Cell adhesion molecule regulation of nucleocytoplasmic trafficking

Bio-Fabrication of Human Amniotic Membrane Zinc Oxide Nanoparticles and the Wet/Dry HAM Dressing Membrane for Wound Healing

The preparation of unique wet and dry wound dressing products derived from unprocessed human amniotic membrane (UP-HAM) is described. The UP-HAM was decellularized, and the constituent proteins were cross-linked and stabilized before being trimmed and packed in sterile Nucril-coated laminated aluminium foil pouches with isopropyl alcohol to manufacture processed wet human amniotic membrane (PW-HAM). The dry type of PD-HAM was prepared by decellularizing the membrane, UV irradiating it, lyophilizing/freeze-drying it, sterilizing it, and storing it at room temperature. The UP-HAM consists of a translucent yellowish mass of flexible membranes with an average thickness of 42 μm. PW-HAM wound dressings that had been processed, decellularized, and dehydrated had a thinner average thickness of 30 μm and lacked nuclear-cellular structures. Following successful decellularization, discrete bundle of fibrous components in the stromal spongy layers, microvilli and reticular ridges were still evident on the surface of the processed HAM, possibly representing the location of the cells that had been removed by the decellularization process. Both wet and dry HAM wound dressings are durable, portable, have a shelf life of 3–5 years, and are available all year. A slice of HAM dressing costs 1.0 US$/cm². Automation and large-scale HAM membrane preparation, as well as storage and transportation of the dressings, can all help to establish advanced technologies, improve the efficiency of membrane production, and reduce costs. Successful treatment of wounds to the cornea of the eye was achieved with the application of the HAM wound dressings. The HAM protein analysis revealed 360 μg proteins per gram of tissue, divided into three main fractions with MWs of 100 kDa, 70 kDa, and 14 kDa, as well as seven minor proteins, with the 14 kDa protein displaying antibacterial properties against human pathogenic bacteria. A wide range of antibacterial activity was observed after treatment with 75 μg/ml zinc oxide nanoparticles derived from human amniotic membrane proteins (HAMP-ZnO NP), including dose-dependent biofilm inhibition and inhibition of Gram-positive (S. aureus, S. mutans, E. faecalis, and L. fusiformis) and Gram-negative bacteria (S. sonnei, P. aeruginosa, P. vulgaris, and C. freundii).

Structural and Functional Equivalency Between Lyopreserved and Cryopreserved Chorions with Viable Cells

Objective: Clinical studies have demonstrated that the use of cryopreserved amnion or trophoblast (TR)-free chorion, containing viable cells, in the treatment of chronic wounds results in high rate of wound closure. Recently, a new lyopreservation method has been developed for preservation of amnion that also retains the endogenous viable cells. The objective of this study was to use this method for lyopreservation of TR-free chorionic membrane (viable lyopreserved chorionic membrane [VLCM]) and compare it with the viable cryopreserved chorionic membrane (VCCM). A second objective was to investigate the immunogenicity of chorion, an important question that has not been fully addressed.

Approach: Chorion immunogenicity was tested in vitro in a mixed lymphocyte reaction and lipopolysaccharide (LPS) challenge assay, and in vivo in a mouse subcutaneous pocket implantation model. VLCM tissue structure was assessed histologically, growth factor content by multiplex assay, and cell viability by LIVE/DEAD cell fluorescent staining. Inhibition of tumor necrosis factor α secretion by LPS-activated THP-1 cells and endothelial cell tubule formation assays were performed to evaluate the anti-inflammatory and proangiogenic properties, respectively. An in vivo rabbit abdominal adhesion model was used to evaluate the antifibrotic properties.

Results: Chorionic membrane without trophoblast (CM) was shown to be nonimmunogenic. Tissue architecture, growth factors, and cell viability of fresh CM were maintained in VLCM and VCCM. In vitro studies showed that anti-inflammatory and angiogenic properties were retained in VLCM. Furthermore, VLCM prevents formation of postsurgical adhesions in a rabbit abdominal surgical adhesion model.

Innovation: Characterization of structural and functional properties of VLCM is reported for the first time.

Conclusion: Similar to VCCM, VLCM retains native components of fresh CM, including collagen-rich extracellular matrix, growth factors, and viable cells. In vitro and in vivo models demonstrate that VLCM is anti-inflammatory, proangiogenic and antifibrotic. Results of this study support the structural and functional equivalency between VLCM and VCCM.

