Defective apical extrusion signaling contributes to aggressive tumor hallmarks

Mechanism of efferocytosis in atherosclerosis

Atherosclerosis (AS) is a chronic inflammatory vascular disease that occurs in the intima of large and medium-sized arteries with the immune system's involvement. It is a common pathological basis for high morbidity and mortality of cardiovascular diseases. Abnormal proliferation of apoptotic cells and necrotic cells leads to AS plaque expansion, necrotic core formation, and rupture. In the early stage of AS, macrophages exert an efferocytosis effect to engulf and degrade apoptotic, dead, damaged, or senescent cells by efferocytosis, thus enabling the regulation of the organism. In the early stage of AS, macrophages rely on this effect to slow down the process of AS. However, in the advanced stage of AS, the efferocytosis of macrophages within the plaque is impaired, which leads to the inability of macrophages to promptly remove the apoptotic cells (ACs) from the organism promptly, causing exacerbation of AS. Moreover, upregulation of CD47 expression in AS plaques also protects ACs from phagocytosis by macrophages, resulting in a large amount of residual ACs in the plaque, further expanding the necrotic core. In this review, we discussed the molecular mechanisms involved in the process of efferocytosis and how efferocytosis is impaired and regulated during AS, hoping to provide new insights for treating AS.

Loss of EGF receptor polarity enables homeostatic imbalance in epithelial-cell models

The polarized distribution of membrane proteins into apical and basolateral domains provides the basis for specialized functions of epithelial tissues. The EGF receptor (EGFR) plays important roles in embryonic development, adult-epithelial tissue homeostasis, and growth and survival of many carcinomas. Typically targeted to basolateral domains, there is also considerable evidence of EGFR sorting plasticity but very limited knowledge regarding domain-specific EGFR substrates. Here we have investigated effects of selective EGFR mistargeting because of inactive-basolateral sorting signals on epithelial-cell homeostatic responses to growth-induced stress in MDCK cell models. Aberrant EGFR localization was associated with multilayer formation, anchorage-independent growth, and upregulated expression of the intermediate filament-protein vimentin characteristically seen in cells undergoing epithelial-to-mesenchymal transition. EGFRs were selectively retained following their internalization from apical membranes, and a signaling pathway involving the signaling adaptor Gab1 protein and extracellular signal-regulated kinase ERK5 had an essential role integrating multiple responses to growth-induced stress. Our studies highlight the potential importance of cellular machinery specifying EGFR polarity in epithelial pathologies associated with homeostatic imbalance.

Bidirectional multiciliated cell extrusion is controlled by Notch-driven basal extrusion and Piezo1-driven apical extrusion

Xenopus embryos are covered with a complex epithelium containing numerous multiciliated cells (MCCs). During late-stage development, there is a dramatic remodeling of the epithelium that involves the complete loss of MCCs. Cell extrusion is a well-characterized process for driving cell loss while maintaining epithelial barrier function. Normal cell extrusion is typically unidirectional, whereas bidirectional extrusion is often associated with disease (e.g. cancer). We describe two distinct mechanisms for MCC extrusion, a basal extrusion driven by Notch signaling and an apical extrusion driven by Piezo1. Early in the process there is a strong bias towards basal extrusion, but as development continues there is a shift towards apical extrusion. Importantly, response to the Notch signal is age dependent and governed by the maintenance of the MCC transcriptional program such that extension of this program is protective against cell loss. In contrast, later apical extrusion is regulated by Piezo1, such that premature activation of Piezo1 leads to early extrusion while blocking Piezo1 leads to MCC maintenance. Distinct mechanisms for MCC loss underlie the importance of their removal during epithelial remodeling.

Deciphering the Role of p60AmotL2 in Epithelial Extrusion and Cell Detachment

Preserving an accurate cell count is crucial for maintaining homeostasis. Apical extrusion, a process in which redundant cells are eliminated by neighboring cells, plays a key role in this regard. Recent studies have revealed that apical extrusion can also be triggered in cells transformed by oncogenes, suggesting it may be a mechanism through which tumor cells escape their microenvironment. In previous work, we demonstrated that p60AmotL2 modulates the E-cadherin function by inhibiting its connection to radial actin filaments. This isoform of AmotL2 is expressed in invasive breast and colon tumors and promotes invasion in vitro and in vivo. Transcriptionally regulated by c-Fos, p60AmotL2 is induced by local stress signals such as severe hypoxia. In this study, we investigated the normal role of p60AmotL2 in epithelial tissues. We found that this isoform is predominantly expressed in the gut, where cells experience rapid turnover. Through time-lapse imaging, we present evidence that cells expressing p60AmotL2 are extruded by their normal neighboring cells. Based on these findings, we hypothesize that tumor cells exploit this pathway to detach from normal epithelia and invade surrounding tissues.

Apoptotic extracellular vesicle formation via local phosphatidylserine exposure drives efficient cell extrusion

Cell extrusion is a universal mode of cell removal from tissues, and it plays an important role in regulating cell numbers and eliminating unwanted cells. However, the underlying mechanisms of cell delamination from the cell layer are unclear. Here, we report a conserved execution mechanism of apoptotic cell extrusion. We found extracellular vesicle (EV) formation in extruding mammalian and Drosophila cells at a site opposite to the extrusion direction. Lipid-scramblase-mediated local exposure of phosphatidylserine is responsible for EV formation and is crucial for executing cell extrusion. Inhibition of this process disrupts prompt cell delamination and tissue homeostasis. Although the EV has hallmarks of an apoptotic body, its formation is governed by the mechanism of microvesicle formation. Experimental and mathematical modeling analysis illustrated that EV formation promotes neighboring cells' invasion. This study showed that membrane dynamics play a crucial role in cell exit by connecting the actions of the extruding cell and neighboring cells.

Clonal interactions in cancer: integrating quantitative models with experimental and clinical data

Tumors consist of different genotypically distinct subpopulations-or subclones-of cells. These subclones can influence neighboring clones in a process called "clonal interaction." Conventionally, research on driver mutations in cancer has focused on their cell-autonomous effects that lead to an increase in fitness of the cells containing the driver. Recently, with the advent of improved experimental and computational technologies for investigating tumor heterogeneity and clonal dynamics, new studies have shown the importance of clonal interactions in cancer initiation, progression, and metastasis. In this review we provide an overview of clonal interactions in cancer, discussing key discoveries from a diverse range of approaches to cancer biology research. We discuss common types of clonal interactions, such as cooperation and competition, its mechanisms, and the overall effect on tumorigenesis, with important implications for tumor heterogeneity, resistance to treatment, and tumor suppression. Quantitative models-in coordination with cell culture and animal model experiments-have played a vital role in investigating the nature of clonal interactions and the complex clonal dynamics they generate. We present mathematical and computational models that can be used to represent clonal interactions and provide examples of the roles they have played in identifying and quantifying the strength of clonal interactions in experimental systems. Clonal interactions have proved difficult to observe in clinical data; however, several very recent quantitative approaches enable their detection. We conclude by discussing ways in which researchers can further integrate quantitative methods with experimental and clinical data to elucidate the critical-and often surprising-roles of clonal interactions in human cancers.

