PD is a progressive neurodegenerative disorder that affects approximately 1% of the population beyond 65 years old [1]. A study of mortality among PD patients showed an odds ratio of 2.5 compared with age-matched subjects [2]. The diagnosis of PD continues to be based on presenting signs and symptoms. Dyskinesia is the most obvious clinical symptom and often starts in one extremity and worsens with stress, fatigue and cold. Bradykinesia is usually the most troublesome symptom. Patients refer to slowness in performing their daily activities. Rigidity of muscles on passive movement, including joints, is also a characteristic of PD [3].

The primary brain abnormality found in all patients is a degeneration of nigrostratial dopaminergic neurons in the substantia nigra, which leads to the depletion of dopamine (DA) with consequent loss of neuronal systems responsible of motor functions, and the formation of intracytoplasmic inclusions called Lewy bodies (LBs) in remaining neurons [4].

It is thought that the cause of idiopathic PD may be an interaction of environmental and genetic factors [4]. More typically, PD is sporadic when there is no family history of disease although a study has suggested a significant contribution of heredability to the development of late-onset PD [5]. In pedigrees with autosomal-recessive early-onset parkinsonism, a wide variety of mutations to the parkin gene (Park-2 gene) were found in families in which at least one member developed the symptoms [6]. Also, a number of hereditable genetic autosomal-dominant mutations have been found in the α-syn gene (also known as SNCA), besides other genes, as rare cause of PD, helping in understanding the disease [5]. α-syn has received much attention because it is the major component of LBs. In addition, α-syn pathologies accumulate throughout the central nervous system (CNS) in areas that also undergo progressive neurodegeneration, leading to dementia and other behavioural impairments as well as parkinsonism [7].

Although significant advances have been made in understanding the pathophysiology of this disease, there is no curative treatment and only symptoms can be controlled. The management of PD is designed to improve the patient’s quality of life. Symptomatic therapy is based totally on the requirements of the individual patient and must be re-evaluated as the condition evolves. Neuroprotective therapy is currently unavailable, in spite of the initial promising data from the DATATOP study performed with the monoamino oxidase (MAO) inhibitor selegiline, definitive neuroprotective action has yet to be demonstrated and its actions can be off set by its side effects that may include nausea, dizziness, insomnia and cognitive impairment. Actually, the American Academy of Neurology suggests that there is currently insufficient evidence to recommend selegiline as a neuroprotective treatment [8].

l-Dopa is presently the most efficacious treatment for PD [9]. l-dopa is converted into DA within the nigrostratial neurons by the enzyme aromatic l-amino acid decaboxylase. This enzyme is a rate-limiting in DA synthesis in PD, but not in healthy individuals, and pyridoxal phosphate (vitamin B6) is a required cofactor for the decarboxylation which may be administered together with l-dopa as pyridoxine. Conversion of l-dopa to DA likewise occurs systemically, outside the brain, in peripheral tissues, and thereby may induce adverse effects. It is hence standard clinical practice to co-administer a peripheral DOPA decarboxylase inhibitor that is restricted from entering brain, such as carbidopa or benserazide, and often a catechol-O-methyl transferase (COMT) inhibitor, to prevent peripheral tissue synthesis of DA. Essentially, all patients require l-dopa at some stage of the disease. However, careful l-dopa titration is essential since it may induce dyskinesias and other l-dopa side effects. The most common is the “wearing-off” phenomenon or shortening of sustainable pharmacological action. This occurs when the symptoms of PD, attenuated by the treatment, become more intense before the next dose. The response usually is an increase of l-dopa dose frequency and addition of alternative therapies, such as the COMT inhibitor entacapone, DA agonists, amantadine or selegiline [10] (see below). The major inconvenience of l-dopa is dyskinesias or involuntary movements related to the drug. The addition of amantadine usually attenuates dyskinesias [11]. Although l-dopa is associated with motor complications, it must be acknowledged that survival has been considerably prolonged in PD since its introduction. Moreover, as in healthy animals, individuals misdiagnosed with PD and exposed to long-term l-dopa treatment did not show signs of parkinsonism [12,13].

