Platelet factor 4 modulates fibroblast growth factor 2 (FGF-2) activity and inhibits FGF-2 dimerization

Platelet factor 4 (PF-4) inhibits angiogenesis in vitro and in vivo. The mechanism of inhibition is poorly understood. We have investigated the mechanism of inhibition by examining the interaction of PF-4 and the fibroblast growth factor-2 (FGF-2)/fibroblast growth factor receptor (FGFR) system. PF-4 inhibited the binding of FGF-2 to high-affinity and low-affinity binding sites in murine microvascular endothelial cells (LEII cells) and proliferation. Maximum inhibition of binding to endothelial FGF receptors was observed at PF-4 concentrations between 5 and 10 microg/mL (half maximum inhibition at 0.6 micro/mL), and proliferation was completely inhibited at 2 microg/mL. At this concentration, PF-4 reduced internalization of 125I-FGF-2 by threefold and delayed degradation. To gain insight into the mechanism of inhibition, we have analyzed the interaction of PF-4 with FGF-2/FGFR by using mutant heparan sulfate-deficient Chinese hamster ovary (CHO) cells transfected with the FGFR-1 cDNA (CHOm-FGFR-1) and by examining the direct interaction with FGF-2. In the absence of heparin, PF-4 inhibited binding of 125I-FGF-2 to CHOm-FGFR-1 cells in a concentration-dependent manner, although not completely. In the presence of heparin, PF-4 abolished totally the stimulatory effect of heparin. Furthermore, PF-4 complexed to FGF-2 and inhibited endogenous or heparin-induced FGF-2 dimerization. These results indicate that PF-4 interacts with FGF-2 by complex formation, inhibiting FGF-2 dimerization, binding to FGF receptors, and internalization. This mechanism most likely contributes to the antiangiogenic properties of PF-4.

A natural motif approach to protein design: a synthetic leucine zipper peptide mimics the biological function of the platelet factor 4 protein

The design of smaller functional mimics of large proteins has long been an important challenge. In this study we use the natural leucine zipper as a structural template to design a 31-residue peptide analog that mimics the function of the larger platelet factor 4 (PF4) protein. The heparin binding activity of PF4 has been introduced into an unrelated leucine zipper sequence only by virtue of incorporating four lysines of PF4. Circular dichroism and binding experiments have shown that the designed leucine zipper peptide adopts a stable helical conformation and shows significant PF4-like heparin binding activity. These results strongly suggest that the lysine residues play an important role in the binding of PF4 to heparin. The de novo generation of the PF4 function in a designed leucine zipper peptide demonstrates that the leucine zipper motif is a useful scaffold for the design of functional peptides and proteins.

Fibronectin, platelet factor 4 and beta-thromboglobulin in endemic hepatosplenic schistosomiasis: relation to acute hematemesis

Some platelet alpha-granule contents were assessed in parallel with other markers of hemostatic imbalance in 50 patients with hepatosplenic schistosomiasis (15 patients with compensated hepatosplenomegaly, 15 patients with advanced hepatic fibrosis and ascites and 20 patients during an acute attack of hematemesis from ruptured esophageal varices). Platelet factor 4 (PF4), beta-thromboglobulin (beta-TG), fibronectin (FN), prothrombin fragment 1 + 2, thrombin-antithrombin (TAT) complexes, fibrin degradation products (FbDP) and D-dimer were assessed in schistosomal patients compared to controls (15 healthy subjects). A significant increase in both thrombin (high TAT and prothrombin fragment 1 + 2 levels) and plasmin (high FbDP and D-dimer levels) generation was detected in decompensated patients establishing the presence of a steady state of low-grade disseminated intravascular coagulation, with and without overt bleeding, in these patients. A decrease in plasma FN concentration was found in diseased groups compared to controls. The reduction in plasma levels of FN paralleled the defective liver function and matched the relative decrease in tissue FN in liver specimens of decompensated patients suggesting that FN levels can be used to evaluate the pathological staging of the disease. A significant increase in beta-TG and PF4 levels was noted in decompensated patients with ascites and/or acute hematemesis compared both to controls and compensated patients reflecting platelet alpha-granule release and consequently increased in vivo platelet activation which may initiate and/or perpetuate the pathophysiological mechanisms of the hemostatic imbalance underlying the hemorrhagic diathesis in hepatosplenic schistosomiasis.