Expanding roles of centrosome abnormalities in cancers

Centrosome abnormalities are hallmarks of human cancers. Structural and numerical centrosome abnormalities correlate with tumor aggressiveness and poor prognosis, implicating that centrosome abnormalities could be a cause of tumorigenesis. Since Boveri made his pioneering recognition of the potential causal link between centrosome abnormalities and cancer more than a century ago, there has been significant progress in the field. Here, we review recent advances in the understanding of the causes and consequences of centrosome abnormalities and their connection to cancers. Centrosome abnormalities can drive the initiation and progression of cancers in multiple ways. For example, they can generate chromosome instability through abnormal mitosis, accelerating cancer genome evolution. Remarkably, it is becoming clear that the mechanisms by which centrosome abnormalities promote several steps of tumorigenesis are far beyond what Boveri had initially envisioned. We highlight various cancer-promoting mechanisms exerted by cells with centrosome abnormalities and how these cells possessing oncogenic potential can be monitored. [BMB Reports 2023; 56(4): 216-224].

Epithelial RAC1-dependent cytoskeleton dynamics controls cell mechanics, cell shedding and barrier integrity in intestinal inflammation

OBJECTIVE

Increased apoptotic shedding has been linked to intestinal barrier dysfunction and development of inflammatory bowel diseases (IBD). In contrast, physiological cell shedding allows the renewal of the epithelial monolayer without compromising the barrier function. Here, we investigated the role of live cell extrusion in epithelial barrier alterations in IBD.

DESIGN

Taking advantage of conditional GGTase and RAC1 knockout mice in intestinal epithelial cells (Pggt1b ^iΔIEC and Rac1 ^iΔIEC mice), intravital microscopy, immunostaining, mechanobiology, organoid techniques and RNA sequencing, we analysed cell shedding alterations within the intestinal epithelium. Moreover, we examined human gut tissue and intestinal organoids from patients with IBD for cell shedding alterations and RAC1 function.

RESULTS

Epithelial Pggt1b deletion led to cytoskeleton rearrangement and tight junction redistribution, causing cell overcrowding due to arresting of cell shedding that finally resulted in epithelial leakage and spontaneous mucosal inflammation in the small and to a lesser extent in the large intestine. Both in vivo and in vitro studies (knockout mice, organoids) identified RAC1 as a GGTase target critically involved in prenylation-dependent cytoskeleton dynamics, cell mechanics and epithelial cell shedding. Moreover, inflamed areas of gut tissue from patients with IBD exhibited funnel-like structures, signs of arrested cell shedding and impaired RAC1 function. RAC1 inhibition in human intestinal organoids caused actin alterations compatible with arresting of cell shedding.

CONCLUSION

Impaired epithelial RAC1 function causes cell overcrowding and epithelial leakage thus inducing chronic intestinal inflammation. Epithelial RAC1 emerges as key regulator of cytoskeletal dynamics, cell mechanics and intestinal cell shedding. Modulation of RAC1 might be exploited for restoration of epithelial integrity in the gut of patients with IBD.

Bidirectional multiciliated cell extrusion is controlled by Notch driven basal extrusion and Piezo 1 driven apical extrusion

Xenopus embryos are covered with a complex epithelium containing numerous multiciliated cells (MCCs). During late stage development there is a dramatic remodeling of the epithelium that involves the complete loss of MCCs. Cell extrusion is a well-characterized process for driving cell loss while maintaining epithelial barrier function. Normal cell extrusion is typically unidirectional whereas bidirectional extrusion is often associated with disease (e.g. cancer). We describe two distinct mechanisms for MCC extrusion, a basal extrusion driven by Notch signaling and an apical extrusion driven by Piezo1. Early in the process there is a strong bias towards basal extrusion, but as development continues there is a shift towards apical extrusion. Importantly, receptivity to the Notch signal is age-dependent and governed by the maintenance of the MCC transcriptional program such that extension of this program is protective against cell loss. In contrast, later apical extrusion is regulated by Piezo 1 such that premature activation of Piezo 1 leads to early extrusion while blocking Piezo 1 leads to MCC maintenance. Distinct mechansms for MCC loss underlie the importance of their removal during epithelial remodeling.

Summay Statement

Cell extrusion typically occurs unidirectionally. We have identified a single population of multiciliated cells that extrudes bidirectionally: Notch-driven basal extrusion and Piezo 1-mediated apical extrusion.

Cell polarity and extrusion: How to polarize extrusion and extrude misspolarized cells?

The barrier function of epithelia is one of the cornerstones of the body plan organization of metazoans. It relies on the polarity of epithelial cells which organizes along the apico-basal axis the mechanical properties, signaling as well as transport. This barrier function is however constantly challenged by the fast turnover of epithelia occurring during morphogenesis or adult tissue homeostasis. Yet, the sealing property of the tissue can be maintained thanks to cell extrusion: a series of remodeling steps involving the dying cell and its neighbors leading to seamless cell expulsion. Alternatively, the tissue architecture can also be challenged by local damages or the emergence of mutant cells that may alter its organization. This includes mutants of the polarity complexes which can generate neoplastic overgrowths or be eliminated by cell competition when surrounded by wild type cells. In this review, we will provide an overview of the regulation of cell extrusion in various tissues focusing on the relationship between cell polarity, cell organization and the direction of cell expulsion. We will then describe how local perturbations of polarity can also trigger cell elimination either by apoptosis or by cell exclusion, focusing specifically on how polarity defects can be directly causal to cell elimination. Overall, we propose a general framework connecting the influence of polarity on cell extrusion and its contribution to aberrant cell elimination.

ECM-transmitted shear stress induces apoptotic cell extrusion in early breast gland development

Epithelial cells of human breast glands are exposed to various mechanical ECM stresses that regulate tissue development and homeostasis. Mechanoadaptation of breast gland tissue to ECM-transmitted shear stress remained poorly investigated due to the lack of valid experimental approaches. Therefore, we created a magnetic shear strain device that enabled, for the first time, to analyze the instant shear strain response of human breast gland cells. MCF10A-derived breast acini with basement membranes (BM) of defined maturation state and basoapical polarization were used to resemble breast gland morphogenesis in vitro. The novel biophysical tool was used to apply cyclic shear strain with defined amplitudes (≤15%, 0.2 Hz) over 22 h on living spheroids embedded in an ultrasoft matrix (<60 Pa). We demonstrated that breast spheroids gain resistance to shear strain, which increased with BM maturation and basoapical polarization. Most intriguingly, poorly developed spheroids were prone to cyclic strain-induced extrusion of apoptotic cells from the spheroid body. In contrast, matured spheroids were insensitive to this mechanoresponse—indicating changing mechanosensing or mechanotransduction mechanisms during breast tissue morphogenesis. Together, we introduced a versatile tool to study cyclic shear stress responses of 3D cell culture models. It can be used to strain, in principle, all kinds of cell clusters, even those that grow only in ultrasoft hydrogels. We believe that this approach opens new doors to gain new insights into dynamic shear strain-induced mechanobiological regulation circuits between cells and their ECM.

