The endoplasmic-sarcoplasmic reticulum of smooth muscle: immunocytochemistry of vas deferens fibers reveals specialized subcompartments differently equipped for the control of Ca2+ homeostasis.

Cryosection immunofluorescence and immunogold labeling with antibodies against specific markers were used in rat vas deferens smooth muscle fibers to reveal the molecular arrangement of the endomembrane system (referred to variously in the text as ER or sarcoplasmic reticulum [SR]; S-ER or ER/SR) known to participate in the control of Ca2+ homeostasis. The lumenal ER chaperon, immunoglobulin binding protein (BiP), as well as protein disulfide isomerase, and calreticulin, a Ca2+ binding protein expressed by most eukaryotic cells, appeared to be evenly distributed throughout the entire system (i.e., within [a] the nuclear envelope and the few rough-surfaced cisternae clustered near the nucleus; [b] single elements scattered around in the contractile cytoplasm; and [c] numerous, heterogeneous, mainly smooth-surfaced elements concentrated in the peripheral cytoplasm, part of which is in close apposition to the plasmalemma). All other structures, including nuclei, mitochondria, Golgi complex, and surface caveolae were unlabeled. An even distribution throughout the endomembrane system appeared also for the proteins recognized by anti-ER membrane antibodies. In contrast, calsequestrin (the protein that in striated muscles is believed to be the main actor of the rapidly exchanging Ca2+ storage within the lumen of the sarcoplasmic reticulum) was found preferentially clustered at discrete lumenal sites, most often within peripheral smooth-surfaced elements of moderate electron density. Within these elements dual labeling revealed intermixing of calsequestrin with the other lumenal ER proteins. Moreover, the calsequestrin-rich elements were enriched also in the receptor for inositol 1,4,5-trisphosphate, the second messenger that induces Ca2+ release from intracellular stores. These results document the previously hypothesized molecular heterogeneity of the smooth muscle endomembrane system, particularly in relation to the rapid storage and release of Ca2+.

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.

To identify intracellular Ca2+ stores, we have mapped (by cryosection immunofluorescence and immunogold labeling) the distribution in the chicken cerebellar cortex of an essential component, the main low affinity-high capacity Ca2+ binding protein which in this tissue has been recently shown undistinguishable from muscle calsequestrin (Volpe, P., B. H. Alderson-Lang, L. Madeddu, E. Damiani, J. H. Collins, and A. Margreth. 1990. Neuron. 5:713-721). Appreciable levels of the protein were found exclusively within Purkinje neurons, distributed to the cell body, the axon, and the elaborate dendritic tree, with little labeling, however, of dendritic spines. At the EM level the protein displayed a dual localization: within the ER (rough- and smooth-surfaced cisternae, including the cisternal stacks recently shown [in the rat] to be highly enriched in receptors for inositol 1,4,5-triphosphate) and, over 10-fold more concentrated, within a population of moderately dense, membrane-bound small vacuoles and tubules, identified as calciosomes. These latter structures were widely distributed both in the cell body (approximately 1% of the cross-sectional area, particularly concentrated near the Golgi complex) and in the dendrites, up to the entrance of the spines. The distribution of calsequestrin was compared to those of another putative component of the Ca2+ stores, the membrane pump Ca2+ ATPase, and of the ER resident lumenal protein, Bip. Ca2+ ATPase was expressed by both calciosomes and regular ER cisternae, but excluded from cisternal stacks; Bip was abundant within the ER lumena (cisternae and stacks) and very low within calciosomes (average calsequestrin/Bip immunolabeling ratios were approximately 0.5 and 36.5 in the two types of structure, respectively). These results suggest that ER cisternal stacks do not represent independent Ca2+ stores, but operate coordinately with the adjacent, lumenally continuous ER cisternae. The ER and calciosomes could serve as rapidly exchanging Ca2+ stores, characterized however by different properties, in particular, by the greater Ca2+ accumulation potential of calciosomes. Hypotheses of calciosome biogenesis (directly from the ER or via the Golgi complex) are discussed.