Glycosaminoglycans enhance megakaryocytopoiesis by modifying the activities of hematopoietic growth regulators

We have previously reported that heparin is capable of stimulating in vitro and in vivo megakaryocytopoiesis in mice and has a thrombopoietic effect when given in chronic immune thrombocytopenic purpura and that heparin and several other glycosaminoglycans (GAGs) promote the growth of human megakaryoblastic cell lines in the presence of serum. We show here that GAGs, including heparan sulfate (HS), chondroitin sulfate (CS), dermatan sulfate (DS), and hyaluronic acid (HA), also stimulate in vitro growth of murine megakaryocyte progenitors and augment the diameter of individual megakaryocytes in the presence of serum. However, in a serum-free agar system, the GAGs alone had no effect on megakaryocyte colony formation, suggesting that GAGs cooperate with some serum factor(s) to exert their activity. We also show that heparin significantly potentiates the megakaryocytopoietic activity of C-Mpl ligand and interleukin (IL)-6 but not IL3, GM-CSF, SCF, and Epo. In addition, the GAGs significantly neutralize the inhibitory action of platelet factor 4 (PF4) and transforming growth factor beta 1 (TGF beta 1) on megakaryocyte colony growth. These results demonstrate a stimulating activity of GAGs on megakaryocytopoiesis by modifying the activity of several growth-regulating factors.

[Positive and negative regulation of megakaryocytopoiesis]

Megakaryocytopoiesis concerns the commitment of hematopoietic stem cells towards the megakaryocyte lineage, the proliferation and maturation of megakaryocyte progenitors, leading to platelet formation. Normal megakaryocytopoiesis requires an equilibrium between positive and negative regulators. Thrombopoietin, recently cloned, is the main growth factor for the megakaryocyte lineage, enhancing all the steps of megakaryocyte development: proliferation, maturation, ploidisation, platelet formation. Several other factors have a direct (interleukin 3, 6, 11, 13, GMCSF, erythropoietin) or indirect (interleukin 1, stem cell factor) positive effect. Factors inhibiting megakaryocytopoiesis are synthetized by the megakaryocytes themselves (platelet factor 4, transforming growth factor beta), or have many other effects (alpha-interferon, thrombin) or correspond to a novel molecule (anagrelide). Heparin and other glycosaminoglycans positively modulate megakaryocytopoiesis by acting synergistically with some growth factors and by neutralizing some inhibitors. These progresses in the knowledge of normal and pathological megakaryocytopoiesis should lead to considerable therapeutic advances.

Platelets prime PMN via released PF4: mechanism of priming and synergy with GM-CSF

Platelet-PMN interactions have been extensively studied and a spectrum of possible effects has been demonstrated. However, the physiological relevance of many of the observed in vitro phenomena remains obscure. Here we report a novel, and potentially pathophysiologically important, mechanism by which platelets can enhance PMN reactivity. We first observed that addition of platelets to PMN suspensions enhanced the chemiluminescence response of PMN to FMLP. This enhancement occurred without augmentation of superoxide generation and did not involve mutual platelet-PMN adhesion. The soluble material responsible was biochemically and immunologically identified as PF4 derived from platelet alpha-granules. The alpha-granule release was shown to be selective and required minimal platelet stimulation. Since the PF4 effect did not influence NADPH oxidase activation, it differed markedly from that of other priming agents such as GM-CSF. Further studies showed that the PF4 effect was attributable entirely to the surface translocation and secretion of primary granule myeloperoxidase. There was marked synergy between PF4 and GM-CSF and both were required for maximal potentiation of PMN reactivity. These results demonstrate that PF4 and GM-CSF employ different pathways in PMN priming. The ease with which platelets could release PF4 at sites of vessel-wall damage and inflammation suggests that platelet-PMN interaction via PF4 is likely to be of major pathophysiological importance.

Platelet and coagulation factor variations induced in vitro by polyethylene terephthalate (Dacron) coated with pyrolytic carbon

The haemocompatibility of polyethylene terephthalate (Dacron) coated with pyrolytic carbon was examined in vitro, evaluating its capability of inducing adhesion and platelet activation, and of modifying the intrinsic coagulation pathway. Platelet adhesion was evaluated by counting platelets before and after in vitro contact of human plasma with the material under examination. Platelet activation was evaluated by determining platelet factor 4 (PF4) and thromboxane B2. Intrinsic coagulation pathway alterations were studied by determining activated partial thromboplastin time (APTT) and the activity of single factors. The results obtained show that pyrolytic carbon-coated Dacron induces platelet adhesion, reduction in platelet volume and lower increase in thromboxane production than that obtained after contact with uncoated Dacron. Pyrolytic carbon-coated Dacron does not induce PF4 release, contrary to uncoated Dacron induces a significant release. Moreover, pyrolytic carbon-coated Dacron, induces a significant extension of APTT by reducing the activity of intrinsic pathway factors, particularly factor XI.

