NCAM 180 Acting via a Conserved C-Terminal Domain and MLCK Is Essential for Effective Transmission with Repetitive Stimulation

SummaryNCAM 180 isoform null neuromuscular junctions are unable to effectively mobilize and exocytose synaptic vesicles and thus exhibit periods of total transmission failure during high-frequency repetitive stimulation. We have identified a highly conserved C-terminal (KENESKA) domain on NCAM that is required to maintain effective transmission and demonstrate that it acts via a pathway involving MLCK and probably myosin light chain (MLC) and myosin II. By perfecting a method of introducing peptides into adult NMJs, we tested the hypothesized role of proteins in this pathway by competitive disruption of protein-protein interactions. The effects of KENESKA and other peptides on MLCK and MLC activation and on failures in both wild-type and NCAM 180 null junctions supported this pathway, and serine phosphorylation of KENESKA was critical. We propose that this pathway is required to replenish synaptic vesicles utilized during high levels of exocytosis by facilitating myosin-driven delivery of synaptic vesicles to active zones or their subsequent exocytosis

Cholinergic Input Is Required during Embryonic Development to Mediate Proper Assembly of Spinal Locomotor Circuits

SummaryRhythmic limb movements are controlled by pattern-generating neurons within the ventral spinal cord, but little is known about how these locomotor circuits are assembled during development. At early stages of embryogenesis, motor neurons are spontaneously active, releasing acetylcholine that triggers the depolarization of adjacent cells in the spinal cord. To investigate whether acetylcholine-driven activity is required for assembly of the central pattern-generating (CPG) circuit, we studied mice lacking the choline acetyltransferase (ChAT) enzyme. Our studies show that a rhythmically active spinal circuit forms in ChAT mutants, but the duration of each cycle period is elongated, and right-left and flexor-extensor coordination are abnormal. In contrast, blocking acetylcholine receptors after the locomotor network is wired does not affect right-left or flexor-extensor coordination. These findings suggest that the cholinergic neurotransmitter pathway is involved in configuring the CPG during a transient period of development

A Latent Propriospinal Network Can Restore Diaphragm Function After High Cervical Spinal Cord Injury

Spinal cord injury (SCI) above cervical level 4 disrupts descending axons from the medulla that innervate phrenic motor neurons, causing permanent paralysis of the diaphragm. Using an ex vivo preparation in neonatal mice, we have identified an excitatory spinal network that can direct phrenic motor bursting in the absence of medullary input. After complete cervical SCI, blockade of fast inhibitory synaptic transmission caused spontaneous, bilaterally coordinated phrenic bursting. Here, spinal cord glutamatergic neurons were both sufficient and necessary for the induction of phrenic bursts. Direct stimulation of phrenic motor neurons was insufficient to evoke burst activity. Transection and pharmacological manipulations showed that this spinal network acts independently of medullary circuits that normally generate inspiration, suggesting a distinct non-respiratory function. We further show that this “latent” network can be harnessed to restore diaphragm function after high cervical SCI in adult mice and rats

Development - Editorial overview

The purpose of neural development is to assemble functional circuits that control behavior, a process that requires both the proper specification of neuronal identities and the establishment of appropriate patterns of connectivity between distinct neuronal subtypes. Given the vast number of neurons in the brain, the even larger number of specific connections, and the relatively small number of genes, the problem of how to accomplish this appears daunting. Nevertheless, considerable progress has been made over the past several years in deciphering a number of the general cellular and molecular mechanisms that are used to assemble the nervous system. This issue of Current Opinion in Neurobiology focuses primarily on the problems of neuronal identity specification and circuit formation, as they relate both to early development and to the emergence of complex behaviors. We have endeavored to strike a balance between providing updates on fast-moving fields of central importance and including areas that received relatively less coverage in the review literature but that are emerging as exciting new frontiers. The result is a somewhat idiosyncratic and certainly noncomprehensive collection of reviews that reflects our own biased perceptions of what is interesting. Consequently, there are major areas and approaches not represented, with our chief sin of omission being the absence of reviews explicitly focused on invertebrate genetic systems; however, several of the reviews draw on insights provided by such systems, and many of the advances in vertebrate neural development derive from concepts originally revealed in Drosophila. For the purposes of this overview, we have divided the reviews into four groups. The first group deals with neurogenesis, the diversification of neurons and muscles, and some aspects of patterning. The second group concerns the mechanisms that guide the selection of targets by growing axons. The third group surveys the relative contributions of electrical activity and neurotrophins to circuit formation. Finally, the fourth group deals with the role of social interactions between animals and its influence on the development of learned behaviors

Development of the major pathways for neurite outgrowth in the chick hindlimb

To elucidate mechanisms that may control development of the gross anatomical nerve pattern, motoneuron outgrowth into the chick hindlimb was examined using orthograde labeling, scanning and transmission electron microscopy, and Alcian blue staining. Results show that growth cones are not guided by contact with oriented extracellular fibrils, aligned mesenchyme cells, the myotome, or the vasculature. Pathways are not delineated by cell-free space or channels of lower cell density; however, densely packed mesenchyme may form barriers that channel outgrowth. In addition, abundant mesenchymal cell death was seen at the nerve front. This cell death may provide space that encourages growth cone advancement. Pathways often lie along interfaces between areas that stain darkly and lightly with Alcian blue, which specifically stains glycosaminoglycans, and growth cones never penetrate areas that stain intensely, such as the pelvic girdle, which is known to be a barrier to outgrowth. Leading growth cones form specialized contacts with mesenchyme cells, but the predominant contacts are interneuronal. It is proposed that the anatomical pattern of outgrowth is determined by the distribution of preferred substrata, the most preferred substratum being other neurites. Further, neurites tend to prefer loose mesenchyme to dense mesenchyme or areas rich in glycosaminoglycans