Aplysia ror forms clusters on the surface of identified neuroendocrine cells

Non-canonical WNT5A-ROR signaling: New perspectives on an ancient developmental pathway

Deciphering non-canonical WNT signaling has proven to be both fascinating and challenging. Discovered almost 30 years ago, non-canonical WNT ligands signal independently of the transcriptional co-activator β-catenin to regulate a wide range of morphogenetic processes during development. The molecular and cellular mechanisms that underlie non-canonical WNT function, however, remain nebulous. Recent results from various model systems have converged to define a core non-canonical WNT pathway consisting of the prototypic non-canonical WNT ligand, WNT5A, the receptor tyrosine kinase ROR, the seven transmembrane receptor Frizzled and the cytoplasmic scaffold protein Dishevelled. Importantly, mutations in each of these signaling components cause Robinow syndrome, a congenital disorder characterized by profound tissue morphogenetic abnormalities. Moreover, dysregulation of the pathway has also been linked to cancer metastasis. As new knowledge concerning the WNT5A-ROR pathway continues to grow, modeling these mutations will likely provide crucial insights into both the physiological regulation of the pathway and the etiology of WNT5A-ROR-driven diseases.

Cellular and molecular mechanisms implicated in the dual role of ROR2 in cancer

ROR1 and ROR2 are Wnt receptors that are critical for β-catenin-independent Wnt pathways and have been linked to processes driving tumor progression, such as cell proliferation, survival, invasion, and therapy resistance. Both receptors have garnered interest as potential therapeutic targets since they are largely absent in adult tissue, are overexpressed in several cancers, and, as members of the receptor tyrosine kinase family, are easier to target than all other components of the pathway. Unlike ROR1 which always promotes tumorigenesis, ROR2 has a very complex role in cancer acting either to promote or inhibit tumor progression in different tumor types. In the present article, we summarize the findings on ROR2 expression in cancer patients and its impact on clinical outcome. Further, we review the biological processes and signaling pathways regulated by ROR2 that explain its dual role in cancer. Finally, we describe the ongoing strategies to target ROR2 in cancer.

Genome-wide survey of single-nucleotide polymorphisms reveals fine-scale population structure and signs of selection in the threatened Caribbean elkhorn coral, Acropora palmata

The advent of next-generation sequencing tools has made it possible to conduct fine-scale surveys of population differentiation and genome-wide scans for signatures of selection in non-model organisms. Such surveys are of particular importance in sharply declining coral species, since knowledge of population boundaries and signs of local adaptation can inform restoration and conservation efforts. Here, we use genome-wide surveys of single-nucleotide polymorphisms in the threatened Caribbean elkhorn coral, Acropora palmata, to reveal fine-scale population structure and infer the major barrier to gene flow that separates the eastern and western Caribbean populations between the Bahamas and Puerto Rico. The exact location of this break had been subject to discussion because two previous studies based on microsatellite data had come to differing conclusions. We investigate this contradiction by analyzing an extended set of 11 microsatellite markers including the five previously employed and discovered that one of the original microsatellite loci is apparently under selection. Exclusion of this locus reconciles the results from the SNP and the microsatellite datasets. Scans for outlier loci in the SNP data detected 13 candidate loci under positive selection, however there was no correlation between available environmental parameters and genetic distance. Together, these results suggest that reef restoration efforts should use local sources and utilize existing functional variation among geographic regions in ex situ crossing experiments to improve stress resistance of this species.

Diverse roles for the ror-family receptor tyrosine kinases in neurons and glial cells during development and repair of the nervous system

