The timeline of lamellar basement membrane changes during equine laminitis development

A review of cellular and molecular mechanisms in endocrinopathic, sepsis-related and supporting limb equine laminitis

Equine laminitis has both fascinated and frustrated veterinary researchers and clinicians for many years. The recognition that many ponies suffering from pasture-associated laminitis have an insulin-dysregulated phenotype (endocrinopathic laminitis, EL) and that prolonged insulin and glucose infusions can experimentally induce laminar pathology and functional failure are seminal discoveries in this field. Researchers have studied the molecular basis for disease pathogenesis in models of EL, sepsis-related laminitis and supporting limb laminitis and generated much data over the last 15 years. This review attempts to synthesise those data, drawing comparisons between models and naturally occurring laminitis. A hypothesis is proposed that the basal epithelial cell stress is a central event in each category of laminitis. Furthermore, in naturally occurring pasture-associated laminitis, pathways that predominate in each type of laminitis contribute to laminar lamellar pathology to varying extents. Based on the molecular mechanisms determined in experimental models, interactions between these pathways are identified.

Selection signatures for local and regional adaptation in Chinese Mongolian horse breeds reveal candidate genes for hoof health

BACKGROUND

Thousands of years of natural and artificial selection since the domestication of the horse has shaped the distinctive genomes of Chinese Mongolian horse populations. Consequently, genomic signatures of selection can provide insights into the human-mediated selection history of specific traits and evolutionary adaptation to diverse environments. Here, we used genome-wide SNPs from five distinct Chinese Mongolian horse populations to identify genomic regions under selection for the population-specific traits, gait, black coat colour, and hoof quality. Other global breeds were used to identify regional-specific signatures of selection.

RESULTS

We first identified the most significant selection peak for the Wushen horse in the region on ECA23 harbouring DMRT3, the major gene for gait. We detected selection signatures encompassing several genes in the Baicha Iron Hoof horse that represent good biological candidates for hoof health, including the CSPG4, PEAK1, EXPH5, WWP2 and HAS3 genes. In addition, an analysis of regional subgroups (Asian compared to European) identified a single locus on ECA3 containing the ZFPM1 gene that is a marker of selection for the major domestication event leading to the DOM2 horse clade.

CONCLUSIONS

Genomic variation at these loci in the Baicha Iron Hoof may be leveraged in other horse populations to identify animals with superior hoof health or those at risk of hoof-related pathologies. The overlap between the selection signature in Asian horses with the DOM2 selection peak raises questions about the nature of horse domestication events, which may have involved a prehistoric clade other than DOM2 that has not yet been identified.

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.

The effect of continuous digital hypothermia on lamellar energy metabolism and perfusion during laminitis development in two experimental models

BACKGROUND

Continuous digital hypothermia (CDH) prevents lamellar failure in the euglycaemic hyperinsulinaemic clamp (EHC) and oligofructose (OF) laminitis models, but the mechanisms remain unclear.

OBJECTIVES

To evaluate the effects of CDH on lamellar energy metabolism and perfusion in healthy horses and during EHC and OF laminitis models.

STUDY DESIGN

In vivo experiment.

METHODS

Archived samples were used from Standardbred geldings that received no treatment (CON) (n = 8) or underwent EHC (n = 8) or OF (n = 6) laminitis models. Both forelimbs were instrumented with a lamellar microdialysis system, and one forelimb was cooled (CDH) with the other maintained at ambient temperature (AMB). Microdialysate was collected every 6 hours and analysed for glucose, lactate and pyruvate concentrations and lactate to pyruvate ratio (L:P). Microdialysis urea clearance was used to estimate lamellar tissue perfusion. Data were analysed using a mixed-effects linear regression model.

RESULTS

Glucose did not change in CDH limbs relative to AMB in CON (P = .3), EHC (P = .3) or OF (P = .6) groups. There was a decrease in lactate (P < .001) and pyruvate (P < .01) in CDH limbs relative to AMB in all groups. L:P decreased in CON CDH relative to CON AMB (P < .001) but was not different in EHC (P = .6) and OF (P = .07) groups. Urea clearance decreased in CDH limbs relative to AMB in CON (P = .002) and EHC (P < .001), but not in OF (P = .4).

MAIN LIMITATIONS

The EHC model may not mimic natural endocrinopathic laminitis.

