Letting the calcium flow

Two calcium-binding proteins, CaBP1 and CaBP2, cooperate to keep calcium channels in the hair cells of the inner ear open.

Clonorchis sinensis calcium-binding protein Cs16 causes acute hepatic injury possibly by reprogramming the metabolic pathway of bone marrow-derived monocytes

Introduction

Clonorchis sinensis infection results in various complications in the liver and biliary systems and is a neglected tropical disease in Eastern Asia. In this study, we report that C. sinensis calcium-binding protein Cs16 activates host immune cells and induces immunopathology in liver.

Methods

Immunohistochemistry was used to detect the localization of Cs16 in C. sinensis adult worms. ELISA was used to detect the serum levels of anti-Cs16 IgG antibody in infected humans and mice. Bile duct injection model was used to figure out the role of Cs16 in vivo. RT-qPCR and ELISA were used to detect the cytokine production from Cs16-treated BMMs in vitro. Seahorse assay was used to detect the metabolic pathway of Cs16-treated BMMs in vitro.

Result

Cs16 localizes in the tegument and gut of C. sinensis. Humans and mice with C. sinensis infection exhibited increased levels of anti-Cs16-specific antibody. Using the bile duct injection technique, we found that Cs16 induced obvious inflammation and hepatic necrosis in vivo. Cs16 treatment caused the upregulation of inflammatory cytokines in innate immune cells. Moreover, Cs16-treated monocytes relied more on the glycolytic metabolic pathway.

Discussion

Our findings suggest that Cs16 is a potential pathogenic factor derived from C. sinensis adult worm. By reprogramming the metabolic pathway of innate immune cells, Cs16 triggers pro-inflammatory responses in the liver, and therefore, Cs16 is a potential target for the prevention and treatment of clonorchiasis.

Calnexin, More Than Just a Molecular Chaperone

Calnexin is a type I integral endoplasmic reticulum (ER) membrane protein with an N-terminal domain that resides in the lumen of the ER and a C-terminal domain that extends into the cytosol. Calnexin is commonly referred to as a molecular chaperone involved in the folding and quality control of membrane-associated and secreted proteins, a function that is attributed to its ER- localized domain with a structure that bears a strong resemblance to another luminal ER chaperone and Ca²⁺-binding protein known as calreticulin. Studies have discovered that the cytosolic C-terminal domain of calnexin undergoes distinct post-translational modifications and interacts with a variety of proteins. Here, we discuss recent findings and hypothesize that the post-translational modifications of the calnexin C-terminal domain and its interaction with specific cytosolic proteins play a role in coordinating ER functions with events taking place in the cytosol and other cellular compartments.

Can Bioactive Compounds in Beetroot/Carrot Juice Have a Neuroprotective Effect? Morphological Studies of Neurons Immunoreactive to Calretinin of the Rat Hippocampus after Exposure to Cadmium

Cadmium ions (Cd²⁺) penetrate the blood-brain barrier and can, among other effects, influence intracellular calcium metabolism, leading to neurodegeneration. In the presented work, we estimated the effect of Cd²⁺ on the expression of calretinin in the neurons of the rat hippocampus and analyzed the reverse effect of freshly pressed beetroot/carrot juice in this context. In the 12-week lasting experiment, 32 8-week-old male Wistar rats were divided into four experimental groups (n = 8): the control group (C) received pure tap water; the Cd group (Cd)-received Cd²⁺ dissolved in tap water (5 mg Cd²⁺/kg b.w.); and two groups received beetroot/carrot juice: the BCJ group was administered only juice, and the Cd + BCJ group received juice with the addition of Cd²⁺ (5 mg Cd²⁺/kg b.w.). The exposition to low doses of Cd²⁺ caused a significant decrease in calretinin-immunoreactive (Cr-IR) neurons compared to the non-exposed groups. Moreover, the addition of Cd²⁺ to tap water reduced the numbers and length of Cr-IR nerve fibers. The negative effect of Cd²⁺ was significantly attenuated by the simultaneous supplementation of beetroot/carrot juice (Cd + BCJ). The study showed that the bioactive compounds in the beetroot/carrot juice can modulate Ca²⁺ levels in neurons, and thus, potentially act as a neuroprotective factor against neuronal damage.

Structural Basis for the Functional Diversity of Centrins: A Focus on Calcium Sensing Properties and Target Recognition

Centrins are a family of small, EF hand-containing proteins that are found in all eukaryotes and are often complexed with centrosome-related structures. Since their discovery, centrins have attracted increasing interest due to their multiple, diverse cellular functions. Centrins are similar to calmodulin (CaM) in size, structure and domain organization, although in contrast to CaM, the majority of centrins possess at least one calcium (Ca²⁺) binding site that is non-functional, thus displaying large variance in Ca²⁺ sensing abilities that could support their functional versatility. In this review, we summarize current knowledge on centrins from both biophysical and structural perspectives with an emphasis on centrin-target interactions. In-depth analysis of the Ca²⁺ sensing properties of centrins and structures of centrins complexed with target proteins can provide useful insight into the mechanisms of the different functions of centrins and how these proteins contribute to the complexity of the Ca²⁺ signaling cascade. Moreover, it can help to better understand the functional redundancy of centrin isoforms and centrin-binding proteins.