The cross-talk between soluble "Find me" and "Keep out" signals as an initial step in regulating efferocytosis

The rapid clearance of apoptotic cells (ACs), known as efferocytosis, prompts the inhibition of inflammatory responses and autoimmunity and maintains homeostatic cell turnover by controlling the release of intracellular contents. The fast clearance of ACs requires professional and nonprofessional phagocytic cells that can accurately and promptly recognize ACs and migrate towards them. Cells undergoing apoptosis alarm their presence by releasing special soluble chemotactic factors, such as lactoferrin, that act as "Find me," "Keep out," or "Stay away" signals to recruit phagocytic cells, such as macrophages or prevent granulocyte migration. Efferocytosis effectively serves to prevent damage-associated molecular pattern release and secondary necrosis and inhibit inflammation/autoimmunity at the very first step. Since less attention has been given to the cross-talk and balance of "Find me" and "Keep out" signals released from ACs in efferocytosis, we set out to investigate the current knowledge of the roles of "Find me" and "Keep out" signals in the efferocytosis process.

Ha-RasV12-Induced Multilayer Cellular Aggregates Is Mediated by Rac1 Activation Rather Than YAP Activation

We demonstrate that Ha-RasV12 overexpression induces the nuclear translocation of Hippo effector Yes-associated protein (YAP) in MDCK cells via the hippo-independent pathway at the confluent stage. Ha-RasV12 overexpression leads to the downregulation of Caveolin-1 (Cav1) and the disruption of junction integrity. It has been shown that the disruption of actin belt integrity causes YAP nuclear translocation in epithelial cells at high density. Therefore, we hypothesized that Ha-RasV12-decreased Cav1 leads to the disruption of cell junction integrity, which subsequently facilitates YAP nuclear retention. We revealed that Ha-RasV12 downregulated Cav1 through the ERK pathway. Furthermore, the distribution and expression of Cav1 mediated the cell junction integrity and YAP nuclear localization. This suggests that the downregulation of Cav1 induced by Ha-RasV12 disrupted the cell junction integrity and promoted YAP nuclear translocation. We further indicated the consequence of Ha-RasV12-induced YAP activation. Surprisingly, the activation of YAP is not required for Ha-RasV12-induced multilayer cellular aggregates. Instead, Ha-RasV12 triggered the ERK-Rac pathway to promote cellular aggregate formation. Moreover, the overexpression of constitutively active Rac is sufficient to trigger cellular aggregation in MDCK cells at the confluent stage. This highlights that Rac activity is essential for cellular aggregates.

Impact of Epithelial Cell Shedding on Intestinal Homeostasis

The gut barrier acts as a first line of defense in the body, and plays a vital role in nutrition and immunoregulation. A layer of epithelial cells bound together via intercellular junction proteins maintains intestinal barrier integrity. Based on a tight equilibrium between cell extrusion and cell restitution, the renewal of the epithelium (epithelial turnover) permits the preservation of cell numbers. As the last step within the epithelial turnover, cell shedding occurs due to the pressure of cell division and migration from the base of the crypt. During this process, redistribution of tight junction proteins enables the sealing of the epithelial gap left by the extruded cell, and thereby maintains barrier function. Disturbance in cell shedding can create transient gaps (leaky gut) or cell accumulation in the epithelial layer. In fact, numerous studies have described the association between dysregulated cell shedding and infection, inflammation, and cancer; thus epithelial cell extrusion is considered a key defense mechanism. In the gastrointestinal tract, altered cell shedding has been observed in mouse models of intestinal inflammation and appears as a potential cause of barrier loss in human inflammatory bowel disease (IBD). Despite the relevance of this process, there are many unanswered questions regarding cell shedding. The investigation of those mechanisms controlling cell extrusion in the gut will definitely contribute to our understanding of intestinal homeostasis. In this review, we summarized the current knowledge about intestinal cell shedding under both physiological and pathological circumstances.

Stromal architecture directs early dissemination in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDA) is an extremely metastatic and lethal disease. Here, in both murine and human PDA, we demonstrate that extracellular matrix architecture regulates cell extrusion and subsequent invasion from intact ductal structures through tumor-associated collagen signatures (TACS). This results in early dissemination from histologically premalignant lesions and continual invasion from well-differentiated disease, and it suggests TACS as a biomarker to aid in the pathologic assessment of early disease. Furthermore, we show that pancreatitis results in invasion-conducive architectures, thus priming the stroma prior to malignant disease. Analysis in potentially novel microfluidic-derived microtissues and in vivo demonstrates decreased extrusion and invasion following focal adhesion kinase (FAK) inhibition, consistent with decreased metastasis. Thus, data suggest that targeting FAK or strategies to reengineer and normalize tumor microenvironments may have roles not only in very early disease, but also for limiting continued dissemination from unresectable disease. Likewise, it may be beneficial to employ stroma-targeting strategies to resolve precursor diseases such as pancreatitis in order to remove stromal architectures that increase risk for early dissemination.

KRas-transformed epithelia cells invade and partially dedifferentiate by basal cell extrusion

Metastasis is the main cause of carcinoma-related death, yet we know little about how it initiates due to our inability to visualize stochastic invasion events. Classical models suggest that cells accumulate mutations that first drive formation of a primary mass, and then downregulate epithelia-specific genes to cause invasion and metastasis. Here, using transparent zebrafish epidermis to model simple epithelia, we can directly image invasion. We find that KRas-transformation, implicated in early carcinogenesis steps, directly drives cell invasion by hijacking a process epithelia normally use to promote death-cell extrusion. Cells invading by basal cell extrusion simultaneously pinch off their apical epithelial determinants, endowing new plasticity. Following invasion, cells divide, enter the bloodstream, and differentiate into stromal, neuronal-like, and other cell types. Yet, only invading KRas^V12 cells deficient in p53 survive and form internal masses. Together, we demonstrate that KRas-transformation alone causes cell invasion and partial dedifferentiation, independently of mass formation.

