Nanofibrillar scaffolds induce preferential activation of Rho GTPases in cerebral cortical astrocytes.

Cerebral cortical astrocyte responses to polyamide nanofibrillar scaffolds versus poly-L-lysine (PLL)-functionalized planar glass, unfunctionalized planar Aclar coverslips, and PLL-functionalized planar Aclar surfaces were investigated by atomic force microscopy and immunocytochemistry. The physical properties of the cell culture environments were evaluated using contact angle and surface roughness measurements and compared. Astrocyte morphological responses, including filopodia, lamellipodia, and stress fiber formation, and stellation were imaged using atomic force microscopy and phalloidin staining for F-actin. Activation of the corresponding Rho GTPase regulators was investigated using immunolabeling with Cdc42, Rac1, and RhoA. Astrocytes cultured on the nanofibrillar scaffolds showed a unique response that included stellation, cell-cell interactions by stellate processes, and evidence of depression of RhoA. The results support the hypothesis that the extracellular environment can trigger preferential activation of members of the Rho GTPase family, with demonstrable morphological consequences for cerebral cortical astrocytes.

ISG15 disrupts cytoskeletal architecture and promotes motility in human breast cancer cells.

The interferon-stimulated gene 15 (ISG15) pathway is highly elevated in breast cancer; however, very little is known about how the ISG15 pathway contributes to breast tumorigenesis. In the current study, using the gene disruption approach, we demonstrate that both ISG15 and UbcH8 (ISG15-specific conjugating enzyme) disrupt F-actin architecture and formation of focal adhesions in ZR-75-1 breast cancer cells. In addition, ISG15 and UbcH8 promote breast cancer cell migration. We also demonstrate that ISG15 inhibits ubiquitin/26S proteasome-mediated turnover of proteins implicated in tumor cell motility, invasion and metastasis. Together, our results suggest that the aberrant activation of the ISG15 pathway confers a motile phenotype to breast cancer cells by disrupting cell architecture and stabilizing proteins involved in cell motility, invasion and metastasis. Because the cellular architecture is conserved and the ISG15 pathway is constitutively activated in tumor cells of different lineages, it is reasonable to assume that our observations in breast cancer must hold true for many other tumors.

Salicylic acid-derived poly(anhydride-ester) electrospun fibers designed for regenerating the peripheral nervous system.

Continuous biomaterial advances and the regenerating potential of the adult human peripheral nervous system offer great promise for restoring full function to innervated tissue following traumatic injury via synthetic nerve guidance conduits (NGCs). To most effectively facilitate nerve regeneration, a tissue engineering scaffold within a conduit must be similar to the linear microenvironment of the healthy nerve. To mimic the native nerve structure, aligned poly(lactic-co-glycolic acid)/bioactive polyanhydride fibrous substrates were fabricated through optimized electrospinning parameters with diameters of 600 ± 200 nm. Scanning electron microscopy images show fibers with a high degree of alignment. Schwann cells and dissociated rat dorsal root ganglia demonstrated elongated and healthy proliferation in a direction parallel to orientated electrospun fibers with significantly longer Schwann cell process length and neurite outgrowth when compared to randomly orientated fibers. Results suggest that an aligned polyanhydride fiber mat holds tremendous promise as a supplement scaffold for the interior of a degradable polymer NGC. Bioactive salicylic acid-based polyanhydride fibers are not limited to nerve regeneration and offer exciting promise for a wide variety of biomedical applications.

Microscale plasma-initiated patterning of electrospun polymer scaffolds.

Microscale plasma-initiated patterning (µPIP) is a novel micropatterning technique used to create biomolecular micropatterns on polymer surfaces. The patterning method uses a polydimethylsiloxane (PDMS) stamp to selectively protect regions of an underlying substrate from oxygen plasma treatment resulting in hydrophobic and hydrophilic regions. Preferential adsorption of the biomolecules onto either the plasma-exposed (hydrophilic) or plasma-protected (hydrophobic) regions leads to the biomolecular micropatterns. In the current work, laminin-1 was applied to an electrospun polyamide nanofibrillar matrix following plasma treatment. Radial glial clones (neural precursors) selectively adhered to these patterned matrices following the contours of proteins on the surface. This work demonstrates that textured surfaces, such as nanofibrillar scaffolds, can be micropatterned to provide external chemical cues for cellular organization.