Increased Calbindin D28k Expression via Long-Term Alternate-Day Fasting Does Not Protect against Ischemia-Reperfusion Injury: A Focus on Delayed Neuronal Death, Gliosis and Immunoglobulin G Leakage

Calbindin-D28k (CB), a calcium-binding protein, mediates diverse neuronal functions. In this study, adult gerbils were fed a normal diet (ND) or exposed to intermittent fasting (IF) for three months, and were randomly assigned to sham or ischemia operated groups. Ischemic injury was induced by transient forebrain ischemia for 5 min. Short-term memory was examined via passive avoidance test. CB expression was investigated in the Cornu Ammonis 1 (CA1) region of the hippocampus via western blot analysis and immunohistochemistry. Finally, histological analysis was used to assess neuroprotection and gliosis (microgliosis and astrogliosis) in the CA1 region. Short-term memory did not vary significantly between ischemic gerbils with IF and those exposed to ND. CB expression was increased significantly in the CA1 pyramidal neurons of ischemic gerbils with IF compared with that of gerbils fed ND. However, the CB expression was significantly decreased in ischemic gerbils with IF, similarly to that of ischemic gerbils exposed to ND. The CA1 pyramidal neurons were not protected from ischemic injury in both groups, and gliosis (astrogliosis and microgliosis) was gradually increased with time after ischemia. In addition, immunoglobulin G was leaked into the CA1 parenchyma from blood vessels and gradually increased with time after ischemic insult in both groups. Taken together, our study suggests that IF for three months increases CB expression in hippocampal CA1 pyramidal neurons; however, the CA1 pyramidal neurons are not protected from transient forebrain ischemia. This failure in neuroprotection may be attributed to disruption of the blood–brain barrier, which triggers gliosis after ischemic insults.

Calcium Binding Protein Ncs1 Is Calcineurin Regulated in Cryptococcus neoformans and Essential for Cell Division and Virulence

Intracellular calcium (Ca2+) is crucial for signal transduction in Cryptococcus neoformans, the major cause of fatal fungal meningitis. The calcineurin pathway is the only Ca2+-requiring signaling cascade implicated in cryptococcal stress adaptation and virulence, with Ca2+ binding mediated by the EF-hand domains of the Ca2+ sensor protein calmodulin. In this study, we identified the cryptococcal ortholog of neuronal calcium sensor 1 (Ncs1) as a member of the EF-hand superfamily. We demonstrated that Ncs1 has a role in Ca2+ homeostasis under stress and nonstress conditions, as the ncs1Δ mutant is sensitive to a high Ca2+ concentration and has an elevated basal Ca2+ level. Furthermore, NCS1 expression is induced by Ca2+, with the Ncs1 protein adopting a punctate subcellular distribution. We also demonstrate that, in contrast to the case with Saccharomyces cerevisiae, NCS1 expression in C. neoformans is regulated by the calcineurin pathway via the transcription factor Crz1, as NCS1 expression is reduced by FK506 treatment and CRZ1 deletion. Moreover, the ncs1Δ mutant shares a high temperature and high Ca2+ sensitivity phenotype with the calcineurin and calmodulin mutants (cna1Δ and cam1Δ), and the NCS1 promoter contains two calcineurin/Crz1-dependent response elements (CDRE1). Ncs1 deficiency coincided with reduced growth, characterized by delayed bud emergence and aberrant cell division, and hypovirulence in a mouse infection model. In summary, our data show that Ncs1 has a significant role as a Ca2+ sensor in C. neoformans, working with calcineurin to regulate Ca2+ homeostasis and, consequently, promote fungal growth and virulence.IMPORTANCECryptococcus neoformans is the major cause of fungal meningitis in HIV-infected patients. Several studies have highlighted the important contributions of Ca2+ signaling and homeostasis to the virulence of C. neoformans Here, we identify the cryptococcal ortholog of neuronal calcium sensor 1 (Ncs1) and demonstrate its role in Ca2+ homeostasis, bud emergence, cell cycle progression, and virulence. We also show that Ncs1 function is regulated by the calcineurin/Crz1 signaling cascade. Our work provides evidence of a link between Ca2+ homeostasis and cell cycle progression in C. neoformans.

Schistosoma japonicum SjE16.7 Protein Promotes Tumor Development via the Receptor for Advanced Glycation End Products (RAGE)

Schistosome infection contributes to cancer development, but the mechanisms are still not well-understood. SjE16.7 is an EF-hand calcium-binding protein secreted from Schistosoma japonicum eggs. It is a neutrophil attractant and macrophage activator and, as such, plays an important role in the inflammatory granuloma response in schistosomiasis. Here, we show that SjE16.7 binds to host cells by interacting with receptors for advanced glycation end products (RAGE). This ligation leads to activation of the NF-κB signaling pathway, an increase in the generation of reactive oxygen species, and production of the pro-inflammatory cytokines IL-6 and TNF-α. Using a mouse model of colorectal cancer, we demonstrate that intraperitoneal injection of SjE16.7 promotes colorectal cancer progression along with systemic myeloid cell accumulation. Thus, our results identify a new helminth antigen contributing to tumor development in the mammalian host.

Conformation and Domain Movement Analysis of Human Matrix Metalloproteinase-2: Role of Associated Zn2+ and Ca2+ Ions

Matrix metaloproteinase-2 (MMP-2) is an extracellular Zn²⁺ protease specific to type I and IV collagens. Its expression is associated with several inflammatory, degenerative, and malignant diseases. Conformational properties, domain movements, and interactions between MMP-2 and its associated metal ions were characterized using a 1.0 µs molecular dynamics simulation. Dihedral principle component analysis revealed ten families of conformations with the greatest degree of variability occurring in the link region connecting the catalytic and hemopexin domains. Dynamic cross-correlation analysis indicated domain movements corresponding to the opening and closing of the hemopexin domain in relation to the fibronectin and catalytic domains facilitated by the link region. Interaction energies were calculated using the molecular mechanics Poisson Boltzman surface area-interaction entropy (MMPBSA-IE) analysis method and revealed strong binding energies for the catalytic Zn²⁺ ion 1, Ca²⁺ ion 1, and Ca²⁺ ion 3 with significant conformational stability at the binding sites of Zn²⁺ ion 1 and Ca²⁺ ion 1. Ca²⁺ ion 2 diffuses freely away from its crystallographically defined binding site. Zn²⁺ ion 2 plays a minor role in conformational stability of the catalytic domain while Ca²⁺ ion 3 is strongly attracted to the highly electronegative sidechains of the Asp residues around the central β-sheet core of the hemopexin domain; however, the interacting residue sidechain carboxyl groups are outside of Ca²⁺ ion 3′s coordination sphere.

