BDNF‐dependent phrenic motor facilitation: role of protein synthesis

Abstract

Moderate acute intermittent hypoxia (mAIH) induces a form of respiratory motor plasticity known as phrenic long‐term facilitation (pLTF) — a persistent increase in inspiratory phrenic motor output that requires cervical spinal serotonin receptor activation and new synthesis of brain‐derived neurotrophic factor (BDNF). One fundamental difference between short‐term (minutes) and long‐term (days) synaptic plasticity is the reliance on transcription versus translation of genes required for protein synthesis. Although it is known that mAIH‐induced pLTF requires new synthesis of BDNF protein (Baker‐Herman et al. 2004), it is unknown if this form of plasticity also requires gene transcription. In turn, neurotrophins such as BDNF regulate synaptic transmission via rapid protein synthesis‐independent versus delayed protein synthesis‐dependent mechanisms. Although BDNF protein synthesis is necessary for mAIH‐induced pLTF, and cervical spinal application of exogenous BDNF is sufficient to elicit phrenic motor facilitation, it is unknown if phrenic motor facilitation requires new‐protein synthesis downstream from BDNF/TrkB signaling. Thus, we tested the hypotheses that: 1) mAIH‐induced pLTF requires transcriptional regulation of new protein synthesis, and 2) if translational regulation of new protein synthesis is necessary for BDNF‐induced phrenic motor facilitation. Experiments were performed on anesthetized, mechanically ventilated adult male Sprague Dawley rats. To inhibit spinal transcription, rats were pre‐treated with cervical spinal intrathecal injections of the transcription inhibitor triptolide, and then exposed to mAIH (3, 5‐min hypoxic episodes, PaO2 35–55 mmHg; 5 min intervals). To explore the requirement for new protein synthesis downstream from BDNF/TrkB signaling, a widely used translation inhibitor (emetine), was injected into the intrathecal space prior to BDNF injections. Intrathecal cervical spinal triptolide injections had no discernible effect on mAIH‐induced pLTF, demonstrating that pLTF does not require gene transcription. On the other hand, pre‐treatment with emetine blocked mAIH‐induced pLTF, confirming that pLTF requires new protein synthesis. Whereas cervical spinal BDNF injections induced phrenic motor facilitation similar to pLTF, its magnitude was unaffected by emetine pre‐treatment. Thus, BDNF/TrkB elicits phrenic motor facilitation via a mechanism that does not require additional protein synthesis. Collectively, our findings suggest that pLTF requires new synthesis of BDNF protein via translation‐dependent, transcription‐independent mechanisms, whereas additional new protein synthesis is not required downstream from BDNF/TrkB signaling. Thus, BDNF‐induced phrenic motor facilitation likely occurs exclusively via covalent modifications of existing proteins, presumably phosphorylation events mediated via BDNF/TrkB dependent activation of PKC‐q (Agosto‐Marlin & Mitchell, 2017).Support or Funding InformationNIH HL69064, HL111598 and the UF McKnight Brain InstituteThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.