An apicobasal gradient of Rac activity determines protrusion form and position

Dispensable role of Rac1 and Rac3 after cochlear hair cell specification

Rac small GTPases play important roles during embryonic development of the inner ear; however, little is known regarding their function in cochlear hair cells (HCs) after specification. Here, we revealed the localization and activation of Racs in cochlear HCs using GFP-tagged Rac plasmids and transgenic mice expressing a Rac1-fluorescence resonance energy transfer (FRET) biosensor. Furthermore, we employed Rac1-knockout (Rac1-KO, Atoh1-Cre;Rac1flox/flox) and Rac1 and Rac3 double KO (Rac1/Rac3-DKO, Atoh1-Cre;Rac1flox/flox;Rac3-/-) mice, under the control of the Atoh1 promoter. However, both Rac1-KO and Rac1/Rac3-DKO mice exhibited normal cochlear HC morphology at 13 weeks of age and normal hearing function at 24 weeks of age. No hearing vulnerability was observed in young adult (6-week-old) Rac1/Rac3-DKO mice even after intense noise exposure. Consistent with prior reports, the results from Atoh1-Cre;tdTomato mice confirmed that the Atoh1 promoter became functional only after embryonic day 14 when the sensory HC precursors exit the cell cycle. Taken together, these findings indicate that although Rac1 and Rac3 contribute to the early development of sensory epithelia in cochleae, as previously shown, they are dispensable for the maturation of cochlear HCs in the postmitotic state or for hearing maintenance following HC maturation. KEY MESSAGES: Mice with Rac1 and Rac3 deletion were generated after HC specification. Knockout mice exhibit normal cochlear hair cell morphology and hearing. Racs are dispensable for hair cells in the postmitotic state after specification. Racs are dispensable for hearing maintenance after HC maturation.

Dock2 generates characteristic spatiotemporal patterns of Rac activity to regulate neutrophil polarisation, migration and phagocytosis

Introduction

Rac-GTPases and their Rac-GEF activators play important roles in neutrophil-mediated host defence. These proteins control the adhesion molecules and cytoskeletal dynamics required for neutrophil recruitment to inflamed and infected organs, and the neutrophil effector responses that kill pathogens.

Methods

Here, we used live cell TIRF-FRET imaging in neutrophils from Rac-FRET reporter mice with deficiencies in the Rac-GEFs Dock2, Tiam1 or Prex1/Vav1 to evaluate if these proteins activate spatiotemporally distinct pools of Rac, and to correlate patterns of Rac activity with the neutrophil responses they control.

Results

All the GEFs were required for neutrophil adhesion, and Prex1/Vav1 were important during spreading and for the velocity of migration during chemotaxis. However, Dock2 emerged as the prominent regulator of neutrophil responses, as this GEF was required for neutrophil polarisation and random migration, for migration velocity during chemokinesis, for the likelihood to migrate and for the speed of migration and of turning during chemotaxis, as well as for rapid particle engulfment during phagocytosis. We identified characteristic spatiotemporal patterns of Rac activity generated by Dock2 which correlate with the importance of the Rac-GEF in these neutrophil responses. We also demonstrate a requirement for Dock2 in neutrophil recruitment during aseptic peritonitis.

Discussion

Collectively, our data provide a first direct comparison of the pools of Rac activity generated by different types of Rac-GEFs, and identify Dock2 as a key regulator of polarisation, migration and phagocytosis in primary neutrophils.

