Parkinson's Disease: Therapeutic Targeting of Toll-Like Receptor

Background: Microglia play opposing roles in Parkinson's disease (PD) pathogenesis: in early stage they exert neuroprotection via alpha-synuclein (α-syn) phagocytosis, while as the disease progresses, they fail in α-syn clearance and induce neuroinflammation and neurodegeneration. Both α-syn clearance and inflammation are TLR-mediated: specific TLRs promote α-syn clearance in the early stage, while the same TLRs chronically activated by accumulated α-syn initiate a pro-inflammatory cascade leading to degenerative changes in neurons. These emerging knowledges allow to select specific immunomodulators, which, acting as agonists or antagonists to specific TLRs, can lead to the stop of the neuroinflammatory cascade. Among the different immunomodulators TLR-targeting, there are specific molecules, including neutralizing nanobodies or smallest molecules, able to cross the blood brain barrier (BBB). These immunomodulators could represent a promising approach, since they exhibit high affinity to TLRs, can be given to patients as an inhaled drug, a skin patch or a pill. Recently, small interfering RNA-selective compounds have proved significant inpredictive models for new therapeutic approaches. In addition, small interfering RNA-selective compounds, studied in predictive modeling, could successfully be used in PD therapy. These TLR-targeting drugs have shown fewer side effects and lower or no toxicity compared to drugs with anti-inflammatory effects commonly used in PD treatment. Results: Agonists There are currently many TLR agonists in clinical trials either alone or in combination with specific antigens to treat cancer, allergies, and viral infections. Among these: TLR2, TLR3 and TLR4 Agonists MPLA (TLR4) licensed for use as a vaccine adjuvant. It acts stimulating TRAM/TRIF transduction pathway, and deactivating Mal/MyD88 signalling, therefore acting as a partial agonist of the receptor; AMPLIGEN, (TLR3) which is a synthetic mismatched polyI:polyC dsRNA (polyI:poly C12U). It is in Phase III for chronic fatigue syndrome and in Phase II for HIV and cancer treatment. This compound induces the up-regulation of various proteins in CFS, including interferon; PolyI:C pretreatment (TLR3) in simulated cerebral ischemia models has been shown to exert neuroprotective and antiinflammatory effects, even though it also shows side effects;   TLR7, TLR8 and TLR9 Agonists IMIQUIMOD and GARDIQUIMOD, which acta s selective TLR7 ligands, and autophagy inducers; IMO-2125, in clinical assessment to prevent and treat α-syn deposition (TLR9). Antagonists Amplification of neuroinflammation is also due to the upregulation of TLRs in response to extracellular α-syn. It has been shown that α-syn, acting as a DAMP for microglia, increases the expression of TLR1,-2,-3 and 7. Some of the several classes of TLR antagonists developed could be used to limit specific or excessive inflammatory response in the late stages of PD.  TLR2 and TLR4 Antagonists IBUDILAST, which is considered as a phosphodiesterase-4 inhibitor, capable of antagonizing TLR4 activity, could suppress production of proinflammatory cytokines, such as TNF-alpha and IL-6, and induce the anti-inflammatory cytokine IL-10. The anti-inflammatory properties of this compound have been shown to lead to inhibition of glial cell activation and, subsequently, to attenuation of neuroinflammation, even if the exact mechanism of IL-10 induction is unknown; CPN10 (chaperonin 10) molecule, able to inhibit TLR4 downstream signalling cascade, is in Phase II clinical trial for RA treatment, psoriasis and MS. Further studies indicate that Cpn10 inhibits TLR4-mediated production of Nf-kB, as well as TNF-alpha and IL-6. Discussion: The main feature of PD is rappresented by the progressive decline of dopaminergic neurons in substantia nigra pars compacta (SNpc) and the presence of eosinophilic intracellular proteinaceous and lipidic inclusions, called "Lewy Bodies"(LBs), in surviving neurons. The basic components of LBs are misfolded alpha-synuclein (α-syn), neurofilament proteins, and ubiquitin. Recent evidence has shown that misfolded α-syn directly activates microglia, triggering the production of proinflammatory molecules and oxidative stress resulting in neurodegeneration. Microglia are considered "the brain macrophages", able to shift from a surveillance mode to a reactive mode, thus acting as immune effectors cells, producing proinflammatory cytokines. Microglial activation is, in part, induced by Pattern Recognition Receptors (PRRs), such as Toll-like Receptors (TLRs), that are expressed in immune cells, including microglia and astrocytes. TLRs are capable of detecting microbial products Pathogen Associated Molecular Patterns (PAMPs) or endogenous molecules Damage Associated Molecular Patterns (DAMPs), initiating the inflammatory response and contributing to oxidative stress, mediated by the release of cytokines, nitric oxide (NO) and other reactive oxygen species (ROS), which, in turn, may adversely impact on nearby neurons.. All of this may adversely impact on nearby neurons. In PD, TLR-dependent microglial activation plays a neuroprotective role in the early stages, whereas it can play a detrimental, proinflammatory role, in the late stages. More specifically, in the early stages of PD, TLRs play a physiological role, in clearing α-sin, by phagocytosis. However, as the disease progresses, misfolded α-syn begins to accumulate to form aggregates, which prevent α-syn degradation and turnovee, through the impairment of microglial clearance and this also induces microglia-mediated production of proinflammatory citokines, NO and ROS, which may be toxic to neurons. Conclusions: In PD, both α-syn clearance and neuroinflammation are TLR-mediated, anti-inflammatory agents targeting TLR could have neuroprotective effects in the middle-late stages of the disease. Another winning point of TLR-targeting drugs is that they show fewer side effects and lower or no toxicity, compared with that of commonly used drugs in PD treatment. In conclusion, although there has not been a new drug approved for PD treatment in many years, current investigation regarding TLR targeting shows promising expectations. Further research is needed to finally reach the objective of blocking neuroinflammation and progression of the disease.