Akt phosphorylation of BAD couples survival signals to the cell-intrinsic death machinery

Growth factors can promote cell survival by activating the phosphatidylinositide-3'-OH kinase and its downstream target, the serine-threonine kinase Akt. However, the mechanism by which Akt functions to promote survival is not understood. We show that growth factor activation of the PI3'K/Akt signaling pathway culminates in the phosphorylation of the BCL-2 family member BAD, thereby suppressing apoptosis and promoting cell survival. Akt phosphorylates BAD in vitro and in vivo, and blocks the BAD-induced death of primary neurons in a site-specific manner. These findings define a mechanism by which growth factors directly inactivate a critical component of the cell-intrinsic death machinery.

Quantitative reverse transcription-polymerase chain reaction to study mRNA decay: comparison of endpoint and real-time methods

Four quantitative reverse transcription-PCR (RT-PCR) methods were compared to evaluate the time course of mRNA formation and decay. Mouse fibroblasts (NIH 3T3) transfected with the human beta-globin open reading frame/c-myc 3'-untranslated region chimeric gene under control of the c-fos promoter (fos-glo-myc) were used for serum-inducible transcription. The amount of fos-glo-myc mRNA, relative to beta-actin, was measured by quantitative, RT-PCR at various times following the addition of serum to serum-starved fibroblasts transfected with the chimeric gene. Both endpoint (band densitometry and probe hybridization) and real-time (SYBR green and TaqMan) PCR methods were used to assay the identical cDNA. The real-time methods produced a 4- to 5-log dynamic range of amplification, while the dynamic range of the endpoint assays was 1-log. The real-time and probe hybridization assays produced a comparable level of sensitivity that was considerably greater than band densitometry. The coefficient of variation from 22 replicate PCR reactions was 14.2 and 24.0% for the SYBR green and TaqMan detection, respectively, and 44.9 and 45.1% for the band densitometry and probe hybridization assays, respectively. The rank order for the values of r(2) obtained from the linear regression of the first-order mRNA decay plots was SYBR green > TaqMan > probe hybridization > band densitometry. Real-time PCR is more precise and displays a greater dynamic range than endpoint PCR. Among the real-time methods, SYBR green and TaqMan assays produced comparable dynamic range and sensitivity while SYBR green detection was more precise and produced a more linear decay plot than TaqMan detection.

PAK kinases are directly coupled to the PIX family of nucleotide exchange factors

The PAK family of kinases are regulated through interaction with the small GTPases Cdc42 and Rac1, but little is known of the signaling components immediately upstream or downstream of these proteins. We have purified and cloned a new class of Rho-p21 guanine nucleotide exchange factor binding tightly through its N-terminal SH3 domain to a conserved proline-rich PAK sequence with a Kd of 24 nM. This PAK-interacting exchange factor (PIX), which is widely expressed and enriched in Cdc42- and Rac1-driven focal complexes, is required for PAK recruitment to these sites. PIX can induce membrane ruffling, with an associated activation of Rac1. Our results suggest a role for PIX in Cdc42-to-Rac1 signaling, involving the PIX/PAK complex.

Adipocyte differentiation and gene expression

The major function of adipocytes is to store triacylglycerol in periods of energy excess and to mobilize this energy during times of deprivation. The short-term control of these lipogenic and lipolytic processes is carefully modulated by hormonal signals from the bloodstream, which provide an inventory of the body's metabolic state. Long-term changes in fat storage needs are accomplished by altering both the size and number of fat cells within the body because terminally differentiated adipocytes cannot divide. Alterations in the number of fat cells within the body must be accomplished by the differentiation of preadipocytes, which act as the renewable source of adipocytes. Our understanding of the events that occur during preadipocyte differentiation has advanced considerably in the last few years and has relied mainly on the use of tissue culture models of adipogenesis. This article will discuss the various models used for studying the preadipocyte differentiation process, with the mouse 3T3-L1 cell culture line described in detail. We focus on those genetic events that link effectors to induction of adipocyte gene expression.