An Update on Molecular Pathways Regulating Vasculogenic Mimicry in Human Osteosarcoma and Their Role in Canine Oncology

Canine tumors are valuable comparative models for human counterparts, especially to explore novel biomarkers and to understand pathways and processes involved in metastasis. Vasculogenic mimicry (VM) is a unique property of malignant cancer cells which promote metastasis. Thus, it represents an opportunity to investigate both the molecular mechanisms and the therapeutic targets of a crucial phenotypic malignant switch. Although this biological process has been largely investigated in different human cancer types, including osteosarcoma, it is still largely unknown in veterinary pathology, where it has been mainly explored in canine mammary tumors. The presence of VM in human osteosarcoma is associated with poor clinical outcome, reduced patient survival, and increased risk of metastasis and it shares the main pathways involved in other type of human tumors. This review illustrates the main findings concerning the VM process in human osteosarcoma, search for the related current knowledge in canine pathology and oncology, and potential involvement of multiple pathways in VM formation, in order to provide a basis for future investigations on VM in canine tumors.

Uncovering the 'sphinx' of sphingosine 1-phosphate signalling: from cellular events to organ morphogenesis

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid metabolite, functioning as a signalling molecule in diverse cellular processes. Over the past few decades, studies of S1P signalling have revealed that the physiological activity of S1P largely depends on S1P metabolizing enzymes, transporters and receptors on the plasma membrane, as well as on the intracellular proteins that S1P binds directly to. In addition to its roles in cancer signalling, immunity and inflammation, a large body of evidence has identified a close link of S1P signalling with organ morphogenesis. Here we discuss the vital role of S1P signalling in orchestrating various cellular events during organ morphogenesis through analysing each component along the extracellular and intracellular S1P signalling axes. For each component, we review advances in our understanding of S1P signalling and function from the upstream regulators to the downstream effectors and from cellular behaviours to tissue organization, primarily in the context of morphogenetic mechanisms. S1P-mediated vesicular trafficking is also discussed as a function independent of its signalling function. A picture emerges that reveals a multifaceted role of S1P-dependent pathways in the development and maintenance of organ structure and function.

EPHA2-dependent outcompetition of KRASG12D mutant cells by wild-type neighbors in the adult pancreas

As we age, our tissues are repeatedly challenged by mutational insult, yet cancer occurrence is a relatively rare event. Cells carrying cancer-causing genetic mutations compete with normal neighbors for space and survival in tissues. However, the mechanisms underlying mutant-normal competition in adult tissues and the relevance of this process to cancer remain incompletely understood. Here, we investigate how the adult pancreas maintains tissue health in vivo following sporadic expression of oncogenic Kras (KrasG12D), the key driver mutation in human pancreatic cancer. We find that when present in tissues in low numbers, KrasG12D mutant cells are outcompeted and cleared from exocrine and endocrine compartments in vivo. Using quantitative 3D tissue imaging, we show that before being cleared, KrasG12D cells lose cell volume, pack into round clusters, and E-cadherin-based cell-cell adhesions decrease at boundaries with normal neighbors. We identify EphA2 receptor as an essential signal in the clearance of KrasG12D cells from exocrine and endocrine tissues in vivo. In the absence of functional EphA2, KrasG12D cells do not alter cell volume or shape, E-cadherin-based cell-cell adhesions increase and KrasG12D cells are retained in tissues. The retention of KRasG12D cells leads to the early appearance of premalignant pancreatic intraepithelial neoplasia (PanINs) in tissues. Our data show that adult pancreas tissues remodel to clear KrasG12D cells and maintain tissue health. This study provides evidence to support a conserved functional role of EphA2 in Ras-driven cell competition in epithelial tissues and suggests that EphA2 is a novel tumor suppressor in pancreatic cancer.

Pulsatile contractions promote apoptotic cell extrusion in epithelial tissues

Extrusion is a mechanism used to eliminate unfit, excess, or dying cells from epithelial tissues. The initial events guiding which cells will be selectively extruded from the epithelium are not well understood. Here, we induced damage in a subset of epithelial cells in the developing zebrafish and used time-lapse imaging to examine cell and cytoskeletal dynamics leading to extrusion. We show that cell extrusion is preceded by actomyosin contractions that are pulsatile. Our data show that pulsatile contractions are induced by a junctional to medial re-localization of myosin. Analysis of cell area during contractions revealed that cells pulsing with the longest duration and highest amplitude undergo progressive area loss and extrude. Although pulses were driven by local increases in tension, damage to many cells promoted an overall decrease in the tensile state of the epithelium. We demonstrate that caspase activation leads to sphingosine-1-phosphate enrichment that controls both tissue tension and pulses to dictate areas of extrusion. These data suggest that the kinetics of pulsatile contractions define a key behavioral difference between extruding and non-extruding cells and are predictive of extrusion. Altogether, our study provides mechanistic insight into how localized changes in physical forces are coordinated to remove defective cells for homeostatic maintenance of living epithelial tissues.

Entosis and apical cell extrusion constitute a tumor-suppressive mechanism downstream of Matriptase

Armistead et al. show that in a bilayered epithelium in vivo, apical cell extrusion of basal cells is achieved via their engulfment by surface cells. In zebrafish hai1a mutants, this constitutes a tumor-suppressive mechanism, revealing a double face of Matriptase.

The type II transmembrane serine protease Matriptase 1 (ST14) is commonly known as an oncogene, yet it also plays an understudied role in suppressing carcinogenesis. This double face is evident in the embryonic epidermis of zebrafish loss-of-function mutants in the cognate Matriptase inhibitor Hai1a (Spint1a). Mutant embryos display epidermal hyperplasia, but also apical cell extrusions, during which extruding outer keratinocytes carry out an entosis-like engulfment and entrainment of underlying basal cells, constituting a tumor-suppressive effect. These counteracting Matriptase effects depend on EGFR and the newly identified mediator phospholipase D (PLD), which promotes both mTORC1-dependent cell proliferation and sphingosine-1-phosphate (S1P)–dependent entosis and apical cell extrusion. Accordingly, hypomorphic hai1a mutants heal spontaneously, while otherwise lethal hai1a amorphs are efficiently rescued upon cotreatment with PLD inhibitors and S1P. Together, our data elucidate the mechanisms underlying the double face of Matriptase function in vivo and reveal the potential use of combinatorial carcinoma treatments when such double-face mechanisms are involved.

