The biosynthetic pathway of FK506 and its engineering: from past achievements to future prospects

Generation of Bioactivity-Tailored FK506/FK520 Analogs by CRISPR Editing in Streptomyces tsukubaensis

For a potential application of FK506 in the treatment of acute kidney failure only the FKBP12 binding capability of the compound is required, while the immunosuppressive activity via calcineurin binding is considered as a likely risk to the patients. The methoxy groups at C13 and C15 are thought to have significant influence on the immunosuppressive activity of the molecule. Consequently, FK506 analogs with different functionalities at C13 and C15 were generated by targeted CRISPR editing of the AT domains in module 7 and 8 of the biosynthetic assembly line in Streptomyces tsukubaensis. In addition, the corresponding FK520 (C21 ethyl derivative of FK506) analogs could be obtained by media adjustments. The compounds were tested for their bioactivity in regards to FKBP12 binding, BMP potentiation and calcineurin sparing. 15-desmethoxy FK506 was superior to the other tested analogs as it did not inhibit calcineurin but retained high potency towards FKBP12 binding and BMP potentiation.

Optimization of the precursor supply for an enhanced FK506 production in Streptomyces tsukubaensis

Tacrolimus (FK506) is a macrolide widely used as immunosuppressant to prevent transplant rejection. Synthetic production of FK506 is not efficient and costly, whereas the biosynthesis of FK506 is complex and the level produced by the wild type strain, Streptomyces tsukubaensis, is very low. We therefore engineered FK506 biosynthesis and the supply of the precursor L-lysine to generate strains with improved FK506 yield. To increase FK506 production, first the intracellular supply of the essential precursor lysine was improved in the native host S. tsukubaensis NRRL 18488 by engineering the lysine biosynthetic pathway. Therefore, a feedback deregulated aspartate kinase AskSt* of S. tsukubaensis was generated by site directed mutagenesis. Whereas overexpression of AskSt* resulted only in a 17% increase in FK506 yield, heterologous overexpression of a feedback deregulated AskCg* from Corynebacterium glutamicum was proven to be more efficient. Combined overexpression of AskCg* and DapASt, showed a strong enhancement of the intracellular lysine pool following increase in the yield by approximately 73% compared to the wild type. Lysine is coverted into the FK506 building block pipecolate by the lysine cyclodeaminase FkbL. Construction of a ∆fkbL mutant led to a complete abolishment of the FK506 production, confirming the indispensability of this enzyme for FK506 production. Chemical complementation of the ∆fkbL mutant by feeding pipecolic acid and genetic complementation with fkbL as well as with other lysine cyclodeaminase genes (pipAf, pipASt, originating from Actinoplanes friuliensis and Streptomyces pristinaespiralis, respectively) completely restored FK506 production. Subsequently, FK506 production was enchanced by heterologous overexpression of PipAf and PipASp in S. tsukubaensis. This resulted in a yield increase by 65% compared to the WT in the presence of PipAf from A. friuliensis. For further rational yield improvement, the crystal structure of PipAf from A. friuliensis was determined at 1.3 Å resolution with the cofactor NADH bound and at 1.4 Å with its substrate lysine. Based on the structure the Ile91 residue was replaced by Val91 in PipAf, which resulted in an overall increase of FK506 production by approx. 100% compared to the WT.

A periodontal tissue regeneration strategy via biphasic release of zeolitic imidazolate framework-8 and FK506 using a uniaxial electrospun Janus nanofiber