Cytoneme-like protrusion formation induced by LAR is promoted by receptor dimerization

Actin-based protrusions called cytonemes are reported to function in cell communication by supporting events such as morphogen gradient establishment and pattern formation. Despite the crucial roles of cytonemes in cell signaling, the molecular mechanism for cytoneme establishment remains elusive. In this study, we showed that the leukocyte common antigen-related (LAR) receptor protein tyrosine phosphatase plays an important role in cytoneme-like protrusion formation. Overexpression of LAR in HEK293T cells induced the formation of actin-based protrusions, some of which exceeded 200 µm in length and displayed a complex morphology with branches. Upon focusing on the regulation of LAR dimerization or clustering and the resulting regulatory effects on LAR phosphatase activity, we found that longer and more branched protrusions were formed when LAR dimerization was artificially induced and when heparan sulfate was applied. Interestingly, although the truncated form of LAR lacking phosphatase-related domains promoted protrusion formation, the phosphatase-inactive forms did not show clear changes, suggesting that LAR dimerization triggers the formation of cytoneme-like protrusions in a phosphatase-independent manner. Our results thus emphasize the importance of LAR and its dimerization in cell signaling.

This article has an associated First Person interview with the first author of the paper.

Summary: We showed that the formation of cytoneme-like protrusions, which function in cell signaling, is induced by LAR and clarified that it is LAR dimerization which promotes protrusion formation.

Regulatory mechanisms of cytoneme-based morphogen transport

During development and tissue homeostasis, cells must communicate with their neighbors to ensure coordinated responses to instructional cues. Cues such as morphogens and growth factors signal at both short and long ranges in temporal- and tissue-specific manners to guide cell fate determination, provide positional information, and to activate growth and survival responses. The precise mechanisms by which such signals traverse the extracellular environment to ensure reliable delivery to their intended cellular targets are not yet clear. One model for how this occurs suggests that specialized filopodia called cytonemes extend between signal-producing and -receiving cells to function as membrane-bound highways along which information flows. A growing body of evidence supports a crucial role for cytonemes in cell-to-cell communication. Despite this, the molecular mechanisms by which cytonemes are initiated, how they grow, and how they deliver specific signals are only starting to be revealed. Herein, we discuss recent advances toward improved understanding of cytoneme biology. We discuss similarities and differences between cytonemes and other types of cellular extensions, summarize what is known about how they originate, and discuss molecular mechanisms by which their activity may be controlled in development and tissue homeostasis. We conclude by highlighting important open questions regarding cytoneme biology, and comment on how a clear understanding of their function may provide opportunities for treating or preventing disease.

Rac1 promotes kidney collecting duct integrity by limiting actomyosin activity

A polarized collecting duct (CD), formed from the branching ureteric bud (UB), is a prerequisite for an intact kidney. The small Rho GTPase Rac1 is critical for actin cytoskeletal regulation. We investigated the role of Rac1 in the kidney collecting system by selectively deleting it in mice at the initiation of UB development. The mice exhibited only a mild developmental phenotype; however, with aging, the CD developed a disruption of epithelial integrity and function. Despite intact integrin signaling, Rac1-null CD cells had profound adhesion and polarity abnormalities that were independent of the major downstream Rac1 effector, Pak1. These cells did however have a defect in the WAVE2–Arp2/3 actin nucleation and polymerization apparatus, resulting in actomyosin hyperactivity. The epithelial defects were reversible with direct myosin II inhibition. Furthermore, Rac1 controlled lateral membrane height and overall epithelial morphology by maintaining lateral F-actin and restricting actomyosin. Thus, Rac1 promotes CD epithelial integrity and morphology by restricting actomyosin via Arp2/3-dependent cytoskeletal branching.

CG7379 and ING1 suppress cancer cell invasion by maintaining cell-cell junction integrity

Approximately 90% of cancer-related deaths can be attributed to a tumour's ability to spread. We have identified CG7379, the fly orthologue of human ING1, as a potent invasion suppressor. ING1 is a type II tumour suppressor with well-established roles in the transcriptional regulation of genes that control cell proliferation, response to DNA damage, oncogene-induced senescence and apoptosis. Recent work suggests a possible role for ING1 in cancer cell invasion and metastasis, but the molecular mechanism underlying this observation is lacking. Our results show that reduced expression of CG7379 promotes invasion in vivo in Drosophila, reduces the junctional localization of several adherens and septate junction components, and severely disrupts cell-cell junction architecture. Similarly, ING1 knockdown significantly enhances invasion in vitro and disrupts E-cadherin distribution at cell-cell junctions. A transcriptome analysis reveals that loss of ING1 affects the expression of several junctional and cytoskeletal modulators, confirming ING1 as an invasion suppressor and a key regulator of cell-cell junction integrity.