The Ror-family of receptor tyrosine kinases (RTKs) are involved critically in tissue genesis and organogenesis during development. In mammals, Ror1 and Ror2, members of the Ror-family RTKs, have been shown to mediate cell polarity, migration, proliferation, and differentiation through the activation of noncanonical Wnt signaling by acting as receptors or co-receptors for Wnt5a. Nematodes bearing mutations within the cam-1 gene, encoding a Ror2 ortholog, exhibit defects in various developmental processes of the nervous system, including neuronal cell migration, polarization, axonal extension, and synaptic transmission. In mice, Ror2 and/or Ror1 are also shown to play roles in regulating neurite extension, synapse formation, and synaptic transmission of hippocampal neurons, indicating that the Ror-family RTKs have evolutionarily conserved functions at least in part in neurons during development. Furthermore, Ror2 and/or Ror1 are expressed in neural stem/progenitor cells of the developing brain and in astrocytes of the adult brain after injury, and they play important roles in regulating cell proliferation under these different contexts. In this article, we overview recent advances in our understanding of the roles of the Ror-family RTKs in the development and repair of the nervous system and discuss their potential for therapeutic targets to neurodegenerative diseases. Developmental Dynamics 247:24-32, 2018. © 2017 Wiley Periodicals, Inc.

ROR-Family Receptor Tyrosine Kinases

ROR-family receptor tyrosine kinases form a small subfamily of receptor tyrosine kinases (RTKs), characterized by a conserved, unique domain architecture. ROR RTKs are evolutionary conserved throughout the animal kingdom and act as alternative receptors and coreceptors of WNT ligands. The intracellular signaling cascades activated downstream of ROR receptors are diverse, including but not limited to ROR-Frizzled-mediated activation of planar cell polarity signaling, RTK-like signaling, and antagonistic regulation of WNT/β-Catenin signaling. In line with their diverse repertoire of signaling functions, ROR receptors are involved in the regulation of multiple processes in embryonic development such as development of the axial and paraxial mesoderm, the nervous system and the neural crest, the axial and appendicular skeleton, and the kidney. In humans, mutations in the ROR2 gene cause two distinct developmental syndromes, recessive Robinow syndrome (RRS; MIM 268310) and dominant brachydactyly type B1 (BDB1; MIM 113000). In Robinow syndrome patients and animal models, the development of multiple organs is affected, whereas BDB1 results only in shortening of the distal phalanges of fingers and toes, reflecting the diversity of functions and signaling activities of ROR-family RTKs. In this chapter, we give an overview on ROR receptor structure and function. We discuss their signaling functions and role in vertebrate embryonic development with a focus on those developmental processes that are affected by mutations in the ROR2 gene in human patients.

Wnt5a induces ROR1/ROR2 heterooligomerization to enhance leukemia chemotaxis and proliferation

Evolutionarily conserved receptor tyrosine kinase–like orphan receptor-1 and -2 (ROR1/2) are considered distinct receptors for Wnt5a and are implicated in noncanonical Wnt signaling in organogenesis and cancer metastasis. We found that Wnt5a enhanced proliferation and migration of chronic lymphocytic leukemia (CLL) cells and that these effects were blocked by the humanized anti-ROR1 mAb cirmtuzumab (UC-961). Treatment of CLL cells with Wnt5a induced ROR1 to oligomerize with ROR2 and recruit guanine exchange factors (GEFs), which activated Rac1 and RhoA; siRNA-mediated silencing of either ROR1 or ROR2 or treatment with UC-961 inhibited these effects. Using the ROR1-deficient CLL cell line MEC1, we demonstrated that ectopic ROR1 expression induced ROR1/ROR2 heterooligomers, which recruited GEFs, and enhanced proliferation, cytokine-directed migration, and engraftment potential of MEC1 cells in immune-deficient mice. Notably, treatment with UC-961 inhibited engraftment of ROR1+ leukemia cells in immune-competent ROR1-transgenic mice. Molecular analysis revealed that the extracellular Kringle domain is required for ROR1/ROR2 heterooligomerization and the cysteine-rich domain or intracellular proline-rich domain is required for Wnt5a-induced recruitment of GEFs to ROR1/ROR2. This study identifies an interaction between ROR1 and ROR2 that is required for Wnt5a signaling that promotes leukemia chemotaxis and proliferation.

