Generation and dynamics of an endogenous, self-generated signaling gradient across a migrating tissue

Self-extinguishing relay waves enable homeostatic control of human neutrophil swarming

Neutrophils collectively migrate to sites of injury and infection. How these swarms are coordinated to ensure the proper level of recruitment is unknown. Using an ex vivo model of infection, we show that human neutrophil swarming is organized by multiple pulsatile chemoattractant waves. These waves propagate through active relay in which stimulated neutrophils trigger their neighbors to release additional swarming cues. Unlike canonical active relays, we find these waves to be self-terminating, limiting the spatial range of cell recruitment. We identify an NADPH-oxidase-based negative feedback loop that is needed for this self-terminating behavior. We observe near-constant levels of neutrophil recruitment over a wide range of starting conditions, revealing surprising robustness in the swarming process. This homeostatic control is achieved by larger and more numerous swarming waves at lower cell densities. We link defective wave termination to a broken recruitment homeostat in the context of human chronic granulomatous disease.

Incoherent collective cell chemotaxis underlies organ dysmorphia in a model of branchio-oto-renal syndrome

Mutations in eya1 cause branchio-oto-renal syndrome (BOR) in humans and the equivalent condition in animal models. BOR is characterized by multi-organ malformations. To better understand the role of Eya1 in organogenesis we used the zebrafish posterior lateral-line primordium. This multicellular tissue moves from head-to-tail at a constant velocity via the simultaneous action of two chemokine receptors, Cxcr4b and Ackr3b (formerly cxcr7b). We found that loss of eya1 strongly reduces the expression of ackr3b , disrupting the coherent motion of the primordium and leading to lateral-line truncations. These findings point to abnormal collective cell chemotaxis as the origin of organ dysmorphia in BOR.

The evolution of developmental biology through conceptual and technological revolutions

Developmental biology-the study of the processes by which cells, tissues, and organisms develop and change over time-has entered a new golden age. After the molecular genetics revolution in the 80s and 90s and the diversification of the field in the early 21st century, we have entered a phase when powerful technologies provide new approaches and open unexplored avenues. Progress in the field has been accelerated by advances in genomics, imaging, engineering, and computational biology and by emerging model systems ranging from tardigrades to organoids. We summarize how revolutionary technologies have led to remarkable progress in understanding animal development. We describe how classic questions in gene regulation, pattern formation, morphogenesis, organogenesis, and stem cell biology are being revisited. We discuss the connections of development with evolution, self-organization, metabolism, time, and ecology. We speculate how developmental biology might evolve in an era of synthetic biology, artificial intelligence, and human engineering.

Pulses of RhoA signaling stimulate actin polymerization and flow in protrusions to drive collective cell migration

In animals, cells often move as collectives to shape organs, close wounds, or-in the case of disease-metastasize. To accomplish this, cells need to generate force to propel themselves forward. The motility of singly migrating cells is driven largely by an interplay between Rho GTPase signaling and the actin network. Whether cells migrating as collectives use the same machinery for motility is unclear. Using the zebrafish posterior lateral line primordium as a model for collective cell migration, we find that active RhoA and myosin II cluster on the basal sides of the primordium cells and are required for primordium motility. Positive and negative feedbacks cause RhoA and myosin II activities to pulse. These pulses of RhoA signaling stimulate actin polymerization at the tip of the protrusions and myosin-II-dependent actin flow and protrusion retraction at the base of the protrusions and deform the basement membrane underneath the migrating primordium. This suggests that RhoA-induced actin flow on the basal sides of the cells constitutes the motor that pulls the primordium forward, a scenario that likely underlies collective migration in other contexts.

Non-genetic adaptation by collective migration

Collective behaviors require coordination of individuals. Thus, a population must adjust its phenotypic distribution to adapt to changing environments. How can a population regulate its phenotypic distribution? One strategy is to utilize specialized networks for gene regulation and maintaining distinct phenotypic subsets. Another involves genetic mutations, which can be augmented by stress-response pathways. Here, we studied how a migrating bacterial population regulates its phenotypic distribution to traverse across diverse environments. We generated isogenic Escherichia coli populations with varying distributions of swimming behaviors and observed their phenotype distributions during migration in liquid and porous environments. Surprisingly, we found that during collective migration, the distributions of swimming phenotypes adapt to the environment without mutations or gene regulation. Instead, adaptation is caused by the dynamic and reversible enrichment of high-performing swimming phenotypes within each environment. This adaptation mechanism is supported by a recent theoretical study, which proposed that the phenotypic composition of a migrating population results from a balance between cell growth generating diversity and collective migration eliminating the phenotypes that are unable to keep up with the migrating group. Furthermore, by examining chemoreceptor abundance distributions during migration towards different attractants, we found that this mechanism acts on multiple chemotaxis-related traits simultaneously. Our findings reveal that collective migration itself can enable cell populations with continuous, multi-dimensional phenotypes to flexibly and rapidly adapt their phenotypic composition to diverse environmental conditions.

Significance statement

Conventional cell adaptation mechanisms, like gene regulation and random phenotypic switching, act swiftly but are limited to a few traits, while mutation-driven adaptations unfold slowly. By quantifying phenotypic diversity during bacterial collective migration, we discovered an adaptation mechanism that rapidly and reversibly adjusts multiple traits simultaneously. By dynamically balancing the elimination of phenotypes unable to keep pace with generation of diversity through growth, this process enables populations to tune their phenotypic composition based on the environment, without the need for gene regulation or mutations. Given the prevalence of collective migration in microbes, cancers, and embryonic development, non-genetic adaptation through collective migration may be a universal mechanism for populations to navigate diverse environments, offering insights into broader applications across various fields.

Organizing collective cell migration through guidance by followers

Morphogenesis, wound healing, and some cancer metastases rely on the collective migration of groups of cells. In these processes, guidance and coordination between cells and tissues are critical. While strongly adherent epithelial cells have to move collectively, loosely organized mesenchymal cells can migrate as individual cells. Nevertheless, many of them migrate collectively. This article summarizes how migratory reactions to cell-cell contacts, also called "contact regulation of locomotion" behaviors, organize mesenchymal collective cell migration. It focuses on one recently discovered mechanism called "guidance by followers", through which a cell is oriented by its immediate followers. In the gastrulating zebrafish embryo, during embryonic axis elongation, this phenomenon is responsible for the collective migration of the leading tissue, the polster, and its guidance by the following posterior axial mesoderm. Such guidance of migrating cells by followers ensures long-range coordination of movements and developmental robustness. Along with other "contact regulation of locomotion" behaviors, this mechanism contributes to organizing collective migration of loose populations of cells.

