Annexin a6 modulates chick cranial neural crest cell emigration

Functional Diversification of SRSF Protein Kinase to Control Ubiquitin-Dependent Neurodevelopmental Signaling

Conserved protein kinases with core cellular functions have been frequently redeployed during metazoan evolution to regulate specialized developmental processes. The Ser/Arg (SR)-rich splicing factor (SRSF) protein kinase (SRPK), which is implicated in splicing regulation, is one such conserved eukaryotic kinase. Surprisingly, we show that SRPK has acquired the capacity to control a neurodevelopmental ubiquitin signaling pathway. In mammalian embryonic stem cells and cultured neurons, SRPK phosphorylates Ser-Arg motifs in RNF12/RLIM, a key developmental E3 ubiquitin ligase that is mutated in an intellectual disability syndrome. Processive phosphorylation by SRPK stimulates RNF12-dependent ubiquitylation of nuclear transcription factor substrates, thereby acting to restrain a neural gene expression program that is aberrantly expressed in intellectual disability. SRPK family genes are also mutated in intellectual disability disorders, and patient-derived SRPK point mutations impair RNF12 phosphorylation. Our data reveal unappreciated functional diversification of SRPK to regulate ubiquitin signaling that ensures correct regulation of neurodevelopmental gene expression.

Diverse Roles of Annexin A6 in Triple-Negative Breast Cancer Diagnosis, Prognosis and EGFR-Targeted Therapies

The calcium (Ca2+)-dependent membrane-binding Annexin A6 (AnxA6), is a multifunctional, predominantly intracellular scaffolding protein, now known to play relevant roles in different cancer types through diverse, often cell-type-specific mechanisms. AnxA6 is differentially expressed in various stages/subtypes of several cancers, and its expression in certain tumor cells is also induced by a variety of pharmacological drugs. Together with the secretion of AnxA6 as a component of extracellular vesicles, this suggests that AnxA6 mediates distinct tumor progression patterns via extracellular and/or intracellular activities. Although it lacks enzymatic activity, some of the AnxA6-mediated functions involving membrane, nucleotide and cholesterol binding as well as the scaffolding of specific proteins or multifactorial protein complexes, suggest its potential utility in the diagnosis, prognosis and therapeutic strategies for various cancers. In breast cancer, the low AnxA6 expression levels in the more aggressive basal-like triple-negative breast cancer (TNBC) subtype correlate with its tumor suppressor activity and the poor overall survival of basal-like TNBC patients. In this review, we highlight the potential tumor suppressor function of AnxA6 in TNBC progression and metastasis, the relevance of AnxA6 in the diagnosis and prognosis of several cancers and discuss the concept of therapy-induced expression of AnxA6 as a novel mechanism for acquired resistance of TNBC to tyrosine kinase inhibitors.

In Ovo Electroporation of Plasmid DNA and Morpholinos into Specific Tissues During Early Embryogenesis

In ovo electroporation enables transfection of non-viral plasmid DNA and/or morpholinos to fluorescently label and/or perturb gene function in cells of interest. However, targeted electroporation into specific subregions of the embryo can be challenging due to placement and size limitations of the electrodes. Here we describe the basic techniques for in ovo electroporation in the chick embryo and suggest parameters to electroporate cells within different target tissues that with some modifications may be applicable to a wide range of developmental stages and other embryo model organisms.

Implication of calcium activated RasGRF2 in Annexin A6-mediated breast tumor cell growth and motility

The role of AnxA6 in breast cancer and in particular, the mechanisms underlying its contribution to tumor cell growth and/or motility remain poorly understood. In this study, we established the tumor suppressor function of AnxA6 in triple negative breast cancer (TNBC) cells by showing that loss of AnxA6 is associated with early onset and rapid growth of xenograft TNBC tumors in mice. We also identified the Ca²⁺ activated RasGRF2 as an effector of AnxA6 mediated TNBC cell growth and motility. Activation of Ca²⁺ mobilizing oncogenic receptors such as epidermal growth factor receptor (EGFR) in TNBC cells or pharmacological stimulation of Ca²⁺ influx led to activation, subsequent degradation and altered effector functions of RasGRF2. Inhibition of Ca²⁺ influx or overexpression of AnxA6 blocked the activation/degradation of RasGRF2. We also show that AnxA6 acts as a scaffold for RasGRF2 and Ras proteins and that its interaction with RasGRF2 is modulated by GTP and/or activation of Ras proteins. Meanwhile, down-regulation of RasGRF2 and treatment of cells with the EGFR-targeted tyrosine kinase inhibitor (TKI) lapatinib strongly attenuated the growth of otherwise EGFR-TKI resistant AnxA6 high TNBC cells. These data not only suggest that AnxA6 modulated Ca²⁺ influx and effector functions of RasGRF2 underlie at least in part, the AnxA6 mediated TNBC cell growth and/or motility, but also provide a rationale to target Ras-driven TNBC with EGFR targeted therapies in combination with inhibition of RasGRF2.

