Effect of chronic cold exposure on tyrosine hydroxylase mRNA in rat adrenal gland.

The effect of chronic stress on the levels of tyrosine hydroxylase (TH) RNA in rat adrenal gland was investigated by RNA-DNA hybridization using a cloned TH cDNA probe. Results of dot-blot hybridization experiments and northern analysis demonstrate that exposure of animals to cold for 1 week results in a 4-5-fold increase in the relative abundance of TH mRNA. This increase in TH mRNA level may underlie the increase in adrenal TH activity that is known to occur when rats are exposed to such cold stress.

cDNA clone for the alpha subunit of the acetylcholine receptor from the mouse muscle cell line BC3H-1.

Sequences from a gene coding for mouse acetylcholine receptor alpha subunit have been inserted into a recombinant plasmid and cloned in Escherichia coli. mRNAs for acetylcholine receptors occur in low abundance in vertebrate muscle. To clone the mouse alpha-subunit cDNA, we made use of (i) a cell line, BC3H-1, that overproduces the alpha-subunit mRNA and (ii) a polysome fractionation procedure that results in enrichment of alpha-subunit mRNA. Polyadenylylated RNA was used to construct a cDNA library of 750 recombinant clones. Acetylcholine receptor-specific sequences were identified by hybrid-selected translation, followed by monoclonal antibody precipitation and peptide mapping of the translation product. One clone (pA59) that fit these criteria was found in the first 80 isolates. It had a 700-base-pair insert that was excised with Pst I. Blot-hybridization experiments with nick-translated pA59 DNA showed that BC3H-1 cells contain 100-1,000 times more alpha-subunit mRNA than does newborn or adult mouse muscle. Blot hybridization of restriction digests of mouse liver DNA revealed that pA59 is homologous to a very small number (probably one) of genomic sequences.

Characterization of the mRNA for mouse muscle acetylcholine receptor alpha subunit by quantitative translation in vitro.

The nicotinic acetylcholine receptor of mammalian skeletal muscle is a multisubunit membrane glycoprotein whose synthesis is regulated by developmental and physiological cues. We report here the identification and characterization of the primary translation product of alpha subunit mRNA. The alpha subunit synthesized in rabbit reticulocyte lysate is approximately 2000 larger in apparent molecular weight than the native alpha subunit polypeptide found in acetylcholine receptor. Evidence from peptide maps and the effect of co-translational incubation with dog pancreas microsomes suggests that the in vitro product differs in two ways from native alpha subunit: 1) it is synthesized with an NH2-terminal signal peptide which is removed in vivo, and 2) the in vitro product is not glycosylated. We have characterized the alpha subunit mRNA activity by using a quantitative the membrane-bound polysome fraction. It is poly(A+) and approximately 2000 nucleotides long. Finally, we have shown that in BC3H-1 cells, alpha subunit mRNA is regulated developmentally. We detected a 10-fold increase in the relative abundance of alpha subunit mRNA in cells which had undergone the transition from log phase growth to differentiated myoblast.

The regulation of acetylcholine receptor expression in mammalian muscle.