Pulses of RhoA Signaling Stimulate Actin Polymerization and Flow in Protrusions to Drive Collective Cell Migration

In animals, cells often move as collectives to shape organs, close wounds, or-in the case of disease-metastasize. To accomplish this, cells need to generate force to propel themselves forward. The motility of singly migrating cells is driven largely by an interplay between Rho GTPase signaling and the actin network (Yamada and Sixt, 2019). Whether cells migrating as collectives use the same machinery for motility is unclear. Using the zebrafish posterior lateral line primordium as a model for collective cell migration, we find that active RhoA and myosin II cluster on the basal sides of the primordium cells and are required for primordium motility. Positive and negative feedbacks cause RhoA and myosin II activities to pulse. These pulses of RhoA signaling stimulate actin polymerization at the tip of the protrusions and myosin II-dependent actin flow and protrusion retraction at the base of the protrusions, and deform the basement membrane underneath the migrating primordium. This suggests that RhoA-induced actin flow on the basal sides of the cells constitutes the motor that pulls the primordium forward, a scenario that likely underlies collective migration in other-but not all (Bastock and Strutt, 2007; Lebreton and Casanova, 2013; Matthews et al., 2008)-contexts.

CCR7 acts as both a sensor and a sink for CCL19 to coordinate collective leukocyte migration

Immune responses rely on the rapid and coordinated migration of leukocytes. Whereas it is well established that single-cell migration is often guided by gradients of chemokines and other chemoattractants, it remains poorly understood how these gradients are generated, maintained, and modulated. By combining experimental data with theory on leukocyte chemotaxis guided by the G protein-coupled receptor (GPCR) CCR7, we demonstrate that in addition to its role as the sensory receptor that steers migration, CCR7 also acts as a generator and a modulator of chemotactic gradients. Upon exposure to the CCR7 ligand CCL19, dendritic cells (DCs) effectively internalize the receptor and ligand as part of the canonical GPCR desensitization response. We show that CCR7 internalization also acts as an effective sink for the chemoattractant, dynamically shaping the spatiotemporal distribution of the chemokine. This mechanism drives complex collective migration patterns, enabling DCs to create or sharpen chemotactic gradients. We further show that these self-generated gradients can sustain the long-range guidance of DCs, adapt collective migration patterns to the size and geometry of the environment, and provide a guidance cue for other comigrating cells. Such a dual role of CCR7 as a GPCR that both senses and consumes its ligand can thus provide a novel mode of cellular self-organization.

Competition between chemoattractants causes unexpected complexity and can explain negative chemotaxis

Negative chemotaxis, where eukaryotic cells migrate away from repellents, is important throughout biology, for example, in nervous system patterning and resolution of inflammation. However, the mechanisms by which molecules repel migrating cells are unknown. Here, we use predictive modeling and experiments with Dictyostelium cells to show that competition between different ligands that bind to the same receptor leads to effective chemorepulsion. 8-CPT-cAMP, widely described as a simple chemorepellent, is inactive on its own and only repels cells when it acts in combination with the attractant cAMP. If cells degrade either competing ligand, the pattern of migration becomes more complex; cells may be repelled in one part of a gradient but attracted elsewhere, leading to populations moving in different directions in the same assay or converging in an arbitrary place. More counterintuitively still, two chemicals that normally attract cells can become repellent when combined. Computational models of chemotaxis are now accurate enough to predict phenomena that have not been anticipated by experiments. We have used them to identify new mechanisms that drive reverse chemotaxis, which we have confirmed through experiments with real cells. These findings are important whenever multiple ligands compete for the same receptors.

Receptors, enzymes and self-attraction as autocrine generators and amplifiers of chemotaxis and cell steering

Cells create their own steering cues, or modify cues from their outside, for a number of reasons. These include generating optimal, legible directional information; probing their environments for information to help decide an optimal route; symmetry breaking; generating new patterns and complexity; and bringing together collectives such as neutrophil swarms. Recent advances include more mechanisms of self-steering, in particular by using cell-generated mechanical cues, and gradients of respired oxygen. An increasing number of cell types are being found to use self-steering, in particular immune cells responding to chemokines and mesodermal cells during gastrulation. Finally, receptor modification has emerged as an important limit on the range of neutrophil swarming, allowing cells to monitor other areas as well as coming together. Self-steering is thus emerging as a dominant feature of cell motility.

Neuronal chemokine concentration gradients mediate effects of embryonic ethanol exposure on ectopic hypocretin/orexin neurons and behavior in zebrafish

Embryonic ethanol exposure in zebrafish and rats, while stimulating hypothalamic hypocretin/orexin (Hcrt) neurons along with alcohol consumption and related behaviors, increases the chemokine receptor Cxcr4 that promotes neuronal migration and may mediate ethanol's effects on neuronal development. Here we performed a more detailed anatomical analysis in zebrafish of ethanol's effects on the Cxcl12a/Cxcr4b system throughout the entire brain as it relates to Hcrt neurons developing within the anterior hypothalamus (AH) where they are normally located. We found that ethanol increased these Hcrt neurons only in the anterior part of the AH and induced ectopic Hcrt neurons further anterior in the preoptic area, and these effects along with ethanol-induced behaviors were completely blocked by a Cxcr4 antagonist. Analysis of cxcl12a transcripts and internalized Cxcr4b receptors throughout the brain showed they both exhibited natural posterior-to-anterior concentration gradients, with levels lowest in the posterior AH and highest in the anterior telencephalon. While stimulating their density in all areas and maintaining these gradients, ethanol increased chemokine expression only in the more anterior and ectopic Hcrt neurons, effects blocked by the Cxcr4 antagonist. These findings demonstrate how increased chemokine expression acting along natural gradients mediates ethanol-induced anterior migration of ectopic Hcrt neurons and behavioral disturbances.

Collective chemotaxis in a Voronoi model for confluent clusters

Collective chemotaxis, where single cells cannot climb a biochemical signaling gradient but clusters of cells can, has been observed in different biological contexts, including confluent tissues where there are no gaps or overlaps between cells. Although particle-based models have been developed that predict important features of collective chemotaxis, the mechanisms in those models depend on particle overlaps, and so it remains unclear if they can explain behavior in confluent systems. Here, we develop an open-source code that couples a two-dimensional Voronoi simulation for confluent cell mechanics to a dynamic chemical signal that can diffuse, advect, and/or degrade and use the code to study potential mechanisms for collective chemotaxis in cellular monolayers. We first study the impact of advection on collective chemotaxis and delineate a regime where advective terms are important. Next, we investigate two possible chemotactic mechanisms, contact inhibition of locomotion and heterotypic interfacial tension, and demonstrate that both can drive collective chemotaxis in certain parameter regimes. We further demonstrate that the scaling behavior of cluster motion is well captured by simple analytic theories.

