The neuroprotective role of miR-221 in PD: Insights from in
vitro and in vivo Studies
miR-221 plays a crucial role in promoting neuronal survival and
protecting against oxidative stress in PD through several pathways
(Figure 2 ). Autophagy is a process that helps maintain healthy
cells by removing damaged or nonfunctional organelles and recycling
their materials (Vargas, Hamasaki, Kawabata, Youle, & Yoshimori, 2023).
It also destroys pathogens and removes long-lived proteins and
aggregates to maintain cell homeostasis (Kocaturk & Gozuacik, 2018).
There are three types of autophagy observed in mammalian cells:
macroautophagy, chaperone-mediated autophagy, and microautophagy (Sato
et al., 2016). This process is important for keeping cells healthy and
has been linked to PD (Cerri & Blandini, 2019). When autophagy is not
functioning properly, it can contribute to the development of PD (Cheung
& Ip, 2009). Regulating autophagy is a proposed treatment strategy for
PD (Rakowski, Porębski, & Grzelak, 2022). Another process called
mitophagy, which helps remove damaged mitochondria from cells, may also
play a role in protecting brain cells in PD (Goiran, Eldeeb, Zorca, &
Fon, 2022).
A long non-coding RNA called LncRNA-SNHG1 is significantly increased in
PD (H. Wang, Wang, Zhang, & Zhao, 2021). It has been shown to increase
the formation of protein clumps called α-Syn aggregates and promote
inflammation in the brain (L. Zhao & Wang, 2019b). mTOR is an enzyme
that has a key role in controlling cellular growth, division, and
viability (Ali et al., 2022; Barzegar Behrooz et al., 2022). It is a
portion of two different protein complexes, mTORC1 and mTORC2, which
have different targets and functions within the cell (Jhanwar-Uniyal,
Dominguez, Mohan, Tobias, & Gandhi, 2022). Signals such as growth
factors and nutrient levels activate mTORC1 which controls processes
such as protein production, cell growth, and autophagy (Movahhed et al.,
2022). mTORC2 is activated by growth factors and controls cell survival,
metabolism, and the organization of the cell’s internal structure (Y.
Sun et al., 2023). When mTOR signaling is not functioning properly, it
can facilitate the development of various diseases, including cancer,
diabetes, and neurological disorders (Russo, Citraro, Constanti, & De
Sarro, 2012; Suhara, Baba, Shimada, Higa, & Matsui, 2017; Zou, Tao, Li,
& Zhu, 2020). p27 is a protein that helps control cell division by
inhibiting the activity of certain enzymes called cyclin-dependent
kinases (Guiley et al., 2019). Functioning as a tumor suppressor, it
prevents uncontrolled cell proliferation and has been linked to various
types of cancer (Razavipour, Harikumar, & Slingerland, 2020). It has
been demonstrated that p27 takes part in controlling autophagy and
apoptosis in PD (Lv, Wang, Zhong, & Huang, 2020). LC3 II is a protein
found on the membranes of autophagosomes, which are structures involved
in autophagy (R. Wang et al., 2022). The level of LC3 II is related to
the number of autophagosomes in a cell (Kabeya et al., 2000). A study by
Qian and colleagues suggests that reducing levels of lncRNA-SNHG1 can
increase autophagy and protect against cell death in PD by affecting the
miR-221/222/p27/mTOR signaling pathway. They found that reducing
lncRNA-SNHG1 levels increased the number of autophagosomes and reduced
cell damage in a model of PD. This was due to decreased interaction
between lncRNA-SNHG1 and miR-221/222, which resulted in lower levels of
p27 and increased activity of the mTOR pathway. Blocking miR-221/222
reversed the effects of reducing lncRNA-SNHG1 levels on p27 and mTOR.
These results suggest that targeting the interaction between
lncRNA-SNHG1, miR-221/222, p27, and mTOR could be a potential treatment
strategy for PD (Qian et al., 2019).
NPTX2 is a protein concentrated in certain brain tissues and cells
affected by PD (Moran et al., 2008; Shao, Shan, Ru, & Ma, 2020). HOTAIR
is a type of lncRNA that is associated with various diseases, including
PD, thyroid cancer, and gallbladder cancer (H.-m. Li et al., 2017; M.-z.
