Discussion and conclusions
Montelukast has been associated with some neuropsychiatric side effects,
especially among children. Our previous work suggests that these adverse
effects may be associated with a dysregulation of the
hypothalamic-pituitary-adrenal axis and an increase of oxidative stress
in children’s brain (Marques et al. , 2022d). Since older people
have a mature brain, they should be less susceptible to these effects,
which could explain the decreased incidence of adverse drug reactions
(ADRs) in adults. Moreover, specific montelukast effects in the older
brain may underlie the proposed, yet mechanistically elusive,
repurposing of the drug for the management of neurodegenerative
disorders. In the current work, the same proteomics dataset generated in
our previous work was interrogated from a repurposing perspective, to
identify altered proteins known to be involved in the pathophysiology of
Alzheimer’s disease.
The up-regulation of Nceh1 and HIVEP3 in the mouse brain suggests that
MTK contributes to AD management. Indeed, while Nceh1 has been
associated with a protective role against α‑synuclein toxicity (Zhanget al. , 2017) and has been found to have decreased levels in the
blood of AD patients (Mandas et al. , 2012), HIVEP3 is
underexpressed in a double transgenic APP/PS1 mouse model of early-onset
AD (Islam et al. , 2021). Likewise, the hippocampus up-regulation
of Psen1 mimics the effect observed upon γ‑secretase inhibition
(Campanari et al. , 2014; Sogorb-Esteve et al. , 2018). This
is in agreement with the observed up-regulation, in the PFC, of the
reticulon-3 protein, a known inhibitor of the anterograde transport of
the BACE1 subunit of β‑secretase, thus contributing to an improved
clearance of amyloid deposits (Deng et al. , 2013; Kume et
al. , 2009).
The insulin-degrading enzyme, up-regulated in mouse PFC, is involved in
the degradation of α‑synuclein aggregates, and its dysregulation has
been related to the occurrence of type 2 diabetes mellitus, whereby the
patients have a high risk of developing AD or PD (Sousa et al. ,
2021). Neprilysin was also found to be up-regulated in the PFC of
MTK-treated mice. It has been proposed that the up-regulation of these
two enzymes could be a promising therapeutic strategy for
neurodegenerative disorders (Nalivaeva and Turner, 2019). It is
noteworthy that the glucagon-like peptide 1 receptor, which has been
associated with improved memory and learning in mice (During et
al. , 2003), was also overexpressed in MTK-treated mature chicken embryo
neurons supporting the hypothesis of an improvement of the amyloid
clearance process caused by MTK.
The lower levels of nicastrin, coupled to the altered levels of Psen1
(another component of the γ‑secretase complex) in the PFC of MTK‑treated
mice, also indicate that MTK may be involved in the inhibition of
γ‑secretase, leading to a decrease of amyloid production. Factoring in
the altered AD presenilin-dependent pathway, dominated by an
up-regulation of the Wnt pathways, observed in treated neurons, MTK
appears to have the potential to overcome the synaptic degeneration and
cognitive deficits characteristic of AD. It is noteworthy that Wnt
proteins are systematically found to be down-regulated in AD patients,
and these lower levels are associated to activation of the amyloidogenic
pathway (Palomer et al. , 2019). Thus, the proteomic shifts
associated with MTK have the potential to counterbalance a number of
pro-amyloidogenic mechanisms involved in AD’s etiology.
5-LOX is involved in the synthesis of cysteinyl leukotrienes, which have
a pro-inflammatory effect antagonized by MTK (Marques et al. ,
2022c). Interestingly, while 5-LOX levels are found to be higher in AD
patients (Li et al. , 2017), the observed lower 5-LOX levels in
the mouse hippocampus also suggest that MTK is able to, at least
partially, restore the altered phenotype of AD. This is further
corroborated by the observed up-regulation of the 14-3-3 protein family
in the prefrontal cortex of treated mice. While decreased levels of the
14-3-3 proteins are associated with several AD and PD hallmarks (Guet al. , 2020), their MTK-driven overexpression is quite
promising, since the modulation of these isoforms has been described as
a potential target for neurodegenerative disorders (Pair and Yacoubian,
2021).
From a central metabolism standpoint, the ageing process is
characterized by a lowering of cell basal metabolic rate, particularly
in the case of neurons (Yan et al. , 2020). HK1 levels are
consistently found to be decreased in AD patients (Yan et al. ,
2020), whereby the observed increase of HK1 levels caused by MTK could
play a neuroprotective role in the brain. This HK1 increase was observed
in the PFC of treated mice, together with the up-regulation of the
tricarboxylic acid (TCA) cycle enzymes malate dehydrogenase and
isocitrate dehydrogenase. From a metabolic perspective, the observed
overexpression of the glucagon-like peptide 1 receptor in chicken embryo
neurons is a further link between the brain energy pathways and AD, as
this protein is also involved in the clearance of amyloid and synuclein
aggregates. Notably, the glucagon-like peptide 1 receptor has been
proposed as a therapeutic target for neurodegenerative disorders
(Athauda and Foltynie, 2016; Li et al. , 2010). The MTK-induced
up-regulation of this receptor observed in our work, and its mechanistic
implications, may thus provide, at least in part, the missing rationale
for the repurposing of MTK in these diseases. It is also noteworthy that
the proteomic alterations of TCA cycle enzymes reported herein are in
agreement with the TCA changes we observed previously at a metabolomics
level (Marques et al. , 2022d).
Taken together, the proteomics data reanalysed herein clearly indicate
that montelukast is associated with a shift towards amyloid clearance,
coupled to an up-regulation of specific enzymes involved in energy
production (Figure 3 ). These are two fundamental biological
processes that sustain MTK’s repurposing towards AD management, which is
currently being assessed in various clinical trials (for example,
ClinicalTrials.gov Identifiers NCT03991988 and NCT03402503). This wide
anti-AD activity of montelukast is reminiscent of reports from studies
on valproic acid, which has been shown to increase neprilysin expression
and activity in hypoxic rats and in model mice, and is speculated to
enhance amyloid clearance in AD patients (Nalivaeva et al. , 2012;
Nalivaeva and Turner, 2019; Wang et al. , 2014). Overall, central
nervous system-acting drugs of different classes offer new potential
strategies for the management of AD and other neurological diseases. To
be fulfilled, these promising approaches require further investigation
of the biochemical mechanisms underlying such beneficial side effects.