Changes in kinesin distribution and phosphorylation occur during regulated secretion in pancreatic acinar cells

Exocytosis, dependent on Ca2+ release from Ca2+ stores, is regulated by Ca2+ microdomains

The relationship between the cellular Ca2+ signal and secretory vesicle fusion (exocytosis) is a key determinant of the regulation of the kinetics and magnitude of the secretory response. Here, we have investigated secretion in cells where the exocytic response is controlled by Ca2+ release from intracellular Ca2+ stores. Using live-cell two-photon microscopy that simultaneously records Ca2+signals and exocytic responses, we provide evidence that secretion is controlled by changes in Ca2+ concentration [Ca2+] in relatively large-volume microdomains. Our evidence includes: (1) long latencies (>2 seconds) between the rise in [Ca2+] and exocytosis, (2) observation of exocytosis all along the lumen and not clustered around Ca2+ release hot-spots, (3) high affinity (Kd=1.75 microM) Ca2+dependence of exocytosis, (4) significant reduction in exocytosis in the presence of cytosolic EGTA, (5) spatial exclusion of secretory granules from the cell membrane by the endoplasmic reticulum, and (6) inability of local Ca2+ responses to trigger exocytosis. These results strongly indicate that the control of exocytosis, triggered by Ca2+ release from stores, is through the regulation of cytosolic[Ca2+] within a microdomain.

The role of kinesin, dynein and microtubules in pancreatic secretion

The regulated secretion of pancreatic zymogens depends on a functional cytoskeleton and intracellular vesicle transport. To study the dynamics of tubulin and its motor proteins dynein and kinesin during secretion in pancreatic acinar cells, we infused rats with 0.1 mug/kg/h caerulein. Electron and fluorescence microscopy detected neither dynein nor kinesin at the apical secretory pole, nor on the surface of mature zymogen granules. After 30 min of secretagogue stimulation, kinesin and the Golgi marker protein 58 K were reallocated towards the apical plasma membrane and association of kinesin with tubulin was enhanced. Disruption of acinar cell microtubules had no effect on initial caerulein-induced amylase release but completely blocked secretion during a second stimulus. Our results suggest that mature zymogen granule exocytosis is independent of intact microtubules, kinesin and dynein. However, microtubule-dependent mechanisms seem to be important for the replenishment of secretory vesicles by redistribution of Golgi elements towards the apical cell pole.

How peptide hormone vesicles are transported to the secretion site for exocytosis

Post-Golgi transport of peptide hormone-containing vesicles from the site of genesis at the trans-Golgi network to the release site at the plasma membrane is essential for activity-dependent hormone secretion to mediate various endocrinological functions. It is known that these vesicles are transported on microtubules to the proximity of the release site, and they are then loaded onto an actin/myosin system for distal transport through the actin cortex to just below the plasma membrane. The vesicles are then tethered to the plasma membrane, and a subpopulation of them are docked and primed to become the readily releasable pool. Cytoplasmic tails of vesicular transmembrane proteins, as well as many cytosolic proteins including adaptor proteins, motor proteins, and guanosine triphosphatases, are involved in vesicle budding, the anchoring of the vesicles, and the facilitation of movement along the transport systems. In addition, a set of cytosolic proteins is also necessary for tethering/docking of the vesicles to the plasma membrane. Many of these proteins have been identified from different types of (neuro)endocrine cells. Here, we summarize the proteins known to be involved in the mechanisms of sorting various cargo proteins into regulated secretory pathway hormone-containing vesicles, movement of these vesicles along microtubules and actin filaments, and their eventual tethering/docking to the plasma membrane for hormone secretion.

