Phospholipase C beta 4 is involved in modulating the visual response in mice

Golgi cell dendrites are restricted by Purkinje cell stripe boundaries in the adult mouse cerebellar cortex

Despite the general uniformity in cellular composition of the adult cerebellar cortex, there is a complex underlying pattern of parasagittal stripes of Purkinje cells with characteristic molecular phenotypes and patterns of connectivity. It is not known whether interneuron processes are restricted at stripe boundaries. To begin to address the issue, three strategies were used to explore how cerebellar Golgi cell dendrites are organized with respect to parasagittal stripes: first, double immunofluorescence staining combining anti-neurogranin to identify Golgi cell dendrites with the Purkinje cell compartmentation antigens zebrin II/aldolase C, HNK-1, and phospholipase Cbeta4; second, zebrin II immunohistochemistry combined with a rapid Golgi-Cox impregnation procedure to reveal Golgi cell dendritic arbors; third, stripe antigen expression was used on sections of a GlyT2-EGFP transgenic mouse in which reporter expression is prominent in Golgi cell dendrites. In each case, the dendritic projections of Golgi cells were studied in the vicinity of Purkinje cell stripe boundaries. The data presented here show that the dendrites of a cerebellar interneuron, the Golgi cell, respect the fundamental cerebellar stripe cytoarchitecture.

Purkinje cell subtype specification in the cerebellar cortex: early B-cell factor 2 acts to repress the zebrin II-positive Purkinje cell phenotype

The mammalian cerebellar cortex is highly compartmentalized. First, it is subdivided into four transverse expression domains: the anterior zone (AZ), the central zone (CZ), the posterior zone (PZ), and the nodular zone (NZ). Within each zone, the cortex is further subdivided into a symmetrical array of parasagittal stripes. The most extensively studied compartmentation antigen is zebrin II/aldolase c, which is expressed by a subset of Purkinje cells forming parasagittal stripes. Stripe phenotypes are specified early in cerebellar development, in part through the action of early B-cell factor 2 (Ebf2), a member of the atypical helix-loop-helix transcription factor family Collier/Olf1/EBF. In the murine cerebellum, Ebf2 expression is restricted to the zebrin II-immunonegative (zebrin II-) Purkinje cell population. We have identified multiple cerebellar defects in the Ebf2 null mouse involving a combination of selective Purkinje cell death and ectopic expression of multiple genes normally restricted to the zebrin II- subset. The nature of the cerebellar defect in the Ebf2 null is different in each transverse zone. In contrast to the ectopic expression of genes characteristic of the zebrin II+ Purkinje cell phenotype, phospholipase Cbeta4 expression, restricted to zebrin II- Purkinje cells in control mice, is well maintained, and the normal number of stripes is present. Taken together, these data suggest that Ebf2 regulates the expression of genes associated with the zebrin II+ Purkinje cell phenotype and that the zebrin II- Purkinje cell subtype is specified independently.

The Reelin receptors Apoer2 and Vldlr coordinate the patterning of Purkinje cell topography in the developing mouse cerebellum

The adult cerebellar cortex is comprised of reproducible arrays of transverse zones and parasagittal stripes of Purkinje cells. Adult stripes are created through the perinatal rostrocaudal dispersion of embryonic Purkinje cell clusters, triggered by signaling through the Reelin pathway. Reelin is secreted by neurons in the external granular layer and deep cerebellar nuclei and binds to two high affinity extracellular receptors on Purkinje cells-the Very low density lipoprotein receptor (Vldlr) and apolipoprotein E receptor 2 (Apoer2). In mice null for either Reelin or double null for Vldlr and Apoer2, Purkinje cell clusters fail to disperse. Here we report that animals null for either Vldlr or Apoer2 individually, exhibit specific and parasagittally-restricted Purkinje cell ectopias. For example, in mice lacking Apoer2 function immunostaining reveals ectopic Purkinje cells that are largely restricted to the zebrin II-immunonegative population of the anterior vermis. In contrast, mice null for Vldlr have a much larger population of ectopic Purkinje cells that includes members from both the zebrin II-immunonegative and -immunopositive phenotypes. HSP25 immunoreactivity reveals that in Vldlr null animals a large portion of zebrin II-immunopositive ectopic cells are probably destined to become stripes in the central zone (lobules VI–VII). A small population of ectopic zebrin II-immunonegative Purkinje cells is also observed in animals heterozygous for both receptors (Apoer2^+/−: Vldlr^+/−), but no ectopia is present in mice heterozygous for either receptor alone. These results indicate that Apoer2 and Vldlr coordinate the dispersal of distinct, but overlapping subsets of Purkinje cells in the developing cerebellum.

The sense of smell, its signalling pathways, and the dichotomy of cilia and microvilli in olfactory sensory cells

Smell is often regarded as an ancillary perception in primates, who seem so dominated by their sense of vision. In this paper, we will portray some aspects of the significance of olfaction to human life and speculate on what evolutionary factors contribute to keeping it alive. We then outline the functional architecture of olfactory sensory neurons and their signal transduction pathways, which are the primary detectors that render olfactory perception possible. Throughout the phylogenetic tree, olfactory neurons, at their apical tip, are either decorated with cilia or with microvilli. The significance of this dichotomy is unknown. It is generally assumed that mammalian olfactory neurons are of the ciliary type only. The existence of so-called olfactory microvillar cells in mammals, however, is well documented, but their nature remains unclear and their function orphaned. This paper discusses the possibility, that in the main olfactory epithelium of mammals ciliated and microvillar sensory cells exist concurrently. We review evidence related to this hypothesis and ask, what function olfactory microvillar cells might have and what signalling mechanisms they use.

Daily oscillation of phospholipase C beta4 in the mouse suprachiasmatic nucleus

An endogenous biological clock located in the hypothalamic suprachiasmatic nucleus (SCN) regulates the timing of an organism's physiology and behavior. A variety of receptors are found on SCN pacemaker cells which permit the clock mechanism to respond to extra- and intra-SCN chemical messengers. A subset of these receptors is coupled to G-proteins, which when bound, lead to the activation of a variety of intracellular signaling cascades. One common signaling pathway employs the phosphotidylinositol-specific phospholipase C enzyme to increase intracellular calcium levels. A specific isoform of this enzyme, phospholipase C beta4, is of particular interest to circadian biologists because in its absence, mice display a circadian phenotype. Moreover, it has been shown to be associated with receptor types that are involved in clock resetting. Despite compelling data that this enzyme could be a critical component of an intracellular signaling pathway in the SCN, no study to date has investigated the possible oscillation of phospholipase C in any mammalian tissue. In the present study, we analyzed the temporal variation in the number of phospholipase C beta4 immunoreactive cells in the SCN. Herein, we show that PLCbeta4 levels oscillate in the SCN of mice housed in a light:dark photoperiod. Protein levels reached a significant peak during the early night and a trough during the day. The oscillation was considerably damped in the SCN of mice housed in constant dark conditions indicating the cycle is photoperiod-dependent. These data are critical to understanding the temporal regulation of a variety of inputs to the mammalian central circadian clock.

