Communication between the cell membrane and the nucleus: Role of protein compartmentalization

Acute and short-term fluctuations in gravity are associated with changes in circulatory plasma protein levels

Gravitational changes between micro- and hypergravity cause several adaptations and alterations in the human body. Besides muscular atrophy and immune system impairment, effects on the circulatory system have been described, which can be associated with a wide range of blood biomarker changes. This study examined nine individuals (seven males, two females) during a parabolic flight campaign (PFC). Thirty-one parabolas were performed in one flight day, resulting in ~22 s of microgravity during each parabola. Each participant was subjected to a single flight day with a total of 31 parabolas, totaling 11 min of microgravity during one parabolic flight. Before and after (1 hour (h) and 24 h), the flights blood was sampled to examine potential gravity-induced changes of circulating plasma proteins. Proximity Extension Assay (PEA) offers a proteomic solution, enabling the simultaneous analysis of a wide variety of plasma proteins. From 2925 unique proteins analyzed, 251 (8.58%) proteins demonstrated a differential regulation between baseline, 1 h and 24 h post flight. Pathway analysis indicated that parabolic flights led to altered levels of proteins associated with vesicle organization and apoptosis up to 24 h post microgravity exposure. Varying gravity conditions are associated with poorly understood physiological changes, including stress responses and fluid shifts. We provide a publicly available library of gravity-modulated circulating protein levels illustrating numerous changes in cellular pathways relevant for inter-organ function and communication.

Identification and Characterization of Novel Matrix-Derived Bioactive Peptides: A Role for Collagenase from Santyl® Ointment in Post-Debridement Wound Healing?

Debridement, the removal of diseased, nonviable tissue, is critical for clinicians to readily assess wound status and prepare the wound bed for advanced therapeutics or downstream active healing. Removing necrotic slough and eschar through surgical or mechanical methods is less specific and may be painful for patients. Enzymatic debridement agents, such as Clostridial collagenase, selectively and painlessly degrade devitalized tissue. In addition to its debriding activities, highly-purified Clostridial collagenase actively promotes healing, and our past studies reveal that extracellular matrices digested with this enzyme yield peptides that activate cellular migratory, proliferative and angiogenic responses to injury in vitro, and promote wound closure in vivo. Intriguingly, while collagenase Santyl® ointment, a sterile preparation containing Clostridial collagenases and other non-specific proteases, is a well-accepted enzymatic debridement agent, its role as an active healing entity has never been established. Based on our previous studies of pure Clostridial collagenase, we now ask whether the mixture of enzymes contained within Santyl® produces matrix-derived peptides that promote cellular injury responses in vitro and stimulate wound closure in vivo. Here, we identify novel collagen fragments, along with collagen-associated peptides derived from thrombospondin-1, multimerin-1, fibronectin, TGFβ-induced protein ig-h3 and tenascin-C, generated from Santyl® collagenase-digested human dermal capillary endothelial and fibroblastic matrices, which increase cell proliferation and angiogenic remodeling in vitro by 50-100% over controls. Using an established model of impaired healing, we further demonstrate a specific dose of collagenase from Santyl® ointment, as well as the newly-identified and chemically-synthesized ECM-derived peptides significantly increase wound re-epithelialization by 60-100% over saline-treated controls. These results not only confirm and extend our earlier studies using purified collagenase- and matrix-derived peptides to stimulate healing in vitro and in vivo, but these Santyl®-generated, matrix-derived peptides may also represent exciting new opportunities for creating advanced wound healing therapies that are enabled by enzymatic debridement and potentially go beyond debridement.

The likely role of proteolytic enzymes in unwanted differentiation of stem cells in culture

Tissue engineering aims at developing the necessary technological strategies for replacement or regeneration tissues. However, the number of cells required is much greater than the number obtained from a cell source. Expanding the cells' number in cell culture for a long period is required until the necessary amount of cells is obtained. While in culture, cells often go unwanted differentiation. Little attention has been given to the use of proteolytic enzymes in cell passage. Review the importance of extracellular matrix and surface proteins for cell behavior and the possible mechanisms of cellular changes that may occur due to the use of proteolytic enzymes is an essential issue. Possible alternative to replace these enzymes in cell passage has also been developed.

