Glucocorticoid Receptors (GR) in Mitochondria: The Role in Chronic Stress

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Chronic stress has been shown to be associated clinically with formation of depression in patients and hormones are known to mediate certain clinical manifestations of mood disorders. Chronic restraint stress induces a morphological reorganization in the areas of rodent brain, effects which are also accompanied by behavioral changes. Although the precise mechanisms underlying these effects remain to be elucidated, increasing data suggests that an alteration in neuroprotection and mitochondrial functions may play an important role in regulating various forms of synaptic and neural plasticity; we have sought to investigate the mitochondrial functions regulated by hormones during chronic stress. Cortical neuronal cultures were established in order to determine the localization and function of glucocorticoid receptors in the mitochondria. Glucocorticoid receptors translocated into mitochondria after 1.5 hour treatment with low concentration (100 nM) and high concentration (1,000 nM) of corticosterone in cultured cortical neurons. Consistent with the enhancement of mitochondrial function, mitochondrially encoded gene cytochrome oxidase I (COXI) (has GRE in its promoter region) expression was also increased in mitochondria fraction after 24 hours treatment. However, after three days of treatment, 1uM corticosterone resulted in a decrease in GR levels in mitochondria and 100nM corticosterone treatment did not. Similarly, mitochondrial membrane potential were enhanced after one day treatment with high (1uM) and low (100nM) concentration of corticosterone in a similar extend, and only high concentration (1uM) significantly decreased in mitochondrial membrane potential after 3 day treatment in comparison to the 100nM corticosterone. In addition, mitochondrial oxidation were enhanced after one day treatment with high (1uM) and low (100nM) concentration of corticosterone in a similar extend, and only high concentration (1uM) significantly decreased in mitochondrial membrane potential after 3 day treatment in comparison to the 100nM corticosterone and untreated control. To determine the situation under chronic stress, we found that glucocorticoid receptor levels in mitochondria were significantly decreased in the mitochondrial fraction from prefrontal cortex tissue after chronic stress, suggesting a similar change after high concentration and long-term corticosterone treatment in vitro. These studies may provide additional insights into the mechanisms by which glucocorticoid regulate mitochondrial function and neuronal signaling. Furthermore, this research also has the potential to contribute to a more complete understanding of the mechanisms by which chronic stress and hormones regulate cellular plasticity and resilience and to the future development of improved therapeutics. Glucocorticoids play an important biphasic role in modulating neural plasticity, with low doses enhancing neural plasticity and spatial memory behavior, whereas chronic, higher doses producing inhibition. We undertook a series of experiments to elucidate the mechanisms underlying these biphasic effects, and found that glucocorticoid receptors (GRs) formed a complex with the anti-apoptotic protein Bcl-2 in response to corticosterone treatment, and translocated with Bcl-2 into mitochondria after acute treatment with low or high doses of corticosterone in primary cortical neurons. However, after three days of treatment, high, but not low, corticosterone resulted in a decrease in GR and Bcl-2 levels in mitochondria. In addition, three independent measures of mitochondrial functionmitochondrial calcium holding capacity, mitochondrial oxidation, and membrane potentialwere also regulated by long-term corticosterone treatment in an inverted U-shape. This regulation of mitochondrial function by corticosterone correlated with neuroprotection: that is, treatment with low doses of corticosterone demonstrated a neuroprotective effect, whereas treatment with high doses of corticosterone enhanced kainic acid (KA)-induced toxicity of cortical neurons. As with the in vitro studies, Bcl-2 levels in the mitochondria of the prefrontal cortex were significantly decreased, along with GR levels, after long-term treatment with high dose corticosterone. These findings have the potential to contribute to a more complete understanding of the mechanisms by which glucocorticoids and chronic stress regulate cellular plasticity and resilience, and to inform the future development of improved therapeutics.