Solution NMR structure of the Ca2+-bound N-terminal domain of CaBP7: a regulator of golgi trafficking

Calcium-binding protein 7 expressed in muscle negatively regulates age-related degeneration of neuromuscular junctions in mice

The neuromuscular junction (NMJ) forms centrally in myotubes and, as the only synapse between motor neuron and myotube, are indispensable for motor activity. The midmuscle formation of NMJs, including midmuscle-restricted expression of NMJ-related genes, is governed by the muscle-specific kinase (MuSK). However, mechanisms underlying MuSK-mediated signaling are unclear. Here, we find that the Calcium-binding protein 7 (Cabp7) gene shows midmuscle-restricted expression, and muscle-specific depletion of Cabp7 in mice accelerated age-related NMJ degeneration, muscle weakness/atrophy, and motor dysfunction. Surprisingly, forced expression in muscle of CIP, an inhibitory peptide of the negative regulator of NMJ formation cyclin-dependent kinase 5 (Cdk5), restored NMJ integrity and muscle strength, and healed muscle atrophy in muscle-specific Cabp7-deficient mice, which showed increased muscle expression of the Cdk5 activator p25. These findings together demonstrate that MuSK-mediated signaling induces muscle expression of Cabp7, which suppresses age-related NMJ degeneration likely by attenuating p25 expression, providing insights into prophylactic/therapeutic intervention against age-related motor dysfunction.

Calcium Sensors in Neuronal Function and Dysfunction

Calcium signaling in neurons as in other cell types can lead to varied changes in cellular function. Neuronal Ca²⁺ signaling processes have also become adapted to modulate the function of specific pathways over a wide variety of time domains and these can have effects on, for example, axon outgrowth, neuronal survival, and changes in synaptic strength. Ca²⁺ also plays a key role in synapses as the trigger for fast neurotransmitter release. Given its physiological importance, abnormalities in neuronal Ca²⁺ signaling potentially underlie many different neurological and neurodegenerative diseases. The mechanisms by which changes in intracellular Ca²⁺ concentration in neurons can bring about diverse responses is underpinned by the roles of ubiquitous or specialized neuronal Ca²⁺ sensors. It has been established that synaptotagmins have key functions in neurotransmitter release, and, in addition to calmodulin, other families of EF-hand-containing neuronal Ca²⁺ sensors, including the neuronal calcium sensor (NCS) and the calcium-binding protein (CaBP) families, play important physiological roles in neuronal Ca²⁺ signaling. It has become increasingly apparent that these various Ca²⁺ sensors may also be crucial for aspects of neuronal dysfunction and disease either indirectly or directly as a direct consequence of genetic variation or mutations. An understanding of the molecular basis for the regulation of the targets of the Ca²⁺ sensors and the physiological roles of each protein in identified neurons may contribute to future approaches to the development of treatments for a variety of human neuronal disorders.

The tegumental allergen-like proteins of Schistosoma mansoni: A biochemical study of SmTAL4-TAL13

Schistosoma mansoni, like other trematodes, expresses a number of unusual calcium binding proteins which consist of an EF-hand domain joined to a dynein light chain-like (DLC-like) domain by a flexible linker. These proteins have been implicated in host immune responses and drug binding. Three members of this protein family from S. mansoni (SmTAL1, SmTAL2 and SmTAL3) have been well characterised biochemically. Here we characterise the remaining family members from this species (SmTAL4-13). All of these proteins form homodimers and all except SmTAL5 bind to calcium and manganese ions. SmTAL9, 10 and 11 also bind to magnesium ions. The antischistosomal drug, praziquantel interacts with SmTAL4, 5 and 8. Some family members also bind to calmodulin antagonists such as chlorpromazine and trifluoperazine. Molecular modelling suggests that all ten proteins adopt similar overall folds with the EF-hand and DLC-like domains folding discretely. Bioinformatics analyses suggest that the proteins may fall into two main categories: (i) those which bind calcium ions reversibly at the second EF-hand and may play a role in signalling (SmTAL1, 2, 8 and 12) and (ii) those which bind calcium ions at the first EF-hand and may play either signalling or structural roles (SmTAL7, 9, 10 and 13). The remaining proteins include those which do not bind calcium ions (SmTAL3 and 5) and three other proteins (SmTAL4, 6 and 11). The roles of these proteins are less clear, but they may also have structural roles.

