Intracellular Ca2+ stores in chicken Purkinje neurons: differential distribution of the low affinity-high capacity Ca2+ binding protein, calsequestrin, of Ca2+ ATPase and of the ER lumenal protein, Bip

Intracellular Ca2+ stores of rat cerebellum: heterogeneity within and distinction from endoplasmic reticulum

Rat cerebellum microsomes were subfractionated on isopycnic linear sucrose (20-42%)-density gradients. The distribution of endoplasmic reticulum (ER) markers (RNA, signal-sequence receptor alpha, calnexin, calreticulin, the immunoglobulin-binding protein Bip) and markers of intracellular rapidly exchanging Ca2+ stores [Ca2+ channels sensitive to either Ins(1,4,5)P3 or ryanodine) was investigated biochemically and immunologically. The comparison indicates that: (a) vesicles bearing the InsP3 receptor were separated from those bearing the ryanodine receptor; (b) ER markers, i.e. Bip, calnexin, signal-sequence receptor alpha, RNA, did not sediment as either InsP3 or ryanodine receptors did; (c) calreticulin, an intralumenal low-affinity high-capacity Ca(2+)-binding protein, had a widespread distribution, similar to that of Bip and calnexin, and was present in Purkinje, granule, Golgi and stellate neurons, as indicated by immunofluorescent labelling of cerebellum cortex cryosections. The present results show that the ER is not a homogeneous entity, and that Ca2+ stores are heterogeneous insofar as InsP3 receptors and ryanodine receptors are segregated, either to discrete intracellular organelles or to specialized ER subcompartments.

Tridimensional organization of Purkinje neuron cisternal stacks, a specialized endoplasmic reticulum subcompartment rich in inositol 1,4,5-trisphosphate receptors

Stacks of regularly spaced, flat, smooth-surfaced endoplasmic reticulum cisternae frequently observed in both the cell body and dendrites of cerebellar Purkinje neurons, were previously shown by immunocytochemistry to be highly enriched in receptors for the second messenger, inositol 1,4,5-trisphosphate. Morphometric analyses have been carried out on randomly selected thin section images of rat Purkinje neurons to reveal the tridimensional organization of these structures. Individual stacked cisternae (on the average approximately 3.5 per stack) were shown to be separated from each other by a 23.5 nm space occupied by perpendicular bridges, approximately 20 nm in diameter, most probably composed by two apposed receptor homotetramer molecules, inserted into the parallel membranes in their hydrophobic domains. In the stacked membranes the density of the bridges was approximately 500 microns -2, corresponding to approximately 15% of the surface area. The lateral distribution of bridges was not random, but revealed regular distances that might correspond to unoccupied receptor slots. In each stack, the external cisternae were often in direct lumenal continuity with conventional elements of the endoplasmic reticulum, whereas the internal cisternae were not. Since continuities between stacked cisternae were never observed, the results indicate that the internal cisternae are at least transitorily discrete, i.e. they are not in permanent lumenal continuity with the rest of the endoplasmic reticulum. To our knowledge this is the first demonstration of a physical subcompartmentalization of the latter endomembrane system in a non-mitotic cells. A model for the biogenesis of cisternal stacks, based on the head-to-head binding and lateral interaction of the inositol 1,4,5-trisphosphate receptor molecules in the plane of the interacting membranes, is proposed and critically discussed.

Differential distribution of the alternative forms of the sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase, SERCA2b and SERCA2a, in the avian brain

Cellular distribution of the two forms of SERCA2 was examined in adult chicken brain. Four regions of the brain were analyzed with three immunological reagents: a monoclonal antibody that recognizes both forms of SERCA2, and two antisera which are specific for the two alternative forms, SERCA2b or SERCA2a. Cerebellar Purkinje cells express predominantly SERCA2b but also low levels of SERCA2a, as has been reported for mammals. The nucleus isthmo-opticus, nucleus magnocellularis cochlearis, and nucleus laminaris all express high levels of SERCA2 but with different ratios of SERCA2b and SERCA2a. These immunohistochemical results were supported by in situ hybridization analysis. Therefore, it appears that regions within the brain have specific requirements for the two forms of SERCA2. This suggests functional significance for the alternative forms SERCA2b and SERCA2a, and possible functions are discussed.

