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RESEARCH
INTERESTS :
My research mainly focuses on the studies
of soft materials (including phospholipids, polymers, surfactants) through
structural characterization to understand the chemical physics of the
systems in interest, which has important potential for practical
applications. Below are the three research topics of my current
interests: 1.) developing self-assembled, uniform-sized, stable
phospholipid unilamellar vesicles (ULVs)/liposomes with targeting functions for
diagnostic and therapeutic purpose, 2.) formulating
highly alignable model biomimetic membranes in bulk solutions, where
physiologically relevant conditions (including pH, temperature, hydration
and ionic strength) can be easily achieved or adjusted and 3.) characterizing the structures of polymer hydrogels, polymer thin films and water-soluble
proteins.
- Self-Assembled
Targeting ULVs as Carriers for Therapeutic Drugs And Imaging Probes
Phospholipids,
the basic constituents of cell membranes, comprising a hydrophilic head
group and one or more hydrophobic tails (hydrocarbon chains) can naturally
form a bilayered structure. In an aqueous solution, most lipids
self-assemble into impermeable spherical shells – vesicles/liposomes,
capable of encapsulating molecules and releasing them in a controlled
manner. Because of the biocompatibility, they have been used as
carriers for therapeutic drugs or diagnostic contrast agents. One of
the most well-known commercial applications is Doxil®,
an anticancer drug for chemotherapy. Small unilamellar
(single-bilayered) vesicles have a higher loading efficiency, longer body
circulation time and less liver accumulation than those administrated by
large multi-lamellar vesicles/liposomes (MLVs), naturally formed in common
lipids. Therefore, it usually involves labor-intensive multi-stage
extrusions of MLV solutions to produce small ULVs. Moreover, ruptured
and clogged filters are problematic to the mass production of ULVs.
We
have developed stable, self-assembled uniform-nanosized
ULVs (20 nm < diameter < 50 nm) from phospholipid mixtures. The spontaneous formation needs no
extrusion, making the mass production of ULVs possible. These ULVs are capable of
incorporating peptides, encapsulating water-soluble molecules as well as
demonstrating thermo controlled-release mechanisms. Moreover, the formation of ULVs is
robust even after a replacement of 50% of the lipids with other amphiphilic
molecules. Fig 1 shows the scheme of ideal
targeting ULV carriers, which incorporate with poly-ethylene glycol (PEG),
targeting antibody and payloads for therapy and/or imaging. Our
research in collaboration with scientists at NRC-Institute for Biological
Sciences (NRC-IBS) has also successfully developed liposomes for targeting
cancer. Our preliminary result of animal tests shows the active ULVs
co-developed with the have superior targeting efficacy to that of the
passive ones (Fig 2).
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Fig.1 The proposed
structure of targeting liposomes for both diagnostic and therapeutic
purposes.
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Fig.2 Comparison of the
self-assembled non-targeting and targeting liposomes carrying an MRI
contrast agent containing Gd-DTPA.
Targeting bi-modal imaging payload to a xenograft
tumor using liposomes loaded with Gd and the
near-infrared probe and functionalized with the antibody (developed at
NRC-IBS), which recognizes EGFR. The glioblastoma
cell line over-expressing EGFR (U87MG-EGFR) was inoculated into left
flank region of nude mice and grown for 10 days to form a small (1 mm
diameter) xenograft tumor. Non-targeted and
targeted ULVs carrying Gd and optical probe
were injected, and animals were imaged using explore Optix
after 24 h. Tumors were excised, sectioned and stained with
anti-EGFR Ab. Significant intracellular red probe signal was observed
around the tumor region in the case of targeting liposomes. The Gd concentration was measured in same tumor sections
and other organs using ICP-MS. Gd biodistribution shows a significant accumulation of
the contrast agent in the tumor of mice injected with targeted ULVs.
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- Alignable
Model Membranes in Physiologically Relevant Conditions
Membrane or integral proteins make up about one third of known proteins and
their functions are strongly related to their native conformation.
However, up to now, most of their global structures have not been resolved
because of the difficulty to crystallize the proteins on the membranes.
Alignable model membranes capable of associating with proteins in their
native state therefore serve an important function to decouple in-plane and
out-of-plane structures through x-ray, neutron diffractions or NMR study
(Fig 3). One of the most common methods is to mechanically align the biomembrane on a flat substrate, which however is far
from physiologically relevant and is difficult to change the physical
parameters of the system such as hydration, pH, ionic
strength.