Phospholipase Cβ3 is distributed in both somatodendritic and axonal compartments and localized around perisynapse and smooth endoplasmic reticulum in mouse Purkinje cell subsets

Phospholipase Cbeta3 (PLCbeta3) and PLCbeta4 are the two major isoforms in cerebellar Purkinje cells (PCs), displaying reciprocal expression across the cerebellum. Here, we examined subcellular distribution of PLCbeta3 in the mouse cerebellum by producing specific antibody. PLCbeta3 was detected as a particulate pattern of immunostaining in various PC elements. Like PLCbeta4, PLCbeta3 was richly distributed in somatodendritic compartments, where it was colocalized with molecules constituting the metabotropic glutamate receptor (mGluR1) signalling pathway, i.e. mGluR1alpha, G alpha q/G alpha 11 subunits of G q protein, inositol 1,4,5-trisphosphate receptor IP3R1, Homer1, protein kinase C PKCgamma, and diacylglycerol lipase DAGLalpha. Unlike PLCbeta4, PLCbeta3 was also distributed at low to moderate levels in PC axons, which were intense for IP3R1 and PKCgamma, low for G alpha q/G alpha 11, and negative for mGluR1alpha, Homer1, and DAGLalpha. By immunoelectron microscopy, PLCbeta3 was preferentially localized around the smooth endoplasmic reticulum in spines, dendrites, and axons of PCs, and also accumulated at the perisynapse of parallel fibre-PC synapses. Consistent with the ultrastructural localization, PLCbeta3 was biochemically enriched in the microsomal and postsynaptic density fractions. These results suggest that PLCbeta3 plays a major role in mediating mGluR1-dependent synaptic transmission, plasticity, and integration in PLCbeta3-dominant PCs, through eliciting Ca2+ release, protein phosphorylation, and endocannabinoid production at local somatodendritic compartments. Because PLCbeta3 can be activated by G betagamma subunits liberated from Gi/o and Gs proteins as well, axonal PLCbeta3 seems to modulate the conduction of action potentials through mediating local Ca2+ release and protein phosphorylation upon activation of a variety of G protein-coupled receptors other than mGluR1.

Phospholipase cβ4 expression reveals the continuity of cerebellar topography through development

Mediolateral boundaries divide the mouse cerebellar cortex into four transverse zones, and within each zone the cortex is further subdivided into a symmetrical array of parasagittal stripes. Various expression markers reveal this complexity, and detailed maps have been constructed based on the differential expression of zebrin II/aldolase C in a Purkinje cell subset. Recently, phospholipase (PL) Cbeta4 expression in adult mice was shown to be restricted to, and coextensive with, the zebrin II-immunonegative Purkinje cell subset. The Purkinje cell expression of PLCbeta4 during embryogenesis and postnatal development begins just before birth in a subset of Purkinje cells that are clustered to form a reproducible array of parasagittal stripes. Double label and serial section immunocytochemistry revealed that the early PLCbeta4-immunoreactive clusters in the neonate are complementary to those previously identified by neurogranin expression. The PLCbeta4 expression pattern can be traced continuously from embryo to adult, revealing the continuity of the topographical map from perinatal to adult cerebella. The only exception, as has been seen for other antigenic markers, is that transient PLCbeta4 expression (which subsequently disappears) is seen in some Purkinje cell stripes during the second postnatal week. Furthermore, the data confirm that some adult Purkinje cell stripes are composite in origin, being derived from two or more distinct embryonic clusters. Thus, the zone and stripe topography of the cerebellum is conserved from embryo to adult, confirming that the early- and late-antigenic markers share a common cerebellar topography.

Scotopic visual signaling in the mouse retina is modulated by high-affinity plasma membrane calcium extrusion

Transmission of visual signals at the first retinal synapse is associated with changes in calcium concentration in photoreceptors and bipolar cells. We investigated how loss of plasma membrane Ca2+ ATPase isoform 2 (PMCA2), the calcium transporter isoform with the highest affinity for Ca2+/calmodulin, affects transmission of rod- and cone-mediated responses. PMCA2 expression in the neuroblast layer was observed soon after birth; in the adult, PMCA2 was expressed in inner segments and synaptic terminals of rod photoreceptors, in rod bipolar cells, and in most inner retinal neurons but was absent from cones. To determine the role of PMCA2 in retinal signaling, we compared morphology and light responses of retinas from control mice and deafwaddler dfw2J mice, which lack functional PMCA2 protein. The cytoarchitecture of retinas from control and dfw2J mice was indistinguishable at the light microscope level. Suction electrode recordings revealed no difference in the sensitivity or amplitude of outer segment light responses of control and dfw2J rods. However, rod-mediated ERG b-wave responses in dfw2J mice were approximately 45% smaller and significantly slower than those of control mice. Furthermore, recordings from individual rod bipolar cells showed that the sensitivity of transmission at the rod output synapse was reduced by approximately 50%. No changes in the amplitude or timing of cone-mediated ERG responses were observed. These results suggest that PMCA2-mediated Ca2+ extrusion modulates the amplitude and timing of the high-sensitivity rod pathway to a much greater extent than that of the cone pathway.

Synaptically driven endocannabinoid release requires Ca2+-assisted metabotropic glutamate receptor subtype 1 to phospholipase Cbeta4 signaling cascade in the cerebellum

Endocannabinoids mediate retrograde signaling and modulate synaptic transmission in various regions of the CNS. Depolarization-induced elevation of intracellular Ca2+ concentration causes endocannabinoid-mediated suppression of excitatory/inhibitory synaptic transmission. Activation of G(q/11)-coupled receptors including group I metabotropic glutamate receptors (mGluRs) also causes endocannabinoid-mediated suppression of synaptic transmission. However, precise mechanisms of endocannabinoid production initiated by physiologically relevant synaptic activity remain to be determined. To address this problem, we made whole-cell recordings from Purkinje cells (PCs) in mouse cerebellar slices and examined their excitatory synapses arising from climbing fibers (CFs) and parallel fibers (PFs). We first characterized three distinct modes to induce endocannabinoid release by analyzing CF to PC synapses. The first mode is strong activation of mGluR subtype 1 (mGluR1)-phospholipase C (PLC) beta4 cascade without detectable Ca2+ elevation. The second mode is Ca2+ elevation to a micromolar range without activation of the mGluR1-PLCbeta4 cascade. The third mode is the Ca2+-assisted mGluR1-PLCbeta4 cascade that requires weak mGluR1 activation and Ca2+ elevation to a submicromolar range. By analyzing PF to PC synapses, we show that the third mode is essential for effective endocannabinoid release from PCs by excitatory synaptic activity. Furthermore, our biochemical analysis demonstrates that combined weak mGluR1 activation and mild depolarization in PCs effectively produces 2-arachidonoylglycerol (2-AG), a candidate of endocannabinoid, whereas either stimulus alone did not produce detectable 2-AG. Our results strongly suggest that under physiological conditions, excitatory synaptic inputs to PCs activate the Ca2+-assisted mGluR1-PLCbeta4 cascade, and thereby produce 2-AG, which retrogradely modulates synaptic transmission to PCs.

Synaptic transmission at retinal ribbon synapses

The molecular organization of ribbon synapses in photoreceptors and ON bipolar cells is reviewed in relation to the process of neurotransmitter release. The interactions between ribbon synapse-associated proteins, synaptic vesicle fusion machinery and the voltage-gated calcium channels that gate transmitter release at ribbon synapses are discussed in relation to the process of synaptic vesicle exocytosis. We describe structural and mechanistic specializations that permit the ON bipolar cell to release transmitter at a much higher rate than the photoreceptor does, under in vivo conditions. We also consider the modulation of exocytosis at photoreceptor synapses, with an emphasis on the regulation of calcium channels.

