Myo-inositol monophosphatase is an activated target of calbindin D28k

Emerging role of inositol monophosphatase in cancer

Inositol monophosphatase (IMPase) is an enzyme with two homologs-IMPA1 and IMPA2-that is responsible for dephosphorylating myo-inositol monophosphate to generate myo-inositol. IMPase has been extensively studied in neuropsychiatric diseases and is regarded as a susceptibility gene. Recently, emerging evidence has implied that IMPase is linked to cancer development and progression and correlates with patient survival outcomes. Interestingly, whether it acts as a tumor-promoter or tumor-suppressor is inconsistent among different research studies. In this review, we summarize the latest findings on IMPase in cancer, focusing on exploring the underlying mechanisms for its pro- and anticancer roles. In addition, we discuss the potential methods of IMPase regulation in cancer cells and the possible approaches for IMPase intervention in clinical practice.

Exploring Calbindin-IMPase fusion proteins structure and activity

Calbindin-D28k is a calcium binding protein that is highly expressed in the mammalian central nervous system. It has been reported that calbindin-D28k binds to and increases the activity of inositol Monophosphatase (IMPase). This is an enzyme that is involved in the homeostasis of the Inositol trisphosphate signalling cascade by catalysing the final dephosphorylation of inositol and has been implicated in the therapeutic mechanism of lithium treatment of bipolar disorder. Previously studies have shown that calbindin-D28k can increase IMPase activity by up to 250 hundred-fold. A preliminary in silico model was proposed for the interaction.

Here, we aimed at exploring the shape and properties of the calbindin-IMPase complex to gain new insights on this biologically important interaction. We created several fusion constructs of calbindin-D28k and IMPase, connected by flexible amino acid linkers of different lengths and orientations to fuse the termini of the two proteins together. The resulting fusion proteins have activities 200%–400% higher the isolated wild-type IMPase. The constructs were characterized by small angle X-ray scattering to gain information on the overall shape of the complexes and validate the previous model. The fusion proteins form a V-shaped, elongated and less compact complex as compared to the model. Our results shed new light into this protein-protein interaction.

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The interaction between calbindin-D28k and the enzyme IMPase is studies using fusion proteins.

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The fusion proteins have activities 200%–400% higher than wild-type IMPase.

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The calbindin-D28k and the enzyme IMPase fusion proteins have a V-shaped structure.

Comorbidity of osteoporosis and Alzheimer's disease: Is `AKT `-ing on cellular glucose uptake the missing link?

Osteoporosis and Alzheimer's disease (AD) are both degenerative diseases. Osteoporosis often proceeds cognitive deficits, and multiple studies have revealed common triggers that lead to energy deficits in brain and bone. Risk factors for osteoporosis and AD, such as obesity, type 2 diabetes, aging, chemotherapy, vitamin deficiency, alcohol abuse, and apolipoprotein Eε4 and/or Il-6 gene variants, reduce cellular glucose uptake, and protective factors, such as estrogen, insulin, exercise, mammalian target of rapamycin inhibitors, hydrogen sulfide, and most phytochemicals, increase uptake. Glucose uptake is a fine-tuned process that depends on an abundance of glucose transporters (Gluts) on the cell surface. Gluts are stored in vesicles under the plasma membrane, and protective factors cause these vesicles to fuse with the membrane, resulting in presentation of Gluts on the cell surface. This translocation depends mainly on AKT kinase signaling and can be affected by a range of factors. Reduced AKT kinase signaling results in intracellular glucose deprivation, which causes endoplasmic reticulum stress and iron depletion, leading to activation of HIF-1α, the transcription factor necessary for higher Glut expression. The link between diseases and aging is a topic of growing interest. Here, we show that diseases that affect the same biochemical pathways tend to co-occur, which may explain why osteoporosis and/or diabetes are often associated with AD.

Cytosolic Ca2+ Buffers Are Inherently Ca2+ Signal Modulators

For precisely regulating intracellular Ca²⁺ signals in a time- and space-dependent manner, cells make use of various components of the "Ca²⁺ signaling toolkit," including Ca²⁺ entry and Ca²⁺ extrusion systems. A class of cytosolic Ca²⁺-binding proteins termed Ca²⁺ buffers serves as modulators of such, mostly short-lived Ca²⁺ signals. Prototypical Ca²⁺ buffers include parvalbumins (α and β isoforms), calbindin-D9k, calbindin-D28k, and calretinin. Although initially considered to function as pure Ca²⁺ buffers, that is, as intracellular Ca²⁺ signal modulators controlling the shape (amplitude, decay, spread) of Ca²⁺ signals, evidence has accumulated that calbindin-D28k and calretinin have additional Ca²⁺ sensor functions. These other functions are brought about by direct interactions with target proteins, thereby modulating their targets' function/activity. Dysregulation of Ca²⁺ buffer expression is associated with several neurologic/neurodevelopmental disorders including autism spectrum disorder (ASD) and schizophrenia. In some cases, the presence of these proteins is presumed to confer a neuroprotective effect, as evidenced in animal models of Parkinson's or Alzheimer's disease.

Loss-of-function mutation in inositol monophosphatase 1 (IMPA1) results in abnormal synchrony in resting-state EEG

Background

Dysregulation of the inositol cycle is implicated in a wide variety of human diseases, including developmental defects and neurological diseases. A homozygous frameshift mutation in IMPA1, coding for the enzyme inositol monophosphatase 1 (IMPase), has recently been associated with severe intellectual disability (ID) in a geographically isolated consanguineous family in Northeastern Brazil (Figueredo et al., 2016). However, the neurophysiologic mechanisms that mediate the IMPA1 mutation and associated ID phenotype have not been characterized. To this end, resting EEG (eyes-open and eyes-closed) was collected from the Figueredo et al. pedigree. Quantitative EEG measures, including mean power, dominant frequency and dominant frequency variability, were investigated for allelic associations using multivariate family-based association test using generalized estimating equations.

Results

We found that the IMPA1 mutation was associated with relative decreases in frontal theta band power as well as altered alpha-band variability with no regional specificity during the eyes-open condition. For the eyes-closed condition, there was altered dominant theta frequency variability in the central and parietal regions.

Conclusions

These findings represent the first human in vivo phenotypic assessment of brain function disturbances associated with a loss-of-function IMPA1 mutation, and thus an important first step towards an understanding the pathophysiologic mechanisms of intellectual disability associated with the mutation that affects this critical metabolic pathway.

The X-ray structure of human calbindin-D28K: an improved model

Calbindin-D28K is a widely expressed calcium-buffering cytoplasmic protein that is involved in many physiological processes. It has been shown to interact with other proteins, suggesting a role as a calcium sensor. Many of the targets of calbindin-D28K are of therapeutic interest: for example, inositol monophosphatase, the putative target of lithium therapy in bipolar disorder. Presented here is the first crystal structure of human calbindin-D28K. There are significant deviations in the tertiary structure when compared with the NMR structure of rat calbindin-D28K (PDB entry 2g9b), despite 98% sequence identity. Small-angle X-ray scattering (SAXS) indicates that the crystal structure better predicts the properties of calbindin-D28K in solution compared with the NMR structure. Here, the first direct visualization of the calcium-binding properties of calbindin-D28K is presented. Four of the six EF-hands that make up the secondary structure of the protein contain a calcium-binding site. Two distinct conformations of the N-terminal EF-hand calcium-binding site were identified using long-wavelength calcium single-wavelength anomalous dispersion (SAD). This flexible region has previously been recognized as a protein-protein interaction interface. SAXS data collected in both the presence and absence of calcium indicate that there are no large structural differences in the globular structure of calbindin-D28K between the calcium-loaded and unloaded proteins.

