Age- and brain region-dependent α-synuclein oligomerization is attributed to alterations in intrinsic enzymes regulating α-synuclein phosphorylation in aging monkey brains

Single-cell genomics identifies distinct B1 cell developmental pathways and reveals aging-related changes in the B-cell receptor repertoire

Background

B1 cells are self-renewing innate-like B lymphocytes that provide the first line of defense against pathogens. B1 cells primarily reside in the peritoneal cavity and are known to originate from various fetal tissues, yet their developmental pathways and the mechanisms underlying maintenance of B1 cells throughout adulthood remain unclear.

Results

We performed high-throughput single-cell analysis of the transcriptomes and B-cell receptor repertoires of peritoneal B cells of neonates, young adults, and elderly mice. Gene expression analysis of 31,718 peritoneal B cells showed that the neonate peritoneal cavity contained many B1 progenitors, and neonate B cell specific clustering revealed two trajectories of peritoneal B1 cell development, including pre-BCR dependent and pre-BCR independent pathways. We also detected profound age-related changes in B1 cell transcriptomes: clear difference in senescence genetic program was evident in differentially aged B1 cells, and we found an example that a B1 subset only present in the oldest mice was marked by expression of the fatty-acid receptor CD36. We also performed antibody gene sequencing of 15,967 peritoneal B cells from the three age groups and discovered that B1 cell aging was associated with clonal expansion and two B1 cell clones expanded in the aged mice had the same CDR-H3 sequence (AGDYDGYWYFDV) as a pathogenically linked cell type from a recent study of an atherosclerosis mouse model.

Conclusions

Beyond offering an unprecedent data resource to explore the cell-to-cell variation in B cells, our study has revealed that B1 precursor subsets are present in the neonate peritoneal cavity and dissected the developmental pathway of the precursor cells. Besides, this study has found the expression of CD36 on the B1 cells in the aged mice. And the single-cell B-cell receptor sequencing reveals B1 cell aging is associated with clonal expansion.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-022-00795-6.

Polo-Like Kinase 2: From Principle to Practice

Polo-like kinase (PLK) 2 is an evolutionarily conserved serine/threonine kinase that shares the n-terminal kinase catalytic domain and the C-terminal Polo Box Domain (PBD) with other members of the PLKs family. In the last two decades, mounting studies have focused on this and tried to clarify its role in many aspects. PLK2 is essential for mitotic centriole replication and meiotic chromatin pairing, synapsis, and crossing-over in the cell cycle; Loss of PLK2 function results in cell cycle disorders and developmental retardation. PLK2 is also involved in regulating cell differentiation and maintaining neural homeostasis. In the process of various stimuli-induced stress, including oxidative and endoplasmic reticulum, PLK2 may promote survival or apoptosis depending on the intensity of stimulation and the degree of cell damage. However, the role of PLK2 in immunity to viral infection has been studied far less than that of other family members. Because PLK2 is extensively and deeply involved in normal physiological functions and pathophysiological mechanisms of cells, its role in diseases is increasingly being paid attention to. The effect of PLK2 in inhibiting hematological tumors and fibrotic diseases, as well as participating in neurodegenerative diseases, has been gradually recognized. However, the research results in solid organ tumors show contradictory results. In addition, preliminary studies using PLK2 as a disease predictor and therapeutic target have yielded some exciting and promising results. More research will help people better understand PLK2 from principle to practice.

Intermediates of α-synuclein aggregation: Implications in Parkinson's disease pathogenesis

Cytoplasmic deposition of aberrantly misfolded α-synuclein (α-Syn) is a common feature of synucleinopathies, including Parkinson's disease (PD). However, the precise pathogenic mechanism of α-Syn in synucleinopathies remains elusive. Emerging evidence has suggested that α-Syn may contribute to PD pathogenesis in several ways; wherein the contribution of fibrillar species, for exerting toxicity and disease transmission, cannot be neglected. Further, the oligomeric species could be the most plausible neurotoxic species causing neuronal cell death. However, understanding the structural and molecular insights of these oligomers are very challenging due to the heterogeneity and transient nature of the species. In this review, we discuss the recent advancements in understanding the formation and role of α-Syn oligomers in PD pathogenesis. We also summarize the different types of α-Syn oligomeric species and potential mechanisms to exert neurotoxicity. Finally, we address the possible ways to target α-Syn as a promising approach against PD and the possible future directions.