CONCLUSIONS

CDH caused a marked decrease in lamellar glucose metabolism (CON, EHC and OF) and perfusion (CON and EHC) without affecting lamellar glucose concentration. Although cellular energy failure is not a primary pathophysiological event in EHC and OF laminitis models, CDH may act by limiting energy supply to pathologic cellular processes whilst preserving those critical to lamellar homoeostasis.

Are There Shared Mechanisms in the Pathophysiology of Different Clinical Forms of Laminitis and What Are the Implications for Prevention and Treatment?

Laminitis is a consequence of primary disease processes elsewhere in the body. The key pathophysiologic events are insulin dysregulation in endocrinopathic laminitis, ischemia in supporting limb laminitis, and inflammation in sepsis-related laminitis. These apparently disparate mechanisms converge to cause lamellar attachment failure through epithelial cell adhesion loss and stretch, possibly mediated by common growth factor signaling pathways. Tissue damage through mechanical distraction, inflammation, pain, and a proliferative epithelial healing response are features of acute laminitis regardless of the cause. Preventive and treatment strategies based on knowledge of these unique and common mechanistic events are likely to improve clinical outcomes.

Detection of endoplasmic reticulum stress and the unfolded protein response in naturally-occurring endocrinopathic equine laminitis

Background

Laminitis is often associated with endocrinopathies that cause hyperinsulinemia and is also induced experimentally by hyperinsulinemia, suggesting that insulin initiates laminitis pathogenesis. Hyperinsulinemia is expected to activate pro-growth and anabolic signaling pathways. We hypothesize that chronic over-stimulation of these pathways in lamellar tissue results in endoplasmic reticulum stress, contributing to tissue pathology, as it does in human metabolic diseases. We tested this hypothesis by asking whether lamellar tissue from horses with naturally-occurring endocrinopathic laminitis showed expression of protein markers of endoplasmic reticulum stress.

Results

Three markers of endoplasmic reticulum stress, spliced XBP1, Grp78/BiP and Grp94, were upregulated 2.5–9.5 fold in lamellar tissues of moderately to severely laminitic front limbs (n = 12) compared to levels in controls (n = 6–7) measured by immunoblotting and densitometry. Comparing expression levels between laminitic front limbs and less affected hind limbs from the same horses (paired samples from 7 to 8 individual horses) demonstrated significantly higher expression for both spliced XBP1 and Grp78/BiP in the laminitic front limbs, and a similar trend for Grp94. Expression levels of the 3 markers were minimal in all samples of the control (n = 6–7) or hind limb groups (n = 7–8). Immunofluorescent localizations were used to identify cell types expressing high levels of Grp78/BiP, as an indicator of endoplasmic reticulum stress. Grp78/BiP expression was highly elevated in suprabasal epidermal keratinocytes and only observed in laminitic front limbs (10/12 laminitic samples, compared to 0/7 in sections from the hind limbs and 0/5 of controls).

Conclusions

These data demonstrate that the endoplasmic reticulum stress pathway is active in naturally occurring cases of laminitis and is most active within a subset of epidermal keratinocytes. These data provide the rationale for further study of endoplasmic reticulum stress in experimental models of laminitis and the links between laminitis and human diseases sharing activation of this stress pathway. Pharmacological options to manipulate the endoplasmic reticulum stress pathway under investigation for human disease could be applicable to laminitis treatment and prevention should this pathway prove to be a driver of disease progression.

Electronic supplementary material

The online version of this article (10.1186/s12917-018-1748-x) contains supplementary material, which is available to authorized users.

Paradigm shifts in understanding equine laminitis

Laminitis, one of the most debilitating conditions of all equids, is now known to be the result of several systemic disease entities. This finding, together with other recent developments in the field of laminitis research, have provoked a rethink of our clinical and research strategies for this condition. First, laminitis is now considered to be a clinical syndrome associated with systemic disease (endocrine disease, sepsis or systemic inflammatory response syndrome, SIRS) or altered weight bearing rather than being a discrete disease entity. Next, laminitis associated with endocrine disease (endocrinopathic laminitis) is now believed to be the predominant form in animals presenting (primarily) for lameness. Third, the designation of laminitis as a primary and severe basement membrane pathology now requires revision. Instead, current data now proposes a variable subclinical phase associated with gross changes in the hoof capsule, with stretching and elongation of the lamellar cells an early and key event in the pathophysiology. These findings have fuelled new mechanistic hypotheses and research directions that will be discussed, together with their implications for future clinical management.