Guided tissue regeneration (GTR) strategies are an effective approach to repair periodontal defects by using GTR membranes. However, commercial GTR membranes still have limitations in periodontal tissue regeneration owing to lack of antibacterial and osteogenic properties. The development of novel Janus nanofibers with biphasic release characteristics based on the therapeutic needs of GTR is essential to tackle this issue. Here, we developed a multifunctional Janus nanofiber via uniaxial electrospinning, with zeolitic imidazolate framework-8 nanoparticle (ZIF-8 NP) loading in the hydrophilic polyvinylpyrrolidone (PVP) part and FK506 embedding in the hydrophobic polycaprolactone (PCL) part. The release of Zn²⁺ conformed to the Ritger-Peppas kinetics which could effectively prevent bacterial infection, and the release profile of FK506 was fitted to a first-order equation which could provide persistent osteogenic stimulation for osteogenesis. The periodontal tissue regeneration data from a rat periodontitis model revealed that the multifunctional electrospun Janus nanofibers could be used as an effective bioplatform to restore alveolar bone impairment, compared with the control group. In summary, the Janus nanofibers with biphasic release characteristics quickly exert antibacterial function as well as continuously provide a microenvironment beneficial to the osteogenesis process, demonstrating its great potential for GTR treatment in dental clinic applications.

Genome mining for drug discovery: progress at the front end

Microbial genome mining for drug discovery and development has been accelerating in recent years, driven by technical advancements in genome sequencing, bioinformatics, metabolomics/metabologenomics, and synthetic biology. Microbial genome mining is a multistep process that starts with the sequencing of microbes that encode multiple secondary metabolites and identifying new and novel secondary metabolite biosynthetic gene clusters (BGCs) to pursue. The initial steps in the process are critical for the overall success, and they encompass the most innovative new technologies to revitalize natural product discovery. As microbial genome mining has matured in recent years, unvalidated conjectures about what microbes to pursue, how to identify legitimate secondary metabolite BGCs, and how to sequence DNA to satisfactory levels of completion have been identified. The solutions to correct the misconceptions around these topics are beginning to be implemented.

Streptomyces tsukubensis VKM Aс-2618D-an Effective Producer of Tacrolimus

The Streptomyces sp. VKM Ac-2618D strain has been identified, and its morphological and physiological features have been studied in relation to the production of the immunosuppressant tacrolimus. The phenotypic variability of the strain was analyzed, and a dissociant with a high level of tacrolimus production was selected. Based on a comprehensive study of morphological, physiological, and chemotaxonomic properties and on phylogenetic analysis, the strain was named Streptomyces tsukubensis VKM Ac-2618D. The strain genome contains the full version of the tacrolimus biosynthetic gene cluster. The advantages of fed-batch cultivation mode for tacrolimus biosynthesis are shown. The results broaden the understanding of the characteristics of polyketide biosynthesis and can be used in the development of technology for tacrolimus production.

Improvement of FK506 production via metabolic engineering-guided combinational strategies in Streptomyces tsukubaensis

Background

FK506, a macrolide mainly with immunosuppressive activity, can be produced by various Streptomyces strains. However, one of the major challenges in the fermentation of FK506 is its insufficient production, resulting in high fermentation costs and environmental burdens. Herein, we tried to improve its production via metabolic engineering-guided combinational strategies in Streptomyces tsukubaensis.

Results

First, basing on the genome sequencing and analysis, putative competitive pathways were deleted. A better parental strain L19-2 with increased FK506 production from 140.3 to 170.3 mg/L and a cleaner metabolic background was constructed. Subsequently, the FK506 biosynthetic gene cluster was refactored by in-situ promoter-substitution strategy basing on the regulatory circuits. This strategy enhanced transcription levels of the entire FK506 biosynthetic gene cluster in a fine-tuning manner and dramatically increased the FK506 production to 410.3 mg/mL, 1.41-fold higher than the parental strain L19-2 (170.3 mg/L). Finally, the FK506 production was further increased from 410.3 to 603 mg/L in shake-flask culture by adding L-isoleucine at a final concentration of 6 g/L. Moreover, the potential of FK506 production capacity was also evaluated in a 15-L fermenter, resulting in the FK506 production of 830.3 mg/L.

Conclusion

From the aspects of competitive pathways, refactoring of the FK506 biosynthetic gene cluster and nutrients-addition, a strategy for hyper-production and potentially industrial application of FK506 was developed and a hyper-production strain L19-9 was constructed. The strategy presented here can be generally applicable to other Streptomyces for improvement of FK506 production and streamline hyper-production of other valuable secondary metabolites.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01660-w.

