Successful synaptic transmission at the neuromuscular junction depends on the precise alignment of the nerve terminals with the postsynaptic specialization of the muscle fiber. It is increasingly apparent that this precision is achieved during development and maintained in the adult through signals exchanged between motoneurons and their target muscle fibers that serve to coordinate their spatial and temporal differentiation. Several aspects of neuronal differentiation appear to be dependent on retrograde signals from the target and studies about synaptic modulation have now focused attention on the characterization of proteins that mediate retrograde signals regulating the organization and function of nerve terminals. According to the published evidences, we find Insulin-like growth factor-I (IGF-I ) might be one of these potential factors.
The acute application of IGF-I, a factor which has been addressed to widely express in developing myocyte, dose-dependently enhances the spontaneous acetylcholine secretion at developing neuromuscular synapses in Xenopus cell culture using whole-cell patch clamp recording. The IGF-I-induced potentiating effect is not abolished when calcium is eliminated from culture medium or bath application of pharmacological calcium channel blocker cadmium, indicating calcium influx through voltage-activated calcium channels are not required. We further define the roles of intracellular Ca2+ stores in IGF-I-induced synaptic potentiation. To approach this problem, Ca2+-ATPase inhibitor thapsigargin were initially used to deplete internal Ca2+ stores. IGF-I no longer elicited any changes in SSC frequency in thapsigargin-treated synapses suggesting that an increase in [Ca2+]i due to Ca2+ release from intracellular Ca2+ stores may contribute to the facilitation of transmitter release induced by IGF-I. Application of membrane-permeable inhibitors of IP3-induced Ca2+ release 2-aminoethoxydiphenyl borate (2-APB) or Xestospongin C (XeC) effectively occluded the increase of SSC frequency elicited by IGF-I. Furthermore, pretreatment of the cultures with ryanodine receptor antagonist 8-(dethylamino) octyl 3, 4, 5-trimethoxybenzoate (TMB-8) also blocked the IGF-I effects indicating that IGF-I activates IP3 and/or ryanodine pathway to initiate calcium release from intracellular stores which subsequently potentiate transmitter release. Treating cells with inhibitors of phosphoinositide-3 kinase (wortmannin and LY294002) and Phospholipase C-g (U73122), but not inhibitor of MAP kinase (PD98059) abolishes IGF-1-induced potentiation of synaptic transmission. Inhibition of Ca2+/calmodulin-dependent protein kinase II (CaMKII) by KN-62 effectively blocks the effect of IGF-I. Taken collectively, our results obtained suggest that IGF-I potentiates neurotransmitter secretion by stimulating Ca2+ release from IP3 and ryanodine sensitive intracellular calcium stores via activate PI3 and/or PLC-g signaling cascades, which leading to an activation of CaMKII-dependent transmitter release.
| Identifer | oai:union.ndltd.org:NSYSU/oai:NSYSU:etd-0709102-225241 |
| Date | 09 July 2002 |
| Creators | Tsai, Feng-Ru |
| Contributors | Jau-Cheng Liou, none, Hung-Tu Huang, wen-mei Fu |
| Publisher | NSYSU |
| Source Sets | NSYSU Electronic Thesis and Dissertation Archive |
| Language | Cholon |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.lib.nsysu.edu.tw/ETD-db/ETD-search/view_etd?URN=etd-0709102-225241 |
| Rights | off_campus_withheld, Copyright information available at source archive |
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