Addendum: Cryptic phosphorylation in nucleoside natural product biosynthesis.

A Correction to this paper has been published: https://doi.org/10.1038/s41589-021-00867-7.

Cryptic phosphorylation in nucleoside natural product biosynthesis.

Kinases are annotated in many nucleoside biosynthetic gene clusters but generally are considered responsible only for self-resistance. Here, we report an unexpected 2'-phosphorylation of nucleoside biosynthetic intermediates in the nikkomycin and polyoxin pathways. This phosphorylation is a unique cryptic modification as it is introduced in the third of seven steps during aminohexuronic acid (AHA) nucleoside biosynthesis, retained throughout the pathway's duration, and is removed in the last step of the pathway. Bioinformatic analysis of reported nucleoside biosynthetic gene clusters indicates the presence of cryptic phosphorylation in other pathways and the importance of functional characterization of kinases in nucleoside biosynthetic pathways in general. This study also functionally characterized all of the enzymes responsible for AHA biosynthesis and revealed that AHA is constructed via a unique oxidative C-C bond cleavage reaction. The results indicate a divergent biosynthetic mechanism for three classes of antifungal nucleoside natural products.

Conserved Mechanism of 2'-Phosphorylation-Aided Amide Ligation in Peptidyl Nucleoside Biosynthesis.

Peptidyl nucleoside antifungals, represented by nikkomycins and polyoxins, consist of an unusual six-carbon nucleoside [aminohexuronic acid (AHA)] ligated to a nonproteinogenic amino acid via an amide bond. A recent study suggested that AHA is biosynthesized through cryptic phosphorylation, where a 2'-phosphate is introduced early in the pathway and required to form AHA. However, whether 2'-phosphorylation is necessary for the last step of biosynthesis, the formation of the amide bond between AHA and nonproteinogenic amino acids, remains ambiguous. Here, we address this question with comprehensive in vitro and in vivo characterizations of PolG and NikS, which together provide strong evidence that amide ligation proceeds with 2'-phosphorylated substrates in both pathways. Our results suggest that 2'-phosphorylation is retained for the entirety of both nikkomycin and polyoxin biosynthesis, providing important insights into how cryptic phosphorylation assists with nucleoside natural product biosynthesis.

Development of a Tape-Stripping Liquid Chromatography-Mass Spectrometry Method for Evaluating Skin Deposition of Topical Tazarotene.

BACKGROUND: Standard in vitro and ex vivo techniques for evaluating topical drug penetration utilize skin that is physiologically unlike living skin and cannot assess intra-dermal drug deposition. We combined tape stripping with liquid chromatography-heated electrospray ionization-mass spectrometry (LC-ESI-MS) to assess in vivo deposition of tazarotene after application of 0.1% cream and 0.045% lotion formulations. METHODS: Ten healthy adult female participants had ~0.1 g of tazarotene lotion and cream applied to two sites each on opposite forearms. After 3 and 6 hours, 21 tape strips were used to sample progressively deeper skin layers. LC-ESI-MS was used to determine percent recovery of the applied tazarotene dose and the concentration of tazarotene recovered from even-numbered tape strips. RESULTS: At both sampling times, percent recovery was slightly higher with tazarotene 0.045% lotion versus 0.1% cream, though tazarotene concentrations were approximately 2-fold higher for cream than lotion at both superficial and deep skin layers. Absolute differences in tazarotene concentrations between 0.1% cream and 0.045% lotion decreased in progressively deeper skin layers, from 0.8 μg/mL at tape strip 2 to 0.09 μg/mL at tape strip 20 (6 hours post-application). CONCLUSIONS: Tape stripping plus LC-ESI-MS—a consistent, accurate, and non-invasive method for assessing drug delivery into layers of living skin—can be used to assess in vivo deposition of topical formulations. Results from this study, when combined with clinical data, suggest that small tazarotene-deposition differences between lotion and cream in deeper skin layers are not clinically relevant; however, lower-dose 0.045% lotion may minimize tazarotene skin exposure versus 0.1% cream, potentially resulting in a more favorable tolerability profile. J Drugs Dermatol. 2021;20(10):1105-1111. doi:10.36849/JDD.6211.