The receptor tyrosine kinase ROR1--an oncofetal antigen for targeted cancer therapy

Targeted cancer therapies have emerged as new treatment options for various cancer types. Among targets, receptor tyrosine kinases (RTKs) are among the most promising. ROR1 is a transmembrane RTK of importance during the normal embryogenesis for the central nervous system, heart, lung and skeletal systems, but is not expressed in normal adult tissues. However, ROR1 is overexpressed in several human malignancies and may act as a survival factor for tumor cells. Its unique expression by malignant cells may provide a target for novel therapeutics including monoclonal antibodies (mAbs) and small molecule inhibitors of tyrosine kinases (TKI) for the treatment of cancer. Promising preclinical results have been reported in e.g. chronic lymphocytic leukemia, pancreatic carcinoma, lung and breast cancer. ROR1 might also be an interesting oncofetal antigen for active immunotherapy. In this review, we provide an overview of the ROR1 structure and functions in cancer and highlight emerging therapeutic options of interest for targeting ROR1 in tumor therapy.

ROR1, an embryonic protein with an emerging role in cancer biology

Receptor tyrosine kinase-like orphan receptor 1 (ROR1) is a member of the ROR family consisting of ROR1 and ROR2. RORs contain two distinct extracellular cysteine-rich domains and one transmembrane domain. Within the intracellular portion, ROR1 possesses a tyrosine kinase domain, two serine/threonine-rich domains and a proline-rich domain. RORs have been studied in the context of embryonic patterning and neurogenesis through a variety of homologs. These physiologic functions are dichotomous based on the requirement of the kinase domain. A growing literature has established ROR1 as a marker for cancer, such as in CLL and other blood malignancies. In addition, ROR1 is critically involved in progression of a number of blood and solid malignancies. ROR1 has been shown to inhibit apoptosis, potentiate EGFR signaling, and induce epithelial-mesenchymal transition (EMT). Importantly, ROR1 is only detectable in embryonic tissue and generally absent in adult tissue, making the protein an ideal drug target for cancer therapy.

Wnt signaling through the Ror receptor in the nervous system

The receptor tyrosine kinase-like orphan receptor (Ror) proteins are conserved tyrosine kinase receptors that play roles in a variety of cellular processes that pattern tissues and organs during vertebrate and invertebrate development. Ror signaling is required for skeleton and neuronal development and modulates cell migration, cell polarity, and convergent extension. Ror has also been implicated in two human skeletal disorders, brachydactyly type B and Robinow syndrome. Rors are widely expressed during metazoan development including domains in the nervous system. Here, we review recent progress in understanding the roles of the Ror receptors in neuronal migration, axonal pruning, axon guidance, and synaptic plasticity. The processes by which Ror signaling execute these diverse roles are still largely unknown, but they likely converge on cytoskeletal remodeling. In multiple species, Rors have been shown to act as Wnt receptors signaling via novel non-canonical Wnt pathways mediated in some tissues by the adapter protein disheveled and the non-receptor tyrosine kinase Src. Rors can either activate or repress Wnt target expression depending on the cellular context and can also modulate signal transduction by sequestering Wnt ligands away from their signaling receptors. Future challenges include the identification of signaling components of the Ror pathways and bettering our understanding of the roles of these pleiotropic receptors in patterning the nervous system.

Ror-family receptor tyrosine kinases regulate maintenance of neural progenitor cells in the developing neocortex

The Ror-family of receptor tyrosine kinases (RTKs), Ror1 and Ror2, have been shown to play crucial roles in the developmental morphogenesis by acting as receptors or co-receptors to mediate Wnt5a-induced signaling. Although Ror1, Ror2, and Wnt5a are expressed in the developing brain, little is known about their roles in the neural development. Here we show that Ror1, Ror2, and their ligand Wnt5a are highly expressed in neocortical neural progenitor cells (NPCs). siRNA-mediated suppression of Ror1, Ror2, or Wnt5a in cultured NPCs isolated from embryonic neocortex results in the reduction of βIII-tubulin-positive neurons that are produced from NPCs possibly through the generation of T-box brain 2 (Tbr2)-positive intermediate progenitors. BrdU-labeling experiments further reveal that proportion of proliferative and neurogenic NPCs, that are positive for neural progenitor cell marker (Pax6), but negative for glial cell marker (glial fibrillary acidic protein; GFAP), is reduced within a few days in culture following knockdown of these molecules, suggesting that Ror1, Ror2, and Wnt5a regulate neurogenesis through the maintenance of NPCs. Moreover, we show that Dishevelled2 (Dvl2) is involved in Wnt5a–Ror1 and Wnt5a–Ror2 signalings in NPCs, and that suppressed expression of Dvl2 indeed reduces the proportion of proliferative and neurogenic NPCs. Interestingly, suppressed or forced expression of either Ror1 or Ror2 in NPCs in the developing neocortex results in their precocious or delayed differentiation into neurons, respectively. Collectively, these results indicate that Wnt5a–Ror1 and Wnt5a–Ror2 signalings play roles in maintaining proliferative and neurogenic NPCs during neurogenesis of the developing neocortex.

