Liprin-α1 regulates breast cancer cell invasion by affecting cell motility, invadopodia and extracellular matrix degradation

Migration of cells and degradation of the extracellular matrix (ECM) are required for efficient tumor cell invasion, but the underlying molecular mechanisms are only partially known. The PPFIA1 gene for liprin-α1 is frequently amplified in human breast cancers. We recently demonstrated that liprin-α1 is an important regulator of cell edge dynamics during motility. We show, herein, that the liprin-α1 protein is highly expressed in human breast tumors. Functional analysis shows that liprin-α1 is specifically required for the migration and invasion of highly invasive human breast cancer MDA-MB-231 cells. We used time-lapse analysis to demonstrate defects in the motility of liprin-α1-depleted cells that include a striking instability of the lamellipodia. Liprin-α1 levels altered by either RNA interference or overexpression affected also cell spreading and the number of invadopodia per cell, but not the density of invadopodia per unit of surface area. On the other hand, silencing of liprin-α1 inhibited the degradation of the ECM, whereas its overexpression enhanced degradation, resulting in significant negative or positive effects, respectively, on the area of degradation per invadopodium. Transfection of fluorescent-labeled cortactin revealed that depletion of liprin-α1 also affected the assembly and disassembly of invadopodia, with decrease of their lifetime. Our results strongly support a novel important role of liprin-α1 in the regulation of human tumor cell invasion.

Molecular mechanisms regulating the subcellular localization of p95-APP1 between the endosomal recycling compartment and sites of actin organization at the cell surface

Cell migration requires coordination between adhesion, actin organization and membrane traffic. Rac and ARF6 have been shown to cooperate for the organization of actin at the cell surface. Recently, the GIT family of ARF-GAPs has been identified, which includes proteins that can functionally interact with both ARF and Rac GTPases. The p95-APP1 protein is a member of this family, isolated as part of a multi-molecular complex interacting with GTP-Rac. Our previous work has indicated that this protein may be part of the machinery redirecting membrane recycling towards sites of protrusion during locomotion. By analyzing the distribution and the effects of truncated forms of p95-APP1, we show here that the lack of the ARF-GAP domain of p95-APP1 dramatically shifts its localization to large vesicles. The use of several markers of the endocytic pathway has revealed that the truncated p95-APP1 localizes specifically to a Rab11-, transferrin receptor-positive compartment. Other markers are excluded from the p95-APP1-positive vesicles, while known components of the multi-molecular complex colocalize with truncated p95-APP1 in this compartment. Coexpression of a constitutively active form of Rac induces the redistribution of the truncated constructs and of the associated PIX, PAK, and paxillin to peripheral sites of Rac-mediated actin organization, and the disassembly of the large Rab11-positive vesicles. Together, the data presented indicate that p95-APP1 is part of a complex that shuttles between the plasma membrane and the endocytic recycling compartment, and suggest that the dynamic redistribution of the p95-APP1-containing complex is mediated both by the ARF-GAP domain, and by the recruitment of the complex at the cell surface at sites of Rac activation.

Cell migration: GAPs between membrane traffic and the cytoskeleton

During cell migration, coordination between membrane traffic, cell substrate adhesion and actin reorganization is required for protrusive activity to occur at the leading edge. Actin organization is regulated by Rho family GTPases and, with a contribution from the endocytic cycle, serves to extend the cell front. The details of the molecular mechanisms that direct membrane traffic at sites of adhesion and rearrange actin at the cell edge are still unknown. However, recent findings show that a number of multi-domain proteins characterized by an ArfGAP domain interact with both actin-regulating and integrin-binding proteins, as well as affecting Rac-mediated protrusive activity and cell migration. Some of these proteins have been shown to localize to endocytic compartments and to have a role in regulating endocytosis. Given the participation of Arf proteins in regulating membrane traffic, one appealing hypothesis is that the ArfGAPs act as molecular devices that coordinate membrane traffic and cytoskeletal reorganization during cell motility.

p95-APP1 links membrane transport to Rac-mediated reorganization of actin

Motility requires protrusive activity at the cellular edge, where Rho family members regulate actin dynamics. Here we show that p95-APP1 (ArfGAP-putative, Pix-interacting, paxillin-interacting protein 1), a member of the GIT1/PKL family, is part of a complex that interacts with Rac. Wild-type and truncated p95-APP1 induce actin-rich protrusions mediated by Rac and ADP-ribosylation factor 6 (Arf6). Distinct p95-APP1-derived polypeptides have different distributions, indicating that p95-APP1 cycles between the cell surface and endosomes. Our results show that p95-APP1 functionally interacts with Rac and localizes to endosomal compartments, thus identifying p95-APP1 as a molecular link between actin organization, adhesion, and membrane transport during cell motility.

