Pang-Yu Hsueh

Biodegradation of elastin-like polypeptide nanoparticles

Protein polymers are repetitive polypeptides produced by ribosomal biosynthetic pathways; furthermore, they offer emerging opportunities in drug and biopharmaceutical delivery. As for any polymer, biodegradation is one of the most important determinants affecting how a protein polymer can be utilized in the body. This study was designed to characterize the proteolytic biodegradation for a library of protein polymers derived from the human tropoelastin, the Elastin‐like polypeptides (ELPs). ELPs are of particular interest for controlled drug delivery because they reversibly transition from soluble to insoluble above an inverse phase transition temperature (Tt). More recently, ELP block copolymers have been developed that can assemble into micelles; however, it remains unclear if proteases can act on these ELP nanoparticles. For the first time, we demonstrate that ELP nanoparticles can be degraded by two model proteases and that comparable proteolysis occurs after cell uptake into a transformed culture of murine hepatocytes. Both elastase and collagenase endopeptidases can proteolytically degrade soluble ELPs. To our surprise, the ELP phase transition was protective against collagenase but not to elastase activity. These findings enhance our ability to predict how ELPs will biodegrade in different physiological microenvironments and are essential to develop protein polymers into biopharmaceuticals.

Design and cellular internalization of genetically engineered polypeptide nanoparticles displaying adenovirus knob domain

Hepatocytes and acinar cells exhibit high-efficiency, fiber-dependent internalization of adenovirus; however, viral capsids have unpredictable immunological effects and are challenging to develop into targeted drug carriers. To exploit this internalization pathway and minimize the use of viral proteins, we developed a simple gene product that self assembles nanoparticles decorated with the knob domain of adenovirus serotype 5 fiber protein. The most significant advantages of this platform include: (i) compatibility with genetic engineering; (ii) no bioconjugate chemistry is required to link fusion proteins to the nanoparticle surface; and (iii) it can direct the reversible assembly of large nanoparticles, which are monodisperse, multivalent, and biodegradable. These particles are predominantly composed from diblock copolymers of elastin-like polypeptide (ELP). ELPs have unique phase transition behavior, whereby they self-assemble above a transition temperature that is simple to control. The diblock ELP described contains two motifs with distinct transition temperatures, which assemble nanoparticles at physiological temperatures. Analysis by non-denaturing-PAGE demonstrated that the purified knob-ELP formed trimers or dimers, which is a property of the native knob/fiber protein. Dynamic light scattering indicated that the diblock copolymer, with or without knob, is able to self assemble into nanoparticles ~40 nm in diameter. To examine the functionality of knob-ELP, their uptake was assessed in a hepatocyte cell-line that expresses the receptor for adenovirus serotype 5 fiber and knob, the coxsackievirus and adenovirus receptor (CAR). Both plain ELP and knob-ELP were bound to the outside of hepatocytes; however, the knob-ELP fusion protein exhibits more internalization and localization to lysosomes of hepatocytes. These findings suggest that functional fusion proteins may only minimally influence the assembly temperature and diameter of ELP nanoparticles. These results are a proof-of-principal that large fusion proteins (>10 kDa) can be assembled by diblock ELPs without the need for bioconjugate chemistry, which greatly simplifies the design and evaluation of targeted drug carriers.

A quantitative recipe for engineering protein polymer nanoparticles

Protein polymers can assemble switchable nanostructures with emerging applications as biomaterials and nanomedicines. For example, above a critical micelle temperature (CMT) some elastin-like polypeptide (ELP) diblock copolymers assemble spherical nanoparticles, which may modulate cellular internalization and in vivo biodistribution. To achieve engineering-level control over their properties, this report explores a comprehensive library of ELP monoblock and diblock polymers. For the first time, we report that a surprisingly high core molecular weight is required for stable nanoparticle formation; furthermore, nanoparticle size depends on polymer molecular weight. A mathematical model was developed to characterize four ELP monoblock libraries and to predict the phase behavior of corresponding diblock copolymers. The CMT was almost entirely dependent on the hydrophobic core ELP, while the bulk phase transition temperature (Tt,bulk ) depends predominantly on the hydrophilic block. Nanoparticle assembly was accompanied by a conversion in secondary structure of the hydrophobic block from random coil and beta-sheets to type-2 β turns. For the first time, this report enables the rational design of ELP protein polymer nanoparticles with physico-chemico properties that will be suitable for biological applications.