Promiscuity of a modular polyketide synthase towards natural and non-natural extender units.

Combinatorial biosynthesis approaches that involve modular type I polyketide synthases (PKSs) are proven strategies for the synthesis of polyketides. In general however, such strategies are usually limited in scope and utility due to the restricted substrate specificity of polyketide biosynthetic machinery. Herein, a panel of chemo-enzymatically synthesized acyl-CoA's was used to probe the promiscuity of a polyketide synthase. Promiscuity determinants were dissected, revealing that the KS is remarkably tolerant to a diverse array of extender units, while the AT likely discriminates between extender units that are native to the producing organism. Our data provides a clear blueprint for future enzyme engineering efforts, and sets the stage for harnessing extender unit promiscuity by employing various in vivo polyketide diversification strategies.

Enhanced Skin Deposition of Betamethasone Dipropionate from a Novel Formulation and Drug Delivery Technology.

INTRODUCTION: Effective topical drug delivery is the essence of dermatologic treatment. The drug must be applied to the skin surface, be released from the vehicle, enter the stratum corneum, traverse the epidermis, and enter the dermis pharmacologically intact. New advances have improved emulsion-type formulation and drug delivery technology by encapsulating dispersed oil droplets in a robust multimolecular aqueous film of surfactants, oil, and water, enabling a multifold decrease in surfactant concentration compared to conventional creams. In the research reported here, we studied this new concept, termed polyaphron dispersion (PAD) technology, by comparing skin delivery of betamethasone dipropionate from a novel oil-in-water emulsion system of calcipotriene and betamethasone dipropionate (CAL/BDP) cream to that from a traditional topical suspension (CAL/BDP TS) utilizing in vitro and in vivo detection methods. METHODS: The amount of BDP released from the CAL/BDP cream and CAL/BDP TS was evaluated using both in vitro Franz cell analysis and in vivo human tape stripping from ten female human volunteers after a single application of CAL/BDP cream or CAL/BDP TS. For the tape stripping analysis, 20 circular tape strips were taken from forearm application sites at 1, 2, 4, and 8 h after application and analyzed for the amount of BDP in the tape strip using liquid chromatography-mass spectrometry (LC-MS). RESULTS: The in vitro Franz cell analysis demonstrated that the cumulative amount of BDP that diffused through the epidermis was statistically significantly greater for the CAL/BDP cream compared to the CAL/BDP TS at all time points. In addition, consistently higher amounts of BDP were recovered following CAL/BDP cream application than following CAL/BDP TS application at 1, 2, 4, and 8 h following application utilizing the in vivo tape stripping technique. CONCLUSION: The novel PAD technology-based cream formulation delivered more BDP into the upper stratum corneum and lower epidermis than a traditional topical suspension.

Fixed-combination halobetasol propionate and tazarotene topical lotion decreases TNF-α and IL-17A levels in psoriatic lesions.

OBJECTIVE: Fixed-combination halobetasol propionate (0.01%) and tazarotene (0.045%) lotion (HP/TAZ) is approved for the treatment of plaque psoriasis in adults, with a demonstrated efficacy and safety profile in phase 3 trials. This study examined the effect of HP/TAZ on the reduction of tumor necrosis factor alpha (TNF-α) and interleukin 17 A (IL-17A) and its correlation to psoriasis improvement. MATERIALS AND METHODS: Ten adults with mild-to-moderate plaque psoriasis and 2 symmetrical plaques self-applied HP/TAZ (treated plaque) or vehicle lotion (untreated plaque) for 12 weeks. At baseline and each study visit (weeks 2, 4, 8, and 12), Investigator's Global Assessment (IGA) score and erythema, scaling, and induration were assessed. Additionally, D-squame tape strips were utilized to quantify TNF-α and IL-17A in target lesions by enzyme-linked immunosorbent assay. RESULTS: Significant improvements in mean IGA score in HP/TAZ-treated compared with untreated plaques were evident at week 2 and maintained through week 12 (p < 0.003). HP/TAZ significantly reduced TNF-α levels at weeks 4 through 12 (p < 0.03) and IL-17A levels at weeks 2 through 8 (p < 0.05) in treated compared with untreated plaques. CONCLUSIONS: HP/TAZ was highly effective in treating psoriasis plaques and, although HP/TAZ is not a biologic, effectively reduced cytokine-associated inflammatory markers that drive psoriatic disease.