A self-generated Toddler gradient guides mesodermal cell migration

The sculpting of germ layers during gastrulation relies on the coordinated migration of progenitor cells, yet the cues controlling these long-range directed movements remain largely unknown. While directional migration often relies on a chemokine gradient generated from a localized source, we find that zebrafish ventrolateral mesoderm is guided by a self-generated gradient of the initially uniformly expressed and secreted protein Toddler/ELABELA/Apela. We show that the Apelin receptor, which is specifically expressed in mesodermal cells, has a dual role during gastrulation, acting as a scavenger receptor to generate a Toddler gradient, and as a chemokine receptor to sense this guidance cue. Thus, we uncover a single receptor-based self-generated gradient as the enigmatic guidance cue that can robustly steer the directional migration of mesoderm through the complex and continuously changing environment of the gastrulating embryo.

Structures of atypical chemokine receptor 3 reveal the basis for its promiscuity and signaling bias

Both CXC chemokine receptor 4 (CXCR4) and atypical chemokine receptor 3 (ACKR3) are activated by the chemokine CXCL12 yet evoke distinct cellular responses. CXCR4 is a canonical G protein-coupled receptor (GPCR), whereas ACKR3 is intrinsically biased for arrestin. The molecular basis for this difference is not understood. Here, we describe cryo-EM structures of ACKR3 in complex with CXCL12, a more potent CXCL12 variant, and a small-molecule agonist. The bound chemokines adopt an unexpected pose relative to those established for CXCR4 and observed in other receptor-chemokine complexes. Along with functional studies, these structures provide insight into the ligand-binding promiscuity of ACKR3, why it fails to couple to G proteins, and its bias toward β-arrestin. The results lay the groundwork for understanding the physiological interplay of ACKR3 with other GPCRs.

Dual role for CXCL12 signaling in semilunar valve development

Cxcl12-null embryos have dysplastic, misaligned, and hyperplastic semilunar valves (SLVs). In this study, we show that CXCL12 signaling via its receptor CXCR4 fulfills distinct roles at different stages of SLV development, acting initially as a guidance cue to pattern cellular distribution within the valve primordia during the endocardial-to-mesenchymal transition (endoMT) phase and later regulating mesenchymal cell proliferation during SLV remodeling. Transient, anteriorly localized puncta of internalized CXCR4 are observed in cells undergoing endoMT. In vitro, CXCR4+ cell orientation in response to CXCL12 requires phosphatidylinositol 3-kinase (PI3K) signaling and is inhibited by suppression of endocytosis. This dynamic intracellular localization of CXCR4 during SLV development is related to CXCL12 availability, potentially enabling activation of divergent downstream signaling pathways at key developmental stages. Importantly, Cxcr7-/- mutants display evidence of excessive CXCL12 signaling, indicating a likely role for atypical chemokine receptor CXCR7 in regulating ligand bioavailability and thus CXCR4 signaling output during SLV morphogenesis.

Hypoxia triggers collective aerotactic migration in Dictyostelium discoideum

Using a self-generated hypoxic assay, we show that the amoeba Dictyostelium discoideum displays a remarkable collective aerotactic behavior. When a cell colony is covered, cells quickly consume the available oxygen (O₂) and form a dense ring moving outwards at constant speed and density. To decipher this collective process, we combined two technological developments: porphyrin-based O₂ -sensing films and microfluidic O₂ gradient generators. We showed that Dictyostelium cells exhibit aerotactic and aerokinetic response in a low range of O₂ concentration indicative of a very efficient detection mechanism. Cell behaviors under self-generated or imposed O₂ gradients were modeled using an in silico cellular Potts model built on experimental observations. This computational model was complemented with a parsimonious ‘Go or Grow’ partial differential equation (PDE) model. In both models, we found that the collective migration of a dense ring can be explained by the interplay between cell division and the modulation of aerotaxis.

Going your own way: Self-guidance mechanisms in cell migration

How cells and tissues migrate from one location to another is a question of significant biological and medical relevance. Migration is generally thought to be controlled by external hardwired guidance cues, which cells follow by polarizing their internal locomotory machinery in the imposed direction. However, a number of recently discovered 'self-guidance' mechanisms have revealed that migrating cells have more control over the path they follow than previously thought. Here, directional information is generated by the migrating cells themselves via a dynamic interplay of cell-intrinsic and -extrinsic regulators. In this review, we discuss how self-guidance can emerge from mechanisms acting at different levels of scale and how these enable cells to rapidly adapt to environmental challenges.

Cell dispersal by localized degradation of a chemoattractant

Chemotaxis, the guided motion of cells by chemical gradients, plays a crucial role in many biological processes. In the social amoeba Dictyostelium discoideum, chemotaxis is critical for the formation of cell aggregates during starvation. The cells in these aggregates generate a pulse of the chemoattractant, cyclic adenosine 3',5'-monophosphate (cAMP), every 6 min to 10 min, resulting in surrounding cells moving toward the aggregate. In addition to periodic pulses of cAMP, the cells also secrete phosphodiesterase (PDE), which degrades cAMP and prevents the accumulation of the chemoattractant. Here we show that small aggregates of Dictyostelium can disperse, with cells moving away from instead of toward the aggregate. This surprising behavior often exhibited oscillatory cycles of motion toward and away from the aggregate. Furthermore, the onset of outward cell motion was associated with a doubling of the cAMP signaling period. Computational modeling suggests that this dispersal arises from a competition between secreted cAMP and PDE, creating a cAMP gradient that is directed away from the aggregate, resulting in outward cell motion. The model was able to predict the effect of PDE inhibition as well as global addition of exogenous PDE, and these predictions were subsequently verified in experiments. These results suggest that localized degradation of a chemoattractant is a mechanism for morphogenesis.

Self-organized cell migration across scales - from single cell movement to tissue formation

Self-organization is a key feature of many biological and developmental processes, including cell migration. Although cell migration has traditionally been viewed as a biological response to extrinsic signals, advances within the past two decades have highlighted the importance of intrinsic self-organizing properties to direct cell migration on multiple scales. In this Review, we will explore self-organizing mechanisms that lay the foundation for both single and collective cell migration. Based on in vitro and in vivo examples, we will discuss theoretical concepts that underlie the persistent migration of single cells in the absence of directional guidance cues, and the formation of an autonomous cell collective that drives coordinated migration. Finally, we highlight the general implications of self-organizing principles guiding cell migration for biological and medical research.