Overexpression of calreticulin increases the Ca2+ capacity of rapidly exchanging Ca2+ stores and reveals aspects of their lumenal microenvironment and function.

A molecularly tagged form of calreticulin (CR), a low affinity-high capacity Ca2+ binding protein that resides in the ER lumen, was transiently transfected into HeLa cells to specifically modify the Ca2+ buffering capacity of the intracellular Ca2+ stores. Fluorescence and confocal microscope immunocytochemistry revealed the tagged protein to be expressed by over 40% of the cells and to overlap in its distribution the endogenous CR yielding a delicate cytoplasmic network, i.e., the typical pattern of ER. In contrast, no signal was observed associated with the plasmalemma (marked by ConA) and within the nucleus. One- and two-dimensional Western blots revealed the transfected to exceed the endogenous CR of approximately 3.5-fold and to maintain its Ca2+ binding ability, whereas the expression of other ER proteins was unchanged. Ca2+ homeostasis in the transfected cells was investigated by three parallel approaches: (a) 45Ca equilibrium loading of cell populations; (b) [Ca2+]c measurement with fura-2 followed by quantitative immunocytochemistry of single cells and iii) [Ca2+]c measurement of cell population upon cotransfection with the Ca(2+)-sensitive photoprotein, aequorin. The three approaches revealed different aspects of Ca2+ homeostasis, yielding results which were largely complementary. In particular, the following conclusions were established: (a) both endogenous and transfected CR participate in Ca2+ buffering within the IP3-sensitive, rapidly exchanging, Ca2+ stores; the other pools of the cells were in contrast unaffected by CR transfection; (b) the Ca2+ capacity of the stores is not the main limiting factor of individual IP3-mediated Ca2+ release responses triggered by receptor agonists; (c) in control cells, the contribution of CR to Ca2+ buffering within the IP3-sensitive stores accounts for approximately 45% of the total, the rest being probably contributed by the other lumenal (and also membrane) Ca2+ binding proteins; (d) the free [Ca2+] within the lumen of the IP3-sensitive stores, revealed by the degree of Ca2+ binding to the transfected CR protein, amounts to values in (or approaching) the millimolar range; and (e) Ca2+ influx across the plasmalemma activated by depletion of the stores is directly dependent on the lumenal [Ca2+].

Overexpression of calsequestrin in L6 myoblasts: formation of endoplasmic reticulum subdomains and their evolution into discrete vacuoles where aggregates of the protein are specifically accumulated.

Calsequestrin (CSQ), the major low-affinity Ca(2+)-binding glycoprotein of striated muscle fibers, is concentrated to yield aggregates that occupy the lumen of the terminal cisternae of the sarcoplasmic reticulum (SR). When infected or transfected into L6 myoblast, the protein is also concentrated, however, in dense vacuoles apparently separate from the endoplasmic reticulum (ER). CSQ-rich cells appear otherwise normal; in particular, neither other proteins involved in Ca2+ homeostasis nor ER chaperones are increased. The CSQ dense vacuoles are shown herein to be specialized ER subdomains as demonstrated by 1) the endoglycosidase H sensitivity of their CSQ and 2) two markers, calreticulin and calnexin (but not others, protein disulfide isomerase and BiP), intermixed with the vacuole content. Their formation is shown to start with the aggregation of CSQ at discrete sites of the ER lumen. When cells were transfected with both CSQ and calreticulin, only the first gave rise to vacuoles; the second remained diffusely distributed within the ER lumen. The possibility that CSQ aggregation is an artifact of overexpression appears unlikely because 1) within dense vacuoles CSQ molecules are not disulfide cross-linked, 2) their turnover is relatively slow (t = 12 h), and 3) segregated CSQ is bound to large amounts of Ca2+. Transfection of a tagged CSQ into cells already overexpressing the protein revealed the continuous import of the newly synthesized protein into preassembled vacuoles. The tendency to aggregation appears, therefore, as a property contributing to the segregation of CSQ within the ER lumen and to its accumulation within specialized subdomains. The study of L6 cells expressing CSQ-rich vacuoles might thus ultimately help to unravel mechanisms by which the complexity of the sarcoplasmic reticulum is established in muscle fibers.