Annexin A6 controls neuronal membrane dynamics throughout chick cranial sensory gangliogenesis

Cranial sensory ganglia are components of the peripheral nervous system that possess a significant somatosensory role and include neurons within the trigeminal and epibranchial nerve bundles. Although it is well established that these ganglia arise from interactions between neural crest and neurogenic placode cells, the molecular basis of ganglia assembly is still poorly understood. Members of the Annexin protein superfamily play key roles in sensory nervous system development throughout metazoans. Annexin A6 is expressed in chick trigeminal and epibranchial placode cell-derived neuroblasts and neurons, but its function in cranial ganglia formation has not been elucidated. To this end, we interrogated the role of Annexin A6 using gene perturbation studies in the chick embryo. Our data reveal that placode cell-derived neuroblasts with reduced Annexin A6 levels ingress and migrate normally to the ganglionic anlage, where neural crest cell corridors correctly form around them. Strikingly, while Annexin A6-depleted placode cell-derived neurons still express mature neuronal markers, they fail to form two long processes, which are considered morphological features of mature neurons, and no longer innervate their designated targets due to the absence of this bipolar morphology. Moreover, overexpression of Annexin A6 causes some placode cell-derived neurons to form extra protrusions alongside these bipolar processes. These data demonstrate that the molecular program associated with neuronal maturation is distinct from that orchestrating changes in neuronal morphology, and, importantly, reveal Annexin A6 to be a key membrane scaffolding protein during sensory neuron membrane biogenesis. Collectively, our results provide novel insight into mechanisms underscoring morphological changes within placode cell-derived neurons that are essential for cranial gangliogenesis.

Annexin A6-A multifunctional scaffold in cell motility

Annexin A6 (AnxA6) belongs to a highly conserved protein family characterized by their calcium (Ca²⁺)-dependent binding to phospholipids. Over the years, immunohistochemistry, subcellular fractionations, and live cell microscopy established that AnxA6 is predominantly found at the plasma membrane and endosomal compartments. In these locations, AnxA6 acts as a multifunctional scaffold protein, recruiting signaling proteins, modulating cholesterol and membrane transport and influencing actin dynamics. These activities enable AnxA6 to contribute to the formation of multifactorial protein complexes and membrane domains relevant in signal transduction, cholesterol homeostasis and endo-/exocytic membrane transport. Hence, AnxA6 has been implicated in many biological processes, including cell proliferation, survival, differentiation, inflammation, but also membrane repair and viral infection. More recently, we and others identified roles for AnxA6 in cancer cell migration and invasion. This review will discuss how the multiple scaffold functions may enable AnxA6 to modulate migratory cell behavior in health and disease.

Differential expression pattern of Annexin A6 in chick neural crest and placode cells during cranial gangliogenesis

The cranial trigeminal and epibranchial ganglia are components of the peripheral nervous system that possess an important somatosensory role. These ganglia arise from the intermixing and coalescence of two different migratory cell types, neural crest cells and neurogenic placodes cells, and thus typify the phenomena of cell migration and intercellular interactions for their creation. The underlying molecular mechanisms of ganglia formation, however, are still poorly understood. To address this, we have analyzed the spatio-temporal expression profile of Annexin A6 during chick gangliogenesis, as Annexin proteins play important, conserved roles in ganglia development and physiology. We observe Annexin A6 protein in cranial neural crest cells prior to, during and after their emergence from the neural tube. Fully migratory cranial neural crest cells, however, are devoid of Annexin A6. Interestingly, we note Annexin A6 protein in trigeminal and epibranchial placode cells as these cells ingress from the ectoderm to initiate ganglia formation. This expression is also maintained in the sensory placodes later on when they coalesce with neural crest cells to assemble the cranial ganglia. These results suggest that the dynamic expression of Annexin A6 in various embryonic cell types may allow Annexin A6 to serve distinct functions throughout embryonic development.

Migratory neural crest cell αN-catenin impacts chick trigeminal ganglia formation

Neural crest cells are an embryonic cell population that is crucial for proper vertebrate development. Initially localized to the dorsal neural folds, premigratory neural crest cells undergo an epithelial-to-mesenchymal transition (EMT) and migrate to their final destinations in the developing embryo. Together with epidermally-derived placode cells, neural crest cells then form the cranial sensory ganglia of the peripheral nervous system. Our prior work has shown that αN-catenin, the neural subtype of the adherens junction α-catenin protein, regulates cranial neural crest cell EMT by controlling premigratory neural crest cell cadherin levels. Although αN-catenin down-regulation is critical for initial neural crest cell EMT, a potential role for αN-catenin in later neural crest cell migration, and formation of the cranial ganglia, has not been examined. In this study, we show for the first time that migratory neural crest cells that will give rise to the cranial trigeminal ganglia express αN-catenin and Cadherin-7. αN-catenin loss- and gain-of-function experiments reveal effects on the migratory neural crest cell population that include subsequent defects in trigeminal ganglia assembly. Moreover, αN-catenin perturbation in neural crest cells impacts the placode cell contribution to the trigeminal ganglia and also changes neural crest cell Cadherin-7 levels and localization. Together, these results highlight a novel function for αN-catenin in migratory neural crest cells that form the trigeminal ganglia.

