Distribution of hippocalcin mRNA and immunoreactivity in rat brain

The development of neural visinin-like Ca(2+)-binding protein 2 immunoreactivity in the rat neocortex and hippocampus

Neural visinin-like Ca(2+)-binding protein 2 (NVP2) immunoreactivity in the rat neocortex and hippocampus was barely detectable by immunoblot analysis on postnatal day 1 (P1), but increased during postnatal weeks 2-3, reaching a plateau on P28. Immunohistochemical analysis revealed moderate immunoreactivity firstly on P7 in some subsets of the hippocampal interneurons and in the hippocampal pyramidal cells and dentate granule cells. Immunoreactivity of the interneurons decreased during postnatal weeks 2-3 and disappeared by P28. In contrast, immunoreactivity of the cortical and hippocampal pyramidal cells and dentate granule cells abruptly increased during postnatal week 2. The distinctly immunoreactive cells were distributed throughout the neocortex, especially in the cortical plate, and the stratum pyramidale of Ammon's horn and granular layer of the dentate gyrus on P14. Immunoreactivity was homogeneously concentrated in the cell bodies and proximal dendrites at this stage, whereas thereafter immunoreactivity in the neuropil gradually increased, and underwent a relative decrease in the cell bodies. By P28, the higher and granular immunoreactivity in the neuropil covered whole layers of the neocortex, Ammon's horn and the dentate gyrus, the same as in adults. Differential expression of NVP2 in different neuron populations may reflect the differential functional consequences for neuronal development.

Recoverin, a calcium-binding protein in photoreceptors

Recoverin is a Ca2+-binding protein found primarily in vertebrate photoreceptors. The proposed physiological function of recoverin is based on the finding that recoverin inhibits light-stimulated phosphorylation of rhodopsin. Recoverin interacts with rod outer segment membranes in a Ca2+-dependent manner. This interaction requires N-terminal acylation of recoverin. Four types of fatty acids have been detected on the N-terminus of recoverin, but the functional significance of this heterogeneous acylation is not yet clear.

Future directions for rhodopsin structure and function studies

NMR (nuclear magnetic resonance) may be useful for determining the structure of retinal and its environment in rhodopsin, but not for determining the complete protein structure. Aggregation and low yield of fragments of rhodopsin may make them difficult to study by NMR. A long-term multidisciplinary attack on rhodopsin structure is required.

More answers about cGMP-gated channels pose more questions

Our understanding of the molecular properties and cellular role of cGMP-gated channels in outer segments of vertebrate photo-receptors has come from over a decade of studies which have continuously altered and refined ideas about these channels. Further examination of this current view may lead to future surprises and further refine the understanding of cGMP-gated channels.

Cyclic nucleotides as regulators of light-adaptation in photoreceptors

Cyclic nucleotides can regulate the sensitivity of retinal rods to light through phosducin. The phosphorylation state of phosducin determines the amount of G available for activation by Rho*. Phosducin phosphorylation is regulated by cyclic nucleotides through their activation of cAMP-dependent protein kinase. The regulation of phosphodiesterase activity by the noncatalytic cGMP binding sites as well as Ca2+/calmodulin dependent regulation of cGMP binding to the cation channel are also discussed.

Long term potentiation and CaM-sensitive adenylyl cyclase: Long-term prospects

The type I CaM-sensitive adenylyl cyclase is in a position to integrate signals from multiple inputs, consistent with the requirements for mediating long term potentiation (LTP). Biochemical and genetic evidence supports the idea that this enzyme plays an important role inc LTP. However, more work is needed before we will be certain of the role that CaM-sensitive adenylyl cyclases play in LTP.

Modulation of the cGMP-gated channel by calcium

Calcium acting through calmodulin has been shown to regulate the affinity of cyclic nucleotide-gated channels expressed in cell lines. But is calmodulin the Ca-sensor that normally regulates these channels?

How many light adaptation mechanisms are there?

The generally positive response to our target article indicates that most of the commentators accept our contention that light adaptation consists of multiple and possibly redundant mechanisms. The commentaries fall into three general categories. The first deals with putative mechanisms that we chose not to emphasize. The second is a more extended discussion of the role of calcium in adaptation. Finally, additional aspects of cGMP involvement in adaptation are considered. We discuss each of these points in turn.