Signalling through phospholipase C beta 4 is not essential for midbrain dopaminergic neuron survival

The most prominent progressive neurodegenerative movement disorder, Parkinson's disease, is attributed to selective loss of dopamine neurons in the substantia nigra pars compacta, resulting in severe deficiency of dopamine. The homeo-domain gene, Pit x 3, is essential for proper development of midbrain dopaminergic neurons in the substantia nigra pars compacta and might be involved in midbrain dopaminergic survival pathways. The mGluR1-signaling downstream-effector phospholipase C beta 4 was identified in a suppression subtractive hybridization screen comparing wild-type and Pit x 3-deficient Aphakia midbrain dopaminergic neurons. Expression pattern analysis revealed that phospholipase C beta 4 was expressed in midbrain dopaminergic neurons of the substantia nigra pars compacta and part of the ventral tegmental area, whereas expression of mGluR1alpha was predominantly observed in the more vulnerable midbrain dopaminergic neurons in the lateral substantia nigra pars compacta. However, clear expression of phospholipase C beta 4 in spared midbrain dopaminergic neurons of Aphakia mice located in the ventral tegmental area, indicated that induction and maintenance of phospholipase C beta 4 expression is Pit x 3-independent in these neurons. Furthermore, we report here a normal distribution of midbrain dopaminergic cell bodies and axonal projection to the striatum in phospholipase C beta 4-/- mice, indicating that signaling of phospholipase C beta 4 is not essential for the survival of midbrain dopaminergic neurons.

Serca isoform expression in the mammalian retina

The sarcoplasmic-endoplasmic reticulum calcium ATPase (SERCA) is a key intracellular calcium transporter, which regulates cellular calcium concentration [Ca2+] by transporting Ca2+ ions from the cytosol into the endoplasmic reticulum. SERCA-mediated Ca2+ sequestration controls proper folding of newly synthesized proteins within the ER as well as the timing and spatial patterning of depolarization-evoked Ca2+ responses in the cytoplasm. To understand the spatial and temporal properties of Ca2+ homeostasis in retinal neurons better, I studied expression and distribution of all three SERCA isoforms in the mouse retina using isoform-specific antibodies. No immunostaining was observed with the SERCA1 antibody. SERCA2 was expressed in photoreceptor inner segments, amacrine and ganglion cells of the mouse retina. Similar SERCA2 localization was observed in adult rat, macaque and ground squirrel retinas. Analysis of distribution of SERCA2 immunofluorescence in the developing mouse retina revealed prominent SERCA2 signals throughout postnatal development. The N89 antibodys used to identify the SERCA3 isoforms labelled cone outer segments, inner segments of photoreceptors and cell processes in the inner nuclear layer of the mouse retina. These results imply that the SERCA2 isoform controls Ca2+ sequestration into the endoplasmic reticulum in most classes of retinal neuron. A potential role for SERCA3 in cone function is suggested.

Signaling complex formation of phospholipase Cβ4 with metabotropic glutamate receptor type 1α and 1,4,5-trisphosphate receptor at the perisynapse and endoplasmic reticulum in the mouse brain

Upon activation of cell surface receptors coupled to the Gq subclass of G proteins, phospholipase C (PLC) beta hydrolyses membrane phospholipid to yield a pair of second messengers, inositol 1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol. PLCbeta4 has been characterized as the isoform enriched in cerebellar Purkinje cells (PCs) and the retina and involved in motor and visual functions. Here we examined cellular and subcellular distributions of PLCbeta4 in adult mouse brains. Immunohistochemistry showed that high levels of PLCbeta4 were detected in the somatodendritic domain of neuronal populations expressing the metabotropic glutamate receptor (mGluR) type 1alpha, including olfactory periglomerular cells, neurons in the bed nucleus anterior commissure, thalamus, substantia nigra, inferior olive, and unipolar brush cells and PCs in the cerebellum. Low to moderate levels were detected in many other mGluR1alpha-positive neurons and in a few mGluR1alpha-negative neurons. In PCs, immunogold electron microscopy localized PLCbeta4 to the perisynapse, at which mGluR1alpha is concentrated, and to the smooth endoplasmic reticulum in dendrites and spines, an intracellular Ca2+ store gated by IP3 receptors. In the cerebellum, immunoblot demonstrated its concentrated distribution in the post-synaptic density and microsomal fractions, where mGluR1alpha and type 1 IP3 receptor were also greatly enriched. Furthermore, PLCbeta4 formed coimmunoprecipitable complexes with mGluR1alpha, type 1 IP3 receptor and Homer 1. These results suggest that PLCbeta4 is preferentially localized in the perisynapse and smooth endoplasmic reticulum as a component of the physically linked phosphoinositide signaling complex. This close molecular relationship might provide PLCbeta4 with a high-fidelity effector function to mediate various neuronal responses under physiological and pathophysiological conditions.

Expression of calcium transporters in the retina of the tiger salamander (Ambystoma tigrinum)

Changes in intracellular calcium concentration, [Ca2+]i, modulate the flow of visual signals across all stages of processing in the retina, yet the identities of Ca2+ transporters responsible for these changes are still largely unknown. In the current study, the distribution of plasma membrane and intracellular Ca2+ transporters in the retina of tiger salamander, a model system for physiological studies of retinal function, was determined. Plasma membrane calcium ATPases (PMCAs), responsible for high-affinity Ca2+ extrusion, were highly expressed in the salamander retina. PMCA isoforms 1, 2, and 4 were localized to photoreceptors, whereas the inner retina expressed all four isoforms. PMCA3 was expressed in a sparse population of amacrine and ganglion neurons, whereas PMCA2 was expressed in most amacrine and ganglion cells. Na+/Ca2+ exchangers, a high-capacity Ca2+ extrusion system, were expressed in the outer plexiform layer and in a subset of inner nuclear and ganglion layer cells. Intracellular Ca2+ store transporters were also represented prominently. SERCA2a, a splice variant of the sarcoplasmic-endoplasmic Ca2+ ATPase, was found mostly in photoreceptors, whereas SERCA2b was found in the majority of retinal neurons and in glial cells. The predominant endoplasmic reticulum (ER) Ca2+ channels in the salamander retina are represented by the isoform 2 of the IP3 receptor family and the isoform 2 of the ryanodine receptor family. These results indicate that Ca2+ transporters in the salamander retina are expressed in a cell type-specific manner.

A potentially critical role of phospholipases in central nervous system ischemic, traumatic, and neurodegenerative disorders

Phospholipases are a diverse group of enzymes whose activation may be responsible for the development of injury following insult to the brain. Amongst the numerous isoforms of phospholipase proteins expressed in mammals are 19 different phospholipase A2's (PLA2s), classified functionally as either secretory, calcium dependent, or calcium independent, 11 isozymes belonging to three structural groups of PLC, and 3 PLD gene products. Many of these phospholipases have been identified in selected brain regions. Under normal conditions, these enzymes regulate the turnover of free fatty acids (FFAs) in membrane phospholipids affecting membrane stability, fluidity, and transport processes. The measurement of free fatty acids thus provides a convenient method to follow phospholipase activity and their regulation. Phospholipase activity is also responsible for the generation of an extensive list of intracellular messengers including arachidonic acid metabolites. Phospholipases are regulated by many factors including selective phosphorylation, intracellular calcium and pH. However, under abnormal conditions, excessive phospholipase activation, along with a decreased ability to resynthesize membrane phospholipids, can lead to the generation of free radicals, excitotoxicity, mitochondrial dysfunction, and apoptosis/necrosis. This review evaluates the critical contribution of the various phospholipases to brain injury following ischemia and trauma and in neurodegenerative diseases.