Nuclear localization of orphan receptor protein kinase (Ror1) is mediated through the juxtamembrane domain

Background

Several receptor tyrosine kinases (RTKs) such as EGFR, FGFR, TRK, and VEGFR are capable of localizing in the cell nucleus in addition to their usual plasma membrane localization. Recent reports also demonstrate that nuclear-localized RTKs have important cellular functions such as transcriptional activation. On the basis of preliminary bioinformatic analysis, additional RTKs, including receptor tyrosine kinase-like orphan receptor 1 (Ror1) were predicted to have the potential for nuclear subcellular localization. Ror1 is a receptor protein tyrosine kinase that modulates neurite growth in the central nervous system. Because the nuclear localization capability of the Ror1 cytoplasmic domain has not been reported, we examined the cellular expression distribution of this region.

Results

The Ror1 cytoplasmic region was amplified and cloned into reporter constructs with fluorescent tags. Following transfection, the nuclear distribution patterns of transiently expressed fusion proteins were observed. Serial deletion constructs were then used to map the juxtamembrane domain of Ror1 (aa_471-513) for this nuclear translocation activity. Further site-directed mutagenesis suggested that a KxxK-16 aa-KxxK sequence at residues 486-509 is responsible for the nuclear translocation interaction. Subsequent immunofluorescence analysis by cotransfection of Ran and Ror1 implied that the nuclear translocation event of Ror1 might be mediated through the Ran pathway.

Conclusions

We have predicted several RTKs that contain the nuclear localization signals. This is the first report to suggest that the juxtamembrane domain of the Ror1 cytoplasmic region mediates the translocation event. Ran GTPase is also implicated in this event. Our study might be beneficial in future research to understand the Ror1 biological signaling pathway.

Ror-family receptor tyrosine kinases in noncanonical Wnt signaling: their implications in developmental morphogenesis and human diseases

The Ror-family receptor tyrosine kinases (RTKs) play crucial roles in the development of various organs and tissues. In mammals, Ror2, a member of the Ror-family RTKs, has been shown to act as a receptor or coreceptor for Wnt5a to mediate noncanonical Wnt signaling. Ror2- and Wnt5a-deficient mice exhibit similar abnormalities during developmental morphogenesis, reflecting their defects in convergent extension movements and planar cell polarity, characteristic features mediated by noncanonical Wnt signaling. Furthermore, mutations within the human Ror2 gene are responsible for the genetic skeletal disorders dominant brachydactyly type B and recessive Robinow syndrome. Accumulating evidence demonstrate that Ror2 mediates noncanonical Wnt5a signaling by inhibiting the beta-catenin-TCF pathway and activating the Wnt/JNK pathway that results in polarized cell migration. In this article, we review recent progress in understanding the roles of noncanonical Wnt5a/Ror2 signaling in developmental morphogenesis and in human diseases, including heritable skeletal disorders and tumor invasion.

