Molecular links between the cytoskeleton and membranes

Three-dimensional systems for in vitro follicular culture: overview of alginate-based matrices

The in vitro culture of ovarian follicles has provided critical insight into the biology of the follicle and its enclosed oocyte and the physical interaction and communication between the theca and granulosa cells and the oocyte that is necessary to produce meiotically competent oocytes. Various two-dimensional (2D) and three-dimensional (3D) culture systems have been developed to evaluate the effect of growth factors, hormones, extracellular matrix components and culture conditions on follicle development and oocyte growth and maturation. Among these culture systems, 3D systems make it possible to maintain follicle structure and support communication between the various cell compartments within the follicle. In this review article, we will discuss the three main approaches to ovarian follicle culture: 2D attachment systems, 3D floating systems and 3D encapsulated systems. We will specifically emphasise the development of and advances in alginate-based encapsulated systems for in vitro follicle culture.

Melanoma invasion - current knowledge and future directions

The acquisition of invasive behaviour is the key transition in the progression of benign melanocyte hyperplasia to life threatening melanoma. Understanding this transition and the mechanisms of invasion are the key to understanding why malignant melanoma is such a devastating disease and will aid treatment strategies. Underlying the invasive behaviour is increased cell motility caused by changes in cytoskeletal organization and altered contacts with the extra-cellular matrix (ECM). In addition, changes in the interactions of melanoma cells with keratinocytes and fibroblasts enable them to survive and proliferate outside their normal epidermal location. Proteomic and genomic initiatives are greatly increasing our knowledge of which gene products are deregulated in invasive and metastatic melanoma; however, the next challenge is to understand how these genes promote the invasion of melanoma cells. In recent years new models have been developed that more closely recapitulate the conditions of melanoma invasion in vivo. It is hoped that these models will give us a better understanding of how the genes implicated in melanoma progression affect the motility of melanoma cells and their interactions with the ECM, stromal cells and blood vessels. This review will summarise our current understanding of melanoma invasion and focus on the new model systems that can be used to study melanoma.

Global genetic analysis

The introduction of molecular markers in genetic analysis has revolutionized medicine. These molecular markers are genetic variations associated with a predisposition to common diseases and individual variations in drug responses. Identification and genotyping a vast number of genetic polymorphisms in large populations are increasingly important for disease gene identification, pharmacogenetics and population-based studies. Among variations being analyzed, single nucleotide polymorphisms seem to be most useful in large-scale genetic analysis. This review discusses approaches for genetic analysis, use of different markers, and emerging technologies for large-scale genetic analysis where millions of genotyping need to be performed.

Transient receptor potential ion channels and animal sensation: lessons from Drosophila functional research

Ion channels of the transient receptor potential (TRP) superfamily are non-selective cationic channels with six transmembrane domains. The TRP channel made its first debut as a light-gated Ca2+ channel in Drosophila. Recently, research on animal sensation in Drosophila disclosed other members of the TRP family that are required for touch sensation and hearing as well as the sensation of painful stimuli.

Quantitative Analysis of Nucleic Acids - the Last Few Years of Progress

DNA and RNA quantifications are widely used in biological and biomedical research. In the last ten years, many technologies have been developed to enable automated and high-throughput analyses. In this review, we first give a brief overview of how DNA and RNA quantifications are carried out. Then, five technologies (microarrays, SAGE, differential display, real time PCR and real competitive PCR) are introduced, with an emphasis on how these technologies can be applied and what their limitations are. The technologies are also evaluated in terms of a few key aspects of nucleic acids quantification such as accuracy, sensitivity, specificity, cost and throughput.

A Drosophila mechanosensory transduction channel

Mechanosensory transduction underlies a wide range of senses, including proprioception, touch, balance, and hearing. The pivotal element of these senses is a mechanically gated ion channel that transduces sound, pressure, or movement into changes in excitability of specialized sensory cells. Despite the prevalence of mechanosensory systems, little is known about the molecular nature of the transduction channels. To identify such a channel, we analyzed Drosophila melanogaster mechanoreceptive mutants for defects in mechanosensory physiology. Loss-of-function mutations in the no mechanoreceptor potential C (nompC) gene virtually abolished mechanosensory signaling. nompC encodes a new ion channel that is essential for mechanosensory transduction. As expected for a transduction channel, D. melanogaster NOMPC and a Caenorhabditis elegans homolog were selectively expressed in mechanosensory organs.

