Several studies in term and pre-term infants have investigated the rhythmic pattern of non-nutritive sucking (NNS) indicating correlations between the quantitative measures derived from sucking pressure variation and/or electromyographic (EMG) recordings and a range of factors that include age, perinatal stress and sequelae. In the human fetus, NNS has been reported from 13 weeks of gestation and has been studied using real-time Doppler ultrasonography exclusively. The present study indicates that NNS in fetus can be reliably recorded and quantified using non-invasive biomagnetic measurements that have been recently introduced as an investigational tool for the assessment of fetal neurophysiologic development. We show that source separation techniques, such as independent component analysis, applied to the high-resolution multichannel recordings allow the segregation of an explicit waveform that represents the biomagnetic equivalent of the ororhythmic sucking pressure variation or EMG signal recorded in infants. This enables the morphological study of NNS patterning over different temporal scales, from the global quantitative measures to the within burst fine structure characterization, in correlation with the fetal cardiac rhythm.
Coupling between G-protein-coupled receptors (GPCRs) and the G proteins is a key step in cellular signaling. Despite extensive experimental and computational studies, the mechanism of specific GPCR-G protein coupling remains poorly understood. This has greatly hindered effective drug design of GPCRs that are primary targets of ~1/3 of currently marketed drugs. Here, we have employed all-atom molecular simulations using a robust Gaussian accelerated molecular dynamics (GaMD) method to decipher the mechanism of the GPCR-G protein interactions. Adenosine receptors (ARs) were used as model systems based on very recently determined cryo-EM structures of the A1AR and A2AAR coupled with the Gi and Gs proteins, respectively. Changing the Gi protein to the Gs led to increased fluctuations in the A1AR and agonist adenosine (ADO), while agonist 5'-N-ethylcarboxamidoadenosine (NECA) binding in the A2AAR could be still stabilized upon changing the Gs protein to the Gi. Free energy calculations identified one stable low-energy conformation for each of the ADO-A1AR-Gi and NECA-A2AAR-Gs complexes as in the cryo-EM structures, similarly for the NECA-A2AAR-Gi complex. In contrast, the ADO agonist and Gs protein sampled multiple conformations in the ADO-A1AR-Gs system. GaMD simulations thus indicated that the ADO-bound A1AR preferred to couple with the Gi protein to the Gs, while the A2AAR could couple with both the Gs and Gi proteins, being highly consistent with experimental findings of the ARs. More importantly, detailed analysis of the atomic simulations showed that the specific AR-G protein coupling resulted from remarkably complementary residue interactions at the protein interface, involving mainly the receptor transmembrane 6 helix and the Gα α5 helix and α4-β6 loop. In summary, the GaMD simulations have provided unprecedented insights into the dynamic mechanism of specific GPCR-G protein interactions at an atomistic level, which is expected to facilitate future drug design efforts of the GPCRs.
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