Immunohistochemical localization of the inositol 1,4,5-trisphosphate receptor in the human nervous system

A monoclonal antibody raised against the mouse cerebellar inositol trisphosphate receptor was used to study the immunohistochemical localization of this protein in the human central nervous system. As in the brain of rodents, strong immunoreactivity was found in dendrites, axon and cell bodies of Purkinje cells, as well as in nerve endings in the cerebellar and vestibular nuclei. Cerebellar efferent fibres were the only positive structures demonstrated in the brainstem and no immunostaining could be detected in the spinal cord or dorsal root ganglia. By contrast, numerous immunoreactive neurons were present in several telencephalic and diencephalic structures, including the brain cortex, hippocampus, basal ganglia, basal forebrain, amygdala and thalamus. Immunostaining of these brain neurons was weaker than that found in Purkinje cells and was evident in cell bodies and dendrites. Thus, the human brain contains a molecule cross-reacting with the mouse inositol trisphosphate receptor protein that is expressed in a pattern similar to that found in rodents. These findings can be of great importance for understanding the function of this protein in normal brain and its modifications in neuropathological disorders.

Differences in the subcellular localization of calreticulin and organellar Ca(2+)-ATPase in neurons

It has become clear that calcium is an important mediator in the transduction of signals due to ligand binding to cell surface receptors. Cytosolic calcium is typically maintained at low levels in both muscle and non-muscle cells and intracellular sequestering of calcium appears to be important in this process. The identification of intracellular calcium pools has been the subject of much recent study, and it has been proposed that such pools would contain three components: a calcium-activated pump or Ca(2+)-ATPase, a calcium channel such as the inositol trisphosphate receptor or ryanodine receptor, and a high-capacity calcium-binding protein such as calsequestrin or calreticulin. We report here on the localization of two components, the organellar Ca(2+)-ATPase (SERCA) and calreticulin, in neuronal tissues. Using immunofluorescence and subcellular fractionation, we have found that for the most part, these two proteins do not co-localize in neuron cell bodies, dendrites, or axons; but may co-localize at the axon terminal.

Establishment of a Chinese hamster ovary cell line that expresses grp78 antisense transcripts and suppresses A23187 induction of both GRP78 and GRP94

GRP78, a 78,000 dalton protein residing in the endoplasmic reticulum, is postulated to play important roles in protein folding and cell survival during calcium and other physiological stress. Here we describe the construction of an eukaryotic expression vector for the constitutive expression of grp78 antisense RNA and the creation of a CHO cell line, 78WO, which expresses high levels of the grp78 antisense RNA through amplification of the stably transfected antisense vector. We observed that whereas 78WO maintains a basal level of GRP78 similar to that of control cells, GRP78 is no longer inducible by A23187. The 78WO cells have undergone a compensatory increase in grp78 transcription such that the effects of antisense are cancelled out at the protein level under nonstressed conditions. In these same cells, GRP94, a 94,00 dalton ER protein, is also rendered noninducible by A23187. This provides the first evidence that the regulation of two ER proteins might be coupled such that the failure to induce GRP78 results in the down-regulation of GRP94. The 78WO cell line grows with a doubling time of about 26 hr and exhibits decreased tolerance to A23187, suggesting the GRPs contribute to cell viability under calcium stress. The establishment of this cell line, which can be stably maintained, will provide a useful tool for testing whether the induction of the GRPs is important for protein folding or transport and whether their enhanced synthesis is the cause or consequence of a variety of physiological adaptations.

Plasma membrane Ca(2+)-ATPase isoforms: distribution of mRNAs in rat brain by in situ hybridization

Several mRNAs which encode for isoforms of the plasma membrane Ca(2+)-transport ATPase (PMCA) are present in adult rat brain. Using in situ hybridization with antisense oligonucleotide probes we found complex patterns of specific hybridization for three isoforms (PMCA1-3). Each rat brain region studied exhibited a distinct pattern of expression of isoforms. PMCA1 mRNA, which is widely distributed in rat tissues, was highest in CA1 pyramidal cells of hippocampus and very low in hypothalamic nuclei, cerebellum and choroid plexus. PMCA2 mRNA was highest in Purkinje cells of cerebellum and low in caudate-putamen, hypothalamic nuclei, habenula and choroid plexus. The highest levels of PMCA3 mRNA were found in habenula and choroid plexus. The PMCA1-3 isoforms appeared to be expressed primarily in neurons since hybridization was detected neither in white matter nor in regions rich in astrocytes. In different regions, different levels of expression of each PMCA mRNA may underlie specialized requirements for calcium homeostasis in specific neurons.