Suppressed Calbindin Levels in Hippocampal Excitatory Neurons Mediate Stress-Induced Memory Loss

Calbindin modulates intracellular Ca²⁺ dynamics and synaptic plasticity. Reduction of hippocampal calbindin levels has been implicated in early-life stress-related cognitive disorders, but it remains unclear how calbindin in distinct populations of hippocampal neurons contributes to stress-induced memory loss. Here we report that early-life stress suppressed calbindin levels in CA1 and dentate gyrus (DG) neurons, and calbindin knockdown in adult CA1 or DG excitatory neurons mimicked early-life stress-induced memory loss. In contrast, calbindin knockdown in CA1 interneurons preserved long-term memory even after an acute stress challenge. These results indicate that the dysregulation of calbindin in hippocampal excitatory, but not inhibitory, neurons conveys susceptibility to stress-induced memory deficits. Moreover, calbindin levels were downregulated by early-life stress through the corticotropin-releasing hormone receptor 1-nectin3 pathway, which in turn reduced inositol monophosphatase levels. Our findings highlight calbindin as a molecular target of early-life stress and an essential substrate for memory.

Expression and colocalization patterns of calbindin-D28k, calretinin and parvalbumin in the rat hypothalamic arcuate nucleus

Calcium binding proteins (CaBPs) form a diverse group of molecules that function as signal transducers or as intracellular buffers of Ca(2+) concentration. They have been extensively used to histochemically categorize cell types throughout the brain. One region which has not yet been characterized with regard to CaBP expression is the hypothalamic arcuate nucleus, which plays a vital role in neuroendocrine control and the central regulation of energy metabolism. Using in situ hybridization and immunofluorescence, we have investigated the cellular distribution of the three CaBPs, calbindin-D28k (CB), calretinin (CR) and parvalbumin (PV) in the rat arcuate nucleus. Both mRNA and immunoreactivity was detected in the arcuate nucleus for CB - located in the medial aspects - and CR - located ventrolaterally. No PV mRNA was detected in the arcuate nucleus. Immunofluorescence results for PV were ambiguous; while one antibody detected a group of cell somata, a different antibody failed to visualize any arcuate nucleus cell profiles. Using double-labeling, neither of the examined CaBPs were observed in cells immunoreactive for the signaling molecules agouti gene-related protein, tyrosine hydroxylase, neurotensin, growth hormone-releasing hormone, somatostatin, enkephalin, dynorphin or galanin. We did, however, observe CB- and CR-immunoreactivity, in two distinct populations of neurons immunoreactive for the melanocortin peptide α-melanocyte-stimulating hormone. These data identify distinct subpopulations of arcuate neurons defined by their expression of CaBPs and provide further support for differentiation between subpopulations of anorexigenic melanocortin neurons.

Inhibition of inositol monophosphatase (IMPase) at the calbindin-D28k binding site: molecular and behavioral aspects

Bipolar-disorder (manic-depressive illness) is a severe chronic illness affecting ∼1% of the adult population. It is treated with mood-stabilizers, the prototypic one being lithium-salts (lithium), but it has life threatening side-effects and a significant number of patients fail to respond. The lithium-inhibitable enzyme inositol-monophosphatase (IMPase) is one of the viable targets for lithium's mechanism of action. Calbindin-D28k (calbindin) up-regulates IMPase activity. The IMPase-calbindincomplex was modeled using the program MolFit. The in-silico model indicated that the 55-66 amino-acid segment of IMPase anchors calbindin via Lys59 and Lys61 with a glutamate in between (Lys-Glu-Lys motif) and that the motif interacts with residues Asp24 and Asp26 of calbindin. We found that differently from wildtype calbindin, IMPase was not activated by mutated calbindin in which Asp24 and Asp26 were replaced by alanine. Calbindin's effect was significantly reduced by a linear peptide with the sequence of amino acids 58-63 of IMPase (peptide 1) and by six amino-acid linear peptides including at least part of the Lys-Glu-Lys motif. The three amino-acid peptide Lys-Glu-Lys or five amino-acid linear peptides containing this motif were ineffective. Mice administered peptide 1 intracerebroventricularly exhibited a significant anti-depressant-like reduced immobility in the forced-swim test. Based on the sequence of peptide 1, and to potentially increase the peptide's stability, cyclic and linear pre-cyclic analog peptides were synthesized. One cyclic peptide and one linear pre-cyclic analog peptide inhibited calbindin-activated brain IMPase activity in-vitro. Our findings may lead to the development of molecules capable of inhibiting IMPase activity at an alternative site than that of lithium.

Role of calbindin-D28K in estrogen treatment for Parkinson's disease

Studies have shown that estrogen has neuroprotective effects on the nigrostriatal system. The present study established a Parkinson's disease model in C57BL/6 mice by intraperitoneal injection of 1-methyl-4-phenyl-1,2,3,6-tetrapyridine. The mice were subjected to 17β estradiol injection into the lateral ventricle. Immunofluorescence double staining showed that estrogen increased tyrosine hydroxylase and calbindin-D28K expression and co-expression in dopaminergic neurons of midbrain substantia nigra pars compacta of model mice. Behavior experiments showed that estrogen improved swimming and hanging behaviors in this mouse model of Parkinson's disease.

Human myo-inositol monophosphatase 2 rescues the nematode thermotaxis mutant ttx-7 more efficiently than IMPA1: functional and evolutionary considerations of the two mammalian myo-inositol monophosphatase genes

Mammals express two myo-inositol monophosphatase (IMPase) genes, IMPA1/Impa1 and IMPA2/Impa2. In this study, we compared the spatial expression patterns of the two IMPase gene transcripts and proteins in mouse tissues. Results indicated discrete expression of the two IMPase genes and their protein products in various organs, including the brain. In Caenorhabditis elegans, loss of the IMPase gene, ttx-7, disrupts cellular polarity in RIA neurons, eliciting abnormal thermotaxis behavior. We performed a rescue experiment in mutant nematodes using mammalian IMPases. Human IMPA2 rescued the abnormal behavioral phenotype in the ttx-7 mutants more efficiently than IMPA1. These results raise a question about the phylogenetic origin of IMPases and the biological roles of mammalian IMPase 2 in mammals. Impa2 knockout mice generated in our laboratory, exhibited neither behavioral abnormalities nor a significant reduction in myo-inositol content in the brain and other examined tissues. Given the ability of human IMPA2 to rescue the ttx-7 mutant, and its genetic association with multiple neuropsychiatric disorders, close scrutiny of IMPA2 function and the evolutionary origin of IMPase genes is warranted.

Three functional facets of calbindin D-28k

Many neurons of the vertebrate central nervous system (CNS) express the Ca²⁺ binding protein calbindin D-28k (CB), including important projection neurons like cerebellar Purkinje cells but also neocortical interneurons. CB has moderate cytoplasmic mobility and comprises at least four EF-hands that function in Ca²⁺ binding with rapid to intermediate kinetics and affinity. Classically it was viewed as a pure Ca²⁺ buffer important for neuronal survival. This view was extended by showing that CB is a critical determinant in the control of synaptic Ca²⁺ dynamics, presumably with strong impact on plasticity and information processing. Already 30 years ago, in vitro studies suggested that CB could have an additional Ca²⁺ sensor function, like its prominent acquaintance calmodulin (CaM). More recent work substantiated this hypothesis, revealing direct CB interactions with several target proteins. Different from a classical sensor, however, CB appears to interact with its targets both, in its Ca²⁺-loaded and Ca²⁺-free forms. Finally, CB has been shown to be involved in buffered transport of Ca²⁺, in neurons but also in kidney. Thus, CB serves a threefold function as buffer, transporter and likely as a non-canonical sensor.

