The 62E early-late puff of Drosophila contains D-spinophilin, an ecdysone-inducible PDZ-domain protein dynamically expressed during metamorphosis

Presynaptic spinophilin tunes neurexin signalling to control active zone architecture and function

Assembly and maturation of synapses at the Drosophila neuromuscular junction (NMJ) depend on trans-synaptic neurexin/neuroligin signalling, which is promoted by the scaffolding protein Syd-1 binding to neurexin. Here we report that the scaffold protein spinophilin binds to the C-terminal portion of neurexin and is needed to limit neurexin/neuroligin signalling by acting antagonistic to Syd-1. Loss of presynaptic spinophilin results in the formation of excess, but atypically small active zones. Neuroligin-1/neurexin-1/Syd-1 levels are increased at spinophilin mutant NMJs, and removal of single copies of the neurexin-1, Syd-1 or neuroligin-1 genes suppresses the spinophilin-active zone phenotype. Evoked transmission is strongly reduced at spinophilin terminals, owing to a severely reduced release probability at individual active zones. We conclude that presynaptic spinophilin fine-tunes neurexin/neuroligin signalling to control active zone number and functionality, thereby optimizing them for action potential-induced exocytosis.

Synaptic assembly depends on trans-synaptic Neurexin/Neuroligin signalling. Here, Muhammad et al. show that Spinophilin, a pre-synaptic scaffolding protein, interacts with Neurexin, in competition with Syd-1, to regulate the formation and function of synaptic active zones at Drosophila neuromuscular junctions.

Structural basis for spinophilin-neurabin receptor interaction

Neurabin and spinophilin are neuronal scaffolding proteins that play important roles in the regulation of synaptic transmission through their ability to target protein phosphatase 1 (PP1) to dendritic spines where PP1 dephosphorylates and inactivates glutamate receptors. However, thus far, it is still unknown how neurabin and spinophilin themselves are targeted to these membrane receptors. Spinophilin and neurabin contain a single PDZ domain, a common protein-protein interaction recognition motif, which are 86% identical in sequence. We report the structures of both the neurabin and spinophilin PDZ domains determined using biomolecular NMR spectroscopy. These proteins form the canonical PDZ domain fold. However, despite their high degree of sequence identity, there are distinct and significant structural differences between them, especially between the peptide binding pockets. Using two-dimensional 1H-15N HSQC NMR analysis, we demonstrate that C-terminal peptide ligands derived from glutamatergic AMPA and NMDA receptors and cytosolic proteins directly and differentially bind spinophilin and neurabin PDZ domains. This peptide binding data also allowed us to classify the neurabin and spinophilin PDZ domains as the first identified neuronal hybrid class V PDZ domains, which are capable of binding both class I and II peptides. Finally, the ability to bind to glutamate receptor subunits suggests that the PDZ domains of neurabin and spinophilin are important for targeting PP1 to C-terminal phosphorylation sites in AMPA and NMDA receptor subunits.

Spinophilin: from partners to functions

Spinophilin/neurabin 2 has been isolated independently by two laboratories as a protein interacting with protein phosphatase 1 (PP1) and F-actin. Gene analysis and biochemical approaches have contributed to define a number of distinct modular domains in spinophilin that govern protein-protein interactions such as two F-actin-, three potential Src homology 3 (SH3)-, a receptor- and a PP1-binding domains, a PSD95/DLG/zo-1 (PDZ) and three coiled-coil domains, and a potential leucine/isoleucine zipper (LIZ) motif. More than 30 partner proteins of spinophilin have been discovered, including cytoskeletal and cell adhesion molecules, enzymes, guanine nucleotide exchange factors (GEF) and regulator of G-protein signalling protein, membrane receptors, ion channels and others proteins like the tumour suppressor ARF. The physiological relevance of some of these interactions remains to be demonstrated. However, spinophilin structure suggests that the protein is a multifunctional protein scaffold that regulates both membrane and cytoskeletal functions. Spinophilin plays important functions in the nervous system where it is implicated in spine morphology and density regulation, synaptic plasticity and neuronal migration. Spinophilin regulates also seven-transmembrane receptor signalling and may provide a link between some of these receptors and intracellular mitogenic signalling events dependent on p70(S6) kinase and Rac G protein-GEF. Strikingly a role for spinophilin in cell growth was demonstrated and this effect was enhanced by its interaction with ARF. Here we review the current knowledge of the protein partners of spinophilin and present the available data that are contributing to the appreciation of spinophilin functions.

Gene dosage and selective expression modify phenotype in a Drosophila model of human mitochondrial disease

Human mitochondrial disease manifests with a wide range of clinical phenotypes of varying severity. To create a model for these disorders, we have manipulated the Drosophila gene technical knockout, encoding mitoribosomal protein S12. Various permutations of endogenous and transgenic alleles create a range of phenotypes, varying from larval developmental arrest through to mild neurological defects in the adult, and also mimic threshold effects associated with human mtDNA disease. Nuclear genetic background influences mutant phenotype by a compensatory mechanism affecting mitochondrial RNA levels. Selective expression of the wild-type allele indicates critical times and cell-types in development, in which mitochondrial protein synthesis deficiency leads to specific phenotypic outcomes.