Plasma membrane signaling in HIV-1 infection

Plasma membrane is a multifunctional structure that acts as the initial barrier against infection by intracellular pathogens. The productive HIV-1 infection depends upon the initial interaction of virus and host plasma membrane. Immune cells such as CD4+ T cells and macrophages contain essential cell surface receptors and molecules such as CD4, CXCR4, CCR5 and lipid raft components that facilitate HIV-1 entry. From plasma membrane HIV-1 activates signaling pathways that prepare the grounds for viral replication. Through viral proteins HIV-1 hijacks host plasma membrane receptors such as Fas, TNFRs and DR4/DR5, which results in immune evasion and apoptosis both in infected and uninfected bystander cells. These events are hallmark in HIV-1 pathogenesis that leads towards AIDS. The interplay between HIV-1 and plasma membrane signaling has much to offer in terms of viral fitness and pathogenicity, and a better understanding of this interplay may lead to development of new therapeutic approaches. This article is part of a Special Issue entitled: Viral Membrane Proteins - Channels for Cellular Networking.

Three-dimensional culture of human breast epithelial cells: the how and the why

Organs are made of the organized assembly of different cell types that contribute to the architecture necessary for functional differentiation. In those with exocrine function, such as the breast, cell-cell and cell-extracellular matrix (ECM) interactions establish mechanistic constraints and a complex biochemical signaling network essential for differentiation and homeostasis of the glandular epithelium. Such knowledge has been elegantly acquired for the mammary gland by placing epithelial cells under three-dimensional (3D) culture conditions.Three-dimensional cell culture aims at recapitulating normal and pathological tissue architectures, hence providing physiologically relevant models to study normal development and disease. The specific architecture of the breast epithelium consists of glandular structures (acini) connected to a branched ductal system. A single layer of basoapically polarized luminal cells delineates ductal or acinar lumena at the apical pole. Luminal cells make contact with myoepithelial cells and, in certain areas at the basal pole, also with basement membrane (BM) components. In this chapter, we describe how this exquisite organization as well as stages of disorganization pertaining to cancer progression can be reproduced in 3D cultures. Advantages and limitations of different culture settings are discussed. Technical designs for induction of phenotypic modulations, biochemical analyses, and state-of-the-art imaging are presented. We also explain how signaling is regulated differently in 3D cultures compared to traditional two-dimensional (2D) cultures. We believe that using 3D cultures is an indispensable method to unravel the intricacies of human mammary functions and would best serve the fight against breast cancer.

Tissue polarity-dependent control of mammary epithelial homeostasis and cancer development: an epigenetic perspective

The basoapical organization of monolayered epithelia is defined by the presence of hemidesmosomes at the basal cellular pole, where the cell makes contacts with the basement membrane, and tight junctions at the opposite apical pole. In the mammary gland, tight junctions seal cell-cell contacts against the lumen and separate the apical and basolateral cell membranes. This separation is critical to organize intracellular signaling pathways and the cytoskeleton. The study of the impact of the highly organized apical pole, and notably apical polarity regulators (Crb complex, Par complex, and Scrib, Dlg, Lgl proteins) and tight junction proteins on cell phenotype and gene expression has revealed an intricate relationship between apical polarity and the cell nucleus. The goal of this review is to highlight the role of the apical pole of the tissue polarity axis in the epigenetic control of tissue phenotype. The organization of the apical pole and its importance in mammary homeostasis and tumorigenesis will be emphasized before presenting how apical polarity proteins impact gene expression indirectly, by influencing signal transduction and the location of transcription regulators, and directly, by participating in chromatin-associated complexes. The relationship between apical polarity and cell nucleus organizations might explain how apical polarity proteins could switch from nuclear repressors to nuclear promoters of cancerous behavior following alterations in the apical pole. The impact of apical polarity proteins on epigenetic mechanisms of gene expression will be discussed in light of increased evidence supporting a role for apical polarity in the fate of breast neoplasms.

Contributions of extracellular matrix signaling and tissue architecture to nuclear mechanisms and spatial organization of gene expression control

Post-translational modification of histones, ATP-dependent chromatin remodeling, and DNA methylation are interconnected nuclear mechanisms that ultimately lead to the changes in chromatin structure necessary to carry out epigenetic gene expression control. Tissue differentiation is characterized by a specific gene expression profile in association with the acquisition of a defined tissue architecture and function. Elements critical for tissue differentiation, like extracellular stimuli, adhesion and cell shape properties, and transcription factors all contribute to the modulation of gene expression and thus, are likely to impinge on the nuclear mechanisms of epigenetic gene expression control. In this review, we analyze how these elements modify chromatin structure in a hierarchical manner by acting on the nuclear machinery. We discuss how mechanotransduction via the structural continuum of the cell and biochemical signaling to the cell nucleus integrate to provide a comprehensive control of gene expression. The role of nuclear organization in this control is highlighted, with a presentation of differentiation-induced nuclear structure and the concept of nuclear organization as a modulator of the response to incoming signals.

