Role of α- and β-Synucleins in the Axonal Pathology of Parkinson's Disease and Related Synucleinopathies

Alpha-synucleinopathy reduces NMNAT3 protein levels and neurite formation that can be rescued by targeting the NAD+ pathway

Parkinson's disease is characterized by the deposition of α-synuclein, which leads to synaptic dysfunction, the loss of neuronal connections and ultimately progressive neurodegeneration. Despite extensive research into Parkinson's disease pathogenesis, the mechanisms underlying α-synuclein-mediated synaptopathy have remained elusive. Several lines of evidence suggest that altered nicotinamide adenine dinucleotide (NAD+) metabolism might be causally related to synucleinopathies, including Parkinson's disease. NAD+ metabolism is central to the maintenance of synaptic structure and function. Its synthesis is mediated by nicotinamide mononucleotide adenylyltransferases (NMNATs), but their role in Parkinson's disease is not known. Here we report significantly decreased levels of NMNAT3 protein in the caudate nucleus of patients who have died with Parkinson's disease, which inversely correlated with the amount of monomeric α-synuclein. The detected alterations were specific and significant as the expression levels of NMNAT1, NMNAT2 and sterile alpha and TIR motif containing 1 (SARM1) were not significantly different in Parkinson's disease patients compared to controls. To test the functional significance of these findings, we ectopically expressed wild-type α-synuclein in retinoic acid-differentiated dopaminergic SH-SY5Y cells that resulted in decreased levels of NMNAT3 protein plus a neurite pathology, which could be rescued by FK866, an inhibitor of nicotinamide phosphoribosyltransferase that acts as a key enzyme in the regulation of NAD+ synthesis. Our results establish, for the first time, NMNAT3 alterations in Parkinson's disease and demonstrate in human cells that this phenotype together with neurite pathology is causally related to α-synucleinopathy. These findings identify alterations in the NAD+ biosynthetic pathway as a pathogenic mechanism underlying α-synuclein-mediated synaptopathy.

Alterations in Sub-Axonal Architecture Between Normal Aging and Parkinson's Diseased Human Brains Using Label-Free Cryogenic X-ray Nanotomography

Gaining insight to pathologically relevant processes in continuous volumes of unstained brain tissue is important for a better understanding of neurological diseases. Many pathological processes in neurodegenerative disorders affect myelinated axons, which are a critical part of the neuronal circuitry. Cryo ptychographic X-ray computed tomography in the multi-keV energy range is an emerging technology providing phase contrast at high sensitivity, allowing label-free and non-destructive three dimensional imaging of large continuous volumes of tissue, currently spanning up to 400,000 μm3. This aspect makes the technique especially attractive for imaging complex biological material, especially neuronal tissues, in combination with downstream optical or electron microscopy techniques. A further advantage is that dehydration, additional contrast staining, and destructive sectioning/milling are not required for imaging. We have developed a pipeline for cryo ptychographic X-ray tomography of relatively large, hydrated and unstained biological tissue volumes beyond what is typical for the X-ray imaging, using human brain tissue and combining the technique with complementary methods. We present four imaged volumes of a Parkinson's diseased human brain and five volumes from a non-diseased control human brain using cryo ptychographic X-ray tomography. In both cases, we distinguish neuromelanin-containing neurons, lipid and melanic pigment, blood vessels and red blood cells, and nuclei of other brain cells. In the diseased sample, we observed several swellings containing dense granular material resembling clustered vesicles between the myelin sheaths arising from the cytoplasm of the parent oligodendrocyte, rather than the axoplasm. We further investigated the pathological relevance of such swollen axons in adjacent tissue sections by immunofluorescence microscopy for phosphorylated alpha-synuclein combined with multispectral imaging. Since cryo ptychographic X-ray tomography is non-destructive, the large dataset volumes were used to guide further investigation of such swollen axons by correlative electron microscopy and immunogold labeling post X-ray imaging, a possibility demonstrated for the first time. Interestingly, we find that protein antigenicity and ultrastructure of the tissue are preserved after the X-ray measurement. As many pathological processes in neurodegeneration affect myelinated axons, our work sets an unprecedented foundation for studies addressing axonal integrity and disease-related changes in unstained brain tissues.

Alpha-Synuclein Physiology and Pathology: A Perspective on Cellular Structures and Organelles

Alpha-synuclein (α-syn) is localized in cellular organelles of most neurons, but many of its physiological functions are only partially understood. α-syn accumulation is associated with Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy as well as other synucleinopathies; however, the exact pathomechanisms that underlie these neurodegenerative diseases remain elusive. In this review, we describe what is known about α-syn function and pathophysiological changes in different cellular structures and organelles, including what is known about its behavior as a prion-like protein. We summarize current knowledge of α-syn and its pathological forms, covering its effect on each organelle, including aggregation and toxicity in different model systems, with special interest on the mitochondria due to its relevance during the apoptotic process of dopaminergic neurons. Moreover, we explore the effect that α-syn exerts by interacting with chromatin remodeling proteins that add or remove histone marks, up-regulate its own expression, and resume the impairment that α-syn induces in vesicular traffic by interacting with the endoplasmic reticulum. We then recapitulate the events that lead to Golgi apparatus fragmentation, caused by the presence of α-syn. Finally, we report the recent findings about the accumulation of α-syn, indirectly produced by the endolysosomal system. In conclusion, many important steps into the understanding of α-syn have been made using in vivo and in vitro models; however, the time is right to start integrating observational studies with mechanistic models of α-syn interactions, in order to look at a more complete picture of the pathophysiological processes underlying α-synucleinopathies.