Role of thalamic phospholipase C[beta]4 mediated by metabotropic glutamate receptor type 1 in inflammatory pain

Phospholipase C (PLC) beta4, one of the four isoforms of PLCbetas, is the sole isoform expressed in the mouse ventral posterolateral thalamic nucleus (VPL), a key station in pain processing. The mouse thalamus also has been shown to express a high level of metabotropic glutamate receptor type 1 (mGluR1), which stimulates PLCbetas through activation of Galphaq/11 protein. It is therefore expected that the thalamic mGluR1-PLCbeta4 cascade may play a functional role in nociceptive transmission. To test this hypothesis, we first studied behavioral responses to various nociceptive stimuli in PLCbeta4 knock-out mice. We performed the formalin test and found no difference in the pain behavior in the first phase of the formalin test, which is attributed to acute nociception, between PLCbeta4 knock-out and wild-type mice. Consistent with this result, acute pain responses in the hot plate and tail flick tests were also unaffected in the PLCbeta4 knock-out mice. However, the nociceptive behavior in the second phase of the formalin test, resulting from the tissue inflammation, was attenuated in PLCbeta4 knock-out mice. In the dorsal horn of the spinal cord where PLCbeta1 and PLCbeta4 mRNAs are expressed, no difference was found between the wild-type and knock-out mice in the number of Fos-like immunoreactive neurons, which represent neuronal activity in the second phase in the formalin test. Thus, it is unlikely that spinal PLCbeta4 is involved in the formalin-induced inflammatory pain. Next, we found that pretreatment with PLC inhibitors, mGluR1 antagonists, or both, by either intracerebroventricular or intrathalamic injection, attenuated the formalin-induced pain behavior in the second phase in wild-type mice. Furthermore, activation of mGluR1 at the VPL enhanced pain behavior in the second phase in the wild-type mice. In contrast, PLCbeta4 knock-out mice did not show such enhancement, indicating that mGluR1 is connected to PLCbeta4 in the VPL. Finally, in parallel with the behavioral results, we showed in an electrophysiological study that the time course of firing discharges in VPL corresponds well to that of pain behavior in the formalin test in both wild-type and PLCbeta4 knock-out mice. These findings indicate that the thalamic mGluR1-PLCbeta4 cascade is indispensable for the formalin-induced inflammatory pain by regulating the response of VPL neurons.

Role of thalamic phospholipase C[beta]4 mediated by metabotropic glutamate receptor type 1 in inflammatory pain

Phospholipase C (PLC) beta4, one of the four isoforms of PLCbetas, is the sole isoform expressed in the mouse ventral posterolateral thalamic nucleus (VPL), a key station in pain processing. The mouse thalamus also has been shown to express a high level of metabotropic glutamate receptor type 1 (mGluR1), which stimulates PLCbetas through activation of Galphaq/11 protein. It is therefore expected that the thalamic mGluR1-PLCbeta4 cascade may play a functional role in nociceptive transmission. To test this hypothesis, we first studied behavioral responses to various nociceptive stimuli in PLCbeta4 knock-out mice. We performed the formalin test and found no difference in the pain behavior in the first phase of the formalin test, which is attributed to acute nociception, between PLCbeta4 knock-out and wild-type mice. Consistent with this result, acute pain responses in the hot plate and tail flick tests were also unaffected in the PLCbeta4 knock-out mice. However, the nociceptive behavior in the second phase of the formalin test, resulting from the tissue inflammation, was attenuated in PLCbeta4 knock-out mice. In the dorsal horn of the spinal cord where PLCbeta1 and PLCbeta4 mRNAs are expressed, no difference was found between the wild-type and knock-out mice in the number of Fos-like immunoreactive neurons, which represent neuronal activity in the second phase in the formalin test. Thus, it is unlikely that spinal PLCbeta4 is involved in the formalin-induced inflammatory pain. Next, we found that pretreatment with PLC inhibitors, mGluR1 antagonists, or both, by either intracerebroventricular or intrathalamic injection, attenuated the formalin-induced pain behavior in the second phase in wild-type mice. Furthermore, activation of mGluR1 at the VPL enhanced pain behavior in the second phase in the wild-type mice. In contrast, PLCbeta4 knock-out mice did not show such enhancement, indicating that mGluR1 is connected to PLCbeta4 in the VPL. Finally, in parallel with the behavioral results, we showed in an electrophysiological study that the time course of firing discharges in VPL corresponds well to that of pain behavior in the formalin test in both wild-type and PLCbeta4 knock-out mice. These findings indicate that the thalamic mGluR1-PLCbeta4 cascade is indispensable for the formalin-induced inflammatory pain by regulating the response of VPL neurons.

Light adaptation through phosphoinositide-regulated translocation of Drosophila visual arrestin

Photoreceptor cells adapt to bright or continuous light, although the molecular mechanisms underlying this phenomenon are incompletely understood. Here, we report a mechanism of light adaptation in Drosophila, which is regulated by phosphoinositides (PIs). We found that light-dependent translocation of arrestin was defective in mutants that disrupt PI metabolism or trafficking. Arrestin bound to PIP(3) in vitro, and mutation of this site delayed arrestin shuttling and resulted in defects in the termination of the light response, which is normally accelerated by prior exposure to light. Disruption of the arrestin/PI interaction also suppressed retinal degeneration caused by excessive endocytosis of rhodopsin/arrestin complexes. These findings indicate that light-dependent trafficking of arrestin is regulated by direct interaction with PIs and is required for light adaptation. Since phospholipase C activity is required for activation of Drosophila phototransduction, these data point to a dual role of PIs in phototransduction.

Electrophysiological analysis of visual function in mutant mice

The mouse has become a key animal model for ocular research. This situation reflects the fact that genes implicated in human retinal disorders or in mammalian retinal function may be readily manipulated in the mouse. Visual electrophysiology provides a means to examine retinal function in mutant mice, and stimulation and recording protocols have been developed that allow the activity of many classes of retinal neurons to be examined and which take into account unique features of the mouse retina. Here, we review the mouse visual electrophysiology literature, covering techniques used to record the mouse electroretinogram and visual evoked potential, and how these have been applied to characterize the functional implications of gene mutation or manipulation in the mouse retina.