Nucleocytoplasmic trafficking and transcription effects of huntingtin in Huntington's disease

There are nine genetic neurodegenerative diseases caused by a similar genetic defect, a CAG DNA triplet-repeat expansion in the disease gene's open reading frame resulting in a polyglutamine expansion in the disease proteins. Despite the commonality of polyglutamine expansion, each of the polyglutamine diseases manifest as unique diseases, with some similarities, but important differences. These differences suggest that the context of the polyglutamine expansion is important to the mechanism of pathology of the disease proteins. Therefore, it is becoming increasingly paramount to understand the normal functions of these polyglutamine disease proteins, which include huntingtin, the polyglutamine-expanded protein in Huntington's disease (HD). Transcriptional dysregulation is seen in HD. Here we discuss the role of normal huntingtin in transcriptional regulation and misregulation in Huntington's disease in relation to potentially analogous model systems, and to other polyglutamine disease proteins. Huntingtin has functional roles in both the cytoplasm and the nucleus. One commonality of activity of polyglutamine disease proteins is at the level of protein dynamics and ability to import and export to and from the nucleus. Knowing the temporal location of huntingtin protein in response to signaling and neuronal communication could lead to valuable insights into an important trigger of HD pathology.

Colocalization of c-Src (pp60src) and bone morphogenetic protein 2/4 expression during mandibular distraction osteogenesis: in vivo evidence of their role within an integrin-mediated mechanotransduction pathway

Craniofacial distraction osteogenesis (DO) is an evolving reconstructive technique with expanding applications for the treatment of bony deficiencies of the facial skeleton. Mechanical force has been known to play a fundamental role in modulating sustained osteogenic response and therefore is believed to function as a critical regulator of DO. We hypothesize that key clustering components of an integrin-mediated signaling pathway, including c-Src (pp60), are necessary for mediating the response to mechanical force. The specific aim of this study is to demonstrate up-regulation of a key focal adhesion molecule, c-Src, selectively in new bone formation subject to the mechanical forces of distraction and to demonstrate a lack of that same up-regulation in new bone formation associated with simple fracture healing. An additional specific aim is to demonstrate colocalization of c-Src expression and bone morphogenetic protein (BMP 2/4) expression during mandibular DO. Using a rat model of mandibular DO, c-Src and BMP 2/4 expression were evaluated in critical size defects, subcritical size defects, and mandibles undergoing gradual distraction. Osseous regeneration was observed in the course of gradual distraction; this process was associated with increased expression of c-Src. Furthermore, the presence of BMP 2/4 closely approximated c-Src expression spatially and temporally, suggesting a link between cytoplasmic focal adhesion activation and the resultant nuclear regulation of osteogenic protein expression. In significant contradistinction, minimal c-Src expression was found in the subcritical-sized defects where the fractures healed secondarily but where no gradual distraction was performed. Instead, the new bone formation inherent in the secondarily healed subcritical-sized defects demonstrated expected BMP 2/4 expression but was devoid of an up-regulation of c-Src. Finally, as expected, minimal expression of both c-Src and BMP was found in fibrous nonunion specimens. C-src expression was observed during gradual distraction; furthermore, minimal c-Src expression was visualized during subacute and critical-size defect fracture healing. C-Src expression also closely approximated BMP expression during DO. These findings that c-Src expression is found primarily only during conditions of cyclic distraction forces strongly implicates that mechanical force during gradual distraction is associated with c-Src expression. These results provide in vivo support for previous in vitro evidence that mechanical force profoundly influences osseous regeneration during distraction osteogenesis by means of a c-Src dependent mechanotransduction pathway, resulting in increased expression of osteogenic proteins, including BMP 2/4.

Structural and functional roles of desmin in mouse skeletal muscle during passive deformation

Mechanical interactions between desmin and Z-disks, costameres, and nuclei were measured during passive deformation of single muscle cells. Image processing and continuum kinematics were used to quantify the structural connectivity among these structures. Analysis of both wild-type and desmin-null fibers revealed that the costamere protein talin colocalized with the Z-disk protein alpha-actinin, even at very high strains and stresses. These data indicate that desmin is not essential for mechanical coupling of the costamere complex and the sarcomere lattice. Within the sarcomere lattice, significant differences in myofibrillar connectivity were revealed between passively deformed wild-type and desmin-null fibers. Connectivity in wild-type fibers was significantly greater compared to desmin-null fibers, demonstrating a significant functional connection between myofibrils that requires desmin. Passive mechanical analysis revealed that desmin may be partially responsible for regulating fiber volume, and consequently, fiber mechanical properties. Kinematic analysis of alpha-actinin strain fields revealed that knockout fibers transmitted less shear strain compared to wild-type fibers and experienced a slight increase in fiber volume. Finally, linkage of desmin intermediate filaments to muscle nuclei was strongly suggested based on extensive loss of nuclei positioning in the absence of desmin during passive fiber loading.

