nisin BDNF is intricately implicated in energy homeostasis H
BDNF is intricately implicated in energy homeostasis (Hamdouchi et al., 2016; Schwartz and Mobbs, 2012), regulation of mood (Sartorius et al., 2015) and adult neurogenesis (Taliaz et al., 2010; Waterhouse et al., 2012), our primary focus was to evaluate the effects of GPR40 stimulation on BDNF expression in the hippocampus. In line with other studies (Fox et al., 2013; Xiang et al., 2015), we found decreased BDNF expression in the hippocampus of HFD and db/db mice. This decrease in expression of BDNF could be due to the decrease of GPR40 expression during diabesity, which may lead to cognitive impairment. To confirm this, we demonstrated that chronic stimulation of GPR40 by DHA or GW9508 normalized BDNF expression in the hippocampus, which correlated with improved cognitive performance in the active avoidance paradigm. Since GPR40 has been shown to induce hippocampal neurogenesis in primates (Boneva and Yamashima, 2012), it is quite likely that DHA activates GPR40 expressed in the dentate gyrus of the hippocampus and increases neurogenesis, which are widely known pathways of cognitive enhancement and antidepressant action (Anacker and Hen, 2017). Given the critical role of AMPA-mediated glutamatergic signaling in cognition (Derkach et al., 2007), we investigated the effect of GPR40 signaling on pGluR1 (pS831 and pS845) levels in our model system. Our finding of increased pGluR1 (pS831 and pS845) due to DHA and GW9508 treatment suggests that, indeed, GPR40 is involved in glutamatergic signaling, and therefore in cognition. Although several studies have shown the beneficial effects of DHA on cognitive impairments (Grosso et al., 2014; Sharma et al., 2012), the molecular target of DHA in the nisin has not been elucidated until now. While our data suggest that GPR40 mediates some effects of DHA, we cannot exclude DHA effects that are independent of GPR40, such as inflammation and neurodevelopment (Bazinet and Laye, 2014; Dyall, 2015; Lauritzen et al., 2016). Furthermore, DHA also exhibits beneficial effects on mood and cognition through retinoid X receptors (Lukiw et al., 2005), neuroprotective and anti-inflammatory effects via metabolites such as neuroprotectin D1 and resolvin D1 (Lukiw et al., 2005; Yan et al., 2013), more studies are needed to completely delineate DHA mediated effects of GPR40 in brain. Since DHA has been shown to bind both GPR40 and GPR120 (Brown et al., 2005), our observations of complete blockade of DHA-induced calcium flux, BDNF expression, pP38 and pERK expression in primary cortical neurons by the GPR40-specific antagonist, GW1100 (IC50 = 1 μM at GPR40 v/s no activity at GPR120 up to 10 μM; (Briscoe et al., 2006)), or by GPR40 knockdown, suggest that beneficial effects of DHA in the brain are mediated by GPR40. Moreover, GPR120 specific antagonist AH7614 (pIC50 = 7.1 and 4.6 for human GPR120 and GPR40, respectively; (Sparks et al., 2014)) didn't affect DHA induced pERK expression in primary cortical neurons, that further supports our hypothesis that brain specific beneficial effects of DHA are most likely via GPR40, not GPR120. Our results also revealed that GPR40-dependent stimulation of BDNF involved both the P38-MAPK and ERK-MAPK pathways. Given the complex as well as heterogeneous etiology of cognitive decline and broad expression pattern of GPR40 in the brain, it is challenging to exactly pinpoint the target neurocircuitry for the beneficial effects of DHA or GW9508 on cognition. Further, it is very likely that DHA mediated increase of BDNF could modulate metabolic hormones and neuropeptides such as leptin, insulin and ghrelin (Liao et al., 2012; Marchelek-Mysliwiec et al., 2015; Russo et al., 2017), and thereby alleviate cognitive deficit in diabesity. However, we have not investigated this aspect of DHA induced GPR40 signaling, a limitation of this study. Nevertheless, our observations that acute stimulation of GPR40 with GW9508 prevented scopolamine-induced amnesia, as measured by the novel object recognition paradigm, and that i.c.v. treatment with GPR40 antagonist blocked the beneficial cognitive effect of DHA and increased BDNF expression in the hippocampus, suggests that GPR40 plays a role in the effects of DHA on learning and memory. These results support our hypothesis that decreased GPR40 expression/signaling is an underlying mechanism for impaired cognitive functions in diabesity. However, because GW9508 and GW1100 also affect GPR120, another PUFA receptor expressed in the microglia cells (Dragano et al., 2017), we cannot exclude the role of GPR120 in some of the beneficial effects of DHA. So, further studies with GPR120 and GPR40 specific antagonists are warranted to discern the contribution of GPR40 and GPR120 in DHA-mediated CNS-specific beneficial effects. In addition, it would be important to evaluate other indices associated with the improvements in memory and BDNF associated with GPR40 modulation, such as neuroinflammatory effects that have been suggested by others (Dragano et al., 2017; Vauzour et al., 2015). Nevertheless, this study for the first time reveals another facet of GPR40 signaling in the brain, which is the regulation of BDNF expression and modulation of comorbid cognitive impairments in diabesity. Altogether, these findings further support the notion that GPR40 is an attractive target for the treatment of cognitive disabilities that are quite common across several CNS disorders.