Could Mini Antibodies Block Parkinson's Disease Progression?
Most readers are probably aware of the implication of the protein alpha-synuclein (aSyn) in Parkinson’s disease (PD). In pathological conditions, this naturally unfolded protein tends to form aggregates within neurons and contributes to the cellular dysfunction and degeneration. Therapeutic strategies aiming to reduce the amount of aSyn in the brain, or inhibit its aggregation, could therefore be beneficial to slow the disease progression. Immunotherapy is an interesting tool in this regard because antibodies can recognize specific proteins (or protein species) and neutralize them in various ways. Immunotherapy can be active or passive. Vaccination corresponds to active immunization and involves the stimulation of the immune system. Passive immunotherapy refers to the administration of exogenous antibodies and is often the preferred approach in this context for safety reasons. However, systemic administration of full-length antibodies (IgGs) has certain disadvantages including poor blood-brain barrier penetration, risks of inflammation and repetitive dosage. The recent interruption of two advanced clinical trials of monoclonal antibodies against aSyn for Parkinson’s disease may in part reflect these difficulties.
To circumvent these challenges, we developed a combined immunotherapy and gene therapy approach that allows the permanent brain expression of small single-chain antibodies called scFvs. scFvs – for single-chain variable fragments, are made by linking the variable region of the light and the heavy chain of an IgG with a flexible peptide linker (Figure 1).
Since they are made with the antigen-binding region of the IgG, scFvs retain the same binding specificity but are 6 times smaller, thus having a better diffusion in tissues. They are also devoid of a Fc region, which interacts with receptors on immune cells. This makes scFvs less immunogenic than IgGs. Their small size also allows viral vector encapsulation for intracellular delivery. In our case, the AAVCap-B10 was used because of its ability to cross the blood-brain barrier in adult mice and marmosets (Goertsen et al., Nature Neuroscience, 2022). We generated one scFv that binds to all forms of aSyn, and one that binds strictly to phosphorylated aSyn, a species that is specifically found in pathological aggregates. Our AAV-scFvs were injected intravenously in a mouse model of synucleinopathy that was previously described (Luk et al, Journal of Experimental Medicine, 2012). Briefly, transgenic mice overexpressing a mutated human aSyn (M83 line) were injected in the dorsal striatum with aSyn preformed fibrils (PFF). In the mice that received an unspecific scFv, motor symptoms and paralysis occurred between 8 and 12 weeks after PFF injection, and phosphorylated aSyn aggregates were vastly present in their brain and spinal cord. On the contrary, the mice that were treated with our scFvs remained healthy for the duration of the study, and their brain pathology was significantly decreased. Preliminary data suggests that the scFvs can prevent the entry of aSyn fibrils inside neurons, and inhibits fibrillization of aSyn monomers. These two mechanisms would explain the significant decrease in aSyn pathology within the brain of the treated mice.
To our knowledge, our study is the first to demonstrate the potential of the systemic delivery of virally encoded scFvs in an aggressive synucleinopathy mouse model. In the future, we wish to validate our results in a non-human primate model, to eventually bring this approach to clinical trials. Generally speaking, a better understanding of the roles of each aSyn species (oligomers, fibrils, phosphorylated, truncated, etc.) in cellular toxicity and disease progression would aid the development of more effective immunotherapeutic tools. Overall, our data strongly supports the development of this type of treatment options for Parkinson’s disease.
Anne-Marie Castonguay, Ph.D. is a researcher at Martin Lévesque’s laboratory, CERVO Brain Research Center, Université Laval, Québec, Canada. She was an poster presenter at the WPC 2023 in Barcelona.
Ideas and opinions expressed in this post reflect that of the author solely. They do not reflect the opinions or positions of the World Parkinson Coalition®