Vacuolar H+-ATPase binding to microfilaments: regulation in response to phosphatidylinositol 3-kinase activity and detailed characterization of the actin-binding site in subunit B

Vacuolar H(+)-ATPase (V-ATPase) binds microfilaments, and that interaction may be mediated by an actin binding domain in subunit B of the enzyme. To test for possible physiologic functions of the actin binding activity of V-ATPase, early responses of resorbing osteoclasts to inhibition of phosphatidylinositol 3-kinase activity by wortmannin and LY294002 were examined. Rapid co-localization between V-ATPase and F-actin was demonstrated by immunocytochemistry, and corresponding association between V-ATPase and F-actin in immunoprecipitations and pelleting assays was detected. This response was reversed as osteoclasts recovered resorptive activity after inhibitors were removed. By expressing and characterizing fusion proteins containing segments of the actin-binding amino-terminal regions of the B subunits of V-ATPase, we mapped the actin-binding site to a 44-amino acid domain. An 11-amino acid segment with a sequence similar to the actin-binding site of human profilin I was detected within this region. 13-Mers containing these profilin-like segments bound actin in fluorescent anisotropy studies and competed with profilin for binding to actin. Using site-directed mutagenesis, the 11-amino acid profilin-like actin-binding motifs (amino acids 49-59 of B1 and 55-65 of B2) were replaced with an 11-amino acid spacer with a sequence based on the homologous sequence from subunit B of Pyrococcus horikoshii, an organism that lacks an actin cytoskeleton. These substitutions eliminated the actin-binding activity of the B subunit fusion proteins. In summary, binding between V-ATPase and F-actin in osteoclasts occurs in response to blocking phosphatidylinositol 3-kinase activity. This response was fully reversible. The actin binding activities of the B subunits of V-ATPase required 11-amino acid actin-binding motifs that are similar in sequence to the actin-binding site of mammalian profilin I.

Type III intermediate filament proteins interact with four-way junction DNA and facilitate its cleavage by the junction-resolving enzyme T7 endonuclease I

The isolation from proliferating mouse and human embryo fibroblasts of SDS-stable crosslinkage products of vimentin with DNA fragments containing inverted repeats capable of cruciform formation under superhelical stress and the competitive effect of a synthetic Holliday junction on the binding of cytoplasmic intermediate filament (cIF) proteins to supercoiled DNA prompted a detailed investigation of the proteins' capacity to associate with four-way junction DNA and to influence its processing by junction-resolving endonucleases. Electrophoretic mobility shift analysis of reaction products obtained from vimentin and Holliday junctions under varying ionic conditions revealed efficient complex formation of the filament protein not only with the unstacked, square-planar configuration of the junctions but also with their coaxially stacked X-conformation. Glial fibrillary acidic protein (GFAP) was less efficient and desmin virtually inactive in complex formation. Electron microscopy showed binding of vimentin tetramers or octamers almost exclusively to the branchpoint of the Holliday junctions under physiological ionic conditions. Even at several hundredfold molar excess, sequence-related single- and double-stranded DNAs were unable to chase Holliday junctions from their complexes with vimentin. Vimentin also stimulated bacteriophage T7 endonuclease I in introducing single-strand cuts diametrically across the branchpoint and thus in the resolution of the Holliday junctions. This effect is very likely due to vimentin-induced structural distortion of the branchpoint, as suggested by the results of hydroxyl radical footprinting of Holliday junctions in the absence and the presence of vimentin. Moreover, vimentin, and to a lesser extent GFAP and desmin, interacted with the cruciform structures of inverted repeats inserted into a supercoiled vector plasmid, thereby changing their configuration via branch migration and sensibilizing them to processing by T7 endonuclease I. This refers to both plasmid relaxation caused by unilateral scission and, particularly, linearization via bilateral scission at primary and cIF protein-induced secondary cruciform branchpoints that were identified by T7 endonuclease I footprinting. cIF proteins share these activities with a variety of other architectural proteins interacting with and structurally modulating four-way DNA junctions. In view of the known and hypothetical functions of four-way DNA junctions and associated protein factors in DNA metabolism, cIF proteins as complementary nuclear matrix proteins may play important roles in such nuclear matrix-associated processes as DNA replication, recombination, repair, and transcription, with special emphasis on both the preservation and evolution of the genome.