Phenotypic stability of the human MG-63 osteoblastic cell line at different passages

One of the most popular cell lines in osteogenesis studies is the human osteoblastic line MG-63. For cell biological investigation, it is important that the cells remain stable in their phenotype over several passages in cell culture. MG-63 cells can be used to provide fundamental insights into cell--material interaction. The aim of this study is to present a systematic characterization of the physiological behavior of MG-63 cells in the range of passages 5-30. Significant cell physiology processes during the first 24 h, including cell morphology, availability of adhesion receptors, cell cycle phases, as well as the expression of the signaling proteins Akt, GSK3a/b, IkB-α, ERK1/2, p38-MAPK, and intracellular calcium ion mobilization, remained stable over the entire range of passages P5-P30. Due to these stable characteristics in a wide range of cell culture passages, MG-63 cells can be considered as a suitable in vitro model to analyze the biocompatibility and biofunctionality of implant materials.

Deconstructing signaling in three dimensions

Cells in vivo exist within the context of a multicellular tissue, where their behavior is governed by homo- and heterotypic cell-cell interactions, the material properties of the extracellular matrix, and the distribution of various soluble and physical factors. Most methods currently used to study and manipulate cellular behavior in vitro, however, sacrifice physiological relevance for experimental expediency. The fallacy of such approaches has been highlighted by the recent development and application of three-dimensional culture models to cell biology, which has revealed striking phenotypic differences in cell survival, migration, and differentiation in genetically identical cells simply by varying culture conditions. These perplexing findings beg the question of what constitutes a three-dimensional culture and why cells behave so differently in two- and three-dimensional culture formats. In the following review, we dissect the fundamental differences between two- and three-dimensional culture conditions. We begin by establishing a basic definition of what "three dimensions" means at different biological scales and discuss how dimensionality influences cell signaling across different length scales. We identify which three-dimensional features most potently influence intracellular signaling and distinguish between conserved biological principles that are maintained across culture conditions and cellular behaviors that are sensitive to microenvironmental context. Finally, we highlight state-of-the-art molecular tools amenable to the study of signaling in three dimensions under conditions that facilitate deconstruction of signaling in a more physiologically relevant manner.

The clinical applications of human amnion in plastic surgery

Since the early 1900s, human amnion has been applied to a wide variety of clinical scenarios including burns, chronic ulcers, dural defects, intra-abdominal adhesions, peritoneal reconstruction, genital reconstruction, hip arthroplasty, tendon repair, nerve repair, microvascular reconstruction, corneal repair, intra-oral reconstruction and reconstruction of the nasal lining and tympanic membrane. Amnion epithelial and mesenchymal cells have been shown to contain a variety of regulatory mediators that result in the promotion of cellular proliferation, differentiation and epithelialisation and the inhibition of fibrosis, immune rejection, inflammation and bacterial invasion. The full repertoire of biological factors that these cells synthesise, store and release and the mechanisms by which these factors exert their beneficial effects are only now being fully appreciated. Although many commercially available biological and synthetic alternatives to amnion exist, ethical, religious, and financial constraints may limit the widespread utilisation of these products. Amnion is widely available, economical and is easy to manipulate, process and store. Although many clinical applications are of historical interest only, amnion offers an alternative source of multi-potent or pluripotent stem cells and therefore may yet have a great deal to offer the plastic surgery and regenerative medicine community. It is the purpose of this article to review the clinical applications of human amnion relevant to plastic surgery.

αν and β1 Integrins mediate Aβ-induced neurotoxicity in hippocampal neurons via the FAK signaling pathway

