had only a mild effect.

Similar results had been obtained in an earlier, distinct study, again using HCV as a model (Wakita and Wands 1994). Once again, antisense oligonucleotides that were targeted to regions within the 5’ UTR and to the AUG codon resulted in the arrest o f mRNA translation, whereas oligonucleotides targeted to regions downstream o f the AUG were ineffective.

In a cell-free study using the rabbit reticulocyte lysate system (Blake et a l 1985), it was found that antisense oligonucleotides targeted to the 5’ UTR and initiation codon regions of the 13-globin mRNA prevented the translation o f both a and 6-globin

mRNAs whereas antisense oligonucleotides targeted to coding regions had little or no effect. It was suggested by these authors that a helix-destabilising activity associated with the reticulocyte ribosomes was able to disrupt the secondary structure during the elongation step but not during the initiation step o f translation. Thus oligonucleotides bound to the coding region o f the mRNA would be expected to be unable to prevent translation in the reticulocyte systems. Conversely, when the same antisense oligonucleotides were used in the cell-free wheat germ system, despite the oligonucleotides targeted to the AUG region being more efficient inhibitors o f translation, all the oligonucleotides, regardless o f their binding site on the mRNA, were effective at preventing translation o f the globin mRNAs. It has been suggested by the authors that the ability o f anti sense oligonucleotides targeted to coding regions o f the

ixiRNA to inhibit translation in the wheat germ system and not the reticulocyte system, reflects some difference in the mechanism of translation between the two systems. Possibly the wheat germ ribosomes, unlike the reticulocyte ribosomes, have a reduced ability to disrupt oligonucleotide-mRNA complexes during the ribosome elongation step of translation. This could be in part due to the lower temperature at which wheat germ translation reactions are carried out compared to those in the reticulocyte system (25-30°C in the wheat germ system compared with 37°C in the reticulocyte system). Although not alluded to in the above study, it is possible that in the wheat germ system, which is sometimes found to contain a higher level of RNAse H activity than reticulocyte derived systems, the antisense oligonucleotides targeted to the coding region o f the mRNA were able to direct RNAse H activity, resulting in mRNA cleavage and subsequent degradation.

RNAse H activity has also been shown to play a role in studies using Xenopus oocytes as an environment in which to study the effects o f antisense oligonucleotides (Cazenave et al. 1987). Upon injection o f antisense oligonucleotides targeted to the coding region of exogenous rabbit 6-globin mRNA, the 6-globin transcripts were

found to disappear. Truncated transcripts were rarely detected due in part to the instability o f the RNA fragments that were subsequently processed by RNAses. The presence o f RNAse H and antisense oligonucleotides together, can lead to irreversible effects as once cleaved, mRNA can no longer support translation (Cazenave et al. 1987, Helene and Toulme 1990). Since RNAse H does not cleave the oligonucleotide, provided it is not itself degraded by other nucleases, it can proceed to induce the degradation o f multiple transcripts. RNAse H would seem to act therefore, as an amplifier o f the antisense effect.

From the studies described above, it would appear that the site to which antisense oligonucleotides should be targeted in order to prevent translation, can depend to a certain extent on which system is being used for study. In all cell-free systems, the niost effective region to which antisense oligonucleotides should be targeted in order to prevent mRNA translation, appears to be the area upstream and surrounding the AUG start site of translation. In rabbit reticulocyte systems, oligonucleotides which are

directed to the coding regions o f the mRNA are ineffective in preventing translation. This suggests that the unwindase activity possessed by ribosomes is only efficient during the elongation phase o f translation and during initiation. Conversely, in the wheat germ system antisense oligonucleotides targeted within the coding region o f the mRNA, may also be effective in preventing protein expression. This may be due, in part, to the higher level o f RNAse H activity that is sometimes reported to exist in these cell-ffee systems. If RNAse H activity is the reason why such antisense oligonucleotides prevent mRNA translation, it is via an indirect mechanism. Rather than by preventing translation p er se, mRNAs cleaved by RNAse H are degraded and are therefore not available for translation (Cazenave et al. 1987, Helene and Toulme 1990).

antisense oligor antisense oligo. antisense oligor antisense oligo. antisense oligo. antisense oligo.

