Laminitis is a devastatingly painful, life-threatening disease of the equine digit. Initiating causes include sepsis/systemic inflammatory response syndrome (SIRS) related conditions, metabolic syndrome/endocrine dysfunction, and severe contralateral limb lameness. The pathogenesis of laminitis has yet to be completely elucidated; however, several theories on the predominate mechanism exist. Recent evidence shows a prominent proinflammatory response very early in the developmental phases of laminitis induced experimentally by administration of black walnut extract (BWE). As dysregulation of the inflammatory response is suspected to play an important role in sepsis/SIRS and subsequent multiple organ failure in humans, a similar mechanism has been proposed to occur in the horse, with the hoof being the predominate organ that fails. Laminar failure occurs when there is dysadhesion between the epidermal and dermal lamina at the basement membrane.
The studies conducted were performed to determine if inflammation in the front and hind limb lamina occur after administration of a carbohydrate overload (CHO), which is a model that more closely resembles clinical cases of laminitis occurring from sepsis, to determine if phosphorylation of STAT proteins play a role in the inflammatory response of the lamina, and to determine if other organs have a similar inflammatory response as the lamina after CHO administration. Additional studies were performed to develop an understanding of how the equine epidermal cell responds to bacterial ligands known to be produced in the cecum after CHO administration and to develop a technique for isolation of RNA specifically from the equine laminar basal epithelial cell (LBEC) using laser capture microdissection (LCM).
Results of these studies demonstrate that inflammation does occur in the front and hind lamina after CHO administration; however, the response occurs at the onset of lameness in these horses which is different from the BWE model where it occurs at the early developmental phase. STAT3, but not STAT1, phosphorylation occurs in the lamina and is most likely a direct result of the profound IL-6 response that has been documented to occur in the lamina from horses receiving both BWE and CHO. Inflammation does occur in the liver, lung and kidney after CHO administration; however, a different pattern of inflammatory gene expression occurs in each tissue with the lamina overall having the greatest response of any organ evaluated. Equine epidermal epithelial cells in culture respond to the gram negative bacterial components, LPS and flagellin, by producing proinflammatory cytokines, but do not response to the bacterial component of gram positive organisms, which is similar to what is observed clinically where horses with sepsis resulting from a gram negative source are much more likely to develop laminitis.
Overall, inflammation appears to be involved in the development of laminitis. In addition, it is likely the laminar epithelial cell plays a much more active role, not only structurally but also producing and responding to inflammatory mediators. The methods developed to isolate RNA from the LCM technique will allow for extensive study of these cells in context of disease, thereby providing essential information from future studies.