The extracellular matrix glycoprotein tenascin-C is beneficial for spinal cord regeneration.

Tenascin-C (TNC), a major component of the extracellular matrix, is strongly upregulated after injuries of the central nervous system (CNS) but its role in tissue repair is not understood. Both regeneration promoting and inhibiting roles of TNC have been proposed considering its abilities to both support and restrict neurite outgrowth in vitro. Here, we show that spontaneous recovery of locomotor functions after spinal cord injury is impaired in adult TNC-deficient (TNC(-/-)) mice in comparison to wild-type (TNC(+/+)) mice. The impaired recovery was associated with attenuated excitability of the plantar Hoffmann reflex (H-reflex), reduced glutamatergic input, reduced sprouting of monaminergic axons in the lumbar spinal cord and enhanced post-traumatic degeneration of corticospinal axons. The degeneration of corticospinal axons in TNC(-/-) mice was normalized to TNC(+/+) levels by application of the alternatively spliced TNC fibronectin type III homologous domain D (fnD). Finally, overexpression of TNC-fnD via adeno-associated virus in wild-type mice improved locomotor recovery, increased monaminergic axons sprouting, and reduced lesion scar volume after spinal cord injury. The functional efficacy of the viral-mediated TNC indicates a potentially useful approach for treatment of spinal cord injury.

Design and evaluation of novel polyanhydride blends as nerve guidance conduits.

Implantable biodegradable nerve guidance conduits (NGCs) have the potential to align and support regenerating cells, as well as prevent scar formation. In this study in vitro bioassays and in vivo material evaluations were performed using a nerve guidance conduit material made from a novel polyanhydride blend. In vitro cytotoxicity studies with both fibroblasts and primary chick neurons demonstrated that the proposed polyanhydride blend was non-cytotoxic. Subcutaneous implantation for 7days in rats resulted in an initial fibrin matrix, minimal macrophage presence and angiogenesis in the surrounding tissues. Nerve guidance conduits fabricated from the proposed polyanhydride blend material may serve as favorable biocompatible tissue engineering devices.

Increased FGF-2 secretion and ability to support neurite outgrowth by astrocytes cultured on polyamide nanofibrillar matrices.

An electrospun nonwoven matrix of polyamide nanofibers was employed as a new model for the capillary basement membrane at the blood-brain barrier (BBB). The basement membrane separates astrocytes from endothelial cells and is associated with growth factors, such as fibroblast growth factor-2 (FGF-2). FGF-2 is produced by astrocytes and induces specialized functions in endothelial cells, but also has actions on astrocytes. To investigate potential autocrine actions of FGF-2 at the BBB, astrocytes were cultured on unmodified nanofibers or nanofibers covalently modified with FGF-2. The former assumed an in vivo-like stellate morphology that was enhanced in the presence of cross-linked FGF-2. Furthermore, astrocyte monolayers established on unmodified nanofibers were more permissive for neurite outgrowth when cultured with an overlay of neurons than similar monolayers established on standard tissue culture surfaces, while astrocytes cultured on FGF-2-modifed nanofibers were yet more permissive. The observed differences were due in part to progressively increasing amounts of FGF-2 secreted by the astrocytes into the medium; hence FGF-2 increases its own expression in astrocytes to modulate astrocyte-neuron interactions. Soluble FGF-2 was unable to replicate the effects of cross-linked FGF-2. Nanofibers alone up-regulated FGF-2, albeit to a lesser extent than nanofibers covalently modified with FGF-2. These results underscore the importance of both surface topography and growth factor presentation on cellular function. Moreover, these results indicate that FGF-2-modified nanofibrillar scaffolds may demonstrate utility in tissue engineering applications for replacement and regeneration of lost tissue following central nervous system (CNS) injury or disease.

Covalently attached FGF-2 to three-dimensional polyamide nanofibrillar surfaces demonstrates enhanced biological stability and activity.