Snail induces epithelial cell extrusion by regulating RhoA contractile signalling and cell-matrix adhesion

Cell extrusion is a morphogenetic process that is implicated in epithelial homeostasis and elicited by stimuli ranging from apoptosis to oncogenic transformation. To explore whether the morphogenetic transcription factor Snail (SNAI1) induces extrusion, we inducibly expressed a stabilized Snail6SA transgene in confluent MCF-7 monolayers. When expressed in small clusters (less than three cells) within otherwise wild-type confluent monolayers, Snail6SA expression induced apical cell extrusion. In contrast, larger clusters or homogenous cultures of Snail6SA cells did not show enhanced apical extrusion, but eventually displayed sporadic basal delamination. Transcriptomic profiling revealed that Snail6SA did not substantively alter the balance of epithelial and mesenchymal genes. However, we identified a transcriptional network that led to upregulated RhoA signalling and cortical contractility in cells expressing Snail6SA Enhanced contractility was necessary, but not sufficient, to drive extrusion, suggesting that Snail collaborates with other factors. Indeed, we found that the transcriptional downregulation of cell-matrix adhesion cooperates with contractility to mediate basal delamination. This provides a pathway for Snail to influence epithelial morphogenesis independently of classic epithelial-to-mesenchymal transition.

Symmetry Breaking and Epithelial Cell Extrusion

Cell extrusion is a striking morphological event found in epithelia and endothelia. It is distinguished by two symmetry-breaking events: a loss of planar symmetry, as cells are extruded in either apical or basal directions; and loss of mechanochemical homogeneity within monolayers, as cells that are fated to be extruded become biochemically and mechanically distinct from their neighbors. Cell extrusion is elicited by many diverse events, from apoptosis to the expression of transforming oncogenes. Does the morphological outcome of extrusion reflect cellular processes that are common to these diverse biological phenomena? To address this question, in this review we compare the progress that has been made in understanding how extrusion is elicited by epithelial apoptosis and cell transformation.

Inflammatory Responses during Tumour Initiation: From Zebrafish Transgenic Models of Cancer to Evidence from Mouse and Man

The zebrafish is now an important model organism for cancer biology studies and provides unique and complementary opportunities in comparison to the mammalian equivalent. The translucency of zebrafish has allowed in vivo live imaging studies of tumour initiation and progression at the cellular level, providing novel insights into our understanding of cancer. Here we summarise the available transgenic zebrafish tumour models and discuss what we have gleaned from them with respect to cancer inflammation. In particular, we focus on the host inflammatory response towards transformed cells during the pre-neoplastic stage of tumour development. We discuss features of tumour-associated macrophages and neutrophils in mammalian models and present evidence that supports the idea that these inflammatory cells promote early stage tumour development and progression. Direct live imaging of tumour initiation in zebrafish models has shown that the intrinsic inflammation induced by pre-neoplastic cells is tumour promoting. Signals mediating leukocyte recruitment to pre-neoplastic cells in zebrafish correspond to the signals that mediate leukocyte recruitment in mammalian tumours. The activation state of macrophages and neutrophils recruited to pre-neoplastic cells in zebrafish appears to be heterogenous, as seen in mammalian models, which provides an opportunity to study the plasticity of innate immune cells during tumour initiation. Although several potential mechanisms are described that might mediate the trophic function of innate immune cells during tumour initiation in zebrafish, there are several unknowns that are yet to be resolved. Rapid advancement of genetic tools and imaging technologies for zebrafish will facilitate research into the mechanisms that modulate leukocyte function during tumour initiation and identify targets for cancer prevention.

The stressful tumour environment drives plasticity of cell migration programmes, contributing to metastasis

Tumours evolve to cope with environmental stresses or challenges such as nutrient starvation, depletion of survival factors, and unbalanced mechanical forces. The uncontrolled growth and aberrant deregulation of core cell homeostatic pathways induced by genetic mutations create an environment of stress. Here, we explore how the adaptations of tumours to the changing environment can drive changes in the motility machinery of cells, affecting migration, invasion, and metastasis. Tumour cells can invade individually or collectively, or they can be extruded out of the surrounding epithelium. These mechanisms are thought to be modifications of normal processes occurring during development or tissue repair. Therefore, tumours may activate these pathways in response to environmental stresses, enabling them to survive in hostile environments and spread to distant sites. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

aPKCi triggers basal extrusion of luminal mammary epithelial cells by tuning contractility and vinculin localization at cell junctions

This study shows that an oncogenic mammary epithelial cell surrounded by normal cells can extrude basally in vivo and invade surrounding tissues without formation of a primary tumor. Here, we show that overexpression of the key polarity protein atypical protein kinase C ι (aPKCi) is sufficient for triggering basally oriented epithelial cell extrusion and early cell invasion into the mammary gland stroma. Moreover, we highlight the importance of the difference between the mechanical properties of aPKCi-overexpressing cells and those of the normal surrounding cells associated with the decrease of vinculin at the cell junction, which triggers cell segregation, the first step toward promoting and controlling the direction of cell extrusion.

Metastasis is the main cause of cancer-related deaths. How a single oncogenic cell evolves within highly organized epithelium is still unknown. Here, we found that the overexpression of the protein kinase atypical protein kinase C ι (aPKCi), an oncogene, triggers basally oriented epithelial cell extrusion in vivo as a potential mechanism for early breast tumor cell invasion. We found that cell segregation is the first step required for basal extrusion of luminal cells and identify aPKCi and vinculin as regulators of cell segregation. We propose that asymmetric vinculin levels at the junction between normal and aPKCi⁺ cells trigger an increase in tension at these cell junctions. Moreover, we show that aPKCi⁺ cells acquire promigratory features, including increased vinculin levels and vinculin dynamics at the cell–substratum contacts. Overall, this study shows that a balance between cell contractility and cell–cell adhesion is crucial for promoting basally oriented cell extrusion, a mechanism for early breast cancer cell invasion.

The Emerging Link between Centrosome Aberrations and Metastasis

Centrosome aberrations are commonly observed in human tumors and correlate with tumor aggressiveness and poor prognosis. Extra centrosomes drive mitotic errors that have been implicated in promoting tumorigenesis in mice. However, centrosome aberrations can also disrupt tissue architecture and confer invasive properties that may facilitate the dissemination of metastatic cells. Recent work has shown that centrosome defects facilitate invasion through cell-autonomous and non-cell-autonomous mechanisms, suggesting cancer cells can benefit from centrosome aberrations present in a subset of the tumor cell population. Here, we discuss how centrosome defects promote invasive behaviors that may contribute to initial steps in the metastatic cascade.

Structural centrosome aberrations sensitize polarized epithelia to basal cell extrusion

Centrosome aberrations disrupt tissue architecture and may confer invasive properties to cancer cells. Here we show that structural centrosome aberrations, induced by overexpression of either Ninein-like protein (NLP) or CEP131/AZI1, sensitize polarized mammalian epithelia to basal cell extrusion. While unperturbed epithelia typically dispose of damaged cells through apical dissemination into luminal cavities, certain oncogenic mutations cause a switch in directionality towards basal cell extrusion, raising the potential for metastatic cell dissemination. Here we report that NLP-induced centrosome aberrations trigger the preferential extrusion of damaged cells towards the basal surface of epithelial monolayers. This switch in directionality from apical to basal dissemination coincides with a profound reorganization of the microtubule cytoskeleton, which in turn prevents the contractile ring repositioning that is required to support extrusion towards the apical surface. While the basal extrusion of cells harbouring NLP-induced centrosome aberrations requires exogenously induced cell damage, structural centrosome aberrations induced by excess CEP131 trigger the spontaneous dissemination of dying cells towards the basal surface from MDCK cysts. Thus, similar to oncogenic mutations, structural centrosome aberrations can favour basal extrusion of damaged cells from polarized epithelia. Assuming that additional mutations may promote cell survival, this process could sensitize epithelia to disseminate potentially metastatic cells.

