blocks cytokinesis, as it results in the production of multinucleate cells. Immunolocalization studies revealed that Nir2 is mainly localized in the Golgi apparatus in interphase cells, but it is recruited to the cleavage furrow and the midbody during cytokinesis. Nir2 colocalizes with the small GTPase RhoA in the cleavage furrow and the midbody, and it associates with RhoA in mitotic cells. Its N-terminal region, which contains a phosphatidylinositol transfer domain and a novel Rho-inhibitory domain (Rid), is required for normal cytokinesis, as overexpression of an N-terminal-truncated mutant blocks cytokinesis completion. Time-lapse videomicroscopy revealed that this mutant normally initiates cytokinesis but fails to complete it, due to cleavage furrow regression, while Rid markedly affects cytokinesis due to abnormal contractility. Rid-expressing cells exhibit aberrant ingression and ectopic cleavage sites; the cells fail to segregate into daughter cells and they form a long unseparated bridge-like cytoplasmic structure. These results provide new insight into the cellular functions of Nir2 and introduce it as a novel regulator of cytokinesis.

Nir2, a novel regulator of cell morphogenesis

Cell morphogenesis requires dynamic reorganization of the actin cytoskeleton, a process that is tightly regulated by the Rho family of small GTPases. These GTPases act as molecular switches by shuttling between their inactive GDP-bound and active GTP-bound forms. Here we show that Nir2, a novel protein related to Drosophila retinal degeneration B (RdgB), markedly affects cell morphology through a novel Rho-inhibitory domain (Rid) which resides in its N-terminal region. Rid exhibits sequence homology with the Rho-binding site of formin-homology (FH) proteins and leads to an apparent loss of F-actin staining when ectopically expressed in mammalian cells. We also show that Rid inhibits Rho-mediated stress fiber formation and lysophosphatidic acid-induced RhoA activation. Biochemical studies demonstrated that Nir2, via Rid, preferentially binds to the inactive GDP-bound form of the small GTPase Rho. Microinjection of antibodies against Nir2 into neuronal cells markedly attenuates neurite extension, whereas overexpression of Nir2 in these cells attenuates Rho-mediated neurite retraction. These results implicate Nir2 as a novel regulator of the small GTPase Rho in actin cytoskeleton reorganization and cell morphogenesis.

The mammalian retinal degeneration B2 gene is not required for photoreceptor function and survival

The retinal degeneration B (rdgB) gene in Drosophila is essential for photoreceptor function and survival. The rdgB mutant fly exhibits an abnormal electroretinogram and a light-dependent photoreceptor degeneration. The function of RdgB is not fully understood, but the presence of a phosphatidylinositol transfer protein domain suggests a possible role in phosphatidylinositol metabolism and signaling. Two mammalian homologs, M-RdgB1 and M-RdgB2, are known. While M-RdgB1 is widely expressed, M-RdgB2 is found primarily in the retina and the dentate gyrus. Functional conservation between the Drosophila and mammalian RdgBs was demonstrated by the ability of both M-RdgBs to rescue the photoreceptor phenotype in rdgB mutant flies through transgenic expression. To investigate the role of M-RdgB2 in the mammalian retina, we disrupted the m-rdgB2 gene in mice by gene targeting. The homozygous knockout mice are fertile and apparently healthy. By light microscopy, immunocytochemistry and electroretinograms, mice up to 18 months of age showed normal photoreceptor function and survival. The inner retinal neurons were also examined by immunolabeling with a number of cell-specific markers and no apparent defects were found in the major cell populations. We conclude that M-rdgB2 is not essential for phototransduction and photoreceptor survival. Thus, m-rdgB2 is not a candidate gene for human retinal degenerations. Whether M-rdgB2 has a role in visual processing in the inner retina, or whether it is required for hippocampal function, remains to be determined.