Generation and live imaging of tumors with specific genotypes in the living fly pupa

This protocol describes the step-by-step generation of tumors with specific genotypes on the dorsal thorax epithelium of the fly. This in vivo system allows the imaging of tumor cell morphology and behavior in high spatial and temporal resolution. Phenotypes such as cell invasion, cell division, and tumor size can be quantified and compared to specific controls or to the neighboring wild-type tissue. Thus, this model allows the study of conserved genes that enhance or suppress epithelial tumor progression.

For complete details on the use and execution of this protocol, please refer to Canales Coutiño et al. (2020).

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Tumors with specific genotypes can be genetically generated on the back of the fly

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Tumors can be imaged in real time in the living fly pupa

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The system is flexible, allowing a combination of mutant alleles and UAS-transgenes

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This system is ideal for both screening and gene characterization

Sequential oncogenic mutations influence cell competition

At the initial stage of carcinogenesis, newly emerging transformed cells are often eliminated from epithelial layers via cell competition with the surrounding normal cells. For instance, when surrounded by normal cells, oncoprotein RasV12-transformed cells are extruded into the apical lumen of epithelia. During cancer development, multiple oncogenic mutations accumulate within epithelial tissues. However, it remains elusive whether and how cell competition is also involved in this process. In this study, using a mammalian cell culture model system, we have investigated what happens upon the consecutive mutations of Ras and tumor suppressor protein Scribble. When Ras mutation occurs under the Scribble-knockdown background, apical extrusion of Scribble/Ras double-mutant cells is strongly diminished. In addition, at the boundary with Scribble/Ras cells, Scribble-knockdown cells frequently undergo apoptosis and are actively engulfed by the neighboring Scribble/Ras cells. The comparable apoptosis and engulfment phenotypes are also observed in Drosophila epithelial tissues between Scribble/Ras double-mutant and Scribble single-mutant cells. Furthermore, mitochondrial membrane potential is enhanced in Scribble/Ras cells, causing the increased mitochondrial reactive oxygen species (ROS). Suppression of mitochondrial membrane potential or ROS production diminishes apoptosis and engulfment of the surrounding Scribble-knockdown cells, indicating that mitochondrial metabolism plays a key role in the competitive interaction between double- and single-mutant cells. Moreover, mTOR (mechanistic target of rapamycin kinase) acts downstream of these processes. These results imply that sequential oncogenic mutations can profoundly influence cell competition, a transition from loser to winner. Further studies would open new avenues for cell competition-based cancer treatment, thereby blocking clonal expansion of more malignant populations within tumors.

Hydrogen bond effects in multimode nuclear dynamics of acetic acid observed via resonant x-ray scattering

A theoretical and experimental study of the gas phase and liquid acetic acid based on resonant inelastic x-ray scattering (RIXS) spectroscopy is presented. We combine and compare different levels of theory for an isolated molecule for a comprehensive analysis, including electronic and vibrational degrees of freedom. The excitation energy scan over the oxygen K-edge absorption reveals nuclear dynamic effects in the core-excited and final electronic states. The theoretical simulations for the monomer and two different forms of the dimer are compared against high-resolution experimental data for pure liquid acetic acid. We show that the theoretical model based on a dimer describes the hydrogen bond formation in the liquid phase well and that this bond formation sufficiently alters the RIXS spectra, allowing us to trace these effects directly from the experiment. Multimode vibrational dynamics is accounted for in our simulations by using a hybrid time-dependent stationary approach for the quantum nuclear wave packet simulations, showing the important role it plays in RIXS.

