Role of hippocalcin in Ca2+ -induced activation of phospholipase D

Phospholipase D1 Ablation Disrupts Mouse Longitudinal Hippocampal Axis Organization and Functioning

Phosphatidic acid (PA) is a signaling lipid involved in the modulation of synaptic structure and functioning. Based on previous work showing a decreasing PA gradient along the longitudinal axis of the rodent hippocampus, we asked whether the dorsal hippocampus (DH) and the ventral hippocampus (VH) are differentially affected by PA modulation. Here, we show that phospholipase D1 (PLD1) is a major hippocampal PA source, compared to PLD2, and that PLD1 ablation affects predominantly the lipidome of the DH. Moreover, Pld1 knockout (KO) mice show specific deficits in novel object recognition and social interaction and disruption in the DH-VH dendritic arborization differentiation in CA1/CA3 pyramidal neurons. Also, Pld1 KO animals present reduced long-term depression (LTD) induction and reduced GluN2A and SNAP-25 protein levels in the DH. Overall, we observe that PLD1-derived PA reduction leads to differential lipid signatures along the longitudinal hippocampal axis, predominantly affecting DH organization and functioning.

Quercetin Reduces Ischemic Brain Injury by Preventing Ischemia-induced Decreases in the Neuronal Calcium Sensor Protein Hippocalcin

Calcium acts as a second messenger that mediates physiologic functions, such as metabolism, cell proliferation, and apoptosis. Hippocalcin is a neuronal calcium sensor protein that regulates intracellular calcium concentration. Moreover, it prevents neuronal cell death from oxidative stress. Quercetin has excellent antioxidant properties and preventative effects. We studied modulation of hippocalcin expression by quercetin treatment in cerebral ischemic injury and glutamate-induced neuronal cell damage. Focal cerebral ischemia was induced by permanent middle cerebral artery occlusion (pMCAO). Male Sprague-Dawley rats were injected with vehicle or quercetin (10 mg/kg) 1 h prior to pMCAO, and cerebral cortical tissues were isolated 24 h after pMCAO. Quercetin improved pMCAO-induced neuronal movement deficit and infarction. pMCAO induced a decrease in hippocalcin expression in the cerebral cortex. However, quercetin treatment attenuated this pMCAO-induced decrease. In cultured hippocampal cells, glutamate excitotoxicity dramatically increased the intracellular calcium concentration, whereas quercetin alleviated intracellular calcium overload. Moreover, Western blot and immunocytochemical studies showed reduction of hippocalcin expression in glutamate-exposed cells. Quercetin prevented this glutamate-induced decrease. Furthermore, caspase-3 expression in hippocalcin siRNA transfection conditions is higher than caspase-3 expression in un-transfection conditions. Quercetin treatment attenuated the increase of caspase-3. Taken together, these results suggest that quercetin exerts a preventative effect through attenuation of intracellular calcium overload and restoration of down-regulated hippocalcin expression during ischemic injury.

Hippocalcin Promotes Neuronal Differentiation and Inhibits Astrocytic Differentiation in Neural Stem Cells

Hippocalcin (HPCA) is a calcium-binding protein that is restricted to nervous tissue and contributes to neuronal activity. Here we report that, in addition to inducing neurogenesis, HPCA inhibits astrocytic differentiation of neural stem cells. It promotes neurogenesis by regulating protein kinase Cα (PKCα) activation by translocating to the membrane and binding to phosphoinositide-dependent protein kinase 1 (PDK1), which induces PKCα phosphorylation. We also found that phospholipase D1 (PLD1) is implicated in the HPCA-mediated neurogenesis pathway; this enzyme promotes dephosphorylation of signal transducer and activator of transcription 3 (STAT3[Y705]), which is necessary for astrocytic differentiation. Moreover, we found that the SH2-domain-containing tyrosine phosphatase 1 (SHP-1) acts upstream of STAT3. Importantly, this SHP-1-dependent STAT3-inhibitory mechanism is closely involved in neurogenesis and suppression of gliogenesis by HPCA. Taken together, these observations suggest that HPCA promotes neuronal differentiation through activation of the PKCα/PLD1 cascade followed by activation of SHP-1, which dephosphorylates STAT3(Y705), leading to inhibition of astrocytic differentiation.