High-resolution calcium mapping of the endoplasmic reticulum-Golgi-exocytic membrane system. Electron energy loss imaging analysis of quick frozen-freeze dried PC12 cells.

The calcium pools segregated within the endoplasmic reticulum, Golgi complex, exocytic, and other organelles are believed to participate in the regulation of a variety of cell functions. Until now, however, the precise intracellular distribution of the element had not been established. Here, we report about the first high-resolution calcium mapping obtained in neurosecretory PC12 cells by the imaging mode of the electron energy loss spectroscopy technique. The preparation procedure used included quick freezing of cell monolayers, followed by freeze-drying, fixation with OSO4 vapors, resin embedding, and cutting of very thin sections. Conventional electron microscopy and high-resolution immunocytochemistry revealed a high degree of structural preservation, a condition in which inorganic elements are expected to maintain their native distribution. Within these cells, calcium signals of nucleus, cytosol, and most mitochondria remained below detection, whereas in other organelles specific patterns were identified. In the endoplasmic reticulum, the distribution was heterogeneous with strongly positive cisternae (more often the nuclear envelope and stacks of parallel elements that are frequent in quick frozen preparations) lying in the proximity of or even in direct continuity with other, apparently negative cisternae. The Golgi complexes were labeled strongly and uniformly in all cisternae and part of their vesicles, with no appreciable differences along the cis-trans axis. Weaker or negative signals were recorded from the trans-Golgi network elements and from scattered vesicles, whereas in contrast secretion granules were strongly positive for calcium. These results are discussed in relation to the existing knowledge about the mechanisms of calcium transport in the variations organelles, and about the processes and functions regulated by organelle lumenal calcium in eukaryotic cells.

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.

Intracellular Ca2+ stores in neurons. Identification and functional aspects.

Various aspects of the rapidly exchanging intracellular Ca2+ stores of neurons and nerve cells are reviewed: their multiplicity, with separate sensitivity to either the second messenger, inositol 1,4,5-trisphosphate, or ryanodine-caffeine (the latter stores are probably activated via Ca(2+)-induced Ca2+ release); their control of the plasma membrane Ca2+ permeability, via the activation of a peculiar type of cation channels; their ability to sustain localized heterogeneities of the [Ca2+]i that could be of physiological key-importance. Finally, the molecular composition of these stores is discussed. They are shown (by high resolution immunocytochemistry and subcellular fractionation) to express: i) a Ca2+ ATPase responsible for the accumulation of the cation; ii) Ca2+ binding protein(s) of low affinity and high capacity to keep Ca2+ stored; and iii) a Ca2+ channel, activated by either one of the mechanisms mentioned above, to release Ca2+ to the cytosol. Results obtained in Purkinje neurons document the heterogeneity of the stores and the strategical distribution of the corresponding organelles (calciosomes; specialized portions of the ER) within the cell body, dendrites and dendritic spines.

Ageing affects cytosolic Ca2+ binding proteins and synaptic markers in the retina but not in cerebral cortex neurons of the rat.

Two cerebral cortex areas (frontal and occipital) and the retina of rats varying in age from 0.4 to 30 months were investigated for the expression levels and distribution of two cytosolic high affinity Ca2+ binding proteins, calbindin-28 and calretinin, and of two presynaptic protein markers. Of these latter proteins, one is integral (synaptophysin) the other peripheral (synapsin I) to the synaptic vesicle membranes. In the cortex areas, no significant changes of the markers were observed, except for a drop of calretinin from 0.4 to 2 months, probably related to a stage of neuronal development. In the retina, calbindin-28 decreased progressively during ageing (-40% at 30 months) while calretinin remained unchanged. Concomitantly, the two synaptic vesicle proteins dropped, synaptophysin > 50% and synapsin I > 85%. The role of these changes in sustaining the functional alterations previously described in the retina of aged animals remains to be investigated.

