Diacylglycerol Kinase Defect in a Drosophila Retinal Degeneration Mutant rdgA

Knockdown of Dehydrodolichyl Diphosphate Synthase in the Drosophila Retina Leads to a Unique Pattern of Retinal Degeneration

Dehydrodolichyl diphosphate synthase (DHDDS) is a ubiquitously expressed enzyme that catalyzes cis-prenyl chain elongation to produce the poly-prenyl backbone of dolichol. It appears in all tissues including the nervous system and it is a highly conserved enzyme that can be found in all animal species. Individuals who have biallelic missense mutations in the DHDDS gene are presented with non-syndromic retinitis pigmentosa with unknown underlying mechanism. We have used the Drosophila model to compromise DHDDS ortholog gene (CG10778) in order to look for cellular and molecular mechanisms that, when defective, might be responsible for this retinal disease. The Gal4/UAS system was used to suppress the expression of CG10778 via RNAi-mediated-knockdown in various tissues. The resulting phenotypes were assessed using q-RT-PCR, transmission-electron-microscopy (TEM), electroretinogram, antibody staining and Western blot analysis. Targeted knockdown of CG10778-mRNA in the early embryo using the actin promoter or in the developing wings using the nub promoter resulted in lethality, or wings loss, respectively. Targeted expression of CG10778-RNAi using the glass multiple reporter (GMR)-Gal4 driver (GMR-DHDDS-RNAi) in the larva eye disc and pupal retina resulted in a complex phenotype: (a) TEM retinal sections revealed a unique pattern of retinal-degeneration, where photoreceptors R2 and R5 exhibited a nearly normal structure of their signaling-compartment (rhabdomere), but only at the region of the nucleus, while all other photoreceptors showed retinal degeneration at all regions. (b) Western blot analysis revealed a drastic reduction in rhodopsin levels in GMR-DHDDS-RNAi-flies and TEM sections showed an abnormal accumulation of endoplasmic reticulum (ER). To conclude, compromising DHDDS in the developing retina, while allowing formation of the retina, resulted in a unique pattern of retinal degeneration, characterized by a dramatic reduction in rhodopsin protein level and an abnormal accumulation of ER membranes in the photoreceptors cells, thus indicating that DHDDS is essential for normal retinal formation.

Exploring Excitotoxicity and Regulation of a Constitutively Active TRP Ca2+ Channel in Drosophila

Unregulated Ca²⁺ influx affects intracellular Ca²⁺ homoeostasis, which may lead to neuronal death. In Drosophila, following the activation of rhodopsin the TRP Ca²⁺ channel is open to mediate the light-dependent depolarization. A constitutively active TRP channel triggers the degeneration of Trp^P365 /+ photoreceptors. To explore retinal degeneration, we employed a multidisciplinary approach including live imaging using GFP tagged actin and arrestin 2. Importantly, we demonstrate that the major rhodopsin (Rh1) was greatly reduced before the onset of rhabdomere degeneration; a great reduction of Rh1 affects the maintenance of rhabdomere leading to degeneration of photoreceptors. Trp^P365 /+ also led to the up-regulation of CaMKII, which is beneficial as suppression of CaMKII accelerated retinal degeneration. We explored the regulation of TRP by investigating the genetic interaction between Trp^P365 /+ and mutants affecting the turnover of diacylglycerol (DAG). We show a loss of phospholipase C in norpA^P24 exhibited a great reduction of the DAG content delayed degeneration of Trp^P365 /+ photoreceptors. In contrast, knockdown or mutations in DAG lipase (InaE) that is accompanied by slightly reduced levels of most DAG but an increased level of DAG 34:1, exacerbated retinal degeneration of Trp^P365 /+. Together, our findings support the notion that DAG plays a role in regulating TRP. Interestingly, DAG lipase is likely required during photoreceptor development as Trp^P365 /+; inaE^N125 double mutants contained severely degenerated rhabdomeres.

PE homeostasis rebalanced through mitochondria-ER lipid exchange prevents retinal degeneration in Drosophila

The major glycerophospholipid phosphatidylethanolamine (PE) in the nervous system is essential for neural development and function. There are two major PE synthesis pathways, the CDP-ethanolamine pathway in the endoplasmic reticulum (ER) and the phosphatidylserine decarboxylase (PSD) pathway in mitochondria. However, the role played by mitochondrial PE synthesis in maintaining cellular PE homeostasis is unknown. Here, we show that Drosophila pect (phosphoethanolamine cytidylyltransferase) mutants lacking the CDP-ethanolamine pathway, exhibited alterations in phospholipid composition, defective phototransduction, and retinal degeneration. Induction of the PSD pathway fully restored levels and composition of cellular PE, thus rescued the retinal degeneration and defective visual responses in pect mutants. Disrupting lipid exchange between mitochondria and ER blocked the ability of PSD to rescue pect mutant phenotypes. These findings provide direct evidence that the synthesis of PE in mitochondria contributes to cellular PE homeostasis, and suggest the induction of mitochondrial PE synthesis as a promising therapeutic approach for disorders associated with PE deficiency.