Induction of peroxisome proliferator-activated receptor gamma during the conversion of 3T3 fibroblasts into adipocytes is mediated by C/EBPbeta, C/EBPdelta, and glucocorticoids

The differentiation of 3T3 preadipocytes into adipocytes is accompanied by a transient induction of C/EBPbeta and C/EBPdelta expression in response to treatment of the cells with methylisobutylxanthine (MIX) and dexamethasone (DEX), respectively. In this report, we demonstrate that peroxisome proliferator-activated receptor gamma (PPARgamma) expression in 3T3-L1 preadipocytes is induced by MIX and DEX, suggesting that C/EBPbeta and C/EBPdelta may be involved in this process. Using a tetracycline-responsive expression system, we have recently shown that the conditional ectopic expression of C/EBPbeta in NIH 3T3 fibroblasts (beta2 cells) in the presence of DEX activates the synthesis of peroxisome PPARgamma mRNA. Subsequent exposure of these cells to PPAR activators stimulates their conversion into adipocytes; however, neither the expression of C/EBPbeta nor exposure to DEX alone is capable of inducing PPARgamma expression in the beta2 cell line. We find that unlike the case for 3T3 preadipocytes, C/EBPdelta is not induced by DEX in these 3T3 fibroblasts and therefore is not relaying the effect of this glucocorticoid to the PPARgamma gene. To define the role of glucocorticoids in regulating PPARgamma expression and the possible involvement of C/EBPdelta, we have established an additional set of NIH 3T3 cell lines expressing either C/EBPdelta alone (delta23 cells) or C/EBPdelta and C/EBPbeta together (beta/delta39 cells), using the tetracycline-responsive system. Culture of these cells in tetracycline-deficient medium containing DEX, MIX, insulin, and fetal bovine serum shows that the beta/delta39 cells express PPARgamma and aP2 mRNAs at levels that are almost equivalent to those observed in fully differentiated 3T3-L1 adipocytes. These levels are approximately threefold higher than their levels of expression in the beta2 cells. Despite the fact that these beta/delta39 cells produce abundant amounts of C/EBPbeta and C/EBPdelta (in the absence of tetracycline), they still require glucocorticoids to attain maximum expression of PPARgamma mRNA. Furthermore, the induction of PPARgamma mRNA by exposure of these cells to DEX occurs in the absence of ongoing protein synthesis. The delta23 cells, on the other hand, are not capable of activating PPARgamma gene expression when exposed to the same adipogenic inducers. Finally, attenuation of ectopic C/EBPbeta production at various stages during the differentiation process results in a concomitant inhibition of PPARgamma and the adipogenic program. These data strongly suggest that the induction of PPARgamma gene expression in multipotential mesenchymal stem cells (NIH 3T3 fibroblasts) is dependent on elevated levels of C/EBPbeta throughout the differentiation process, as well as an initial exposure to glucocorticoids. C/EBPdelta may function by synergizing with C/EBPbeta to enhance the level of PPARgamma expression.

Measurement of mechanical tractions exerted by cells in three-dimensional matrices

Quantitative measurements of cell-generated forces have heretofore required that cells be cultured on two-dimensional substrates. We describe a technique to quantitatively measure three-dimensional traction forces exerted by cells fully encapsulated in well-defined elastic hydrogel matrices. Using this approach we measured traction forces for several cell types in various contexts and revealed patterns of force generation attributable to morphologically distinct regions of cells as they extend into the surrounding matrix.

Microtubule growth activates Rac1 to promote lamellipodial protrusion in fibroblasts

Microtubules are involved in actin-based protrusion at the leading-edge lamellipodia of migrating fibroblasts. Here we show that the growth of microtubules induced in fibroblasts by removal of the microtubule destabilizer nocodazole activates Rac1 GTPase, leading to the polymerization of actin in lamellipodial protrusions. Lamellipodial protrusions are also activated by the rapid growth of a disorganized array of very short microtubules induced by the microtubule-stabilizing drug taxol. Thus, neither microtubule shortening nor long-range microtubule-based intracellular transport is required for activating protrusion. We suggest that the growth phase of microtubule dynamic instability at leading-edge lamellipodia locally activates Rac1 to drive actin polymerization and lamellipodial protrusion required for cell migration.

Expression and function of PPARgamma in rat and human vascular smooth muscle cells

BACKGROUND

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is activated by fatty acids, eicosanoids, and insulin-sensitizing thiazolidinediones (TZDs). The TZD troglitazone (TRO) inhibits vascular smooth muscle cell (VSMC) proliferation and migration in vitro and in postinjury intimal hyperplasia.