Ror1-Ror2 complexes modulate synapse formation in hippocampal neurons

Ror1 and Ror2, a small family of tyrosine kinase receptors, have been implicated in multiple aspects of brain development in C. elegans and X. laevis. More recently, we have shown that these receptors modulate the rate of neurite elongation in cultured rat hippocampal neurons. However, no information is available regarding a potential role of these receptors in other developmental milestones in mammalian central neurons. Neither is the identity known of the Ror ligand(s) and/or the signal transduction pathway(s) in which they participate. Here we report that the down regulation of either Ror1 or Ror2 led to a significant decrease in synapse formation in cultured hippocampal neurons. Simultaneous targeting of Ror proteins, however, did not result in an additive phenotype. Our results also indicated that Ror1 and Ror2 physically interact in the mouse brain, suggesting that they might function as heterodimers in central neurons. In addition, these Ror complexes interacted with Wnt-5a mediating its effects on synaptogenesis. Together, these data suggest that Ror proteins play a key role in Wnt-5a-activated signaling pathways leading to synapse formation in the mammalian CNS.

Ror receptor tyrosine kinases: orphans no more

Receptor tyrosine kinase-like orphan receptor (Ror) proteins are a conserved family of tyrosine kinase receptors that function in developmental processes including skeletal and neuronal development, cell movement and cell polarity. Although Ror proteins were originally named because the associated ligand and signaling pathway were unknown, recent studies in multiple species have now established that Ror proteins are Wnt receptors. Depending on the cellular context, Ror proteins can either activate or repress transcription of Wnt target genes and can modulate Wnt signaling by sequestering Wnt ligands. New evidence implicates Ror proteins in planar cell polarity, an alternative Wnt pathway. Here, we review the progress made in understanding these mysterious proteins and, in particular, we focus on their function as Wnt receptors.

Cloning and expression pattern of chicken Ror2 and functional characterization of truncating mutations in Brachydactyly type B and Robinow syndrome

Ror2 is a receptor tyrosine kinase mutated in the human syndromes Brachydactyly type B (BDB) and recessive Robinow syndrome (RS). In this study, we used the chick as a model to investigate the role of Ror2 in skeletogenesis and to elucidate the functional consequences of Ror2 mutations. For this purpose, we cloned chicken Ror2 and analyzed its expression pattern at various embryonic stages by in situ hybridization and immunolabeling. We document expression of cRor2 in several organs, including mesonephros, heart, nervous system, intestine and cartilage. The high conservation of expression when compared with the mouse underlines the validity of the chick as a model system. Using replication-competent retroviral vector-mediated overexpression, we analyzed the functional consequences of truncating BDB and RS mutations in the developing chick limb. Overexpression of Ror2 mutants led to a disturbance of growth plate architecture and a severe block of chondrocyte differentiation, demonstrating the functional importance of Ror2 in skeletogenesis.

Tracing the evolution and function of the Trk superfamily of receptor tyrosine kinases

Most growth factors and their receptors have been strongly conserved during evolution. In contrast, Trks (Tropomyosin-related kinases) and related receptors in the Trk superfamily, Rors (receptor tyrosine kinase-like orphan receptors), Musks (muscle specific kinases) and Ddrs (discoidin domain receptor family), appear to be ancient, but their function has been lost in multiple lineages and the roles for the receptors have been modified over time. We will trace the evolution of the Trk superfamily and discuss possible conserved functional roles, including a unifying theme of target recognition by growing axons. We present an analogy between the evolution of G-protein-coupled receptors and receptor tyrosine kinases (RTKs), proposing that an important driving force for the divergence of receptors is the ease of divergence of their ligands.

Neurite extension in central neurons: a novel role for the receptor tyrosine kinases Ror1 and Ror2

Neurite elongation and branching are key cellular events during brain development as they underlie the formation of a properly wired neuronal network. Here we report that the receptor tyrosine kinases Ror1 and Ror2 modulate the growth of neurites as well as their branching pattern in hippocampal neurons. Upon Ror1 or Ror2 suppression using antisense oligonucleotides or RNA interference (RNAi), neurons extended shorter and less branched minor processes when compared to those in control cells. In addition, Ror-depleted cells elongated longer, albeit less branched, axons than seen in control cells. Conversely, Ror overexpression both in non-neuronal cells and in hippocampal neurons resulted in the enhanced extension of short and highly branched processes. These phenotypes were accompanied by changes in the microtubule-associated proteins MAP1B and MAP2. Taken together, these results support a novel role for Ror receptors as modulators of neurite extension in central neurons.