Regulation of Membrane Turnover by Phosphatidic Acid: Cellular Functions and Disease Implications

Phosphatidic acid (PA) is a simple glycerophospholipid with a well-established role as an intermediate in phospholipid biosynthesis. In addition to its role in lipid biosynthesis, PA has been proposed to act as a signaling molecule that modulates several aspects of cell biology including membrane transport. PA can be generated in eukaryotic cells by several enzymes whose activity is regulated in the context of signal transduction and enzymes that can metabolize PA thus terminating its signaling activity have also been described. Further, several studies have identified PA binding proteins and changes in their activity are proposed to be mediators of the signaling activity of this lipid. Together these enzymes and proteins constitute a PA signaling toolkit that mediates the signaling functions of PA in cells. Recently, a number of novel genetic models for the analysis of PA function in vivo and analytical methods to quantify PA levels in cells have been developed and promise to enhance our understanding of PA functions. Studies of several elements of the PA signaling toolkit in a single cell type have been performed and are presented to provide a perspective on our understanding of the biochemical and functional organization of pools of PA in a eukaryotic cell. Finally, we also provide a perspective on the potential role of PA in human disease, synthesizing studies from model organisms, human disease genetics and analysis using recently developed PLD inhibitors.

Human eye conditions: insights from the fly eye

The fruit fly Drosophila melanogaster has served as an excellent model to study and understand the genetics of many human diseases from cancer to neurodegeneration. Studying the regulation of growth, determination and differentiation of the compound eyes of this fly, in particular, have provided key insights into a wide range of diseases. Here we review the regulation of the development of fly eyes in light of shared aspects with human eye development. We also show how understanding conserved regulatory pathways in eye development together with the application of tools for genetic screening and functional analyses makes Drosophila a powerful model to diagnose and characterize the genetics underlying many human eye conditions, such as aniridia and retinitis pigmentosa. This further emphasizes the importance and vast potential of basic research to underpin applied research including identifying and treating the genetic basis of human diseases.

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.

Phospholipase D activity couples plasma membrane endocytosis with retromer dependent recycling

During illumination, the light-sensitive plasma membrane (rhabdomere) of Drosophila photoreceptors undergoes turnover with consequent changes in size and composition. However, the mechanism by which illumination is coupled to rhabdomere turnover remains unclear. We find that photoreceptors contain a light-dependent phospholipase D (PLD) activity. During illumination, loss of PLD resulted in an enhanced reduction in rhabdomere size, accumulation of Rab7 positive, rhodopsin1-containing vesicles (RLVs) in the cell body and reduced rhodopsin protein. These phenotypes were associated with reduced levels of phosphatidic acid, the product of PLD activity and were rescued by reconstitution with catalytically active PLD. In wild-type photoreceptors, during illumination, enhanced PLD activity was sufficient to clear RLVs from the cell body by a process dependent on Arf1-GTP levels and retromer complex function. Thus, during illumination, PLD activity couples endocytosis of RLVs with their recycling to the plasma membrane thus maintaining plasma membrane size and composition.

DOI:http://dx.doi.org/10.7554/eLife.18515.001

Certain cells in the eye contain a receptor protein known as rhodopsin that enables them to detect light. Rhodopsin is found in distinct patches on the membrane surrounding each of these “photoreceptor” cells and the number of rhodopsin molecules present controls how sensitive the cell is to light. In humans, vitamin A deficiency or genetic defects can decrease the number of rhodopsin molecules on the membrane, leading to difficulty in seeing in dim light.

Fruit fly eyes also contain rhodopsin. Exposure to normal levels of light triggers parts of the membranes of fly photoreceptor cells to detach and move into the interior of the cell. These internalized pieces of membrane have two possible fates: they can either be destroyed or recycled back to the cell surface. This membrane turnover adjusts the size of the membrane surrounding the cell and the number of rhodopsin molecules in it to regulate the cell’s sensitivity to light. It is crucial that turnover is tightly regulated in order to maintain the integrity of the cell membrane. However, it is not clear how the process is regulated during light exposure.

Thakur et al. set out to address this question in fruit flies. The experiments show that an enzyme called phospholipase D is activated when photoreceptors are exposed to light. Active phospholipase D – which generates a molecule called phosphatidic acid – coordinates the internalization of pieces of membrane with the recycling of rhodopsin back to the cell surface. Thakur et al. generated fly mutants that lacked phospholipase D and in these animals the internalized rhodopsin was not transported back to the cell membrane. This caused the membrane to shrink in size and decreased the number of rhodopsin molecules in it. As a result, the photoreceptor cells became less sensitive to light.

The findings of Thakur et al. show that in response to normal levels of light, phospholipase D balances membrane internalization and recycling to maintain the size and rhodopsin composition of the membrane. Future challenges will be to work out exactly how phospholipase D is activated and how phosphatidic acid tunes membrane internalization and recycling.