METHODS AND RESULTS

Rat and human VSMCs express mRNA and nuclear receptors for PPARgamma1. Three PPARgamma ligands, the TZDs TRO and rosiglitazone and the prostanoid 15-deoxy-Delta(12,14)-prostaglandin J2 (15d-PGJ2), all inhibited VSMC proliferation and migration. PPARgamma is upregulated in rat neointima at 7 days and 14 days after balloon injury and is also present in early human atheroma and precursor lesions.

CONCLUSIONS

Pharmacological activation of PPARgamma expressed in VSMCs inhibits their proliferation and migration, potentially limiting restenosis and atherosclerosis. These receptors are upregulated during vascular injury.

Dimensional and mechanical dynamics of active and stable edges in motile fibroblasts investigated by using atomic force microscopy

The atomic force microscope (AFM) was employed to investigate the extension and retraction dynamics of protruding and stable edges of motile 3T3 fibroblasts in culture. Such dynamics closely paralleled the results of earlier studies employing video microscopy that indicated that the AFM force-mapping technique does not appreciably perturb these dynamics. Force scans permitted height determinations of active and stable edges. Whereas the profiles of active edges are flat with average heights of 0.4-0.8 micrometer, stable edges smoothly ascend to 2-3 micrometers within about 6 micrometers of the edge. In the region of the leading edge, the height fluctuates up to 50% (SD) of the mean value, much more than the stable edge; this fluctuation presumably reflects differences in underlying cytoskeletal activity. In addition, force mapping yields an estimate of the local Young's modulus or modulus of elasticity (E, the cortical stiffness). This stiffness will be related to "cortical tension," can be accurately calculated for the stable edges, and is approximately 12 kPa in this case. The thinness of the leading edge precludes accurate estimation of the E values, but within 4 micrometers of the margin it is considerably smaller than that for stable edges, which have an upper limit of 3-5 kPa. Although blebbing cannot absolutely be ruled out as a mechanism of extension, the data are consistent with an actin polymerization and/or myosin motor mechanism in which the average material properties of the extending margin would be nearly constant to the edge. Because the leading edge is softer than the stable edge, these data also are consistent with the notion that extension preferentially occurs in regions of lower cortical tension.

Micromechanical mapping of live cells by multiple-particle-tracking microrheology

This paper introduces the method of live-cell multiple-particle-tracking microrheology (MPTM), which quantifies the local mechanical properties of living cells by monitoring the Brownian motion of individual microinjected fluorescent particles. Particle tracking of carboxylated microspheres imbedded in the cytoplasm produce spatial distributions of cytoplasmic compliances and frequency-dependent viscoelastic moduli. Swiss 3T3 fibroblasts are found to behave like a stiff elastic material when subjected to high rates of deformations and like a soft liquid at low rates of deformations. By analyzing the relative contributions of the subcellular compliances to the mean compliance, we find that the cytoplasm is much more mechanically heterogeneous than reconstituted actin filament networks. Carboxylated microspheres embedded in cytoplasm through endocytosis and amine-modified polystyrene microspheres, which are microinjected or endocytosed, often show directed motion and strong nonspecific interactions with cytoplasmic proteins, which prevents computation of local moduli from the microsphere displacements. Using MPTM, we investigate the mechanical function of alpha-actinin in non-muscle cells: alpha-actinin-microinjected cells are stiffer and yet mechanically more heterogeneous than control cells, in agreement with models of reconstituted cross-linked actin filament networks. MPTM is a new type of functional microscopy that can test the local, rate-dependent mechanical and ultrastructural properties of living cells.

Myosin II filament assemblies in the active lamella of fibroblasts: their morphogenesis and role in the formation of actin filament bundles

