Beyond Pathology: α‐Synuclein Homeostasis and Three Principles to Guide Research

A 56-year-old man presents with parkinsonism. Brain imaging reveals calcium deposits in the basal ganglia. His clinician, concluding that the calcifications triggered his symptoms, prescribes calcium chelation therapy. This faux clinical vignette illustrates a fallacy in inductive reasoning. While the observations (brain calcifications, parkinsonism) are accurate, the omission of additional evidence leads to a flawed causal inference: calcifications → parkinsonism. Most clinicians would recognize these calcifications as markers of an underlying calcium metabolism disorder, hypoparathyroidism, expanding the logical framework to hypoparathyroidism → hypocalcemia → [calcifications] → parkinsonism, and prescribe calcium and calcitriol supplementation (or PTH replacement) to increase calcium, not to lower it. Fictitious clinician: Treating a disorder characterized by excessive calcification with more calcium? Wouldn't that worsen the disorder? The dominant "proteinopathy" model of α-synuclein (αSyn) in Parkinson's disease (PD) follows a similarly flawed logic: Pathological αSyn → PD. As a result, research efforts have focused on eliminating the "accumulation" of pathological αSyn,1 with animal models extensively used to evaluate how αSyn is "toxic."2 Increasing monomeric αSyn has not been in the research radar, not even as what might be expected from anti-aggregation interventions in the pipeline. Today's clinician-scientists, akin to the fictitious clinician in our vignette, advocate for "pathology chelation," an agent to bind and remove the pathology. If it fails, they assume that chelation wasn't aggressive enough or did not target all "toxic αSyn species"3—and a better chelator is pursued. (In case it is unclear: the reports of negative clinical trials of interventions directly or indirectly lowering αSyn, such as nilotinib, venglustat, minzasolmin, or cinpanemab, have been explained as artifacts of inadequate dosing, insufficient target engagement, or treatment initiated too late. We keep on searching for better anti-αSyn molecules, improved trials, earlier patients, without questioning the relevance of lowering αSyn).4-6 Lewy pathology does not arise de novo. It represents a biophysical transition of αSyn from their monomeric (or "intrinsically disordered" native state, which exhibits various structures depending on its interactions) to their aggregated, insoluble, cross-beta pleated amyloid state (Lewy pathology).7 Here we use "monomeric" to denote any functional configuration of αSyn. For instance, the physiological regulation of mitochondrial bioenergetics depends on the ability of monomeric αSyn to interact with ATP synthase, which is lost when it transitions into pathological αSyn, resulting in oxidative stress.8 Monomeric and oligomeric αSyn expression increases in neurons of the gastrointestinal tract upon a viral infection (norovirus infection in the duodenum of children) to facilitate a homeostatic immune response.9 The resulting increase in pathological αSyn is a marker of the virus-induced acute and chronic inflammation in the intestinal wall, not its vehicle of toxicity. The transition of monomeric to pathological αSyn can also be catalyzed by other infections, best documented with Escherichia coli.10 It is conceivable that if a particular pathogenic exposure continues unabated, and monomeric αSyn levels fail to keep up, its consumption into pathological αSyn deplete the monomeric αSyn below a compensation threshold and disease begins, a mechanism experimentally demonstrated using SARS-CoV-2 and HSV-1 as exposures.11 Thus, an alternative sequence of events is worth considering: Pathogenic exposure → ↑ Monomeric αSyn → [Pathological αSyn] → ↓ Monomeric αSyn → PD. Pathological αSyn represents a consequence of the monomeric αSyn response to a pathogenic exposure—a marker of protein reactivity, not a maker of disease (Fig. 1). Fictitious clinician: But the bulk of research overwhelmingly supports the conclusion that pathological αSyn is toxic! αSyn is widely distributed throughout in the body, predominantly in neurons and oligodendrocytes, but also in skin melanocytes, colon, liver, kidney, testis, bone marrow, lymph nodes, and even in immune cells and red blood cells.12 Given that αSyn has been evolutionarily redundant and preserved across the animal kingdom since prehistoric genomes,13 isn't it more likely that the misfolding of monomeric αSyn into pathological αSyn is a homeostatic reaction against a pathogenic exposure rather than a mechanism to drive disease? The preservation of αSyn homeostasis might require a certain level of functioning protein. Protein dyshomeostasis occurs when the pool of soluble, functional monomers is reduced as they transition into aggregated, insoluble proteins. Early research indicated that there was no apoptotic activity in the substantia nigra in the presence of αSyn and much of it in its absence, suggesting that the physiological expression of αSyn "played a role in protection or restoration of neurons which survive."14 Only one human autopsy study has quantified both the monomeric and pathological forms of αSyn, showing they are inversely correlated: the more the pathological αSyn, the less the monomeric αSyn.15 Similarly, one animal study using the αSyn preformed fibrils (PFF) mouse model quantified both monomeric and pathological αSyn, finding that brain regions with high levels of pathological αSyn had low levels of monomeric αSyn.16 If the brain cannot produce enough of this protein in response to the toxic, biological, or infectious challenges that force their soluble-to-insoluble, monomeric-to-pathological phase transition, levels of the normal peptide drop below a compensation threshold. In PD, total αSyn levels in cerebrospinal fluid (CSF) correlate with disease progression: the lower the levels, the faster the progression to key milestones.17 Conversely, pathological αSyn (Lewy bodies and Lewy neurites) does not correlate with neurodegeneration or clinical