DOI:http://dx.doi.org/10.7554/eLife.18515.002

The GTP- and Phospholipid-Binding Protein TTD14 Regulates Trafficking of the TRPL Ion Channel in Drosophila Photoreceptor Cells

Recycling of signaling proteins is a common phenomenon in diverse signaling pathways. In photoreceptors of Drosophila, light absorption by rhodopsin triggers a phospholipase Cβ-mediated opening of the ion channels transient receptor potential (TRP) and TRP-like (TRPL) and generates the visual response. The signaling proteins are located in a plasma membrane compartment called rhabdomere. The major rhodopsin (Rh1) and TRP are predominantly localized in the rhabdomere in light and darkness. In contrast, TRPL translocates between the rhabdomeral plasma membrane in the dark and a storage compartment in the cell body in the light, from where it can be recycled to the plasma membrane upon subsequent dark adaptation. Here, we identified the gene mutated in trpl translocation defective 14 (ttd14), which is required for both TRPL internalization from the rhabdomere in the light and recycling of TRPL back to the rhabdomere in the dark. TTD14 is highly conserved in invertebrates and binds GTP in vitro. The ttd14 mutation alters a conserved proline residue (P75L) in the GTP-binding domain and abolishes binding to GTP. This indicates that GTP binding is essential for TTD14 function. TTD14 is a cytosolic protein and binds to PtdIns(3)P, a lipid enriched in early endosome membranes, and to phosphatidic acid. In contrast to TRPL, rhabdomeral localization of the membrane proteins Rh1 and TRP is not affected in the ttd14^P75L mutant. The ttd14^P75L mutation results in Rh1-independent photoreceptor degeneration and larval lethality suggesting that other processes are also affected by the ttd14^P75L mutation. In conclusion, TTD14 is a novel regulator of TRPL trafficking, involved in internalization and subsequent sorting of TRPL into the recycling pathway that enables this ion channel to return to the plasma membrane.

Protein trafficking in neurons occurs throughout the lifetime of a cell and includes the internalization and redistribution of plasma membrane proteins. Regulated protein trafficking controls the equipment of the plasma membrane with receptors and ion channels and thereby attenuates or enhances neuronal function. Defects in recycling of plasma membrane proteins can cause detrimental neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease and Down´s syndrome. In Drosophila photoreceptors, the TRPL ion channel, together with the TRP channel, mediates vision and light-dependently shuttles between an endomembrane storage compartment and the apical plasma membrane. Here, we report the identification of a mutation in the ttd14 gene that inhibits TRPL-trafficking in both directions and also results in photoreceptor degeneration. The TTD14 protein contains a region with weak homology to a PX-domain, which is also found in proteins that sort cargo in the endosome and enable protein recycling. We characterize TTD14 as a new regulator of photoreceptor maintenance and ion channel trafficking that binds to GTP and PtdIns(3)P, a phospholipid enriched in early endosomes.

The plant non-specific phospholipase C gene family. Novel competitors in lipid signalling

Non-specific phospholipases C (NPCs) were discovered as a novel type of plant phospholipid-cleaving enzyme homologous to bacterial phosphatidylcholine-specific phospholipases C and responsible for lipid conversion during phosphate-limiting conditions. The six-gene family was established in Arabidopsis, and growing evidence suggests the involvement of two articles NPCs in biotic and abiotic stress responses as well as phytohormone actions. In addition, the diacylglycerol produced via NPCs is postulated to participate in membrane remodelling, general lipid metabolism and cross-talk with other phospholipid signalling systems in plants. This review summarises information concerning this new plant protein family and focusses on its sequence analysis, biochemical properties, cellular and tissue distribution and physiological functions. Possible modes of action are also discussed.

Mutants in phospholipid signaling attenuate the behavioral response of adult Drosophila to trehalose

In Drosophila melanogaster, gustatory receptor genes (Grs) encode putative G-protein-coupled receptors (GPCRs) that are expressed in gustatory receptor neurons (GRNs). One of the Gr genes, Gr5a, encodes a receptor for trehalose that is expressed in a subset of GRNs. Although a role for the G protein, Gsα, has been shown in Gr5a-expressing taste neurons, there is the residual responses to trehalose in Gsα mutants which could suggest additional transduction mechanisms. Expression and genetic analysis of the heterotrimeric G-protein subunit, Gq, shown here suggest involvement of this Gα subunit in trehalose perception in Drosophila. A green fluorescent protein reporter of Gq expression is detected in gustatory neurons in the labellum, tarsal segments, and wing margins. Animals heterozygous for dgq mutations and RNA interference-mediated knockdown of dgq showed reduced responses to trehalose in the proboscis extension reflex assay and feeding behavior assay. These defects were rescued by targeted expression of the wild-type dgqα transgene in the GRNs. These data together with observations from other mutants in phospholipid signaling provide insights into the mechanisms of taste transduction in Drosophila.

Phosphoinositide 3-kinase signaling in the vertebrate retina

The phosphoinositide (PI) cycle, discovered over 50 years ago by Mabel and Lowell Hokin, describes a series of biochemical reactions that occur on the inner leaflet of the plasma membrane of cells in response to receptor activation by extracellular stimuli. Studies from our laboratory have shown that the retina and rod outer segments (ROSs) have active PI metabolism. Biochemical studies revealed that the ROSs contain the enzymes necessary for phosphorylation of phosphoinositides. We showed that light stimulates various components of the PI cycle in the vertebrate ROS, including diacylglycerol kinase, PI synthetase, phosphatidylinositol phosphate kinase, phospholipase C, and phosphoinositide 3-kinase (PI3K). This article describes recent studies on the PI3K-generated PI lipid second messengers in the control and regulation of PI-binding proteins in the vertebrate retina.