Estradiol induces differential neuronal phenotypes by activating estrogen receptor alpha or beta

Estrogens are female sex steroids that have a plethora of effects on a wide range of tissues. These effects are mediated through two well characterized intracellular receptors: estrogen receptor alpha and beta (ERalpha and ERbeta, respectively). Because of their high structural homology, it has been argued whether these two receptors may elicit differential biochemical events in estrogen target cells. Here we examine the effect of 17beta-estradiol-dependent activation of ERalpha and ERbeta on neurite sprouting, a well known consequence of this sex hormone action in neural cells. In SK-N-BE neuroblastoma cells transfected with ERalpha or ERbeta, 17beta-estradiol induces two distinct morphological phenotypes. ERalpha activation results in increased length and number of neurites, whereas ERbeta activation modulates only neurite elongation. By the use of chimeric receptors we demonstrate that the presence of both transcription activation functions located in the NH2-terminus and COOH-terminus of the two ER proteins are necessary for maintaining the differential biological activity reported. ERalpha-dependent, but not ERbeta-dependent, morphological changes are observed only in the presence of the active form of the small G protein Rac1B. Our data provide the first clear evidence that, in a given target cell, ERalpha and ERbeta may play distinct biological roles and support the hypothesis that 17beta-estradiol activates selected intracellular signaling pathways depending on the receptor subtype bound.

Rac GTPases localize at sites of actin reorganization during dynamic remodeling of the cytoskeleton of normal embryonic fibroblasts

Rac GTP-binding proteins are implicated in the dynamic organization of the actin cytoskeleton, and the mechanisms utilized for this purpose are not understood yet. In this paper we have analysed the effects of the expression of Rac proteins on the organization of the cytoskeleton, and their subcellular distribution in chicken embryo fibroblasts. In these cells, overexpression of wild-type Rac GTPases induces disassembly of stress fibers, and production of long, highly branched actin-rich protrusions, with consequent dramatic changes in cell morphology. The formation of these protrusions is mediated by adhesion to the substrate, and is prevented by incubation with anti-(beta)1 function-blocking antibodies. Rac-mediated cell shape changes require a wild-type GTPase, since expression of constitutively active V12-Rac proteins affects actin organization differently in these cells, without causing alterations in their morphology. Localization studies performed on ventral plasma membranes from fibroblasts transfected with wild-type or mutant GTPases show codistribution of Rac along stress fibers, before their disassembly and the formation of the actin-rich protrusions. These data show a link between Rac protein distribution, and their effects on the actin cytoskeleton. Altogether, our results are indicative of an active role of Rac proteins in stress fiber disassembly, and show that Rac, which can cycle its bound nucleotide, produces unique dynamic effects on actin organization.

A cell-free system to study regulation of focal adhesions and of the connected actin cytoskeleton

Assembly and modulation of focal adhesions during dynamic adhesive processes are poorly understood. We describe here the use of ventral plasma membranes from adherent fibroblasts to explore mechanisms regulating integrin distribution and function in a system that preserves the integration of these receptors into the plasma membrane. We find that partial disruption of the cellular organization responsible for the maintenance of organized adhesive sites allows modulation of integrin distribution by divalent cations. High Ca2+ concentrations induce quasi-reversible diffusion of beta1 integrins out of focal adhesions, whereas low Ca2+ concentrations induce irreversible recruitment of beta1 receptors along extracellular matrix fibrils, as shown by immunofluorescence and electron microscopy. Both effects are independent from the presence of actin stress fibers in this system. Experiments with cells expressing truncated beta1 receptors show that the cytoplasmic portion of beta1 is required for low Ca2+-induced recruitment of the receptors to matrix fibrils. Analysis with function-modulating antibodies indicates that divalent cation-mediated receptor distribution within the membrane correlates with changes in the functional state of the receptors. Moreover, reconstitution experiments show that purified alpha-actinin colocalizes and redistributes with beta1 receptors on ventral plasma membranes depleted of actin, implicating binding of alpha-actinin to the receptors. Finally, we found that recruitment of exogenous actin is specifically restricted to focal adhesions under conditions in which new actin polymerization is inhibited. Our data show that the described system can be exploited to investigate the mechanisms of integrin function in an experimental setup that permits receptor redistribution. The possibility to uncouple, under cell-free conditions, events involved in focal adhesion and actin cytoskeleton assembly should facilitate the comprehension of the underlying molecular mechanisms.

