Function of coated membranes and multivesicular bodies during membrane regulation in stimulated exocrine pancreas cells

Agonist-induced vesiculation of the Golgi apparatus in pancreatic acinar cells

BACKGROUND & AIMS

The pancreatic acinar cell is known to regulate exocytosis, total protein synthesis, and secretory protein transport in response to a secretory stimulus. Whether secretory vesicle formation also is regulated is unclear. In this study, we determined whether agonist stimulation induces morphologic alterations in the acinar cell Golgi apparatus, and we evaluated the role of the vesicle severing protein dynamin.

METHODS

Changes in Golgi structural integrity by examining the distribution of various Golgi and TGN lipid and protein markers in live and fixed cells on stimulation with cholecystokinin were noted in a primary pancreatic acinar cell model. Multiple dynamin reagents were used to examine the distribution and function of this molecular pinchase in resting and stimulated cells.

RESULTS

Regulated secretion in acinar cells induced (1) marked fragmentation of the trans-Golgi network (TGN) that corresponded temporally with an increase in cytoplasmic calcium whereas pre-TGN compartments of the Golgi and regions of the TGN involved in the generation of constitutively trafficking vesicles were unaffected by agonist, and (2) significant recruitment of dynamin to the acinar cell Golgi apparatus that appeared to potentiate fragmentation of the TGN.

CONCLUSIONS

These results suggest that the TGN is a dynamic organelle that fragments in response to cholecystokinin stimulation, a process that may contribute to zymogen granule formation.

Morphometric studies of secretory granule formation in mouse pancreatic acinar cells. Dissecting the early structural changes following pilocarpine injection

Secretory granule formation in pancreatic acinar cells is known to involve massive membrane flow. In previous studies we have undertaken morphometry of the regranulation mechanism in these cells and in mast cells as a model for cellular membrane movement. In our current work, electron micrographs of pancreatic acinar cells from ICR mice were taken at several time points after extensive degranulation induced by pilocarpine injection in order to investigate the volume changes of rough endoplasmic reticulum (RER), nucleus, mitochondria and autophagosomes. At 2-4 h after stimulation, when the pancreatic cells demonstrated a complete loss of granules, this was accompanied by an increased proportion of autophagosomal activity. This change primarily reflected a greatly increased proportion of profiles retaining autophagic vacuoles containing recognisable cytoplasmic structures such as mitochondria, granule profiles and fragments of RER. The mitochondrial structures reached a significant maximal size 4 h following injection (before degranulation 0.178 +/- 0.028 microm3; at 4 h peak value, 0.535 +/- 0.109 microm3). Nucleus size showed an early volume increase approaching a maximum value 2 h following degranulation. The regranulation span was thus divided into 3 stages. The first was the membrane remodelling stage (0-2 h). During this period the volume of the RER and secretory granules was greatly decreased. At the intermediate stage (2-4 h) a significant increase of the synthesis zone was observed within the nucleus. The volume of the mitochondria was increasing. At the last step, the major finding was a significant granule accumulation in parallel with an active Golgi zone.

Mannose 6-phosphate receptors are sorted from immature secretory granules via adaptor protein AP-1, clathrin, and syntaxin 6-positive vesicles

The occurrence of clathrin-coated buds on immature granules (IGs) of the regulated secretory pathway suggests that specific transmembrane proteins are sorted into these buds through interaction with cytosolic adaptor proteins. By quantitative immunoelectron microscopy of rat endocrine pancreatic beta cells and exocrine parotid and pancreatic cells, we show for the first time that the mannose 6-phosphate receptors (MPRs) for lysosomal enzyme sorting colocalize with the AP-1 adaptor in clathrin-coated buds on IGs. Furthermore, the concentrations of both MPR and AP-1 decline by approximately 90% as the granules mature. Concomitantly, in exocrine secretory cells lysosomal proenzymes enter and then are sorted out of IGs, just as was previously observed in beta cells (Kuliawat, R., J. Klumperman, T. Ludwig, and P. Arvan. 1997. J. Cell Biol. 137:595-608). The exit of MPRs in AP-1/clathrin-coated buds is selective, indicated by the fact that the membrane protein phogrin is not removed from maturing granules. We have also made the first observation of a soluble N-ethylmaleimide-sensitive factor attachment protein receptor, syntaxin 6, which has been implicated in clathrin-coated vesicle trafficking from the TGN to endosomes (Bock, J.B., J. Klumperman, S. Davanger, and R.H. Scheller. 1997. Mol. Biol. Cell. 8:1261-1271) that enters and then exits the regulated secretory pathway during granule maturation. Thus, we hypothesize that during secretory granule maturation, MPR-ligand complexes and syntaxin 6 are removed from IGs by AP-1/clathrin-coated vesicles, and then delivered to endosomes.

