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M F Brink

Publications and source records attributed to M F Brink.

14 recordsLinked to original sources
PubMed2000

Developing efficient strategies for the generation of transgenic cattle which produce biopharmaceuticals in milk.

At the close of the millennium, a revolution in the treatment of disease is taking shape due to the emergence of new therapies based on human recombinant proteins. The ever-growing demand for such pharmaceutical proteins is an important driving force for the development of safe and large-scale production platforms. Since the efficacy of a human protein is generally dependent on both its amino acid composition as well as various post-translational modifications, many recombinant human proteins can only be obtained in a biologically active conformation when produced in mammalian cells. Hence, mammalian cell culture systems are often used for expression. However, this approach is generally known for limited production capacity and high costs. In contrast, the production of (human) recombinant proteins in milk of transgenic farm animals, particularly cattle, presents a safe alternative without the constraint of limited protein output. Moreover, compared to cell culture, production in milk is very cost-effective. Although transgenic farm animal technology was still in its infancy a decade ago, today it is on the verge of fulfilling its potential of providing therapeutic proteins that can not be produced otherwise in sufficient quantities or at affordable cost. Since 1989, we have been at the forefront of this development, as illustrated by the birth of Herman, the first transgenic bull. In this communication, we will present an overview of approaches we have taken over the years to generate transgenic founder animals and production herds. Our initial strategies were based on microinjection; at the time the only viable option to generate transgenic cattle. Recently, we have adopted a more powerful approach founded on the application of nuclear transfer. As we will illustrate, this strategy presents a breakthrough in the overall efficiency of generating transgenic animals, product consistency, and time of product development.

Animals↗
PubMed1995

Specialized ribosomes: highly specific translation in vivo of a single targetted mRNA species.

In previous studies [Hui and de Boer, Proc. Natl. Acad. Sci. USA 84 (1987) (1987) 4762-4766; Hui et al., Methods Enzymol. 153 (1987) 432-452], it was shown that efficient translation of the human growth hormone mRNA (hGH) species having an altered Shine-Dalgarno (SD) sequence, 5'-GUGUG-3', depends on the presence of specialized (spc) ribosomes containing the modified anti-SD (ASD) sequence, 5'-CACAC-3', near the 3' end of their 16S rRNA. In spite of the altered ASD sequence, spc ribosomes were not found to be committed exclusively to the translation of the hGH mRNA; no more than 30% of the total amount of protein synthesized by such ribosomes was hGH. Once we replace the coding sequence of the hGH mRNA with that of chloramphenicol acetyltransferase (CAT), the specificity of spc ribosomes for translation of a single targetted mRNA, relative to the endogenous mRNAs, is greatly enhanced; an estimated 80% of the total amount of protein synthesized by spc ribosomes is CAT. Using the inducible spc ribosome system containing the cat gene, we show that, upon induction, spc ribosomes accumulate in large excess over the number needed for optimal translation of the targetted cat mRNA. Despite the excess, only few spc ribosomes initiate translation on a limited number of endogenous mRNAs. The excessive accumulation of spc ribosomes, which are predominantly present as free 30S subunits, is neither deleterious to the cells, nor does it lead to a feedback inhibition of the synthesis of wild-type ribosomes.

Chloramphenicol O-Acetyltransferase↗
PubMed1994

Specialized ribosomes allow for the study of mutations in functionally important regions in 16 S rRNA, without affecting cell growth. The identification of functional regions in the central domain of 16S rRNA.

In order to identify the sequences in the central domain of 16 S rRNA of Escherichia coli that are important for ribosome function, we have generated random mutations using a PCR-based mutagenesis technique. We show that the effects of such mutations on ribosomal activity can be analyzed in vivo utilizing the specialized ribosome system. With this system the effect of rRNA mutations on ribosomal activity can be studied by measuring the translation of a modified CAT-mRNA by specialized ribosomes. Specialized ribosomes do not translate the endogenous mRNAs and, therefore, are expected to constitute a non-essential pool of ribosomes within the cell. In total, we have isolated 28 different clones harboring specialized ribosomes with single or multiple point mutations. We demonstrate that for none of these clones was cell growth retarded, even though some of the mutations severely impaired the activity of the specialized ribosomes, to as low as 3% of the wild-type level. For all mutants, their individual activities ranged between 3% and 100% of that of the wild-type activity. Comparison of several mutants indicates that mutations within the hairpin loops 787-795 and 898-901 strongly reduce the ribosomal activity. We also present evidence that the single-stranded region centered around residue A815 may be involved in maintaining translational accuracy.