Cell Proliferation and Collective Cell Migration During Zebrafish Lateral Line System Development Are Regulated by Ncam/Fgf-Receptor Interactions

The posterior lateral line system (pLLS) of aquatic animals comprises small clustered mechanosensory organs along the side of the animal. They develop from proneuromasts, which are deposited from a migratory primordium on its way to the tip of the tail. We here show, that the Neural Cell Adhesion Molecule Ncam1b is an integral part of the pathways initiating and regulating the development of the pLLS in zebrafish. We find that morpholino-knockdowns of ncam1b (i) reduce cell proliferation within the primordium, (ii) reduce the expression of Fgf target gene erm, (iii) severely affect proneuromast formation, and (iv) affect primordium migration. Ncam1b directly interacts with Fgf receptor Fgfr1a, and a knockdown of fgfr1a causes similar phenotypic changes as observed in ncam1b-morphants. We conclude that Ncam1b is involved in activating proliferation by triggering the expression of erm. In addition, we demonstrate that Ncam1b is required for the expression of chemokine receptor Cxcr7b, which is crucial for directed primordial migration. Finally, we show that the knockdown of ncam1b destabilizes proneuromasts, suggesting a further function of Ncam1b in strengthening the cohesion of proneuromast cells.

Nile tilapia CXCR4, the receptor of chemokine CXCL12, is involved in host defense against bacterial infection and chemotactic activity

CXC chemokine receptor 4 (CXCR4), a member of seven-transmembrane (7-TM) G-protein-coupled receptor superfamily, is the receptor of the CXC chemokine ligand 12 (CXCL12), and plays important roles in host defense and inflammation. In the current study, we cloned and identified a homolog of CXCR4 from Nile tilapia (Oreochromis niloticus), designated as OnCXCR4. The open reading frame of OnCXCR4 is 1149 bp encoding a peptide of 382 amino acids, and the predicted molecular weight is 42.65 kDa OnCXCR4 shares common features of CXCR4 family, including a 7-TM domain and a characteristic CXC motif (containing CYC). Expression analysis showed that OnCXCR4 constitutively expresses in various tested tissues of Nile tilapia, with the highest level in the anterior kidney. When stimulated with Streptococcus agalactiae, Aeromonas hydrophila, Poly(I:C), or LPS in vivo and in vitro, the expression of OnCXCR4 was significantly regulated. AMD3100, a CXCR4 antagonist, could not only inhibit the chemotactic activity of the recombinant OnCXCL12 protein on the leukocytes from anterior kidney, but also reduce the expression of OnCXCR4 significantly. Taken together, these results of our study above indicate that OnCXCR4 may play important roles in host defense against bacterial infectionin in Nile tilapia, and being a receptor of OnCXCL12 to exert functions.

Tetraspanin Cd9b and Cxcl12a/Cxcr4b have a synergistic effect on the control of collective cell migration

Collective cell migration is essential for embryonic development and homeostatic processes. During zebrafish development, the posterior lateral line primordium (pLLP) navigates along the embryo flank by collective cell migration. The chemokine receptors, Cxcr4b and Cxcr7b, as well as their cognate ligand, Cxcl12a, are essential for this process. We corroborate that knockdown of the zebrafish cd9 tetraspanin orthologue, cd9b, results in mild pLL abnormalities. Through generation of CRISPR and TALEN mutants, we show that cd9a and cd9b function partially redundantly in pLLP migration, which is delayed in the cd9b single and cd9a; cd9b double mutants. This delay led to a transient reduction in neuromast numbers. Loss of both Cd9a and Cd9b sensitized embryos to reduced Cxcr4b and Cxcl12a levels. Together these results provide evidence that Cd9 modulates collective cell migration of the pLLP during zebrafish development. One interpretation of these observations is that Cd9 contributes to more effective chemokine signalling.

Zebrafish Posterior Lateral Line primordium migration requires interactions between a superficial sheath of motile cells and the skin

The Zebrafish Posterior Lateral Line primordium migrates in a channel between the skin and somites. Its migration depends on the coordinated movement of its mesenchymal-like leading cells and trailing cells, which form epithelial rosettes, or protoneuromasts. We describe a superficial population of flat primordium cells that wrap around deeper epithelialized cells and extend polarized lamellipodia to migrate apposed to the overlying skin. Polarization of lamellipodia extended by both superficial and deeper protoneuromast-forming cells depends on Fgf signaling. Removal of the overlying skin has similar effects on superficial and deep cells: lamellipodia are lost, blebs appear instead, and collective migration fails. When skinned embryos are embedded in Matrigel, basal and superficial lamellipodia are recovered; however, only the directionality of basal protrusions is recovered, and migration is not rescued. These observations support a key role played by superficial primordium cells and the skin in directed migration of the Posterior Lateral Line primordium.

Involvement of Cxcl12a/Cxcr4b Chemokine System in Mediating the Stimulatory Effect of Embryonic Ethanol Exposure on Neuronal Density in Zebrafish Hypothalamus

BACKGROUND

Embryonic exposure to ethanol (EtOH) produces marked disturbances in neuronal development and alcohol-related behaviors, with low-moderate EtOH doses stimulating neurogenesis without producing apoptosis and high doses having major cytotoxic effects while causing gross morphological abnormalities. With the pro-inflammatory chemokine system, Cxcl12, and its main receptor Cxcr4, known to promote processes of neurogenesis, we examined here this neuroimmune system in the embryonic hypothalamus to test directly if it mediates the stimulatory effects low-moderate EtOH doses have on neuronal development.

METHODS

We used the zebrafish (Danio rerio) model, which develops externally and allows one to investigate the developing brain in vivo with precise control of dose and timing of EtOH delivery in the absence of maternal influence. Zebrafish were exposed to low-moderate EtOH doses (0.1, 0.25, 0.5% v/v), specifically during a period of peak hypothalamic development from 22 to 24 hours postfertilization, and in some tests were pretreated from 2 to 22 hpf with the Cxcr4 receptor antagonist, AMD3100. Measurements in the hypothalamus at 26 hpf were taken of cxcl12a and cxcr4b transcription, signaling, and neuronal density using qRT-PCR, RNAscope, and live imaging of transgenic zebrafish.

RESULTS

Embryonic EtOH exposure, particularly at the 0.5% dose, significantly increased levels of cxcl12a and cxcr4b mRNA in whole embryos, number of cxcl12a and cxcr4b transcripts in developing hypothalamus, and internalization of Cxcr4b receptors in hypothalamic cells. Embryonic EtOH also caused an increase in the number of hypothalamic neurons and coexpression of cxcl12a and cxcr4b transcripts within these neurons. Each of these stimulatory effects of EtOH in the embryo was blocked by pretreatment with the Cxcr4 antagonist AMD3100.

CONCLUSIONS

These results provide clear evidence that EtOH's stimulatory effects at low-moderate doses on the number of hypothalamic neurons early in development are mediated, in part, by increased transcription and intracellular activation of this chemokine system, likely due to autocrine signaling of Cxcl12a at its Cxcr4b receptor within the neurons.

