Contactus adherens, a special type of plaque-bearing adhering junction containing M-cadherin, in the granule cell layer of the cerebellar glomerulus

Mapping proteomic composition of excitatory postsynaptic sites in the cerebellar cortex

Functions of the cerebellar cortex, from motor learning to emotion and cognition, depend on the appropriate molecular composition at diverse synapse types. Glutamate receptor distributions have been partially mapped using immunogold electron microscopy. However, information is lacking on the distribution of many other components, such as Shank2, a postsynaptic scaffolding protein whose cerebellar dysfunction is associated with autism spectrum disorders. Here, we used an adapted Magnified Analysis of the Proteome, an expansion microscopy approach, to map multiple glutamate receptors, scaffolding and signaling proteins at single synapse resolution in the cerebellar cortex. Multiple distinct synapse-selective distribution patterns were observed. For example, AMPA receptors were most concentrated at synapses on molecular layer interneurons and at climbing fiber synapses, Shank1 was most concentrated at parallel fiber synapses on Purkinje cells, and Shank2 at both climbing fiber and parallel fiber synapses on Purkinje cells but little on molecular layer interneurons. Our results are consistent with gene expression data but also reveal input-selective targeting within Purkinje cells. In specialized glomerular structures of the granule cell layer, AMPA receptors as well as most other synaptic components preferentially targeted to synapses. However, NMDA receptors and the synaptic GTPase activating protein SynGAP preferentially targeted to extrasynaptic sites. Thus, glomeruli may be considered integrative signaling units through which mossy fibers differentially activate synaptic AMPA and extrasynaptic NMDA receptor complexes. Furthermore, we observed NMDA receptors and SynGAP at adherens junctions, suggesting a role in structural plasticity of glomeruli. Altogether, these data contribute to mapping the cerebellar 'synaptome'.

Ultrastructural Characteristics of Finger-Like Membrane Protrusions in Cell Competition

A small number of oncogenic mutated cells sporadically arise within the epithelial monolayer. Newly emerging Ras- or Src-transformed epithelial cells are often apically eliminated during competitive interactions between normal and transformed cells. Our recent electron microscopy (EM) analyses revealed that characteristic finger-like membrane protrusions are formed at the interface between normal and RasV12-transformed cells via the cdc42–formin-binding protein 17 (FBP17) pathway, potentially playing a positive role in intercellular recognition during apical extrusion. However, the spatial distribution and ultrastructural characteristics of finger-like protrusions remain unknown. In this study, we performed both X–Y and X–Z EM analyses of finger-like protrusions during the apical extrusion of RasV12-transformed cells. Quantification of the distribution and widths of the protrusions showed comparable results between the X–Y and X–Z sections. Finger-like protrusions were observed throughout the cell boundary between normal and RasV12 cells, except for apicalmost tight junctions. In addition, a non-cell-autonomous reduction in protrusion widths was observed between RasV12 cells and surrounding normal cells under the mix culture condition. In the finger-like protrusions, intercellular adhesions via thin electron-dense plaques were observed, implying that immature and transient forms of desmosomes, adherens junctions or unknown weak adhesions were distributed. Interestingly, unlike RasV12-transformed cells, Src-transformed cells form fewer evident protrusions, and FBP17 in Src cells is dispensable for apical extrusion. Collectively, these results suggest that the dynamic reorganization of intercellular adhesions via finger-like protrusions may positively control cell competition between normal and RasV12-transformed cells. Furthermore, our data indicate a cell context–dependent diversity in the modes of apical extrusion.

Actin filament association at adherens junctions

The adherens junction (AJ) is a cadherin-based and actin filament associated cell-to-cell junction. AJs can contribute to tissue morphogenesis and homeostasis and their association with actin filaments is crucial for the functions. There are three types of AJs in terms of the mode of actin filament/AJ association. Among many actin-binding proteins associated with AJs, α-catenin is one of the most important actin filament/AJ linkers that functions in all types of AJs. Although α-catenin in cadherin-catenin complex appears to bind to actin filaments within cells, it fails to bind to actin filaments in vitro mysteriously. Recent report revealed that α-catenin in the complex can bind to actin filaments in vitro when forces are applied to the filament. In addition to force-sensitive vinculin binding, α-catenin has another force-sensitive property of actin filament-binding. Elucidation of its significance and the molecular mechanism is indispensable for understanding AJ formation and maintenance during tissue morphogenesis, function and repair. J. Med. Invest. 64: 14-19, February, 2017.

Protection against de novo methylation is instrumental in maintaining parent-of-origin methylation inherited from the gametes

Identifying loci with parental differences in DNA methylation is key to unraveling parent-of-origin phenotypes. By conducting a MeDIP-Seq screen in maternal-methylation free postimplantation mouse embryos (Dnmt3L-/+), we demonstrate that maternal-specific methylation exists very scarcely at midgestation. We reveal two forms of oocyte-specific methylation inheritance: limited to preimplantation, or with longer duration, i.e. maternally imprinted loci. Transient and imprinted maternal germline DMRs (gDMRs) are indistinguishable in gametes and preimplantation embryos, however, de novo methylation of paternal alleles at implantation delineates their fates and acts as a major leveling factor of parent-inherited differences. We characterize two new imprinted gDMRs, at the Cdh15 and AK008011 loci, with tissue-specific imprinting loss, again by paternal methylation gain. Protection against demethylation after fertilization has been emphasized as instrumental in maintaining parent-of-origin methylation inherited from the gametes. Here we provide evidence that protection against de novo methylation acts as an equal major pivot, at implantation and throughout life.

Cadherins in brain morphogenesis and wiring

Cadherins are Ca(2+)-dependent cell-cell adhesion molecules that play critical roles in animal morphogenesis. Various cadherin-related molecules have also been identified, which show diverse functions, not only for the regulation of cell adhesion but also for that of cell proliferation and planar cell polarity. During the past decade, understanding of the roles of these molecules in the nervous system has significantly progressed. They are important not only for the development of the nervous system but also for its functions and, in turn, for neural disorders. In this review, we discuss the roles of cadherins and related molecules in neural development and function in the vertebrate brain.

Cadherin-actin interactions at adherens junctions

The adherens junction (AJ) is a major cell-cell junction that mediates cell recognition, adhesion, morphogenesis, and tissue integrity. Although AJs transmit forces generated by actomyosin from one cell to another, AJs have long been considered as an area where signal transduction from cadherin ligation takes place through cell adhesion. Through the efforts to understand embryonic or cellular morphogenesis, dynamic interactions between the AJ and actin filaments have become crucial issues to be addressed since actin association is essential for AJ development, remodeling and function. Here, I provide an overview of cadherin-actin interaction from morphological aspects and of possible molecular mechanisms revealed by recent studies.

Myogenic regulatory factors regulate M-cadherin expression by targeting its proximal promoter elements

M- and N-cadherin are members of the Ca(2+)-dependent cell-cell adhesion molecule family. M-cadherin is expressed predominantly in developing skeletal muscles and has been implicated in terminal myogenic differentiation, particularly in myoblast fusion. N-cadherin-mediated cell-cell adhesion also plays an important role in skeletal myogenesis. In the present study, we found that both genes were differentially expressed in C2C12 and Sol8 myoblasts during myogenic differentiation and that the expression of M-cadherin was preferentially enhanced in slow-twitch muscle. Interestingly, most MRFs (myogenic regulatory factors) significantly activated the promoter of M-cadherin, but not that of N-cadherin. In line with this, overexpression of MyoD in C3H10T1/2 fibroblasts strongly induced endogenous M-cadherin expression. Promoter analysis in silico and in vitro identified an E-box (from -2 to +4) abutting the transcription initiation site within the M-cadherin promoter that is bound and differentially activated by different MRFs. The activation of the M-cadherin promoter by MRFs was also modulated by Bhlhe40 (basic helix-loop-helix family member e40). Finally, chromatin immunoprecipitation proved that MyoD as well as myogenin binds to the M-cadherin promoter in vivo. Taken together, these observations identify a molecular mechanism by which MRFs regulate M-cadherin expression directly to ensure the terminal differentiation of myoblasts.

