Exploring the role of the equine intestinal microbiota in hypoglycin A intoxication: player or witness? Insights from in vivo and in vitro studies.

Methylenecyclopropylalanine, also known as hypoglycin A, is a protoxin responsible for severe intoxications in both humans and animals. In equids, hypoglycin A was the first protoxin identified as being involved in atypical myopathy: a seasonal intoxication with a high mortality rate. These protoxins are not inherently toxic but are activated through a two-step enzymatic process involving first branched-chain amino acid aminotransferase and subsequently the branched-chain -keto acid dehydrogenase complex. This metabolic conversion results in the formation of methylenecyclopropylacetyl-CoA which disrupt lipid metabolism, leading to an accumulation of acylcarnitines in the serum of affected horses. These toxic metabolites are eliminated via the blood and urine after conjugation with carnitine and glycine, respectively.
Studies in humans, equids, and other herbivores suggest a link between gut microbiota and HGA poisoning. To explore this hypothesis, two studies were performed using in vivo and in vitro models.
The first in vivo study compared faecal samples from affected, cograzing and control horses. The latter showed a different microbial profile with a higher -diversity, especially evenness, compared to other groups. Moreover, some genera were significantly different such as Fibrobacter or NK4A214_group and variations in faecal bacterial composition were significantly correlated with blood levels of methylenecyclopropylacetyl-carnitine and C14:1, indicating that they mainly reflect the host’s energy imbalance.
The in vitro model represented six descending colons of horses. Hypoglycin A was added in three of these fermenters while the other three were used as a control group. Two additional fermenters were prepared with the nutritive media without any faecal incubation to assess the stability of hypoglycin A. Hypoglycin A remained stable in the nutritive media while a significant decrease was recorded in the fermenters due to faecal microbiota without the production of toxic metabolites; these observations suggested a protective role of the intestinal microbiota. Furthermore, significant differences were found in the α-diversity and β-diversity between hypoglycin A treated fermenters and control fermenters with a more balanced representation of bacterial populations in the latter group.
Overall, these findings show that the equine gut microbiota is affected by hypoglycin A and may play a partially protective role by reducing protoxin levels. However, since amino acid absorption occurs before the hindgut in horses, hypoglycin A can enter the bloodstream before microbial degradation, explaining their high susceptibility to atypical myopathy. Future studies using labelled HGA and targeting specific detoxifying bacteria could open new preventive strategies through probiotics or dietary interventions.