Activation of fibroblast growth factor receptors (FGFRs) requires the formation of a ternary complex between fibroblast growth factors (FGFs), FGFRs, and heparan sulfate proteoglycans, which are all located on the cell surface and the basement membrane (BM)/extracellular matrix (ECM). Heparan sulfate proteoglycans appear to stabilize FGFs by inhibiting the rapid degradation of FGFs normally observed in solution. Because of the pivotal role of FGFs in proliferative and developmental pathways, a number of recent studies have attempted to engineer microenvironments to stabilize growth factors for use in applications in tissue culture and regenerative medicine. In this communication, we demonstrate that covalent linkage of FGF-2 to nanofibrillar surfaces (defined as covalently bound FGF-2) composed of a network of polyamide nanofibers resulted in the maintenance of the biological efficacy of FGF-2 when stored dry for at least 6 months at 25 degrees C or 4 degrees C. Moreover, covalently bound FGF-2 was more potent than FGF-2 in solution when measured in cellular assays of proliferation and viability using a variety of cell types. Covalently bound FGF-2 also strongly activated FGFR, extracellular signal-regulated kinase (ERK1/2), and c-fos. Hence cell-signaling molecules can be incorporated into a synthetic nanofibrillar surface, providing a novel means to enhance their stability and biological activity.

Role of DNA topoisomerase IIbeta in neurite outgrowth.

Failure to establish neuromuscular junctions is a major phenotype of top2beta knockout mice. However, the precise mechanism for this defect is not known. In the current study, we have investigated the role of TopIIbeta in cultured neurons. We showed that the TopII inhibitor ICRF-193 significantly blocked neurite outgrowth and growth cone formation in cultured cerebellar granule neurons (CGNs), dorsal root ganglions (DRGs) and cortical neurons (CNs). In addition, ICRF-193 also blocked neurite outgrowth and growth cone formation of PC12 cells undergoing NGF-induced differentiation. Isolated cortical neurons from top2beta knockout embryos elaborated shorter neurites than did those from their wild type counterparts, confirming the role of TopIIbeta in neurite outgrowth. Together, these results demonstrate a critical role of TopIIbeta in neurite outgrowth in cultured neurons. Furthermore, we demonstrated that neurons derived from top2beta knockout mice failed to form contacts with muscle cells in co-cultures. These results suggest that the defect in establishing neuromuscular junctions in top2beta knockout mice could be due to the lack of TopIIbeta-mediated neurite outgrowth.

Morphology, cytoskeletal organization, and myosin dynamics of mouse embryonic fibroblasts cultured on nanofibrillar surfaces.

Growth of cells in tissue culture is generally performed on two-dimensional (2D) surfaces composed of polystyrene or glass. Recent work, however, has shown that such 2D cultures are incomplete and do not adequately represent the physical characteristics of native extracellular matrix (ECM)/basement membrane (BM), namely dimensionality, compliance, fibrillarity, and porosity. In the current study, a three-dimensional (3D) nanofibrillar surface composed of electrospun polyamide nanofibers was utilized to mimic the topology and physical structure of ECM/BM. Additional chemical cues were incorporated into the nanofibrillar matrix by coating the surfaces with fibronectin, collagen I, or laminin-1. Results from the current study show an enhanced response of primary mouse embryonic fibroblasts (MEFs) to culture on nanofibrillar surfaces with more dramatic changes in cell spreading and reorganization of the cytoskeleton than previously observed for established cell lines. In addition, the cells cultured on nanofibrillar and 2D surfaces exhibited differential responses to the specific ECM/BM coatings. The localization and activity of myosin II-B for MEFs cultured on nanofibers was also compared. A dynamic redistribution of myosin II-B was observed within membrane protrusions. This was previously described for cells associated with nanofibers composed of collagen I but not for cells attached to 2D surfaces coated with monomeric collagen. These results provide further evidence that nanofibrillar surfaces offer a significantly different environment for cells than 2D substrates.

Inhibition of human corneal epithelial production of fibrotic mediator TGF-beta2 by basement membrane-like extracellular matrix.