All Roads Lead to Directional Cell Migration

Directional cell migration normally relies on a variety of external signals, such as chemical, mechanical, or electrical, which instruct cells in which direction to move. Many of the major molecular and physical effects derived from these cues are now understood, leading to questions about whether directional cell migration is alike or distinct under these different signals, and how cells might be directed by multiple simultaneous cues, which would be expected in complex in vivo environments. In this review, we compare how different stimuli are spatially distributed, often as gradients, to direct cell movement and the mechanisms by which they steer cells. A comparison of the downstream effectors of directional cues suggests that different external signals regulate a common set of components: small GTPases and the actin cytoskeleton, which implies that the mechanisms downstream of different signals are likely to be closely related and underlies the idea that cell migration operates by a common set of physical principles, irrespective of the input.

Seeing around corners: Cells solve mazes and respond at a distance using attractant breakdown

During development and metastasis, cells migrate large distances through complex environments. Migration is often guided by chemotaxis, but simple chemoattractant gradients between a source and sink cannot direct cells over such ranges. We describe how self-generated gradients, created by cells locally degrading attractant, allow single cells to navigate long, tortuous paths and make accurate choices between live channels and dead ends. This allows cells to solve complex mazes efficiently. Cells' accuracy at finding live channels was determined by attractant diffusivity, cell speed, and path complexity. Manipulating these parameters directed cells in mathematically predictable ways; specific combinations can even actively misdirect them. We propose that the length and complexity of many long-range migratory processes, including inflammation and germ cell migration, means that self-generated gradients are needed for successful navigation.

Doevendans P, Schiffelers RM, Lei Z, Sluijter JP. Cellular and Molecular Mechanism of Cardiac Regeneration: A Comparison of Newts, Zebrafish, and Mammals

Cardiovascular disease is the leading cause of death worldwide. Current palliative treatments can slow the progression of heart failure, but ultimately, the only curative treatment for end-stage heart failure is heart transplantation, which is only available for a minority of patients due to lack of donors' hearts. Explorative research has shown the replacement of the damaged and lost myocardium by inducing cardiac regeneration from preexisting myocardial cells. Lower vertebrates, such as the newt and zebrafish, can regenerate lost myocardium through cardiomyocyte proliferation. The preexisting adult cardiomyocytes replace the lost cells through subsequent dedifferentiation, proliferation, migration, and re-differentiation. Similarly, neonatal mice show complete cardiac regeneration post-injury; however, this regenerative capacity is remarkably diminished one week after birth. In contrast, the adult mammalian heart presents a fibrotic rather than a regenerative response and only shows signs of partial pathological cardiomyocyte dedifferentiation after injury. In this review, we explore the cellular and molecular responses to myocardial insults in different adult species to give insights for future interventional directions by which one can promote or activate cardiac regeneration in mammals.

Neutrophil Swarming in Damaged Tissue Is Orchestrated by Connexins and Cooperative Calcium Alarm Signals

Neutrophils are major inflammatory cells that rapidly infiltrate wounds to provide antimicrobial functions. Within the damaged tissue, neutrophil migration behavior often switches from exploratory patrolling to coordinated swarming, giving rise to dense clusters that further disrupt tissue architecture. This aggregation response is self-organized by neutrophil paracrine chemoattractant signaling (most notably of the inflammatory mediator leukotriene B4 [LTB4]). The coordination mechanism and possible evolutionary benefits of neutrophil swarms are elusive. Here, we show that neutrophil swarms require mutual reinforcement of damage signaling at the wound core. New biosensors and live imaging in zebrafish revealed that neutrophil chemoattractant synthesis is triggered by a sustained calcium flux upon contact with necrotic tissue that requires sensing of the damage signal ATP. This "calcium alarm" signal rapidly propagates in the nascent neutrophil cluster in a contact-dependent manner via connexin-43 (Cx43) hemichannels, which are mediators of active ATP release. This enhances chemoattractant biosynthesis in the growing cluster, which is instrumental for coordinated motion and swarming. Inhibition of neutrophil Cx43 compromises clearance of wound-colonizing P. aeruginosa bacteria and exacerbates infection-induced morbidity. Thus, cooperative production of alarm signals among pioneer clustering neutrophils fuels the growth of dense antimicrobial cell masses that effectively seal off breached tissue barriers from opportunistic pathogens.

Cadherin-Mediated Cell Coupling Coordinates Chemokine Sensing across Collectively Migrating Cells

The directed migration of cells sculpts the embryo, contributes to homeostasis in the adult, and, when dysregulated, underlies many diseases [1, 2]. During these processes, cells move singly or as a collective. In both cases, they follow guidance cues, which direct them to their destination [3-6]. In contrast to single cells, collectively migrating cells need to coordinate with their neighbors to move together in the same direction. Recent studies suggest that leader cells in the front sense the guidance cue, relay the directional information to the follower cells in the back, and can pull the follower cells along [7-19]. In this manner, leader cells steer the collective and set the collective's overall speed. However, whether follower cells also participate in steering and speed setting of the collective is largely unclear. Using chimeras, we analyzed the role of leader and follower cells in the collectively migrating zebrafish posterior lateral line primordium. This tissue expresses the chemokine receptor Cxcr4 and is guided by the chemokine Cxcl12a [20-23]. We find that leader and follower cells need to sense the attractant Cxcl12a for efficient migration, are coupled to each other through cadherins, and require coupling to pull Cxcl12a-insensitive cells along. Analysis of cell dynamics in chimeric and protein-depleted primordia shows that Cxcl12a-sensing and cadherin-mediated adhesion contribute jointly to direct migration at both single-cell and tissue levels. These results suggest that all cells in the primordium need to sense the attractant and adhere to each other to coordinate their movements and migrate with robust directionality.

Chemokine receptor CXCR7 non-cell-autonomously controls pontine neuronal migration and nucleus formation

Long distance tangential migration transports neurons from their birth places to distant destinations to be incorporated into neuronal circuits. How neuronal migration is guided during these long journeys is still not fully understood. We address this issue by studying the migration of pontine nucleus (PN) neurons in the mouse hindbrain. PN neurons migrate from the lower rhombic lip first anteriorly and then turn ventrally near the trigeminal ganglion root towards the anterior ventral hindbrain. Previously we showed that in mouse depleted of chemokine receptor CXCR4 or its ligand CXCL12, PN neurons make their anterior-to-ventral turn at posteriorized positions. However, the mechanism that spatiotemporally controls the anterior-to-ventral turning is still unclear. Furthermore, the role of CXCR7, the atypical receptor of CXCL12, in pontine migration has yet to be examined. Here, we find that the PN is elongated in Cxcr7 knockout due to a broadened anterior-to-ventral turning positions. Cxcr7 is not expressed in migrating PN neurons en route to their destinations, but is strongly expressed in the pial meninges. Neuroepithelium-specific knockout of Cxcr7 does not recapitulate the PN phenotype in Cxcr7 knockout, suggesting that CXCR7 acts non-cell-autonomously possibly from the pial meninges. We show further that CXCR7 regulates pontine migration by modulating CXCL12 protein levels.

