Background: A number of studies have separately shown that the neuregulin1 (NRG1)/ErbB4 system and NMDA-type glutamate receptors (NMDARs) are involved in several aspects of neuronal migration. In addition, intracellular calcium fluctuations play central roles in neuronal motility. Stable expression of the tyrosine kinase receptor ErbB4 promotes migratory activity in the neural progenitor cell line ST14A upon NRG1 stimulation. In this work we analyzed the potential interactions between the NRG1/ErbB4 system and NMDARs in the ST14A migratory process as well as its calcium dependence.
Results: RT-PCR studies have shown that both native ST14A cells (non-expressing ErbB4), as well as ErbB4-transfected cells express low levels of a restricted number of NMDAR subunits: NR1, NR2C, NR2D and NR3B. The resulting NMDAR would form Ca(2+) channels characterized by low Mg(2+)-sensitivity and low Ca(2+)-permeability, generating small, long-lasting currents. Ca(2+)-imaging experiments showed slow [Ca(2+)](i) increases in 45% of the cells following 8 μM NMDA stimulation. Basal migration of ErbB4-transfected ST14A cells was unaffected by 18 hrs NMDA incubation. However, over the same incubation time, NMDA was able to significantly enhance NRG1-induced migration. Pre-incubation with the intracellular calcium chelator BAPTA-AM reduced both NRG1- and NRG1/NMDA-stimulated migration, suggesting the involvement of Ca(2+) in these processes. NRG1 stimulation of ErbB4-transfected ST14A cells induced a sustained, long-lasting increase in [Ca(2+)](i), in 99% of the cells. These intracellular Ca(2+) signals could be ascribed to both release from intracellular stores and influx from the extracellular medium trough a mechanism of store-operated calcium entry (SOCE). Short-time co-incubation of NMDA and NRG1 did not substantially modify the NRG1-induced intracellular calcium signals.
Conclusions: In summary, NRG1 stimulation of the ErbB4 receptor exerts a sustained [Ca(2+)](i) increase in ST14A neural progenitors; NRG1-induced migration is Ca(2+)-dependent and can be positively modulated by activation of the NMDA receptor.