Critical Effects on Akt Signaling in Adult Zebrafish Brain Following Alterations in Light Exposure.

Evidence from human and animal studies indicate that disrupted light cycles leads to alterations of the sleep state, poor cognition, and the risk of developing neuroinflammatory and generalized health disorders. Zebrafish exhibit a diurnal circadian rhythm and are an increasingly popular model in studies of neurophysiology and neuropathophysiology. Here, we investigate the effect of alterations in light cycle on the adult zebrafish brain: we measured the effect of altered, unpredictable light exposure in adult zebrafish telencephalon, homologous to mammalian hippocampus, and the optic tectum, a significant visual processing center with extensive telencephalon connections. The expression of heat shock protein-70 (HSP70), an important cell stress mediator, was significantly decreased in optic tectum of adult zebrafish brain following four days of altered light exposure. Further, pSer473-Akt (protein kinase B) was significantly reduced in telencephalon following light cycle alteration, and pSer9-GSK3ß (glycogen synthase kinase-3ß) was significantly reduced in both the telencephalon and optic tectum of light-altered fish. Animals exposed to five minutes of environmental enrichment showed significant increase in pSer473Akt, which was significantly attenuated by four days of altered light exposure. These data show for the first time that unpredictable light exposure alters HSP70 expression and dysregulates Akt-GSK3ß signaling in the adult zebrafish brain.

Mitochondrial viability in mouse and human postmortem brain.

Neuronal function in the brain requires energy in the form of ATP, and mitochondria are canonically associated with ATP production in neurons. The electrochemical gradient, which underlies the mitochondrial transmembrane potential (DeltaPsi(mem)), is harnessed for ATP generation. Here we show that DeltaPsi(mem) and ATP-production can be engaged in mitochondria isolated from human brains up to 8.5 h postmortem. Also, a time course of postmortem intervals from 0 to 24 h using mitochondria isolated from mouse cortex reveals that DeltaPsi(mem) in mitochondria can be reconstituted beyond 10 h postmortem. It was found that complex I of the mitochondrial electron transport chain was affected adversely with increasing postmortem intervals. Mitochondria isolated from postmortem mouse brains maintain the ability to produce ATP, but rates of production decreased with longer postmortem intervals. Furthermore, we show that postmortem brain mitochondria retain their DeltaPsi(mem) and ATP-production capacities following cryopreservation. Our finding that DeltaPsi(mem) and ATP-generating capacity can be reinitiated in brain mitochondria hours after death indicates that human postmortem brains can be an abundant source of viable mitochondria to study metabolic processes in health and disease. It is also possible to archive these mitochondria for future studies.

The basal flux of Akt in the mitochondria is mediated by heat shock protein 90.

Akt is a known client protein of heat shock protein 90 (HSP90). We have found that HSP90 is responsible for Akt accumulation in the mitochondria in unstimulated cells. Treatment of SH-SY5Y neuroblastoma cells and human embryonic kidney cells with the HSP90 inhibitors novobiocin and geldanamycin caused substantial decreases in the level of Akt in the mitochondria without affecting the level of Akt in the cytosol. Moreover, intracerebroventricular injection of novobiocin into mice brains decreased Akt levels in cortical mitochondria. Knockdown of HSP90 expression with short interfering RNA also caused a significant decrease in Akt levels in the mitochondria without affecting total Akt levels. Using a mitochondrial import assay it was found that Akt is transported into the mitochondria. Furthermore, it was found that the mitochondrial import of Akt was independent of Akt activation as both an unmodified Akt and constitutively active mutant Akt; both readily accumulated in the mitochondria in an HSP90-dependent manner. Interestingly, incubation of isolated mitochondria with constitutively active Akt caused visible alterations in mitochondrial morphology, including pronounced remodeling of the mitochondrial matrix. This effect was blocked when Akt was mostly excluded from the mitochondria with novobiocin treatment. These results indicate that the level of Akt in the mitochondria is dependent on HSP90 chaperoning activity and that Akt import can cause dynamic changes in mitochondrial configuration.

Unregulated mitochondrial GSK3beta activity results in NADH: ubiquinone oxidoreductase deficiency.

GSK3beta is prominent for its role in apoptosis signaling and has been shown to be involved in Parkinson's disease (PD) pathogenesis. The overall effects of GSK3beta activity on cell fate are well-established, but the effects of mitochondrial GSK3beta activity on mitochondrial function and cell fate are unknown. Here we selectively expressed constitutively active GSK3beta within the mitochondria and found that this enhanced the apoptosis signaling activated by the PD-mimetic NADH:ubiquinone oxidoreductase (complex I) inhibitors 1-methyl-4-phenylpyridinium ion (MPP+) and rotenone. Additionally, expression of GSK3beta in the mitochondria itself caused a significant decrease in complex I activity and ATP production. Increased mitochondrial a GSK3beta activity also increased reactive oxygen species production and perturbed the mitochondrial morphology. Conversely, chemical inhibitors of GSK3beta inhibited MPP+- and rotenone-induced apoptosis, and attenuated the mitochondrial GSK3beta-mediated impairment in complex I. These results indicate that unregulated mitochondrial GSK3beta activity can mimic some of the mitochondrial insufficiencies found in PD pathology.