Immunohistochemical study on the distribution of sarcoplasmic reticulum calcium ATPase in various human tissues using novel monoclonal antibodies

Novel monoclonal antibodies were raised against sarcoplasmic reticulum calcium (Ca2+)-ATPase of human skeletal muscle. Immunohistochemical analysis demonstrated that these antibodies, designated 6F5 and 7F10, bind Ca(2+)-ATPase of non-muscle tissue of the adult including parathyroid, islets of Langerhans, anterior lobe of the pituitary gland and photoreceptor cells of the retina as well as skeletal muscle. A positive reaction was also found for fetal tissues including skeletal muscle, heart, chondrocytes and peripheral nerves. Our results for distribution suggest that Ca(2+)-ATPase is strongly expressed in the tissues and cells in which signal transduction is actively carried out by Ca2+ release from the cytoplasmic Ca2+ pool.

Subcompartments of the endoplasmic reticulum

The endoplasmic reticulum (ER) is the largest continuous endomembrane structure in the cytoplasm. It may be viewed as a series of unique subcompartments. In this review, we examine the rough ER, nuclear envelope and several smooth ER subcompartments. Consideration is given to the characteristic properties and functions of the ER and its domains, and to the formation and maintenance of subcompartments. Associations within the ER membrane bilayer, and with constituents of the cytoplasm and the ER lumen, contribute to the formation of domains and lead to the establishment of subcompartments that reflect specialized functions and vary according to the physiologic state and phenotype of the individual cell. Although the structural complexity of some ER subcompartments (such as the sarcoplasmic reticulum) is highly elaborate, the ER remains a dynamic organelle, subject to assembly and disassembly, capable of extensive remodelling and active in exchange with other organelles through mechanisms of membrane transport.

Membrane trafficking in neurons

Neurons possess an unusually extensive Golgi apparatus and exhibit a variety of active endocytic-like processes. The Golgi apparatus and the endocytic phenomena both contribute, probably in multiple overlapping ways, to the genesis and fate of the membrane systems in axons and terminals.

Programs for visualization in three-dimensional microscopy

Three-dimensional data representing biological structures can be derived using several methods, including serial section reconstruction, optical sectioning, and tomography. The investigation, comprehension, and communication of structural relationships to others is greatly facilitated by computer-based visualization procedures. We describe SYNU, a suite of programs developed for interactive investigation of three-dimensional structure and for the production of high-quality three-dimensional images and animations. We illustrate the capabilities of SYNU in applications to biological data obtained by confocal light microscopy, serial section, and high-resolution electron microscopy from investigations at the cellular, subcellular, and molecular levels.

The endoplasmic reticulum-Golgi intermediate compartment

The recent identification of an endoplasmic reticulum-Golgi intermediate compartment has added to the complexity of the structural and functional organization of the early secretory pathway. Protein sorting along the endoplasmic reticulum-Golgi pathway depends on different signals and mechanisms, some of which guarantee recycling from various levels of the Golgi apparatus to biosynthetically earlier compartments.

The endoplasmic reticulum-sarcoplasmic reticulum connection: distribution of endoplasmic reticulum markers in the sarcoplasmic reticulum of skeletal muscle fibers

The skeletal muscle sarcoplasmic reticulum (SR) was investigated for the presence of well-known endoplasmic reticulum (ER) markers: the lumenal protein BiP and a group of membrane proteins recognized by an antibody raised against ER membrane vesicles. Western blots of SR fractions revealed the presence of BiP in fast- and slow-twitch muscles of the rabbit as well as in rat and chicken muscles. Analyses of purified SR subfractions, together with cryosection immunofluorescence and immunogold labeling, revealed BiP evenly distributed within the longitudinal SR and the terminal cisternae. Within the terminal cisternae BiP appeared not to be mixed with calsequestrin but to be distributed around the aggregates of the latter Ca2+ binding protein. Of the various membrane markers only calnexin (91 kDa) was found to be distributed within both SR subfractions, whereas the other markers (apparent molecular masses of 64 kDa and 58 kDa and a doublet around 28 kDa) were concentrated in the terminal cisternae. These results suggest that the SR is a specialized ER subcompartment in which general markers, such as the ones we have investigated, coexist with the major SR proteins specifically responsible for Ca2+ uptake, storage, and release. The differential distribution of the ER markers reveals new aspects of the SR molecular structure that might be of importance for the functioning of the endomembrane system.