Presence of cytoskeleton proteins in parotid glands and their roles during secretion

Amylase secretion is induced by the accumulation of cAMP in response to beta-adrenergic stimulation and by the augmentation of intracellular Ca2+ in response to muscarinic-cholinergic stimulation in rat parotid glands. The roles of cytoskeleton and motor proteins in the secretory process are not yet known. We examined the effects of cytoskeleton-modulating reagents on the amylase release induced by isoproterenol (IPR) and carbamylcholine (Cch) in rat parotid acinar cells. The amylase release induced by Cch was decreased by the microtubule-disrupting reagent colchicine (Colch) and the myosin ATPase inhibitor 2,3-butanediene monoxime (BDM), but the release induced by IPR was not. The actin filament-stabilizing reagent jasplakinolide (Jasp) and actin filament-disrupting reagent cytochalasin D (CytoD) decreased the amylase release induced by both the beta-adrenergic and the muscarinic-cholinergic stimulants. Pretreatment with CytoD affected the shape of the acinar cells, which showed an intermediate state between the fusion of the secretory granules with the apical membrane and the retrieval of the membranes only after stimulation with IPR. Myosin and Dynein/dynactin complex were detected in the secretory granule membrane fraction. We concluded from this study that the cytoskeleton played different roles in the beta-adrenergic and the muscarinic-cholinergic secretory processes.

Regulation of peptidergic vesicle mobility by secretagogues

Neuropeptides are released into the extracellular space from large secretory granules. In order to reach their release sites, these granules are translocated on microtubules and thought to interact with filamentous actin as they approach the cell membrane. We have used a green fluorescent protein-tagged neuropeptide prohormone (prepro-orphanin FQ) to visualize vesicle trafficking dynamics in NS20Y cells and cultures of primary hippocampal neurons. We found that the majority of secretory granules were mobile and accumulated at both the tips of neurites as well as other apparently specialized cellular sites. We also used live-cell imaging to test the notion that peptidergic vesicle mobility was regulated by secretagogues. We show that treatment with forskolin appeared to increase vesicle rates of speed, while depolarization with high K+ had no effect, even though both treatments stimulated neuropeptide secretion. In cultured hippocampal neurons the green fluorescent protein-tagged secretory vesicles were routed to both dendrites and axons, indicating that peptidergic vesicle transport was not polarized. Basal peptidergic vesicle mobility rates in hippocampal neurons were the same as those in NS20Y cells. Taken together, these studies suggest that secretory vesicle mobility is regulated by specific classes of secretagogues and that neuropeptide containing secretory vesicles may be released from dendritic structures.

Hedgehog-stimulated phosphorylation of the kinesin-related protein Costal2 is mediated by the serine/threonine kinase fused

The Hedgehog (Hh) signaling molecule is required for the development of numerous tissues in Drosophila. Within the cell, Hh signal transduction utilizes a large protein complex consisting of the Fused (Fu), Costal2 (Cos2), and Cubitis interruptus (Ci) proteins, but the functional interactions between these proteins are still largely uncharacterized. Using a baculovirus system, we demonstrate that the serine/threonine kinase Fu phosphorylates the kinesin-like protein Cos2 when coexpressed with Cos2. Coexpression of Cos2 and a kinase-inactive version of Fu eliminates the majority of Cos2 phosphorylation. We then show that the primary Fu-induced phosphorylation site of Cos2 is serine 572, whereas serine 931 is phosphorylated to a lesser extent. Mutation of serine 572 to alanine eliminates most, but not all, specific phosphopeptides of Cos2 when coexpressed with Fu. We also demonstrate that the phosphorylation pattern of Cos2 produced by baculovirus coexpression with kinase-dead Fu is almost identical to the phosphorylation pattern of Cos2 isolated from unstimulated S2 cells. Finally, the phosphorylation pattern of Cos2 produced by baculovirus coinfection with wild-type Fu is almost identical to that of Cos2 isolated from S2 cells stimulated by Hh, indicating that phosphorylation of serines 572 and 931 is a genuine Hh signaling event. This study clarifies the unique functions of Fu and Cos2 in Hh signal transduction and identifies only the second known phosphorylation site of a kinesin-like molecule.

