Polyphosphate: Hidden Ally Against Neurodegeneration and Amyloid Toxicity

In recent years, research has underscored the significant impact of amyloids—insoluble protein fibrils—on both bacterial biofilms and human neurodegenerative diseases like Alzheimer’s and Parkinson’s. Amyloid proteins, typically soluble, have the unique ability to transform into cross-β-sheet fibrils, a structure central to the fibril formation process. This transition is crucial to understanding disease progression, as it’s the accumulation of toxic oligomers, rather than the fully formed amyloid fibrils, that appears to exert neurotoxic effects. Surprisingly, studies have illuminated an unanticipated but fascinating player in amyloid formation and stability: polyphosphate, a polymer consisting of up to a thousand phosphate units.

Polyphosphate (polyP) is not only abundant across various organisms but has been detected in different cellular compartments, playing protective roles in stressful conditions. In particular, polyP’s interactions with proteins reveal its potential as a scaffold that facilitates the transition of amyloidogenic proteins to a β-sheet structure, thereby accelerating fibril formation. This accelerative influence of polyP holds promise in both enhancing bacterial biofilm resilience and alleviating amyloid toxicity in neurodegenerative cells. This article delves into the roles of polyP in amyloid processes, emphasizing its capacity as a conserved, cytoprotective modifier.

Accelerating Biofilm Formation in E. coli: PolyP’s Role in Curli Fibrils

Polyphosphate’s influence on biofilm formation in bacteria such as uropathogenic E. coli (UPEC) is particularly striking. Biofilm formation in pathogenic strains relies on curli fibrils, cross-β sheet structures produced by the CsgA protein. CsgA proteins are usually degraded if they fail to interact with specific nucleators on the bacterial surface. However, when polyP is present, it substantially accelerates CsgA fibril formation, enhancing bacterial resilience.

In experiments, short polyP chains markedly accelerated the fibrillation process of CsgA, with optimal effects observed within physiologically relevant concentrations. Notably, even a mutant UPEC strain unable to synthesize polyP exhibited delayed biofilm formation, highlighting polyP’s necessity for efficient fibril development in biofilms. This acceleration is significant because it underscores polyP’s role as a stabilizing agent, hastening the transformation of CsgA from its disordered state to a structured β-sheet configuration. By bolstering biofilm integrity, polyP may improve the pathogen’s survival and resistance, presenting potential challenges in combating infections.

Disease-Related Amyloid Formation: PolyP’s Broad Impact on Neurodegenerative Amyloids

PolyP’s presence across tissues, including the brain, suggests its relevance beyond bacterial systems. It can expedite the fibrillation of proteins associated with neurodegenerative diseases, such as α-synuclein (involved in Parkinson’s) and amyloid beta (Aβ) peptides associated with Alzheimer’s. Typically, these proteins require specific conditions to form fibrils, a process inherently slow and reliant on factors like artificial nucleation. Yet, polyP remarkably shortens the lag phase for α-synuclein, encouraging fibril formation even at physiological concentrations.

PolyP demonstrates a preference for longer chain lengths, which are more effective in accelerating amyloidogenic processes compared to shorter chains. This length-dependent efficiency points to polyP’s role not just as a general polyanion but as a selective agent influencing amyloid transformation. Interestingly, even in complex environments like brain homogenates, polyP retained its efficacy, signaling its robustness as a modifier in vivo. These insights position polyP as a potentially powerful ally in modulating disease-relevant amyloidogenic processes.

Structural Influence on Amyloids: PolyP Alters Amyloid Morphology and Stability

A significant discovery is polyP’s ability to alter the morphology and seeding capacity of amyloid fibrils. Amyloid fibrils formed with polyP have distinct structural features, notably fewer filament breaks and a more compact single-stranded appearance. This structural difference not only affects the stability of amyloids but also their seeding potential—polyP-bound α-synuclein fibrils were less effective in promoting further fibril formation, a property that might reduce secondary nucleation and amyloid spread.