αν and β1 integrins mediate Aβ-induced neurotoxicity in primary hippocampal neurons. We treated hippocampal neurons with 2.5 µg/mL 17E6 and 5 µg/mL ab58524, which are specific αν and β1 integrin antagonists, respectively, for 42 h prior to 10 µM Aβ treatment. Next, we employed small interfering RNA (siRNA) to silence focal adhesion kinase (FAK), a downstream target gene of integrins. The siRNAs were designed with a target sequence, an MOI of 10 and the addition of 5 µg/mL polybrene. Under these conditions, the neurons were transfected and the apoptosis of different cell types was detected. Moreover, we used real-time PCR and Western blotting analyses to detect the expression of FAK and ρFAK genes in different cell types and investigated the underlying mechanism and signal pathway by which αν and β1 integrins mediate Aβ-induced neurotoxicity in hippocampal neurons. An MTT assay showed that both 17E6 and ab58524 significantly increased cell viability compared with the Aβ-treated neurons (P<0.01 and P<0.05, respectively). However, this protective effect was markedly attenuated after transfection with silencing FAK (siFAK). Moreover, TUNEL immunostaining and flow cytometry indicated that both 17E6 and ab58524 significantly protected hippocampal neurons against apoptosis induced by Aβ (P<0.05) compared with the Aβ-treated cells. However, this protective effect was reversed with siFAK treatment. Both the gene and protein expression of FAK increased after Aβ treatment. Interestingly, as the gene and protein levels of FAK decreased, the ρFAK protein expression markedly increased. Furthermore, both the gene and protein expression of FAK and ρFAK were significantly diminished. Thus, we concluded that both αν and β1 integrins interfered with Aβ-induced neurotoxicity in hippocampal neurons and that this mechanism partially contributes to the activation of the Integrin-FAK signaling pathway.

FLT3-driven redox-modulation of Ezrin regulates leukaemic cell migration

The concept of reactive oxygen species (ROS) being produced via the activation of specific oncogenes provides a basis for generating genomic instability and pro-survival signalling in tumour cells. The purpose of this study was to identify downstream targets of NADPH oxidase (Nox)-derived ROS signalling in acute myeloid leukaemia cells, by performing a proteomic analysis utilizing two-dimensional phosphotyrosine immunoblotting. The majority of the targets identified were cytoskeletal-associated proteins including Ezrin, a known regulator of the cytoskeleton, which was examined further. The study demonstrated that inhibition of Nox enzymes, using diphenyleneiodonium chloride in the acute myeloid leukaemia cell line MOLM-13, resulted in a decrease in Ezrin tyrosine phosphorylation and also triggered a shift in Ezrin sub-cellular localization as detected by immunofluorescence. The change in Ezrin localization coincided with altered cell morphology, observed using scanning electron microscopy and a decreased ability to migrate through a polycarbonate transwell membrane. Similar effects were observed upon inhibition of the oncogenic receptor tyrosine kinase FLT3 using the staurosporine derivate PKC412, implicating a role for FLT3 as an upstream regulator of Ezrin. Our results indicate that FLT3 drives production of ROS by Nox, which stimulates changes in Ezrin tyrosine phosphorylation and localization via redox regulation of Src. Furthermore, inhibition of FLT3 signalling leads to alterations in MOLM-13 cell morphology and has a significant influence on cell motility.

Dentin phosphoprotein (DPP) activates integrin-mediated anchorage-dependent signals in undifferentiated mesenchymal cells

Dentin phosphoprotein (DPP), a major noncollagenous protein of the dentin matrix, is a highly acidic protein that binds Ca(2+) avidly and is thus linked to matrix mineralization. Here, we demonstrate that the RGD domain in DPP can bind to integrins on the cell surface of undifferentiated mesenchymal stem cells and pulp cells. This coupling generates intracellular signals that are channeled along cytoskeletal filaments and activate the non-receptor tyrosine kinase focal adhesion kinase, which plays a key role in signaling at sites of cellular adhesion. The putative focal adhesion kinase autophosphorylation site Tyr(397) is phosphorylated during focal adhesion assembly induced by DPP on the substrate. We further demonstrate that these intracellular signals propagate through the cytoplasm and activate anchorage-dependent ERK signaling. Activated ERK translocates to the nucleus and phosphorylates the transcription factor ELK-1, which in turn coordinates the expression of downstream target genes such as DMP1 and dentin sialoprotein (DSP). These studies suggest a novel paradigm demonstrating that extracellular DPP can induce intracellular signaling that can be propagated to the nucleus and thus alter gene activities.