Cytoplasmic dynein-associated structures move bidirectionallyin vivo

Intracellular organelle transport is driven by motors that act upon microtubules or microfilaments. The microtubulebased motors, cytoplasmic dynein and kinesin, are believed to be responsible for retrograde and anterograde transport of intracellular cargo along microtubules. Many vesicles display bidirectional movement; however, the mechanism regulating directionality is unresolved. Directional movement might be accomplished by alternative binding of different motility factors to the cargo. Alternatively,different motors could associate with the same cargo and have their motor activity regulated. Although several studies have focused on the behavior of specific types of cargoes, little is known about the traffic of the motors themselves and how it correlates with cargo movement. To address this question, we studied cytoplasmic dynein dynamics in living Dictyostelium cells expressing dynein intermediate chain-green fluorescent protein (IC-GFP) fusion in an IC-null background. Dynein-associated structures display fast linear movement along microtubules in both minus-end and plus-end directions, with velocities similar to that of dynein and kinesin-like motors. In addition, dynein puncta often rapidly reverse their direction. Dynein stably associates with cargo moving in both directions as well as with those that rapidly reverse their direction of movement, suggesting that directional movement is not regulated by altering motor-cargo association but rather by switching activity of motors associated with the cargo. These observations suggest that both plus- and minus-end-directed motors associate with a given cargo and that coordinated regulation of motor activities controls vesicle directionality.

Expression and regulation of calpain in rat pancreatic acinar cells

INTRODUCTION

Calpains, cytosolic Ca(2+)-dependent cysteine proteases, are expressed in a variety of mammalian cells and have been found to participate in stimulus-secretion coupling in platelets and alveolar cells.

AIMS

In pancreatic acinar cells, expression of calpains and their role in the secretory process have not yet been elucidated. Both subjects, therefore, were examined in the current study.

METHODOLOGY

mu-calpain and m-calpain were detected immunochemically. Calpain activation was measured by fluorescence spectrophotometry and single-cell fluorometry using Suc-Leu-Leu-Val-Tyr-AMC as substrate. Amylase secretion and cell damage, characterized by lactate dehydrogenase release, were measured by colorimetric assays.

RESULTS

Immunochemistry revealed cytoplasmic localization of both calpain isoforms. Immediately after increasing the cytosolic Ca(2+) concentration with ionomycin, a marked dose-dependent protease activation and cellular damage were observed. Inhibition of ionomycin-mediated enzyme activation through preincubation of cells with Ca(2+)-free medium, BAPTA-AM, or Z-Leu-Leu-Tyr-CHN(2) significantly reduced cell injury. Cholecystokinin (100 pM) also induced proteolytic activity, preceding cholecystokinin-stimulated amylase secretion. Protease activity and amylase release were significantly inhibited by Z-Leu-Leu-Tyr-CHN(2 ) retreatment.

CONCLUSION

Calpains are expressed in pancreatic acinar cells and may participate in stimulus-secretion coupling. In addition, our study indicates that pathologic calpain activation may contribute to Ca(2+)-mediated acinar cell damage.

CRHSP-28 regulates Ca(2+)-stimulated secretion in permeabilized acinar cells

CRHSP-28 is a Ca(2+)-regulated heat-stable phosphoprotein, abundant in the apical cytoplasm of epithelial cells that are specialized in exocrine protein secretion. To define a functional role for the protein in pancreatic secretion, recombinant CRHSP-28 (rCRHSP-28) was introduced into streptolysin-O-permeabilized acinar cells, and amylase secretion in response to elevated Ca(2+) was determined. Secretion was enhanced markedly by rCRHSP-28 over a time course that closely corresponded with the loss of the native protein from the intracellular compartment. No effects of rCRHSP-28 were detected until approximately 50% of the native protein was lost from the cytosol. Secretion was enhanced by rCRHSP-28 over a physiological range of Ca(2+) concentrations with 2-3-fold increases in amylase release occurring in response to low micromolar levels of free Ca(2+). Further, rCRHSP-28 augmented secretion in a concentration-dependent manner with minimal and maximal effects occurring at 1 and 25 microg/ml, respectively. Covalent cross-linking experiments demonstrated that native CRHSP-28 was present in a 60-kDa complex in cytosolic fractions and in a high molecular mass complex in particulate fractions, consistent with the slow leak rate of the protein from streptolysin-O-permeabilized cells. Probing acinar lysates with rCRHSP-28 in a gel-overlay assay identified two CRHSP-28-binding proteins of 35 (pp35) and 70 kDa (pp70). Interestingly, preparation of lysates in the presence of 1 mm Ca(2+) resulted in a marked redistribution of both proteins from a cytosolic to a Triton X-100-insoluble fraction, suggesting a Ca(2+)-sensitive interaction of these proteins with the acinar cell cytoskeleton. In agreement with our previous study immunohistochemically localizing CRHSP-28 around secretory granules in acinar cells, gel-overlay analysis revealed pp70 copurified with acinar cell secretory granule membranes. These findings demonstrate an important cell physiological function for CRHSP-28 in the Ca(2+)-regulated secretory pathway of acinar cells.

