Lisa M. Moscoso

Defective Neuromuscular Synaptogenesis in Agrin-Deficient Mutant Mice

During neuromuscular synapse formation, motor axons induce clustering of acetylcholine receptors (AChRs) in the muscle fiber membrane. The protein agrin, originally isolated from the basal lamina of the synaptic cleft, is synthesized and secreted by motoneurons and triggers formation of AChR clusters on cultured myotubes. We show here postsynaptic AChR aggregates are markedly reduced in number, size, and density in muscles of agrin-deficient mutant mice. These results support the hypothesis that agrin is a critical organizer of postsynaptic differentiation does occur in the mutant, suggesting the existence of a second-nerve-derived synaptic organizing signal. In addition, we show that intramuscular nerve branching and presynaptic differentiation are abnormal in the mutant, phenotypes which may reflect either a distinct effect of agrin or impaired retrograde signaling from a defective postsynaptic apparatus.

Rapsyn is required for MuSK signaling and recruits synaptic components to a MuSK-containing scaffold.

Agrin-induced clustering of acetylcholine receptors (AChRs) in the postsynaptic membrane is a key step in synaptogenesis at the neuromuscular junction. The receptor tyrosine kinase MuSK is a component of the agrin receptor, while the cytoplasmic protein rapsyn is necessary for the clustering of AChRs and all other postsynaptic membrane components studied to date. We show here that MuSK remains concentrated at synaptic sites in rapsyn-deficient mutant mice, suggesting that MuSK forms a primary structural scaffold to which rapsyn attaches other synaptic components. Using nonmuscle cells, we show that rapsyn-MuSK interactions are mediated by the ectodomain of MuSK, suggesting the existence of a transmembrane intermediate. In addition to rapsyns structural role, we demonstrate that it is required for an early step in MuSK signaling, AChR phosphorylation. This signaling requires the kinase domain of MuSK, but not its ectodomain. Thus, MuSK may interact with rapsyn in multiple ways to play both structural and signaling roles in agrin-induced differentiation.

Regulation of the acetylcholine receptor ϵ subunit gene by recombinant ARIA: An in vitro model for transynaptic gene regulation

Structural specialization of the postsynaptic skeletal muscle membrane is in part mediated by the motor neuron-induced transcriptional regulation of synaptic muscle nuclei. ARIA, a factor that stimulates production of acetylcholine receptors (AChRs), is a candidate signaling molecule for such regulation. Here we examine the transynaptic inducing potential of this polypeptide factor. ARIA immunoreactivity is detectable at synaptic sites in vivo. In vitro, recombinant heregulin beta 1 (rHRG beta 1), the human homolog of ARIA, induces expression of the AChR epsilon gene, the subunit most sensitive to synaptic input. The inducing property of rHRG beta 1 is demonstrated most dramatically in primary muscle cultures from transgenic mice bearing an epsilon promoter-nuclear lacZ reporter transgene. Transient transfection experiments using the Sol 8 muscle cell line indicate that sequences that confer responsiveness to ARIA are located within a 150 bp epsilon subunit promoter region and are E box-independent. These results suggest that ARIA performs a vital role by directing spatially restricted gene expression at the neuromuscular junction.

Mice lacking alpha-calcitonin gene-related peptide exhibit normal cardiovascular regulation and neuromuscular development.

alpha-Calcitonin gene-related peptide (alphaCGRP) is a pleiotropic peptide neuromodulator that is widely expressed throughout the Central and peripheral nervous systems. CGRP has been implicated in a variety of physiological processes including peripheral vasodilation, cardiac acceleration nicotinic acetylcholine receptor (AChR) synthesis and function, testicular descent, nociception, carbohydrate metabolism, gastrointestinal motility, neurogenic inflammation, and gastric acid secretion. To provide a better understanding of the physiological role(s) mediated by this peptide neurotransmitter, we have generated alphaCGRP-null mice by targeted modification in embryonic stem cells. Mice lacking alpha CGRP expression demonstrate no obvious phenotypic differences from their wild-type littermates. Detailed analysis of systemic cardiovascular function revealed no differences between control and mutant mice regarding heart rate and blood pressure under basal or exercise-induced conditions and subsequent to pharmacological manipulation. Characterization of neuromuscular junction in morphology including nicotinic receptor localization, terminal sprouting in response to denervation, developmental regulation of AChR subunit expression, and synapse elimination also revealed no differences in alphaCGRP-deficient animals. These results suggest that alphaCGRP is not required for the systemic regulation of cardiovascular hemodynamics or development of the neuromuscular junction.

N-CAM, 43K-rapsyn, and S-laminin mRNAs are concentrated at synaptic sites in muscle fibers.

Several components of the postsynaptic apparatus are found highly concentrated at the motor endplate. Studies of the acetylcholine receptor have shown that selective transcription of its genes by synaptic nuclei contributes to its synaptic accumulation. We used the method of in situ hybridization to study the distribution of mRNAs encoding three other proteins localized to the motor endplate. We found preferential synaptic accumulation of mRNAs for a membrane-associated cell adhesion molecule (N-CAM) and for an acetylcholine receptor-associated cytoskeletal protein (43K-rapsyn). In contrast, RNAs encoding proteins present throughout the muscle were distributed all along the muscle fiber. RNA encoding a protein concentrated in synaptic basal lamina, s-laminin (laminin beta 2), was intermediate in distribution, detectable extrasynaptically but more abundant synaptically. Our data suggest that selective transcription by synaptic nuclei is a general mechanism that contributes to the concentration of specific proteins in the postsynaptic apparatus at the neuromuscular junction.

Development of a pediatric hospitalist sedation service: training and implementation.

OBJECTIVE There is growing demand for safe and effective procedural sedation in pediatric facilities nationally. Currently, these needs are being met by a variety of providers and sedation techniques, including anesthesiologists, pediatric intensivists, emergency medicine physicians, and pediatric hospitalists. There is currently no consensus regarding the training required by non-anesthesiologists to provide safe sedation. We will outline the training method developed at St. Louis Childrens Hospital. METHODS In 2003, the Division of Pediatric Anesthesia at St. Louis Childrens Hospital approached the Division of Pediatric Hospitalist Medicine as a resource to provide pediatric sedation outside of the operating room. Over the last seven years, Pediatric Hospitalist Sedation services have evolved into a three-tiered system of sedation providers. The first tier provides sedation services in the emergency unit (EU) and the Center for After Hours Referral for Emergency Services (CARES). The second tier provides sedation throughout the hospital including the EU, CARES, inpatient units, Ambulatory Procedure Center (APC), and Pediatric Acute Wound Service (PAWS); it also provides night/weekend sedation call for urgent needs. The third tier provides sedation in all of the second-tier locations, as well as utilizing propofol in the APC. RESULTS This training program has resulted in a successful pediatric hospitalist sedation service. Based on fiscal year 2009 billing data, the division performed 2,471 sedations. We currently have 43 hospitalists providing Tier-One sedation, 18 Tier-Two providers, and six Tier-Three providers. CONCLUSIONS A pediatric hospitalist sedation service with proper training and oversight can successfully augment sedation provided by anesthesiologists.