Construction of an adenovirus type 7a E1A- vector

A strategy for constructing replication-defective adenovirus vectors from non-subgroup C viruses has been successfully demonstrated with adenovirus type 7 strain a (Ad7a) as the prototype. An E1A-deleted Ad7a reporter virus expressing the chloramphenicol acetyltransferase (CAT) gene from the cytomegalovirus promoter enhancer was constructed with DNA fragments isolated from Ad7a, an Ad7a recombination reporter plasmid, and the 293 cell line. The Ad7a-CAT virus particle transduces A549 cells as efficiently as Ad5-based vectors. Intravenous infections in a murine model indicate that the Ad7a-CAT virus infects a variety of tissues, with maximal levels of CAT gene expression found in the liver. The duration of Ad7a-CAT transgene expression in the liver was maximally maintained 2 weeks postinfection, with a decline to baseline activity by the week 4 postinfection. Ad7a-CAT represents the first example of a non-subgroup C E1A- adenovirus gene transfer vector.

Tumor necrosis factor alpha plays a central role in immune-mediated clearance of adenoviral vectors

Adenovirus (Ad) gene transfer vectors are rapidly cleared from infected hepatocytes in mice. To determine which effector mechanisms are responsible for elimination of the Ad vectors, we infected mice that were genetically compromised in immune effector pathways [perforin, Fas, or tumor necrosis factor alpha (TNF-alpha)] with the Ad vector, Ad5-chloramphenicol acetyl transferase (CAT). Mice were sacrificed at 7-60 days postinfection, and the levels of CAT expression in the liver determined by a quantitative enzymatic assay. When the livers of infected mice were harvested 28 days postinfection, the levels of CAT expression revealed that the effectors most important for the elimination of the Ad vector were TNF-alpha > Fas > perforin. TNF-alpha did not have a curative effect on infected hepatocytes, as the administration of TNF-alpha to infected severe combined immunodeficient mice or to infected cultures in vitro had no specific effect on virus persistence. However, TNF-alpha-deficient mice demonstrated a striking reduction in the leukocytic infiltration early on in the infection, suggesting that TNF-alpha deficiency resulted in impaired recruitment of inflammatory cells to the site of inflammation. In addition, the TNF-deficient mice had a significantly reduced humoral immune response to virus infection. These results demonstrate a dominant role of TNF-alpha in elimination of Ad gene transfer vectors. This result is particularly important because viral proteins that disable TNF-alpha function have been removed from most Ad vectors, rendering them highly susceptible to TNF-alpha-mediated elimination.

Sequence-mediated regulation of adenovirus gene expression by repression of mRNA accumulation

Gene expression in complex transcription units can be regulated at virtually every step in the production of mature cytoplasmic mRNA, including transcription initiation, elongation, termination, pre-mRNA processing, nucleus-to-cytoplasm mRNA transport, and alterations in mRNA stability. We have been characterizing alternative poly(A) site usage in the adenovirus major late transcription unit (MLTU) as a model for regulation at the level of pre-mRNA 3'-end processing. The MLTU contains five polyadenylation sites (L1 through L5). The promoter proximal site (L1) functions as the dominant poly(A) site during the early stage of adenovirus infection and in plasmid transfections when multiple poly(A) sites are present at the 3' end of a reporter plasmid. In contrast, stable mRNA processed at all five poly(A) sites is found during the late stage of adenovirus infection, after viral DNA replication has begun. Despite its dominance during early infection, L1 is a comparatively poor substrate for 3'-end RNA processing both in vivo and in vitro. In this study we have investigated the basis for the early L1 dominance. We have found that mRNA containing an unprocessed L1 poly(A) site is compromised in its ability to enter the steady-state pool of stable mRNA. This inhibition, which affects either the nuclear stability or nucleus-to-cytoplasm transport of the pre-mRNA, requires a cis-acting sequence located upstream of the L1 poly(A) site.