In specific clinical situations, lower potency drugs are used. As listed in Table 1, anticholinergics (benztropine, biperiden, diphenhydramine, ethopropazine, orphenadrine, procyclidine and trihexyphenidyl) provide mild symptomatic treatment and may be beneficial to treat tremors [14]. Unfortunately, many patients experience cognitive change following anticholinergics, and therefore they are generally restricted to younger patients. Amantadine, a N-methyl-d-aspartate receptor antagonist, also provides mild symptomatic benefit in early stages of disease. This agent is relatively inexpensive, with a low incidence of adverse effects but is also associated with confusion in older patients [10]. DA agonists (mainly bromocriptine, pramipexole, ropinerole and pergolide) directly stimulate DA receptors and do not need to be metabolized into an active drug. They may hence have potential advantages over l-dopa by having a longer half-life, a longer duration of symptomatic action and, most importantly, DA agonists are rarely associated with motor fluctuations and dyskinesias [15]. Nevertheless, patients may succumb to other side effects, such as hallucinations, hypotension, anxiety, depression, bladder dysfunction, insomnia, edema and cognitive impairment.

Several clinical trials have shown a decrease of dystonia and dyskinesia, and a similar clinical benefit when patients with early stage PD were treated with the DA receptor agonist bromocriptine, or l-dopa alone [16]. A similar result was obtained by using ropinirole plus l-dopa, if required [17]. These data support the use of DA agonists in early PD together smaller doses of l-dopa to minimize complications related to the latter [15]. In addition, several in vitro and animal studies might indicate a role for neuroprotection for DA agonists [18–21], although clinical benefit in humans is to prove. Other DA agonists, apomorphine, lisuride and carbegoline are also available, however, the choice of one them taking into account their effectiveness and/or non-motor symptoms remains unclear.

In addition, COMT is a selective and widely distributed enzyme involved in the catabolism of l-dopa. Tolcapone and entacapone are selective and potent COMT inhibitors that slow the metabolism of l-dopa, thus prolonging its effects [22]. Tolcapone has been shown to be an effective adjunct in the treatment of PD in Phase II and III clinical trials, improving motor fluctuations and reducing l-dopa requirements. Rare reports of severe hepatotoxicity, however, have limited tolcapone implementation in the treatment of PD [23].

The currently available therapies for PD, as in Table 1, are symptomatic and become less effective over time. Therefore, in order to identify potential neuroprotective agents available for testing, the Committee to Identify Neuroprotective Agents in Parkinson’s (CINAPS), supported by the National Institute of Neurologic Disorders and Stroke (NINDS), conducted a systematic assessment of presently available pharmacologic agents. From a list of 59 potential neuroprotective agents, the Committee identified 12 agents that are currently available and should be considered priority agents for further investigation in PD [24] (see Table 2, which includes neuroprective agents).

As mentioned above, PD neurodegeneration is accompanied by the presence of LBs and Lewy neuritic inclusions (LNs) in surviving dopaminergic neurons [25], in which the main component derives from fibrillar aggregates of α-syn [26,27], although α-syn inclusions are also found in non-dopaminergic neurons (called neuronal cytoplasmic inclusions), in glial cells (glial cytoplasmic inclusions, GCIs) and as axonal spheroids [28].

The main evidence of the relevant role of α-syn in PD came from the discovery of three point mutations in the α-syn gene that can cause hereditable early-onset PD [29] in rare pedigrees. These mutations are A53T (change Ala in position 53 to Thr) [30], A30P (change Ala in position 30 to Pro) [31] and E46K (change Glu in position 46 to Lys) [32]. All three occur within the N-terminal side of the protein and are able to accelerate the α-syn oligomeric aggregation process and protofibril formation [33] faster than wild-type (wt) α-syn. Therefore, the insoluble fibrillization rate was also higher (with the exception of the A30P mutation) than in wt variant [34,35], leading to pathologic inclusions, such as LBs and LNs.