The morphogenesis of myosin II structures in active lamella undergoing net protrusion was analyzed by correlative fluorescence and electron microscopy. In rat embryo fibroblasts (REF 52) microinjected with tetramethylrhodamine-myosin II, nascent myosin spots formed close to the active edge during periods of retraction and then elongated into wavy ribbons of uniform width. The spots and ribbons initially behaved as distinct structural entities but subsequently aligned with each other in a sarcomeric-like pattern. Electron microscopy established that the spots and ribbons consisted of bipolar minifilaments associated with each other at their head-containing ends and arranged in a single row in an "open" zig-zag conformation or as a "closed" parallel stack. Ribbons also contacted each other in a nonsarcomeric, network-like arrangement as described previously (Verkhovsky and Borisy, 1993. J. Cell Biol. 123:637-652). Myosin ribbons were particularly pronounced in REF 52 cells, but small ribbons and networks were found also in a range of other mammalian cells. At the edge of the cell, individual spots and open ribbons were associated with relatively disordered actin filaments. Further from the edge, myosin filament alignment increased in parallel with the development of actin bundles. In actin bundles, the actin cross-linking protein, alpha-actinin, was excluded from sites of myosin localization but concentrated in paired sites flanking each myosin ribbon, suggesting that myosin filament association may initiate a pathway for the formation of actin filament bundles. We propose that zig-zag assemblies of myosin II filaments induce the formation of actin bundles by pulling on an actin filament network and that co-alignment of actin and myosin filaments proceeds via folding of myosin II filament assemblies in an accordion-like fashion.

Tsg101: a novel tumor susceptibility gene isolated by controlled homozygous functional knockout of allelic loci in mammalian cells

Using a novel strategy that enables the isolation of previously unknown genes encoding selectable recessive phenotypes, we identified a gene (tsg101) whose homozygous functional disruption produces cell transformation. Antisense RNA from a transactivated promoter introduced randomly into transcribed genes throughout the genome of mouse 3T3 fibroblasts was used to knock out alleles of chromosomal genes adjacent to promoter inserts, generating clones that grew in 0.5% agar and formed metastatic tumors in nude mice. Removal of the transactivator restored normal growth. The protein encoded by tsg101 cDNA encodes a coiled-coil domain that interacts with stathmin, a cytosolic phosphoprotein implicated previously in tumorigenesis. Overexpression of tsg101 antisense transcripts in naive 3T3 cells resulted in cell transformation and increased stathmin-specific mRNA.

In vitro gene delivery to hepatocytes with galactosylated polyethylenimine

A hepatocyte-directed vector has been developed; it includes several key features thought to favor in vivo gene delivery to the liver: electrostatically neutral particles which avoid nonspecific binding to other cells, to the extracellular matrix, and to complement proteins; asialoglycoprotein receptor-mediated endocytosis which may address the complexes to the perinuclear region; and polyethylenimine (PEI)-mediated endosome buffering and swelling as an escape mechanism to the cytoplasm. This system is based on a 5% galactose-bearing polyethylenimine (PEI-gal) polymer which is condensed with plasmid DNA to neutrality. Murine (BNL CL.2) and human (HepG2) hepatocyte-derived cell lines were transfected 10(4)-10(5)-fold more efficiently than murine fibroblasts (3T3), whether transfection was assessed globally (luciferase expression from the cell extract) or following histochemical staining (beta-galactosidase). Under these conditions, over 50% of the hepatocytes were selectively transfected in the presence of 10% serum. Transfection was suppressed by removal of the targeting galactose residues, by their replacement with glucose, or by the addition of excess asialofetuin. Thus, results from comparative and competitive experiments indicate the asialoglycoprotein receptor is involved in transfection of hepatocytes with neutral PEI-gal/DNA complexes.

Induction of p21WAF1/CIP1 and cyclin D1 expression by the Src oncoprotein in mouse fibroblasts: role of activated STAT3 signaling

While the activated viral Src oncoprotein, v-Src, induces uncontrolled cell growth, the mechanisms underlying cell cycle deregulation by v-Src have not been fully defined. Previous studies demonstrated that v-Src induces constitutively active STAT3 signaling that is required for cell transformation and recent data have implicated STAT3 in the transcriptional control of critical cell cycle regulators. Here we show in mouse fibroblasts stably transformed by v-Src that mRNA and protein levels of p21 (WAF1/CIP1), cyclin D1, and cyclin E are elevated. Using reporter constructs in transient-transfection assays, the cyclin D1 and p21 promoters were both found to be transcriptionaly induced by v-Src in a STAT3-dependent manner. The kinase activities of cyclin D/CDK4, 6 and cyclin E/CDK2 complexes were only slightly elevated, consistent with the findings that coordinate increases in p21, cyclin D1 and cyclin E resulted in an increase in cyclin/CDK/p21 complexes. Similar results were obtained in NIH3T3 and BALB/c 3T3 cells stably transformed by v-Src, indicating that these regulatory events associated with STAT3 signaling represent common mechanisms independent of cell line or clonal variation. These findings suggest that STAT3 has an essential role in the regulation of critical cell cycle components in v-Src transformed mouse fibroblasts.