The key target of neuroprotection after the onset of ischemic stroke: secretory pathway Ca(2+)-ATPase 1

The regulatory mechanisms of cytoplasmic Ca(2+) after myocardial infarction-induced Ca(2+) overload involve secretory pathway Ca(2+)-ATPase 1 and the Golgi apparatus and are well understood. However, the effect of Golgi apparatus on Ca(2+) overload after cerebral ischemia and reperfusion remains unclear. Four-vessel occlusion rats were used as animal models of cerebral ischemia. The expression of secretory pathway Ca(2+)-ATPase 1 in the cortex and hippocampus was detected by immunoblotting, and Ca(2+) concentrations in the cytoplasm and Golgi vesicles were determined. Results showed an overload of cytoplasmic Ca(2+) during ischemia and reperfusion that reached a peak after reperfusion. Levels of Golgi Ca(2+) showed an opposite effect. The expression of Golgi-specific secretory pathway Ca(2+)-ATPase 1 in the cortex and hippocampus decreased before ischemia and reperfusion, and increased after reperfusion for 6 hours. This variation was similar to the alteration of calcium in separated Golgi vesicles. These results indicate that the Golgi apparatus participates in the formation and alleviation of calcium overload, and that secretory pathway Ca(2+)-ATPase 1 tightly responds to ischemia and reperfusion in nerve cells. Thus, we concluded that secretory pathway Ca(2+)-ATPase 1 plays an essential role in cytosolic calcium regulation and its expression can be used as a marker of Golgi stress, responding to cerebral ischemia and reperfusion. The secretory pathway Ca(2+)-ATPase 1 can be an important neuroprotective target of ischemic stroke.

Modulation of phosphatidylinositol 4-phosphate levels by CaBP7 controls cytokinesis in mammalian cells

For more than 25 years, lysosomes have been known to cluster at the intercellular bridge during cytokinesis, but why has remained a mystery. This study provides evidence that phosphoinositide metabolism is important for this clustering and that lysosome activity is required for cytokinesis.

Calcium and phosphoinositide signaling regulate cell division in model systems, but their significance in mammalian cells is unclear. Calcium-binding protein-7 (CaBP7) is a phosphatidylinositol 4-kinaseIIIβ (PI4KIIIβ) inhibitor required during cytokinesis in mammalian cells, hinting at a link between these pathways. Here we characterize a novel association of CaBP7 with lysosomes that cluster at the intercellular bridge during cytokinesis in HeLa cells. We show that CaBP7 regulates lysosome clustering and that PI4KIIIβ is essential for normal cytokinesis. CaBP7 depletion induces lysosome mislocalization, extension of intercellular bridge lifetime, and cytokinesis failure. These data connect phosphoinositide and calcium pathways to lysosome localization and normal cytokinesis in mammalian cells.

Sense and specificity in neuronal calcium signalling

Changes in the intracellular free calcium concentration ([Ca²⁺]i) in neurons regulate many and varied aspects of neuronal function over time scales from microseconds to days. The mystery is how a single signalling ion can lead to such diverse and specific changes in cell function. This is partly due to aspects of the Ca²⁺ signal itself, including its magnitude, duration, localisation and persistent or oscillatory nature. The transduction of the Ca²⁺ signal requires Ca²⁺binding to various Ca²⁺ sensor proteins. The different properties of these sensors are important for differential signal processing and determine the physiological specificity of Ca(2+) signalling pathways. A major factor underlying the specific roles of particular Ca²⁺ sensor proteins is the nature of their interaction with target proteins and how this mediates unique patterns of regulation. We review here recent progress from structural analyses and from functional analyses in model organisms that have begun to reveal the rules that underlie Ca²⁺ sensor protein specificity for target interaction. We discuss three case studies exemplifying different aspects of Ca²⁺ sensor/target interaction. This article is part of a special issue titled the 13th European Symposium on Calcium.