Ten years of research on the role of BVES/ POPDC1 in human disease: a review

Since the blood vessel epicardial substance or Popeye domain-containing protein 1 (BVES/POPDC1) was first identified in the developing heart by two independent laboratories in 1999, an increasing number of studies have investigated the structure, function, and related diseases of BVES/POPDC1. During the first 10 years following the discovery of BVES/POPDC1, studies focused mainly on its structure, expression patterns, and functions. Based on these studies, further investigations conducted over the previous decade examined the role of BVES/POPDC1 in human diseases, such as colitis, heart diseases, and human cancers. This review provides an overview of the structure and expression of BVES/POPDC1, mainly focusing on its potential role and mechanism through which it is involved in human cancers.

Unconventional Ways to Live and Die: Cell Death and Survival in Development, Homeostasis, and Disease

Balancing cell death and survival is essential for normal development and homeostasis and for preventing diseases, especially cancer. Conventional cell death pathways include apoptosis, a form of programmed cell death controlled by a well-defined biochemical pathway, and necrosis, the lysis of acutely injured cells. New types of regulated cell death include necroptosis, pyroptosis, ferroptosis, phagoptosis, and entosis. Autophagy can promote survival or can cause death. Newly described processes of anastasis and resuscitation show that, remarkably, cells can recover from the brink of apoptosis or necroptosis. Important new work shows that epithelia achieve homeostasis by extruding excess cells, which then die by anoikis due to loss of survival signals. This mechanically regulated process both maintains barrier function as cells die and matches rates of proliferation and death. In this review, we describe these unconventional ways in which cells have evolved to die or survive, as well as the contributions that these processes make to homeostasis and cancer.

The forces and fates of extruding cells

Cell extrusion drives most epithelial cell death while maintaining a functional epithelial barrier. To extrude, a cell produces a lipid signal that triggers the neighboring cells to reorganize actin and myosin basally to squeeze the extruding cell out apically from the barrier. More studies continue to reveal other signals and mechanisms controlling apical extrusion. New developmental studies are uncovering mechanisms controlling basal extrusion, or ingression, which occurs when apical extrusion is defective or during de-differentiation in development. Here, we review recent advances in epithelial extrusion, focusing particularly on forces exerted upon extruding cells and their various later fates ranging from cell death, normal development, and cancer.

Impact of Centrosome Aberrations on Chromosome Segregation and Tissue Architecture in Cancer

Centrosomes determine the disposition of microtubule networks and thereby contribute to regulate cell shape, polarity, and motility, as well as chromosome segregation during cell division. Additionally, centrioles, the core components of centrosomes, are required for the formation of cilia and flagella. Mutations in genes coding for centrosomal and centriolar proteins are responsible for several human diseases, foremost ciliopathies and developmental disorders resulting in small brains (primary microcephaly) or small body size (dwarfism). Moreover, a long-standing postulate implicates numerical and/or structural centrosome aberrations in the etiology of cancer. In this review, we will discuss recent work on the role of centrosome aberrations in the promotion of genome instability and the disruption of tissue architecture, two hallmarks of human cancers. We will emphasize recent studies on the impact of centrosome aberrations on the polarity of epithelial cells cultured in three-dimensional spheroid models. Collectively, the results from these in vitro systems suggest that different types of centrosome aberrations can promote invasive behavior through different pathways. Particularly exciting is recent evidence indicating that centrosome aberrations may trigger the dissemination of potentially metastatic cells through a non-cell-autonomous mechanism.

Cell Extrusion: A Stress-Responsive Force for Good or Evil in Epithelial Homeostasis

Epithelial tissues robustly respond to internal and external stressors via dynamic cellular rearrangements. Cell extrusion acts as a key regulator of epithelial homeostasis by removing apoptotic cells, orchestrating morphogenesis, and mediating competitive cellular battles during tumorigenesis. Here, we delineate the diverse functions of cell extrusion during development and disease. We emphasize the expanding role for apoptotic cell extrusion in exerting morphogenetic forces, as well as the strong intersection of cell extrusion with cell competition, a homeostatic mechanism that eliminates aberrant or unfit cells. While cell competition and extrusion can exert potent, tumor-suppressive effects, dysregulation of either critical homeostatic program can fuel cancer progression.

JAK/STAT signaling is necessary for cell monosis prior to epithelial cell apoptotic extrusion

Epithelial cell extrusion is crucial for proper development and tissue homeostasis. High-resolution 3D reconstruction and 4D imaging, combined with genetic analyis, have allowed us to reveal the highly-sterotyped morphogenetic events controlled by JAK/STAT signaling in a developmentally-programmed case of epithelial cell extrusion. Specialized somatic cells, Polar Cells (PCs), are produced in excess and then undergo apoptotic elimination from the follicular epithelium in the Drosophila ovary. We show that supernumerary PCs are first systematically enveloped by PC neighbors on all sides, first laterally, then apically in conjunction with highly-reinforced adherens junctions, and finally basally. The PC to be removed thus loses all contact with follicle cells, germline cells and the basement membrane in a process we have called cell 'monosis', for 'isolation' in Greek. PC monosis takes several hours, and always precedes, and is independent of, activation of apoptosis. JAK/STAT signaling is necessary within the surrounding follicular epithelium for PC monosis. Minutes after monosis is complete, PC apoptotic corpses are formed and extruded laterally within the epithelium, in contrast to the apical and basal extrusions described to date. These apoptotic corpses are engulfed and eliminated by surrounding follicle cells, which are thus acting as non-professional phagocytes. This study therefore shows the non cell-autonomous impact of an epithelium, via JAK/STAT signaling activation, on cell morphogenesis events leading to apoptotic extrusion. It is likely that the use of high-resolution 3D and 4D imaging, which allows for better spatio-temporal understanding of morphogenetic events, will reveal that cell monosis and lateral extrusion within an epithelium are pertinent for other cases of epithelial cell extrusion as well.