Specificity and structural requirements of phospholipase C-beta stimulation by Rho GTPases versus G protein beta gamma dimers

Phospholipase C-beta(2) (PLC beta(2)) is activated both by heterotrimeric G protein alpha- and beta gamma- subunits and by Rho GTPases. In this study, activated Rho GTPases are shown to stimulate PLC beta isozymes with the rank order of PLC beta(2) > PLC beta(3) > or = PLC beta(1). The sensitivity of PLC beta isozymes to Rho GTPases was clearly different from that observed for G protein beta gamma dimers, which decreased in the following order: PLC beta(3) > PLC beta(2) > PLC beta(1) for beta(1)gamma(1/2) and PLC beta(2) > PLC beta(1) >>> PLC beta(3) for beta(5)gamma(2). Rac1 and Rac2 were found to be more potent and efficacious activators of PLC beta(2) than was Cdc42Hs. The stimulation of PLC beta(2) by Rho GTPases and G protein beta gamma dimers was additive, suggesting that PLC beta(2) activation can be augmented by independent regulation of the enzyme by the two stimuli. Using chimeric PLC beta(1)-PLC beta(2) enzymes, beta gamma dimers, and Rho GTPases are shown to require different regions of PLC beta(2) to mediate efficient stimulation of the enzyme. Although the catalytic subdomains X and Y of PLC beta(2) were sufficient for efficient stimulation by beta gamma, the presence of the putative pleckstrin homology domain of PLC beta(2) was absolutely required for the stimulation of the enzyme by Rho GTPases. Taken together, these results identify Rho GTPases as novel PLC beta regulators, which mediate PLC beta isozyme-specific stimulation and are potentially involved in coordinating the activation of PLC beta(2) by extracellular mediators in intact cells.

G protein subunit G gamma 13 is coexpressed with G alpha o, G beta 3, and G beta 4 in retinal ON bipolar cells

We investigated the expression of Ggamma13, a recently discovered G protein subunit, and a selection of Gbeta subunits in retinal bipolar cells, by using a transgenic mouse strain in which green fluorescent protein is strongly expressed in a single type of cone bipolar cell. The cells have ON morphology, and patch-clamp recordings in slices confirmed that they are of the physiological ON type. Immunohistochemistry showed that Ggamma13 is expressed in rod bipolar cells and ON cone bipolar cells, where it is colocalized in the dendrites with Galphaomicron. ON and OFF cone bipolar cells and rod bipolar cells were identified among dissociated cells by their green fluorescence and/or distinct morphology. Hybridization of single-cell polymerase chain reaction products with cDNA probes for G protein subunits Gbeta1 to 5 showed that Gbeta3, Gbeta4, and Ggamma13 are coexpressed in ON bipolar cells but not present in OFF bipolar cells. Gbeta1, 2, and 5 are expressed in partially overlapping subpopulations of cone bipolar cells. Ggamma13 and Gbeta3 and/or Gbeta4, thus, seem selectively to participate in signal transduction by ON bipolar cells.

Calcium regulation in photoreceptors

In this review we describe some of the remarkable and intricate mechanisms through which the calcium ion (Ca2+) contributes to detection, transduction and synaptic transfer of light stimuli in rod and cone photoreceptors. The function of Ca2+ is highly compartmentalized. In the outer segment, Ca2+ controls photoreceptor light adaptation by independently adjusting the gain of phototransduction at several stages in the transduction chain. In the inner segment and synaptic terminal, Ca2+ regulates cells' metabolism, glutamate release, cytoskeletal dynamics, gene expression and cell death. We discuss the mechanisms of Ca2+ entry, buffering, sequestration, release from internal stores and Ca2+ extrusion from both outer and inner segments, showing that these two compartments have little in common with respect to Ca2+ homeostasis. We also investigate the various roles played by Ca2+ as an integrator of intracellular signaling pathways, and emphasize the central role played by Ca2+ as a second messenger in neuromodulation of photoreceptor signaling by extracellular ligands such as dopamine, adenosine and somatostatin. Finally, we review the intimate link between dysfunction in photoreceptor Ca2+ homeostasis and pathologies leading to retinal dysfunction and blindness.

Genetic animal models for retinal degeneration

Inherited retinal degenerations are a common cause of blindness in Western countries. A mechanism for most retinal degenerations is still unknown; hence, a suitable treatment for most of these diseases has yet to be found. Before one can rationally design a treatment, it is necessary to understand the pathway from a gene mutation to the phenotype in patients. Animal models are crucial to understand this process and to develop a treatment. Some naturally occurring animal models are known. However, over the past few years, transgenic engineering has allowed the generation of a rapidly growing number of animal models. In this review, we give an overview of the broad variety of genetic animal models for retinal degeneration.

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.

Phospholipase Cbeta4 and protein kinase Calpha and/or protein kinase CbetaI are involved in the induction of long term depression in cerebellar Purkinje cells

Activation of the type-1 metabotropic glutamate receptor (mGluR1) signaling pathway in the cerebellum involves activation of phospholipase C (PLC) and protein kinase C (PKC) for the induction of cerebellar long term depression (LTD). The PLC and PKC isoforms that are involved in LTD remain unclear, however. One previous study found no change in LTD in PKCgamma-deficient mice, thus, in the present study, we examined cerebellar LTD in PLCbeta4-deficient mice. Immunohistochemical and Western blot analyses of cerebellum from wild-type mice revealed that PLCbeta1 was expressed weakly and uniformly, PLCbeta2 was not detected, PLCbeta3 was expressed predominantly in caudal cerebellum (lobes 7-10), and PLCbeta4 was expressed uniformly throughout. In PLCbeta4-deficient mice, expression of total PLCbeta, the mGluR1-mediated Ca(2+) response, and LTD induction were greatly reduced in rostral cerebellum (lobes 1-6). Furthermore, we used immunohistochemistry to localize PKCalpha, -betaI, -betaII, and -gamma in mouse cerebellar Purkinje cells during LTD induction. Both PKCalpha and PKCbetaI were found to be translocated to the plasmamembrane under these conditions. Taken together, these results suggest that mGluR1-mediated activation of PLCbeta4 in rostral cerebellar Purkinje cells induced LTD via PKCalpha and/or PKCbetaI.

Deficient long-term synaptic depression in the rostral cerebellum correlated with impaired motor learning in phospholipase C beta4 mutant mice

Long-term depression (LTD) at parallel fibre-Purkinje cell synapse of the cerebellum is thought to be a cellular substrate for motor learning. LTD requires activation of metabotropic glutamate receptor subtype 1 (mGluR1) and its downstream signalling pathways, which invariably involves phospholipase Cbetas (PLCbetas). PLCbetas consist of four isoforms (PLCbeta1-4) among which PLCbeta4 is the major isoform in most Purkinje cells in the rostral cerebellum (lobule 1 to the rostral half of lobule 6). We studied mutant mice deficient in PLCbeta4, and found that LTD was deficient in the rostral but not in the caudal cerebellum of the mutant. Basic properties of parallel fibre-Purkinje cell synapses and voltage-gated Ca2+ channel currents appeared normal. The mGluR1-mediated Ca2+ release induced by repetitive parallel fibre stimulation was absent in the rostral cerebellum of the mutant, suggesting that their LTD lesion was due to the defect in the mGluR1-mediated signalling in Purkinje cells. Importantly, the eyeblink conditioning, a simple form of discrete motor learning, was severely impaired in PLCbeta4 mutant mice. Wild-type mice developed the conditioned eyeblink response, when pairs of the conditioned stimulus (tone) and the unconditioned stimulus (periorbital shock) were repeatedly applied. In contrast, PLCbeta4 mutant mice could not learn the association between the conditioned and unconditioned stimuli, although their behavioural responses to the tone or to the periorbital shock appeared normal. These results strongly suggest that PLCbeta4 is essential for LTD in the rostral cerebellum, which may be required for the acuisition of the conditioned eyeblink response.

