Understanding the age-dependent mitochondrial function in astrocytes after spinal cord injury via bi-directional manipulation of activity

Project Summary In the United States, there are >17,000 new cases of spinal cord injury (SCI) every year, and ~300,000 people living with chronic SCI. To date, no FDA-approved treatment improves functional recovery. SCI is increasingly occurring in the aging populations, accompanied by an age-dependent decline in recovery. However, the lack of studies that incorporate age as a key biological variable has become a major obstacle in translating preclinical therapies into successfully treating the aging SCI population. The functional decline of mitochondria associated with normal aging suggests that improving mitochondrial function following SCI could result in better recovery. However, non-selectively enhancing mitochondria activity can have divergent effects depending on age and effected cell types, impairing recovery in young mice while promoting it in older mice. While the cellular and molecular mediators of this age-dependent effect are unknown, this suggests that more uniform effects may be achieved by targeting the decline in mitochondria function in a cell specific manner. New data suggest that mitochondrial activity in astrocytes is impaired with age, and that in vitro reducing mitochondrial activity in young astrocytes increases the injury size, and conversely, activation of mitochondria reduces the lesion size. This is of high importance because acute astroglial scar formation after SCI is beneficial, reducing the spread of inflammation and protecting spared neural tissue. Preliminary in vivo data suggests a reduction in acute astroglial scar formation with age and an increase in inflammation markers and lesion size, associated with a reduction in functional recovery. Remarkably, pilot data show an age- dependent changes in molecular pathways involved in astrogliosis including the reduction of STAT3 (signal transducer and activator of transcription) and increase of PIAS3 (Protein inhibitor of activated STAT3), both having opposite roles on mitochondrial activities. Thus, the central hypothesis is that the age-dependent decline in astrocytic mitochondrial functions impairs astroglial scar formation and reduces functional recovery after SCI with age. This hypothesis will be tested in three related, but independent, aims: The overall objective of this project is to demonstrate that promoting mitochondrial activity in astrocytes reduces lesion size and promotes recovery after SCI in aged animals. These objectives will be achieved by reducing mitochondrial function in astrocytes in vitro and in vivo, using genetic (Ndufs4flox;Ai14) and pharmacological strategies (Aim 1), by increasing mitochondrial activity using PGC-1α overexpression in astrocytes and drugs promoting mitochondrial functions chosen from a new High Content Screen assay (Aim 2), and by establishing the sub-cellular roles played by STAT3 and PIAS3 in modulating mitochondrial function in astrocytes using in vitro and in vivo gain and loss of function (transgenic mice STAT3flox, PIAS3flox, SOCS3flox; overexpression of nuclear or mitochondrial STAT3 and PIAS3) and in vivo transient mRNA expression after SCI (Aim 3).