Geometry as a factor for tissue growth: towards shape optimization of tissue engineering scaffolds

Scaffolds for tissue engineering are usually designed to support cell viability with large adhesion surfaces and high permeability to nutrients and oxygen. Recent experiments support the idea that, in addition to surface roughness, elasticity and chemistry, the macroscopic geometry of the substrate also contributes to control the kinetics of tissue deposition. In this study, a previously proposed model for the behavior of osteoblasts on curved surfaces is used to predict the growth of bone matrix tissue in pores of different shapes. These predictions are compared to in vitro experiments with MC3T3-E1 pre-osteoblast cells cultivated in two-millimeter thick hydroxyapatite plates containing prismatic pores with square- or cross-shaped sections. The amount and shape of the tissue formed in the pores measured by phase contrast microscopy confirms the predictions of the model. In cross-shaped pores, the initial overall tissue deposition is twice as fast as in square-shaped pores. These results suggest that the optimization of pore shapes may improve the speed of ingrowth of bone tissue into porous scaffolds.

Differential effect of saturated and unsaturated free fatty acids on the generation of monocyte adhesion and chemotactic factors by adipocytes: dissociation of adipocyte hypertrophy from inflammation

OBJECTIVE

Obesity is associated with monocyte-macrophage accumulation in adipose tissue. Previously, we showed that glucose-stimulated production by adipocytes of serum amyloid A (SAA), monocyte chemoattractant protein (MCP)-1, and hyaluronan (HA) facilitated monocyte accumulation. The current objective was to determine how the other major nutrient, free fatty acids (FFAs), affects these molecules and monocyte recruitment by adipocytes.

RESEARCH DESIGN AND METHODS

Differentiated 3T3-L1, Simpson-Golabi-Behmel syndrome adipocytes, and mouse embryonic fibroblasts were exposed to various FFAs (250 micromol/l) in either 5 or 25 mmol/l (high) glucose for evaluation of SAA, MCP-1, and HA regulation in vitro.

RESULTS

Saturated fatty acids (SFAs) such as laurate, myristate, and palmitate increased cellular triglyceride accumulation, SAA, and MCP-1 expression; generated reactive oxygen species (ROS); and increased nuclear factor (NF) kappaB translocation in both 5 and 25 mmol/l glucose. Conversely, polyunsaturated fatty acids (PUFAs) such as arachidonate, eicosapentaenate, and docosahexaenate (DHA) decreased these events. Gene expression could be dissociated from triglyceride accumulation. Although excess glucose increased HA content, SFAs, oleate, and linoleate did not. Antioxidant treatment repressed glucose- and palmitate-stimulated ROS generation and NFkappaB translocation and decreased SAA and MCP-1 expression and monocyte chemotaxis. Silencing toll-like receptor-4 (TLR4) markedly reduced SAA and MCP-1 expression in response to palmitate but not glucose. DHA suppressed NFkappaB translocation stimulated by both excess glucose and palmitate via a peroxisome prolifterator-activated receptor (PPAR) gamma-dependent pathway.

CONCLUSIONS

Excess glucose and SFAs regulate chemotactic factor expression by a mechanism that involves ROS generation, NFkappaB, and PPARgamma, and which is repressed by PUFAs. Certain SFAs, but not excess glucose, trigger chemotactic factor expression via a TLR4-dependent pathway.

Integration of surface modification and 3D fabrication techniques to prepare patterned poly(L-lactide) substrates allowing regionally selective cell adhesion

Regeneration of organizationally complex tissue requires regulation of spatial distributions of particular cell types in three dimensions. In this paper we demonstrate an integration of polymer processing and selective polymer surface modification using methods suitable for construction of three-dimensional polymer scaffolds which may aid such cell organization. Specifically, the surfaces of degradable polyesters were modified with poly(ethylene-oxide) (PEO)-poly(propylene-oxide) (PPO) copolymers using a process compatible with a solid free-form fabrication technique, the 3DP printing process. We demonstrate inhibition of cell (hepatocyte and fibroblast) adhesion to regions of two-dimensional poly(lactide) (PLA) substrates modified with PEO-PPO-PEO copolymers. We further show that PEO-PPO-PEO-modified surfaces which are not adhesive for hepatocytes or fibroblasts can be made selectively adhesive for hepatocytes by covalent linkage of a carbohydrate ligand specific for the hepatocyte asialoglycoprotein receptor to the PEO chain ends. Our approach may be generally useful for creating regionally selective, microarchitectured scaffolds fabricated from biodegradable polymers, for spatial organization of diverse cell types.