Interestingly, several clinical features may be distinguished among A53T carriers vs. idiopathic PD patients, such as a slightly earlier onset, a faster disease progression, a lower tremor prevalence, and dementia [36]. By contrast, patients with the A30P mutation resemble idiopathic PD. The E46K patients exhibit dementia with Lewy Bodies (DLB) and hallucinations, as well as parkinsonism. This mutation, which substitutes a dicarboxylic amino acid, glutamic acid, with a basic amino acid such as lysine in a much conserved area of the protein, is likely to produce a severe disturbance of protein function [32]. On the other hand, genetic polymorphisms in the α-syn gene appear to confer risk for sporadic PD [37]. In fact, in the Japanese population there are several polymorphisms in the intron 1 that may be associated with PD [38]. Furthermore, duplication or triplication of the α-syn gene has been reported in familial forms of PD [39], and could implicate gene dosage effects in the PD pathogenesis [40].

In agreement with genetic and clinical findings, transgenic animal models have revealed an association between α-syn and the disease. In Drosophila, for example, when wt- α-syn, A30P or A53T mutants are overexpressed, a motor dysfunction, selective loss of DA neurons and presence of filamentous intraneuronal inclusions that contain α-syn occur [41]. In contrast, α-syn knock-out mice possess an abnormal activity of DA neurons with reduced levels of DA detected in the striatum. This implies that the protein may play a role in the regulation of neurotransmitter release [42].

The importance of the link between α-syn and PD, together with the discovery of detectable levels of α-syn in CSF and in human plasma, suggesting that α-syn is released from affected dopaminergic neurons [43], indicates that α-syn could serve as a marker for early diagnosis of PD. In this regard, an Enzyme-Linked Immuno Sorbent Assay (ELISA) based method has recently been developed to detect oligomeric α-syn in CSF and plasma and could serve as a diagnostic tool for PD and related diseases [44].

α-Syn belongs to the synuclein family, which includes β-syn and γ-syn [45]. α-syn and β-syn are predominantly expressed in brain at presynaptic terminals, particularly in the neocortex, hippocampus, striatum, thalamus and cerebellum [46,47]. γ-syn is highly expressed in several areas in the brain, particularly in the substantia nigra, and has been found to be overexpressed in some breast and ovarian tumors [48]. α-syn homologues have been found in several mammals, but not in lower organisms such as Escherichia coli, yeasts, Drosophila or Caenorhabditis elegans.

α-Syn is a small protein (14 kDa) composed of 140 aminoacids (aa). It is a soluble, acidic, resistant to high temperatures and natively unfolded protein with an extended structure that is mainly composed of random coils [49]. However, it acquires secondary structure elements upon interaction with ligands and proteins, adopting a partially folded conformation [50]. A report suggests that unfolded states play a functional role in vesicle fusion in all eukaryotics, bringing membrane surfaces to facilitate fusion, and, after binding, an ordered structure is acquired [51].

The α-syn sequence can be subdivided in three domains (Figure 1). The highly conserved N-terminal domain (residues 1–65) is unordered in solution and includes seven copies of an unusual 11 aa repeat that displays variations of a KTKEGV consensus sequence that may shift to an α-helical structure under certain conditions [52,53]. α-syn can interact with synthetic phospholipid vesicles through this domain [54], suggesting that the protein may be membrane-associated [55], becoming α-helix conformation after binding [56].

The A53T and A30P mutations do not affect the overall structure of α-syn, which remains unfolded, but diminish hydrophobicity of the N-terminal domain; thereby somewhat reducing its ability to form α-helices. The predisposition to form β-sheet structures is enhanced [50], which renders it more prone to aggregation. Whereas A53T does not affect vesicle binding, A30P is characterized by a decrease in lipid binding [33].

The central hydrophobic domain of α-syn (residues 66–95) is known as NAC. This peptide portion has been implicated in AD pathogenesis and is the second major component of brain AD amyloid plaques [57]. It is responsible for protein-protein interactions and confers on α-syn the ability to undergo a conformational change from a random coil to an aggregation-prone β-sheet structure [58] leading to the formation of amyloid-like fibrils [59]. An in vitro study demonstrated that aged NAC, dissolved in solution for seven days, is more toxic than fresh NAC, suggesting that cytotoxicity depends on prior aggregation [60]. The NAC region also contains a phosphorylation site on Ser-87.

