Functionalization of 3D scaffolds with protein-releasing biomaterials for intracellular delivery

Toxicity Profiling of Bacterial Inclusion Bodies in Human Caco-2 Cells

Bacterial inclusion bodies (IBs) are discrete macromolecular complexes that appear in recombinant prokaryotic cells under stress conditions. These structures are often discarded for biotechnological uses given the difficulty in recovering proteins of interest from them in a soluble form. However, recent approaches have revealed the potential of these protein clusters as biomaterials to promote cell growth and as protein depots for the release of recombinant proteins for biotechnological and biomedical applications. Although these kinds of natural supramolecular complexes have attracted great interest, no comprehensive study of their toxicity in cell cultures has been carried out. In this study, caco-2 cells were exposed to natural IBs, soluble protein-only nanoparticles (NPs), and non-assembled versions of the same protein for comparative purposes. Cytotoxicity, oxidative stress, and genotoxicity were analyzed for all these protein formats. Natural IBs and soluble protein formats demonstrated their safety in eukaryotic cells. No cytotoxicity, genotoxicity, or oxidative stress was detected in caco-2 cells exposed to the protein samples in any of the experimental conditions evaluated, which covered protein concentrations used in previous biological activity assays. These conditions evaluated the activity of protein samples obtained from three prokaryotic hosts [Escherichia coli and the endotoxin-free expression systems Lactococcus lactis and ClearColi® BL21 (DE3)]. Our results demonstrate that natural IBs and soluble protein nanoparticles are non-toxic materials for eukaryotic cells and that this may represent an interesting alternative to the classical unassembled format of recombinant proteins for certain applications in biotechnology and biomedicine.

Polylactide, Processed by a Foaming Method Using Compressed Freon R134a, for Tissue Engineering

Fabricating polymeric scaffolds using cost-effective manufacturing processes is still challenging. Gas foaming techniques using supercritical carbon dioxide (scCO₂) have attracted attention for producing synthetic polymer matrices; however, the high-pressure requirements are often a technological barrier for its widespread use. Compressed 1,1,1,2-tetrafluoroethane, known as Freon R134a, offers advantages over CO₂ in manufacturing processes in terms of lower pressure and temperature conditions and the use of low-cost equipment. Here, we report for the first time the use of Freon R134a for generating porous polymer matrices, specifically polylactide (PLA). PLA scaffolds processed with Freon R134a exhibited larger pore sizes, and total porosity, and appropriate mechanical properties compared with those achieved by scCO₂ processing. PLGA scaffolds processed with Freon R134a were highly porous and showed a relatively fragile structure. Human mesenchymal stem cells (MSCs) attached to PLA scaffolds processed with Freon R134a, and their metabolic activity increased during culturing. In addition, MSCs displayed spread morphology on the PLA scaffolds processed with Freon R134a, with a well-organized actin cytoskeleton and a dense matrix of fibronectin fibrils. Functionalization of Freon R134a-processed PLA scaffolds with protein nanoparticles, used as bioactive factors, enhanced the scaffolds' cytocompatibility. These findings indicate that gas foaming using compressed Freon R134a could represent a cost-effective and environmentally friendly fabrication technology to produce polymeric scaffolds for tissue engineering approaches.

Tolerability to non-endosomal, micron-scale cell penetration probed with magnetic particles

The capability of HeLa cells to internalize large spherical microparticles has been evaluated by using inorganic, magnetic microparticles of 1 and 2.8 µm of diameter. In both absence but especially under the action of a magnet, both types of particles were uptaken, in absence of cytotoxicity, by a significant percentage of cells, in a non-endosomal process clearly favored by the magnetic field. The engulfed particles efficiently drive inside the cells chemically associated proteins such as GFP and human alpha-galactosidase A, without any apparent loss of protein functionalities. While 1 µm particles are completely engulfed, at least a fraction of 2.8 µm particles remain embedded into the cell membrane, with only a fraction of their surface in cytoplasmic contact. The detected tolerance to endosomal-independent cell penetration of microscale objects is not then restricted to organic, soft materials (such as bacterial inclusion bodies) as previously described, but it is a more general phenomenon also applicable to inorganic materials. In this scenario, the use of magnetic particles in combination with external magnetic fields can represent a significant improvement in the internalization efficiency of such agents optimized as drug carriers. This fact offers a wide potential in the design and engineering of novel particulate vehicles for therapeutic, diagnostic and theragnostic applications.