Innate immune mechanisms dominate elimination of adenoviral vectors following in vivo administration

To evaluate the contribution of the innate immune component of host defense in clearing the genome of adenovirus (Ad) vectors following in vivo administration, the Ad vectors AdCMV.beta gal (expressing beta-galactosidase) or AdCMV.Null (expressing no gene) were administered intravenously to immunocompetent or immunodeficient mice, and the amount of vector genome was quantified in the liver. Strikingly, 90% of vector DNA was eliminated within 24 hr. There was no increase in vector DNA in other tissues over this period, suggesting that rapid clearance of vector genome resulted from local degradation. After 24 hr, vector elimination was slow, with only 9% of the initial amount of vector genome cleared over the subsequent 3 weeks. Importantly, early phase (0-24 hr) elimination of vector DNA was independent of the transgene and similar in immunocompetent and nude animals. These observations suggest two phases of Ad vector elimination: a previously recognized late, immune-related elimination, and the early, innate immune elimination described in the present study. The early phase of vector loss is, by far, the dominant mechanism, an observation that has implications in developing strategies to maintain persistent expression of the newly transferred gene following in vivo gene therapy.

Circumvention of anti-adenovirus neutralizing immunity by administration of an adenoviral vector of an alternate serotype

Effective gene transfer and expression following repetitive administration of adenoviral (Ad) vectors in experimental animals is limited by anti-Ad neutralizing antibodies. Knowing that anti-Ad humoral immunity is serotype-specific, we hypothesized that anti-Ad neutralizing immunity could be circumvented using Ad vectors of different serotypes (Ad2, Ad5) within the same subgroup (C) to transfer and express beta-glucuronidase (beta glu) in the lung. Sprague-Dawley rats received an intratracheal administration of either Ad2 beta glu or Ad5 beta glu, and, 14 days later, repeat administration of either the same vector or a vector of a different serotype. Analysis of serum and bronchoalveolar lavage fluid following initial vector administration demonstrated systemic and local serotype-specific neutralizing antibodies. For both the Ad2 and Ad5 vectors, beta glu expression 24 hr following the second administration of the same serotype was < 30% of that of naive animals. In contrast, beta glu expression 24 hr following second administration of a different serotype Ad vector was similar to expression at 24 hr of naive animals receiving a single administration (Ad5 beta glu followed by Ad2 beta glu, as well as Ad2 beta glu followed by Ad5 beta glu; p > 0.2 both comparisons). Although the alternative serotype bypassed anti-Ad neutralizing immunity, persistence of expression was reduced compared to that following administration to naive animals. Compatible with this observation, systemic administration of the same vectors to C57B1/6 mice demonstrated induction of cytotoxic T lymphocytes directed against the beta glu transgene, as well as products of the Ad genome. Interestingly, intratracheal administration of vectors with different serotypes and different transgenes to rats resulted in longer expression (but still not normalized) compared to that achieved with vectors of different serotypes but the same transgene. These observations demonstrate that alternate use of Ad vectors from different serotypes within the same subgroup can circumvent anti-Ad humoral immunity to permit effective gene transfer after repeat administration, although the chronicity of expression is limited, likely by cellular immune process directed against both the transgene and viral gene products expressed by the vector.

Adenovirus-mediated gene delivery into neuronal precursors of the adult mouse brain

Precursor cells found in the subventricular zone (SVZ) of the adult brain can undergo cell division and migrate long distances before differentiating into mature neurons. We have investigated the possibility of introducing genes stably into this population of cells. Replication-defective adenoviruses were injected into the SVZ of the lateral ventricle of adult mice. The adenoviruses carried a cDNA for the LacZ reporter or the human p75 neurotrophin receptor, for which species-specific antibodies are available. Injection of the viruses into the SVZ led to efficient labeling of neuronal precursors. Two months after viral injection, infected cells were detected in the olfactory bulb, a significant distance from the site of injection. Labeled periglomerular and granular neurons with extensive dendritic arborization were found in the olfactory bulb. These results demonstrate that foreign genes can be efficiently introduced into neuronal precursor cells. Furthermore, adenovirus-directed infection can lead to long-term stable gene expression in progenitor cells found in the adult central nervous system.

