Intracellular Ca²⁺ stores modulate SOCCs and NMDA receptors via tyrosine kinases in rat hippocampal neurons

The regulation of intracellular Ca²⁺ signalling by phosphorylation processes remains poorly defined, particularly with regards to tyrosine phosphorylation. Evidence from non-excitable cells implicates tyrosine phosphorylation in the activation of so-called store-operated Ca²⁺ channels (SOCCs), but their involvement in neuronal Ca²⁺ signalling is still elusive. In the present study, we determined the role of protein tyrosine kinases (PTKs) and tyrosine phosphatases (PTPs) in the coupling between intracellular Ca²⁺ stores and SOCCs in neonatal rat hippocampal neurons by Fura-2 Ca²⁺ imaging. An early Ca²⁺ response from intracellular stores was triggered with thapsigargin, and followed by a secondary plasma membrane Ca²⁺ response. This phase was blocked by the non-specific Ca²⁺ channel blocker NiCl and the SOCC blocker, 2-aminoethoxydiphenyl borate (2-APB). Interestingly, two structurally distinct PTK inhibitors, genistein and AG126, also inhibited this secondary response. Application of the PTP inhibitor sodium orthovanadate (OV) also activated a sustained and tyrosine kinase dependent Ca²⁺ response, blocked by NiCl and 2-APB. In addition, OV resulted in a Ca²⁺ store dependent enhancement of NMDA responses, corresponding to, and occluding the signalling pathway for group I metabotropic glutamate receptors (mGluRs). This study provides first evidence for tyrosine based phospho-regulation of SOCCs and NMDA signalling in neurons.