Biology subjects

McCalpin, S. D.

Publications and source records attributed to McCalpin, S. D..

4 recordsLinked to original sources

bioRxiv2026

Saponin Chemistry Controls Anionic Lipid Tolerance and Divalent Metal Ion Responses in Magnetically Alignable Bicelles

Saponin-phospholipid bicelles have emerged as promising membrane-mimetic systems for anisotropy-based NMR studies, but their utility depends on their ability to accommodate physiologically relevant lipid compositions and ionic environments. Here, we systematically investigated how saponin chemistry governs three key properties of magnetically alignable bicelles: tolerance to anionic lipid incorporation, responsiveness to lanthanide-induced alignment reorientation, and stability in the presence of divalent metal ions. Using 31P NMR spectroscopy as a sensitive probe of phase behavior and alignment, we compared bicelles formed with glycyrrhizic acid (GA), hederacoside C (HC), and crude Quillaja saponins (CQS). While all saponins effectively solubilized the anionic lipid DMPG, only HC supported magnetically aligned bicelles with high anionic lipid fractions (up to 70%), whereas GA was limited to low incorporation (10%). Both GA- and HC-based bicelles underwent lanthanide-induced alignment flipping, though with significant spectral broadening and intermediate alignment states indicative of increased heterogeneity. Divalent cation effects were strongly ion- and saponin-dependent; HC bicelles were robust to the presence of Ca2+ but were destabilized by Mg2+, while GA bicelles were disrupted by both ions at low concentrations. Together, these results demonstrate that saponin identity critically determines bicelle compatibility with charged lipids and ionic conditions, establishing design principles for tailoring saponin-based bicelles as versatile, biomimetic alignment media for membrane-protein structural studies.

biophysics↗

bioRxiv2025

Anionic lipid catalyzes the generation of cytotoxic insulin oligomers

Misfolding and aggregation of proteins into amyloidogenic assemblies are key features of several metabolic and neurodegenerative diseases. Human insulin has long been known to form amyloid fibrils under various conditions, which affects its bioavailability and function. Clinically, insulin aggregation at recurrent injection sites poses a challenge for diabetic patients who rely on insulin therapy. Furthermore, decreased responsiveness to insulin in type 2 diabetic (T2D) patients may lead to its overproduction and accumulation as aggregates. Earlier reports have reported that various factors such as pH, temperature, agitation, and the presence of lipids or other proteins influence insulin aggregation. Our present study aims to elucidate the effects of non-micellar anionic DMPG (1,2-dimyristoyl-sn-glycero-3-phosphoglycerol) lipids on insulin aggregation. Distinct pathways of insulin aggregation and intermediate formation were observed in the presence of DMPG using a ThT fluorescence assay. The formation of soluble intermediates, alongside large insulin fibrils, was observed in insulin incubated with DMPG via TEM, DLS and NMR, as opposed to insulin aggregates generated without lipids. 13C magic angle spinning solid-state NMR and FTIR experiments indicated that lipids do not alter the conformation of insulin fibrils but do alter the time scale of motion of aromatic and aliphatic sidechains. Furthermore, the soluble intermediates were found to be more cytotoxic as compared to fibrils generated with or without lipids. Overall, our study elucidates the importance of anionic lipids in dictating the pathways and intermediates associated with insulin aggregation. These findings could be useful in determining various approaches to avoid toxicity and enhance the effectiveness of insulin in therapeutic applications.

biophysics↗

bioRxiv2024

Differential Effects of Ganglioside Lipids on the Conformation and Aggregation of Islet Amyloid Polypeptide

Despite causing over 1 million deaths annually, Type 2 Diabetes (T2D) currently has no curative treatments. Aggregation of the islet amyloid polypeptide (hIAPP) into amyloid plaques plays an important role in the pathophysiology of T2D and thus presents a target for therapeutic intervention. The mechanism by which hIAPP aggregates contributes to the development of T2D is unclear but are proposed to involve disruption of cellular membranes. However, nearly all research on hIAPP-lipid interactions has focused on anionic phospholipids, which are primarily present in the cytosolic face of plasma membranes. We seek here to characterize the effects of three gangliosides, the dominant anionic lipids in the outer leaflet of the plasma membrane, on the aggregation, structure, and toxicity of hIAPP. Our results show a dual behavior that depends on the molar ratio between the gangliosides and hIAPP. For each ganglioside, a low lipid:peptide ratio enhances hIAPP aggregation and alters the morphology of hIAPP fibrils, while a high ratio eliminates aggregation and stabilizes an -helix-rich hIAPP conformation. A more negative lipid charge more efficiently promotes aggregation, and a larger lipid headgroup improves inhibition of aggregation. hIAPP also alters the phase transitions of the lipids, favoring spherical micelles over larger tubular micelles. We discuss our results in the context of available lipid surface area for hIAPP binding and speculate on a role for gangliosides in facilitating toxic hIAPP aggregation.

biophysics↗

bioRxiv2023

On-Pathway Oligomer of Human Islet Amyloid Polypeptide Induced and Stabilized by Mechanical Rotation During MAS NMR

Intermediates along the fibrillation pathway are generally considered to be the toxic species responsible for the pathologies of amyloid diseases. However, structural studies of these species have been hampered by heterogeneity and poor stability in standard aqueous conditions. Here, we report a novel methodology for producing stable, on-pathway oligomers of the human Type-2 Diabetes-associated islet amyloid polypeptide (hIAPP, or amylin) using the mechanical forces associated with magic angle spinning (MAS). The species were a heterogeneous mixture of globular and short rod-like species with significant {beta}-sheet content and the capability of seeding hIAPP fibrillation. We used MAS NMR to demonstrate that the nature of the species was sensitive to sample conditions including peptide concentration, ionic strength, and buffer. The methodology should be suitable for studies of other aggregating systems.

biophysics↗