Gene therapy, regulatory mechanisms, and protein function in vision

Hereditary retinal degeneration due to mutations in visual genes may be amenable to therapeutic interventions that modulate, either positively or negatively, the amount of protein product. Some of the proteins involved in phototransduction are rapidly moved by a lightdependent mechanism between the inner segment and the outer segment in rod photoreceptor cells, and this phenomenon is important in phototransduction.

A novel protein family of neuronal modulators

A number of proteins homologous to recoverin have been identified in the brains of the several vertebrate species. The brainderived members originally contain four EF-hand domains, but NH2- terminal domain is aberrant. Many of these proteins inhibited light-induced rhodopsin phosphorylation at high [Ca2+], suggesting that the brain-derived members may act as a Ca2+-sensitive modulator of receptor phosphorylation, as recoverin does.

The structure of rhodopsin and mechanisms of visual adaptation

Rapidly advancing studies on rhodopsin have focused on new strategies for crystallization of this integral membrane protein for x-ray analysis and on alternative methods for structural determination from nuclear magnetic resonance data. Functional studies of the interactions between the apoprotein and its chromophore have clarified the role of the chromophore in deactivation of opsin and in photoactivation of the pigment.

Crucial steps in photoreceptor adaptation: Regulation of phosphodiesterase and guanylate cyclase activities and Ca2+-buffering

This commentary discusses the balance of phosphodiesterase and guanylate cyclase activities in vertebrate photoreceptors at moderate light intensities. The rate of cGMP hydrolysis and synthesis seem to equal each other. Ca2+ as regulator of both enzyme activities is also effectively buffered in photoreceptor cells by cytoplasmic buffer components.

The atomic structure of visual rhodopsin: How and when?

Strong arguments are presented by Hargrave suggesting that the crystallization of visual rhodopsin for high resolution analysis by X-ray crystallography or electron microscopy is feasible. However, the effort needed to achieve this goal will most likely exceed the resources of a single laboratory and a concerted approach to the research is necessary.

Molecular insights gained from covalently tethering cGMP to the ligand-binding sites of retinal rod cGMP-gated channels

A photoaffinity analog of cGMP has been used to biochemically identify a new ligand-binding subunit of the retinal rod cGMP-activated ion channel, as well as amino acids in contact with cGMP in the original subunit. Covalent tethering of this probe to channels in excised menbrane patches has revealed a functional heteogeneity in the ligand-binding sites that may arise from the two biochemically identified subunits.

Genetic and functional complexity of inherited retinal degeneration

Recent findings emphasize the complexity, both genetic and functional, of the manifold genes and mutations causing inherited retinal degeneration in humans. Knowledge of the genetic bases of these diseases can contribute to design of rational therapy, as well as elucidating the function of each gene product in normal visual processes.

Channel structure and divalent cation regulation of phototransduction

The identification of additional subunits of the cGMP-gated cation channel suggests exciting questions about their regulatory roles and about structure/functional relationships. How do the different subunits interact? How is the complex assembled into the plasma membrane? Divalent cations have been implicated in the regulation of adaptation. One often overlooked cation is magnesium. Could this ion play a role in phototransduction?

Structure of the cGMP-gated channel

The subunit structure of the cGMP-gated cation channel of rod photoreceptors is rapidly being defined, and in the process the mode of regulation by Ca2+-calmodulin unraveled. Intriguingly, early results suggest that additional subunits of unknown function are associated with the channel and remain to be identified.

Linking genotypes with phenotypes in human retinal degenerations: Implications for future research and treatment

Although undoubtedly it will be incomplete by the time it is published, the target article by Daiger et al. organizes mutations in genes that produce retinal degenerations in humans into categories of clinically relevant phenotypes. Such classifications should help us understand the link between altered photoreceptor cell proteins and subsequent cell death, and they may yield insight into methods for preventing consequent blindness.

Genetic and clinical heterogeneity in tapetal retinal dystrophies

Large scale DNA-mutation screening in patients with hereditary retinal diseases greatly enhances our knowledge about retinal function and diseases. Scientists, clinicians, patients, and families involved with retinal disorders may directly benefit from these developments. However, certain aspects of this expanding knowledge, such as the correlation between genotype and phenotype, may be much more complicated than we expect at present.

The determination of rhodopsin structure may require alternative approaches

The structure of rhodopsin is a subject of intense interest. Solving the structure by traditional methods has proved exceedingly challenging. It may therefore be useful to confront the problem by a combination of alternate techniques. These include FTIR (Fourier transform infrared spectroscopy) and AFM (atomic force microscopy) on the intact protein. Furthermore, additional insights may be gained through structural investigations of discrete rhodopsin domains.