Overexpression of a neural-specific rho family GTPase, cRac1B, selectively induces enhanced neuritogenesis and neurite branching in primary neurons

Rho family GTPases have been implicated in cytoskeletal reorganization during neuritogenesis. We have recently identified a new gene of this family, cRac1B, specifically expressed in the chicken developing nervous system. This GTPase was overexpressed in primary neurons to study the role of cRac1B in the development of the neuronal phenotype. Overexpression of cRac1B induced an increment in the number of neurites per neuron, and dramatically increased neurite branching, whereas overexpression of the highly related and ubiquitous cRac1A GTPase did not evidently affect neuronal morphology. Furthermore, expression of an inactive form of cRac1B strikingly inhibited neurite formation. The specificity of cRac1B action observed in neurons was not observed in fibroblasts, where both GTPases produced similar effects on cell morphology and actin organization, indicating the existence of a cell type-dependent specificity of cRac1B function. Molecular dissection of cRac1B function by analysis of the effects of chimeric cRac1A/cRac1B proteins showed that the COOH-terminal portion of cRac1B is essential to induce increased neuritogenesis and neurite branching. Considering the distinctive regulation of cRac1B expression during neural development, our data strongly support an important role of cRac1B during neuritogenesis, and they uncover new mechanisms underlying the functional specificity of distinct Rho family GTPases.

Differential expression of distinct members of Rho family GTP-binding proteins during neuronal development: identification of Rac1B, a new neural-specific member of the family

Previous studies on small GTP-binding proteins of the Rho family have revealed their involvement in the organization of cell actin cytoskeleton. The function of these GTPases during vertebrate development is not known. With the aim of understanding the possible role of these proteins during neuronal development, we have cloned and sequenced five members expressed in developing chick neural retinal cells. We have identified four chicken genes, cRhoA, cRhoB, cRhoC, and cRac1A, homologous to known human genes, and a novel Rac gene, cRac1B. Analysis of the distribution of four of the identified transcripts in chicken embryos shows for the first time high levels of expression of Rho family genes in the vertebrate developing nervous system, with distinct patterns of distribution for the different transcripts. In particular, cRhoA and cRac1A gene expression appeared ubiquitous in the whole embryo, and the cRhoB transcript was more prominent in populations of neurons actively extending neurites, whereas the newly identified cRac1B gene was homogeneously expressed only in the developing nervous system. Temporal analysis of the expression of the five genes suggests a correlation with the morphogenetic events occurring within the developing retina and the retinotectal pathway. Expression of an epitope-tagged cRac1B in retinal neurons showed a diffuse distribution of the protein in the cell body and along neurites. Taken as a whole, our results suggest important roles for ubiquitous and neural-specific members of the Rho family in the acquisition of the mature neuronal phenotype.

Preferential localization of tyrosine-phosphorylated paxillin in focal adhesions

Focal adhesions are sites for integrin-mediated attachment of cultured cells to the extracellular matrix. Localization studies have shown that focal adhesions can be stained by antiphosphotyrosine antibodies, but the role of tyrosine-phosphorylated proteins in focal adhesions is not known. By using ventral plasma membranes prepared from chicken embryo fibroblasts spread on the substrate, we present evidence for the preferential localization of a minor pool of tyrosine-phosphorylated paxillin in focal adhesions. Ventral plasma membranes showed an enrichment in beta 1-integrins, and in several tyrosine-phosphorylated polypeptides, while focal adhesion proteins like vinculin and paxillin, although localized to focal adhesions in ventral plasma membranes, were not particularly enriched in these preparations compared to whole cell lysates. Biochemical and morphological analysis of ventral plasma membranes showed a dramatic increase in the level of tyrosine-phosphorylation of the pool of paxillin localized to the adhesive sites, when compared to the paxillin present in whole cell lysates. The observed preferential localization of tyrosine-phosphorylated paxillin to focal adhesions may represent a general mechanism to compartmentalize focal adhesion components from large non-phosphorylated, cytosolic pools.