Roles of phospholipase Cbeta4 in synapse elimination and plasticity in developing and mature cerebellum

The beta isoforms of phospholipase C (PLCbetas) are thought to mediate signals from metabotropic glutamate receptor subtype 1 (mGluR1) that is crucial for the modulation of synaptic transmission and plasticity. Among four PLCbeta isoforms, PLCbeta4 is one of the two major isoforms expressed in cerebellar Purkinje cells. The authors have studied the roles of PLCbeta4 by analyzing PLCbeta4 knockout mice, which are viable, but exhibit locomotor ataxia. Their cerebellar histology, parallel fiber synapse formation, and basic electrophysiology appear normal. However, developmental elimination of multiple climbing fiber innervation is clearly impaired in the rostral portion of the cerebellar vermis, where PLCbeta4 mRNA is predominantly expressed in the wild-type mice. In the adult, long-term depression is deficient at parallel fiber to Purkinje cell synapses in the rostral cerebellum of the PLCbeta4 knockout mice. The impairment of climbing fiber synapse elimination and the loss of long-term depression are similar to those seen in mice defective in mGluR1, Galphaq, or protein kinase C. Thus, the authors' results strongly suggest that PLCbeta4 is part of a signaling pathway, including the mGluR1, Galphaq and protein kinase C, which is crucial for both climbing fiber synapse elimination in the developing cerebellum and long-term depression induction in the mature cerebellum.

Calcium responses to thyrotropin-releasing hormone, gonadotropin-releasing hormone and somatostatin in phospholipase css3 knockout mice

These studies examined the importance of phospholipase Cbeta (PLCbeta) in the calcium responses of pituitary cells using PLCbeta3 knockout mice. Pituitary tissue from wild-type mice contained PLCbeta1 and PLCbeta3 but not PLCbeta2 or PLCbeta4. Both Galphaq/11 and Gbetagamma can activate PLCbeta3, whereas only Galphaq/11 activates PLCss1 effectively. In knockout mice, PLCbeta3 was absent, PLCbeta1 was not up-regulated, and PLCbeta2 and PLCbeta4 were not expressed. Since somatostatin inhibited influx of extracellular calcium in pituitary cells from wild-type and PLCbeta3 knockout mice, the somatostatin signal pathway was intact. However, somatostatin failed to increase intracellular calcium in pituitary cells from either wild-type or knockout mice under a variety of conditions, indicating that it did not stimulate PLCbeta3. In contrast, somatostatin increased intracellular calcium in aortic smooth muscle cells from wild-type mice, although it evoked no calcium response in cells from PLCbeta3 knockout animals These results show that somatostatin, like other Gi/Go-linked hormones, can stimulate a calcium transient by activating PLCbeta3 through Gbetagamma, but this response does not normally occur in pituitary cells. The densities of Gi and Go, as well as the relative concentrations of PLCbeta1 and PLCbeta3, were similar in cells that responded to somatostatin with an increase in calcium and pituitary cells. Calcium responses to 1 nM and 1 microM TRH and GnRH were identical in pituitary cells from wild-type and PLCbeta3 knockout mice, as were responses to other Gq-linked agonists. These results show that in pituitary cells, PLCbeta1 is sufficient to transmit signals from Gq-coupled hormones, whereas PLCbeta3 is required for the calcium-mobilizing actions of somatostatin observed in smooth muscle cells.

Adaptation in vertebrate photoreceptors

When light is absorbed within the outer segment of a vertebrate photoreceptor, the conformation of the photopigment rhodopsin is altered to produce an activated photoproduct called metarhodopsin II or Rh(*). Rh(*) initiates a transduction cascade similar to that for metabotropic synaptic receptors and many hormones; the Rh(*) activates a heterotrimeric G protein, which in turn stimulates an effector enzyme, a cyclic nucleotide phosphodiesterase. The phosphodiesterase then hydrolyzes cGMP, and the decrease in the concentration of free cGMP reduces the probability of opening of channels in the outer segment plasma membrane, producing the electrical response of the cell. Photoreceptor transduction can be modulated by changes in the mean light level. This process, called light adaptation (or background adaptation), maintains the working range of the transduction cascade within a physiologically useful region of light intensities. There is increasing evidence that the second messenger responsible for the modulation of the transduction cascade during background adaptation is primarily, if not exclusively, Ca(2+), whose intracellular free concentration is decreased by illumination. The change in free Ca(2+) is believed to have a variety of effects on the transduction mechanism, including modulation of the rate of the guanylyl cyclase and rhodopsin kinase, alteration of the gain of the transduction cascade, and regulation of the affinity of the outer segment channels for cGMP. The sensitivity of the photoreceptor is also reduced by previous exposure to light bright enough to bleach a substantial fraction of the photopigment in the outer segment. This form of desensitization, called bleaching adaptation (the recovery from which is known as dark adaptation), seems largely to be due to an activation of the transduction cascade by some form of bleached pigment. The bleached pigment appears to activate the G protein transducin directly, although with a gain less than Rh(*). The resulting decrease in intracellular Ca(2+) then modulates the transduction cascade, by a mechanism very similar to the one responsible for altering sensitivity during background adaptation.

Structure, function, and control of phosphoinositide-specific phospholipase C

Phosphoinositide-specific phospholipase C (PLC) subtypes beta, gamma, and delta comprise a related group of multidomain phosphodiesterases that cleave the polar head groups from inositol lipids. Activated by all classes of cell surface receptor, these enzymes generate the ubiquitous second messengers inositol 1,4, 5-trisphosphate and diacylglycerol. The last 5 years have seen remarkable advances in our understanding of the molecular and biological facets of PLCs. New insights into their multidomain arrangement and catalytic mechanism have been gained from crystallographic studies of PLC-delta(1), while new modes of controlling PLC activity have been uncovered in cellular studies. Most notable is the realization that PLC-beta, -gamma, and -delta isoforms act in concert, each contributing to a specific aspect of the cellular response. Clues to their true biological roles were also obtained. Long assumed to function broadly in calcium-regulated processes, genetic studies in yeast, slime molds, plants, flies, and mammals point to specific and conditional roles for each PLC isoform in cell signaling and development. In this review we consider each subtype of PLC in organisms ranging from yeast to mammals and discuss their molecular regulation and biological function.

Immunological characterization of 130 kDa phospholipase C-beta 4 isozyme in rat cerebellar Purkinje cells

We have developed a monoclonal antibody, beta4-22-9 that recognizes an epitope on pleckstrin homology (PH) domain (1-136 amino acids) located at the amino terminus of rat 130 kDa phospholipase C-beta4 (PLC-beta4). Tissue distribution of 130 kDa PLC-beta4 was determined with this monoclonal antibody. One hundred and thirty kiloDalton PLC-beta4 was exclusively expressed in the nervous tissues including cerebellum and cerebrum, especially at very high levels in cerebellum. The expression of 130 kDa PLC-beta4 in cerebellum was gradually increased from 5 days after birth to adulthood. In the adult cerebellum, especially high level of 130 kDa PLC-beta4 was localized on the periphery of Purkinje cells in the patch-like pattern. These data implies that 130 kDa PLC-beta4 may a role in cerebellar Purkinje cells.