A thermo-responsive protein treatment for dry eyes

Millions of Americans suffer from dry eye disease, and there are few effective therapies capable of treating these patients. A decade ago, an abundant protein component of human tears was discovered and named lacritin (Lacrt). Lacrt has prosecretory activity in the lacrimal gland and mitogenic activity at the corneal epithelium. Similar to other proteins placed on the ocular surface, the durability of its effect is limited by rapid tear turnover. Motivated by the rationale that a thermo-responsive coacervate containing Lacrt would have better retention upon administration, we have constructed and tested the activity of a thermo-responsive Lacrt fused to an elastin-like polypeptide (ELP). Inspired from the human tropoelastin protein, ELP protein polymers reversibly phase separate into viscous coacervates above a tunable transition temperature. This fusion construct exhibited the prosecretory function of native Lacrt as illustrated by its ability to stimulate β-hexosaminidase secretion from primary rabbit lacrimal gland acinar cells. It also increased tear secretion from non-obese diabetic (NOD) mice, a model of autoimmune dacryoadenitis, when administered via intra-lacrimal injection. Lacrt ELP fusion proteins undergo temperature-mediated assembly to form a depot inside the lacrimal gland. We propose that these Lacrt ELP fusion proteins represent a potential therapy for dry eye disease and the strategy of ELP-mediated phase separation may have applicability to other diseases of the ocular surface.

Imbalanced Rab3D versus Rab27 increases cathepsin S secretion from lacrimal acini in a mouse model of Sjögren's Syndrome

The mechanism responsible for the altered spectrum of tear proteins secreted by lacrimal gland acinar cells (LGAC) in patients with Sjögrens Syndrome (SS) remains unknown. We have previously identified increased cathepsin S (CTSS) activity as a unique characteristic of tears of patients with SS. Here, we investigated the role of Rab3D, Rab27a, and Rab27b proteins in the enhanced release of CTSS from LGAC. Similar to patients with SS and to the male nonobese diabetic (NOD) mouse model of SS, CTSS activity was elevated in tears of mice lacking Rab3D. Findings of lower gene expression and altered localization of Rab3D in NOD LGAC reinforce a role for Rab3D in suppressing excess CTSS release under physiological conditions. However, CTSS activity was significantly reduced in tears of mice lacking Rab27 isoforms. The reliance of CTSS secretion on Rab27 activity was supported by in vitro findings that newly synthesized CTSS was detected in and secreted from Rab27-enriched secretory vesicles and that expression of dominant negative Rab27b reduced carbachol-stimulated secretion of CTSS in cultured LGAC. High-resolution 3D-structured illumination microscopy revealed microdomains of Rab3D and Rab27 isoforms on the same secretory vesicles but present in different proportions on different vesicles, suggesting that changes in their relative association with secretory vesicles may tailor the vesicle contents. We propose that a loss of Rab3D from secretory vesicles, leading to disproportionate Rab27-to-Rab3D activity, may contribute to the enhanced release of CTSS in tears of patients with SS.

Tear-mediated delivery of nanoparticles through transcytosis of the lacrimal gland.

Rapid clearance from the tears presents a formidable obstacle to the delivery of peptide drugs to the eye surface. This impedes therapies for ocular infections, wound healing, and dry-eye disease that affect the vision of millions worldwide. To overcome this challenge, this manuscript explores a novel strategy to reach the ocular surface via receptor-mediated transcytosis across the lacrimal gland (LG), which produces the bulk of human tears. The LG abundantly expresses the coxsackievirus and adenovirus receptor (CAR); furthermore, we recently reported a peptide-based nanoparticle (KSI) that targets CAR on liver cells. This manuscript reports the unexpected finding that KSI both targets and transcytoses into the LG acinar lumen, which drains to tear ducts. When followed using ex vivo live cell imaging KSI rapidly accumulates in lumen formed by LG acinar cells. LG transduction with a myosin Vb tail, which is dominant negative towards transcytosis, inhibits lumenal accumulation. Transcytosis of KSI was confirmed in vivo by confocal and TEM imaging of LG tissue following administration of KSI nanoparticles. These findings suggest that it is possible to target nanomaterials to the tears by targeting certain receptors on the LG. This design strategy represents a new opportunity to overcome barriers to ocular delivery.

Use of nucleofection to efficiently transfect primary rabbit lacrimal gland acinar cells

Lacrimal gland acinar cells are an important cell type to study due to their role in production and release of tear proteins, a function essential for ocular surface integrity and normal visual acuity. However, mechanistic studies are often limited by problems with transfection using either plasmid DNA or siRNA. Although various gene delivery methods are available, many have been unproductive due to consistently low transfection efficiencies. We have developed a method using nucleofection that can result in 50% transfection efficiency and 60% knockdown efficiency for plasmid DNA and siRNA, respectively. These results are vastly improved relative to previous studies, demonstrating that nucleofection offers an efficient transfection technique for primary lacrimal gland acinar cells.