Bioengineering of Anti-Inflammatory Natural Products

Inflammatory processes occur as a generic response of the immune system and can be triggered by various factors, such as infection with pathogenic microorganisms or damaged tissue. Due to the complexity of the inflammation process and its role in common diseases like asthma, cancer, skin disorders or Alzheimer's disease, anti-inflammatory drugs are of high pharmaceutical interest. Nature is a rich source for compounds with anti-inflammatory properties. Several studies have focused on the structural optimization of natural products to improve their pharmacological properties. As derivatization through total synthesis is often laborious with low yields and limited stereoselectivity, the use of biosynthetic, enzyme-driven reactions is an attractive alternative for synthesizing and modifying complex bioactive molecules. In this minireview, we present an outline of the biotechnological methods used to derivatize anti-inflammatory natural products, including precursor-directed biosynthesis, mutasynthesis, combinatorial biosynthesis, as well as whole-cell and in vitro biotransformation.

Biosynthesis of Nonimmunosuppressive ProlylFK506 Analogues with Neurite Outgrowth and Synaptogenic Activity

Separating the immunosuppressive activity of FK506 (1) from its neurotrophic activity is required to develop FK506 analogues as drugs for the treatment of neuronal diseases. Two new FK506 analogues, 9-deoxo-36,37-dihydro-prolylFK506 (2) and 9-deoxo-31-O-demethyl-36,37-dihydro-prolylFK506 (3) containing a proline moiety instead of the pipecolate ring at C-1 and modifications at the C-9/C-31 and C-36-C-37 positions, respectively, were biosynthesized, and their biological activities were evaluated. The proline substitution in 9-deoxo-36,37-dihydroFK506 and 9-deoxo-31-O-demethyl-36,37-dihydroFK506 reduced immunosuppressive activity by more than 120-fold, as previously observed. Compared with FK506 (1), 2 and 3 exhibited ∼1.2 × 105- and 2.2 × 105-fold reductions in immunosuppressive activity, respectively, whereas they retained almost identical neurite outgrowth activity. Furthermore, these compounds significantly increased the strength of synaptic transmission, confirming that replacement of the pipecolate ring with a proline is critical to reduce the strong immunosuppressive activity of FK506 (1) while enhancing its neurotrophic activity.

Bluemomycin, a new naphthoquinone derivative from Streptomyces sp. with antimicrobial and cytotoxic properties

Streptomyces is one of the most prolific producers of economically important bioactive compounds used against several illnesses; it has also been found to produce industrially useful enzymes. In this study, Streptomyces sp. (ERINLG-201) was isolated from the soil sample of Kodanad forest (Southern Western Ghats), The Nilgiris, Tamil Nadu, India. ERINLG-201 isolate showed promising antibacterial activity against tested Gram-positive and Gram-negative bacteria which was confirmed by perpendicular 'T' streak method. Secondary metabolites of ERINLG-201 isolate exhibited promising antibacterial activity against tested Gram-positive and Gram-negative bacteria which was confirmed by disc diffusion method using the ethylacetate extract. Further, the ethylacetate extract of ERINLG-201 (15 g) was packed in column chromatography over silica gel and eluted; it resulted in isolation of a new naphthoquinone derivative named bluemomycin from the active fraction. Bluemomycin showed promising antibacterial activity against Gram-negative bacteria and clinical isolates at least concentration (6.25 µg/mL). Cytotoxic studies of bluemomycin showed promising activity against A549, Skvo-3 and HepG2 cell lines with IC₅₀ values of 5.9, 24.2 and 11 µM, respectively.