Genetic biomarkers for ALS disease in transgenic SOD1(G93A) mice

The pathophysiological mechanisms of both familial and sporadic Amyotrophic Lateral Sclerosis (ALS) are unknown, although growing evidence suggests that skeletal muscle tissue is a primary target of ALS toxicity. Skeletal muscle biopsies were performed on transgenic SOD1^G93A mice, a mouse model of ALS, to determine genetic biomarkers of disease longevity. Mice were anesthetized with isoflurane, and three biopsy samples were obtained per animal at the three main stages of the disease. Transcriptional expression levels of seventeen genes, Ankrd1, Calm1, Col19a1, Fbxo32, Gsr, Impa1, Mef2c, Mt2, Myf5, Myod1, Myog, Nnt, Nogo A, Pax7, Rrad, Sln and Snx10, were tested in each muscle biopsy sample. Total RNA was extracted using TRIzol Reagent according to the manufacturer's protocol, and variations in gene expression were assayed by real-time PCR for all of the samples. The Pearson correlation coefficient was used to determine the linear correlation between transcriptional expression levels throughout disease progression and longevity. Consistent with the results obtained from total skeletal muscle of transgenic SOD1^G93A mice and 74-day-old denervated mice, five genes (Mef2c, Gsr, Col19a1, Calm1 and Snx10) could be considered potential genetic biomarkers of longevity in transgenic SOD1^G93A mice. These results are important because they may lead to the exploration of previously unexamined tissues in the search for new disease biomarkers and even to the application of these findings in human studies.

Differential alterations of synaptic plasticity in dentate gyrus and CA1 hippocampal area of Calbindin-D28K knockout mice

Regulation of the intracellular calcium concentration ([Ca(2+)](i)) is of critical importance for synaptic function. Therefore, neurons buffer [Ca(2+)](i) using intracellular Ca(2+)-binding proteins (CaBPs). Previous evidence suggests that Calbindin-D(28K) (CB), an abundantly expressed endogenous fast CaBP, plays an important role in neuronal survival, motor coordination, spatial learning paradigms and some forms of synaptic plasticity. In the present study, the role of CB in synaptic transmission and plasticity was further investigated using extracellular recordings of synaptic activity in cell- and dendritic layers of dentate gyrus (DG) and CA1 area in hippocampal slices from wild-type, heterozygous and homozygous CB knockout mice. The results demonstrate a consistent failure to maintain long-term potentiation (LTP) in hippocampal DG and CA1 area of knockout mice. Compared to wild-type mice, the paired-pulse ratio of EPSPs recorded in DG is significantly lower in slices from knockout mice, whereas it is significantly higher in CA1 area. The amplitude of the population spike recorded in CA1 area of wild-type mice steadily increases following tetanic stimulation, whereas it steadily decreases in knockout mice. The combined results demonstrate that the absence of CB results in an impairment of LTP maintenance in both hippocampal DG and CA1 area, whereas paired-pulse facilitation and cellular excitability in CA1 area are differentially affected. These results support the role of CB as a critical determinant for several forms of synaptic plasticity in hippocampal DG and CA1 area. It is hypothesized that CB functions as a postsynaptic Ca(2+) buffer as well as a presynaptic Ca(2+) sensor.

The renaissance of Ca2+-binding proteins in the nervous system: secretagogin takes center stage

Effective control of the Ca(2+) homeostasis in any living cell is paramount to coordinate some of the most essential physiological processes, including cell division, morphological differentiation, and intercellular communication. Therefore, effective homeostatic mechanisms have evolved to maintain the intracellular Ca(2+) concentration at physiologically adequate levels, as well as to regulate the spatial and temporal dynamics of Ca(2+)signaling at subcellular resolution. Members of the superfamily of EF-hand Ca(2+)-binding proteins are effective to either attenuate intracellular Ca(2+) transients as stochiometric buffers or function as Ca(2+) sensors whose conformational change upon Ca(2+) binding triggers protein-protein interactions, leading to cell state-specific intracellular signaling events. In the central nervous system, some EF-hand Ca(2+)-binding proteins are restricted to specific subtypes of neurons or glia, with their expression under developmental and/or metabolic control. Therefore, Ca(2+)-binding proteins are widely used as molecular markers of cell identity whilst also predicting excitability and neurotransmitter release profiles in response to electrical stimuli. Secretagogin is a novel member of the group of EF-hand Ca(2+)-binding proteins whose expression precedes that of many other Ca(2+)-binding proteins in postmitotic, migratory neurons in the embryonic nervous system. Secretagogin expression persists during neurogenesis in the adult brain, yet becomes confined to regionalized subsets of differentiated neurons in the adult central and peripheral nervous and neuroendocrine systems. Secretagogin may be implicated in the control of neuronal turnover and differentiation, particularly since it is re-expressed in neoplastic brain and endocrine tumors and modulates cell proliferation in vitro. Alternatively, and since secretagogin can bind to SNARE proteins, it might function as a Ca(2+) sensor/coincidence detector modulating vesicular exocytosis of neurotransmitters, neuropeptides or hormones. Thus, secretagogin emerges as a functionally multifaceted Ca(2+)-binding protein whose molecular characterization can unravel a new and fundamental dimension of Ca(2+)signaling under physiological and disease conditions in the nervous system and beyond.

Knockdown of MTDH sensitizes endometrial cancer cells to cell death induction by death receptor ligand TRAIL and HDAC inhibitor LBH589 co-treatment

Understanding the molecular underpinnings of chemoresistance is vital to design therapies to restore chemosensitivity. In particular, metadherin (MTDH) has been demonstrated to have a critical role in chemoresistance. Over-expression of MTDH correlates with poor clinical outcome in breast cancer, neuroblastoma, hepatocellular carcinoma and prostate cancer. MTDH is also highly expressed in advanced endometrial cancers, a disease for which new therapies are urgently needed. In this present study, we focused on the therapeutic benefit of MTDH depletion in endometrial cancer cells to restore sensitivity to cell death. Cells were treated with a combination of tumor necrosis factor-α-related apoptosis-inducing ligand (TRAIL), which promotes death of malignant cells of the human reproductive tract, and histone deacetylase (HDAC) inhibitors, which have been shown to increase the sensitivity of cancer cells to TRAIL-induced apoptosis. Our data indicate that depletion of MTDH in endometrial cancer cells resulted in sensitization of cells that were previously resistant in response to combinatorial treatment with TRAIL and the HDAC inhibitor LBH589. MTDH knockdown reduced the proportion of cells in S and increased cell arrest in G2/M in cells treated with LBH589 alone or LBH589 in combination with TRAIL, suggesting that MTDH functions at the cell cycle checkpoint to accomplish resistance. Using microarray technology, we identified 57 downstream target genes of MTDH, including calbindin 1 and galectin-1, which may contribute to MTDH-mediated therapeutic resistance. On the other hand, in MTDH depleted cells, inhibition of PDK1 and AKT phosphorylation along with increased Bim expression and XIAP degradation correlated with enhanced sensitivity to cell death in response to TRAIL and LBH589. These findings indicate that targeting or depleting MTDH is a potentially novel avenue for reversing therapeutic resistance in patients with endometrial cancer.

Cytosolic Ca2+ buffers

"Ca(2+) buffers," a class of cytosolic Ca(2+)-binding proteins, act as modulators of short-lived intracellular Ca(2+) signals; they affect both the temporal and spatial aspects of these transient increases in [Ca(2+)](i). Examples of Ca(2+) buffers include parvalbumins (α and β isoforms), calbindin-D9k, calbindin-D28k, and calretinin. Besides their proven Ca(2+) buffer function, some might additionally have Ca(2+) sensor functions. Ca(2+) buffers have to be viewed as one of the components implicated in the precise regulation of Ca(2+) signaling and Ca(2+) homeostasis. Each cell is equipped with proteins, including Ca(2+) channels, transporters, and pumps that, together with the Ca(2+) buffers, shape the intracellular Ca(2+) signals. All of these molecules are not only functionally coupled, but their expression is likely to be regulated in a Ca(2+)-dependent manner to maintain normal Ca(2+) signaling, even in the absence or malfunctioning of one of the components.