Cellular interactions in tissue factor expression by blood monocytes

Tissue factor (TF) is the cellular receptor for factor VIIa. In complex with TF, factor VIIa initiates coagulation by activation of factor IX and factor X. TF is normally not in contact with blood, but is expressed on extravascular cells which forms a haemostatic envelope ready to activate coagulation when vascular integrity is disrupted. However, under pathological conditions monocytes, but probably not endothelial cells, are stimulated to express TF activity on the surface of the cells and may thereby trigger activation of blood coagulation. For several years we have observed some individuals (high responders) with a very high response to lipopolysaccharide (LPS) as judged by induction of TF activity in their monocytes in whole blood. This phenomenon has been shown to be mediated by a P-selectin dependent interaction between granulocytes, platelets and monocytes where the release of platelet factor 4 (PF4) plays an important role. It is concluded that cellular interactions play a central role in the expression of TF activity in circulating monocytes.

Role of C-X-C chemokines as regulators of angiogenesis in lung cancer

Lung cancer is the leading cause of malignancy-related mortality in the U.S. and is predicted to increase over the remainder of this decade. Despite attempts to advance early diagnosis and use combination therapies, the clinical response of this cancer yields an overall 5-year survival rate of less than 15%. Clearly, new strategies for therapy are indicated. Although carcinogenesis is complex, tumor growth beyond 1-2 mm3 is dependent on angiogenesis. One of the potential mechanisms that allows for tumorigenesis is dysregulation of the balance of angiogenic and angiostatic factors that favors net neovascularization within the primary tumor. Numerous studies have investigated the role of a variety of molecules in the regulation of angiogenesis. Recently, interleukin-8 (IL-8), a member of the C-X-C chemokine family, has been found to be an angiogenic factor. In contrast, platelet factor 4 (PF4), another C-X-C chemokine, has been shown to have angiostatic properties. It is interesting that the major structural difference between IL-8 and PF4 is the presence of the NH2-terminal ELR (Glu-Leu-Arg) motif that precedes the first cysteine amino acid residue of IL-8 and is important in ligand/receptor interactions. We hypothesize that angiogenesis associated with tumorigenesis is dependent on members of the C-X-C chemokine family acting as either angiogenic or angiostatic factors. This paradigm predicts that the biological balance in the expression of these C-X-C chemokines dictates whether the neoplasm grows and develops metastatic potential or regresses. In this review we discuss our recent laboratory findings that support this contention and suggest that further elucidation of the biology of C-X-C chemokines in the context of neovascularization of nonsmall cell lung cancer will permit novel targeted therapy aimed specifically at attenuating tumor growth and metastasis.

Three-dimensional structures of alpha and beta chemokines

Members of the chemokine family of proteins play a key role in the orchestration of the immune response. This family has been further divided into two subfamilies, alpha and beta, based on sequence, function, and chromosomal location. To date, the three-dimensional structures of two members of the alpha subfamily, interleukin-8 (IL-8) and platelet factor 4, and one member of the beta subfamily, human macrophage inflammatory protein-1 beta (hMIP-1 beta), have been solved by either NMR or X-ray crystallography. In this review, we discuss their three-dimensional structures and their possible relationship to function. The structures of the monomers are very similar, as expected from the significant degree of sequence identity between these proteins. The quaternary structures of the alpha and beta chemokines, however, are entirely distinct and the dimer interface is formed by a completely different set of residues. Whereas the IL-8 dimer is globular, the hMIP-1 beta dimer is elongated and cylindrical. Platelet factor 4 is a tetramer comprising a dimer of dimers of the IL-8 type. The IL-8 dimer comprises a six stranded anti-parallel beta-sheet, three strands contributed by each subunit, on top of which lie two antiparallel helices separated by approximately 14 A, and the symmetry axis is located between residues 26 and 26' (equivalent to residue 29 of hMIP-1 beta) at the center of strands beta 1 and beta 1'.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural requirements of platelet chemokines for neutrophil activation

