Uptake and intracellular transport of acidic fibroblast growth factor: evidence for free and cytoskeleton-anchored fibroblast growth factor receptors

Transcriptome analysis in comparing carcass and meat quality traits of Jiaxing Black Pig and Duroc × Duroc × Berkshire × Jiaxing Black Pig crosses

This study compares two typical strains: Chinese local excellent meat quality of Jiaxing Black (JXB) Pig and quadratic crossbred pig strain Duroc × Duroc × Berkshire × Jiaxing Black (DDBJ). It was found that between the two pig strains, carcass traits and meat quality traits differed significantly. This is exemplified by the leanness and dressing out percent of DDBJ that were significantly higher than JXB pigs of the same age (P < 0.05) and the better growth rate of DDBJ pigs as to JXB pigs was shown by quantifying muscle proliferation and differentiation of longissimus dorsi muscle employing Hematoxylin and Eosin staining of longissimus dorsi muscle. Nutrients such as inosinic acid, intramuscular fat, and free amino acids in the longissimus dorsi muscle were significantly higher in JXB pigs than DDBJ pigs (p < 0.0001); saturated fatty acids were higher in JXB than in DDBJ pigs (p = 0.0097); essential amino acids and fresh taste amino acids (serine, glutamic acid, proline, glycine, alanine) of JXB pigs was higher than that of DDBJ pigs (p < 0.0001) and amino acids in longissimus dorsi muscle of JXB pigs surpasses the amino acid concentration of DDBJ pigs (p < 0.0001), thus showing the superiority of JXB in terms of meat quality. However, the content of polyunsaturated fatty acids, which is responsible for poor meat quality, was significantly higher in the longissimus dorsi muscle of DDBJ pig than JXB pigs (p < 0.0001); RNA-seq analysis of 5 biological replicates from two of the strains was performed. The screening of 164 up-regulated genes and 183 down-regulated genes found in longissimus dorsi muscle of DDBJ was done and the results identified differentially expressed genes related to muscle development, adipogenesis, amino acid metabolism, fatty acid metabolism and inosine synthesis. In conclusion, the study identified functional genes, elucidated the mechanisms associated with carcass quality traits, meat quality traits and other related traits, and provided means of genetic enhancement to improve meat quality traits and carcass traits in Chinese commercial pigs.

Ebulin l Is Internalized in Cells by Both Clathrin-Dependent and -Independent Mechanisms and Does Not Require Clathrin or Dynamin for Intoxication

Ebulin l is an A-B toxin, and despite the presence of a B chain, this toxin displays much less toxicity to cells than the potent A-B toxin ricin. Here, we studied the binding, mechanisms of endocytosis, and intracellular pathway followed by ebulin l and compared it with ricin. COS-1 cells and HeLa cells with inducible synthesis of a mutant dynamin (K44A) were used in this study. The transport of these toxins was measured using radioactively or fluorescently labeled toxins. The data show that ebulin l binds to cells to a lesser extent than ricin. Moreover, the expression of mutant dynamin does not affect the endocytosis, degradation, or toxicity of ebulin l. However, the inhibition of clathrin-coated pit formation by acidification of the cytosol reduced ebulin l endocytosis but not toxicity. Remarkably, unlike ricin, ebulin l is not transported through the Golgi apparatus to intoxicate the cells and ebulin l induces apoptosis as the predominant cell death mechanism. Therefore, after binding to cells, ebulin l is taken up by clathrin-dependent and -independent endocytosis into the endosomal/lysosomal system, but there is no apparent role for clathrin and dynamin in productive intracellular routing leading to intoxication.

Sulfated syndecan 1 is critical to preventing cellular senescence by modulating fibroblast growth factor receptor endocytosis

Cellular senescence can be triggered by various intrinsic and extrinsic stimuli. We previously reported that silencing of 3'-phosphoadenosine 5'-phosphosulfate synthetase 2 (PAPSS2) induces cellular senescence through augmented fibroblast growth factor receptor 1 (FGFR1) signaling. However, the exact molecular mechanism connecting heparan sulfation and cellular senescence remains unclear. Here, we investigated the potential involvement of heparan sulfate proteoglycans (HSPGs) in augmented FGFR1 signaling and cellular senescence. Depletion of several types of HSPGs revealed that cells depleted of syndecan 1 (SDC1) exhibited typical senescence phenotypes, and those depleted of PAPSS2-, SDC1-, or heparan sulfate 2-O sulfotransferase 1 (HS2ST1) showed decreased FGFR1 internalization along with hyperresponsiveness to and prolonged activation of fibroblast growth factor 2 (FGF2)-stimulated FGFR1- v-akt murine thymoma viral oncogene homolog (AKT) signaling. Clathrin- and caveolin-mediated FGFR1 endocytosis contributed to cellular senescence through the FGFR1-AKT-p53-p21 signaling pathway. Dynasore treatment triggered senescence phenotypes, augmented FGFR1-AKT-p53-p21 signaling, and decreased SDC1 expression. Finally, the replicatively and prematurely senescent cells were characterized by decreases of SDC1 expression and FGFR1 internalization, and an increase in FGFR1-AKT-p53-p21 signaling. Together, our results demonstrate that properly sulfated SDC1 plays a critical role in preventing cellular senescence through the regulation of FGFR1 endocytosis.

NEGR1 and FGFR2 cooperatively regulate cortical development and core behaviours related to autism disorders in mice

Autism spectrum disorders are neurodevelopmental conditions with diverse aetiologies, all characterized by common core symptoms such as impaired social skills and communication, as well as repetitive behaviour. Cell adhesion molecules, receptor tyrosine kinases and associated downstream signalling have been strongly implicated in both neurodevelopment and autism spectrum disorders. We found that downregulation of the cell adhesion molecule NEGR1 or the receptor tyrosine kinase fibroblast growth factor receptor 2 (FGFR2) similarly affects neuronal migration and spine density during mouse cortical development in vivo and results in impaired core behaviours related to autism spectrum disorders. Mechanistically, NEGR1 physically interacts with FGFR2 and modulates FGFR2-dependent extracellular signal-regulated kinase (ERK) and protein kinase B (AKT) signalling by decreasing FGFR2 degradation from the plasma membrane. Accordingly, FGFR2 overexpression rescues all defects due to Negr1 knockdown in vivo. Negr1 knockout mice present phenotypes similar to Negr1-downregulated animals. These data indicate that NEGR1 and FGFR2 cooperatively regulate cortical development and suggest a role for defective NEGR1-FGFR2 complex and convergent downstream ERK and AKT signalling in autism spectrum disorders.

