Medin Oligomer Membrane Pore Formation: A Potential Mechanism of Vascular Dysfunction

Abstract

Medin, a 50-amino-acid cleavage product of the milk fat globule-EGF factor 8 protein, is one of the most common forms of localized amyloid found in the vasculature of individuals older than 50 years. Medin induces endothelial dysfunction and vascular inflammation, yet despite its prevalence in the human aorta and multiple arterial beds, little is known about the nature of its pathology. Medin oligomers have been implicated in the pathology of aortic aneurysm, aortic dissection, and more recently, vascular dementia. Recent in vitro biomechanical measurements found increased oligomer levels in aneurysm patients with altered aortic wall integrity. Our results suggest an oligomer-mediated toxicity mechanism for medin pathology. Using lipid bilayer electrophysiology, we show that medin oligomers induce ionic membrane permeability by pore formation. Pore activity was primarily observed for preaggregated medin species from the growth-phase and rarely for lag-phase species. Atomic force microscopy (AFM) imaging of medin aggregates at different stages of aggregation revealed the gradual formation of flat domains resembling the morphology of supported lipid bilayers. Transmission electron microscopy images showed the coexistence of compact oligomers, largely consistent with the AFM data, and larger protofibrillar structures. Circular dichroism spectroscopy revealed the presence of largely disordered species and suggested the presence of β -sheets. This observation and the significantly lower thioflavin T fluorescence emitted by medin aggregates compared to amyloid- β fibrils, along with the absence of amyloid fibers in the AFM and transmission electron microscopy images, suggest that medin aggregation into pores follows a nonamyloidogenic pathway. In silico modeling by molecular dynamics simulations provides atomic-level structural detail of medin pores with the CN p NC barrel topology and diameters comparable to values estimated from experimental pore conductances.

Cerebrovascular medin is associated with Alzheimer's disease and vascular dementia

“…14 Similar to Aβ, medin oligomers were recently found to form membrane pores that could disrupt cellular homeostasis. 31 Medin's in vivo effects have not been studied as far as we know and much remains to be learned, including whether the association found in this current study between medin on the one hand and AD, VaD, and pathologic lesions associated with AD and VaD on the other hand represent causal or incidental relationships. A potential causal relationship is supported by studies showing that endothelial dysfunction and inflammation, known pathologic responses to medin, are prominent in AD and VaD.…”

“…In both 2D and 3D chip cell culture models, exposure of astrocytes to medin‐treated endothelial cells or endothelial cell media caused pro‐inflammatory activation with increased astrocyte production of IL‐8 14 . Similar to Aβ, medin oligomers were recently found to form membrane pores that could disrupt cellular homeostasis 31 . Medin's in vivo effects have not been studied as far as we know and much remains to be learned, including whether the association found in this current study between medin on the one hand and AD, VaD, and pathologic lesions associated with AD and VaD on the other hand represent causal or incidental relationships.…”

Light, Water, and Melatonin: The Synergistic Regulation of Phase Separation in Dementia

“…Cerebral arteriole medin is regarded as a novel biomarker for AD and VaD [ 91 ]. Even though the glycoprotein lactadherin has multiple, important physiological functions [ 92 ] including phagocytosis [ 93 ], angiogenesis [ 94 ], and mucosal repair [ 95 ], medin aggregates alter cellular homeostasis, causing microvascular endothelial dysfunction by inducing permeability via the formation of pores in lipid membranes that result in upregulated ionic current flow [ 96 , 97 ], a mechanism not dissimilar to how Aβ peptides form calcium ion channels in lipid bilayer membranes [ 98 , 99 ]. However, the conditions that trigger the cleavage of medin from lactadherin causing medin to self-assemble into pathogenic, insoluble fibrils remain unclear [ 82 , 83 ].…”

Inflammation, Nitro-Oxidative Stress, Impaired Autophagy, and Insulin Resistance as a Mechanistic Convergence Between Arterial Stiffness and Alzheimer’s Disease

“…In addition, specific cleavage of the MFG-E8 protein leads to the formation of the amyloid protein, medin. This protein is deposited in the aortic media of the majority of European-Americans aged 50 and older ( Wang et al, 2013 ; Migrino et al, 2017 ; Younger et al, 2020 ). Thus, the amyloidogenic MFG-E8/medin complex might accompany arterial stiffness with advanced age.…”

“…Several interventions using statins, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, angiotensin receptor/neprilysin inhibitors, β-blockers, diuretic agents, calcium-channel blockers, and hemodialysis showed the feasibility to manipulate the APP/Aβ turnover, aggregation of Aβ1–40 peptides or blockage of its inflammatory properties ( Stakos et al, 2020 ). Furthermore, the age-associated amyloidogenic protein medin has been associated with vascular and cerebrovascular inflammation in recent years as it provokes endothelial dysfunction ( Migrino et al, 2017 ; Degenhardt et al, 2020 ; Migrino et al, 2020 ; Younger et al, 2020 ). In this way, medin could also be considered an important inflammatory biomarker in the early-stages of the arterial stiffness and/or AD pathology.…”