severity. Some studies even suggest the opposite: in autopsy-confirmed PD cohorts, the more Lewy bodies in the substantia nigra, the more nigral neurons18; also the more amyloid and tau pathologies, the longer people lived.19 This explains why a positive αSyn-seed amplification assay (αSyn-SAA), which identifies pathological αSyn, does not correlate with disease severity, track progression, or distinguish among various PD subtypes.20 We performed a simple PubMed search to illustrate the imbalance in αSyn research. Since the term was introduced in 1999, "synucleinopathy" has appeared in 1841 papers, whereas "synuclein homeostasis" has appeared in only 30 and "synuclein dyshomeostasis" in just one. Despite the nearly universal focus on pathological α-synuclein, only 23 publications (1.2%) have acknowledged monomeric α-synuclein, only two mentioned a tetrameric form, and just six (0.3%) quantified it. By contrast, "synuclein oligomers" appeared in the titles of 208 articles, 103 of which included the adjective "toxic," often without actually testing for toxicity. This reflects the prevailing assumption that pathological α-synuclein represents "late-stage aggregation."21 Yet oligomers are not a stable "early-stage" species, but rather a short-lived intermediate in the transition from monomers to fibrils.22 The extensive body of work attempting to prove oligomer toxicity overlooks the fact that their very existence depends on a progressively shrinking pool of αSyn monomers. Moreover, for oligomers to appear "toxic," a particular model, preparation condition, and assay sensitivity are required.23 One could therefore conclude that if "oligomeric toxicity" is only demonstrable under narrow experimental conditions, then oligomers are unlikely to be inherently toxic. One additional problem of logic. The growing literature on "toxic oligomers" exposes a deep internal inconsistency in the claim that pathological αSyn is harmful. Lewy bodies are portrayed as both "cytotoxic"24 and protective by sequestering αSyn oligomers into fibrils to reduce toxicity.25 By this reasoning, a greater accumulation of pathological αSyn should be regarded as beneficial rather than harmful. The large bulk of basic science and clinical research in PD has for too long focused on pathological αSyn as both a marker and maker of disease, illustrated in many diagrams as "accumulation." What is missing is the other side—monomeric αSyn depletion. Occam's razor dictates that what we gain matters in proliferative disorders and what we lose matters in degenerative ones. Why has PD been defined by what is gained rather than what is lost? Pathological αSyn "goes up" only in relative terms: it represents the tombstones of monomeric αSyn, casualties of the perennial war against aging, which is most often won. Indeed, most pathology-filled brains in brain banks come from individuals without a history of neurological symptoms. For movement disorders neurologists, the central issue is not merely theoretical. How we frame αSyn shapes the way we interpret clinical research, communicate with patients, and decide whether to lend our sites and patients to experimental therapies. When "removing pathology" is treated as an unquestioned good, negative trials are explained away rather than taken as examinations of the hypothesis. Clinicians, as the intermediaries between research and patients, should be clear-eyed: when speaking with patients and caregivers, we must acknowledge that this dominant idea of our field may not be true: the idea that once a normal protein such as monomeric αSyn transitions into pathological αSyn in response to a pathogenic exposure, it becomes toxic. When considering whether to participate in new clinical trials, site investigators should ask not only whether the molecule is promising but whether the hypothesis it tests is sound. Otherwise, we risk perpetuating research cycles that test better drugs, earlier patients, or bigger cohorts, while leaving unaddressed the foundational question: whether αSyn depletion rather than accumulation drives disease. (1) Research project: A. Conception, B. Organization, C. Execution; (2) Statistical Analysis: A. Design, B. Execution, C. Review and Critique; (3) Manuscript: A. Writing of the first draft, B. Review and Critique. A.J.E.: 3A, 3B. A.J.L.: 3B. Ethical Compliance Statement: No IRB permission required: this is a Viewpoint. Declaration of patient consent: Informed patient consent was not necessary for this work. Journal's Ethical Publication Guidelines. We confirm that we have read the Journal's position on issues involved in ethical publication and affirm that this work is consistent with those guidelines. Funding Sources and Conflicts of Interest: No specific funding was received for this work. The authors declare that there are no conflicts of interest relevant to this work. Financial Disclosures for the Previous 12 Months: AJE has received grant support from the NIH and the Michael J. Fox Foundation; personal compensation as a consultant/scientific advisory board member for Mitsubishi Tanabe Pharma America (formerly, Neuroderm), Amneal, Acorda, Abbvie, Bial, Supernus (formerly, USWorldMeds), NeuroDiagnostics, Inc. (SYNAPS Dx), Intrance Medical Systems, Inc., Merz, Praxis Precision Medicines, Citrus Health, and Herantis Pharma; Data Safety Monitoring Board (chair) of AskBio; and publishing royalties from Lippincott Williams & Wilkins, Cambridge University Press, and Springer. He is co-inventor of the patent "Compositions and methods for treatment and/or prophylaxis of proteinopathies." He cofounded REGAIN Therapeutics to fund preclinical studies but relinquished the right to any personal income from future treatments. He serves on the editorial boards of the Journal of Parkinson's Disease, Journal of Alzheimer's Disease, European Journal of Neurology, Movement Disorders Clinical Practice, and JAMA Neurology. AJL has a consultancy agreement with Britannia Pharmaceuticals and has received honoraria from Bial and Convatec. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.