Regulation of Caveolin-1 and Junction Proteins by bFGF Contributes to the Integrity of Blood-Spinal Cord Barrier and Functional Recovery

The blood-spinal cord barrier (BSCB) plays important roles in the recovery of spinal cord injury (SCI), and caveolin-1 is essential for the integrity and permeability of barriers. Basic fibroblast growth factor (bFGF) is an important neuroprotective protein and contributes to the survival of neuronal cells. This study was designed to investigate whether bFGF is beneficial for the maintenance of junction proteins and the integrity of the BSCB to identify the relations with caveolin-1 regulation. We examined the integrity of the BSCB with Evans blue dye and fluorescein isothiocyanate-dextran extravasation, measured the junction proteins and matrix metalloproteinases, and evaluated the locomotor function recovery. Our data indicated that bFGF treatment improved the recovery of BSCB and functional locomotion in contusive SCI model rats, reduced the expression and activation of matrix metalloproteinase-9, and increased the expressions of caveolin-1 and junction proteins, including occludin, claudin-5, p120-catenin, and β-catenin. In the brain, in microvascular endothelial cells, bFGF treatment increased the levels of junction proteins, caveolin-1 small interfering RNA abolished the protective effect of bFGF under oxygen-glucose deprivation conditions, and the expression of fibroblast growth factor receptor 1 and co-localization with caveolin-1 decreased significantly, which could not be reversed by bFGF treatment. These findings provide a novel mechanism underlying the beneficial effects of bFGF on the BSCB and recovery of SCI, especially the regulation of caveolin-1.

FGF21 promotes endothelial cell angiogenesis through a dynamin-2 and Rab5 dependent pathway

Binding of angiogenic molecules with cognate receptor tyrosine kinases (RTK) is required for angiogenesis however the precise link between RTK binding, endocytosis, and signaling requires further investigation. Here, we use FGFR1 as a model to test the effects of the large GTPase and endocytosis regulatory molecule dynamin-2 on angiogenic signaling in context of distinct FGF ligands. In vitro, overexpression of dominant negative dynamin-2 (DynK44A) attenuates FGFR1 activation of Erk and tubulogenesis by FGF2. Furthermore, we identify FGF21, a non-classical, FGF ligand implicated in diverse human pathologies as an angiogenic molecule acting through FGFR1 and β-Klotho coreceptor. Disruption of FGFR1 activation of ERK by FGF21 is achieved by perturbation of the function of both dynamin-2 and Rab5 GTPase. In vivo, mice harboring endothelial selective overexpression of DynK44A, show impaired angiogenesis in response to FGF21. In conclusion, dynamin dependent endocytosis of FGFR1 is required for in vitro and in vivo angiogenesis in response to FGF2 and the non-classical FGF ligand, FGF21. These studies extend our understanding of the relationships between RTK binding, internalization, endosomal targeting, and angiogenic signaling.

Endocytosis of Fgf8 is a double-stage process and regulates spreading and signaling

Tightly controlled concentration gradients of morphogens provide positional information and thus regulate tissue differentiation and morphogenesis in multicellular organisms. However, how such morphogenetic fields are formed and maintained remains debated. Here we show that fibroblast growth factor 8 (Fgf8) morphogen gradients in zebrafish embryos are established and maintained by two essential mechanisms. Firstly, Fgf8 is taken up into the cell by clathrin-mediated endocytosis. The speed of the uptake rate defines the range of the morphogenetic gradient of Fgf8. Secondly, our data demonstrate that after endocytosis the routing of Fgf8 from the early endosome to the late endosome shuts down signaling. Therefore, intracellular endocytic transport regulates the intensity and duration of Fgf8 signaling. We show that internalization of Fgf8 into the early endosome and subsequent transport towards the late endosome are two independent processes. Therefore, we hypothesize that Fgf8 receiving cells control both, the propagation width and the signal strength of the morphogen.

Sorting of the FGF receptor 1 in a human glioma cell line

Fibroblast growth factor receptor 1 (FGFR1) is a receptor tyrosine kinase promoting tumor growth in a variety of cancers, including glioblastoma. Binding of FGFs triggers the intracellular Ras/Raf/ERK signaling pathway leading to cell proliferation. Down-regulation of FGFR1 and, consequently, inactivation of its signaling pathways represent novel treatment strategies for glioblastoma. In this study, we investigated the internalization and endocytic trafficking of FGFR1 in the human glioma cell line U373. Stimulation with FGF-2 induced cell rounding accompanied by increased BrdU and pERK labeling. The overexpression of FGFR1 (without FGF treatment) resulted in enhanced phosphorylated FGFR1 suggesting receptor autoactivation. Labeled ligand (FGF-2-Cy5.5) was endocytosed in a clathrin- and caveolin-dependent manner. About 25 % of vesicles carrying fluorescently tagged FGFR1 represented early endosomes, 15 % transferrin-positive recycling endosomes and 40 % Lamp1-positive late endosomal/lysosomal vesicles. Stimulation with FGF-2 increased the colocalization rate in each of these vesicle populations. The treatment with the lysosomal inhibitor leupeptin resulted in FGFR1 accumulation in lysosomes, but did not enhance receptor recycling as observed in neurons. Analysis of vesicle distributions revealed an accumulation of recycling endosomes in the perinuclear region. In conclusion, the shuttling of receptor tyrosine kinases can be directly visualized by overexpression of fluorescently tagged receptors which respond to ligand stimulation and follow the recycling and degradation pathways similarly to their endogenous counterparts.

Evidence for the interaction of fibroblast growth factor-2 with the lymphatic endothelial cell marker LYVE-1

LYVE-1 (lymphatic vessel endothelial hyaluronan receptor-1) is a homolog of the hyaluronan receptor CD44, and one of the most widely used markers of lymphatic endothelial cells in normal and tumor tissues. However, the physiologic role of LYVE-1 in the lymphatic system still remains unclear. It is well established that fibroblast growth factor 2 (FGF2) induces lymphangiogenesis. Based on the known interaction between FGF2 and CD44 and based on the structural similarity of CD44 and LYVE-1, we investigated whether FGF2 might interact with LYVE-1. We found that FGF2 is able to bind LYVE-1 using AlphaScreen, or after surface-immobilization or in solution. FGF2 binds to LYVE-1 with a higher affinity than any other known LYVE-1–binding molecules, such as hyaluronan or PDGF-BB. Glycosylation of LYVE-1 is important for FGF2 binding. Furthermore, FGF2 interacts with LYVE-1 when overexpressed in CHO cells. Soluble LYVE-1 and knockdown of LYVE-1 in lymphatic endothelial cells impaired FGF2 signaling and functions. In addition, FGF2 but not VEGF-C-induced in vivo lymphangiogenesis, was also inhibited. Conversely, FGF2 also modulates LYVE-1 expression in cells and ex vivo. Thus, our data demonstrate a functional relationship to the interaction between FGF2 and LYVE-1.