Endocytic internalization in yeast and animal cells: similar and different

The internalization step of endocytosis has been the focus of several laboratories during the last forty years. Unlike some other budding events in the cell, many fundamental questions regarding the molecular machinery involved in the mechanism of budding itself still remain unsolved. Over the last few years the general picture of the field has quickly evolved from the originally simplistic view which postulated that clathrin polymerization is the major force driving budding at the plasma membrane. Refinement of the assays and molecular markers to measure endocytosis in animal cells has shown that other factors in addition to the clathrin coat are required and that endocytosis can also take place through clathrin-independent mechanisms. At the same time, recent introduction of genetic approaches to study endocytosis has accelerated the identification of molecules required for this process. The isolation of endocytosis mutants in budding yeast has been especially fruitful in this respect. Preliminary comparison of the results obtained in yeast and animal cells did not seem to coincide, but further progress in both systems now suggests that part of the divergence originally seen may be due to the particular experimental approaches used rather than fundamental differences in endocytic mechanisms. In this review we present a short historical overview on the advances made in yeast and animal cells regarding the study of endocytosis, underlining both emerging similarities and still interesting differences.

Regulation of CD44-protein 4.1 interaction by Ca2+ and calmodulin. Implications for modulation of CD44-ankyrin interaction

Erythrocyte membrane skeletal protein 4.1 isoforms have been identified in a variety of non-erythroid cells. However, interactions between protein 4.1 and its binding partners in non-erythroid cell membranes are poorly understood. In the erythrocyte membrane, protein 4.1 binds to the cytoplasmic domain of band 3 and, through this interaction, modulates ankyrin binding to band 3. The sequences LRRRY or IRRRY in band 3 mediate the interaction between band 3 and protein 4.1. The cytoplasmic domain of CD44, a transmembrane glycoprotein found in erythroid as well as non-erythroid cells, has internal sequences SRRRC and QKKKL. We wanted to determine if protein 4.1 binds to CD44 in a fashion analogous to its binding to band 3 and through this interaction modulates ankyrin binding to CD44. We report here that protein 4.1 binds to the cytoplasmic domain of CD44 with a dissociation constant on the order of 10(-7) M and that Ca2+ and calmodulin reduce the affinity of this interaction. Furthermore, although independent binding of both protein 4.1 and ankyrin to CD44 could be documented, binding of protein 4.1 prevented subsequent ankyrin binding. These studies have enabled us to identify a potentially important functional role for protein 4.1 in modulating ankyrin binding to CD44.

End4p/Sla2p interacts with actin-associated proteins for endocytosis in Saccharomyces cerevisiae

end4-1 was isolated as a temperature-sensitive endocytosis mutant. We cloned and sequenced END4 and found that it is identical to SLA2/MOP2. This gene is required for growth at high temperature, viability in the absence of Abp1p, polarization of the cortical actin cytoskeleton, and endocytosis. We used a mutational analysis of END4 to correlate in vivo functions with regions of End4p and we found that two regions of End4p participate in endocytosis but that the talin-like domain of End4p is dispensable. The N-terminal domain of End4p is required for growth at high temperature, endocytosis, and actin organization. A central coiled-coil domain of End4p is necessary for formation of a soluble sedimentable complex. Furthermore, this domain has an endocytic function that is redundant with the function(s) of ABP1 and SRV2. The endocytic function of Abp1p depends on its SH3 domain. In addition we have isolated a recessive negative allele of SRV2 that is defective for endocytosis. Combined biochemical, functional, and genetic analysis lead us to propose that End4p may mediate endocytosis through interaction with other actin-associated proteins, perhaps Rvs167p, a protein essential for endocytosis.