Widespread tissue distribution of rabbit calreticulin, a non-muscle functional analogue of calsequestrin

Calreticulin was identified in a variety of rabbit tissues by Western blot analysis. Indirect immunofluorescence studies on cultured cells or frozen sections from the corresponding tissues revealed that the protein was distributed to the endoplasmic reticulum or sarcoplasmic reticulum. Calreticulin was found to be an abundant calcium-binding protein in non-muscle and smooth muscle cells and a constituent calcium-binding protein in cardiac and skeletal muscle. From the immunoblot data, calreticulin may exist as an isoform in rabbit neural retina. The present study establishes the ubiquity of calreticulin in intracellular calcium binding.

Immunocytochemical analysis of the transfer of vesicular stomatitis virus G glycoprotein from the intermediate compartment to the Golgi complex

We performed an immunocytochemical analysis to study the transfer of a marker protein (G glycoprotein coded by vesicular stomatitis virus ts 045 strain) from the intermediate compartment to the Golgi stacks in infected Vero cells. The intermediate compartment seemed to consist of about 30-40 separate units of clustered small vesicles and short tubules. The units contained Rab2 protein and were spread throughout the cytoplasm, with a ratio of about 6:4 in the peripheral versus perinuclear site. Time-course experiments revealed a progressive transfer of G glycoprotein from the intermediate compartment to the Golgi stacks, while the tubulo-vesicular units did not appear to change their intracellular distribution. Moreover, the labeling density of peripheral and perinuclear units decreased in parallel during the transfer. These results support the notion that the intermediate compartment is a station in the secretory pathway, and that a vesicular transport connects this station to the Golgi complex.

The endoplasmic reticulum of Purkinje neuron body and dendrites: molecular identity and specializations for Ca2+ transport

Immunofluorescence and immunogold labeling, together with sucrose gradient separation and Western blot analysis of microsomal subfractions, were employed in parallel to probe the endoplasmic reticulum in the cell body and dendrites of rat cerebellar Purkinje neurons. Two markers, previously investigated in non-nerve cells, the membrane protein p91 (calnexin) and the lumenal protein BiP, were found to be highly expressed and widely distributed to the various endoplasmic reticulum sections of Purkinje neurons, from the cell body to dendrites and dendritic spines. An antibody (denominated anti-rough-surfaced endoplasmic reticulum), which recognized two membrane proteins, p14 and p40, revealed a similar immunogold labeling pattern. However, centrifugation results consistent with a widespread distribution were obtained for p14 only, while p40 was concentrated in the rough microsome-enriched subfractions. The areas enriched in the inositol 1,4,5-triphosphate receptor and thus presumably specialized in Ca2+ transport (stacks of multiple smooth-surfaced cisternae; the dendritic spine apparatus) also exhibited labeling for BiP and p91, and were positive for the anti-rough-surfaced endoplasmic reticulum antibody (presumably via the p14 antigen). Additional antibodies, that yielded inadequate immunocytochemical signals, were employed only by Western blotting of the microsomal subfractions, while the ryanodine receptor was studied by specific binding. The latter receptor and the Ca2+ ATPase, known in other species to be concentrated in Purkinje neurons, exhibited bimodal distributions with a peak in the light and another in the heavy subfractions. A similar distribution was also observed with another lumenal protein, protein disulfide isomerase. Taken as a whole, the results that we have obtained suggest the existence in the endoplasmic reticulum of Purkinje neurons of two levels of organization; the first identified by widespread, probably general markers (BiP, p91, possibly p14 and others), the second by specialization markers, such as the inositol 1,4,5-triphosphate receptor and, possibly, p40, which appear restricted to areas where specific functions appear to be localized.

