Inositol 1,3,4,5-tetrakisphosphate-induced Ca2+ sequestration into bovine adrenal-medullary secretory vesicles

Calcium Signaling and Exocytosis in Adrenal Chromaffin Cells

At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca2+concentration ([Ca2+]c) depends on at least four efficient regulatory systems: 1) plasmalemmal calcium channels, 2) endoplasmic reticulum, 3) mitochondria, and 4) chromaffin vesicles. Different mammalian species express different levels of the L, N, P/Q, and R subtypes of high-voltage-activated calcium channels; in bovine and humans, P/Q channels predominate, whereas in felines and murine species, L-type channels predominate. The calcium channels in chromaffin cells are regulated by G proteins coupled to purinergic and opiate receptors, as well as by voltage and the local changes of [Ca2+]c. Chromaffin cells have been particularly useful in studying calcium channel current autoregulation by materials coreleased with catecholamines, such as ATP and opiates. Depending on the preparation (cultured cells, adrenal slices) and the stimulation pattern (action potentials, depolarizing pulses, high K+, acetylcholine), the role of each calcium channel in controlling catecholamine release can change drastically. Targeted aequorin and confocal microscopy shows that Ca2+entry through calcium channels can refill the endoplasmic reticulum (ER) to nearly millimolar concentrations, and causes the release of Ca2+(CICR). Depending on its degree of filling, the ER may act as a sink or source of Ca2+that modulates catecholamine release. Targeted aequorins with different Ca2+affinities show that mitochondria undergo surprisingly rapid millimolar Ca2+transients, upon stimulation of chromaffin cells with ACh, high K+, or caffeine. Physiological stimuli generate [Ca2+]cmicrodomains in which the local subplasmalemmal [Ca2+]crises abruptly from 0.1 to ∼50 μM, triggering CICR, mitochondrial Ca2+uptake, and exocytosis at nearby secretory active sites. The fact that protonophores abolish mitochondrial Ca2+uptake, and increase catecholamine release three- to fivefold, support the earlier observation. This increase is probably due to acceleration of vesicle transport from a reserve pool to a ready-release vesicle pool; this transport might be controlled by Ca2+redistribution to the cytoskeleton, through CICR, and/or mitochondrial Ca2+release. We propose that chromaffin cells have developed functional triads that are formed by calcium channels, the ER, and the mitochondria and locally control the [Ca2+]cthat regulate the early and late steps of exocytosis.

Mechanism regulating nuclear calcium signalingThis paper is one of a selection of papers published in this Special Issue, entitled The Nucleus: A Cell Within A Cell

Although the outer nuclear membrane is continuous with the endoplasmic reticulum, it is possible to isolate nuclei both intact and free from endoplasmic reticulum contaminants. The outer and the inner nuclear membranes can be purified free from cross-contamination. Evidence in support of autonomous regulation of nuclear calcium signaling relies upon the investigations with isolated nuclei. Mechanisms for generating calcium signaling in the nucleus have been identified. Two calcium transporting systems, an ATP-dependant nuclear Ca2+-ATPase and an IP4-mediated inositol 1,3,4,5-tetrakisphosphate receptor, are located on the outer nuclear membrane. Thus, ATP and IP4, depending on external free calcium concentrations, are responsible for filling the nuclear envelope calcium pool. The inositol 1,4,5-trisphosphate receptor is located on the inner nuclear membrane with its ligand binding domain facing toward the nucleoplasm. Likewise, the ryanodine receptor is located on the inner nuclear membrane and its ligand cADP-ribose is generated within the nucleus. A 120 kDa protein fragment of nuclear PLC-γ1 is stimulated in vivo by epidermal growth factor nuclear signaling coincident with the time course of nuclear membrane epidermal growth factor receptor activation. Stimulated 120 kDa protein fragment interacts with PIKE, a nuclear GTPase, and together they form a complex with PI[3]kinase serving as a module for nuclear PI[3]K stimulation. Thus, the nucleus has its own IP3generating system.

Structure-function relationships of the mouse Gap1m. Determination of the inositol 1,3,4,5-tetrakisphosphate-binding domain

Gap1(IP4BP), one of a member of Ras GTPase-activating proteins, has been identified as a specific inositol 1,3,4,5-tetrakisphosphate (IP4)-binding protein (Cullen, P. J., Hsuan, J. J., Truong, O., Letcher, A. J., Jackson, T. R., Dawson, A. P., and Irvine, R. F. (1995) Nature 386, 527-530). In this paper we describe Gap1(m), which is closely related to Gap1(IP4BP), to also be an IP4-binding protein and show that the pleckstrin homology domain (PH) is the central IP4-binding domain by expressing fragments of the mouse Gap1(m) in Escherichia coli as fusion proteins and examining their activities. However, in addition to the PH domain, an adjacent GAP-related domain and carboxyl terminus are required for high affinity specific IP4 binding. The PH domain is highly conserved in the Gap1 family and also has striking homology to the amino-terminal region of Bruton's tyrosine kinase. Substitution of Cys for Arg at position 628 in the PH domain corresponding to the mutation of Bruton's tyrosine kinase observed in X-linked immunodeficiency mice results in a dramatic reduction of IP4 binding activity as well as phospholipid binding capacity of Gap1(m). This mutant also showed the GAP activity against Ha-Ras to be similar to that of the wild type Gap1(m). Our results suggest that the PH domain of Gap1(m) functions as a modulatory domain of GAP activity by binding IP4 and phospholipids.