Diacylglycerol kinases as sources of phosphatidic acid

There are ten mammalian diacylglycerol kinases (DGKs) whose primary role is to terminate diacylglycerol (DAG) signaling. However, it is becoming increasingly apparent that DGKs also influence signaling events through their product, phosphatidic acid (PA). They do so in some cases by associating with proteins and then modifying their activity by generating PA. In other cases, DGKs broadly regulate signaling events by virtue of their ability to provide PA for the synthesis of phosphatidylinositols (PtdIns).

Drosophila mutants in phospholipid signaling have reduced olfactory responses as adults and larvae

In this paper, we show that mutants in the gene stambhA (stmA), which encodes a putative phosphatidylinositol 4,5 bisphosphate-diacylglycerol lipase, exhibit a significant reduction in the amplitudes of odor-evoked responses recorded from the antennal surface of adult Drosophila. This lends support to previously published findings that olfactory transduction in Drosophila requires a phospholipid intermediate. Mutations in stmA also affect the olfactory behavior response of larvae. Moreover, there is a requirement for G(q)alpha and phospholipase Cbeta function in larval olfaction. The results suggest that larval olfactory transduction, like that of the adult, utilizes a phospholipid second messenger, generated by the activation of G(q)alpha and Plcbeta21c, and modulated by the stmA gene product.

Phototransduction and retinal degeneration in Drosophila

Drosophila visual transduction is the fastest known G-protein-coupled signaling cascade and has therefore served as a genetically tractable animal model for characterizing rapid responses to sensory stimulation. Mutations in over 30 genes have been identified, which affect activation, adaptation, or termination of the photoresponse. Based on analyses of these genes, a model for phototransduction has emerged, which involves phosphoinoside signaling and culminates with opening of the TRP and TRPL cation channels. Many of the proteins that function in phototransduction are coupled to the PDZ containing scaffold protein INAD and form a supramolecular signaling complex, the signalplex. Arrestin, TRPL, and G alpha(q) undergo dynamic light-dependent trafficking, and these movements function in long-term adaptation. Other proteins play important roles either in the formation or maturation of rhodopsin, or in regeneration of phosphatidylinositol 4,5-bisphosphate (PIP2), which is required for the photoresponse. Mutation of nearly any gene that functions in the photoresponse results in retinal degeneration. The underlying bases of photoreceptor cell death are diverse and involve mechanisms such as excessive endocytosis of rhodopsin due to stable rhodopsin/arrestin complexes and abnormally low or high levels of Ca2+. Drosophila visual transduction appears to have particular relevance to the cascade in the intrinsically photosensitive retinal ganglion cells in mammals, as the photoresponse in these latter cells appears to operate through a remarkably similar mechanism.

Regulation of Drosophila TRPC channels by protein and lipid interactions

The Drosophila TRPC channels TRP and TRPL are the founding members of the TRP superfamily of ion channels, proteins likely to be important components of calcium influx pathways. The activation of these channels in the context of fly phototransduction is one of the few in vivo models for TRPC channel activation and has served as a paradigm for understanding TRPC function. TRP and TRPL are activated by G-protein coupled PI(4,5)P(2) hydrolysis through a mechanism in which IP(3) receptor mediated calcium release seems dispensable. Recent analysis has provided compelling evidence that the accurate turnover of PI(4,5)P(2) generated lipid messengers in essential for regulating TRP and TRPL activity. TRP channels also appear to exist in the context of a macromolecular complex containing key components involved in activation such as phospholipase Cbeta and protein kinase C. This complex may be important for activation. The role of these protein and lipid elements in regulating TRP and TRPL activity is discussed in this review.

lazaro encodes a lipid phosphate phosphohydrolase that regulates phosphatidylinositol turnover during Drosophila phototransduction

An essential step in Drosophila phototransduction is the hydrolysis of phosphatidylinositol 4,5 bisphosphate PI(4,5)P2 by phospholipase Cbeta (PLCbeta) to generate a second messenger that opens the light-activated channels TRP and TRPL. Although the identity of this messenger remains unknown, recent evidence has implicated diacylglycerol kinase (DGK), encoded by rdgA, as a key enzyme that regulates its levels, mediating both amplification and response termination. In this study, we demonstrate that lazaro (laza) encodes a lipid phosphate phosphohydrolase (LPP) that functions during phototransduction. We demonstrate that the synergistic activity of laza and rdgA regulates response termination during phototransduction. Analysis of retinal phospholipids revealed a reduction in phosphatidic acid (PA) levels and an associated reduction in phosphatidylinositol (PI) levels. Together our results demonstrate the contribution of PI depletion to the rdgA phenotype and provide evidence that depletion of PI and its metabolites might be a key signal for TRP channel activation in vivo.

Functional INAD complexes are required to mediate degeneration in photoreceptors of the Drosophila rdgA mutant

The TRP family of ion channels mediates a wide range of calcium-influx phenomena in eukaryotic cells. Many members of this family are activated downstream of phosphoinositide hydrolysis but the subsequent steps that lead to TRP channel activation in vivo remain unclear. Recently, the lipid products of phosphoinositide hydrolysis (such as diacylglycerol and its metabolites) have been implicated in activating TRP channels in both Drosophila and mammals. In Drosophila photoreceptors, lack of diacylglycerol kinase (DGK) activity (encoded by rdgA) leads to both constitutive TRP-channel activity and retinal degeneration. In this study, using a novel forward-genetic screen, we identified InaD, a multivalent PDZ domain protein as a suppresser of retinal degeneration in rdgA mutants. We show that InaD suppresses rdgA and that the rescue is correlated with reduced levels of phospholipase Cbeta (PLCbeta), a key enzyme for TRP channel activation. Furthermore, we show that light, Gq and PLCbeta all modulate retinal degeneration in rdgA. The results demonstrate a previously unknown requirement for a balance of PLCbeta and DGK activity for retinal degeneration in rdgA. They also suggest a key role for the lipid products of phosphoinositide hydrolysis in the activation of TRP channels in vivo.