Molecular characterization of the mouse Tem1/endosialin gene regulated by cell density in vitro and expressed in normal tissues in vivo

Human tumor endothelial marker 1/endosialin (TEM1/endosialin) was recently identified as a novel tumor endothelial cell surface marker potentially involved in angiogenesis, although no specific function for this novel gene has been assigned so far. It was reported to be expressed in tumor endothelium but not in normal endothelium with the exception of perhaps the corpus luteum. Here we describe the cDNA and genomic sequences for the mouse Tem1/endosialin homolog, the identification and characterization of its promoter region, and an extensive characterization of its expression pattern in murine and human tissues and murine cell lines in vitro. The single copy gene that was mapped to chromosome 19 is intronless and encodes a 92-kDa protein that has 77.5% overall homology to the human protein. The remarkable findings are 1) this gene is ubiquitously expressed in normal human and mouse somatic tissues and during development, and 2) its expression at the mRNA level is density-dependent and up-regulated in serum-starved cells. In vitro, its expression is limited to cells of embryonic, endothelial, and preadipocyte origin, suggesting that the wide distribution of its expression in vivo is due to the presence of vascular endothelial cells in all the tissues. The ubiquitous expression in vivo is in contrast to previously reported expression limited to corpus luteum and highly angiogenic tissues such as tumors and wound tissue.

Protein-tyrosine kinase Pyk2 mediates endothelin-induced p38 MAPK activation in glomerular mesangial cells

Endothelin-1 (ET-1), a member of a family of 21 amino acid peptides possessing vasoconstrictor properties, is known to stimulate mesangial cell proliferation. In this study, ET-1 (100 nm) induced a rapid activation of p21(ras) in human glomerular mesangial cells (HMC). Inhibition of Src family tyrosine kinase activation with [4-Amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine] or chelation of intracellular free calcium with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester significantly decreased ET-1dependent p21(ras) activation and suggested the involvement of the cytoplasmic proline-rich tyrosine kinase Pyk2. We have observed that Pyk2 was expressed in HMC and was tyrosine-phosphorylated within 5 min of ET-1 treatment. ET-1-induced activation of Pyk2 was further confirmed using phospho-specific anti-Pyk2 antibodies. Surprisingly, Src kinase activity was required upstream of ET-1-induced autophosphorylation of Pyk2. To determine whether Pyk2 autophosphorylation mediated ET-1-dependent p21(ras) activation, adenovirus-mediated transfer was employed to express a dominant-negative form of Pyk2 (CRNK). CRNK expression inhibited ET-1-induced endogenous Pyk2 autophosphorylation, but did not abolish ET-1-mediated increases in GTP-bound p21(ras) levels. ET-1-induced activation of the p38 MAPK (but not ERK) pathway was inhibited in HMC and in rat glomerular mesangial cells expressing the dominant-negative form of Pyk2. These findings suggest that the engagement of Pyk2 is important for ET-1-mediated p38 MAPK activation and hence the biological effect of this peptide in mesangial cells.

Structure of a multifunctional protein. Mammalian phosphatidylinositol transfer protein complexed with phosphatidylcholine

Eukaryotic phosphatidylinositol transfer protein is a ubiquitous multifunctional protein that transports phospholipids between membrane surfaces and participates in cellular phospholipid metabolism during signal transduction and vesicular trafficking. The three-dimensional structure of the alpha-isoform of rat phosphatidylinositol transfer protein complexed with one molecule of phosphatidylcholine, one of its physiological ligands, has been determined to 2.2 A resolution by x-ray diffraction techniques. A single beta-sheet and several long alpha-helices define an enclosed internal cavity in which a single molecule of the phospholipid is accommodated with its polar head group in the center of the protein and fatty acyl chains projected toward the surface. Other structural features suggest mechanisms by which cytosolic phosphatidylinositol transfer protein interacts with membranes for lipid exchange and associates with a variety of lipid and protein kinases.

Cloning and characterization of a human beta,beta-carotene-15,15'-dioxygenase that is highly expressed in the retinal pigment epithelium

Retinoids play a critical role in vision, as well as in development and cellular differentiation. beta,beta-Carotene-15,15'-dioxygenase (Bcdo), the enzyme that catalyzes the oxidative cleavage of beta,beta-carotene into two retinal molecules, plays an important role in retinoid synthesis. We report here the first cloning of a mammalian Bcdo. Human BCDO encodes a protein of 547 amino acid residues that demonstrates 68% identity with chicken Bcdo. It is expressed highly in the retinal pigment epithelium (RPE) and also in kidney, intestine, liver, brain, stomach, and testis. The gene spans approximately 20 kb, is composed of 11 exons and 10 introns, and maps to chromosome 16q21-q23. A mouse orthologue was also identified, and its predicted amino acid sequence is 83% identical with human BCDO. Biochemical analysis of baculovirus expressed human BCDO demonstrates the predicted beta,beta-carotene-15,15'-dioxygenase activity. The expression pattern of BCDO suggests that it may provide a local supplement to the retinoids available to photoreceptors, as well as a supplement to the retinoid pools utilized elsewhere in the body. In addition, the finding that many of the enzymes involved in retinoid metabolism are mutated in retinal degenerations suggests that BCDO may also be a candidate gene for retinal degenerative disease.