Rac1 Signaling in Amygdala Astrocytes Regulates Fear Memory Acquisition and Retrieval

The importance of astrocytes in behavior control is increasingly appreciated, but little is known about the effects of their dynamic activity in regulating learning and memory. In the present study, we constructed AAVs of photoactivatable and photoinactivatable Ras-related C3 botulinum toxin substrate 1 (Rac1) under the mGFAP promoter, which enabled the manipulation of Rac1 activity in astrocytes by optical stimulation in free-moving mice. We found that both up-regulation and down-regulation of astrocytic Rac1 activity in the basolateral amygdala (BLA) attenuated memory acquisition in a fear conditioning mouse model. Meanwhile, neuronal activation in the BLA induced by memory acquisition was inhibited under both the up- and down-regulation of astrocytic Rac1 activity during training. In terms of the impact on fear memory retrieval, we found both up- and down-regulation of BLA astrocytic Rac1 activity impaired memory retrieval of fear conditioning and memory retrieval-induced neuronal activation. Notably, the effect of astrocytic Rac1 on memory retrieval was reversible. Our results demonstrate that the normal activity of astrocytic Rac1 is necessary for the activation of neurons and memory formation. Both activation and inactivation of astrocytic Rac1 activity in the BLA reduced the excitability of neurons, and thereby impaired fear memory acquisition and retrieval.

A Genetic Analysis of Tumor Progression in Drosophila Identifies the Cohesin Complex as a Suppressor of Individual and Collective Cell Invasion

Metastasis is the leading cause of death for patients with cancer. Consequently it is imperative that we improve our understanding of the molecular mechanisms that underlie progression of tumor growth toward malignancy. Advances in genome characterization technologies have been very successful in identifying commonly mutated or misregulated genes in a variety of human cancers. However, the difficulty in evaluating whether these candidates drive tumor progression remains a major challenge. Using the genetic amenability of Drosophila melanogaster we generated tumors with specific genotypes in the living animal and carried out a detailed systematic loss-of-function analysis to identify conserved genes that enhance or suppress epithelial tumor progression. This enabled the discovery of functional cooperative regulators of invasion and the establishment of a network of conserved invasion suppressors. This includes constituents of the cohesin complex, whose loss of function either promotes individual or collective cell invasion, depending on the severity of effect on cohesin complex function.

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Screen identifies genes that affect tumor behavior in a wide variety of ways

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A functionally validated network of invasion-suppressor genes was generated

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Loss of cohesin complex function can promote individual or collective cell invasion

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The fly pupal notum is an excellent in vivo system to study tumor progression

Cytonemes, Their Formation, Regulation, and Roles in Signaling and Communication in Tumorigenesis

Increasing evidence during the past two decades shows that cells interconnect and communicate through cytonemes. These cytoskeleton-driven extensions of specialized membrane territories are involved in cell–cell signaling in development, patterning, and differentiation, but also in the maintenance of adult tissue homeostasis, tissue regeneration, and cancer. Brain tumor cells in glioblastoma extend ultralong membrane protrusions (named tumor microtubes, TMs), which contribute to invasion, proliferation, radioresistance, and tumor progression. Here we review the mechanisms underlying cytoneme formation, regulation, and their roles in cell signaling and communication in epithelial cells and other cell types. Furthermore, we discuss the recent discovery of glial cytonemes in the Drosophila glial cells that alter Wingless (Wg)/Frizzled (Fz) signaling between glia and neurons. Research on cytoneme formation, maintenance, and cell signaling mechanisms will help to better understand not only physiological developmental processes and tissue homeostasis but also cancer progression.

The cytoneme connection: direct long-distance signal transfer during development

During development, specialized cells produce signals that distribute among receiving cells to induce a variety of cellular behaviors and organize tissues. Recent studies have highlighted cytonemes, a type of specialized signaling filopodia that carry ligands and/or receptor complexes, as having a role in signal dispersion. In this Primer, we discuss how the dynamic regulation of cytonemes facilitates signal transfer in complex environments. We assess recent evidence for the mechanisms for cytoneme formation, function and regulation, and postulate that contact between cytoneme membranes promotes signal transfer as a new type of synapse (morphogenetic synapsis). Finally, we reflect on the fundamental unanswered questions related to understanding cytoneme biology.