Interactions of FK506 and Rapamycin With FK506 Binding Protein 12 in Opportunistic Human Fungal Pathogens

Over the past few decades advances in modern medicine have resulted in a global increase in the prevalence of fungal infections. Particularly people undergoing organ transplants or cancer treatments with a compromised immune system are at an elevated risk for lethal fungal infections such as invasive candidiasis, aspergillosis, cryptococcosis, etc. The emergence of drug resistance in fungal pathogens poses a serious threat to mankind and it is critical to identify new targets for the development of antifungals. Calcineurin and TOR proteins are conserved across eukaryotes including pathogenic fungi. Two small molecules FK506 and rapamycin bind to FKBP12 immunophilin and the resulting complexes (FK506-FKBP12 and rapamycin-FKBP12) target calcineurin and TOR, respectively in both humans and fungi. However, due to their immunosuppressive nature these drugs in the current form cannot be used as an antifungal. To overcome this, it is important to identify key differences between human and fungal FKBP12, calcineurin, and TOR proteins which will facilitate the development of new small molecules with higher affinity toward fungal components. The current review highlights FK506/rapamycin-FKBP12 interactions with calcineurin/TOR kinase in human and fungi, and development of non-immunosuppressive analogs of FK506, rapamycin, and novel small molecules in inhibition of fungal calcineurin and TOR kinase.

Development of Non-Immunosuppressive FK506 Derivatives as Antifungal and Neurotrophic Agents

FK506, also known as tacrolimus, is a clinically important immunosuppressant drug and has promising therapeutic potentials owing to its antifungal, neuroprotective, and neuroregenerative activities. To generate various FK506 derivatives, the structure of FK506 has been modified by chemical methods or biosynthetic pathway engineering. Herein, we describe the mode of the antifungal action of FK506 and the structure-activity relationship of FK506 derivatives in the context of immunosuppressive and antifungal activities. In addition, we discuss the neurotrophic mechanism of FK506 known to date, along with the neurotrophic FK506 derivatives with significantly reduced immunosuppressive activity. This review suggests the possibility to generate novel FK506 derivatives as antifungal as well as neuroregenerative/neuroprotective agents.

Harnessing calcineurin-FK506-FKBP12 crystal structures from invasive fungal pathogens to develop antifungal agents

Calcineurin is important for fungal virulence and a potential antifungal target, but compounds targeting calcineurin, such as FK506, are immunosuppressive. Here we report the crystal structures of calcineurin catalytic (CnA) and regulatory (CnB) subunits complexed with FK506 and the FK506-binding protein (FKBP12) from human fungal pathogens (Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans and Coccidioides immitis). Fungal calcineurin complexes are similar to the mammalian complex, but comparison of fungal and human FKBP12 (hFKBP12) reveals conformational differences in the 40s and 80s loops. NMR analysis, molecular dynamic simulations, and mutations of the A. fumigatus CnA/CnB-FK506-FKBP12-complex identify a Phe88 residue, not conserved in hFKBP12, as critical for binding and inhibition of fungal calcineurin. These differences enable us to develop a less immunosuppressive FK506 analog, APX879, with an acetohydrazine substitution of the C22-carbonyl of FK506. APX879 exhibits reduced immunosuppressive activity and retains broad-spectrum antifungal activity and efficacy in a murine model of invasive fungal infection.

Biosynthesis of Nonimmunosuppressive FK506 Analogues with Antifungal Activity

A reduction in the strong immunosuppressive activity of FK506 (1) is essential for developing this compound as an antifungal agent. Seven new FK506 analogues modified at both the FK506-binding protein 12- and the calcineurin-binding regions were biosynthesized. 9-DeoxoFK520 (7) exhibited a >900-fold reduction in the in vitro immunosuppressive activity but maintained significant antifungal activity, indicating that the C-9 and C-21 positions are critical for separation of immunosuppressive and antifungal activities. 7 exhibited robust synergistic antifungal activity with fluconazole. FK506 (1) is a 23-membered macrolide produced by several Streptomyces species and is used as an immunosuppressive drug to prevent the rejection of transplanted organs. FK506 has also exhibited antifungal, neuroprotective, and neuroregenerative activities. In humans, FK506 binds to FK506-binding protein (FKBP) 12, and the resulting FKBP12-FK506 complex interacts with a Ca²⁺-calmodulin-dependent phosphatase, calcineurin (CaN). Inactivation of CaN by forming the FKBP12-FK506-CaN ternary complex prevents the activation of nuclear factor of activated T cells (NF-AT), inhibiting the production of interleukin-2 and subsequent T-cell proliferation. This CaN signaling pathway also plays a critical role in the growth and pathogenesis of major fungal pathogens such as Cryptococcus neoformans, Candida albicans, and Aspergillus fumigatus. Therefore, the synthesis of FK506 analogues that can discriminate human FKBP12/CaN from its fungal counterparts may separate antifungal activity from the immunosuppressive activity, thereby allowing the development of a novel antifungal agent.