Three additional inositol 1,4,5-trisphosphate receptors: molecular cloning and differential localization in brain and peripheral tissues

Three inositol 1,4,5-trisphosphate receptor (IP3R) cDNAs, designated IP3R-II, -III, and -IV, were cloned from a mouse placenta cDNA library. All three display strong homology in membrane-spanning domains M7 and M8 to the originally cloned cerebellar IP3R-I, with divergences predominantly in cytoplasmic domains. Levels of mRNA for the three additional IP3Rs in general are substantially lower than for IP3R-I, though in the gastrointestinal tract the levels of IP3R-III may be comparable to IP3R-I. Cerebellar Purkinje cells express at least two and possibly three distinct IP3Rs, suggesting heterogeneity of IP3 action within a single cell.

Calcium binding proteins in the sarcoplasmic/endoplasmic reticulum of muscle and nonmuscle cells

In this paper we review some of the large quantities of information currently available concerning the identification, structure and function of Ca(2+)-binding proteins of endoplasmic and sarcoplasmic reticulum membranes. The review places particular emphasis on identification and discussion of Ca2+ 'storage' proteins in these membranes. We believe that the evidence reviewed here supports the contention that the Ca(2+)-binding capacity of both calsequestrin and calreticulin favor their contribution as the major Ca(2+)-binding proteins of muscle and nonmuscle cells, respectively. Other Ca(2+)-binding proteins discovered in both endoplasmic reticulum and sarcoplasmic reticulum membranes probably contribute to the overall Ca2+ storage capacity of these membrane organelles, and they also play other important functional role such as posttranslational modification of newly synthesized proteins, a cytoskeletal (structural) function, or movement of Ca2+ within the lumen of the sarcoplasmic/endoplasmic reticulum towards the storage sites.

Mammalian stress response: induction of the glucose-regulated protein family

The glucose-regulated protein family consists of a set of stress-inducible proteins localized in the endoplasmic reticulum. Since their discovery in 1977, significant advances in our understanding of their structure, function and regulation have been made. Recent findings concerning the physiological roles played by the glucose-regulated proteins, and their regulations at the transcriptional, post-transcriptional, translational and post-translational levels are summarized.

Intracellular calcium: molecules and pools

The complex nature of intracellular calcium storage pools has been examined at many levels in the past year. Additional molecules associated with calcium stores have been identified and their localization examined. The convergence of molecular biology, cell biology and biochemistry has now allowed the details of calcium signalling to be meaningfully explored.

s-cyclophilin is retained intracellularly via a unique COOH-terminal sequence and colocalizes with the calcium storage protein calreticulin

Cyclophilins (cyclosporin A-binding proteins) are conserved, ubiquitous, and abundant proteins that accelerate the isomerization of XaaPro peptide bonds and the refolding of proteins in vitro. s-Cyclophilin is a member of the cyclophilin family with unique NH2- and COOH-terminal extensions, and with a signal sequence. We now report that s-cyclophilin is retained in the cell, and that the conserved s-cyclophilin-specific COOH-terminal extension VEKPFAIAKE is sufficient to direct a secretory protein to s-cyclophilin containing structures. Antibodies to s-cyclophilin-specific peptides were produced and the location of the protein was determined by an immunocytochemical study at the light microscopic level. s-Cyclophilin colocalized with the Ca(2+)-binding protein calreticulin and, to a lesser extent, with the microsomal Ca(2+)-ATPase in the myogenic cell line L6, and with the Ca(2+)-binding protein calsequestrin in skeletal muscle. In activated platelets, s-cyclophilin immunoreactivity was detected in a ring-like structure that might correspond to the Ca(2+)-storing and -releasing dense tubular network. In spreading cells, s-cyclophilin containing vesicular structures accumulated at actin-rich protrusion sites. While s-cyclophilin consistently codistributed with Ca2+ storage site markers, the distribution of s-cyclophilin immunoreactivity was not identical to that of ER markers. To determine whether the COOH-terminal extension of s-cyclophilin was involved in its intracellular transport we added this sequence to the COOH-terminus of the secretory protein glia-derived nexin. Appropriate constructs were expressed transiently in cultured cells and proteins were detected with specific antibodies. We found that glia-derived nexin with the COOH-terminal sequence VEKPFAIAKE (but not with the control sequence GLVVMNIT) colocalized with endogenous s-cyclophilin, indicating that the sequence contained retention information. These results indicate that s-cyclophilin is a retained component of an intracellular organelle and that it may accumulate in specialized portions of the ER, and possibly in calciosomes. Because of its conserved structure, widespread distribution, and abundance s-cyclophilin may be a useful marker to study the biogenesis and distribution of ER subcompartments.