Organization of NMDA receptors at extrasynaptic locations

NMDA receptors are found in neurons both at synapses and in extrasynaptic locations. Extrasynaptic locations are poorly characterized. Here we used preembedding immunoperoxidase and postembedding immunogold electron microscopy and fluorescence light microscopy to characterize extrasynaptic NMDA receptor locations in dissociated hippocampal neurons in vitro and in the adult and postnatal hippocampus in vivo. We found that extrasynaptic NMDA receptors on neurons in vivo and in vitro were usually concentrated at points of contact with adjacent processes, which were mainly axons, axon terminals, or glia. Many of these contacts were shown to contain adhesion factors such as cadherin and catenin. We also found associations of extrasynaptic NMDA receptors with the membrane associated guanylate kinase (MAGUKs), postsynaptic density (PSD)-95 and SAP102. Developmental differences were also observed. At postnatal day 2 in vivo, extrasynaptic NMDA receptors could often be found at sites with distinct densities whereas dense material was seen only rarely at sites of extrasynaptic NMDA receptors in the adult hippocampus in vivo. This difference probably indicates that many sites of extrasynaptic NMDA receptors in early postnatal ages represent synapse formation or possibly sites for synapse elimination. At all ages, as suggested in both in vivo and in vitro studies, extrasynaptic NMDA receptors on dendrites or the sides of spines may form complexes with other proteins, in many cases, at stable associations with adjacent cell processes. These associations may facilitate unique functions for extrasynaptic NMDA receptors.

The junctions that don't fit the scheme: special symmetrical cell-cell junctions of their own kind

Immunocytochemical, electron-, and immunoelectron-microscopical studies have revealed that, in addition to the four major "textbook categories" of cell-cell junctions (gap junctions, tight junctions, adherens junctions, and desmosomes), a broad range of other junctions exists, such as the tiny puncta adhaerentia minima, the taproot junctions (manubria adhaerentia), the plakophilin-2-containing adherens junctions of mesenchymal or mesenchymally derived cell types including malignantly transformed cells, the composite junctions (areae compositae) of the mature mammalian myocardium, the cortex adhaerens of the eye lens, the interdesmosomal "sandwich" or "stud" junctions in the subapical layers of stratified epithelia and the tumors derived therefrom, and the complexus adhaerentes of the endothelial and virgultar cells of the lymph node sinus. On the basis of their sizes and shapes, other morphological criteria, and their specific molecular ensembles, these junctions and the genes that encode them cannot be subsumed under one of the major categories mentioned above but represent special structures in their own right, appear to serve special functions, and can give rise to specific pathological disorders.

Alterations in CDH15 and KIRREL3 in patients with mild to severe intellectual disability

Cell-adhesion molecules play critical roles in brain development, as well as maintaining synaptic structure, function, and plasticity. Here we have found the disruption of two genes encoding putative cell-adhesion molecules, CDH15 (cadherin superfamily) and KIRREL3 (immunoglobulin superfamily), by a chromosomal translocation t(11;16) in a female patient with intellectual disability (ID). We screened coding regions of these two genes in a cohort of patients with ID and controls and identified four nonsynonymous CDH15 variants and three nonsynonymous KIRREL3 variants that appear rare and unique to ID. These variations altered highly conserved residues and were absent in more than 600 unrelated patients with ID and 800 control individuals. Furthermore, in vivo expression studies showed that three of the CDH15 variations adversely altered its ability to mediate cell-cell adhesion. We also show that in neuronal cells, human KIRREL3 colocalizes and interacts with the synaptic scaffolding protein, CASK, recently implicated in X-linked brain malformation and ID. Taken together, our data suggest that alterations in CDH15 and KIRREL3, either alone or in combination with other factors, could play a role in phenotypic expression of ID in some patients.

Biological interpretations of transcriptomic profiles in mammalian oocytes and embryos

The characterization of gene-expression profiles in oocytes and embryos is critical to understand the influence of genetic and environmental factors on preimplantation and fetal development. Numerous gene-expression microarray studies using different platforms and species are offering insights into the biological processes extensively represented among the genes exhibiting differential expression. Major advances on understanding the direct relationship between gene expression and developmental competence are being reported. Integration of information across studies using meta-analysis techniques can increase the precision and accuracy to identify expression profiles associated with embryo development. Gene network and pathway analyses are offering insights into gene interactions and expression profiles of embryos. All these advances are cementing the way toward a comparative and systems approach to understanding the complex processes underlying vertebrate development.

Advancing the understanding of the embryo transcriptome co-regulation using meta-, functional, and gene network analysis tools

Embryo development is a complex process orchestrated by hundreds of genes and influenced by multiple environmental factors. We demonstrate the application of simple and effective meta-study and gene network analyses strategies to characterize the co-regulation of the embryo transcriptome in a systems biology framework. A meta-analysis of nine microarray experiments aimed at characterizing the effect of agents potentially harmful to mouse embryos improved the ability to accurately characterize gene co-expression patterns compared with traditional within-study approaches. Simple overlap of significant gene lists may result in under-identification of genes differentially expressed. Sample-level meta-analysis techniques are recommended when common treatment levels or samples are present in more than one study. Otherwise, study-level meta-analysis of standardized estimates provided information on the significance and direction of the differential expression. Cell communication pathways were highly represented among the genes differentially expressed across studies. Mixture and dependence Bayesian network approaches were able to reconstruct embryo-specific interactions among genes in the adherens junction, axon guidance, and actin cytoskeleton pathways. Gene networks inferred by both approaches were mostly consistent with minor differences due to the complementary nature of the methodologies. The top-down approach used to characterize gene networks can offer insights into the mechanisms by which the conditions studied influence gene expression. Our work illustrates that further examination of gene expression information from microarray studies including meta- and gene network analyses can help characterize transcript co-regulation and identify biomarkers for the reproductive and embryonic processes under a wide range of conditions.

Structure and function of desmosomes

Desmosomes are prominent adhesion sites that are tightly associated with the cytoplasmic intermediate filament cytoskeleton providing mechanical stability in epithelia and also in several nonepithelial tissues such as cardiac muscle and meninges. They are unique in terms of ultrastructural appearance and molecular composition with cell type-specific variations. The dynamic assembly properties of desmosomes are important prerequisites for the acquisition and maintenance of tissue homeostasis. Disturbance of this equilibrium therefore not only compromises mechanical resilience but also affects many other tissue functions as becomes evident in various experimental scenarios and multiple diseases.

Characterization of a novel type of adherens junction in meningiomas and the derived cell line HBL-52

Adhering junctions are generally grouped into desmosomes and adherens junctions based on their ultrastructural appearance and molecular composition. The armadillo-protein plakoglobin is common to both types of junctions, which are otherwise composed of mutually exclusive proteins. This view is based on observations in epithelial tissues but cannot easily be transferred to other cell types and tissues, as has become apparent during the last decade with the identification of new junctional proteins and the investigation of further non-epithelial junctions. Using a broad array of well-characterized specific antibodies against key junctional proteins in immunoblot reactions, high-resolution double-label laser scanning confocal microscopy, and immunoelectron microscopy, we describe a new type of adherens junction in human meningiomas and the human meningioma cell line HBL-52. This novel junction has a unique composition of proteins not found in any other tissue; it contains the desmosomal armadillo-protein plakophilin 2 together with the classic proteins of "epithelial" adherens junctions, i.e., E-cadherin (in some instances replaced by N-cadherin), alpha-catenin, beta-catenin, plakoglobin, and p120(ctn). Ultrastructurally, it is formed between two or three neighboring cells. For pragmatic reasons, we suggest the name "meningeal junction" for this new structure.

Expression of classical cadherins in the cerebellar anlage: quantitative and functional aspects

During central nervous system (CNS) development, cell migration precedes and is key to the integration of diverse sets of cells. Mechanistically, CNS histogenesis is realized through a balanced interplay of cell-cell and cell-matrix adhesion molecules. Here, we summarize experiments that probe the developmental expression and potential significance of a set of cadherins, including M-, N- and R-cadherin, for patterning of the cerebellar cortex. We established a transgenic marker that allows cerebellar granule cells to be followed from the neuroblast stage to their final, postmitotic settlement. In conjunction with flow cytometry, this allowed us to derive a quantitative view of cadherin expression in differentiating granule cells and relate it to the expression of the same cadherins in cerebellar inhibitory interneuronal precursors. In vitro reaggregation analysis supports a role for cadherins in cell sorting and migration within the nascent cerebellar cortex that may be rationalized within the context of the differential adhesion hypothesis (Foty, R.A. and Steinberg, M.S., 2005. The differential adhesion hypothesis: a direct evaluation. Dev. Biol. 278, 255-263.).