Expression and permeation properties of the K(+) channel Kir7.1 in the retinal pigment epithelium

Bovine Kir7.1 clones were obtained from a retinal pigment epithelium (RPE)-subtracted cDNA library. Human RPE cDNA library screening resulted in clones encoding full-length human Kir7.1. Northern blot analysis indicated that bovine Kir7.1 is highly expressed in the RPE. Human Kir7.1 channels were expressed in Xenopus oocytes and studied using the two-electrode voltage-clamp technique. The macroscopic Kir7.1 conductance exhibited mild inward rectification and an inverse dependence on extracellular K+ concentration ([K+]o). The selectivity sequence based on permeability ratios was K+ (1.0) approximately Rb+ (0.89) > Cs+ (0.013) > Na+ (0.003) approximately Li+ (0.001) and the sequence based on conductance ratios was Rb+ (9.5) >> K+ (1.0) > Na+ (0.458) > Cs+ (0.331) > Li+ (0.139). Non-stationary noise analysis of Rb+ currents in cell-attached patches yielded a unitary conductance for Kir7.1 of approximately 2 pS. In whole-cell recordings from freshly isolated bovine RPE cells, the predominant current was a mild inwardly rectifying K+ current that exhibited an inverse dependence of conductance on [K+]o. The selectivity sequence based on permeability ratios was K+ (1.0) approximately Rb+ (0.89) > Cs+ (0.021) > Na+ (0.003) approximately Li+ (0.002) and the sequence based on conductance ratios was Rb+ (8.9) >> K+ (1.0) > Na+ (0.59) > Cs+ (0.23) > Li+ (0.08). In cell-attached recordings with Rb+ in the pipette, inwardly rectifying currents were observed in nine of 12 patches of RPE apical membrane but in only one of 13 basolateral membrane patches. Non-stationary noise analysis of Rb+ currents in cell-attached apical membrane patches yielded a unitary conductance for RPE Kir of approximately 2 pS. On the basis of this molecular and electrophysiological evidence, we conclude that Kir7.1 channel subunits comprise the K+ conductance of the RPE apical membrane.

Studying human neurodegenerative diseases in flies and worms

Invertebrate models of several human neurodegenerative diseases have recently been described. These models faithfully replicate key neuropathological features of the human disorders. Because the basic cell biology of the nervous system is very similar in vertebrates and invertebrates, the sophisticated and rapid genetic analysis feasible in Drosophila and C. elegans promises significant insight into human neurodegenerative syndromes. In addition, the short lifespan, small size, and ease of culturing make worms and flies ideal for drug testing.

Phospholipid transfer proteins and physiological functions

Issues of how cells generate and maintain unique lipid compositions in distinct intracellular membrane systems remain the subject of much study. A ubiquitous class of soluble proteins capable of transporting phospholipid monomers from membrane to membrane across an aqueous milieu has been thought to define part of the mechanism by which lipids are sorted in cells. Progress in the study of these phospholipid transfer proteins (PLTPs) raises questions regarding their physiological functions in cells and the mechanisms by which these proteins execute them. It is now clear that across the eukaryotic kingdom, members of this protein family exert essential roles in the regulation of phospholipid metabolism and central aspects of phospholipid-mediated signaling. Indeed, it is now known that dysfunction of specific PLTPs defines the basis of inherited diseases in mammals, and this list is expected to grow. Phospholipid transfer proteins, their biochemical properties, and the emerging clues regarding their physiological functions are reviewed.

Visual transduction in Drosophila

The Drosophila phototransduction cascade has emerged as an attractive paradigm for understanding the molecular mechanisms underlying visual transduction, as well as other G protein-coupled signaling cascades that are activated and terminated with great rapidity. A large collection of mutants affecting the fly visual cascade have been isolated, and the nature and function of many of the affected gene products have been identified. Virtually all of the proteins, including those that were initially classified as novel, are highly related to vertebrate homologs. Recently, it has become apparent that most of the proteins central to Drosophila phototransduction are coupled into a supramolecular signaling complex, signalplex, through association with a PDZ-containing scaffold protein. The characterization of this complex has led to a re-evaluation of the mechanisms underlying the activation and deactivation of the phototransduction cascade.