Serotonin induces or inhibits neuritic regeneration of leech CNS neurons depending on neuronal identity

Recovery of motor function after central nervous system (CNS) injury is dependent on the regeneration capacity of the nervous system, which is a multifactorial process influenced, among other things, by the role of neuromodulators such as serotonin. The neurotransmitter serotonin can promote neuronal regeneration but there are also reports of it causing restriction, so it is important to clarify these divergent findings in order to understand the direct scope and side effects of potential pharmacological treatments. We evaluated the effect of serotonin on the extent of neuritic outgrowth and morphology of three different neuronal types in the leech Haementeria officinalis during their regeneration in vitro: Retzius interneurons (Rz), annulus erector (AE) motoneurons, and anterolateral number 1 (AL1) CNS neurons. Neurons were isolated and cultured in L15 medium, with or without serotonin. Growth parameters were registered and quantified, and observed differences were analyzed. The addition of serotonin was found to induce AL1 neurons to increase their average growth dramatically by 8.3-fold (P=0.02; n=5), and to have no clear effect on AE motoneurons (P=0.44; n=5). For Rz interneurons, which normally do not regenerate their neurites, the addition of concanavaline-A causes substantial growth, which serotonin was found to inhibit on average by 98% (P=0.02; n=5). The number of primary neurites and their branches were also affected. These results reveal that depending on the neuronal type, serotonin can promote, inhibit, or have no effect on neuronal regeneration. This suggests that after CNS injury, non-specific pharmacological treatments affecting serotonin may have different effects on different neuronal populations.

From top to bottom: Cell polarity in Hedgehog and Wnt trafficking

Spatial organization of membrane domains within cells and cells within tissues is key to the development of organisms and the maintenance of adult tissue. Cell polarization is crucial for correct cell-cell signalling, which, in turn, promotes cell differentiation and tissue patterning. However, the mechanisms linking internal cell polarity to intercellular signalling are just beginning to be unravelled. The Hedgehog (Hh) and Wnt pathways are major directors of development and their malfunction can cause severe disorders like cancer. Here we discuss parallel advances into understanding the mechanism of Hedgehog and Wnt signal dissemination and reception. We hypothesize that cell polarization of the signal-sending and signal-receiving cells is crucial for proper signal spreading and activation of the pathway and, thus, fundamental for development of multicellular organisms.

Myelinating Schwann Cell Polarity and Mechanically-Driven Myelin Sheath Elongation

Myelin sheath geometry, encompassing myelin sheath thickness relative to internodal length, is critical to optimize nerve conduction velocity and these parameters are carefully adjusted by the myelinating cells in mammals. In the central nervous system these adjustments could regulate neuronal activities while in the peripheral nervous system they lead to the optimization and the reliability of the nerve conduction velocity. However, the physiological and cellular mechanisms that underlie myelin sheath geometry regulation are not yet fully elucidated. In peripheral nerves the myelinating Schwann cell uses several molecular mechanisms to reach and maintain the correct myelin sheath geometry, such that myelin sheath thickness and internodal length are regulated independently. One of these mechanisms is the epithelial-like cell polarization process that occurs during the early phases of the myelin biogenesis. Epithelial cell polarization factors are known to control cell size and morphology in invertebrates and mammals making these processes critical in the organogenesis. Correlative data indicate that internodal length is regulated by postnatal body growth that elongates peripheral nerves in mammals. In addition, the mechanical stretching of peripheral nerves in adult animals shows that myelin sheath length can be increased by mechanical cues. Recent results describe the important role of YAP/TAZ co-transcription factors during Schwann cell myelination and their functions have linked to the mechanotransduction through the HIPPO pathway and the epithelial polarity factor Crb3. In this review the molecular mechanisms that govern mechanically-driven myelin sheath elongation and how a Schwann cell can modulate internodal myelin sheath length, independent of internodal thickness, will be discussed regarding these recent data. In addition, the potential relevance of these mechanosensitive mechanisms in peripheral pathologies will be highlighted.