The morphology of excitatory central synapses: from structure to function

Synapses are the key elements for signal transduction and plasticity in the brain. For a better understanding of the functional signal cascades underlying synaptic transmission, a quantitative morphological analysis of the pre- and postsynaptic structures that represent morphological correlates for synaptic transmission is important. In particular, realistic values of the number, distribution, and geometry of synaptic contacts and the organization of the pool of synaptic vesicles provide important constraints for realistic models and numerical simulations of those parameters of synaptic transmission that, at present, are still not accessible to experiment. Although all synapses are composed of almost the same structural elements, the composition of these elements within a given synapse and the microcircuit in which they are embedded are the deciding factors determining its function. One possible way to analyze these structures is by computer-assisted three-dimensional reconstructions of synapses and their subsequent quantitative analysis based on ultrathin serial sections. The present review summarizes and discusses the morphology of five central excitatory synapses that are quantitatively well described: (1) a giant synapse, the so-called Calyx of Held, in the medial nucleus of the trapezoid body in the auditory brain stem, (2) the mossy fiber terminal establishing synapses with multiple cerebellar granule cell dendrites, (3) the mossy fiber bouton in the hippocampus predominantly terminating on proximal dendrites of CA3 pyramidal neurons, (4) the climbing fiber-Purkinje cell synapse in the cerebellum, and (5) cortical input synapses on the basal dendrites of layer 5 pyramidal cells. The detailed morphological description of these synaptic structures may help to define the morphological correlates of the functional parameters of synaptic transmission, such as the readily releasable pool of synaptic vesicles, of release, and of the variability of quantal size and might therefore explain the existing differences in the function between individual synapses embedded in different microcircuits.

The area composita of adhering junctions connecting heart muscle cells of vertebrates. II. Colocalizations of desmosomal and fascia adhaerens molecules in the intercalated disk

Using immunofluorescence histochemistry and immunoelectron microscopy on sections through myocardiac tissues of diverse mammalian (human, cow, rat, mouse) and fish species we show that both desmosomal and fascia adhaerens proteins identified by gel electrophoresis and immunoblot occur in the area composita, the by far major type of plaque-bearing junctions of the intercalated disks (IDs) connecting cardiomyocytes. Specifically, we demonstrate that desmoplakin and the other desmosomal proteins occur in these junctions, together with N-cadherin, cadherin-11, alpha- and beta-catenin as well as vinculin, afadin and proteins p120(ctn), ARVCF, p0071, and ZO-1, suggestive of colocalization. We conclude that the predominant type of adhering junction present in IDs is a junction sui generis, termed area composita, that is characterized by an unusually high molecular complexity and an intimate association of molecules of both ensembles, the desmosomal one and the fascia adhaerens category. We discuss possible myocardium-specific, complex-forming interactions between members of the two ensembles and the relevance of our findings for the formation and functioning of the heart and for the understanding of hereditary and other cardiomyopathies. We further propose to use this highly characteristic area composita ensemble of molecules as cardiomyocyte markers for the monitoring of cardiomyogenesis, cardiomyocyte regeneration and possible cardiomyocyte differentiation from mesenchymal stem cells.

Organization of dye-coupled cerebellar granule cells labeled from afferent vestibular and dorsal root fibers in the frogRana esculenta

Application of neurobiotin to the nerves of individual labyrinthine organs and dorsal root fibers of limb-innervating segments of the frog resulted in labeling of granule cells in the cerebellum showing a significant overlap with a partial segregation in the related areas of termination. In different parts of the cerebellum, various combinations of different canal and otolith organ-related granule cells have been discerned. The difference in the extension of territories of vertical canals vs. horizontal canals may reflect their different involvement in the vestibuloocular and vestibulospinal reflex. Dye-coupled cells related to the lagenar and saccular neurons were localized in more rostral parts of the cerebellum, whereas cells of the utricle were represented only in its caudal half. This separation is supportive of the dual function of the lagena and the saccule. The territories of granule cells related to the cervical and lumbar segments of the spinal cord were almost completely separated along the rostrocaudal axis of cerebellum, whereas their territories were almost entirely overlapping in the mediolateral and ventrodorsal directions. The partial overlap of labyrinthine organ-related and dorsal root fiber-related granule cells are suggestive of a convergence of sensory modalities involved in the sense of balance. We propose that the afferent input of vestibular and proprioceptive fibers mediated by gap junctions to the cerebellar granule cells subserve one of the possible morphological correlates of a very rapid modification of the motor activity in the vestibulocerebellospinal neuronal circuit.

Synthesis of junctional proteins in metastasizing colon cancer cells

Various authors have reported reduced synthesis of epithelial junctional proteins during dedifferentiation, tumorigenesis and metastasis in a great variety of tumors. Consequently, it is generally accepted that loss of adhesive molecules and adhesion structures is implicated in the development of an invasive phenotype and poor patient prognosis. Colon carcinomas, on the other hand, were shown to behave differently as synthesis of main adhesive proteins continues despite the development of an invasive phenotype. In this study we used cultured cells grown under conditions that inhibited intercellular adhesion (low Ca2+ concentration) and compared these results with data obtained from metastasizing colon cancer cells (signet ring cell carcinoma). Characterization of these proteins and their structures were performed by immunoprecipitations, Western blot analysis, immunohistochemistry, pre-embedding immuno-electron microscopy, and a new method to perform immuno-electron microscopy on paraffin-embedded material, which we present in this paper. We demonstrate that synthesis carries on for both, the desmosomal and the proteins of the zonula adhaerens. While, however, the assembly of desmosomal structures in the form of half-desmosomes at the cell surface continues, those of the zonula adhaerens did not. Instead E-cadherin was found, although associated with alpha-catenin, beta-catenin, and plakoglobin, evenly distributed at the plasma membrane of the cultured cells and also at the surface of the dissociated tumor cells. We conclude from our observations that continued expression and synthesis of junctional proteins do not necessarily contribute to the suppression of tumor invasion and metastasis of colon cancer.

Modeling of slow glutamate diffusion and AMPA receptor activation in the cerebellar glomerulus

Synaptic conductances are influenced markedly by the geometry of the space surrounding the synapse since the transient glutamate concentration in the synaptic cleft is determined by this geometry. Our paper is an attempt to understand the reasons for slow glutamate diffusion in the cerebellar glomerulus, a structure situated around the enlarged mossy fiber terminal in the cerebellum and surrounded by a glial sheath. For this purpose, analytical expressions for glutamate diffusion in the glomerulus were considered in models with two-, three-, and fractional two-three-dimensional (2D-3D) geometry with an absorbing boundary. The time course of average glutamate concentration in the synaptic cleft of the mossy fiber-granule cell connection was calculated for both direct release of glutamate from the same synaptic unit, and for cumulative spillover of glutamate from neighboring release sites. Several kinetic schemes were examined, and the parameters of the diffusion models were estimated by identifying theoretical activation of AMPA receptors with direct release and spillover components of published experimental AMPA receptor-mediated EPSCs. For model selection, the correspondence of simulated paired-pulse ratio and EPSC increase after prevention of desensitization to experimental values were also taken into consideration. Our results suggest at least a 7- to 10-fold lower apparent diffusion coefficient of glutamate in the porous medium of the glomerulus than in water. The modeling of glutamate diffusion in the 2D-3D geometry gives the best fit of experimental EPSCs. We show that it could be only partly explained by normal diffusion of glutamate in the complex geometry of the glomerulus. We assume that anomalous diffusion of glutamate occurs in the glomerulus. A good match of experimental estimations and theoretical parameters, obtained in the simulations that use an approximation of anomalous diffusion by a solution for fractional Brownian motion, confirms our assumption.