Na-Ca + K exchanger and Ca2+ homeostasis in retinal rod outer segments: Inactivation of the Ca2+ efflux mode and possible involvement of intracellular Ca2+ stores in Ca2+ homeostasis

Inactivation of the Ca2+ extrusion mode of the retinal rod Na- Ca + K exchanger is suggested to be the mechanism that prevents lowering of cytosolic free Ca2+ to < 1 nM when rod cells are saturated for a prolonged time under bright light conditions. Under these conditions, Ca2+ fluxes across disk membranes can contribute significantly to Ca2+ homeostasis in rods.

Nuclear magnetic resonance studies on the structure and function of rhodopsin

Magic angle spinning (MAS) NMR methods provide a means of obtaining high resolution structural data on rhodopsin and its photoin termediates. Current work has focused on the structure of the retinal chromophore and its interactions with surrounding protein charges. The recent development of MAS NMR methods for measuring internuclear distances with a resolution of ∼0.2 will complement diffraction methods for addressing key mechanistic questions.

Glutamate accumulation in the photoreceptor-presumed final common path of photoreceptor cell death

Genetic abnormalities of three factors related to the photoreceptor mechanism have been reported in both animal models and humans. Apoptotic mechanism has also been suggested as a final common pathway of photoreceptor cell death. Our findings of increased level of glutamate in photoreceptor cells in rds mice suggest that amino acid might mediate between these two pathological mechanisms.

Unique lipids and unique properties of retinal proteins

Amino-terminal heteroacylation has been identified in retinal proteins including recoverin and α subunit of G-protein, transducin. The tissue-specific modification seems to mediate not only a proteinmembrane interaction but also a specific protein-protein interaction. The mechanism generating the heterogeneity and its physiological role are still unclear, but an interesting idea for the latter postulates a fine regulation of the signal transduction pathway by distinct N-acyl groups.

Further insight into the structural and regulatory properties of the cGMP-gated channel

Recent studies from several different laboratories have provided further insight into structure-function relationships of cyclic nucleotide-gated channel and in particular the cCMPgated channel of rod photoreceptors. Site-directed mutagenesis and rod-olfactory chimeria constructs have defined important amino acids and peptide segments of the channel that are important in ion blockage, ligand specificity, and gating properties. Molecular cloning studies have indicated that cyclic nucleotide-gated channels consist of two subunits that are required to reproduce the properties of the native channels. Biochemical analysis of the cGMP-gated channel of rodcells have indicated that the 240 kDa protein that co-purifies with the 63 kDa channel subunit contains both the previously cloned second subunit of the channel and a glutamic acid-rich protein. The regulatory properties of the cGMP-gated channel from rod cells has also been studied in more detail. Studies indicate that the beta subunit of the cGMP-gated channel of rod cells contains the binding site for calmodulin. Interaction of calmodulin with the channel alters the apparent affinity of the channel for cGMP in all in vitro systems that have been studied. The significance of these recent studies are discussed in relation to the commentaries on the target article.

Unsolved issues in S-modulin/recoverin study

S-Modulin is a frog homolog of recoverin. The function and the underlying mechanism of the action of these proteins are now understood in general. However, there remain some unsolved issues including; two distinct effects of S-modulin; Ca2+-dependent binding of S-modulin to membranes and a possible target protein; S-modulin-like proteins in other neurons. These issues are considered in this commentary.

Mechanisms of photoreceptor degenerations

The candidate gene approach has identified many causes of photoreceptor rod cell death in retinitis pigmentosa. Some mutations lead to increased cyclicGMP concentrations in rods. Rod photoreceptors are also particularly susceptible to some mutations in housekeeping genes. Although many more cases of macular degeneration than retinitis pigmentosa occur each year, there is much less known about both genetic and sporadic forms of this disease.

Reduced cytoplasmic calcium concentration may be both necessary and sufficient for photoreceptor light adaptation

Light adaptation is modulated almost exclusively by changes in intracellular Ca2+ concentration, and other Ca2+-independent mechanisms are likely to play only a minor role. Changes in Ca2+i may be not only necessary for light adaptation to take place but sufficient to cause it.