Ma et al., 2014; S. Wang, Zhang, Guo, Rong, & Liu, 2017). It can affect
gene expression by interacting with proteins that modify DNA structure
and with proteins that control gene activity (Spokoini-Stern et al.,
2020). Lang and colleagues proposed that HOTAIR participated in
promoting autophagy in certain brain cells affected by PD in mice. They
found that HOTAIR levels were increased in the brains of mice with
chemically induced PD and that reducing HOTAIR levels decreased
autophagy and improved movement in these mice. They also discovered that
HOTAIR affects the levels of the NPTX2 protein by interacting with
miR-221-3p and that NPTX2 plays a role in regulating autophagy in PD.
These findings indicate that targeting the interaction between HOTAIR,
miR-221-3p, and NPTX2 could be a potential treatment strategy for PD
(Lang et al., 2020).
Crocin is a natural pigment found in saffron that gives it its bright
red color (Pan, Wen, Ma, Qin, & Feng, 2022). It possesses antioxidant,
neuroprotective, and anti-inflammatory effects according to studies
(Kermanshahi et al., 2020; Nam et al., 2010). Research has looked at the
prospect of crocin as a treatment for various diseases, including PD,
AD, and cancer (Ahmed et al., 2020; Veisi et al., 2020). PI3K enzyme has
a crucial part in transmitting signals within cells that control cell
growth, division, differentiation, and survival (Juntilla & Koretzky,
2008). It is activated by signals from outside the cell, such as growth
factors and hormones, and converts one type of molecule (PIP2) into
another (PIP3) (Luo, Manning, & Cantley, 2003). PIP3 then activates
other signaling molecules within the cell, such as Akt and mTOR, which
control various cellular processes (Park et al., 2010). Akt is an enzyme
that has a crucial part in transmitting signals within cells that
control cell growth, division, differentiation, and survival (Dickson &
Rhodes, 2004; Macintyre et al., 2011). Akt is stimulated by various
signaling components, such as growth factors, cytokines, and hormones,
and it phosphorylates downstream signaling molecules, such as mTOR,
GSK-3β, and FoxO3a, which regulate various cellular processes (B.-G. Li,
Hasselgren, & Fang, 2005; Salama, Abdel-Latif, Abbas, Hekmat, &
Schaalan, 2020). A study by Rania and colleagues showed that crocin, a
natural pigment found in saffron, can protect against PD in rats exposed
to the chemical rotenone. This protective effect is achieved through the
interaction between the PI3K/Akt/mTOR signaling pathway and increased
levels of two miRNAs, miRNA-7 and miRNA-221. The PI3K/Akt/mTOR pathway
is stimulated by crocin, increasing the activity of some proteins while
reducing brain cell death. Crocin also reduces cell death and levels of
a protein called α-syn. These results provide new information about how
crocin may protect against PD (Salama et al., 2020).
PTEN is a tumor suppressor gene that produces a protein that acts as a
phosphatase, removing phosphate groups from specific proteins
(Pérez-Ramírez, Cañadas-Garre, Molina, Faus-Dáder, & Calleja-Hernández,
2015). It helps maintain balance in the PI3K/AKT pathway and may
suppress tumor growth independently of its role in the PI3K pathway
(Dubrovska et al., 2009). As described by the research conducted by Li
et al. miR-221 may have a protective effect in PD. Their research showed
that levels of miR-221 were substantially reduced in cell models of PD
induced by the chemical 6-OHDA. When miR-221 levels were increased, the
cells showed improved viability and growth and a decrease in cell death.
They found that AKT phosphorylation is significant in PD as it is
involved in regulating cell survival and apoptosis. miR-221 promotes AKT
phosphorylation by targeting PTEN, thereby activating the PI3K/AKT
pathway and potentially contributing to the protective role of miR-221
in PD. In summary, miR-221 can be a potential treatment goal for PD (L.
Li et al., 2018).
