MacMARCKS mutation blocks macrophage phagocytosis of zymosan

The major myristoylated PKC substrate (MARCKS) is involved in cell spreading, tyrosine phosphorylation of paxillin, and focal contact formation

The expression of the myristoylated PKC substrate MARCKS is reduced in tumor-derived choroidal melanoma cells (OCM-1). We transfected the OCM-1 cells with MARCKS cDNA and we selected clones with stable overexpression of the protein. Tyrosine phosphorylation of paxillin, a biochemical marker of focal contact formation, was conserved upon serum starvation when MARCKS was overexpressed, while it was almost abolished in the control cells. Immunofluorescent labelling of paxillin and vinculin, another component of focal contact, revealed that these structures were conserved upon serum starvation when MARCKS was overexpressed but not in the control cells. Furthermore, the cell morphology was affected by the ectopic expression of MARCKS, leading to increased spreading and formation of membrane processes. These data suggest the involvement of MARCKS in cell spreading and focal contact formation.

The motility-associated proteins GAP-43, MARCKS, and CAP-23 share unique targeting and surface activity-inducing properties

Local regulation of the cortical cytoskeleton controls cell surface dynamics. GAP-43 and MARCKS are two abundant cytosolic protein kinase C substrates that are anchored to the cell membrane via acyl groups and interact with the cortical cytoskeleton. Each of them has been implicated in several forms of motility involving the cell surface. Although their primary sequences do not reveal significant homologies, GAP-43, MARCKS, and the cortical cytoskeleton-associated protein CAP-23 (in the following, the three proteins will be abbreviated as GMC) share a number of characteristic biochemical and biophysical properties and an unusual amino acid composition. In this study we determined whether GMC may be related functionally. In double-labeling immunocytochemistry experiments GMC accumulated at unique surface-associated structures, where they codistributed. In transfected cells GMC induced the same range of characteristic changes in cell morphology and cell surface activities, including prominent blebs and filopodia. These activities correlated with local accumulation of transgene and had characteristic features of locally elevated actin dynamics, including loss of stress fiber structures, accumulation of beta-(cytosolic) actin at cell surface protrusions, and dynamic blebbing activity. Analysis of appropriate deletion and fusion constructs revealed that the surface accumulation pattern and cell surface activities were correlated and that minimal structural requirements included acylation-mediated targeting to the cell membrane and the presence of a predominantly GMC-type sequence composition. Based on these experiments and on the results of previous studies on GAP-43, MARCKS, and CAP-23, we propose that GMC may define a class of functionally related proteins whose local accumulation promotes actin dynamics and the formation of dynamic structures at the cell periphery. Superimposed on these general properties, differences in the regulation of membrane association and binding properties of effector domains would confer individual properties to each of these proteins.

The faciogenital dysplasia gene product FGD1 functions as a Cdc42Hs-specific guanine-nucleotide exchange factor

The Rho family of small GTP-binding proteins plays important roles in the regulation of actin cytoskeleton organization and cell growth. Activation of these GTPases involves the replacement of bound GDP with GTP, a process catalyzed by the Dbl-like guanine-nucleotide exchange factors, all of which seem to share a putative catalytic motif termed the Dbl homology (DH) domain, followed by a pleckstrin homology (PH) domain. Here we have examined the role of a Dbl-like molecule, the faciogenital dysplasia gene product (FGD1), which when mutated in its Dbl homology domain, cosegregates with the developmental disease Aarskog-Scott syndrome. We report that a polypeptide of FGD1 encompassing the DH and PH domains can bind specifically to the Rho family GTPase Cdc42Hs and stimulates the GDP-GTP exchange of the isoprenylated form of Cdc42Hs. Microinjection of this FGD1 polypeptide into Swiss 3T3 fibroblast cells induces the formation of peripheral actin microspikes, similar to that previously observed when cells were injected with a constitutively active form of Cdc42Hs. This effect of FGD1 on actin organization is readily inhibited by coinjection of a dominant-negative mutant of Cdc42Hs. Examination of NIH 3T3 cells expressing the FGD1 fragment revealed that similar to cells expressing Dbl, two independent elements downstream of Cdc42Hs, the Jun NH2-terminal kinase and the p70 S6 kinase, became activated. Hence, our results indicate that FGD1, through its DH and PH domains, acts as a Cdc42Hs-specific guanine-nucleotide exchange factor and suggest that the Cdc42Hs GTPase may have a role in mammalian development.

