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