The synthesis of functional AChRs can be described as a pathway leading from the translation of subunit mRNAs to the plasma membrane forms of extrajunctional and junctional receptors (Fig. 9). We have not included in this scheme pretranslational steps for the synthesis and processing of RNA coding for receptor subunits because very little is known about such processes. Several aspects of Figure 9 are worthy of note: It is now well established that polypeptide synthesis is initiated on free cytoplasmic polysomes and that once sufficient nascent subunits bearing signal peptides at the amino terminus is formed, polysomes assemble with the membranes of the rough endoplasmic reticulum via a mechanism that employs the signal recognition particle (Anderson et al. 1982). Nascent subunits undergo cotranslational insertion through the rough endoplasmic reticulum membrane, signal peptide removal, and core glycosylation (Anderson and Blobel 1981; Merlie et al. 1981; Anderson et al. 1982; Sebbane et al. 1983). Anderson and Blobel (this volume) have demonstrated that subunits synthesized in vitro and inserted into membrane vesicles do not undergo heterologous subunit-subunit associations. We have shown that alpha- and beta-subunits newly synthesized in vivo are not associated with each other. Our data indicate that the alpha-subunit is initially present in vivo in a conformation that is radically different from its native conformation in the mature receptor complex. We assume that beta-, gamma-, and delta-subunits also are synthesized as conformationally "immature" forms, but verification of this point must await the availability of new monoclonal antibody specificities. Our data indicate that only a fraction of the newly synthesized alpha-subunit undergoes conformational maturation to the 5S species which binds both alpha-bungarotoxin and anti-main immunogenic region monoclonal antibodies. alpha-Subunits synthesized during a 5-minute pulse labeling require 30 minutes for completion of this process. alpha-Subunits that do not undergo conformational maturation are degraded rapidly (t1/2 = 0.5 hr) ( Merlie et al. 1982). Assembly of alpha- and beta-subunits synthesized during a 5-minute pulse labeling lags for approximately 30 minutes and is not complete until 90 minutes. Finally, assembled receptors are transported to the surface and appear in the plasma membrane. These processes occur during expression of AChRs in differentiated myoblasts. We do not know how undifferentiated myogenic cells, in vivo or in tissue culture, differ with regard to any of these steps.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of glycosylation with tunicamycin blocks assembly of newly synthesized acetylcholine receptor subunits in muscle cells.

We have characterized the oligosaccharide chains of the alpha subunit of acetylcholine receptor of the clonal mouse muscle cell line BC3H-1 by their sensitivity to end-beta-N-acetylglucosaminidase H and by comparison of the native glycosylated polypeptide with the nonglycosylated form made in tunicamycin-treated cells. These studies indicate that the native alpha subunit has a single N-asparagine-linked oligosaccharide chain of the "high mannose" or "simple" type. Furthermore, these results considered in light of our previous characterization of the alpha subunit synthesized in vitro suggest that the alpha subunit contains no "complex"-type N-linked oligosaccharide chains. We have investigated the role of glycosylation in the biogenesis of the acetylcholine receptor. Receptor biogenesis in normal cells involves the assembly of newly synthesized alpha subunits into a form active for binding alpha-bungarotoxin. This process is only 30% efficient and is complete by 30 min postsynthesis. When glycosylation is inhibited by tunicamycin, alpha subunit synthesis is inhibited only slightly but assembly into an alpha-bungarotoxin binding species is reduced dramatically.

Acetylcholine receptor synthesis from membrane polysomes.

We have established conditions for the fractionation of cytoplasmic and membrane-bound polyribosomes from the clonal mouse cell line BC3H-1. Polyribosome fractions are obtained in good yield and purity. They are active in protein synthesis when incubated with nuclease-treated rabbit reticulocyte lysates, and we have demonstrated that the cytoplasmic and membrane-bound fractions direct the synthesis of distinctly different sets of proteins. Using immunoprecipitation and sodium dodecyl sulfate gel analysis, we have shown that the membrane-bound but not the cytoplasmic polyribosomes direct the synthesis of two protein species (Mr = 39,000 nd 42,000) which are homologous to the native alpha subunit of acetylcholine receptor. Peptide maps suggest that the two species synthesized in vitro may correspond to the nonglycosylated and glycosylated forms, respectively, of the alpha subunit.

Acetylcholine receptor subunits transit a precursor pool before acquiring alpha-bungarotoxin binding activity.

We have developed methods for the rapid immunoprecipitation of acetylcholine receptor from the clonal mouse cell line BC3H-1. One antiserum, anti-alpha-bungarotoxin, precipitates receptors to which alpha-bungarotoxin is bound. A second antiserum prepared against sodium dodecyl sulfate-denatured receptor precipitates receptor polypeptides independent of toxin binding activity. We have used these sera to analyze the products of synthesis in vivo after short pulses of [35S]methionine. Acetylcholine receptor polypeptides of synthesized during a 5-min pulse require 15 min before they become fully active for alpha-bungarotoxin binding. These experiments identify an early step in the assembly of functional acetylcholine receptor and suggest a novel mechanism by which receptor synthesis may be controlled post-translationally.