Distinct RGK GTPases differentially use α1- and auxiliary β-binding-dependent mechanisms to inhibit CaV1.2/CaV2.2 channels

Ca(V)1/Ca(V)2 channels, comprised of pore-forming α(1) and auxiliary (β,α(2)δ) subunits, control diverse biological responses in excitable cells. Molecules blocking Ca(V)1/Ca(V)2 channel currents (I(Ca)) profoundly regulate physiology and have many therapeutic applications. Rad/Rem/Rem2/Gem GTPases (RGKs) strongly inhibit Ca(V)1/Ca(V)2 channels. Understanding how RGKs block I(Ca) is critical for insights into their physiological function, and may provide design principles for developing novel Ca(V)1/Ca(V)2 channel inhibitors. The RGK binding sites within Ca(V)1/Ca(V)2 channel complexes responsible for I(Ca) inhibition are ambiguous, and it is unclear whether there are mechanistic differences among distinct RGKs. All RGKs bind β subunits, but it is unknown if and how this interaction contributes to I(Ca) inhibition. We investigated the role of RGK/β interaction in Rem inhibition of recombinant Ca(V)1.2 channels, using a mutated β (β(2aTM)) selectively lacking RGK binding. Rem blocked β(2aTM)-reconstituted channels (74% inhibition) less potently than channels containing wild-type β(2a) (96% inhibition), suggesting the prevalence of both β-binding-dependent and independent modes of inhibition. Two mechanistic signatures of Rem inhibition of Ca(V)1.2 channels (decreased channel surface density and open probability), but not a third (reduced maximal gating charge), depended on Rem binding to β. We identified a novel Rem binding site in Ca(V)1.2 α(1C) N-terminus that mediated β-binding-independent inhibition. The Ca(V)2.2 α(1B) subunit lacks the Rem binding site in the N-terminus and displays a solely β-binding-dependent form of channel inhibition. Finally, we discovered an unexpected functional dichotomy amongst distinct RGKs- while Rem and Rad use both β-binding-dependent and independent mechanisms, Gem and Rem2 use only a β-binding-dependent method to inhibit Ca(V)1.2 channels. The results provide new mechanistic perspectives, and reveal unexpected variations in determinants, underlying inhibition of Ca(V)1.2/Ca(V)2.2 channels by distinct RGK GTPases.     

Immunocytochemical localization of methionine enkephalin: preliminary observations.

Antiserum directed against methionine enkephalin (metenkephalin) was used to determine its anatomical distribution in rat brain. Cross reactivity of that antiserum was not detectable against leucine enkephalin (leu-enkephalin), β-lipotropin (β-LPH), β-endorphin or assorted peptide fragments of met-enkephalin; alpha-endorphin was 370 times less active than met-enkephalin. The localization of met-enkephalin was carried out in the presence of excess leu-enkephalin and yet could be blocked with equal amounts of met-enkephalin. Met-enkephalin was detected in several structures in the spinal cord, medulla, pons, mesencephalon, diencephalon and telencephalon. No met-enkephalin was detected in cerebellum or cerebral cortex.     

Pulmonary complications of the acquired immunodeficiency syndrome

Cardiac transplantation has evolved from an experiment to an accepted therapy for severe heart failure. Increasing competition for donor organs mandates a greater emphasis on selection and timing for transplantation and paradoxically forces more reliance on aggressive medical therapy for all patients after evaluation. The growth of recipient and donor pools may enhance the opportunity for assessing histocompatibility, for which distinguishing between autoantibodies and human leukocyte antigen-determined reactivity is important, and some general nonresponders may be detected. Therapy with cyclosporine has improved the outcome after transplantation, but further refinement is needed, perhaps with pharmacologic synergy, to minimize nephrotoxicity and maximize specific immunosuppression. Survival is more than 80% at 1 year, after which the incidence of acute rejection and infection declines and accelerated atherosclerosis becomes prominent. Although resuming employment is not always possible, the overall quality of life is excellent after cardiac transplantation.     

Selective Alterations of Opiate Receptor Subtypes in Mono sodium Glutamate-Treated Rats

Neonatal treatment of rats with monosodium glutamate (MSG) has been demonstrated to destroy cell bodies of neurons in the arcuate nucleus including the brain beta-endorphin (B-END) system. The effects on opiate receptors of the loss of B-END is unknown. Neonatal rats were treated with MSG as previously described. After reaching maturity (7-9 months), MSG-treated rats and litter-matched untreated control rats were decapitated and brains dissected into brain regions. Opiate receptor assays were run with [3H]morphine (mu receptor ligand) and [3H]D-alanine2-D-leucine5 (DADL) enkephalin (delta receptor ligand) for each brain region for both MSG and control rats simultaneously. Scatchard plot analyses showed a selective increase in delta receptors in the thalamus only. No corresponding change in mu receptors in the thalamus was found. The cross-competition IC50 data supported this conclusion, showing a loss in the potency of morphine in displacing [3H]DADL enkephalin in the thalamus of MSG-treated rats. This shift in delta receptors produced an IC50 displacement pattern in thalamus, ordinarily a mu-rich area, similar to that of striatum or cortex, delta-rich areas, again indicating an increase in delta receptors. Similar changes in delta receptors in other brain regions were not found. These results represent one of the few examples of a selective and localized shift in delta with no change in mu sites. Furthermore, the delta increase may reflect an up-regulation of the receptors in thalamus after chronic loss of the endogenous opioid B-END.     