The acidic Glu-rich C-terminal domain (residues 96–140) has no recognized structure and a strong negative charge [52]. Several phosphorylation sites have been identified within this region, on Tyr-125, −133 and −136, and Ser−129 [61]. Its acidic domain (aa 125–140) appears to be critical for the chaperone activity of α-syn [62]. Chaperones are proteins that prevent irreversible protein aggregation and facilitate the correct folding of proteins through binding in vivo. α-syn shares a 40% homology with a chaperone called 14-3-3 [63], suggesting that both proteins may share the same function. Chaperone 14-3-3 is particularly abundant in brain and can prevent apoptosis by binding with the pro-apoptotic protein, BAD [64]. α-Syn selectively interacts with 14-3-3 in substantia nigra, where it accumulates in LBs, leading to a decrease in available levels of 14-3-3 to inhibit apoptosis [65]. Both α-syn and 14-3-3 interact with tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis that is responsible for catalyzing the conversion of the amino acid l-tyrosine to l-dopa. TH activity is stimulated by 14-3-3 and inhibited by α-syn [66].

The structural disposition of α-syn can show under a number of different conformations:

The intrinsic unfolded state under physiologic conditions, both in vitro and in vivo.

Because of this number of possible structural conformations, it seems reasonable to suggest that α-syn is potentially prone to misfold [105]. Fibrillization is a nucleation polymerization process, in which there is an initial lag phase, during which nuclei are formed, followed by the exponential growth of the fibrils, and an equilibrium phase between the protein in solution and the protein in fibrillar form [49]. The fibrillization rate increases with higher α-syn concentration, low pH, high temperature, oxidative stress-inducing compounds such as DA [106], the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrapyridine (MPTP) [107], lipids [54] and pesticides [99]. The insoluble aggregates might represent the major building blocks of synucleinopathies-related inclusions, as LBs, LNs, GCIs and axonal spheroids, all of them observed in PD and related pathologies, as well as in other CNS disorders, including AD.

As stated above, a number of environmental and genetic factors can induce partial folding of α-syn, and therefore these same factors are able to accelerate the fibrillization process. Other aggregation-prone factors are:

Oxidation: the exposure of α-syn to oxidative agents induces the formation of high-order oligomers [74], and both the familial Parkininsonian A30P and A53T mutants have shown an even higher rate of self-assembly [108], providing support for the hypothesis that an impairment of cellular antioxidative mechanisms and/or overproduction of reactive oxygen species (ROS) may cause the initiation and progression of neurodegenerative synucleinopathies [109]. However, all amino acids are susceptible to oxidation [110], methionine being one of the easiest to undergo oxidation to form methionine sulfoxide (MetO). Thus, contrary to expectations, under mild oxidative conditions, when all four Met in α-syn are oxidized to MetO, the oxidized α-syn was found to be more unfolded than the non-oxidized form and less prone to oligomerize and fibrillate. Indeed, it even proved able to inhibit the fibrillization of non-modified α-syn [111].

DA metabolism in nigrostratial neurons can produce ROS and contribute to lipid peroxidation, DNA damage, impairment of mitochondrial function, depletion of reduced glutathione (GSH), and, finally, cell death [112]. Over-expression of α-syn, and especially its mutant forms, enhances the vulnerability of neurons to DA-induced cell death through massive generation of ROS [113]. It has been proved that conjugation of DA with α-syn impedes the protofibril-to-fiber transition and, therefore, potentially more cytotoxic protofibrils may accumulate [114]. Transfection of wt- α-syn, A30P or A53T mutants has been reported to trigger apoptosis of cultured dopaminergic neurons, whereas there was an increase in survival of non-dopaminergic neurons [65]. Inhibition of DA synthesis by blocking TH activity prevented α-syn induced apoptosis.

