Retinoid signaling is involved in governing the waiting period for axons in chick hindlimb

Aberrant early growth of individual trigeminal sensory and motor axons in a series of mouse genetic models of 22q11.2 deletion syndrome

We identified divergent modes of initial axon growth that prefigure disrupted differentiation of the trigeminal nerve (CN V), a cranial nerve essential for suckling, feeding and swallowing (S/F/S), a key innate behavior compromised in multiple genetic developmental disorders including DiGeorge/22q11.2 Deletion Syndrome (22q11.2 DS). We combined rapid in vivo labeling of single CN V axons in LgDel^+/− mouse embryos, a genomically accurate 22q11.2DS model, and 3D imaging to identify and quantify phenotypes that could not be resolved using existing methods. We assessed these phenotypes in three 22q11.2-related genotypes to determine whether individual CN V motor and sensory axons wander, branch and sprout aberrantly in register with altered anterior–posterior hindbrain patterning and gross morphological disruption of CN V seen in LgDel^+/−. In the additional 22q11.2-related genotypes: Tbx1^+/−, Ranbp1^−/−, Ranbp1^+/− and LgDel^+/−:Raldh2^+/−; axon phenotypes are seen when hindbrain patterning and CN V gross morphology is altered, but not when it is normal or restored toward WT. This disordered growth of CN V sensory and motor axons, whose appropriate targeting is critical for optimal S/F/S, may be an early, critical determinant of imprecise innervation leading to inefficient oropharyngeal function associated with 22q11.2 deletion from birth onward.

The developmental and genetic basis of 'clubfoot' in the peroneal muscular atrophy mutant mouse

Genetic factors underlying the human limb abnormality congenital talipes equinovarus ('clubfoot') remain incompletely understood. The spontaneous autosomal recessive mouse 'peroneal muscular atrophy' mutant (PMA) is a faithful morphological model of human clubfoot. In PMA mice, the dorsal (peroneal) branches of the sciatic nerves are absent. In this study, the primary developmental defect was identified as a reduced growth of sciatic nerve lateral motor column (LMC) neurons leading to failure to project to dorsal (peroneal) lower limb muscle blocks. The pma mutation was mapped and a candidate gene encoding LIM-domain kinase 1 (Limk1) identified, which is upregulated in mutant lateral LMC motor neurons. Genetic and molecular analyses showed that the mutation acts in the EphA4-Limk1-Cfl1/cofilin-actin pathway to modulate growth cone extension/collapse. In the chicken, both experimental upregulation of Limk1 by electroporation and pharmacological inhibition of actin turnover led to defects in hindlimb spinal motor neuron growth and pathfinding, and mimicked the clubfoot phenotype. The data support a neuromuscular aetiology for clubfoot and provide a mechanistic framework to understand clubfoot in humans.

In vivo zebrafish morphogenesis shows Cyp26b1 promotes tendon condensation and musculoskeletal patterning in the embryonic jaw

Integrated development of diverse tissues gives rise to a functional, mobile vertebrate musculoskeletal system. However, the genetics and cellular interactions that drive the integration of muscle, tendon, and skeleton are poorly understood. In the vertebrate head, neural crest cells, from which cranial tendons derive, pattern developing muscles just as tendons have been shown to in limb and trunk tissue, yet the mechanisms of this patterning are unknown. From a forward genetic screen, we determined that cyp26b1 is critical for musculoskeletal integration in the ventral pharyngeal arches, particularly in the mandibulohyoid junction where first and second arch muscles interconnect. Using time-lapse confocal analyses, we detail musculoskeletal integration in wild-type and cyp26b1 mutant zebrafish. In wild-type fish, tenoblasts are present in apposition to elongating muscles and condense in discrete muscle attachment sites. In the absence of cyp26b1, tenoblasts are generated in normal numbers but fail to condense into nascent tendons within the ventral arches and, subsequently, muscles project into ectopic locales. These ectopic muscle fibers eventually associate with ectopic tendon marker expression. Genetic mosaic analysis demonstrates that neural crest cells require Cyp26b1 function for proper musculoskeletal development. Using an inhibitor, we find that Cyp26 function is required in a short time window that overlaps the dynamic window of tenoblast condensation. However, cyp26b1 expression is largely restricted to regions between tenoblast condensations during this time. Our results suggest that degradation of RA by this previously undescribed population of neural crest cells is critical to promote condensation of adjacent scxa-expressing tenoblasts and that these condensations are subsequently required for proper musculoskeletal integration.

Axon pathfinding for locomotion

Motor neurons of the spinal cord are responsible for the assembly of neuromuscular connections indispensable for basic locomotion and skilled movements. A precise spatial relationship exists between the position of motor neuron cell bodies in the spinal cord and the course of their axonal projections to peripheral muscle targets. Motor neuron innervation of the vertebrate limb is a prime example of this topographic organization and by virtue of its accessibility and predictability has provided access to fundamental principles of motor system development and neuronal guidance. The seemingly basic binary map established by genetically defined motor neuron subtypes that target muscles in the limb is directed by a surprisingly large number of directional cues. Rather than being simply redundant, these converging signaling pathways are hierarchically linked and cooperate to increase the fidelity of axon pathfinding decisions. A current priority is to determine how multiple guidance signals are integrated by individual growth cones and how they synergize to delineate class-specific axonal trajectories.