Reduced annexin A6 expression promotes the degradation of activated epidermal growth factor receptor and sensitizes invasive breast cancer cells to EGFR-targeted tyrosine kinase inhibitors

Background

The expression of annexin A6 (AnxA6) in AnxA6-deficient non-invasive tumor cells has been shown to terminate epidermal growth factor receptor (EGFR) activation and downstream signaling. However, as a scaffolding protein, AnxA6 may stabilize activated cell-surface receptors to promote cellular processes such as tumor cell motility and invasiveness. In this study, we investigated the contribution of AnxA6 in the activity of EGFR in invasive breast cancer cells and examined whether the expression status of AnxA6 influences the response of these cells to EGFR-targeted tyrosine kinase inhibitors (TKIs) and/or patient survival.

Results

We demonstrate that in invasive BT-549 breast cancer cells AnxA6 expression is required for sustained membrane localization of activated (phosho-Y1068) EGFR and consequently, persistent activation of MAP kinase ERK1/2 and phosphoinositide 3-kinase/Akt pathways. Depletion of AnxA6 in these cells was accompanied by rapid degradation of activated EGFR, attenuated downstream signaling and as expected enhanced anchorage-independent growth. Besides inhibition of cell motility and invasiveness, AnxA6-depleted cells were also more sensitive to the EGFR-targeted TKIs lapatinib and PD153035. We also provide evidence suggesting that reduced AnxA6 expression is associated with a better relapse-free survival but poorer distant metastasis-free and overall survival of basal-like breast cancer patients.

Conclusions

Together this demonstrates that the rapid degradation of activated EGFR in AnxA6-depleted invasive tumor cells underlies their sensitivity to EGFR-targeted TKIs and reduced motility. These data also suggest that AnxA6 expression status may be useful for the prediction of the survival and likelihood of basal-like breast cancer patients to respond to EGFR-targeted therapies.

Cadherin-6B is proteolytically processed during epithelial-to-mesenchymal transitions of the cranial neural crest

The epithelial-to-mesenchymal transition (EMT) is a highly coordinated process underlying both development and disease. Premigratory neural crest cells undergo EMT, migrate away from the neural tube, and differentiate into diverse cell types during vertebrate embryogenesis. Adherens junction disassembly within premigratory neural crest cells is one component of EMT and, in chick cranial neural crest cells, involves cadherin-6B (Cad6B) down-regulation. Whereas Cad6B transcription is repressed by Snail2, the rapid loss of Cad6B protein during EMT is suggestive of posttranslational mechanisms that promote Cad6B turnover. For the first time in vivo, we demonstrate Cad6B proteolysis during neural crest cell EMT, which generates a Cad6B N-terminal fragment (NTF) and two C-terminal fragments (CTF1/2). Coexpression of relevant proteases with Cad6B in vitro shows that a disintegrin and metalloproteinases (ADAMs) ADAM10 and ADAM19, together with γ-secretase, cleave Cad6B to produce the NTF and CTFs previously observed in vivo. Of importance, both ADAMs and γ-secretase are expressed in the appropriate spatiotemporal pattern in vivo to proteolytically process Cad6B. Overexpression or depletion of either ADAM within premigratory neural crest cells prematurely reduces or maintains Cad6B, respectively. Collectively these results suggest a dual mechanism for Cad6B proteolysis involving two ADAMs, along with γ-secretase, during cranial neural crest cell EMT.

Human fetal keratocytes have multipotent characteristics in the developing avian embryo

The human cornea contains stem cells that can be induced to express markers consistent with multipotency in cell culture; however, there have been no studies demonstrating that human corneal keratocytes are multipotent. The objective of this study is to examine the potential of human fetal keratocytes (HFKs) to differentiate into neural crest-derived tissues when challenged in an embryonic environment. HFKs were injected bilaterally into the cranial mesenchyme adjacent to the neural tube and the periocular mesenchyme in chick embryos at embryonic days 1.5 and 3, respectively. The injected keratocytes were detected by immunofluorescence using the human cell-specific marker, HuNu. HuNu-positive keratocytes injected along the neural crest pathway were localized adjacent to HNK-1-positive migratory host neural crest cells and in the cardiac cushion mesenchyme. The HuNu-positive cells transformed into neural crest derivatives such as smooth muscle in cranial blood vessels, stromal keratocytes, and corneal endothelium. However, they failed to form neurons despite their presence in the condensing trigeminal ganglion. These results show that HFKs retain the ability to differentiate into some neural crest-derived tissues. Their ability to respond to embryonic cues and generate corneal endothelium and stromal keratocytes provides a basis for understanding the feasibility of creating specialized cells for possible use in regenerative medicine.