Artur Kania

Requirement of Lim1 for female reproductive tract development

Lim1 encodes a LIM-class homeodomain transcription factor that is essential for head and kidney development. In the developing urogenital system, Lim1 expression has been documented in the Wolffian (mesonephric) duct, the mesonephros, metanephros and fetal gonads. Using, a Lim1 lacZ knock-in allele in mice, we identified a previously unreported urogenital tissue for Lim1 expression, the epithelium of the developing Müllerian duct that gives rise to the oviduct, uterus and upper region of the vagina of the female reproductive tract. Lim1 expression in the Müllerian duct is dynamic, corresponding to its formation and differentiation in females and regression in males. Although female Lim1-null neonates had ovaries they lacked a uterus and oviducts. A novel female mouse chimera assay was developed and revealed that Lim1 is required cell autonomously for Müllerian duct epithelium formation. These studies demonstrate an essential role for Lim1 in female reproductive tract development.

Specification of Motor Axon Trajectory by Ephrin-B:EphB Signaling: Symmetrical Control of Axonal Patterning in the Developing Limb

Studies of the innervation of limb muscles by spinal motor neurons have helped to define mechanisms by which axons establish trajectories to their targets. Related motor axons select dorsal or ventral pathways at the base of the limb, raising the question of how these alternate trajectories are specified. EphA signaling has been proposed to control the dorsal trajectory of motor axons in conjunction with other signaling systems, although the respective contributions of each system to motor axon guidance are unclear. We show that the expression of EphB receptors by motor axons, and ephrin-B ligands by limb mesenchymal cells, directs the ventral trajectory of motor axons. Our findings reveal symmetry in the molecular strategies that establish this aspect of nerve-muscle connectivity. The involvement of ephrin:Eph signaling in guiding both sets of motor axons raises the possibility that other signaling systems function primarily to refine or modulate a core Eph signaling program.

Mechanisms of ephrin–Eph signalling in development, physiology and disease

Eph receptor Tyr kinases and their membrane-tethered ligands, the ephrins, elicit short-distance cell–cell signalling and thus regulate many developmental processes at the interface between pattern formation and morphogenesis, including cell sorting and positioning, and the formation of segmented structures and ordered neural maps. Their roles extend into adulthood, when ephrin–Eph signalling regulates neuronal plasticity, homeostatic events and disease processes. Recently, new insights have been gained into the mechanisms of ephrin–Eph signalling in different cell types, and into the physiological importance of ephrin–Eph in different organs and in disease, raising questions for future research directions.

Ephrin-Mediated cis-Attenuation of Eph Receptor Signaling Is Essential for Spinal Motor Axon Guidance

Axon guidance receptors guide neuronal growth cones by binding in trans to axon guidance ligands in the developing nervous system. Some ligands are coexpressed in cis with their receptors, raising the question of the relative contribution of cis and trans interactions to axon guidance. Spinal motor axons use Eph receptors to select a limb trajectory in response to trans ephrins, while expressing ephrins in cis. We show that changes in motor neuron ephrin expression result in trajectory selection defects mirrored by changes in growth cone sensitivity to ephrins in vitro, arguing for ephrin cis-attenuation of Eph function. Furthermore, the relative contribution of trans-signaling and cis-attenuation is influenced by the subcellular distribution of ephrins to membrane patches containing Eph receptors. Thus, growth cone ephrins are essential for axon guidance in vivo and the balance between cis and trans modes of axon guidance ligand-receptor interaction contributes to the diversity of axon guidance signaling responses.

Lmx1b controls the differentiation and migration of the superficial dorsal horn neurons of the spinal cord.

The differentiation and migration of superficial dorsal horn neurons and subsequent ingrowth of cutaneous afferents are crucial events in the formation of somatosensory circuitry in the dorsal spinal cord. We report that the differentiation and migration of the superficial dorsal horn neurons are regulated by the LIM homeobox gene Lmx1b, and its downstream targets Rnx and Drg11, two transcription factors implicated in the development of dorsal horn circuitry. An analysis of Lmx1b mutants shows that Lmx1b normally acts to maintain the expression of the Ebf genes and to repress the Zic genes. Lmx1b mutants also exhibit the disruption of the cutaneous afferent ingrowth, suggesting that the dorsal horn cells might provide important cues guiding sensory axons into the dorsal spinal cord. Our results thus indicate that Lmx1b has a pivotal role in genetic cascades that control the assembly of circuitry in the superficial dorsal horn.

Foxp1 and lhx1 coordinate motor neuron migration with axon trajectory choice by gating Reelin signalling.