Enhanced tyrosine hydroxylase activity induces oxidative stress, causes accumulation of autotoxic catecholamine metabolites, and augments amphetamine effects in vivo

In Parkinson's disease, dopamine‐containing nigrostriatal neurons undergo profound degeneration. Tyrosine hydroxylase (TH) is the rate‐limiting enzyme in dopamine biosynthesis. TH increases in vitro formation of reactive oxygen species, and previous animal studies have reported links between cytosolic dopamine build‐up and oxidative stress. To examine effects of increased TH activity in catecholaminergic neurons in vivo, we generated TH‐over‐expressing mice (TH‐HI) using a BAC‐transgenic approach that results in over‐expression of TH with endogenous patterns of expression. The transgenic mice were characterized by western blot, qPCR, and immunohistochemistry. Tissue contents of dopamine, its metabolites, and markers of oxidative stress were evaluated. TH‐HI mice had a 3‐fold increase in total and phosphorylated TH levels and an increased rate of dopamine synthesis. Coincident with elevated dopamine turnover, TH‐HI mice showed increased striatal production of H₂O₂ and reduced glutathione levels. In addition, TH‐HI mice had elevated striatal levels of the neurotoxic dopamine metabolites 3,4‐dihydroxyphenylacetaldehyde and 5‐S‐cysteinyl‐dopamine and were more susceptible than wild‐type mice to the effects of amphetamine and methamphetamine. These results demonstrate that increased TH alone is sufficient to produce oxidative stress in vivo, build up autotoxic dopamine metabolites, and augment toxicity.

Age-related increase of alpha-synuclein oligomers is associated with motor disturbances in L61 transgenic mice

The pathogenesis of Parkinson’s disease involves fibrillization and deposition of alpha-synuclein (α-syn) into Lewy bodies. Accumulating evidence suggests that α-syn oligomers are particularly neurotoxic. Transgenic (tg) mice overexpressing wild-type human α-syn under the Thy-1 promoter (L61) reproduce many Parkinson’s disease features, but the pathogenetic relevance of α-syn oligomers in this mouse model has not been studied in detail. Here, we report an age progressive increase of α-syn oligomers in the brain of L61 tg mice. Interestingly, more profound motor symptoms were observed in animals with higher levels of membrane-bound oligomers. As this tg model is X-linked, we also performed subset analyses, indicating that both sexes display a similar age-related increase in α-syn oligomers. However, compared with females, males featured increased brain levels of oligomers from an earlier age, in addition to a more severe behavioral phenotype with hyperactivity and thigmotaxis in the open field test. Taken together, our data indicate that α-syn oligomers are central to the development of brain pathology and behavioral deficits in the L61 tg α-syn mouse model.

Genetic deletion of Polo-like kinase 2 reduces alpha-synuclein serine-129 phosphorylation in presynaptic terminals but not Lewy bodies

Phosphorylation of alpha-synuclein at serine-129 is an important marker of pathologically relevant, aggregated forms of the protein in several important human diseases, including Parkinson's disease, Dementia with Lewy bodies, and Multiple system atrophy. Although several kinases have been shown to be capable of phosphorylating alpha-synuclein in various model systems, the identity of the kinase that phosphorylates alpha-synuclein in the Lewy body remains unknown. One member of the Polo-like kinase family, PLK2, is a strong candidate for being the Lewy body kinase. To examine this possibility, we have used a combination of approaches, including biochemical, immunohistochemical, and in vivo multiphoton imaging techniques to study the consequences of PLK2 genetic deletion on alpha-synuclein phosphorylation in both the presynaptic terminal and preformed fibril-induced Lewy body pathology in mouse cortex. We find that PLK2 deletion reduces presynaptic terminal alpha-synuclein serine-129 phosphorylation, but has no effect on Lewy body phosphorylation levels. Serine-129 mutation to the phosphomimetic alanine or the unphosphorylatable analog aspartate does not change the rate of cell death of Lewy inclusion-bearing neurons in our in vivo multiphoton imaging paradigm, but PLK2 deletion does slow the rate of neuronal death. Our data indicate that inhibition of PLK2 represents a promising avenue for developing new therapeutics, but that the mechanism of neuroprotection by PLK2 inhibition is not likely due to reducing alpha-synuclein serine-129 phosphorylation and that the true Lewy body kinase still awaits discovery.