Cell orientation determines the alignment of cell-produced collagenous matrix

In healing ligaments and tendons, the cells are not aligned and collagen matrix is not organized as in normal tissues. In addition, the mechanical properties of the tissues are abnormal. We hypothesized that the lack of alignment of the collagen matrix results from random orientation of the cells seen in the healing area. To test this hypothesis, a novel in vitro model was used in which the orientation of cells could be controlled via microgrooves, and alignment of the collagen matrix formed by these cells could be easily observed. It is known that cells align uniformly along the direction of microgrooves; therefore MC3T3-E1 cells, which produce large amounts of collagen, were grown on silicone membranes with parallel microgrooves (10 microm wide x 3 microm deep) in the surface. As a control, the same cells were also grown on smooth silicone membranes. Cells on both the microgrooved and smooth silicone surfaces produced a layer of readily visible collagen matrix. Immunohistochemical staining showed that the matrix consisted of abundant type I collagen. Polarized light microscopy of the collagen matrix revealed the collagen fibers to be parallel to the direction of the microgrooves, whereas the collagen matrix produced by the randomly oriented cells on the smooth membranes was disorganized. Thus, the results of this study suggest that the orientation of cells affects the organization of the collagenous matrix produced by the cells. The results also suggest that orienting cells along the longitudinal direction of healing ligaments and tendons may lead to the production of aligned collagenous matrix that more closely represents the uninjured state. This may enhance the mechanical properties of healing ligaments and tendons.

Selective bone cell adhesion on formulations containing carbon nanofibers

Bone cell adhesion on novel carbon nanofibers and polycarbonate urethane/carbon nanofiber (PCU/CNF) composites is investigated in the present in vitro study. Carbon nanofibers have exceptional theoretical mechanical properties (such as high strength to weight ratios) that, along with possessing nanoscale fiber dimensions similar to crystalline hydroxyapatite found in physiological bone, suggest strong possibilities for use as an orthopedic/dental implant material. The effects of select properties of carbon fibers (specifically, dimension, surface energy, and chemistry) on osteoblast, fibroblast, chondrocyte, and smooth muscle cell adhesion were determined in the present in vitro study. Results provided evidence that smaller-scale (i.e., nanometer dimension) carbon fibers promoted osteoblast adhesion. Adhesion of other cells was not influenced by carbon fiber dimensions. Also, smooth muscle cell, fibroblast, and chondrocyte adhesion decreased with an increase in either carbon nanofiber surface energy or simultaneous change in carbon nanofiber chemistry. Moreover, greater weight percentages of high surface energy carbon nanofibers in the PCU/CNF composite increased osteoblast adhesion while at the same time decreased fibroblast adhesion.

Cell type-specific expression of hnRNP proteins

HnRNP proteins are abundant nucleoplasmic pre-mRNA-binding proteins which have important roles in the biogenesis of mRNA. Although hnRNP proteins have been extensively characterized in cultured cell lines, little is known about their expression in animal tissues. Here, we have undertaken a systematic survey of the expression of major hnRNP proteins in mouse tissue using specific monoclonal antibodies. Immunohistochemical staining demonstrated that hnRNP proteins C, L, and U were localized to nuclei in all tissues examined. However, cytoplasmic expression of hnRNP A1, D, F/H, and K was also detected in several tissues, suggesting that these proteins have roles in the cytoplasm as well as the nucleus. Importantly, the relative amounts of different hnRNP proteins varied among cell types. This was especially striking in neuronal and reproductive cells. In the brain, certain neuronal cell types contained more hnRNP proteins than glial cells, perhaps reflecting increased levels of neuronal transcription and RNA processing. In the ovary, oocytes contained exceptionally high concentrations of hnRNP proteins as compared to follicular and stromal cells. In the testis, the expression of hnRNP proteins was generally high and was found to be tightly regulated during spermatogenesis. Specifically, hnRNP A1 was highly expressed only in early spermatogonia and absent in later stages. These findings demonstrate that hnRNP proteins do not exist in a fixed stoichiometry across different cell types. Furthermore, as the relative amounts of pre-mRNA-binding proteins (e.g., A1 and ASF/SF2) can affect alternative splicing patterns, the variations that we have observed could profoundly affect cell-specific gene expression.

