Evidence for calcium mediated conformational changes in calbindin-D28K (the vitamin D-induced calcium binding protein) interactions with chick intestinal brush border membrane alkaline phosphatase as studied via photoaffinity labeling techniques

Molecular mechanisms of vitamin D action

The hormonal metabolite of vitamin D, 1α,25-dihydroxyvitamin D(3) (1,25D), initiates biological responses via binding to the vitamin D receptor (VDR). When occupied by 1,25D, VDR interacts with the retinoid X receptor (RXR) to form a heterodimer that binds to vitamin D responsive elements in the region of genes directly controlled by 1,25D. By recruiting complexes of either coactivators or corepressors, ligand-activated VDR-RXR modulates the transcription of genes encoding proteins that promulgate the traditional functions of vitamin D, including signaling intestinal calcium and phosphate absorption to effect skeletal and calcium homeostasis. Thus, vitamin D action in a particular cell depends upon the metabolic production or delivery of sufficient concentrations of the 1,25D ligand, expression of adequate VDR and RXR coreceptor proteins, and cell-specific programming of transcriptional responses to regulate select genes that encode proteins that function in mediating the effects of vitamin D. For example, 1,25D induces RANKL, SPP1 (osteopontin), and BGP (osteocalcin) to govern bone mineral remodeling; TRPV6, CaBP(9k), and claudin 2 to promote intestinal calcium absorption; and TRPV5, klotho, and Npt2c to regulate renal calcium and phosphate reabsorption. VDR appears to function unliganded by 1,25D in keratinocytes to drive mammalian hair cycling via regulation of genes such as CASP14, S100A8, SOSTDC1, and others affecting Wnt signaling. Finally, alternative, low-affinity, non-vitamin D VDR ligands, e.g., lithocholic acid, docosahexaenoic acid, and curcumin, have been reported. Combined alternative VDR ligand(s) and 1,25D/VDR control of gene expression may delay chronic disorders of aging such as osteoporosis, type 2 diabetes, cardiovascular disease, and cancer.

Cellular function and molecular structure of ecto-nucleotidases

Ecto-nucleotidases play a pivotal role in purinergic signal transmission. They hydrolyze extracellular nucleotides and thus can control their availability at purinergic P2 receptors. They generate extracellular nucleosides for cellular reuptake and salvage via nucleoside transporters of the plasma membrane. The extracellular adenosine formed acts as an agonist of purinergic P1 receptors. They also can produce and hydrolyze extracellular inorganic pyrophosphate that is of major relevance in the control of bone mineralization. This review discusses and compares four major groups of ecto-nucleotidases: the ecto-nucleoside triphosphate diphosphohydrolases, ecto-5'-nucleotidase, ecto-nucleotide pyrophosphatase/phosphodiesterases, and alkaline phosphatases. Only recently and based on crystal structures, detailed information regarding the spatial structures and catalytic mechanisms has become available for members of these four ecto-nucleotidase families. This permits detailed predictions of their catalytic mechanisms and a comparison between the individual enzyme groups. The review focuses on the principal biochemical, cell biological, catalytic, and structural properties of the enzymes and provides brief reference to tissue distribution, and physiological and pathophysiological functions.

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.

Accelerated fat absorption in intestinal alkaline phosphatase knockout mice

Intestinal alkaline phosphatase (IAP) is the most ancestral of the tissue-specific members of the AP gene family. Several studies have suggested an absorptive function for IAP, but in vivo data to this effect have been lacking. We inactivated the mouse IAP gene in embryo-derived stem cells and generated mice homozygous for the null mutation. The mice were macroscopically and histologically normal and fertile and showed no difference from the wild-type controls under normal laboratory conditions. However, when maintained long-term on a high-fat diet, the IAP-deficient mice showed faster body weight gain than did control animals. Histological examination revealed an accelerated transport of fat droplets through the intestinal epithelium and elevation of serum triglyceride levels in the IAP-deficient mice compared to wild-type mice. Our study suggests that IAP participates in a rate-limiting step regulating fat absorption.