Molecular motors and their functions in plants

Molecular motors that hydrolyze ATP and use the derived energy to generate force are involved in a variety of diverse cellular functions. Genetic, biochemical, and cellular localization data have implicated motors in a variety of functions such as vesicle and organelle transport, cytoskeleton dynamics, morphogenesis, polarized growth, cell movements, spindle formation, chromosome movement, nuclear fusion, and signal transduction. In non-plant systems three families of molecular motors (kinesins, dyneins, and myosins) have been well characterized. These motors use microtubules (in the case of kinesines and dyneins) or actin filaments (in the case of myosins) as tracks to transport cargo materials intracellularly. During the last decade tremendous progress has been made in understanding the structure and function of various motors in animals. These studies are yielding interesting insights into the functions of molecular motors and the origin of different families of motors. Furthermore, the paradigm that motors bind cargo and move along cytoskeletal tracks does not explain the functions of some of the motors. Relatively little is known about the molecular motors and their roles in plants. In recent years, by using biochemical, cell biological, molecular, and genetic approaches a few molecular motors have been isolated and characterized from plants. These studies indicate that some of the motors in plants have novel features and regulatory mechanisms. The role of molecular motors in plant cell division, cell expansion, cytoplasmic streaming, cell-to-cell communication, membrane trafficking, and morphogenesis is beginning to be understood. Analyses of the Arabidopsis genome sequence database (51% of genome) with conserved motor domains of kinesin and myosin families indicates the presence of a large number (about 40) of molecular motors and the functions of many of these motors remain to be discovered. It is likely that many more motors with novel regulatory mechanisms that perform plant-specific functions are yet to be discovered. Although the identification of motors in plants, especially in Arabidopsis, is progressing at a rapid pace because of the ongoing plant genome sequencing projects, only a few plant motors have been characterized in any detail. Elucidation of function and regulation of this multitude of motors in a given species is going to be a challenging and exciting area of research in plant cell biology. Structural features of some plant motors suggest calcium, through calmodulin, is likely to play a key role in regulating the function of both microtubule- and actin-based motors in plants.

Regulation of molecular motor proteins

Motor proteins in the kinesin, dynein, and myosin superfamilies are tightly regulated to perform multiple functions in the cell requiring force generation. Although motor proteins within families are diverse in sequence and structure, there are general mechanisms by which they are regulated. We first discuss the regulation of the subset of kinesin family members for which such information exists, and then address general mechanisms of kinesin family regulation. We review what is known about the regulation of axonemal and cytoplasmic dyneins. Recent work on cytoplasmic dynein has revealed the existence of multiple isoforms for each dynein chain, making the study of dynein regulation more complicated than previously realized. Finally, we discuss the regulation of myosins known to be involved in membrane trafficking. Myosins and kinesins may be evolutionarily related, and there are common themes of regulation between these two classes of motors.

Dynein, dynactin, and kinesin II's interaction with microtubules is regulated during bidirectional organelle transport

The microtubule motors, cytoplasmic dynein and kinesin II, drive pigmented organelles in opposite directions in Xenopus melanophores, but the mechanism by which these or other motors are regulated to control the direction of organelle transport has not been previously elucidated. We find that cytoplasmic dynein, dynactin, and kinesin II remain on pigment granules during aggregation and dispersion in melanophores, indicating that control of direction is not mediated by a cyclic association of motors with these organelles. However, the ability of dynein, dynactin, and kinesin II to bind to microtubules varies as a function of the state of aggregation or dispersion of the pigment in the cells from which these molecules are isolated. Dynein and dynactin bind to microtubules when obtained from cells with aggregated pigment, whereas kinesin II binds to microtubules when obtained from cells with dispersed pigment. Moreover, the microtubule binding activity of these motors/dynactin can be reversed in vitro by the kinases and phosphatase that regulate the direction of pigment granule transport in vivo. These findings suggest that phosphorylation controls the direction of pigment granule transport by altering the ability of dynein, dynactin, and kinesin II to interact with microtubules.