Lack of calbindin-D28k alters response of the murine circadian clock to light

A strong stimulus adjusting the circadian clock to the prevailing light-dark cycle is light. However, the circadian clock is reset by light only at specific times of the day. The mechanisms mediating such gating of light input to the CNS are not well understood. There is evidence that Ca(2+) ions play an important role in intracellular signaling mechanisms, including signaling cascades stimulated by light. Therefore, Ca(2+) is hypothesized to play a role in the light-mediated resetting of the circadian clock. Calbindin-D28k (CB; gene symbol: Calb1) is a Ca(2+) binding protein implicated in Ca(2+) homeostasis and sensing. The absence of this protein influences Ca(2+) buffering capacity of a cell, alters spatio-temporal aspects of intracellular Ca(2+) signaling, and hence might alter transmission of light information to the circadian clock in neurons of the suprachiasmatic nuclei (SCN). We tested mice lacking a functional Calb1 gene (Calb1(-/-)) and found an increased phase-delay response to light applied at circadian time (CT) 14 in these animals. This is accompanied by elevated induction of Per2 gene expression in the SCN. Period length and circadian rhythmicity were comparable between Calb1(-/-) and wild-type animals. Our findings indicate an involvement of CB in the signaling pathway that modulates the behavioral and molecular response to light.

A sensitive period of mice inhibitory system to neonatal GABA enhancement by vigabatrin is brain region dependent

Neurodevelopmental disorders, such as schizophrenia and autism, have been associated with disturbances of the GABAergic system in the brain. We examined immediate and long-lasting influences of exposure to the GABA-potentiating drug vigabatrin (GVG) on the GABAergic system in the hippocampus and cerebral cortex, before and during the developmental switch in GABA function (postnatal days P1-7 and P4-14). GVG induced a transient elevation of GABA levels. A feedback response to GABA enhancement was evident by a short-term decrease in glutamate decarboxylase (GAD) 65 and 67 levels. However, the number of GAD65/67-immunoreactive (IR) cells was greater in 2-week-old GVG-treated mice. A long-term increase in GAD65 and GAD67 levels was dependent on brain region and treatment period. Vesicular GABA transporter was insensitive to GVG. The overall effect of GVG on the Cl(-) co-transporters NKCC1 and KCC2 was an enhancement of their synthesis, which was dependent on the treatment period and brain region studied. In addition, a short-term increase was followed by a long-term decrease in KCC2 oligomerization in the cell membrane of P4-14 hippocampi and cerebral cortices. Analysis of the Ca(2+) binding proteins expressed in subpopulations of GABAergic cells, parvalbumin and calbindin, showed region-specific effects of GVG during P4-14 on parvalbumin-IR cell density. Moreover, calbindin levels were elevated in GVG mice compared to controls during this period. Cumulatively, these results suggest a particular susceptibility of the hippocampus to GVG when exposed during days P4-14. In conclusion, our studies have identified modifications of key components in the inhibitory system during a critical developmental period. These findings provide novel insights into the deleterious consequences observed in children following prenatal and neonatal exposure to GABA-potentiating drugs.

Behavioral analyses of transgenic mice harboring bipolar disorder candidate genes, IMPA1 and IMPA2

The inositol depletion hypothesis proposes the inhibition of IMPase (myo-inositol monophosphatase) by lithium, a mood stabilizer, as a mechanism of lithium's efficacy. This hypothesis predicts that the upregulation of this biochemical pathway may underlie the pathophysiology of bipolar disorder. In favor of this idea, IMPA2 encoding IMPase is a candidate susceptibility gene for bipolar disorder and that the risk-conferring single nucleotide polymorphisms enhance the promoter activity of IMPA2. However, it is yet unknown whether such upregulation has a biological role in bipolar disorder. To address this issue, we generated transgenic mice for the two IMPase genes (IMPA1 and IMPA2). The expression levels of the transgene were robust in IMPA2 Tg lines, but moderate in IMPA1 Tg lines, when compared to those of endogenous proteins. The transgenic mice behaved normally under drug-naïve conditions, and did not exhibit signs for manic change when an antidepressant amitriptyline was administrated. Interestingly, the male transgenic mice for IMPA2 exhibited a lithium-resistant phenotype in the forced swim test. The current study, as a whole, did not support a substantial role of the upregulation of IMPase in bipolar disorder, although the lithium-insensitivity trait seen in IMPA2 transgenic mice might represent some aspect relevant to the inositol depletion hypothesis.

The effect of noise on CaMKII activation in a dendritic spine during LTP induction

Activation of calcium-calmodulin dependent protein kinase II (CaMKII) during induction of long-term potentiation (LTP) is a series of complicated stochastic processes that are affected by noise. There are two main sources of noise affecting CaMKII activation within a dendritic spine. One is the noise associated with stochastic opening of N-methyl-d-aspartate (NMDA) receptor channels and the other is the noise associated with the stochastic reaction-diffusion kinetics leading to CaMKII activation. Many models have been developed to simulate CaMKII activation, but there is no fully stochastic model that studies the effect of noise on CaMKII activation. Here we construct a fully stochastic model to study these effects. Our results show that noise has important effects on CaMKII activation variability, with the effect from stochastic opening of NMDA receptor channels being 5-10 times more significant than that from stochastic reactions involving CaMKII. In addition, CaMKII activation levels and the variability of activation are greatly affected by small changes in NMDA receptor channel number at the synapse. One reason LTP induction protocols may require tetanic or repeated burst stimulation is that there is a need to overcome inherent variability to provide sufficiently large calcium signals through NMDA receptor channels; with meaningful physiological stimuli, noise may allow the calcium signal to exceed threshold for CaMKII activation when it might not do so otherwise.

Integrated protein array screening and high throughput validation of 70 novel neural calmodulin-binding proteins

Calmodulin is an essential regulator of intracellular processes in response to extracellular stimuli mediated by a rise in Ca(2+) ion concentration. To profile protein-protein interactions of calmodulin in human brain, we probed a high content human protein array with fluorophore-labeled calmodulin in the presence of Ca(2+). This protein array contains 37,200 redundant proteins, incorporating over 10,000 unique human neural proteins from a human brain cDNA library. We designed a screen to find high affinity (K(D) < or = 1 microm) binding partners of calmodulin and identified 76 human proteins from all intracellular compartments of which 72 are novel. We measured the binding kinetics of 74 targets with calmodulin using a high throughput surface plasmon resonance assay. Most of the novel calmodulin-target complexes identified have low dissociation rates (k(off) < or = 10(-3) s(-1)) and high affinity (K(D) </= 1 mum), consistent with the design of the screen. Many of the identified proteins are known to assemble in neural tissue, forming assemblies such as the spectrin scaffold and the postsynaptic density. We developed a microarray of the identified target proteins with which we can characterize the biochemistry of calmodulin for all targets in parallel. Four novel targets were verified in neural cells by co-immunoprecipitation, and four were selected for exploration of the calmodulin-binding regions. Using synthetic peptides and isothermal titration calorimetry, calmodulin binding motifs were identified in the potassium voltage-gated channel Kv6.1 (residues 474-493), calmodulin kinase-like vesicle-associated protein (residues 302-316), EF-hand domain family member A2 (residues 202-216), and phosphatidylinositol-4-phosphate 5-kinase, type I, gamma (residues 400-415).

Structural characterization of the conformational change in calbindin-D28k upon calcium binding using differential surface modification analyzed by mass spectrometry

Calbindin-D28k is a calcium binding protein with six EF hand domains. Calbindin-D28k is unique in that it functions as both a calcium buffer and a sensor protein. It is found in many tissues, including brain, pancreas, kidney, and intestine, playing important roles in each. Calbindin-D28k is known to bind four calcium ions and upon calcium binding undergoes a conformational change. The structure of apo calbindin-D28k is in an ordered state, transitioning into a disordered state as calcium is bound. Once fully loaded with four calcium ions, it again takes on an ordered state. The solution structure of disulfide-reduced holo-calbindin-D28k has been determined by NMR, while the structure of apo calbindin-D28k has yet to be determined. Differential surface modification of lysine and histidine residues analyzed by mass spectrometry has been used in this study to identify, for the first time, the specific regions of calbindin-D28k undergoing conformational changes between the holo and apo states. Using differential surface modification in combination with mass spectrometry, EF hands 1 and 4 as well as the linkers before EF hand 1 and the linkers between EF hands 4 and 5 and EF hands 5 and 6 were identified as regions of conformational change between apo and holo calbindin-D28k. Under the experimental conditions employed, EF hands 2 and 6, which are known not to bind calcium, were unaffected in either form. EF hand 2 is highly accessible; however, EF hand 6 was determined not to be surface accessible in either form. Previous research has identified a disulfide bond between cysteines 94 and 100 in the holo state. Until now, it was unknown whether this bond also exists in the apo form. Our data confirm the presence of the disulfide bond between cysteines 94 and 100 in the holo form and indicate that there is predominantly no disulfide bond between these residues in the apoprotein.

