TrkB Activation by Brain-derived Neurotrophic Factor Inhibits the G Protein-gated Inward Rectifier Kir3 by Tyrosine Phosphorylation of the Channel

Abstract

G protein-activated inwardly rectifying potassium channels (Kir3) are widely expressed throughout the brain, and regulation of their activity modifies neuronal excitability and synaptic transmission. In this study, we show that the neurotrophin brain-derived neurotrophic factor (BDNF), through activation of TrkB receptors, strongly inhibited the basal activity of Kir3. This inhibition was subunit dependent as functional homomeric channels of either Kir3.1 or Kir3.4 were significantly inhibited, whereas homomeric channels composed of Kir3.2 were insensitive. The general tyrosine kinase inhibitors genistein, Gö 6976, and K252a but not the serine/threonine kinase inhibitor staurosporine blocked the BDNF-induced inhibition of the channel. BDNF was also found to directly stimulate channel phosphorylation because Kir3.1 immunoprecipitated from BDNFstimulated cells showed enhanced labeling by anti-phosphotyrosine-specific antibodies. The BDNF effect required specific tyrosine residues in the amino terminus of Kir3.1 and Kir3.4 channels. Mutations of either Tyr-12, Tyr-67, or both in Kir3.1 or mutation of either Tyr-32, Tyr-53, or both of Kir3.4 channels to phenylalanine significantly blocked the BDNF-induced inhibition. The insensitive Kir3.2 was made sensitive to BDNF by adding a tyrosine (D41Y) and a lysine (P32K) upstream to generate a phosphorylation site motif analogous to that present in Kir3.4. These results suggest that neurotrophin activation of TrkB receptors may physiologically control neuronal excitability by direct tyrosine phosphorylation of the Kir3.1 and Kir3.4 subunits of G protein-gated inwardly rectifying potassium channels.Neurotrophins are a family of growth factors that include nerve growth factor, BDNF, 1 NT3, and NT-4/5 (1) and activate receptor tyrosine kinases (Trk) to regulate neuronal survival and differentiation during brain development (2). Neurotrophins also rapidly modulate neuronal excitability to regulate synaptic plasticity in the hippocampus (3-7), plasticity of spinal cord neurons in models of chronic pain (8), and excitability of cortical neurons (9). The mechanisms of these neuronal effects on excitability are not yet known; however, BDNF was shown to rapidly modulate sodium channels in the CA1 region of the hippocampus (3) and to enhance synaptic currents in hippocampal postsynaptic neurons (6). These studies suggest that BDNF has direct effects on ion channel properties to modulate synaptic activity.The neurotrophin receptors are transmembrane tyrosine kinases, and BDNF activation of the TrkB receptor is known to initiate a cascade of phosphorylation events that activate a complex of signaling proteins (10). Tyrosine kinases directly phosphorylate ion channels to provide rapid regulation of neuronal excitability (11-18). Tyrosine kinase activation by G protein-coupled receptors (19) also suppresses delayed rectifying potassium channels by phosphorylation of a tyrosine residue in the amino terminus of Kv1.2 (20). Similarly, phosphorylation of serine residues in the amino terminus...

Structural elements in the Girk1 subunit that potentiate G protein–gated potassium channel activity

“…Although the precise structural impact of the phenylalanine substitution is unknown, manipulations at this site influence diverse channel properties. Indeed, an S148T substitution in Girk2 yields highly active homomeric channels (20). Perhaps more surprisingly, Girk1(F137S) homomers reach the cell surface and are functional, despite the lack of an endoplasmic reticulum export signal that precludes surface trafficking of Girk1 (21).…”

Brain-Derived Neurotrophic Factor Triggers Transcription-Dependent, Late Phase Long-Term PotentiationIn Vivo

“…Although presynaptic fiber excitability is not affected during HFS-LTP, this has not been investigated in BDNF-LTP (Bliss and Lomo, 1973). BDNF regulation of presynaptic ion channel function is clearly a possibility, because BDNF has been shown to modulate a voltagedependent sodium channel (Kafitz et al, 1999) and the potassiumchannel Kir3 (Rogalski et al, 2000). We therefore determined the minimum stimulus current needed to consistently (in Ͼ90% of trials) evoke an fEPSP.…”

N-terminal Tyrosine Residues within the Potassium Channel Kir3 Modulate GTPase Activity of Gαi

“…The activity of this channel is controlled by a large number of modulators including phosphatidylinositol bisphosphate, Na ϩ , eicosanoids, ATP, Mg 2ϩ , and phosphorylation (1)(2)(3)(4)(5)(6). For example, in a previous study (7), we found that tyrosine phosphorylation of Kir3 resulted in channel inhibition. Brain-derived neurotrophic factor (BDNF) activation of trkB receptors caused the phosphorylation of specific tyrosine residues in the N-terminal domain of Kir3.1 and Kir3.4, reducing basal channel conductance.…”