The expression of equine keratins K42 and K124 is restricted to the hoof epidermal lamellae of Equus caballus

Witch Nails (Krt90whnl): A spontaneous mouse mutation affecting nail growth and development

Numerous single gene mutations identified in humans and mice result in nail deformities with many similarities between the species. A spontaneous, autosomal, recessive mutation called witch nails (whnl) is described here where the distal nail matrix and nail bed undergo degenerative changes resulting in formation of an abnormal nail plate causing mice to develop long, curved nails. This mutation arose spontaneously in a colony of MRL/MpJ-Fas^lpr/J at The Jackson Laboratory. Homozygous mutant mice are recognizable by 8 weeks of age by their long, curved nails. The whnl mutation, mapped on Chromosome 15, is due to a 7-bp insertion identified in the 3’ region of exon 9 in the Krt90 gene (formerly Riken cDNA 4732456N10Rik), and is predicted to result in a frameshift that changes serine 476 to arginine and subsequently introduces 36 novel amino acids into the protein before a premature stop codon (p. Ser476ArgfsTer36). By immunohistochemistry the normal KRT90 protein is expressed in the nail matrix and nail bed in control mice where lesions are located in mutant mice. Immunoreactivity toward equine KRT124, the ortholog of mouse KRT90, is restricted to the hoof lamellae (equine hoof wall and lamellae are homologous to the mouse nail plate and nail bed) and the mouse nail bed. Equine laminitis lesions are similar to those observed in this mutant mouse suggesting that the latter may be a useful model for hoof and nail diseases. This first spontaneous mouse mutation affecting the novel Krt90 gene provides new insight into the normal regulation of the molecular pathways of nail development.

Transcriptome diversity and differential expression in supporting limb laminitis

Laminitis results in impaired tissue integrity and Inflammation of the epidermal and dermal lamellae connecting the hoof capsule to the underlying distal phalanx and causes loss-of-use, poor quality of life and euthanasia in horses. Historically, studies to better understand the etiology of laminitis by documenting changes in gene expression were hampered by the paucity of gene annotation specific to hoof tissues. Next-generation sequencing enables improvements to annotation by incorporating equine- and hoof-specific transcripts. Here we characterize the hoof lamellar tissue transcriptome of naturally occurring supporting limb laminitis (SLL) using archived lamellar tissue from Thoroughbred racehorses consisting of 13 SLL hospital cases and seven age-matched control horses. This was achieved using: 1) Applied transcriptome annotation by long-read sequencing to document transcript diversity and 2) short-read RNA sequencing to document changes in gene expression correlating to the developmental and acute stages of naturally occurring SLL. 1.99Gbp of long-read transcriptome sequencing deeply documented 5067 unique loci, while short read RNA-seq under very stringent quality filters described 66 differentially expressed loci. Functional analysis of these loci revealed alterations in cell replication and growth, stress response and leukocyte recruitment and activation pathways. Differential expression of the Ezrin and TIMP3 genes suggests they may have utility as biomarkers for laminitis disease, while NR1D1 and genes relevant to the inflammasome are promising targets for novel pharmacological treatments.

The Big Picture in Better Understanding the Equine Foot

The multiple topics summarized in this article and discussed in detail in this issue of VCNA are comprehensive and in-depth, exploring concepts and clinical experiences for state-of-the-art care of the equine foot. The research on the equine foot will translate to the clinical setting and with this the compassionate care of the horse.

Continuous digital hypothermia reduces expression of keratin 17 and 1L-17A inflammatory pathway mediators in equine laminitis induced by hyperinsulinemia

The euglycemic hyperinsulinemic clamp model (EHC) of equine endocrinopathic laminitis induces rapid loss of lamellar tissue integrity, disrupts keratinocyte functions, and induces inflammation similar to natural disease. Continuous digital hypothermia (CDH) blocks tissue damage in this experimental model, allowing identification of specific genes or molecular pathways contributing to disease initiation or early progression. Archived lamellar tissues (8 horses, 48 h EHC treatment, including CDH-treated front limbs) were used to measure relative expression levels of genes encoding keratin 17 (KRT17), a stress-induced intermediate filament protein, and genes upregulated downstream of keratin 17 and/or interleukin 17A (IL-17A), as mediators of inflammation. Compared to front or hind limbs at ambient temperature, CDH resulted in significantly lower expression of KRT17, CCL2, CxCL8, PTGS2 (encoding COX2), IL6, TNFα, S100A8 and MMP1. By immunofluorescence, COX2 was robustly expressed in lamellar keratinocytes from ambient limbs, but not in CDH-treated limbs. Genes not significantly reduced by CDH were IL17A, DEFB4B, S100A9 and MMP9. Overall, 8 of 12 genes were expressed at lower levels in the CDH-treated limb. These 8 genes are expressed by wounded or stress-activated keratinocytes in human disease or mouse models, highlighting the role of keratinocytes in equine laminitis.

Interleukin-17A pathway target genes are upregulated in Equus caballus supporting limb laminitis

Supporting Limb Laminitis (SLL) is a painful and crippling secondary complication of orthopedic injuries and infections in horses, often resulting in euthanasia. SLL causes structural alterations and inflammation of the interdigitating layers of specialized epidermal and dermal tissues, the lamellae, which suspend the equine distal phalanx from the hoof capsule. Activation of the interleukin-17A (IL-17A)-dependent inflammatory pathway is an epidermal stress response that contributes to physiologic cutaneous wound healing as well as pathological skin conditions. As a first test of the hypothesis that hoof lamellae of horses diagnosed with SLL also respond to stress by activating the IL-17A pathway, the expression of IL-17A, IL-17 receptor subunit A and 11 IL-17A effector genes was measured by RT-PCR or qPCR. Lamellar tissue was isolated from Thoroughbreds euthanized due to naturally occurring SLL and in age and breed matched non-laminitic controls. By RT-PCR, the IL-17 Receptor A subunit was expressed in both non-laminitic and laminitic tissues, while IL-17A was primarily detectable in laminitic tissues. IL-17A target gene expression was undetectable in non-laminitic samples with the exception of weak detection of DEFB4B, S100A9 and PTSG2. In contrast, all target genes examined, except CCL20, were expressed by some or all laminitic samples. By qPCR, severe acute (n = 7) SLL expressed ~15–100 fold higher levels of DEFB4B and S100A9 genes compared to non-laminitic controls (n = 8). DEFB4B was also upregulated in developmental/subclinical (n = 8) and moderate acute (n = 7) by ~ 5-fold, and in severe chronic (n = 5) by ~15–200 fold. In situ hybridization (DEFB4) and immunofluorescence (calprotectin, a dimer of S100A9/S100A8 proteins) demonstrated expression in keratinocytes, primarily in suprabasal cell layers, from SLL samples. These data demonstrate upregulation of a cohort of IL-17A target genes in SLL and support the hypothesis that similarities in the response to stresses and damage exist between equine and human epidermal tissues.

Genetics and Signaling Pathways of Laminitis

Laminitis is a devastating disease with diverse etiologies and few, if any, effective treatments. Gene expression and hypothesis-generating genomic studies have provided a fresh look at the key molecular players at crucial timepoints in diverse experimental and naturally affected tissues. We summarize findings to date, and propose a unifying model of the laminitis disease process that includes several pathogenesis concepts shared with other diseases of epidermal and epithelial tissues. The value of these new pathways as potential therapeutic targets is exciting but will require careful future work to validate new methods and launch systematic clinical trials.