Bergmann glial S100B activates myo-inositol monophosphatase 1 and Co-localizes to purkinje cell vacuoles in SCA1 transgenic mice

Spinocerebellar ataxia-1 (SCA1) is a late onset neurodegenerative disease caused by the expansion of a polyglutamine repeat within ataxin-1 protein. The toxic effects triggered by mutant ataxin-1 result in degeneration of the neurons in cerebellum, brain stem and spinocerebellar tracts. The targeted overexpression of mutant ataxin-1 in cerebellar Purkinje cells (PCs) of the SCA1 transgenic mice results in the formation of cytoplasmic vacuoles in PCs. These vacuoles appear early on before the onset of behavioral abnormalities. Interestingly, we found that vacoules contain S100B and vimentin proteins, which normally localize to neighboring Bergmann glia (BG). Further, immunohistochemical and specialized silver stain analysis revealed that vacuolar formation is associated with alterations in the morphology of dendritic spines of PCs. To gain insights into the mechanisms of vacuolar formation, we investigated if vacuoles in SCA1 PCs have an autophagic origin or are a consequence of some other event. We examined the expression levels (by Western blotting) of microtubule-associated protein light chain 3 (LC3)-I and LC3-II, and the degradation levels of p62 (a LC3 partner) in the cerebellar fractions prepared from pre-symptomatic SCA1 and age-matched wild-type mice. No p62 degradation was observed; however, LC3-II/(LC3-I + LC3-II) ratios were significantly altered in SCA1 mice indicating changes in the autophagic flux. In addition, LC3 localized to PC vacuoles. Further, we observed a co-localization of myo-inositol monophosphatase 1 (IMPA1) with S100B in PC vacuoles. IMPA1 is present in PC spines and has been implicated in autophagy. In vitro studies using purified IMPA1 and S100B demonstrated that S100B interacted with and activated IMPA1. Both apo and Ca(2+)-bound S100B were found to activate IMPA1, depending on substrate concentration. IMPA1 is regulated by another calcium-binding protein calbindin-D28k (CaB), since we reported earlier that the CaB levels are reduced in SCA1 PCs, the activation of IMPA1 by S100B may modulate CaB-dependent inositol signaling. This may cause BG-PC interface to degenerate resulting in vacuolar formation. In sum, these data indicate that vacuoles appearing early in SCA1 PCs could be developing through some unknown autophagic mechanism.

Spine neck geometry determines spino-dendritic cross-talk in the presence of mobile endogenous calcium binding proteins

Dendritic spines are thought to compartmentalize second messengers like Ca2+. The notion of isolated spine signaling, however, was challenged by the recent finding that under certain conditions mobile endogenous Ca(2+)-binding proteins may break the spine limit and lead to activation of Ca(2+)-dependent dendritic signaling cascades. Since the size of spines is variable, the spine neck may be an important regulator of this spino-dendritic crosstalk. We tested this hypothesis by using an experimentally defined, kinetic computer model in which spines of Purkinje neurons were coupled to their parent dendrite by necks of variable geometry. We show that Ca2+ signaling and calmodulin activation in spines with long necks is essentially isolated from the dendrite, while stubby spines show a strong coupling with their dendrite, mediated particularly by calbindin D28k. We conclude that the spine neck geometry, in close interplay with mobile Ca(2+)-binding proteins, regulates the spino-dendritic crosstalk.

Changes in the effective gravitational field strength affect the state of phosphorylation of stress-related proteins in callus cultures of Arabidopsis thaliana

In a recent study it was shown that callus cell cultures of Arabidopsis thaliana respond to changes in gravitational field strengths by changes in protein expression. Using ESI-MS/MS for proteins with differential abundance after separation by 2D-PAGE, 28 spots which changed reproducibly and significantly in amount (P <0.05) after 2 h of hypergravity (18 up-regulated, 10 down-regulated) could be identified. The corresponding proteins were largely involved in stress responses, including the detoxification of reactive oxygen species (ROS). In the present study, these investigations are extended to phosphorylated proteins. For this purpose, callus cell cultures of Arabidopsis thaliana were exposed to hypergravity (8 g) and simulated weightlessness (random positioning; RP) for up to 30 min, a period of time which yielded the most reliable data. The first changes, however, were visible as early as 10 min after the start of treatment. In comparison to 1 g controls, exposure to hypergravity resulted in 18 protein spots, and random positioning in 25, respectively, with increased/decreased signal intensity by at least 2-fold (P <0.05). Only one spot (alanine aminotransferase) responded the same way under both treatments. After 30 min of RP, four spots appeared, which could not be detected in control samples. Among the protein spots altered in phosphorylation, it was possible to identify 24 from those responding to random positioning and 12 which responded to 8 g. These 12 proteins (8 g) are partly (5 out of 12) the same as those changed in expression after exposure to 2 h of hypergravity. The respective proteins are involved in scavenging and detoxification of ROS (32%), primary metabolism (20.5%), general signalling (14.7%), protein translation and proteolysis (14.7%), and ion homeostasis (8.8%). Together with our recent data on protein expression, it is assumed that changes in gravitational fields induce the production of ROS. Our data further indicate that responses toward RP are more by post-translational protein modulation (most changes in the degree of phosphorylation occur under RP-treatment) than by protein expression (hypergravity).

Zn2+ binding to human calbindin D(28k) and the role of histidine residues

We have studied the binding of Zn2+ to the hexa EF-hand protein, calbindin D(28k)-a strong Ca2+-binder involved in apoptosis regulation-which is highly expressed in brain tissue. By use of radioblots, isothermal titration calorimetry, and competition with a fluorescent Zn2+ chelator, we find that calbindin D(28k) binds Zn2+ to three rather strong sites with dissociation constants in the low micromolar range. Furthermore, we conclude based on spectroscopic investigations that the Zn2+-bound state is structurally distinct from the Ca2+-bound state and that the two forms are incompatible, yielding negative allosteric interaction between the zinc- and calcium-binding events. ANS titrations reveal a change in hydrophobicity upon binding Zn2+. The binding of Zn2+ is compatible with the ability of calbindin to activate myo-inositol monophosphatase, one of the known targets of calbindin. Through site-directed mutagenesis, we address the role of cysteine and histidine residues in the binding of Zn2+. Mutation of all five cysteines into serines has no effect on Zn2+-binding affinity or stoichiometry. However, mutating histidine 80 into a glutamine reduces the binding affinity of the strongest Zn2+ site, indicating that this residue is involved in coordinating the Zn2+ ion in this site. Mutating histidines 5, 22, or 114 has significantly smaller effects on Zn2+-binding affinity.

Crystallization and preliminary crystallographic analysis of human Ca 2+-loaded calbindin-D28k

Calbindin-D28k is a calcium-binding protein that belongs to the troponin C superfamily. It is expressed in many tissues, including brain, intestine, kidney and pancreas, and performs roles as both a calcium buffer and a calcium sensor and carries out diverse physiological functions of importance. In order to resolve the crystal structure of human calbindin-D28k and to gain a better understanding of its biological functions, recombinant human calbindin-D28k was crystallized at 291 K using PEG 3350 as precipitant and a 2.4 A resolution X-ray data set was collected from a single flash-cooled crystal (100 K). The crystal belonged to space group C2, with unit-cell parameters a = 108.1, b = 28.2, c = 70.6 A, beta = 107.8 degrees . The presence of one molecule per asymmetric unit is presumed, corresponding to a Matthews coefficient of 1.75 A(3) Da(-1).

