Lateral motor column axons execute a ternary trajectory choice between limb and body tissues.
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.


CONCLUSIONS
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.
Luria
Victor
V
Department of Genetics and Development, Columbia University, New York, NY 10032, USA. avl@columbia.edu
Laufer
Ed
E
D000818:Animals; D001369:Axons / Q000502:physiology* / Q000648:ultrastructure; D019521:Body Patterning / Q000235:genetics*; D052005:Bone Morphogenetic Protein Receptors, Type I / Q000235:genetics; D002454:Cell Differentiation / Q000235:genetics; D036342:Ephrins / Q000235:genetics; D005121:Extremities / Q000196:embryology* / Q000294:innervation*; D018507:Gene Expression Regulation, Developmental / Q000235:genetics; D020439:Growth Cones / Q000502:physiology / Q000648:ultrastructure; D018398:Homeodomain Proteins / Q000235:genetics; D051741:Kruppel-Like Transcription Factors / Q000235:genetics; D060850:LIM-Homeodomain Proteins; D008648:Mesoderm / Q000166:cytology / Q000502:physiology; D051379:Mice; D018345:Mice, Knockout; D009046:Motor Neurons / Q000166:cytology / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology; D009473:Neuronal Plasticity / Q000235:genetics; D000075722:Promyelocytic Leukemia Zinc Finger Protein; D036081:Receptors, Eph Family / Q000235:genetics; D013116:Spinal Cord / Q000166:cytology / Q000196:embryology*; D014157:Transcription Factors / Q000235:genetics
D036342:Ephrins; D018398:Homeodomain Proteins; D051741:Kruppel-Like Transcription Factors; C112149:LIM homeobox transcription factor 1 beta; D060850:LIM-Homeodomain Proteins; D000075722:Promyelocytic Leukemia Zinc Finger Protein; D014157:Transcription Factors; C092037:ZBTB16 protein, human; D036081:Receptors, Eph Family; D052005:Bone Morphogenetic Protein Receptors, Type I