Misoprostol Inhibits Lipopolysaccharide-Induced Pro-inflammatory Cytokine Production by Equine Leukocytes

Pro-inflammatory cytokines including tumor necrosis factor α (TNFα), IL-1β, IL-6, and IL-8 are potent immune mediators that exacerbate multiple equine diseases such as sepsis and laminitis. Unfortunately, safe and effective cytokine-targeting therapies are lacking in horses; therefore, novel mechanisms of inhibiting cytokine production are critically needed. One potential mechanism for inhibiting cytokine synthesis is elevation of intracellular cyclic AMP (cAMP). In human leukocytes, intracellular cAMP production is induced by activation of E-prostanoid (EP) receptors 2 and 4. These receptors can be targeted by the EP2/4 agonist and prostaglandin E1 analog, misoprostol. Misoprostol is currently used as a gastroprotectant in horses but has not been evaluated as a cytokine-targeting therapeutic. Thus, we hypothesized that misoprostol treatment would inhibit pro-inflammatory cytokine production by lipopolysaccharide (LPS)-stimulated equine leukocytes in an in vitro inflammation model. To test this hypothesis, equine leukocyte-rich plasma (LRP) was collected from 12 healthy adult horses and used to model LPS-mediated inflammatory signaling. LRP was treated with varying concentrations of misoprostol either before (pretreated) or following (posttreated) LPS stimulation. LRP supernatants were assayed for 23 cytokines using an equine-specific multiplex bead immunoassay. Leukocytes were isolated from LRP, and leukocyte mRNA levels of four important cytokines were evaluated via RT-PCR. Statistical differences between treatments were determined using one-way RM ANOVA (Holm-Sidak post hoc testing) or Friedman's RM ANOVA on Ranks (SNK post hoc testing), where appropriate (p < 0.05, n = 3-6 horses). These studies revealed that misoprostol pre- and posttreatment inhibited LPS-induced TNFα and IL-6 protein production in equine leukocytes but had no effect on IL-8 protein. Interestingly, misoprostol pretreatment enhanced IL-1β protein synthesis following 6 h of LPS stimulation, while misoprostol posttreatment inhibited IL-1β protein production after 24 h of LPS stimulation. At the mRNA level, misoprostol pre- and posttreatment inhibited LPS-induced TNFα, IL-1β, and IL-6 mRNA production but did not affect IL-8 mRNA. These results indicate that misoprostol exerts anti-inflammatory effects on equine leukocytes when applied before or after a pro-inflammatory stimulus. However, the effects we observed were cytokine-specific and sometimes differed at the mRNA and protein levels. Further studies are warranted to establish the inhibitory effects of misoprostol on equine cytokine production in vivo.

Histopathological hoof laminar changes in horses with Pituitary Pars Intermedia Adenoma: cases report

ABSTRACTLaminitis in horses is often associated with endocrine disorders, especially the pituitary pars intermedia dysfunction (PPID) in older animals. Morphologic exams of the laminar tissue of the hoof were performed in two horses with suspected PPID, with no clinical signs of laminitis. Changes compatible with laminitis of endocrine origin were observed, such as rounding of the nuclei of the basal cells, thinning and stretching of the secondary epidermal laminae and tissue proliferation. PPID horses with no clinical signs of laminitis may be affected by lesions of the laminar tissue of the hoof that compromise the integrity of the dermal-epidermal junction and may develop clinical symptoms of the disease. It has been suggested that the development stage of endocrine laminitis is longer, but further studies should be conducted to confirm it.

Ultrafiltration of equine digital lamellar tissue

There are no experimentally validated pharmacological means of preventing laminitis; however, locally acting pharmaceutical agents with the potential to prevent laminitis have been identified. Demonstrating therapeutic drug concentrations in lamellar tissue is essential for evaluating the efficacy of these agents. The aim of this study was to develop an experimental technique for repeatedly sampling lamellar interstitial fluid. A technique for placing ultrafiltration probes was developed in vitro using 15 cadaver limbs. Subsequently, lamellar ultrafiltration probes were placed in one forelimb in six living horses. Interstitial fluid was collected continuously from the probes as ultrafiltrate for 4 (n = 4) or 14 days (n = 2). The rate of ultrafiltrate collection was calculated every 12 h. Biochemical analyses were performed on ultrafiltrate collected on night 1 (12-24 h post-implantation) and night 4 (84-96 h post-implantation). Sections surrounding the probe and control tissue from the contralateral limb were harvested, stained with H&E and Masson's trichrome and scored based on the tissue response to the probe. Ultrafiltration probes were placed in the lamellar tissue in all six horses. Ultrafiltrate was collected from these probes at 55 (30-63) μL/h (median [interquartile range]). Fluid production decreased significantly with time from night 3 onwards (P < 0.05). There was no significant change in the constituents of the ultrafiltrate between nights 1 and 4 (P > 0.05). The technique was well tolerated. This study demonstrates that ultrafiltration can be used to sample equine digital lamellar interstitial fluid, and has potential for measuring lamellar drug levels.