Multivesicular Bodies Are an Intermediate Stage in the Formation of Platelet α-Granules

We have used ultrathin cryosectioning and immunogold cytochemistry to study the position of α-granules in the endocytic and biosynthetic pathways in megakaryocytes and platelets. Morphologically, we distinguished three types of granules; so-called multivesicular bodies type I (MVB I) with internal vesicles only, granules with internal vesicles and an electron dense matrix (MVB II), and the α-granules with mainly a dense content and often internal membrane vesicles at their periphery. The MVBs were prominent in cultured megakaryocytes and the megakaryoblastic cell line CHRF-288, but were less numerous in bone marrow megakaryocytes and platelets, whereas α-granules were most prominent in mature bone marrow megakaryocytes and in platelets. The internalization kinetics of bovine serum albumin-gold particles and of fibrinogen positioned the MVB subtypes and α-granules sequentially in the endocytic pathway. MVBs contained the secretory proteins von Willebrand factor (vWF) and β-thromboglobulin (β-TG), the platelet-specific membrane protein P-selectin, and the lysosomal membrane protein CD63. Within the MVBs, endocytosed fibrinogen and endogenous β-TG were restricted to the matrix, while vWF was predominantly associated with internal vesicles. CD63 was also observed in association with internal membrane vesicles in the α-granules. These observations, and the gradual morphologic transition from granules containing vesicles to granules containing predominantly dense material, suggest that MVBs represent a developmental stage in α-granule maturation.

Multivesicular Bodies Are an Intermediate Stage in the Formation of Platelet α-Granules

We have used ultrathin cryosectioning and immunogold cytochemistry to study the position of α-granules in the endocytic and biosynthetic pathways in megakaryocytes and platelets. Morphologically, we distinguished three types of granules; so-called multivesicular bodies type I (MVB I) with internal vesicles only, granules with internal vesicles and an electron dense matrix (MVB II), and the α-granules with mainly a dense content and often internal membrane vesicles at their periphery. The MVBs were prominent in cultured megakaryocytes and the megakaryoblastic cell line CHRF-288, but were less numerous in bone marrow megakaryocytes and platelets, whereas α-granules were most prominent in mature bone marrow megakaryocytes and in platelets. The internalization kinetics of bovine serum albumin-gold particles and of fibrinogen positioned the MVB subtypes and α-granules sequentially in the endocytic pathway. MVBs contained the secretory proteins von Willebrand factor (vWF) and β-thromboglobulin (β-TG), the platelet-specific membrane protein P-selectin, and the lysosomal membrane protein CD63. Within the MVBs, endocytosed fibrinogen and endogenous β-TG were restricted to the matrix, while vWF was predominantly associated with internal vesicles. CD63 was also observed in association with internal membrane vesicles in the α-granules. These observations, and the gradual morphologic transition from granules containing vesicles to granules containing predominantly dense material, suggest that MVBs represent a developmental stage in α-granule maturation.

Different time courses of GTP[gamma-S]-induced exocytosis and current oscillations in isolated mouse pancreatic acinar cells

Exocytosis in isolated mouse pancreatic acinar cells was investigated using the dual-frequency method for measuring membrane capacitance and ionic conductances. Under control conditions, single exo- and endocytotic events could be resolved. The total cell capacitance slightly decreased to 98.7 +/- 0.9% of the initial cell capacitance within 10 min after establishing the whole-cell configuration. When guanosine 5'-O-(3-thiophosphate) (GTP[gamma-S] was added to the patch pipette, stepwise elevations in membrane capacitance occurred and the cell capacitance increased to 106.7 +/- 1.6% within 10 min. Exocytosis was also stimulated by GTP[gamma-S] when a Ca2+-free pipette solution supplemented with 1 to 10 mM ethylenebis(oxonitrilo) tetraacetate (EGTA) was used. Measurement of the DC current component in parallel with AC current analysis was used to isolate components of the Ca2+-dependent Cl- and monovalent cation conductances from the whole-cell conductance. These experiments demonstrate that in GTP[gamma-S]-stimulated pancreatic acinar cells: (1) activation of Cl- currents precedes that of cation currents, and (2) fusion of the zymogen granule membrane with the plasma membrane does not lead to incorporation of active Cl- or nonselective cation channels (>/= 10 pS).

Improving structural integrity of cryosections for immunogold labeling

Cryosections of aldehyde-fixed material prepared according to Tokuyasu are a good substrate for immunocytochemistry. However, structural defects occur that limit the resolution of this approach. We found that the step during which sections are thawed and transferred from the cryochamber to the supporting film on an EM grid is most critical for structural preservation. Surface tension of the transfer medium, on which sections are spread during thawing, can easily damage their structure by overstretching. By substituting a mixture of methylcellulose and sucrose for the conventional sucrose transfer medium, we were able to alleviate the problem of overstretching, thus improving greatly the structural integrity of thin cryosections. Also, material extraction from the sections after thawing causes structural damage, particularly when cross-linking is deficient. Incorporation of uranyl acetate in the transfer medium can then further help to maintain the structural integrity of the sections during the immunolabeling procedure. Excellent ultrastructure was featured in sections picked up and dried directly in methylcellulose/uranyl acetate mixtures. Such preparations can provide new insight into subcellular details and is an efficient back-up for immunolabeled sections in respect of their morphology. Cryosections from fresh frozen tissue can be preserved for immunolabeling by using transfer media that contain fixatives. This approach may have advantages if chemical fixation of tissue is thought to induce morphological artifacts or antigen redistribution.