Damage caused by DA species is mediated by DA auto-oxidation catalyzed by free metals (especially Fe) [115], yielding 6-hydroxydopamine (6-OHDA) or through enzymatic deamination by MAOs to form toxic DA metabolites and hydrogen peroxide (H₂O₂) [116]. Levels of MAO-B appear to be highest in the substantia nigra. H₂O₂ produced as a by-product of DA oxidation and as normal oxygen reduction by MAOs is highly permeable and cannot be converted to water by GSH peroxidase due the low level of available GSH as a reducer [117], allowing H₂O₂ to potentially diffuses out of dopaminergic neurons and damage the neighbouring neurons. Under normal conditions, ROS are kept under control by an efficient antioxidant cascade. This includes the cytosolic copper-zinc superoxide dismutase and the mitochondrial manganese superoxide dismutase, which convert superoxide to oxygen and H₂O₂. The latter, in turn, is removed by catalases and peroxidases. These enzymes are key to scavenge ROS generated by oxidative insults. For example, in a transgenic murine model that overexpressed Cu/Zn superoxide dismutase or GSH peroxidase and was treated with pesticide paraquat, which causes a PD-like profile, the transgenic animals did not show alterations as reductions in locomotor activity, levels of striatal DA and metabolites, or dopaminergic neurons in the substantia nigra, unlike non-transgenic controls in which all of these were affected [118].

Soluble α-syn is able to interact with the DA transporter (DAT) through the NAC domain [119], decreasing the amount of DAT in the PM, to allow for an optimal moderate level of synaptic DA reuptake to be accumulated into vesicles. In the event of α-syn aggregation, a decrease in the level of soluble α-syn results, and leads to the increased PM accumulation of DAT, giving rise to a massive entry of DA into the cell and consequent potential generation of ROS [116]. In accordance, the much more neurotoxic A53T mutant (but not A30P) interacts very weakly with DAT and causes an impairment of vesicular DA storage and release [119].

Interestingly, the dopaminergic neurotoxin MPTP, which causes a PD-like neurodegeneration in rodents, humans and primates, enters into the cell through the DAT as its ionic metabolite MPP⁺. It then targets the mitochondria, inhibits complex I of the electron transport chain, impairs ATP production, induces a loss of mitochondrial membrane potential allowing the release of cytochrome c and generation of ROS, and additionally, increases α-syn mRNA and protein levels. The A53T mutant can enhance the vulnerability of cells to MPP⁺, while α-syn null-mice are resistant to MPP⁺-induced degeneration [120], signifying a fundamental role of α-syn in drug-mediated neurotoxicity.

As mentioned above, oxidation can lead to degeneration of dopaminergic neurons, resulting from the increased level of redox-active metals ions (Cu or Fe) within the substantia nigra, which initiates a cascade of events, such as α-syn oligomerization, mitochondrial dysfunction, cytotoxicity, and a rise of cytosolic free Ca, leading to cell death.

As commented above, duplication or triplication of the α-syn gene has been reported in early-onset PD patients, suggesting a dose-dependence association with the disease, and genetic polymorphisms in the α-syn gene are linked to idiopathic PD by increases in α-syn concentration [40]. Other studies have revealed that the overexpression of α-syn can induce Fe-dependent aggregation [153].

Under physiological conditions, there is an equilibrium between the natively unfolded and the partially folded conformation; therefore a high α-syn concentration may increase the rate of fibrillization due to an increased total concentration of the partially folded fibrillization-prone conformation [105]. Several studies report an increase in α-syn mRNA levels in brains of patients with PD compared to healthy controls [154]. In fact, overexpression of α-syn can generate α-syn immunopositive inclusions, together with alterations in mitochondria, increases in ROS production [155], lysosomal dysfunction [156] and Golgi fragmentation [157], leading to cell death. Of note, all these effects could be partially attenuated by antioxidants [155].

Accordingly, murine models transfected with recombinant viruses that overexpressed wt- α-syn or the A53T mutated form showed the typical features of PD in humans, including a loss of dopaminergic neurons in substantia nigra, α-syn inclusions similar to LBs [158] and a reduction in TH levels. In fact, a correlation between the number of α-syn immunoreactive LBs and α-syn mRNA levels has been found [159].

