Separation of self from non-self in the complement system

The alternative complement pathway is a self-contained and independent recognition and effector pathway that evolved to protect the host from microbes. As such, it must separate self from non-self. Via low grade continuous turnover (tickover) of the pivotal C3 component, the alternative complement pathway is always on guard to defend the host. Activated C3 binds continuously to self tissue and to foreign tissue, if present. There is no apparent discrimination at this initiation step. However, the amplification of C3 deposition on self (but not foreign) tissue, a necessity in establishing the effector functions of this pathway, is inhibited by a series of functionally, structurally and genetically related plasma and membrane glycoproteins which down-regulate complement activation. These regulatory molecules are widely distributed on human tissue. The plasma proteins are preferentially active on fluid-phase components while membrane-bound forms act on cell-bound components. Here, John Atkinson and Timothy Farries discuss these inhibitors of complement activation and suggest that their action explains the ability of the alternative pathway to amplify on foreign tissue but be down-regulated on autologous tissue.

Subtype-selective positive cooperative interactions between brucine analogs and acetylcholine at muscarinic receptors: functional studies

In radioligand binding studies, it has been reported that brucine, N-chloromethyl brucine, and brucine N-oxide increased the affinity of acetylcholine for M1, M3, and M4 muscarinic receptors, respectively, in a manner consistent with the predictions of the ternary complex allosteric model. We now demonstrate an equivalent ability of these three allosteric agents to modulate the actions of acetylcholine in functional studies in membranes and in whole cells. The enhancing actions of brucine and brucine N-oxide on acetylcholine (ACh) potency at M1 and M4 receptors respectively have been confirmed in guanosine-5'-O-(3-[35S]thio)triphosphate, GTPase, cAMP, and intracellular Ca2+ mobilization assays of function. In general, neither the basal nor the maximally stimulated response to ACh is affected. The subtype-selective allosteric effects of N-chloromethyl brucine on M2 and M3 receptors were shown to be qualitatively and quantitatively the same in guanosine-5'-O-(3-[35S]thio)triphosphate functional assays, in terms of both its affinity and cooperativity with ACh, as those found in binding assays. Neutral cooperativity of N-chloromethyl brucine with ACh on M4 receptor function was also observed, thereby demonstrating its "absolute subtype selectivity": a lack of action at any concentration at M4 receptors and an action at M2 and M3 receptors. The enhancing action of N-chloromethyl brucine on neurogenically released ACh binding at M3 receptors was also detected in whole tissue as an increased contraction of the isolated guinea pig ileum to submaximal electrical stimulation. In conclusion, these functional studies confirm that brucine analogs are allosteric enhancers of ACh affinity at certain muscarinic receptor subtypes.

Selective allosteric enhancement of the binding and actions of acetylcholine at muscarinic receptor subtypes

The ternary allosteric model predicts the possibility of discovering molecules with novel and highly subtype-selective modes of action. This approach has been applied to muscarinic receptors. The alkaloid brucine is capable of selectively enhancing by an allosteric mechanism the effects of low but not high concentrations of acetylcholine at only the m1 subtype of muscarinic receptors. A simple derivative of brucine, N-chloromethylbrucine, enhances acetylcholine actions selectively at only m3 receptors. In addition it binds to, but does not affect, the properties of m4 receptors, thereby demonstrating neutral cooperativity and an 'absolute' selectivity of action at m3 receptors over m4 receptors. Brucine N-oxide enhances acetylcholine binding at m3 and m4 receptors and is neutral at m1 and m5 receptors. These findings allow the possibility of developing muscarinic agents that have a novel and highly targeted mode of action; they may act only on a single muscarinic receptor subtype which is functioning sub-optimally and therefore be of use therapeutically in the early stages of Alzheimer's Disease.

Pharmacological characterization of guanine nucleotide exchange reactions in membranes from CHO cells stably transfected with human muscarinic receptors m1-m4

We have studied muscarinic agonist stimulated [35S]GTP gamma S binding and [gamma 32P]GTP hydrolysis (GTPase) in membranes from CHO cells stably transfected with human muscarinic m1-m4 receptors. 'Full' agonists were at least 10-fold more potent at m2 & m4 receptors than at m1 & m3. This pattern was less marked with 'partial' agonists, which had a greater maximal effect at m2 & m4 than at m1 & m3. McN-A343 uniquely was more potent and efficacious at m4 than at m2 receptors. Antagonist affinity constants were estimated by fitting the data from inhibition curves directly to the Schild model. Antagonist affinity estimates were very similar to those measured earlier in binding studies using animal tissues, and confirmed a small degree of m4 selectivity for tropicamide and secoverine. The receptor subtypes activated more than one G-protein subtype; m2 & m4 receptors activated only pertussis (PTX) sensitive G-proteins, while m1 & m3 coupled to both PTX sensitive and insensitive G-proteins. Acetylcholine (ACh) was more potent in stimulating guanine nucleotide exchange in PTX-treated m1 cells than in controls.