Cytoplasmic granule formation in mouse pancreatic acinar cells. Evidence for formation of immature granules (condensing vacuoles) by aggregation and fusion of progranules of unit size, and for reductions in membrane surface area and immature granule volume during granule maturation

We used a computer-assisted morphometry approach to analyze quantitatively the process of cytoplasmic granule formation in mouse pancreatic acinar cells stimulated with pilocarpine to induce secretion. Our findings suggest that each condensing vacuole/immature granule of pancreatic acinar cells is formed by the progressive aggregation of 106 to 128 "unit progranules" of narrowly fixed volume, define a range of 7.7 to 9.2 for the factor of volume condensation between the largest immature granules and the mature unit granule, and predict that the formation of a single mature unit granule by the aggregation and fusion of unit progranules involves a net reduction of at least 95% in the amount of membrane surface area associated with these structures.

Beta-COP localizes mainly to the cis-Golgi side in exocrine pancreas

We examined the distribution of the non-clathrin-coated vesicle-associated coat protein beta-COP in rat exocrine pancreatic cells by immunogold cytochemistry. Labeling for beta-COP was found in the Golgi region (48%) where it was associated with vesicles and buds of approximately 50 nm, showing a characteristic approximately 10-nm-thick coat. The other half of the label was present in the cytoplasm, not associated with visible coats or membranes, with a minor fraction present on small clusters of tubules and vesicles. Clathrin-coated vesicles were typically located at the trans-side of the Golgi complex, and showed a thicker coat of approximately 18 nm. Of the total beta-COP labeling over the Golgi region, 68% occurred on the cis-side, 6% on the cisternae, 17% on the rims of the cisternae, and only 9% on the trans-side. For clathrin these figures were 16, 2, 4, and 78%, respectively. At the cis-Golgi side beta-COP was present in transitional areas (TA), on so-called peripheral elements (PE), consisting of tubules and vesicles located between the cup-shaped transitional elements (TE) of the RER and the cis-most Golgi cisternae. Label for Sec23p was also present in TA but was located closer to the TE, while beta-COP labeled PE were located near the cis-Golgi cisternae. Upon energy depletion, Golgi associated beta-COP was almost exclusively (86%) in spherical aggregates of 200-500 nm in diameter, whereas the cis-side (6%), the cisternae (1%), the rims (4%) and trans-side (3%) of the Golgi complex, were barely labeled; 50% of the total label remained in the cytoplasm. The aggregates were predominantly located at the cis-side of the Golgi stack, next to, but distinct from the Sec23p positive TA, that were devoid of beta-COP and had only a few recognizable vesicles left. Incubation with aluminum fluoride resulted in fragmentation of the Golgi complex into large clusters of beta-COP positive vesicles, while 50% of the label remained in the cytoplasm, as in control cells. After 10 min of Brefeldin A treatment 91% of beta-COP was cytoplasmic and only 7% associated with membranes of the Golgi complex. The total label for beta-COP over exocrine cells remained unchanged during the incubation with either of the drugs, indicating that the drugs induce reallocation of beta-COP. Our data suggest that beta-COP plays a role in membrane transport at the cis-side of the Golgi complex.

Secretory pathways in animal cells: with emphasis on pancreatic acinar cells

Studies over the past three decades have clearly established the existence of at least two distinct pathways for the intracellular transport and release of secretory proteins by animal cells. These have been identified as the regulated and constitutive pathways. Many observations have indicated that in certain cells, such as those of the exocrine pancreas and parotid glands at least, these pathways coexist in the same cells. Although the general scheme of protein transport within these pathways is well established, many fundamental aspects of intracellular transport remain to be unraveled. How are proteins transported through the endoplasmic reticulum? How are the transitional vesicles formed and what are the underlying mechanisms involved in their fusion with the cis-Golgi cisterna? Even the general mode of transfer through the Golgi stack is debated: Is there a diffusion through the stack by flow through intercisternal tubules and openings or is there a vesicle transfer system where membrane quanta hop from one cisterna to the other? What is the fate of secretory proteins in the trans-Golgi area and by what mechanisms is a fraction of newly synthesized molecules of a given secretory protein released spontaneously while the majority of such nascent molecules are diverted into a secretory granule compartment? In this review, we have examined these and other aspects of intracellular transport of secretory proteins using pancreatic acinar cells as our reference model and we present some evidence to support the existence of a paragranular pathway of secretion associated with secretory granule maturation.

Morphometric and stereological analysis of the effects of 17 beta-estradiol on the glandular epithelium of the castrated guinea pig lateral prostate

Upon administration of pharmacological doses of estradiol to castrated guinea pigs, the secretory cells of the lateral prostate underwent hypertrophy which resulted from significant increases in nuclear and cytoplasmic volume. There were quantitative increases in the small highly electron-dense granules and multivesicular bodies when compared with the castrated control. The dramatic increase in the number of highly electron-dense granules probably occurred at the expense of the low electron-dense granules. The average size of the condensing granules and mitochondria decreased significantly after estradiol administration. However, significant increase in the number of mitochondria was detected when compared with the castrated control. Ultrastructural data revealed no significant changes in the absolute dimensions of granular endoplasmic reticulum or of the Golgi complex, suggesting that estradiol exerted no significant stimulatory effects on these organelles. Pharmacological doses of estrogen appear to regulate the expression of secretory granules and multivesicular bodies in the lateral prostate of castrated guinea pigs.

