Developmental and tissue expression patterns of mouse Mpp4 gene

Cone-rod homeobox CRX controls presynaptic active zone formation in photoreceptors of mammalian retina

In the mammalian retina, rod and cone photoreceptors transmit the visual information to bipolar neurons through highly specialized ribbon synapses. We have limited understanding of regulatory pathways that guide morphogenesis and organization of photoreceptor presynaptic architecture in the developing retina. While neural retina leucine zipper (NRL) transcription factor determines rod cell fate and function, cone-rod homeobox (CRX) controls the expression of both rod- and cone-specific genes and is critical for terminal differentiation of photoreceptors. A comprehensive immunohistochemical evaluation of Crx-/- (null), CrxRip/+ and CrxRip/Rip (models of dominant congenital blindness) mouse retinas revealed abnormal photoreceptor synapses, with atypical ribbon shape, number and length. Integrated analysis of retinal transcriptomes of Crx-mutants with CRX- and NRL-ChIP-Seq data identified a subset of differentially expressed CRX target genes that encode presynaptic proteins associated with the cytomatrix active zone (CAZ) and synaptic vesicles. Immunohistochemistry of Crx-mutant retina validated aberrant expression of REEP6, PSD95, MPP4, UNC119, UNC13, RGS7 and RGS11, with some reduction in Ribeye and no significant change in immunostaining of RIMS1, RIMS2, Bassoon and Pikachurin. Our studies demonstrate that CRX controls the establishment of CAZ and anchoring of ribbons, but not the formation of ribbon itself, in photoreceptor presynaptic terminals.

Comparison of proton and electron radiation effects on biological responses in liver, spleen and blood

PURPOSE

To determine whether differences exist between proton and electron radiations on biological responses after total-body exposure.

MATERIALS AND METHODS

ICR mice (n=45) were irradiated to 2 Gray (Gy) using fully modulated 70 MeV protons (0.5 Gy/min) and 21 MeV electrons (3 Gy/min). At 36 h post-irradiation liver gene expression, white blood cell (WBC), natural killer (NK) cell and other analyses were performed.

RESULTS

Oxidative stress-related gene expression patterns were strikingly different for irradiated groups compared to 0 Gy (P<0.05). Proton radiation up-regulated 15 genes (Ctsb, Dnm2, Gpx5, Il19, Il22, Kif9, Lpo, Nox4, Park7, Prdx4, Prdx6, Rag2, Sod3, Srxn1, Xpa) and down-regulated 2 genes (Apoe, Prdx1). After electron irradiation, 20 genes were up-regulated (Aass, Ctsb, Dnm2, Gpx1, Gpx4, Gpx5, Gpx6, Gstk1, Il22, Kif9, Lpo, Nox4, Park7, Prdx3, Prdx4, Prdx5, Rag2, Sod1, Txnrd3, Xpa) and 1 was down-regulated (Mpp4). Of the modified genes, only 11 were common to both forms of radiation. Comparison between the two irradiated groups showed that electrons significantly up-regulated three genes (Gstk1, Prdx3, Scd1). Numbers of WBC and major leukocyte types were low in the irradiated groups (P<0.001 vs. 0 Gy). Hemoglobin and platelet counts were low in the electron-irradiated group (P<0.05 vs. 0 Gy). However, spleens from electron-irradiated mice had higher WBC and lymphocyte counts, as well as enhanced NK cell cytotoxicity, compared to animals exposed to protons (P<0.05). There were no differences between the two irradiated groups in body mass, organ masses, and other assessed parameters, although some differences were noted compared to 0 Gy.

CONCLUSION

Collectively, the data demonstrate that at least some biological effects induced by electrons may not be directly extrapolated to protons.

A mitochondria-localized glutamic acid-rich protein (MGARP/OSAP) is highly expressed in retina that exhibits a large area of intrinsic disorder

Study of retina specific genes would offer insights into retinal diseases and treatment. Based on the information from the gene expression profiles of mouse retinas, we here identified a mitochondria-localized glutamic acid-rich protein (MGARP/OSAP) as one of the highly expressed proteins in retina. Sequence analysis revealed that mouse and rat MGARPs have an extra insertion of four consecutive amino acid repeats at the C-terminus, while other homologues do not. MGARP was demonstrated to be localized to the mitochondria and overexpression of MGARP missing N-terminal region causes severe mitochondrial aggregation, implying an important role of MGARP in maintaining mitochondrial morphology. MGARP is highly expressed in mitochondria-rich layers, including inner segment of the photoreceptor, outer plexiform layer and ganglion cell layers of mouse retina. Far-UV CD spectrum analysis suggested that MGARP exhibits a large area of intrinsic disorder and the unusual position of its Tyr fluorescence suggested that Tyr residues in MGARP might form excimer and exist in an ionized state. These findings implied that MGARP be a good candidate for assembling certain ion channels on mitochondria membrane and have great potential to be involved in retinal energetic metabolism through mitochondria related pathway.