Identification of novel candidate regulators of retinotectal map formation through transcriptional profiling of the chick optic tectum

Information from the retina is carried along the visual pathway with accuracy and spatial conservation as a result of topographically mapped axonal connections. The optic tectum in the midbrain is the primary region to which retinal ganglion cells project their axons in the chick. The two primary axes of the retina project independently onto the tectum using different sets of guidance cues to give rise to the retinotectal map. Specificity of the map is determined by attractive or repulsive interactions between molecular tags that are distributed in gradients in the retina and the tectum. Despite several studies, knowledge of the retinotectal guidance molecules is far from being complete. We screened for all molecules that are expressed differentially along the anterior-posterior and medial-lateral axes of the chick tectum using microarray based transcriptional profiling and identified several novel candidate retinotectal guidance molecules. Two such genes, encoding Wnt5a and Raldh2, the synthesizing enzymes for retinoic acid, were further analyzed for their function as putative regulators of retinotectal map formation. Wnt5a and retinoic acid were found to exhibit differential effects on the growth of axons from retinal explants derived from different quadrants of the retina. This screen also yielded a large number of genes expressed in a lamina-specific manner in the tectum, which may have other roles in tectal development. J. Comp. Neurol. 525:459-477, 2017. © 2016 Wiley Periodicals, Inc.

Impaired development of left anterior heart field by ectopic retinoic acid causes transposition of the great arteries

Background

Transposition of the great arteries is one of the most commonly diagnosed conotruncal heart defects at birth, but its etiology is largely unknown. The anterior heart field (AHF) that resides in the anterior pharyngeal arches contributes to conotruncal development, during which heart progenitors that originated from the left and right AHF migrate to form distinct conotruncal regions. The aim of this study is to identify abnormal AHF development that causes the morphology of transposition of the great arteries.

Methods and Results

We placed a retinoic acid–soaked bead on the left or the right or on both sides of the AHF of stage 12 to 14 chick embryos and examined the conotruncal heart defect at stage 34. Transposition of the great arteries was diagnosed at high incidence in embryos for which a retinoic acid–soaked bead had been placed in the left AHF at stage 12. Fluorescent dye tracing showed that AHF exposed to retinoic acid failed to contribute to conotruncus development. FGF8 and Isl1 expression were downregulated in retinoic acid–exposed AHF, and differentiation and expansion of cardiomyocytes were suppressed in cultured AHF in medium supplemented with retinoic acid.

Conclusions

The left AHF at the early looped heart stage, corresponding to Carnegie stages 10 to 11 (28 to 29 days after fertilization) in human embryos, is the region of the impediment that causes the morphology of transposition of the great arteries.

Meningeal cells and glia establish a permissive environment for axon regeneration after spinal cord injury in newts

Background

Newts have the remarkable ability to regenerate their spinal cords as adults. Their spinal cords regenerate with the regenerating tail after tail amputation, as well as after a gap-inducing spinal cord injury (SCI), such as a complete transection. While most studies on newt spinal cord regeneration have focused on events occurring after tail amputation, less attention has been given to events occurring after an SCI, a context that is more relevant to human SCI. Our goal was to use modern labeling and imaging techniques to observe axons regenerating across a complete transection injury and determine how cells and the extracellular matrix in the injury site might contribute to the regenerative process.

Results

We identify stages of axon regeneration following a spinal cord transection and find that axon regrowth across the lesion appears to be enabled, in part, because meningeal cells and glia form a permissive environment for axon regeneration. Meningeal and endothelial cells regenerate into the lesion first and are associated with a loose extracellular matrix that allows axon growth cone migration. This matrix, paradoxically, consists of both permissive and inhibitory proteins. Axons grow into the injury site next and are closely associated with meningeal cells and glial processes extending from cell bodies surrounding the central canal. Later, ependymal tubes lined with glia extend into the lesion as well. Finally, the meningeal cells, axons, and glia move as a unit to close the gap in the spinal cord. After crossing the injury site, axons travel through white matter to reach synaptic targets, and though ascending axons regenerate, sensory axons do not appear to be among them. This entire regenerative process occurs even in the presence of an inflammatory response.

Conclusions

These data reveal, in detail, the cellular and extracellular events that occur during newt spinal cord regeneration after a transection injury and uncover an important role for meningeal and glial cells in facilitating axon regeneration. Given that these cell types interact to form inhibitory barriers in mammals, identifying the mechanisms underlying their permissive behaviors in the newt will provide new insights for improving spinal cord regeneration in mammals.