Activation of hematopoietic growth factor signal transduction pathways by the human oncogene BCR/ABL

BCR/ABL is a human chimeric oncogene that causes chronic myelogenous leukemia (CML). The BCR/ABL oncogene is generated from the Philadelphia chromosome (Ph) translocation, t(9;22)(q34;q11), and creates a constitutively active tyrosine kinase. There is clonal expansion of hematopoietic stem cells of several different lineages in CML. CML patients in stable phase usually have high white blood counts and immature cells of granulocytic lineages. Stable phase CML evolves to a more aggressive phase typically within 3.5-5 years, where differentiation is blocked and acute leukemia ensues. The transition of CML stable phase to blast phase is reflected in the loss of growth factor requirement of CML cells and correlates with additional cytogenetic alterations. Some biological effects reported in primary CML cells include reduced apoptosis and altered adhesion to fibronectin; however, the cells are dependent on hematopoietic growth factors. On a molecular level, the BCR/ABL translocation is well characterized. However, the actual mechanism of transformation by the BCR/ABL oncogene of hematopoietic cells is largely unknown. Enhancement of the c-ABL tyrosine kinase activity in BCR/ABL appears to be crucial for transformation. This tyrosine kinase activity leads to activation of several signal transduction pathways that are also utilized by hematopoietic growth factors, including steel factor, thrombopoietin, interleukin-3, and granulocyte/macrophage-colony stimulating factor. In several model systems, BCR/ABL has overlapping biological effects with hematopoietic growth factors, and transformation of hematopoietic growth factor-dependent cell lines leads to growth factor independence. In this review, we will describe the molecular and biological abnormalities in CML and several signal transduction mechanisms utilized by BCR/ABL as compared to hematopoietic growth factors.

Neurofibromatosis and associated tumour suppressor genes

Neurofibromatosis 1 and 2 (NF1 and NF2) are autosomal dominantly inherited disorders with close to 100% penetrance. NF1 is one of the most frequent human genetic diseases with an incidence of 1:3000. The incidence of NF2 is about 10 fold lower. NF1 is caused by mutations which inactivate the NF1 gene on chromosome 17q, while the NF2 gene is on chromsome 22. Both genes are tumour suppressor genes. The product of the NF1 gene, called neurofibromin, is a large protein of 2818 amino acids. The protein acts as a negative regulator in the ras signal transduction pathway and may also act downstream of ras. In the cell types that are affected in NF1 patients, the absence of neurofibromin leads to increased proliferation resulting in benign, and in some cases malignant tumours. The product of the NF2 gene is a protein of 595 amino acids. The protein displays in its N-terminal half considerable homology with proteins that are involved in contacts between the cytoskeleton and the cell membrane, and a similar function has been proposed for the NF2 protein. How the absence of the NF2 protein may lead to the development of Schwannomas and meningiomas, which are the major manifestations of NF2 in patients, is not clear at present.

Cytoskeletal differences between stereocilia of the human sperm passageway and microvilli/stereocilia in other locations

BACKGROUND

Stereocilia of the human ductus epididymidis and ductus deferens display unique features in that they arise from an apical cell protrusion (hillock) and contain thick stem portions which are interconnected by cytoplasmic bridges. The molecular basis for this unique fusion and branching pattern is hitherto unknown. These morphologic specialties led us to study the cytoskeleton of male spermway stereocilia with respect to the major proteins that constitute the supportive cytoskeleton of intestinal microvilli and inner ear stereocilia.

METHODS

Samples of the human epididymidis and ductus deferens were studied by immunoblotting and immunocytochemistry at the light and electron microscope levels.

RESULTS

Spermway stereocilia are supported by an internal actin filament bundle crosslinked by fimbrin and associated with the membrane linker molecule ezrin. The stem portions and hillock area are supplied with the crossbridge forming molecule alpha-actinin. Spermway stereocilia differ from brush border microvilli of the intestine, kidney, and ductuli efferentes by the lack of the second bundling protein villin and the unusual expression of alpha-actinin in the stem region. They resemble inner ear stereocilia by the presence of fimbrin and absence of villin, but differ from them by expression of ezrin and alpha-actinin. Thus, the main molecular difference between spermway stereocilia and stereocilia/microvilli of other locations is the presence of alpha-actinin in their stem portion and the hillock area.

CONCLUSIONS

Since alpha-actinin can form crossbridges between adjacent actin filaments (bundles) at longer distances than the other crosslinker of the stereocilium core bundle, fimbrin, we assume that alpha-actinin is essential for both the formation of the stem portions of spermway stereocilia and for the generation of their striking branching pattern. A developmentally regulated temporal sequence of expression of fimbrin and alpha-actinin might control the unique architecture of spermway stereocilia.