Regulation of fibroblast growth factor receptor signalling and trafficking by Src and Eps8

Fibroblast growth factor receptors (FGFRs) mediate a wide spectrum of cellular responses that are crucial for development and wound healing. However, aberrant FGFR activity leads to cancer. Activated growth factor receptors undergo stimulated endocytosis, but can continue to signal along the endocytic pathway. Endocytic trafficking controls the duration and intensity of signalling, and growth factor receptor signalling can lead to modifications of trafficking pathways. We have developed live-cell imaging methods for studying FGFR dynamics to investigate mechanisms that coordinate the interplay between receptor trafficking and signal transduction. Activated FGFR enters the cell following recruitment to pre-formed clathrin-coated pits (CCPs). However, FGFR activation stimulates clathrin-mediated endocytosis; FGF treatment increases the number of CCPs, including those undergoing endocytosis, and this effect is mediated by Src and its phosphorylation target Eps8. Eps8 interacts with the clathrin-mediated endocytosis machinery and depletion of Eps8 inhibits FGFR trafficking and immediate Erk signalling. Once internalized, FGFR passes through peripheral early endosomes en route to recycling and degredative compartments, through an Src- and Eps8-dependent mechanism. Thus Eps8 functions as a key coordinator in the interplay between FGFR signalling and trafficking. This work provides the first detailed mechanistic analysis of growth factor receptor clustering at the cell surface through signal transduction and endocytic trafficking. As we have characterised the Src target Eps8 as a key regulator of FGFR signalling and trafficking, and identified the early endocytic system as the site of Eps8-mediated effects, this work provides novel mechanistic insight into the reciprocal regulation of growth factor receptor signalling and trafficking.

Syndecan 4 regulates FGFR1 signaling in endothelial cells by directing macropinocytosis

Fibroblast growth factor 2 (FGF2) induces endothelial cell migration and angiogenesis through two classes of receptors: receptor tyrosine kinases, such as FGF receptor 1 (FGFR1), and heparan sulfate proteoglycans, such as syndecan 4 (S4). We examined the distinct contributions of FGFR1 and S4 in shaping the endothelial response to FGF2. S4 determined the kinetics and magnitude of FGF2-induced mitogen-activated protein kinase (MAPK) signaling by promoting the macropinocytosis of the FGFR1-S4-FGF2 signaling complex. Internalization of the S4 receptor complex was independent of clathrin and dynamin, proceeded from lipid raft-enriched membranes, and required activation of the guanosine triphosphatases RhoG and Rab5. Genetic knockout of S4, disruption of S4 function, or inhibition of Rab5 led to increased endocytosis and MAPK signaling. These data define the mechanism by which FGFR1 and S4 coordinate downstream signaling upon FGF2 stimulation: FGFR1 initiates MAPK signaling, whereas S4-dependent FGFR1 macropinocytosis modulates the kinetics of MAPK activation. Our studies identify S4 as a regulator of MAPK signaling and address the question of how distinct classes of FGFRs individually contribute to signal transduction in endothelial cells.

Inhibition of fibroblast growth factor receptor 1 endocytosis promotes axonal branching of adult sensory neurons

Fibroblast growth factors (FGFs) promote axon growth during development and regeneration of the nervous system. Among the four types of FGF receptors (FGFRs), FGFR1 is expressed in adult sensory neurons of dorsal root ganglia (DRG), and overexpression of FGFR1 promotes FGF-2-induced elongative axon growth in vitro. Ligand-induced activation of FGFR1 is followed by endocytosis and lysosomal degradation, which leads to the termination of receptor signaling. We previously reported that the lysosomal inhibitor leupeptin enhances FGF-2-induced elongative axon growth of adult DRG neurons overexpressing FGFR1. To better understand the role of subcellular localization of FGFR1 in axon growth, we analyzed the effects of inhibition of endocytosis of FGFR1 on FGF-2-induced neurite outgrowth in PC12 pheochromocytoma cells and adult DRG neurons. The endocytosis inhibitors methyl-β-cyclodextrin (MβCD) and chlorpromazine enhanced surface localization of FGFR1 in PC12 cells and DRG neurons. Furthermore, MβCD and chlorpromazine increased FGF-2-induced neurite outgrowth of PC12 cells and axonal branching of adult DRG neurons overexpressing FGFR1, whereas MβCD inhibited FGF-2-induced axonal elongation. Analysis of the signaling pathways involved in axon morphology revealed that FGF-2-induced phosphorylation of extracellular signal-regulated kinase (ERK) and Akt was increased by inhibition of FGFR1 endocytosis. Together, our results imply that inhibition of FGFR1 endocytosis by MβCD or chlorpromazine promotes FGF-2-induced axonal branching. The results of this study confirm that internalization of FGFR1 controls axon growth and morphology of adult sensory neurons via selective activation of intracellular signaling pathways.

Cell surface heparan sulfates mediate internalization of the PWWP/HATH domain of HDGF via macropinocytosis to fine-tune cell signalling processes involved in fibroblast cell migration

HDGF (hepatoma-derived growth factor) stimulates cell proliferation by functioning on both sides of the plasma membrane as a ligand for membrane receptor binding to trigger cell signalling and as a stimulator for DNA synthesis in the nucleus. Although HDGF was initially identified as a secretory heparin-binding protein, the biological significance of its heparin-binding ability remains to be determined. In the present study we demonstrate that cells devoid of surface HS (heparan sulfate) were unable to internalize HDGF, HATH (N-terminal domain of HDGF consisting of amino acid residues 1-100, including the PWWP motif) and HATH(K96A) (single-site mutant form of HATH devoid of receptor binding activity), suggesting that the binding of HATH to surface HS is important for HDGF internalization. We further demonstrate that both HATH and HATH(K96A) could be internalized through macropinocytosis after binding to the cell surface HS. Interestingly, HS-mediated HATH(K96A) internalization is found to exhibit an inhibitory effect on cell migration and proliferation in contrast with that observed for HATH action on NIH 3T3 cells, suggesting that HDGF exploits the innate properties of both cell surface HS and membrane receptor via the HATH domain to affect related cell signalling processes. The present study indicates that MAPK (mitogen-activated protein kinase) signalling pathways could be affected by the HS-mediated HATH internalization to regulate cell migration in NIH 3T3 fibroblasts, as judged from the differential effect of HATH and HATH(K96A) treatment on the expression level of matrix metalloproteases.