In Search of Effective Treatments Targeting α-Synuclein Toxicity in Synucleinopathies: Pros and Cons

Parkinson's disease (PD), multiple system atrophy (MSA) and Dementia with Lewy bodies (DLB) represent pathologically similar, progressive neurodegenerative disorders characterized by the pathological aggregation of the neuronal protein α-synuclein. PD and DLB are characterized by the abnormal accumulation and aggregation of α-synuclein in proteinaceous inclusions within neurons named Lewy bodies (LBs) and Lewy neurites (LNs), whereas in MSA α-synuclein inclusions are mainly detected within oligodendrocytes named glial cytoplasmic inclusions (GCIs). The presence of pathologically aggregated α-synuclein along with components of the protein degradation machinery, such as ubiquitin and p62, in LBs and GCIs is considered to underlie the pathogenic cascade that eventually leads to the severe neurodegeneration and neuroinflammation that characterizes these diseases. Importantly, α-synuclein is proposed to undergo pathogenic misfolding and oligomerization into higher-order structures, revealing self-templating conformations, and to exert the ability of "prion-like" spreading between cells. Therefore, the manner in which the protein is produced, is modified within neural cells and is degraded, represents a major focus of current research efforts in the field. Given that α-synuclein protein load is critical to disease pathogenesis, the identification of means to limit intracellular protein burden and halt α-synuclein propagation represents an obvious therapeutic approach in synucleinopathies. However, up to date the development of effective therapeutic strategies to prevent degeneration in synucleinopathies is limited, due to the lack of knowledge regarding the precise mechanisms underlying the observed pathology. This review critically summarizes the recent developed strategies to counteract α-synuclein toxicity, including those aimed to increase protein degradation, to prevent protein aggregation and cell-to-cell propagation, or to engage antibodies against α-synuclein and discuss open questions and unknowns for future therapeutic approaches.

Impact of Age and Sex on α-Syn (α-Synuclein) Knockdown-Mediated Poststroke Recovery

BACKGROUND AND PURPOSE

Increased expression of α-Syn (α-Synuclein) is known to mediate secondary brain damage after stroke. We presently studied if α-Syn knockdown can protect ischemic brain irrespective of sex and age.

METHODS

Adult and aged male and female mice were subjected to transient middle cerebral artery occlusion. α-Syn small interfering RNA (siRNA) was administered intravenous at 30 minutes or 3 hour reperfusion. Poststroke motor deficits were evaluated between day 1 and 7 and infarct volume was measured at day 7 of reperfusion.

RESULTS

α-Syn knockdown significantly decreased poststroke brain damage and improved poststroke motor function recovery in adult and aged mice of both sexes. However, the window of therapeutic opportunity for α-Syn siRNA is very limited.

CONCLUSIONS

α-Syn plays a critical role in ischemic brain damage and preventing α-Syn protein expression early after stroke minimizes poststroke brain damage leading to better functional outcomes irrespective of age and sex.

α-Syn oligomers incubated with Parkinson's disease plasma promote neuron damage

Alpha-Synuclein (α-Syn) aggregates represent the major component of Lewy bodies (LBs), a pathologic hallmark of Parkinson's disease (PD). Current reports have assessed the toxicity of oligomeric α-Syn (o-α-Syn) mostly in vitro after the incubation with PBS, which leaves o-α-Syn non-phosphorylated and does not reflect actual physiologic conditions in PD patients. The present study aimed to assess the pathogenic role of o-α-Syn while addressing the above issues using o-α-Syn incubated with PD plasma. Several α-Syn oligomer types were prepared by incubating recombinant human α-Syn with phosphate-buffered saline (PBS), and plasma samples from normal controls (NS) and PD patients. O-α-Syn incubated with PD plasma (o-α-Syn-PD), moderately or highly phosphorylated at serine 129, induced cell death more substantially compared with the PBS and NS groups. PD plasma exhibited reduced PP2A activity and ceramide levels, promoting the phosphorylation of o-α-Syn. In agreement, ceramide addition alleviated o-α-Syn-PD cytotoxicity. In vivo, o-α-Syn-PD significantly reduced dopaminergic neurons in the substantia nigra and could be transferred to the cortex, hippocampus, and other parts of the brain. Mice administered o-α-Syn-PD exhibited significant PD-like dyskinesia changes in a short period of time. Finally, o-α-Syn-PD injection was associated with decreased GCase and PP2A activities in the mouse brain. The above findings provide novel insights into the effect of o-α-Syn on neurodegeneration in PD and dementia with LBs.

