Multilamellar lipid structures and their elastic properties in living systems

Bandara, Sarith R.

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https://hdl.handle.net/2142/115859 Copy

Description

Title

Multilamellar lipid structures and their elastic properties in living systems

Author(s)

Bandara, Sarith R.

Issue Date

2022-06-06

Director of Research (if dissertation) or Advisor (if thesis)

Leal, Cecília

Doctoral Committee Chair(s)

Leal, Cecília

Committee Member(s)

• Espinosa-Marzal, Rosa
• Evans, Christopher
• Harley, Brendan
• Kilian, Kristopher

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• liposomes
• LNPs
• extracellular vesicles
• EVs
• extracellular matrix
• ECM
• hydrogels
• osmotic force
• lipid droplets
• LDs
• multilamellar lipid systems
• small-angle X-ray scattering
• SAXS
• laser scanning confocal microscopy
• LSCM
• polarized optical microscopy
• micropipette aspiration
• obesity

Abstract

Lipids—amphiphilic or hydrophobic molecules directly involved in compartmentalization, metabolism, and transport—are essential for life. Amphiphilic lipids, particularly phospholipids, are the key component of cell membranes and other vesicular entities such as endosomes and extracellular vesicles (EVs), which are involved in the trafficking of useful molecules from one location in the body to another. Not only are they effective at transport, but they are also non-toxic, which has led to their increased prevalence in nanomedicine as drug delivery vehicles. Hydrophobic lipids, particularly glycerolipids and sterol lipids, are the main form in which excess nutrients are stored in the body as lipid droplets (LDs). Evidently, their study has a direct bearing on obesity, a leading cause of death worldwide. While the single lipid bilayer has been thoroughly studied and characterized, studies on multilamellar lipid systems—which are also found extensively in nature—are scarce. This thesis investigates how entities in the body composed of these two amphiphilic and hydrophobic lipid divisions evolve into multilamellar structures under different conditions. For vesicles made of self-assembled amphiphilic lipids, there is a gap in the knowledge of how their structures are affected in the extracellular matrix (ECM). We modeled EVs and lipid-based nanoparticles using liposomes and investigated—using small-angle X-ray scattering (SAXS), laser scanning confocal microscopy (LSCM), and cryogenic transmission electron microscopy—how their structure was affected in an ECM environment, with both pseudo-ECM modeled using synthetic hydrogels and actual ECM. We showed that nanometer-sized liposomes transformed into micrometer-sized aggregates in a hydrogel environment. These aggregates comprised multilamellar vesicles with a mean interlamellar spacing of 5.5 nm. Shielding the liposomes with a corona of poly(ethylene glycol) protected the liposomes from the restructuring effect. We conjectured that the hydrogel environment osmotically drove the unilamellar to multilamellar phase transition. These findings could have major implications in the design of nanomedicines and in understanding the fate and function of EVs within the tissue microenvironment. While cellular LDs composed of hydrophobic lipids are known to increase in size and number under caloric excess, how their structures are affected by obesogenic diets has not been well studied. Understanding LD structure as a function of diet could lead to better therapies that combat obesity. Using SAXS, LSCM, and polarized optical microscopy, we compared the structure of LDs isolated from mice fed diets high in fat and sugar to those isolated from mice fed regular diets. We discovered that LDs have a disordered liquid core and an ordered multilamellar shell, with the multilamellar shell displaying more lamellae for high-calorie diets. We also studied diet-based structural changes of adipocytes and adipose tissue that the LDs were derived from. A natural extension to the study of diet-dependence on the structure of LDs and adipose tissue was the investigation of how their mechanical properties were affected upon caloric excess. While studies on mechanical properties of the endoplasmic reticulum—where LDs are created in the cell—are numerous, mechanical properties of LDs have been virtually unexplored. Using micropipette aspiration, we discovered that the membranes of LDs derived from high-calorie diets demonstrated higher area expansivity moduli (or greater resistance to stretching) compared to those of LDs derived from regular diets. Using atomic force microscopy, we also uncovered that adipose tissue followed a similar trend in Young's moduli, with the tissue from high-calorie diets being stiffer than that from regular diets. These findings could explain why LDs from obese sources, which have greater crystallinity in their outer shell and are more mechanically robust, are harder to metabolize.

Graduation Semester

2022-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Sarith R. Bandara

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