The neurological effects of ghrelin in brain diseases: Beyond metabolic functions

Ghrelin, a peptide released by the stomach that plays a major role in regulating energy metabolism, has recently been shown to have effects on neurobiological behaviors. Ghrelin enhances neuronal survival by reducing apoptosis, alleviating inflammation and oxidative stress, and accordingly improving mitochondrial function. Ghrelin also stimulates the proliferation, differentiation and migration of neural stem/progenitor cells (NS/PCs). Additionally, the ghrelin is benefit for the recovery of memory, mood and cognitive dysfunction after stroke or traumatic brain injury. Because of its neuroprotective and neurogenic roles, ghrelin may be used as a therapeutic agent in the brain to combat neurodegenerative disease. In this review, we highlight the pre-clinical evidence and the proposed mechanisms underlying the role of ghrelin in physiological and pathological brain function.

Pathological and clinical aspects of alpha/beta synuclein in Parkinson's disease and related disorders

Parkinson's disease (PD) and related synucleinopathies are characterized by extensive neuronal cell loss, which is potentially triggered by α-synuclein misfolding and aggregation. Therefore it is reasonable to suggest that treatments targeting α-synuclein could reduce its levels and toxicity, rescue neuronal cells and halt the neurodegeneration process. Several approaches to decrease α-synuclein levels were employed thus far, mainly by using β-synuclein, another protein from the same family, or immunotherapies. These treatments demonstrated some positive results in preclinical studies, which may pave the road to the development of new promising disease-modifying therapies (DMTs). This approach should be further examined in preclinical and clinical settings, together with a clear process in order to advance candidates, enable the ability to define when there are target engagements and to detect what is a meaningful pharmacological response, and how it will be translated in clinical efficacy.

Autophagy and Alpha-Synuclein: Relevance to Parkinson's Disease and Related Synucleopathies

Evidence from human postmortem material, transgenic mice, and cellular/animal models of PD link alpha-synuclein accumulation to alterations in the autophagy lysosomal pathway. Conversely, alpha-synuclein mutations related to PD pathogenesis, as well as post-translational modifications of the wild-type protein, result in the generation of aberrant species that may impair further the function of the autophagy lysosomal pathway, thus generating a vicious cycle leading to neuronal death. Moreover, PD-linked mutations in lysosomal-related genes, such as glucocerebrosidase, have been also shown to contribute to alpha-synuclein accumulation and related toxicity, indicating that lysosomal dysfunction may, in part, account for the neurodegeneration observed in synucleinopathies. In the current review, we summarize findings related to the inter-relationship between alpha-synuclein and lysosomal proteolytic pathways, focusing especially on recent experimental strategies based on the manipulation of the autophagy lysosomal pathway to counteract alpha-synuclein-mediated neurotoxicity in vivo. Pinpointing the factors that regulate alpha-synuclein association to the lysosome may represent potential targets for therapeutic interventions in PD and related synucleinopathies.

Yeast reveals similar molecular mechanisms underlying alpha- and beta-synuclein toxicity

Synucleins belong to a family of intrinsically unstructured proteins that includes alpha-synuclein (aSyn), beta-synuclein (bSyn) and gamma-synuclein (gSyn). aSyn is the most studied member of the synuclein family due to its central role in genetic and sporadic forms of Parkinson's disease and other neurodegenerative disorders known as synucleionopathies. In contrast, bSyn and gSyn have been less studied, but recent reports also suggest that, unexpectedly, these proteins may also cause neurotoxicity. Here, we explored the yeast toolbox to investigate the cellular effects of bSyn and gSyn. We found that bSyn is toxic and forms cytosolic inclusions that are similar to those formed by aSyn. Moreover, we found that bSyn shares similar toxicity mechanisms with aSyn, including vesicular trafficking impairment and induction of oxidative stress. We demonstrate that co-expression of aSyn and bSyn exacerbates cytotoxicity, due to increased dosage of toxic synuclein forms, and that they are able to form heterodimers in both yeast and in human cells. In contrast, gSyn is not toxic and does not form inclusions in yeast cells. Altogether, our findings shed light into the question of whether bSyn can exert toxic effects and confirms the occurrence of aSyn/bSyn heterodimers, opening novel perspectives for the development of novel strategies for therapeutic intervention in synucleinopathies.