Proteomic analysis of the membrane palmitoylated protein-4 (MPP4)-associated protein complex in the retina

Membrane palmitoylated protein-4 (MPP4) is a retina-specific scaffolding protein of the membrane-associated guanylate kinase family that has been implicated in organizing presynaptic protein complexes in the photoreceptor ribbon synapse. To isolate the components of this complex we applied a proteomic approach based on immunoaffinity chromatography with a monoclonal anti-MPP4 antibody followed by two-dimensional electrophoresis and mass spectrometry. Among the identified molecules were previously reported proteins of the MPP4 scaffolding complex including adaptor proteins Veli3 and Psd95. Here we demonstrate a selective association between MPP4 and the Psd95-beta isoform that is mediated by interaction of their N-terminal L27 domains. In addition, we have identified recoverin and Hsc70 as novel associated proteins of the MPP4 multiprotein complex in the retina. This study demonstrates the utility of anti-MPP4 antibody precipitation for the elucidation of the MPP4-associated protein complex, which is essential in understanding its precise role in signal transmission at the photoreceptor synapse.

Comparison of gene expression during in vivo and in vitro postnatal retina development

Retina explants are widely used as a model of neural development. To define the molecular basis of differences between the development of retina in vivo and in vitro during the early postnatal period, we carried out a series of microarray comparisons using mouse retinas. About 75% of 8,880 expressed genes from retina explants kept the same expression volume and pattern as the retina in vivo. Fewer than 6% of the total gene population was changed at two consecutive time points, and only about 1% genes showed more than a threefold change at any time point studied. Functional Gene Ontology (GO) mapping for both changed and unchanged genes showed similar distribution patterns, except that more genes were changed in the GO clusters of response to stimuli and carbohydrate metabolism. Three distinct expression patterns of genes preferentially expressed in rod photoreceptors were observed in the retina explants. Some genes showed a lag in increased expression, some showed no change, and some continued to have a reduced level of expression. An early downregulation of cyclin D1 in the explanted retina might explain the reduction in numbers of precursors in explanted retina and suggests that external factors are required for maintenance of cyclin D1. The global view of gene profiles presented in this study will help define the molecular changes in retina explants over time and will provide criteria to define future changes that improve this model system.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s12177-008-9009-z) contains supplementary material, which is available to authorized users.

Composition and function of the Crumbs protein complex in the mammalian retina

The Crumbs proteins (CRBs) are transmembrane proteins, homologous to Drosophila Crumbs, with a key role in defining the apical membrane domain in photoreceptors as well as in embryonic epithelia. Crumbs proteins are conserved between species and their intracellular domains are involved in organizing a conserved macromolecular protein scaffold with important roles in cell polarity as well as morphogenesis and maintenance of the retina. Mutations in the gene encoding human CRB1, the first one identified out of the three human orthologs, have been associated with a number of retinal dystrophies including Leber amaurosis and retinitis pigmentosa type 12. Although no other mammalian Crumbs complex members as of yet have been associated with retinal degeneration, disruption of different zebrafish and fruitfly orthologs can lead to various retinal defects. The core Crumbs complex localizes apical to the outer limiting membrane, where photoreceptors and Müller glia contact each other. Correct functioning of Crumbs ensures adhesion between these cells by an unknown mechanism. This review summarizes the current view on the composition and function of the Crumbs prsotein complex in the mammalian retina. Recently, a number of new members of the Crumbs protein complex have been identified. These include most members of the membrane palmitoylated protein family (MPP), involved in assembly of macromolecular protein complexes. Some components of the complex are found to exert a function in the photoreceptor synapses and/or at the region of the connecting cilium. Studies using polarized cell cultures or model organisms, like Drosophila and zebrafish, suggest important links of the Crumbs protein complex to several biological processes in the mammalian eye, including retinal patterning, ciliogenesis and vesicular transport.

Mpp4 is required for proper localization of plasma membrane calcium ATPases and maintenance of calcium homeostasis at the rod photoreceptor synaptic terminals

Membrane palmitoylated protein 4 (Mpp4) is a member of the membrane-associated guanylate kinase family. We show that Mpp4 localizes specifically to the plasma membrane of photoreceptor synaptic terminals. Plasma membrane Ca(2+) ATPases (PMCAs), the Ca(2+) extrusion pumps, interact with an Mpp4-dependent presynaptic membrane protein complex that includes Veli3 and PSD95. In mice lacking Mpp4, PMCAs were lost from rod photoreceptor presynaptic membranes. Synaptic ribbons were enlarged, a phenomenon known to correlate with higher Ca(2+). SERCA2 (sarcoplasmic-endoplasmic reticulum Ca(2+) ATPase, type 2), which pumps cytosolic Ca(2+) into intracellular Ca(2+) stores and localizes next to the ribbons, was increased. The distribution of IP(3)RII (InsP(3) receptor, type 2), which releases Ca(2+) from the stores, was shifted away from the synaptic terminals. Synaptic transmission to second-order neurons was maintained but was reduced in amplitude. These data suggest that loss of Mpp4 disrupts a Ca(2+) extrusion mechanism at the presynaptic membranes, with ensuing adaptive responses by the photoreceptor to restore Ca(2+) homeostasis. We propose that Mpp4 organizes a presynaptic protein complex that includes PMCAs and has a role in modulating Ca(2+) homeostasis and synaptic transmission in rod photoreceptors.

