Graduation Year

2013

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Physics

Major Professor

Martin Muschol

Keywords

FTIR, lysozyme, oligomer, posterior pituitary, potassium accumulation

Abstract

This dissertation describes the work done on two distinct projects. In the first part I sought to unravel the mechanisms that underlie the activity-dependent modulation of response in the excitation-secretion coupling of the neurohypophysis. In the second part, I optically monitored and analyzed the secondary structure changes accompanying amyloid fibril formation along multiple pathways, under both denaturing and near-physiological conditions.

Neuronal plasticity plays an important role in regulating various biological systems by modulating release of hormones or neurotransmitters. The changing response to the same stimulus, depending on the context and previous stimulation events, is also the basis of learning and all higher order brain functions. The mechanisms behind this modulation are widely varied, and are often poorly understood in specific tissues. In this work, we examined excitation-secretion coupling in the neurohypophysis, a tissue composed of densely packed axons that secretes the hormones arginine vasopressin and oxytocin. The release of hormones depends not only on the overall level of activity in the gland, but also upon the specifics of the temporal pattern of stimulation. By optically monitoring the electrical activity using voltage sensitive dyes, we were able to investigate this plasticity in the intact gland. Varying extracellular potassium concentration in the bath, increasing interstitial space via hypertonic saline, and retarding potassium reuptake with ouabain all showed that extracellular potassium accumulation drives the depression of excitability. This effect is hidden from glass micro-electrode recordings because of the inevitable damage sustained by the surrounding tissue. Furthermore, no calcium mediated release mechanism played any significant role in the depression. Numerical simulations confirmed the findings and give more insight to the details of the mechanism.

Deposits of amyloid fibrils, long, unbranched polymeric protein aggregates, are the molecular hallmark for a variety of human diseases, including Alzheimer's disease, Parkinson's disease, and type II diabetes. While the amyloid fibrils all share a characteristic cross-beta sheet structure, the proteins that make up the aggregates have no unifying theme in either native structure or function. In this research, I characterized the structural reordering that accompanies this aggregation using Fourier transform infrared spectroscopy (FTIR). Hen egg white lysozyme forms fibrillar aggregates with two distinct morphologies, depending on the growth conditions. At acidic pH with low ionic concentrations, lysozyme forms the fibrils with standard amyloid morphology. These aggregates are long and stiff but with the cross sectional area of a single monomer. At higher salt concentrations, the aggregation follows another pathway, under which oligomers initially form and later assemble into protofibrils. The oligomeric protofibrils are thicker than the monomeric filaments, but are much more curvilinear. These fibrils are not universally recognized as amyloidogenic aggregates. Using FTIR, I showed that both this aggregates are indeed amyloid structures, but that they are structurally distinct. While it is generally accepted that partial unfolding of the protein is a prerequisite for amyloid fibril formation, we found that native protein can be the substrate for amyloid growth when seeded with preformed oligomeric or protofibrillar aggregates. These seeded fibrils grown under near-physiological conditions are structurally indistinguishable from those grown from partially unfolded protein under denaturing conditions. This incorporation and restructuring of native monomers is characteristic of prion-like assembly.

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