LRRK2 Phosphorylation, More Than an Epiphenomenon

Mutations in the Leucine Rich Repeat Kinase 2 (LRRK2) gene are linked to autosomal dominant Parkinson's disease (PD), and genetic variations at the LRRK2 locus are associated with an increased risk for sporadic PD. This gene encodes a kinase that is physiologically multiphosphorylated, including clusters of both heterologous phosphorylation and autophosphorylation sites. Several pieces of evidence indicate that LRRK2's phosphorylation is important for its pathological and physiological functioning. These include a reduced LRRK2 heterologous phosphorylation in PD brains or after pharmacological inhibition of LRRK2 kinase activity as well as the appearance of subcellular LRRK2 accumulations when this protein is dephosphorylated at heterologous phosphosites. Nevertheless, the regulatory mechanisms governing LRRK2 phosphorylation levels and the cellular consequences of changes in LRRK2 phosphorylation remain incompletely understood. In this review, we present current knowledge on LRRK2 phosphorylation, LRRK2 phosphoregulation, and how LRRK2 phosphorylation changes affect cellular processes that may ultimately be linked to PD mechanisms.

Phosphorylated Alpha-Synuclein in Red Blood Cells as a Potential Diagnostic Biomarker for Multiple System Atrophy: A Pilot Study

Diagnosis of multiple system atrophy (MSA) remains a challenge, due to the complexity and overlapping of its symptoms with other Parkinsonian disorders. The critical role of alpha-synuclein (α-syn) in the pathogenesis of MSA makes it an ideal biomarker for the diagnosis of MSA. Although α-syn alterations in cerebrospinal fluid (CSF) and blood plasma have been extensively assessed for the utility in diagnosing MSA, inconsistent results have been obtained, presumably due to the contamination by hemolysis and other confounding factors. In this study, levels of serine 129-phosphorylated α-syn (pS-α-syn), a major pathologic form of α-syn, in red blood cells (RBCs), were measured using ELISA in a Chinese cohort consisting of 107 MSA patients and 220 healthy controls. A significant increase in the levels of pS-α-syn in RBCs (pS-α-syn-RBC) was observed in MSA patients than in healthy controls (14.02 ± 4.02 ng/mg versus 11.89 ± 3.57 ng/mg; p < 0.0001). Receiver operating characteristic curve (ROC) indicated that pS-α-syn-RBC discriminated the patients well from the controls with a sensitivity of 80.37% (95% confidence interval (CI): 71.58%-87.42%), a specificity of 88.64% (95% CI: 83.68%-92.51%), and an area under the curve (AUC) of 0.91 (95% CI: 0.87-0.94). The levels of pS-α-syn-RBC were negatively correlated with RBD-HK scores and differed between MSA-P and MSA-C subtypes (13.27 ± 1.91 versus 12.19 ± 3.04; p=0.025). The difference between subtypes was seen at Hoehn and Yahr stages 3 and 4, and the age at onset (AAO) between 60 and 69 years (p=0.016). The results suggest that pS-α-syn-RBC is increased in MSA patients and can be used as a potential diagnostic biomarker for MSA.

Alpha-synuclein oligomerization and dopaminergic degeneration occur synchronously in the brain and colon of MPTP-intoxicated parkinsonian monkeys

Dopaminergic (DAergic) degeneration and abnormal α-synuclein (α-syn) expression, phosphorylation and aggregation are observed in both the nigrostriatal system (NSS) and enteric nervous system (ENS) of patients with Parkinson's disease (PD). Whether these alterations in α-syn and DAergic neurons occur synchronously in the two nervous systems or follow a process that spreads from the gut to the brain remains a subject of debate. Here, in MPTP-intoxicated cynomolgus monkeys, we showed a parallel DAergic degeneration in the colon as well as in the substantia nigra and striatum (SN/STR), as indicated by reduced expression of tyrosine hydroxylase (TH) and dopamine transporter (DAT). In addition, we observed a simultaneous increase in the concentrations of total, phosphorylated, and oligomeric α-syn in the colon and SN/STR. Moreover, we identified that the above changes in α-syn were associated with an increase in the expression of polo-like kinase 2 (PLK2), an enzyme that promotes α-syn phosphorylation, and a decrease in the activity of protein phosphatase 2A (PP2A), an enzyme that facilitates α-syn dephosphorylation. Because the colonic ENS can be readily analyzed using routine biopsies, the shared pathological features between the colonic ENS and the brain NSS found in this study provide useful information for assessing and understanding the neuropathology in PD patients using colonic biopsies.

