The ryanodine receptors (RyRs) are high-conductance intracellular Ca2+ channels that play a pivotal role in the excitation-contraction coupling of skeletal and cardiac muscles. RyRs are the largest known ion channels, with a homotetrameric organization and approximately 5000 residues in each protomer. Here we report the structure of the rabbit RyR1 in complex with its modulator FKBP12 at an overall resolution of 3.8 Å, determined by single-particle electron cryo-microscopy. Three previously uncharacterized domains, named Central, Handle, and Helical domains, display the armadillo repeat fold. These domains, together with the amino-terminal domain, constitute a network of superhelical scaffold for binding and propagation of conformational changes. The channel domain exhibits the voltage-gated ion channel superfamily fold with distinct features. A negative charge-enriched hairpin loop connecting S5 and the pore helix is positioned above the entrance to the selectivity filter vestibule. The four elongated S6 segments form a right-handed helical bundle that closes the pore at the cytoplasmic border of the membrane. Allosteric regulation of the pore by the cytoplasmic domains is mediated through extensive interactions between the Central domains and the channel domain. These structural features explain high ion conductance by RyRs and the long-range allosteric regulation of channel activities.

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An association between biochemical and biomechanical functional status and MR imaging parameters of articular cartilage was demonstrated. Linear correlations existed between modulus and proteoglycan content in terms of T2. Additionally, proteoglycan loss and collagen loss had differing effects on gadolinium-enhanced T1 when it was expressed as the ratio of T1 after gadolinium enhancement/T1 before gadolinium enhancement.

Gonadotrophin-releasing hormone (GnRH) neurones represent the final output neurones in the neuroendocrine control of reproduction, and gamma-amino butyric acid (GABA) is one of the major players in the regulation of GnRH neurones. GABA inhibits a large proportion of brain neurones in adult animals by acting on A-type GABA receptors (GABA(A)Rs). Two contradictory reports on the action of GABA in the GnRH neurones of adult mice have been published. DeFazio et al. (Mol Endocrinol 2002; 16: 2872) demonstrated that activation of GABA(A)Rs excites the GnRH neurones of adult mice, whereas Han et al. (Endocrinology 2002; 143: 1459) showed that the response to GABA on GnRH neurones switches from depolarisation to hyperpolarisation around puberty in female mice. Therefore, we examined the reversal potential of GABA(A)R currents by means of perforated patch-clamp recording with gramicidin in overnight-cultured GnRH neurones isolated from adult GnRH-enhanced green fluorescent protein transgenic rats. The reversal potential was -26 +/- 1.4 mV (mean +/- SEM, n = 42) in GnRH neurones, whereas it was -57 +/- 2.7 mV (n = 34) in unidentified neurones, and GABA depolarised the GnRH neurones in current-clamp condition. The GABA(A)R currents in rat GnRH neurones were augmented by neurosteroids, allopregnanolone and 3 alpha,21-dihydroxy-5 alpha-pregnan-20-one, at submicromolar concentrations. In addition, the expression patterns of GABA(A)R subunit mRNAs were determined by multi-cell reverse transcription-polymerase chain reaction, which revealed that the alpha2, beta 3, gamma 1 and gamma 2 subunits were dominant and the alpha 6 and gamma 3 subunits were negative in rat GnRH neurones. These results indicate that GABA(A)Rs in the soma of rat GnRH neurones are comprised mainly of alpha2, beta 3 and gamma 1 or gamma 2 subunits and that they are sensitive to neurosteroids; moreover, they suggest that activation of these receptors depolarises GnRH neurones. Thus, GABA and neurosteroids influence the electrical activity of GnRH neurones.

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