A Review of the Microbial Production of Bioactive Natural Products and Biologics

A variety of organisms, such as bacteria, fungi, and plants, produce secondary metabolites, also known as natural products. Natural products have been a prolific source and an inspiration for numerous medical agents with widely divergent chemical structures and biological activities, including antimicrobial, immunosuppressive, anticancer, and anti-inflammatory activities, many of which have been developed as treatments and have potential therapeutic applications for human diseases. Aside from natural products, the recent development of recombinant DNA technology has sparked the development of a wide array of biopharmaceutical products, such as recombinant proteins, offering significant advances in treating a broad spectrum of medical illnesses and conditions. Herein, we will introduce the structures and diverse biological activities of natural products and recombinant proteins that have been exploited as valuable molecules in medicine, agriculture and insect control. In addition, we will explore past and ongoing efforts along with achievements in the development of robust and promising microorganisms as cell factories to produce biologically active molecules. Furthermore, we will review multi-disciplinary and comprehensive engineering approaches directed at improving yields of microbial production of natural products and proteins and generating novel molecules. Throughout this article, we will suggest ways in which microbial-derived biologically active molecular entities and their analogs could continue to inspire the development of new therapeutic agents in academia and industry.

Protective Effects of Nargenicin A1 against Tacrolimus-Induced Oxidative Stress in Hirame Natural Embryo Cells

Tacrolimus is widely used as an immunosuppressant to reduce the risk of rejection after organ transplantation, but its cytotoxicity is problematic. Nargenicin A1 is an antibiotic extracted from Nocardia argentinensis and is known to have antioxidant activity, though its mode of action is unknown. The present study was undertaken to evaluate the protective effects of nargenicin A1 on DNA damage and apoptosis induced by tacrolimus in hirame natural embryo (HINAE) cells. We found that reduced HINAE cell survival by tacrolimus was due to the induction of DNA damage and apoptosis, both of which were prevented by co-treating nargenicin A1 or N-acetyl-l-cysteine, a reactive oxygen species (ROS) scavenger, with tacrolimus. In addition, apoptosis induction by tacrolimus was accompanied by increases in ROS generation and decreases in adenosine triphosphate (ATP) levels caused by mitochondrial dysfunction, and these changes were significantly attenuated in the presence of nargenicin A1, which further indicated tacrolimus-induced apoptosis involved an oxidative stress-associated mechanism. Furthermore, nargenicin A1 suppressed tacrolimus-induced B-cell lymphoma-2 (Bcl-2) down-regulation, Bax up-regulation, and caspase-3 activation. Collectively, these results demonstrate that nargenicin A1 protects HINAE cells against tacrolimus-induced DNA damage and apoptosis, at least in part, by scavenging ROS and thus suppressing the mitochondrial-dependent apoptotic pathway.

Natural product drug discovery in the genomic era: realities, conjectures, misconceptions, and opportunities

Natural product discovery from microorganisms provided important sources for antibiotics, anti-cancer agents, immune-modulators, anthelminthic agents, and insecticides during a span of 50 years starting in the 1940s, then became less productive because of rediscovery issues, low throughput, and lack of relevant new technologies to unveil less abundant or not easily detected drug-like natural products. In the early 2000s, it was observed from genome sequencing that Streptomyces species encode about ten times as many secondary metabolites as predicted from known secondary metabolomes. This gave rise to a new discovery approach—microbial genome mining. As the cost of genome sequencing dropped, the numbers of sequenced bacteria, fungi and archaea expanded dramatically, and bioinformatic methods were developed to rapidly scan whole genomes for the numbers, types, and novelty of secondary metabolite biosynthetic gene clusters. This methodology enabled the identification of microbial taxa gifted for the biosynthesis of drug-like secondary metabolites. As genome sequencing technology progressed, the realities relevant to drug discovery have emerged, the conjectures and misconceptions have been clarified, and opportunities to reinvigorate microbial drug discovery have crystallized. This perspective addresses these critical issues for drug discovery.

