Silk-based delivery systems of bioactive molecules

Abstract

Silks are biodegradable, biocompatible, self-assembling proteins that can also be tailored via genetic engineering to contain specific chemical features, offering utility for drug and gene delivery. Silkworm silk has been used in biomedical sutures for decades and has recently achieved Food and Drug Administration approval for expanded biomaterials device utility. With the diversity and control of size, structure and chemistry, modified or recombinant silk proteins can be designed and utilized in various biomedical application, such as for the delivery of bioactive molecules. This review focuses on the biosynthesis and applications of silk-based multi-block copolymer systems and related silk protein drug delivery systems. The utility of these systems for the delivery of small molecule drugs, proteins and genes is reviewed.

Figure 1

Schematic illustration of the construction of silk-like polymer encoding genes (A) and expression and purification of these silk-like polymers (B). Link: Link sequence reported in Ref [22]. In the construction of silk-like polymer encoding genes (A), restriction enzyme(s), calf intestinal alkaline phosphatase (CIP), and DNA ligase treatments are necessary. For gene delivery, the polylysine sequence is essential to form polyion complexes between the genes and the silk-like polymers. Functional peptides can be chosen based on the application with the silk-like polymers. (B) electroporation or chemical transformation can be used to transform pDNA into E. coli. Expression of the polymer is induced by addition of isopropyl β-D-1-thiogalactopyranoside (IPTG).

Figure 2

(A) Schematic of silk-based pDNA complexes and silk films containing the complexes. Silk-based polyioncomplexes are formed between negatively charged pDNA and positively charged polylysine sequence of silk-polylysine block copolymer. Silk-based polyioncomplexes and films to contain the complexes are prepared for pDNA delivery. (B) AFM height image of pDNA complexes of the recombinant silk. Reproduced with permission from Numata et al., Biomaterials; published by Elsevier, 2009.

Figure 3

Schematic presentation of primary structure of silk-based block copolymer and functions of each module of the copolymer. Functional peptides can be added at the both ends of molecules for targeted delivery or therapeutic effects. Molecular weight and the secondary structures of silk multimer sequences can control sizes, enzyme resistance, degradation rates, and drug/gene release rates. Polylysine sequences affect sizes as well as surface charge (zeta potential) of the silk-based complexes.

Figure 4

Model of receptor-mediated transfection via silk-based cationic block copolymers with ligands or functional peptides. (a) Formation of ion complexes between gene(s) and silk-polylysine block copolymers. (b) Binding of the complex to the cell via specific receptors or membrane proteins such as integrins. (c) Internalization via endocytosis and degradation of the polymers in lysosomes. (d) Trafficking of genes to the nucleus to initiate gene expression after the degradation of the complex. (e) Binding of adenovirus vector to the cell via the coxsackievirus and adenovirus receptor (CAR). (f) Internalization via the receptor-mediated endocytosis, involving interactions between integrins and RGDs in the adenoviral penton capsid protein. (g) Dismantling of capsid and acidification endosome, and subsequent docking at nuclear pore complexes and passage of DNA through nuclear pores via interaction of naked capsid with microtubules and dynein motors.

Figure 5

Silk-based biomaterials processed from silk solution (A). (B and C) Silk scaffolds, (D) SEM image of porous structure of scaffold, (E) SEM image of silk tube, (F) SEM image of polymeric microsphere coated with silk layers [112], (G) silk hydrogel, (H) SEM image of silk electrospun fibers, (I) AFM image of single electrospun fibers of silk, (J) SEM image of silk-based microspheres, (K) SEM image of the surface of silk films, (L) Silk film. Reproduced with permission from Wang et al., Biomaterials; published by Elsevier, 2007.