Ubiquitination of fibroblast growth factor receptor 1 is required for its intracellular sorting but not for its endocytosis

Endocytosis and targeting of growth factor receptors for lysosomal degradation have been associated with ubiquitination of the intracellular part of the receptors. To elucidate the role of receptor ubiquitination in internalization and sorting of fibroblast growth factor receptor (FGFR), we constructed several mutants of FGFR1 in which lysines, potential ubiquitination sites, were substituted for arginines. Substitution of all lysine residues in the intracellular part of FGFR1 resulted in inactivation of the tyrosine kinase domain of the receptor. However, several multilysine FGFR1 mutants, where up to 26 of 29 lysines in the intracellular part of the receptor were mutated, retained tyrosine kinase activity. The active multilysine mutants were poorly ubiquitinated, but internalized normally, indicating that ubiquitination of the receptor is not required for endocytosis. In contrast, degradation of the multilysine mutants was dramatically reduced as the mutants were inefficiently transported to lysosomes but rather sorted to recycling endosomes. The altered sorting resulted in sustained signaling. The duration of FGFR1 signaling seems to be tightly regulated by receptor ubiquitination and subsequent sorting to the lysosomes for degradation.

Keratinocyte growth factor receptor ligands target the receptor to different intracellular pathways

The keratinocyte growth factor receptor (KGFR)/fibroblast growth factor receptor 2b is activated by high-affinity-specific interaction with two different ligands, keratinocyte growth factor (KGF)/fibroblast growth factor (FGF)7 and FGF10/KGF2, which are characterized by an opposite requirement of heparan sulfate proteoglycans and heparin for binding to the receptor. We investigated here the possible different endocytic trafficking of KGFR, induced by the two ligands. Immunofluorescence and immunoelectron microscopy analysis showed that KGFR internalization triggered by either KGF or FGF10 occurs through clathrin-coated pits. Immunofluorescence confocal microscopy using endocytic markers as well as tumor susceptibility gene 101 (TSG101) silencing demonstrated that KGF drives KGFR to the degradative pathway, while FGF10 targets the receptor to the recycling endosomes. Biochemical analysis showed that KGFR is ubiquitinated and degraded after KGF treatment but not after FGF10 treatment, and that the alternative fate of KGFR might depend on the different ability of the receptor to phosphorylate the fibroblast growth factor receptor substrate 2 (FRS2) substrate and to recruit the ubiquitin ligase c-Cbl. The recycling endocytic pathway followed by KGFR upon FGF10 stimulation correlates with the higher mitogenic activity exerted by this ligand on epithelial cells compared with KGF, suggesting that the two ligands may play different functional roles through the regulation of the receptor endocytic transport.

Different abilities of the four FGFRs to mediate FGF-1 translocation are linked to differences in the receptor C-terminal tail

Members of the fibroblast growth factor family bind to one or more of the four closely related membrane-spanning FGF receptors. In addition to signaling through the receptors, exogenous FGF-1 and FGF-2 are endocytosed and translocated to the cytosol and nucleus where they stimulate RNA and DNA synthesis. Here we have studied the ability of the four FGF receptors to facilitate translocation of exogenous FGF-1 to the cytosol and nucleus. FGFR1 and FGFR4 were able to mediate translocation, whereas FGFR2 and FGFR3 completely lacked this ability. By analyzing mutant FGFRs we found that the tyrosine kinase domain could be deleted from FGFR1 without abolishing translocation, whereas the C-terminal tail of the FGFRs, constituted by approximately 50 amino acids downstream of the kinase domain, plays a crucial role in FGF-1 translocation. Three amino acids residues within the C-terminal tail were found to be of particular importance for translocation. For FGFR2, the two amino acid substitutions Q774M and P800H were sufficient to enable the receptor to support FGF-1 translocation. The results demonstrate a striking diversity in function of the four FGFRs determined by their C-terminal domain.

Reduced binding of FGF1 to mutant fibroblast growth factor receptor 3

The activating mutation FGFR3-R248C in the D2-D3 linker region of fibroblast growth factor receptor 3 leads as germline mutation to the neonatal lethal syndrome thanatophoric dysplasia type I (TD1). As somatic mutation it has been found in cancer. We introduced into the murine FGFR3 the mutation R242C that is orthologoues to the human mutation R248C. A strong reduction in binding of the 16 and 18 kDa forms of FGF1 to the mutant receptor was found, highlighting the importance of D2-D3 linker region of FGFR3 in determination of binding affinity to ligands. Another mutant, G374R, introduced into the murine FGFR3, is orthologoues to the human mutant FGFR3-G380R, and leads to achondroplasia (ACH). The binding of the 16 kDa and 18 kDa forms of FGF1 to this mutant receptor was the same as for wild-type FGFR3 in a cell-free system, but it was reduced in living cells. The data indicate a minor changes in conformation of FGFR3-G374R receptors at the cell surface that lead to reduced binding to FGF1.

Lipid rafts serve as a signaling platform for nicotinic acetylcholine receptor clustering

Agrin, a motoneuron-derived factor, and the muscle-specific receptor tyrosine kinase (MuSK) are essential for the acetylcholine receptor (AChR) clustering at the postjunctional membrane. However, the underlying signaling mechanisms remain poorly defined. We show that agrin stimulates a dynamic translocation of the AChR into lipid rafts-cholesterol and sphingolipid-rich microdomains in the plasma membrane. This follows MuSK partition into lipid rafts and requires its activation. Disruption of lipid rafts inhibits MuSK activation and downstream signaling and AChR clustering in response to agrin. Rapsyn, an intracellular protein necessary for AChR clustering, is located constitutively in lipid rafts, but its interaction with the AChR is inhibited when lipid rafts are perturbed. These results reveal that lipid rafts may regulate AChR clustering by facilitating the agrin/MuSK signaling and the interaction between the receptor and rapsyn, both necessary for AChR clustering and maintenance. These results provide insight into mechanisms of AChR cluster formation.

FGF-1 and FGF-2 Require the Cytosolic Chaperone Hsp90 for Translocation into the Cytosol and the Cell Nucleus

Similarly to many protein toxins, the growth factors fibroblast growth factor 1 (FGF-1) and FGF-2 translocate from endosomes into the cytosol. It was recently found that certain toxins are dependent on cytosolic Hsp90 for efficient translocation across the endosomal membrane. We therefore investigated the requirement for Hsp90 in FGF translocation. We found that low concentrations of the specific Hsp90 inhibitors, geldanamycin and radicicol, completely blocked the translocation of FGF-1 and FGF-2 to the cytosol and the nucleus. The drugs did not interfere with the initial binding of FGF-1 to the growth factor receptors at the cell-surface or with the subsequent internalization of the growth factors into endosomes. The activation of known signaling cascades downstream of the growth factor receptors was also not affected by the drugs. The data indicate that the drugs block translocation from endosomes to the cytosol implying that Hsp90 is required for translocation of FGF-1 and FGF-2 across the endosomal membrane.