A genetic screen for dominant modifiers of a cyclin E hypomorphic mutation identifies novel regulators of S-phase entry in Drosophila

Cyclin E together with its kinase partner Cdk2 is a critical regulator of entry into S phase. To identify novel genes that regulate the G1- to S-phase transition within a whole animal we made use of a hypomorphic cyclin E mutation, DmcycEJP, which results in a rough eye phenotype. We screened the X and third chromosome deficiencies, tested candidate genes, and carried out a genetic screen of 55,000 EMS or X-ray-mutagenized flies for second or third chromosome mutations that dominantly modified the DmcycEJP rough eye phenotype. We have focused on the DmcycEJP suppressors, S(DmcycEJP), to identify novel negative regulators of S-phase entry. There are 18 suppressor gene groups with more than one allele and several genes that are represented by only a single allele. All S(DmcycEJP) tested suppress the DmcycEJP rough eye phenotype by increasing the number of S phases in the postmorphogenetic furrow S-phase band. By testing candidates we have identified several modifier genes from the mutagenic screen as well as from the deficiency screen. DmcycEJP suppressor genes fall into the classes of: (1) chromatin remodeling or transcription factors; (2) signaling pathways; and (3) cytoskeletal, (4) cell adhesion, and (5) cytoarchitectural tumor suppressors. The cytoarchitectural tumor suppressors include scribble, lethal-2-giant-larvae (lgl), and discs-large (dlg), loss of function of which leads to neoplastic tumors and disruption of apical-basal cell polarity. We further explored the genetic interactions of scribble with S(DmcycEJP) genes and show that hypomorphic scribble mutants exhibit genetic interactions with lgl, scab (alphaPS3-integrin--cell adhesion), phyllopod (signaling), dEB1 (microtubule-binding protein--cytoskeletal), and moira (chromatin remodeling). These interactions of the cytoarchitectural suppressor gene, scribble, with cell adhesion, signaling, cytoskeletal, and chromatin remodeling genes, suggest that these genes may act in a common pathway to negatively regulate cyclin E or S-phase entry.

Mitochondrial disease in flies

The Drosophila mutant technical knockout (tko), affecting the mitochondrial protein synthetic apparatus, exhibits respiratory chain deficiency and a phenotype resembling various features of mitochondrial disease in humans (paralytic seizures, deafness, developmental retardation). We are using this mutant to analyse the cellular and genomic targets of mitochondrial dysfunction, and to identify ways in which the phenotype can be alleviated. Transgenic expression of wild-type tko in different patterns in the mutant background reveals critical times and cell-types for production of components of the mitochondrial disease-like phenotype. Mitochondrial bioenergy deficit during the period of maximal growth, as well as in specific parts of the nervous system, appears to be most deleterious. Inbreeding of tko mutant lines results in a systematic improvement in all phenotypic parameters tested. The resulting sub-lines can be used for genetic mapping and transcriptomic analysis, revealing clues as to the genes and pathways that can modify mitochondrial disease-like phenotypes in a model metazoan.

Functional diversity of protein phosphatase-1, a cellular economizer and reset button

The protein serine/threonine phosphatase protein phosphatase-1 (PP1) is a ubiquitous eukaryotic enzyme that regulates a variety of cellular processes through the dephosphorylation of dozens of substrates. This multifunctionality of PP1 relies on its association with a host of function-specific targetting and substrate-specifying proteins. In this review we discuss how PP1 affects the biochemistry and physiology of eukaryotic cells. The picture of PP1 that emerges from this analysis is that of a "green" enzyme that promotes the rational use of energy, the recycling of protein factors, and a reversal of the cell to a basal and/or energy-conserving state. Thus PP1 promotes a shift to the more energy-efficient fuels when nutrients are abundant and stimulates the storage of energy in the form of glycogen. PP1 also enables the relaxation of actomyosin fibers, the return to basal patterns of protein synthesis, and the recycling of transcription and splicing factors. In addition, PP1 plays a key role in the recovery from stress but promotes apoptosis when cells are damaged beyond repair. Furthermore, PP1 downregulates ion pumps and transporters in various tissues and ion channels that are involved in the excitation of neurons. Finally, PP1 promotes the exit from mitosis and maintains cells in the G1 or G2 phases of the cell cycle.