Adenovirus type 5 and 7 capsid chimera: fiber replacement alters receptor tropism without affecting primary immune neutralization epitopes

The efficient uptake of adenovirus into a target cell is a function of adenovirus capsid proteins and their interaction with the host cell. The capsid protein fiber mediates high-affinity attachment of adenovirus to the target cell. Although the cellular receptor(s) for adenovirus is unknown, evidence indicates that a single receptor does not function as the attachment site for each of the 49 different serotypes of adenovirus. Sequence variation of the fiber ligand, particularly in the C- terminal knob domain, is associated with serotype-specific binding specificity. Additionally, this domain of fiber functions as a major serotype determinant. Fiber involvement in cell targeting and its function as a target of the host immune response make the fiber gene an attractive target for manipulation, both from the perspective of adenovirus biology and from the perspective of using adenovirus vectors for gene transfer experiments. We have constructed a defective chimeric adenovirus type 5 (Ad5) reporter virus by replacing the Ad5 fiber gene with the fiber gene from Ad7A. Using the chloramphenicol acetyltransferase reporter gene, we have characterized this virus with respect to infectivity both in vitro and in vivo. We have also characterized the role of antifiber antibody in the host neutralizing immune response to adenovirus infection. Our studies demonstrate that exchange of fiber is a strategy that will be useful in characterizing receptor tropism for different serotypes of adenovirus. Additionally, the neutralizing immune response to Ad5 and Ad7 does not differentiate between two viruses that differ only in their fiber proteins. Therefore, following a primary adenovirus inoculation, antibodies generated against fiber do not constitute a significant fraction of the neutralizing antibody population.

Polarity of TRH receptors in transfected MDCK cells is independent of endocytosis signals and G protein coupling

Information concerning the molecular sorting of G protein-coupled receptors in polarized epithelial cells is limited. Therefore, we have expressed the receptor for thyrotropin-releasing hormone (TRH) in Madin-Darby canine kidney (MDCK) cells by adenovirus-mediated gene transfer to determine its distribution in a model cell system and to begin analyzing the molecular information responsible for its distribution. Equilibrium binding of [methyl-3H]TRH to apical and basolateral surfaces of polarized MDCK cells reveals that TRH receptors are expressed predominantly (>80%) on the basolateral cell surface. Receptors undergo rapid endocytosis following agonist binding; up to 80% are internalized in 15 min. A mutant receptor missing the last 59 residues, C335Stop, is poorly internalized (<10%) but is nevertheless basolaterally expressed (>85%). A second mutant TRH receptor, delta218-263, lacks essentially all of the third intracellular loop and is not coupled to G proteins on binding agonist. This receptor internalizes TRH approximately half as efficiently as wild-type TRH receptors but is nevertheless strongly polarized to the basolateral surface (>90%). These results indicate that molecular sequences responsible for basolateral accumulation of TRH receptors can be segregated from signals for ligand-induced receptor endocytosis and coupling to heterotrimeric G proteins.

Ectopic expression of thyrotropin releasing hormone (TRH) receptors in liver modulates organ function to regulate blood glucose by TRH

Maintenance of blood glucose by the liver is normally initiated by extracellular regulatory molecules such as glucagon and vasopressin triggering specific hepatocyte receptors to activate the cAMP or phosphoinositide signal transduction pathways, respectively. We now show that the normal ligand-receptor regulators of blood glucose in the liver can be bypassed using an adenovirus vector expressing the mouse pituitary thyrotropin releasing hormone receptor (TRHR) cDNA ectopically in rat liver in vivo. The ectopically expressed TRHR links to the phosphoinositide pathway, providing a means to regulate liver function with TRH, an extracellular ligand that does not normally affect hepatic function. Administration of TRH to these animals activates the phosphoinositide pathway, resulting in a sustained rise in blood glucose. It should be possible to use this general strategy to modulate the differentiated functions of target organs in a wide variety of pathologic states.

Circumventing the immune response to adenovirus-mediated gene therapy

Adenovirus-mediated gene transfer experiments have demonstrated an exceptional efficiency of virus uptake and gene expression in a variety of in vivo models. Unfortunately, the efficiency of gene delivery is not accompanied by long-term gene expression. Maximal gene expression peaks during the first week of infection followed by a rapid decline to near baseline levels within several weeks. Data from several laboratories implicate host cellular and humoral immune responses as being responsible for the limited duration of expression and for the inability to successfully readminister a gene using adenovirus vectors. In this study we have examined two strategies which, independently or in combination, circumvent aspects of the host immune response against adenovirus-mediated gene therapy. The first strategy explores induction of immune tolerance in the experimental host as a method to increase the duration of gene expression and as a method to allow readministration of adenovirus expression vectors. Our second strategy is directed at the need to readminister adenoviral vectors to immune competent adult animals. We have demonstrated that a sequential exposure of rats to at least two other adenovirus serotypes does not compromise our ability to successfully administer an Ad5-based CAT expression vector. The characterization of serotype-specific neutralizing response indicates that the construction and use of Ad expression vectors from different serotypes will facilitate a useful adenovirus-based strategy allowing multiple administrations of a target gene.