Centrosome-directed translocation of Weibel-Palade bodies is rapidly induced by thrombin, calyculin A, or cytochalasin B in human aortic endothelial cells

To examine the possible role of the cytoskeleton in exocytosis of Weibel-Palade bodies (WPBs), we used double immunofluorescence and electron microscopy to study the spatial relationships between WPBs and main cytoskeletal elements in endothelial cells treated with secretagogue, such as thrombin, or cytoskeleton-damaging agents. Unexpectedly, we have found that WPBs undergo rapid translocation towards the centrosome both in cells treated with thrombin and in those treated with cytochalasin B or calyculin A. Typically, 3 or 5 min after agent addition compact cluster of WPBs became visible near the microtubule-organizing center (MTOC) in most endothelial cells in which a fivefold increase in WPBs localized in close proximity to the mother centriole had been detected. In both thrombin- and cytochalasin-treated cells that exhibit a noticeable depletion in WPBs compared to control cells, WPBs located at the cell periphery were found to colocalize with vimentin intermediate filaments, but not with microtubules. In contrast, there was precise colocalization observed between WPBs and microtubules in calyculin-treated cells in which all WPBs undergo centrosome-directed translocation within 15 min after the agent addition. When vimentin filaments were induced to collapse to a perinuclear location by the microtubule-disrupting agent demecolcine, WPBs also translocated to the perinuclear region, where numerous WPBs were found to be localized within the bundles of intermediate-sized filaments. The data provide the first direct evidence that secretory granules utilize microtubule-based transport system to move in retrograde direction, i.e., away from the plasma membrane, towards the centrosome. We suggest that anterograde movement of WPBs is primarily dependent on their interaction with vimentin intermediate filaments.

Kinesin is involved in regulation of rat pancreatic amylase secretion

BACKGROUND & AIMS

Kinesin has recently been localized to zymogen granules of pancreatic acini and is suggested to participate in exocytosis of exocrine pancreas. We examined the function of kinesin in regulated exocytosis of pancreatic acini in this study.

METHODS

Kinesin function in exocytosis was examined by introducing hexahistidine-tagged recombinant kinesin protein and antikinesin monoclonal antibody into streptolysin-O-permeabilized acini. Intracellular localization of introduced recombinant kinesin was investigated by immunohistochemistry. Interaction between recombinant kinesin and the microtubule network was confirmed by nocodazole pretreatment of acini. Kinesin regulation by secretagogues was investigated by examining their effect on adenosine triphosphatase (ATPase) activity of endogenous kinesin.

RESULTS

Recombinant kinesin enhanced calcium-stimulated amylase release from streptolysin-O-permeabilized acini. Introduced recombinant kinesin was localized to both the microtubule network and zymogen granule. Nocodazole pretreatment of acini abolished the enhancing effect of recombinant kinesin on calcium-stimulated amylase release. Antikinesin antibody inhibited amylase release stimulated by the combination of calcium and cyclic adenosine monophosphate (cAMP) but not that stimulated by calcium alone. Secretin and 8-bromo-cAMP increased ATPase activity of endogenous kinesin.

CONCLUSIONS

Kinesin plays a stimulatory role in regulated exocytosis of pancreatic acini and is involved in stimulus-secretion coupling through a cAMP-dependent pathway.

Exocytosis and movement of zymogen granules observed by VEC-DIC microscopy in the pancreatic tissue en bloc

The dynamic aspects of exocytosis, especially in the normal acinar tissue en bloc, have remained unclear. We visualized exocytosis directly in the tissue of the exocrine pancreas of rodents by video-enhanced contrast-differential interference contrast (VEC-DIC) microscopy to investigate various exocytosis-related rates and the relationship between the movement of granules and exocytotic responses. Stimulation of the tissue with bethanechol or cholecystokinin caused many of the zymogen granules in the apical pole to disappear abruptly. The exocytotic transients of individual granules were completed in 0.48-0.65 s. Granules destined to participate in the exocytotic response moved randomly at velocities of approximately 0.06 microm/s or less during stimulation. In the tissue preparation, granules located far from the apical pole frequently moved back and forth for 1-7 microm without showing exocytosis. Colchicine suppressed this movement and the late phase of the secretory response. Real-time (VEC-DIC) observation of granule dynamics revealed that the initial step of exocytosis was not coupled directly with the microtubule-dependent translocation but with a continuous, slow Brownian fluctuation of granules.