Myristoylation does not modulate the properties of MARCKS-related protein (MRP) in solution

The members of the myristoylated alanine-rich C kinase substrate (MARCKS) family are proteins essential for brain development and phagocytosis. MARCKS proteins bind to actin filaments and calmodulin (CaM) and are phosphorylated by protein kinase C. In order to investigate how these interactions are regulated, we have characterized the properties of both the myristoylated (myr) and unmyristoylated (unmyr) forms of recombinant MARCKS-related protein (MRP), a 20-kDa member of the MARCKS family. Ultracentrifugation and circular dichroic spectroscopy reveal that MRP is an elongated protein, with an axis ratio estimated between 7 and 12 and with an apparent random coil conformation. MRP binds to CaM with high affinity (Kd,myr = 4 nM; Kd,unmyr = 7 nM) and with a second order rate constant, k+1,unmyr, of 1.6 x 10(8) M-1 s-1. In contrast to classical ligands such as the myosin light chain kinase, binding of MRP to CaM does not induce the formation of an alpha-helix in MRP. The catalytic subunit of protein kinase C (PKM) phosphorylates myr MRP with high affinity ([S]0.5 = 3.5 microM), positive cooperativity (nH = 2.5) and a turnover number of 130 min-1. CaM inhibits the phosphorylation of myr MRP with a half-maximum rate of phosphorylation at a [CaM]/[MRP] ratio of 0.7, indicating that CaM might efficiently regulate the phosphorylation of MRP in vivo. Interestingly, Ca2+ inhibits the binding of MRP to CaM as well as its phosphorylation by PKM in the millimolar concentration range, suggesting that MRP has a weak affinity for Ca2+. Finally, unmyr MRP can be stoichiometrically myristoylated by N-myristoyl transferase in vitro. Since neither binding of CaM nor phosphorylation by PKM inhibits myristoylation, the N terminus of unmyr MRP is exposed on the surface of the protein and is well separated from the effector domain. In view of the observations that unmyr and myr MRP do not exhibit significant differences in their properties in solution, the function of myristoylation is most probably to modulate the interactions of MRP with membranes.

Activation of phospholipase D is tightly coupled to the phagocytosis of Mycobacterium tuberculosis or opsonized zymosan by human macrophages

Phagocytosis of Mycobacterium tuberculosis by human mononuclear phagocytes is mediated primarily by complement receptors (CRs) but the transmembrane signaling mechanisms that regulate phagocytosis of the bacterium are unknown. We have analyzed the activation of phospholipase D (PLD) during phagocytosis of the virulent Erdman and attenuated H37Ra strains of M. tuberculosis by human monocyte-derived macrophages (MDMs), radiolabeled with [3H]-lyso-phosphatidylcholine. Phagocytosis of either Erdman or H37Ra M. tuberculosis in the presence of autologous non-immune serum was associated with a 2.5-3-fold increase in phosphatidic acid (PA). Definitive evidence for activation of PLD by M. tuberculosis was provided by markedly increased generation of the PLD-specific product phosphatidylethanol (PEt) (9.9-fold increases in [3H]-PEt for both Erdman and H37Ra strains compared to control, P < 0.001, n = 12), in the presence of 0.5% ethanol. Phagocytosis of opsonized zymosan (OZ), which is also mediated by CRs, was similarly associated with activation of PLD (12.2-fold increase in PEt, P < 0.001, n = 12). The competitive PLD inhibitor 2,3-diphosphoglycerate (2,3-DPG) produced concentration-dependent inhibition of PLD activity stimulated by either M. tuberculosis (-78 +/- 8%) or OZ (-73 +/- 6%). Inhibition of PLD by 2,3-DPG was associated with concentration-dependent reductions in phagocytosis of M. tuberculosis (-74 +/- 4%) and OZ (-68 +/- 5%). Addition of purified PLD from Streptomyces chromofuscus to 2,3-DPG-treated macrophages restored phagocytosis of M. tuberculosis to control levels. Inhibition of M. tuberculosis- or OZ-stimulated PA generation by ethanol was associated with concentration-dependent reductions in phagocytosis of both particles. Incubation of MDMs with either Erdman or H37Ra M. tuberculosis, or OZ, resulted in rapid (onset 1 min) and sustained (60 min) increases in the tyrosine phosphorylation (Tyr-P) of multiple MDM proteins. Prominent Tyr-P was noted in proteins of 150, 95, 72, 56, and 42 kD. The protein tyrosine kinase (PTK) inhibitors genistein and herbimycin A reduced M. tuberculosis-stimulated PLD activity by 66-84%. Inhibition of PLD activity by genistein or herbimycin A was associated with inhibition of phagocytosis of M. tuberculosis and OZ. These data demonstrate that PLD is activated during macrophage phagocytosis of M. tuberculosis or OZ, that PTKs are involved in this stimulation of PLD, and that the extent of phagocytosis of these particles is tightly coupled to activation of PLD.

