Alzheimer’s
disease (AD) patients undergo several neurovascular changes at
different levels. Brain vascular dysregulation is the earliest and
strongest factor during disease progression and is followed by amyloid-β
(Aβ) peptide deposition, glucose metabolism dysregulation, functional
impairment, and gray matter atrophy, in this order [1].
Decreased expression of the low-density lipoprotein receptor-related
protein 1 (LRP-1) and P-glycoprotein (P-gp), as well as upregulation of
the receptor for advanced glycation end products (RAGE), are mechanisms
reported to be changed in AD patients, leading to Aβ accumulation in the
brain [2, 3].
In addition to defective clearance mechanisms, increased endothelial
pinocytosis, decreased number of mitochondria, decreased glucose
transporter (GLUT)-1, and loss of tight/adherents junctions are features
detected in AD [4]. The reduction of the capillary density is also characteristic of the AD brains [5].
This is due to an aberrant angiogenesis with premature pruning of
capillary networks. This defective angiogenesis may be caused by a lack
of angiogenic stimuli and unresponsive endothelium [6]. Although other authors describe increased vascular density in AD [7],
the underlying angiogenic process has pathological characteristics.
Some studies suggest that the promotion of angiogenesis results in
concomitant blood-brain barrier (BBB) disruption and vessel leakiness [7].
Other studies defend that the vascular damage is a consequence of poor
blood perfusion of the brain, leading to hypoperfusion/hypoxia causing
the BBB dysfunction [8].
Other authors argue that the accumulation of Aβ in the walls of the
capillaries can contribute to the reduced brain capillary density in AD
via anti-angiogenic activity [9, 10]. Another observed alteration in AD is the increased thickness of the vascular BM in AD [11].
Since the increase in BM thickness occurs before Aβ deposition, it is
speculated that it functions as a physical barrier to the Aβ clearance
across the BBB [12]. Some studies have related this BM thickening with increased collagen IV content, in AD and aging [13, 14].
Transthyretin
(TTR), a 55-kDa homotetrameric plasma and cerebrospinal fluid (CSF)
protein, transports retinol through binding to the retinol-binding
protein (RBP), which binds at the surface of TTR, and thyroxine (T4),
which binds at a central hydrophobic channel formed at the dimer-dimer
interface [15]. In the CSF, TTR is the main Aβ binding protein [16], providing neuroprotection by avoiding Aβ aggregation [16,17,18,19,20,21,22,23] and toxicity [17, 24], and by participating in Aβ brain efflux at the BBB [25]. TTR is early decreased in AD, both in plasma [26,27,28] and in the CSF [29], probably due to its tetrameric instability [26, 30],
hypothesized to result in accelerated clearance and lower levels. TTR
instability is also a key feature in familial amyloid polyneuropathy
(FAP), a systemic amyloidosis that is usually caused by mutations in
TTR. The amyloidogenic potential of the TTR variants is inversely
correlated with its tetrameric stability [31], and the dissociation of the tetramer into monomers is at the basis of the events that culminate with TTR amyloid formation [32, 33]. TTR stabilization, used as a therapy in FAP [34, 35],
can be achieved through the use of small-molecule compounds sharing
molecular structural similarities with T4 and binding in the T4 central
binding channel [36,37,38]. Although no TTR mutations have been found in AD patients [22], TTR stabilization has also been proposed as a therapeutic strategy to recover its ability to protect in AD [19, 39], and shown beneficial in a mouse model of AD [39, 40].
Iododiflunisal (IDIF), a potent TTR stabilizer, was administered to AD
mice and bound plasma TTR displacing T4, resulting in decreased Aβ
amyloid burden and total Aβ brain levels, and improved cognition [40].
Interestingly, TTR stabilization by IDIF improves TTR-assisted Aβ brain
efflux in vitro and enhanced the expression of LRP-1 in vivo [30]. The formation of TTR-IDIF complexes enhances BBB permeability of both IDIF and TTR, in vivo [41].
TTR
has also been implicated in angiogenesis and the first reports of its
involvement have been described in diseases such as FAP [42]; in diabetic retinopathy (DR) [43, 44], and lately, in cancer [45].
As reported, a study investigated the effect of TTR in angiogenesis by
treating human umbilical vein endothelial cells (HUVECs) with wild-type
(WT) TTR or a common FAP TTR mutant, V30M. The authors concluded that
the TTR mutant inhibited cell migration and decreased survival relative
to the WT TTR, by down-regulating several pro-angiogenic genes for
angiopoietin-2 (Ang-2), vascular endothelial growth factor (VEGF)
receptors 1 and 2, basic fibroblast growth factor (bFGF), and
transforming growth factor-beta 2 (TGF-β2) [42].
In another study, to investigate how TTR affects the development of new
vessels in DR, human retinal microvascular endothelial cells (hRECs)
were cultured with TTR in natural and simulated DR environments
(hyperglycemia and hypoxia). In the DR environment, TTR inhibited cell
proliferation, migration, and tube formation, by repressing the
expression of the pro-angiogenic genes Ang-2 and VEGF receptors 1 and 2 [43].
Conversely, in a low glucose environment, these angiogenesis-related
features were improved by TTR. Recently, it was reported that TTR levels
were increased in human serum of lung cancer patients. Additionally,
TTR was shown able to promote tumor growth by enhancing several lung ECs
functions as permeability, migration, and tube formation [45].
However, TTR potential in angiogenesis has never been addressed in vivo
and the possible participation of TTR in brain angiogenesis and
vascular alterations has never been elucidated.
Taking these
evidences into account, this work aimed at investigating the angiogenic
potential of TTR and at assessing its involvement in the vascular
impairment that occurs in AD.
Jinfeng He1†, 
Zhenxiang Zhan
Fangwang Fu
Qiongqiong Bao