Roles of phospholipid signaling in chemoattractant-induced responses

Chemoattractants, including chemokines, play a central role in regulation of inflammatory reactions by attracting and activating leukocytes. These molecules have been found to regulate metabolism of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) via phospholipase C (PLC) and phosphoinositide 3-kinase (PI3K). Recent studies of mouse lines that lack PLC-(beta)2, PLC-(beta)3, or PI3K(gamma) demonstrate that chemoattractants act through PLC-(beta)2 and PLC-(beta)3 to hydrolyze PtdIns(4,5)P(2) and through PI3K(gamma) to phosphorylate PtdIns(4,5)P(2) in mouse neutrophils. These studies also confirmed the importance and revealed new roles of these signaling pathways in chemoattractant-induced responses.

Phospholipase C-gamma as a signal-transducing element

A ubiquitous signaling event in hormonal responses is the phospholipase C (PLC)-catalyzed hydrolysis of phosphatidylinositol 4, 5-bisphosphate to produce the metabolite second messenger molecules inositol 1,4,5-trisphosphate and diacylglycerol. The former provokes a transient increase in intracellular free Ca(2+), while the latter serves as a direct activator of protein kinase C. In tyrosine kinase-dependent signaling pathways this reaction is mediated by the PLC-gamma isozymes. These are direct substrates of many tyrosine kinases in a wide variety of cell types. The mechanism of PLC-gamma activation involves its association with and phosphorylation by receptor and non-receptor tyrosine kinases, as well as interaction with specialized adaptor molecules and, perhaps, other second messenger molecules. However, the biochemistry of PLC-gamma is at a more advanced state than a clear understanding of exactly how this signaling element functions in the generation of a mitogenic response.

Modulation of the light response by cAMP in Drosophila photoreceptors

Phototransduction in Drosophila is mediated by a G-protein-coupled phospholipase C transduction cascade in which each absorbed photon generates a discrete electrical event, the quantum bump. In whole-cell voltage-clamp recordings, cAMP, as well as its nonhydrolyzable and membrane-permeant analogs 8-bromo-cAMP (8-Br-cAMP) and dibutyryl-cAMP, slowed down the macroscopic light response by increasing quantum bump latency, without changes in bump amplitude or duration. In contrast, cGMP or 8-Br-cGMP had no effect on light response amplitude or kinetics. None of the cyclic nucleotides activated any channels in the plasma membrane. The effects of cAMP were mimicked by application of the non-specific phosphodiesterase inhibitor IBMX and the adenylyl cyclase activator forskolin; zaprinast, a specific cGMP-phosphodiesterase inhibitor, was ineffective. Bump latency was also increased by targeted expression of either an activated G(s) alpha subunit, which increased endogenous adenylyl cyclase activity, or an activated catalytic protein kinase A (PKA) subunit. The action of IBMX was blocked by pretreatment with the PKA inhibitor H-89. The effects of cAMP were abolished in mutants of the ninaC gene, suggesting this nonconventional myosin as a possible target for PKA-mediated phosphorylation. Dopamine (10 microM) and octopamine (100 microM) mimicked the effects of cAMP. These results indicate the existence of a G-protein-coupled adenylyl cyclase pathway in Drosophila photoreceptors, which modulates the phospholipase C-based phototransduction cascade.

Modulation of the light response by cAMP in Drosophila photoreceptors

Phototransduction in Drosophila is mediated by a G-protein-coupled phospholipase C transduction cascade in which each absorbed photon generates a discrete electrical event, the quantum bump. In whole-cell voltage-clamp recordings, cAMP, as well as its nonhydrolyzable and membrane-permeant analogs 8-bromo-cAMP (8-Br-cAMP) and dibutyryl-cAMP, slowed down the macroscopic light response by increasing quantum bump latency, without changes in bump amplitude or duration. In contrast, cGMP or 8-Br-cGMP had no effect on light response amplitude or kinetics. None of the cyclic nucleotides activated any channels in the plasma membrane. The effects of cAMP were mimicked by application of the non-specific phosphodiesterase inhibitor IBMX and the adenylyl cyclase activator forskolin; zaprinast, a specific cGMP-phosphodiesterase inhibitor, was ineffective. Bump latency was also increased by targeted expression of either an activated G(s) alpha subunit, which increased endogenous adenylyl cyclase activity, or an activated catalytic protein kinase A (PKA) subunit. The action of IBMX was blocked by pretreatment with the PKA inhibitor H-89. The effects of cAMP were abolished in mutants of the ninaC gene, suggesting this nonconventional myosin as a possible target for PKA-mediated phosphorylation. Dopamine (10 microM) and octopamine (100 microM) mimicked the effects of cAMP. These results indicate the existence of a G-protein-coupled adenylyl cyclase pathway in Drosophila photoreceptors, which modulates the phospholipase C-based phototransduction cascade.

Genetic alteration of phospholipase C beta3 expression modulates behavioral and cellular responses to mu opioids

Morphine and other micro opioids regulate a number of intracellular signaling pathways, including the one mediated by phospholipase C (PLC). By studying PLC beta3-deficient mice, we have established a strong link between PLC and mu opioid-mediated responses at both the behavioral and cellular levels. Mice lacking PLC beta3, when compared with the wild type, exhibited up to a 10-fold decrease in the ED(50) value for morphine in producing antinociception. The reduced ED(50) value was unlikely a result of changes in opioid receptor number or affinity because no differences were found in whole-brain B(max) and K(d) values for mu, kappa, and delta opioid receptors between wild-type and PLC beta3-null mice. We also found that opioid regulation of voltage-sensitive Ca(2+) channels in primary sensory neurons (dorsal root ganglion) was different between the two genotypes. Consistent with the behavioral findings, the specific mu agonist [D-Ala(2),(Me)Phe(4),Gly(ol)(5)]enkephalin (DAMGO) induced a greater whole-cell current reduction in a greater proportion of neurons isolated from the PLC beta3-null mice than from the wild type. In addition, reconstitution of recombinant PLC protein back into PLC beta3-deficient dorsal root ganglion neurons reduced DAMGO responses to those of wild-type neurons. In neurons of both genotypes, activation of protein kinase C with phorbol esters markedly reduced DAMGO-mediated Ca(2+) current reduction. These data demonstrate that PLC beta3 constitutes a significant pathway involved in negative modulation of mu opioid responses, perhaps via protein kinase C, and suggests the possibility that differences in opioid sensitivity among individuals could be, in part, because of genetic factors.

Disruption of a retinal guanylyl cyclase gene leads to cone-specific dystrophy and paradoxical rod behavior

One of two orphan photoreceptor guanylyl cyclases that are highly conserved from fish to mammals, GC-E (or retGC1) was eliminated by gene disruption. Expression of the second retinal cyclase (GC-F) as well as the numbers and morphology of rods remained unchanged in GC-E null mice. However, rods isolated from such mice, despite having a normal dark current, recovered from a light flash markedly faster. Unexpectedly, the a- and b-waves of electroretinograms (ERG) from dark-adapted null mice were suppressed markedly. Cones, initially present in normal numbers in the retina, disappeared by 5 weeks, based on ERG and histology. Thus, the GC-E-deficient mouse defines a model for cone dystrophy, but it also demonstrates that morphologically normal rods display paradoxical behavior in their responses to light.