Different intracellular trafficking of FGF1 endocytosed by the four homologous FGF receptors

Many growth factors and cytokines bind to more than one receptor, but in many cases the different roles of the separate receptors in signal transduction are unclear. Intracellular sorting of ligand-receptor complexes may modulate the signalling, and we have here studied the intracellular trafficking of ligand bound to receptors for fibroblast growth factors (FGFs). For this purpose, we transfected HeLa cells with any one of the four tyrosine kinase FGF receptors (FGFR1-4). In cells expressing any one of these receptors, externally added FGF1 was localized to sorting/early endosomes after 15 minutes at 37 degrees C. After longer incubation times, FGF1 internalized in cells expressing FGFR1 was localized mainly to late endosomes/lysosomes, similarly to EGF. By contrast, FGF1 internalized in cells expressing FGFR4 followed largely the same intracellular pathway as the recycling ligand, transferrin. In cells expressing FGFR2 or FGFR3, sorting of FGF1 to lysosomes was somewhat less efficient than that observed for FGFR1. Furthermore, FGF1 was more slowly degraded in cells expressing FGFR4 than in cells expressing FGFR1-3 and in addition, internalized FGFR4 as such was more slowly degraded than the other receptors. The data indicate that after endocytosis, FGFR4 and its bound ligand are sorted mainly to the recycling compartment, whereas FGFR1-3 with ligand are sorted mainly to degradation in the lysosomes. Alignment of the amino acid sequence of the intracellular part of the four FGFRs revealed several lysines conserved in FGFR1-3 but absent in FGFR4. Lysines are potential ubiquitylation sites and could thus target a receptor to lysosomes for degradation. Indeed, we found that FGFR4 is less ubiquitylated than FGFR1, which could be the reason for the different sorting of the receptors.

Nuclear Translocation of Cell-Surface Receptors: Lessons from Fibroblast Growth Factor

The nuclear localization of a number of growth factors, cytokine ligands and their receptors has been reported in various cell lines and tissues. These include members of the fibroblast growth factor (FGF), epidermal growth factor and growth hormone families. Accordingly, a number of nuclear functions have begun to emerge for these protein families. The demonstration of functional interactions of these proteins with the nuclear import machinery has further supported their functions as nuclear signal transducers. Here, we review the membrane- trafficking machinery and pathways demonstrated to regulate this cell surface to nucleus-trafficking event and highlight the many remaining unanswered questions. We focus on the FGF family, which is providing many of the clues as to the process of this unusual phenomenon.

Fibroblast growth factor 2 endocytosis in endothelial cells proceed via syndecan-4-dependent activation of Rac1 and a Cdc42-dependent macropinocytic pathway

Full activity of fibroblast growth factors (FGFs) requires their internalization in addition to the interaction with cell surface receptors. Recent studies have suggested that the transmembrane proteoglycan syndecan-4 functions as a FGF2 receptor. In this study we investigated the molecular basis of syndecan endocytosis and its role in FGF2 internalization in endothelial cells. We found that syndecan-4 uptake, induced either by treatment with FGF2 or by antibody clustering, requires the integrity of plasma membrane lipid rafts for its initiation, occurs in a non-clathrin-, non-dynamin-dependent manner and involves Rac1, which is activated by syndecan-4 clustering. FGF2 was internalized in a complex with syndecan-4 in 70 kDa dextran-containing endocytic vesicles. FGF2 and syndecan-4 but not dextran endocytosis were blocked by the dominant negative Rac1 while amiloride and the dominant-negative Cdc42 blocked internalization of dextran in addition to FGF2 and syndecan-4. Taken together, these results demonstrate that FGF2 endocytosis requires syndecan-4 clustering-dependent activation of Rac1 and the intact CDC42-dependent macropinocytic pathway.

Endocytosis controls spreading and effective signaling range of Fgf8 protein

Secreted signaling molecules released from a restricted source are of great importance during embryonic development because they elicit induction, proliferation, differentiation, and patterning events in target cells . Fgf8 is a member of the fibroblast growth factor family with key inductive functions during vertebrate development of, for example, the forebrain , midbrain , cerebellum , heart , inner ear , and mesoderm . However, the mechanism by which the signaling range of Fgf8 is controlled in a field of target cells is unknown. We studied Fgf8 as a potential morphogen in the nascent neuroectoderm of living zebrafish embryos. We find that spreading of epitope-tagged Fgf8 through target tissue is carefully controlled by endocytosis and subsequent degradation in lysosomes, or "restrictive clearance," from extracellular spaces. If internalization is inhibited, Fgf8 protein accumulates extracellularly, spreads further, and activates target gene expression over a greater distance. Conversely, enhanced internalization increases Fgf8 uptake and shortens its effective signaling range. Our results suggest that Fgf8 spreads extracellularly by a diffusion-based mechanism and demonstrate that target cells can actively influence, through endocytosis and subsequent degradation, the availability of Fgf8 ligand to other target cells.

BDNF-induced recruitment of TrkB receptor into neuronal lipid rafts: roles in synaptic modulation

Brain-derived neurotrophic factor (BDNF) plays an important role in synaptic plasticity but the underlying signaling mechanisms remain unknown. Here, we show that BDNF rapidly recruits full-length TrkB (TrkB-FL) receptor into cholesterol-rich lipid rafts from nonraft regions of neuronal plasma membranes. Translocation of TrkB-FL was blocked by Trk inhibitors, suggesting a role of TrkB tyrosine kinase in the translocation. Disruption of lipid rafts by depleting cholesterol from cell surface blocked the ligand-induced translocation. Moreover, disruption of lipid rafts prevented potentiating effects of BDNF on transmitter release in cultured neurons and synaptic response to tetanus in hippocampal slices. In contrast, lipid rafts are not required for BDNF regulation of neuronal survival. Thus, ligand-induced TrkB translocation into lipid rafts may represent a signaling mechanism selective for synaptic modulation by BDNF in the central nervous system.

Phosphorylation-regulated nucleocytoplasmic trafficking of internalized fibroblast growth factor-1

Fibroblast growth factor-1 (FGF-1), which stimulates cell growth, differentiation, and migration, is capable of crossing cellular membranes to reach the cytosol and the nucleus in cells containing specific FGF receptors. The cell entry process can be monitored by phosphorylation of the translocated FGF-1. We present evidence that phosphorylation of FGF-1 occurs in the nucleus by protein kinase C (PKC)delta. The phosphorylated FGF-1 is subsequently exported to the cytosol. A mutant growth factor where serine at the phosphorylation site is exchanged with glutamic acid, to mimic phosphorylated FGF-1, is constitutively transported to the cytosol, whereas a mutant containing alanine at this site remains in the nucleus. The export can be blocked by leptomycin B, indicating active and receptor-mediated nuclear export of FGF-1. Thapsigargin, but not leptomycin B, prevents the appearance of active PKCdelta in the nucleus, and FGF-1 is in this case phosphorylated in the cytosol. Leptomycin B increases the amount of phosphorylated FGF-1 in the cells by preventing dephosphorylation of the growth factor, which seems to occur more rapidly in the cytoplasm than in the nucleus. The nucleocytoplasmic trafficking of the phosphorylated growth factor is likely to play a role in the activity of internalized FGF-1.