GFAP is expressed as a major soluble pool associated with glucagon secretory granules in A-cells of mouse pancreas

To elucidate the role of intermediate filament proteins in endocrine cells, we investigated the expression and subcellular distribution of GFAP in mouse islets of Langerhans. For this purpose, combined immunocytochemical and biochemical analysis with a panel of antibodies was carried out to identify GFAP-immunoreactive cells in mouse endocrine pancreas. Cell fractionation into NP-40-soluble and detergent/high salt-insoluble components was performed to assess whether GFAP was located in the cytosolic and/or cytoskeletal compartments of immunoreactive cells. Immunoelectron microscopic analysis was carried out to determine the subcellular distribution of the protein. Peripheral islet cells were stained with anti-GFAP antiserum. These cells were identified as glucagon-secreting cells by immunocytochemical staining of consecutive sections with anti-somatostatin, anti-GFAP, and anti-glucagon antisera. Western blotting analysis of both NP-40-soluble and detergent/high-salt insoluble fractions of isolated islets of Langerhans allowed detection of GFAP in both cytosolic and cytoskeletal compartments. Interestingly, however, the former location was highly predominant. In addition, immunoelectron microscopy localized GFAP associated with the periphery of secretory granules. On the basis of these results, an intriguing role for GFAP in secretory events should be strongly suspected.(J Histochem Cytochem 48:1233-1242, 2000)

Microtubules regulate local Ca2+ spiking in secretory epithelial cells

The role of the cytoskeleton in regulating Ca(2+) release has been explored in epithelial cells. Trains of local Ca(2+) spikes were elicited in pancreatic acinar cells by infusion of inositol trisphosphate through a whole cell patch pipette, and the Ca(2+)-dependent Cl(-) current spikes were recorded. The spikes were only transiently inhibited by cytochalasin B, an agent that acts on microfilaments. In contrast, nocodazole (5-100 micrometer), an agent that disrupts the microtubular network, dose-dependently reduced spike frequency and decreased spike amplitude leading to total blockade of the response. Consistent with an effect of microtubular disruption, colchicine also inhibited spiking but neither Me(2)SO nor beta-lumicolchicine, an inactive analogue of colchicine, had any effect. The microtubule-stabilizing agent, taxol, also inhibited spiking. The nocodazole effects were not due to complete loss of function of the Ca(2+) signaling apparatus, because supramaximal carbachol concentrations were still able to mobilize a Ca(2+) response. Finally, as visualized by 2-photon excitation microscopy of ER-Tracker, nocodazole promoted a loss of the endoplasmic reticulum in the secretory pole region. We conclude that microtubules specifically maintain localized Ca(2+) spikes at least in part because of the local positioning of the endoplasmic reticulum.

Interaction of a kinesin-like calmodulin-binding protein with a protein kinase

Kinesin-like calmodulin-binding protein (KCBP) is a novel member of the kinesin superfamily that is involved in cell division and trichome morphogenesis. KCBP is unique among all known kinesins in having a myosin tail homology-4 region in the N-terminal tail and a calmodulin-binding region following the motor domain. Calcium, through calmodulin, has been shown to negatively regulate the interaction of KCBP with microtubules. Here we have used the yeast two-hybrid system to identify the proteins that interact with the tail region of KCBP. A protein kinase (KCBP-interacting protein kinase (KIPK)) was found to interact specifically with the tail region of KCBP. KIPK is related to a group of protein kinases specific to plants that has an additional sequence between subdomains VII and VIII of the conserved C-terminal catalytic domain and an extensive N-terminal region. The catalytic domain alone of KIPK interacted weakly with the N-terminal KCBP protein but strongly with full-length KCBP, whereas the noncatalytic region did not interact with either protein. The interaction of KCBP with KIPK was confirmed using coprecipitation assays. Using bacterially expressed full-length and truncated proteins, we have shown that the catalytic domain is capable of phosphorylating itself. The association of KIPK with KCBP suggests regulation of KCBP or KCBP-associated proteins by phosphorylation and/or that KCBP is involved in targeting KIPK to its proper cellular location.

