Can α-synuclein be targeted in novel therapies for Parkinson’s disease?

Keywords::

• α-synuclein
• disease modification
• dopamine
• gene therapy
• Lewy body
• neuroprotection
• Parkinson’s disease
• prion
• protein aggregation
• RNAi
• small molecule

Classical descriptions of the symptoms of Parkinson’s disease (PD) focus on the progressive development of rigidity, tremor and hypokinesia. However, patients also suffer from a range of debilitating non-motor symptoms [1]. PD is primarily a disorder of aging and as the second (behind Alzheimer’s disease) most common neurodegenerative disease, it constitutes a tremendous societal burden. Current therapies are symptomatic and focused on promoting dopamine neurotransmission. With disease progression, their beneficial effects wane and invariably patients develop troubling side effects. A powerful disease-modifying therapy, which can dramatically reduce the rate of disease progression, is clearly needed. Considering that such a treatment would most probably require lifelong administration, the market value would be substantial. Several attempts to develop disease-modifying treatments for PD have failed. Numerous treatment strategies show great promise in cell and animal models of PD, but the translation into the clinic has been unsuccessful. The wrong treatment targets have possibly been used or the designs of the clinical trials have not been optimal [2]. In this article, we discuss whether targeting the protein α-synuclein might lead to disease modification in PD.

Over the past decade, the protein aggregates that develop in the brains of PD patients have received increased attention. Targeting the development of these aggregates would possibly be a fruitful approach to disease modification. The idea that ‘proteinopathy’ plays an important pathogenetic role in neurodegenerative disease is not limited to PD. Protein misfolding and the formation of protein aggregates are clearly prominent in PD, but whether the protein aggregates directly cause neurodegeneration, or are epiphenoma to other neurotoxic events, is yet to be clarified. First described by the neuropathologist Friedrich Lewy almost a century ago, the intraneuronal inclusions found in PD came to bear his name. They are called Lewy bodies when found in the cell bodies of neurons and Lewy neurites when they are smaller and located in neuronal extensions. They are made up of numerous proteins and lipids, and a misfolded variant of α-synuclein constitutes the major component [3]. During normal aging, α-synuclein levels gradually increase in the cell bodies of substantia nigra dopamine neurons [4], which are particularly susceptible to PD. Genome-wide association studies have identified single nucleotide polymorphisms in the α-synuclein gene that predispose to PD [5]. Importantly, three different point mutations in the α-synuclein gene lead to rare autosomal dominant forms of PD, featuring Lewy pathology [6]. In other rare cases, the α-synuclein gene is duplicated or triplicated, leading to inherited neurological disorders that exhibit some parkinsonian features [7].

Taken together, the evidence that misfolded α-synuclein is involved in PD pathogenesis is overwhelming [6]. As mentioned above, however, it is unclear whether the α-synuclein aggregates are toxic or if they represent an attempt by the cells to disarm other potentially more dangerous oligomers or protofibrils. Notwithstanding uncertainties regarding how α-synuclein contributes to cell death in PD, strategies to either reduce the basal levels of α-synuclein or to inhibit the process of aggregation itself are being explored. One suggested strategy is to reduce α-synuclein levels by interfering with factors controlling its transcription [8], or to alternatively reduce its levels post-transcriptionally through the use of RNAi approaches. The latter principle has been tested in human cell culture [9], as well as in vivo in rodents [10] and monkeys [11]. One potential drawback with dramatically reducing α-synuclein levels is that it could potentially damage cells. Relatively little is known about the normal functional role of α-synuclein in neurons and whether markedly reducing the levels of the protein will lead to neuronal dysfunction. Although α-synuclein was recently suggested to play a role in synaptic transmission by interacting with the SNARE complex [12], α-synuclein knock-out mice exhibit no marked behavioral deficits, only mild changes in neurotransmission, a reduction in the rapidly recycling pool of synaptic vesicles and a reduced susceptibility to neurotoxins [6,13]. This must be interpreted with caution because when α-synuclein is already absent during development, other molecules (e.g., other synuclein family members) may adopt α-synuclein functions. Interestingly, recently, contrary to other reports, RNAi-mediated knock down of α-synuclein was reported to cause nigral cell death [14], instead of neuroprotection.

Reducing α-synuclein levels is not the only therapeutic approach directed at the protein. Therapies aimed at dissolving aggregates or preventing the assembly of aggregates with small peptides or molecules are obvious approaches [15]. Other novel approaches include attempts to improve the endogenous cellular defenses against misfolded proteins, for example, by increasing expression of cellular chaperones or augmenting autophagy [16], resulting in less accumulation of aggregated α-synuclein.