During embryonic development of the vertebrate motor system, the same transcription factors that regulate axonal trajectories can also regulate cell body migration, thereby controlling topographic map formation.

Ephrin signalling in the developing nervous system

Ephrin ligands and their Eph receptors hold our attention since their link to axon guidance almost twenty years ago. Since then, they have been shown to be critical for short distance cell-cell interactions in the nervous system. The interest in their function has not abated, leading to ever-more sophisticated studies generating as many surprising answers about their function as new questions. We discuss recent insights into their functions in the developing nervous system, including neuronal progenitor sorting, stochastic cell migration, guidance of neuronal growth cones, topographic map formation, as well as synaptic plasticity.

G-protein coupled receptor BAI3 promotes myoblast fusion in vertebrates.

Significance Myoblast fusion is essential for multinucleated muscle fiber formation. Regulators of fusion have been identified in Drosophila, but the molecular mechanisms controlling vertebrate myoblast fusion are poorly understood. We describe here previously unidentified in vivo functional evidence that the G-protein coupled receptor brain-specific angiogenesis inhibitor (BAI3) is a receptor controlling myoblast fusion in developing vertebrate muscles. We show that BAI3 binds the intracellular effector of myoblast fusion ELMO/DOCK1 and that this binding is obligatory for fusion. Using in vivo manipulations in the chicken embryo, we demonstrate that BAI3 binding to ELMO is essential for myoblast fusion. These experiments identify BAI3 as a long-sought-after receptor controlling vertebrate myoblast fusion. Muscle fibers form as a result of myoblast fusion, yet the cell surface receptors regulating this process are unknown in vertebrates. In Drosophila, myoblast fusion involves the activation of the Rac pathway by the guanine nucleotide exchange factor Myoblast City and its scaffolding protein ELMO, downstream of cell-surface cell-adhesion receptors. We previously showed that the mammalian ortholog of Myoblast City, DOCK1, functions in an evolutionarily conserved manner to promote myoblast fusion in mice. In search for regulators of myoblast fusion, we identified the G-protein coupled receptor brain-specific angiogenesis inhibitor (BAI3) as a cell surface protein that interacts with ELMO. In cultured cells, BAI3 or ELMO1/2 loss of function severely impaired myoblast fusion without affecting differentiation and cannot be rescued by reexpression of BAI3 mutants deficient in ELMO binding. The related BAI protein family member, BAI1, is functionally distinct from BAI3, because it cannot rescue the myoblast fusion defects caused by the loss of BAI3 function. Finally, embryonic muscle precursor expression of a BAI3 mutant unable to bind ELMO was sufficient to block myoblast fusion in vivo. Collectively, our findings provide a role for BAI3 in the relay of extracellular fusion signals to their intracellular effectors, identifying it as an essential transmembrane protein for embryonic vertebrate myoblast fusion.

Eph and ephrin signaling: Lessons learned from spinal motor neurons

In nervous system assembly, Eph/ephrin signaling mediates many axon guidance events that shape the formation of precise neuronal connections. However, due to the complexity of interactions between Ephs and ephrins, the molecular logic of their action is still being unraveled. Considerable advances have been made by studying the innervation of the limb by spinal motor neurons, a series of events governed by Eph/ephrin signaling. Here, we discuss the contributions of different Eph/ephrin modes of interaction, downstream signaling and electrical activity, and how these systems may interact both with each other and with other guidance molecules in limb muscle innervation. This simple model system has emerged as a very powerful tool to study this set of molecules, and will continue to be so by virtue of its simplicity, accessibility and the wealth of pioneering cellular studies.

Genetic Analysis of DSCAM's Role as a Netrin-1 Receptor in Vertebrates

Down syndrome cell adhesion molecule (DSCAM) has mainly been characterized for its function as an adhesion molecule in axon growth and in self-recognition between dendrites of the same neuron. Recently, it has been shown that DSCAM can bind to Netrin-1 and that downregulation of DSCAM expression by siRNAs in chick and rodent spinal cords leads to impaired growth and turning response of commissural axons to Netrin-1. To investigate the effect of complete genetic ablation of DSCAM on Netrin-1-induced axon guidance, we analyzed spinal commissural neurons in DSCAM-null mice and found that they extend axons that reach and cross the floor plate and express apparently normal levels of the Netrin receptors DCC (deleted in colorectal carcinoma) and Neogenin. In vitro, commissural neurons in dorsal spinal cord explants of DSCAM-null embryos show normal outgrowth in response to Netrin-1. We therefore conclude that DSCAM is not required for Netrin-induced commissural axon outgrowth and guidance in mice.