The neuronal actin-binding proteins, neurabin I and neurabin II, recruit specific isoforms of protein phosphatase-1 catalytic subunits

Neurabins are protein phosphatase-1 (PP1) targeting subunits that are highly concentrated in dendritic spines and post-synaptic densities. Immunoprecipitation of neurabin I and neurabin II/spinophilin from rat brain extracts sedimented PP1gamma1 and PP1alpha but not PP1beta. In vitro studies showed that recombinant peptides representing central regions of neurabins also preferentially bound PP1gamma1 and PP1alpha from brain extracts and associated poorly with PP1beta. Analysis of PP1 binding to chimeric neurabins suggested that sequences flanking a conserved PP1-binding motif altered their selectivity for PP1beta and their activity as regulators of PP1 in vitro. Assays using recombinant PP1 catalytic subunits and a chimera of PP1 and protein phosphatase-2A indicated that the C-terminal sequences unique to the PP1 isoforms contributed to their recognition by neurabins. Collectively, the results from several different in vitro assays established the rank order of PP1 isoform selection by neurabins to be PP1gamma1 > PP1alpha > PP1beta. This PP1 isoform selectivity was confirmed by immunoprecipitation of neurabin I and II from brain extracts from wild type and mutant PP1gamma null mice. In the absence of PP1gamma1, both neurabins showed enhanced association with PP1alpha but not PP1beta. These studies identified some of the structural determinants in PP1 and neurabins that together contribute to preferential targeting of PP1gamma1 and PP1alpha to the mammalian synapse.

Targeting protein phosphatase 1 (PP1) to the actin cytoskeleton: the neurabin I/PP1 complex regulates cell morphology

Neurabin I, a neuronal actin-binding protein, binds protein phosphatase 1 (PP1) and p70 ribosomal S6 protein kinase (p70S6K), both proteins implicated in cytoskeletal dynamics. We expressed wild-type and mutant neurabins fused to green fluorescent protein in Cos7, HEK293, and hippocampal neurons. Biochemical and cellular studies showed that an N-terminal F-actin-binding domain dictated neurabin I localization at actin cytoskeleton and promoted disassembly of stress fibers. Deletion of the C-terminal coiled-coil and sterile alpha motif domains abolished neurabin I dimerization and induced filopodium extension. Immune complex assays showed that neurabin I recruited an active PP1 via a PP1-docking sequence,(457)KIKF(460). Mutation of the PP1-binding motif or PP1 inhibition by okadaic acid and calyculin A abolished filopodia and restored stress fibers in cells expressing neurabin I. In vitro and in vivo studies suggested that the actin-binding domain attenuated protein kinase A (PKA) phosphorylation of neurabin I. Modification of a major PKA site, serine-461, impaired PP1 binding. Finally, p70S6K was excluded from neurabin I/PP1 complexes and required the displacement of PP1 for recruitment to neurabin I. These studies provided new insights into the assembly and regulation of a neurabin I/PP1 complex that controls actin rearrangement to promote spine development in mammalian neurons.

Regulator-driven functional diversification of protein phosphatase-1 in eukaryotic evolution

We have used the (nearly) completed eukaryotic genome sequences to trace the evolution of thirteen families of established vertebrate regulators of type-1 protein phosphatases (PP1). Two of these families are present in all lineages of the eukaryotic crown and therefore qualify as candidate primordial regulators that determined the surface of PP1. The set of regulators of PP1 has continued to expand ever since, often in response to functional innovations in different eukaryotic lineages. In particular, the development of metazoan multicellularity was accompanied by an explosive increase in the number of regulators of PP1. The further increase in the functional diversity of PP1 in the vertebrate lineage was mainly achieved by the duplication of genes for regulatory subunits and by the conversion of already existing proteins into regulators of PP1. Unexpectedly, our analysis has also enabled us to classify nine poorly characterized proteins as likely regulators of PP1.

Ecdysone-regulated puff genes 2000

The Ashburner model for the hormonal control of polytene chromosome puffing has provided a strong foundation for understanding the basic mechanisms of steroid-regulated gene expression (Cold Spring Harbor Symp. Quant. Biol. 38 (1974) 655). According to this model, the steroid hormone 20-hydroxyecdysone (referred here as ecdysone) directly induces the expression of a small set of early regulatory genes. These genes, in turn, induce a much larger set of late target genes that play a more direct role in controlling the biological responses to the hormone. The recent characterization of two early puff genes, E63-1 and E23, and three late puff genes, D-spinophilin, L63, and L82, provide further confirmation of the Ashburner model. In addition, these studies provide exciting new directions for our understanding of ecdysone signaling. Overexpression studies of E63-1 implicate this gene in directing calcium-dependent salivary gland glue secretion. In contrast, overexpression of E23 indicates that this ABC transporter family member may negatively regulate ecdysone signaling by actively transporting the hormone out of target cells. Finally, genetic studies of the L63 and L82 late genes reveal unexpected possible functions for ecdysone in controlling developmental timing and growth. This review surveys the recent characterization of these ecdysone-inducible genes and provides an overview of how they expand our understanding of ecdysone functions during development.