In a recent study, researchers Lynn Cassimeris, Julie Engiles and Hannah Galantino-Homer found a similar reaction on the cellular level between humans and horses with endocrinopathies, as published in the journal BMC Veterinary Research. While humans don’t have lamellar cells as horses do, both have animal cells containing endoplasmic reticulum (ER). Metabolic issues arise when the ER is stressed in both humans and horses.

The ER is a continuous membrane within the animal cell that makes hormones and lipids while transporting proteins and carbohydrates throughout the cell. These processes are crucial because they play a significant role in the development of the skeleton and cell reproduction. 

When the body, horse or human, is overloaded with glucose, the endocrine system works overtime. In some cases, this can result in hyperinsulinemia, where there are excess levels of insulin in relation to glucose in the bloodstream. Hyperinsulinemia is expected to stimulate pro-growth and anabolic signaling pathways. Anabolism is the process of creating complex molecules from simple ones for energy storage. When the ER is stressed, hyperinsulinemia over stimulates these processes, creating problems.

It is believed that hyperinsulinemia is the main cause of laminitis, and although humans do not have lamellar cells, cell function mirrors that between humans and horses. Insulin dysfunction in horses results in equine metabolic syndrome (EMS) and pituitary pars intermedia dysfunction (PPID), which mirrors human metabolic syndrome and type 2 diabetes in humans, respectively. The ER stress and the up-regulated protein response (UPR) are present in both humans and horses in insulin-related health problems. In this case, if researchers can find a link in pathways, there might be hope of treating laminitis with human medication typically used for diabetes and other insulin-dysfunction problems.

For the purpose of this study, researchers collected 12 horses with naturally occurring endocrinopathic laminitis (EL), six with EMS and six with PPID, and they collected a control group of eight horses that showed no signs of laminitis. The samples were collected immediately after euthanasia. To begin, researchers extracted 1-2mm of tissue from each limb of each horse to compare which biomarkers were present in the EL group and not the control. 

In the EL group, there were three biomarkers present that did not appear in the control group. The three markers of ER stress were spliced X-box protein binding 1 (XBP1), glucose-regulated protein, also known as binding immunoglobulin protein (Grp78/BiP) and glucose-regulated protein (Grp94). Grp94 was an important marker because it allowed researchers to localize specific cell types within lamellar tissue undergoing ER stress. Immunofluorescent localizations were used to identify cell types expressing high Grp78/BiP to indicate ER stress as well.

When the ER is stressed, the cell is overloaded, causing malfunctions of the ER, resulting in misfolded proteins. These problematic proteins are the biomarkers that appear on the blood and tissue test in the front limbs of horses with EL. 

The biomarkers were all highly marked in the EL group’s front limbs and either not apparent or significant in the EL hind limbs or control group. Further, markers were found specifically in the laminitic feet, at the top layer of cells besides the keratinized axis and hoof wall, below the primary epidermal laminellae (PEL), but not above the PEL. In this case, the research suggests that ER stress and UPR occurs in these specific cells in this area, but the significance of the high levels of the biomarkers is unknown. 

Prolonged stress of the ER can lead to apoptosis, or cell death, in this particular area. The necrotic cells could compromise the structural integrity of the hoof, which is seen in laminitis when the hoof wall begins to separate. 

It is interesting to note that the levels of the biomarkers were higher in the front limbs than in the hind when both pairs of limbs on the same horses should have featured the same level of markers. Hyperinsulinemia affects the body as whole, exposing all limbs to the excess amounts of insulin. The common exposure would make all limbs susceptible to ER stress, but the front limbs were significantly more marked in the results than the hinds.

A few suggestions as to why this occurs falls into the anatomy of the horse. It is estimated that horses bear more weight on their front limbs (60%) than their hinds (40%). Another reason theorized could be due to anatomical and physiological differences between the front and hind limbs that result in varied cell processes. Lastly, it is possible that the front limbs are more susceptible because they demand more energy and load bearing initially, which sends more nutrients to the front limbs, increasing exposure to hyperinsulinemia.

In the study “Neuropathic changes in equine laminitis pain,” researchers studied horses with chronic laminitis and found that many display peripheral nerve degenerative changes and neuropathic pain markers. The use of a medical blocking agent of soluble epoxide hydrolase, a type of enzyme that suppresses the immune system in areas that regulate ER stress, has been used to reverse pain behavior and ER stress in mice. This shows promise for treatment of laminitic cases in horses. 

In another study “Lamellar events related to insulin-like growth factor-1 receptor signaling in two models relevant to endocrinopathic laminitis,” the use of the inhibitor rapamycin was used to help cells dispose of misfolded proteins created under ER stress. These drugs have been previously used in horses to treat musculoskeletal inflammation and EMS. 

Additional drugs used to treat human metabolic syndrome regulate the overabundance of insulin in instances of hyperinsulinemia that triggers ER stress. These medications may be useful in treating horses, either alone or in combination with other treatment.

REFERENCES

Cassimeris L, Engiles JB, Galantino-Homer H. Detection of endoplasmic reticulum stress and the unfolded protein response in naturally-occurring endocrinopathic equine laminitis. BMC Veterinary Research. 2019.

Jones E, Vinuela-Fernandez I, Eager RA, Delaney A, Anderson H, Patel A, et al. Neuropathic changes in equine laminitis pain. Pain. 2007;132:321–31.

Lane HE, Burns TA, Hegedus OC, Watts MR, Weber PS, Woltman KA, Geor RJ, McCutcheon LJ, Eades SC, Mathes LE, Belknap JK. Lamellar events related to insulin-like growth factor-1 receptor signalling in two models relevant to endocrinopathic laminitis. Equine Vet J. 2017;49:643–54.