Integrin-mediated tyrosine phosphorylation and redistribution of paxillin during neuronal adhesion

Integrins are important receptors for neuronal adhesion to laminin, which is one of the best promoters of neurite outgrowth. The present study was carried out to understand some of the intracellular mechanisms which allow integrin-mediated neurite extension on laminin. In chicken retinal neurons, integrin-mediated adhesion to laminin and antibody-induced integrin clustering caused an increase in tyrosine phosphorylation of paxillin and focal adhesion kinase. The kinetics of phosphorylation and dephosphorylation of these proteins were different in neurons plated on laminin, compared to neurons in which the receptors were clustered with anti-integrin antibodies. Analysis of sucrose velocity gradients could not show any association of paxillin and focal adhesion kinase with the integrin receptors. On the other hand, by using digitonin and milder extraction conditions, we found an enrichment of the tyrosine-phosphorylated polypeptides in the cytoskeletal, digitonin-insoluble fraction. Furthermore, neuronal adhesion induced a dramatic increase in the fraction of tyrosine-phosphorylated paxillin recovered with the digitonin-insoluble fraction, suggesting redistribution of this protein following adhesion of neurons to laminin. Localization studies on the detergent-insoluble fraction showed codistribution of both paxillin and focal adhesion kinase with integrins. We also found that paxillin tyrosine phosphorylation, but not paxillin expression, is developmentally regulated in the retina. Our results show that integrin-mediated neuronal adhesion leads to the accumulation of a pool of highly phosphorylated proteins at adhesion sites. There they may be responsible for the reorganization of the cytoskeleton, which underlies the process of neurite extension.

Cytoplasmic topography of focal contacts

To investigate the structure of focal contacts, the cytoplasmic faces of fibroblast membranes were examined in solution by scanning force and immunofluorescence microscopy. Focal contacts were identified in scanning force topographs by correlation with fluorescence images. Finer details were resolved in topographs of the focal contacts than in fluorescence micrographs. Increased separation of ventral plasma membranes from the substrate correlated with the duration of cell culture. The cytoplasmic projections of the focal contacts also increased with the cell culture period. These changes accompanied lateral spreading of fibroblasts during a period of several hours after seeding cells in culture medium.

Differential distribution of two cytoplasmic variants of the alpha 6 beta 1 integrin laminin receptor in the ventral plasma membrane of embryonic fibroblasts

The integrin alpha 6 beta 1 is a receptor involved in the adhesion of several cell types to laminin. By using function-blocking antibodies, we have shown that alpha 6 beta 1 is a functional laminin receptor in chick embryo fibroblasts. We also found that these cells express two variants of the alpha 6 subunit, alpha 6A and alpha 6B, characterized by different cytoplasmic domains. By using indirect immunofluorescence with isoform-specific polyclonal antibodies, we showed that the two isoforms of the alpha 6 subunit distribute differently on the ventral plasma membrane of these cells cultured on laminin-coated substrates. In fact, while the alpha 6A subunit was found codistributing with vinculin in focal contacts, the alpha 6B subunit showed a homogeneously distributed punctate pattern. This difference was particularly evident when preparations of ventral plasma membranes were used for the immunolocalization. Furthermore, when cells were cultured on fibronectin, a substrate not recognized by the alpha 6 beta 1 laminin receptor, the distribution of the two alpha 6 isoforms was similar to that observed on laminin, with alpha 6A still colocalizing with vinculin in focal adhesions. Our results indicate that two forms of the alpha 6 beta 1 laminin receptor coexpressed in the same cells show distinctive distributions, and suggest that receptor occupancy by laminin is not essential for the accumulation of the alpha 6A beta 1 integrin in adhesion plaques.