20 Years of Secretagogin: Exocytosis and Beyond

Calcium is one of the most important signaling factors in mammalian cells. Specific temporal and spatial calcium signals underlie fundamental processes such as cell growth, development, circadian rhythms, neurotransmission, hormonal actions and apoptosis. In order to translate calcium signals into cellular processes a vast number of proteins bind this ion with affinities from the nanomolar to millimolar range. Using classical biochemical methods an impressing number of calcium binding proteins (CBPs) have been discovered since the late 1960s, some of which are expressed ubiquitously, others are more restricted to specific cell types. In the nervous system expression patterns of different CBPs have been used to discern different neuronal cell populations, especially before advanced methods like single-cell transcriptomics and activity recording were available to define neuronal identity. However, understanding CBPs and their interacting proteins is still of central interest. The post-genomic era has coined the term “calciomics,” to describe a whole new research field, that engages in the identification and characterization of CBPs and their interactome. Secretagogin is a CBP, that was discovered 20 years ago in the pancreas. Consecutively it was found also in other organs including the nervous system, with characteristic expression patterns mostly forming cell clusters. Its regional expression and subcellular location together with the identification of protein interaction partners implicated, that secretagogin has a central role in hormone secretion. Meanwhile, with the help of modern proteomics a large number of actual and putative interacting proteins has been identified, that allow to anticipate a much more complex role of secretagogin in developing and adult neuronal cells. Here, we review recent findings that appear like puzzle stones of a greater picture.

Neuronal calcium sensor 1 (NCS1) promotes motility and metastatic spread of breast cancer cells in vitro and in vivo

Increased levels of the calcium-binding protein neuronal calcium sensor 1 (NCS1) predict an unfavorable patient outcome in several aggressive cancers, including breast and liver tumors. Previous studies suggest that NCS1 overexpression facilitates metastatic spread of these cancers. To investigate this hypothesis, we explored the effects of NCS1 overexpression on cell proliferation, survival, and migration patterns in vitro in 2- and 3-dimensional (2/3-D). Furthermore, we translated our results into an in vivo mouse xenograft model. Cell-based proliferation assays were used to demonstrate the effects of overexpression of NCS1 on growth rates. In vitro colony formation and wound healing experiments were performed and 3-D migration dynamics were studied using collagen gels. Nude mice were injected with breast cancer cells to monitor NCS1-dependent metastasis formation over time. We observed that increased NCS1 levels do not change cellular growth rates, but do significantly increase 2- and 3-D migration dynamics in vitro. Likewise, NCS1-overexpressing cells have an increased capacity to form distant metastases and demonstrate better survival and less necrosis in vivo. We found that NCS1 preferentially localizes to the leading edge of cells and overexpression increases the motility of cancer cells. Furthermore, this phenotype is correlated with an increased number of metastases in a xenograft model. These results lay the foundation for exploring the relevance of an NCS1-mediated pathway as a metastatic biomarker and as a target for pharmacologic interventions.-Apasu, J. E., Schuette, D., LaRanger, R., Steinle, J. A., Nguyen, L. D., Grosshans, H. K., Zhang, M., Cai, W. L., Yan, Q., Robert, M. E., Mak, M., Ehrlich, B. E. Neuronal calcium sensor 1 (NCS1) promotes motility and metastatic spread of breast cancer cells in vitro and in vivo.

Axotomy Leads to Reduced Calcium Increase and Earlier Termination of CCL2 Release in Spinal Motoneurons with Upregulated Parvalbumin Followed by Decreased Neighboring Microglial Activation

BACKGROUND

Motoneurons with naturally elevated calcium binding protein content, such as parvalbumin, are more resistant against injury. Furthermore, increase of intracellular calcium, which plays a pivotal role in injury of neurons, could be moderated by elevating their calcium binding proteins.

OBJECTIVE

To test whether by elevating parvalbumin content of motoneurons, activation of neighboring microglial cells, a robust component of the inflammatory reaction after injury, could be influenced.

METHODS

Mice overexpressing neuronal parvalbumin were derived and the spinal motoneurons were challenged by cutting the sciatic nerve. At postoperative days 1, 4, 7, 14 and 21 the change of the chemokine ligand 2 immunostaining in the motoneurons and the activation of microglial cells, measured as alterations in CD11b immunostaining were determined. Calcium level of motoneurons was tested electron microscopically at postoperative day 7.

RESULTS

After axotomy, increased level of chemokine ligand 2 was detected in the lumbar motoneurons. The staining intensity reached its maximum at day 7 and decayed faster in transgenic mice compared to controls. Microglial activation around motoneurons attenuated faster in parvalbumin overexpressing mice, too, but the decrease of microglial activation was delayed compared to the decline of the chemokine ligand 2 signal. At the time when the microglial reaction peaked, no intracellular calcium increase was detected in the motoneurons of transgenic mice, in contrast to the twofold increase in wild type animals.

CONCLUSION

Increased calcium buffering capacity, which augments resistance of motoneurons against calcium-mediated injury, leads to earlier termination of motoneuronal emission of CCL2 followed by a reduction of neighboring microglial activation after axotomy.

Choice of reconstitution protocol modulates the aggregation state of full-length membrane-reconstituted synaptotagmin-1

Synaptotagmin-1 (Syt1) functions as the Ca²⁺ sensor in neuronal exocytosis, and it is routinely incorporated into lipid bilayers along with other components of the fusion machinery in order to reconstruct the in vivo fusion process. Here, we demonstrate that the detergent used to reconstitute full-length Syt1 has a significant effect on the state of the protein in bilayers. When octyl-β-d-glucopyranoside is used to reconstitute the protein, Syt1 is present in an aggregated state that is mediated by the long juxta-membrane linker. EPR spectra from spin labels in the two C2 domains of Syt1 no longer resemble those obtained from a soluble construct containing these domains, and the C2B domain no longer exhibits a Ca²⁺ -dependent membrane insertion. In contrast, when reconstituted using 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate, Syt1 is largely monomeric and the EPR spectra from C2A and C2B resemble those of the soluble construct. This result demonstrates that the choice of detergent used to reconstitute Syt1 can modulate the state of the neuronal Ca²⁺ -sensor.

