A general method for the large-scale isolation of polysomes and messenger RNA applied to MOPC 21 mouse myeloma tumors

Blimp-1, a novel zinc finger-containing protein that can drive the maturation of B lymphocytes into immunoglobulin-secreting cells

We describe a novel gene, Blimp-1 (for B lymphocyte-induced maturation protein), transcripts of which are rapidly induced during the differentiation of B lymphocytes into immunoglobulin secretory cells and whose expression is characteristic of late B and plasma cell lines. The 856 amino acid open reading frame contains five Krüppel-type zinc finger motifs and proline-rich and acidic regions similar to those of known transcription factors. Serological studies show an approximately 100 kd protein that localizes to the nucleus. Stable or transient transfection of Blimp-1 into B cell lymphoma lines leads to the expression of many of the phenotypic changes associated with B cell differentiation into an early plasma cell stage, including induction of J chain message and immunoglobulin secretion, up-regulation of Syndecan-1, and increased cell size and granularity. Thus, Blimp-1 appears to be a pleiotropic regulatory factor capable of at least partially driving the terminal differentiation of B cells.

Ferritin mRNA is found on bound as well as on free polyribosomes in rat heart

Free and endoplasmic reticulum-bound polyribosomes from rat heart were examined for their ferritin mRNA content. A procedure for separation and purification of the two ribosome populations that produced good yields of homogeneous mono- and polyribosomes with no contaminating ultrastructures and gave distinctive sedimentation profiles in 15-50% sucrose gradients was developed. 14C-labeled free and bound polyribosomes added to heart preparations indicated that only 3% of free and 5.5% of bound polyribosomes cross-contaminated the bound and free fractions, respectively. RNA from both polyribosome populations hybridized with [32P]cDNA for rat ferritin. The extent of hybridization with mRNA from endoplasmic reticulum (ER)-derived polyribosomes was much greater than what could be accounted for by cross-contamination with free polyribosomes. This indicates that heart ferritin is synthesized not only on free polyribosomes for internal use in iron storage but also on ER-bound polyribosomes, where it may be destined for secretion into the plasma.

Mouse CD1 is distinct from and co-exists with TL in the same thymus

Human CD1 antigens have a similar tissue distribution and overall structure to (mouse) TL. However recent data from human CD1 suggest that the mouse homologue is not TL. Since no human TL has been conclusively demonstrated, we have analysed the murine CD1 genes. Two closely linked genes are found in a tail to tail orientation and the limited polymorphism found shows that, as in humans, the CD1 genes are not linked to the MHC. Both genes are found to be equally transcribed in the thymus, but differentially in other cell types. The expression in liver, especially, does not parallel CD1 in humans. This demonstrates conclusively that CD1 and TL are distinct and can co-exist in the same thymus. It is paradoxical that despite the structural similarity between mouse and human CD1, the tissue distribution of human CD1 is closer to TL. The possibility of a functional convergence between MHC molecules and CD1 is discussed.

Sequence of a rat immunoglobulin gamma 2c heavy chain constant region cDNA: extensive homology to mouse gamma 3

The complete sequence of the rat immunoglobulin gamma 2c heavy chain constant region has been determined by cDNA cloning and by mRNA sequencing. Comparison with other heavy chain genes reveals a high degree of homology (87%) to mouse gamma 3 and suggests that rat and mouse gamma genes separated from a common set of three ancestral genes.

Cloned murine T200 (Ly-5) cDNA reveals multiple transcripts within B- and T-lymphocyte lineages

The murine T200 family of cell-surface glycoproteins is expressed on hematopoietic lineage cell types in a developmentally regulated manner with different lymphocyte subpopulations expressing cross-reactive but structurally distinct forms. To investigate the differences between these forms and the regulation of these differences, murine T200 cDNA clones were isolated by using a probe obtained by subtractive hybridization. This procedure made use of a T200+ L-cell transfectant and the parent Ltk- cell line. The 1.9-kilobase (kb) cDNA clone was sequenced and found to contain the coding region of the COOH-terminal 331 amino acids and 0.9 kb of 3' untranslated region. Where the sequence overlapped with the rat sequence, 80-90% homology was observed. RNA blot analysis revealed that B-lineage cell lines express either a 5.6-kb or a 6.5-kb mRNA correlating to the size difference of the T200 glycoprotein synthesized. Similarly, in the T-cell lineage a helper T-cell clone and a cytotoxic T-cell clone express T200 transcripts of 5.6 kb and 5.9 kb, respectively, which correlate with the distinct sizes of their T200 glycoproteins. A T200-negative mutant of a T-cell line was found to express full-length T200 mRNA, although at a diminished level.

Regulation of mRNAs for three enzymes in the glial cell model C6 cell line

In the glial cell line C6, regulation of actinomycin D (Act-D)-sensitive translatable polysomal mRNAs of three key enzymes--glycerol phosphate dehydrogenase (GPDH; EC 1.1.1.8) and glutamine synthetase (GS) by glucocorticoids and lactate dehydrogenase (LDH; EC 1.1.1.27) by catecholamines--is described. Though the first two enzymes are hydrocortisone (HC)-inducible, the nature of their response to the hyperacetylating agent sodium butyrate is dramatically different. Furthermore the appearance of GPDH translatable poly(A)+ RNA in HC-induced cells is inhibited by the presence of cycloheximide (CHX), whereas the induction of GS is unaffected by CHX. These observations necessitate further probing into an existing model system to explain the varied mechanisms of induction of these two enzymes by a single inducer. In combination with the third enzyme whose induction by catecholamines is glial specific, we believe that the C6 cell represents the most appropriate cell line for molecular neurobiologists to study the mechanisms of hormone action in glia.

