Estrogen, a Steroid Hormone
Estrogen, a steroid hormone, produced by the ovaries, has been the focus of research for its neurobehavioral and physiological properties. Estrogen has many different functions besides its well documented roles in reproductive development and sexual differentiation (Vrtacnik et al 2014). For example, the importance of estrogen becomes apparently clear during menopause when the endogenous levels of estrogen are drastically reduced. It is during this period that the reduction in estrogen leads to potentially damaging consequences to cardiovascular function and bone density (Yang and Reckelhoff 2011). Furthermore, there is converging evidence that estrogen plays a critical role in the CNS (Woolley 1999).
Alzheimer’s disease (AD) is the most common type of dementia. It is a progressive neurodegenerative disorder that begins with mild cognitive impairment and leads to severely disabling deficits in the advanced stages. The risk for developing AD along with the severity of AD differ between the sexes (Pike, 2017). In fact, the occurrence of AD is 2-3 times higher in women compared to men, with premature menopause increasing the risk of dementia over the long term (Ryan et al 2014). This proposes a link between post-menopausal estrogen loss and AD development (Anderson et al 1999, Ruitenberg et al 2001). In addition, cognitive impairment appears to be larger in women than in men at the same stage of AD and this is likely due to the reduced estrogen levels in post-menopausal women (Laws et al 2016).
Epidemiological studies have proposed that estrogen reduces the risk for AD in healthy women and improves cognitive function in women with AD. In addition, initial investigations have suggested that hormone replacement therapy to manage post-menopausal symptoms connected to reduced estrogen levels, improved cognitive decline associated with AD and reduced AD risk in general (Tang et al.,1996; Pike et al., 2009; Engler-Chiurazzi et al., 2016a, 2016b).
In addition to estrogens neuroprotective role in AD (Simpkins et al., 1994; Henderson, 2006; Lan et al., 2015; Engler-Chiurazzi et al., 2016a) it has also been shown to be beneficial in Parkinson’s disease (Liu and Dluzen, 2007; Parkinson Study Group, 2011; Rodriguez-Perez et al., 2015; Litim et al., 2016), multiple sclerosis (Christianson et al., 2015; Benedek et al., 2016; Kipp et al., 2016), and retinal degeneration such as glaucoma (Cascio et al., 2015).
In AD, microglia and astrocytes are involved in mediating several mechanisms of protection against A? toxicity (Ries and Sastre, 2016). Glial cells express estrogen receptors and are a main target for estrogens neuroprotective properties in a variety of pathologies (Dhandapani and Brann, 2007; Arevalo et al., 2010; Barreto, 2016). The estrogen stimulated glial cells in response to neurotoxic cues in AD reduce inflammation and oxidative stress (Acaz-Fonseca et al., 2014; Villa et al., 2016).
The neuroprotective properties of estrogen on cultured primary neurons that have been challenged with A? is mediated by astrocytes (Sortino et al., 2004). Estrogen has been shown to modulate GFAP expression in mixed glial-neuronal cultures and in ovariectomized mice supplemented with 17?-estradiol for different time extensions (Rozovsky et al., 2002; McAsey et al., 2006). Consequently, increased reactive gliosis is observed in young and aged female mice who have reduced estrogen levels (Struble et al., 2007).
Glaucoma is characterized by the slow neurodegeneration of retinal ganglion cells and their axons resulting in irreversible visual sensitivity loss and blindness (Munemasa and Kitaoka 2013). Increasing evidence suggests that glaucoma represents the accelerated aging of the optic nerve and is a neurodegenerative disease of the central nervous system (Yeni 2013). Strong evidence suggests that the early loss of estrogen leads to premature aging and increased susceptibility of the optic nerve to glaucomatous damage (Vajaranant and Pasquale 2012).
The present study looked at whether estrogen could act as a neuroprotective agent if given to optic nerve head astrocytes undergoing oxidative stress. We analyzed whether estrogen exerts its neuroprotective effects by preventing tau cleavage. Our results showed that estrogen prevented the activation of caspase-3 and the generation of caspase-3-cleaved truncated tau preventing the formation of neurofibrillary tangles (NFT). Estrogen did this by dephosphorylating tau at Ser422 allowing active caspase-3 access to the caspase-3 cleavage site Asp421. The treatment with estrogen also decreased apoptosis in ONHAs undergoing oxidative stress. Together, these results suggest that preventing the decline in estrogen associated with aging might decrease the susceptibility of neurons to oxidative stress.
