Parkinson’s disease (PD) is a chronic neurodegenerative disease affecting 7 to 10 million people worldwide, 1% above 60 years of age and up to 5% above 80. Patients’ quality of life is progressively impaired by motor symptoms, such as bradykinesia, rigidity, and resting tremor, and non-motor symptoms, such as autonomic dysregulation and mood and cognitive disturbances. Available options for PD patients are just symptomatic, as no treatments exist to prevent or delay disease progression, due to our still limited understanding of the events leading to neurodegeneration (Samii et al., 2004).
Research on PD pathogenesis focused until the end of last century on brain damage and dopaminergic neuron death, however in recent years increasing interest was raised by two novel and possibly interconnected areas of investigation which shed new light on peripheral factors contributing to PD pathogenesis: the peripheral immune system and the gut microbiota.
The idea that peripheral immunity might be involved in PD actually dates back to some decades ago, based on the observation of various immunological abnormalities which however could not be specifically associated to the disease itself, to the effects of anti-Parkinson drugs and/or to other factors (Kuhn et al., 1997). Indeed, although the first evidence of T lymphocytes infiltrating the substantia nigra of Parkinsonian brains was published in 1988 (McGeer et al., 1988), it was only more than 15 later that Howard E. Gendelman and co-workers at the University of Nebraska Medical Center, Omaha (NE), showed that modulation of peripheral T cells might be neuroprotective in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-intoxicated mice, an animal model recapitulating the major brain lesions occurring in PD (Benner et al., 2004).
Currently, established peripheral immune traits in PD include altered CD4+ T cell profile, with unbalanced Th1 lineage and increased production of IFN-γ and TNF-α (Kustrimovic et al., 2016 and 2018), altered CD8+ T cell profile, with increased activation and reduced senescence markers (Williams-Gray et al., 2018), a more rapid disease progression associated with a pro-inflammatory cytokine profile in the blood (Williams-Gray et al., 2016), and the ability of helper and cytotoxic T cell from PD patients to recognize as antigens some peptides derived from α-synuclein (α-syn), a key pathogenic protein in PD (Sulzer et al., 2017). Experiments showing T cell-induced neurotoxicity in induced pluripotent stem cell-derived midbrain neurons from PD patients (Sommer et al., 2018) provide now strong impulse to the thorough evaluation of T cell-targeted immunomodulatory treatments to cure PD (see e.g. Storelli et al., 2019).
On the other side, a large number of studies has been published in recent years describing peculiar differences in the gut microbiota of PD patients in comparison to healthy subjects (reviewed in Sampson, 2019). Alterations in the microbial community most consistently associated with PD include increased abundance of the Gram-negative family Enterobacteraceae as well as of the Gram-positive family Lactobacillaceae. Remarkably, it has been recently shown that suppression of the gut microbiota results in reduced neuroinflammation and neurodegeneration in different rodent models of PD (Sampson et al., 2016; Bisht et al., 2014), and in particular that in mice overexpressing α-syn, colonization with microbiota from patients with PD enhances physical impairments compared to microbiota from healthy subjects (Sampson et al., 2016), indicating modulation of the gut microbiota as an attractive opportunity for innovative PD therapeutics.
Researchers working at the University of Montréal and at the McGill University in Quebec, Canada, recently published a study linking intestinal infection, immunity and neurodegeneration, providing mechanistic evidence about how inflammatory processes starting in the gut may ultimately lead to brain damage (Matheoud et al., 2019). The main experiments have been performed in PINK1 KO mice, an animal model developed to assess the effects of PINK1 deficiency (Kitada et al., 2007). PINK1 is an acronym standing for PTEN-induced putative kinase 1, a protein belonging to the protein kinase superfamily and to the Ser/Thr protein kinase family, encoded by the PINK1 gene, localized in the PARK6 locus and responsible for autosomal recessive early‐onset Parkinson’s disease (PD) (Valente et al., 2004).
PINK1 has been shown to protect neurons from stress-induced mitochondrial dysfunction and subsequent apoptosis, an effect that is abrogated by inactivating mutations, thus providing first direct molecular link between mitochondria and PD (Valente et al., 2004). PINK1 phosphorylates parkin, a 465-residue E3 ubiquitin ligase which is critical for degradation of molecules in proteasomes or lysosomes, promoting its recruitment to depolarized mitochondria, eventually resulting in their autophagy (mitophagy) (Vives-Bauza et al., 2010).
In a first set of experiments, researchers tested the ability of Gram-positive and Gram-negative bacteria to induce mitochondrial antigen presentation (MitAP) in the mouse macrophage-like cell line RAW 264.7. Only Gram-negative bacteria resulted in MitAP, which was likely dependent on cell exposure to bacteria LPS and possibly unrelated to mitophagy.
Thereafter, wild-type and PINK1 KO mice were infected by oral gavage with Gram-negative bacteria. The two mouse strains showed no difference in time course and characteristics of infection as well as in the local and systemic inflammatory response were detected between the two strains, suggesting that loss of PINK1 does not affect immune defenses towards the infection. Both strains developed effective immunization as indicated by absence of gut colonization after subsequent rounds of superinfection. Quite interestingly, however, MitAP was increased only in PINK1 KO mice, which developed clonally selected CD8+ cytotoxic T cells specific for mitochondrial antigens, which were detected also in the brain between 2 and 4 weeks post infection. Remarkably, these T cells express the cytokine receptor CX3CR1, which specifically recognizes CX3CL1 (fractalkine), in turn expressed by neurons during inflammation.