Expression and in vitro function of beta 1-integrin laminin receptors in the developing avian ciliary ganglion

In chick development, ciliary ganglion (CG) neurons go through a period of axon extension from approximately embryonic day (E)4 to E8, followed by a period of synaptogenesis and neuronal cell death. By examining the immunohistochemical localization of laminin, in conjunction with Dil labeling of the ciliary nerve projection, we have determined that the pathway taken by these neurons is rich in laminin expression. Therefore, laminins are good candidate molecules for mediating outgrowth of these neurons in vivo. In vitro, the ability of CG neurons to extend neurites on laminin-1 (EHS laminin, alpha 1 beta 1 gamma 1) is maximal up to E8, then declines dramatically. CG neuron outgrowth on laminin-1 requires the activity of beta 1-class integrins. We have used subunit-specific antibodies to determine which of the five beta 1-containing heterodimers known to be laminin receptors (alpha 1 beta 1, alpha 2 beta 1, alpha 6 beta 1, alpha 7 beta 1) are expressed, and which mediate neurite outgrowth. While we could not detect expression of alpha 2 or alpha 7, we have found that alpha 1, alpha 3 beta 1, and alpha 6 beta 1 are expressed on the surface of ciliary ganglion neuron cell bodies and axons, both in vitro and in vivo. Furthermore, antibodies against alpha 3 and alpha 6, but not alpha 1, interfered with CG neurite outgrowth on laminin-1 in vitro. Taken together, these data suggest that interactions of cell surface alpha 3 beta 1 and alpha 6 beta 1 integrins with laminin-1 are likely to mediate growth of CG neurons during pathfinding in vivo.

Identification of a large complex containing the integrin alpha 6 beta 1 laminin receptor in neural retinal cells

Integrin alpha 6 beta 1 is a laminin receptor involved in adhesion and neurite extension of retinal neurons on laminin. The present study was carried out to identify potential interactions between the alpha 6 beta 1 receptor and cellular proteins that may be involved in integrin signaling and function. For this purpose we have used a biochemical approach based on the solubilization of retinal neurons cultured on laminin with nonionic detergents, followed by centrifugation on sucrose velocity gradients. Analysis of the distribution of the alpha 6 and beta 1 integrin subunits in the gradients showed that they migrate as a large complex after extraction of cells with octylglucoside, but not after Triton X-100 extraction. Cytoskeletal proteins known to localize in adhesion plaques did not comigrate with alpha 6 beta 1 in octylglucoside gradients, while a set of polypeptides whose tyrosine phosphorylation was enhanced by culture on laminin colocalized with alpha 6 beta 1 on the gradients after octylglucoside solubilization. Culture of retinal neurons on bovine serum albumin, a nonadhesive substratum, partially affected the gradient distribution of the receptor after octylglucoside extraction. Furthermore, analysis of the gradient distribution of two alternatively spliced isoforms of the alpha 6 subunit, alpha 6-cytoA and alpha 6-cytoB, showed that the effect of non-adhesion on the sedimentation properties of the two integrin alpha 6 isoforms was more dramatic for alpha 6-cytoB than alpha 6-cytoA. These differences in the sedimentation behaviour indicate distinct biochemical properties of the two alpha 6 isoforms that, together with previous observations on their differential distribution in the developing retina, may reflect functional specificities.

Function and spatial distribution in developing chick retina of the laminin receptor alpha 6 beta 1 and its isoforms

We have recently shown that the laminin-binding integrin receptor, alpha 6 beta 1, is prominently expressed in the developing chick retina, and its expression and activity are regulated during development on both retinal ganglion cells and other neural retinal cells. In the present study, we show that antibodies specific for the extracellular portion of the chick alpha 6 subunit dramatically inhibit interactions in vitro between embryonic day 6 neural retinal cells and laminin, showing that alpha 6 beta 1 functions as an important laminin receptor on developing retinal neurons. In previous work, we showed that alpha 6 mRNA levels on retinal ganglion cells decrease dramatically after E6 during the period that RGC axons innervate the optic tectum. In the present study, we show decreases in alpha 6 mRNA are not prevented by ablation of the optic tectum, indicating that tectal contact is not the major cause of this decrease. Within the embryonic retina, the alpha 6 subunit is codistributed, in part, with laminin, suggesting that it functions as a laminin receptor during retina development in vivo. Furthermore, two isoforms of the alpha 6 protein with distinct cytoplasmic domains generated by differential splicing have quite different distribution patterns in the retina, suggesting that these two isoforms may have different functions during retinal development.