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.

'New' functions for 'old' proteins: the role of the calcium-binding proteins calbindin D-28k, calretinin and parvalbumin, in cerebellar physiology. Studies with knockout mice

Calretinin (CR), calbindin D-28k (CB) and parvalbumin (PV) belong to the large family of EF-hand calcium-binding proteins, which comprises more than 200 members in man. Structurally these proteins are characterized by the presence of a variable number of evolutionary well-conserved helix-loop-helix motives, which bind Ca2+ ions with high affinity. Functionally, they fall into two groups: by interaction with target proteins, calcium sensors translate calcium concentrations into signaling cascades, whereas calcium buffers are thought to modify the spatiotemporal aspects of calcium transients. Although CR, CB and PV are currently being considered calcium buffers, this may change as we learn more about their biology. Remarkable differences in their biophysical properties have led to the distinction of fast and slow buffers and suggested functional specificity of individual calcium buffers. Evaluation of the physiological roles of CR, CB and PV has been facilitated by the recent generation of mouse strains deficient in these proteins. Here, we review the biology of these calcium-binding proteins with distinct reference to the cerebellum, since they are particularly enriched in specific cerebellar neurons. CR is principally expressed in granule cells and their parallel fibres, while PV and CB are present throughout the axon, soma, dendrites and spines of Purkinje cells. PV is additionally found in a subpopulation of inhibitory interneurons, the stellate and basket cells. Studies on deficient mice together with in vitro work and their unique cell type-specific distribution in the cerebellum suggest that these calcium-binding proteins have evolved as functionally distinct, physiologically relevant modulators of intracellular calcium transients. Analysis of different brain regions suggests that these proteins are involved in regulating calcium pools critical for synaptic plasticity. Surprisingly, a major role of any of these three calcium-binding proteins as an endogenous neuroprotectant is not generally supported.

Structural and biochemical characterization of neuronal calretinin domain I-II (residues 1-100). Comparison to homologous calbindin D28k domain I-II (residues 1-93)

This study characterizes the calcium-bound CR I-II domain (residues 1-100) of rat calretinin (CR). CR, with six EF-hand motifs, is believed to function as a neuronal intracellular calcium-buffer and/or calcium-sensor. The secondary structure of CR I-II, defined by standard NMR methods on 13C,15N-labeled protein, contains four helices and two short interacting segments of extended structure between the calcium-binding loops. The linker between the two helix-loop-helix, EF-hand motifs is 12 residues long. Limited trypsinolysis at K60 (there are 10 other K/R residues in CR I-II) confirms that the linker of CR I-II is solvent-exposed and that other potential sites are protected by regular secondary structure. 45Ca-overlay of glutathione S-transferase (GST)-CR(1-60) and GST-CR(61-100) fusion proteins confirm that both EF-hands of CR I-II have intrinsic calcium-binding properties. The primary sequence and NMR chemical shifts, including calcium-sensitive glycine residues, also suggest that both EF-hand loops of CR I-II bind calcium. NMR relaxation, analytical ultracentrifugation, chemical cross-linking and NMR translation diffusion measurements indicate that CR I-II exists as a monomer. Calb I-II (the homologous domain of calbindin D28k) has the same EF-hand secondary structures as CR I-II, except that helix B is three residues longer and the linker has only four residues [Klaus, W., Grzesiek, S., Labhardt, A. M., Buckwald, P., Hunziker, W., Gross, M. D. & Kallick, D. A. (1999) Eur. J. Biochem. 262, 933-938]. In contrast, Calb I-II binds one calcium cation per monomeric unit and exists as a dimer. Despite close homology and similar secondary structures, CR I-II and Calb I-II probably have distinct tertiary structure features that suggest different cellular functions for the full-length proteins.