Another useful murine model to study PD is created by the injection of MPTP, which has been reported to enhance the production of α-syn at both mRNA and protein levels, and induce formation of α-syn-positive inclusions in neurons and abnormal locomotor function [160]. Similarly, administration of the pesticide paraquat to mice caused an upregulation of α-syn and formation of α-syn aggregates. A faster PD-like model has been obtained by using an alternative pesticide, rotenone, which is highly lipophilic and readily crosses plasma membranes to allow rapid brain entry [161]. Within brain, it inhibits mitochondrial complex I and increases oxidative stress by ROS production leading to degeneration of dopaminergic neurons and fibrillar α-syn inclusions [162]. It has been demonstrated that at a nM range concentration, α-syn provides neurons protection against serum deprivation, oxidative stress and excitotoxicity, whereas in the μM range, cytotoxicity results [163]. Thus it is clear that there is a strong correlation between α-syn levels and aggregation-derivated toxicity.

Both enzymatic and non-enzymatic catabolism of DA are accelerated by the presence of redox elements [164]. In particular, the high concentration of Fe in substantia nigra may catalyze the conversion of H₂O₂, produced during breakdown of DA, to highly reactive hydroxyl radicals by the Fenton reaction, resulting in oxidative damage [165]. In contrast, the superoxide radical is unreactive, but can serve as reducing agent for oxidized metal ions to produce more hydroxyl radicals from H₂O₂, via a cycle known as the Haber-Weiss reaction [166]. Oxidative stress may enhance Fe levels by deattachment from ferritin (see below), from heme proteins like haemoglobin and cytochrome c by peroxides and from iron-sulfur proteins by ONOO^.- [117]. Iron also catalyzes the conversion of an excess of DA to neuromelanin, an insoluble black-brown pigment that accumulates in all aged dopaminergic neurons. Neuromelanin sequesters redox ions with high affinity for Fe³⁺; however, when bound to an excess of Fe³⁺, neuromelanin tends to become pro-oxidant, reducing Fe³⁺ to Fe²⁺, which is released from neuromelanin and increase the amount of iron available to react with H₂O₂ [167].

In cells, the main route of Fe uptake is through the transferrin receptor (TfR) at the cell surface. Each TfR binds to a Fe-laden transferrin (Tf) molecule and internalizes it via endocytosis. The acidic pH of endosomes causes the dissociation and unloading of Fe from Tf, and is then recycled back to the cell surface. In cytoplasm, most available Fe is sequestered by the iron-storage protein ferritin and a small amount is left free. In the case of Fe deficiency, the levels of TfR in the plasma membrane are increased and ferritin synthesis is downregulated, enhancing the availability of free Fe. When there is too much free Fe, TfR levels are downregulated and ferritin synthesis becomes upregulated.

This homeostasis is regulated at the translational level by two cytoplasmic iron regulatory proteins (IRPs), IRP1 and IRP2 [168; 169], based on their coordinately binding to iron-response elements (IREs) within the 5’-untranslated region (5-UTR) of mRNA ferritin and to the 3-UTR of mRNA TfR. When cellular Fe levels are low, IRPs bind to the ferritin 5-UTR, resulting in a block of ferritin translation and bind to the TfR 3-UTR, stabilizing the mRNA TfR and preventing its endonuclease cleavage [117]. Therefore, there is a decrease in Fe storage, an increase in extracellular import and consequently, an enhancement in cytoplasmic Fe levels. In the presence of excess Fe, IRP2 is degraded [170] and IRP1 inactivated, thereby increasing ferritin levels.

There are several lines of evidence that support the contention that Fe accumulation in a number of degenerative disorders may be a primary event, rather than a consequence of the disease:

Injection of Fe₃Cl into the substantia nigra of rats has been reported to result in a selective decrease of striatal DA, which supports the assumption that Fe initiates dopaminergic neurodegeneration in PD. This decrease was prevented by infusion of the Fe chelator, desferrioxiamine [171].

