Negative regulation of receptor tyrosine kinases: unexpected links to c-Cbl and receptor ubiquitylation

The epidermal growth factor receptor family: biology driving targeted therapeutics

The epidermal growth factor family of receptor tyrosine kinases (ErbBs) plays essential roles in regulating cell proliferation, survival, differentiation and migration. The ErbB receptors carry out both redundant and restricted functions in mammalian development and in the maintenance of tissues in the adult mammal. Loss of regulation of the ErbB receptors underlies many human diseases, most notably cancer. Our understanding of the function and complex regulation of these receptors has fueled the development of targeted therapeutic agents for human malignancies in the last 15 years. Here we review the biology of ErbB receptors, including their structure, signaling, regulation, and roles in development and disease, then briefly touch on their increasing roles as targets for cancer therapy.

The gradient of Gurken, a long-range morphogen, is directly regulated by Cbl-mediated endocytosis

The asymmetric localization of gurken mRNA and post-translational sorting mechanisms are responsible for the polar distribution of Gurken protein in Drosophila. However, endocytosis of Egfr, the receptor for Gurken in the follicle cells, also plays a role in shaping the extracellular gradient of the Gurken morphogen. Previously, we have found that mutation in the Cbl gene caused elevated Egfr signaling along the dorsoventral axis, and resulted in dorsalization phenotypes in embryos and egg shells. Here, we report that overexpression of the Cbl long isoform significantly changed Gurken distribution. Using an HRP-Gurken fusion protein, we demonstrate that internalization of the Gurken-Egfr complex depends on the activity of Cbl. Increased levels of CblL promote the internalization of this complex, leading to the reduction of free ligands. The Gurken-Egfr complex trafficks through the Rab5/Rab7 associated endocytic pathway to the lysosomal degradation compartment for signaling termination. We observe endocytic Gurken not only in the dorsal but also in the ventral follicle cells, which is, to our knowledge, the first visualization of Gurken on the ventral side of egg chambers. Our results show that Gurken travels towards the lateral/posterior of the egg chamber in the absence of Cbl, suggesting that Cbl actively regulates Gurken distribution through promoting endocytosis and subsequent degradation.

c-Cbl facilitates cytoskeletal effects in v-Abl transformed fibroblast through Rac1- and Rap1-mediated signaling

c-Cbl functions as a multifunctional adaptor and an E3 ubiquitin ligase. Several studies have shown that c-Cbl is involved in cytoskeleton-mediated events, but the molecular mechanisms linking c-Cbl to cytoskeletal rearrangements remain to be elucidated. Our previous results indicated that c-Cbl facilitates spreading and migration of v-Abl-transformed NIH 3T3 fibroblasts and suggested that small GTPases play important roles in the cytoskeletal effects of c-Cbl in this system. To elucidate the individual contributions of small GTPases to these effects, we assessed the roles of endogenous Rac1, RhoA and Rap1 in the c-Cbl-dependent spreading and migration of v-Abl-transformed fibroblasts overexpressing c-Cbl, using RNAi. Furthermore, since it has been shown that Rap1 can act as an upstream regulator of Rac1 in inducing cell spreading, we analyzed the interplay between Rap1 and Rac1 in the signaling pathways connecting c-Cbl to the cytoskeletal events. Our results indicate that Rac1 is essential for cell migration and spreading, whereas activation of RhoA exerts a negative effect. We have also shown that Rap1 is essential for cell spreading, although not for migration in our experimental system. Furthermore, we provide evidence that Rap1 is located upstream of Rac1 in one of the signaling pathways that regulate c-Cbl-facilitated cell spreading. Overall, our findings are consistent with the model describing the connection of c-Cbl to the cytoskeletal rearrangements via two pathways, one of which is mediated by PI3K and Rac1, and the other, by CrkL/C3G, Rap1 and Rac1.