Neuronal interactions with the extracellular matrix

The interactions of neurons with extracellular cues are important in directing the formation of precise neuronal networks during the development of the nervous system. This review will focus on recent progress towards the understanding of the molecular machinery involved in the interactions of neurons with the extracellular matrix.

Laminin receptors in the retina: sequence analysis of the chick integrin alpha 6 subunit. Evidence for transcriptional and posttranslational regulation

The integrin alpha 6 beta 1 is a prominent laminin receptor used by many cell types. In the present work, we isolate clones and determine the primary sequence of the chick integrin alpha 6 subunit. We show that alpha 6 beta 1 is a prominent integrin expressed by cells in the developing chick retina. Between embryonic days 6 and 12, both retinal ganglion cells and other retinal neurons lose selected integrin functions, including the ability to attach and extend neurites on laminin. In retinal ganglion cells, we show that this is correlated with a dramatic decrease in alpha 6 mRNA and protein, suggesting that changes in gene expression account for the developmental regulation of the interactions of these neurons with laminin. In other retinal neurons the expression of alpha 6 mRNA and protein remains high while function is lost, suggesting that the function of the alpha 6 beta 1 heterodimer in these cells is regulated by posttranslational mechanisms.

Isolation of exocytic carrier vesicles from BHK cells

Newly synthesized cell surface glycoproteins are transported from the trans-Golgi network (TGN) to the plasma membrane in vesicular carriers. Here we describe a cell-free system in which the formation of these carrier vesicles is reconstituted. Vesicle formation and release occurred specifically from the TGN and were dependent on ATP and cytosol. The released vesicles were isolated by density gradient sedimentation and specific immunoadsorption. Electron microscopy demonstrated that the vesicles had a diameter of 84 +/- 6 nm. The immunoisolated vesicles had a highly simplified protein pattern on two-dimensional gel electrophoresis.

Low cytoplasmic pH inhibits endocytosis and transport from the trans-Golgi network to the cell surface

A fibroblast mutant cell line lacking the Na+/H+ antiporter was used to study the influence of low cytoplasmic pH on membrane transport in the endocytic and exocytic pathways. After being loaded with protons, the mutant cells were acidified at pH 6.2 to 6.8 for 20 min while the parent cells regulated their pH within 1 min. Cytoplasmic acidification did not affect the level of intracellular ATP or the number of clathrin-coated pits at the cell surface. However, cytosolic acidification below pH 6.8 blocked the uptake of two fluid phase markers, Lucifer Yellow and horseradish peroxidase, as well as the internalization and the recycling of transferrin. When the cytoplasmic pH was reversed to physiological values, both fluid phase endocytosis and receptor-mediated endocytosis resumed with identical kinetics. Low cytoplasmic pH also inhibited the rate of intracellular transport from the Golgi complex to the plasma membrane. This was shown in cells infected by the temperature-sensitive mutant ts 045 of the vesicular stomatitis virus (VSV) using as a marker of transport the mutated viral membrane glycoprotein (VSV-G protein). The VSV-G protein was accumulated in the trans-Golgi network (TGN) by an incubation at 19.5 degrees C and was transported to the cell surface upon shifting the temperature to 31 degrees C. This transport was arrested in acidified cells maintained at low cytosolic pH and resumed during the recovery phase of the cytosolic pH. Electron microscopy performed on epon and cryo-sections of mutant cells acidified below pH 6.8 showed that the VSV-G protein was present in the TGN. These results indicate that acidification of the cytosol to a pH less than 6.8 inhibits reversibly membrane transport in both endocytic and exocytic pathways. In all likelihood, the clathrin and nonclathrin coated vesicles that are involved in endo- and exocytosis cannot pinch off from the cell surface or from the TGN below this critical value of internal pH.

Dissection of Semliki Forest virus glycoprotein delivery from the trans-Golgi network to the cell surface in permeabilized BHK cells

In this paper mechanically permeabilized cells have been used to dissect the transport of Semliki Forest virus glycoproteins from the trans-Golgi network to the plasma membrane. Transport from the Golgi complex was monitored by measuring the proteolytic cleavage of the Semliki Forest virus p62 glycoprotein into the E2 and E3 polypeptide chains. Cell surface appearance was measured by the exposure of the exoplasmic domain to antibodies directed against the viral glycoprotein. Both the cleavage of the p62 protein and the transport of the glycoprotein to the cell surface were reconstituted in permeabilized BHK cells when calcium and glucose were present in the medium. Detailed analysis showed that the cleavage of the p62 protein occurred before arrival to the plasma membrane.