"Sero-switch" adenovirus-mediated in vivo gene transfer: circumvention of anti-adenovirus humoral immune defenses against repeat adenovirus vector administration by changing the adenovirus serotype

Recombinant, replication-deficient adenovirus (Ad) vectors have been successfully used to transfer and express the normal human cystic fibrosis transmembrane conductance regulator (CFTR) cDNA in vivo in the respiratory epithelium of experimental animals and humans with cystic fibrosis (CF). Since Ad-directed gene expression wanes over time, repeat administration is necessary to achieve an effective treatment for CF. A major hurdle to such a strategy is the possibility that anti-Ad humoral immunity may prevent gene expression in individuals with pre-existing anti-Ad immunity or following repeat administration. One strategy to circumvent such a problem would be alternating the use of Ad vectors belonging to different subgroups. Neutralizing antibodies developed with the administration of one Ad serotype do not cross-react with an Ad belonging to a second serotype in a manner that blocks infection and gene expression. To test this hypothesis, an immunizing dose of wild-type Ad5 (subgroup C), Ad4 (subgroup E), or Ad30 (subgroup D) was administered intratracheally to experimental animals, followed by an intratracheal administration of a replication-deficient subgroup C-derived vector coding for marker genes (chloramphenicol acetyl transferase or beta-galactosidase) or for the normal human CFTR cDNA. As expected, studies with vectors coding for marker genes or for CFTR cDNA demonstrated that airway administration of a vector does not yield efficient gene transfer, if there has been prior recent airway administration of the same Ad subgroup. In contrast, effective expression from the second administration can be achieved with an adenovirus vector belonging to a subgroup different from the first adenovirus administered. These data support the paradigm of alternating Ad vectors derived from different subgroups as strategy to circumvent anti-Ad humoral immunity, thus permitting the use of Ad vectors as a means to treat the respiratory manifestations of CF.

The impact of developmental stage, route of administration and the immune system on adenovirus-mediated gene transfer

Important aspects of successful adenovirus gene transfer include the amount and persistence of gene expression, the ability to readminister virus and the localization of virus-directed gene expression to target organs. Our objective in this study was to use a single recombinant adenovirus bearing a quantifiable reporter gene [chloramphenicol acetyltransferase (CAT)] to establish the parameters which define the limits of adenovirus gene expression in a rat model. First, we determined how the route of virus administration affected the amount, duration and distribution of expression in different tissues and in rats of different developmental stages. All routes resulted in infection of all tissues tested. Surprisingly, the most efficient and widespread gene transfer was achieved by intracardiac muscle injection. The high levels of CAT protein that can be produced in a liver (< or = 1.7 mg) or a heart (< or = 196 micrograms) 5 days after infection suggest that the amount of gene product will not be a limitation in the use of adenovirus. Following peak activity at 5 days after infection, a gradual decline of CAT expression was observed in all tissues assayed; by 80 days neither CAT activity nor adenovirus DNA were detectable. In addition, adults could not be boosted by a second administration of virus, presumably due to the presence of high levels of neutralizing antibodies. The limited persistence of gene expression could be circumvented when virus was injected into neonates. Blocking T lymphocyte expansion by cyclosporine enhanced the persistence of CAT gene product over a 25-day period in heart and lung but not in liver compared with control animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequence elements upstream of the 3' cleavage site confer substrate strength to the adenovirus L1 and L3 polyadenylation sites