Mitochondria form a filamentous reticular network in hippocampal dendrites but are present as discrete bodies in axons: A three-dimensional ultrastructural study

The fine structure of mitochondria and smooth endoplasmic reticulum (SER) was studied via electron microscopy in dendritic and axonal neuronal segments of hippocampal areas CA1, CA3, and dentate gyrus (DG) of both ground squirrels in normothermic and hibernating conditions, and rats. Ultrathin serial sections of approximately 60 nm (up to 150 per series) were taken and three-dimensional (3D) reconstructions made of dendritic segments, up to 36 microm in length. Mitochondria were demonstrated to be present in filamentous form in every dendrite examined, in each of the hippocampal regions studied, whether in rat or ground squirrel. In addition, apparent continuity between the outer mitochondrial membrane and that of SER was observed by 3D reconstructions of very ultrathin (20 nm) serial sections prepared from dendritic segments. It is believed that SER penetrate into the heads of thin and mushroom spines but mitochondria do not enter the heads of these types of spines in dentate gyrus or CA1 of either rat or ground squirrel. However, in CA3 we have shown here that mitochondria penetrate into the base of the large thorny excrescences. Mushroom dendritic spines (but not thin spines) contained puncta adherentia, formed between pre- and postsynaptic membranes. In contrast to dendrites, the mitochondrial population of axonal processes in the same hippocampal regions were found only in the form of discrete bodies no more than 3 microm in length. The issue of the likely function of this network in dendrites and its potential role in calcium movement is discussed.

Localization of synapsin-I and PSD-95 in developing postnatal rat cerebellar cortex

The comparative localization of two prominent synaptic proteins, synapsin-I (Syn-I) and PSD-95, was investigated in slices of developing (P3-P21) rat cerebellar cortex using double- or triple-label fluorescence immunohistochemistry and confocal microscopy. During the first postnatal week, Syn-I and PSD-95 immunoreactive (IR) puncta were strongly concentrated in the Purkinje cell layer (PCL) where they circumscribed irregularly shaped PC somata, forming pericellular nests that likely correspond to early climbing fiber synapses. PSD-95 and Syn-I puncta also were found along the shafts and at the tips of growing PC dendrite branches labeled with calbindin. During the second postnatal week, synaptic puncta were lost from the PC layer, while many new puncta were added to the molecular layer (ML). At P10, about half of the PCs were circumscribed by PSD-95 or Syn-I puncta, whereas at P14 no PCs were circumscribed. By P14, PSD-95 and Syn-I became most strongly localized to many small puncta in the ML and to large clusters at mossy fiber rosettes in the glomerular layer (GL) where PSD-95 often encircled Syn-I clusters. Some large clusters in the GL contained only PSD-95 or Syn-I, but not both, suggesting differential growth or remodeling of pre- and post-synaptic structures. No PSD-95 staining of pre-synaptic terminal pinceau was observed during the first 3 weeks of postnatal development. Thus, in relation to PCs, there is a developmental shift in PSD-95 localization whereby, first, it is concentrated on PC cell bodies and short dendrites (P3-P7), then it is lost on PC cell bodies (P7-14) and becomes localized almost exclusively to PC dendrites (P14-P21).

A novel cell-cell junction system: the cortex adhaerens mosaic of lens fiber cells

The anucleate prismoid fiber cells of the eye lens are densely packed to form a tissue in which the plasma membranes and their associated cytoplasmic coat form a single giant cell-cell adhesive complex, the cortex adhaerens. Using biochemical and immunoprecipitation methods in various species (cow, pig, rat), in combination with immunolocalization microscopy, we have identified two different major kinds of cortical complex. In one, the transmembrane glycoproteins N-cadherin and cadherin-11 [which also occur in heterotypic ('mixed') complexes] are associated with alpha- and beta-catenin, plakoglobin (proportions variable among species), p120ctn and vinculin. The other complex contains ezrin, periplakin, periaxin and desmoyokin (and so is called the EPPD complex), usually together with moesin, spectrin(s) and plectin. In sections through lens fiber tissue, the short sides of the lens fiber hexagons appear to be enriched in the cadherin-based complexes, whereas the EPPD complexes also occur on the long sides. Moreover, high resolution double-label fluorescence microscopy has revealed, on the short sides, a finer, almost regular mosaicism of blocks comprising the cadherin-based, catenin-containing complexes, alternating with patches formed by the EPPD complexes. The latter, a new type of junctional plaque ensemble of proteins hitherto known only from certain other cell types, must be added to the list of major lens cortex proteins. We here discuss its possible functional importance for the maintenance of lens structure and functions, notably clear and sharp vision.

Dye coupling without gap junctions suggests excitatory connections of gamma-aminobutyric acidergic neurons

After injections of the low-molecular-weight tracer neurobiotin into the preeminential nucleus of the brain of the mormyrid fish Gnathonemus petersii, we observed that retrogradely labeled, large fusiform projection neurons (LFd cells) of the deep granular layer of the electrosensory lobe (ELL) were surrounded by 30-50 labeled satellite granular cells. More superficially located projection cells, including large fusiform cells in the superficial granular layer (LFs) and large ganglionic (LG) cells in the ganglionic layer, were never surrounded by labeled satellites. LFd-satellite cells have a small soma (diameter 5-8 microm), a few small dendrites, and an apical axon that terminates in the plexiform and ganglionic layers of the ELL. They contact LFd projection neurons with dendrodendritic, dendrosomatic, and somatodendritic puncta adhaerentia-like appositions, designated here as "neurapses." In the electron microscope, these contacts resemble synapses without presynaptic vesicles. Because no gap junctions were found between LFd and satellite granule cells, we suggest that the neurapses allow the passage of neurobiotin, though not biocytin or biotinylated dextran amine. These contacts may provide the intermediate substrate for the postulated, but so far unknown, excitatory connection between primary afferent input and LFd projection neurons, via gamma-aminobutyric acid (GABA)-ergic granular cells. We suggest that certain puncta adhaerentia-like contacts might not be only adhesive structures and that LFd-satellite granular cells might both excite LFd projection cells via neuraptic contacts of their dendrites and cell bodies and inhibit more superficial LF and LG cells via their GABAergic axonal synapses. Our results suggest that puncta adhaerentia-like contacts could be responsible in some cases for the electrical coupling found electrophysiologically in local inhibitory circuits.

NO66, a highly conserved dual location protein in the nucleolus and in a special type of synchronously replicating chromatin

It has recently become clear that the nucleolus, the most prominent nuclear subcompartment, harbors diverse functions beyond its classic role in ribosome biogenesis. To gain insight into nucleolar functions, we have purified amplified nucleoli from Xenopus laevis oocytes using a novel approach involving fluorescence-activated cell sorting techniques. The resulting protein fraction was analyzed by mass spectrometry and used for the generation of monoclonal antibodies directed against nucleolar components. Here, we report the identification and molecular characterization of a novel, ubiquitous protein, which in most cell types appears to be a constitutive nucleolar component. Immunolocalization studies have revealed that this protein, termed NO66, is highly conserved during evolution and shows in most cells analyzed a dual localization pattern, i.e., a strong enrichment in the granular part of nucleoli and in distinct nucleoplasmic entities. Colocalizations with proteins Ki-67, HP1alpha, and PCNA, respectively, have further shown that the staining pattern of NO66 overlaps with certain clusters of late replicating chromatin. Biochemical experiments have revealed that protein NO66 cofractionates with large preribosomal particles but is absent from cytoplasmic ribosomes. We propose that in addition to its role in ribosome biogenesis protein NO66 has functions in the replication or remodeling of certain heterochromatic regions.

Cadherin-mediated differential cell adhesion controls slow muscle cell migration in the developing zebrafish myotome

Slow-twitch muscle fibers of the zebrafish myotome undergo a unique set of morphogenetic cell movements. During embryogenesis, slow-twitch muscle derives from the adaxial cells, a layer of paraxial mesoderm that differentiates medially within the myotome, immediately adjacent to the notochord. Subsequently, slow-twitch muscle cells migrate through the entire myotome, coming to lie at its most lateral surface. Here we examine the cellular and molecular basis for slow-twitch muscle cell migration. We show that slow-twitch muscle cell morphogenesis is marked by behaviors typical of cells influenced by differential cell adhesion. Dynamic and reciprocal waves of N-cadherin and M-cadherin expression within the myotome, which correlate precisely with cell migration, generate differential adhesive environments that drive slow-twitch muscle cell migration through the myotome. Removing or altering the expression of either protein within the myotome perturbs migration. These results provide a definitive example of homophilic cell adhesion shaping cellular behavior during vertebrate development.