Dimerizable cationic detergents with a low cmc condense plasmid DNA into nanometric particles and transfect cells in culture

The size of condensed DNA particles is a key determinant for in vivo diffusion and gene delivery to cells. Gene molecules can be individually compacted by cationic thiol detergents into nanometric particles that are stabilized by oxidative conversion of the detergent into a gemini lipid. To reach the other goal, gene delivery, a series of cationic thiol detergents with various chain lengths (C(12)-C(16)) and headgroups (ornithine or spermine) was prepared, using a versatile polymer-supported synthetic strategy. Critical micelle concentrations and thiol oxidation rates of the detergents were measured. The formation and stability of complexes formed with plasmid DNA, as well as the size, xi-potential, morphology, and transfection efficiency of the particles were investigated. Using the tetradecane/ornithine detergent, a solution of 5.5 Kpb plasmid DNA molecules was converted into a homogeneous population of 35 nm particles. The same detergent, once oxidized, exhibited a typical lipid phase internal structure and was capable of effective cell transfection. The particle size did not increase with time. Surprisingly, the gel electrophoretic mobility of the DNA complexes was found to be higher than that of plasmid DNA itself. Favorable in vivo diffusion and intracellular trafficking properties may thus be expected for these complexes.

A novel method for surface modification to promote cell attachment to hydrophobic substrates

The ability to study and regulate cell behavior at a biomaterial interface requires strict control over material surface chemistry. Perhaps the greatest challenge to researchers working in this area is preventing the fouling of a given surface due to uncontrolled protein adsorption. This work describes a method for coupling peptides to hydrophobic materials for the purpose of simultaneously preventing nonspecific protein adsorption and controlling cell adhesion. A hexapeptide containing the ubiquitous RGD cell-adhesion motif was coupled to polystyrene (PS) via a polyethylene oxide (PEO) tether in the form of a modified PEO/PPO/PEO triblock copolymer. Triblocks were adsorbed onto PS at a density of 3.3 +/- (5.14 x 10(-4)) mg/m2 (1.4 x 10(5) +/- 2.12 x 10(1) molecules/microm2), which was determined by isotope 125I labeling. The peptide, GRGDSY, was activated at the N terminus with N-Succinimidyl 3-(2-pyridyldithio) propionate and coupled to immobilized triblocks where the terminal hydroxyls had been converted to sulfhydryl groups. Surface peptide density was measured by amino acid analysis and found to be 1.4 x 10(4) +/- 0.47 x 10(4) molecules/microm2. PS modified with PEO/PPO/PEO copolymers alone was found to be inert to cell adhesion both in the presence of serum proteins and when exposed to activated RGD peptide. In contrast, PS conjugated with RGD via endgroup-activated PEO/PPO/PEO copolymers supported cell adhesion and spreading. The surface coupling scheme reported here should prove valuable for studying cell-ligand interactions under simplified and highly controlled conditions.

Effects of broad frequency vibration on cultured osteoblasts

Bone is subjected in vivo to both high amplitude, low frequency strain, incurred by locomotion, and to low amplitude, broad frequency strain. The biological effects of low amplitude, broad frequency strain are poorly understood. To evaluate the effects of low amplitude strains ranging in frequency from 0 to 50 Hz on osteoblastic function, we seeded MC3T3-E1 cells into collagen gels and applied the following loading protocols for 3 min per day for either 3 or 7 days: (1) sinusoidal strain at 3 Hz, with 0-3000 microstrain peak-to-peak followed by 0.33 s resting time, (2) "broad frequency vibration" of low amplitude strain (standard deviation of 300 microstrain) including frequency components from 0 to 50 Hz, and (3) sinusoidal strain combined with broad frequency vibration (S + V). The cells were harvested on day 4 or 8. We found that the S + V stimulation significantly repressed cell proliferation by day 8. Osteocalcin mRNA was up-regulated 2.6-fold after 7 days of S + V stimulation, and MMP-9 mRNA was elevated 1.3-fold after 3 days of vibration alone. Sinusoidal stimulation alone did not affect the cell responses. No differences due to loading were observed in alkaline phosphatase activity and in mRNA levels of type I collagen, osteopontin, connexin 43, MMPs-1A, -3, -13. These results suggest that osteoblasts are more sensitive to low amplitude, broad frequency strain, and this kind of strain could sensitize osteoblasts to high amplitude, low frequency strain. This suggestion implies a potential contribution of stochastic resonance to the mechanical sensitivity of osteoblasts.

