FOXO transcription factors are mechanosensitive and their regulation is altered with aging in the respiratory pump

The mechanical regulation of the forkhead box O ( FOXO) subclass of transcription factors in the respiratory pump and its implication in aging are completely unknown. We investigated the effects of diaphragm stretch on three FOXO isoforms, Foxo1, Foxo3a, and Foxo4, in normal mice at different ages. We tested the hypotheses that 1) FOXO activities are regulated in response to diaphragm stretch and 2) mechanical properties of aging diaphragm are altered, leading to altered regulation of FOXO with aging. Our results showed that stretch downregulated FOXO DNA- binding activity by a mechanism that required Akt and IKK activation in young mice but that these pathways lost their mechanosensitivity with age. This aberrant regulation of FOXO with aging was associated with altered viscoelasticity, compliance, and extensibility of the aged diaphragm. Curiously, the dramatic decrease of the nuclear content of Foxo1 and Foxo3a, the two isoforms associated with muscle atrophy, with aging correlated with higher basal activation of Akt and IKK signaling in diaphragms of old mice. In contrast, the stability of Foxo4 in the nucleus became dependent on JNK, which is strongly activated in aged diaphragm. This finding suggests that Foxo4 was responsible for the FOXO- dependent transcriptional activity in aging diaphragm. Our data support the hypothesis that aging alters the mechanical properties of the respiratory pump, leading to altered mechanical regulation of the stretch- induced signaling pathways controlling FOXO activities. Our study supports a mechanosensitive signaling mechanism that is responsible for regulation of the FOXO transcription factors by aging.