Calbindin D28k-Immunoreactivity in Human Enteric Neurons

Calbindin (CALB) is well established as immunohistochemical marker for intrinsic primary afferent neurons in the guinea pig gut. Its expression by numerous human enteric neurons has been demonstrated but little is known about particular types of neurons immunoreactive for CALB. Here we investigated small and large intestinal wholemount sets of 26 tumor patients in order to evaluate (1) the proportion of CALB⁺ neurons in the total neuron population, (2) the colocalization of CALB with calretinin (CALR), somatostatin (SOM) and vasoactive intestinal peptide (VIP) and (3) the morphology of CALB⁺ neurons. CALB⁺ neurons represented a minority of myenteric neurons (small intestine: 31%; large intestine: 25%) and the majority of submucosal neurons (between 72 and 95%). In the submucosa, most CALB⁺ neurons co-stained for CALR and VIP (between 69 and 80%) or for SOM (between 20 and 3%). In the myenteric plexus, 85% of CALB⁺ neurons did not co-stain with the other markers investigated. An unequivocal correlation between CALB reactivity and neuronal morphology was found for myenteric type III neurons in the small intestine: uniaxonal neurons with long, slender and branched dendrites were generally positive for CALB. Since also other neurons displayed occasional CALB reactivity, this protein is not suited as an exclusive marker for type III neurons.

Differential Effects of Parkinson's Disease on Interneuron Subtypes within the Human Anterior Olfactory Nucleus

Synucleinopathies (including α-synucleinopathies), which include Parkinson's disease (PD), manifest themsevles early on (stage 1) in the olfactory system; preferentially in the anterior olfactory nucleus (AON). In particular, the non-motor, early manifestations of PD include hyposmia, which is the partial loss of the sense of smell. The neural basis of hyposmia in PD, however, is poorly understood; but the AON appears to be a key structure in the disease's progression. We analyzed whether α-synuclein was involved in the differential interneuron vulnerability associated with PD in the retrobulbar, cortical anterior and cortical posterior divisions of the AON. First, we determined the expression of the calcium binding interneuron markers, calretinin, calbindin and parvalbumin, as well as non-calcium binding interneuron marker, somatostatin, in neuronal cell bodies alone (cells/mm²) and in neuronal cell bodies and neurites (% of area fraction) of post-mortem tissue from PD cases and age-matched controls (n = 4 for each) by immunofluorescent confocal microscopy. Results indicated that parvalbumin expression was upregulated in neuronal cell bodies throughout the anterior olfactory nucleus of PD cases compared with controls. Furthermore, there was increased calbindin, calretinin and parvalbumin expression in the cell bodies and neurites of neurons in the retrobulbar division and also increased parvalbumin expression in the neurites of neurons in the cortical division; calretinin expression was also increased in neuronal cell bodies and neurites in the cortical posterior division. Second, we analyzed the co-localization of the above markers with α-synuclein, with results indicating that α-synuclein co-localized with the calcium-binding proteins, but only partially with somatostatin. Taken together, these results indicate differential expression levels among different neural markers in the divisions of the AON in PD cases and point to several possibilities, among them: possible neuroprotective mechanisms of calcium-binding proteins against α-synuclein; and the differential involvement of somatostatin in α-synuclein-positive cell bodies and neurites.

Thorny ganglion cells in marmoset retina: Morphological and neurochemical characterization with antibodies against calretinin

In primates, over 17 morphological types of retinal ganglion cell have been distinguished by their dendritic morphology and stratification, but reliable markers for specific ganglion cell populations are still rare. The calcium binding protein calretinin is known to be expressed in the inner nuclear and the ganglion cell layer of marmoset retina, however, the specific cell type(s) expressing calretinin in the ganglion cell layer are yet to be determined. Here, we identified calretinin positive retinal ganglion cells in the common marmoset Callithrix jacchus. Double labeling with the ganglion cell marker RBPMS demonstrated that the large majority (80%) of the calretinin positive cells in the ganglion cell layer are ganglion cells, and 20% are displaced amacrine cells. The calretinin positive ganglion cells made up on average 12% of the total ganglion cell population outside of the foveal region and their proportion increased with eccentricity. Prelabeling with antibodies against calretinin and subsequent intracellular injection with DiI revealed that the large majority of the injected cells (n = 74) were either narrow thorny or broad thorny ganglion cells, 14 cells were displaced amacrine cells. Narrow thorny cells were further distinguished into outer and inner stratifying cells. In addition, weakly labeled cells with a large soma were identified as parasol ganglion cells. Our results show that three types of thorny ganglion cells in marmoset retina can be identified with antibodies against calretinin. Our findings are also consistent with the idea that the proportion of wide-field ganglion cell types increases in peripheral retina.

Distinct Neural Properties in the Low-Frequency Region of the Chicken Cochlear Nucleus Magnocellularis

Topography in the avian cochlear nucleus magnocellularis (NM) is represented as gradually increasing characteristic frequency (CF) along the caudolateral-to-rostromedial axis. In this study, we characterized the organization and cell biophysics of the caudolateral NM (NMc) in chickens (Gallus gallus). Examination of cellular and dendritic architecture first revealed that NMc contains small neurons and extensive dendritic processes, in contrast to adendritic, large neurons located more rostromedially. Individual dye-filling study further demonstrated that NMc is divided into two subregions, with NMc2 neurons having larger and more complex dendritic fields than NMc1. Axonal tract tracing studies confirmed that NMc1 and NMc2 neurons receive afferent inputs from the auditory nerve and the superior olivary nucleus, similar to the adendritic NM. However, the auditory axons synapse with NMc neurons via small bouton-like terminals, unlike the large end bulb synapses on adendritic NM neurons. Immunocytochemistry demonstrated that most NMc2 neurons express cholecystokinin but not calretinin, distinct from NMc1 and adendritic NM neurons that are cholecystokinin negative and mostly calretinin positive. Finally, whole-cell current clamp recordings revealed that NMc neurons require significantly lower threshold current for action potential generation than adendritic NM neurons. Moreover, in contrast to adendritic NM neurons that generate a single-onset action potential, NMc neurons generate multiple action potentials to suprathreshold sustained depolarization. Taken together, our data indicate that NMc contains multiple neuron types that are structurally, connectively, molecularly, and physiologically different from traditionally defined NM neurons, emphasizing specialized neural properties for processing low-frequency sounds.