Secretion of gamma-glutamyltranspeptidase by the pancreas: evidence for a membrane shedding process during exocytosis

Gamma Glutamyltranspeptidase (GGT) is a membrane-bound enzyme involved in glutathione metabolism. It is present in rat exocrine pancreas at a level which is only exceeded by the kidney. It has been previously shown that most of the enzyme activity is located in the apical area of the acinar cell, more precisely at the level of zymogen granules and plasma membrane. The aim of the present study was to examine the secretory behavior of that enzyme. Under resting conditions, in vivo, high levels of GGT were found in pancreatic juice and its level was not related to protein concentration. Under secretin infusion, a relatively constant level of GGT was released, and again, there was no correlation between enzyme activity and protein concentration. Following a bolus injection of caerulein, an analog of cholecystokinin, marked and concomitant rises in protein and GGT levels were observed. Ultracentrifugation, as well as gel filtration on Sepharose 4B, demonstrated that the enzyme was not released in a soluble form. This observation is in agreement with in vitro determinations on isolated zymogen granules showing that GGT is totally associated with the ZG membrane and undetect-able in the content of these organelles. The present data show that 1 degree GGT is released from the rat pancreas acinar cells in a particulate form; 2 degree GGT release is elicited by hormonal stimulation coinciding with the exocytotic release of secretory proteins. Our observations lead us to propose that in rat pancreas, ZG membrane fragments are released along with secretory proteins during exocytosis.

Immunohistochemical, electron microscopic, and morphometric studies of human prolactinomas after short-term bromocriptine treatment

Two cases of human prolactinomas after short-term treatment with bromocriptine were studied by means of immunohistochemistry for prolactin, electron microscopy, and morphometry at the ultrastructural level. The results obtained showed an abundance of immunoreactive secretory granules in the tumor cells, a paucity in the electron microscopic images suggesting exocytosis of secretory granules, and no significant changes in the volume density of rough endoplasmic reticulum when compared with untreated (control) tumor cells. These findings strongly suggest that the effects of short-term bromocriptine treatment for lowering serum prolactin levels did not inhibit protein and secretory granule synthesis but rather caused a disturbance in the secretion of prolactin granules. The present morphometrical data suggest the possibility that a reduction in the number of cytoplasmic microtubules might be related to the disturbance of prolactin secretion.

Cortical granule exocytosis is coupled with membrane retrieval in the egg of Brachydanio

Activation of the teleost (Brachydanio) fish egg includes the exocytosis of cortical granules, the construction of a mosaic surface consisting of the unfertilized egg plasma membrane and the limiting membranes of the cortical granules, and the appearance of coated and smooth vesicles in the cytoplasm (Donovan and Hart, '82). Unfertilized and activated eggs were incubated in selected extracellular tracers to (1) determine experimentally if cortical granule exocytosis was coupled with the endocytosis of membrane during the cortical reaction, and (2) establish the intracellular pathway(s) by which internalized vesicles were processed. Unfertilized eggs incubated in dechlorinated tap water or Fish Ringer's solution containing either horseradish peroxidase (HRP; 10 mg/ml), native ferritin (12.5 mg/ml), or cationized ferritin (12.5 mg/ml) were activated as judged by cortical granule breakdown and elevation of the chorion. Cells treated with HRP and native ferritin exhibited a delay in cortical granule exocytosis when compared with water-activated eggs lacking the tracer. Each tracer was internalized through the formation of a coated vesicle from a coated pit. Since coated pits appeared to be topographically restricted to the perigranular membrane domain of the mosaic egg surface, their labeling, particularly with cationized ferritin, strongly suggested that the retrieved membrane was of cortical granule origin. Cationized ferritin and concanavalin A (Con A) coupled with either hemocyanin or ferritin labeled the surface of the unactivated egg and both domains of the mosaic egg surface. Transformation of the deep evacuated cortical granule crypt into later profiles of exocytosis was accompanied by increased Con A binding. Within activated egg cortices, HRP reaction product, native ferritin, and cationized ferritin were routinely localized in smooth vesicles, multivesicular bodies, and autophagic vacuoles. Occasionally, each tracer was found in small coated vesicles adjacent to the Golgi and within Golgi cisternae. The intracellular distribution of HRP, native ferritin, and cationized ferritin suggests that internalized membrane is primarily processed by organelles of the lysosomal compartment. A second and less significant pathway is the Golgi complex.

Zymogen granule size in pancreas of nursing rats

Dramatic depression in granule volume density and size was measured in acinar cells of postnatal rat pancreas following the initiation of feeding. Volume density decreased about threefold from 45% at birth to 16% 2 days thereafter. Mean granule diameter decreased from 1.50 micron to 0.80 micron, an 85% decrease in corresponding granule volume. At the same time, numerical density approximately doubled. At 2 days after birth, cells with smaller granules had lower volume densities, and differences in mean granule volume between cells accounted for most of the differences in volume density. Although the distribution of granule diameter in newborns was lognormal, the distribution at 2 days was heavily skewed to larger sizes. This was the result of skewed distributions within individual cells and not an artifact of sampling. The results corroborate the central role of granule volume in determining changes in the volume density of zymogen granules in the pancreas and suggest that zymogen granules can act as capacitors that can change size as a function of the enzyme contained within.