Regulation of 1,4,5-IP3, 1,3,4,5-IP4 and IP6 binding sites following entorhinal cortex lesions in rat brain

A lesion of the entorhinal cortex produces a loss of more than 80% of the synapses in the outer molecular layer of the hippocampus in the rat. However, this synaptic loss is transient. Beginning a few days after denervation, new synapses are formed, virtually replacing the lost inputs within two months. Synaptic remodelling induced by entorhinal cortex lesion is associated with specific modifications of various neurotransmitters, hormones and growth factors. Many of these substances act at membrane bound-receptors to induce the hydrolysis of phosphatidylinositols generating various inositol phosphates. Some of the key members of this family include inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate and inositol hexakisphosphate which are all associated with the maintenance Ca2+ homeostasis. To investigate the potential roles and/or alterations of inositol phosphates in entorhinal cortex lesions-induced neuronal plasticity, we quantified specific receptor sites for inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate and inositol hexakisphosphate using their respective tritiated ligands, at different periods post-lesion corresponding to the degenerative and subsequent reinnervation phases. [3H]inositol 1,4,5-trisphosphate binding sites are maximally increased (30%) between two and eight days post-lesion in the hippocampal formation on both sides of the lesion. In the cortex, [3H]inositol 1,4,5-trisphosphate binding increased also bilaterally following the lesion. Changes in [3H]inositol 1,3,4,5-tetrakisphosphate binding are delayed and reduced (20% increase) in magnitude compared to these seen for [3H]inositol 1,4,5-trisphosphate binding. The maximal peak in [3H]inositol 1,3,4,5-tetrakisphosphate binding is observed between eight and 14 days after the lesion in the hippocampal formation and the cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential localization and pH dependency of phosphoinositide 1,4,5-IP3, 1,3,4,5-IP4 and IP6 receptors in rat and human brains

It is well established that the inositol lipids mediate signal transduction in several cellular populations. Many neurotransmitters, hormones and growth factors act at plasma membrane receptors to induce the hydrolysis of phosphatidylinositols and hence the generation of various inositol phosphates (IP). The best known member of this family is 1,4,5-IP3, which is associated with the release of Ca2+ from intracellular pools. It has also been proposed that two others inositides, 1,3,4,5-IP4 and IP6, may be involved in Ca2+ homeostasis. In order to study the possible relevance of these various inositides in neuronal tissues, we have localized the respective receptors in rat and human brain under both acidic and basic pH conditions. In the hippocampal formation, [3H]1,3,4,5-IP4 binding sites are concentrated in the hilus and the molecular layer while a clearly different pattern of distribution is seen for [3H]1,4,5-IP3, its highest concentration of labelling being concentrated in the oriens and radiatum laminae. This contrasting profile of distribution is also observed in other brain areas such as the caudate-putamen, the septo-hippocampal area, and the molecular and granular layers of the cerebellum. Moreover, while highest amounts of specific [3H]1,4,5-IP3 binding are obtained at pH 8.5, the opposite is found for [3H]1,3,4,5-IP4, with high binding levels seen under acidic conditions. [3H]IP6 binding sites are broadly distributed with specific labelling concentrated in areas enriched with neuronal perikarya such as the granular cell layer of the dentate gyrus, the pyramidal cell layers of the hippocampus and the granular cell layer of the cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)

pH-dependent association of chromogranin A with secretory vesicle membrane and a putative membrane binding region of chromogranin A

Chromogranin A is a low-affinity, high-capacity Ca2+ binding protein, postulated to be responsible for the Ca2+ buffering role of secretory vesicles, and has been found only in the soluble portions of the vesicular proteins. Contrary to the generally accepted notion of chromogranin A existing as a soluble matrix protein, chromogranin A bound to the secretory vesicle membrane at the intravesicular pH of 5.5 and freed from the membrane when the pH was raised to a more physiological pH of 7.5. Trypsin digestion studies of the vesicle membrane suggested that chromogranin A interacts with the protein component(s) on the intravesicular side of the membrane. Furthermore, in a study using 14 synthetic chromogranin A peptides which represent various portions of chromogranin A, a segment in the N-terminal region (residues 18-37) was shown to bind to the vesicle membrane in a pH-dependent manner. The pH-dependent vesicle membrane binding property of chromogranin A appears to be of fundamental physiological importance with regard to the potential roles of chromogranin A in secretory vesicle biogenesis, particularly in segregating secretory vesicle membranes from others in the trans-Golgi network, and also in transmitting extravesicular signals such as inositol 1,4,5-trisphosphate or inositol 1,3,4,5-tetrakisphosphate for Ca2+ release or uptake to the inside of vesicles.