Lrig1 is an endogenous inhibitor of Ret receptor tyrosine kinase activation, downstream signaling, and biological responses to GDNF

Glial cell line-derived neurotrophic factor (GDNF)/Ret signaling has potent trophic effects on ventral midbrain dopaminergic, motor, sensory, and sympathetic neurons. The molecular mechanisms that restrict Ret receptor tyrosine kinase activation are not well understood. Here, we show that Lrig1, a transmembrane protein containing leucine-rich repeats and Ig-like domains in its extracellular region, acts in a negative feedback loop to regulate the activity of Ret receptor tyrosine kinase. In particular, we demonstrate that Lrig1 is capable of physically interacting with Ret and that Lrig1/Ret association inhibits GDNF binding, recruitment of Ret to lipid rafts, receptor autophosphorylation, and mitogen-activated protein kinase (MAPK) activation in response to GDNF. In neuronal cells, Lrig1 overexpression also inhibits GDNF/Ret-induced neurite outgrowth in a cell-autonomous manner. Downregulation of Lrig1 using small interference RNA knock-down experiments potentiates both neuronal differentiation and MAPK activation in response to GDNF. Together, these results provide an insight into Lrig1 function and establish a new physiological mechanism to restrict signaling and biological responses induced by GDNF and Ret in neuronal cells.

Sprouty proteins, masterminds of receptor tyrosine kinase signaling

Angiogenesis relies on endothelial cells properly processing signals from growth factors provided in both an autocrine and a paracrine manner. These mitogens bind to their cognate receptor tyrosine kinases (RTKs) on the cell surface, thereby activating a myriad of complex intracellular signaling pathways whose outputs include cell growth, migration, and morphogenesis. Understanding how these cascades are precisely controlled will provide insight into physiological and pathological angiogenesis. The Sprouty (Spry) family of proteins is a highly conserved group of negative feedback loop modulators of growth factor-mediated mitogen-activated protein kinase (MAPK) activation originally described in Drosophila. There are four mammalian orthologs (Spry1-4) whose modulation of RTK-induced signaling pathways is growth factor- and cell context-dependent. Endothelial cells are a group of highly differentiated cell types necessary for defining the mammalian vasculature. These cells respond to a plethora of growth factors and express all four Spry isoforms, thus highlighting the complexity that is required to form and maintain vessels in mammals. This review describes Spry functions in the context of endothelial biology and angiogenesis, and provides an update on Spry-interacting proteins and Spry mechanisms of action.

Vitronectin: growth factor complexes hold potential as a wound therapy approach

Topical administration of growth factors has displayed some potential in wound healing, but variable efficacy, high doses, and costs have hampered their implementation. Moreover, this approach ignores the fact that wound repair is driven by interactions between multiple growth factors and extracellular matrix (ECM) proteins. We report herein that complexes comprising IGF and IGF-binding proteins bound to the ECM protein vitronectin (VN) significantly enhance cellular functions relevant to wound repair in human skin keratinocytes in two- and three-dimensional in vitro cell models and are active, even in the presence of wound fluid. Moreover, these responses require activation of both the IGF receptor and the VN-binding alpha(v) integrins. Further, we assessed the complexes as a topical agent in the treatment of deep dermal partial thickness burns in a porcine model. This pilot study revealed that the complexes may hold promise as a wound healing therapy. Critically, the significant responses observed in vitro and the encouraging preliminary data in vivo were obtained with nanogram doses of growth factors. This suggests that coupling delivery of growth factors to ECM proteins such as VN may ultimately prove to be a more effective strategy for developing a wound healing therapy.

The multiple personalities of Alix

Alix is a cytosolic protein in mammalian cells that was originally identified on the basis of its association with pro-apoptotic signaling. More recent evidence has established that Alix has a hand in regulating other cellular mechanisms, including endocytic membrane trafficking and cell adhesion. Although Alix appears to participate directly in these various activities, the role it plays in each process has largely been inferred from the functions of proteins with which it interacts. For example, recruitment of Alix to endosomes is mediated by its N-terminal Bro1 domain, the structure of which was recently solved for its yeast orthologue, Bro1. The diversity of Alix functions is due to its proline-rich C-terminus, which provides multiple protein-binding sites. With this blueprint in hand, we can now ask whether Alix acts simply as an adaptor that links different proteins into networks or, instead, contributes a specific function to distinct molecular machineries.