Peptide binding proclivities of calcium loaded calbindin-D28k

Calbindin-D28k is known to function as a calcium-buffering protein in the cell. Moreover, recent evidence shows that it also plays a role as a sensor. Using circular dichroism and NMR, we show that calbindin-D28k undergoes significant conformational changes upon binding calcium, whereas only minor changes occur when binding target peptides in its Ca(2+)-loaded state. NMR experiments also identify residues that undergo chemical shift changes as a result of peptide binding. The subsequent use of computational protein-protein docking protocols produce a model describing the interaction interface between calbindin-D28k and its target peptides.

Role of tissue transglutaminase type 2 in calbindin-D28k interaction with ataxin-1

Spinocerebellar ataxia-1 (SCA1) is caused by the expansion of a polyglutamine repeats within the disease protein, ataxin-1. The mutant ataxin-1 precipitates as large intranuclear aggregates in the affected neurons. These aggregates may protect neurons from mutant protein and/or trigger neuronal degeneration by encouraging recruitment of other essential proteins. Our previous studies have shown that calcium binding protein calbindin-D28k (CaB) associated with SCAl pathogenesis is recruited to ataxin-l aggregates in Purkinje cells of SCAl mice. Since our recent findings suggest that tissue transglutaminase 2 (TG2) may be involved in crosslinking and aggregation of ataxin-l, the present study was initiated to determine if TG2 has any role in CaB-ataxin-l interaction. The guinea pig TG2 covalently crosslinked purified rat brain CaB. Time dependent progressive increase in aggregation produced large multimers, which stayed on top of the gel. CaB interaction with ataxin-l was studied using HeLa cell lysates expressing GFP and GFP tagged ataxin-l with normal and expanded polyglutamine repeats (Q2, Q30 and Q82). The reaction products were analyzed by Western blots using anti-polyglutamine, CaB or GFP antibodies. CaB interacted with ataxin-1 independent of TG2 as the protein-protein crosslinker DSS stabilized CaB-ataxin-l complex. TG2 crosslinked CaB preferentially with Q82 ataxin-1. The crosslinking was inhibited with EGTA or TG2 inhibitor cystamine. The present data indicate that CaB may be a TG2 substrate. In addition, aggregates of mutant ataxin-l may recruit CaB via TG2 mediated covalent crosslinking, further supporting the argument that ataxin-l aggregates may be toxic to neurons.

Production of authentic SARS-CoV M(pro) with enhanced activity: application as a novel tag-cleavage endopeptidase for protein overproduction

The viral proteases have proven to be the most selective and useful for removing the fusion tags in fusion protein expression systems. As a key enzyme in the viral life-cycle, the main protease (M^pro) is most attractive for drug design targeting the SARS coronavirus (SARS-CoV), the etiological agent responsible for the outbreak of severe acute respiratory syndrome (SARS) in 2003. In this study, SARS-CoV M^pro was used to specifically remove the GST tag in a new fusion protein expression system. We report a new method to produce wild-type (WT) SARS-CoV M^pro with authentic N and C termini, and compare the activity of WT protease with those of three different types of SARS-CoV M^pro with additional residues at the N or C terminus. Our results show that additional residues at the N terminus, but not at the C terminus, of M^pro are detrimental to enzyme activity. To explain this, the crystal structures of WT SARS-CoV M^pro and its complex with a Michael acceptor inhibitor were determined to 1.6 Å and 1.95 Å resolution respectively. These crystal structures reveal that the first residue of this protease is important for sustaining the substrate-binding pocket and inhibitor binding. This study suggests that SARS-CoV M^pro could serve as a new tag-cleavage endopeptidase for protein overproduction, and the WT SARS-CoV M^pro is more appropriate for mechanistic characterization and inhibitor design.

Spatial expression patterns and biochemical properties distinguish a second myo-inositol monophosphatase IMPA2 from IMPA1

Lithium is used in the clinical treatment of bipolar disorder, a disease where patients suffer mood swings between mania and depression. Although the mode of action of lithium remains elusive, a putative primary target is thought to be inositol monophosphatase (IMPase) activity. Two IMPase genes have been identified in mammals, the well characterized myo-inositol monophosphatase 1 (IMPA1) and myo-inositol monophosphatase 2 (IMPA2). Several lines of genetic evidence have implicated IMPA2 in the pathogenesis of not only bipolar disorder but also schizophrenia and febrile seizures. However, little is known about the protein, although it is predicted to have lithium-inhibitable IMPase activity based on its homology to IMPA1. Here we present the first biochemical study comparing the enzyme activity of IMPA2 to that of IMPA1. We demonstrate that in vivo, IMPA2 forms homodimers but no heterodimers with IMPA1. Recombinant IMPA2 exhibits IMPase activity, although maximal activity requires higher concentrations of magnesium and a higher pH. IMPA2 shows significantly lower activity toward myo-inositol monophosphate than IMPA1. We therefore screened for additional substrates that could be more efficiently dephosphorylated by IMPA2, but failed to find any. Importantly, when using myo-inositol monophosphate as a substrate, the IMPase activity of IMPA2 was inhibited at high lithium and restricted magnesium concentrations. This kinetics distinguishes it from IMPA1. We also observed a characteristic pattern of differential expression between IMPA1 and IMPA2 in a selection of tissues including the brain, small intestine, and kidney. These data suggest that IMPA2 has a separate function in vivo from that of IMPA1.

Dual role of calbindin-D28Kin vesicular catecholamine release from mouse chromaffin cells

Calbindin-D(28K) is suggested to play a postsynaptic role in neurotransmission and in the regulation of the intracellular Ca(2+) concentration. However, it is still unclear whether calbindin-D(28K) has a role in the regulation of exocytosis, either as Ca(2+) buffer or as Ca(2+) sensor. Amperometric recordings of catecholamine exocytosis from wild-type and calbindin-D(28K) knockout mouse chromaffin cells reveal a strong reduction in the number of released vesicles, as well as in the amount of neurotransmitter released per fusion event in knockout cells. However, Ca(2+) current recordings and Ca(2+) imaging experiments, including video-rate confocal laser scanning microscopy, revealed that the intracellular Ca(2+) dynamics are remarkably similar in wild-type and knockout cells. The combined results demonstrate that calbindin-D(28K) plays an important and dual role in exocytosis, affecting both release frequency and quantal size, apparently without strong effects on intracellular Ca(2+) dynamics. Consequently, the possibility that calbindin-D(28K) functions not only as a Ca(2+) buffer but also as a modulator of vesicular catecholamine release is discussed.

Functionally improved bone in calbindin-D28k knockout mice

In vitro studies indicate that Calbindin-D28k, a calcium binding protein, is important in regulating the life span of osteoblasts as well as the mineralization of bone extracellular matrix. The recent creation of a Calbindin-D28k knockout mouse has provided the opportunity to study the physiological effects of the Calbindin-D28k protein on bone remodeling in vivo. In this experiment, histomorphometry, microCT, and bend testing were used to characterize bones in Calbindin-D28k KO (knockout) mice. The femora of Calbindin-D28k KO mice had significantly increased cortical bone volume (60.4% +/- 3.1) compared to wild-type (WT) mice (45.4% +/- 4.6). The increased bone volume was due to a decrease in marrow cavity area, and significantly decreased endosteal perimeters (3.397 mm +/- 0.278 in Calbindin-D28k KO mice, and 4.046 mm +/- 0.450 in WT mice). Similar changes were noted in the analysis of the tibias in both mice. The bone formation rates were similar in the femoral and tibial cortical bones of both mice. microCT analysis of the trabecular bone in the tibial plateau indicated that Calbindin-D28k KO mice had an increased bone volume (35.2% +/- 3.1) compared to WT mice (24.7% +/- 4.9) which was primarily due to increased trabecular number (8.99 mm(-1) +/- 0.94 in Calbindin-D28k KO mice compared to 6.75 mm(-1) +/- 0.85 in WT mice). Bone mineral content analysis of the tibias indicated that there is no difference in the calcium or phosphorus content between the Calbindin-D28k KO and WT mice. Cantilever bend testing of the femora demonstrated significantly lower strains in the bones of Calbindin-D28k KO mice (4135 micro strain/kg +/- 1266) compared to WT mice (6973 micro strain/kg +/- 998) indicating that the KO mice had stiffer bones. Three-point bending demonstrated increased failure loads in bones of Calbindin-D28k KO mice (31.6 N +/- 2.1) compared to WT mice (15.0 N +/- 1.7). In conclusion, Calbindin-D28k KO mice had increased bone volume and stiffness indicating that Calbindin-D28k plays an important role in bone remodeling.