In Vitro and In Vivo Assessment of FK506 Analogs as Novel Antifungal Drug Candidates

FK506 (tacrolimus) is an FDA-approved immunosuppressant indicated for the prevention of allograft rejections in patients undergoing organ transplants. In mammals, FK506 inhibits the calcineurin-nuclear factor of activated T cells (NFAT) pathway to prevent T-cell proliferation by forming a ternary complex with its binding protein, FKBP12, and calcineurin. FK506 also exerts antifungal activity by inhibiting calcineurin, which is essential for the virulence of human-pathogenic fungi. Nevertheless, FK506 cannot be used directly as an antifungal drug due to its immunosuppressive action. In this study, we analyzed the cytotoxicity, immunosuppressive activity, and antifungal activity of four FK506 analogs, 31-O-demethyl-FK506, 9-deoxo-FK506, 9-deoxo-31-O-demethyl-FK506, and 9-deoxo-prolyl-FK506, in comparison with that of FK506. The four FK506 analogs generally possessed lower cytotoxicity and immunosuppressive activity than FK506. The FK506 analogs, except for 9-deoxo-prolyl-FK506, had strong antifungal activity against Cryptococcus neoformans and Candida albicans, which are two major invasive pathogenic yeasts, due to the inhibition of the calcineurin pathway. Furthermore, the FK506 analogs, except for 9-deoxo-prolyl-FK506, had strong antifungal activity against the invasive filamentous fungus Aspergillus fumigatus Notably, 9-deoxo-31-O-demethyl-FK506 and 31-O-demethyl-FK506 exhibited robust synergistic antifungal activity with fluconazole, similar to FK506. Considering the antifungal efficacy, cytotoxicity, immunosuppressive activity, and synergistic effect with commercial antifungal drugs, we selected 9-deoxo-31-O-demethyl-FK506 for further evaluation of its in vivo antifungal efficacy in a murine model of systemic cryptococcosis. Although 9-deoxo-31-O-demethyl-FK506 alone was not sufficient to treat the cryptococcal infection, when it was used in combination with fluconazole, it significantly extended the survival of C. neoformans-infected mice, confirming the synergistic in vivo antifungal efficacy between these two agents.

Unraveling Nutritional Regulation of Tacrolimus Biosynthesis in Streptomyces tsukubaensis through omic Approaches

Streptomyces tsukubaensis stands out among actinomycetes by its ability to produce the immunosuppressant tacrolimus. Discovered about 30 years ago, this macrolide is widely used as immunosuppressant in current clinics. Other potential applications for the treatment of cancer and as neuroprotective agent have been proposed in the last years. In this review we introduce the discovery of S. tsukubaensis and tacrolimus, its biosynthetic pathway and gene cluster (fkb) regulation. We have focused this work on the omic studies performed in this species in order to understand tacrolimus production. Transcriptomics, proteomics and metabolomics have improved our knowledge about the fkb transcriptional regulation and have given important clues about nutritional regulation of tacrolimus production that can be applied to improve production yields. Finally, we address some points of S. tsukubaensis biology that deserve more attention.