Deletion mutant of FGFR4 induces onion-like membrane structures in the nucleus

The expression of several deletion mutants of fibroblast growth factor receptor 4 (FGFR4) was studied in COS-1 cells. FGFR4-mutants lacking most of the extracellular region did not efficiently reach the plasma membrane but accumulated in the endoplasmic reticulum (ER) and Golgi body. A mutant FGFR4 lacking the kinase domain as well as most of the extracellular region (DeltaExt/R4Tth) had a distinct intracellular distribution. It localized in part to the nucleus, where it exhibited a striking spotted pattern. Ultrastructural studies showed that the nuclear spots consisted of several layers of membrane that were folded into onion-like structures at the nucleoplasmic side of the nuclear envelope. These intranuclear structures did not contain nuclear pores but were positive for the ER proteins calreticulin and protein disulfide isomerase, in addition to abundant DeltaExt/R4Tth. Formation of the intranuclear structures was sensitive to inhibition of protein kinase C. Live microscopy of a green-fluorescent-protein/DeltaExt/R4Tth fusion protein showed that the intranuclear structures were stable and immobile, suggesting that they function as deposits of the overexpressed mutant and associated membrane. The DeltaExt/R4Tth protein also induced formation of densely packed membrane stacks in the cytosol and we suggest a model were the intranuclear structures are formed by invagination of ER-derived membrane stacks into the nucleus.

Signaling, internalization, and intracellular activity of fibroblast growth factor

The fibroblast growth factor (FGF) family contains 23 members in mammals including its prototype members FGF-1 and FGF-2. FGFs have been implicated in regulation of many key cellular responses involved in developmental and physiological processes. These includes proliferation, differentiation, migration, apoptosis, angiogenesis, and wound healing. FGFs bind to five related, specific cell surface receptors (FGFRs). Four of these have intrinsic tyrosine kinase activity. Dimerization of the receptor is a prerequisite for receptor transphosphorylation and activation of downstream signaling molecules. All members of the FGF family have a high affinity for heparin and for cell surface heparan sulfate proteoglycans, which participate in formation of stable and active FGF-FGFR complexes. FGF-mediated signaling is an evolutionarily conserved signaling module operative in invertebrates and vertebrates. It seems that some members of the family have a dual mode of action. FGF-1, FGF-2, FGF-3, and FGF-11-14 have been found intranuclearly as endogenous proteins. Exogenous FGF-1 and FGF-2 are internalized by receptor-mediated endocytosis, in a clathrin-dependent and -independent way. Internalized FGF-1 and FGF-2 are able to cross cellular membranes to reach the cytosol and the nuclear compartment. The role of FGF internalization and the intracellular activity of some FGFs are discussed in the context of the known signaling induced by FGF.

Defective lysosomal targeting of activated fibroblast growth factor receptor 3 in achondroplasia

Mutations of fibroblast growth factor receptor 3 (FGFR3) are responsible for achondroplasia (ACH) and related dwarfing conditions in humans. The pathogenesis involves constitutive activation of FGFR3, which inhibits proliferation and differentiation of growth plate chondrocytes. Here we report that activating mutations in FGFR3 increase the stability of the receptor. Our results suggest that the mutations disrupt c-Cbl-mediated ubiquitination that serves as a targeting signal for lysosomal degradation and termination of receptor signaling. The defect allows diversion of actively signaling receptors from lysosomes to a recycling pathway where their survival is prolonged, and, as a result, their signaling capacity is increased. The lysosomal targeting defect is additive to other mechanisms proposed to explain the pathogenesis of ACH.

Ligand-induced clathrin-mediated endocytosis of the keratinocyte growth factor receptor occurs independently of either phosphorylation or recruitment of eps15

Keratinocyte growth factor receptor (KGFR) is a receptor tyrosine kinase expressed on epithelial cells. Following ligand binding, KGFR is rapidly activated and internalized by clathrin-mediated endocytosis. Among the possible receptor substrates which could be involved in the regulation of KGFR endocytosis and down-modulation, we analyzed here the eps15 protein in view of the proposed general role of eps15 in regulating clathrin-mediated endocytosis as well as that of eps15 tyrosine phosphorylation in the control of regulated endocytosis. Immunoprecipitation and Western blot analysis showed that activated KGFR was not able to phosphorylate eps15, suggesting that eps15 is not a receptor substrate. Double immunofluorescence and confocal microscopy revealed that activated KGFR, differently from epidermal growth factor receptor (EGFR), did not induce recruitment of eps15 to the cell plasma membrane. Microinjection of a monoclonal antibody directed against the C-terminal DPF domain which contains the AP2 binding region of eps15 led to inhibition of both pathways of receptor-mediated endocytosis, the EGFR ligand-induced endocytosis and the transferrin constitutive endocytosis, but did not appear to block the KGFR ligand-induced internalization. Taken together our results indicate that the clathrin-mediated uptake of KGFR is not mediated by eps15.

Translocation of FGF-1 and FGF-2 across vesicular membranes occurs during G1-phase by a common mechanism

The entry of exogenous fibroblast growth factor 2 (FGF-2) to the cytosolic/nuclear compartment was studied and compared with the translocation mechanism used by FGF-1. To differentiate between external and endogenous growth factor, we used FGF-2 modified to contain a farnesylation signal, a CaaX-box. Because farnesylation occurs only in the cytosol and nucleoplasm, farnesylation of exogenous FGF-2-CaaX was taken as evidence that the growth factor had translocated across cellular membranes. We found that FGF-2 translocation occurred in endothelial cells and fibroblasts, which express FGF receptors, and that the efficiency of translocation was increased in the presence of heparin. Concomitantly with translocation, the 18-kDa FGF-2 was N-terminally cleaved to yield a 16-kDa form. Translocation of FGF-2 required PI3-kinase activity but not transport through the Golgi apparatus. Inhibition of endosomal acidification did not prevent translocation, whereas dissipation of the vesicular membrane potential completely blocked it. The data indicate that translocation occurs from intracellular vesicles containing proton pumps and that an electrical potential across the vesicle membrane is required. Translocation of both FGF-1 and FGF-2 occurred during most of G(1) but decreased shortly before the G(1)-->S transition. A common mechanism for FGF-1 and FGF-2 translocation into cells is postulated.