Calbindin expression in renal tubular epithelial cells. Altered sodium phosphate co-transport in association with cytoskeletal rearrangement

Sodium-phosphate transport in the opossum kidney (OK) cell line was studied in an OK clonal cell line that was transfected with an episomal vector expressing high levels of rat calbindin (28 kDa). High level expression of calbindin buffered the influx of calcium induced by ionomycin by 53% and raised the basal intracellular calcium from 100 +/- 6 to 150 +/- 8 nM. The decrement in sodium phosphate uptake induced by parathyroid hormone or forskolin was identical in the two cell lines. However, phorbol esters (10(-10)-10(-7) M), which decreased sodium phosphate uptake in the parental OK line, increased it in the calbindin-expressing line. Similarly, the parental clone did not respond to phosphate deprivation, while the calbindin-expressing clone did increase phosphate uptake in response to phosphate deprivation. In the calbindin-expressing cells, phorbol 12-myristate 13-acetate or low phosphate medium, which increased phosphate transport, produced actin filament aggregation, dissociation of the myristoylated alanine-rich C kinase substrate protein from sub-apical actin, and membrane-associated tyrosine phosphate staining. Agonists that reduced sodium phosphate uptake (cAMP, parathyroid hormone) did not affect these cellular features. The cytoskeletal rearrangement, redistribution of the myristoylated alanine-rich C kinase substrate protein, and membrane tyrosine phosphorylation are suggested to be involved in the events by which phosphate transport is increased in this cell line.

Pathogenesis of degenerative joint disease in the human temporomandibular joint

The wide range of disease prevalences reported in epidemiological studies of temporomandibular degenerative joint disease reflects the fact that diagnoses are frequently guided by the presence or absence of non-specific signs and symptoms. Treatment is aimed at alleviating the disease symptoms rather than being guided by an understanding of the underlying disease processes. Much of our current understanding of disease processes in the temporomandibular joint is based on the study of other articular joints. Although it is likely that the molecular basis of pathogenesis is similar to that of other joints, additional study of the temporomandibular joint is required due to its unique structure and function. This review summarizes the unique structural and molecular features of the temporomandibular joint and the epidemiology of degenerative temporomandibular joint disease. As is discussed in this review, recent research has provided a better understanding of the molecular basis of degenerative joint disease processes, including insights into: the regulation of cytokine expression and activation, arachidonic acid metabolism, neural contributions to inflammation, mechanisms of extracellular matrix degradation, modulation of cell adhesion in inflammatory states, and the roles of free radicals and heat shock proteins in degenerative joint disease. Finally, the multiple cellular and molecular mechanisms involved in disease initiation and progression, along with factors that may modify the adaptive capacity of the joint, are presented as the basis for the rational design of new and more effective therapy.

Immunomorphological characterization and effects of 12-(S)-HETE on a dynamic intracellular pool of the alpha IIb beta 3-integrin in melanoma cells

In metastatic B16a murine melanoma cells, alpha IIb beta 3 integrin was shown to be one of the key adhesion molecules responsible for matrix adhesion and spreading. Upon stimulation, alpha IIb beta 3 can be upregulated at the cell surface due to translocation of the receptor to the plasma membrane from an intracellular pool. Here we have characterized this integrin pool as a tubulovesicular structure (TVS) corresponding to endosomes. TVS was found to be associated temporarily with microtubules and intermediate filaments especially after protein kinase C (PKC) stimulation with a lipoxygenase metabolite of arachidonic acid, 12-(S)-hydroxyeicosatetraenoic acid [12-(S)-HETE]. After PKC stimulation, the predominantly vesicular TVS became elongated and alpha IIb beta 3 appeared at the apical plasma membrane and microvilli. Disruption of either the microtubules or intermediate filaments prevented the 12-(S)-HETE effect both on vesicular to tubular transition of TVS as well as on surface expression of this integrin. The connection with the Golgi system of the integrin-containing TVS was proved by a Golgi-inhibitor (brefeldin A) pretreatment, which prevented the PKC-stimulation-induced TVS elongation and subsequent receptor-upregulation at the cell surface. After a soluble ligand binding (mAb to the alpha IIb beta 3 complex) the surface receptor endocytosed back to the TVS indicating the presence of a dynamic, cytoskeleton associated integrin pool in melanoma cells.

