Polyhydroxyalkanoates (PHAs) are microbially produced polyesters pursued as tissue-engineering scaffolds, but their inherent hydrophobicity limits cell adhesion and wettability control. Insomphun et al. (ACS Biomater. Sci. Eng. 3(12):3064–3075, 2017) biosynthesized the first PHA carrying side-chain hydroxyl groups — poly(3-hydroxybutyrate-co-3-hydroxyvalerate-co-2,3-dihydroxybutyrate) (PHBVDB) — in recombinant Ralstonia eutropha expressing propionyl-CoA transferase on glycolate, reaching only ~3 mol% 2,3-dihydroxybutyrate (2,3-DHBA), and showed short-term (48 h) human mesenchymal stem cell viability on the scaffold. The authors explicitly left open how to raise 2,3-DHBA incorporation and polymer yield without the growth penalty, how graded 2,3-DHBA composition controls hydrophilicity, mechanics, cell adhesion/proliferation/differentiation and long-term performance, what roles accessory thiolases and reductases (e.g. BktB, PhaB3) play in precursor supply and molecular weight, and how PHBVDB behaves in vivo. No reported work raises 2,3-DHBA content, maps composition to scaffold performance, or tests PHBVDB in vivo, so the problem remains open.

Background

Polyhydroxyalkanoates (PHAs) are polyesters synthesized intracellularly by bacteria as carbon and energy reserves; the best-studied members, poly(3-hydroxybutyrate) (PHB) and its copolymer with 3-hydroxyvalerate (PHBV), are biodegradable and biocompatible and have been widely explored as tissue-engineering scaffold matrices. A persistent limitation for soft-tissue applications is their hydrophobic surface, which correlates with poor protein adsorption and cell adhesion, and their high crystallinity; conventional remedies (plasma treatment, alkaline hydrolysis, chemical grafting) modify only the surface and can be poorly controlled.

One biosynthetic alternative is to install functional groups directly on monomer side chains during fermentation. Numata and co-workers designed such a pathway: glycolate is activated to glycolyl-CoA by propionyl-CoA transferase (Pct, from Megasphaera elsdenii); the β\beta-ketothiolase BktB condenses glycolyl-CoA with acetyl-CoA to give 2-hydroxy-3-ketobutyryl-CoA (plus a 4-hydroxy regioisomer); the reductase PhaB1 converts these to 2,3-dihydroxybutyryl-CoA (2,3-DHBA-CoA, a 2-hydroxyacid thioester analogous to lactyl-CoA) and 3,4-dihydroxybutyryl-CoA; and the PHA synthase PhaC1 incorporates the 2,3-DHBA unit into the growing polyester alongside 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV). The design built on a platform pathway for free 3-hydroxyacid synthesis by Martin et al. (Nat. Commun. 4:1414, 2013). In Escherichia coli this route gave a PHA with only ~1 mol% 2,3-DHBA (Insomphun et al., AMB Express 6:29, 2016), attributed to competition of the native phaAB1 genes for acetyl-CoA and to the poor acceptance of 2-hydroxyacid substrates by PHA synthase; the authors proposed draining less glycolate to the glyoxylate route and trying alternative synthases as future strategies. The 2017 successor study moved the system into Ralstonia eutropha: expressing pct and feeding glycolate, and disrupting phaA (the primary β\beta-ketothiolase), raised the 2,3-DHBA fraction to ~3 mol% in a PHBVDB copolymer. The pendant hydroxyl groups measurably lowered the water contact angle (i.e. increased hydrophilicity), the scaffold's mechanical properties met soft-tissue matrix requirements, and 48 h in vitro assays showed no significant cytotoxicity toward human mesenchymal stem cells (hMSCs, the stromal stem cells of bone marrow used as a standard biocompatibility readout).

Related functional-monomer programs frame what is at stake. For glycolate itself as a backbone comonomer (a 2-hydroxyacid without a pendant hydroxyl), engineered and chimeric PHA synthases — PhaC1Ps(STQK), an engineered Pseudomonas-species synthase, later the chimeric PhaCAR, the first PHA synthase shown to incorporate glycolate units — raised glycolate incorporation from ~15 mol% to ~40 mol% overall (up to 71 mol% in glycolate-rich segments; Arai et al., Front. Bioeng. Biotechnol. 8:612991, 2020), with follow-up terpolymers and mechanistic NMR studies through 2023. A parallel line incorporated a thioether-bearing 2-hydroxyacid, 2-hydroxy-4-methylthiobutyrate, at up to 30.6 mol% and rendered the polymer hydrophilic by post-polymerization oxidation (Mizuno et al., Int. J. Biol. Macromol. 224:840–847, 2023). Chemical routes — e.g. lithium borohydride surface reduction of PHBV (Chaber et al., Materials 15(21):7494, 2022), surveyed in a 2024 review of chemical modification methods — deliver surface hydroxyls without biosynthesis. None of these lines incorporates 2,3-DHBA or establishes how its content governs scaffold performance, so the biosynthetic pendant-hydroxyl route remains at its 2017 state of the art: ~3 mol%, short-term in vitro assays only.

Problem Statement

Establish pendant-hydroxyl PHA (PHBVDB) as a composition-tunable scaffold material for soft-tissue engineering, which requires jointly: (1) biosynthetic access to PHBVDB with substantially higher 2,3-dihydroxybutyrate incorporation than the demonstrated ~3 mol% without the biomass and polymer-yield penalty incurred by pct expression, together with determining what roles accessory thiolases and reductases such as BktB and PhaB3 play in precursor supply, polymer yield, and molecular weight; (2) a composition–property–biological map showing how graded 2,3-DHBA content controls scaffold hydrophilicity (water contact angle), mechanical properties, and cell adhesion, proliferation, and differentiation of human mesenchymal stem cells, extending beyond the present single low-DHBA point and 48 h in vitro assays to long-term in vitro and in vivo performance, including biodegradation behavior, immune response, and functional tissue integration of PHBVDB scaffolds in animal models; and (3) on that basis, strain and cultivation optimization delivering material with reproducible molecular weight, polydispersity, and yield at preparative scale. Components (1)–(3) are coupled stages of one objective — only with materially higher, tunable 2,3-DHBA content at usable yield can the composition-dependent scaffold performance be mapped, and only a mapped performance window justifies scale-up. The specific 2,3-DHBA incorporation level required for component (1) is itself part of what the answer must determine; 'substantially higher' than ~3 mol% is an open-ended comparative, not a fixed threshold.

References

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