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Hippocalcin is required for neuronal differentiation in neural stem cells

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PKCα/PLD1 activation is required for hippocalcin-mediated neuronal differentiation

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Blocking of STAT3(Y705) activity by hippocalcin decreases astrocytic differentiation

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Hippocalcin promotes neurogenesis by inhibiting gliogenesis in neural stem cells

Hippocalcin Is Required for Astrocytic Differentiation through Activation of Stat3 in Hippocampal Neural Precursor Cells

Hippocalcin (Hpca) is a neuronal calcium sensor protein expressed in the mammalian brain. However, its function in neural stem/precursor cells has not yet been studied. Here, we clarify the function of Hpca in astrocytic differentiation in hippocampal neural precursor cells (HNPCs). When we overexpressed Hpca in HNPCs in the presence or absence of bFGF, expression levels of nerve-growth factors such as neurotrophin-3 (NT-3), neurotrophin-4/5 (NT-4/5), and brain-derived neurotrophic factor (BDNF), together with the proneural basic helix loop helix (bHLH) transcription factors NeuroD and neurogenin 1 (Ngn1), increased significantly. In addition, there was an increase in the number of cells expressing glial fibrillary acidic protein (GFAP), an astrocyte marker, and in branch outgrowth, indicating astrocytic differentiation of the HNPCs. Downregulation of Hpca by transfection with Hpca siRNA reduced expression of NT-3, NT-4/5, BDNF, NeuroD, and Ngn1 as well as levels of GFAP protein. Furthermore, overexpression of Hpca increased the phosphorylation of STAT3 (Ser727), and this effect was abolished by treatment with a STAT3 inhibitor (S3I-201), suggesting that STAT3 (Ser727) activation is involved in Hpca-mediated astrocytic differentiation. As expected, treatment with Stat3 siRNA or STAT3 inhibitor caused a complete inhibition of astrogliogenesis induced by Hpca overexpression. Taken together, this is the first report to show that Hpca, acting through Stat3, has an important role in the expression of neurotrophins and proneural bHLH transcription factors, and that it is an essential regulator of astrocytic differentiation and branch outgrowth in HNPCs.

Molecular structure and target recognition of neuronal calcium sensor proteins

BACKGROUND

Neuronal calcium sensor (NCS) proteins, a sub-branch of the calmodulin superfamily, are expressed in the brain and retina where they transduce calcium signals and are genetically linked to degenerative diseases. The amino acid sequences of NCS proteins are highly conserved but their physiological functions are quite distinct. Retinal recoverin and guanylate cyclase activating proteins (GCAPs) both serve as calcium sensors in retinal rod cells, neuronal frequenin (NCS1) modulate synaptic activity and neuronal secretion, K+ channel interacting proteins (KChIPs) regulate ion channels to control neuronal excitability, and DREAM (KChIP3) is a transcriptional repressor that regulates neuronal gene expression.

SCOPE OF REVIEW

Here we review the molecular structures of myristoylated forms of NCS1, recoverin, and GCAP1 that all look very different, suggesting that the sequestered myristoyl group helps to refold these highly homologous proteins into very different structures. The molecular structure of NCS target complexes have been solved for recoverin bound to rhodopsin kinase, NCS-1 bound to phosphatidylinositol 4-kinase, and KChIP1 bound to A-type K+ channels.

MAJOR CONCLUSIONS

We propose the idea that N-terminal myristoylation is critical for shaping each NCS family member into a unique structure, which upon Ca2+-induced extrusion of the myristoyl group exposes a unique set of previously masked residues, thereby exposing a distinctive ensemble of hydrophobic residues to associate specifically with a particular physiological target. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

Visinin-like proteins (VSNLs): interaction partners and emerging functions in signal transduction of a subfamily of neuronal Ca2+ -sensor proteins

The visinin-like protein (VSNL) subfamily, including VILIP-1 (the founder protein), VILIP-2, VILIP-3, hippocalcin, and neurocalcin delta, constitute a highly homologous subfamily of neuronal calcium sensor (NCS) proteins. Comparative studies have shown that VSNLs are expressed predominantly in the brain with restricted expression patterns in various subsets of neurons but are also found in peripheral organs. In addition, the proteins display differences in their calcium affinities, in their membrane-binding kinetics, and in the intracellular targets to which they associate after calcium binding. Even though the proteins use a similar calcium-myristoyl switch mechanism to translocate to cellular membranes, they show calcium-dependent localization to various subcellular compartments when expressed in the same neuron. These distinct calcium-myristoyl switch properties might be explained by specificity for defined phospholipids and membrane-bound targets; this enables VSNLs to modulate various cellular signal transduction pathways, including cyclic nucleotide and MAPK signaling. An emerging theme is the direct or indirect effect of VSNLs on gene expression and their interaction with components of membrane trafficking complexes, with a possible role in membrane trafficking of different receptors and ion channels, such as glutamate receptors of the kainate and AMPA subtype, nicotinic acetylcholine receptors, and Ca(2+)-channels. One hypothesis is that the highly homologous VSNLs have evolved to fulfil specialized functions in membrane trafficking and thereby affect neuronal signaling and differentiation in defined subsets of neurons. VSNLs are involved in differentiation processes showing a tumor-invasion-suppressor function in peripheral organs. Finally, VSNLs play neuroprotective and neurotoxic roles and have been implicated in neurodegenerative diseases.