Mechanically stimulated cytoskeleton rearrangement and cortical contraction in human neutrophils

A mechanical test with micropipets is used to characterize cytoskeleton rearrangement and contraction induced by mechanical stresses in human neutrophils. The yield shear resultant of the cell cortex is on the order of 0.06 to 0.09 mN.m-1. The measured yield shear resultant suggests that the neutrophil cortex is a weakly cross-linked structure. When a tether is pulled out from the cell surface, a polymer structure starts to fill it and spreads out from the cell body. The rate of advancement of the polymerization front is almost constant and, therefore, is not diffusion limited. The measured rate is much smaller than the one of spontaneous actin polymerization, suggesting that the limiting process is either the dissociation of actin monomers from their dimers with the capping proteins or the rate of formation of new nucleation sites or both. Polymerization is also observed after applying sufficient mechanical stresses on a small portion of the cell surface. The polymerization is followed by mass transfer from the cell into the prestressed region and later on by contraction of the main cell body. The pressure generating the flow is located in the prestressed region and most probably is a result of its "swelling" and contraction. The contraction of the main cell body is very similar (in its time dependence and magnitude) to the contraction during phagocytosis. The measured maximum cortical tension is on the order of 0.5 mN.m-1, which for a 3.5-microns diameter pipet corresponds to a maximum contraction force of 11 nN.

The C-terminal domain of alpha-spectrin is structurally related to calmodulin

An alignment of amino acid sequences suggests that the spectrin domain, which contains two EF-hand calcium-binding motifs, is structurally related to calmodulin. It is possible to align approximately 160 residues at the C-terminus of alpha-spectrin with the entire calmodulin sequence. We have expressed this domain in Escherichia coli and purified it. Circular dichroic and nuclear magnetic resonance spectroscopy show that the protein is folded and mostly helical. The conformation of the protein, as monitored spectroscopically, is sensitive to calcium at 0.1-1.0 mM. Equilibrium dialysis shows that there are two binding sites within this domain, with affinities in the 0.5 mM range. The domain can be split into N-terminal and C-terminal halves which fold independently. Only the N-terminal subdomain binds calcium. These data suggest that the C-terminus of alpha-spectrin has a domain with a calmodulin fold and two calcium-binding sites. Sequence alignments suggest that the related domains in alpha-actinin, and possibly in dystrophin, may share the same calmodulin-like structure. However, only non-muscle alpha-actinins appear to have one or two EF-hand(s) with the calcium-binding consensus sequence, and a strict consensus is not found in the muscle alpha-actinins or dystrophins.

Cytoskeletal domains in the activated platelet

Platelets circulate in the blood as discoid cells which, when activated, change shape by polymerizing actin into various structures, such as filopodia and stress fibers. In order to understand this process, it is necessary to determine how many other proteins are involved. As a first step in defining the full complement of actin-binding proteins in platelets, filamentous (F)-actin affinity chromatography was used. This approach identified > 30 different proteins from ADP-activated human blood platelets which represented 4% of soluble protein. Although a number of these proteins are previously identified platelet actin-binding proteins, many others appeared to be novel. Fourteen different polyclonal antibodies were raised against these apparently novel proteins and used to sort them into nine categories based on their molecular weights and on their location in the sarcomere of striated muscle, in fibroblasts and in spreading platelets. Ninety-three percent of these proteins (13 of 14 proteins tested) were found to be associated with actin-rich structures in vivo. Four distinct actin filament structures were found to form during the initial 15 min of activation on glass: filopodia, lamellipodia, a contractile ring encircling degranulating granules, and thick bundles of filaments resembling stress fibers. Actin-binding proteins not localized in the discoid cell became highly concentrated in one or another of these actin-based structures during spreading, such that each structure contains a different complement of proteins. These results present crucial information about the complexity of the platelet cytoskeleton, demonstrating that four different actin-based structures form during the first 15 min of surface activation, and that there remain many as yet uncharacterized proteins awaiting further investigation that are differentially involved in this process.

Platelet talin is phosphorylated by calyculin A

Calyculin A and okadaic acid, potent and cell permeable inhibitors of type 1 and type 2A protein phosphatases, inhibit platelet aggregation and secretion. However, the relationship between phosphatase inhibition and inhibition of platelet function is not well understood. We found that in unstimulated platelets, talin (P235) was phosphorylated at threonine residues by calyculin A. Furthermore, the extent of talin phosphorylation by calyculin A was closely correlated with its inhibition of thrombin-induced platelet aggregation. Since the binding of talin to platelet glycoprotein IIb/IIIa complex has been shown to be affected by its phosphorylation, these results suggest that type 1 and/or type 2A protein phosphatases may play a role in the regulation of membrane-cytoskeleton interaction through dephosphorylation of talin.