Beta cell chromogranin B is partially segregated in distinct granules and can be released separately from insulin in response to stimulation.

AIMS/HYPOTHESIS: We investigated, in three beta cell lines (INS-1E, RIN-5AH, betaTC3) and in human and rodent primary beta cells, the storage and release of chromogranin B, a secretory protein expressed in beta cells and postulated to play an autocrine role. We asked whether chromogranin B is stored together with and discharged in constant ratio to insulin upon various stimuli. METHODS: The intracellular distribution of insulin and chromogranin B was revealed by immunofluorescence followed by three-dimensional image reconstruction and by immunoelectron microscopy; their stimulated discharge was measured by ELISA and immunoblot analysis of homogenates and incubation media. RESULTS: Insulin and chromogranin B, co-localised in the Golgi complex/trans-Golgi network, appeared largely segregated from each other in the secretory granule compartment. In INS-1E cells, the percentage of granules positive only for insulin or chromogranin B and of those positive for both was 66, 7 and 27%, respectively. In resting cells, both insulin and chromogranin B were concentrated in the granule cores; upon stimulation, chromogranin B (but not insulin) was largely redistributed to the core periphery and the surrounding halo. Strong stimulation with a secretagogue mixture induced parallel release of insulin and chromogranin B, whereas with 3-isobutyl-1-methylxantine and forskolin +/- high glucose release of chromogranin B predominated. Weak, Ca(2+)-dependent stimulation with ionomycin or carbachol induced exclusive release of chromogranin B, suggesting a higher Ca(2+) sensitivity of the specific granules. CONCLUSIONS/INTERPRETATION: The unexpected complexity of the beta cell granule population in terms of heterogeneity, molecular plasticity and the differential discharge, could play an important role in physiological control of insulin release and possibly also in beta cell pathology.

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.

Spermiogenesis in Xenopus laevis: from late spermatids to spermatozoa.

Spermatogenesis is a complex morphogenetic process in which microfilaments and microtubules have been shown to play an important role. The last steps of Xenopus spermatogenesis, i.e., the corkscrew shaping of the sperm head, have been followed to study actin and microtubule distribution by conventional and immunoelectron microscopy. During sperm head morphogenesis, actin is absent in the elongating spermatids, but it is present in the Sertoli cells where results localized at the periphery of their cytoplasm that surrounds the developing germ cells. Sertoli cell actin and microtubules may assist the elongation and the shaping of the spermatids and function in maintaining the Sertoli-spermatid association.

Cytosolic Ca2+ binding proteins during rat brain ageing: loss of calbindin and calretinin in the hippocampus, with no change in the cerebellum.

The expression of two cytosolic, high affinity Ca(2+)-binding proteins, calbindin-28 and calretinin, has been investigated in the cerebellum and hippocampus of young and old rats (from 12 days to 30 months) by combining immunofluorescence and Western blotting. Three markers, calreticulin (the major Ca2+ binding protein within the lumen of the endoplasmic reticulum), MAP-2 (a microtubule binding protein concentrated in neuronal dendrites) and synaptophysin (an integral protein of synaptic vesicles), were studied in parallel. In the cerebellar cortex a rise from 12 to 60 days was observed with calbindin-28 and, especially, calretinin, concentrated in the Purkinje and granule neurons, respectively. The level of expression of the two proteins subsequently remained high and the distribution was unchanged, even in the cerebellum of old animals. A completely different pattern was observed in the hippocampus. Here calretinin, present especially in fibres and interneurons, was abundant in the young, decreased in the adult and reached low values in the old rats. Calbindin-28 accumulated during growth, especially in a subpopulation of CA1 pyramidal cells and in the mossy fibres of CA3, then declined, although irregularly, during ageing. These changes of the two proteins were more marked in the dorsal and central parts than in the ventral part of the hippocampus. In the same brain areas the levels of expression of the three additional markers and their distribution within neurons and synapses were unchanged by ageing.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification, kinetic properties and intracellular localization of the (Ca(2+)-Mg2+)-ATPase from the intracellular stores of chicken cerebellum.