Quantitative electron microscopic observations on Paneth cells of germfree and ex-germfree Wistar rats

Ultrastructural changes of Paneth cells of germfree (GF) rats which had been inoculated with bacteria-containing feces from conventionally-reared (SPF) rats were quantitatively examined. 12 and 24 h after inoculation, the Paneth cells showed a striking decrease in the number of secretory granules and the occurrence of large vacuoles. Phagosomes containing bacteria were not seen. After 4 days, the secretory granules reaccumulated and smooth-surfaced apical vesicles increased in number. It is discussed that the large vacuoles may be related to membrane-retrieval events following the massive extrusion of secretory granules whereas the apical vesicles appear to serve this function when exocytosis is not pronounced. In addition to the large secretory granules ca. 10% of Paneth cell profiles contained a few dense-cored vesicles measuring about 150 nm in diameter which resemble peptidergic neurosecretory granules.

Circadian changes of the rat pancreas acinar cell. A quantitative morphological investigation

The dynamics of the physiologic secretory cycle of the rat pancreatic acinar cells along a 24-h span with an alternating 12 : 12 light to dark cycle have been quantitated by morphometric methods. Significant circadian changes could be detected in cell volume and the volumes and surface areas of secretory granules and several organelles involved in the secretory process. During the dark period, which is the activity and feeding period, there is a decrease in the size of cells and individual zymogen granules as well as in the amount of secretory material. This is paralleled by a drop in the number of ribosomes and Golgi vesicles (after a transient increase) and a loss of cytoplasmic membranes. Most of the cell's zymogen content is refilled during the first hours of the light period, when cellular autophagy reaches maximal values.

Poststimulatory endocytosis, microvesicle repopulation and changes in smooth endoplasmic reticulum in nerve endings of the median eminence superfused in vitro

Mediobasal hypothalami of adult rats were superfused in vitro. A single 5 min pulse of 60 mM KCl-containing medium was infused, followed by either 15, 30, 45, 60 or 75 min superfusions with standard medium. In some experiments, 5 or 10% dextran was added followed by a 15 min recovery. Morphologically, two recovery phases were recognized. The early phase (15-30 min) was characterized by two features: (1) A clear-cut increase in the quantity of large, pleomorphic vacuoles occupying the axoplasm of nerve endings; these vacuoles were observed to be connected to caveolae of the same diameter in the axolemma and they were either coated or uncoated. (2) Progressive increase in the quantity of microvesicles (synaptic vesicles) from an initial depleted state. The vacuoles were found to contain dextran aggregates. Microvesicle-like protrusions bulged from the membrane of vacuoles. The late phase, from 45 min poststimulation onward, was typically identified after the appearance of tubules of smooth endoplasmic reticulum at the most distal segments of the nerve terminals. During this period, large vacuoles tended to decrease in quantity. Granular vesicles remained scant during the entire observation period. Images suggesting formation of microvesicles from tubules of smooth endoplasmic reticulum were observed. These results open the possibility that endocytosis of patches of membranes forming large vacuoles be an important mechanism for retrieving the membranes belonging to microvesicles and granular vesicles. Some of these large vacuoles may contribute to the early regeneration of microvesicles. More microvesicles could later be produced from the smooth endoplasmic reticulum.

The Golgi apparatus and lysosomes of rat pancreatic acinar cells following refeeding

The short term effects of refeeding on the Golgi apparatus and lysosomes of the rat exocrine pancreas were evaluated by ultrastructural, morphometric and cytochemical methods. Ten minutes after refeeding, there was a significant enlargement of Golgi cisternae and a significant increase, compared with the controls, in the number of condensing vacuoles and lysosomes. These modifications were accompanied by the appearance of acid phosphatase activity in stacked Golgi cisternae (as well as GERL) of some cells. One hour after refeeding, there were about the same numbers of condensing vacuoles and lysosomes as in the controls; Golgi cisternae were still significantly enlarged, compared with the controls, but they were no longer reactive for acid phosphatase. In both fasting and refed animals, acid phosphatase activity was demonstrable in tubular lysosomes. The data are interpreted in terms both of membrane disposal and recycling, leading to enhanced formation of zymogen granules, during physiologically stimulated secretion.

Quantitative determination of the intracellular fate of internalized plasma membrane in dissociated pituitary prolactin cells utilizing a radioiodinated cationic ferritin probe (CFI*) and electron microscopic autoradiography

Dissociated anterior pituitary cells derived from estrogen-treated female rats were incubated with radioiodinated cationic ferritin (CFI) for 2 min and subsequently in the absence of CFI for varying periods of time up to 3 hr in order to quantitate, using electron microscopic autoradiography, the distribution of retrieved plasma membrane in these cells. Following a 2-min incubation with CFI, autoradiographic grains were found to be associated almost exclusively with the plasma membrane. With increasing periods of incubation in the absence of CFI, grain-density analysis revealed increasing levels of CFI in multiple intracellular organelles. The levels of CFI were greatest for the lysosomes, intermediate for the mature secretory granules, and least for the Golgi cisternae and immature secretory granules. These findings are consistent with the idea that a portion of the retrieved plasma membrane is degraded in lysosomes and that the remainder is recycled to organelles comprising the secretory pathway to be reutilized in successive waves of the secretory cycle.