The cytoskeleton of digestive epithelia in health and disease

The mammalian cell cytoskeleton consists of a diverse group of fibrillar elements that play a pivotal role in mediating a number of digestive and nondigestive cell functions, including secretion, absorption, motility, mechanical integrity, and mitosis. The cytoskeleton of higher-eukaryotic cells consists of three highly abundant major protein families: microfilaments (MF), microtubules (MT), and intermediate filaments (IF), as well as a growing number of associated proteins. Within digestive epithelia, the prototype members of these three protein families are actins, tubulins, and keratins, respectively. This review highlights the important structural, regulatory, functional, and unique features of the three major cytoskeletal protein groups in digestive epithelia. The emerging exciting biological aspects of these protein groups are their involvement in cell signaling via direct or indirect interaction with a growing list of associated proteins (MF, MT, IF), the identification of several disease-causing mutations (IF, MF), the functional role that they play in protection from environmental stresses (IF), and their functional integration via several linker proteins that bridge two or potentially all three of these groups together. The use of agents that target specific cytoskeletal elements as therapeutic modalities for digestive diseases offers potential unique areas of intervention that remain to be fully explored.

Rab5 regulates motility of early endosomes on microtubules

The small GTPase Rab5 regulates membrane docking and fusion in the early endocytic pathway. Here we reveal a new role for Rab5 in the regulation of endosome interactions with the microtubule network. Using Rab5 fused to green fluorescent protein we show that Rab5-positive endosomes move on microtubules in vivo. In vitro, Rab5 stimulates both association of early endosomes with microtubules and early-endosome motility towards the minus ends of microtubules. Moreover, similarly to endosome membrane docking and fusion, Rab5-dependent endosome movement depends on the phosphatidylinositol-3-OH kinase hVPS34. Thus, Rab5 functionally links regulation of membrane transport, motility and intracellular distribution of early endosomes.

Motor and cargo interactions

The movements of intracellular cargo along microtubules within cells are often saltatory or of short duration. Further, calculations of the fraction of membrane vesicles that are moving at any period, indicate that active motor complexes are rare. From observations of normal vesicle traffic in cells, there appears to be position-dependent activation of motors and a balance of traffic in the inward and outward directions. In-vitro binding of motors to cargo is observed under many conditions but motility is not. Multi-component complexes appear to be involved in producing active organelle movements by a graded activation system that is highly localized in the cell. The basis of the activation of motility of the organelle motor complexes is still unknown but phosphorylation has been implicated in many systems. In the case of the motor-binding protein, kinectin, it has been linked to active organelle movements powered by conventional kinesin. From the coiled-coil structure of kinectin and the coiled-coil tail of kinesin, it is postulated that a coiled-coil assembly is responsible for the binding interaction. Many other cargoes are transported but the control of transport will be customized for each function, such as axonemal rafts or cytoskeletal complexes. Each function will have to be analyzed separately and motor activity will need to be integrated into the specific aspects of the function.

Cytoplasmic dynein and dynactin as likely candidates for microtubule-dependent apical targeting of pancreatic zymogen granules

The critical role of microtubules in vectorial delivery of post-Golgi carrier vesicles to the apical cell surface has been established for various polarized epithelial cell types. In the present study we used secretory granules of the rat and chicken pancreas, termed zymogen granules, as model system for apically bound post-Golgi carrier vesicles that underlie the regulated exocytotic pathway. We found that targeting of zymogen granules to the apical cell surface requires an intact microtubule system which contains its colchicine-resistant organizing center and, thus, the microtubular minus ends close to the apical membrane domain. Purified zymogen granules and their membranes were found to be associated with cytoplasmic dynein intermediate and heavy chain and to contain the major components of the dynein activator complex, dynactin, i.e. p150Glued, p62, p50, Arp1, and beta-actin. Kinesin heavy chain and the kinesin receptor, 160 kD kinectin, were not detected as components of zymogen granules. Immunofluorescence staining showed a zymogen granule-like distribution for dynein and dynactin (p150Glued, p62, p50, Arpl) in the apical cytoplasm, whereas kinesin and kinectin were largely concentrated in the basal half of the cells in a pattern similar to the distribution of calreticulin, a component of the endoplasmic reticulum. Secretory granules of non-polarized chromaffin cells of the bovine adrenal medulla, that are assumed to underlie microtubular plus end targeting from the Golgi apparatus to the cell periphery, were not found to be associated with dynein or dynactin. To our knowledge, this is the first demonstration of major components of the dynein-dynactin complex associated with the membrane of a biochemically and functionally well-defined organelle which is considered to underlie a vectorial minus end-driven microtubular transport critically involved in precise delivery of digestive enzymes to the apically located acinar lumen.