Rolling blackout, a newly identified PIP2-DAG pathway lipase required for Drosophila phototransduction

The rolling blackout (rbo) gene encodes an integral plasma membrane lipase required for Drosophila phototransduction. Photoreceptors are enriched for the RBO protein, and temperature-sensitive rbo mutants show reversible elimination of phototransduction within minutes, demonstrating an acute requirement for the protein. The block is activity dependent, indicating that the action of RBO is use dependent. Conditional rbo mutants show activity-dependent depletion of diacylglycerol and concomitant accumulation of phosphatidylinositol phosphate and phosphatidylinositol 4,5-bisphosphate within minutes of induction, suggesting rapid downregulation of phospholipase C (PLC) activity. The RBO requirement identifies an essential regulatory step in G-protein-coupled, PLC-dependent inositol lipid signaling mediating activation of TRP and TRPL channels during phototransduction.

Nuclear localization of diacylglycerol kinase zeta in neurons

Diacylglycerol kinase (DGK) is involved in intracellular signal transduction as a regulator of levels of diacylglycerol which leads to protein kinase C activation. Previous studies have revealed that DGK consists of a family of isozymes in mammalian species and that most if not all of them show abundant expression in the central nervous system, suggesting the importance of this enzyme in neuronal function. Among the isozymes, DGK zeta (previously also known as DGK-IV for the rat clone) has unique structural features, such as four ankyrin-like repeats and a nuclear localization signal (NLS), and shows intense mRNA expression in neurons of the olfactory bulb, hippocampus and cerebral and cerebellar cortices (Goto, K. & Kondo, H. (1996), Proc. Natl Acad. Sci. USA, 93, 11196-11201). However, previous studies have given conflicting results about whether or not DGK zeta localizes to the nucleus in these cells. In this study, we have used immunohistochemistry with specific antibodies in brain tissues and cDNA transfection into primary cultured neurons to address this question. We have shown that, while DGK zeta is primarily a nuclear protein in neurons, it can also be cytoplasmic in some conditions, and the subcellular location depends not only on the cell type but also on the developmental state or growth conditions of the cell. In addition, we have used deletion mutants to show that nuclear transport of DGK zeta depends on a cooperative interaction between the NLS and the C-terminal region including ankyrin repeats in a manner which suggests that the NLS is a cryptic site whose exposure is regulated by the C-terminal region. Together, these results support the hypothesis that the localization of DGK zeta may be regulated by differential expression of these various proteins which interact with its C-terminal region.

Photoreceptor degeneration and Ca2+ influx through light-activated channels of Drosophila

We discuss in this chapter the role of Ca2+ homeostasis in maintaining the structural integrity of photoreceptor cells in Drosophila. Both insufficient and excessive amounts of Ca2+ in photoreceptor cells appear to lead to cell degeneration. Because one of the two classes of light-sensitive channels in Drosophila photoreceptors is highly Ca2+-permeable, how well this class of channels functions can profoundly affect Ca2+ homeostasis. We will begin by reviewing Drosophila phototransduction, emphasizing what is known about the mechanism of activation of light-sensitive channels. We will then describe Ca2+ entry through light-sensitive channels and the presumed mechanisms by which too little and too much Ca2+ entry can both cause photoreceptor degeneration. We will conclude the chapter with discussions of two examples of mutations known to cause unregulated Ca2+ entry through light-sensitive channels, leading to massive photoreceptor degeneration.

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.

TRP gating is linked to the metabolic state and maintenance of the Drosophila photoreceptor cells

The Drosophila light-activated channel TRP is the founding member of a large and diverse family of channel proteins that is conserved throughout evolution. In spite of much progress, the gating mechanism of TRP channels is still unknown. However, recent studies have shown multi-faceted functions of the Drosophila light-sensitive TRP channel that may shed light on TRP gating. Accordingly, metabolic stress, which leads to depletion of cellular ATP, reversibly activates the Drosophila TRP and TRPL channels in the dark in a constitutive manner. In several Drosophila mutants, constitutive activity of TRP channels lead to a rapid retinal degeneration in the dark, while genetic elimination of TRP protects the cells from degeneration. Additional studies have shown that TRPL translocates in a light-dependent manner between the signaling membranes and the cell body. This light-activated translocation is accompanied by reversible morphological changes leading to partial and reversible collapse of the microvillar signaling membranes into the cytosol, which allows turnover of signaling molecules. These morphological changes are also blocked by genetic elimination of TRP channels. The link of TRP gating to the metabolic state and maintenance of cells makes cells expressing TRP extremely vulnerable to metabolic stress via a mechanism that may underlie retinal degeneration and neuronal cell death upon malfunction.