Neuroprotective effects of ischemic preconditioning on hippocampal CA1 pyramidal neurons through maintaining calbindin D28k immunoreactivity following subsequent transient cerebral ischemia

Ischemic preconditioning elicited by a non-fatal brief occlusion of blood flow has been applied for an experimental therapeutic strategy against a subsequent fatal ischemic insult. In this study, we investigated the neuroprotective effects of ischemic preconditioning (2-minute transient cerebral ischemia) on calbindin D28k immunoreactivity in the gerbil hippocampal CA1 area following a subsequent fatal transient ischemic insult (5-minute transient cerebral ischemia). A large number of pyramidal neurons in the hippocampal CA1 area died 4 days after 5-minute transient cerebral ischemia. Ischemic preconditioning reduced the death of pyramidal neurons in the hippocampal CA1 area. Calbindin D28k immunoreactivity was greatly attenuated at 2 days after 5-minute transient cerebral ischemia and it was hardly detected at 5 days post-ischemia. Ischemic preconditioning maintained calbindin D28k immunoreactivity after transient cerebral ischemia. These findings suggest that ischemic preconditioning can attenuate transient cerebral ischemia-caused damage to the pyramidal neurons in the hippocampal CA1 area through maintaining calbindin D28k immunoreactivity.

Structural determinants of adhesion by Protocadherin-19 and implications for its role in epilepsy

Non-clustered δ-protocadherins are homophilic cell adhesion molecules essential for the development of the vertebrate nervous system, as several are closely linked to neurodevelopmental disorders. Mutations in protocadherin-19 (PCDH19) result in a female-limited, infant-onset form of epilepsy (PCDH19-FE). Over 100 mutations in PCDH19 have been identified in patients with PCDH19-FE, about half of which are missense mutations in the adhesive extracellular domain. Neither the mechanism of homophilic adhesion by PCDH19, nor the biochemical effects of missense mutations are understood. Here we present a crystallographic structure of the minimal adhesive fragment of the zebrafish Pcdh19 extracellular domain. This structure reveals the adhesive interface for Pcdh19, which is broadly relevant to both non-clustered δ and clustered protocadherin subfamilies. In addition, we show that several PCDH19-FE missense mutations localize to the adhesive interface and abolish Pcdh19 adhesion in in vitro assays, thus revealing the biochemical basis of their pathogenic effects during brain development.

DOI:http://dx.doi.org/10.7554/eLife.18529.001

As the brain develops, its basic building blocks – cells called neurons – need to form the correct connections with one another in order to give rise to neural circuits. A mistake that leads to the formation of incorrect connections can result in a number of disorders or brain abnormalities.

Proteins called cadherins that are present on the surface of neurons enable them to stick to their correct partners like Velcro. One of these proteins is called Protocadherin-19. However, it was not fully understood how this protein forms an adhesive bond with other Protocadherin-19 molecules, or how some of the proteins within the cadherin family are able to distinguish between one another.

Cooper et al. used X-ray crystallography to visualize the molecular structure of Protocadherin-19 taken from zebrafish in order to better understand the adhesive bond that these proteins form with each other. In addition, the new structure showed the sites of the mutations that cause a form of epilepsy in infant females. From this, Cooper et al. could predict how the mutations would disrupt Protocadherin-19’s shape and function.

The structures revealed that Protocadherin-19 molecules from adjacent cells engage in a “forearm handshake” to form the bond that connects neurons. Some of the mutations that cause epilepsy occur in the region responsible for this Protocadherin-19 forearm handshake. Laboratory experiments confirmed that these mutations impair the formation of the adhesive bond, revealing the molecular basis for some of the mutations that underlie Protocadherin-19-female-limited epilepsy.

Other cadherin molecules may interact via a similar forearm handshake; this could be investigated in future experiments. It also remains to be discovered how brain wiring depends on Protocadherin-19 adhesion in animal development, and how altering these proteins can rewire developing brain circuits.

DOI:http://dx.doi.org/10.7554/eLife.18529.002

The C-terminal calcium-sensitive disordered motifs regulate isoform-specific polymerization characteristics of calsequestrin

Calsequestrin (CASQ) exists as two distinct isoforms CASQ1 and CASQ2 in all vertebrates. Although the isoforms exhibit unique functional characteristic, the structural basis for the same is yet to be fully defined. Interestingly, the C-terminal region of the two isoforms exhibit significant differences both in length and amino acid composition; forming Dn-motif and DEXn-motif in CASQ1 and CASQ2, respectively. Here, we investigated if the unique C-terminal motifs possess Ca(2+)-sensitivity and affect protein function. Sequence analysis shows that both the Dn- and DEXn-motifs are intrinsically disordered regions (IDRs) of the protein, a feature that is conserved from fish to man. Using purified synthetic peptides, we show that these motifs undergo distinctive Ca(2+)-mediated folding suggesting that these disordered motifs are Ca(2+)-sensitivity. We generated chimeric proteins by swapping the C-terminal portions between CASQ1 and CASQ2. Our studies show that the C-terminal portions do not play significant role in protein folding. An interesting finding of the current study is that the switching of the C-terminal portion completely reverses the polymerization kinetics. Collectively, these data suggest that these Ca(2+)-sensitivity IDRs located at the back-to-back dimer interface influence isoform-specific Ca(2+)-dependent polymerization properties of CASQ.

Identification and molecular characterization of 48 kDa calcium binding protein as calreticulin from finger millet (Eleusine coracana) using peptide mass fingerprinting and transcript profiling

BACKGROUND

Attempts were made to identify and characterize the calcium binding proteins (CaBPs) in grain filling stages of finger millet using proteomics, bioinformatics and molecular approaches.