Using recombinantly expressed proteins and synthetic peptides, we examined the structural/functional features of the platelet chemokines, neutrophil-activating peptide-2 (NAP-2) and platelet factor 4 (PF4); that were important in their activation of neutrophils. Previous studies with the chemokine interleukin-8 (IL-8) had shown that the N-terminal region preceding the first cysteine residue was critical in defining neutrophil-activating properties. We examined whether NAP-2 and PF4 had similar structural requirements. In the Ale-glu-leu-arg (AELR) N-terminus of NAP-2, substitution of E or R abolished Ca2+ mobilization and elastase secretion. Unlike the parent molecule PF4, AELR/PF4, the hybrid formed by replacing the N-terminal sequence of PF4 before the first cysteine residue with the homologous sequence of NAP-2, stimulated Ca2+ mobilization and elastase secretion. Furthermore, the effect of amino acid substitutions in the ELR motif differed from those seen with NAP-2 in that conserved substitutions of E or R in NAP-2 abolished activity, but only reduced neutrophil activation in the hybrid. These studies show that just as with IL-8, the N-termini of NAP-2 and PF4 are critical for high-level neutrophil-activating function. Desensitization studies provided information on receptor binding. NAP-2, which binds almost exclusively to the type 2 IL-8 receptor (IL-8R), did not desensitize neutrophils to activation by IL-8 because IL-8 could bind to and activate via both type 1 and 2 IL-8R. AELR/PF4 appears to bind to both types of receptors because it desensitized neutrophils to NAP-2 activation; but was not desensitized by NAP-2, and because it desensitized to and was desensitized by IL-8. Thus, although NAP-2 and AELR/PF4 share approximately 60% amino acid homology, they have different receptor affinities. Studies were performed to define the role of the C-termini of these platelet chemokines in receptor binding. Heparin and a monoclonal antibody specific for the heparin-binding domain of PF4 both inhibited Ca2+ mobilization and elastase release, further suggesting that the C-terminus of these chemokines is important in receptor binding. Synthetic NAP-2(51-70) failed to mobilize Ca2+, whereas PF4(47-70) and PF4(58-70) induced Ca2+ mobilization and secretion of elastase at high concentrations. Pertussis toxin inhibited neutrophil activation by 40% to 50%, establishing a role for G-protein-coupled receptors such as the IL-8Rs in activation by the PF4 C-terminal peptides.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular piracy of chemokine receptors by herpesviruses

To succeed as a biological entity, viruses must exploit normal cellular functions and elude the host immune system; they often do so by molecular mimicry. One way that mimicry may occur is when viruses copy and modify host genes. The best studied examples of this are the oncogenes of RNA retroviruses, but a growing number of examples are also known for DNA viruses. So far they all come from just two groups of DNA viruses, the herpesviruses and poxviruses, and the majority of examples are for genes whose products regulate immune responses, such as cytokines, cytokine receptors, and complement control proteins. This review will focus on human and herpesvirus receptors for chemokines, a family of leukocyte chemoattractant and activating factors that are thought to be important mediators of inflammation. Although the biological roles of the viral chemokine receptor homologues are currently unknown, their connection to specific sets of chemokines has suggested a number of possible functions.

Heparin neutralization by platelet-rich thrombi. Role of platelet factor 4

BACKGROUND

Platelets contain several factors that inhibit heparin. This study was designed to assess the heparin-neutralizing activity present in acute, platelet-rich arterial thrombi formed at sites of arterial injury in animals.

METHODS AND RESULTS

Platelet-rich thrombi (n = 3) were induced in pig coronary arteries by balloon catheter-mediated arterial injury. Soluble extracts were prepared from each thrombus and assayed for the capacity to inhibit heparin in an in vitro clotting assay (activated partial thromboplastin time). Mean heparin-neutralizing activity was 28 U of heparin neutralized per milliliter of thrombus, indicating that 1 vol of coronary thrombus completely inhibited the heparin present in 140 vols of therapeutically anticoagulated (0.2 U heparin/mL) plasma. In contrast, thrombus extracts had no effect on the anticoagulant activity of hirudin, a direct-acting thrombin inhibitor. The heparin-neutralizing activity present in coronary thrombi bound to heparin-agarose and was eluted from it by 1.4 mol/L NaCl, suggesting that platelet factor 4 mediated the antiheparin effect of thrombi. Consistent with this hypothesis, a murine monoclonal antibody to rabbit platelet factor 4 nearly completely inhibited the heparin-neutralizing activity present in rabbit thrombi (n = 3) generated by carotid artery injury.