Clinically relevant morphological structures in breast cancer represent transcriptionally distinct tumor cell populations with varied degrees of epithelial-mesenchymal transition and CD44+CD24- stemness

Intratumor morphological heterogeneity in breast cancer is represented by different morphological structures (tubular, alveolar, solid, trabecular, and discrete) and contributes to poor prognosis; however, the mechanisms involved remain unclear. In this study, we performed 3D imaging, laser microdissection-assisted array comparative genomic hybridization and gene expression microarray analysis of different morphological structures and examined their association with the standard immunohistochemistry scorings and CD44⁺CD24^- cancer stem cells. We found that the intratumor morphological heterogeneity is not associated with chromosomal aberrations. By contrast, morphological structures were characterized by specific gene expression profiles and signaling pathways and significantly differed in progesterone receptor and Ki-67 expression. Most importantly, we observed significant differences between structures in the number of expressed genes of the epithelial and mesenchymal phenotypes and the association with cancer invasion pathways. Tubular (tube-shaped) and alveolar (spheroid-shaped) structures were transcriptionally similar and demonstrated co-expression of epithelial and mesenchymal markers. Solid (large shapeless) structures retained epithelial features but demonstrated an increase in mesenchymal traits and collective cell migration hallmarks. Mesenchymal genes and cancer invasion pathways, as well as Ki-67 expression, were enriched in trabecular (one/two rows of tumor cells) and discrete groups (single cells and/or arrangements of 2-5 cells). Surprisingly, the number of CD44⁺CD24^- cells was found to be the lowest in discrete groups and the highest in alveolar and solid structures. Overall, our findings indicate the association of intratumor morphological heterogeneity in breast cancer with the epithelial-mesenchymal transition and CD44⁺CD24^- stemness and the appeal of this heterogeneity as a model for the study of cancer invasion.

Tissue-Intrinsic Tumor Hotspots: Terroir for Tumorigenesis

Epithelial tissues are highly organized systems with a remarkable homeostatic ability to maintain morphology through regulation of cellular proliferation and tissue integrity. This robust self-organizing system is progressively disrupted during tumor development. Recent studies of conserved tumor-suppressor genes in Drosophila showed how protumor cells deviate from the robustly organized tissue microenvironment to take the first steps into becoming aggressive tumors. Here we review the 'tumor hotspot' hypothesis that explains how the tissue-intrinsic local microenvironment has a pivotal role in the initial stage of tumorigenesis in Drosophila epithelia and discuss comparable mechanisms in mammalian tissues.

Apoptosis in Cellular Society: Communication between Apoptotic Cells and Their Neighbors

Apoptosis is one of the cell-intrinsic suicide programs and is an essential cellular behavior for animal development and homeostasis. Traditionally, apoptosis has been regarded as a cell-autonomous phenomenon. However, recent in vivo genetic studies have revealed that apoptotic cells actively influence the behaviors of surrounding cells, including engulfment, proliferation, and production of mechanical forces. Such interactions can be bidirectional, and apoptosis is non-autonomously induced in a cellular community. Of note, it is becoming evident that active communication between apoptotic cells and living cells contributes to physiological processes during tissue remodeling, regeneration, and morphogenesis. In this review, we focus on the mutual interactions between apoptotic cells and their neighbors in cellular society and discuss issues relevant to future studies of apoptosis.

Both the anti- and pro-apoptotic functions of villin regulate cell turnover and intestinal homeostasis

In the small intestine, epithelial cells are derived from stem cells in the crypts, migrate up the villus as they differentiate and are ultimately shed from the villus tips. This process of proliferation and shedding is tightly regulated to maintain the intestinal architecture and tissue homeostasis. Apoptosis regulates both the number of stem cells in the crypts as well as the sloughing of cells from the villus tips. Previously, we have shown that villin, an epithelial cell-specific actin-binding protein functions as an anti-apoptotic protein in the gastrointestinal epithelium. The expression of villin is highest in the apoptosis-resistant villus cells and lowest in the apoptosis-sensitive crypts. In this study we report that villin is cleaved in the intestinal mucosa to generate a pro-apoptotic fragment that is spatially restricted to the villus tips. This cleaved villin fragment severs actin in an unregulated fashion to initiate the extrusion and subsequent apoptosis of effete cells from the villus tips. Using villin knockout mice, we validate the physiological role of villin in apoptosis and cell extrusion from the gastrointestinal epithelium. Our study also highlights the potential role of villin’s pro-apoptotic function in the pathogenesis of inflammatory bowel disease, ischemia-reperfusion injury, enteroinvasive bacterial and parasitic infections.

Alix-mediated assembly of the actomyosin-tight junction polarity complex preserves epithelial polarity and epithelial barrier

Maintenance of epithelial cell polarity and epithelial barrier relies on the spatial organization of the actin cytoskeleton and proper positioning/assembly of intercellular junctions. However, how these processes are regulated is poorly understood. Here we reveal a key role for the multifunctional protein Alix in both processes. In a knockout mouse model of Alix, we identified overt structural changes in the epithelium of the choroid plexus and in the ependyma, such as asymmetrical cell shape and size, misplacement and abnormal beating of cilia, blebbing of the microvilli. These defects culminate in excessive cell extrusion, enlargement of the lateral ventricles and hydrocephalus. Mechanistically, we find that by interacting with F-actin, the Par complex and ZO-1, Alix ensures the formation and maintenance of the apically restricted actomyosin-tight junction complex. We propose that in this capacity Alix plays a role in the establishment of apical-basal polarity and in the maintenance of the epithelial barrier.

Epithelial cell extrusion: Pathways and pathologies

To remove dying or unwanted cells from an epithelium while preserving the barrier function of the layer, epithelia use a unique process called cell extrusion. To extrude, the cell fated to die emits the lipid Sphingosine 1 Phosphate (S1P), which binds the G-protein-coupled receptor Sphingosine 1 Phosphate receptor 2 (S1P₂) in the neighboring cells that activates Rho-mediated contraction of an actomyosin ring circumferentially and basally. This contraction acts to squeeze the cell out apically while drawing together neighboring cells and preventing any gaps to the epithelial barrier. Epithelia can extrude out cells targeted to die by apoptotic stimuli to repair the barrier in the face of death or extrude live cells to promote cell death when epithelial cells become too crowded. Indeed, because epithelial cells naturally turn over by cell death and division at some of the highest rates in the body, epithelia depend on crowding-induced live cell extrusion to preserve constant cell numbers. If extrusion is defective, epithelial cells rapidly lose contact inhibition and form masses. Additionally, because epithelia act as the first line of defense in innate immunity, preservation of this barrier is critical for preventing pathogens from invading the body. Given its role in controlling constant cell numbers and maintaining barrier function, a number of different pathologies can result when extrusion is disrupted. Here, we review mechanisms and signaling pathways that control epithelial extrusion and discuss how defects in these mechanisms can lead to multiple diseases. We also discuss tactics pathogens have devised to hijack the extrusion process to infect and colonize epithelia.