The signal peptide of pro-opiomelancortin: validation of a specific radioimmunoassay

The N-terminus portion of the POMC leader sequence (signal peptide) was synthesized, and an antiserum was raised against it. A radioimmunoassay was developed which is effective at a dilution of 1:500,000, and sensitive at less than 1 fmole/tube. Since leader sequences often exhibit structural homologies, and since synthetic peptides are not readily available, we resorted to an unusual procedure to establish specificity. This involved extraction of pituitary RNA, cell-free translation to produce the pre-prohormones, and purification by B-END and signal antibody affinity columns. The eluates were then tested by SDS gel electrophoresis and by multiple immunoprecipitations. All results showed that the signal antibody captured a single molecular species, approximately 30,000 in MW, which was also captured by the B-END column, and was immunoprecipitable by B-END and ACTH antisera. It therefore appears that this antibody selectively measures the POMC leader sequence and should be valuable in measuring the newly synthesized pre-prohormone.     

Enkephalinase: Selective inhibitors and partial characterization

There are at least two types of enzymes in brain, endopeptidases and aminopeptidases, which metabolize enkephalins. Evidence is presented to suggest that enkephalinase, an endopeptidase cleaving at the Gly-Phe bond, is specific for the endogenous enkephalinergic system. Selective inhibitors are described for each enzyme. These are parachloromercuriphenylsulfonic acid and puromycin in the case of aminopeptidases and various enkephalin fragments in the case of enkephalinase. Some characteristics of the two types of enzymes are described. Enkephalinase has many properties in common with the well-characterized brain angiotensin-converting enzyme. These two enzymes, however, behaved differently when tested for chloride dependance, for activity in several buffers and for susceptibility to specific inhibitors.     

Hydrophobic Residues of the D2 Dopamine Receptor Are Important for Binding and Signal Transduction

Abstract: Dopamine receptors belong to the seven transmembrane helix-containing, G protein-coupled receptor superfamily. Mutagenesis studies suggest that dopamine and its analogues interact with aspartate-114 in helix 3 and two helix 5 serines (194 and 197) of the D₂ receptor. In addition to these amino acids, hydrophobic residues within the receptor core may be important not only for binding but also for receptor activation. Described is a site-directed mutagenesis investigation into the roles of these hydrophobic residues in the long isoform of the human D₂ receptor. Replacement of helix 6 phenylalanines (389 or 390) with alanines resulted in disrupted binding to several agonists and antagonists and impaired inhibition of adenylyl cyclase activity. Replacement of the helix 5 phenylalanine-198 with an alanine selectively disrupted [³H]N-0437 binding, whereas the affinities for other agonists and antagonists remained unchanged. This mutant remained functionally intact when stimulated with dopamine or bromocriptine. Replacement of the helix 7 phenylalanine-411 or the helix 6 leucine-387 with alanines produced receptors that bound agonists well but were unable to inhibit adenylyl cyclase. Based on these data, two conserved helix 6 phenylalanines (389 and 390) appear to be crucial for ligand binding, and phenylalanine-411 in helix 7 and leucine-387 in helix 6 may be important for propagating conformational changes from the agonist binding site(s) to G protein coupling domain(s) of the D₂ receptor.     

Effects of Chronic Morphine Treatment on β-Endorphin-Related Peptides in the Caudal Medulla and Spinal Cord

Abstract: The effects of chronic morphine treatment on β-endorphin (βE)-immunoreactive (βE-ir) peptide levels were determined in the rat caudal medulla and different areas of the spinal cord. Seven days of morphine pelleting had no effect on total βE-ir peptides in the caudal medulla. In contrast, it significantly increased βE-ir peptide concentrations in the cervical and thoracic regions of the spinal cord compared with placebo-pelleted controls, whereas in the lumbosacral region this trend did not reach statistical significance. Injections of the opiate receptor antagonist naloxone 1 h before the rats were killed had no effect on the morphine-induced increases in the cord. Chromatographic analyses revealed that enzymatic processing of βE-related peptides in the spinal cord seemed unaffected by the morphine and/or naloxone treatments. In light of previous data showing that morphine down-regulates βE biosynthesis in the hypothalamus, the present results suggest that the regulation of βE-ir peptides in the spinal cord is distinct from that found in other CNS areas. These data provide support for previous results suggesting that βE-expressing neurons may be intrinsic to the spinal cord.