Cloning and characterization of a novel human phosphatidylinositol transfer protein, rdgBbeta

The various PITP, retinal degeneration B (rdgB), and amino-terminal domain interacting receptor (Nir) phosphatidylinositol transfer proteins can be divided into two structural families. The small, soluble PITP isoforms contain only a phosphatidylinositol transfer domain and have been implicated in phosphoinositide signaling and vesicle trafficking. In contrast, the rdgB proteins, which include Nir2 and Nir3, contain an amino-terminal PITP-like domain, an acidic, Ca(2+)-binding domain, six putative transmembrane domains, and a conserved carboxyl-terminal domain. However, the biological function of rdgB proteins is unclear. Here, we report the isolation of a cDNA encoding a novel rdgB protein, mammalian rdgBbeta (MrdgBbeta). The 38-kDa MrdgBbeta protein contains an amino-terminal PITP-like domain and a short carboxyl-terminal domain. In contrast to other rdgB-like proteins, MrdgBbeta contains no transmembrane motifs or the conserved carboxyl-terminal domain. Using Northern and reverse transcription-polymerase chain reaction analysis, we demonstrate that MrdgBbeta mRNA is ubiquitously expressed. Immunofluorescence analysis of ectopic MrdgBbeta showed cytoplasmic staining, and the ability of recombinant MrdgBbeta to transfer phosphatidylinositol in vitro was similar to other PITP-like domains. Although early reports found functional degeneracy in vitro, the identification of a fifth mammalian PITP-like protein with a unique domain organization and widespread expression supports more recent results that suggest that different PITP-like domains have distinct functions in vivo.

A neuronal-specific mammalian homolog of the Drosophila retinal degeneration B gene with expression restricted to the retina and dentate gyrus

Mutations in the Drosophila retinal degeneration B (rdgB) gene cause a rapid loss of the electrophysiological light response and subsequent light-enhanced photoreceptor degeneration. The rdgB gene encodes a protein with an N-terminal phosphatidylinositol transfer protein domain, a large C-terminal segment, and several hydrophobic regions thought to multiply span the subrhabdomeric cisternal membrane. A mammalian rdgB homolog (m-rdgB1) was previously identified and shown to exhibit widespread tissue distribution and functionally rescue the Drosophila rdgB mutant phenotypes. We describe a second mammalian rdgB homolog (m-rdgB2) that possesses 46% amino acid identity to Drosophila RdgB and 56% identity to M-RdgB1. M-RdgB2 possesses a neuronal-specific expression pattern, with high levels in the retina and the dentate gyrus mossy fibers and dendritic field. Using M-RdgB2-specific antibodies and subcellular fractionation, we demonstrate that M-RdgB2 is not an integral membrane protein but is stably associated with a particulate fraction through protein-protein interactions. Although transgenic expression of M-RdgB2 in rdgB2 null mutant flies suppressed the retinal degeneration, it failed to fully restore the electrophysiological light response. Because transgenic expression of M-RdgB2 does not restore the wild-type phenotype to rdgB2 mutant flies to the same extent as M-RdgB1, functional differences likely exist between the two M-RdgB homologs.

Involvement of PITPnm, a mammalian homologue of Drosophila rdgB, in phosphoinositide synthesis on Golgi membranes

Phosphatidylinositol transfer protein (PITP) is involved in phospholipase C-mediated signaling and membrane trafficking. We previously reported cloning and characterization of a gene encoding for membrane-bound PITP, named PITPnm, that is a mammalian homologue of the Drosophila retinal degeneration B (rdgB) gene (Aikawa, Y., Hara, H., and Watanabe, T. (1997) Biochem. Biophys. Res. Commun. 236, 559-564). Here we report the subcellular localization of PITPnm protein and provide evidence for its involvement in phosphatidylinositol 4-phosphate (PtdIns 4-P) synthesis. PITPnm is an integral membrane protein that largely localized in close association with membranes of Golgi vacuoles and the endoplasmic reticulum (ER). The amino terminus region of PITPnm was exposed to cytoplasmic side. Interaction with various phosphoinositides was observed in the amino terminus region spanning from 196 amino acids to 257 amino acids of PITPnm. At the amino terminus regions of 1-372 amino acids, PITPnm formed a complex with type III PtdIns 4-kinase. The transmembrane and carboxyl-terminal portions (residues 418-1242) functioned to retain the PITPnm in the Golgi vacuole. These results suggest that PITPnm plays a role in phosphoinositide synthesis on the Golgi vacuoles and possibly in the PtdIns signaling pathway in mammalian cells.