Anther link between Fe and oxidative stress comes from the discovery of upregulation of ferritin and downregulation of TfR in response to conditions of heightened oxidative stress, leading to IRP inactivation [178], and inversely, overexpression of ferritin decreased the oxidative levels [179]. However, there are reports where increases of H₂O₂ level enhanced the activation of IRPs [180]. In a study, when the Fe concentration was increased, IRP2 activity disminished, as expected, but IRP1 activity decreased in the beginning and later aberrantly enhanced, indicating the presence of complex feedback loops to mitigate Fe-induced oxidative damage [181]. Another study assessed that there was neither increase in ferritin levels under excess of Fe nor alteration in the IRP-IRE system in PD brains, maybe induced by the sequestering of Fe by α-syn [182].

Another example of IRP-IRE mediated regulation is the regulation of mRNA levels of mitochondrial complex I, which is compromised in PD [183]. The mitochondrial electron transport chain is the major source of free radicals in vivo, therefore dysfunction of IRP regulation of complex I or IRE-containing tricarboxilic acid cycle enzymes, that provide substrates for the electron transport chain, would impair mitochondrial function and would lead to exacerbated levels of oxidative stress. Also, an IRE motif has been located in the 5-UTR of murine and human erythroid-specific delta-aminolevulinic acid synthase (eALAS) mRNA which encodes the first, and possibly rate limiting, enzyme of the heme biosynthetic pathway [184], and for which translation is controlled by IRPs.

Fe-oxidative stress has also been shown in several studies to promote α-syn aggregation [153], maybe due to alterations in secondary structure leading to partially folded states, more susceptible to oligomerization [98]. Fe has been identified as a component of LBs [185], showing the tight relationship among Fe, oxidative stress, α-syn and PD. Indeed, in accordance with and adding to the described literature, an IRE has recently been discovered within the 5-UTR of the α-syn mRNA [186].

In PD, α-syn aggregation is the typical hallmark, and it has been demonstrated that MPTP-induction and α-syn overexpression triggers Fe-mediated α-syn oligomerization, because Fe chelation reduced the toxicity exerted by MPTP. It is known that certain neurotoxins, such as 6-OHDA, are selective for dopaminergic neurons, and cause a PD-like clinical profile and, meaningfully, this neurotoxicity only can be exerted through Fe mediation, since again Fe chelation was able to revert this effect.

The presence of an IRE within the 5-UTR of the α-syn gene and the importance of α-syn in PD, clearly indicates a role for Fe in the pathogenesis of the disorder. Thus, α-syn levels are critical to hold Fe homeostasis, and an impairment in the IRE-mediated regulation system of α-syn can lead to overexpression, to a misfunction in regulation of Fe storage, and consequently to Fe-mediated oxidative stress, α-syn aggregation, dopaminergic neuronal death, DA depletion, and finally to PD symptoms. Indeed, the oxidative insult is not limited to these neurons in substantia nigra, and can expand to other areas of the brain leading to massive neuronal death. For this reason, it is frequent to find dementia in patients affected by PD.

Duplication or triplication of the α-syn gene leads to α-syn overexpression and aggregation, perhaps by altering the equilibrium between IRE-containing α-syn and IRE-containing Fe regulatory protein system with regard to the recruitment of IRPs, although even a subtle change in α-syn concentration could have the same effect. A shift in α-syn isoform ratio towards the 112 aa isoform, that is more prone to aggregate than the full-length protein, also could occur in vivo.

Synucleinopathies are additionally related to AD, since neurofibrillary tangles of protein tau, a hallmark of AD, are in many cases found co-localized with LBs [198]. Moreover, APP mRNA also encodes an IRE, revealing the critical importance of Fe homeostasis in neurodegenerative processes and the main role of the IRE translational regulatory system in the CNS.

In conclusion, the search for new therapeutic agents for PD able to regulate increasing α-syn levels by binding to its IRE might retard Fe-induced neurotoxicity, as well as avoid the deleterious effects of α-syn overproduction and aggregation. Ever since the first-line of treatment of PD, with l-dopa that was developed 50 years ago, there has been no other drug that has proved to be sufficiently efficacious to substitute for it, despite its side effects and short duration of efficacy. Hence, in the future these potential new translation blocker drugs could widen the available clinical options for PD treatment by providing the opportunity for arresting and/or reversing the progression of not only this disease but also other related neurodegenerative synucleinopathies.