A toolbox to study epidermal cell types in zebrafish

Epithelia provide a crucial protective barrier for our organs and are also the sites where the majority of carcinomas form. Most studies on epithelia and carcinomas use cell culture or organisms where high-resolution live imaging is inaccessible without invasive techniques. Here, we introduce the developing zebrafish epidermis as an excellent in vivo model system for studying a living epithelium. We developed tools to fluorescently tag specific epithelial cell types and express genes in a mosaic fashion using five Gal4 lines identified from an enhancer trap screen. When crossed to a variety of UAS effector lines, we can now track, ablate or monitor single cells at sub-cellular resolution. Using photo-cleavable morpholino oligonucleotides that target gal4, we can also express genes in a mosaic fashion at specific times during development. Together, this system provides an excellent in vivo alternative to tissue culture cells, without the intrinsic concerns of culture conditions or transformation, and enables the investigation of distinct cell types within living epithelial tissues.

p120 Catenin Suppresses Basal Epithelial Cell Extrusion in Invasive Pancreatic Neoplasia

Aberrant regulation of cellular extrusion can promote invasion and metastasis. Here, we identify molecular requirements for early cellular invasion using a premalignant mouse model of pancreatic cancer with conditional knockout of p120 catenin (Ctnnd1). Mice with biallelic loss of p120 catenin progressively develop high-grade pancreatic intraepithelial neoplasia (PanIN) lesions and neoplasia accompanied by prominent acute and chronic inflammatory processes, which is mediated, in part, through NF-κB signaling. Loss of p120 catenin in the context of oncogenic Kras also promotes remarkable apical and basal epithelial cell extrusion. Abundant single epithelial cells exit PanIN epithelium basally, retain epithelial morphology, survive, and display features of malignancy. Similar extrusion defects are observed following p120 catenin knockdown in vitro, and these effects are completely abrogated by the activation of S1P/S1pr2 signaling. In the context of oncogenic Kras, p120 catenin loss significantly reduces expression of genes mediating S1P/S1pr2 signaling in vivo and in vitro, and this effect is mediated at least, in part, through activation of NF-κB. These results provide insight into mechanisms controlling early events in the metastatic process and suggest that p120 catenin and S1P/S1pr2 signaling enhance cancer progression by regulating epithelial cell invasion. Cancer Res; 76(11); 3351-63. ©2016 AACR.

Do not let death do us part: 'find-me' signals in communication between dying cells and the phagocytes

The turnover and clearance of cells is an essential process that is part of many physiological and pathological processes. Improper or deficient clearance of apoptotic cells can lead to excessive inflammation and autoimmune disease. The steps involved in cell clearance include: migration of the phagocyte toward the proximity of the dying cells, specific recognition and internalization of the dying cell, and degradation of the corpse. The ability of phagocytes to recognize and react to dying cells to perform efficient and immunologically silent engulfment has been well-characterized in vitro and in vivo. However, how apoptotic cells themselves initiate the corpse removal and also influence the cells within the neighboring environment during clearance was less understood. Recent exciting observations suggest that apoptotic cells can attract phagocytes through the regulated release of 'find-me' signals. More recent studies also suggest that these find-me signals can have additional roles outside of phagocyte attraction to help orchestrate engulfment. This review will discuss our current understanding of the different find-me signals released by apoptotic cells, how they may be relevant in vivo, and their additional roles in facilitating engulfment.

Type IV Collagen Controls the Axogenesis of Cerebellar Granule Cells by Regulating Basement Membrane Integrity in Zebrafish

Granule cells (GCs) are the major glutamatergic neurons in the cerebellum, and GC axon formation is an initial step in establishing functional cerebellar circuits. In the zebrafish cerebellum, GCs can be classified into rostromedial and caudolateral groups, according to the locations of their somata in the corresponding cerebellar lobes. The axons of the GCs in the caudolateral lobes terminate on crest cells in the dorsal hindbrain, as well as forming en passant synapses with Purkinje cells in the cerebellum. In the zebrafish mutant shiomaneki, the caudolateral GCs extend aberrant axons. Positional cloning revealed that the shiomaneki (sio) gene locus encodes Col4a6, a subunit of type IV collagen, which, in a complex with Col4a5, is a basement membrane (BM) component. Both col4a5 and col4a6 mutants displayed similar abnormalities in the axogenesis of GCs and retinal ganglion cells (RGCs). Although type IV collagen is reported to control axon targeting by regulating the concentration gradient of an axonal guidance molecule Slit, Slit overexpression did not affect the GC axons. The structure of the BM surrounding the tectum and dorsal hindbrain was disorganized in the col4a5 and col4a6 mutants. Moreover, the abnormal axogenesis of the caudolateral GCs and the RGCs was coupled with aberrant BM structures in the type IV collagen mutants. The regrowth of GC axons after experimental ablation revealed that the original and newly formed axons displayed similar branching and extension abnormalities in the col4a6 mutants. These results collectively suggest that type IV collagen controls GC axon formation by regulating the integrity of the BM, which provides axons with the correct path to their targets.

The cerebellum is involved in motor coordination and motor learning. Granule cells are the major excitatory neurons in the cerebellum. It is largely unknown how the formation of cerebellar neural circuits, including the elaboration of granule cell axons, is controlled. We investigated a zebrafish mutant shiomaneki, in which some of the granule cells have abnormal axons. We identified collagen (col) 4a6 as the gene responsible for the mutant phenotype. Col4a6 forms a complex with Col4a5, which is a component of the basement membrane. We found that mutants of both col4a5 and col4a6 showed similar axonal abnormalities in both the granule cells and the retinal ganglion cells, and that the basement membrane structure surrounding the central nervous system was disrupted in these mutants. Furthermore, the abnormalities in granule cell axon formation were coupled with aberrant basement membrane structures in the col4a6 mutants. These data suggest that type IV collagen controls the axon formation of some types of neurons by establishing and/or maintaining the integrity of the basement membrane, which provides axons with the correct path to their targets. These findings may explain some aspects of a human disorder, Alport syndrome, which is caused by mutations in type IV collagen genes.

The Sound of Silence: Signaling by Apoptotic Cells

Apoptosis is a carefully choreographed process of cellular self-destruction in the absence of inflammation. During the death process, apoptotic cells actively communicate with their environment, signaling to both their immediate neighbors as well as distant sentinels. Some of these signals direct the anti-inflammatory immune response, instructing specific subsets of phagocytes to participate in the limited and careful clearance of dying cellular debris. These immunomodulatory signals can also regulate the activation state of the engulfing phagocytes. Other signals derived from apoptotic cells contribute to tissue growth control with the common goal of maintaining tissue integrity. Derangements in these growth control signals during prolonged apoptosis can lead to excessive cell loss or proliferation. Here, we highlight some of the most intriguing signals produced by apoptotic cells during the course of normal development as well as during physiological disturbances such as atherosclerosis and cancer.