Base Sequence↗
PubMed1994

Spectinomycin interacts specifically with the residues G1064 and C1192 in 16S rRNA, thereby potentially freezing this molecule into an inactive conformation.

The upper stem of helix 34, consisting of the base-paired sequences C1063G1064U1065 and A1191C1192G1193, is suggested to be involved in the binding of spectinomycin. In E. coli 16S rRNA, each of the three mutations at position C1192 confers resistance to spectinomycin. In chloroplast ribosomes from tobacco plants and algae, resistance is conferred by single mutations at positions 1064, 1191, and 1193 (E. coli numbering). Since each of these mutations disrupt any of the three basepairs in the upper stem of helix 34, it has been postulated that spectinomycin can bind to this region and inhibit protein synthesis, only if its nucleotides are basepaired. We have tested this hypothesis by introducing disruptive and compensatory mutations that alter the basepair G1064-C1192. Using the specialized ribosome system, the translational activity of such mutants was determined, in the absence and presence of spectinomycin. We show that any of the three disruptive mutations A1064, C1064, and U1064 confer resistance, in accordance with the model for spectinomycin binding. Compensatory mutations A1064U1192, C1064G1192, and U1064A1192, however, maintained the resistance. This indicates that a basepaired conformation as such is not sufficient for spectinomycin binding, but rather that a G-C pair at positions 1064 and 1192 is required. In addition, we find that the translational activity of specialized ribosomes containing the mutations C1064G1192 is 5-fold lower compared to that of ribosomes containing any of the other mutations introduced, regardless whether spectinomycin is present or not. Since the introduction of C1064G1192 is expected to increase the stability of the upper stem of helix 34, we suggest that these mutations impair ribosome function by preventing the (transient) disruption of the upper stem. By analogy, we speculate that spectinomycin blocks protein synthesis by stabilizing the upper stem. In both cases, the 30S subunit would be frozen into an inactive conformation.

Base Sequence↗
PubMed1993

Separation of mutant and wild-type ribosomes based on differences in their anti Shine-Dalgarno sequence.

We describe a system to isolate 30S ribosomal subunits which contain targeted mutations in their 16S rRNA. The mutations of interest should be present in so-called specialized 30S subunits which have an anti-Shine-Dalgarno sequence that is altered from 5' ACCUCC to 5' ACACAC. These plasmid-encoded specialized 30S subunits are separated from their chromosomally encoded wild-type counterparts by affinity chromatography that exploits the different Shine-Dalgarno complementarity. An oligonucleotide complementary to the 3' end of wild-type 16S rRNA and attached to a solid phase matrix retains the wild-type 30S subunits. The flow-through of the column contains close to 100% mutant 30S subunits. Toeprinting assays demonstrate that affinity column treatment does not cause significant loss of activity of the specialized particles in initiation complex formation, whereas elongation capacity as determined by poly(Phe) synthesis is only slightly decreased. The method described offers an advantage over total reconstitution from in vitro transcribed mutant 16S rRNA since our 30S subunits contain the naturally occurring base modifications in their 16S rRNA.

Base Sequence↗
PubMed1993

Formation of the central pseudoknot in 16S rRNA is essential for initiation of translation.

The postulated central pseudoknot formed by regions 9-13/21-25 and 17-19/916-918 of 16S rRNA of Escherichia coli is phylogenetically conserved in prokaryotic as well eukaryotic species. This pseudoknot is located at the center of the secondary structure of the 16S rRNA and connects the three major domains of this molecule. We have introduced mutations into this pseudoknot by changing the base-paired residues C18 and G917, and the effect of such mutations on the ribosomal activity was studied in vivo, using a 'specialized' ribosome system. As compared with ribosomes having the wild-type pseudoknot, the translational activity of ribosomes containing an A, G or U residue at position 18 was dramatically reduced, while the activity of mutant ribosomes having complementary bases at positions 18 and 917 was at the wild-type level. The reduced translational activity of those mutants that are incapable of forming a pseudoknot was caused by their inability to form 70S ribosomal complexes. These results demonstrate that the potential formation of a central pseudoknot in 16S rRNA with any base-paired residues at positions 18 and 917 is essential to complete the initiation process.

Base Composition↗