RESULTS

A distinctly observed blue color band of 48 kDa stained by Stains-all was eluted and analyzed as calreticulin (CRT) using nano liquid chromatography-tandem mass spectrometry (nano LC-MS). Based on the top hits of peptide mass fingerprinting results, conserved primers were designed for isolation of the CRT gene from finger millet using calreticulin sequences of different cereals. The deduced nucleotide sequence analysis of 600 bp amplicon showed up to 91% similarity with CRT gene(s) of rice and other plant species and designated as EcCRT1. Transcript profiling of EcCRT1 showed different levels of relative expression at different stages of developing spikes. The higher expression of EcCRT1 transcripts and protein were observed in later stages of developing spikes which might be due to greater translational synthesis of EcCRT1 protein during seed maturation in finger millet.

CONCLUSIONS

Preferentially higher synthesis of this CaBP during later stages of grain filling may be responsible for the sequestration of calcium in endoplasmic reticulum of finger millet grains.

Nuclear Calcium Buffering Capacity Shapes Neuronal Architecture

Calcium-binding proteins (CaBPs) such as parvalbumin are part of the cellular calcium buffering system that determines intracellular calcium diffusion and influences the spatiotemporal dynamics of calcium signals. In neurons, CaBPs are primarily localized to the cytosol and function, for example, in nerve terminals in short-term synaptic plasticity. However, CaBPs are also expressed in the cell nucleus, suggesting that they modulate nuclear calcium signals, which are key regulators of neuronal gene expression. Here we show that the calcium buffering capacity of the cell nucleus in mouse hippocampal neurons regulates neuronal architecture by modulating the expression levels of VEGFD and the complement factor C1q-c, two nuclear calcium-regulated genes that control dendrite geometry and spine density, respectively. Increasing the levels of nuclear calcium buffers by means of expression of a nuclearly targeted form of parvalbumin fused to mCherry (PV.NLS-mC) led to a reduction in VEGFD expression and, as a result, to a decrease in total dendritic length and complexity. In contrast, mRNA levels of the synapse pruning factor C1q-c were increased in neurons expressing PV.NLS-mC, causing a reduction in the density and size of dendritic spines. Our results establish a close link between nuclear calcium buffering capacity and the transcription of genes that determine neuronal structure. They suggest that the development of cognitive deficits observed in neurological conditions associated with CaBP deregulation may reflect the loss of necessary structural features of dendrites and spines.

Ca(2+) binding protein S100A1 competes with calmodulin and PIP2 for binding site on the C-terminus of the TPRV1 receptor

Transient receptor potential vanilloid 1 ion channel (TRPV1) belongs to the TRP family of ion channels. These channels play a role in many important biological processes such as thermosensation and pain transduction. The TRPV1 channel was reported to be also involved in nociception. Ca(2+) ions are described to participate in the regulation of TRP channels through the interaction with Ca(2+)-binding proteins, such as calmodulin or S100A1. Calmodulin is involved in the Ca(2+)-dependent regulation of TRPV1 via its binding to the TRPV1 C-terminal region. However, the role of the Ca(2+)-binding protein S100A1 in the process of TRP channel regulation remains elusive. Here we characterized a region on the TRPV1 C-terminus responsible for the interaction with S100A1 using biochemical and biophysical tools. We found that this region overlaps with previously identified calmodulin and PIP2 binding sites and that S100A1 competes with calmodulin and PIP2 for this binding site. We identified several positively charged residues within this region, which have crucial impact on S100A1 binding, and we show that the reported S100A1-TRPV1 interaction is calcium-dependent. Taken together, our data suggest a mechanism for the mutual regulation of PIP2 and the Ca(2+)-binding proteins S100A1 and calmodulin to TRPV1.

Ca(2+)-binding protein expression in primary human thyrocytes

We recently identified several Ca(2+)-binding proteins (CaBP) from the S100 and annexin family to be regulated by TSH in FRTL-5 cells. Here, we study the regulation of S100A4, S100A6 and ANXA2 in primary human thyrocytes (PHT) derived from surrounding tissues (ST), cold benign thyroid nodules (CTN) and autonomously functioning thyroid nodules (AFTN). We investigated the expression and regulation of CaBP and the effect of their expression on Ca(2+) and TSHR signaling. We used an approach that accounts for the potential of an individual PHT culture to proliferate or to express thyroid differentiation features by assessing the expression of FOS and TPO. We found a strong correlation between the regulation of CaBP and the proliferation-associated transcription factor gene FOS. PKA and MEK1/2 were regulators of ANXA2 expression, while PI3-K and triiodothyronine were additionally involved in S100 regulation. The modulated expression of CaBP was reflected by changes in ATP-elicited Ca(2+) signaling in PHT. S100A4 increased the ratio of subsequent Ca(2+) responses and showed a Ca(2+) buffering effect, while ANXA2 affected the first Ca(2+) response to ATP. Overexpression of S100A4 led to a reduced activation of NFAT by TSH. Using S100A4 E33Q, D63N, F72Q and Y75K mutants we found that the effects of S100A4 expression on Ca(2+) signaling are mediated by protein interaction. We present evidence that TSH has the ability to fine-tune Ca(2+) signals through the regulation of CaBP expression. This represents a novel putative cross-regulating mechanism in thyrocytes that could affect thyrocyte signaling and physiology.

Identification of calcium binding sites on calsequestrin 1 and their implications for polymerization

Biophysical studies have shown that each molecule of calsequestrin 1 (CASQ1) can bind about 70-80 Ca(2+) ions. However, the nature of Ca(2+)-binding sites has not yet been fully characterized. In this study, we employed in silico approaches to identify the Ca(2+) binding sites and to understand the molecular basis of CASQ1-Ca(2+) recognition. We built the protein model by extracting the atomic coordinates for the back-to-back dimeric unit from the recently solved hexameric CASQ1 structure (PDB id: ) and adding the missing C-terminal residues (aa350-364). Using this model we performed extensive 30 ns molecular dynamics simulations over a wide range of Ca(2+) concentrations ([Ca(2+)]). Our results show that the Ca(2+)-binding sites on CASQ1 differ both in affinity and geometry. The high affinity Ca(2+)-binding sites share a similar geometry and interestingly, the majority of them were found to be induced by increased [Ca(2+)]. We also found that the system shows maximal Ca(2+)-binding to the CAS (consecutive aspartate stretch at the C-terminus) before the rest of the CASQ1 surface becomes saturated. Simulated data show that the CASQ1 back-to-back stacking is progressively stabilized by the emergence of an increasing number of hydrophobic interactions with increasing [Ca(2+)]. Further, this study shows that the CAS domain assumes a compact structure with an increase in Ca(2+) binding, which suggests that the CAS domain might function as a Ca(2+)-sensor that may be a novel structural motif to sense metal. We propose the term "Dn-motif" for the CAS domain.