Plasma membrane retrieval in neoplastic pancreatic acinar cells

The dynamics of plasma membrane retrieval has been studied in the pancreatic acinar carcinoma in order to determine if neoplastic cells exhibiting a heterogeneity of cytodifferentiation states retain the capacity to interiorize and recycle plasma membrane. To this end, the plasma membranes of neoplastic pancreatic acinar cells were labeled with radioiodinated cationic ferritin (125I-CF), and the fate of the tracer was monitored by quantitative electron microscopic autoradiography. The undifferentiated granule-deficient cells of the tumor internalized membrane-bound 125I-CF and sequestered it predominantly in lysosomes. By contrast, the differentiated granule-rich cells internalized significantly more membrane-bound 125I-CF, and the tracer was localized in secretory granules and in lysosomes. The data suggest that neoplastic cells retain the capability of retrieving plasma membrane and that the dynamics of the process is correlated with the state of cytodifferentiation of the neoplastic cells.

Increase in zymogen granule volume accounts for increase in volume density during prenatal development of pancreas

The sudden increase in volume density of zymogen granules in acinar cells of the fetal rat pancreas was examined with particular attention to the respective roles of granule size and number in this event. Volume density increased some twelvefold, from about 3% of cytoplasmic volume at 17 days to about 45% at 20 days, following a sigmoidal pattern in which the greatest rate of increase occurred during day 18. This increase in volume density was primarily the result of an increase in granule volume. Zymogen granule diameter increased from 0.55 micron at 17 days to 1.20 micron at 20 days, an order of magnitude increase in average granule volume. The total number of granules in the tissue increased in proportion to the increase in organ weight (cell number and size), but changes in the number of granules per unit cytoplasmic volume were minor (+ 40%) in comparison to the increase in volume density. The distribution of granule diameter was roughly normal and unimodal at each time interval, and the increase in average diameter over time was marked by an increase in the upper limit of the size distribution and an increased percentage of large granules. The size of condensing vacuoles also increased during this period, and their distributions were roughly coextensive with those seen for zymogen granules at the same time. The potential origins of changes in granule size are discussed, as well as the important effect that size has on the number of granules observed in "two-dimensional" tissue sections viewed in the electron microscope. If size is not considered in our estimates, then we underestimate the numerical density in cells with small granules compared to those with large granules. The results indicate the central role of granule size, as opposed to number, in determining granule volume density in the embryonic pancreas.

Structural changes in nerve endings of rat median eminence superfused with media rich in potassium ions

In vitro fragments of male rat mediobasal hypothalami were superfused with Krebs--Ringer solution in the presence or absence of CaCl2. Infusions containing up to 60 mM potassium chloride were applied, at the end of which tissues were fixed in osmium tetroxide and prepared for transmission electron microscopy. Control superfusions were run in parallel. Quantitative measurements performed on electron micrographs of the outermost palisade region showed significant (20-30%) increase in caliber of axon endings after intensive potassium ion stimulation. Ultrastructurally, widespread depletion of granular vesicles and microvesicles was found. Vesicle shift to the outer zone of the terminals, formation of membrane-bound tubules of the same diameter as microvesicles, and images of attachment and collapse of vesicles into the axolemma were found, particularly after 1 min stimulation. These findings were interpreted as consistent with exocytosis. Longer stimulations were followed by the appearance of large pleomorphic vacuoles that are probably the result of post-exocytotic membrane retrieval. Axon enlargement and vesicle depletion were absent in specimens superfused with calcium-free medium containing high potassium. The functional significance of these ultrastructural changes are interpreted as supporting the hypothesis that exocytosis of calcium-loaded microvesicles can contribute to extrude this ion from median eminence nerve endings during secretion.

Exocrine glands and protein secretion: a stereological viewpoint

The exocrine pancreas and the parotid salivary gland have been widely used as models for studying the synthesis, intracellular transport and discharge of exportable proteins. This article briefly reviews quantitative morphological (stereological) studies which have been made of these glands and assesses their contribution to our understanding of the secretory process. A general stereological profile of these glands is presented and the way in which their morphology changes during development is outlined. Detailed consideration is given to the granule population of the cells, particularly the way in which granules are formed and discharged as a result of secretory stimuli. The membrane content of secretory cells and membrane dynamics during the secretory cycle are also examined. Throughout, the emphasis is placed on the interpretation of stereological data rather than on the methods themselves.

Secretory process in Brunner's glands during recovery from stimulation with a single dose of pilocarpine

The secretory pathway and kinetics of the secretory process were studied in Brunner's glands of mice after stimulation of secretion with a parasympatho-mimetic drug. Adult male mice were injected with pilocarpine, while unstimulated controls received saline. The animals were subsequently administered an intravenous injection of 3H-threonine, and tissue was prepared for electron microscope autoradiography at intervals ranging from 5 minutes to 2 hours after injection of the radioactive precursor. Stimulation with pilocarpine resulted in discharge of secretory granules, which was reflected in a significantly lower percentage of the cell volume occupied by granules. In both control and stimulated mice, at 5 minutes after injection of 3H-threonine, the highest percentage of silver grains was found over the rough endoplasmic reticulum. The proportion of silver grains over the rough endoplasmic reticulum declined at later intervals, and a peak of labeling was observed over the Golgi apparatus at 1 hour. Labeling of the secretory granules increased in the 1- and 2-hour samples from both control and stimulated mice, although the relative concentration of radioactivity in both Golgi-associated and apical secretory granules was greater in stimulated than control glands at 1 hour. The results suggest that the secretory protein produced by Brunner's glands was synthesized by the rough endoplasmic reticulum and transported to the Golgi apparatus where secretory granules were formed in both stimulated and control glands. Depletion of secretory granules by prior stimulation resulted in no change in the kinetics of arrival of radioactivity in the cell organelles involved in the secretory process. However, the drainage of the radioactive label from the rough endoplasmic reticulum was significantly slower in the stimulated glands than in the controls. The existence of two subcompartments within the rough endoplasmic reticulum is among the possible explanations for the latter observation.