The microsomal fraction of chicken cerebellum expresses a large amount of Ca(2+)-ATPase (105 kDa), which is phosphorylated by ATP in the presence of Ca2+. The Ca(2+)-ATPase activity is highly sensitive to temperature and to the presence of detergents. This ATPase has kinetic properties similar to those of chicken skeletal-muscle sarcoplasmic reticulum, as (i) it is activated by low (microM) and inhibited by high (mM) Ca2+ concentrations, (ii) it shows biphasic activation with ATP and (iii) it is inhibited by vanadate. However, the vanadate-sensitivity is at least 10 times greater than that observed in chicken skeletal or cardiac sarcoplasmic-reticulum Ca(2+)-ATPases. Thus, despite cross-reacting with antibodies against the cardiac and skeletal isoforms, the cerebellar microsomal Ca(2+)-ATPase appears to be distinct from both muscle enzymes. The Ca(2+)-ATPase is concentrated in, but not exclusive to, Purkinje neurons. In Purkinje neurons the Ca(2+)-ATPase appears to be expressed throughout the cell body, the dendritic tree (and the spines) and the axons. At the electron-microscope level the Ca(2+)-ATPase is found in smooth and rough endoplasmic-reticulum cisternae as well as in other, yet unidentified, smooth-surfaced structures.

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.

Neurosecretion competence, an independently regulated trait of the neurosecretory cell phenotype.

Neurosecretion competence is intended as the ability of neurosecretory cells to express dense and clear vesicles discharged by regulated exocytosis (neurotransmitter release). Such a property, which so far has never been studied independently, is investigated here by a heterotypic cell fusion approach, using a clone of rat pheochromocytoma PC12 cells totally incompetent for neurosecretion that still largely maintains its typical molecular and cellular phenotype. When fused with wild-type partners of various species (rat, human) and specialization (PC12, neuroblastoma SH-SY5Y, HeLa), the defective cells reacquire their competence as revealed by the expression of their secretion-specific proteins. Fused wild-type cells therefore appear able to complement defective cells by providing them with factor(s) inducing the reactivation of their secretory program. The mechanism of action of these factors may consist not in a coordinate unblocking of transcription but in the prevention of a rapid post-transcriptional degradation of the mRNAs for secretion-specific genes.

Intracellular Ca2+ stores in chick cerebellum Purkinje neurons: ontogenetic and functional studies.

The molecular composition of intracellular Ca2+ stores in developing chicken cerebellum Purkinje neurons from embryonic day 11 (E11) to posthatching day 2 (P2) was studied by immunocytochemistry using specific antibodies for three molecular constituents, the receptor (R) and/or channel sensitive to inositol 1,4,5-trisphosphate (IP3), Ca(2+)-adenosinetriphosphatase (ATPase), and calsequestrin (CS). CS, IP3R, and Ca(2+)-ATPase were first detected by light-microscopic immunofluorescence in migrating Purkinje cells at E11-E12 and throughout late phases of embryonic development. Ontogenesis of CS, IP3R, and Ca(2+)-ATPase accompanied well-defined stages of cerebellum histogenesis and cytogenesis and was accomplished before hatching. High-resolution immunogold electronmicroscopy revealed that, at E18-P1, CS was still largely distributed to the endoplasmic reticulum (ER) lumen and began to be segregated to ER subcompartments (calciosomes) only by P2, whereas the IP3R was concentrated into ER cisternal stacks as early as E18. Both ionotropic and metabotropic plasma membrane receptors were present in dissociated single chicken Purkinje cells from E16 onward, as indicated by measurements of membrane currents (whole cell recording mode) and of cytoplasmic Ca2+ transients monitored with the cell-trappable fluorescent indicator fura 2-acetoxymethyl ester, respectively. Cytoplasmic Ca2+ transients were detected after either activation of glutamate metabotropic receptors, i.e., evidence of IP3-sensitive Ca2+ channels, or application of caffeine, i.e., evidence of ryanodine-sensitive Ca2+ channels. Intracellular Ca2+ stores appear to be functional during embryonic development.