The genetic kaleidoscope of vision

Site-specific phenotypic effects of the 73 known alleles in the rhodopsin gene that cause retinal degeneration are difficult to interpret because most alleles are documented in only one case or one family, which means variation in effects could actually arise from interactions with other loci. However, sample sizes necessary to detect epistatic interaction may place an answer to this question beyond our grasp.

Evidence that the type I adenylyl cyclase may be important for neuroplasticity: Mutant mice deficient in the gene for type I adenylyl cyclase show altered behavior and LTP

The regulatory properties of the neurospecific, type I adenylyl cyclase and its distribution within brain have suggested that this enzyme may be important for neuroplasticity. To address this issue, the murine, Ca2+ -stimulated adenylyl cyclase (type I), was inactivated by targeted mutagenesis. Ca2+ -stimulated adenylyl cyclase activity was reduced 40% to 60% in the hippocampus, neocortex, and cerebellum. Long term potentiation in the CA1 region of the hippocampus from mutants was perturbed relative to controls. Both the initial slope and maxim um extent of changes in synaptic response were reduced. Although mutant mice learned to find a hidden platform normally in the Morris water task, they did not display a preference for the region where the platform had been when it was removed. The behavioral phenotype of these mice is very similar to that exhibited by mice which have been surgically lesioned in the hippocampus. These results indicate that disruption of the gene for the type I adenylyl cyclase produces changes in spatial memory and indicate that the cAMP signal transduction pathway may play an important role for synaptic plasticity.

Calcium/calmodulin-sensitive adenylyl cyclase as an example of a molecular associative integrator

Evidence suggests that the Ca2+/calmodulin-sensitive adenylyl cyclase may play a key role in neural plasticity and learning in Aplysia, Drosophila, and mammals. This dually-regulated enzyme has been proposed as a possible site of stimulus convergence during associative learning. This commentary discusses the evidence that is required to demonstrate that a protein in a second messenger cascade actually functions as a molecular site of associative integration. It also addresses the issue of how a dually-regulated protein could contribute to the temporal pairing requirements of classical conditioning: that relationship between stimuli display both temporal contiguity and predictability.

The key to rhodopsin function lies in the structure of its interface with transducin

Light activated rhodopsin functions by catalyzing the exchange of GTP for GDP on numerous copies of transducin. Peptide mapping has shown that at least six regions, three on rhodopsin and three on the transducin alpha subunit, are involved in the active interface between the two proteins. The most informative structural studies of rhodopsin should include focus on the transducin interaction.

Hippocalcin in rat retina. Comparison with calbindin-D28k, calretinin and neurocalcin

The post-natal developmental expression in rat retina of four calcium-binding proteins belonging to the calmodulin-troponin-C family was investigated by immunohistochemistry using anti-calbindin-D28k, anti-calretinin, anti-hippocalcin and anti-neurocalcin polyclonal antibodies on paraffin sections from Wistar rat retinae aged from post-natal days 1 (P1), 5 (P5), 10 (P10), 20 (P20) to adulthood (8 weeks). Immunoblot using anti-hippocalcin and homogenates proteins from retina, cerebellar cortex, hippocampus and cerebellum was also performed. Hippocalcin immunoreactivity in adult rat retina was demonstrated by both immunohistochemistry and Western blot. During post-natal development, calbindin-D28k, calretinin and neurocalcin immunoreactivity were detected at P1 in ganglion cells, whereas hippocalcin immunoreactivity was seen later at P5 in this cell layer. In the amacrine cell layer, neurocalcin immunoreactivity was detected at P5 and hippocalcin at P10. Calbindin-D28k was labelling the immature horizontal cell, calretinin was detected in nearly all ganglion cells and in some amacrine cells since P1. These three calcium-binding proteins do not seem to play a role in synaptogenesis which takes place later. We confirmed that calbindin-D28k appeared to be a good marker for horizontal cells. The presence of hippocalcin, a myristoylated calcium-binding protein belonging to the recovering subfamily and previously localized in few brain areas has been detected for the first time in retina.