The E3 ligase Aip4/Itch ubiquitinates and targets ErbB‐4 for degradation

The ErbB-4 receptors are unique in the EGFR/ErbB family for the ability to associate with WW domain-containing proteins. To identify new ligands of the cytoplasmic tail of ErbB-4, we panned a brain cDNA phage library with ErbB-4 peptides containing sequence motifs corresponding to putative docking sites for class-I WW domains. This approach led to identification of AIP4/Itch, a member of the Nedd4-like family of E3 ubiquitin protein ligases, as a protein that specifically interacts with and ubiquitinates ErbB-4 in vivo. Interaction with the ErbB-4 receptors occurs via the WW domains of AIP4/Itch. Functional analyses demonstrate that AIP4/Itch is recruited to the ErbB-4 receptor to promote its polyubiquitination and degradation, thereby regulating stability of the receptor and access of receptor intracellular domains to the nuclear compartment. These findings expand our understanding of the mechanisms contributing to the integrity of the ErbB signaling network and mechanistically link the cellular ubiquitination pathway of AIP4/Itch to the ErbB-4 receptor.

Chemically diverse toxicants converge on Fyn and c-Cbl to disrupt precursor cell function

Identification of common mechanistic principles that shed light on the action of the many chemically diverse toxicants to which we are exposed is of central importance in understanding how toxicants disrupt normal cellular function and in developing more effective means of protecting against such effects. Of particular importance is identifying mechanisms operative at environmentally relevant toxicant exposure levels. Chemically diverse toxicants exhibit striking convergence, at environmentally relevant exposure levels, on pathway-specific disruption of receptor tyrosine kinase (RTK) signaling required for cell division in central nervous system (CNS) progenitor cells. Relatively small toxicant-induced increases in oxidative status are associated with Fyn kinase activation, leading to secondary activation of the c-Cbl ubiquitin ligase. Fyn/c-Cbl pathway activation by these pro-oxidative changes causes specific reductions, in vitro and in vivo, in levels of the c-Cbl target platelet-derived growth factor receptor-α and other c-Cbl targets, but not of the TrkC RTK (which is not a c-Cbl target). Sequential Fyn and c-Cbl activation, with consequent pathway-specific suppression of RTK signaling, is induced by levels of methylmercury and lead that affect large segments of the population, as well as by paraquat, an organic herbicide. Our results identify a novel regulatory pathway of oxidant-mediated Fyn/c-Cbl activation as a shared mechanism of action of chemically diverse toxicants at environmentally relevant levels, and as a means by which increased oxidative status may disrupt mitogenic signaling. These results provide one of a small number of general mechanistic principles in toxicology, and the only such principle integrating toxicology, precursor cell biology, redox biology, and signaling pathway analysis in a predictive framework of broad potential relevance to the understanding of pro-oxidant–mediated disruption of normal development.

Chemically different toxins (lead, methylmercury, and paraquat) each cause the intracellular environment to become more oxidized, and thereby activate a common pathway that suppresses signaling from growth factor receptors that may be associated with developmental impairments.

Discovering general principles underlying the effects of toxicant exposure on biological systems is one of the central challenges of toxicological research. We have discovered a previously unrecognized regulatory pathway on which chemically diverse toxicants converge, at environmentally relevant exposure levels, to disrupt the function of progenitor cells of the developing central nervous system. We found that the ability of low levels of methylmercury, lead, and paraquat to make progenitor cells more oxidized causes activation of an enzyme called Fyn kinase. Activated Fyn then activates another enzyme (c-Cbl) that modifies specific proteins—receptors that are required for cell division and survival—to initiate the proteins' degradation. By enhancing degradation of these receptors, their downstream signaling functions are repressed. Analysis of developmental exposure to methylmercury provided evidence that this same pathway is activated in vivo by environmentally relevant toxicant levels. The remarkable sensitivity of progenitor cells to low levels of toxicant exposure, and the discovery of the redox/Fyn/c-Cbl pathway as a mechanism by which small increases in oxidative status can markedly alter cell function, provide a novel and specific means by which exposure to chemically diverse toxicants might perturb normal development. In addition, the principles revealed in our studies appear likely to have broad applicability in understanding the regulation of cell function by alterations in redox balance, regardless of how they might be generated.

LRIG inhibitors of growth factor signalling - double-edged swords in human cancer?

The leucine-rich repeats and immunoglobulin-like domains (LRIG) proteins are newly discovered negative regulators of growth factor signalling and proposed tumour suppressors. They antagonise signalling by interacting with growth factor receptors and by enhancing their ubiquitylation and degradation. Data on the expression of LRIG in human cancer have recently begun to accumulate; however, not all data appear consistent with the notion that the LRIG proteins always function as tumour suppressors. In the present review, we argue that the LRIG proteins could be double-edged swords, promoting or suppressing human cancer depending on cellular context.