Relationship between surrounding tissue morphology and directional collective migration of the posterior lateral line primordium in zebrafish

Collective cell migration, in which cells assemble and move together, is an essential process in embryonic development, wound healing and cancer metastasis. Chemokine signaling guides cell assemblies to their destinations. In zebrafish posterior lateral line primordium (PLLP), a model system for collective cell migration, it has been proposed that the chemokine ligand Cxcl12a secreted from muscle pioneer cells (MPs) and muscle fast fibers (MFFs), which are distributed along with the horizontal midline, binds to the receptor Cxcr4b in PLLP and that Cxcl12a-Cxcr4b signaling guides the anterior-to-posterior migration of PLLP along the horizontal midline. However, how the surrounding tissues affect PLLP migration remains to be elucidated. Here, we investigated the relationship between the PLLP and the surrounding tissues and found that a furrow between the dorsal and ventral myotomes is generated by Sonic hedgehog (Shh) signaling-dependent MP and MFF differentiation and that the PLLP migrates in this furrow. When transient inhibition of Shh signaling impaired both the furrow formation and differentiation of cxcl12a-expressing MPs/MFFs, directional PLLP migration was severely perturbed. Furthermore, when differentiated MPs and MFFs were ablated by femtosecond laser irradiations, the furrow remained and PLLP migration was relatively unaffected. These results suggest that the furrow formation between the dorsal and ventral myotomes is associated with the migratory behavior of PLLP.

A liposome/gelatin methacrylate nanocomposite hydrogel system for delivery of stromal cell-derived factor-1α and stimulation of cell migration

Chronic, non-healing skin and soft tissue wounds are susceptible to infection, difficult to treat clinically, and can severely reduce a patient's quality of life. A key aspect of this issue is the impaired recruitment of mesenchymal stem cells (MSCs), which secrete regenerative cytokines and modulate the phenotypes of other effector cells that promote healing. We have engineered a therapeutic delivery system that can controllably release the pro-healing chemokine stromal cell derived factor-1α (SDF-1α) to induce the migration of MSCs. In order to protect the protein cargo from hydrolytic degradation and control its release, we have loaded SDF-1α in anionic liposomes (lipoSDF) and embedded them in gelatin methacrylate (GelMA) to form a nanocomposite hydrogel. In this study, we quantify the release of SDF-1α from our hydrogel system and measure the induced migration of MSCs in vitro via a transwell assay. Lastly, we evaluate the ability of this system to activate intracellular signaling in MSCs by using Western blots to probe for the phosphorylation of key proteins in the mTOR pathway. To our knowledge, this is the first study to report the delivery of liposomal SDF-1α using a nanocomposite approach. The results of this study expand on our current understanding of factors that can be modified to affect MSC behavior and phenotype. Furthermore, our findings contribute to the development of new hydrogel-based therapeutic delivery strategies for clinical wound healing applications. STATEMENT OF SIGNIFICANCE: Chronic, non-healing wounds promote an inflammatory environment that inhibits the migration of mesenchymal stem cells (MSCs), which secrete pro-healing and regenerative cytokines. The goal of this project is to apply principles of tissue engineering to achieve controllable release of the pro-healing chemokine SDF-1α to modulate the intracellular signaling and migratory behavior of MSCs. In this work, we introduce a nanocomposite strategy to tailor the release of SDF-1α using a liposome/gelatin methacrylate hydrogel approach. We are the first group to report the delivery of liposomal SDF-1α using this strategy. Our findings aim to further elucidate the role of MSCs in directing wound healing and guide the development of immunomodulatory and therapeutic delivery strategies for clinical wound healing applications.

Mechanistic models of PLC/PKC signaling implicate phosphatidic acid as a key amplifier of chemotactic gradient sensing

Chemotaxis of fibroblasts and other mesenchymal cells is critical for embryonic development and wound healing. Fibroblast chemotaxis directed by a gradient of platelet-derived growth factor (PDGF) requires signaling through the phospholipase C (PLC)/protein kinase C (PKC) pathway. Diacylglycerol (DAG), the lipid product of PLC that activates conventional PKCs, is focally enriched at the up-gradient leading edge of fibroblasts responding to a shallow gradient of PDGF, signifying polarization. To explain the underlying mechanisms, we formulated reaction-diffusion models including as many as three putative feedback loops based on known biochemistry. These include the previously analyzed mechanism of substrate-buffering by myristoylated alanine-rich C kinase substrate (MARCKS) and two newly considered feedback loops involving the lipid, phosphatidic acid (PA). DAG kinases and phospholipase D, the enzymes that produce PA, are identified as key regulators in the models. Paradoxically, increasing DAG kinase activity can enhance the robustness of DAG/active PKC polarization with respect to chemoattractant concentration while decreasing their whole-cell levels. Finally, in simulations of wound invasion, efficient collective migration is achieved with thresholds for chemotaxis matching those of polarization in the reaction-diffusion models. This multi-scale modeling framework offers testable predictions to guide further study of signal transduction and cell behavior that affect mesenchymal chemotaxis.

Cell movement directed by external gradients of chemical composition is critical for immune responses, wound healing, and development. Although theoretical concepts explaining how shallow external gradients might definitively polarize a cell’s motility have been offered over the past two decades, mathematical models cast in terms of defined molecules and mechanisms are uncommon in this context. Based on both recent and older insights from the literature, we offer mechanistic models that are able to explain experimentally observed polarization of signal transduction elicited by shallow attractant gradients. A novel insight of our models is the implicated role of phosphatidic acid, a membrane lipid produced by at least two enzymatic pathways, in two positive feedback loops that amplify signal transduction locally. In separate simulations, we explored the implications of polarization for efficient cell invasion during wound healing. We expected that the ability to polarize in response to shallow gradients would enhance the speed of wound invasion, but an unexpected finding is that this property can promote intermittent polarization throughout the wound.

ACKR3 Regulation of Neuronal Migration Requires ACKR3 Phosphorylation, but Not β-Arrestin

Phosphorylation of heptahelical receptors is thought to regulate G protein signaling, receptor endocytosis, and non-canonical signaling via recruitment of β-arrestins. We investigated chemokine receptor functionality under phosphorylation-deficient and β-arrestin-deficient conditions by studying interneuron migration in the embryonic cortex. This process depends on CXCL12, CXCR4, G protein signaling and on the atypical CXCL12 receptor ACKR3. We found that phosphorylation was crucial, whereas β-arrestins were dispensable for ACKR3-mediated control of CXCL12 levels in vivo. Cortices of mice expressing phosphorylation-deficient ACKR3 exhibited a major interneuron migration defect, which was accompanied by excessive activation and loss of CXCR4. Cxcl12-overexpressing mice mimicked this phenotype. Excess CXCL12 caused lysosomal CXCR4 degradation, loss of CXCR4 responsiveness, and, ultimately, similar motility defects as Cxcl12 deficiency. By contrast, β-arrestin deficiency caused only a subtle migration defect mimicked by CXCR4 gain of function. These findings demonstrate that phosphorylation regulates atypical chemokine receptor function without β-arrestin involvement in chemokine sequestration and non-canonical signaling.

