Establishment of Neurospora crassa as a host for heterologous protein production using a human antibody fragment as a model product

Background: Filamentous fungi are commonly used as production hosts for bulk enzymes in biotechnological applications. Their robust and quick growth combined with their ability to secrete large amounts of protein directly into the culture medium makes fungi appealing organisms for the generation of novel production systems. The red bread mold Neurospora crassa has long been established as a model system in basic research. It can be very easily genetically manipulated and a wealth of molecular tools and mutants are available. In addition, N. crassa is very fast growing and non-toxic. All of these features point to a high but so far untapped potential of this fungus for biotechnological applications. In this study, we used genetic engineering and bioprocess development in a design-build-test-cycle process to establish N. crassa as a production host for heterologous proteins. Results: The human antibody fragment HT186-D11 was fused to a truncated version of the endogenous enzyme glucoamylase (GLA-1), which served as a carrier protein to achieve secretion into the culture medium. A modular expression cassette was constructed and tested under the control of different promoters. Protease activity was identified as a major limitation of the production strain, and the effects of different mutations causing protease deficiencies were compared. Furthermore, a parallel bioreactor system (1 L) was employed to develop and optimize a production process, including the comparison of different culture media and cultivation parameters. After successful optimization of the production strain and the cultivation conditions an exemplary scale up to a 10 L stirred tank reactor was performed. Conclusions: The data of this study indicate that N. crassa is suited for the production and secretion of heterologous proteins. Controlling expression by the optimized promoter Pccg1nr in a fourfold protease deletion strain resulted in the successful secretion of the heterologous product with estimated yields of 3 mg/L of the fusion protein. The fungus could easily be cultivated in bioreactors and a first scale-up was successful. The system holds therefore much potential, warranting further efforts in optimization