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Mechanisms Of Pathology In Equine Laminitis: Versican Depletion From Basal Epithelial Cells And Suppressed Canonical Wnt Signaling

Laminitis is a crippling disease of horses resulting from faiure of the digital laminae, which suspend the distal pedal bone and hence the axial skeleton within the hoof capsule. This disease affects about 2% of all horses in US and there is no effective therapeutic agent. Failure of the laminae results from detachment of the epidermal and demal layers of the tissue which allows the distal phalanx to rotate and sink within the hoof capsule. These layers are joined at a basement membrane which is anchored to epithelial cells of the epidermal tissue by integrins. My studies focus on laminitis associated changes in this junctionl complex. The experimental model for my research is healthy horses administered a pro-laminitis gastric bolus of corn starch/wood flour gruel (CHO). Here, I have shown that: (i) In the laminae of healthy horses laminar basal epithelial cells are packed with the metalloproteinase ADAMTS-4 and its proteoglycan substrates versican and aggrecan. Suprabasal epithelial cells are also packed with aggrecan but not versican, making versican a specific basal epithelial cell differentiation antigen; (ii) A pro-laminitis bolus of starch gruel elevates ADAMTS-4 gene and protein expression in the laminae leading to the elevated cleavage of versican; (iii) in the laminae of horses with acute laminitis versican is depleted from the basal epithelial cells by elevated ADAMTS-4 cleavage and suppressed versican gene expression, which is significantly and positively correlated with a decline in β-catenin and integrin β4 gene expression, and (iv) decline in β-catenin gene and protein expression in laminitic laminae results from depressed canonical Wnt signaling, manifest as a reduced expression of positive regulators (Wnt 4, FzD4, LRP6, PP1, Akt2 and β-catenin) and the up-regulation of negative regulators (Axin1, CKIαand GSK3β). Taken together, the studies show that suppressed canonical Wnt signaling in laminitic horses results in loss of β-catenin and integrin β4, which respectively are required components of adherens junctions and hemidesmosomes, leading to the detachment of the laminar basal epithelial cells from each other and from the basement membrane and thus, destabilizing the epidermal:dermal junction.
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