Tight junction-related structures in the absence of a lumen: Occludin, claudins and tight junction plaque proteins in densely packed cell formations of stratified epithelia and squamous cell carcinomas

Tight junctions (TJs), hallmark structures of one-layered epithelia and of endothelia, are of central biological importance as intramembranous "fences" and as hydrophobic "barriers" between lumina represented by liquid- or gas-filled spaces on the one hand and the mesenchymal space on the other. They have long been thought to be absent from stratified epithelia. Recently, however, constitutive TJ proteins and TJ-related structures have also been identified in squamous stratified epithelia, including the epidermis, where they occur in special positions, most prominently in the uppermost living epidermal cell layer, the stratum granulosum. Much to our surprise, however, we have now also discovered several major TJ proteins (claudins 1 and 4, occludin, cingulin, symplekin, protein ZO-1) and TJ-related structures in specific positions of formations of epithelium-derived tissues that lack any lumen and do not border on luminal or body surfaces. Using immunohistochemistry and electron microscopy we have localized TJ proteins and structures in peripheral cells of the Hassall's corpuscles of human and bovine thymi as well as in specific central formations of tumor nests in squamous cell carcinomas, including the so-called "horn pearls". Such structures have even been found in carcinoma metastases. In carcinomas, they often seem to separate certain tumor regions from others or from stroma. The structural significance and the possible functional relevance of the locally restricted synthesis of TJ proteins and of the formations of TJ-related structures are discussed. It is proposed to include the determination of the presence or absence of such proteins and structures in the diagnostic program of tumor pathology.

De novo formation of desmosomes in cultured cells upon transfection of genes encoding specific desmosomal components

Desmosomes are cell junctions and cytoskeleton-anchoring structures of epithelia, the myocardium, and dendritic reticulum cells of lymphatic follicles whose major components are known. Using cultured HT-1080 SL-1 fibrosarcoma-derived cells and transfection of cDNAs encoding specific desmosomal components, we have determined a minimum ensemble of proteins sufficient to introduce de novo structures, which, by morphology and functional competence, are indistinguishable from authentic desmosomes. In a more refined analysis, the influence of the desmosomal proteins desmoplakin (Dp), plakoglobin (Pg), and plakophilin 2 (Pp2) on the lateral clustering of the desmosomal transmembrane-glycoprotein desmoglein 2 (Dsg) was examined. We found that for efficient clustering of desmoglein 2 and desmosome structure formation, all three major plaque proteins-desmoplakin, plakoglobin, and plakophilin 2- were necessary. Furthermore, in this cell model, plakophilin 2 was capable of directing desmoplakin to adhaerens junctions (AJ), whereas plakoglobin was crucial for the segregation of desmosomal and AJ components. These results are discussed with respect to the variability in cell junction composition observed in various nonepithelial tissues.

Three-dimensional reconstruction of a calyx of Held and its postsynaptic principal neuron in the medial nucleus of the trapezoid body

The three-dimensional morphology of the axosomatic synaptic structures between a calyx of Held and a principal neuron in the medial nucleus of the trapezoid body (MNTB) in the brainstem of young postnatal day 9 rats was reconstructed from serial ultrathin sections. In the apposition zone between the calyx and the principal neuron two types of membrane specializations were identified: synaptic contacts (SCs) with active zones (AZs) and their associated postsynaptic densities (PSDs) constituted approximately 35% (n = 554) of the specializations; the remaining 65% (n = 1010) were puncta adherentia (PA). Synaptic contacts comprised approximately 5% of the apposition area of presynaptic and postsynaptic membranes. A SC had an average area of 0.100 microm(2), and the nearest neighbors were separated, on average, by 0.59 microm. Approximately one-half of the synaptic vesicles in the calyx were clustered within a distance of 200 nm of the AZ membrane area, a cluster consisting of approximately 60 synaptic vesicles (n = 52 SCs). Approximately two synaptic vesicles per SC were "anatomically docked." Comparing the geometry of the synaptic structure with its previously studied functional properties, we find that during a single presynaptic action potential (AP) (1) approximately 35% of the AZs release a transmitter quantum, (2) the number of SCs and anatomically docked vesicles is comparable with the low estimates of the readily releasable pool (RRP) of quanta, and (3) the broad distribution of PSD areas [coefficient of variation (CV) = 0.9] is likely to contribute to the large variability of miniature EPSC peaks. The geometry of the reconstructed synapse suggests that each of the hundreds of SCs is likely to contribute independently to the size and rising phase of the EPSC during a single AP.

Three-dimensional reconstruction of a calyx of Held and its postsynaptic principal neuron in the medial nucleus of the trapezoid body

The three-dimensional morphology of the axosomatic synaptic structures between a calyx of Held and a principal neuron in the medial nucleus of the trapezoid body (MNTB) in the brainstem of young postnatal day 9 rats was reconstructed from serial ultrathin sections. In the apposition zone between the calyx and the principal neuron two types of membrane specializations were identified: synaptic contacts (SCs) with active zones (AZs) and their associated postsynaptic densities (PSDs) constituted approximately 35% (n = 554) of the specializations; the remaining 65% (n = 1010) were puncta adherentia (PA). Synaptic contacts comprised approximately 5% of the apposition area of presynaptic and postsynaptic membranes. A SC had an average area of 0.100 microm(2), and the nearest neighbors were separated, on average, by 0.59 microm. Approximately one-half of the synaptic vesicles in the calyx were clustered within a distance of 200 nm of the AZ membrane area, a cluster consisting of approximately 60 synaptic vesicles (n = 52 SCs). Approximately two synaptic vesicles per SC were "anatomically docked." Comparing the geometry of the synaptic structure with its previously studied functional properties, we find that during a single presynaptic action potential (AP) (1) approximately 35% of the AZs release a transmitter quantum, (2) the number of SCs and anatomically docked vesicles is comparable with the low estimates of the readily releasable pool (RRP) of quanta, and (3) the broad distribution of PSD areas [coefficient of variation (CV) = 0.9] is likely to contribute to the large variability of miniature EPSC peaks. The geometry of the reconstructed synapse suggests that each of the hundreds of SCs is likely to contribute independently to the size and rising phase of the EPSC during a single AP.

Tight junctions and compositionally related junctional structures in mammalian stratified epithelia and cell cultures derived therefrom

The occurrence of extended tight junction (TJ) structures, including zonulae occludentes (ZO), and the spatial arrangement of TJ proteins in stratified mammalian epithelia has long been controversially discussed. Therefore, we have systematically examined the localization of TJ proteins in diverse stratified epithelial tissues (e.g., epidermis, heel pad, snout, gingiva, tongue, esophagus, exocervix, vagina, urothelium, cornea) of various species (human, bovine, rodents) as well as in human cell culture lines derived from stratified epithelia, by electron microscopy as well as by immunocytochemistry at both the light and the electron microscopic level, using antibodies to TJ proteins such as occludin, claudins 1 and 4, protein ZO-1, cingulin and symplekin. We have found an unexpected diversity of TJ-related structures of which only those showing colocalization with the most restricted transmembrane TJ marker protein, occludin, are presented here. While in epidermis and urothelium occludin is restricted to the uppermost living cell layer, TJ-related junctions are abundant in the upper third or even in the majority of the suprabasal cell layers in other stratified epithelia. Interfollicular epidermis contains, in the stratum granulosum, extended, probably continuous ZO-like structures which can also be traced at least through the Henle cell layer of hair follicles. Similar apical ZO-like structures have been seen in the upper living cell layers of all other stratified epithelia and cell cultures examined, but in most of them we have noticed, in addition, junctional regions showing relatively broad, ribbon-like membrane contacts which in cross-section often appear pentalaminar, with an electron-dense middle lamella ("lamellated TJs", coniunctiones laminosae). In suprabasal layers of several stratified epithelia we have further observed TJ protein-containing junctions of variable sizes which are characterized by a 10-30-nm dense lamina interposed between the two membranes ("sandwich junctions"; iuncturae structae). Moreover, we have often observed variously sized regions in which the intermembrane distance is rather regularly bridged by short rod-like elements ("cross-bridged cell walls"; parietes transtillati), often in close vicinity of TJ-related structures or desmosomes. The significance of these structures and their possible biological importance are discussed.