Phospholipid mixtures – “bicelles”, composed of long- and short-chain phospholipids,
have been extensively used as biomimetic substrates in NMR studies to study
membrane proteins, of which the native conformations retain as associating
with bicelles. Bicelles weakly align in a magnetic field with the
bilayer normal perpendicular to the field. When slightly doped with
certain lanthanide ions, bicelles form a smectic
liquid crystalline phase and can be aligned in bulk solutions in a magnetic
field with their bilayer normal || the field. We have systematically
investigated the structure and alignment of bicellar mixtures and found
that highly aligned lipid membranes in solutions are
achievable simply through a macroscopic (in mm scale) confinement and a weak
shear flow.
No
lanthanide ions or strong magnetic fields are needed. This
discovery provides a simple way of aligning membrane proteins in
physiologically relevant environment without doping unnecessary ions,
making the study both the in-plane and out-of-plane structures of membrane
proteins possible.
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Fig 3.
Aligned membrane crystallizes membrane proteins with in-plane and
out-of-plane structures.
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- Other
Research Projects on Soft Materials
I
have also been conducting many neutron scattering experiments to
characterize structures of varieties of soft materials including polymers,
surfactants and proteins in collaboration with university and government
researchers. The following are a few examples of using neutron scattering
to have important breakthrough in resolving nano-scale structures.
- Block Copolymers as Proton Exchange Membranes
(PEMs) and Polymer Hydrogel
PEM is known as the heart of fuel
cells where protons transport from the anode to the cathode. Thus,
ion exchange capacity, proton conductance, mechanical strength, and
chemical and dimensional stabilities are important parameters of PEM
materials. Collaborating with scientists at the NRC-Institute for
Chemical Process and Environmental Technology (NRC-ICPET) who synthesized
novel perfluorinated comb-shaped diblock copolymers with comparable properties to those
of Nafion, a bench-marked PEM, we have
successfully shown inter-connected ellipsoidal channels in these novel
materials using small angle neutron scattering (SANS). Another collaboration with the researchers (Prof. Wankei
Wan and Prof. Jeff Hutter) at the University of
Western Ontario on polymer study is to investigate the structure of
polyvinyl alcohol (PVA), which forms hydrogel by
physical crosslinking through cycles of
freeze-and-thaw. This hydrogel exhibits
anisotropic mechanical properties similar to those of arterial wall
composed of the cardiovascular tissues, therefore a good candidate for
cardiovascular tissue replacement. The combinational analysis of SANS
and Ultra-SANS (USANS) is able to resolve the structures of the material
over almost 3 decades of length scales (from mm to nm).
- Morphologies of Surfactants in Aqueous Solutions
Like
phospholipids, surfactants have rich morphologies in aqueous
solutions. SANS is a powerful tool to characterize the structures of
surfactant micelles or aggregates. Using SANS, an unusual structural
transformation in a non-charged Gemini surfactant (Surfynol,
Air Product Chemical Corp.) solution from low-concentration large clusters
(> 100 nm) to high-concentration micelles (< 3 nm) has been
discovered. Recently, we have also used SANS to investigate the
morphologies of an environmentally friendly biosurfactant,
rhamnolipid as a function of pH values and varieties of heavy metal ions
through collaboration with Prof. Catherine Mulligan’s group at Concordia Univeristy. The result shows that unilamellar
vesicles and micelles are obtained at acidic and basic conditions,
respectively. Since rhamnolipid enhances the solubility of organic
compounds in water, it can be used as de-contaminating agent to remove organic
contaminations in soil.
- Density Profile & Hydration of Biocompatible
Polymer Thin Films in Water
Biocompatible
polymer thin film coated on the surface of implanted organs or tissues can
reduce/prevent protein fouling that possibly causes immunological rejection
in the host body. Our recent collaboration with a research group
(Prof. Shiping Zhu) at McMaster University has
successfully employed neutron reflectometry to resolve the density (or
hydration) profiles across biocompatible polymer thin films (polymethacrylate with side chains of phosphorylcholine and oligo
ethylene glycol, respectively) immersing in water as a function of film
thickness and grafting density. Future study will focus on the depth
of proteins adsorbed onto the ill-functioned polymer thin films to
understand the mechanism of protein fouling.
- Structural Characterization of Water-Soluble
Protein
The functions of proteins
are closely related to their folding states. SANS is a powerful tool to
probe the global structures of aggregates in solution. Recently, we
have employed SANS to investigate three folding states of an enzyme
protein, pepsin – (a) low-pH native folding state, (b) high-pH non-active
unfolding state, and (c) low-pH refolding state partially recovered from
high-pH in collaboration with Dr. Ricky Yada’s
group (University of Guelph).
Curriculum Vitae
Publications and Patents
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