A biphasic pattern of gene expression during mouse retina development

BACKGROUND

Between embryonic day 12 and postnatal day 21, six major neuronal and one glia cell type are generated from multipotential progenitors in a characteristic sequence during mouse retina development. We investigated expression patterns of retina transcripts during the major embryonic and postnatal developmental stages to provide a systematic view of normal mouse retina development,

RESULTS

A tissue-specific cDNA microarray was generated using a set of sequence non-redundant EST clones collected from mouse retina. Eleven stages of mouse retina, from embryonic day 12.5 (El2.5) to postnatal day 21 (PN21), were collected for RNA isolation. Non-amplified RNAs were labeled for microarray experiments and three sets of data were analyzed for significance, hierarchical relationships, and functional clustering. Six individual gene expression clusters were identified based on expression patterns of transcripts through retina development. Two developmental phases were clearly divided with postnatal day 5 (PN5) as a separate cluster. Among 4,180 transcripts that changed significantly during development, approximately 2/3 of the genes were expressed at high levels up until PN5 and then declined whereas the other 1/3 of the genes increased expression from PN5 and remained at the higher levels until at least PN21. Less than 1% of the genes observed showed a peak of expression between the two phases. Among the later increased population, only about 40% genes are correlated with rod photoreceptors, indicating that multiple cell types contributed to gene expression in this phase. Within the same functional classes, however, different gene populations were expressed in distinct developmental phases. A correlation coefficient analysis of gene expression during retina development between previous SAGE studies and this study was also carried out.

CONCLUSION

This study provides a complementary genome-wide view of common gene dynamics and a broad molecular classification of mouse retina development. Different genes in the same functional clusters are expressed in the different developmental stages, suggesting that cells might change gene expression profiles from differentiation to maturation stages. We propose that large-scale changes in gene regulation during development are necessary for the final maturation and function of the retina.

Mpp4 recruits Psd95 and Veli3 towards the photoreceptor synapse

Membrane-associated guanylate kinase (MAGUK) proteins function as scaffold proteins contributing to cell polarity and organizing signal transducers at the neuronal synapse membrane. The MAGUK protein Mpp4 is located in the retinal outer plexiform layer (OPL) at the presynaptic plasma membrane and presynaptic vesicles of photoreceptors. Additionally, it is located at the outer limiting membrane (OLM) where it might be involved in OLM integrity. In Mpp4 knockout mice, loss of Mpp4 function only sporadically causes photoreceptor displacement, without changing the Crumbs (Crb) protein complex at the OLM, adherens junctions or synapse structure. Scanning laser ophthalmology revealed no retinal degeneration. The minor morphological effects suggest that Mpp4 is a candidate gene for mild retinopathies only. At the OPL, Mpp4 is essential for correct localization of Psd95 and Veli3 at the presynaptic photoreceptor membrane. Psd95 labeling is absent of presynaptic membranes in both rods and cones but still present in cone basal contacts and dendritic contacts. Total retinal Psd95 protein levels are significantly reduced which suggests Mpp4 to be involved in Psd95 turnover, whereas Veli3 proteins levels are not changed. These protein changes in the photoreceptor synapse did not result in an altered electroretinograph. These findings suggest that Mpp4 coordinates Psd95/Veli3 assembly and maintenance at synaptic membranes. Mpp4 is a critical recruitment factor to organize scaffolds at the photoreceptor synapse and is likely to be associated with synaptic plasticity and protein complex transport.

Membrane-associated guanylate kinase proteins MPP4 and MPP5 associate with Veli3 at distinct intercellular junctions of the neurosensory retina

MPP4 and MPP5 are closely related members of the p55-subfamily of membrane-associated guanylate kinases (MAGUKs) known to mediate the assembly of protein complexes at the plasma membrane of cell-cell junctions. Both MPP4 and MPP5 have been implicated in retinal function; however, their specific roles in the cellular mechanisms underlying vision are largely unknown. Here, we generated specific poly- and monoclonal antibodies against the two proteins and show that MPP4 and MPP5 are localized at distinct sites of cell-cell contact in the mouse retina. While MPP4 is a component of the synaptic terminals of photoreceptors, MPP5 exclusively localizes to apical membrane domains of the outer limiting membrane (OLM) junctions. The vertebrate homologs of Caenorhabditis elegans lin-7, Veli1, -2, and -3, have previously been identified as putative binding partners of MPP5. In this study, we show that MPP4 directly interacts with the Veli proteins via L27 heterodimerization in vitro. In addition, two of the three Veli isoforms, Veli1 and -3, are demonstrated to be expressed in the mouse retina. Immunofluorescence microscopy reveals extensive colocalization of Veli3 with both MPP4 and MPP5. This association of Veli3 with either MPP4 or MPP5 suggests that the MAGUKs recruit Veli3 and its binding partners to different cellular regions of the retina where they may participate in the organization of specialized intercellular junctions.