Hic-5 communicates between focal adhesions and the nucleus through oxidant-sensitive nuclear export signal

hic-5 was originally isolated as an H(2)O(2)-inducible cDNA clone whose product was normally found at focal adhesions. In this study, we found that Hic-5 accumulated in the nucleus in response to oxidants such as H(2)O(2). Other focal adhesion proteins including paxillin, the most homologous to Hic-5, remained in the cytoplasm. Mutation analyses revealed that the C- and N-terminal halves of Hic-5 contributed to its nuclear localization in a positive and negative manner, respectively. After the finding that leptomycin B (LMB), an inhibitor of nuclear export signal (NES), caused Hic-5 to be retained in the nucleus, Hic-5 was demonstrated to harbor NES in the N-terminal, which was sensitive to oxidants, thereby regulating the nuclear accumulation of Hic-5. NES consisted of a leucine-rich stretch and two cysteines with a limited similarity to Yap/Pap-type NES. In the nucleus, Hic-5 was suggested to participate in the gene expression of c-fos. Using dominant negative mutants, we found that Hic-5 was actually involved in endogenous c-fos gene expression upon H(2)O(2) treatment. Hic-5 was thus proposed as a focal adhesion protein with the novel aspect of shuttling between focal adhesions and the nucleus through an oxidant-sensitive NES, mediating the redox signaling directly to the nucleus.

A model for mechanotransduction in bone cells: the load-bearing mechanosomes

The skeleton's response to mechanical force, or load, has significance to space travel, the treatment of osteoporosis, and orthodontic appliances. How bone senses and processes load remains largely unknown. The cellular basis of mechanotransduction, however, likely involves the integration of diffusion-controlled signaling pathways with a solid-state scaffold linking the cell membrane to the genes. Here, we integrate various concepts from models of connective membrane skeleton proteins, cellular tensegrity, and nuclear matrix architectural transcription factors, to describe how a load-induced deformation of bone activates a change in the skeletal genetic program. We propose that mechanical information is relayed from the bone to the gene in part by a succession of deformations, changes in conformations, and translocations. The load-induced deformation of bone is converted into the deformation of the sensor cell membrane. This, in turn, drives conformational changes in membrane proteins of which some are linked to a solid-state signaling scaffold that releases protein complexes capable of carrying mechanical information, "mechanosomes", into the nucleus. These mechanosomes translate this information into changes in the geometry of the 5' regulatory region of target gene DNA altering gene activity; bending bone ultimately bends genes. We identify specific candidate proteins fitting the profile of load-signaling mechanosomes.

Vascular smooth muscle cell phenotypic modulation in culture is associated with reorganisation of contractile and cytoskeletal proteins

Smooth muscle cells (SMC) exhibit a functional plasticity, modulating from the mature phenotype in which the primary function is contraction, to a less differentiated state with increased capacities for motility, protein synthesis, and proliferation. The present study determined, using Western analysis, double-label immunofluorescence and confocal microscopy, whether changes in phenotypic expression of rabbit aortic SMC in culture could be correlated with alterations in expression and distribution of structural proteins. "Contractile" state SMC (days 1 and 3 of primary culture) showed distinct sorting of proteins into subcellular domains, consistent with the theory that the SMC structural machinery is compartmentalised within the cell. Proteins specialised for contraction (alpha-SM actin, SM-MHC, and calponin) were highly expressed in these cells and concentrated in the upper central region of the cell. Vimentin was confined to the body of the cell, providing support for the contractile apparatus but not co-localising with it. In line with its role in cell attachment and motility, beta-NM actin was localised to the cell periphery and basal cortex. The dense body protein alpha-actinin was concentrated at the cell periphery, possibly stabilising both contractile and motile apparatus. Vinculin-containing focal adhesions were well developed, indicating the cells' strong adhesion to substrate. In "synthetic" state SMC (passages 2-3 of culture), there was decreased expression of contractile and adhesion (vinculin) proteins with a concomitant increase in cytoskeletal proteins (beta-non-muscle [NM] actin and vimentin). These quantitative changes in structural proteins were associated with dramatic changes in their distribution. The distinct compartmentalisation of structural proteins observed in "contractile" state SMC was no longer obvious, with proteins more evenly distributed throughout the cytoplasm to accommodate altered cell function. Thus, SMC phenotypic modulation involves not only quantitative changes in contractile and cytoskeletal proteins, but also reorganisation of these proteins. Since the cytoskeleton acts as a spatial regulator of intracellular signalling, reorganisation of the cytoskeleton may lead to realignment of signalling molecules, which, in turn, may mediate the changes in function associated with SMC phenotypic modulation.