Endocytosis, endosome trafficking, and the regulation of Drosophila development

Endocytosis and endosome trafficking regulate cell signaling in unexpected ways. Here we review the contribution that Drosophila research has made to this exciting field. In addition to attenuating signaling, endocytosis shapes morphogen gradients, activates ligands, and regulates spatially receptor activation within a single cell. Moreover, some receptors signal from within endosomes, and the ability of a specific type of endosome to form controls the ability of cells to signal. Experiments in Drosophila reveal that through regulation of a variety of cell signaling pathways, endocytosis controls cell patterning and cell fate.

Gastrointestinal hormones cause rapid c-Met receptor down-regulation by a novel mechanism involving clathrin-mediated endocytosis and a lysosome-dependent mechanism

The activated c-Met receptor has potent effects on normal tissues and tumors. c-Met levels are regulated by hepatocyte growth factor (HGF); however, it is unknown if they can be regulated by gastrointestinal (GI) hormones. c-Met is found in many GI tissues/tumors that possess GI hormone receptors. We studied the effect of GI hormones on c-Met in rat pancreatic acini, which possess both receptors. CCK-8, carbachol, and bombesin, but not VIP/secretin, decreased c-Met. CCK-8 caused rapid and potent c-Met down-regulation and abolished HGF-induced c-Met and Gab1 tyrosine phosphorylation, while stimulating c-Met serine phosphorylation. The effect of cholecystokinin (CCK) was also seen in intact acini using immunofluorescence, in a biotinylated fraction representing membrane proteins, in single acinar cells, in Panc-1 tumor cells, and in vivo in rats injected with CCK. CCK-8 did not decrease cell viability or overall responsiveness. GF109203X, thapsigargin, or their combination partially reversed the effect of CCK-8. In contrast to HGF-induced c-Met down-regulation, the effect of CCK was decreased by a lysosome inhibitor (concanamycin) but not the proteasome inhibitor lactacystin. Inhibitors of clathrin-mediated endocytosis blocked the effect of CCK. HGF but not CCK-8 caused c-Met ubiquitination. These results show CCK and other GI hormones can cause rapid c-Met down-regulation, which occurs by a novel mechanism. These results could be important for c-Met regulation in normal as well as in neoplastic tissue in the GI tract.

Triad3A Regulates Ubiquitination and Proteasomal Degradation of RIP1 following Disruption of Hsp90 Binding

Toll-like receptors (TLRs) play a crucial role in innate immunity by recognizing microbial pathogens. Triad3A is an E3 ubiquitin-protein ligase that interacts with the Toll/interleukin-1 receptor domain of TLRs and promotes their proteolytic degradation. In the present study, we further investigated its activity on signaling molecules downstream of TLRs and tumor necrosis factor (TNF) receptor 1. Triad3A promoted down-regulation of two TIR domain-containing adapter proteins, TIRAP and TRIF, as well as a RIP1 but had no effect on other adapter molecules in either the TLRs or TNF-alpha signaling pathways. Multiple sequence alignment analysis suggested that RIP1 contains a TIR homologous domain, and mutation of amino acid residues in this domain identified three residues critical for its interaction with Triad3A. Moreover, Triad3A acted as a negative regulator in TNF-alpha signaling. Reduction of Triad3A expression by small interference RNAs rendered cells hyperresponsive to TNF-alpha stimulation. Conversely, overexpression of Triad3A in cells blocked TNF-alpha-induced cell activation. This negative regulation was effected independently of changes in the cellular protein level of RIP1. Further studies indicated that RIP1 formed a complex with Triad3A and heat shock protein 90 (Hsp90), which is a chaperone protein capable of maintaining the stability of its client proteins. Treatment of cells with geldanamycin to disrupt the Hsp90 complex led to proteasomal degradation of RIP1. Depletion of Triad3A by small interference RNA treatment inhibited geldanamycin-activated ubiquitination and proteolytic degradation of RIP1. These results suggest that Triad3A is an E3 ubiquitin-protein ligase to RIP1 and that Hsp90 and Triad3A cooperatively maintain the homeostasis of RIP1.

