The potential therapeutic actions of GHSR in the central

The potential therapeutic actions of GHSR in the central nervous system has been in the limelight lately. Peripheral administration of GHSR agonists have been shown to inhibit oxidative stress, apoptosis, proinflammatory cytokine production, microglia activation, mitochondrial dysfunction, and excitotoxicity both in vivo and in vitro [16], [67], [68], [69], [70], [71], as well as exert neuroprotective effects against hippocampal and cortical neuronal death [68], [72], [73], [74]. Through these actions, GHSR has thus been linked to neuroprotection in several CNS diseases such as stroke [75], Alzheimer disease [72], [76], Parkinson disease [11], [12], [13], [69], [77], multiple sclerosis [78], [79], epilepsy [80], [81], and spinal cord injury [82]. Nevertheless, GHSRs have been shown to display higher constitutive basal activity in the absence of ligand [83], suggesting that (1) GHSR-mediated pathways do not need acyl ghrelin gene products to be activated but are modulated by them; (2) the existence of other(s) endogenous ligand(s) able to regulate GHSR activity in the midbrain and other Cimetidine sale areas. In different cell types and tissues, acyl-ghrelin binds GHSR to activate several intracellular cascades, including ERK 1/2, PI3K/Akt/mTOR, and AMPK signaling pathways, thus mediating different physiological functions. The first described pathway associated with GHSR activation was the inositol 1,4,5-trisphosphate/diacylglycerol (IP3/DAG) pathway, leading to GH release in the anterior pituitary. In other non-neuronal tissues and cell types, the binding of acyl-ghrelin to GHSR activates either the phospholipase C/inositol trisphosphate or the adenylate cyclase/protein kinase A pathways, resulting in an increase in intracellular calcium levels. These activation of these different signaling pathways could be due to different endogenous ligands, differential concentration of acyl ghrelin [84], and/or differential GHSR dimerization to other receptors (as described for dopamine, serotonin, melanocortin receptors, among others) [85], [86], [87], [88]. In SN DA cells, mitochondrial function is regulated to some extent by 5′-AMP-activated protein kinase (AMPK), a ubiquitous enzyme that is considered a master sensor of intracellular energy stress that plays a crucial role in adaptive responses to falling energy levels (e.g., from low nutrient availability or cellular stress). AMPK signaling is activated by GHSR and declines in PD patients [89]. Ghrelin-mediated activation of AMPK is associated with neuroprotection of SN DA cells [13], [14]. Thus, activating AMPK may be an effective neuroprotective strategy to help restore energy balance and redox homeostasis in SN DA neurons, but other signaling pathways might also be involved in GHSR-mediated neuroprotection and must be considered. GHSR-mediated activation of AMPK is also observed in other brain regions, including the arcuate nucleus of the hypothalamus [90]. Further characterization of GHSR-mediated signaling pathways is required to support the development of clinical therapeutics targeting GHSR in SN DA cells to promote survival. Oxidative stress, reactive gliosis, and T-cell infiltration are major pathological features in human PD and are recapitulated in the mouse MPTP model [91]. Oxidative stress defines an imbalance between the levels of reactive oxygen species (ROS) and reactive nitrogen species (RNS) produced and the ability of a biological system to detoxify those reactive intermediates. Generation of ROS and RNS involves the activity of mitochondria and are impacted by mitochondrial dynamics. In SN DA neurons, oxidative stress can not only compromise protein function but also generate lipid peroxidation [92] and DNA fragmentation, ultimately leading to cell death. In addition, impairment of dopamine synthesis has been observed as a result of oxidation of tyrosine hydroxylase (TH) in the MPTP model [93]. In PD, the roles of oxidative stress and reactive gliosis are supported by postmortem brain tissue analyses (reviewed in [91]). Overall, activated glial cells contribute to increase local oxidative stress [94] and increase proinflammatory cytokine production [91]. Reactive gliosis happens as a consequence of cellular stress and can be divided in two major features: microglia activation and reactivation of astroglia. Specifically in the MPTP model, microglial activation precedes the loss of DA neurons and upreglation of proinflammatory cytokines [95]. Interventions that halt microglial activation have a direct impact on SN DA neuroprotection [96]. Reactive astrogliosis and T-cell infiltration in the SN are also present in the MPTP model [97], [98], and neuroprotection of SN DA cell is achieved with strategies that tackle either one of those features [99], [100], [101]. Here, we demonstrated that Dln101 treatment was correlated with decreases in TH+ cell oxidative stress, and microglia activation, as measured by decreased CD68 expression in SN region, similar to observed in interventions in monkeys [102]. We also reported prevention of upregulation of proinflammatory genes such as TNFα and the inflammasome component Nrlp3, both of which have been associated with MPTP-induced loss of SN DA neurons [103], [104]. Moreover, Dln101 ability to activate GHSR might have a deeper relevance to neuroprotection beyond regulation of mitochondrial dynamics, once GHSR signaling has been linked to negative control of neuroinflammation [104], [105], which is believed to sustain and exacerbate the loss of the DA neurons in PD. Taken together, our data suggest that Dln101 can act in different components of the pathophysiology of SN DA neurodegeneration, perhaps providing a better outcome in clinical therapies for PD.