Gut Microbiomes in HD

Two of the most common non-neurological features of Huntington disease (HD) are progressive weight loss and metabolic dysfunction. However, a small proportion of HD patients are pathologically overweight, despite having similar CAG repeat lengths as pathologically underweight patients. The investigators hypothesize this spectrum of weight abnormalities may be caused by HD-related metabolic dysfunction. Pathological weight loss is recapitulated in transgenic HD model mice expressing fragments of human huntingtin (HTT), either transgenically3,4 or knocked-in to a portion of the mouse HD homolog (Hdh) gene5. Conversely, pathological weight gain is recapitulated in transgenic HD model mice expressing full-length human HTT either along with the full complement of Hdh6,7 or in Hdh-null backgrounds8,9. In Hdh-null background transgenic HD model mice, which are pathologically overweight, circadian feeding is disrupted, despite maintenance of naturally nocturnal circadian activity. Interestingly, circadian feeding patterns are restored by suppression of brain HTT (unpublished Dr. Amber Southwell), suggesting that HTT plays a role in circadian feeding regulation. Furthermore, when circadian feeding patterns are artificially restored with scheduled feeding, striatal HTT is temporarily suppressed, while metabolic markers and body weight are normalized (unpublished Dr. Amber Southwell). Together, this demonstrates that HTT is involved in gut-brain feedback, but since HTT suppression during scheduled feedings is transient, while metabolic effects are lasting, HTT is likely not the master regulator of this feedback loop. Instead, the gut microbiome may influence this pathway, possibly contributing to the onset and/or progression of HD.