PURPOSE: Transforming growth factor (TGF)-beta2 is a major epithelial mediator of fibrotic marker expression during corneal repair in mice. Production of TGF-beta2 protein by cultured rabbit corneal epithelial cells is reduced by plating on a basement membrane-like extracellular matrix extract (Matrigel; BD Biosciences, Bedford, MA). The goal of the present study was to understand further the nature of Matrigel regulation. METHODS: TGF-beta2 protein, mRNA, and gene transcriptional promotion were characterized in cultured human corneal epithelial cells. RESULTS: TGF-beta2 production was inhibited by Matrigel at the level of mRNA accumulation and activity of the gene transcriptional promoter. This effect of Matrigel was not explained by (1) growth factor contaminants, as growth-factor reduced Matrigel also inhibited TGF-beta2; (2) independent matrix components, as the pure forms of the major ECM components laminin and collagen IV did not reproduce the effect; or (3) inhibition of a constitutive TGF-beta2 autocrine feedback loop, as addition of exogenous TGF-beta2 increased p-Smad3 and restored TGF-beta2 mRNA levels. In addition, Matrigel's ability to reduce TGF-beta2 was not explained by its geometry, as TGF-beta2 production was not inhibited by plating cells on a synthetic nanofiber matrix with a three-dimensional topography similar to Matrigel. Matrigel caused a reduction of ezrin, a member of the ezrin-radixin-moesin (ERM) family, which plays a role in establishing polarity of epithelial cells in tissues through the Rho signaling pathway. CONCLUSIONS: These findings indicate that Matrigel inhibits TGF-beta2 gene expression and point to a mechanism dependent on Matrigel composition and structure. The capacity of Matrigel to reduce ezrin is consistent with this idea and directs the focus of future studies toward the ERM/Rho pathway.

Living in three dimensions: 3D nanostructured environments for cell culture and regenerative medicine.

Research focused on deciphering the biochemical mechanisms that regulate cell proliferation and function has largely depended on the use of tissue culture methods in which cells are grown on two-dimensional (2D) plastic or glass surfaces. However, the flat surface of the tissue culture plate represents a poor topological approximation of the more complex three-dimensional (3D) architecture of the extracellular matrix (ECM) and the basement membrane (BM), a structurally compact form of the ECM. Recent work has provided strong evidence that the highly porous nanotopography that results from the 3D associations of ECM and BM nanofibrils is essential for the reproduction of physiological patterns of cell adherence, cytoskeletal organization, migration, signal transduction, morphogenesis, and differentiation in cell culture. In vitro approximations of these nanostructured surfaces are therefore desirable for more physiologically mimetic model systems to study both normal and abnormal functions of cells, tissues, and organs. In addition, the development of 3D culture environments is imperative to achieve more accurate cell-based assays of drug sensitivity, high-throughput drug discovery assays, and in vivo and ex vivo growth of tissues for applications in regenerative medicine.

Three-dimensional nanofibrillar surfaces promote self-renewal in mouse embryonic stem cells.

The regulation of mouse embryonic stem cell (mESC) fate is controlled by the interplay of signaling networks that either promote self-renewal or induce differentiation. Leukemia inhibitory factor (LIF) is a cytokine that is required for stem cell renewal in mouse but not in human embryonic stem cells. However, feeder layers of embryonic fibroblasts are capable of inducing stem cell renewal in both cell types, suggesting that the self-renewal signaling pathways may also be promoted by other triggers, such as alternative cytokines and/or chemical or physical properties of the extracellular matrix (ECM) secreted by feeder fibroblasts. We have recently used a synthetic polyamide matrix (Ultra-Web) whose three-dimensional (3D) nanofibrillar organization resembles the ECM/basement membrane. Growth of mESCs on this nanofibrillar surface greatly enhanced proliferation and self-renewal in comparison with growth on tissue culture surfaces without nanofibers, despite the presence of LIF in both systems. Enhanced proliferation and self-renewal of the stem cells on nanofibrillar surfaces were correlated with the activation of the small GTPase Rac, the activation of phosphoinositide 3-kinase (PI3K) pathway, and the enhanced expression of Nanog, a homeoprotein required for maintenance of pluripotency. Inhibitors of PI3K reduced the expression level of Nanog in mESCs cultured on 3D nanofibrillar surfaces. These results provide support for the view that the three-dimensionality of the culture surface may function as a cue for the activation of Rac and PI3K signaling pathways, resulting in stem cell proliferation and self-renewal.

