**8. Estrogen, mitochondria, and neurodegeneration**

Neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD) are characterized by the progressive deterioration and death of specific neuron types in the brain [69]. It is well known that mitochondrial dysfunction plays a pathogenic role in neurodegeneration [69]. AD is characterized by atrophy of the cortex and hippocampus. Additionally, increases in glucose metabolism and a reduction in the activities of mitochondrial Complexes I, III, and IV in the brain have been reported [69, 70]. Plaques containing amyloid beta and tau protein can become incorporated in the remaining brain cells [69]. These inclusions are known to increase oxidative stress and mitochondrial dysfunction.

In PD, the loss of dopaminergic neurons in the substantia nigra contributes to the development of motor disturbances. The protein alpha synuclein becomes entangled with these neurons, resulting in ROS production, mitochondrial dysfunction, and neuronal cell death [71]. A decrease in mitochondrial Complex I activity has been observed in postmortem brains of PD patients that is linked to a reduction in oxidative phosphorylation [72]. *In vivo* and *in vitro* models of PD utilize Complex I inhibitors, such as rotenone, to induce PD-like systems to study disease mechanisms [73, 74]. Most PD cases are of unknown etiology; however, data suggest that defects in the expression of the mitochondrial clearance genes parkin, pink1, and DJ-1 may be an underlying mechanism [69].

Gender differences have been identified in the pathology of both AD and PD [75, 76]. These include alterations in brain weight, regional atrophy, distribution of white and gray matter, cerebral blood flow, expression of neurotransmitter transporters and receptors, and age of onset [77]. In the case of PD, there is also an increased incidence of the disease in men compared to women [78, 79]. The gender difference in neurodegenerative phenotypes has led to the hypothesis that estrogen exerts neuroprotective effects and that these effects are mediated at the level of the mitochondrion [80]. In mouse spinal cord neurons, estrogen treatment increased mRNA levels of nuclear-encoded mitochondrial electron transport chain genes ND1, CytB, Cox2, and ATP6 [80]. ERα has been identified in the mitochondria of endothelial cells in the brain and forebrain. In these cells, estrogen increased expression of cytochrome C and reduced ROS formation [81]. ERβ has been localized to hippocampal mitochondria [17, 81, 82]. Hippocampal cells isolated from ERβ KO mice had lower mitochondrial membrane potential and were more resistant to oxidative stress compared to control mice [83]. Taken together, these data suggest opposing effects of ERα and ERβ in the brain. More cell-type specific studies are needed to better understand the role of these ERs in the brain and to help clarify these opposing views.

Estrogen also interacts with mitochondrial proteins in a non-genomic manner. Under *in vitro* conditions, estrogen had no effect on sodium dependent calcium influx from mitochondria isolated from synaptosomes and increased mitochondrial calcium efflux [84]. Thus, estrogen prevented mitochondrial calcium overload. Higher levels of cytoplasmic calcium increase mitochondrial ATP production and cause neuron-specific changes in cellular signaling. In aged, post-reproductive rodents, loss of estrogen is associated with a decrease in brain weight and a concomitant increase in the utilization of ketone bodies and fatty acids [85]. This was associated with a decrease in metabolic substrates for mitochondrial ATP production that was further decreased in an AD mouse model [85]. Taken together, we and others propose that reductions in estrogen levels that cause decreased mitochondrial function during the postmenopausal period may explain the increased incidence of AD in women at this stage.

Estrogen preserves mitochondrial structure/function by upregulating the mitochondrial antioxidant enzyme MnSOD in the brain of female rodents [37, 86, 87]. In SK-N-SH neuroblastoma cells, estrogen inhibits the effects of the mitochondrial Complex II inhibitor, 3-nitroprionic acid (3-NPA), by preserving mitochondrial ATP production and inhibiting the 3-NPA induced hydrogen peroxide and peroxynitrite formation [88]. These data suggest that estrogen also plays an anti-oxidant role in the brain. This has led many to hypothesize that the anti-oxidant effects of estrogen may play a role in slowing disease pathogenesis.

Estrogen also regulates mitochondrial dynamics in astrocytes in a gender-dependent manner. It reduces expression of the fusion protein Mfn1 in astrocytes isolated from male rodents but has no effect on astrocytes obtained from females [80]. Treatment of cortical primary astrocytes with estrogen increases the expression of fission (Dyn 1 and Fis 1) and fusion (Mfn2) proteins to a greater extent in female mice than males. The upregulation of both fission and fusion proteins suggests that mitochondrial network is more dynamic in females than males. Although the exact mechanisms and reasons for the differences in fission and fusion regulation between male and female rodents are unknown, these responses may explain the sexual dimorphism seen in neurodegenerative diseases and other pathologies.
