Seibutsu Butsuri

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A brief account of structure and function including remaining research subjects of ion channels, which are the key molecules for understanding brain and neural functions, are given in this review. In specific terms, structure and function of five types of channels are summarized. The mechanism of selective ion permeation of larger K⁺ ion faster than smaller Na⁺ ion through K⁺ channel, which had been long puzzling question in electrophysiology, was revealed but it is still unclear that other ion selectivity could be explained by same mechanism. Structure of acetylcholine receptor in a condition similar with native membrane was analyzed but detailed gating mechanism is expected to be analyzed by Freeze-trapping method. Historically important structure analysis of glutamate receptor was achieved and the analysis revealed importance of asymmetric structure of the homo-tetramer. A plug model, which might be crucially important for gating mechanism of gap junction channel, was proposed based on structure analysis of connexin. Structure of water channel expressed in brain was analyzed but its functional studies are expected.

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The human ability to sense gravity and sounds relies on specialized vestibular and auditory organs, respectively, in our inner ear. In the fly, the ability to hear has been ascribed to the antenna, whereas the gravity sensor had remained unidentified. We found that the fly has implemented both sensory modalities into a single system, the Johnston’s organ, which houses specialized clusters of mechanosensory neurons. Each cluster monitors specific movements of the antenna and feeds into distinct neural pathways that are reminiscent of the vestibular and auditory pathways, respectively, in our brain.

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Atmospheric oxygen is generated by water oxidation at the MnCa cluster of the protein-pigment complex, photosystem II located in the thylakoid membrane. At the heart of photosynthetic reaction centers (RCs) are pairs of chlorophylla, P680 in photosystem II and P700 in photosystem I of cyanobacteria, algae, or plants, and a pair of bacteriochlorophylla, P870 in purple bacterial RCs. These pairs differ greatly in their redox potentials for one-electron oxidation, E_m. For P680, E_m is 1100-1200 mV, but for P700 and P870, E_m is only 500 mV. Calculations with the linearized Poisson-Boltzmann equation reproduce these measured E_m differences successfully.

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There exist several cross talks between histamine H1 receptor and other types of receptor, especially in their effects on H1 receptor expression. Muscarinic, IL-4, and H1 receptor stimulation increases (up-regulates) H1 receptor level through the activation of H1 receptor gene transcription, whereas β2-adrenergic receptor stimulation decreases (down-regulates) H1 receptors through degradation of H1 receptor proteins and the inhibition of H1 receptor gene transcription. This review describes the significance of these cross talks in regulating the H1 receptor level and discusses their physiological and pathophysiological roles.

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