Three-dimensional nanofibrillar surfaces covalently modified with tenascin-C-derived peptides enhance neuronal growth in vitro.

Current methods to promote growth of cultured neurons use two-dimensional (2D) glass or polystyrene surfaces coated with a charged molecule (e.g. poly-L-lysine (PLL)) or an isolated extracellular matrix (ECM) protein (e.g. laminin-1). However, these 2D surfaces represent a poor topological approximation of the three-dimensional (3D) architecture of the assembled ECM that regulates neuronal growth in vivo. Here we report on the development of a new 3D synthetic nanofibrillar surface for the culture of neurons. This nanofibrillar surface is composed of polyamide nanofibers whose organization mimics the porosity and geometry of the ECM. Neuronal adhesion and neurite outgrowth from cerebellar granule, cerebral cortical, hippocampal, motor, and dorsal root ganglion neurons were similar on nanofibers and PLL-coated glass coverslips; however, neurite generation was increased. Moreover, covalent modification of the nanofibers with neuroactive peptides derived from human tenascin-C significantly enhanced the ability of the nanofibers to facilitate neuronal attachment, neurite generation, and neurite extension in vitro. Hence the 3D nanofibrillar surface provides a physically and chemically stabile cell culture surface for neurons and, potentially, an exciting new opportunity for the development of peptide-modified matrices for use in strategies designed to encourage axonal regrowth following central nervous system injury.

Neurite guidance by the FnC repeat of human tenascin-C: neurite attraction vs. neurite retention.

The alternatively spliced fibronectin type-III repeat C of human tenascin-C (fnC) provides directional cues to elongating neurites in vitro. When given a choice at an interface with poly L-lysine (PLL), rat cerebellar granule neurites preferentially crossed onto fnC (defined herein as neurite attraction) whereas neurites originating on fnC preferentially remained on fnC (defined as neurite retention). Guidance motifs were further refined using synthetic peptides spanning the sequence of fnC. We found that a peptide with amino acid sequence DINPYGFTVSWMASE was sufficient to attract and retain neurites. Peptides with alterations in NPYG facilitated neurite retention but not attraction and, conversely, molecules with alterations in ASE facilitated neurite attraction but not retention. Hence neurite attraction and neurite retention mediated by fnC are separable events that can be independently regulated. This property may prove valuable for the strategic design of peptide reagents for use in strategies to facilitate directed axonal regrowth following CNS injury.

Requirement of activated Cdc42-associated kinase for survival of v-Ras-transformed mammalian cells.

Activated Cdc42-associated kinase (ACK) has been shown to be an important effector molecule for the small GTPase Cdc42. We have shown previously an essential role for Cdc42 in the transduction of Ras signals for the transformation of mammalian cells. In this report, we show that the ACK-1 isoform of ACK plays a critical role in transducing Ras-Cdc42 signals in the NIH 3T3 cells. Overexpression of a dominant-negative (K214R) mutant of ACK-1 inhibits Ras-induced up-regulation of c-fos and inhibits the growth of v-Ras-transformed NIH 3T3 cells. Using small interfering RNA, we knocked down the expression of ACK-1 in both v-Ha-Ras-transformed and parental NIH 3T3 cells and found that down-regulation of ACK-1 inhibited cell growth by inducing apoptosis only in v-Ha-Ras-transformed but not parental NIH 3T3 cells. In addition, we studied the effect of several tyrosine kinase inhibitors and found that PD158780 inhibits the kinase activity of ACK-1 in vitro. We also found that PD158780 inhibits the growth of v-Ha-Ras-transformed NIH 3T3 cells. Taken together, our results suggest that ACK-1 kinase plays an important role in the survival of v-Ha-Ras-transformed cells, suggesting that ACK-1 is a novel target for therapies directed at Ras-induced cancer.

A synthetic nanofibrillar matrix promotes in vivo-like organization and morphogenesis for cells in culture.