Bio-functionalized star PEG-coated PVDF surfaces for cytocompatibility-improved implant components

Unmodified and GRGDS peptide-modified six arm PEG star based hydrogels (Star PEG) have been applied as a multifunctional, easy to handle coating system for textile polyvinylidene fluoride (PVDF) structures, which prevent unspecific protein and cell adsorption and control-specific cell adhesion. The reactive isocyanate-terminated Star PEG has been successfully applied to ammonia-plasma treated two- and three-dimensional PVDF surfaces. Easy modification of the surface hydrogel by mixing in of GRGDS peptide during the coating step or subsequent coupling of GRGDS was determined by TOF-SIMS. Unmodified and GRGDS-functionalized hydrogel surfaces show distinct protein repellency, as demonstrated by fluorescence microscopy after incubation with fluorescent labeled proteins and Surface MALDI-TOF-Mass Spectroscopy. Cell culture experiments with primary human dermal fibroblasts, primary fetal rat fibroblasts, and human osteoblasts on GRGDS and/or KRSR Star PEG-modified two- and three-dimensional substrates show advancement in cell adhesion and proliferation compared with untreated PVDF surfaces, whereas pure star PEG-coated surfaces show no cell adhesion. The combination of protein and cell repellent properties with specific biofunctionality and easy application of the coatings will enable their application for 3D-scaffolds.

Paxillin and hydrogen peroxide-inducible clone 5 expression and distribution in control and Alzheimer disease hippocampi

Hydrogen peroxide-inducible clone 5 (Hic-5) and paxillin are members of the Group III LIM domain protein family that localize to both cell nuclei and focal adhesions and link integrin-mediated signaling to the actin cytoskeleton. Prior in vitro studies have implicated paxillin in beta-amyloid-induced cell death, but little is known about the expression and function of Hic-5 and paxillin in the brain. We performed a blinded retrospective cross-sectional study of Hic-5 and paxillin expression in the hippocampi of Alzheimer disease (AD) and control subjects using immunohistochemistry and laser scanning confocal microscopy. The analysis included assessment of the expression of phosphorylated isoforms of paxillin that reflect activation of distinct signaling pathways. We found changes in the subcellular distribution of Hic-5, paxillin, and specific phosphorylated isoforms of paxillin in the AD brains. The Hic-5 and phosphorylated isoforms of paxillin colocalized with neurofibrillary tangles. Paxillin was predominantly found in reactive astrocytes in the AD hippocampi, and activated paxillin was also detected in granulovacuolar degeneration bodies in AD. These data indicate that these important scaffolding proteins that link various intracellular signaling pathways to the extracellular matrix are modified and have altered subcellular distribution in AD.

Focal adhesions regulate Abeta signaling and cell death in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder that results from a loss of synaptic transmission and ultimately cell death. The presenting pathology of AD includes neuritic plaques composed of beta-amyloid peptide (Abeta) and neurofibrillary tangles composed of hyperphosphorylated tau, with neuronal loss in specific brain regions. However, the mechanisms that induce neuronal cell loss remain elusive. Focal adhesion (FA) proteins assemble into intracellular complexes involved in integrin-mediated communication between the extracellular matrix and the actin cytoskeleton, regulating many cell physiological processes including the cell cycle. Interestingly, recent studies report that integrins bind to Abeta fibrils, mediating Abeta signal transmission from extracellular sites of Abeta deposits into the cell and ultimately to the nucleus. In this review, we will discuss the Abeta induced integrin/FA signaling pathways that mediate cell cycle activation and cell death.

Bio-mimetic composite matrix that promotes endothelial cell growth for modification of biomaterial surface

The incidence of thrombogenesis and occlusion of cardiovascular implants is likely to be reduced by endothelial cell (EC) growth promotion on biomaterials used for device fabrication. However, proper signaling between the matrix proteins deposited on the device surface and the cells grown on it is a prime requirement for growth and function. It was demonstrated earlier that a composition of matrix proteins that include fibrin, fibronectin, gelatin, and growth factors maintain a steady proliferation potential and prolong the survival of endothelial cells in vitro. In this study, assessment of the same matrix to prevent EC from dedifferentiation during in vitro culture and to promote endothelialization of biomaterials used for fabrication of cardiovascular implants is carried out. Up/down regulation of m-RNA expression for a prothrombotic molecule-plasminogen activator inhibitor (PAI), and two antithrombotic molecules- nitric oxide synthetase (eNOS) and tissue plasminogen activator (t-PA) are chosen as the indicators of cell dedifferentiation during cell culture and passaging. Immunostaining for vinculin and actin demonstrated that composite coating on biomaterials improves focal adhesion and cytoskeletal organization that increases the quality of EC grown on it. EC proliferation, measured by (3)H-thymidine uptake, on all bare materials was poor and high incidence of cell apoptosis was noticed within 72 h in culture, whereas once coated with composite all materials showed good proliferation and survival. The results suggest that the designed composition of biomimetic adhesive proteins and growth factors is suitable for EC growth, survival, and functional integrity, thus making it suitable for cardiovascular tissue engineering that requires in vitro EC culture.

