Nature Neuroscience: Solving the mystery of how the liver affects the progression of Alzheimer's disease!

*For medical professionals only

apoE, a apolipoprotein, has been attracting the attention
of scientists for decades as a star molecule in Alzheimer's disease (AD) research.

There are three subtypes of apoE, 2, 3 and 4, of which apoE4 is the most important genetic risk factor
for AD.
apoE4 exacerbates the aggregation of amyloid Aβ, reduces dendritic density, and affects synaptic plasticity by reducing synaptic glutamate receptors
.

However, apoE is expressed in both the central nervous system and the periphery, mainly by astrocytes in the brain and mainly by hepatocytes in the periphery
.
So Alzheimer's disease, as a central nervous system disease, is apoE from the periphery also a causative factor of the disease? Do different subtypes of apoE have the same pathogenic effect? What is their pathogenic mechanism?

Recently, scientists have given the answer
.

A research team led by Mayo Clinic Guojun Bu and Chia-Chen Liu has published important findings in Nature Neuroscience [1].

They found that peripheral apoE was also a causative factor
in Alzheimer's disease.
And the effects of different subtypes of apoE (apoE3 and apoE4) on brain function and pathological outcomes are completely opposite
.

Specifically, liver-derived apoE4 exacerbated brain amyloid pathology by impairing cerebrovascular function and synaptic plasticity, reducing memory and cognition in mice, and increasing blood vessel-related glial cell proliferation; Peripheral apoE3 works the opposite
.

Screenshot of the front page of the paper

The most important pathological feature of AD is amyloid Aβ protein deposition
.
AD is one of the most common progressive neurodegenerative diseases, and its incidence is related to a variety of factors such as genetics and environment [2].

Epidemiological studies have shown that apoE4 significantly increases the risk of AD and increases AD incidence
in a genetically dose-dependent manner.
It has been documented that carrying one apoE4 allele increases the risk of AD by a factor of 3 to 4, while carrying two apoE4 alleles increases the risk by a factor of 9 to 15 [3].

Compared with the common apoE2 or apoE3, a large number of studies have found that apoE4 not only regulates the expression levels of amyloid plaques, tau protein and TDP43 in the brain of AD patients, but also affects normal brain function and accelerates aging-related cognitive decline in non-dementia individuals [4].

The results of a liver transplantation study showed that the apoE subtype status in the recipient's plasma was consistent with the genotype of the donor, while the apoE in the cerebrospinal fluid remained unchanged and consistent with the recipient [5].

This suggests that the peripheral apoE and the central nervous system are physically separated due to the presence of the
blood-brain barrier.

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However, since both peripheral and brain can express apoE independently, it is difficult to separate the central apoE and separately assess the effect of peripheral apoE isomers on central nervous system function and its specific contribution
to AD disease progression.

To answer this question, the researchers used liver-specific expression cre mice (Alb-Cre) on the basis of apoE all-knock mice to establish a mouse model
that only expressed apoE3 (iE3/Cre+) or apoE4 (iE4/Cre⁺) in the liver, but did not express any apoE subtypes in the brain.

Subsequently, the researchers examined
the cognitive behavior and long-term enhancement (LTP) associated with learning and memory in iE3/Cre+ and iE4/Cre⁺ mice through behavioral and electrophysiological experiments, respectively.

The results showed that the memory and synaptic plasticity of neurons were enhanced in iE3/Cre⁺ mice, while the memory function of iE4/Cre⁺ mice was impaired, and synaptic plasticity was also reduced
.
This suggests that liver-derived apoE3 promotes brain function, while apoE4 disrupts brain function
.

Recording of excitatory postsynaptic potentials in CA1 regions of iE3/Cre+ and iE4/Cre⁺ mouse hippocampal brain slices

Since peripheral apoE4 can damage the brain, this suggests that apoE4 may have broken the blood-brain barrier
.

Considering that blood-brain barrier dysfunction is associated with the pathogenesis of AD [6], the researchers tested cerebral vascular permeability
in mice.
The results showed that iE4/Cre⁺ mice showed a significant increase in cerebral vascular permeability, decreased expression of tight junction protein (CLDN5) and scaffolding protein (ZO1) on endothelial cells, and leakage
of the blood-brain barrier.

In addition, through two-photon imaging, the researchers also found that liver-derived apoE4 significantly inhibited the speed of cerebral blood flow in the cerebral arteries, reduced the frequency of cerebral artery vasodilation, increased amplitude, and reduced the number of
vascular branches.

These results suggest that peripheral apoE4 expression may negatively affect
the brain by impairing the integrity and function of cerebral blood vessels.

