Indoles from commensal microbiota promote mobility during aging

Abstract: Aging is associated with loss of mobility and infirmity, which accounts for 23% of the total global burden of

disease in people aged 60-years and older and will only increase over the next 20-years. Frailty is associated with dysbiosis

23, 41-43, but it is unclear what microbiota products contribute to health or frailty. We identified indole and its derivatives as molecules secreted by benevolent commensal bacteria that act across diverse phyla (C. elegans, Drosophila and mice) to

augment healthspan and allow animals to live better for longer1. Indoles derived from commensal bacteria act via the Aryl hydrocarbon receptor (Ahr) to promote healthy intestinal homeostasis20, augment intestinal barrier integrity and limit systemic inflammation1, 20, 23, as well as promote DNA repair and genome integrity in stem cells and germ cells22.

Preliminary and published data1 also suggest that as animals age, indoles limit the loss of muscle proteins concomitant

with loss of mobility. Citing our work as the starting point1, a recent metabolomic study was initiated using samples from the Arivale and Osteoporotic Fractures in Men [MrOS] cohorts and found that loss of indole producing bacteria and

reduced plasma indole levels correlated with decreased mobility in aged humans44, 45. We hypothesize that indoles act via Ahr to (i) promote stability of muscle proteins essential for the contractile apparatus by promoting chaperone-mediated refolding of damaged proteins, and by regulating proteasome activity to promote loss of irreparable proteins and limit

aggregation of damaged proteins. We have identified other indole-regulated process in muscle that may act in conjunction with proteostatic mechanisms to promote mobility in the aged. These include regulation of genes associated

with mitochondrial respiratory function and efficiency, which fuels muscle activity but declines with age46, 47. Together, we hypothesize that these mechanisms maintain muscle mass and function and thereby promote mobility in the aged. Aim 1

will determine the mechanism by which indole limits age-dependent loss of existing assembled myofibrils via upregulating the levels of the myosin chaperone UNC-45 and additional chaperones and proteasome components. Aim 2 will develop address whether indole affects additional cellular pathways by using genetic approaches to identify which indole-

regulated genes are critical for maintaining mobility during aging. Gene ontogeny (GO) analysis to date has shown that

indole upregulates genes essential for mitochondrial function in aging animals. Thus, Aim 3 will test the hypothesis that

indole maintains or improves mitochondrial function in aged animals so as to fuel muscle activity and/or limit production of factors that disrupt proteostasis (e.g. ROS). We use genetic and biochemical analysis in the nematode Caenorhabditis

elegans, whose striated muscles exhibit significant structural conservation with mammals. Importantly, as C. elegans age,

they exhibit loss of mobility and changes in proteostasis, including loss of the contractile apparatus and mitochondrial function, characteristic features of sarcopenia in mammals. Here, we also propose to corroborate our worm results in aged mice. Together, these studies will provide mechanistic information on how indoles derived from commensal bacteria

counteract sarcopenia during aging. These mechanistic studies are essential to developing indoles as biomarkers for frailty and health, and as possible therapeutics to treat muscle wasting, including sarcopenia in elderly people.