Analysis of a cortical cytoskeletal structure: a role for ezrin-radixin-moesin (ERM proteins) in the marginal band of chicken erythrocytes

We are studying how the cytoskeleton determines cell shape, using a simple model system, the marginal band of chicken erythrocytes. We previously identified a minor component of the marginal band by a monoclonal antibody, called 13H9 (Birgbauer and Solomon (1989). J. Cell Biol. 109, 1609–1620; Goslin et al. (1989). J. Cell Biol. 109, 1621–1631). mAb 13H9 also binds to the leading edges of fibroblasts and to neuronal growth cones and recognizes the cytoskeletal protein ezrin. In recent years, two proteins with a high degree of homology to ezrin were identified: moesin and radixin, together comprising the ERM protein family. We now show that the contiguous epitope sufficient for mAb 13H9 binding is a sequence present in each of the ERM proteins, as well as the product of the gene associated with neurofibromatosis 2, merlin or schwannomin. We used biochemical and immunological techniques, as well as PCR to characterize the expression and localization of the ERM proteins in chicken erythrocytes. The results demonstrate that radixin is the major ERM protein associated with the cytoskeleton. Both ezrin and radixin localize to the position of the marginal band. Our results suggest that the ERM proteins play functionally conserved roles in quite diverse organelles.

Comparative effects of hepatocyte growth factor and epidermal growth factor on motility, morphology, mitogenesis, and signal transduction of primary rat hepatocytes

Hepatocyte growth factor (HGF) and epidermal growth factor (EGF) are major hepatocyte mitogens, but HGF, also known as scatter factor (SF), has also been shown as a potent motogen for epithelial and endothelial cells. The mechanisms by which HGF is a stronger motogen compared to other mitogens are not understood. Here we report a comparative study of the effect of the two growth factors on cultured primary rat hepatocytes regarding their differential effects on morphology, mitogenicity, and motility as well as the phosphorylation of cytoskeletal-associated proteins. Using three different motility assays, both HGF and EGF increased the motility of hepatocytes, but HGF consistently elicited a significantly greater motility response than EGF. Additionally, HGF induced a more flattened, highly spread morphology compared to EGF. To examine if HGF and EGF phosphorylated different cytoskeletal elements as signal transduction targets in view of the observed variation in morphology and motility, primary cultures of 32P-loaded rat hepatocytes were stimulated by either HGF or EGF for up to 60 min. Both mitogens rapidly stimulated four isoforms of MAP kinase with similar kinetics and also rapidly facilitated the phosphorylation of cytoskeletal-associated F-actin. Two cytoskeletal-associated proteins, however, were observed to undergo rapid phosphorylation by HGF and not EGF during the time points described. One protein of 28 kDa was observed to become phosphorylated fivefold over controls, while the EGF-stimulated cells showed only a slight increase in the phosphorylation of this protein. Another protein with an apparent mwt of 42 kDa was phosphorylated 20-fold at 1 min and remained phosphorylated over 50-fold over control up to the 60 min time point. This protein was observed to become phosphorylated by EGF only after 10 min, and to a lesser extent (20-fold). Taken together, the data suggest that HGF and EGF stimulate divergent as well as redundant signal transduction pathways in the hepatocyte cytoskeleton, and this may result in unique HGF- or EGF-specific motility, morphology, and mitogenicity in hepatocytes.

Accumulation of actin in subsets of pioneer growth cone filopodia in response to neural and epithelial guidance cues in situ

Directed outgrowth of neural processes must involve transmission of signals from the tips of filopodia to the central region of the growth cone. Here, we report on the distribution and dynamics of one possible element in this process, actin, in live growth cones which are reorienting in response to in situ guidance cues. In grasshopper embryonic limbs, pioneer growth cones respond to at least three types of guidance cues: a limb axis cue, intermediate target cells, and a circumferential band of epithelial cells. With time-lapse imaging of intracellularly injected rhodamine-phalloidin and rhodamine-actin, we monitored the distribution of actin during growth cone responses to these cues. In distal limb regions, accumulation of actin in filopodia and growth cone branches accompanies continued growth, while reduction of actin accompanies withdrawal. Where growth cones are reorienting to intermediate target cells, or along the circumferential epithelial band, actin selectively accumulates in the proximal regions of those filopodia that have contacted target cells or are extending along the band. Actin accumulations can be retrogradely transported along filopodia, and can extend into the central region of the growth cone. These results suggest that regulation and translocation of actin may be a significant element in growth cone steering.