Hormonal depletion during aging has been linked to the development of neurodegenerative diseases such as AD (Hogervorst et al., 2001; Morley, 2001; Moffat et al., 2004; Rosario et al., 2004). In addition, epidemiologic studies have suggested that in post-menopausal women hormone replacement therapy can reduce the risk of AD, delay disease onset, and improve cognitive function (Tang et al., 1996; Kawas et al., 1997; Paganini-Hill and Henderson, 1996; Baldereschi et al., 1998; Brinton, 2001; Polo-Kantola and Erkkola, 2001). However, the mechanism of how estrogen acts as a neuroprotective agent has not been fully elucidated. The results presented here indicate that estrogen blocks caspase-3 activation and regulates the phosphorylation state of tau at Ser422. Furthermore, this suggests that estrogen prevents tau cleavage leading to the generation of toxic fragments.
Previous work has shown that estrogen has protective effects against various stimuli. For instance, estrogen can protect against the apoptosis-inducing agents staurosporine, A?, and hydrogen peroxide (Weaver et al., 1997; Honda et al., 2001; Negishi et al., 2003; Sur et al., 2003). Two mechanisms by which estrogen could act in preventing apoptosis have been identified. First, estrogen increased the expression of Bcl-xL, an anti-apoptotic protein, in hippocampal neurons, preventing apoptosis (Pike, 1999). Secondly, estrogen could act on the caspase-3 pathway and reduce A?-induced apoptosis (Jover et al., 2002; Celsi et al., 2004). In AD, the neuroprotective ability of estrogen works on many levels. In addition to its ability to work against neuroinflammation and oxidative stress, preclinical data has shown that estrogen can work on the level of A? and tau.
The neuroprotective mechanisms of how estrogen targets tau have become of high interest. Tau is a microtubule stabilizing protein that is regulated by multiple phosphorylation events that control its activity and turnover (Johnson, 2006; Arendt et al., 2016). In pathological conditions, tau phosphorylation can become dysregulated leading to the build-up of hyperphosphorylated tau, which can aggregate into neurofibrillary tangles. This then can activate multiple signaling cascades that eventually lead to death. This makes tau responsible for a variety of neurodegenerative disorders known as tauopathies, with AD being one of the more prominent ones (Morris et al., 2011; Arendt et al., 2016). In primary rat cortical neurons and SH-SY5Y neuronal cells estrogen was shown to promote dephosphorylation of tau. Interestingly, male and female rat cortical neurons showed different sensitivity to estrogen, with the neurons derived from females showing a greater effect (Alvarez-de-la-Rosa et al., 2005, Zhang and Simpkins, 2010).
More recently, the importance of tau cleavage in the pathological process of AD has come to the forefront. Tau cleavage mediated by caspase-3 or calpain appeared to be early events and came before tau hyperphosphorylation (Gamblin et al., 2003; Park and Ferreira, 2005). In addition, caspase-3-truncated tau has been detected prior to the formation of apoptosis and neurofibrillary tangles (Ugolini et al., 1997; Rissman et al., 2004). Tau has also been shown to be proteolytically regulated by calpain. Tau cleavage by calpain produces a 17-kDa fragment and was detected before tau hyperphosphorylation in cultured hippocampal neurons (Park and Ferreira, 2005).
How cleaved tau causes neuronal apoptosis and/or neuronal degeneration is not clear. One potential mechanism involves the generation of truncated tau forms that could assemble into filaments. The formation of tau filaments could then interfere with different cellular processes. This seems to be the case with caspase-3-mediated tau truncation (Gamblin et al., 2003).
Our results extend previous observations describing estrogens neuroprotective effects. A previous study suggested that the sex hormones estrogen and testosterone worked differently as neuroprotective agents. It provided data for a role of testosterone as a neuroprotective hormone in neurons. When looking at how they regulate protease activity, their results show that testosterone and dihydrotestosterone inhibited calpain activation and the generation of the 17-kDa tau fragment, but did not work on caspase-3 activation and tau truncation at Asp421 like estrogen did. Taken together, this indicated that estrogen and testosterone have independent pathways that mediate the neuroprotective effects of these sex hormones.
In short, we show that estrogen is neuroprotective in ONHAs and can inhibit caspase-3 activation and reduce tau cleavage thereby preventing NFT formation. We also noticed that 2-hour preincubation like 18-hour was effective in reducing caspase-3 activation and tau proteolytic cleavage, although the longer preincubation was more beneficial. A previous study showed that the neuroprotective properties of estrogen and testosterone on hippocampal neurons with A?-induced neurotoxicity was effective not only when preincubated at 24 hours, but was also effective when preincubated for 2 hours prior to A? addition (Ferreira, 2007).
Based on our data,Tau plays an important role in the degeneration of the neural retina. Our data indicate that delivery of sex hormones (estrogen) have potential therapeutic value and appear to target Tau and the caspase-3 pathway. Studying the pathogenesis of retinal Tau can increase our knowledge of vital mechanisms involved in neuronal and cellular degeneration.”