Isolation of a fraction enriched in the trans-Golgi network from baby hamster kidney cells

Incubation of cultured cells at 20 degrees C blocks the transport of newly synthesized plasma membrane proteins, and the proteins accumulate intracellularly in a terminally glycosylated form. When baby hamster kidney cells are infected with the ts O45 mutant of vesicular stomatitis virus, and incubated at 20 degrees C, the terminally glycosylated spike glycoprotein G of the virus accumulates in the membranes of a tubular network localized on the trans side of the Golgi cisternae, the trans-Golgi network (TGN). We have used the G protein of ts O45 as a marker for the TGN and isolated a TGN fraction using a combination of conventional cell fractionation techniques and immunoisolation. The TGN was separated from the bulk of the endoplasmic reticulum, mitochondria, lysosomes, plasma membrane, and endosomes, while the activity of trans-Golgi marker galactosyltransferase copurified with the G protein. Using G protein as the TGN marker we have determined that the TGN was enriched 25-fold in the final fraction relative to the total homogenate. Several polypeptides (Mr 75,000, 87,000, 92,000, and 120,000) copurified with the G protein in the isolated TGN fraction and most likely represent resident markers of the compartment.

Purification and characterization of two plasma membrane domains from ejaculated bull spermatozoa

Plasma membranes were detached from ejaculated bull spermatozoa by a brief sonication in a moderately hypotonic medium, and the released plasma membranes were partially purified by differential centrifugation. The resulting fraction was enriched 8- and 15-fold in alkaline phosphatase and 5' nucleotidase activities, respectively, compared with the starting sonicated spermatozoa. This total plasma membrane fraction was separated into two distinct fractions by equilibrium density centrifugation on a continuous linear sucrose gradient. Two peaks of light scattering material were formed at densities of 1.117 and 1.148 g/ml. The denser peak contained most of the protein of the plasma membrane fraction, whereas nearly all the concanavalin A binding activity was found in the lighter peak. The two bands had distinctly different polypeptide compositions when analyzed by SDS PAGE. Polyclonal antibodies were raised in rabbits against a major integral membrane glycoprotein of each fraction (Mr of 92,000 in the light peak and 98,000 in the dense peak). The two antigens were detected on the surface of intact spermatozoa by indirect immunofluorescence microscopy. The 92-kD protein (present in the lighter band) was detected only on the plasma membrane of the acrosomal and anterior postacrosomal regions of the head. The 98-kD antigen, present in the heavier band, was localized to the surface of the postacrosomal region of the head, to the principal piece of the tail, and to the connecting piece between the head and tail. The exclusive localization of the 92-kD polypeptide to the surface of the anterior portion of the head was confirmed by immunoelectron microscopy. These data show that the two fractions isolated on the sucrose gradient originate from different regions of the sperm cell plasma membrane.

Chemotherapy does not affect the development of inter-Sertoli junctions in childhood leukaemia

The inter-Sertoli junctions of children aged between 5 and 12 years, affected by acute lymphoblastic leukaemia, were analyzed in sections and freeze-fracture replicas. The testicular biopsies were performed at the end of therapy, when patients were in continuous remission for over 30 months. Chemotherapy does not seem to affect the development of junctions that were studied in sections and freeze fracture. Two age groups were considered (I, 5 to 8 years; II, 9 to 12 years). In age group I, oval Sertoli cells were connected by occasionally focal points of fusion, which in replicas appeared as scattered, interrupted ridges on the P face and grooves on the corresponding E face. In age group II Sertoli cells presented cytoplasmic extensions and interdigitations. Tight junctions appeared close to one another in conventional sections. Freeze fracture evidenced extensive although isolated areas formed by intervining strands. Lanthanum penetrated freely the intercellular spaces and gap junctions were observed in both age groups. The results suggest that tight junctions formation is initiated long before puberty; a progression in the complexity of the strand organization is present as the tubules mature; the strands reorganize in parallel and continuous rows only at puberty.