Endocytic pathways at the lateral and basal cell surfaces of exocrine acinar cells

In parotid acinar cells, horseradish peroxidase (HRP) administered via the main excretory duct is endocytosed from the apical cell surface in smooth C- or ring-shaped vesicles (Oliver, C. and A. R. Hand. 1979. J. Cell Biol. 76:207). These vesicles ultimately fuse with lysosomes adjacent to the Golgi apparatus. The present investigation extends these findings and examines the uptake and fate of intravenously injected HRP from the lateral and basal cell surfaces of resting and stimulated parotid and pancreatic acinar cells from rats and mice. Isoproterenol and pilocarpine were used to stimulate the parotid gland and the pancreas, respectively. HRP was internalized in smooth and coated vesicles primarily in areas of membrane infoldings. Both the number of coated vesicles and the amount of tracer internalized increased markedly following secretagogue administration. In both resting and stimulated cells, the HRP was rapidly sequestered in a unique system of basally located lysosomes that possess trimetaphosphatase activity, but not acid phosphatase activity. At 1-3 h after HRP administration, reaction product was also found in multivesicular bodies, vesicles, and lysosomes adjacent to the Golgi apparatus. With time, more HRP was localized in Golgi-associated lysosomes. By 6-7 h, tubules in the apical cytoplasm of stimulated cells contained HRP reaction product. When native ferritin was administered retrogradely and HRP injected intravenously, both tracers could be localized in the same lysosome after 4-5 h, indicating that material taken in from all cell surfaces mixes in Golgi-associated lysosomes. The results of this study suggest that two separate and distinct endocytic pathways exist in exocrine acinar cells: one involves membrane retrieval from the apical cell surface; and the other is a stimulation-dependent process at the lateral and basal cell surfaces.

Uptake and intracellular distribution of ferritin in the rat distal convoluted tubule

The purpose of this study was to determine the uptake and intracellular distribution of anionic ferritin (AF) and cationic ferritin (CF) in the distal convoluted tubule (DCT) of the rat kidney. Male Sprague-Dawley rats were prepared for micropuncture, and individual distal tubules were perfused with 5 or 10 mg/ml of AF or 0.1 or 0.5 mg/ml of CF in isotonic saline for either 30 sec or 3 min. The tubules were fixed by perfusion with 6.25% glutaraldehyde either immediately or at different time intervals after exposure to ferritin. Electron microscopy of tubules fixed immediately after perfusion showed no binding of AF to the apical cell membrane, and only traces of AF were observed in small apical structures. In contrast, CF was extensively bound to the apical cell membrane and located in apical vesicles and tubules, and in multivesicular bodies. Occasionally, CF was observed in Golgi vesicles and cisternae. Sixty min after perfusion with ferritin, traces of AF were present in multivesicular bodies and lysosome-like structures. Thirty and 60 min after perfusion, large concentrations of CF were located in multivesicular bodies and lysosome-like bodies. This study reveals that in the DCT, CF is bound to the apical cell membrane and taken up into the tubule cells, whereas only traces of AF are taken up, indicating that the charge of a protein molecule may determine whether or not the protein is reabsorbed by the DCT. The demonstration of CF in the Golgi complex is compatible with the existence of membrane recycling in cells of the DCT.

The uptake of cationised ferritin and its subsequent redistribution by gallbladder epithelium in vivo

Cationised ferritin was injected into the gallbladders of guinea pigs at operation and its subsequent fate studied with an electron microscope over a 24-hr period. The apical cell membrane of the epithelium was tagged and showed patching. The cationised ferritin was internalised into small vesicles, some derived from coated pits, within 5 min. Other vesicles were found at the same time near the lateral cellular membrane. Cationised ferritin was found in lysosomes and in residual bodies in large quantities and occasionally in multivesicular bodies and vesicles near the Golgi apparatus. Clumps of (cationised) ferritin appeared in the lateral intercellular space, 2 hr after exposure, increasing in number with time up to 5 hr. Similar clumps also were found between the basal cell membrane and the lamina rara interna of the basement membrane in a fairly regular pattern. By 5 hr the marker was beginning to pass through the basement membrane. The process was largely complete by 10 hr. Twenty-four hours after injection of cationised ferritin, virtually no marker was found in either the epithelium or underlying connective tissue.

Testosterone-dependent changes in vivo and in vitro in the structure of the renal glomeruli of the teleost Gasterosteus aculeatus L

During the reproductive period of the male stickleback, structural and functional changes of the kidney take place, both in the glomeruli and in the renal tubule cells. The structural changes in the glomeruli involve almost all glomerular components and point to a reduction of the glomerular filtration rate. The purpose of this study was to investigate whether these changes are controlled by testosterone, indirectly or directly. In vivo experiments demonstrated that exposures of immature castrates for 8 days to methyltestosterone leads to activation of mesangial cells and podocytes, to slight expansion of mesangial matrix and slight thickening of basal lamina. Observations on cultured renal tissue showed that two androgens, 11-ketotestosterone and methyltestosterone, are also able to stimulate the secretory activity of podocytes and mesangial cells in vitro. The results therefore indicate, that most glomerular changes in male sticklebacks during the reproductive season are directly effected by testosterone.