Nonclassical transport proteins and peptides: an alternative to classical macromolecule delivery systems

The number of peptides and proteins known to exhibit nonclassical transport activity has increased significantly in recent years. In most cases, these entities have been studied in relation to their ability to deliver high molecular weight compounds, including proteins and DNA, for the ultimate purpose of developing new drug delivery strategies. In this review, an overview of the various types of vectors is presented. The in vitro and in vivo delivery successes of this technology, as well as preliminary therapeutic efforts, are described. Although a comprehensive mechanism of nonclassical transport has not yet been clearly established, we propose a straightforward model based on the cationic nature of the vectors and the need for lack of highly organized structure. In this hypothesis we suggest that the movement of polycations is mediated by a network of extra- and intracellular polyanions while transport across the bilayer is facilitated by cation-pi interactions between the vectors' basic groups and aromatic amino acid side chains in the bilayer spanning helices of membrane proteins.

Inactivation of ether lipid biosynthesis causes male infertility, defects in eye development and optic nerve hypoplasia in mice

Although known for almost 80 years, the physiological role of plasmalogens (PLs), the major mammalian ether lipids (ELs), is still enigmatic. Humans that lack ELs suffer from rhizomelic chondrodysplasia punctata (RCDP), a peroxisomal disorder usually resulting in death in early childhood. In order to learn more about the functions of ELs, we generated a mouse model for RCDP by a targeted disruption of the dihydroxyacetonephosphate acyltransferase gene. The mutant mice revealed multiple abnormalities, such as male infertility, defects in eye development, cataract and optic nerve hypoplasia, some of which were also observed in RCDP. Mass spectroscopic analysis demonstrated the presence of highly unsaturated fatty acids including docosahexaenoic acid (DHA) in brain PLs and the occurrence of PLs in lipid raft microdomains (LRMs) isolated from brain myelin. In mutants, PLs were completely absent and the concentration of brain DHA was reduced. The marker proteins flotillin-1 and F3/contactin were found in brain LRMs in reduced concentrations. In addition, the gap junctional protein connexin 43, known to be recruited to LRMs and essential for lens development and spermatogenesis, was down-regulated in embryonic fibroblasts of the EL-deficient mice. Free cholesterol, an important constituent of LRMs, was found in these fibroblasts to be accumulated in a perinuclear compartment. These data suggest that the EL-deficient mice allow the identification of new phenotypes not related so far to EL-deficiency (male sterility, defects in myelination and optic nerve hypoplasia) and indicate that PLs are required for the correct assembly and function of LRMs.

Heparan sulphate glycosaminoglycans derived from endothelial cells and smooth muscle cells differentially modulate fibroblast growth factor-2 biological activity through fibroblast growth factor receptor-1

Fibroblast growth factor (FGF) signalling is involved in a wide range of important biological activities with differential effects in various cell types. The activity of FGF is modulated by heparin/heparan sulphate-like glycosaminoglycans (HSGAGs), found both in the extracellular matrix and on the cell surface. HSGAGs affect FGF signalling by interacting with both the growth factor and the FGF receptor (FGFR). In this study we sought to investigate whether HSGAGs at the cell surface of bovine aortic endothelial cells (BAEC) and smooth muscle cells (SMC) can differentially modulate FGF signalling in these cell types and modulate their differential response to FGF. We find that SMC and BAEC express the same FGFR isoforms and bind FGF2 with equal affinity at the cell surface, yet FGF has a markedly higher proliferative effect on SMC than on BAEC. Isolated HSGAGs from these two cell types were found to elicit distinct patterns of proliferation in chlorate-treated cells. Furthermore, examination of focal sequences reveals that HSGAGs from SMC, but not those from BAEC, retain the sulphation pattern necessary to induce FGF2 activity. As such, the differences in FGF2-mediated proliferation can be explained by the distinct cell surface HSGAGs of the two cell types. We conclude that the focal sequences of cell surface HSGAGs from SMC and BAEC govern, at least in part, the differential activity of FGF2 on these two cell types.

Transport of exogenous growth factors and cytokines to the cytosol and to the nucleus

In recent years a number of growth factors, cytokines, protein hormones, and other proteins have been found in the nucleus after having been added externally to cells. This review evaluates the evidence that translocation takes place and discusses possible mechanisms. As a demonstration of the principle that extracellular proteins can penetrate cellular membranes and reach the cytosol, a brief overview of the penetration mechanism of protein toxins with intracellular sites of action is given. Then problems and pitfalls in attempts to demonstrate the presence of proteins in the cytosol and in the nucleus as opposed to intracellular vesicular compartments are discussed, and some new approaches to study this are described. A detailed overview of the evidence for translocation of fibroblast growth factor, HIV-Tat, interferon-gamma, and other proteins where there is evidence for intracellular action is given, and translocation mechanisms are discussed. It is concluded that although there are many pitfalls, the bulk of the experiments indicate that certain proteins are indeed able to enter the cytosol and nucleus. Possible roles of the internalized proteins are discussed.

FGF3 Attached to a Phosholipid Membrane Anchor Gains a High Transforming Capacity

NIH3T3 cells transformed by mouse FGF3-cDNA (DMI cells) selected for their ability to grow as anchorage-independent colonies in soft agar and in defined medium lacking growth factors exhibit a highly transformed phenotype. We have used dominant negative (DN) fibroblast growth factor (FGF) receptor 2 (FGFR2) isoforms to block the FGF response in DMI cells. When the DN-FGFR was expressed in DMI cells, their transformed phenotype can be reverted. The truncated FGFR2(IIIb), the high affinity FGFR for FGF3, is significantly more efficient at reverting the transformed phenotype as the IIIc isoform, reaffirming the notion that the affinity of the ligand to the DN-FGFR2 isoform determines the effect. Heparin or heparan sulfate displaces FGF3 from binding sites on the cell surface inhibiting the growth of DMI cells and reverts the transformed phenotype (). However, the presence of heparin is necessary to induce a mitogenic response in NIH3T3 cells when stimulated with soluble purified mouse FGF3. We have investigated the importance of cell surface binding of FGF3 for its ability to transform NIH3T3 cells by creating an FGF3 mutant anchored to the membrane via glycosylphosphatidylinositol (GPI). The GPI anchor renders the cell surface association of FGF3 independent from binding to heparan sulfate-proteoglycan of the cell surface membrane. Attachment of a GPI anchor to FGF3 also confers a much higher transforming potential to the growth factor. Even more, the purified GPI-attached FGF3 is as much transforming as the secreted protein acting in an autocrine mode. Because NIH3T3 cells do not express the high affinity tyrosine kinase FGF receptors for FGF3, these findings suggest that FGF3 attached to GPI-linked heparan sulfate-proteoglycan may have a broader biological activity as when bound to transmembrane or soluble heparan sulfate-proteoglycan.