Self-Generated Gradients Yield Exceptionally Robust Steering Cues

Chemotaxis is a widespread mechanism that allows migrating cells to steer to where they are needed. Attractant gradients may be imposed by external sources, or self-generated, when cells create their own steep local gradients by breaking down a prevalent, broadly distributed attractant. Here we show that chemotaxis works far more robustly toward self-generated gradients. Cells can only respond efficiently to a restricted range of attractant concentrations; if attractants are too dilute, their gradients are too shallow for cells to sense, but if they are too high, all receptors become saturated and cells cannot perceive spatial differences. Self-generated gradients are robust because cells maintain the attractant at optimal concentrations. A wave can recruit varying numbers of steered cells, and cells can take time to break down attractant before starting to migrate. Self-generated gradients can therefore operate over a greater range of attractant concentrations, larger distances, and longer times than imposed gradients. The robustness is further enhanced at low cell numbers if attractants also act as mitogens, and at high attractant concentrations if the enzymes that break down attractants are themselves induced by constant attractant levels.

Single cell migration profiling on a microenvironmentally tunable hydrogel microstructure device that enables stem cell potency evaluation

Cell migration is a key function in a myriad of physiological events and disease conditions. Efficient, quick and descriptive profiling of migration behaviour in response to different treatments or conditions is highly desirable in a series of applications, ranging from fundamental studies of the migration mechanism to drug discovery and cell therapy. This investigation applied the use of methacrylamide gelatin (GelMA) to microfabricate migration lanes based on GelMA hydrogel with encapsulated migration stimuli and structural stability under culture medium conditions, providing the possibility of tailoring the microenvironment during cell-based assays. The actual device provides 3D topography, cell localization and a few step protocol, allowing the quick evaluation and quantification of individual migrated distances of a cell sample by an ImageJ plugin for automated microscopy processing. The detailed profiling of migration behaviour given by the new device has demonstrated a broader assay sensitivity compared to other migration assays and higher versatility to study cell migration in different settings of applications. In this study, parametric information extracted from the migration profiling was successfully used to develop predictive models of immunosuppressive cell function that could be applied as a potency test for mesenchymal stem cells.

Chemokine Receptors and Phagocyte Biology in Zebrafish

Phagocytes are highly motile immune cells that ingest and clear microbial invaders, harmful substances, and dying cells. Their function is critically dependent on the expression of chemokine receptors, a class of G-protein-coupled receptors (GPCRs). Chemokine receptors coordinate the recruitment of phagocytes and other immune cells to sites of infection and damage, modulate inflammatory and wound healing responses, and direct cell differentiation, proliferation, and polarization. Besides, a structurally diverse group of atypical chemokine receptors (ACKRs) are unable to signal in G-protein-dependent fashion themselves but can shape chemokine gradients by fine-tuning the activity of conventional chemokine receptors. The optically transparent zebrafish embryos and larvae provide a powerful in vivo system to visualize phagocytes during development and study them as key elements of the immune response in real-time. In this review, we discuss how the zebrafish model has furthered our understanding of the role of two main classes of chemokine receptors, the CC and CXC subtypes, in phagocyte biology. We address the roles of the receptors in the migratory properties of phagocytes in zebrafish models for cancer, infectious disease, and inflammation. We illustrate how studies in zebrafish enable visualizing the contribution of chemokine receptors and ACKRs in shaping self-generated chemokine gradients of migrating cells. Taking the functional antagonism between two paralogs of the CXCR3 family as an example, we discuss how the duplication of chemokine receptor genes in zebrafish poses challenges, but also provides opportunities to study sub-functionalization or loss-of-function events. We emphasize how the zebrafish model has been instrumental to prove that the major determinant for the functional outcome of a chemokine receptor-ligand interaction is the cell-type expressing the receptor. Finally, we highlight relevant homologies and analogies between mammalian and zebrafish phagocyte function and discuss the potential of zebrafish models to further advance our understanding of chemokine receptors in innate immunity and disease.

Dynamic Buffering of Extracellular Chemokine by a Dedicated Scavenger Pathway Enables Robust Adaptation during Directed Tissue Migration

How tissues migrate robustly through changing guidance landscapes is poorly understood. Here, quantitative imaging is combined with inducible perturbation experiments to investigate the mechanisms that ensure robust tissue migration in vivo. We show that tissues exposed to acute "chemokine floods" halt transiently before they perfectly adapt, i.e., return to the baseline migration behavior in the continued presence of elevated chemokine levels. A chemokine-triggered phosphorylation of the atypical chemokine receptor Cxcr7b reroutes it from constitutive ubiquitination-regulated degradation to plasma membrane recycling, thus coupling scavenging capacity to extracellular chemokine levels. Finally, tissues expressing phosphorylation-deficient Cxcr7b migrate normally in the presence of physiological chemokine levels but show delayed recovery when challenged with elevated chemokine concentrations. This work establishes that adaptation to chemokine fluctuations can be "outsourced" from canonical GPCR signaling to an autonomously acting scavenger receptor that both senses and dynamically buffers chemokine levels to increase the robustness of tissue migration.

NetLogo agent-based models as tools for understanding the self-organization of cell fate, morphogenesis and collective migration of the zebrafish posterior Lateral Line primordium

Interactions between primordium cells and their environment determines the self-organization of the zebrafish posterior Lateral Line primordium as it migrates under the skin from the ear to the tip of the tail forming and depositing neuromasts to spearhead formation of the posterior Lateral Line sensory system. In this review we describe how the NetLogo agent-based programming environment has been used in our lab to visualize and explore how self-generated chemokine gradients determine collective migration, how the dynamics of Wnt signaling can be used to predict patterns of neuromast deposition, and how previously defined interactions between Wnt and Fgf signaling systems have the potential to determine the periodic formation of center-biased Fgf signaling centers in the wake of a shrinking Wnt system. We also describe how NetLogo was used as a database for storing and visualizing the results of in toto lineage analysis of all cells in the migrating primordium. Together, the models illustrate how this programming environment can be used in diverse ways to integrate what has been learnt from biological experiments about the nature of interactions between cells and their environment, and explore how these interactions could potentially determine emergent patterns of cell fate specification, morphogenesis and collective migration of the zebrafish posterior Lateral Line primordium.

