Genetics and function of proteins involved in neurodegeneration (APP, presenilin, tau)
A general description of Alzheimer disease (AD) and relevant projects
Alzheimer disease (AD), the most common cause of dementia in the
aged, is a neurodegenerative disorder caused by severe neuronal loss and
synaptic abnormalities in the hippocampus and neocortical regions of
the brain. Neuropathologically, the disease is defined by brain
accumulation of amyloid plaques (APs) and neurofibrillary tangles
(NFTs). AD is a serious humanitarian and financial problem as it
afflicts a large number of individuals and is very expensive to care for
its victims. Most AD cases are classified as sporadic (SAD) because
they lack obvious genetic etiologies. A small percent of all cases
however, segregates within families (FAD) suggesting genetic etiologies.
Most FAD mutations map on three genes encoding the Amyloid Precursor
Protein (APP), presenilin1 (PS1) and presenilin2 (PS2). Close to 200 FAD
mutations map on the gene encoding PS1 while about 20 FAD mutations are
found in APP and 10 in PS2. PSs are functional components of the
γ-secretase proteolytic complexes that process many receptor proteins
and APP. PS/γ-secretase processing of APP yields Aβ peptides that
aggregate to form APs used to define AD. The mechanisms by which FAD
mutants promote neuronal death and AD remain unclear. A common theory
suggests that AD is caused by Aβ peptides and its derivatives, but
inhibitors of Aβ production or anti-Aβ antibodies show no therapeutic
value (for recent review see Robakis 2011). Other evidence indicates
that FAD mutants affect cellular functions including inhibition of
signal transduction and gene expression. Since SAD and FAD have similar
clinical and neuropathological phenotypes, understanding how genetic
mutations promote neurodegeneration and AD will also help our
understanding of SAD.
Role of PS and FAD mutants in neoroprotection
To examine the effects of FAD mutations on neurodegeneration, we
study the genetics, molecular biology, and biological functions of wild
type (WT) and mutant PSs and APP. We found that PS1 FAD mutants cause a
loss of γ-secretase cleavage function at ε-sites of substrates
manifested by both, decreased production of cytosolic peptides that
function in cell signaling and accumulation of γ-secretase substrates.
These data support a theory that PS FAD mutations promote neurotoxicity
by inhibiting γ-secretase-catalyzed ε-cleavage of substrates thus
reducing cell signaling while causing accumulation of membrane-bound
cytotoxic peptides (Marambaud et al. 2003; Georgakopoulos et al., 2006;
Litters et al., 2007; Barthet et al., 2012a). We also reported that PS
mediates brain-derived neurotrophic factor (BDNF)-dependent
neuroprotection against toxic insults such as oxidative stress and
glutamate toxicity (Barthet et al., 2012b). We currently examine the
mechanisms of the PS-dependent neuroprotection and the effects of FAD
mutants on trophic factor-mediated nueroprotection of cortical neurons
using both in vitro primary neuronal cultures and in vivo transgenic
mouse models.
PSs and miRNAs in glucose deprivation and AD
Emerging evidence indicates that PSs regulate expression of microRNAs
(miRs), non-coding small RNAs that suppress mRNA translation. In our
lab we found that PS1/γ-secretase regulates expression of miR-212 that
targets the mRNA of PEA15, a protein known as a promoter of cancer cell
survival under glucose deprivation. We observed that absence of PS1
sensitizes neurons to glucose deprivation and increases expression of
miR-212 while decreasing PEA-15. Furthermore, we found that
PS/γ-secretase regulates neuronal survival by suppressing expression of
miR-212. Based on our data, we hypothesize that increased expression of
miR-212 and resultant decrease in PEA15 expression contribute to
increased vulnerability of PS1 null neurons under reduced glucose, a
condition commonly found in AD. In our Center we currently examine the
relationship between PS1, miR-212 and its target protein PEA15 and
examine their effects on neuronal survival under stress conditions. In
addition, we ask how PS/γ-secretase affects miR-212 and whether PS
modulates the glucose deprivation-induced survival signaling of PEA15.
We also ask whether expression of miR-212 and its target PEA15 changes
in brains of AD patients and in PS FAD mutant knockin animal models.
Brain vascular abnormalities and AD
Evidence in the last decade implicates cerebral microvasculature
abnormalities in the genesis of AD neuropathology. Additional literature
shows that the EphB4/ephrinB2 system regulates development/function of
vascular systems. Binding of extracellular EphB4 receptor to
transmembrane ephrinB2 ligand protein on surface of endothelial cells
stimulates angiogenesis/growth of new vessels from existing vasculature.
Thus, treatment of endothelial cells with EphB4 stimulates cell
sprouting and tube formation, processes considered crucial initial steps
in angiogenesis. We found that the EphB4-induced sprouting/tube
formation depends on γ-secretase activity and that EphB4 stimulates
γ-secretase processing of ephrinB2 producing peptide ephrinB2/CTF2
(Georgakopoulos et al., 2006; 2011). Our data suggest that
EphB4/ephrinB2 regulates angiogenesis through PS/γ-secretase. In support
of this hypothesis, we found that peptide ephrinB2/CTF2 stimulates
sprouting of endothelial cells in vitro. In our lab, we explore the
mechanisms via which the EphB4/ephrinB2 and PS1/γ-secretase systems
promote angiogenesis and ask whether these mechanisms are altered in AD.
Since PS1 FAD mutants affect the ε cleavage of γ-secretase substrates
thus decreasing production of ephrinB2/CTF2 (see above), we ask whether
FAD mutants alter the EphB4/ephrinB2-dependent angiogenesis. In our work
we use both in vtro cell systems and mouse transgenic models.
Role of Progranulin in Frontotemporal lobar degeneration (FTLD)
Recent reports show that progranulin (PGRN) gene null or missense
mutations are linked to frontotemporal lobar degeneration (FTLD), a form
of dementia characterized by severe neuronal loss in the frontal and
temporal brain regions of adult patients. The nature of these mutations
suggests that survival of certain neuronal brain populations need
expression of both functional alleles of PGRN (haploinsufficiency). We
found that PRGN stimulates phosphorylation and activation of neuronal
MEK/ERK/p90RSK and PI3K/Akt cell survival pathways and rescues cortical
neurons from cell death induced by glutamate or oxidative stresses. Our
data showed that extracellular PRGN acts as a neuroptotective factor
supporting the hypothesis that in FTLD reduction of PGRN results in
decreased survival signaling and neuroprotection against excitotoxicity
and other stresses, leading to accelerated neuronal death (Xu et al.,
2011). We are currently working on the identification of receptors that
mediate the cellular effects of PRGN and its mutants involved in FTLD.
Current Grant Support: This research center is supported by four NIH grants and a grant from the Alzheimer's Association.
Robakis Laboratory