DNA

X

RNA

X

mRNA

X

mRNA

X

X

X

initiation at cap site ribosome subunit assembly at AUG (start) site sliding of ribosome

PROTEIN

NUCLEUS

CYTOPLASM

Figure 1,8: Diagram dem onstrating the events which can be in terru p ted by the use o f antisense oligonucleotides. The arrows indicate the possible sites o f action o f the antisense oligonucleotide (ODN). This diagram was taken from Baertschi e t a l 1994.

1,84: Ribozymes

In recent years there has been a growing interest in using ribozymes to cleave target RNA (Woolf 1995). Ribozymes are naturally occuring RNA molecules that possess catalytic activity and can cleave either themselves or other RNA molecules. Ribozymes therefore represent an alternative to using antisense oligonucleotides that activate RNAse H activity as a way o f cleaving target RNA sequences. In a report by Haseloff and Gerlach (1988), parameters have been suggested for the design o f synthetic ribozymes with new sequence-specific endoribonuclease activities which could be used as an alternative to antisense oligonucleotides. In self-cleaving ribozymes the cleavage reaction results from RNA conformation bringing reactive groups into close proximity. The sites o f cleavage are specific and associated with domains o f conserved sequence and secondary structure (HaselofiF and Gerlach 1988). Self-catalysed cleavage o f these RNAs is generally an intramolecular reaction, that is to say a single molecule contains all the RNA-encoded functions required for cleavage. The self-cleaving domain is separate fi’om the substrate domain. The cloning and in vitro mutagenesis o f the self­ cleaving domain has resulted in an RNA molecule which does not possess a substrate region and will therefore only cleave other RNA molecules which contain the correct substrate region (Haseloff and Gerlach 1988). Thus, mutation o f the cleavage domain results in the cleavage o f a selected region o f target RNA. Because in naturally occurring self-cleavage, the site in the RNA substrate is preceeded on the 5’side by a GUC sequence which is highly conserved, this sequence must be located in target RNA (Haseloff and Gerlach 1988).

1-9: Aims and objectives of this project

The aim of this project was primarily to investigate the structure/function relationships of the LH/CG receptor, with a particular emphasis on understanding more fully the process of desensitisation o f this receptor in testicular Leydig cells. The work was to be carried out using a murine Leydig tumour cell line (MAIO). It was intended to approach this project fi'om several different angles, using a number o f methods. Some of these techniques were not routinely used in the department prior to this project and had first to be established.

Despite the similarity between the 6 2AR and other GPCRs, it was unclear as to whether

the LH/CG receptor underwent desensitisation as a result o f receptor phosphorylation. Previous work in this department had been successful in raising antipeptide antibodies to the rat LH/CG receptor (Pallikaros et al. 1995). It was therefore proposed to firstly establish an immunopurification protocol using these antibodies. With this achieved, the second aim was to subject MAIO cells to various treatments in order to desensitise (or mimic desensitisation) in the presence o f [^^P], and using immunopurification techniques, to establish whether the MAI 0 LH/CG receptor was phosphorylated when desensitised. It would also be possible to ascertain whether this receptor was co- immunopurified with other proteins and if so, the phosphorylation status o f these proteins. In addition to immunopurification analysis o f the MAIO LH/CG receptor, these antibodies were examined for the ability to recognise the respective antigens in immunocytochemistry and immunohistochemistry experiments. If the antibodies did prove successful in antigen recognition, these techniques would provide information regarding the expression o f the LH/CG receptor in different cells and tissues. It could also be possible to study the intracellular trafiQcking o f the LH/CG receptor during processes such as desensitisation and down-regulation by confocal microscopy.

