Distinct B Cell Receptor Functions Are Determined by Phosphorylation

Background: Neuronal topographic map formation requires appropriate selection of axonal trajectories at intermediate choice points prior to target innervation. Axons of neurons in the spinal cord lateral motor column (LMC), as defined by a transcription factor code, are thought to innervate limb target tissues exclusively. Axons of the medial and lateral LMC divisions appear to execute a binary decision at the base of the limb as they choose between ventral and dorsal limb trajectories. The cellular logic that guides motor axon trajectory choices into non-limb tissues such as the ventral flank remains unclear. Results: We determined the spinal cord motor column origin of motor nerves that innervate ventral flank tissues at hindlimb level. We found unexpectedly that a subset of medial LMC axons innervates ventral non-limb mesenchyme at hindlimb level, rather than entering ventral limb mesenchyme. We also found that in a conditional BmprIa mutant where all ventral hindlimb mesenchyme is converted to a dorsal identity, all medial LMC axons are redirected into the ventral flank, while lateral LMC axons innervate the bidorsal limb. Conclusion: We have found that medial LMC neurons innervate both ventral flank and limb targets. While normally only a subset of medial LMC axons innervate the flank, all are capable of doing so. Furthermore, LMC axons execute a ternary, rather than binary, choice at the base of the limb between ventral flank, ventral limb and dorsal limb trajectories. When making this choice, medial and lateral LMC axons exhibit different and asymmetric relative preferences for these three trajectories. These data redefine the LMC as a motor column that innervates both limb and body tissues. Background The precision of neural circuits requires stereotypic patterns of neuronal connectivity, which are often organized as topographic maps. Studies of how the point-to-point connections between spinal cord motor neurons and their targets are generated have revealed much about how such maps are constructed [1-3]. One principle to emerge from these studies is that functionally related neurons extend axons along shared trajectories that are precisely and accurately subdivided at intermediate choice points [4,5]. Deciphering the logic underlying these trajectory choices is thus critical to understanding how these connectivity patterns are established. Published: 2 July 2007 Neural Development 2007, 2:13 doi:10.1186/1749-8104-2-13 Received: 12 January 2007 Accepted: 2 July 2007 This article is available from: http://www.neuraldevelopment.com/content/2/1/13 © 2007 Luria and Laufer; licensee BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.