Most of the hosts used to produce the 151 recombinant pharmaceuticals so far approved for human use by the Food and Drug Administration (FDA) and/or by the European Medicines Agency (EMEA) are microbial cells, either bacteria or yeast. This fact indicates that despite the diverse bottlenecks and obstacles that microbial systems pose to the efficient production of functional mammalian proteins, namely lack or unconventional post-translational modifications, proteolytic instability, poor solubility and activation of cell stress responses, among others, they represent convenient and powerful tools for recombinant protein production. The entering into the market of a progressively increasing number of protein drugs produced in non-microbial systems has not impaired the development of products obtained in microbial cells, proving the robustness of the microbial set of cellular systems (so far Escherichia coli and Saccharomyces cerevisae) developed for protein drug production. We summarize here the nature, properties and applications of all those pharmaceuticals and the relevant features of the current and potential producing hosts, in a comparative way.

Diabetes, growth or clotting disorders are among the spectrum of human diseases related to protein absence or malfunction. Since these pathologies cannot be yet regularly treated by gene therapy, the administration of functional proteins produced ex vivo is required. As both protein extraction from natural producers and chemical synthesis undergo inherent constraints that limit regular large-scale production, recombinant DNA technologies have rapidly become a choice for therapeutic protein production. The spectrum of organisms exploited as recombinant cell factories has expanded from the early predominating Escherichia coli to alternative bacteria, yeasts, insect cells and especially mammalian cells, which benefit from metabolic and protein processing pathways similar to those in human cells. Up to date, around 650 protein drugs have been worldwide approved, among which about 400 are obtained by recombinant technologies. Other 1300 recombinant pharmaceuticals are under development, with a clear tendency towards engineered versions with improved performance and new functionalities regarding the conventional, plain protein species. This trend is exemplified by the examination of the contemporary protein-based drugs developed for cancer treatment.

Autoregulation of the ciliary beat frequency (CBF) has been proposed as the mechanism used by epithelial ciliated cells to maintain the CBF and prevent the collapse of mucociliary transport under conditions of varying mucus viscosity. Despite the relevance of this regulatory response to the pathophysiology of airways and reproductive tract, the underlying cellular and molecular aspects remain unknown. Hamster oviductal ciliated cells express the transient receptor potential vanilloid 4 (TRPV4) channel, which is activated by increased viscous load involving a phospholipase A₂–dependent pathway. TRPV4-transfected HeLa cells also increased their cationic currents in response to high viscous load. This mechanical activation is prevented in native ciliated cells loaded with a TRPV4 antibody. Application of the TRPV4 synthetic ligand 4α-phorbol 12,13-didecanoate increased cationic currents, intracellular Ca²⁺, and the CBF in the absence of a viscous load. Therefore, TRPV4 emerges as a candidate to participate in the coupling of fluid viscosity changes to the generation of the Ca²⁺ signal required for the autoregulation of CBF.

Bacterial inclusion bodies (IBs) are functional, non-toxic amyloids occurring in recombinant bacteria showing analogies with secretory granules of the mammalian endocrine system. The scientific interest in these mesoscale protein aggregates has been historically masked by their status as a hurdle in recombinant protein production. However, progressive understanding of how the cell handles the quality of recombinant polypeptides and the main features of their intriguing molecular organization has stimulated the interest in inclusion bodies and spurred their use in diverse technological fields. The engineering and tailoring of IBs as functional protein particles for materials science and biomedicine is a good example of how formerly undesired bacterial byproducts can be rediscovered as promising functional materials for a broad spectrum of applications.

Hypertension is the most prevalent risk factor for cardiovascular diseases, present in almost 30% of adults. A key element in the control of vascular tone is the large-conductance, Ca²⁺-dependent K⁺ (BK) channel. The BK channel in vascular smooth muscle is formed by an ion-conducting α subunit and a regulatory β₁ subunit, which couples local increases in intracellular Ca²⁺ to augmented channel activity and vascular relaxation. Our large population-based genetic epidemiological study has identified a new single-nucleotide substitution (G352A) in the β₁ gene (KCNMB1), corresponding to an E65K mutation in the protein. This mutation results in a gain of function of the channel and is associated with low prevalence of moderate and severe diastolic hypertension. BK-β_1E65K channels showed increased Ca²⁺ sensitivity, compared with wild-type channels, without changes in channel kinetics. In conclusion, the BK-β_1E65K channel might offer a more efficient negative-feedback effect on vascular smooth muscle contractility, consistent with a protective effect of the K allele against the severity of diastolic hypertension.

The vertebrate transient receptor potential cationic channel TRPV4 has been proposed as an osmo- and mechanosensor channel. Studies using knock-out animal models have further emphasized the relevance of the TRPV4 channel in the maintenance of the internal osmotic equilibrium and mechanosensation. However, at the cellular level, there is still one important question to answer: does the TRPV4 channel generate the Ca²⁺ signal in those cells undergoing a Ca²⁺-dependent regulatory volume decrease (RVD) response? RVD in human airway epithelia requires the generation of a Ca²⁺ signal to activate Ca²⁺-dependent K⁺ channels. The RVD response is lost in airway epithelia affected with cystic fibrosis (CF), a disease caused by mutations in the cystic fibrosis transmembrane conductance regulator channel. We have previously shown that the defective RVD in CF epithelia is linked to the lack of swelling-dependent activation of Ca²⁺-dependent K⁺ channels. In the present study, we show the expression of TRPV4 in normal human airway epithelia, where it functions as the Ca²⁺ entry pathway that triggers the RVD response after hypotonic stress, as demonstrated by TRPV4 antisense experiments. However, cell swelling failed to trigger Ca²⁺ entry via TRPV4 channels in CF airway epithelia, although the channel's response to a specific synthetic activator, 4α-phorbol 12,13-didecanoate, was maintained. Furthermore, RVD was recovered in CF airway epithelia treated with 4α-phorbol 12,13-didecanoate. Together, these results suggest that defective RVD in CF airway epithelia might be caused by the absence of a TRPV4-mediated Ca²⁺ signal and the subsequent activation of Ca²⁺-dependent K⁺ channels.