Characteristics and function of Ca(2+)- and inositol 1,4,5-trisphosphate-releasable stores of Ca2+ in neurons

Molecular, biochemical and physiological evidence for the existence of releasable Ca2+ stores in neurons is strong. There are two separate molecules that function as release channels from those Ca2+ stores, the RyanR and InsP3R, and both have multiple regulatory sites for positive and negative control. Perhaps most intriguing is the biphasic, concentration-dependent action of cytosolic Ca2+ on both channels, first to stimulate release then, at higher concentration, to depress release. Whether the InsP3R and RyanR channels regulate Ca2+ release from different or identical functional compartments will need to be defined for each neuron type and perhaps even for each intracellular region within neurons since the evidence for functional separation of stores is mixed. The identification of Ca2+ storage and releasing capacity throughout all subcellular regions of neurons and the increasing evidence for a role for Ca2+ stores in neuronal plasticity suggests that the further characterization of the functional properties of Ca2+ stores will be an increasingly important and expanding area of interest in neurobiology.

Purification and characterization of calsequestrin from chicken cerebellum

Chicken cerebellum microsomal fractions contain a protein tentatively identified as calsequestrin (CS) (Volpe et al., Neuron 5, 713-721, 1990). Here we report, for the first time, the purification of cerebellum CS from whole tissue homogenate by DEAE-Cellulose chromatography and Ca(2+)-dependent elution from phenyl-Sepharose. The purified cerebellum CS displays the shift and increase in intrinsic fluorescence characteristic of skeletal muscle CS, and is shown to be a high-capacity, low-affinity Ca2+ binding protein (Kd = 1 mM).

Exocytotic and endocytotic membrane traffic in neurons

Because neurons are highly polarized and capable of various modes of neurosecretion the exocytotic and endocytotic membrane traffic in these cells is more complex than in other eukaryotic cells. Progress in our understanding of neuronal membrane traffic and organelle biogenesis has come from recently discovered analogies to epithelial and endocrine cells.

Structure and function of inositol trisphosphate receptors

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) is a soluble intracellular messenger formed rapidly after activation of a variety of cell-surface receptors that stimulate phosphoinositidase C activity. The initial response to Ins(1,4,5)P3 is a rapid Ca2+ efflux from nonmitochondrial intracellular stores which are probably specialized subcompartments of the endoplasmic reticulum, although their exact identities remain unknown. This initial response is followed by more complex Ca2+ signals: regenerative Ca2+ waves propagate across the cell, repetitive Ca2+ spikes occur, and stimulated Ca2+ entry across the plasma membrane contributes to the sustained Ca2+ signal. The mechanisms underlying these complex Ca2+ signals are unknown, although Ins(1,4,5)P3 is clearly involved. The intracellular receptor that mediates Ins(1,4,5)P3-stimulated Ca2+ mobilization has been purified and functionally reconstituted, and its amino acid sequence deduced from its cDNA sequence. These studies demonstrate that the Ins(1,4,5)P3 receptor has an integral Ca2+ channel separated from the Ins(1,4,5)P3 binding site by a long stretch of residues some of which form binding sites for allosteric regulators, and some of which are substrates for phosphorylation. In this review, we discuss the ligand recognition characteristics of Ins(1,4,5)P3 receptors, and their functional properties in their native environment and after purification, and we relate these properties to what is known of the structure of the receptor. In addition to regulation by Ins(1,4,5)P3, the Ins(1,4,5)P3 receptor is subject to many additional regulatory influences which include Ca2+, adenine nucleotides, pH and phosphorylation by protein kinases. Many of the functional and structural characteristics of the Ins(1,4,5)P3 receptor show striking similarities to another intracellular Ca2+ channel, the ryanodine receptor. These properties of the Ins(1,4,5)P3 are discussed, and their possible roles in contributing to the complex Ca2+ signals evoked by extracellular stimuli are considered.