Adhesion-dependent redistribution of MAP kinase and MEK promotes muscarinic receptor-mediated signaling to the nucleus

The mitogen-activated protein kinases (MAPKs) are activated by extracellular signals, and translocate to the nucleus where they modulate transcription. Integrin-mediated cell adhesion to extracellular matrix (ECM) proteins is required for efficient transmission of MAPK-based signals initiated by growth factors. However, the modulation of G protein-coupled receptor (GPCR) signaling by adhesion is less well understood. In the present study, we assessed the impact of cell adhesion on MAPK activation by muscarinic M3 receptors. The muscarinic agonist carbachol more efficiently promoted stress fiber formation and tyrosine phosphorylation of focal adhesion-associated proteins in M3 receptor-expressing cells adherent to fibronectin or collagen type I, as compared to polylysine. Overall MAPK activation was robust in cells adherent to all three substrata. However, total levels of MAPK and mitogen-activated protein kinase kinase (MEK) in the nucleus were significantly greater in cells adherent to ECM proteins for 2.5 h, and levels of activated MAPK and MEK in the nuclei of these cells were higher following carbachol stimulation, relative to levels in cells adherent to polylysine. MEK inhibitors did not prevent adhesion-dependent translocation of MAPK and MEK to the nucleus, and increased nuclear phospho-MEK levels in carbachol-stimulated cells. The results suggest that adhesion of cells to ECM triggers the redistribution of MAPK and MEK to the nucleus, possibly as a result of the cytoskeletal rearrangements that accompany cell spreading. This may represent a mechanism for priming the nucleus with MEK and MAPK, leading to more rapid and pronounced increases in intranuclear phospho-MAPK upon GPCR stimulation.

Cell cycle-dependent nucleocytoplasmic shuttling of the neurofibromatosis 2 tumour suppressor merlin

The neurofibromatosis 2 tumour suppressor merlin/schwannomin is structurally related to the ezrin-radixin-moesin family of proteins, which anchor actin cytoskeleton to specific membrane proteins and participate in cell signalling. Merlin inhibits cell growth with a yet unknown mechanism. As most tumour suppressors are linked to cell cycle control, we investigated merlin's behaviour during cell cycle. In glioma and osteosarcoma cells, endogenous merlin was targeted to the nucleus in a cell cycle-specific manner. Merlin accumulated perinuclearly at the G2/M phase, and shifted to the nucleus at early G1. During mitosis, merlin localized to mitotic spindles and at the contractile ring. Nuclear merlin was strongly reduced in confluent cells. Blocking of the CRM1/exportin nuclear export pathway led to accumulation of merlin in the nucleus. Activation of the p21-activated kinase or protein kinase A, which result in phosphorylation of merlin, did not affect its nuclear localization. Merlin regulates the activity of extracellular signal-regulated kinase 2 (ERK2) and nuclear localization of both proteins was induced by cell adhesion. Unlike ERK2, nuclear localization of merlin was not, however, dependent on intact actin cytoskeleton. These results link merlin to events related to cell cycle control and may help to resolve its tumour suppressor function.

Potential oncogenic action of tenascin-C in tumorigenesis

The prominent expression of tenascin-C in the stroma of most solid tumors, first observed in the mid 1980s, implicates tenascin-C in tumorigenesis. This is also supported by in vitro experiments that demonstrate the capacity of tenascin-C to stimulate tumor growth by various mechanisms including promotion of proliferation, escaping immuno-surveillance and positively influencing angiogenesis. However, tumorigenesis in tenascin-C knock-out mice is not significantly different from that observed in control animals. Perhaps this is not unexpected if one considers that tenascin-C may act as an oncogene. The potential role of tenascin-C in tumorigenesis through its oncogenic action on cellular signaling will be discussed in this review, including how tenascin-C mediated tumor cell detachment might affect genome stability.