Manipulating the expression of SARP family regulator BulZ and its target gene product to increase tacrolimus production

Tacrolimus (FK506), an effective immunosuppressant, is widely used in the treatment of autoimmune diseases. In this study, we identified that BulZ, a Streptomyces antibiotic regulatory protein (SARP) family regulator, acted as a positive regulator for spore differentiation and tacrolimus production. A knockout of bulZ resulted in a 47.5% decrease of tacrolimus production and a delay of spore differentiation. Using quantitative real-time PCR (qRT-PCR) analysis and electrophoretic mobility shift assays (EMSAs), it was found that BulZ directly activated the transcriptions of bulZ and bulS2, a putative γ-butyrolactone (GBL) synthetase, and bulS2 was shown to play a positive role in tacrolimus biosynthesis. Meanwhile, BulZ was able to indirectly regulate the transcriptions of the cluster-linked activator genes tcs7 and fkbN, as well as the GBL receptor gene bulR1. STSU_RS22595, which encoded a WhiB family transcriptional regulator, was found to be a previously unknown potential target gene of BulZ based on a whole-genome search of the conserved sequence (5'-TSVAVVVNVNBTSRAGNN-3') of the SARP-binding motifs. Although overexpression of STSU_RS22595 did not result in an obvious enhancement of tacrolimus yield, STSU_RS22595 might have important effects on the spore differentiation process. Finally, co-overexpression of bulZ and its target gene bulS2 improved tacrolimus production by 36% compared to the control strain, reaching 324 mg/L. The insights obtained in this study will help further elucidate the regulatory mechanism of tacrolimus biosynthesis and provide new avenues for further improvement of tacrolimus production.

Streptomyces tsukubaensis as a new model for carbon repression: transcriptomic response to tacrolimus repressing carbon sources

In this work, we identified glucose and glycerol as tacrolimus repressing carbon sources in the important species Streptomyces tsukubaensis. A genome-wide analysis of the transcriptomic response to glucose and glycerol additions was performed using microarray technology. The transcriptional time series obtained allowed us to compare the transcriptomic profiling of S. tsukubaensis growing under tacrolimus producing and non-producing conditions. The analysis revealed important and different metabolic changes after the additions and a lack of transcriptional activation of the fkb cluster. In addition, we detected important differences in the transcriptional response to glucose between S. tsukubaensis and the model species Streptomyces coelicolor. A number of genes encoding key players of morphological and biochemical differentiation were strongly and permanently downregulated by the carbon sources. Finally, we identified several genes showing transcriptional profiles highly correlated to that of the tacrolimus biosynthetic pathway regulator FkbN that might be potential candidates for the improvement of tacrolimus production.

Type I interferons in the pathogenesis and treatment of canine diseases

Type I interferons (IFNs) such as IFN-α, IFN-β, IFN-ε, IFN-κ, and IFN-ω represent cytokines, which are deeply involved in the regulation and activation of innate and adaptive immune responses. They possess strong antiviral, antiproliferative, and immunomodulatory activities allowing their use in the therapy of different viral diseases, neoplasms, and immune-mediated disorders, respectively. Initially, treatment strategies were based on nonspecific inducers of type I IFNs, which were soon replaced by different recombinant proteins. Drugs with type I IFNs as active agents are currently used in the treatment of hepatitis B and C virus infection, lymphoma, myeloid leukemia, renal carcinoma, malignant melanoma, and multiple sclerosis in humans. In addition, recombinant feline IFN-ω has been approved for the treatment of canine parvovirus, feline leukemia virus, and feline immunodeficiency virus infections. However, the role of type I IFNs in the pathogenesis of canine diseases remains largely undetermined so far, even though some share pathogenic mechanisms and clinical features with their human counterparts. This review summarizes the present knowledge of type I IFNs and down-stream targets such as Mx and 2',5'-oligoadenylate synthetase proteins in the pathogenesis of infectious and immune-mediated canine diseases. Moreover, studies investigating the potential use of type I IFNs in the treatment of canine lymphomas, melanomas, sarcomas, and carcinomas, canine distemper virus, parvovirus, and papillomavirus infections as well as immune-mediated keratoconjunctivitis sicca and atopic dermatitis are presented. A separate chapter is dedicated to the therapeutic potential of IFN-λ, a type III IFN, in canine diseases. However, further future studies are still needed to unravel the exact functions of the different subtypes of type I IFNs and their target genes in healthy and diseased dogs and the full potential action of type I IFNs as treatment strategy.