Laird C. Sheldahl

The regimen of estrogen replacement can alter the consequences of estrogen therapy and stressors. To determine the long-term effects and interaction of these systems on the brain and periphery, adult female rats were infused with lipopolysaccharide (LPS) into the fourth ventricle of the brain for 4 weeks, and ovariectomized rats were administered either constant or pulsed regimens of estrogen replacement (17β-estradiol) until sacrifice at 8 weeks. Constant, but not pulsed, estrogen replacement reduced ERα and increased HSP90, HSP70, and PR_B uterine protein levels. Both estrogen regimens increased ERβ, HSP27, and PR_A uterine proteins. Both regimens reduced hypothalamic levels of ERα, but not ERβ, HSP, or PR. No changes were observed in the hippocampus. Long-term brain infusion of LPS activated microglia and reduced body weight, but did not alter corticosterone or nitrotyrosine levels. LPS infusion into intact rats suppressed uterine weight, increased ERα and decreased HSP90 in the uterus. LPS did not alter uterine weight in ovariectomized rats treated with constant or pulsed estrogen. Together, these data suggest the timing of estrogen replacement and neuroinflammatory stressors can profoundly affect uterine and hypothalamic steroid receptor expression and may be important parameters to consider in the post-menopausal intervention with estrogen.

Estrogen is neuroprotective in a large number of models in vivo and in vitro. Its application in hormone replacement therapy has proven to be more complicated, necessitating better understanding of how estrogen signals in the brain. Estrogen binds to estrogen receptors to regulate gene transcription, and activates a number of rapid signaling cascades from the plasma membrane. These rapid signaling cascades have been shown to play important roles in mediating the neuroprotective effects of estrogen. This review covers evidence that understanding and targeting the membrane effects of estrogen has emerged as an important area in the design of novel neuroprotective drugs.

The effects of estrogen therapy can differ depending on the regimen of estrogen administration. In addition, estrogen can modulate the effects of stressors. To examine the interaction between these systems, we infused adult female rats with li- popolysaccharide (LPS) into the fourth ventricle of the brain for 6 d and compared the effects of constant and pulsed es- trogen replacement. Constant, but not pulsed, estrogen treat- ment reduced estrogen receptor- (ER) protein by 90% in the uterus and increased heat-shock proteins 70 and 90 by 74 and 48%, respectively, whereas progesterone receptor levels in- creased in all ovariectomized rats receiving estrogen replace- ment. In contrast to the uterine decline in ER, no changes in ERwereobservedinthehypothalamusorhippocampus,and ER levels were unchanged in all regions tested. Brain infu- sion of LPS did not alter these proteins but increased the number of activated microglia in the thalamus and reduced body weight in all rats as well as activated the hypothalamic- pituitary-adrenal axis in ovariectomized rats, as determined by elevations in circulating corticosterone and progesterone. Estrogen treatments did not alter these markers, and no dif- ferences were observed in cortical choline acetyltransferase activity or nitrotyrosine for any of the treatment groups. The currentstudyfoundanunexpectedincreaseinuterineweight in lipopolysaccharide-infused rats treated with constant, but not pulsed, estrogen. This report suggests that constant and pulsedregimensofestrogenadministrationproducedifferent effects and that stress may be an important factor in the post- menopausal intervention with estrogen. (Endocrinology 148: 232-240, 2007)

Estrogen reduces brain injury after experimental cerebral ischemia in part through a genomic mechanism of action. Using DNA microarrays, we analyzed the genomic response of the brain to estradiol, and we identified a transcript, cocaine- and amphetamine-regulated transcript (CART), that is highly induced in the cerebral cortex by estradiol under ischemic conditions. Using in vitro and in vivo models of neural injury, we confirmed and characterized CART mRNA and protein up-regulation by estradiol in surviving neurons, and we demonstrated that i.v. administration of a rat CART peptide is protective against ischemic brain injury in vivo . We further demonstrated binding of cAMP response element (CRE)-binding protein to a CART promoter CRE site in ischemic brain and rapid activation by CART of ERK in primary cultured cortical neurons. The findings suggest that CART is an important player in estrogen-mediated neuroprotection and a potential therapeutic agent for stroke and other neurodegenerative diseases.

Numerous preclinical studies suggest that gonadal steroids, particularly estrogen, may be neuroprotective against insult or disease progression. This paper reviews the mechanisms contributing to estrogen-mediated neuroprotection. Rapid signaling pathways, such as MAPK, P13K, Akt, and PKC, are required for estrogen's ability to provide neuroprotection. These rapid signaling pathways converge on genomic pathways to modulate transcription of E2-responsive genes via ERE-dependent and ERE-independent mechanisms. It is clear that both rapid signaling and transcription are important for estrogen's neuroprotective effects. A mechanistic understanding of estrogen-mediated neuroprotection is crucial for the development of therapeutic interventions that enhance quality of life withou deleterious side effects.

Wnt ligands and Frizzled (Fz) receptors have been shown to activate multiple intracellular signaling pathways. Activation of the Wnt–β-catenin pathway has been described in greatest detail, but it has been reported that Wnts and Fzs also activate vertebrate planar cell polarity (PCP) and Wnt–Ca²⁺ pathways. Although the intracellular protein Dishevelled (Dsh) plays a dual role in both the Wnt–β-catenin and the PCP pathways, its potential involvement in the Wnt–Ca²⁺ pathway has not been investigated. Here we show that a Dsh deletion construct, XDshΔDIX, which is sufficient for activation of the PCP pathway, is also sufficient for activation of three effectors of the Wnt–Ca²⁺ pathway: Ca²⁺ flux, PKC, and calcium/calmodulin-dependent protein kinase II (CamKII). Furthermore, we find that interfering with endogenous Dsh function reduces the activation of PKC by Xfz7 and interferes with normal heart development. These data suggest that the Wnt–Ca²⁺ pathway utilizes Dsh, thereby implicating Dsh as a component of all reported Fz signaling pathways.

Familial exudative vitreoretinopathy (FEVR) is a hereditary ocular disorder characterized by a failure of peripheral retinal vascularization. Loci associated with FEVR map to 11q13-q23 (EVR1; OMIM 133780, ref. 1), Xp11.4 (EVR2; OMIM 305390, ref. 2) and 11p13-12 (EVR3; OMIM 605750, ref. 3). Here we have confirmed linkage to the 11q13-23 locus for autosomal dominant FEVR in one large multigenerational family and refined the disease locus to a genomic region spanning 1.55 Mb. Mutations in FZD4, encoding the putative Wnt receptor frizzled-4, segregated completely with affected individuals in the family and were detected in affected individuals from an additional unrelated family, but not in normal controls. FZD genes encode Wnt receptors, which are implicated in development and carcinogenesis. Injection of wildtype and mutated FZD4 into Xenopus laevis embryos revealed that wildtype, but not mutant, frizzled-4 activated calcium/calmodulin-dependent protein kinase II (CAMKII) and protein kinase C (PKC), components of the Wnt/Ca(2+) signaling pathway. In one of the mutants, altered subcellular trafficking led to defective signaling. These findings support a function for frizzled-4 in retinal angiogenesis and establish the first association between a Wnt receptor and human disease.