Structure, binding interface and hydrophobic transitions of Ca2+-loaded calbindin-D(28K)

Calbindin-D(28K) is a Ca2+-binding protein, performing roles as both a calcium buffer and calcium sensor. The NMR solution structure of Ca2+-loaded calbindin-D(28K) reveals a single, globular fold consisting of six distinct EF-hand subdomains, which coordinate Ca2+ in loops on EF1, EF3, EF4 and EF5. Target peptides from Ran-binding protein M and myo-inositol monophosphatase, along with a new target from procaspase-3, are shown to interact with the protein on a surface comprised of alpha5 (EF3), alpha8 (EF4) and the EF2-EF3 and EF4-EF5 loops. Fluorescence experiments reveal that calbindin-D(28K) adopts discrete hydrophobic states as it binds Ca2+. The structure, binding interface and hydrophobic characteristics of Ca2+-loaded calbindin-D(28K) provide the first detailed insights into how this essential protein may function. This structure is one of the largest high-resolution NMR structures and the largest monomeric EF-hand protein to be solved to date.

Detecting cryptic epitopes created by nanoparticles

As potential applications of nanotechnology and nanoparticles increase, so too does the likelihood of human exposure to nanoparticles. Because of their small size, nanoparticles are easily taken up into cells (by receptor-mediated endocytosis), whereupon they have essentially free access to all cellular compartments. Similarly to macroscopic biomaterial surfaces (that is, implants), nanoparticles become coated with a layer of adsorbed proteins immediately upon contact with physiological solutions (unless special efforts are taken to prevent this). The process of adsorption often results in conformational changes of the adsorbed protein, which may be affected by the larger curvature of nanoparticles compared with implant surfaces. Protein adsorption may result in the exposure at the surface of amino acid residues that are normally buried in the core of the native protein, which are recognized by the cells as "cryptic epitopes." These cryptic epitopes may trigger inappropriate cellular signaling events (as opposed to being rejected by the cells as foreign bodies). However, identification of such surface-exposed epitopes is nontrivial, and the molecular nature of the adsorbed proteins should be investigated using biological and physical science methods in parallel with systems biology studies of the induced alterations in cell signaling.

140 Mouse Brain Proteins Identified by Ca2+-Calmodulin Affinity Chromatography and Tandem Mass Spectrometry

Calmodulin is an essential Ca2+-binding protein that binds to a variety of targets that carry out critical signaling functions. We describe the proteomic characterization of mouse brain Ca2+-calmodulin-binding proteins that were purified using calmodulin affinity chromatography. Proteins in the eluates from four different affinity chromatography experiments were identified by 1-DE and in-gel digestion followed by LC-MS/MS. Parallel experiments were performed using two related control-proteins belonging to the EF-hand family. After comparing the results from the different experiments, we were able to exclude a significant number of proteins suspected to bind in a nonspecific manner. A total of 140 putative Ca2+-calmodulin-binding proteins were identified of which 87 proteins contained calmodulin-binding motifs. Among the 87 proteins that contained calmodulin-binding motifs, 48 proteins have not previously been shown to interact with calmodulin and 39 proteins were known calmodulin-binding proteins. Many proteins with ill-defined functions were identified as well as a number of proteins that at the time of the analysis were described only as ORFs. This study provides a functional framework for studies on these previously uncharacterized proteins.

Characterization of calretinin I-II as an EF-hand, Ca2+, H+-sensing domain

Calretinin, a neuronal protein with well-defined calcium-binding properties, has a poorly defined function. The pH dependent properties of calretinin (CR), the N-terminal (CR I-II), and C-terminal (CR III-VI) domains were investigated. A drop in pH within the intracellular range (from pH 7.5 to pH 6.5) leads to an increased hydrophobicity of calcium-bound CR and its domains as reported by fluorescence spectroscopy with the hydrophobic probe 2-(p-toluidino)-6-naphthalenesulfonic acid (TNS). The TNS data for the N- and C-terminal domains of CR are additive, providing further support for their independence within the full-length protein. Our work concentrated on CR I-II, which was found to have hydrophobic properties similar to calmodulin at lower pH. The elution of CR I-II from a phenyl-Sepharose column was consistent with the TNS data. The pH-dependent structural changes were further localized to residues 13-28 and 44-51 using nuclear magnetic resonance spectroscopy chemical shift analysis, and there appear to be no large changes in secondary structure. Protonation of His 12 and/or His 27 side chains, coupled with calcium chelation, appears to lead to the organization of a hydrophobic pocket in the N-terminal domain. CR may sense and respond to calcium, proton, and other signals, contributing to conflicting data on the proteins role as a calcium sensor or calcium buffer.

Calbindin D28k targets myo-inositol monophosphatase in spines and dendrites of cerebellar Purkinje neurons

The Ca(2+)-binding protein calbindin D28k (CB) is vital for the normal function of the central nervous system but its specific functional role is largely unclear. CB is typically described as a mobile Ca(2+)buffer that shapes the spatiotemporal extent of cellular Ca(2+)signals. Recent biochemical data, however, indicate that CB also has characteristics of a Ca(2+) sensor and activates myo-inositol monophosphatase (IMPase), a key enzyme of the inositol-1,4,5-trisphosphate signaling cascade and an assumed target of mood-stabilizing drugs in the treatment of bipolar disorder. Here, we show that CB interacts with IMPase in cerebellar Purkinje neurons, a cell type well known to rely on inositol-1,4,5-trisphosphate-dependent synaptic integration. Quantification of the mobility of dye-labeled CB with two-photon fluorescence recovery after photobleaching revealed that a substantial fraction of CB is immobilized in spines and dendrites, but not in axons. Immobilization occurs over several seconds, is increased by suprathreshold synaptic activity, and can be relieved by a synthetic peptide that resembles the putative CB-binding site of IMPase, indicating that CB binds to immobilized IMPase. Measurements of the apparent diffusion coefficients of CB imply that CB does not interact with cytosolic IMPase or that the latter is present only in minute amounts in the spiny dendrites of Purkinje neurons. Our results suggest that CB acts as an activity-dependent sensor that targets membrane/cytoskeleton-bound IMPase in central neurons.

Deamidation and disulfide bridge formation in human calbindin D28k with effects on calcium binding

Calbindin D(28k) (calbindin) is a cytoplasmic protein expressed in the central nervous system, which is implied in Ca(2+) homeostasis and enzyme regulation. A combination of biochemical methods and mass spectrometry has been used to identify post-translational modifications of human calbindin. The protein was studied at 37 degrees C or 50 degrees C in the presence or absence of Ca(2+). One deamidation site was identified at position 203 (Asn) under all conditions. Kinetic experiments show that deamidation of Asn 203 occurs at a rate of 0.023 h(-1) at 50 degrees C for Ca(2+)-free calbindin. Deamidation is slower for the Ca(2+)-saturated protein. The deamidation process leads to two Asp iso-forms, regular Asp and iso-Asp. The form with regular Asp 203 binds four Ca(2+) ions with high affinity and positive cooperativity, i.e., in a very similar manner to non-deamidated protein. The form with beta-aspartic acid (or iso-Asp 203) has reduced affinity for two or three sites leading to sequential Ca(2+) binding, i.e., the Ca(2+)-binding properties are significantly perturbed. The status of the cysteine residues was also assessed. Under nonreducing conditions, cysteines 94 and 100 were found both in reduced and oxidized form, in the latter case in an intramolecular disulfide bond. In contrast, cysteines 187, 219, and 257 were not involved in any disulfide bonds. Both the reduced and oxidized forms of the protein bind four Ca(2+) ions with high affinity in a parallel manner and with positive cooperativity.

