New Paper! Open Access!
What “Witch Nails” in mice tells us about equine hoof biology and laminitis
For many years, the gold standard for determining the biological function of a protein has been to see how mutation or removal of the protein through artificial genetic engineering or natural mutation impacts the structure, physiology, or behavior (“phenotype”) of the organism carrying the genetic mutation. This mode of investigation is not practical for studies of equine laminitis and hoof biology, for obvious reasons, and to date, there have been few published studies on nail unit biology (claw and associated anatomical structures) in the laboratory animal model of choice, the mouse.
John Sundberg of The Jackson Laboratory of Bar Harbor, Maine, Robert Rice of UC Davis, Hannah Galantino-Homer of the University of Pennsylvania School of Veterinary Medicine (Penn Vet) and co-authors have reported in PLOS One on a genetic mutation of a keratin gene in mice that provides insight into the biology and development of the claw of the mouse and equine hoof (Sundberg, 2022). The mutated protein is a keratin, KRT90, and is expressed in the nail bed of the claw, which is, like the hoof lamellae, the tissue under the claw that connects the claw to the distal phalangeal bone of the digit. The genetic mutation does not eliminate KRT90, but it does change the structure of part of the protein that is important in formation of structures that comprise the “skeleton” of keratinocytes. Like a skeleton, the cell skeleton, or cytoskeleton, is essential for the ability of cells and the tissues they form to resist mechanical forces and maintain structural integrity. Mice with the KRT90 mutation develop what the authors have named the “witch nail” phenotype. The claws of these mice are long and curved, the nail bed is thickened, and the connection between the nail bed and underlying digit is more readily broken, resulting in sloughing of claws in some cases.
The Galantino-Homer lab had previously produced and characterized monoclonal antibodies to equine KRT124, which is the equine equivalent of KRT90 of mice (Armstrong et al., 2019). Those studies had shown that KRT124 is tightly restricted to the hoof lamellae where it pairs with KRT42 to form the most abundant cytoskeletal proteins of the hoof lamellae (Carter et al., 2010), suggesting that KRT124 might have a specialized role in the mechanical suspension of the horse’s distal phalangeal bone (or “coffin bone”) within the hoof capsule. This suspensory apparatus of the distal phalanx is an essential feature of equine anatomy as well as the structure that fails in horses with laminitis. John Sundberg contacted Hannah Galantino-Homer at Penn Vet in early 2020 about the potential relevance of the witch nails mutation to equine laminitis. They noted that there are interesting parallels between the pathology of the nail bed/lamellae in the mouse mutant and in horses with chronic laminitis. The current study used the antibodies against equine KRT124 to demonstrate that KRT90 localizes to the nail bed in wild type mice. This localization and the pathology observed in the witch nail mutants support the idea that KRT90/KRT124 is critical for the form and function of the nail unit.
Interestingly, humans, who do not bear weight on their nails, have lost the ability to express the KRT90 gene and therefore do not have this keratin. The mouse mutation is still useful as a model of nail disease since it allows investigators to examine the downstream impact of nail bed fragility and damage on nail growth, mechanical integrity, inflammatory processes, and secondary effects on the adjacent distal phalangeal bone.
Armstrong, C., Cassimeris, L., Santos, C. D. S., Micoogullari, Y., Wagner, B., Babasyan, S., Brooks, S., & Galantino-Homer, H. (2019). The expression of equine keratins K42 and K124 is restricted to the hoof epidermal lamellae of Equus caballus. PLoS One, 14(9), e0219234. https://doi.org/https://doi.org/10.1371/journal.pone.0219234
Carter, R. A., Shekk, V., de Laat, M. A., Pollitt, C. C., & Galantino-Homer, H. L. (2010). Novel keratins identified by quantitative proteomic analysis as the major cytoskeletal proteins of equine (Equus caballus) hoof lamellar tissue. J Anim Sci, 88(12), 3843-3855.
Sundberg, J. P. Galantino-Homer, H.; Fairfield, H.; Ward-Bailey, P.F.; Harris, B.S.; Berry, M.; Pratt, C.H.; Gott, N.E.; Bechtold, L.S.; Kaplan, P.R.; Durbin-Johnson, B.P.; Rocke, D.M.; Rice, R.H. (2022). Witch Nails (Krt90whnl): A spontaneous mouse mutation affecting nail growth and development. PLoS One, 17(11), e0277284. https://doi.org/10.1371/journal.pone.0277284