The purpose of this study was to design a synthetic nanofibrillar matrix that more accurately models the porosity and fibrillar geometry of cell attachment surfaces in tissues. The synthetic nanofibrillar matrices are composed of nanofibers prepared by electrospinning a polymer solution of polyamide onto glass coverslips. Scanning electron and atomic force microscopy showed that the nanofibers were organized into fibrillar networks reminiscent of the architecture of basement membrane, a structurally compact form of the extracellular matrix (ECM). NIH 3T3 fibroblasts and normal rat kidney (NRK) cells, when grown on nanofibers in the presence of serum, displayed the morphology and characteristics of their counterparts in vivo. Breast epithelial cells underwent morphogenesis to form multicellular spheroids containing lumens. Hence the synthetic nanofibrillar matrix described herein provides a physically and chemically stable three-dimensional surface for ex vivo growth of cells. Nanofiber-based synthetic matrices could have considerable value for applications in tissue engineering, cell-based therapies, and studies of cell/tissue function and pathology.

Three dimensional nanofibrillar surfaces induce activation of Rac.

Studies to define the mechanisms by which the extracellular matrix (ECM) activates Rho GTPases within the cell have generally focused on the chemistry of the macromolecules comprising the ECM. Considerably less information is available to assess the role of the physical structure of the ECM, particularly its three dimensional (3D) geometry. In this report, we examined the effect of 3D surfaces on the activation states of Rho GTPases within NIH 3T3 fibroblasts and normal rat kidney cells. Cells were cultured on synthetic 3D surfaces comprised of polyamide nanofibers. In contrast to results using two dimensional tissue culture surfaces, growth of both cell types on 3D nanofibrillar surfaces resulted in a preferential and sustained activation of the small GTPase Rac. These results support the growing view that in addition to chemical composition, the three dimensionality and nanofibrillar architecture of ECM may represent another essential element in signal transduction pathways and cellular physiology.

Neurite outgrowth by the alternatively spliced region of human tenascin-C is mediated by neuronal alpha7beta1 integrin.

The region of tenascin-C containing only alternately spliced fibronectin type-III repeat D (fnD) increases neurite outgrowth by itself and also as part of tenascin-C. We previously localized the active site within fnD to an eight amino acid sequence unique to tenascin-C, VFDNFVLK, and showed that the amino acids FD and FV are required for activity. The purpose of this study was to identify the neuronal receptor that interacts with VFDNFVLK and to investigate the hypothesis that FD and FV are important for receptor binding. Function-blocking antibodies against both alpha7 and beta1 integrin subunits were found to abolish VFDNFVLK-mediated process extension from cerebellar granule neurons. VFDNFVLK but not its mutant, VSPNGSLK, induced clustering of neuronal beta1 integrin immunoreactivity. This strongly implicates FD and FV as important structural elements for receptor activation. Moreover, biochemical experiments revealed an association of the alpha7beta1 integrin with tenascin-C peptides containing the VFDNFVLK sequence but not with peptides with alterations in FD and/or FV. These findings are the first to provide evidence that the alpha7beta1 integrin mediates a response to tenascin-C and the first to demonstrate a functional role for the alpha7beta1 integrin receptor in CNS neurons.

Cdc42-dependent nuclear translocation of non-receptor tyrosine kinase, ACK.

Ras signals for the transformation of mammalian cells are apparently transduced through Rho GTPases. The Rho GTPase family member Cdc42 generates independent signals that regulate the rearrangement of the actin cytoskeleton and the transcription of genes. However, the molecular mechanism of signal transduction from Cdc42 to the nucleus remains to be understood. The non-receptor tyrosine kinases ACK-1 and ACK-2 have been found to bind specifically to Cdc42. In this paper we studied whether ACKs transduce Cdc42 signals to the nucleus directly, or through other cytoplasmic proteins. Using immunocytochemistry and Western blot analysis, we found a nuclear localization of ACKs in semi-confluent glioblastoma (U251) cells, as opposed to a cytosolic localization in confluent cells. In agreement with the nuclear localization, a putative nuclear export signal was identified in ACK-1 and ACK-2. Furthermore, the interaction of Cdc42 with ACKs was shown to be essential for the nuclear localization of ACKs. Overexpression of ACK42 (a Cdc42 binding domain of ACK) inhibited cell growth and movement, indicating that Cdc42 signals are transduced to the nucleus through ACKs. This is the first report providing evidence of a novel role for ACKs in transducing Cdc42 signals directly to the nucleus.