Michael A. Romanos

Cloning, distribution and functional analysis of the type III sodium channel from human brain.

The type III voltage‐gated sodium channel was cloned from human brain. The full‐length cDNA has 89% identity with rat type III, and the predicted protein (1951 amino acids) has 55 differences. The expression pattern of human type III mRNA was determined in adult brain tissue and, in contrast to rat, was detected in many regions, including caudate nucleus, cerebellum, hippocampus and frontal lobe. The human type III channel was stably expressed in Chinese hamster ovary (CHO) cells and its biophysical properties compared to the human type II channel using identical conditions. The voltage dependence and kinetics of activation were found to be similar to that of type II. The kinetics of inactivation of the two human subtypes were also similar. However, type III channels inactivated at more hyperpolarized potentials and were slower to recover from inactivation than type II. When expressed in human embryonic kidney (HEK293T) cells, type III channels produced currents with a prominent persistent component, which were similar to those reported for rat type II [Ma et al. (1997) Neuron, 19, 443–452]. However, unlike type II, this was prominent even in the absence of coexpressed G‐proteins, suggesting type III may adopt this gating mode more readily. The distinct properties of the channel, together with its wide distribution in adult brain, suggest that in humans, type III may have important physiological roles under normal, and perhaps also pathological conditions.

Recombinant Bordetella pertussis pertactin (P69) from the yeast Pichia pastoris: high-level production and immunological properties

Acellular whooping cough vaccines are based on pertussis toxoid but their effectiveness may be increased by the addition of other Bordetella pertussis antigens. We expressed the immunogenic outer membrane protein pertactin (P69) from B. pertussis to high levels in multi-copy transformants of the industrial yeast Pichia pastoris. In high-density fermentations, engineered P. pastoris yielded greater than 3 g of the protein per litre of culture. Purified recombinant pertactin was able to stimulate the incomplete protection afforded by toxoid to the level of the whole-cell vaccine, as shown by the Kendrick test, supporting its inclusion in future acellular vaccines.

Molecular cloning, distribution and functional analysis of the NAV1.6. Voltage-gated sodium channel from human brain

We have cloned and expressed the full-length human Na(V)1.6 sodium channel cDNA. Northern analysis showed that the hNa(V)1.6 gene, like its rodent orthologues, is abundantly expressed in adult brain but not other tissues including heart and skeletal muscle. Within the adult brain, hNa(V)1.6 mRNA is widely expressed with particularly high levels in the cerebellum, occipital pole and frontal lobe. When stably expressed in human embryonic kidney cells (HEK293), the hNa(V)1.6 channel was found to be very similar in its biophysical properties to human Na(V)1.2 and Na(V)1.3 channels [Eur. J. Neurosci. 12 (2000) 4281-4289; Pflügers Arch. 441 (2001) 425-433]. Only relatively subtle differences were observed, for example, in the voltage dependence of gating. Like hNa(V)1.3 channels, hNa(V)1.6 produced sodium currents with a prominent persistent component when expressed in HEK293 cells. These persistent currents were similar to those reported for the rat Na(V)1.2 channel [Neuron 19 (1997) 443-452], although they were not dependent on over-expression of G protein betagamma subunits. These data are consistent with the proposal that Na(V)1.6 channels may generate the persistent currents observed in cerebellar Purkinje neurons [J. Neurosci. 17 (1997) 4157-4536]. However, in our hNa(V)1.6 cell line we have been unable to detect the resurgent currents that have also been described in Purkinje cells. Although Na(V)1.6 channels have been implicated in producing these resurgent currents [Neuron 19 (1997) 881-891], our data suggest that this may require modification of the Na(V)1.6 alpha subunit by additional factors found in Purkinje neurons but not in HEK293 cells.

BacMam Recombinant Baculoviruses in G Protein–Coupled Receptor Drug Discovery

With completion of the sequencing of the human and mouse genomes, the primary sequences of close to 400 non-olfactory G protein-coupled receptors (GPCRs) have been determined. There are intensive efforts within the pharmaceutical industry to discover and develop new therapeutic agents acting via GPCRs. In addition, there is a concerted effort to identify potential new drug targets from the remaining 150+orphan GPCRs through the identification of their ligands. Access to functionally expressed recombinant receptors underpins both of these key drug discovery activities. Typically, GPCR drug discovery screening activities are carried out using mammalian cell lines stably expressing the target of interest. The influx of new receptor sequences originating from genomic sequencing efforts has caused a shift toward wider applications of transient rather than stable expression systems, especially in support of assays for orphan receptor ligand screening. Recombinant baculoviruses in which the polyhedrin promoter has been replaced with a mammalian promoter, termed BacMam viruses, were originally designed as potential new gene therapy delivery vehicles. This same technology offers numerous advantages as a transient expression system in the assay of membrane-expressed drug targets, including GPCRs. Data presented show that BacMam can be used rapidly to generate robust and pharmacologically authentic GPCR assays in several formats, with the potential to transform drug discovery screening processes for this gene family.