Time-dependent changes of calbindin D-28K and parvalbumin immunoreactivity in the hippocampus of rats with streptozotocin-induced type 1 diabetes

The hippocampus is affected by various stimuli that include hyperglycemia, depression, and ischemia. Calcium-binding proteins (CaBPs) have protective roles in the response to such stimuli. However, little is known about the expression of CaBPs under diabetic conditions. This study was conducted to examine alterations in the physiological parameters with type 1 diabetes induced with streptozotocin (STZ) as well as time-dependent changes in the expression of two CaBPs changes of were being evaluated. Rats treated with STZ (70 mg/kg) had high blood glucose levels (>21.4 mmol/L) along with increased food intake and water consumption volumes compared to the sham controls. In contrast, body weight of the animals treated with STZ was significantly reduced compared to the sham group. CB-specific immunoreactivity was generally increased in the hippocampal CA1 region and granule cell layer of the dentate gyrus (DG) 2 weeks after STZ treatment, but decreased thereafter in these regions. In contrast, the number of PV-immunoreactive neurons and fibers was unchanged in the hippocampus and DG 2 weeks after STZ treatment. However, this number subsequently decreased over time. These results suggest that CB and PV expression is lowest 3 weeks after STZ administration, and these deficits lead to disturbances in calcium homeostasis.

S100 calcium binding proteins and ion channels

S100 Ca(2+)-binding proteins have been associated with a multitude of intracellular Ca(2+)-dependent functions including regulation of the cell cycle, cell differentiation, cell motility and apoptosis, modulation of membrane-cytoskeletal interactions, transduction of intracellular Ca(2+) signals, and in mediating learning and memory. S100 proteins are fine tuned to read the intracellular free Ca(2+) concentration and affect protein phosphorylation, which makes them candidates to modulate certain ion channels and neuronal electrical behavior. Certain S100s are secreted from cells and are found in extracellular fluids where they exert unique extracellular functions. In addition to their neurotrophic activity, some S100 proteins modulate neuronal electrical discharge activity and appear to act directly on ion channels. The first reports regarding these effects suggested S100-mediated alterations in Ca(2+) fluxes, K(+) currents, and neuronal discharge activity. Recent reports revealed direct and indirect interactions with Ca(2+), K(+), Cl(-), and ligand activated channels. This review focuses on studies of the physical and functional interactions of S100 proteins and ion channels.

Calcium influx and DREAM protein are required for GnRH gene expression pulse activity

Recent evidence using GT1-7 cells indicates that GnRH pulsatility depends on exocytotic-release and gene transcription events. To determine whether calcium or DREAM may play a role in linking these processes, we used an L-type Ca(2+)-blocker (nimodipine) and found that not only GnRH gene expression (GnRH-GE) pulse activity was abolished but also that binding of proteins to OCT1BS-a (essential site for GnRH-GE pulses) was reduced. We further found that only EF-hand forms of DREAM were expressed in GT1-7 and that DREAM was part of the complex binding to OCT1BS-a. Finally, microinjection of DREAM antibody into cells abolished GnRH-GE pulses demonstrating its importance in pulsatility. These results reveal that calcium and DREAM may bridge cytoplasmic and nuclear events enabling temporal coordination of intermittent activity. Expression of DREAM in various cell types coupled with the universal role of calcium raise the possibility that these factors may play similar role in other secretory cells.

Effects of Phosphorylation in Chlamydomonas Centrin Ser 167

Centrin is a conserved calcium binding protein belonging to the EF-hand superfamily with two independent structural domains. This protein is found to be phosphorylated near the carboxyl terminal end. Our goal was to perform a novel comparative study of phosphorylated and unphosphorylated centrin by Fourier transform infrared (FT-IR) spectroscopy, two-dimensional correlation spectroscopy (2D-COS) analysis and differential scanning calorimetry (DSC). To achieve this goal, we have bacterially expressed, isolated, purified and phosphorylated centrin. We verified the extent of phosphorylation to be >97% for centrin by MALDI MS analysis and determined the absence of aggregated protein. The thermal denaturation temperature and ΔCp were determined to be T(m) = 112.1 °C (ΔCp = 7.8 Kcal/mole/ΔC) and T(m) = 111.0°C (ΔCp = 5.0 Kcal/mole/°C) for holo-centrin and phosphorylated centrin, respectively. We have also described the molecular dynamics leading up to the thermal denaturation of the protein: for holo-centrin the vibrational modes associated with the calcium binding sites aspartates and glutamates, loops then the arginines, followed by the structured backbone vibrational modes the α-helix at 1635 cm(-1) then β-sheet and finally the more exposed α-helix at 1650 cm(-1); while for phosphorylated centrin aspartate, glutamate and arginine, followed by the backbone associated vibrational modes α-helix (1650 cm(-1)), loop then the β-sheet (1633 cm(-1)) and finally the α-helix (1637 cm(-1)). Therefore, the effect on domain stability due to phosphorylation at Ser(167) was observed in the loops as well as the α-helix at 1650 cm(-1).