The adenovirus major late transcription unit is a well-characterized transcription unit which relies heavily on alternative pre-mRNA processing to generate distinct populations of mRNA during the early and late stages of viral infection. In the early stage of infection, two major late transcription unit mRNA transcripts are generated through use of the first (L1) of five available poly(A) sites (L1 through L5). This contrasts with the late stage of infection when as many as 45 distinct mRNAs are generated, with each of the five poly(A) sites being used. In previous work characterizing elements involved in alternative poly(A) site use, we showed that the L1 poly(A) site is processed less efficiently than the L3 poly(A) site both in vitro and in vivo. Because of the dramatic difference in processing efficiency and the role processing efficiency plays in production of steady-state levels of mRNA, we have identified the sequence elements that account for the differences in L1 and L3 poly(A) site processing efficiency. We have found that the element most likely to be responsible for poly(A) site strength, the GU/U-rich downstream element, plays a minor role in the different processing efficiencies observed for the L1 and L3 poly(A) sites. The sequence element most responsible for inefficient processing of the L1 poly(A) site includes the L1 AAUAAA consensus sequence and those sequences which immediately surround the consensus hexanucleotide. This region of the L1 poly(A) site contributes to an inability to form a stable processing complex with the downstream GU/U-rich element. In contrast to the L1 element, the L3 poly(A) site has a consensus hexanucleotide and surrounding sequences which can form a stable processing complex in cooperation with the downstream GU/U-rich element. The L3 poly(A) site is also aided by the presence of sequences upstream of the hexanucleotide which facilitate processing efficiency. The sequence UUCUUUUU, present in the L3 upstream region, is shown to enhance processing efficiency as well as stable complex formation (shown by increased binding of the 64-kDa cleavage stimulatory factor subunit) and acts as a binding site for heterogeneous nuclear ribonucleoprotein C proteins.

Expression of thyrotropin-releasing hormone receptors by adenovirus-mediated gene transfer reveals that thyrotropin-releasing hormone desensitization is cell specific

Biological studies of seven-transmembrane region G protein-coupled receptors have been restricted by available techniques for gene transfer into mammalian cells. We have created a highly efficient adenovirus-based expression vector for the thyrotropin-releasing hormone (TRH) receptor (TRH-R), AdCMVmTRHR, to circumvent difficulties encountered when transient or stable plasmid expression systems are used. We show that infection with AdCMVmTRHR results in fully functional TRH-Rs, which can be expressed in a broad range of mammalian cell types, including those resistant to conventional transient transfection. TRH-Rs can be expressed at high levels, up to 2 x 10(6) receptors/cell. Expression in several cell lines in culture reveals that rapid TRH-R desensitization by TRH and phorbol 12-myristate 13-acetate is cell type specific. The versatility of adenovirus-mediated gene transfer and expression of TRH-Rs not only facilitates in vitro studies of TRH-R biology but also provides a valuable in vivo expression vector capable of extending TRH-R studies to animal model systems.

Thyrotropin-releasing hormone (TRH) receptor number determines the size of the TRH-responsive phosphoinositide pool. Demonstration using controlled expression of TRH receptors by adenovirus mediated gene transfer

We use an adenovirus vector, AdCMVmTRHR, to express thyrotropin-releasing hormone (TRH) receptors (TRH-Rs) to determine whether the size of the hormone-responsive phosphoinositide pool in mammalian cells is directly related to receptor number. Infection of HeLa cells with increasing numbers of AdCMVmTRHR caused time-dependent graded expression of TRH-Rs. Measurement of cytoplasmic free Ca2+ in individual cells permitted quantitation of the fraction of cells responsive to TRH. Infection with 100 AdCMVmTRHR particles/cell or more led to TRH responsiveness in > or = 90% of HeLa cells. Measurement of prelabeled phosphoinositides hydrolyzed during prolonged TRH stimulation assesses the size of the TRH-responsive pool. In cells infected with AdCMVmTRHR for 24 h, the size of the TRH-responsive phosphoinositide pool increased with increasing TRH-R expression. The TRH-responsive pool also increased with time after infection as the number of TRH-Rs increased. Similar observations were made in GHY and KB cells. These data confirm our previous suggestion (Cubitt, A. B., Geras-Raaka, E., and Gershengorn, M. C. (1990) Biochem. J. 271, 331-336) that there are hormone-responsive and -unresponsive pools of cellular phosphoinositides and that the maximal size of the TRH-responsive pool is directly related to the number of TRH-Rs.