Production of Active Recombinant Hyaluronidase Inclusion Bodies from Apis mellifera in E. coli Bl21(DE3) and characterization by FT-IR Spectroscopy

The bacterium E. coli is one of the most important hosts for recombinant protein production. The benefits are high growth rates, inexpensive media, and high protein titers. However, complex proteins with high molecular weight and many disulfide bonds are expressed as inclusion bodies (IBs). In the last decade, the overall perception of these IBs being not functional proteins changed, as enzyme activity was found within IBs. Several applications for direct use of IBs are already reported in literature. While fluorescent proteins or protein tags are used for determination of IB activity to date, direct measurements of IB protein activity are scacre. The expression of recombinant hyaluronidase from Apis mellifera in E. coli BL21(DE3) was analyzed using a face centered design of experiment approach. Hyaluronidase is a hard to express protein and imposes a high metabolic burden to the host. Conditions giving a high specific IB titer were found at 25 °C at low specific substrate uptake rates and induction times of 2 to 4 h. The protein activity of hyaluronidase IBs was verified using (Fourier transform) FT-IR spectroscopy. Degradation of the substrate hyaluronan occurred at increased rates with higher IB concentrations. Active recombinant hyaluronidase IBs can be immediately used for direct degradation of hyaluronan without further down streaming steps. FT-IR spectroscopy was introduced as a method for tracking IB activity and showed differences in degradation behavior of hyaluronan dependent on the applied active IB concentration.

Bacterial inclusion bodies are industrially exploitable amyloids

Understanding the structure, functionalities and biology of functional amyloids is an issue of emerging interest. Inclusion bodies, namely protein clusters formed in recombinant bacteria during protein production processes, have emerged as unanticipated, highly tunable models for the scrutiny of the physiology and architecture of functional amyloids. Based on an amyloidal skeleton combined with varying amounts of native or native-like protein forms, bacterial inclusion bodies exhibit an unusual arrangement that confers mechanical stability, biological activity and conditional protein release, being thus exploitable as versatile biomaterials. The applicability of inclusion bodies in biotechnology as enriched sources of protein and reusable catalysts, and in biomedicine as biocompatible topographies, nanopills or mimetics of endocrine secretory granules has been largely validated. Beyond these uses, the dissection of how recombinant bacteria manage the aggregation of functional protein species into structures of highly variable complexity offers insights about unsuspected connections between protein quality (conformational status compatible with functionality) and cell physiology.

Bacterial Inclusion Bodies: Discovering Their Better Half

Bacterial inclusion bodies (IBs) are functional, non-toxic amyloids occurring in recombinant bacteria showing analogies with secretory granules of the mammalian endocrine system. The scientific interest in these mesoscale protein aggregates has been historically masked by their status as a hurdle in recombinant protein production. However, progressive understanding of how the cell handles the quality of recombinant polypeptides and the main features of their intriguing molecular organization has stimulated the interest in inclusion bodies and spurred their use in diverse technological fields. The engineering and tailoring of IBs as functional protein particles for materials science and biomedicine is a good example of how formerly undesired bacterial byproducts can be rediscovered as promising functional materials for a broad spectrum of applications.

Bacterial mimetics of endocrine secretory granules as immobilized in vivo depots for functional protein drugs

In the human endocrine system many protein hormones including urotensin, glucagon, obestatin, bombesin and secretin, among others, are supplied from amyloidal secretory granules. These granules form part of the so called functional amyloids, which within the whole aggregome appear to be more abundant than formerly believed. Bacterial inclusion bodies (IBs) are non-toxic, nanostructured functional amyloids whose biological fabrication can be tailored to render materials with defined biophysical properties. Since under physiological conditions they steadily release their building block protein in a soluble and functional form, IBs are considered as mimetics of endocrine secretory granules. We have explored here if the in vivo implantation of functional IBs in a given tissue would represent a stable local source of functional protein. Upon intratumoral injection of bacterial IBs formed by a potent protein ligand of CXCR4 we have observed high stability and prevalence of the material in absence of toxicity, accompanied by apoptosis of CXCR4⁺ cells and tumor ablation. Then, the local immobilization of bacterial amyloids formed by therapeutic proteins in tumors or other tissues might represent a promising strategy for a sustained local delivery of protein drugs by mimicking the functional amyloidal architecture of the mammals’ endocrine system.

Nanostructured recombinant cytokines: A highly stable alternative to short-lived prophylactics

Cytokines have been widely used as adjuvants and therapeutic agents in treatments of human diseases. Despite their recognized potential as drugs, the medical use of cytokines has considerable drawbacks, mainly related to their low stability and short half-life. Such intrinsic limitations imply the administration of high doses, often prompting toxicity, undesirable side effects and greater production costs. Here, we describe a new category of mechanically stable nanostructured cytokines (TNFα and CCL4/MIP-1β) that resist harsh physicochemical conditions in vitro (pH and temperature), while maintaining functionality. These bio-functional materials are produced in recombinant cell factories through cost-effective and fully scalable processes. Notably, we demonstrate their prophylactic potential in vivo showing they protect zebrafish from a lethal infection by Pseudomonas aeruginosa.