Apoptosis is a natural mechanism observed in multicellular organisms,
facilitating the elimination of damaged or unnecessary cells from the
body (D’arcy, 2019). This process is tightly regulated through intricate
signaling pathways and involves the activation of caspases, a class of
enzymes that act in targeted molecule degradation and subsequent
cellular breakdown (Boice & Bouchier-Hayes, 2020). Apoptosis, or
”programmed cell death,” is a mechanism that involves the Bax/caspase-3
signaling pathway (Zamanian et al., 2017; Zhu et al., 2022). BAX is a
member of the Bcl-2 family of proteins, which are part of the apoptosis
regulators and either promote or inhibit cell death. Its primary
location is in the cytoplasm of cells. During apoptosis, Bax undergoes a
conformational change and translocates from the cytoplasm to the
mitochondria (Ansari, Ball, Wase, Novak, & Haqqi, 2021). Caspase-3, in
turn, acts as a protease that selectively cleaves specific substrates,
leading to the dismantling of the cell (P. Yadav, Yadav, Jain, &
Vaidya, 2021). Bim is a pro-apoptotic protein that participates in
controlling how cells undergo apoptosis. It belongs to the Bcl-2 family
of proteins, which are essential for regulating the apoptosis of neurons
(Kim et al., 2017; Linseman et al., 2002). It has roles in the
regulation of numerous types of neurons, such as sympathetic neurons,
cerebellar granule neurons, and cortical neurons (Linseman et al.,
2002). Bim is regulated by transcriptional factors such as FOXO, Egr-1,
and the JNK/c-Jun pathway (Xie et al., 2011). About PD, the expression
of Bim is controlled by the activation of the JNK/c-jun signaling
pathway and the decrease in its translocation to the mitochondrial field
(Hu et al., 2019). According to the research, there was a notable
decrease in miR-221 levels in both the blood of PD patients and in the
brain tissue of mice with a chemically induced form of the disease. The
study showed that introducing additional miR-221 prevented the death of
dopamine-producing neurons in the mice and influenced the Bax/caspase-3
pathway by interacting with Bim, a protein that promotes cell death.
These are evidence that miR-221 could be a potential target for
developing treatments to protect dopamine-producing neurons and
alleviate symptoms of PD (Yao et al., 2023).
DJ-1 is a gene that produces a protein that helps protect cells from
oxidative stress (Kahle, Waak, & Gasser, 2009). Changes in the DJ-1
gene have accompanied the primary development of PD (Kahle et al.,
2009). DJ-1 has been shown to regulate the thioredoxin/ASK1 complex,
which protects against oxidative damage, and to adjust miR-221 levels to
promote neuronal survival in the face of oxidative stress (Oh et al.,
2018). DJ-1 has been shown to help prevent cellular demise by protecting
against oxidative stress and it also protects nerve cells due to its
movement to the mitochondria driven by cysteine-sulfinic acid
(Canet-Avilés et al., 2004). Oh et al. discovered that DJ-1, a gene
product associated with PD, promotes neuronal preservation in the face
of oxidative stress. Their study found that when miR-221 was
overexpressed in SH-SY5Y cells, cell viability increased significantly
following treatment with MPP+, a dopaminergic neurotoxin that induces
oxidative stress. Specifically, they found that DJ-1 adjusts miR-221
levels to shield the neurons from oxidative stress. They also discovered
that when DJ-1 was knocked down, neurite outgrowth decreased and
susceptibility to cell death caused by oxidative stress increased, while
overexpression of miR-221 reduced these effects. Additionally, they
found that miR-221 targets transcripts of apoptotic proteins, including
BIM, FOXO3a, BMF, and BNIP3L, which are predicted to be suppressed by
miR-221 (Oh et al., 2018). In conclusion, DJ-1 and miR-221 could be
anticipated endpoints for the development of therapies for PD.
The SNCA gene is associated with PD and produces a protein called α-syn.
Changes in the SNCA gene have been associated with inherited types of
PD. (Deng & Yuan, 2014). This protein is a key constituent of the
protein aggregates known as Lewy bodies, which are present in the brains
of individuals with PD (Fields, Bengoa-Vergniory, & Wade-Martins,
2019). The PARK2 gene is also associated with PD and produces a protein
called parkin. Changes in the PARK2 gene have been associated with
early-onset forms of PD (Nuytemans, Theuns, Cruts, & Van Broeckhoven,
2010). Parkin helps break down damaged or misfolded proteins in cells
(Kilarski et al., 2012). When parkin doesn’t function properly, toxic
proteins can build up in cells which can lead to the development of PD
(Kilarski et al., 2012). Tatura et al. reported that miR-221 is
upregulated in their study and its upregulation is consistent with the
observed decrease in SNCA transcript levels. Additionally, the
downregulation of miR-221 is consistent with its potential role in the
onset and development of PD (Tatura et al., 2016).
