Analysis of hemisphere-dependent effects of unilateral intrastriatal injection of α-synuclein pre-formed fibrils on mitochondrial protein levels, dynamics, and function

Poration of mitochondrial membranes by amyloidogenic peptides and other biological toxins

Mitochondria are essential organelles known to serve broad functions, including in cellular metabolism, calcium buffering, signaling pathways and the regulation of apoptotic cell death. Maintaining the integrity of the outer (OMM) and inner mitochondrial membranes (IMM) is vital for mitochondrial health. Cardiolipin (CL), a unique dimeric glycerophospholipid, is the signature lipid of energy-converting membranes. It plays a significant role in maintaining mitochondrial architecture and function, stabilizing protein complexes and facilitating efficient oxidative phosphorylation (OXPHOS) whilst regulating cytochrome c release from mitochondria. CL is especially enriched in the IMM and at sites of contact between the OMM and IMM. Disorders of protein misfolding, such as Alzheimer's and Parkinson's diseases, involve amyloidogenic peptides like amyloid-β, tau and α-synuclein, which form metastable toxic oligomeric species that interact with biological membranes. Electrophysiological studies have shown that these oligomers form ion-conducting nanopores in membranes mimicking the IMM's phospholipid composition. Poration of mitochondrial membranes disrupts the ionic balance, causing osmotic swelling, loss of the voltage potential across the IMM, release of pro-apoptogenic factors, and leads to cell death. The interaction between CL and amyloid oligomers appears to favour their membrane insertion and pore formation, directly implicating CL in amyloid toxicity. Additionally, pore formation in mitochondrial membranes is not limited to amyloid proteins and peptides; other biological peptides, as diverse as the pro-apoptotic Bcl-2 family members, gasdermin proteins, cobra venom cardiotoxins and bacterial pathogenic toxins, have all been described to punch holes in mitochondria, contributing to cell death processes. Collectively, these findings underscore the vulnerability of mitochondria and the involvement of CL in various pathogenic mechanisms, emphasizing the need for further research on targeting CL-amyloid interactions to mitigate mitochondrial dysfunction.

Alpha-Synuclein Effects on Mitochondrial Quality Control in Parkinson's Disease

The maintenance of healthy mitochondria is essential for neuronal survival and relies upon mitochondrial quality control pathways involved in mitochondrial biogenesis, mitochondrial dynamics, and mitochondrial autophagy (mitophagy). Mitochondrial dysfunction is critically implicated in Parkinson's disease (PD), a brain disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra. Consequently, impaired mitochondrial quality control may play a key role in PD pathology. This is affirmed by work indicating that genes such as PRKN and PINK1, which participate in multiple mitochondrial processes, harbor PD-associated mutations. Furthermore, mitochondrial complex-I-inhibiting toxins like MPTP and rotenone are known to cause Parkinson-like symptoms. At the heart of PD is alpha-synuclein (αS), a small synaptic protein that misfolds and aggregates to form the disease's hallmark Lewy bodies. The specific mechanisms through which aggregated αS exerts its neurotoxicity are still unknown; however, given the vital role of both αS and mitochondria to PD, an understanding of how αS influences mitochondrial maintenance may be essential to elucidating PD pathogenesis and discovering future therapeutic targets. Here, the current knowledge of the relationship between αS and mitochondrial quality control pathways in PD is reviewed, highlighting recent findings regarding αS effects on mitochondrial biogenesis, dynamics, and autophagy.

α-Synuclein triggers cofilin pathology and dendritic spine impairment via a PrPC-CCR5 dependent pathway

Cognitive dysfunction and dementia are critical symptoms of Lewy Body dementias (LBD). Specifically, alpha-synuclein (αSyn) accumulation in the hippocampus leading to synaptic dysfunction is linked to cognitive deficits in LBD. Here, we investigated the pathological impact of αSyn on hippocampal neurons. We report that either αSyn overexpression or αSyn pre-formed fibrils (PFFs) treatment triggers the formation of cofilin-actin rods, synapse disruptors, in cultured hippocampal neurons and in the hippocampus of synucleinopathy mouse models and of LBD patients. In vivo, cofilin pathology is present concomitantly with synaptic impairment and cognitive dysfunction. Rods generation prompted by αSyn involves the co-action of the cellular prion protein (PrPC) and the chemokine receptor 5 (CCR5). Importantly, we show that CCR5 inhibition, with a clinically relevant peptide antagonist, reverts dendritic spine impairment promoted by αSyn. Collectively, we detail the cellular and molecular mechanism through which αSyn disrupts hippocampal synaptic structure and we identify CCR5 as a novel therapeutic target to prevent synaptic impairment and cognitive dysfunction in LBD.