Translation of the human C3b/C4b receptor mRNA in a cell-free system and by Xenopus oocytes

The C3b/C4b complement receptor (CR1) is a large, single-chain integral membrane glycoprotein present on erythrocytes, leukocytes, glomerular podocytes, and splenic dendritic-reticular cells that mediates the binding of complement-coated particles and immune complexes. CR1 is unusual in that it is polymorphic in size with the four allelic variants having molecular weights of 190,000, 220,000, 250,000, and 280,000 (SDS-PAGE, reducing conditions). The in vitro translation of the common (Mr 220,000) allelic variant CR1 has been achieved by using mRNA in lysates of rabbit reticulocytes and in Xenopus oocytes. HL-60, a promyelocytic human leukemic cell line, was treated with DMSO to induce differentiation and synthesis of CR1. Poly(A+) RNA was purified from these cells by column chromatography on oligo(dT)-cellulose. In the rabbit reticulocyte system, no CR1 was detected unless the translation mixture was denatured. In the presence of methylmercuric hydroxide, the CR1 translation product, unlike most translation products, had the same molecular weight in gel electrophoresis as the high-mannose-containing pro-CR1 and was 15-20K larger than nonglycosylated CR1. This suggests that a cotranslational modification of CR1 structure occurs, probably involving a proteolytic cleavage event. When poly(A+) RNA was translated in Xenopus oocytes, CR1 could be detected by treatment of oocytes with anti-CR1 monoclonal antibody followed by fluorescein-conjugated goat anti-mouse IgG. CR1 was diffusely distributed but preferentially localized to the vegetal surface. The molecular weight of this product, identified in immunoprecipitates of lysates of [35S]methionine-labeled oocytes, was identical with that of CR1 of HL-60.

Additional forms of human decay-accelerating factor (DAF)

Decay-accelerating factor (DAF) of human erythrocytes is a glycoprotein with a Mr of 65,000 that is anchored in the membrane via a glycolipid tail. During the purification of DAF, two lower m.w. forms were noted. DAF-A had an Mr of 63,000, and DAF-B had an Mr of 55,000. In a fluid phase assay, both forms accelerated the decay of the classical and the alternative C3 convertases with a specific activity similar to that of DAF. However, the decay-accelerating activity for the cell-bound C3 convertases was abolished, suggesting that neither could insert into E membranes and therefore that the glycolipid tail is altered. Analysis by molecular sieve high-pressure liquid chromatography demonstrated that DAF-A eluted with a Mr of approximately 450,000, similar to native DAF, and was thus in an aggregated form. In contrast, DAF-B eluted as a monomer with a Mr of approximately 60,000. DAF-A, but not DAF-B, bound to a hydrophobic column. To further characterize these two forms, surface-labeled human erythrocytes were incubated with phosphatidyl inositol-specific phospholipase C or papain. The phospholipase inefficiently released a form of DAF that was slightly larger (Mr of 64,000) than DAF-A. Papain efficiently released a 55,000 fragment that had the same Mr as DAF-B. To determine if DAF was cleaved by endogenous enzymes, surface-labeled erythrocytes were incubated with leukocytes. The kinetics of the leukocyte-induced degradation was similar to those observed with papain, and the released fragment aligned on seizing gels with the papain-derived fragment. We hypothesize that endogenous phospholipases and proteases cleave DAF to produce fragments similar to DAF-A and DAF-B, respectively.

Additional forms of human decay-accelerating factor (DAF)

Decay-accelerating factor (DAF) of human erythrocytes is a glycoprotein with a Mr of 65,000 that is anchored in the membrane via a glycolipid tail. During the purification of DAF, two lower m.w. forms were noted. DAF-A had an Mr of 63,000, and DAF-B had an Mr of 55,000. In a fluid phase assay, both forms accelerated the decay of the classical and the alternative C3 convertases with a specific activity similar to that of DAF. However, the decay-accelerating activity for the cell-bound C3 convertases was abolished, suggesting that neither could insert into E membranes and therefore that the glycolipid tail is altered. Analysis by molecular sieve high-pressure liquid chromatography demonstrated that DAF-A eluted with a Mr of approximately 450,000, similar to native DAF, and was thus in an aggregated form. In contrast, DAF-B eluted as a monomer with a Mr of approximately 60,000. DAF-A, but not DAF-B, bound to a hydrophobic column. To further characterize these two forms, surface-labeled human erythrocytes were incubated with phosphatidyl inositol-specific phospholipase C or papain. The phospholipase inefficiently released a form of DAF that was slightly larger (Mr of 64,000) than DAF-A. Papain efficiently released a 55,000 fragment that had the same Mr as DAF-B. To determine if DAF was cleaved by endogenous enzymes, surface-labeled erythrocytes were incubated with leukocytes. The kinetics of the leukocyte-induced degradation was similar to those observed with papain, and the released fragment aligned on seizing gels with the papain-derived fragment. We hypothesize that endogenous phospholipases and proteases cleave DAF to produce fragments similar to DAF-A and DAF-B, respectively.