The Cbl family proteins: ring leaders in regulation of cell signaling

The proto-oncogenic protein c-Cbl was discovered as the cellular form of v-Cbl, a retroviral transforming protein. This was followed over the years by important discoveries, which identified c-Cbl and other Cbl-family proteins as key players in several signaling pathways. c-Cbl has donned the role of a multivalent adaptor protein, capable of interacting with a plethora of proteins, and has been shown to positively influence certain biological processes. The identity of c-Cbl as an E3 ubiquitin ligase unveiled the existence of an important negative regulatory pathway involved in maintaining homeostasis in protein tyrosine kinase (PTK) signaling. Recent years have also seen the emergence of novel regulators of Cbl, which have provided further insights into the complexity of Cbl-influenced pathways. This review will endeavor to provide a summary of current studies focused on the effects of Cbl proteins on various biological processes and the mechanism of these effects. The major sections of the review are as follows: Structure and genomic organization of Cbl proteins; Phosphorylation of Cbl; Interactions of Cbl; Localization of Cbl; Mechanism of effects of Cbl: (a) Ubiquitylation-dependent events: This section elucidates the mechanism of Cbl-mediated downregulation of EGFR and details the PTK and non-PTKs targeted by Cbl. In addition, it addresses the functional requirements for E3 Ubiquitin ligase activity of Cbl and negative regulation of Cbl-mediated downregulation of PTKs, (b) Adaptor functions: This section discusses the mechanisms of adaptor functions of Cbl in mitogen-activated protein kinase (MAPK) activation, insulin signaling, regulation of Ras-related protein 1 (Rap1), PI-3' kinase signaling, and regulation of Rho-family GTPases and cytoskeleton; Biological functions: This section gives an account of the diverse biological functions of Cbl and includes the role of Cbl in transformation, T-cell signaling and thymus development, B-cell signaling, mast-cell degranulation, macrophage functions, bone development, neurite growth, platelet activation, muscle degeneration, and bacterial invasion; Conclusions and perspectives.

A soluble ectodomain of LRIG1 inhibits cancer cell growth by attenuating basal and ligand-dependent EGFR activity

Leucine-rich repeats and immunoglobulin-like domains-1 (LRIG1) is a transmembrane protein with an ectodomain containing 15 leucine-rich repeats (LRRs) homologous to mammalian decorin and the Drosophila kekkon1 gene. In this study, we demonstrate that a soluble ectodomain of LRIG1, containing only the LRRs, inhibits ligand-independent epidermal growth factor receptor (EGFR) activation and causes growth inhibition of A431, HeLa and MDA-468 carcinoma cells. In contrast, cells that do not express detectable levels of EGFR fail to respond to soluble LRIG1. However, when a functional EGFR gene is introduced in these cells, they become growth-inhibited by soluble LRIG1 protein. Furthermore, we demonstrate the existence of high-affinity (K(d)=10 nM) binding sites on the A431 cells that can be competitively displaced (up to 75%) by molar excess of EGF. Even more powerful effects are obtained with a chimeric proteoglycan harboring the N-terminus of decorin, substituted with a single glycosaminoglycan chain, fused to the LRIG1 ectodomain. Both proteins also inhibit ligand-dependent activation of the EGFR and extracellular signal-regulated protein kinase 1/2 signaling in a rapid and dose-dependent manner. These results suggest a novel mechanism of action evoked by a soluble ectodomain of LRIG1 protein that could modulate EGFR signaling and its growth-promoting activity. Attenuation of EGFR activity without physical downregulation of the receptor could represent a novel therapeutic approach toward malignancies in which EGFR plays a primary role in tumor growth and survival.

Glycosaminoglycan modification of neuropilin-1 modulates VEGFR2 signaling

Neuropilin-1 (NRP1) is a co-receptor for vascular endothelial growth factor (VEGF) that enhances the angiogenic signals cooperatively with VEGFR2. VEGF signaling is essential for physiological and pathological angiogenesis through its effects on vascular endothelial cells (ECs) and smooth muscle cells (SMCs), but the mechanisms coordinating this response are not well understood. Here we show that a substantial fraction of NRP1 is proteoglycan modified with either heparan sulfate or chondroitin sulfate on a single conserved Ser. The composition of the NRP1 glycosaminoglycan (GAG) chains differs between ECs and SMCs. Glycosylation increased VEGF binding in both cell types, but the differential GAG composition of NRP1 mediates opposite responsiveness to VEGF in ECs and SMCs. Finally, NRP1 expression and its GAG modification post-transcriptionally regulate VEGFR2 protein expression. These findings indicate that GAG modification of NRP1 plays a critical role in modulating VEGF signaling, and may provide new insights into physiological and pathological angiogenesis.

