Crosstalk between oxidative stress and inflammation in Alzheimer-like dementia

Trisomy of Chr 21, commonly known as Down syndrome (DS), is the most prevailing genetic cause of intellectual disability with an incidence of 1:700 births. DS individuals, after the age of 40, develop a type of dementia that closely resembles that of Alzheimer’s disease (AD) with deposition of senile plaques containing Aβ and neurofibrillary tangles (NFTs) composed of hyperphoshorylated tau. Inflammation and oxidative stress (OS) are known to occur in DS and AD brains in response to Aβ plaques and NFTs. Furthermore, several studies demonstrated also the involvement of brain insulin resistance (BIR) in the progression of AD-like pathology. Therefore, we believe that OS, inflammation and BIR could contribute to the severity of AD and DS pathology and act as potential accelerators of AD pathology. Within this scenario, we focused our attention on the dysfunction of molecular pathways that are closely related with increased OS and that might be involved in the development of Alzheimer-like dementia including autophagy (the major cellular pathway responsible for the removal of Aβ and tau aggregates), insulin signaling and inflammation. Recent studies from our laboratory reported, in human DS samples, an early accumulation of protein oxidative damage concomitant with the alteration of mTOR/ autophagy signaling. However, the relationship between OS and mTOR/autophagy signaling is intricate and needs to be clarified (Project 1). Furthermore, increasing evidence supports the involvement of inflammation-related miRNAs in neurodegenerative diseases and because miR146a and miR155 are key regulators of the microglia-mediated inflammatory response we hypothesized that a dysregulation of these miRNAs may occur in DS and AD pathology (Project 2). Finally, BVR-A being a direct target of IR kinase activity and once IR-phosphorylated is able to phosphorylate IRS1 on Ser inhibitory, and because we previously demonstrated that OS induces impairment of BVR-A in human AD brain, we investigate if BVR-A dysregulation could be associated with the onset of BIR in AD (Project 3). Aim of this work In order to clarify the intricate relationship between increased OS and alteration of mTOR/autophagy signaling we analyzed the integrity of the mTOR axis in DS mouse model (Ts65Dn) at different ages and we evaluated the effects of autophagy inhibition (by rapamycin treatment) on protein oxidative damage in SH-SY5Y cell line (Project 1). To elucidate the potential link between deregulation of inflammation-related miRNAs, neurobehavioral deficits and AD pathology; we investigated the expression and cellspecific distribution of both miR146a and miR155 in the developing hippocampus from controls, patients with DS and adults with DS-AD pathology. In addition, we evaluated the levels of these miRNAs, their putative targets as well as the levels of IL-1β in human hippocampus from sporadic AD (sAD) at different stages of the disease and in Ts65Dn mice and in experimental models AD mouse (APP/PS1) (Project 2). To make clear the potential involvement of BVR-A dysregulation in the onset of BIR we analyzed the age dependent changes of (i) BVR-A protein levels and activation, (ii) total OS markers levels (PC, HNE, 3-NT) as well as (iii) IR/IRS1 levels and activation in triple transgenic mouse model of AD (3xTg-AD). Furthermore, ad hoc invitro experiments have been performed to clarify the contribution of oxidative/nitrosative stress on insulin resistance. Finally, because mTOR is able to phosphorylate IRS1 we investigated also if the insulin resistance could be associated with mTOR hyper-activation (Project 3). Results Our results show that: i) defects of mTOR signaling contribute to the buildup of protein oxidative damage which characterizes AD and DS neuropathology; ii) the involvement of both miR146a and miR155 in the hippocampus during brain development and their dysregulation in DS and AD; iii) the OS-induced inactivation of BVR-A promote BIR possibly trhough hyper-activation of mTOR. In conclusion, this study provides the evidence of a central role of mTOR in the neurodegenerative process. Indeed, hyperactivation of mTOR signaling impairs autophagy, insulin signaling and, indirectly, the inflammation-related miRNAs function.