Cryptic Phosphorylation-Mediated Divergent Biosynthesis of High-Carbon Sugar Nucleoside Antifungals.

Establishing a general biosynthetic scheme for natural products is critical for a broader understanding of natural product biosynthesis and the structural prediction of metabolites based on genome sequence information. High-carbon sugar nucleoside antimicrobials are an underexplored class of natural products with unique structures and important biological activities. Recent studies on C6 sugar nucleoside antifungal natural products, such as nikkomycins and polyoxins, revealed a novel biosynthetic mechanism involving cryptic phosphorylation. However, the generality of this biosynthetic mechanism remained unexplored. We here report in vitro characterization of the biosynthesis of a C7 sugar nucleoside antifungal, malayamycin A. Our results demonstrate that the malayamycin biosynthetic enzymes specifically accept 2'-phosphorylated biosynthetic intermediates, suggesting that cryptic phosphorylation-mediated biosynthesis is conserved beyond C6 sugar nucleosides. Furthermore, the results suggest a generalizable divergent biosynthetic mechanism for high-carbon sugar nucleoside antifungals. In this model, C6 and C7 sugar nucleoside biosyntheses proceed via a common C8 sugar nucleoside precursor, and the sugar size is determined using the functions of α-ketoglutarate (α-KG)-dependent dioxygenases (NikI/PolD for C6 sugar nucleosides and MalI for C7 sugar nucleosides). These results provide an important guidance for the future genome-mining discovery of high-carbon sugar nucleoside antimicrobials.

Synthesis of the Siderophore Coelichelin and Its Utility as a Probe in the Study of Bacterial Metal Sensing and Response.

A convergent total synthesis of the siderophore coelichelin is described. The synthetic route also provided access to acetyl coelichelin and other congeners of the parent siderophore. The synthetic products were evaluated for their ability to bind ferric iron and promote growth of a siderophore-deficient strain of the Gram-negative bacterium Pseudomonas aeruginosa under iron restriction conditions. The results of these studies indicate coelichelin and several derivatives serve as ferric iron delivery vehicles for P. aeruginosa.

Poly specific trans-acyltransferase machinery revealed via engineered acyl-CoA synthetases.

Polyketide synthases construct polyketides with diverse structures and biological activities via the condensation of extender units and acyl thioesters. Although a growing body of evidence suggests that polyketide synthases might be tolerant to non-natural extender units, in vitro and in vivo studies aimed at probing and utilizing polyketide synthase specificity are severely limited to only a small number of extender units, owing to the lack of synthetic routes to a broad variety of acyl-CoA extender units. Here, we report the construction of promiscuous malonyl-CoA synthetase variants that can be used to synthesize a broad range of malonyl-CoA extender units substituted at the C2-position, several of which contain handles for chemoselective ligation and are not found in natural biosynthetic systems. We highlighted utility of these enzymes by probing the acyl-CoA specificity of several trans-acyltransferases, leading to the unprecedented discovery of poly specificity toward non-natural extender units, several of which are not found in naturally occurring biosynthetic pathways. These results reveal that polyketide biosynthetic machinery might be more tolerant to non-natural substrates than previously established, and that mutant synthetases are valuable tools for probing the specificity of biosynthetic machinery. Our data suggest new synthetic biology strategies for harnessing this promiscuity and enabling the regioselective modification of polyketides.