Molecular analysis of a novel Drosophila diacylglycerol kinase, DGKepsilon

Diacylglycerol kinase plays a central role in the metabolism of diacylglycerol by converting diacylglycerol into phosphatidic acid thus initiating resynthesis of phosphatidylinositols. Diacylglycerol is a known second messenger reversibly activating protein kinase C. In addition, diacylglycerol is a potential precursor for polyunsaturated fatty acids. We describe the identification and molecular analysis of a novel type III Drosophila diacylglycerol kinase isoform, DGKepsilon. Drosophila DGKepsilon is mapped to the cytological position 49C1-3. DGKepsilon mRNA is 1.9 kb in length and is broadly distributed throughout development in different cells, primordia and organs, including testes. In embryogenesis, the transcripts are enriched in the cells, which are in S-phase or undergoing endoreplication. Comparison of the Drosophila DGKepsilon with the human homologue revealed that the first zinc finger-like motif is specific for the type III isoform. Although the testis-specific diacylglycerol kinase activity is dependent upon the dose of DGKepsilon gene, the deletion of DGKepsilon does not modulate the total cellular diacylglycerol level. In spite of a proposed key role of diacylglycerol kinase in termination of the diacylglycerol signal, overexpression of a DGKepsilon transgene in flies under the control of a yeast upstream activating sequence promoter does not disrupt normal development in Drosophila.

Constitutive activity of the light-sensitive channels TRP and TRPL in the Drosophila diacylglycerol kinase mutant, rdgA

Mutations in the Drosophila retinal degeneration A (rdgA) gene, which encodes diacylglycerol kinase (DGK), result in early onset retinal degeneration and blindness. Whole-cell recordings revealed that light-sensitive Ca2+ channels encoded by the trp gene were constitutively active in rdgA photoreceptors. Early degeneration was rescued in rdgA;trp double mutants, lacking TRP channels; however, the less Ca2+-permeable light-sensitive channels (TRPL) were constitutively active instead. No constitutive activity was seen in rdgA;trpI;trp mutants lacking both classes of channel, although, like rdgA;trp, these still showed a residual slow degeneration. Responses to light were restored in rdgA;trp but deactivated abnormally slowly, indicating that DGK is required for response termination. The findings suggest that early degeneration in rdgA is caused by uncontrolled Ca2+ influx and support the proposal that diacylglycerol or its metabolites are messengers of excitation in Drosophila photoreceptors.

The cloning and characterization of a novel human diacylglycerol kinase, DGKiota

Diacylglycerol (DAG) plays a central role in both the synthesis of complex lipids and in intracellular signaling; diacylglycerol kinase (DGK) catalyzes the phosphorylation of DAG, which yields phosphatidic acid. A family of DGKs has been identified in multicellular organisms over the past few years, but the physiological function(s) of this diversity is not clear. One clue has come from the Drosophila DGK2, rdgA, since mutations in this gene cause retinal degeneration. We isolated a novel DGK, which we designated DGKiota, from human retina and brain libraries. DGKiota contains two cysteine-rich repeats, a region similar to the phosphorylation site domain of myristoylated alanine-rich C kinase substrate, a conserved catalytic domain, and four ankyrin repeats at its C terminus. By primary structure, it is most similar to human DGKzeta and Drosophila rdgA. An >12-kilobase mRNA for DGKiota was detected only in brain and retina among the tissues examined. In cells transfected with the DGKiota cDNA, we detected an approximately 130-kDa protein by immunoassay, and activity assays demonstrated that it encodes a functional DAG kinase. The protein was found to be in both the cytoplasm and nucleus with the localization controlled by PKC isoforms alpha and gamma. The gene encoding DGKiota was localized to human chromosome 7q32.3-33, which is known to be a locus for an inherited form of retinitis pigmentosa. These results have defined a novel isoform of DAG kinase, which may have important cellular functions in the retina and brain.

Alternations of diacylglycerol kinase in streptozotocin-induced diabetic rats

Dysfunction of organs has been reported in diabetic rats, suggesting an association with changes in intracellular signal transduction pathways including phosphatidylinositol (PI) turnover. Diacylglycerol (DG) kinase catalyses the phosphorylation of DG, which is considered to play a major physiological role in the metabolism of the intracellular messenger DG. However, no relation between DG kinase activity and any disease in mammalian tissue has been reported to date. In the present study, we investigated whether the changes in DG kinase activity are related to diabetes. Basal resting level of DG kinase activity changed in tissue isolated from diabetic rats. Decreases in resting activity detected in aorta and kidney and agonist-induced responses differed between these tissues. Submaximal increases in basal activity also were detected in vas deferens and hepatocytes. These changes in DG kinase activity resemble the functional changes associated with complications of diabetes, suggesting that changes in PI turnover followed by DG kinase activity are a key element in the complications. It is the first study about the changes in DG kinase activity in mammalian disease.