Hippocalcin expression in the brain of the Snell dwarf mutant mouse

To determine factors contributing to the expression of the brain-derived protein, hippocalcin, we mapped its distribution in the brain of Snell pituitary dwarf mutant mice (dw) by immunohistochemical and immunoblot methods. Our findings are as follows. (1) In the hippocampus, hippocalcin immunoreactivity was found in the cell body and dendrites of pyramidal neurons of the normal controls and dw mice, although the intensity of immunoreactivity in the dw mice was lower. (2) In the cerebellum, hippocalcin immunoreactivity was strongly expressed in the Purkinje cell body of both the control and dw mice. However, the Purkinje cell dendrites were found to be more intensely stained in the dw mice than in the normal controls. (3) In the dw cerebral cortex, the pyramidal neurons of layers II to VI strongly expressed hippocalcin, whereas its expression in the controls was weak. (4) The amount of hippocalcin in the dw hippocampus was less than in the normal controls, whereas the amount in the dw cerebral cortex and cerebellum was greater. These results indicate that the developmental expression of hippocalcin in the dw brain is affected by the retarded maturation of the neuronal network due to the deficient hormonal state (the lack of growth and thyroid hormones).

Molecular cloning of a novel calcium-binding protein structurally related to hippocalcin from human brain and chromosomal mapping of its gene

A cDNA clone (hHLP2) encoding a novel calcium-binding protein structurally related to hippocalcin has been isolated from the human hippocampus cDNA library. The primary structure consists of 193 amino acids, and contains three EF-hand structures and a possible NH2-terminal myristoylation site. A single transcript at a position corresponding to 1.7 kilobases was detected only in the brain. The hHLP2 gene was mapped to human chromosome 2.

Expression of hippocalcin in the developing rat brain

Expression of hippocalcin in the developing rat brain was investigated by a combination of Northern blot, in situ hybridization, immunoblot and immunohistochemical methods. In the hippocampus, hippocalcin mRNA and immunoreactivity first appeared in the CA3 pyramidal cells on embryonic day 19 (E19) and postnatal day 1 (P1), respectively, and extended throughout Ammon's horn. After P14, the hippocampal pyramidal cells, especially in the CA1 region, maintained the highest expression level among the brain regions. The dentate granule cells expressed a small amount of hippocalcin mRNA and immunoreactivity from P7 and maintained a low level through the developmental stages. In the cerebral cortex, hippocalcin mRNA and immunoreactivity appeared in the pyramidal cells of the piriform cortex from P1 and P4, respectively. Their expression extended throughout the cerebral cortex and reached the maximum level on P14, and then declined gradually with age to half of the maximum level by adults. In the cerebellum, a few Purkinje cells expressed a small amount of hippocalcin mRNA and immunoreactivity on P7. Their expression became evident in most of the Purkinje cells on P14 and increased gradually by P28. Then, their expression declined with age; however, the immunoreactivity was concentrated in the cell bodies and proximal segments of the dendrites in adults. These results suggest that the expression of hippocalcin mRNA and protein is strictly controlled by both the cell type and the developmental process and that hippocalcin plays a role in neuronal differentiation in the early stages of development and may relate to other neuronal function in the adult brain.

Immunohistochemical localization of neural visinin-like Ca(2+)-binding protein 2 in adult rat brain

The distribution of neural visinin-like Ca(2+)-binding protein 2 (NVP2) in adult rat brain was analyzed by immunoblot and immunohistochemical methods. NVP2 immunoreactivity was expressed intensely in the pyramidal cells of the CA1 and 2 regions of Ammon's horn, the granule cells of the dentate gyrus and the pyramidal cells of the cerebral cortex layers II to VI, moderately in the pyramidal-shaped cells of the anterior olfactory nucleus and large spindle-shaped cells of the globus pallidus, and weakly in neurons in the nucleus accumbens and the anterior and dorsomedial thalamus. In most cell types, NVP2 immunoreactivity was located in the cytoplasm and plasma membrane of the cell bodies and dendrites.

Hippocalcin: a calcium-binding protein of the EF-hand superfamily dominantly expressed in the hippocampus

Hippocalcin is a recently identified Ca(2+)-binding protein with three EF-hand structures, dominantly expressed in the hippocampal pyramidal layer. The complete amino acid sequence of hippocalcin deduced from the cDNA is composed of 195 residues, has a calculated molecular mass of 22,574 daltons, and has a striking sequence homology to those of visinin, recoverin, S-modulin, neurocalcins and neural visinin-like proteins. Hippocalcin binds 3 mol of Ca2+ per mol of protein at submicromolar Ca2+ levels, and associates the plasma membrane in a Ca(2+)-dependent manner. Hippocalcin is myristoylated at its NH2-terminal glycine residue, and this modification is a key event in terms of its membrane-association property.