Iterative type I polyketide synthases involved in enediyne natural product biosynthesis

Combinatorial metabolic engineering of Streptomyces sp. CB03234-S for the enhanced production of anthraquinone-fused enediyne tiancimycins

BACKGROUND

Anthraquinone-fused enediynes (AFEs) are excellent payloads for antibody-drug conjugates (ADCs). The yields of AFEs in the original bacterial hosts are extremely low. Multiple traditional methods had been adopted to enhance the production of the AFEs. Despite these efforts, the production titers of these compounds are still low, presenting a practical challenge for their development. Tiancimycins (TNMs) are a class of AFEs produced by Streptomyces sp. CB03234. One of their salient features is that they exhibit rapid and complete cell killing ability against various cancer cell lines.

RESULTS

In this study, a combinatorial metabolic engineering strategy guided by the CB03234-S genome and transcriptome was employed to improve the titers of TNMs. First, re-sequencing of CB03234-S (Ribosome engineered mutant strains) genome revealed the deletion of a 583-kb DNA fragment, accounting for about 7.5% of its genome. Second, by individual or combined inactivation of seven potential precursor competitive biosynthetic gene clusters (BGCs) in CB03234-S, a double-BGC inactivation mutant, S1009, was identified with an improved TNMs titer of 28.2 ± 0.8 mg/L. Third, overexpression of five essential biosynthetic genes, including two post-modification genes, and three self-resistance auxiliary genes, was also conducted, through which we discovered that mutants carrying the core genes, tnmE or tnmE10, exhibited enhanced TNMs production. The average TNMs yield reached 43.5 ± 2.4 mg/L in a 30-L fermenter, representing an approximately 360% increase over CB03234-S and the highest titer among all AFEs to date. Moreover, the resulting mutant produced TNM-W, a unique TNM derivative with a double bond instead of a common ethylene oxide moiety. Preliminary studies suggested that TNM-W was probably converted from TNM-A by both TnmE and TnmE10.

CONCLUSIONS

Based on the genome and transcriptome analyses, we adopted a combined metabolic engineering strategy for precursor enrichment and biosynthetic pathway reorganization to construct a high-yield strain of TNMs based on CB03234-S. Our study establishes a solid basis for the clinical development of AFE-based ADCs.

Polyketide Synthase and Nonribosomal Peptide Synthetase Gene Clusters in Type Strains of the Genus Phytohabitans

(1) Background: Phytohabitans is a recently established genus belonging to rare actinomycetes. It has been unclear if its members have the capacity to synthesize diverse secondary metabolites. Polyketide and nonribosomal peptide compounds are major secondary metabolites in actinomycetes and expected as a potential source for novel pharmaceuticals. (2) Methods: Whole genomes of Phytohabitans flavus NBRC 107702^T, Phytohabitans rumicis NBRC 108638^T, Phytohabitans houttuyneae NBRC 108639^T, and Phytohabitans suffuscus NBRC 105367^T were sequenced by PacBio. Polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS) gene clusters were bioinformatically analyzed in the genome sequences. (3) Results: These four strains harbored 10, 14, 18 and 14 PKS and NRPS gene clusters, respectively. Most of the gene clusters were annotated to synthesis unknown chemistries. (4) Conclusions: Members of the genus Phytohabitans are a possible source for novel and diverse polyketides and nonribosomal peptides.

Fabclavine diversity in Xenorhabdus bacteria

The global threat of multiresistant pathogens has to be answered by the development of novel antibiotics. Established antibiotic applications are often based on so-called secondary or specialized metabolites (SMs), identified in large screening approaches. To continue this successful strategy, new sources for bioactive compounds are required, such as the bacterial genera Xenorhabdus or Photorhabdus. In these strains, fabclavines are widely distributed SMs with a broad-spectrum bioactivity. Fabclavines are hybrid SMs derived from nonribosomal peptide synthetases (NRPS), polyunsaturated fatty acid (PUFA), and polyketide synthases (PKS). Selected Xenorhabdus and Photorhabdus mutant strains were generated applying a chemically inducible promoter in front of the suggested fabclavine (fcl) biosynthesis gene cluster (BGC), followed by the analysis of the occurring fabclavines. Subsequently, known and unknown derivatives were identified and confirmed by MALDI-MS and MALDI-MS2 experiments in combination with an optimized sample preparation. This led to a total number of 22 novel fabclavine derivatives in eight strains, increasing the overall number of fabclavines to 32. Together with the identification of fabclavines as major antibiotics in several entomopathogenic strains, our work lays the foundation for the rapid fabclavine identification and dereplication as the basis for future work of this widespread and bioactive SM class.