CONCLUSIONS

Extracts prepared from platelet-rich arterial thrombi significantly inhibit the in vitro anticoagulant potency of heparin but not of hirudin. This antiheparin effect appears to be mediated by platelet factor 4. These results are consistent with the hypothesis that localized inhibition of heparin at sites of platelet activation may reduce its antithrombotic efficacy. In addition, they suggest an additional mechanism for the apparent superiority of hirudin over heparin as a thrombin inhibitor at sites of arterial injury.

Control of megakaryocyte development: from basic data to clinical results

Megakaryocyte development is a multi-stage cell process which is positively regulated by a number of cytokines and negatively controlled mainly by megakaryocyte-platelet derived molecules. The glycosaminoglycans (GAGs) also play a regulatory role in this process. Understanding of this cell process and its regulation is of both basic and clinical significance in hematology and oncology. Future studies should focus on the pathophysiology of megakaryocytopoietic disorders and the clinical application of growth-regulating factors and GAGs.

Modulation of eosinophil chemotaxis by interleukin-5

Eosinophilia and eosinophil function are regulated by cytokines such as granulocyte/macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and interleukin-5 (IL-5). We have investigated the modulatory role of IL-5 on N-formyl-methionyl-leucyl-phenylalanine (FMLP), neutrophil-activating factor (NAF/IL-8), platelet factor 4 (PF4), and cytokine-induced chemotaxis of eosinophils from normal individuals. These eosinophils show a small chemotactic response toward PF4 but not to NAF/IL-8 and FMLP. Preincubation of eosinophils with low concentrations of IL-5 caused significantly increased responses toward PF4 and induced a significant chemotactic response toward FMLP and NAF/IL-8. In marked contrast, IL-5 (or IL-3) priming of eosinophils from normal donors resulted in a strong inhibition of GM-CSF-induced chemotaxis. A similar decrease in the chemotactic response toward GM-CSF was observed in eosinophils derived from allergic asthmatic individuals. This finding suggests that the latter eosinophils may have had a prior exposure to IL-5 (or IL-3). Washing of the cells after priming did not abrogate the inhibition of the GM-CSF response. Our data indicate that at low concentrations IL-5 is an important modulator of eosinophil chemotaxis, causing selective upregulation or downregulation of chemotactic responses toward different agents.

Platelet factor 4 selectively inhibits binding of TGF-beta 1 to the type I TGF-beta 1 receptor

A low molecular weight inhibitor of TGF-beta 1 binding was detected in partially purified human platelet extracts by using Hep 3B hepatoma cells in the binding assays. The inhibitory protein was purified to homogeneity and was identified as platelet factor 4 on the basis of its amino acid sequence. TGF-beta 1 binding to Hep 3B cells was almost completely inhibited by 100 nM concentrations of platelet factor 4, but TGF-beta 1 binding to NRK 49F fibroblasts was inhibited only slightly. Affinity cross-linking experiments revealed that these differences in the inhibition of TGF-beta 1 binding by platelet factor 4 were due to differences in the complements of TGF-beta 1 binding proteins present on these two cell types. In Hep 3B cells the majority of bound TGF-beta 1 was cross-linked to a complex which had an apparent molecular weight of 70 kDa. TGF-beta 1 binding to this protein was the most sensitive to inhibition by platelet factor 4. Based on its size and TGF-beta 1 binding properties, we believe this protein is the type I TGF-beta 1 receptor. Hep 3B cells also had a high-affinity TGF-beta 1 binding protein which appeared as an 80 kDa complex, and which we believe to be the type II TGF-beta 1 receptor. TGF-beta 1 binding to this protein was not inhibited by platelet factor 4. TGF-beta 1 was also cross-linked to complexes of higher molecular weights in Hep 3B cells, but it was not clear whether any of them represented the type III TGF-beta 1 receptor. In NRK 49F cells, the majority of bound TGF-beta 1 was cross-linked to a high molecular weight complex which probably represented the type III TGF-beta 1 receptor. NRK 49F cells also had type I TGF-beta 1 receptors and platelet factor 4 inhibited binding to these receptors in the NRK cells. Since the type I receptor contributed only a small percentage of total TGF-beta 1 binding, however, the overall effects of platelet factor 4 on TGF-beta 1 binding to NRK 49F cells were negligible. We were unable to demonstrate specific or saturable binding of platelet factor 4 to Hep 3B cells using either direct binding or affinity cross-linking assays. Thus, it is not clear whether platelet factor 4 inhibits TGF-beta 1 binding by competition for binding to the type I receptor. Modest concentrations of TGF-beta 1 reduced the adherence of Hep 3B cells to tissue culture dishes.(ABSTRACT TRUNCATED AT 400 WORDS)

