High-level expression of recombinant human nerve growth factor beta in milk of nontransgenic rabbits

Challenges and progress of neurodrug: bioactivities, production and delivery strategies of nerve growth factor protein

Nerve growth factor (NGF) is a vital cytokine that plays a crucial role in the development and regeneration of the nervous system. It has been extensively studied for its potential therapeutic applications in various neural diseases. However, as a protein drug, limited natural source seriously hinders its translation and clinical applications. Conventional extraction of NGF from mouse submandibular glands has a very high cost and potentially induces immunogenicity; total synthesis and semi-synthesis methods are alternatives, but have difficulty in obtaining correct protein structure; gene engineering of plant cells is thought to be non-immunogenic, bioactive and economical. Meanwhile, large molecular weight, high polarity, and negative electrical charge make it difficult for NGF to cross the blood brain barrier to reach therapeutic targets. Current delivery strategies mainly depend on the adenovirus and cell biodelivery, but the safety and efficacy remain to be improved. New materials are widely investigated for the controllable, safe and precise delivery of NGF. This review illustrates physiological and therapeutic effects of NGF for various diseases. Moreover, new progress in production and delivery technologies for NGF are summarized. Bottlenecks encountered in the development of NGF as therapeutics are also discussed with the countermeasures proposed.

The transgenic animal platform for biopharmaceutical production

The recombinant production of therapeutic proteins for human diseases is currently the largest source of innovation in the pharmaceutical industry. The market growth has been the driving force on efforts for the development of new therapeutic proteins, in which transgenesis emerges as key component. The use of the transgenic animal platform offers attractive possibilities, residing on the low production costs allied to high productivity and quality of the recombinant proteins. Although many strategies have evolved over the past decades for the generation of transgenic founders, transgenesis in livestock animals generally faces some challenges, mainly due to random transgene integration and control over transgene copy number. But new developments in gene editing with CRISPR/Cas system promises to revolutionize the field for its simplicity and high efficiency. In addition, for the final approval of any given recombinant protein for animal or human use, the production and characterization of bioreactor founders and expression patterns and functionality of the proteins are technical part of the process, which also requires regulatory and administrative decisions, with a large emphasis on biosafety. The approval of two mammary gland-derived recombinant proteins for commercial and clinical use has boosted the interest for more efficient, safer and economic ways to generate transgenic founders to meet the increasing demand for biomedical proteins worldwide.

Adeno-associated-virus-mediated transduction of the mammary gland enables sustained production of recombinant proteins in milk

Biopharming for the production of recombinant pharmaceutical proteins in the mammary gland of transgenic animals is an attractive but laborious alternative compared to mammalian cell fermentation. The disadvantage of the lengthy process of genetically modifying an entire animal could be circumvented with somatic transduction of only the mammary epithelium with recombinant, replication-defective viruses. While other viral vectors offer very limited scope for this approach, vectors based on adeno-associated virus (AAV) appear to be ideal candidates because AAV is helper-dependent, does not induce a strong immune response and has no association with disease. Here, we sought to test the suitability of recombinant AAV (rAAV) for biopharming. Using reporter genes, we showed that injected rAAV efficiently transduced mouse mammary cells. When rAAV encoding human myelin basic protein (hMBP) was injected into the mammary glands of mice and rabbits, this resulted in the expression of readily detectable protein levels of up to 0.5 g/L in the milk. Furthermore we demonstrated that production of hMBP persisted over extended periods and that protein expression could be renewed in a subsequent lactation by re-injection of rAAV into a previously injected mouse gland.

Expression systems and species used for transgenic animal bioreactors

Transgenic animal bioreactors can produce therapeutic proteins with high value for pharmaceutical use. In this paper, we compared different systems capable of producing therapeutic proteins (bacteria, mammalian cells, transgenic plants, and transgenic animals) and found that transgenic animals were potentially ideal bioreactors for the synthesis of pharmaceutical protein complexes. Compared with other transgenic animal expression systems (egg white, blood, urine, seminal plasma, and silkworm cocoon), the mammary glands of transgenic animals have enormous potential. Compared with other mammalian species (pig, goat, sheep, and cow) that are currently being studied as bioreactors, rabbits offer many advantages: high fertility, easy generation of transgenic founders and offspring, insensitivity to prion diseases, relatively high milk production, and no transmission of severe diseases to humans. Noticeably, for a small- or medium-sized facility, the rabbit system is ideal to produce up to 50 kg of protein per year, considering both economical and hygienic aspects; rabbits are attractive candidates for the mammary-gland-specific expression of recombinant proteins. We also reviewed recombinant proteins that have been produced by targeted expression in the mammary glands of rabbits and discussed the limitations of transgenic animal bioreactors.

High level expression of recombinant human antithrombin in the mammary gland of rabbits by adenoviral vectors infection

Expression of recombinant pharmaceutical proteins in the mammalian mammary gland is of great interest for the medical industry. This study was designed to express recombinant human antithrombin (rhAT) in the mammary gland of rabbits by adenovirus vectors infection. Replication-defective adenovirus encoding human antithrombin complementary DNA (cDNA) was constructed and directly infused into the mammary gland of rabbits via the teat canal. The milk serum was collected from the infected mammary gland 48 h post-infection and subjected to Western blot analysis, Enzyme-linked immunosorbent assay (ELISA), and antithrombotic activity assay. In this way, the target protein was verified, and a high expression level of rhAT up to 4.8 g/L was obtained, and antithrombotic activity of the rhAT was not different than that of a standard human antithrombin protein (p > 0.05). Compared to previous attempts to produce human antithrombin in the mammary gland of transgenic animals or fractionation the plasma of blood donors, the method for rhAT expression we established would reduce production cost and further increase production efficacy.

Subunit influenza vaccine candidate based on CD154 fused to HAH5 increases the antibody titers and cellular immune response in chickens

World Health Organization has a great concern about the spreading of avian influenza virus H5N1. To counteract its massive spread, poultry vaccination is highly recommended together with biosecurity measures. In our study, a recombinant vaccine candidate based on the fusion of extracellular segments of hemagglutinin (HA) H5 of avian influenza virus and chicken CD154 (HACD) is tested with the aim of enhancing humoral and cellular immune responses in chickens. Protein expression was carried out by transducing several mammalian cell lines with recombinant adenoviral vectors. HACD purification was assessed by three distinct purification protocols: immunoaffinity chromatography by elution at acidic pH or with a chaotropic agent and size exclusion chromatography. Humoral and cellular immune responses were measured using the hemagglutination inhibition assay and the semiquantitative real time PCR, respectively. The results showed that humoral response against HACD was significantly higher than the obtained with HA alone after booster (P<0.01, P<0.05). From HACD molecules purified by distinct protocols, only the obtained by size exclusion chromatography generated hemagglutinationin-inhibition activity. IFN-γ levels indicated that cellular immune response was significantly higher with HACD, in its pure or impure form, compared to its counterpart HA (P<0.01). These data demonstrate that HACD is able to significantly enhance humoral and cellular immune responses against HA antigen, which make this fusion protein a promising subunit vaccine candidate against H5N1 virus outbreaks.