Pathobiochemical aspects of cytoskeleton components

This review summarizes pathobiochemical aspects of diseases, in which cytoskeletal components play a crucial role in pathogenesis. An attempt to classify the disorders on the basis of phenotypic changes that occur in microfilaments, intermediate filaments and microtubuli was unsuccessful. Three groups of disorders are presented: 1. cytoplasmic inclusions in specific diseases (merely descriptive); 2. diseases with genetic defects in cytoskeletal proteins (a chain of causality from defect to phenotype, in some cases with large gaps); 3. diseases with suspected involvement of cytoskeleton (hypothetical causal chain). Microfilaments are involved in certain pathogenetic processes on account of defects in their associated proteins; in Duchenne muscular dystrophy, dystrophin is defective, while the defective protein in Rett syndrome is synapsin. Defects in spectrin and membrane anchor proteins lead to disorders of the red cell membrane skeleton (congenital haemolytic anaemias). Intermediate filaments accumulate in some types of cytoplasmic inclusions, together with ubiquitin (Mallory bodies, desmin accumulation in some myopathies and others). A pathogenetic interpretation of this phenomenon is lacking. A genetic defect in certain types of keratin is the cause of epidermolysis bullosa. Interesting preliminary results are reviewed that reveal the crucial role of cytoskeletal components in a further group of diseases (intrahepatic cholestasis, Alzheimer disease, pemphigus). These disorders are currently under investigation, or are of theoretical interest with respect to the cytoskeleton. Specific reactions of cytoskeletal components in serum, which might be used diagnostically, have not been found.

Actin microfilaments play a critical role in endocytosis at the apical but not the basolateral surface of polarized epithelial cells

Treatment with cytochalasin D, a drug that acts by inducing the depolymerization of the actin cytoskeleton, selectively blocked endocytosis of membrane bound and fluid phase markers from the apical surface of polarized MDCK cells without affecting the uptake from the basolateral surface. Thus, in MDCK cell transformants that express the VSV G protein, cytochalasin blocked the internalization of an anti-G mAb bound to apical G molecules, but did not reduce the uptake of antibody bound to the basolateral surface. The selective effect of cytochalasin D on apical endocytosis was also demonstrated by the failure of the drug to reduce the uptake of 125I-labeled transferrin, which occurs by receptor-mediated endocytosis, via clathrin-coated pits, almost exclusively from the basolateral surface. The actin cytoskeleton appears to play a critical role in adsorptive as well as fluid phase apical endocytic events, since treatment with cytochalasin D prevented the apical uptake of cationized ferritin, that occurs after the marker binds to the cell surface, as well as uptake of Lucifer yellow, a fluorescent soluble dye. Moreover, the drug efficiently blocked infection of the cells with influenza virus, when the viral inoculum was applied to the apical surface. On the other hand, it did not inhibit the basolateral uptake of Lucifer yellow, nor did it prevent infection with VSV from the basolateral surface, or with influenza when this virus was applied to monolayers in which the formation of tight junctions had been prevented by depletion of calcium ions. EM demonstrated that cytochalasin D leads to an increase in the number of coated pits in the apical surface where it suppresses the pinching off of coated vesicles. In addition, in drug-treated cells cationized ferritin molecules that were bound to microvilli were not cleared from the microvillar surface, as is observed in untreated cells. These findings indicate that there is a fundamental difference in the process by which endocytic vesicles are formed at the two surfaces of polarized epithelial cells and that the integrity and/or the polymerization of actin filaments are required at the apical surface. Actin filaments in microvilli may be part of a mechanochemical motor that moves membrane components along the microvillar surface towards intermicrovillar spaces, or provides the force required for converting a membrane invagination or pit into an endocytic vesicle within the cytoplasm.