In subsequent experiments, researchers showed that both LPS and IFN-γ induce MHC class I molecules on dopaminergic neurons, which makes cells vulnerable to CD8+ T cell-induced cytotoxicity, and that in co-culture experiments neurons from PINK1 KO mice stimulated mitochondrial antigen-specific CD8+ T cells, suggesting the occurrence of MitAP. CD8+ T cell-induced cytotoxicity was shown to be specific for dopaminergic neurons. Based on these results, researchers suggest that infection with Gram-negative bacteria in a genetically susceptible milieu like that provided by PINK1 KO mice results in clonal expansion of mitochondrial antigen-specific CD8+ T cells which then migrate to the brain and potentially attack dopaminergic neurons.
This possibility is supported by the development of motor impairment in infected PINK1 KO mice, which began four months after infection and was mostly reversed by administration of the dopamine precursor l-DOPA, as well as by the more than 40% reduced density of dopaminergic neuron axon terminals in both the dorsal and ventral striatum of these animals. Reduction of dopaminergic terminals was observed 6 months after the infection and recovered after 12 months accompanied by decrease of CD8+ T cell infiltration, indicating the involvement of autoimmune mechanisms in neuron damage.
The notion that Gram-negative bacteria in the gut may trigger signals eventually leading to damage of dopaminergic neurons in the brain actually is not completely new. Indeed, it was recently shown that in mice oral administration of the Gram-negative P. mirabilis, which belongs to the Enterobacteriaceae commonly increased in the feces of PD mice models, results in exacerbated striatal dopaminergic neuronal damage and motor deficits after suboptimal doses of the neurotoxin MPTP (Choi et al., 2018). Remarkably, P. mirabilis induced dopaminergic neuronal damage even in the absence of MPTP, and its LPS could also induce pathologic aggregation of α-synuclein in the brain as well as in the colon (Choi et al., 2018). Interestingly, α-synuclein aggregation in mice is induced also by E. coli, possibly through exposure to the extracellular bacterial amyloid protein curli (Chen et al., 2016). As a whole, it is suggested that LPS action may occur at different levels, both in the brain and in the gut, and may involve distinct mechanisms, as apparently not all Gram-negative bacteria elicit the same detrimental effects in the various animal models. Moreover, other bacterial products may exert detrimental effects, as in the case of E. coli (Chen et al., 2016).
Nonetheless, dysbiosis associated to PD is not limited to increased abundance of Gram-negative families, but includes also Gram-positive families such as the Lactobacillaceae (Sampson, 2019), a bacterial strain inducing Th1-type systemic immune responses which may possibly resemble the complex complex Th1 bias recently described in the peripheral blood of PD patients (Kustrimovic et al., 2018). Gut microbiota seems therefore to be able to affect the processes leading to PD pathogenesis acting through different and possibly complementary mechanisms. It must be considered in this regard that the dual-hit hypothesis of PD pathogenesis postulates the origin of α-syn aggregation in the enteric nervous system (Hawkes et al., 2019), and that rodent models have been recently developed and characterized where prion-like spread of pathologic α-syn in brain occurs after injection of pathological α-syn preformed fibrils into the duodenal and pyloric muscularis layer, eventually resulting in loss of dopaminergic neurons and motor and non-motor symptoms resembling human PD pathology (Kim et al., 2019).
We recently briefly revised existing evidence linking PD with the vermiform appendix (and appendectomy), a topic which is attracting increasing interest since in the gastrointestinal tract the appendix is the region richest in α-syn (Gray et al., 2014), proposing that, in the context of a Th1-biased environment, antigenic recognition of aggregated α-syn by T cells in the appendix, as well as possibly in other in gut-associated lymphoid tissues, may trigger PD pathogenesis (Cosentino et al., 2019).
In summary, according to presently available evidence the key points of the gut-brain connection in PD may be summarized as follows:
(i) the gastrointestinal tract and in particular the appendix are rich in α-syn, and many local factors including the gut microbiota and inflammatory processes may promote pathological α-syn aggregation;
(ii) pathological α-syn may in turn:
(iia) spread from the gut to brain via the vagus nerve in a prion-like fashion, directly resulting in neuroinflammation and neuronal damage;
(iib) undergo antigenic recognition by T cells homing into the gut wall, driving detrimental T cell responses;
(iii) activated α-syn-specific T cells could thereafter home into the brain, promoting neuroinflammation and directly attacking dopaminergic neurons.
The emerging role of peripheral immunity and of the gut microbiota in the pathogenesis and progression of PD provides unprecedented opportunities for the development of novel disease-modifying therapeutics. Would the contribution of (auto)immune mechanisms in PD be confirmed, candidate drugs for PD could include drugs already validated for diseases such as psoriasis, rheumatoid arthritis and inflammatory bowel disease (Storelli et al., 2019) as well as less conventional approaches for the modulation of immunity (Cosentino et al., 2017). On the other side, preclinical evidence suggests that modulation of the gut microbiota may provide clinical benefit in PD, for example by means of fecal microbial transplantation (Sun et al., 2018) or using probiotics (Magistrelli et al., 2019).
MarcoCosentino1, Cristoforo Comi1,2, Franca Marino1 , (1) Center of Research in Medical Pharmacology, University of Insubria, Varese, VA, Italy, and (2) Movement Disorders Centre, Neurology Unit, Department of Translational Medicine, University of Piemonte Orientale, Novara, Italy; Correspondence: Marco Cosentino, Phone: +39 0332 217410, Fax: +39 0332 217409; email: firstname.lastname@example.org
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