Disruption of a retinal guanylyl cyclase gene leads to cone-specific dystrophy and paradoxical rod behavior

One of two orphan photoreceptor guanylyl cyclases that are highly conserved from fish to mammals, GC-E (or retGC1) was eliminated by gene disruption. Expression of the second retinal cyclase (GC-F) as well as the numbers and morphology of rods remained unchanged in GC-E null mice. However, rods isolated from such mice, despite having a normal dark current, recovered from a light flash markedly faster. Unexpectedly, the a- and b-waves of electroretinograms (ERG) from dark-adapted null mice were suppressed markedly. Cones, initially present in normal numbers in the retina, disappeared by 5 weeks, based on ERG and histology. Thus, the GC-E-deficient mouse defines a model for cone dystrophy, but it also demonstrates that morphologically normal rods display paradoxical behavior in their responses to light.

A unique isoform of phospholipase Cbeta4 highly expressed in the cerebellum and eye

We report a unique isoform of PLCbeta4 in rat, PLCbeta4c, that has an additional 37-nucleotide exon inserted between nucleotides 3459-3460 of the previously published PLCbeta4a coding sequence. This insertion results in replacement of 22 amino acid residues at the carboxyl terminal tail of PLCbeta4a with 41 unique residues. A human EST for PLCbeta4 also contains this exon and this exon was mapped to within a 5.5 kb intron of the human PLCbeta4 gene. PLCbeta4c is the third PLCbeta4 isoform to be identified which has a unique carboxyl-terminal tail. PLCbeta4b differs from PLCbeta4a by truncation 162 amino acid residues from the carboxyl terminus which are replaced with 10 distinct amino acid residues. Reverse transcription-polymerase chain reaction experiments show that both PLCbeta4a and PLCbeta4c mRNA are expressed throughout the rat brain and that PLCbeta4c mRNA is highly expressed in the eye and cerebellum. RNase protection assays demonstrate that both PLCbeta4a and PLCbeta4c transcripts are abundant in the cerebellum. The different carboxyl terminal tails of PLCbeta4 isoforms may allow for differential targeting and subcellular localization, contributing to regulation of PLC beta4-mediated signal transduction.

Phospholipase cbeta4 is specifically involved in climbing fiber synapse elimination in the developing cerebellum

Elimination of excess climbing fiber (CF)-Purkinje cell synapses during cerebellar development involves a signaling pathway that includes type 1 metabotropic glutamate receptor, Galphaq, and the gamma isoform of protein kinase C. To identify phospholipase C (PLC) isoforms involved in this process, we generated mice deficient in PLCbeta4, one of two major isoforms expressed in Purkinje cells. PLCbeta4 mutant mice are viable but exhibit locomotor ataxia. Their cerebellar histology, parallel fiber synapse formation, and basic electrophysiology appear normal. However, developmental elimination of multiple CF innervation clearly is impaired in the rostral portion of the cerebellar vermis, in which PLCbeta4 mRNA is predominantly expressed. By contrast, CF synapse elimination is normal in the caudal cerebellum, in which low levels of PLCbeta4 mRNA but reciprocally high levels of PLCbeta3 mRNA are found. These results indicate that PLCbeta4 transduces signals that are required for CF synapse elimination in the rostral cerebellum.

Targeted disruption of the mouse phospholipase C beta3 gene results in early embryonic lethality

In order to investigate the biological function of phosphatidylinositol-specific phospholipase C (PLC) we generated mutant mice by gene targeting. Homozygous inactivation of PLCbeta3 is lethal at embryonic day 2.5. These mutants show poor embryonic organization as well as reduced numbers of cells. Identical phenotypes were recorded in homozygous mutants generated from two independently targeted embryonic stem cell clones. Heterozygous mutant mice, however, are viable and fertile for at least two generations. We also showed that mouse PLCbeta3 is expressed in unfertilized eggs, 3-cell and egg cylinder stages of embryos. In conclusion, these results indicate that PLCbeta3 expression is essential for early mouse embryonic development.

Families of phosphoinositide-specific phospholipase C: structure and function

A large number of extracellular signals stimulate hydrolysis of phosphatidylinositol 4,5-bisphosphate by phosphoinositide-specific phospholipase C (PI-PLC). PI-PLC isozymes have been found in a broad spectrum of organisms and although they have common catalytic properties, their regulation involves different signalling pathways. A number of recent studies provided an insight into domain organisation of PI-PLC isozymes and contributed towards better understanding of the structural basis for catalysis, cellular localisation and molecular changes that could underlie the process of their activation.

Expression of a novel murine phospholipase D homolog coincides with late neuronal development in the forebrain

Members of the phospholipase D (PLD) superfamily are defined by the conserved HXKXXXXD motif, which is essential for the catalytic function of mammalian PLD. PLD enzymes are thought to play roles in signal transduction and membrane vesicular trafficking in mammalian cells. Here we describe a 54-kDa novel murine polypeptide (designated SAM-9) that is predicted to be a membrane-associated member of the PLD superfamily. SAM-9 shares 40, 30, and 29% amino acid identity with potential orthologs, in vaccinia virus, Caenorhabditis elegans, and Dictyostelium discoideum, respectively, and belongs to a subclass of PLD homologs in which the second HXKXXXXD motif is imperfect and harbors a conserved Asp to Glu substitution. The sam-9 gene has more than eight exons, and the two HXKXXXXD motifs are encoded by two highly conserved exons. The expression of the sam-9 gene is greater in the brain than in non-nervous tissue and appears to be predominantly of neuronal origin. sam-9 expression is pronounced in mature neurons of the forebrain and appears to be turned on at late stages of neurogenesis as revealed by in situ hybridization analysis of sam-9 expression during postnatal development of the hippocampal formation and the primary somatosensory cortex.

A cytosolic, galphaq- and betagamma-insensitive splice variant of phospholipase C-beta4

Phospholipase C (PLC)-beta4 has been considered to be a mammalian homolog of the NorpA PLC, which is responsible for visual signal transduction in Drosophila. We reported previously the cloning of a cDNA encoding rat phospholipase C-beta4 (PLC-beta4) (Kim, M. J., Bahk, Y. Y., Min, D. S., Lee, S. J., Ryu, S. H., and Suh, P.-G. (1993) Biochem. Biophys. Res. Commun. 194, 706-712). We report now the isolation and characterization of a splice variant (PLC-beta4b). PLC-beta4b is identical to the 130-kDa PLC-beta4 (PLC-beta4a) except that the carboxyl-terminal 162 amino acids of PLC-beta4a are replaced by 10 distinct amino acids. The existence of PLC-beta4b transcripts in the rat brain was demonstrated by reverse transcription-polymerase chain reaction analysis. Immunological analysis using polyclonal antibody specific for PLC-beta4b revealed that this splice variant exists in rat brain cytosol. To investigate functional differences between the two forms of PLC-beta4, transient expression studies in COS-7 cells were conducted. We found that PLC-beta4a was localized mainly in the particulate fraction of the cell, and it could be activated by Galphaq, whereas PLC-beta4b was localized exclusively in the soluble fraction, and it could not be activated by Galphaq. In addition, both PLC-beta4a and PLC-beta4b were not activated by G-protein betagamma-subunits purified from rat brain. These results suggest that PLC-beta4b may be regulated by a mechanism different from that of PLC-beta4a, and therefore it may play a distinct role in PLC-mediated signal transduction.