The cell adhesion molecule M-cadherin is not essential for muscle development and regeneration

M-cadherin is a classical calcium-dependent cell adhesion molecule that is highly expressed in developing skeletal muscle, satellite cells, and cerebellum. Based on its expression pattern and observations in cell culture, it has been postulated that M-cadherin may be important for the fusion of myoblasts to form myotubes, the correct localization and function of satellite cells during muscle regeneration, and the specialized architecture of adhering junctions in granule cells of cerebellar glomeruli. In order to investigate the potential roles of M-cadherin in vivo, we generated a null mutation in mice. Mutant mice were viable and fertile and showed no gross developmental defects. In particular, the skeletal musculature appeared essentially normal. Moreover, muscle lesions induced by necrosis were efficiently repaired in mutant mice, suggesting that satellite cells are present, can be activated, and are able to form new myofibers. This was also confirmed by normal growth and fusion potential of mutant satellite cells cultured in vitro. In the cerebellum of M-cadherin-lacking mutants, typical contactus adherens junctions were present and similar in size and numbers to the equivalent junctions in wild-type animals. However, the adhesion plaques in the cerebellum of these mutants appeared to contain elevated levels of N-cadherin compared to wild-type animals. Taken together, these observations suggest that M-cadherin in the mouse serves no absolutely required function during muscle development and regeneration and is not essential for the formation of specialized cell contacts in the cerebellum. It seems that N-cadherin or other cadherins can largely compensate for the lack of M-cadherin.

Molecular characterization of Calymmin, a novel notochord sheath-associated extracellular matrix protein in the zebrafish embryo

During the screening of a zebrafish postsomitogenesis embryo cDNA library, we have identified a cDNA corresponding to a novel type of protein localized to the notochordal sheath-associated extracellular matrix (ECM) of the embryo. The 4.049-kb mRNA encodes a predicted polypeptide of 1,207 amino acids (122 kDa, pI 10.50) with a potential signal peptide of 20 amino acids. After the signal peptide, the mature protein consists of 1,187 amino acids (119 kDa, pI 10.46), for which the name "Calymmin" (from Greek chialphalambdanumumualpha, to envelop, to cover) is proposed. The Calymmin mRNA is highly and transiently expressed by the notochord cells of the embryo from the 10- to 12-somite stage to the pharyngula period (13 and 24 hours postfertilization, respectively), and light and electron microscopical immunolocalization analysis revealed that the protein was specifically localized within a granular and filamentous layer of the ECM compartment surrounding the notochord. In zebrafish no tail mutants (ntl(tc41)), in which the notochord precursor cells are present but fail to differentiate, the Calymmin protein was not detected, confirming the notochord origin of Calymmin. These results indicate that Calymmin is a novel constitutive protein of the ECM compartment associated to the perinotochordal sheath in the zebrafish embryo, which is specifically expressed by the differentiating notochord cells.

Organization and formation of the tight junction system in human epidermis and cultured keratinocytes

Occludin and several proteins of the claudin family have been identiried in simple epithelia and in endothelia as major and structure-determining transmembrane proteins clustered in the barrier-forming tight junctions (TJ), where they are associated with a variety of TJ plaque proteins, including protein ZO-1. To examine whether TJ also occur in the squamous stratified epithelium of the interfollicular human epidermis we have applied several microscopic and biochemical techniques. Using RT-PCR techniques, we have identiried mRNAs encoding protein ZO-1, occludin and claudins 1, 4, 7, 8, 11, 12, and 17 in both tissues, skin and cultured keratinocytes, whereas claudins i and 10 have only been detected in skin tissue. By immunocytochemistry we have localized claudin-1, occludin and protein ZO-1 in distinct plasma membrane structures representing cell-cell attachment zones. While claudin-1 occurs in plasma membranes of all living cell layers, protein ZO-1 is concentrated in or even restricted to the uppermost layers, and occludin is often detected only in the stratum granulosum. Using electron microscopy, typical TJ structures ("kissing points") as well as some other apparently related junctional structures have been detected in the stratum granulosum, interspersed between desmosomes. Modes and patterns of TJ formation have also been studied in experimental model systems, e.g., during wound healing and stratification as well as in keratinocyte cultures during Ca2+-induced stratification. We conclude that the epidermis contains in the stratum granulosum a continuous zonula occludens-equivalent structure with typical TJ morphology and molecular composition, characterized by colocalization of occludin, claudins and TJ plaque proteins. In addition, cell-cell contact structures and certain TJ proteins can also be detected in other epidermal cell layers in specific cell contacts. The pattern of formation and possible functions of epidermal TJ and related structures are discussed.

A novel epithelial keratin, hK6irs1, is expressed differentially in all layers of the inner root sheath, including specialized huxley cells (Flügelzellen) of the human hair follicle

In this study we have characterized a novel human type II keratin, hK6irs1, which is specifically expressed in the inner root sheath of the hair follicle. This keratin represents the ortholog of the recently described mouse inner root sheath keratin mK6irs. The two keratins were highly related and migrated at the same height as keratin 6 in two-dimensional gel electrophoresis. Both RNA in situ hybridization and indirect immunofluorescence studies of human hair follicles demonstrated hK6irs1 expression in the Henle and Huxley layers as well as in the cuticle of the inner root sheath. In all three layers, the expression of hK6irs1 mRNA and protein began simultaneously in adjacent cells of the lowermost bulb above the germinative cell pool. Higher up in the follicle, the detection limits for both hK6irs1 mRNA and protein precisely coincided with the asynchronous onset of abrupt terminal differentiation of the Henle layer, inner root sheath cuticle, and Huxley layer. Mainly above the level of terminal Henle cell differentiation, both indirect immunofluorescence and immunoelectron microscopy revealed the occurrence of distinct Huxley cells that developed pseudopodal hK6irs1-positive extensions passing through the fully keratinized Henle layer. These outwardly protruding foot processes abutted upon cells of the companion layer, with which they were connected by numerous desmosomes. These specialized Huxley cells have previously been termed "Flügelzellen", which means "winged cells", with reference to their characteristic foot processes. We provide evidence that, together with Henle cells, Flügelzellen ensure the maintenance of a continuous desmosomal anchorage of the companion layer along the entire inner root sheath. This tightly connected companion layer/inner root sheath unit provides an optimal molding and guidance of the growing hair shaft.

NMDA receptors and PSD-95 are found in attachment plaques in cerebellar granular layer glomeruli

N-methyl-D-aspartate (NMDA) receptors mediate long-term changes in excitatory synapses in response to glutamate release. In the cerebellar granular layer, most glutamatergic synapses are formed between mossy terminals and granule cell dendrites, which together with some other components, make up complex glomerular structures. Glomeruli contain numerous attachment plaques (or puncta adherentia), which are sites of adhesion between cells. These structures are found mainly between granule cell dendrites, and probably help maintain the integrity of glomeruli. Attachment plaques contain adhesive proteins such as cadherins. In this study, we show that NMDA receptors are common at these attachment plaques, in addition to being found at synapses. We used four different antibodies to the NMDA receptor subunit, NR1, and another to NR2A/B. In contrast, labelling for an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) glutamate receptor antibody was seen only in a few attachment plaques, although AMPA receptors were seen frequently at glomerular synapses. We also show that substantial levels of the NMDA receptor-associated protein, PSD-95, are found in both synapses and attachment plaques. One way that NMDA receptors mediate changes in synapses is through effects on synaptic cadherins, which change their adhesive properties in response to NMDA receptor activation and consequently may alter synaptic function. The presence of NMDA receptors in attachment plaques suggests that these receptors mediate changes in the adhesive properties of these plaques, similar to this function in synapses.

Filamentous actin is concentrated in specific subpopulations of neuronal and glial structures in rat central nervous system

This paper is the second in a series of studies on the light and electron microscopic distribution of filamentous actin (F-actin) in the rat central nervous system (CNS) using phalloidin tagged with the fluorophore eosin followed by fluorescence photooxidation. A previous report described the selective localization of high concentrations of F-actin in subpopulations of dendritic spines in hippocampus, cerebellum and neostriatum. Dendritic spines were the most intensely stained structures in the CNS, but several other structures were notable for their consistent staining for F-actin. Although the majority of cell bodies, axons and large dendrites were unlabeled, mossy fibers and Schaffer collaterals in the hippocampal formation, basket cell axons in the cerebellar pinceau, and granule cell dendrites in the glomeruli of the cerebellar cortex routinely showed strong F-actin labeling. Staining was observed in all three glial cell types. Labeling was consistently observed in the astrocytic processes surrounding the Purkinje cell soma and primary dendrite. Intense but sporadic staining was observed in the perinodal glia of the Node of Ranvier. A few examples of labeled oligodendrocyte processes were also seen in the neostriatum. Labeling was observed in microglia in every brain region examined, although the labeling was present in the lumen of the endoplasmic reticulum and the nuclear membrane, leading to questions about its specificity. Perycites apposed to the blood vessels also showed very consistent labeling. Our results suggest that selected structures in the adult CNS in addition to dendritic spines are enriched in F-actin.