PolyP’s stabilizing influence extends to the amyloids’ proteolytic resistance. Fibrils formed in the presence of polyP showed greater resistance to proteinase K digestion, suggesting a tighter, less accessible structure. This protective effect, coupled with reduced molecular shedding (where mature fibrils disassemble and release toxic oligomers), implies that polyP-stabilized fibrils may mitigate the toxic consequences typically associated with amyloid build-up. In essence, polyP reshapes the physical properties of amyloids, making them more stable and less prone to exacerbating cellular toxicity.

PolyP’s Protective Effects Against Amyloid Cytotoxicity in Cells

PolyP’s stabilizing effects are not confined to test tubes. Studies using neuroblastoma cells revealed that polyP-modified α-synuclein fibrils were less toxic compared to unmodified fibrils. These fibrils neither disrupted cell morphology nor induced cell death, underscoring polyP’s role as a cytoprotective agent. Even after prolonged incubation, polyP-associated fibrils maintained cell viability, an effect mirrored in rat adrenal gland cells as well.

This protection extends to secreted amyloids, where polyP prevented toxicity from α-synuclein released by HeLa cells into the extracellular environment. Neuroblastoma cells exposed to this polyP-treated media showed no morphological changes, unlike those exposed to untreated amyloids. PolyP, therefore, diminishes amyloid toxicity by reducing the likelihood of harmful interactions between fibrils and surrounding cells.

Implications for Alzheimer’s Disease: In Vivo and Cellular Models

In Alzheimer’s disease models, polyP mitigated toxicity in SH-SY5Y cells exposed to secreted Aβ peptides, suggesting a broader applicability to Alzheimer’s pathology. In Caenorhabditis elegans models expressing amyloid-beta, polyP pretreatment delayed paralysis onset, offering evidence of polyP’s potential protective benefits in living organisms. Even more compelling is the observation of reduced polyP levels in aged mouse models of Alzheimer’s, aligning with human studies that correlate declining polyP with neurodegenerative susceptibility. This depletion might exacerbate vulnerability to amyloid toxicity, underscoring the need for interventions that restore or mimic polyP’s function.

PolyP’s Mechanistic Versatility in Amyloid Stabilization and Disease Modulation

PolyP’s unique structure offers profound versatility in interacting with amyloids. As one of the most densely negatively charged molecules, polyP likely binds positively charged regions of amyloids, stabilizing β-sheet conformations essential for fibril formation. This interaction transforms polyP from a mere scaffold to an influential nucleator in amyloid processes, decreasing toxic oligomer accumulation by accelerating fibril formation.

Its impact varies by protein: while polyP drastically enhances fibril formation in α-synuclein, its effect on Aβ fibrillation is more moderate. This variability underscores polyP’s selective efficacy, hinting that its optimal function may depend on the client protein’s intrinsic stability and structural composition.

PolyP’s role in cytoprotection offers promise for developing therapeutic interventions. By stabilizing amyloids, polyP reduces toxic intermediates, diminishes molecular shedding, and strengthens fibril morphology, all of which collectively safeguard cells from amyloid-induced damage. The fact that polyP levels decrease with age further emphasizes its therapeutic potential for neurodegenerative diseases. Future studies may reveal how polyP-based therapies could reinvigorate cellular defenses against age-related amyloid toxicity.

Harnessing PolyP’s Potential for Therapeutic Advancement

The revelations around polyphosphate’s roles in amyloid stabilization, nucleation, and cytoprotection suggest it could be an invaluable component in future neurodegenerative therapies. By accelerating amyloid transformation into stable fibrils, polyP limits toxic oligomer formation, reshaping the landscape of amyloid-related diseases. Its potential to influence amyloid diseases positively makes polyP an intriguing target for drug development, especially for conditions like Alzheimer’s and Parkinson’s where conventional treatments have shown limited efficacy. As research progresses, polyP’s unique biochemical properties may offer innovative pathways to confront the challenges of neurodegeneration.

Study DOI: https://doi.org/10.1016/j.molcel.2016.07.016

Engr. Dex Marco Tiu Guibelondo, B.Sc. Pharm, R.Ph., B.Sc. CpE

Editor-in-Chief, PharmaFEATURES

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