Membrane dynamics in the parotid acinar cell during regranulation: a stereological study following isoprenaline-induced secretion

Recently weaned male rabbits were injected either with 150 micrograms/kg isoprenaline in saline containing 0.01 M ascorbic acid or simply with the drug vehicle. Groups of drug-injected animals were killed at various time after injection. Parotid gland tissue samples from all animals were fixed, embedded and thin sectioned, and micrographs were prepared at standard magnification. Estimations of membrane areas of each membrane type in parotid acinar cells were made. It was found that in animals killed 2 hours after induced secretion apical area was larger than in controls. In animals killed at successively later times the apical area was progressively less. No elevation of any internal smooth membrane areas was ascertained at any sampling time, though the areas of rough endoplasmic reticulum in 2-12 hour samples were larger. It is suggested that excess apical membrane, though probably removed by interiorization, is afterwards disassembled in side the cell to create fresh macromolecular building units (protein molecules), perhaps after passing through the Golgi apparatus. This cryptic pool of building units can provide about 900 micrometers2 of secretion granule membrane per cell, the supply apparently being exhausted in the first eight hours after degranulation, whilst granule numbers are being increased. Thereafter, apparently, limited granule fusion occurs, so that ultimately the cellular complement of secretion granule membrane comes to enclose a greater volume of secretory product, though the average granule number per cell is small.

Ultrastructure of the chondrocytes and extracellular matrix of the swarm rat chondrosarcoma

The Swarm chondrosarcoma, a transplantable tumor maintained in rats, has a structure similar to hyaline cartilage. The extracellular matrix contains narrow, banded collagen fibrils and proteoglycan matrix granules. The cells are filled with organelles typically involved in synthesis and secretion, including an extensive rough endoplasmic reticulum with dilated cisternae, a large Golgi apparatus dispersed throughout the cell, and secretory vacuoles which fill directly from Golgi saccules. There is only one class of secretory vacuoles, suggesting that the secretory products, collagen and proteoglycan, are packaged and secreted together. The cells also contain unusual endoplasmic reticulum, many coated vesicles and multivesicular bodies, and abnormal mitochondria. The large amount of tissue available from the tumor, and its characterization biochemically (Choi et al., '71; Oegema et al., '75) and morphologically as cartilage, make the tumor valuable as a model tissue for studies of synthesis and secretion of extracellular matrix materials.

Recovery of surface membrane in anterior pituitary cells. Variations in traffic detected with anionic and cationic ferritin

Cells dissociated from rat anterior pituitaries were incubated with native or cationized ferritin (CF) to trace the fate of surface membrane. Native ferritin, which did not bind to the cell surface, was taken up in small amounts by bulk-phase endocytosis and was found increasingly (over 1-2 h) concentrated in lysosomes. CF at 100-fold less concentrations bound rapidly to the cell membrane, was taken up by endocytosis in far greater amounts, and within 15-60 min was found increasingly within multiple stacked Golgi cisternae, around forming secretion granules, and within elements of GERL, as well as within lysosomes. The findings demonstrate that the fate of the tracer--and presumably also that of the surface membrane--varies with the same molecule differing only in net charge: vesicles carrying anionic ferritin (net negative charge) fuse only with elements of the lysosomal system whereas those carrying CF (net positive charge) can fuse not only with elements of the lysosomal system, but also with elements along the secretory pathway (Golgi cisternae and condensing granules) as well.

Cellular responses to surface binding and internalization of concanavalin A. An electron microscopic investigation on the problem of membrane cycling

Monolayer cultures of normal and diethylnitrosamine-transformed rat liver cells were labeled in situ with Con A-HRP or ferritin-conjugated Con A. Ligand-induced redistribution with simultaneous internalization of labeled membrane areas occurred in normal as well as in transformed cells when they were reincubated with PBS at 37 degrees C for different periods of time (from 5 min up to 3 hrs). Compared to normal cells, these afore mentioned processes were accelerated in transformed cells. Internalization in normal and transformed cells resulted in a recycling of labeled plasma membrane areas in the Golgi region with the label being finally accumulated in elements which correspond mostly, but not exclusively, to GERL. Then formation of phagolysosomes and multivesiculated bodies occurred whose labeled content was exocytized after fusion with the plasma membrane. This suggested that the internalized plasma membrane areas were at least partly degraded. The relabeling of some parts of the plasma membrane by extruded lysosomal content indicates that at least some Con A molecules are still biological active. Membrane internalization by endocytosis after binding of Con A obviously causes an increased of membrane biogenesis and exocytosis, thus compensating for membrane removal. This is suggested by the vacuolization and enlargement of unlabeled (not in recycling involved) Golgi apparatus. It may indicate a differential functional role of the Golgi apparatus in membrane turnover in the same cell. The fusion of phagolysosomes with the plasma membrane and the insertion of phagolysosomal membrane into the plasma membrane might be another compensatory mechanism.