Expression and activity of collagenases in the digital laminae of horses with carbohydrate overload-induced acute laminitis

Background

Matrix metalloproteinases (MMP) are hypothesized to degrade structurally important components of the laminar extracellular matrix (ECM) in horses with laminitis.

Objective

To compare levels of expression of stromelysin‐1 (MMP‐3), collagenases (MMP‐1, ‐13), and membrane type‐MMPs (MMP‐14, ‐15, ‐16), and the distribution of their ECM substrates, in laminae of healthy horses and horses with carbohydrate overload laminitis.

Animals

Twenty‐five adult horses.

Methods

Gene and protein expression were determined in extracts of laminae using real‐time quantitative polymerase chain reaction and Western blotting after sodium dodecylsulfate polyacrylamide gel electrophoresis. Distribution of MMP‐13 and ECM components was determined using indirect immunofluorescent microscopy of nonfixed frozen sections. ECM morphology was assessed by hematoxylin and eosin staining.

Results

Of the genes studied, only those encoding MMP‐1 and ‐13 were upregulated in CHO‐induced laminitis; MMP‐1 at Obel grade (OG)1 lameness and MMP‐13 at OG3 lameness. Laminar MMP‐1 was present as 52 kDa proenzyme only. MMP‐13 was present as pro‐ (61 kDa) and processed (48 kDa) enzyme. MMP‐13 localized to the basal epithelium of the secondary epidermal laminae and its increased expression were accompanied by the appearance in secondary dermal laminae (SDL) of multiple foci that were devoid of collagen I, fibronectin, chondroitin and keratan sulfate glycosaminoglycans, and eosin‐staining material.

Conclusions and Clinical Relevance

MMP‐13 is upregulated in laminae of horses with CHO‐induced OG3 lameness and, by degrading components of the ECM, may contribute to the formation of ECM‐free lesions (gaps or tears) that appear in the SDL with OG3 lameness.

A review of recent advances and current hypotheses on the pathogenesis of acute laminitis

With the increasing number of studies being published on the different experimental models used to induce and study acute laminitis, the pathophysiological events associated with these various models (i.e. starch overload, oligofructose overload, black walnut extract and hyperinsulinaemia) can be compared more realistically. Within this review, the mechanisms for metabolic vs. inflammatory laminitis are discussed, and the question of how pasture laminitis may fit into any of the proposed mechanisms is addressed.

The timeline of metalloprotease events during oligofructose induced equine laminitis development

REASON FOR PERFORMING STUDY

The role of matrix metalloproteases (MMPs) and the timeline of proteolysis during laminitis development are incompletely understood.

OBJECTIVES

To determine the temporal progression of selected MMPs and protease regulators during laminitis development.

METHODS

Five clinically normal Standardbred horses received, via nasogastric intubation, an oligofructose (OF) bolus (10 g/kg bwt). Laminitis induction proceeded for 48 h followed by euthanasia. Lamellar biopsies were obtained prior to dosing and at intervals during the treatment period for analysis (12, 18, 24, 30 and 36 h and at 48 h following euthanasia). Tissue samples were analysed by real-time PCR, zymography and western blotting.

RESULTS

Activation of proMMP-2 occurs either simultaneously or at least 12 h following lamellar basement membrane (BM) damage, while no activation of proMMP-9 is seen during OF laminitis induction. Aggrecanase gene expression increased initially at 12-18 h post OF dosing, similar to BM changes. Gene expression of TIMP-2, a MMP regulator, decreases during laminitis development.

CONCLUSIONS

The MMP-2/MT1-MMP complex may not play a major role in initiating lamellar BM damage. Aggrecanase and TIMP-2 gene expression appear related to BM lamellar changes.

POTENTIAL RELEVANCE

MMPs, historically thought to cause laminitis, do not appear to play an initiating role in the lamellar lesion. Other host derived proteases and degradation of alternative lamellar matrix components need to be considered.