Drebrin particles: components in the ensemble of proteins regulating actin dynamics of lamellipodia and filopodia

Drebrin, an actin-binding 70-kDa protein with an unusually slow SDS-PAGE mobility corresponding to approximately 120 kDa, containing a proline-rich, profilin-binding motif, had originally been reported from neuronal cells, but recently has also been found in diverse other kinds of tissues and cell lines. In biochemical analyses of various cells and tissues, employing gel filtration, sucrose gradient centrifugation, immunoprecipitation and -blotting, we have identified distinct states of soluble drebrin: a approximately 4S monomer, an 8S, ca. 217-kDa putative trimer, a 13S and a > 20S oligomer. In the 8S particles only [35S]methionine-labelled drebrin but no other actin-binding protein has been detected in stoichiometric amounts. By immunofluorescence and immunoelectron microscopy, drebrin-positive material often appeared as "granules" up to 400 nm in diameter, in some cell types clustered near the Golgi apparatus or in lamellipodia, particularly at leading edges, or in dense-packed submembranous masses at tips (acropodia) or ruffles of leading edges, in filopodia and at plaques of adhering junctions. We conclude that these drebrin complexes and drebrin-rich structures allow the build-up and maintenance of high local drebrin concentrations in strategic positions for the regulation of actin filament assembly, thereby contributing to cell motility and morphology, in particular local changes of plasticity and the formation of protrusions.

Formation of a normal epidermis supported by increased stability of keratins 5 and 14 in keratin 10 null mice

The expression of distinct keratin pairs during epidermal differentiation is assumed to fulfill specific and essential cytoskeletal functions. This is supported by a great variety of genodermatoses exhibiting tissue fragility because of keratin mutations. Here, we show that the loss of K10, the most prominent epidermal protein, allowed the formation of a normal epidermis in neonatal mice without signs of fragility or wound-healing response. However, there were profound changes in the composition of suprabasal keratin filaments. K5/14 persisted suprabasally at elevated protein levels, whereas their mRNAs remained restricted to the basal keratinocytes. This indicated a novel mechanism regulating keratin turnover. Moreover, the amount of K1 was reduced. In the absence of its natural partner we observed the formation of a minor amount of novel K1/14/15 filaments as revealed by immunogold electron microscopy. We suggest that these changes maintained epidermal integrity. Furthermore, suprabasal keratinocytes contained larger keratohyalin granules similar to our previous K10T mice. A comparison of profilaggrin processing in K10T and K10(-/-) mice revealed an accumulation of filaggrin precursors in the former but not in the latter, suggesting a requirement of intact keratin filaments for the processing. The mild phenotype of K10(-/-) mice suggests that there is a considerable redundancy in the keratin gene family.

Molecular diversity of plaques of epithelial-adhering junctions

In biochemical and immunocytochemical comparisons of adhering junctions of different epithelia, we have observed differences in molecular composition not only between the intermediate filament-attached desmosomes and the actin filaments-anchoring adherens junctions but also between desmosomes of different tissues and of different strata in the same stratified epithelium. In addition we now report cell type-specific differences of molecular composition and immunoreactivity in both desmosomes and adherens junctions of certain simple epithelia. Whereas the zonula adhaerens of human intestinal and colonic epithelial cells, and of carcinomas derived therefrom, contains the additional armadillo-type plaque protein ARVCF, this protein has not been detected in the zonula adhaerens of hepatocytes. Similarly, plakophilin 3 is present in the desmosomal plaques of intestinal and colonic cells but appears to be absent from the hepatocytic desmosomes. We suggest that these profound compositional differences in the junctions of related simple epithelia are correlated to functional differences of the specific type of epithelium.

Cadherins in the central nervous system

The central nervous system (CNS) is divided into diverse embryological and functional compartments. The early embryonic CNS consists of a series of transverse subdivisions (neuromeres) and longitudinal domains. These embryonic subdivisions represent histogenetic fields in which neurons are born and aggregate in distinct cell groups (brain nuclei and layers). Different subsets of these aggregates become selectively connected by nerve fiber tracts and, finally, by synapses, thus forming the neural circuits of the functional systems in the CNS. Recent work has shown that 30 or more members of the cadherin family of morphoregulatory molecules are differentially expressed in the developing and mature brain at almost all stages of development. In a regionally specific fashion, most cadherins studied to date are expressed by the embryonic subdivisions of the early embryonic brain, by developing brain nuclei, cortical layers and regions, and by fiber tracts, neural circuits and synapses. Each cadherin shows a unique expression pattern that is distinct from that of other cadherins. Experimental evidence suggests that cadherins contribute to CNS regionalization, morphogenesis and fiber tract formation, possibly by conferring preferentially homotypic adhesiveness (or other types of interactions) between the diverse structural elements of the CNS. Cadherin-mediated adhesive specificity may thus provide a molecular code for early embryonic CNS regionalization as well as for the development and maintenance of functional structures in the CNS, from embryonic subdivisions to brain nuclei, cortical layers and neural circuits, down to the level of individual synapses.

H-1 parvovirus-associated replication bodies: a distinct virus-induced nuclear structure

We have identified a nuclear structure that is induced after infection with the autonomous parvovirus H-1. Using fluorescence microscopy, we observed that the major nonstructural protein (NS1) of H-1 virus which is essential for viral DNA amplification colocalized with virus-specific DNA sequences and sites of ongoing viral DNA replication in distinct nuclear bodies which we designated H-1 parvovirus-associated replication bodies (H-1 PAR-bodies). In addition, two cellular proteins were shown to accumulate in H1 PAR-bodies: (i) the proliferating cell nuclear antigen (PCNA) which is essential for chromosomal and parvoviral replication and (ii) the NS1-interacting small glutamine-rich TPR-containing protein (SGT), suggesting a role for the latter in parvoviral replication and/or gene expression. Since many DNA viruses target preexisting nuclear structures, known as PML-bodies, for viral replication and gene expression, we have determined the localization of H-1 PAR- and PML-bodies by double-fluorescence labeling and confocal microscopy and found them to be spatially unrelated. Furthermore, H-1 PAR-bodies did not colocalize with other prominent nuclear structures such as nucleoli, coiled bodies, and speckled domains. Electron microscopy analysis revealed that NS1, as detected by indirect immunogold labeling, was localized in ring-shaped electron-dense nuclear structures corresponding in size and frequency to H-1 PAR-bodies. These structures were also clearly visible without immunogold labeling and could be detected only in infected cells. Our results suggest that H-1 virus does not target known nuclear bodies for DNA replication but rather induces the formation of a novel structure in the nucleus of infected cells.

Cadherin-catenin complexes during zebrafish oogenesis: heterotypic junctions between oocytes and follicle cells

During vertebrate oogenesis, the germ cells and associated somatic cells remain connected by a variety of adhering junctional complexes. However, the molecular composition of these cellular structures is largely unknown. To identify the proteins forming the heterotypic adherens junctions between oocytes and follicle cells in the zebrafish (Danio rerio), the cDNAs encoding alphaE-catenin and plakoglobin were isolated. Using these cDNAs, in combination with the previously isolated beta-catenin cDNA, and antibodies specific for alpha- and beta-catenin, plakoglobin, and N- and E-cadherin, we found differences in catenin and plakoglobin gene expression during oogenesis. The immunolocalization of these plaque proteins, as well as of cadherins, in the ovarian follicle indicated an enrichment of alpha- and beta-catenin and of E-cadherin-like protein(s) in the oocyte cortex, notably at sites of oocyte-follicle cell contacts, suggesting the presence of hitherto unknown heterotypic adherens junctions between these cells. By contrast, plakoglobin and N-cadherin localization was restricted to cell-cell contacts in the follicle cell layer. During oocyte maturation, mRNAs for alphaE- and beta-catenin and plakoglobin accumulated, and all three plaque-forming proteins were stored in unfertilized eggs, either in complexed forms with cadherins or as free cytoplasmic pools. These findings suggest possible roles of these junctional proteins during early embryogenesis.

