Activity-dependent expression of NT-3 in muscle cells in culture: implications in the development of neuromuscular junctions

Although activity-dependent expression of neurotrophins has been studied extensively in the CNS, its physiological role during synapse development is not well established. At the developing neuromuscular junction in culture, exogenous application of the neurotrophin BDNF or NT-3 has been shown to acutely potentiate synaptic transmission and chronically promote synapse maturation. Using the same cell culture model, we have investigated activity-dependent neurotrophin expression in muscle cells and its role in developing neuromuscular synapses. Membrane depolarization, elicited by either depolarizing agents or repetitive electric stimulation, rapidly and specifically increased the levels of NT-3 mRNA in developing Xenopus laevis muscle cells in culture. NT-3 gene expression also was enhanced by acetylcholine (ACh), the neurotransmitter that causes muscle membrane depolarization. The effects of depolarization were mediated by increasing intracellular calcium concentration. Moreover, factor(s) induced by membrane depolarization appeared to enhance synaptic transmission at the developing neuromuscular junction. The frequency of spontaneous synaptic currents (SSCs) recorded from neuromuscular synapses was increased significantly after treatment with conditioned medium from depolarized muscle cultures. The amplitude, rise time, and decay time of SSCs were not affected, indicating a presynaptic action of the conditioned medium. The effects of the conditioned medium were blocked, partially, by the NT-3 scavenger TrkC-IgG, suggesting that the potentiation of synaptic efficacy was attributable, at least in part, to elevated NT-3 as a consequence of muscle depolarization. Thus, activity-dependent expression of muscle NT-3 may contribute to the development of the neuromuscular synapse.

Expression of the GABAA receptor delta subunit is selectively modulated by depolarization in cultured rat cerebellar granule neurons

The levels of several GABAA receptor subunit mRNAs increase as cerebellar granule neurons migrate to their adult positions and receive excitatory mossy fiber input. Despite the temporal similarity of these increases in transcript expression in vivo, studies in cultured granule neurons demonstrated that the subunit mRNAs are differentially regulated. To address the possibility that neuronal activity regulates transcript expression, GABAA receptor subunit mRNA levels were assessed in cultured granule neurons grown in chemically defined, serum-free medium containing either nondepolarizing (5 mM) or depolarizing (25 mM) KCl concentrations. Whereas the delta subunit mRNA was almost undetectable in cultures maintained in nondepolarizing medium, an eightfold increase occurred between days 2 and 4 in cultures grown in depolarizing medium. Furthermore, delta subunit transcript expression was reduced by 76 +/- 6% when neurons in depolarizing medium were switched into nondepolarizing medium. The importance of depolarization in the initiation and maintenance of subunit transcript expression in granule neurons was selective for the GABAA receptor delta subunit. These changes in transcript expression involved calcium entry through L-type calcium channels. Nifedipine treatment (1 microM) both reduced intracellular calcium and decreased delta subunit mRNA expression by 79 +/- 4%. Furthermore, inhibition of Ca2+/calmodulin-dependent protein kinases (CaM kinases) by KN-62 (1 microM) also reduced delta subunit transcript expression. These studies demonstrate that KCl-induced depolarization, a condition that mimics the effects of neuronal activity, selectively modulates GABAA receptor delta subunit mRNA expression through a pathway involving calcium entry and activation of a CaM kinase.