Mammalian phosphatidylinositol transfer proteins: emerging roles in signal transduction and vesicular traffic

Phosphatidylinositol transfer proteins (PITP) are abundant cytosolic proteins found in all mammalian cells. Two cytosolic isoforms of 35 and 36 kDa (PITP alpha and PITP beta) have been identified which share 77% identity. These proteins are characterized by having a single phospholipid binding site which exhibits dual headgroup specificity. The preferred lipid that can occupy the site can be either phosphatidylinositol (PI) or phosphatidylcholine (PC). In addition, PITP beta can also bind sphingomyelin. A second characteristic of these proteins is the ability to transfer PI and PC (or SM) from one membrane compartment to another in vitro. The function of PITP in mammalian cells has been examined mainly using reconstitution studies utilizing semi-intact cells or cell-free systems. From such analyses, a requirement for PITP has been identified in phospholipase C-mediated phosphatidylinositol bisphosphate (PI(4,5)P2) hydrolysis, in phosphoinositide 3-kinase catalyzed PIP3 generation, in regulated exocytosis, in the biogenesis of secretory granules and vesicles and in intra-golgi transport. Studies aimed at elucidating the mechanism of action of PITP in each of these seemingly disparate processes have yielded a singular theme: the activity of PITP stems from its ability to transfer PI from its site of synthesis to sites of cellular activity. This function was predicted from its in vitro characteristics. The second feature of PITP that was not predicted is the ability to stimulate the local synthesis of several phosphorylated forms of PI including PI(4)P, PI(4,5)P2, PI(3)P, PI(3,4,5)P3 by presenting PI to the lipid kinases involved in phosphoinositide synthesis. We conclude that PITP contributes in multiple aspects of cell biology ranging from signal transduction to membrane trafficking events where a central role for phosphoinositides is recognized either as a substrate or as an intact lipid signalling molecule.

Cloning and characterization of a secreted frizzled-related protein that is expressed by the retinal pigment epithelium

The Wnt/frizzled cell signaling pathway has been implicated in the determination of polarity in a number of systems, including the Drosophila retina. The vertebrate retina develops from an undifferentiated neuroepithelium into an organized and laminated structure that demonstrates a high degree of polarity at both the tissue and cellular levels. In the process of searching for molecules that are preferentially expressed by the vertebrate retinal pigment epithelium (RPE), we identified secreted frizzled-related protein 5 (SFRP5), a member of the SFRP family that appears to act by modulating Wnt signal transduction. SFRP5 is highly expressed by RPE cells, and is also expressed in the pancreas. Within the retina, the related molecule SFRP2 is expressed specifically by cells of the inner nuclear layer. Thus, photoreceptors are likely to be bathed by two opposing gradients of SFRP molecules. Consistent with SFRP5 's postulated role in modulating Wnt signaling in the retina, it inhibits the ability of Xwnt-8 mRNA to induce axis duplication in Xenopus embryos. The human SFRP5 gene consists of three coding exons and it maps to chromosome 10q24.1; human SFRP2 maps to 4q31.3. Based on the biology and complementary expression patterns of SFRP2 and SFRP5, we suggest that they may be involved in determining the polarity of photoreceptor, and perhaps other, cells in the retina.

Identification of a novel family of targets of PYK2 related to Drosophila retinal degeneration B (rdgB) protein

The protein tyrosine kinase PYK2 has been implicated in signaling pathways activated by G-protein-coupled receptors, intracellular calcium, and stress signals. Here we describe the molecular cloning and characterization of a novel family of PYK2-binding proteins designated Nirs (PYK2 N-terminal domain-interacting receptors). The three Nir proteins (Nir1, Nir2, and Nir3) bind to the amino-terminal domain of PYK2 via a conserved sequence motif located in the carboxy terminus. The primary structures of Nirs reveal six putative transmembrane domains, a region homologous to phosphatidylinositol (PI) transfer protein, and an acidic domain. The Nir proteins are the human homologues of the Drosophila retinal degeneration B protein (rdgB), a protein implicated in the visual transduction pathway in flies. We demonstrate that Nirs are calcium-binding proteins that exhibit PI transfer activity in vivo. Activation of PYK2 by agents that elevate intracellular calcium or by phorbol ester induce tyrosine phosphorylation of Nirs. Moreover, PYK2 and Nirs exhibit similar expression patterns in several regions of the brain and retina. In addition, PYK2-Nir complexes are detected in lysates prepared from cultured cells or from brain tissues. Finally, the Nir1-encoding gene is located at human chromosome 17p13.1, in proximity to a locus responsible for several human retinal diseases. We propose that the Nir and rdgB proteins represent a new family of evolutionarily conserved PYK2-binding proteins that play a role in the control of calcium and phosphoinositide metabolism downstream of G-protein-coupled receptors.