Immunolocalization of Drosophila eye-specific diacylgylcerol kinase, rdgA, which is essential for the maintenance of the photoreceptor

The Drosophila retinal degeneration A (rdgA) mutant has photoreceptor cells that degenerate within a week after eclosion. The degeneration starts with the disruption of the subrhabdomeric cisternae (SRC), which are the organelles essential for the transport of phospholipids to the photoreceptive membranes. Our previous biochemical and molecular studies suggested that the rdgA gene encodes an eye-specific diacylglycerol kinase (DGK). In this study, we show that retinal degeneration is prevented by the introduction of the eye-DGK gene in the rdgA mutant genome, suggesting that the DGK activity is crucial for the maintenance of the photoreceptor. Furthermore, by immunohistochemical analysis, we have demonstrated that the rdgA protein is predominantly associated with the SRC, suggesting that the conversion from diacylglycerol (DG) to phosphatidic acid (PA) most actively occurs in SRC. The analysis of the eyes of mutants homozygous for rdgA and eye-protein kinase C mutations indicates that retinal degeneration is caused by the deficiency of PA rather than excessive accumulation of DG. From these data, we conclude that the production of PA in the SRC membranes is essential for the maintenance of the photoreceptor.

Molecular cloning of a diacylglycerol kinase isozyme predominantly expressed in rat retina

We have cloned and characterized a new diacylglycerol kinase (DGK) isozyme which is expressed in the retina and the brain of rat. The cDNA contains an open reading frame of 567 amino acid residues with a predicted protein of 64 kDa and shows very high homology to human DGK epsilon. The new DGK isozyme contains two distinctive zinc-finger structures and a putative catalytic domain. This DGK expressed predominantly in the inner and outer nuclear layers of retina. This expression pattern is different from those of the previously cloned DGKs including the human DGK epsilon, suggesting that this DGK isozyme has potential importance in visual functions as was the case in Drosophila retinal cells.

Regulation of eukaryotic phosphatidylinositol-specific phospholipase C and phospholipase D

This review focuses on two phospholipase activities involved in eukaryotic signal transduction. The action of the phosphatidylinositol-specific phospholipase C enzymes produces two well-characterized second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. This discussion emphasizes recent advances in elucidation of the mechanisms of regulation and catalysis of the various isoforms of these enzymes. These are especially related to structural information now available for a phospholipase C delta isozyme. Phospholipase D hydrolyzes phospholipids to produce phosphatidic acid and the respective head group. A perspective of selected past studies is related to emerging molecular characterization of purified and cloned phospholipases D. Evidence for various stimulatory agents (two small G protein families, protein kinase C, and phosphoinositides) suggests complex regulatory mechanisms, and some studies suggest a role for this enzyme activity in intracellular membrane traffic.

Cloning and characterization of a glucocorticoid-induced diacylglycerol kinase

Diacylglycerol kinase (DGK) plays a key role in cellular processes by regulating the intracellular concentration of the second messenger diacylglycerol. We screened a hamster DDT1 smooth muscle cell library and isolated a unique, glucocorticoid-inducible cDNA with substantial homology to known DGKs. DGK activity was increased in lysates of insect cells infected with recombinant baculovirus containing this cDNA. Antibodies raised against expressed sequences recognized a glucocorticoid-inducible 130-140-kDa protein on immunoblots of DDT1 cell lysates. Thus, this sequence appears to be a new member of the DGK family that we refer to as DGKeta. Homology to other DGKs was apparent in domains that are thought to be important for DGK function including the cysteine-rich motifs and potential catalytic domains. DGKeta shares substantial homology with DGKdelta including the N-terminal pleckstrin homology domain. The tissue distribution of DGKeta message (determined by ribonuclease protection assays) and protein (determined by immunoblots) was broader than reported for other DGKs, indicating that DGKeta may play a more general role in regulating cellular DG levels than other DGKs. Heterogeneity among DGK family members indicates that individual DGKs may have unique functions.

Regulation of PLC-mediated signalling in vivo by CDP-diacylglycerol synthase

CDP-diacylglycerol synthase (CDS) is an enzyme required for the regeneration of the signalling molecule phosphatidylinositol-4,5-bisphosphate (PtdlnsP2) from phosphatidic acid. A photo-receptor cell-specific isoform of CDS from Drosophila is a key regulator of phototransduction, a G-protein-coupled signalling cascade mediated by phospholipase C. cds mutants cannot sustain a light-activated current as a result of depletion of PtdlnsP2. Overexpression of CDS increases the amplitude of the light response, demonstrating that availability of PtdlnsP2 is a determinant in the gain of this pathway. cds mutants undergo light-dependent retinal degeneration which can be suppressed by a mutation in phospholipase C. Thus, enzymes involved in PtdlnsP2 metabolism regulate phosphoinositide-mediated signalling cascades in vivo.

Defective glia in the Drosophila brain degeneration mutant drop-dead

To understand better the cellular basis of late-onset neuronal degeneration, we have examined the brain of the drop-dead mutant of Drosophila. This mutant carries an X-chromosomal recessive mutation that causes severe behavioral defects and brain degeneration, manifested a few days after emergence of the adult. Analysis of genetically mosaic flies has indicated that the focus of the drop-dead mutant phenotype is in the brain and that the gene product is non-cell autonomous. We examined the adult drop-dead mutant brain prior to onset of symptoms and found that many glial cells have stunted processes, whereas neuronal morphology is essentially normal. Adult mutant glial cells resemble immature glia found at an earlier stage of normal brain development. These observations suggest that defective glia in the drop-dead brain may disrupt adult nervous system function, contributing to progressive brain degeneration and death. The normal drop-dead gene product may prevent brain degeneration by providing a necessary glial function.