Signaling regulation during gastrulation: Insights from mouse embryos and in vitro systems

Gastrulation is the process whereby cells exit pluripotency and concomitantly acquire and pattern distinct cell fates. This is driven by the convergence of WNT, BMP, Nodal and FGF signals, which are tightly spatially and temporally controlled, resulting in regional and stage-specific signaling environments. The combination, level and duration of signals that a cell is exposed to, according its position within the embryo and the developmental time window, dictates the fate it will adopt. The key pathways driving gastrulation exhibit complex interactions, which are difficult to disentangle in vivo due to the complexity of manipulating multiple signals in parallel with high spatiotemporal resolution. Thus, our current understanding of the signaling dynamics regulating gastrulation is limited. In vitro stem cell models have been established, which undergo organized cellular differentiation and patterning. These provide amenable, simplified, deconstructed and scalable models of gastrulation. While the foundation of our understanding of gastrulation stems from experiments in embryos, in vitro systems are now beginning to reveal the intricate details of signaling regulation. Here we discuss the current state of knowledge of the role, regulation and dynamic interaction of signaling pathways that drive mouse gastrulation.

RAMP3 determines rapid recycling of atypical chemokine receptor-3 for guided angiogenesis

Receptor-activity-modifying proteins (RAMPs) are single transmembrane-spanning proteins which serve as molecular chaperones and allosteric modulators of G-protein-coupled receptors (GPCRs) and their signaling pathways. Although RAMPs have been previously studied in the context of their effects on Family B GPCRs, the coevolution of RAMPs with many GPCR families suggests an expanded repertoire of potential interactions. Using bioluminescence resonance energy transfer-based and cell-surface expression approaches, we comprehensively screen for RAMP interactions within the chemokine receptor family and identify robust interactions between RAMPs and nearly all chemokine receptors. Most notably, we identify robust RAMP interaction with atypical chemokine receptors (ACKRs), which function to establish chemotactic gradients for directed cell migration. Specifically, RAMP3 association with atypical chemokine receptor 3 (ACKR3) diminishes adrenomedullin (AM) ligand availability without changing G-protein coupling. Instead, RAMP3 is required for the rapid recycling of ACKR3 to the plasma membrane through Rab4-positive vesicles following either AM or SDF-1/CXCL12 binding, thereby enabling formation of dynamic spatiotemporal chemotactic gradients. Consequently, genetic deletion of either ACKR3 or RAMP3 in mice abolishes directed cell migration of retinal angiogenesis. Thus, RAMP association with chemokine receptor family members represents a molecular interaction to control receptor signaling and trafficking properties.

Leader cells in collective chemotaxis: Optimality and trade-offs

Clusters of cells can work together in order to follow a signal gradient, chemotaxing even when single cells do not. Cells in different regions of collectively migrating neural crest streams show different gene expression profiles, suggesting that cells may specialize to leader and follower roles. We use a minimal mathematical model to understand when this specialization is advantageous. In our model, leader cells sense the gradient with an accuracy that depends on the kinetics of ligand-receptor binding, while follower cells follow the cluster's direction with a finite error. Intuitively, specialization into leaders and followers should be optimal when a few cells have more information than the rest of the cluster, such as in the presence of a sharp transition in chemoattractant concentration. We do find this-but also find that high levels of specialization can be optimal in the opposite limit of very shallow gradients. We also predict that the best location for leaders may not be at the front of the cluster. In following leaders, clusters may have to choose between speed and flexibility. Clusters with only a few leaders can take orders of magnitude more time to reorient than all-leader clusters.

Emerging roles of atypical chemokine receptor 3 (ACKR3) in normal development and physiology

The discovery that atypical chemokine receptors (ACKRs) can initiate alternative signaling pathways rather than classical G-protein coupled receptor (GPCR) signaling has changed the paradigm of chemokine receptors and their roles in modulating chemotactic responses. The ACKR family has grown over the years, with discovery of new functions and roles in a variety of pathophysiological conditions. However, the extent to which these receptors regulate normal physiology is still continuously expanding. In particular, atypical chemokine receptor 3 (ACKR3) has proven to be an important receptor in mediating normal biological functions, including cardiac development and migration of cortical neurons. In this review, we illustrate the versatile and intriguing role of ACKR3 in physiology.

Matrix metalloproteinase 1 modulates invasive behavior of tracheal branches during entry into Drosophila flight muscles

Tubular networks like the vasculature extend branches throughout animal bodies, but how developing vessels interact with and invade tissues is not well understood. We investigated the underlying mechanisms using the developing tracheal tube network of Drosophila indirect flight muscles (IFMs) as a model. Live imaging revealed that tracheal sprouts invade IFMs directionally with growth-cone-like structures at branch tips. Ramification inside IFMs proceeds until tracheal branches fill the myotube. However, individual tracheal cells occupy largely separate territories, possibly mediated by cell-cell repulsion. Matrix metalloproteinase 1 (MMP1) is required in tracheal cells for normal invasion speed and for the dynamic organization of growth-cone-like branch tips. MMP1 remodels the CollagenIV-containing matrix around branch tips, which show differential matrix composition with low CollagenIV levels, while Laminin is present along tracheal branches. Thus, tracheal-derived MMP1 sustains branch invasion by modulating the dynamic behavior of sprouting branches as well as properties of the surrounding matrix.

Shootins mediate collective cell migration and organogenesis of the zebrafish posterior lateral line system

The zebrafish sensory posterior lateral line is an excellent model system to study collective cell migration and organogenesis. Shootin1 is a cytoplasmic protein involved in neuronal polarization and axon guidance. Previous studies have shown that shootin1 couples actin filament retrograde flow with extracellular adhesive substrates at the leading edge of axonal growth cones, thereby producing mechanical force for the migration and guidance of axonal growth cones. However, the functions of shootin in peripheral cells remain unknown. Here we identified two novel shootin family members, shootin2 and shootin3. In zebrafish, shootin1 and shootin3 are expressed in the posterior lateral line primordium (PLLP) and neuromasts during embryonic development. A shootin1 mutant displayed a reduced speed of PLLP migration, while shootin1;shootin3 double mutation inhibited cell proliferation in the PLLP. Furthermore, our results suggest that shootin1 and shootin3 positively regulate the number of neuromasts and the number of cells in deposited neuromasts. Our study demonstrates that shootins mediate collective cell migration of the posterior lateral line primordium and formation of neuromasts in zebrafish.

Collective cell migration: general themes and new paradigms

Collective cell migration plays essential roles in embryogenesis and also contributes to disease states. Recent years have seen immense progress in understanding mechanisms and overarching concepts of collective cell migration. Self-organization of moving groups emerges as an important common feature. This includes self-generating gradients, internal chemotaxis or mechanotaxis and contact-dependent polarization within migrating cell groups. Here, we will discuss these concepts and their applications to classical models of collective cell migration. Further, we discuss new models and paradigms of collective cell migration and elaborate on open questions and future challenges. Answering these questions will help to expand our appreciation of this exciting theme in developmental cell biology and contribute to the understanding of disease states.