TfR2, or transferrin receptor 2, is a protein that helps regulate the
balance of iron in the body (Piperno et al., 2004). It acts as a carrier
for transferrin, a molecule that binds to iron and transports it to
different parts of the body (Robb & Wessling-Resnick, 2004). TfR2
shares some similarities with another transferrin receptor protein
called TfR1, but its specific function in iron regulation is not yet
fully understood (Kleven, Jue, & Enns, 2018). TfR2 is present in
various tissues, like the liver as well as pancreas, and brain, and
mutations in the TfR2 gene have been detected in a rare genetic disorder
called hereditary hemochromatosis type 3, which is characterized by iron
overload in the body (Bao et al., 2016; Yamamoto et al., 2002). Asci et
al. used various techniques, including bioinformatics tools and miRNA
inhibitors, to identify and manipulate miRNAs that may be involved in
regulating TfR2 expression. They found that inhibiting endogenous levels
of miR-221 in SH-SY5Y cells increases the expression of TfR2, and both
the mRNA and protein expression of TfR2 can be reduced by miR-221. The
research also showed that microRNA 221 interacts with a specific part of
the TfR2 messenger RNA molecule and reduces its activity. This suggests
that miR-221 can decrease the production of TfR2 protein. In conclusion,
miR-221 may help adjust TfR2 levels in certain cells and could provide
insight into the development of PD (Asci et al., 2013).
Paraoxonase 1 (PON1) is an enzyme that helps protect against oxidative
stress (Kotur-Stevuljevic et al., 2015). PD is characterized by a
disproportion between acetylcholine (ACh) and dopamine (DA) in the CNS
(Rizzi & Tan, 2017). This is mainly due to the degeneration of
dopaminergic neurons in the substantia nigra (Lehéricy, Sharman, Santos,
Paquin, & Gallea, 2012). Acetylcholinesterase can be attacked and
inhibited by free radicals such as peroxide and organophosphate
substances, which causes an increase in acetylcholine and worsens the
imbalance between ACh and DA (Al-Baggou, Naser, & Mohammad, 2011;
Pearson & Patel, 2016). To block the imbalance between ACh and DA, the
PON1 enzyme suppresses these free radicals (Menini & Gugliucci, 2014).
Arylesterase, (ARE) is an enzyme that is involved in the metabolism of
organophosphate compounds (Piras et al., 2021). According to a report by
Ghita and Deeb, serum levels of miRNAs (miR-214, miR-141, and miR-221)
and antioxidants (uric acid, PON1, and ARE) were significantly lower in
cases with PD in comparison to healthy individuals (Ghit & Deeb, 2022).
UPDRS-III is utilized to evaluate the movement-related symptoms of PD,
including shaking, stiffness, slowed movement, and balance issues (M. Lu
et al., 2020). The scale has a range of 0 to 108, where a higher score
signifies more severe motor symptoms (Thobois et al., 2002). UPDRS-V is
utilized to evaluate the daily living activities and independence of
individuals with PD (Shu, Qian, & Wang, 2020). The scale has a range of
0 to 100, where a higher score signifies greater dependence on others
for performing daily activities (Tibar et al., 2018).
The Area Under the Curve (AUC) value ranges from 0 to 1, with 0.5
indicating a test with no discriminatory ability (equivalent to random
chance) and 1 indicating a perfect test that can accurately distinguish
between the two groups (Walter, 2005; Wolever, 2004). Ma et al.
discovered that serum miR-221 was considerably reduced in individuals
with PD and had a positive correlation with UPDRS-III and UPDRS-V
scores, showing its potential as an indicator of PD severity. The AUC
value for predicting PD using serum miR-221 was 0.787, suggestive of its
prospective use as a biomarker for diagnosing PD. These findings suggest
that downregulated serum miR-221 might be considered a biomarker for the
assessment of PD (W. Ma et al., 2016).