Quantitative determination of adenovirus-mediated gene delivery to rat cardiac myocytes in vitro and in vivo

To optimize the use of modified adenoviruses as vectors for gene delivery to the myocardium, we have characterized infection of cultured fetal and adult rat cardiac myocytes in vitro and of adult cardiac myocytes in vivo by using a replication-defective adenovirus carrying the chloramphenicol acetyltransferase (CAT) reporter gene driven by the cytomegalovirus promoter (AdCMVCATgD). In vitro, virtually all fetal or adult cardiocytes express the CAT gene when infected with 1 plaque-forming unit of virus per cell. CAT enzymatic activity can be detected in these cells as early as 4 hr after infection, reaching near-maximal levels at 48 hr. In fetal cells, CAT expression was maintained without a loss in activity for at least 1 week. Using in vitro studies as a guide, we introduced the AdCMVCATgD virus directly into adult rat myocardium and compared the expression results obtained from virus injection with those obtained by direct injection of pAdCMVCATgD plasmid DNA. The amount of CAT activity resulting from adenovirus infection of the myocardium was orders of magnitude higher than that seen from DNA injection and was proportional to the amount of input virus. Immunostaining for CAT protein in cardiac tissue sections following adenovirus injection demonstrated large numbers of positive cells, reaching nearly 100% of the myocytes in many regions of the heart. Expression of genes introduced by adenovirus peaked at 5 days but was still detectable 55 days following infection. Adenoviruses are therefore a very useful tool for high-efficiency gene transfer into the cardiovascular system.

3' RNA processing efficiency plays a primary role in generating termination-competent RNA polymerase II elongation complexes

In several mammalian transcription units, a transcription termination mechanism in which efficient termination is dependent on the presence of an intact 3' RNA processing site has been identified. The mouse beta maj-globin transcription unit is one such example, in which an intact poly(A) site is required for efficient transcription termination. It is now evident that 3' mRNA processing sites are not always processed with the same efficiency. In this study, we characterized several pre-mRNAs as substrates for the 3' mRNA processing reaction of cleavage and polyadenylation. We then determined whether poly(A) sites which vary in processing efficiency support a poly(A) site-dependent termination event. The level of processing efficiency was determined in vitro by assays measuring the efficiency of the pre-mRNA cleavage event and in vivo by the level of poly(A) site-dependent mRNA and gene product expression generated in transient transfection assays. The beta maj globin pre-mRNA is very efficiently processed. This efficient processing correlates with its function in termination assays using recombinant adenovirus termination vectors in nuclear run-on assays. When the beta maj globin poly(A) site was replaced by the L1 poly(A) site of the adenovirus major late transcription unit (Ad-ml), which is a poor processing substrate, termination efficiency decreased dramatically. When the beta maj globin poly(A) site was replaced by the Ad-ml L3 poly(A) site, which is 10- to 20-fold more efficiently processed than the Ad-ml L1 poly(A) site, termination efficiency remained high. Termination is therefore dependent on the yield of the processing event. We then tested chimeric poly(A) sites containing the L3 core AAUAAA but varied downstream GU-rich elements. The change in downstream GU-rich elements affected processing efficiency in a manner which correlated with termination efficiency. These experiments provide evidence that the efficiency of 3' processing complex formation is directly correlated to the efficiency of RNA polymerase II termination at the 3' end of a mammalian transcription unit.

Characterization of the mouse beta maj globin transcription termination region: a spacing sequence is required between the poly(A) signal sequence and multiple downstream termination elements

For the majority of mRNA encoding eukaryotic transcription units, there is little or no knowledge of the elements responsible for transcription termination or how they may interact with RNA polymerase. In this report, we have used recombinant adenovirus reporter vectors to characterize the mouse beta maj globin sequence elements that cause transcription termination. Within the globin 3' termination region, we have identified at least three sequence elements which induce significant levels of transcription termination (> 50%). The smallest functionally active element (64% termination) is 69 bp in length. The natural arrangement of these elements results in a cumulative termination which is greater than 90%. Recognition of the termination elements by RNA polymerase II depends on the presence of a functional poly(A) signal sequence. We demonstrate that efficient transcription termination depends on appropriate spacing between the poly(A) signal sequence and the termination element.

Varied poly(A) site efficiency in the adenovirus major late transcription unit

Regulation of adenovirus major late transcription unit (MLTU) mRNA biosynthesis involves poly(A) site selection between five available sites, L1 through L5. The 5' proximal site completely dominates during early infection, whereas all five sites are used during late infection with L3 being favored slightly over the others. Previous studies have shown this early to late poly(A) switch will occur in the absence of MLTU-specific splicing patterns and hinges in large part on the character of the first poly(A) site. We have used in vitro assays to characterize basic features of the L1 and L3 pre-mRNAs which may help define how processing at poly(A) sites is controlled. We have found that L1 is 5-10-fold less efficient than L3 as a substrate for RNA cleavage. A primary difference between the L1 and L3 sites lies in the kinetics of their use, with cleavage at L3 occurring at twice the rate of cleavage at L1. In addition, L1 is 20-fold less effective than L3 in competing for processing factors. To investigate the sequence elements that contribute to poly(A) site efficiency, we have used competition assays in which the competitor RNAs lack upstream or downstream elements.