Intranasal Administration of GRP78 Protein (HSPA5) Confers Neuroprotection in a Lactacystin-Induced Rat Model of Parkinson's Disease

The accumulation of misfolded and aggregated α-synuclein can trigger endoplasmic reticulum (ER) stress and the unfolded protein response (UPR), leading to apoptotic cell death in patients with Parkinson's disease (PD). As the major ER chaperone, glucose-regulated protein 78 (GRP78/BiP/HSPA5) plays a key role in UPR regulation. GRP78 overexpression can modulate the UPR, block apoptosis, and promote the survival of nigral dopamine neurons in a rat model of α-synuclein pathology. Here, we explore the therapeutic potential of intranasal exogenous GRP78 for preventing or slowing PD-like neurodegeneration in a lactacystin-induced rat model. We show that intranasally-administered GRP78 rapidly enters the substantia nigra pars compacta (SNpc) and other afflicted brain regions. It is then internalized by neurons and microglia, preventing the development of the neurodegenerative process in the nigrostriatal system. Lactacystin-induced disturbances, such as the abnormal accumulation of phosphorylated pS129-α-synuclein and activation of the pro-apoptotic GRP78/PERK/eIF2α/CHOP/caspase-3,9 signaling pathway of the UPR, are substantially reversed upon GRP78 administration. Moreover, exogenous GRP78 inhibits both microglia activation and the production of proinflammatory cytokines, tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), via the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling pathway in model animals. The neuroprotective and anti-inflammatory potential of exogenous GRP78 may inform the development of effective therapeutic agents for PD and other synucleinopathies.

Alpha-synuclein-associated changes in PINK1-PRKN-mediated mitophagy are disease context dependent

Alpha-synuclein (αsyn) aggregates are pathological features of several neurodegenerative conditions including Parkinson disease (PD), dementia with Lewy bodies, and multiple system atrophy (MSA). Accumulating evidence suggests that mitochondrial dysfunction and impairments of the autophagic-lysosomal system can contribute to the deposition of αsyn, which in turn may interfere with health and function of these organelles in a potentially vicious cycle. Here we investigated a potential convergence of αsyn with the PINK1-PRKN-mediated mitochondrial autophagy pathway in cell models, αsyn transgenic mice, and human autopsy brain. PINK1 and PRKN identify and selectively label damaged mitochondria with phosphorylated ubiquitin (pS65-Ub) to mark them for degradation (mitophagy). We found that disease-causing multiplications of αsyn resulted in accumulation of the ubiquitin ligase PRKN in cells. This effect could be normalized by starvation-induced autophagy activation and by CRISPR/Cas9-mediated αsyn knockout. Upon acute mitochondrial damage, the increased levels of PRKN protein contributed to an enhanced pS65-Ub response. We further confirmed increased pS65-Ub-immunopositive signals in mouse brain with αsyn overexpression and in postmortem human disease brain. Of note, increased pS65-Ub was associated with neuronal Lewy body-type αsyn pathology, but not glial cytoplasmic inclusions of αsyn as seen in MSA. While our results add another layer of complexity to the crosstalk between αsyn and the PINK1-PRKN pathway, distinct mechanisms may underlie in cells and brain tissue despite similar outcomes. Notwithstanding, our finding suggests that pS65-Ub may be useful as a biomarker to discriminate different synucleinopathies and may serve as a potential therapeutic target for Lewy body disease.

Advances in understanding the function of alpha-synuclein: implications for Parkinson's disease

The critical role of alpha-synuclein in Parkinson's disease represents a pivotal discovery. Some progress has been made over recent years in identifying disease-modifying therapies for Parkinson's disease that target alpha-synuclein. However, these treatments have not yet shown clear efficacy in slowing the progression of this disease. Several explanations exist for this issue. The pathogenesis of Parkinson's disease is complex and not yet fully clarified and the heterogeneity of the disease, with diverse genetic susceptibility and risk factors and different clinical courses, adds further complexity. Thus, a deep understanding of alpha-synuclein physiological and pathophysiological functions is crucial. In this review, we first describe the cellular and animal models developed over recent years to study the physiological and pathological roles of this protein, including transgenic techniques, use of viral vectors and intracerebral injections of alpha-synuclein fibrils. We then provide evidence that these tools are crucial for modelling Parkinson's disease pathogenesis, causing protein misfolding and aggregation, synaptic dysfunction, brain plasticity impairment and cell-to-cell spreading of alpha-synuclein species. In particular, we focus on the possibility of dissecting the pre- and postsynaptic effects of alpha-synuclein in both physiological and pathological conditions. Finally, we show how vulnerability of specific neuronal cell types may facilitate systemic dysfunctions leading to multiple network alterations. These functional alterations underlie diverse motor and non-motor manifestations of Parkinson's disease that occur before overt neurodegeneration. However, we now understand that therapeutic targeting of alpha-synuclein in Parkinson's disease patients requires caution, since this protein exerts important physiological synaptic functions. Moreover, the interactions of alpha-synuclein with other molecules may induce synergistic detrimental effects. Thus, targeting only alpha-synuclein might not be enough. Combined therapies should be considered in the future.

Mitochondrial signaling on innate immunity activation in Parkinson disease

Parkinson's disease (PD) is a neurodegenerative disease characterized by the accumulation of alpha-synuclein (aSyn) in the nigrostriatal pathway that is followed by severe neuroinflammatory response. PD etiology is still puzzling; however, the mitocentric view might explain the vast majority of molecular findings not only in the brain, but also at systemic level. While neuronal degeneration is tightly associated with mitochondrial dysfunction, the causal role between aSyn accumulation and mitochondrial dysfunction still requires further investigation. Moreover, mitochondrial dysfunction can elicit an inflammatory response that may be transmitted locally but also in a long range through systemic circulation. Furthermore, mitochondrial-driven innate immune activation may involve the synthesis of antimicrobial peptides, of which aSyn poses as a good candidate. While there is still a need to clarify disease-elicited mechanisms and how aSyn has the ability to modulate mitochondrial and cellular dysfunction, recent studies provide insightful views on mitochondria-inflammation axis in PD etiology.