Baculoviruses and mammalian cell-based assays for drug screening.

Publisher Summary A few years ago, a new use for recombinant baculoviruses was revealed with the appearance of two publications demonstrating the delivery of recombinant gene expression cassettes containing reporter genes under control of mammalian cell-active promoters to mammalian cells primarily of liver origin. Subsequent work has shown that a number of cell types are susceptible to transduction. This system, referred to as “BacMam,” has found utility in a number of laboratories. This chapter focuses on the application of BacMam for configuring cell-based assays for automated screening of chemical libraries. Examples have been provided here from a wide range of target molecules demonstrating that the ease and versatility of BacMam-mediated gene delivery make it an excellent alternative to stable cell lines for the development of cell-based assays for drug screening. Inhibitors of histone deacetylase have been shown to enhance levels of gene expression in BacMam-transduced CHO cells, as have transcriptional transactivators that act on the CMV promoter. The report of an avian adenovirus gene product that acts as a histone deacetylase inhibitor and enhances gene expression in CHO cells suggests that modifications may be made to CHO cells to obtain new derivatives that act as efficient hosts for BacMam transduction. BacMam affords the ability to tailor an assay to better mimic a response seen in native tissues. The chapter shows that transduction of primary cells with BacMam viruses that direct the expression of a selectable marker and immortalization genes, such as large T antigen, may provide an efficient and gentle method to establish cell lines from primary tissues. Implementing BacMam-based assays on a large scale requires attention to the logistical considerations associated with handling of large numbers of biological agents discussed in this chapter. These considerations include: optimized virus production methods, reproducible virus titration, quality control assays, long-term storage/inventory systems, and distribution.

Production of a phosphorylated GST::HPV-6 E7 fusion protein using a yeast expression vector and glutathione S-transferase fusions

A Saccharomyces cerevisiae GAL7 expression vector for the production of protein fusions to glutathione S-transferase (GST) has been constructed. Using this vector, a GST fusion to human papillomavirus type 6 (HPV-6) E7 protein was produced and purified by affinity chromatography in a single step, at a yield of 2 micrograms/ml of culture. The E7 portion of the fusion protein was phosphorylated, in contrast to the same product made in Escherichia coli. Therefore, yeast GST vectors may be of specific use in producing phosphoproteins, or proteins with other eukaryotic post-translational modifications, in preparative amounts for in vitro analysis.

A high capacity assay for inhibitors of human papillomavirus DNA replication.

The discovery of antiviral compounds against human papillomaviruses (HPV) has been hindered by the difficulties in culturing virus in vitro or assaying stable HPV DNA replication. However, plasmids containing the HPV replication origin replicate transiently upon co-transfection with HPV E1 and E2 expression vectors. We have adapted this assay using secreted alkaline phosphatase (SAP) as a reporter for rapid analysis of DNA copy number. Use of the S V40 early promoter in controlling SAP expression was critical in ensuring both a strong signal and copy number dependence: the stronger β-actin promoter inhibited replication, while the weaker SV40 late promoter yielded very low levels of SAP. The precise configuration of the E1 and E2 expression vectors also was critical, most pre-existing vectors did not support efficient replication and SAP secretion. The extent of DNA replication and SAP secretion were both proportional to the amount of E1/E2 vector used in transfections; under optimal conditions SAP increased 100-fold during replication. The assay has been developed for compound screening in 96-well plates and several inhibitors have been identified. Quantitative Southern blot analysis has shown that most of these inhibit HPV DNA replication rather than SAP accumulation or activity, and several are under test hi models of viral replication. The assay also provides a rapid system for functional analysis of the HPV E1, E2 genes and the replication origin.

Culture of yeast for the production of heterologous proteins.

This unit describes culture of the yeast strains Saccharomyces cerevisiae and Pichia pastoris for the production of foreign proteins. The protocols listed here for S. cerevisiae are for three widely used types of promoter: galactose‐regulated (GAL1, GAL7, GAL10), glucose‐repressible (e.g., ADH2), and constitutive glycolytic (e.g., PGK or GAPDH). Minor variations to each can be made depending on the selection system used. The P. pastoris expression system uses integrating vectors with the methanol‐regulated AOX1 promoter and HIS4 selection marker; although transformants are stable, they are generally grown in minimal selective medium. Methods are described for small‐scale S. cerevisiae and P. pastoris cultures and also for high‐density fermentations with these yeasts. A simple feeding strategy based on calculated feed rates is provided for S. cerevisiae and yields cell densities of 10 to 30 g/liter. In contrast, with P. pastoris, basic fermenter equipment is used to obtain extremely high‐density cultures (e.g., 130 g/liter). Finally, a describes small‐scale preparation of protein extracts.