Thrombospondin-4, an extracellular matrix protein expressed in the developing and adult nervous system promotes neurite outgrowth

Extracellular matrix (ECM) molecules are involved in multiple aspects of cell-to-cell signaling during development and in the adult. In nervous system development, specific recognition processes, e.g., during axonal pathfinding and synaptogenesis involve modulation and signaling by ECM components. Much less is known about their presence and possible roles in the adult nervous system. We now report that thrombospondin-4 (TSP-4), a recently discovered member of the TSP gene family is expressed by neurons, promotes neurite outgrowth, and accumulates at the neuromuscular junction and at certain synapse-rich structures in the adult. To search for muscle genes that may be involved in neuromuscular signaling, we isolated cDNAs induced in adult skeletal muscle by denervation. One of these cDNAs coded for the rat homologue of TSP-4. In skeletal muscle, it was expressed by muscle interstitial cells. The transcript was further detected in heart and in the developing and adult nervous system, where it was expressed by a wide range of neurons. An antiserum to the unique carboxyl-terminal end of the protein allowed to specifically detect TSP-4 in transfected cells in vitro and on cryostat sections in situ. TSP-4 associated with ECM structures in vitro and in vivo. In the adult, it accumulated at the neuromuscular junction and at synapse-rich structures in the cerebellum and retina. To analyze possible activities of TSP-4 towards neurons, we carried out coculture experiments with stably transfected COS cells and motor, sensory, or retina neurons. These experiments revealed that TSP-4 was a preferred substrate for these neurons, and promoted neurite outgrowth. The results establish TSP-4 as a neuronal ECM protein associated with certain synapse-rich structures in the adult. Its activity towards embryonic neurons in vitro and its distribution in vivo suggest that it may be involved in local signaling in the developing and adult nervous system.

The novel catenin p120cas binds classical cadherins and induces an unusual morphological phenotype in NIH3T3 fibroblasts

p120cas (CAS) is a tyrosine kinase substrate whose phosphorylation has been implicated in cell transformation by Src and in ligand-induced signaling through the EGF, PDGF, and CSF-1 receptors. More recently, CAS has been shown to associate with E-cadherin and its cofactors (catenins), molecules that are involved in cell adhesion. Although both CAS and beta-catenin contain armadillo repeat domains (Arm domains), the amino acid identity between these proteins in this region is only 22%, and it is not yet clear whether CAS will emulate other catenins by associating with other members of the cadherin family. Here we report that in addition to binding E-cadherin, wild-type CAS associated with N-cadherin and P-cadherin. Transient transfection of cloned CAS isoforms into MDCK epithelial cells indicated that CAS1 and CAS2 isoforms are equally capable of binding to E-cadherin even though these cells preferentially express CAS2 isoforms. In addition, CAS colocalized with N-cadherin in NIH3T3 cells and analysis of CAS mutants in vivo indicated that the CAS-N-cadherin interaction requires an intact CAS Arm domain. The data suggest that CAS-cadherin interactions in general are dictated by the conserved armadillo repeats and are not heavily influenced by sequences added outside the Arm domain by alternative splicing. Interestingly, overexpression of CAS in NIH3T3 cells induced a striking morphological phenotype characterized by the presence of long dendrite-like processes. This branching phenotype was specific for CAS, since (i) overexpression of the structurally similar beta-catenin had little effect on cell morphology, and (ii) the branching was abolished by deletions in the CAS Arm domain. Our data indicate that, like other catenins, CAS is a cofactor for multiple members of the cadherin family. However, the dramatically distinct phenotype exhibited by fibroblasts overexpressing CAS, versus beta-catenin, support recent data suggesting that these catenins have fundamentally different and possibly opposing roles in cadherin complexes.