Syk tyrosine kinase is required for immunoreceptor tyrosine activation motif-dependent actin assembly

Clustering of several multisubunit receptors on hematopoetic cells results in a signaling cascade involving the phosphorylation of immunoreceptor tyrosine activation motifs, or "ITAMs," and actin polymerization. Recent experiments indicate that direct clustering of the ITAM-binding protein, p72(syk) (Syk), is capable of transmitting a phagocytic signal in COS cells (Greenberg, S., Chang, P., Wang, D., Xavier, R., and Seed, B.(1996) Proc. Natl. Acad. Sci. U. S. A. 93, 1103-1107). However, the possibility of redundant signaling pathways makes it difficult to test the requirement for Syk in ITAM-dependent actin polymerization in hematopoetic cells. We developed a model system to study ITAM-dependent actin assembly. DT40 lymphocytes were transfected with fusion proteins encoding the transmembrane and cytosolic domains of the ITAM-containing gamma subunit of Fc receptors. Clustering the gamma-containing fusion proteins with IgG-coated erythrocytes triggered submembranous actin assembly. This response depended on an intact ITAM, was absent in cell lines that had been engineered to lack Syk, and was augmented in cell lines that stably overexpressed Syk. These experiments demonstrate an absolute requirement for Syk tyrosine kinase in ITAM-dependent actin assembly in transfected lymphocytes.

Disruption of the MacMARCKS gene prevents cranial neural tube closure and results in anencephaly

MacMARCKS is a member of the MARCKS family of protein kinase C (PKC) substrates. Biochemical evidence demonstrates that these proteins integrate calcium and PKC-dependent signals to regulate actin structure at the membrane. We report here that deletion of the MacMARCKS gene prevents cranial neural tube closure in the developing brain, resulting in anencephaly. This suggests a central role for MacMARCKS and the PKC signal transduction pathway in the folding of the anterior neural plate during the early phases of brain formation, and supports the hypothesis that actin-based motility directs cranial neural tube closure.

Role of MacMARCKS in integrin-dependent macrophage spreading and tyrosine phosphorylation of paxillin

The cellular function of the MARCKS family of protein kinase C substrates is unknown. In this report, we present evidence that indicates a role for MacMARCKS, a member of the MARCKS family, in the integrin-dependent signal transduction pathways in macrophages. Using a dominant negative mutant of MacMARCKS, we showed that MacMARCKS participates in several integrin-dependent macrophage functions, including the phorbol ester-stimulated macrophage spreading, a process involving multiple integrins. The dominant negative mutant also blocks macrophage spreading on immune complex-coated surfaces, a process again requiring beta2 integrin. More direct evidence of the role of MacMARCKS in the integrin-dependent pathway is the ablation of macrophage binding to complement iC3b-coated sheep erythrocytes by MacMARCKS mutant, suggesting an effect of this mutant on the avidity of complement receptor 3, a member of the beta2 integrin family. To further evaluate the possible mechanism of MacMARCKS function, the integrin-dependent tyrosine phosphorylation of paxillin was examined. Concomitant with the inhibition of macrophage spreading and rosette formation, MacMARCKS mutant also inhibits integrin-dependent tyrosine phosphorylation of paxillin. Furthermore, immunofluorescent microscopy data showed that MacMARCKS and paxillin colocalize in the membrane ruffles at the leading edge of the spreading cells, providing a potential site and opportunity for MacMARCKS to participate in the regulation of integrin-dependent tyrosine phosphorylation of paxillin. Together, these data strongly suggest that MacMARCKS plays a role in integrin-dependent signal transduction pathways in macrophages.

Lipopolysaccharide stimulates differential expression of myristoylated protein kinase C substrates in murine microglia

Microglia rapidly respond to lipoplysaccharide (LPS) by transformation from resting to active states and secretion of several neuro- and immuno-regulators including tumour necrosis factor alpha (TNF-alpha), interleukin 1 beta (IL-1 beta), and interleukin 6 (IL-6). With longer LPS treatment, microglia are converted to reactive or phagocytic states with characteristics similar to macrophages in inflammation and injury processes. We have investigated LPS-mediated changes in two myristoylated substrates of protein kinase C (PKC): MARCKS (myristoylated alaninerich C kinase substrate) and MRP (MARCKS-related protein). Within 6 hours of addition, LPS induced a twofold increase in [3H]myristoylated and immunoreactive MARCKS protein and a sevenfold increase in MRP. The differential effect of LPS on expression of MRP vs. MARCKS was even more dramatic at the level of transcription: S1 nuclease protection assays revealed a 40-fold increase in MRP mRNA levels (maximum at 4-6 hours), whereas a threefold increase was observed for MARCKS. TNF alpha and colony-stimulating factor 1 (CSF-1), two cytokines which are induced by LPS, did not reproduce the observed effect of LPS on MARCKS and MRP gene transcription. CSF-1 also induced differential transcription of MRP, but of lower magnitude (threefold) and more sustained than by LPS. Accordingly, these two substrates for PKC are differentially up-regulated by LPS, apparently independent of TNF alpha or CSF-1.

Mechanisms of phagocytosis

Recent advances in research on phagocytosis include a better appreciation of the cross-talk between phagocytic receptors, the definition of multiple signaling domains within these receptors, and a deeper understanding of the downstream effector pathways leading to actin polymerization and particle internalization. Phagosome maturation in macrophages proceeds via a series of membrane fusion and fission events, which modify the phagosome in small increments, and appears to be regulated, in part, by GTP-binding proteins and perhaps by protein kinase C. The isolation of dysphagic mutants of Dictyostelium discoideum presages the identification of new genes required for phagocytosis.