Isolation and characterization of Drosophila retinal degeneration B suppressors

The Drosophila retinal degeneration B protein (RdgB) is a novel integral membrane phosphatidylinositol transfer protein required for photoreceptor cell viability and light response. We isolated one intragenic suppressor (rdgBsu100) and four autosomal suppressors of the hypomorphic rdgBKS222 retinal degeneration phenotype. The rdgBsu100 suppressor dramatically slowed rdgBKS222's photoreceptor degeneration without significantly improving the electroretinogram (ERG) light response. One autosomal recessive suppressor [su(rdgB)69] significantly slowed rdgBKS222 retinal degeneration and restored the ERG light response near to that of the wild type. Unlike all the previously characterized rdgB suppressors, the four new autosomal suppressors do not affect the ERG light response in rdgB+ flies. Only Su(rdgB)116 exhibited a mutant phenotype in a rdgB+ background, which was smaller R1-6 rhabdomeres. We also examined the extent to which two previously identified visual transduction mutations suppressed rdgB retinal degeneration. Absence of one of the light-activated calcium channels (trpCM) slowed the onset of rdgB-dependent degeneration. However, loss of protein kinase C (inaC209), which blocks photoreceptor cell deactivation, desensitization, and light adaptation, failed to suppress rdgB degeneration under normal light conditions. This demonstrates that TRP activity, but not INAC, is required for rapid rdgB-dependent degeneration.

Isolation and chromosomal localization of two human CDP-diacylglycerol synthase (CDS) genes

Phototransduction in Drosophila is a phosphoinositide-mediated signaling pathway. Phosphatidylinositol 4,5-bisphosphate (PIP2) plays a central role in this process, and its levels are tightly regulated. A photoreceptor-specific form of the enzyme CDP-diacylglycerol synthase (CDS), which catalyzes the formation of CDP-diacylglycerol from phosphatidic acid, is a key regulator of the amount of PIP2 available for signaling. cds mutants develop light-induced retinal degeneration. As part of a search for novel genes that may be involved in eye disease in human, using Drosophila phototransduction genes as a model system, two human CDP-diacylglycerol synthase genes (CDS1 and CDS2) were cloned and sequenced. Radiation hybrid panel mapping and fluorescence in situ hybridization were used to localize the genes to chromosomes 4q21 and 20p13. As yet, no known retinal diseases map to either of these regions.

Structure and mapping of the G protein gamma3 subunit gene and a divergently transcribed novel gene, gng3lg

The mammalian nervous system is rich in signaling mediated by heterotrimeric (alphabetagamma) G proteins. As an initial step to define the roles that particular gamma subunit types play in signaling, we have begun to clone and characterize those genes that encode gamma subunits enriched within neural tissue. In the present study, we have isolated and characterized the mouse gamma3 subunit gene (Gng3). The gamma3 subunit is expressed abundantly in the brain and at low levels in testes. Gng3 is composed of three exons spanning approximately 1.4 kb. A comparison of Gng3 with the gene structure for five other gamma subtypes indicates that although these proteins are diverse at the amino acid level, their exon-intron boundaries are conserved. Sequence analysis of the 5' flanking region of Gng3 revealed the presence of a novel gene, the gamma3 linked gene (Gng3lg). Gng3 and Gng3lg are organized in a head-to-head fashion with major transcription initiation sites separated by approximately 133 bp. Sequence analysis of a Gng3lg cDNA clone revealed an open reading frame encoding a 410-amino-acid protein of unknown function. Gng3lg transcripts are expressed in a variety of tissues including both brain and testes. Using an interspecific backcross panel, we localized both Gng3 and Gng3lg to the same locus on chromosome 19. The orientation, close proximity, and expression pattern of these two genes raise the distinct possibility that shared regulatory elements are used to control their expression.