Molecular characterization of a nonsecreting myeloma mutant

The nonsecreting myeloma mutant, NSII/1, derived from MOPC 21 a mouse myeloma cell line, does not produce L chains and expresses a smaller than normal gamma 1 heavy chain, H* (Cowan, Secher and Milstein, J. Mol. Biol. 1974. 90: 691). The free H* is not secreted, but secretion can be rescued in hybrids expressing a light chain. By sequencing L and H chain-cloned cDNA derived from NSII/1 we find only one light chain mRNA expressed which belongs to an aberrantly rearranged kappa gene. We also find that the H* heavy chain is the result of a point mutation. H* mRNA contains an A instead of the G found in wild type at position 1125. The corresponding tryptophan 406 becomes a chain termination, giving rise to a 67 amino acid deletion at the C-terminus. The H* is core-glycosylated, but its state of assembly is abnormal: H* forms high molecular weight, S-S-bonded complexes. The results are discussed with reference to the putative toxic properties of free heavy chains and the failure of some chains to be secreted.

The regulation of protein synthesis in heart muscle under normal conditions and in the adriamycin-cardiomyopathy

The regulation of cardiac protein synthesis, in particular messenger-RNA (mRNA) and polyribosome metabolism, has been investigated in normal rat heart muscle and in the adriamycin-cardiomyopathy by using newly developed methods for the isolation, characterization and in-vitro translation of cardiac polyribosomes and mRNA. The obtained data allow the following conclusions: 1. Normal heart muscle has a high content of polyribosomes (865 micrograms/g) and of mRNA (20-60 micrograms/g), and thus a high rate of protein synthesis. 2. The level of cardiac polyribosomes and mRNA is strictly age-dependent and much higher in young animals (2-3 x). This corresponds to a higher cardiac protein synthesis rate in young animals with a growing heart muscle, and shows that the protein-synthetic reserves of heart muscle decrease sharply with age. 3. Withdrawal of food for 1-3 days results in a pronounced decrease (-50% to -70%) of cardiac polyribosomes and mRNA, demonstrating that the cardiac protein synthesis reacts very sensitively to conditions of starving. 4. The cardiac polyribosomes and mRNA are unevenly distributed in the myocyte. The bulk of these substances is present in the cardiac microsomes, and much less is found in nuclei, myofibrils, mitochondria and in the post-microsomal fraction (=cell-sap) of the cardiac muscle. This shows that the major intracellular site of cardiac protein synthesis is the microsomal fraction of the myocyte. 5. A pool of untranslated mRNA was demonstrated to be present in the cell-sap of the myocyte. This mRNA is to some extent translatable in-vitro and appears to represent mRNA sub-pools with two functions: a) mRNA which is partially broken down or in the process of being broken down, and b) intact mRNA which could have a "reserve-function", e.g., by being utilized to increase cardiac protein synthesis under certain conditions. 6. A method of quantitating small amounts of cardiac mRNA (25-50 ng) has been developed which makes it possible to estimate the mRNA content of cardiac biopsies. 7. These methods were utilized to study the relevance of changes in RNA- and protein synthesis in the development of the adriamycin-cardiomyopathy. It appears that severe decreases in cardiac mRNA and polyribosome levels are a key factor in the pathogenesis of the adriamycin-cardiomyopathy. These decreases are probably caused by the direct binding of adriamycin to cardiac DNA and lead themselves to a persisting decrease in cardiac protein synthesis which in view of the short half-lives of the cardiac contractile proteins (5-12 days) causes a gradual loss of cardiac structure and function.

Immunoglobulin heavy chain genes: demethylation accompanies class switching

The methylation of immunoglobulin heavy chain genes was examined before and after class switching, by using the Hpa II/Msp I restriction mapping technique. The mu, delta, and gamma 1 genes all are methylated in cells that do not express them but are demethylated when they are expressed. In particular, the delta gene remains methylated, and thus presumably untranscribed, in a cell line that probably represents an early stage of B-cell differentiation and produces only mu heavy chains. Because mu and delta RNAs are cotranscribed from a single complex transcription unit at a later stage of B-cell differentiation, this finding implies that the mu-plus-delta complex transcription unit is of variable length.

Mouse immunoglobulin D: construction and characterization of a cloned delta chain cDNA

TEPC 1017 is a BALB/c plasmacytoma that synthesizes IgD in large enough amounts to permit the isolation of mRNA for mouse delta chains. cDNA has been prepared from this mRNA, and an 880-base-pair fragment of it has been cloned by recombinant DNA techniques. The hybridization selection technique has been used to show that this cDNA clone specifically binds only mRNA that can be translated into immunoprecipitable delta chains. the sequence of a portion of this clone has been determined and, when translated, shows homology with the C delta 3 of a human myeloma protein. Using this cDNA clone as a probe, we have found that several different-sized delta RNAs are present in TEPC 1017 and in another IgD-secreting plasmacytoma, TEPC 1033.

Polyribosomes in isolated liver cells. Preparative procedures, effects of incubation and correlation with protein synthesis

Methods have been derived which permit the isolation of undergraded polyribosomes from isolated rat liver cells. Under the conditions used the polyribosome profile of hepatocytes immediately after isolation was essentially identical with that from intact liver. However, during incubation of cells in complex physiological media there was a progressive dissociation of polyribosomes. The addition of a variety of factors that produce reaggregation of polyribosomes in rat liver in vivo did not prevent dissociation during cell incubations. Although large polyribosomes were lost most rapidly, the albumin-synthesizing capacity of isolated cells was not selectively lost when compared with total protein synthesis. The significance of these results for the use of isolated hepatocytes in the study of liver protein synthesis is discussed.