The arm-repeat protein NPRAP (neurojungin) is a constituent of the plaques of the outer limiting zone in the retina, defining a novel type of adhering junction

In the retina, special plaque-bearing adhering junctions are aligned to form a planar system (the "outer limiting zone," OLZ) of heterotypic connections between the photoreceptor cells and the surrounding glial cells ("Müller cells"), together with homotypic junctions. In the plaques of these junctions, which contain N-cadherin-and possibly also related cadherins-we have identified, by immunolocalization techniques, a recently discovered neural tissue-specific protein, neurojungin, a member of the plakoglobin/armadillo protein family. In these plaques we have also detected other adherens plaque proteins, such as alpha- and beta-catenin, protein p120, and vinculin, as well as proteins known as constituents of tight junction plaques, such as symplekin and protein ZO-1, and the desmosomal plaque protein plakophilin 2. This unusual combination of proteins and the demonstrated absence of plakoglobin define the OLZ junctions as a new and distinct category of adhering junction, which probably has special architectural functions.

Multiple cadherin mRNA expression and developmental regulation of a novel cadherin in the developing mouse eye

Cadherins form a large family of transmembrane glycoproteins whose members include the classical cadherins, the desmosomal cadherins, and the protocadherins. The classical cadherins mediate homophilic cell-cell adhesion and are key regulators of many morphogenetic processes. More than a dozen classical cadherins are expressed in both the developing and the mature central nervous system. Although individual cadherins have been identified in the retina of various species, we wished to determine the range of cadherins expressed at distinct developmental stages in the mouse retina. Using a PCR-based cloning strategy, we detected 10 different classical cadherin mRNAs of both type I and type II subtypes. The most abundant cDNA was that encoding the type II cadherin, Cadherin-11. The other type II cadherins detected were VE- and T2-cadherin and Cadherin-6 and -12. Four type I cadherins, N-, R-, P-, and E-cadherin, were also present. One cadherin cDNA encoded a novel cadherin, called EY-cadherin for cloned from eye. EY-cadherin is most closely related to human Cadherin-14 (93% identical). EY-cadherin mRNA was detected in the adult mouse eye, brain, and testis with a 20-fold increase in expression levels in the embryonic head from E11 to E19 and a 50-fold increase in expression levels in the postnatal eye from PN1 to PN16. Multiple cadherin gene expression is consistent with the hypothesis that different cadherins regulate morphogenetic processes, such as neuronal migration and lamination, and determine the specific interneuronal connections found in the mature retina.

The M-cadherin catenin complex interacts with microtubules in skeletal muscle cells: implications for the fusion of myoblasts

M-cadherin, a calcium-dependent intercellular adhesion molecule, is expressed in skeletal muscle cells. Its pattern of expression, both in vivo and in cell culture as well as functional studies, have implied that M-cadherin is important for skeletal muscle development, in particular the fusion of myoblasts into myotubes. M-cadherin formed complexes with the catenins in skeletal muscle cells similar to E-cadherin in epithelial cells. This suggested that the muscle-specific function of the M-cadherin catenin complex might be mediated by additional interactions with yet unidentified cellular components, especially cytoskeletal elements. These include the microtubules which also have been implicated in the fusion process of myoblasts. Here we present evidence that the M-cadherin catenin complex interacts with microtubules in myogenic cells by using three independent experimental approaches. (1) Analysis by laser scan microscopy revealed that the destruction of microtubules by nocodazole leads to an altered cell surface distribution of M-cadherin in differentiating myogenic cells. In contrast, disruption of actin filaments had little effect on the surface distribution of M-cadherin. (2) M-cadherin antibodies coimmunoprecipitated tubulin from extracts of nocodazole-treated myogenic cells but not of nocodazole-treated epithelial cells ectopically expressing M-cadherin. Vice versa, tubulin antibodies coimmunoprecipitated M-cadherin from extracts of nocodazole-treated myogenic cells but not of nocodazole-treated M-cadherin-expressing epithelial cells. (3) M-cadherin and the catenins, but not a panel of control proteins, were copolymerized with tubulin from myogenic cell extracts even after repeated cycles of assembly and disassemly of tubulin. Moreover, neither M-cadherin nor E-cadherin could be found in a complex with microtubules in epithelial cells ectopically expressing M-cadherin. Our data are consistent with the idea that the interaction of M-cadherin with microtubules might be essential to keep the myoblasts aligned during fusion, a process in which both M-cadherin and microtubules have been implicated.

Loss of beta1 integrin function results in a retardation of myogenic, but an acceleration of neuronal, differentiation of embryonic stem cells in vitro

Integrin cell surface receptors play an important role for cell adhesion, migration, and differentiation during embryonic development by mediating cell-cell and cell-matrix interactions. Less is known about the function of integrins during commitment and lineage determination of early embryogenesis. Homozygous inactivation of the beta1 integrin gene results in embryonal death in mice around the time of implantation. In vitro, differentiation of embryonic stem (ES) cells which lack beta1 integrin (beta1-/-) into the cardiogenic lineage is delayed and results in a disordered cellular specification (Fässler et al., J. Cell Sci. 109, 2989-2999, 1996). To analyze beta1 integrin function during myogenesis and neurogenesis we studied differentiation of beta1-/- ES cells via embryoid bodies into skeletal muscle and neuronal cells in vitro. beta1-/- cells showed delayed and reduced myogenic differentiation compared to wildtype and heterozygous (beta1+/-) ES cells. RT-PCR analysis demonstrated delayed expression of skeletal muscle-specific genes in the absence of beta1 integrin. Immunofluorescence studies with antibodies against the sarcomeric proteins myosin heavy chain, titin, nebulin, and slow C-protein showed that myotubes formed, but their number was reduced and the assembly of sarcomeric structures was retarded. In contrast, neuronal cells differentiating from beta1-/- ES cells appeared earlier than wildtype and heterozygous (beta1+/-) ES cells. This was shown by the accelerated expression of neuron-specific genes and an increased number of neuronal cells in beta1-/- embryoid bodies. However, neuronal outgrowth was retarded in the absence of beta1 integrin. No functional difference between wildtype and beta1-/- cells was found with respect to secretion of gamma-aminobutyric acid, the main neurotransmitter of ES cell-derived neuronal cells. The lineage-specific effects of loss of beta1 integrin function, that is the inhibition of mesodermal and acceleration of neuroectodermal differentiation, were supported by differential expression of genes encoding lineage-specific transcription factors (Brachyury, Pax-6, Mash1) and signaling molecules (BMP-4 and Wnt-1). Because of the reduced and delayed expression of the BMP-4 encoding gene in beta1-/- cells, we analyzed in wildtype and beta1-/- cells the regulatory role of exogenously added BMP-4 on the expression of the mesodermal and neuronal marker genes, Brachyury and wnt-1, respectively. The data suggest that BMP-4 plays a regulatory role during differentiation of wildtype and beta1-/- cells by modifying mesodermal and neuronal pathways. The reduced expression of BMP-4 in beta1-/- cells may account for the accelerated neuronal differentiation in beta1-/- ES cells.

Induction of a regular nuclear lattice by overexpression of NuMA

Transient overexpression of nuclear mitotic apparatus protein (NuMA) in HeLa cells results in ordered lattices which can fill the nucleus and which are stable to detergent extraction. Electron microscopy reveals a quasi-hexagonal organization with an average spacing between the vertices of approximately 170 nm and short 6-nm-diameter rods connecting the vertices. Overexpression of a NuMA construct with an in-frame addition in the coiled-coil domain shows hexagons with the spacing increased by 42% while constructs with deletions in the coiled-coil domain yield hexagons with the spacing decreased by 40 and 19%. NuMA constructs truncated at residue 2005 or 2030 in the tail domain cause a drastic reorganization of nuclear components with relocation of the DNA, histone H1, and nucleoli to the nuclear rim. A construct lacking the head and much of the coiled-coil region also affects nuclear organization. In contrast, NuMA constructs truncated at residue 1950 or 1935 which lack the nuclear localization signal display normal nuclear structure but form cytoplasmic aggregates which also display hexagonal organization. Immunoelectron microscopy confirms that the nuclear lattices are built from NuMA. We discuss the importance of the different domains of NuMA for building the ordered in vivo lattices and whether NuMA could play a structural role in the architecture of the normal interphase nucleus.