Ltrk is differentially expressed in developing and adult neurons of theLymnaea central nervous system

The Trk receptor family plays diverse roles in both development and plasticity of the vertebrate nervous system. Ltrk is a related receptor that is expressed in the CNS of the mollusk Lymnaea, although little is known of its cellular distribution. This study provides three independent lines of evidence (based on RT-PCR, in situ hybridization, and immunohistochemistry) that Ltrk is universally expressed by neurons and dorsal body cells of both the juvenile and the adult Lymnaea CNS. The highest level of expression by neuronal somata occurs in the late juvenile stage, whereas axon collaterals express high levels throughout the animal's life span. Our data support multifunctional roles for Ltrk that parallel those of its mammalian counterparts.

ApTrkl, a Trk-like Receptor, Mediates Serotonin- Dependent ERK Activation and Long-Term Facilitation in Aplysia Sensory Neurons

The Trk family of receptor tyrosine kinases plays a role in synaptic plasticity and in behavioral memory in mammals. Here, we report the discovery of a Trk-like receptor, ApTrkl, in Aplysia. We show that it is expressed in the sensory neurons, the locus for synaptic facilitation, which is a cellular model for memory formation. Serotonin, the facilitatory neurotransmitter, activates ApTrkl, which, in turn, leads to activation of ERK. Finally, inhibiting the activation of ApTrkl with the Trk inhibitor K252a or using dsRNA to inhibit ApTrkl blocks the serotonin-mediated activation of ERK in the cell body, as well as the cell-wide long-term facilitation induced by 5-HT application to the cell body. Thus, transactivation of the receptor tyrosine kinase ApTrkl by serotonin is an essential step in the biochemical events leading to long-term facilitation in Aplysia.

Characterization of a novel molluskan tyrosine kinase receptor that inhibits neurite regeneration

Receptor tyrosine kinases play many important roles in neuronal signaling including regulating neurite outgrowth. We have identified a novel receptor tyrosine kinase, neurite outgrowth regulating kinase (nork) from Aplysia californica. A fragment of this kinase was also identified in another mollusk, Lymnaea. The kinase domain is equally homologous to the Ret (rearranged during transformation) and fibroblast growth factor receptor families, but the extracellular domain is entirely novel, suggesting that it binds a nonconserved ligand. Overexpression of neurite outgrowth regulating kinase, but not a kinase dead form, causes a reduction in neurite outgrowth of Aplysia sensory neurons. Thus, we have identified a novel receptor tyrosine kinase implicated in regulating neurite outgrowth.

Differential subcellular localization of Ror tyrosine kinase receptors in cultured astrocytes

Ror1 and Ror2 belong to a family of tyrosine kinase receptors that are highly conserved among species. They are expressed throughout the organism, including the central nervous system. In the present study, we analyzed the expression and subcellular localization of Ror1 and Ror2 in astrocytes by means of reverse transcription-polymerase chain reaction, Western blot analysis, and immunocytochemistry. Our results indicated that both Ror1 and Ror2 are readily detectable in cultured astrocytes. They also showed that Ror1 and Ror2 are associated with different components of the cytoskeleton. While Ror1 co-localized with F-actin along stress fibers, Ror2 partially co-localized with microtubules. In addition, our results suggest that Ror1 and Ror2 undergo different posttranslational modifications in cultured astrocytes. Ror1 is highly glycosylated in these cells. In contrast, no glycosylation was detected in Ror2. Taken together, these results suggest distinct roles for these tyrosine kinase receptors in astrocytes.