Depletion of FGF acts as a lateral inhibitory factor in lung branching morphogenesis in vitro

Previous studies have shown that the interaction of positive and inhibitory signals plays a crucial role during lung branching morphogenesis. We found that in mesenchyme-free conditions, the lung epithelium exerted a lateral inhibitory effect on the neighbouring epithelium via depletion of fibroblast growth factor 1 (FGF1). Contrary to previous suggestions, bone morphogenetic protein 4 could not substitute for the inhibitory effect. Based on of this observation, we used a reaction-diffusion model of the substrate-depletion type to represent the initial phase of in vitro branching morphogenesis of lung epithelium, with depletion of FGF playing the role of lateral inhibitor. The model was able to account for the effects of the FGF1 concentration, extracellular matrix degradation and different subtypes of FGF on morphogenesis of the lung bud epithelia. These results suggest that the depletion of FGF may be a key regulatory component in initial phase of branching morphogenesis of the lung bud epithelium in vitro.

Syndecan-4: dispensable or indispensable?

Studies examining the role of the cell-surface heparan sulfate proteoglycan syndecan-4 have yielded a plethora of information regarding its role in both cell-matrix and growth-factor mediated signaling events. Many of the initial conclusions drawn from such research placed syndecan-4 in a keystone position within various signaling pathways though the generation of syndecan-4 null mice have surprised many in the field by indicating otherwise. These contradictory results place researchers in the frustrating and yet exhilarating position of still asking the question, "What role does syndecan-4 play in life?"

Active site blockade of factor VIIa alters its intracellular distribution

Factor VIIa binding to tissue factor on cell surfaces not only triggers the coagulation cascade but also induces various intracellular responses that may contribute to many pathophysiological processes. Active site-inhibited factor VIIa, similar to factor VIIa, binds to tissue factor on cell surfaces and subsequently gets internalized and degraded. At present, it is unknown whether factor VIIa and active site-inhibited factor VIIa undergo a similar intracellular processing. The data presented herein show that although a fraction of both the internalized factor VIIa and active site-inhibited factor VIIa recycle back to the cell surface, the amount of active site-inhibited factor VIIa recycled back to the cell surface was substantially higher than that of factor VIIa. Furthermore, internalized factor VIIa and not active site-inhibited factor VIIa associates with nuclear fractions. Factor VIIa associated with the nuclear fraction was intact and functionally active. In contrast to factor VIIa, tissue factor is not found in the nuclear fraction. Additional studies show that the internalized factor VIIa specifically associates with cytoskeletal proteins, actin, and tubulin. In summary, the present data reveal that despite the common pathway of tissue factor-mediated processing, considerable differences exist in the trafficking of factor VIIa and active site-inhibited factor VIIa in fibroblasts.

Cell-cycle dependent tyrosine phosphorylation on mortalin regulates its interaction with fibroblast growth factor-1

We previously reported that endogenously expressed, intracellularly localized fibroblast growth factor (FGF)-1 interacts with mortalin. Here we report that FGF-1 added to the culture medium of quiescent BALB/c3T3 cells is taken up by the cells and interacts with mortalin in the cells in a regulated manner. Although both the internalized FGF-1 and mortalin were present at high levels throughout the FGF-1-initiated cell cycle, their interaction became apparent only in late G1 phase. Interestingly, mortalin was preferentially tyrosine phosphorylated at the same time, and when its normally weak phosphorylation in early G1 phase was augmented by treating the cells with vanadate, a strong interaction between mortalin and FGF-1 was established. Conversely, when phosphorylated mortalin was treated with tyrosine phosphatase, its interaction with FGF-1 was abrogated. These results indicate that FGF-1 taken up by cells preferentially interacts with mortalin in late G1 phase of the cell cycle, and that tyrosine phosphorylation of mortalin regulates this interaction.

Potential use of non-classical pathways for the transport of macromolecular drugs

Since an increasing number of drug delivery strategies utilising proteins and peptides exhibiting 'non-classical' transport activities have been proposed, studies have begun to establish underlying functional relationships between different vectors. These attempts to find common factors have been hampered by a lack of biophysical data for the various potential protein and peptide transporters, as well as by the structural and functional diversity of the group as a whole. We describe the various types of vectors being considered for use and the preliminary therapeutic successes that have been achieved. Additionally, the various models that have been proposed for non-classical import and export are outlined and discussed in relation to therapeutic delivery. Possible future developments are also discussed.

Requirement for C-terminal end of fibroblast growth factor receptor 4 in translocation of acidic fibroblast growth factor to cytosol and nucleus

The ability of COS cells to bind and internalise acidic fibroblast growth factor (aFGF) was studied after transient transfection of the cells with wild-type and mutated fibroblast growth factor receptor 4. In one case the tyrosine kinase of the receptor was inactivated by a point mutation in the active site, whereas in other cases parts of the receptor were deleted to remove various parts of the cytoplasmic domain. In all cases the receptors were expressed at the cell surface at a high level and the cells bound labelled growth factor efficiently and internalised it by endocytosis. Translocation of externally added aFGF across cellular membranes to reach the cytosol and nucleus was measured as transport of labelled growth factor to the nuclear fraction obtained by centrifugation, by farnesylation of growth factor modified to carry a CAAX motif, and by phosphorylation of the growth factor at a site specific for protein kinase C. Whereas both full-length receptors (with and without an active kinase domain) facilitated translocation of the growth factor to the cytosol and nucleus, as assessed by these methods, the mutants of the receptor where the C terminus was deleted, were unable to do so. In contrast, a receptor containing only the 57 most C-terminal amino acids of the cytoplasmic domain in addition to the juxtamembrane, transmembrane and extracellular domains, was in fact able to mediate translocation of aFGF to the cytosol. These data indicate that information contained in the C terminus of the receptor is required for translocation.

Requirement of phosphatidylinositol 3-kinase activity for translocation of exogenous aFGF to the cytosol and nucleus

Acidic fibroblast growth factor (aFGF) is a potent mitogen for many cells. Exogenous aFGF is able to enter the cytosol and nucleus of sensitive cells. There are indications that both activation of the receptor tyrosine kinase and translocation of aFGF to the nucleus are of importance for mitogenesis. However, the mechanism of transport of aFGF from the cell surface to the nucleus is poorly understood. In this work we demonstrate that inhibition of phosphatidylinositol (PI) 3-kinase by chemical inhibitors and by expression of a dominant negative mutant of PI 3-kinase blocks translocation of aFGF to the cytosol and nucleus. Translocation to the cytosol and nucleus was monitored by cell fractionation, by farnesylation of aFGF modified to contain a farnesylation signal, and by phosphorylation by protein kinase C of aFGF added externally to cells. If aFGF is fused to diphtheria toxin A-fragment, it can be artificially translocated from the cell surface to the cytoplasm by the diphtheria toxin pathway. Upon further incubation, the fusion protein enters the nucleus due to a nuclear localization sequence in aFGF. We demonstrate here that upon inhibition of PI 3-kinase the fusion protein remains in the cytosol. We also provide evidence that the phosphorylation status of the fusion protein does not regulate its nucleocytoplasmic distribution.