Microtubules and signal transduction

Although molecular components of signal transduction pathways are rapidly being identified, how elements of these pathways are positioned spatially and how signals traverse the intracellular environment from the cell surface to the nucleus or to other cytoplasmic targets are not well understood. The discovery of signaling molecules that interact with microtubules (MTs), as well as the multiple effects on signaling pathways of drugs that destabilize or hyperstabilize MTs, indicate that MTs are likely to be critical to the spatial organization of signal transduction. MTs themselves are also affected by signaling pathways and this may contribute to the transmission of signals to downstream targets.

The subapical actin cytoskeleton regulates secretion and membrane retrieval in pancreatic acinar cells

We examined the effects of disruption of the actin cytoskeleton by cytochalasin D (cytoD) on basal and carbamylcholine-stimulated exocytosis and on compensatory membrane retrieval in pancreatic acinar cells. Although the involvement of actin in exocytosis is reasonably well established, its role in these coupled processes is not understood. Our findings suggested that cytoD inhibited stimulated secretion of amylase. However, morphometry revealed that exocytosis had occurred: the number of zymogen granules decreased, the size of the lumen increased, and large vacuolar structures continuous with the lumen formed into which amylase accumulated. Large amounts of amylase were released to the medium on removal of secretagogue and cytoD, suggesting that the subapical actin network provides contractile forces that expel the lumenal contents. Strikingly, we observed that at the apical pole of the cells where exocytosis occurred, cytoD induced an accumulation of membrane invaginations into a vastly enlarged apical membrane. These pits were often surrounded by a clathrin-like coat. Concomitantly, AP-2-, clathrin-, dynamin- and caveolin-like immunoreactivity concentrated around the enlarged lumina, suggesting that incorporation of zymogen granule membrane into the apical plasma membrane triggered the recruitment of these proteins. After wash out of cytoD and carbamylcholine and reformation of the subapical actin cytoskeleton, the coated invaginations largely disappeared in association with a reduction in lumenal size, and relocation of clathrin, AP-2, dynamin and caveolin into the cell. We suggest that the actin terminal web also controls compensatory membrane retrieval following exocytosis.

The actin-myosin cytoskeleton mediates reversible agonist-induced membrane blebbing

Suprastimulation of pancreatic acinar cells with specific agonists inhibits zymogen secretion and induces the formation of large basolateral blebs. Currently the molecular mechanisms that mediate this dramatic morphologic response are undefined. Further, it is unclear if blebbing represents a terminal or reversible event. Using computer-enhanced video microscopy of living acini we have found that these large blebs form rapidly (within 2–3 minutes) and exhibit ameboid undulations. They are induced by small increases in agonist concentration and require an energy-dependent phosphorylation event. Remarkably, the blebs are rapidly absorbed when agonist levels are reduced, indicating that blebbing is a reversible response to a physiological stimulus, not a terminal event. Morphological methods show that these dramatic changes in cell shape are accompanied by a marked reorganization of actin and myosin II at the basolateral domain. During 30 minutes of suprastimulation, both basolateral actin and myosin II gradually increase to form a ring centered at the necks of the blebs. Immunocytochemical and biochemical studies with a phospho-specific antibody to the myosin regulatory light chain reveal an activation of myosin II in suprastimulated acini that is completely absent in resting cells. Studies using cytoskeletal antagonistic drugs indicate that bleb formation and motility require actin remodeling concomitant with an activation of myosin II. This aberrant activation and reorganization of the actin-myosin cytoskeleton is likely to have detrimental effects on acinar cell function. Additionally, this mechanism of bleb formation may be conserved among other forms of physiological blebbing events.