Crystal Structure of Thioesterase SgcE10 Supporting Common Polyene Intermediates in 9- and 10-Membered Enediyne Core Biosynthesis

Enediynes are potent natural product anticancer antibiotics, and are classified as 9- or 10-membered according to the size of their enediyne core carbon skeleton. Both 9- and 10-membered enediyne cores are biosynthesized by the enediyne polyketide synthase (PKSE), thioesterase (TE), and PKSE-associated enzymes. Although the divergence between 9- and 10-membered enediyne core biosynthesis remains unclear, it has been observed that nascent polyketide intermediates, tethered to the acyl carrier protein (ACP) domain of PKSE, could be released by TE in the absence of the PKSE-associated enzymes. In this study, we determined the crystal structure of SgcE10, the TE that participates in the biosynthesis of the 9-membered enediyne C-1027. Structural comparison of SgcE10 with CalE7 and DynE7, two TEs that participate in the biosynthesis of the 10-membered enediynes calicheamicin and dynemicin, respectively, revealed that they share a common α/β hot-dog fold. The amino acids involved in both substrate binding and catalysis are conserved among SgcE10, CalE7, and DynE7. The volume and the shape of the substrate-binding channel and active site in SgcE10, CalE7, and DynE7 confirm that TEs from both 9- and 10-membered enediyne biosynthetic machineries bind the linear form of similar ACP-tethered polyene intermediates. Taken together, these findings further support the proposal that the divergence between 9- and 10-membered enediyne core biosynthesis occurs beyond PKSE and TE catalysis.

Therapeutic potential of PACAP for neurodegenerative diseases

Pituitary adenylate cyclase activating polypeptide (PACAP) is widely expressed in the central and peripheral nervous system. PACAP can initiate multiple signaling pathways through binding with three class B G-protein coupled receptors, PAC1, VPAC1 and VPAC2. Previous studies have revealed numerous biological activities of PACAP in the nervous system. PACAP acts as a neurotransmitter, neuromodulator and neurotrophic factor. Recently, its neuroprotective potential has been demonstrated in numerous in vitro and in vivo studies. Furthermore, evidence suggests that PACAP might move across the blood-brain barrier in amounts sufficient to affect the brain functions. Therefore, PACAP has been examined as a potential therapeutic method for neurodegenerative diseases. The present review summarizes the recent findings with special focus on the models of Alzheimer's disease (AD) and Parkinson's disease (PD). Based on these observations, the administered PACAP inhibits pathological processes in models of AD and PD, and alleviates clinical symptoms. It thus offers a novel therapeutic approach for the treatment of AD and PD.

Genome-based analysis of non-ribosomal peptide synthetase and type-I polyketide synthase gene clusters in all type strains of the genus Herbidospora

Background

The genus Herbidospora comprises actinomycetes belonging to the family Streptosporangiaceae and currently contains five recognized species. Although other genera of this family often produce bioactive secondary metabolites, Herbidospora strains have not yet been reported to produce secondary metabolites. In the present study, to assess their potential as secondary metabolite producers, we sequenced the whole genomes of the five type strains and searched for the presence of their non-ribosomal peptide synthetase (NRPS) and type-I polyketide synthase (PKS) gene clusters. These clusters are involved in the major secondary metabolite–synthetic pathways in actinomycetes.

Results

The genome sizes of Herbidospora cretacea NBRC 15474^T, Herbidospora mongoliensis NBRC 105882^T, Herbidospora yilanensis NBRC 106371^T, Herbidospora daliensis NBRC 106372^T and Herbidospora sakaeratensis NBRC 102641^T were 8.3, 9.0, 7.9, 8.5 and 8.6 Mb, respectively. They contained 15–18 modular NRPS and PKS gene clusters. Thirty-two NRPS and PKS pathways were identified, among which 9 pathways were conserved in all 5 strains, 8 were shared in 2–4 strains, and the remaining 15 were strain-specific. We predicted the chemical backbone structures of non-ribosomal peptides and polyketides synthesized by these gene clusters, based on module number and domain organization of NRPSs and PKSs. The relationship between 16S rRNA gene sequence-based phylogeny of the five strains and the distribution of their NRPS and PKS gene clusters were also discussed.

Conclusions

The genomes of Herbidospora strains carry as many NRPS and PKS gene clusters, whose products are yet to be isolated, as those of Streptomyces. Herbidospora members should synthesize large and diverse metabolites, many of whose chemical structures are yet to be reported. In addition to those conserved within this genus, each strain possesses many strain-specific gene clusters, suggesting the diversity of these pathways. This diversity could be accounted for by genus-level vertical inheritance and recent acquisition of these gene clusters during evolution. This genome analysis suggested that Herbidospora strains are an untapped and attractive source of novel secondary metabolites.

TAM receptor deficiency affects adult hippocampal neurogenesis

The Tyro3, Axl and Mertk (TAM) subfamily of receptor protein tyrosine kinases functions in cell growth, differentiation, survival, and most recently found, in the regulation of immune responses and phagocytosis. All three receptors and their ligands, Gas6 (growth arrest-specific gene 6) and protein S, are expressed in the central nervous system (CNS). TAM receptors play pivotal roles in adult hippocampal neurogenesis. Loss of these receptors causes a comprised neurogenesis in the dentate gyrus of adult hippocampus. TAM receptors have a negative regulatory effect on microglia and peripheral antigen-presenting cells, and play a critical role in preventing overproduction of pro-inflammatory cytokines detrimental to the proliferation, differentiation, and survival of adult neuronal stem cells (NSCs). Besides, these receptors also play an intrinsic trophic function in supporting NSC survival, proliferation, and differentiation into immature neurons. All these events collectively ensure a sustained neurogenesis in adult hippocampus.