Expression and subcellular localization of Ror tyrosine kinase receptors are developmentally regulated in cultured hippocampal neurons

Ror1 and Ror2 are two novel receptor tyrosine kinases that have been implicated in neuronal differentiation in Caenorhabditis elegans. As a first step toward elucidating their role in the mammalian brain, we analyzed their expression and localization patterns in hippocampal neurons. Our results showed that both receptors are expressed from early stages of development and that their protein levels peak during periods of active synapse formation. Immunocytochemical analysis indicated that Ror1 and Ror2 are highly concentrated in the growth cones of immature neurons and are present throughout the somatodendritic compartment of mature hippocampal cells. Further analysis indicated that they are present not only in the cell membrane but also in Triton- and saponin-insoluble fractions, suggesting that they may be associated with both the cytoskeleton and membrane-bound organelles. Taken collectively, our results suggest that Ror1 and Ror2 might play a role during early stages of development in mammalian central neurons.

The receptor tyrosine kinase Ror2 associates with the melanoma-associated antigen (MAGE) family protein Dlxin-1 and regulates its intracellular distribution

The mammalian Ror family receptor tyrosine kinases, Ror1 and Ror2, play crucial roles in developmental morphogenesis. Although the functions of Ror1 and Ror2 are redundant, Ror2 exhibits more specific functions during development. We show that when expressed in mammalian cells, Ror2, but not Ror1, associates with the melanoma-associated antigen (MAGE) family protein, Dlxin-1, which is known to bind to the homeodomain proteins Msx2 and Dlx5 and regulate their transcriptional functions. This association requires the cytoplasmic C-terminal region of Ror2, containing proline-rich and serine/threonine-rich domains, and the C-terminal necdin homology domain of Dlxin-1. Interestingly, the cytoplasmic C-terminal region of Ror2 is missing in patients with brachydactyly type B. Interestingly, transient expression and immunohistochemical analyses reveal that both Dlxin-1 and Msx2 are co-localized in the nuclei in the absence of Ror2. In the presence of Ror2, Dlxin-1 is co-localized with Ror2 at the membranous compartments and Msx2 is retained in the nuclei. It was also found that the majority of cellular Dlxin-1 is retained in the membrane fractions of wild-type but not Ror2-/- mouse embryonic fibroblasts. Furthermore, we show that transcriptional activity of Msx2, irrespective of Ror2 kinase activity, is regulated by ectopic expression of Ror2 using a reporter plasmid containing the WIP element. Thus, Ror2 sequesters Dlxin-1 in membranous compartments, thereby affecting the transcriptional function of Msx2.

Expression and function of the Ror-family receptor tyrosine kinases during development: lessons from genetic analyses of nematodes, mice, and humans

Receptor tyrosine kinases (RTKs) play crucial roles in various developmental processes. Ror-family RTKs are characterized by the intracellular tyrosine kinase domains, highly related to those of the Trk-family RTKs, and by the extracellular Frizzled-like cysteine-rich domains (CRDs) and Kringle domains. Rors are evolutionally conserved among Caenorhabditis elegans, Aplysia, Drosophila melanogaster, Xenopus, mice, and humans. In D. melanogaster and mammals, pairs of structurally related Rors are found, while a single Ror protein is identified in C. elegans or Aplysia. In Aplysia and D. melanogaster, Rors are expressed exclusively in developing nervous systems. On the other hand, rather widespread expression of Rors was observed in C. elegans and mammals. Mutations in Ror of C. elegans cause inappropriate axon outgrowth as well as defects in cell migration and asymmetric cell division. It has also been reported that the nematode Ror possesses kinase-dependent and kinase-independent functions. Mouse Rors, Ror1, and Ror2, are expressed mainly in migrating neural crest cells and mesenchymal cells, and Ror2-deficient mice exhibit skeletal abnormalities and ventricular septal defects in the heart. Although Ror1-deficient mice exhibit no apparent skeletal or cardiac abnormalities, Ror1/Ror2 double mutant mice show markedly enhanced skeletal and cardiac abnormalities compared with Ror2 mutant mice, indicating genetic interaction of Ror1 and Ror2. In humans, mutations within Ror2 have been found in two genetic skeletal disorders, recessive Robinow syndrome and dominant Brachydactyly type B (BDB), further emphasizing critical functions of Ror2 during developmental morphogenesis. In this article, we also discuss the signaling machinery mediated by Ror-family RTKs with a particular emphasis on our recent structure-function analyses of Ror-family RTKs.