Eliminating viscosity challenges in continuous cultivation of yeast producing a GLP-1 like peptide.

Background: The emergence of GLP-1s for the treatment of diabetes, obesity and other diseases has led to increased focus on finding efficient ways to produce the peptides in sufficient amounts to satisfy the ever-increasing demand. Although the use of microbial hosts constitutes the cheapest, easiest and safest way to produce these peptides in high volumes, process challenges still exist that reduce the production capacity. One of the main production challenges is the high viscosity of cultivation broths, which reduces the mass and oxygen transfer, thereby creating substrate and oxygen gradients that potentially lead to unwanted secondary metabolism and eventually compromises capacity.

Results: The methodology used to identify the underlying factors of highly viscous broths during the recombinant production of GLP-1 precursors in S. cerevisiae in continuous cultivation is presented. Two root causes leading to highly viscous broths were uncovered and solutions identified. The first one is found in the soluble fraction of the broth and relates to the aggregation of GLP-1 precursor molecules that leads to highly viscous, shear thinning cultivation broths. The cultivation conditions under which the aggregation occurs and the consequences for both cultivation and product recovery are discussed. The second source of viscosity is found in the insoluble fraction of the cultivation broth and relates to cell aggregation due to Amn1p dependent incomplete separation of mother and daughter cells. This type of cell aggregation causes formation of cell clumps and leads to high viscosity cultivation broths with mild shear thickening properties.

Conclusions: To eliminate the GLP-1 peptide related viscosity, a new generation of yeast host strains that tolerates cultivation at increased pH values, above those that cause GLP-1 precursor aggregation, were utilized. In the case of the cell derived viscosity, yeast strains carrying either a deletion of the AMN1 gene or integration of the non-clumping AMN1^D368V gene variant were employed. The implementation of these changes led to a scalable cultivation process characterized by a significant improved oxygen mass transfer attributed to the low viscosity and Newtonian behaviour of the cultivation broth.