The endoplasmic reticulum-sarcoplasmic reticulum connection. II. Postnatal differentiation of the sarcoplasmic reticulum in skeletal muscle fibers.

The postnatal differentiation of sarcoplasmic reticulum (SR) of rabbit skeletal muscles (the slow-twitch soleus and the fast-twitch adductor muscles) was monitored between Days 1 and 12 by following on Western blots the expression and accumulation of molecular markers specific not only for the muscle endomembrane system, i.e., calsequestrin (CS) and the ryanodine-sensitive Ca2+ release channel, but also for the endoplasmic reticulum (ER) at large, i.e., BiP, calnexin (CN) and calreticulin. Our results demonstrate that SR development, documented by the increase of the SR fractional volume, terminal cisternae proliferation, and reorientation of triads, is accompanied by the accumulation of the SR-specific proteins and also of CN, with no change of the other ER general markers. Moreover, the distribution of two of the markers, BiP and CS, was investigated by immunocytochemistry at both the light and the electron microscope level. At Day 1 CS was found to be concentrated both within the few recognizable triad terminal cisternae and within the lumen of numerous, apparently discrete cisternae and tubules, widely scattered throughout both the contractile and the subplasmalemmal areas of the cytoplasm. These structures remain evident until Day 12, when most triad junctions have acquired proper configuration, composition and orientation. BiP, on the other hand, appears widely distributed within the ER/SR of the fibers. From the early stages of postnatal development it does colocalize with the Ca2+ binding protein in the lumen of the CS-rich structures and appears also within the longitudinal SR and the conventional ER cisternae.

Heterogeneity of microsomal Ca2+ stores in chicken Purkinje neurons.

Chicken cerebellum microsomes were subfractionated on isopycnic, linear sucrose (15-50%) density gradients. The distribution of four markers of intracellular, rapidly-exchanging Ca2+ stores, i.e. the Ca2+ pump, the receptors for inositol 1,4,5-trisphosphate (IP3) and ryanodine (Ry), and calsequestrin (CS, an intralumenal, high capacity Ca2+ binding protein) was investigated biochemically and immunologically. In the cerebellum, high levels of these markers are expressed by one of the cell types, the Purkinje neuron. Heavy subfractions were enriched in both CS and Ry receptor, intermediate subfractions in the IP3 receptor, while the Ca2+ pump was present in both intermediate and heavy subfractions. Intact cells and pelleted subfractions were examined by conventional and immuno-electron microscopy (immunogold labeling of ultrathin cryosections with anti-CS and anti-IP3 receptor antibodies). Of the strongly CS-labeled, moderately dense-cored vacuoles (calciosomes) recently described in chicken Purkinje neurons only partly exhibited labeling for the IP3 receptor as well, and the rest appeared negative. The latter were enriched in a heavy subfraction of the gradient where Ry receptors were also concentrated, whereas the CS-rich vacuoles in an intermediate subfraction were almost always IP3 receptor-positive. The population of CS-rich calciosomes of chicken Purkinje neurons appears therefore to be molecularly heterogeneous, with a part responsive to IP3 and the rest possibly sensitive to Ry.

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.