In order to further investigate the molecular mechanism by which liver-derived apoE4 can damage the brain, Bu's team isolated the vascular-related cell population in the brains of iE3/Cre+ and iE4/Cre⁺ mice, and found that peripheral apoE4 significantly downregulated genes related to vascular endothelial function and blood-brain barrier integrity through single-cell sequencing technology (scRNA-seq), Ahr), damage astrocyte endum, downregulate AQP4 protein
.

The volcano plot shows the upregulated and downregulated genes of astrocytes in iE4/Cre⁺ mice compared with the control group (left).

Fluorescence co-staining diagram of astrocyte GFAP and AQP4 protein in brain slices of iE4/Cre⁺ mice and control mice (right)

In addition, peripheral apoE4 also upregulates genes associated with endothelial cell innate immune responses (Cxcl14, Ly6c1, Ly6a), as well as genes associated with antigen processing (H2-K1, Psmc6), vascular endothelial growth factor (Vegfa), matrix remodeling and migration (Vwa1, Mgp, Spock2), oxidative stress (Prnp), etc.
, which may be jointly involved in impaired the integrity of the blood-brain barrier and cerebrovascular function
.

Recent studies have shown that replacing young blood can counteract age-related cognitive decline [7,8].

To investigate how young plasma with different apoE isotypes affected age-related cognitive function, Bu's team also exchanged plasma from young iE3/Cre+ and iE4/Cre⁺ mice into older wild-type mice
, respectively.

Interestingly, they found that iE3/Cre⁺-derived plasma significantly improved cognitive function in aged mice, reduced vascular-associated astrocyte and microglial proliferation, and increased AQP4 expression
.
The old mice that replaced the plasma from iE4/Cre⁺ young mice not only did not improve cognition and reduce gliosis, but significantly aggravated the leakage
of the blood-brain barrier.

Two-photon imaging showing the integrity of the blood-brain barrier in mice (left);

Fluorescence co-staining view of astrocyte GFAP or microglia Iba1 with endothelial marker Glut1 (right)

Finally, Bu's team also crossed iE3/Cre+ and iE4/Cre⁺ mice with classic AD model mouse APP/PS1 mice to explore the effect of
peripheral apoE subtypes on Aβ protein deposition in the brain.

They found that Aβ plaque deposition in the brain of APP/iE3 mice was significantly reduced, PSD95 expression of postsynaptic dense eggs, which are closely related to synaptic function, was increased, and astrocyte proliferation was also significantly reduced
.
In contrast, the changes in APP/iE4 mice significantly exacerbated the pathology
of amyloid in the brain.

APP/iE3 and APP/iE4 mouse brain slices were stained for Aβ

In conclusion, Bu's team found that apoE4 expressed by the liver promotes endothelial cell immune response, while increasing vascular-related gliosis, impairing cerebrovascular function and synaptic plasticity, and aggravating brain amyloid pathology, thereby damaging brain function and cognitive behavior
in mice.

In addition, Bu's team confirmed that young plasma of apoE3 instead of apoE4 improved cognitive performance in aged mice, with beneficial effects on
brain function.
Since apoE-targeting drugs in AD therapy may involve regulating both brain and peripheral apoE, Bu's team study provides mechanistic insights
into how to target apoE subtypes in the periphery of AD patients through individualized therapies.

References:

[1] Liu, C.
C.
et al.
Peripheral apoE4 enhances Alzheimer’s pathology and impairs cognition by compromising cerebrovascular function.
Nature Neuroscience.
25, 1020-1033 (2022).

[2] Guo, T.
et al.
Molecular and cellular mechanisms underlying the pathogenesis of Alzheimer’s disease.
Mol.
Neurodegener.
15, 40 (2020).

[3] Farrer, L.
A.
et al.
Effects of age, sex, and ethnicity on the association between apolipoprotein E genotype and Alzheimer disease.
a meta-analysis.
apoe and Alzheimer disease meta analysis consortium.
JAMA 278, 1349–1356 (1997).

[4] Hyman, B.
T.
et al.
Apolipoprotein E and cognitive change in an elderly population.
Ann.
Neurol.
40, 55–66 (1996).

[5] Linton, M.
F.
et al.
Phenotypes of apolipoprotein B and apolipoprotein E after liver transplantation.
J.
Clin.
Invest.
88, 270–281 (1991).

[6] Di Marco, L.
Y.
, Farkas, E.
, Martin, C.
, Venneri, A.
& Frangi, A.
F.
Is vasomotion in cerebral arteries impaired in Alzheimer’s disease? J.
Alzheimers Dis.
46, 35–53 (2015).

[7] Middeldorp, J.
et al.
Preclinical assessment of young blood plasma for Alzheimer disease.
JAMA Neurol.
73, 1325–1333 (2016).

[8] Villeda, S.
A.
et al.
Young blood reverses age-related impairments in cognitive function and synaptic plasticity in mice.
Nat.
Med 20, 659–663 (2014).

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