Because rat Leydig cells and mouse Leydig cells have been reported to undergo desensitisation in response to activators o f PKC and PKA differently (see review by Rommerts and Cooke 1988, West and Cooke 1991a, West et a l 1991), it was postulated that differences may exist between the phosphoiylation consensus sequences o f rats and mice. Whilst the mouse ovarian sequence data had been published (Gudermann et al. 1992a), the sequence o f the MAIO LH/CG receptor was not known. Because the MAIO cell was neoplastic in nature, it was proposed to confirm that the receptor sequence was similar to that found in the murine ovarian LH/CG receptor, if not identical, particularly with respect to the postulated phosphorylation consensus sequences. It was important that we could be certain that the MAIO LH/CG receptor would respond as other murine LH/CG receptors in later desensitisation studies. The cloning o f the MAIO LH/CG receptor would also provide a convenient way in which we could create truncated LH/CG receptors. These truncated receptors were required in order that the effects o f putative truncations

predicted to result from the use o f an antisense strategy [see Chapter Five'] could be mimicked. This would provide a way in which to control for the potential, non-specific effects o f incubating cells with antisense oligonucleotides and would also provide information as to how antisense strategies work.

Previous work in the department had established an antisense strategy which had been reported to generate truncated LH/CG receptors in MAIO cells (West and Cooke 1991b). Another o f the aims o f this project was to continue the antisense work, as the generation o f mutated LH/CG receptors would provide information about the structure/function relationships o f this receptor. O f particular interest was to establish which regions o f the LH/CG receptor were important for functions such as G-protein coupling and desensitisation and/or phosphorylation. Mutations which were predicted to be caused by the presence o f antisense oligonucleotides targeted to particular regions o f the LH/CG receptor would be recreated by digesting the cloned MAIO LH/CG receptor cDNA with restriction enzymes and transfected into a mammalian cell line. In this way we could confirm that the effects o f the antisense strategy were as a result o f creating LH/CG receptor truncations and not through other non-specific mechanisms. However, due to problems that were encountered during preliminary experiments, it was realised that another experimental system would have to be designed before this work could progress further. To this end, the aim o f the work evolved to include the establishment o f cell-free methods in which to first test antisense oligonucleotides for effectiveness. Once suitable antisense oligonucleotides had been established as effective in the cell-free systems, it was intended to return to using the cultured MAIO cells to ascertain whether the results could be repeated.

Finally, the identity o f the protein kinase(s) anticipated to be involved LH/CG receptor phosphorylation/desensitisation were yet to have been elucidated. Evidence was accumulating to suggest that other GPCRs could couple to more than one signal transducing system (Liu et a l 1992, Chabre et a l 1992 Van Sande et a l 1993, Gudermann et a l 1992 a and b), and it had been suggested that the LH/CG receptor Itself may also be coupled to the PLC/ IP3 pathway in addition to the adenylyl

y.

to involve PKC activity, it was proposed that inhibitors o f PKC should be used in cell culture experiments, in order to establish if this protein had a role in Leydig cell signal transduction/desensitisation.

5iimmarv of aims

1. To develop an immunopurification protocol such that the phosphorylation state of the desensitised MAIO LH/CG receptor could be established.

2. To assess whether existing LH/CG receptor antibodies had the potential to be used in immunocytochemistry experiments and therefore also confocal microscopy. This would provide a way o f following the LH/CG receptor during processes such as desensitisation and down-regulation.

3. To clone and sequence the MAIO LH/CG receptor to establish the sequence o f putative phosphorylation consensus sequences and to permit the synthesis o f truncated LH/CG receptors for clarification o f the results anticipated fi'om the use o f an antisense strategy.

4. To continue work using the antisense strategy in order to learn more about the structure/function relationships o f the LH/CG receptor and also the mechanisms by which an àntisense strategy is thought to work. (This rapidly became modified to include the establishment o f a cell-fi*ee system o f LH/CG receptor synthesis in which to first assess the effectiveness o f antisense oligonucleotides in preventing LH/CG

receptor synthesis).

5. To investigate the putative role o f protein kinases such as PKC in LH/CG receptor phosphorylation/desensitisation via the use o f protein kinase inhibitors.