Interaction of calbindin D28k and inositol monophosphatase in human postmortem cortex: possible implications for bipolar disorder

OBJECTIVES

Therapeutically relevant concentrations of lithium (Li) exert an uncompetitive inhibition on inositol monophosphatase (IMPase). It has recently been shown that calbindin D28k (calbindin) activates IMPase. Purified calbindin attaches to a specific amino acid sequence on purified IMPase enhancing its activity by several hundred fold. We studied whether calbindin activates IMPase in postmortem human brain crude homogenate, whether differences in calbindin levels between lymphocytes and brain may be responsible for our previous finding of reduced IMPase activity in lymphocytes but not brain of bipolar patients, and whether calbindin protein levels are altered in postmortem brain from bipolar patients versus control subjects and schizophrenic and major depressive patients.

METHODS

IMPase activity in human postmortem brain specimens with or without 10 microM human recombinant calbindin was quantified spectrophotometrically in an enzyme-linked immunosorbent assay (ELISA) reader. Calbindin protein levels in postmortem brain were determined using Western blot analysis.

RESULTS

Supplementation of human recombinant calbindin to postmortem human brain crude homogenate enhanced IMPase activity by 3.5-fold. No difference in postmortem temporal cortex calbindin protein levels was found between bipolar patients versus comparison groups. Two-fold higher calbindin protein levels were found in Li-treated bipolar patients compared with other bipolar patients. Subchronic Li treatment in mice did not affect brain calbindin protein levels significantly. Chronic Li treatment reduced calbindin protein levels in the frontal cortex but not in the hippocampus.

CONCLUSIONS

Calbindin is a physiological activator of IMPase in human brain. Protein levels of calbindin are not altered in postmortem temporal cortex of bipolar patients.

Prevention of glucocorticoid-induced apoptosis in osteocytes and osteoblasts by calbindin-D28k

This study show for the first time that calbindin-D28k can prevent glucocorticoid-induced bone cell death. The anti-apoptotic effect of calbindin-D28k involves inhibition of glucocorticoid induced caspase 3 activation as well as ERK activation.

INTRODUCTION

Recent studies have indicated that deleterious effects of glucocorticoids on bone involve increased apoptosis of osteocytes and osteoblasts. Because the calcium-binding protein calbindin-D28k has been reported to be anti-apoptotic in different cell types and in response to a variety of insults, we investigated whether calbindin-D28k could protect against glucocorticoid-induced cell death in bone cells.

MATERIALS AND METHODS

Apoptosis was induced by addition of dexamethasone (dex; 10-6 M) for 6 h to MLO-Y4 osteocytic cells as well as to osteoblastic cells. Apoptosis percentage was determined by examining the nuclear morphology of transfected cells. Caspase 3 activity was evaluated in bone cells and in vitro. SELDI mass spectrometry (MS) was used to examine calbindin-D28k-caspase 3 interaction. Phosphorylation of calbindin-D28k was examined by 32P incorporation as well as by MALDI-TOF MS. ERK activation was determined by Western blot.

RESULTS

The pro-apoptotic effect of dex in MLO-Y4 cells was completely inhibited in cells transfected with calbindin-D28k cDNA (5.6% apoptosis in calbindin-D28k transfected cells compared with 16.2% apoptosis in vector-transfected cells, p < 0.05). Similar results were observed in osteoblastic cells. We found that dex-induced apoptosis in bone cells was accompanied by an increase in caspase 3 activity. This increase in caspase 3 activity was inhibited in the presence of calbindin-D28k. In vitro assays indicated a concentration-dependent inhibition of caspase 3 by calbindin-D28k (Ki = 0.22 microM). Calbindin-D28k was found to inhibit caspase 3 specifically because the activity of other caspases was unaffected by calbindin-D28k. The anti-apoptotic effect of calbindin-D28k in response to dex was also reproducibly associated with an increase in the phosphorylation of ERK 1 and 2, suggesting that calbindin-D28k affects more than one signal in the glucocorticoid-induced apoptotic pathway.

CONCLUSION

Calbindin-D28k, a natural non-oncogenic protein, could be an important target in the therapeutic intervention of glucocorticoid-induced osteoporosis.

Calbindin D28k, parvalbumin, and calretinin immunoreactivity in the main and accessory olfactory bulbs of the gray short-tailed opossum,Monodelphis domestica

The vertebrate main and accessory olfactory bulbs (MOB and AOB) are the first synaptic sites in the olfactory pathways. The MOB is a cortical structure phylogenetically well conserved in its laminar structure and overall synaptic organization, while the AOB has significant species variation in size. In order to better understand signal processing in the two olfactory systems and the species differences, immunocytochemical staining and analysis were done of the neuronal expression patterns of the calcium-binding proteins calbindin D28k (CB), parvalbumin (PV), and calretinin (CR) in the MOB and AOB in a marsupial species, the gray short-tailed opossum, Monodelphis domestica. In the MOB, antibody to CB labeled periglomerular cells, superficial short axon cells / Van Gehuchten cells; antibody to PV labeled Van Gehuchten cells; and antibody to CR immunostained periglomerular cells, superficial short axon cells / Van Gehuchten cells, and granule cells. In the AOB, CB immunoreactivity was detected in periglomerular cells and a subpopulation of granule cells; antibody to PV labeled the superficial short axon cells / Van Gehuchten cells and granule cells; and antibody to CR labeled a small number of periglomerular cells, superficial short axon cells / Van Gehuchten cells, and granule cells. These results showed that the patterns of CB, PV, and CR expression differ in the opossum main and accessory olfactory bulbs and differ from that in other animal species. These varying patterns of neuronal immunostaining may be related to the different functions of the main and accessory olfactory bulbs and to the differing signal processing features.

Calbindin D28K immunoreactive neurons in vomeronasal organ and their projections to the accessory olfactory bulb in the rat

The vomeronasal system is a nasal chemosensory system involved in pheromone detection. The chemosensory receptor neurons are located in the sensory epithelium of the vomeronasal organ (VNO). Their axons terminate in the glomeruli of the accessory olfactory bulb (AOB). In this study, we examined the expression of calbindin D28k (CB) in the rat VNO and AOB. In the VNO, a subpopulation of receptor neurons in the middle layer of the sensory epithelium was immunostained with antibodies to CB. Their axons could be traced to terminate in a group of glomeruli in the anterior half of the AOB glomerular layer. This group of CB-immunostained glomeruli in the anterior half of the AOB included a few large glomeruli close to the boundary between the anterior and posterior halves of the AOB, and several small glomeruli scattered in the anterior region of the AOB glomerular layer. The positions of the CB-immunostained glomeruli in the AOB, especially those close to the anterior-posterior boundary, were similar in the two bulbs and in different rats. No sex difference was found. A developmental study showed that the CB-immunoreactive receptor neurons in the middle layer of the VNO sensory epithelium and CB-immunoreactive glomeruli in the anterior AOB were present on the 14th postnatal day and older. The distribution pattern of the CB-immunostained receptor neurons and their localized projection suggest the possibility that these neurons may express the same or functionally related pheromone receptor genes.

Calbindin D28K interacts with Ran-binding protein M: identification of interacting domains by NMR spectroscopy

Calbindin D(28K) is an EF-hand containing protein that plays a vital role in neurological function. We now show that calcium-loaded calbindin D(28K) interacts with Ran-binding protein M, a protein known to play a role in microtubule function. Using NMR methods, we show that a peptide, LASIKNR, derived from Ran-binding protein M, interacts with several regions of the calcium-loaded protein including the amino terminus and two other regions that exhibit conformational exchange on the NMR timescale. We suggest that the interaction between calbindin D(28K) and Ran-binding protein M may be important in calbindin D(28K) function.

The calcium-signalling saga: tap water and protein crystals

Of the approximately 1,400 grams of calcium that are in the human body, less than 10 grams manage to escape being trapped in the skeleton and teeth. These few grams might be an insignificant quantity, but they are extraordinarily significant qualitatively. They circulate in the blood and extracellular spaces, and penetrate cells to regulate their most important activities.