Growth promoting signaling by tenascin-C [corrected]

Tenascin-C is an adhesion-modulating extracellular matrix molecule that is highly expressed in tumor stroma and stimulates tumor cell proliferation. Adhesion of T98G glioblastoma cells to a fibronectin substratum is inhibited by tenascin-C. To address the mechanism of action, we performed a RNA expression analysis of T89G cells grown in the presence or absence of tenascin-C and found that tenascin-C down-regulates tropomyosin-1. Upon overexpression of tropomyosin-1, cell spreading on a fibronectin/tenascin-C substratum was restored, indicating that tenascin-C destabilizes actin stress fibers through down-regulation of tropomyosin-1. Tenascin-C also increased the expression of the endothelin receptor type A and stimulated the corresponding mitogen-activated protein kinase signaling pathway, which triggers extracellular signal-regulated kinase 1/2 phosphorylation and c-Fos expression. Tenascin-C additionally caused down-regulation of the Wnt inhibitor Dickkopf 1. In consequence, Wnt signaling was enhanced through stabilization of beta-catenin and stimulated the expression of the beta-catenin target Id2. Finally, our in vivo data derived from astrocytoma tissue arrays link increased tenascin-C and Id2 expression with high malignancy. Because increased endothelin and Wnt signaling, as well as reduced tropomyosin-1 expression, are closely linked to transformation and tumorigenesis, we suggest that tenascin-C specifically modulates these signaling pathways to enhance proliferation of glioma cells.

Nuclear ERM (ezrin, radixin, moesin) proteins: regulation by cell density and nuclear import

The highly conserved ERM (ezrin-radixin-moesin) family of proteins function as molecular linkers between the actin cytoskeleton and transmembrane receptors. We now provide unequivocal evidence that full-length endogenous ezrin and moesin also localise to the nucleus in two independent mammalian cell lines. All three ERM family members can localise to the nucleus upon exogenous expression of their GFP-tagged counterparts, suggesting a common family trend. Furthermore, Dmoesin, the Drosophila ERM homologue, is present in the nucleus of an insect cell line and can localise to the nucleus when exogenously expressed in MDCK cells. The nuclear localisation of endogenous ezrin and moesin is regulated by cell density and is resistant to detergent extraction, suggesting tight association with nuclear structures. Furthermore, phosphorylation in the actin-binding domain is not a prerequisite for nuclear localisation. We have identified a specific nuclear localisation sequence, which is conserved and functional in all ERM family members, implying specific regulated nuclear import. Although the precise nuclear function of the ERM proteins is unknown, these data provide further evidence that an increasing number of cytoskeletal components directly link the plasma membrane with nuclear events.

PIAS1-mediated sumoylation of focal adhesion kinase activates its autophosphorylation

Focal adhesion kinase (FAK) is a protein tyrosine kinase enriched in focal adhesions, which plays a critical role in integrin-dependent cell motility and survival. The crucial step in its activation is autophosphorylation on Tyr-397, which promotes the recruitment of several enzymes including Src family kinases and the activation of multiple signaling pathways. We found in a yeast two-hybrid screen that the N-terminal domain of FAK interacted with protein inhibitor of activated STAT1 (PIAS1). This interaction was confirmed and shown to be direct using in vitro assays. PIAS1 was co-immunoprecipitated with FAK from transfected cells and brain extracts. PIAS1 has recently been recognized as a small ubiquitin-like modifier (SUMO) ligase. In the presence of PIAS1 and SUMO-1, FAK was sumoylated in intact cells, whereas PYK2, a closely related enzyme, was not. Sumoylation occurred on Lys-152, a residue conserved in FAK during evolution. Sumoylated FAK, like PIAS1, was recovered predominantly from the nuclear fraction. Sumoylation did not require the catalytic activity or autophosphorylation of FAK. In contrast, sumoylation increased dramatically the ability of FAK to autophosphorylate in intact cells and in immune precipitate kinase assays. Endogenous FAK was sumoylated in the presence of PIAS1 and SUMO-1 independently of cell adhesion, and autophosphorylation of sumoylated FAK was persistently increased in suspended cells. These observations show that sumoylation controls the activity of a protein kinase and suggest that FAK may play a novel role in signaling between the plasma membrane and the nucleus.