Hippocalcin functions as a calcium sensor in hippocampal LTD

It is not fully understood how NMDAR-dependent LTD causes Ca(2+)-dependent endocytosis of AMPARs. Here we show that the neuronal Ca(2+) sensor hippocalcin binds the beta2-adaptin subunit of the AP2 adaptor complex and that along with GluR2 these coimmunoprecipitate in a Ca(2+)-sensitive manner. Infusion of a truncated mutant of hippocalcin (HIP(2-72)) that lacks the Ca(2+) binding domains prevents synaptically evoked LTD but has no effect on LTP. These data indicate that the AP2-hippocalcin complex acts as a Ca(2+) sensor that couples NMDAR-dependent activation to regulated endocytosis of AMPARs during LTD.

Hippocalcin-deficient mice display a defect in cAMP response element-binding protein activation associated with impaired spatial and associative memory

Hippocalcin is a member of the neuronal calcium sensor (NCS) protein family that is highly expressed in hippocampal pyramidal cells and moderately expressed in the neurons of cerebral cortex, cerebellum and striatum. Here we examined the physiological roles of hippocalcin using targeted gene disruption. Hippocalcin-deficient (-/-) mice displayed no obvious structural abnormalities in the brain including hippocampal formation at the light microscopic level. Deletion of hippocalcin did not result in up-regulation of the hippocalcin-related proteins; neural visinin-like Ca(2+)-binding proteins (NVP) 1, 2, and 3. The synaptic excitability of hippocampal CA1 neurons appeared to be normal, as estimated by the shape of field excitatory postsynaptic potentials elicited by single- and paired-pulse stimuli, and by tetanic stimulation. However, N-methyl-d-aspartate stimulation- and depolarization-induced phosphorylation of cAMP-response element-binding protein (CREB) was significantly attenuated in -/- hippocampal neurons, suggesting an impairment in an activity-dependent gene expression cascade. In the Morris water maze test, the performance of -/- mice was comparable to that of wild-type littermates except in the probe test, where -/- mice crossed the previous location of the platform significantly less often than +/+ mice. Hippocalcin-deficient mice were also impaired on a discrimination learning task in which they needed to respond to a lamp illuminated on the left or right side to obtain food reinforcement. No abnormalities were observed in motor activity, anxiety behavior, or fear learning. These results suggest that hippocalcin plays a crucial role in the Ca(2+)-signaling pathway that underlies long-lasting neural plasticity and that leads to spatial and associative memory.

The neuronal calcium-sensor proteins

Changes in intracellular free Ca(2+) concentration ([Ca(2+)](i)) affect many different aspects of neuronal function ranging from millisecond regulation of ion channels to long term changes in gene expression. These effects of Ca(2+) are transduced by Ca(2+)-binding proteins that act as Ca(2+) sensors by binding Ca(2+), undergoing a conformational change and then modifying the function of additional target proteins. Mammalian species express 14 members of the neuronal calcium sensor (NCS) family of EF hand-containing Ca(2+)-binding proteins which are expressed mainly in photoreceptor cells or neurons. Many of the NCS proteins are membrane targeted through their N-terminal myristoylation either constitutively or following exposure of the myristoyl group after Ca(2+) binding (the Ca(2+)/myristoyl switch). The NCS proteins have been implicated in a wide range of functional roles in neuronal regulation, several of which have been confirmed though molecular genetic analyses.

Evidence for different functional properties of the neuronal calcium sensor proteins VILIP-1 and VILIP-3: from subcellular localization to cellular function

The visinin-like-proteins VILIP-1 and -3 are EF-hand calcium-binding proteins and belong to the family of neuronal calcium sensor (NCS) proteins. Members of this family are involved in the calcium-dependent regulation of signal transduction cascades mainly in the nervous system. VILIP-1 and VILIP-3 are expressed in different populations of neuronal cells. To gain insights into the different functional characteristics of VILIP-1 and -3, we have compared the localization of the proteins in intact cells and the calcium-dependent membrane association in subcellular fractions. Furthermore, we have investigated the different functional properties of the two proteins in activating cGMP signal pathways and have defined different sets of protein interaction partners. Our data indicate that VILIP-3, which is mainly expressed in Purkinje cells, and VILIP-1, which is expressed in granule cells in the cerebellum, show a different calcium-dependent subcellular localization, may activate different cellular signaling pathways, and thus have signaling functions which seem to be cell-type specific.