Novel Mouse Model Reveals Distinct Activity-Dependent and –Independent Contributions to Synapse Development

The balanced action of both pre- and postsynaptic organizers regulates the formation of neuromuscular junctions (NMJ). The precise mechanisms that control the regional specialization of acetylcholine receptor (AChR) aggregation, guide ingrowing axons and contribute to correct synaptic patterning are unknown. Synaptic activity is of central importance and to understand synaptogenesis, it is necessary to distinguish between activity-dependent and activity-independent processes. By engineering a mutated fetal AChR subunit, we used homologous recombination to develop a mouse line that expresses AChR with massively reduced open probability during embryonic development. Through histological and immunochemical methods as well as electrophysiological techniques, we observed that endplate anatomy and distribution are severely aberrant and innervation patterns are completely disrupted. Nonetheless, in the absence of activity AChRs form postsynaptic specializations attracting motor axons and permitting generation of multiple nerve/muscle contacts on individual fibers. This process is not restricted to a specialized central zone of the diaphragm and proceeds throughout embryonic development. Phenotypes can be attributed to separate activity-dependent and -independent pathways. The correct patterning of synaptic connections, prevention of multiple contacts and control of nerve growth require AChR-mediated activity. In contrast, myotube survival and acetylcholine-mediated dispersal of AChRs are maintained even in the absence of AChR-mediated activity. Because mouse models in which acetylcholine is entirely absent do not display similar effects, we conclude that acetylcholine binding to the AChR initiates activity-dependent and activity-independent pathways whereby the AChR modulates formation of the NMJ

Transcription profiling of HCN-channel isotypes throughout mouse cardiac development

Hyperpolarization-activated ion channels, encoded by four mammalian genes (HCN1-4), contribute in an important way to the cardiac pacemaker current If. Here, we describe the transcription profiles of the four HCN genes, the NRSF, KCNE2 and Kir2.1 genes from embryonic stage E9.5 dpc to postnatal day 120 in the mouse. Embryonic atrium and ventricle revealed abundant HCN4 transcription but other HCN transcripts were almost absent. Towards birth, HCN4 was downregulated in the atrium and almost vanished from the ventricle. After birth, however, HCN isotype transcription changed remarkably, showing increased levels of HCN1, HCN2 and HCN4 in the atrium and of HCN2 and HCN4 in the ventricle. HCN3 showed highest transcription at early embryonic stages and was hardly detectable thereafter. At postnatal day 10, HCN4 was highest in the sinoatrial node, being twofold higher than HCN1 and fivefold higher than HCN2. In the atrium, HCN4 was similar to HCN1 and sevenfold higher than HCN2. In the ventricle, in contrast, HCN2 was sixfold higher than HCN4, while HCN1 was absent. Subsequently all HCN isotype transcripts declined to lower adult levels, while ratios of HCN isotypes remained stable. In conclusion, substantial changes of HCN isotype transcription throughout cardiac development suggest that a regulated pattern of HCN isotypes is required to establish and ensure a stable heart rhythm. Furthermore, constantly low HCN transcription in adult myocardium may be required to prevent atrial and ventricular arrhythmogenesis

AChR channel conversion and AChR−adjusted neuronal survival during embryonic development

We generated knock−in mice that express GFP−labeled embryonic−type acetylcholine receptors (AChR) to follow postsynaptic differentiation and innervation during embryonic development and to visualize the postnatally occurring channel conversion from embryonic− to adult−type AChR. The dynamics of AChRγ/AChRε conversion at the neuromuscular junction indicates that muscle−specific programs of receptor subunit gene transcription control AChR replacement. While conversion proceeds from peripheral to central regions for individual endplates, it does not occur simultaneously for all endplates. Although GFP−labeled receptors were expressed at reduced levels, the localization of postsynaptic sites and synapse formation was not noticeably altered. However, these alterations correlated with a striking reduction of motoneuron programmed cell death, transient increase of neurite growth and axon branching. This animal model refines the view on reciprocal neuromuscular signaling and suggests that motoneuron survival and axon branching are directly regulated by AChR function to enable optimal innervation and timing of neurally evoked muscle contraction

Time Lapse in Vivo Visualization of Developmental Stabilization of Synaptic Receptors at Neuromuscular Junctions

The lifetime of nicotinic acetylcholine receptors (AChRs) in neuromuscular junctions (NMJs) is increased from <1 day to >1 week during early postnatal development. However, the exact timing of AChR stabilization is not known, and its correlation to the concurrent embryonic to adult AChR channel conversion, NMJ remodeling, and neuromuscular diseases is unclear. Using a novel time lapse in vivo imaging technology we show that replacement of the entire receptor population of an individual NMJ occurs end plate-specifically within hours. This makes it possible to follow directly in live animals changing stabilities of end plate receptors. In three different, genetically modified mouse models we demonstrate that the metabolic half-life values of synaptic AChRs increase from a few hours to several days after postnatal day 6. Developmental stabilization is independent of receptor subtype and apparently regulated by an intrinsic muscle-specific maturation program. Myosin Va, an F-actin-dependent motor protein, is also accumulated synaptically during postnatal development and thus could mediate the stabilization of end plate AChR