A third novel therapeutic approach targeting α-synuclein is coupled with the hypothesis that PD might involve a prion-like pathogenesis. Recently, Lewy bodies were reported to appear in embryonic neurons transplanted into PD patients, one decade or more after the surgery [17,18]. The findings suggested that misfolded α-synuclein might have transferred from the host brain to the grafted neurons and acted as seeds that recruited α-synuclein from the recipient cells [19], leading to the formation of Lewy bodies in a small proportion (typically 2–5%) of the grafted neurons. Although other molecular mechanisms might play a role (for a discussion, see [19]), the past 2 years have seen several experimental studies supporting a prion-like hypothesis. Thus, α-synuclein can transfer from one culture to another [20–23], and once inside the new host neuron, the imported α-synuclein can oligomerize with α-synuclein from the host cell as a first step towards the formation of Lewy aggregates [22]. The entry of extracellular α-synuclein into cultured cells relies on endocytosis [20,22] and uptake of exosomes released from neighboring cells [24]. Experiments using specifically designed laboratory techniques to promote the entry of large quantities of misfolded α-synuclein into cultured neurons have even demonstrated the formation of large Lewy-body-like aggregates [25]. Importantly, α-synuclein can even transmit from the host brain into murine grafted cells when implanted into transgenic mice overexpressing human α-synuclein [22,23].

The obvious question that arises is: how are these findings relevant to the progression of neuropathology in the brains of PD patients who have not been grafted? Interestingly, based on a series of post-mortem brain examinations, Braak and colleagues suggested that Lewy pathology progressively spreads through six well-defined neuropathology stages [26]. The first of these two stages do not yet involve Lewy pathology in the substantia nigra and represent a ‘pre-motor’ stage of the disorder. Individuals with these putatively early stages of neuropathology have been suggested to be those who display non-motor symptoms, such as olfactory dysfunction, constipation and rapid eye movement sleep behavior disorders and who would have gone on to develop the motor symptoms of PD 5–10 years later [27]. Braak and coworkers have proposed a ‘dual-hit’ hypothesis, suggesting that α-synuclein misfolding is triggered in olfactory structures and in enteric nerves, originating in the dorsal motor nucleus of the vagal nerve and innervating the wall of the gut [28]. The appearance of aggregated α-synuclein then slowly spreads to other brain regions in a topographically stereotypic fashion that has been suggested to follow long unmyelinated pathways. This continuous spreading of Lewy pathology could then contribute to the development of many of the non-motor symptoms that emerge with advancing disease, for example, depression, cognitive dysfunction and hallucinations. Braak and colleagues proposed that, for example, a neurotropic virus could be the culprit, spreading from neuron to neuron and causing α-synuclein aggregation, but they provided no supporting data [28]. Today, knowing that misfolded α-synuclein can transfer from one neuron to another, and the seed misfolding in the recipient cell, it is tempting to suggest that it is intercellular transmission of α-synuclein itself that underlies spreading of Lewy pathology in the PD brain [29]. Therefore, new therapeutic strategies targeting intercellular transfer of α-synuclein, either by inhibiting the release of the protein into the extracellular space or inhibiting its uptake in neighboring neurons, might prove valuable. Experimental immunization therapy, with antibodies directed against α-synuclein, can retard disease development in a transgenic mouse model of PD overexpressing α-synuclein [30]. One effect of the antibodies could be to bind to α-synuclein when it is in the extracellular space and prevent it from being taken up by neighboring cells.

Although much evidence suggests that α-synuclein is an interesting therapeutic target in PD, future clinical trials need good biomarkers that reflect α-synuclein misfolding in the brain [2]. Future development of ligands that can bind to misfolded α-synuclein and be imaged using PET may allow detection of Lewy pathology, even before the onset of motor symptoms in PD. Such a surrogate marker of progression would be an invaluable tool for clinical trials aiming to achieve disease modification in PD.

Financial & competing interests disclosure

Both authors are active in the Strong Research Environment Multipark (Multidisciplinary research in Parkinson’s disease at Lund and Gothenburg Universities). Patrik Brundin is a paid consultant for Neuronova AB, H Lundbeck A/S and TEVA Pharmaceuticals. He is a cofounder of the biotechnology companies ParkCell AB, Acousort AB and Neurprotex AB. Roger Olsson is a fulltime consultant at ACADIA Pharmaceuticals. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.