Integrating mechanical and biological control of cell proliferation through bioinspired multieffector materials

In nature, cells respond to complex mechanical and biological stimuli whose understanding is required for tissue construction in regenerative medicine. However, the full replication of such bimodal effector networks is far to be reached. Engineering substrate roughness and architecture allows regulating cell adhesion, positioning, proliferation, differentiation and survival, and the external supply of soluble protein factors (mainly growth factors and hormones) has been long applied to promote growth and differentiation. Further, bioinspired scaffolds are progressively engineered as reservoirs for the in situ sustained release of soluble protein factors from functional topographies. We review here how research progresses toward the design of integrative, holistic scaffold platforms based on the exploration of individual mechanical and biological effectors and their further combination.

Cellular uptake and intracellular fate of protein releasing bacterial amyloids in mammalian cells

Bacterial Inclusion Bodies (IBs) are amyloidal protein deposits that functionally mimic secretory granules from the endocrine system. When formed by therapeutically relevant proteins, they complement missing intracellular activities in jeopardized cell cultures, offering an intriguing platform for protein drug delivery in substitutive therapies. Despite the therapeutic potential of IBs, their capability to interact with eukaryotic cells, cross the cell membrane and release their functional building blocks into the cytosolic space remains essentially unexplored. We have systematically dissected the process by which bacterial amyloids interact with mammalian cells. An early and tight cell membrane anchorage of IBs is followed by cellular uptake of single or grouped IBs of variable sizes by macropinocytosis. Although an important fraction of the penetrating particles is led to lysosomal degradation, biologically significant amounts of protein are released into the cytosol. In addition, our data suggest the involvement of the bacterial cell folding modulator DnaK in the release of functional proteins from these amyloidal reservoirs. The mechanisms supporting the internalization of disintegrable protein nanoparticles revealed here offer clues to implement novel approaches for protein drug delivery based on controlled protein packaging as bacterial IBs.

Functional protein aggregates: just the tip of the iceberg

An increasing number of both prokaryotic and eukaryotic cell types are being adapted as platforms for recombinant protein production. The overproduction of proteins in such expression systems leads to the formation of insoluble protein-based aggregates. Although these protein clusters have been poorly studied in most of the eukaryotic systems, aggregates formed in E. coli, named inclusion bodies (IBs), have been deeply characterized in the last decades. Contrary to the general belief, an important fraction of the protein embedded in IB is functional, showing promise in biocatalysis, regenerative medicine and cell therapy. Thus, the exploration of all these functional protein clusters would largely expand their potential in both pharma and biotech industry.

A novel bio-functional material based on mammalian cell aggresomes

Aggresomes are protein aggregates found in mammalian cells when the intracellular protein degradation machinery is over-titered. Despite that they abound in cells producing recombinant proteins of biomedical and biotechnological interest, the physiological roles of these protein clusters and the functional status of the embedded proteins remain basically unexplored. In this work, we have determined for the first time that, like in bacterial inclusion bodies, deposition of recombinant proteins into aggresomes does not imply functional inactivation. As a model, human α-galactosidase A (GLA) has been expressed in mammalian cells as enzymatically active, mechanically stable aggresomes showing higher thermal stability than the soluble GLA version. Since aggresomes are easily produced and purified, we propose these particles as novel functional biomaterials with potential as carrier-free, self-immobilized catalyzers in biotechnology and biomedicine.

Responsive cell–material interfaces

Major design aspects for novel biomaterials are driven by the desire to mimic more varied and complex properties of a natural cellular environment with man-made materials. The development of stimulus responsive materials makes considerable contributions to the effort to incorporate dynamic and reversible elements into a biomaterial. This is particularly challenging for cell–material interactions that occur at an interface (biointerfaces); however, the design of responsive biointerfaces also presents opportunities in a variety of applications in biomedical research and regenerative medicine. This review will identify the requirements imposed on a responsive biointerface and use recent examples to demonstrate how some of these requirements have been met. Finally, the next steps in the development of more complex biomaterial interfaces, including multiple stimuli-responsive surfaces, surfaces of 3D objects and interactive biointerfaces will be discussed.

Production of functional inclusion bodies in endotoxin-free Escherichia coli

Escherichia coli is the workhorse for gene cloning and production of soluble recombinant proteins in both biotechnological and biomedical industries. The bacterium is also a good producer of several classes of protein-based self-assembling materials such as inclusion bodies (IBs). Apart from being a relatively pure source of protein for in vitro refolding, IBs are under exploration as functional, protein-releasing materials in regenerative medicine and protein replacement therapies. Endotoxin removal is a critical step for downstream applications of therapeutic proteins. The same holds true for IBs as they are often highly contaminated with cell-wall components of the host cells. Here, we have investigated the production of IBs in a recently developed endotoxin-free E. coli strain. The characterization of IBs revealed this mutant as a very useful cell factory for the production of functional endotoxin-free IBs that are suitable for the use at biological interfaces without inducing endotoxic responses in human immune cells.