Sequences regulating temporal poly(A) site switching in the adenovirus major late transcription unit

Temporal regulation of poly(A) site choice occurs in an adenovirus recombinant encoding a miniature version of the major late transcription unit with two poly(A) sites, L1 and L3. Using deletion mutagenesis, we have looked directly for cis-acting elements regulating poly(A) site choice in this recombinant. From this work, we draw two main conclusions. First, elements other than the AAUAAA and downstream sequences of the L1 poly(A) site are required for temporal regulation of poly(A) site choice during infection. Second, these regions function in two distinct modes during infection. The two regions enhance selection of the L1 poly(A) site in an additive manner during an early infection, but deletion of either element abolishes the switch in poly(A) site choice during a late infection. This work documents the first example of a regulatory element downstream of a core poly(A) region.

Regulation of antibody secretion by hybridoma cells. II. Mechanism of idiotype-induced suppression of antibody secretion by hybridoma cells

We have previously reported that antibody secretion by B.22 hybridoma cells can be suppressed in an MHC-restricted manner, by idiotype-specific T cells. It was shown that T cells of both helper and suppressor phenotypes are involved, and that the suppression is mediated by soluble factors. In the present paper, we have characterized the effects of T-cell-mediated suppression at the level of B.22 antibody mRNA expression and stability. Nuclear run-on analysis comparing suppressed and control B.22 cells indicates no change in the transcription rates of heavy and light chains. Northern blot analysis demonstrates that steady-state levels of heavy and light chain mRNAs are also unchanged. Thus, the suppression of antibody secretion by B.22 cells probably occurs at the levels of translation or secretion.

Regulation of poly(A) site selection in adenovirus

We have investigated the mechanisms involved in the early-to-late RNA-processing switch which regulates the mRNA species generated from the adenovirus major late transcription unit (MLTU). In particular, polyadenylation choice mechanisms were characterized by using a reconstructed adenovirus E1A gene as a site for insertion of MLTU poly(A) regulation signals (L1 and L3). Adenovirus constructs containing the variant poly(A) recognition elements were used to compare E1A poly(A) signal utilization with wild-type MLTU (L1 to L5) utilization. In both early and late stages of infection, either polyadenylation site (L1 or L3) is capable of being utilized when presented as the only operational poly(A) site. In an early infection, a virus which contains multiple elements presented in tandem (L13) uses the first poly(A) site, L1, preferentially (ratio of L1 to L3, 8:1) in both E1A and MLTU loci. Transcription termination is not involved in restricting the utilization of the downstream L3 site. In a late infection, when each of the five MLTU poly(A) sites is used, a switch also occurs for the E1AL13 construct, with utilization of both the L1 and L3 poly(A) sites. The switch from early to late was not the result of altered processing factors in the late infection, as demonstrated by superinfecting the E1AL13 construct into cells which had already entered a late stage of infection. The superinfecting virus gave an L1-only phenotype; therefore, a cis mechanism is involved in adenovirus poly(A) regulation.

A poly(A) addition site and a downstream termination region are required for efficient cessation of transcription by RNA polymerase II in the mouse beta maj-globin gene

Sequence elements within the mouse beta maj-globin transcription unit required for efficient termination of transcription by RNA polymerase II have been delineated. To facilitate nascent-chain analysis of termination, the DNA segment in which transcription ceases was introduced into the adenovirus chromosome within its E1A transcription unit. Two beta-globin DNA elements were required to effect efficient termination: an upstream sequence that includes two poly(A) addition signals and a downstream region previously shown to be where RNA synthesis stops. The role of poly(A) addition in termination was established by introduction of several single base pair substitutions into the AATAAA polyadenylylation motifs. These mutations inhibited both polyadenylylation and termination within the beta-globin DNA segment. Therefore, poly(A) addition appears to be a prerequisite for efficient termination.