Regulation of sprouty stability by Mnk1-dependent phosphorylation

Sprouty (Spry) proteins are negative feedback modulators of receptor tyrosine kinase pathways in Drosophila melanogaster and mammals. Mammalian Spry proteins have been shown to undergo tyrosine and serine phosphorylation in response to growth factor stimulation. While several studies have addressed the function of tyrosine phosphorylation of Spry, little is known about the significance of Spry serine phosphorylation. Here we identify mitogen-activated protein kinase-interacting kinase 1 (Mnk1) as the kinase that phosphorylates human Spry2 (hSpry2) on serines 112 and 121. Mutation of these serine residues to alanine or inhibition of Mnk1 activity increases the rate of ligand-induced degradation of hSpry2. Conversely, enhancement of serine phosphorylation achieved through either the inhibition of cellular phosphatases or the expression of active Mnk1 results in the stabilization of hSpry2. Previous studies have shown that growth factor stimulation induces the proteolytic degradation of hSpry2 by stimulating tyrosine phosphorylation on hSpry2, which in turn promotes c-Cbl binding and polyubiquitination. A mutant of hSpry2 that is deficient in serine phosphorylation displays enhanced tyrosine phosphorylation and c-Cbl binding, indicating that serine phosphorylation stabilizes hSpry2 by exerting an antagonistic effect on tyrosine phosphorylation. Moreover, loss of serine phosphorylation and the resulting enhanced degradation of hSpry2 impair its capacity to antagonize fibroblast growth factor-induced extracellular signal-regulated kinase activation. Our results imply that Mnk1-mediated serine phosphorylation of hSpry2 constitutes a regulatory mechanism to extend the temporal range of Spry activity.

Sending the right signals: regulating receptor kinase activity

Knowledge of the functions of plant receptor-like-kinases (RLKs) is increasing rapidly, but how their cytoplasmic signalling activity is regulated and how signals are transduced to cytoplasmic or nuclear proteins remain important questions. Recent studies, particularly of the BRASSINOSTEROID INSENSITIVE1 RLK, have begun to shed light on the mechanistic details of RLK activation, including the possible role of ligand binding. Studies of this and other RLKs have also highlighted the potential importance of hetero-oligomerisation and receptor internalisation in RLK signalling. Finally, a range of potential regulatory proteins and putative downstream signalling substrates have been identified for various RLKs. Despite some similarities with animal receptor kinase signalling systems, mechanisms that affect the intracellular behaviour, regulation and interactions of RLKs appear to be very diverse, potentially explaining how signalling specificity is maintained at the cytoplasmic level.

E3 ubiquitin ligases as regulators of membrane protein trafficking and degradation

Ubiquitination is a regulated post-translational modification that conjugates ubiquitin (Ub) to lysine residues of target proteins and determines their intracellular fate. The canonical role of ubiquitination is to mediate degradation by the proteasome of short-lived cytoplasmic proteins that carry a single, polymeric chain of Ub on a specific lysine residue. However, protein modification by Ub has much broader and diverse functions involved in a myriad of cellular processes. Monoubiquitination, at one or multiple lysine residues of transmembrane proteins, influences their stability, protein-protein recognition, activity and intracellular localization. In these processes, Ub functions as an internalization signal that sends the modified substrate to the endocytic/sorting compartments, followed by recycling to the plasma membrane or degradation in the lysosome. E3 ligases play a pivotal role in ubiquitination, because they recognize the acceptor protein and hence dictate the high specificity of the reaction. The multitude of E3s present in nature suggests their nonredundant mode of action and the need for their controlled regulation. Here we give a short account of E3 ligases that specifically modify and regulate membrane proteins. We emphasize the intricate network of interacting proteins that contribute to the substrate-E3 recognition and determine the substrate's cellular fate.

Signaling to cytoskeletal dynamics during chemotaxis

We review insights in signaling pathways controlling cell polarization and cytoskeletal organization during chemotactic movement in Dictyostelium amoebae and neutrophils. We compare and contrast these insights with our current understanding of pathways controlling chemotactic movements in more-complex multicellular developmental contexts.