Phospholipids in Drosophila heads: effects of visual mutants and phototransduction manipulations

A procedure was developed to label phospholipids in Drosophila heads by feeding radioactive phosphate (32Pi). High-performance thin-layer chromatography showed label incorporation into various phospholipids. After 24 h of feeding, major phospholipids labeled were phosphatidylethanolamine (PE), 47%; phosphatidylcholine (PC), 24%; and phosphatidylinositol (PI), 12%. Drosophila heads have virtually no sphingomyelin as compared with mammalian tissues. Notable label was in ethanolamine plasmalogen, lysophosphatidylethanolamine, lysophosphatidylcholine and lysophosphatidylinositol. Less than 1% of the total label was in phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. Other lipids labeled included phosphatidylserine, phosphatidic acid and some unidentified lipids. A time course (3-36 h) study revealed a gradual decrease in proportion of labeled PI, an increase in proportion of labeled PC and no obvious change in labeled PE. There were no significant differences in phospholipid labeling comparing the no receptor potential (norpA) visual mutant and wild type under light vs. dark conditions. However, overall 32P labeling was higher in the wild type fed in the light as compared to the dark and to norpA either in light or dark. This suggests that functional vision facilitates incorporation of label. Differences in phospholipid labeling were observed between young and aged flies, particularly in lysophospholipids and poly-PI, implicating phospholipase A2 function in recycling. v Manipulations such as the outer rhabdomeres absent and eyes absent mutants and carotenoid deprivation failed to yield notable differences in phospholipid labeling pattern, suggesting that phospholipids important to vision may constitute only a minor portion of the total labeled pool in the head.

Delayed appearance of G-protein coupled signal transduction system in developing cerebellar Purkinje cell dendrites

To investigate the relation between the function of Ca(2+)-activated K+ channels and phosphoinositide turnover, we have examined the physiological and pharmacological characteristics of ionotropic and metabotropic quisqualate (QA) receptors in rat cerebellar Purkinje cells during development using the slice-patch method combined with Ca2+ imaging. The typical response to QA obtained from a rat on postnatal day (PND) 21 consisted of three components: (1) a fast inactivating inward current, (2) a slow inward current, and (3) a slow outward current. The slow inward current was abolished in Ca(2+)-free medium, while the fast inactivating inward current and the slow outward current remained unaffected. The slow outward current which appeared to be activated via a metabotropic receptor was not observed in the Purkinje cell of PND 7 rat, in which dendrites were poorly developed but its amplitude increased linearly with PND. QA caused significant increases in [Ca2+]i in the fully developed dendritic region of the Purkinje cells even in Ca(2+)-free medium, suggesting a dendritic localization of the metabotropic receptors.

Phototransduction in Drosophila: a paradigm for the genetic dissection of sensory transduction cascades

A combination of molecular, genetic and physiological studies is providing fundamental insight into the function and regulation of the phototransduction cascade. The availability of Drosophila mutants with defects in visual physiology allows for an in vivo dissection of this complex sensory signal transduction process.

Partial purification and characterization of membrane-associated diacylglycerol kinase of Drosophila heads

A membrane-associated diacylglycerol kinase of Drosophila heads was purified to near homogeneity from the KCl extract of Drosophila heads. The purification procedure involved chromatography on Q-Sepharose, ammonium sulfate fractionation, Superose 12, hydroxyapatite and ATP-agarose. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of fractions after the ATP-agarose column chromatography showed that only a 115 kDa protein correlated well with the enzyme activity. The apparent Km values of partially purified DG kinase were 220 microM for ATP and 540 microM for diolein, respectively. The activity of the DG kinase was inhibited by deoxycholate and was not activated by Ca2+.

The regulatory role of EF-hand motifs of pig 80K diacylglycerol kinase as assessed using truncation and deletion mutants

To elucidate the regulatory function of EF-hand motifs of pig 80K diacylglycerol (DG) kinase, we constructed and expressed several truncation and deletion mutants of the enzyme in E. coli or COS-7 cells. The bacterially expressed EF-hand region could bind Ca2+ and was suggested to undergo conformational change like calmodulin. A mutant enzyme lacking EF-hands lost Ca(2+)-binding activity, but could be fully activated by phosphatidylserine (PS) or deoxycholate in the absence of Ca2+. The full activation of the wild-type enzyme by PS, on the other hand, was totally dependent on Ca2+. Further, the wild-type enzyme expressed in COS-7 cells was exclusively soluble, whereas the EF-hand-deleted mutant was considerably associated with the membranes. The results suggest that under Ca(2+)-free condition, the EF-hand masks the PS-binding site of the DG kinase, and that the Ca(2+)-binding results in the exposure of the PS-binding site through the conformational change of the EF-hand region.

Diacylglycerol kinase: a key modulator of signal transduction?

Diacylglycerol kinase (DGK) plays a central role in the metabolism of diacylglycerol released as a second messenger in agonist-stimulated cells. The major purified form of the enzyme (80 kDa DGK) is highly abundant in lymphocyte cytosol and may become membrane-associated via phosphorylation by protein kinase C. In addition, there are several kinase subspecies immunologically distinct from the 80 kDa enzyme, which differ markedly in their responses to several compounds such as sphingosine and R59022. Thus, further work on each enzyme species is needed to define the function of DGK in stimulated cells.