Neutrophil chemotactic factors

Polymorphonuclear leukocytes (neutrophils) are recruited to inflammatory sites by a variety of soluble mediators (chemoattractants) that stimulate neutrophil directed migration (chemotaxis). Many neutrophil chemoattractants such as neutrophil activating proteins, leukotriene B4 (LTB4), platelet activating factor, and complement-derived C5a, are generated endogenously by host cells or enzymatic cleavage of host proteins. Other chemoattractants such as N-formyl peptides are generated exogenously by bacteria that invade the host. Oxidative modification of methionine residues or changes in the amino acid sequence of peptide chemoattractants dramatically alter their chemoattractive properties. Many of the well-defined neutrophil chemotactic factors and studies of their structure-function relationships will be reviewed.

Amplification of platelet response during acute inflammation in rats

Enhanced aggregation of platelets was observed in platelet-rich plasma, but not in washed platelet suspension (WPS), during acute inflammation in rats. Incubation of WPS with inflammatory plasma increased the aggregatory response to ADP, but the plasma itself did not cause aggregation of platelets. It potentiated the aggregatory response of normal platelets, when platelets were stimulated with arachidonic acid, thrombin, calcium ionophore A23187 or phorbol-12-myristate-13-acetate. Pretreatment of rats with indomethacin (10 mg/kg) did not prevent the increased aggregation response of platelets due to inflammation. This response was not due to any of the biogenic amines nor was it due to platelet factor 4. The activity of the inflammatory plasma was reduced when it was incubated with phospholipase A2, indicating the involvement of platelet-activating factor (PAF). The activity was present both in the lipid and the protein fraction of the inflammatory plasma. The results indicate that a substance(s) released in the circulation during inflammation renders the platelets hyperactive. This substance appears to be a protein which is present in the inflammatory plasma and acts together with PAF to cause increased aggregation of platelets.

[Blood coagulating and anticoagulant properties of the tissues in experimental chronic alcoholism in albino rats]

A study was made of thromboplastin, antiheparin and antithrombin activity of the brain, heart and liver tissues of rats in experimental chronic alcoholization. Appreciable changes in the tissue factors of coagulation according to the dyscoagulemia type depending on the duration and gravity of alcoholic intoxication may aggravate hemostatic disorders.

Clonal segregation of multiple and overlapping matrix adhesive responses in dorsal root neuronal derivative cells

Clones of F11 hybrid (neuroblastoma X dorsal root neuron) cells have been tested for adherence and neurite outgrowth on three different substrata on which the parental cells display some competence--plasma fibronectin (pFN) with its multiple receptors, cholera toxin subunit B(CTB) as a model ganglioside GM1-binding substratum, and platelet factor-4 (PF4) as a model proteoglycan-binding substratum. This paradigm tests for independently segregating and overlapping mechanisms of neuritogenesis via transmembrane processes in pluripotent hybrid cells based on random loss of chromosomes contributed by the parent neural cells. For the nine clones tested, attachment was significantly lower on CTB but much higher on PF4 for all clones when compared to their attachment on pFN. Supplementation of cells with GM1 stimulated attachment of only two clones (on all three substrata). Neurite outgrowth was observed in a substratum-specific pattern and varied from 0 to greater than 60% on pFN; on CTB and PF4 substrata, the patterns were similar to each other but differed markedly from the pattern on pFN. In contrast, PF4- and CTB-directed neurites differed morphologically from each other while sharing some characteristics with neurites on pFN. Supplementation with GM1 or GT1b, but not GD1a, was inhibitory for neurite outgrowth in certain clones. Cycloheximide pretreatment distinguished several classes of clones based on inhibition of neuritogenesis. While most clones on pFN were unaffected, all clones on CTB and PF4 displayed significant and comparable degrees of inhibition, suggesting the sharing of some protein intermediate(s) on these substrata. Exposure to cycloheximide only during the active period of neuritogenesis generated higher percentages and longer neurites for all clones, indicating a widely-used negative regulation mechanism. Based on substratum type and cycloheximide protocols, these data have resolved six or more different transmembrane signalling processes for generating different classes of neurites. Some mechanisms have been segregated into individual clones while others overlap in other clones, providing cell systems for biochemical and molecular biological dissection of these processes.