Calcium transport across the dental enamel epithelium

Dental enamel is the most highly calcified tissue in mammals, and its formation is an issue of fundamental biomedical importance. The enamel-forming cells must somehow supply calcium in bulk yet avoid the cytotoxic effects of excess calcium. Disrupted calcium transport could contribute to a variety of developmental defects in enamel, and the underlying cellular machinery is a potential target for drugs to improve enamel quality. The mechanisms used to transport calcium remain unclear despite much progress in our understanding of enamel formation. Here, current knowledge of how enamel cells handle calcium is reviewed in the context of findings from other epithelial calcium-transport systems. In the past, most attention has focused on approaches to boost the poor diffusion of calcium in cytosol. Recent biochemical findings led to an alternative proposal that calcium is routed through high-capacity stores associated with the endoplasmic reticulum. Research areas needing further attention and a working model are also discussed. Calcium-handling mechanisms in enamel cells are more generally relevant to the understanding of epithelial calcium transport, biomineralization, and calcium toxicity avoidance.

Ca(2+)- and H(+)-dependent conformational changes of calbindin D(28k)

Calbindin D(28k) is a member of a large family of intracellular Ca(2+) binding proteins characterized by EF-hand structural motifs. Some of these proteins are classified as Ca(2+)-sensor proteins, since they are involved in transducing intracellular Ca(2+) signals by exposing a hydrophobic patch on the protein surface in response to Ca(2+) binding. The hydrophobic patch serves as an interaction site for target enzymes. Other members of this group are classified as Ca(2+)-buffering proteins, because they remain closed after Ca(2+) binding and participate in Ca(2+) buffering and transport functions. ANS (8-anilinonaphthalene-1-sulfonic acid) binding and affinity chromatography on a hydrophobic column suggested that both the Ca(2+)-free and Ca(2+)-loaded form of calbindin D(28k) have exposed hydrophobic surfaces. Since exposure of hydrophobic surface is unfavorable in the aqueous intracellular milieu, calbindin D(28k) most likely interacts with other cellular components in vivo. A Ca(2+)-induced conformational change was readily detected by several optical spectroscopic methods. Thus, calbindin D(28k) shares some of the properties of Ca(2+)-sensor proteins. However, the Ca(2+)-induced change in exposed hydrophobic surface was considerably less pronounced than that in calmodulin. The data also shows that calbindin D(28k) undergoes a rapid and reversible conformational change in response to a H(+) concentration increase within the physiological pH range. The pH-dependent conformational change was shown to reside mainly in EF-hands 1-3. Urea-induced unfolding of the protein at pH 6, 7, and 8 showed that the stability of calbindin D(28k) was increased in response to H(+) in the range examined. The results suggest that calbindin D(28k) may interact with targets in a Ca(2+)- and H(+)-dependent manner.

Ataxia and altered dendritic calcium signaling in mice carrying a targeted null mutation of the calbindin D28k gene

Intracellular calcium-binding proteins are abundantly expressed in many neuronal populations. Previous evidence suggests that calcium-binding proteins can modulate various neuronal properties, presumably by their action as calcium buffers. The importance of calcium-binding proteins for nervous system function in an intact integrated system is, however, less clear. To investigate the physiological role of a major endogenous calcium-binding protein, calbindin D28k (calbindin) in vivo, we have generated calbindin null mutant mice by gene targeting. Surprisingly, calbindin deficiency does not affect general parameters of development and behavior or the structure of the nervous system at the light microscopic level. Null mutants are, however, severely impaired in tests of motor coordination, suggesting functional deficits in cerebellar pathways. Purkinje neurons, the only efferent of the cerebellar cortex, and inferior olive neurons, the source of the climbing fiber afferent, have previously been shown to express calbindin. Correlated with this unusual type of ataxia, confocal calcium imaging of Purkinje cells in cerebellar slices revealed marked changes of synaptically evoked postsynaptic calcium transients. Their fast, but not their slow, decay component had larger amplitudes in null mutant than in wild-type mice. We conclude that endogenous calbindin is of crucial importance for integrated nervous system function.