Evidence for a role of the molecular chaperone clusterin in systemic amyloidosis
Greene, Michael J
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The systemic amyloidoses are a diverse collection of multi-organ diseases that feature misfolding and aggregation of an amyloidogenic protein leading to the shared, common manifestation of extracellular tissue amyloid fibril deposition. As the pathologic mechanisms are complex, molecular, biochemical and biophysical studies of these disease processes are needed to identify factors other than the amyloidogenic protein that may be important. The investigations presented in this thesis have explored the involvement of clusterin (CLU), a plasma protein, in transthyretin (TTR) and immunoglobulin light chain (LC) forms of systemic amyloidosis. CLU, ubiquitous in biological fluids, is an extracellular chaperone that is related to heat shock proteins. We hypothesized that CLU is part of a proteostatic mechanism to remove TTR or LC proteins from circulation by complexing with these amyloidogenic proteins to prevent proteotoxic stresses. Further, we posited that certain aspects of CLU biology, genetics, and biochemistry could be exploited for diagnostic or therapeutic strategies in the systemic amyloidoses. Analyses of amyloid deposits in SSA (senile systemic amyloidosis), ATTR (familial TTR-related amyloidosis), and AL (primary LC amyloidosis) tissues was accomplished using light and electron microscopic techniques. Results of these studies indicated that CLU is a component of TTR and LC amyloid deposits. The potential of CLU as a biomarker for amyloidosis was also investigated; plasma circulating CLU levels in patient sera was measured for correlation with disease status and CLU genetic variation within our patient population. To investigate the underlying biophysical and biochemical associations of CLU with amyloid fibril proteins, we purified CLU from pooled human serum and tested the ability of the protein to bind, stabilize, and prevent TTR misfolding and amyloid fibril formation. Direct binding of CLU to TTR oligomers was demonstrated using surface plasmon resonance. As observed by circular dichroism spectroscopy, CLU was capable of stabilizing TTR secondary structure. By quantitative Congo red binding, we showed that CLU inhibited TTR amyloid fibril formation. Together, these results provide evidence that CLU is a common element of SSA, ATTR, and AL amyloidosis pathology, can function as a circulating molecular chaperone, and is part of a proteostatic mechanism in amyloid fibril formation.
Thesis (Ph.D.)--Boston University