Molecular heterogeneity of the actin filament cytoskeleton associated with microvilli of photoreceptors, Müller's glial cells and pigment epithelial cells of the retina

The present study addressed the question as to whether the four different actin-associated proteins that are associated with the actin core bundle in intestinal microvilli (i.e. villin, fimbrin, myosin I and ezrin) are essential components of all microvilli of the body. The retina provides an excellent example of a tissue supplied with three different sets of microvilli, namely those of Müller's glial cells (Müller baskets), photoreceptors (calycal processes), and pigment epithelial cells. The main outcome of this study is that none of these microvilli contain all four actin-associated proteins present in intestinal microvilli. Müller cell microvilli contain villin, ezrin and myosin I (95 kDa isoform) but not fimbrin. Calycal processes of photoreceptors contain fimbrin but not villin, myosin I and ezrin. Finally, microvilli of pigment epithelial cells are positive for ezrin but not for villin, fimbrin and myosin I. Because of limited cross-reactivities of the antibodies to myosin I and ezrin, the myosin I data refer to the chicken retina whereas the findings with anti-ezrin were obtained with the rat retina. A further outcome of this study is that the actin filament core bundles in microvilli of chicken pigment epithelial cells are presumed to contain a crosslinking protein, which is not immunologically related to either villin, fimbrin or myosin I of the intestinal brush border.

Cytoskeleton--plasma membrane interactions

Proteins at the boundary between the cytoskeleton and the plasma membrane control cell shape, delimit specialized membrane domains, and stabilize attachments to other cells and to the substrate. These proteins also regulate cell locomotion and cytoplasmic responses to growth factors and other external stimuli. This diversity of cellular functions is matched by the large number of biochemical mechanisms that mediate the connections between membrane proteins and the underlying cytoskeleton, the so-called membrane skeleton. General organizational themes are beginning to emerge from examination of this biochemical diversity.

Osmotic stress and the yeast cytoskeleton: phenotype-specific suppression of an actin mutation

In the yeast Saccharomyces cerevisiae, actin filaments function to direct cell growth to the emerging bud. Yeast has a single essential actin gene, ACT1. Diploid cells containing a single copy of ACT1 are osmosensitive (Osms), i.e., they fail to grow in high osmolarity media (D. Shortle, unpublished observations cited by Novick, P., and D. Botstein. 1985. Cell. 40:415-426). This phenotype suggests that an underlying physiological process involving actin is osmosensitive. Here, we demonstrate that this physiological process is a rapid and reversible change in actin filament organization in cells exposed to osmotic stress. Filamentous actin was stained using rhodamine phalloidin. Increasing external osmolarity caused a rapid loss of actin filament cables, followed by a slower redistribution of cortical actin filament patches. In the recovery phase, cables and patches were restored to their original levels and locations. Strains containing an act1-1 mutation are both Osms and temperature-sensitive (Ts) (Novick and Botstein, 1985). To identify genes whose products functionally interact with actin in cellular responses to osmotic stress, we have isolated extragenic suppressors which revert only the Osms but not the Ts phenotype of an act1-1 mutant. These suppressors identify three genes, RAH1-RAH3. Morphological and genetic properties of a dominant suppressor mutation suggest that the product of the wild-type allele, RAH3+, is an actin-binding protein that interacts with actin to allow reassembly of the cytoskeleton following osmotic stress.

A yeast gene (BEM1) necessary for cell polarization whose product contains two SH3 domains

Cell polarization requires that a cellular axis or cell-surface site be chosen and that the cytoskeleton be organized with respect to it. Details of the link between the cytoskeleton and the chosen axis or site are not clear. Cells of the yeast Saccharomyces cerevisiae exhibit cell polarization in two phases of their life cycle, during vegetative growth and during mating, which reflects responses to intracellular and extracellular signals, respectively. Here we describe the isolation of two mutants defective specifically in cell polarization in response to peptide mating pheromones. The mutants carry special alleles (denoted bem1-s) of the BEM1 gene required for cell polarization during vegetative growth. Unlike other bem1 mutants, the bem1-s mutants are normal for vegetative growth. Complete deletion of BEM1 leads to the defect in polarization of vegetative cells seen in bem1 mutants. The predicted sequence of the BEM1 protein (Bem1p) reveals two copies of a domain (denoted SH3) that is found in many proteins associated with the cortical cytoskeleton and which may mediate binding to actin or some other component of the cell cortex. The sequence of Bem1p and the properties of mutants defective in this protein indicate that it may link the cytoskeleton to morphogenetic determinants on the cell surface.