Localization of neuropeptide Y1 receptor immunoreactivity in the rat retina and the synaptic connectivity of Y1 immunoreactive cells

Neuropeptide Y (NPY), an inhibitory neuropeptide expressed by a moderately dense population of wide-field amacrine cells in the rat retina, acts through multiple (Y1-y6) G-protein-coupled receptors. This study determined the cellular localization of Y1 receptors and the synaptic connectivity of Y1 processes in the inner plexiform layer (IPL) of the rat retina. Specific Y1 immunoreactivity was localized to horizontal cell bodies in the distal inner nuclear layer and their processes in the outer plexiform layer. Immunoreactivity was also prominent in cell processes located in strata 2 and 4, and puncta in strata 4 and 5 of the IPL. Double-label immunohistochemical experiments with calbindin, a horizontal cell marker, confirmed Y1 immunostaining in all horizontal cells. Double-label immunohistochemical experiments, using antibodies to choline acetyltransferase and vesicular acetylcholine transporter to label cholinergic amacrine cell processes, demonstrated that Y1 immunoreactivity in strata 2 and 4 of the IPL was localized to cholinergic amacrine cell processes. Electron microscopic studies of the inner retina showed that Y1-immunostained amacrine cell processes and puncta received synaptic inputs from unlabeled amacrine cell processes (65.2%) and bipolar cell axon terminals (34.8%). Y1-immunoreactive amacrine cell processes most frequently formed synaptic outputs onto unlabeled amacrine cell processes (34.0%) and ganglion cell dendrites (54.1%). NPY immunoreactivity in the rat retina is distributed primarily to strata 1 and 5 of the IPL, and the present findings, thus, suggest that NPY acts in a paracrine manner on Y1 receptors to influence both horizontal and amacrine cells.

Calreticulin is essential for cardiac development

Calreticulin is a ubiquitous Ca2+ binding protein, located in the endoplasmic reticulum lumen, which has been implicated in many diverse functions including: regulation of intracellular Ca2+ homeostasis, chaperone activity, steroid-mediated gene regulation, and cell adhesion. To understand the physiological function of calreticulin we used gene targeting to create a knockout mouse for calreticulin. Mice homozygous for the calreticulin gene disruption developed omphalocele (failure of absorption of the umbilical hernia) and showed a marked decrease in ventricular wall thickness and deep intertrabecular recesses in the ventricular walls. Transgenic mice expressing a green fluorescent protein reporter gene under the control of the calreticulin promoter were used to show that the calreticulin gene is highly activated in the cardiovascular system during the early stages of cardiac development. Calreticulin protein is also highly expressed in the developing heart, but it is only a minor component of the mature heart. Bradykinin-induced Ca2+ release by the InsP3-dependent pathway was inhibited in crt-/- cells, suggesting that calreticulin plays a role in Ca2+ homeostasis. Calreticulin-deficient cells also exhibited impaired nuclear import of nuclear factor of activated T cell (NF-AT3) transcription factor indicating that calreticulin plays a role in cardiac development as a component of the Ca2+/calcineurin/NF-AT/GATA-4 transcription pathway.

Calbindin D28k blocks the proapoptotic actions of mutant presenilin 1: reduced oxidative stress and preserved mitochondrial function

Mutations in the presenilin 1 (PS-1) gene account for many cases of early-onset autosomal dominant inherited forms of Alzheimer's disease. Recent findings suggest that PS-1 mutations may sensitize neurons to apoptosis induced by trophic factor withdrawal and exposure to amyloid beta-peptide (Abeta). We now report that overexpression of the calcium-binding protein calbindin D28k prevents apoptosis in cultured neural cells expressing mutant PS-1 (L286V and M146V missense mutations). Elevations of the intracellular Ca2+ concentration and generation of reactive oxygen species induced by Abeta, and potentiated by mutant PS-1, were suppressed in calbindin-overexpressing cells. Impairment of mitochondrial function by Abeta (which preceded apoptosis) was exacerbated by PS-1 mutations and was largely prevented by calbindin. These findings suggest that PS-1 mutations render neurons vulnerable to apoptosis by a mechanism involving destabilization of cellular calcium homeostasis, which leads to oxidative stress and mitochondrial dysfunction.

Projections of chemically identified myenteric neurons of the small and large intestine of the mouse

The projections of different subpopulations of myenteric neurons in the mouse small and large intestine were examined by combining immunohistological techniques with myotomy and myectomy operations. The myotomies were used to examine the polarity of neurons projecting within the myenteric plexus and showed that neurons containing immunoreactivity for nitric oxide synthase (NOS), vasoactive intestinal peptide (VIP), calbindin and 5-HT projected anally, while neurons with substance P (SP)-immunoreactivity projected orally, in both the small and large intestine. Neurons containing neuropeptide Y (NPY)- and calretinin-immunoreactivity projected locally. In the large intestine, GABA-immunoreactive neurons projected both orally and anally, with more axons tending to project anally. Myectomy operations revealed that circular muscle motor neurons containing NOS/VIP/ +/-NPY and calretinin neurons projected anally both in the small and large intestine, while SP-immunoreactive circular muscle motor neurons projected orally. In the large intestine, GABA-IR circular muscle motor neurons projected both orally and anally. This study showed that although some neurons, such as the NOS/VP inhibitory motor neurons and interneurons, SP excitatory motor neurons and 5-HT interneurons had similar projections to those in other species, the projections of other chemical classes of neurons in the mouse intestine differed from those reported in other species.

Calcium-dependent regulator protein: localization in mitotic apparatus of eukaryotic cells

Calcium-dependent regulator protein is a low molecular weight (17,000), thermostable, calcium binding protein which is structurally homologous to skeletal muscle troponin C. This protein is present in all nonmuscle cells and has been shown to decorate stress fibers in interphase cells by indirect immunofluorescence. Using this procedure we have investigated the distribution of the protein during mitosis of eukaryotic cells. As the cells enter prophase, the distinct cytoplasmic localization disappears commensurate with the dissolution of the cytoskeleton. The regulator protein seems to be randomly distributed throughout the prophase cell, including the region around the condensed chromosomes. However, at prometaphase, it is localized in association with the half-spindles of the mitotic apparatus. Through metaphase and most of anaphase, the protein remains localized between the chromosomes and the poles of the spindle. During late anaphase the protein is also found in the interzone region but rapidly condenses into two small regions, one on each side of the midbody that separates the daughter cells. The regulator protein is not localized in the cleavage furrow during telophase, whereas actin is demonstrable in this region. Indeed, placement of the protein during mitosis is distinct from both that of actin and that of tubulin. The localization of calcium-dependent regulator protein during mitosis suggests that it may mediate the calcium effects on the mitotic apparatus and thus play a role in chromosome movement.