Leucine Carboxyl Methyltransferase Downregulation and Protein Phosphatase Methylesterase Upregulation Contribute Toward the Inhibition of Protein Phosphatase 2A by α-Synuclein

The pathology of Parkinson's disease (PD) is characterized by intracellular neurofibrillary tangles of phosphorylated α-synuclein (α-syn). Protein phosphatase 2A (PP2A) is responsible for α-syn dephosphorylation. Previous work has demonstrated that α-syn can regulate PP2A activity. However, the mechanisms underlying α-syn regulation of PP2A activity are not well understood. In this study, we found that α-syn overexpression induced increased α-syn phosphorylation at serine 129 (Ser129), and PP2A inhibition, in vitro and in vivo. α-syn overexpression resulted in PP2A demethylation. This demethylation was mediated via downregulated leucine carboxyl methyltransferase (LCMT-1) expression, and upregulated protein phosphatase methylesterase (PME-1) expression. Furthermore, LCMT-1 overexpression, or PME-1 inhibition, reversed α-syn-induced increases in α-syn phosphorylation and apoptosis. In addition to post-translational modifications of the catalytic subunit, regulatory subunits are involved in the regulation of PP2A activity. We found that the levels of regulatory subunits which belong to the PPP2R2 subfamily, not the PPP2R5 subfamily, were downregulated in the examined brain regions of transgenic mice. Our work identifies a novel mechanism to explain how α-syn regulates PP2A activity, and provides the optimization of PP2A methylation as a new target for PD treatment.

Targeting AMPK Signaling as a Neuroprotective Strategy in Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder. It is characterized by the accumulation of intracellular α-synuclein aggregates and the degeneration of nigrostriatal dopaminergic neurons. While no treatment strategy has been proven to slow or halt the progression of the disease, there is mounting evidence from preclinical PD models that activation of 5'-AMP-activated protein kinase (AMPK) may have broad neuroprotective effects. Numerous dietary supplements and pharmaceuticals (e.g., metformin) that increase AMPK activity are available for use in humans, but clinical studies of their effects in PD patients are limited. AMPK is an evolutionarily conserved serine/threonine kinase that is activated by falling energy levels and functions to restore cellular energy balance. However, in response to certain cellular stressors, AMPK activation may exacerbate neuronal atrophy and cell death. This review describes the regulation and functions of AMPK, evaluates the controversies in the field, and assesses the potential of targeting AMPK signaling as a neuroprotective treatment for PD.

Sigma-1 receptor knockout increases α-synuclein aggregation and phosphorylation with loss of dopaminergic neurons in substantia nigra

Sigma-1 receptor (σ1R) is expressed in dopaminergic neurons of substantia nigra. Here, we show that σ1R knockout (σ1R-/-) mice, at age 6-12 months, appeared with age-related loss of dopaminergic neurons and decline of motor coordination. Levels of α-synuclein (αSyn) oligomers and fibrillar αSyn in substantia nigra of σ1R-/- mice were age-dependently increased without the changes in αSyn monomers. The phosphorylation of αSyn monomers or oligomers in dopaminergic neurons was enhanced in σ1R-/- mice. Levels of phosphorylated eIF2a and C/EBP homologous protein expression were elevated in σ1R-/- mice with decline of proteasome activity. Inhibition of endoplasmic reticulum stress by salubrinal recovered the αSyn phosphorylation and proteasome activity and prevented early oligomerization of αSyn in σ1R-/- mice. Rifampicin reduced the late increase of αSyn oligomers in σ1R-/- mice. Rifampicin or salubrinal could reduce the loss of dopaminergic neurons in σ1R-/- mice and improved their motor coordination. The results indicate that the σ1R deficiency through enhanced aggregation and phosphorylation of αSyn causes the loss of dopaminergic neurons leading to the decline of motor coordination.

PINK1 suppresses alpha-synuclein-induced neuronal injury: a novel mechanism in protein phosphatase 2A activation

Alpha-synuclein (α-Syn) and phosphatase and tensin homolog deleted on chromosome ten (PTEN)-induced putative kinase (PINK) 1 are proteins found in Lewy bodies, which are a pathological hallmark of Parkinson's disease (PD). PINK1 overexpression suppresses α-Syn-induced phenotypes and increases lifespan and health in an animal model of PD. It has been suggested that the two proteins regulate protein phosphatase (PP) 2A activity, but the underlying mechanisms and neuroprotective action of PP2A against PD-associated pathology are unknown. We found that α-Syn overexpression in SK-N-SH neuroblastoma cells and primary cortical neurons caused mitochondrial dysfunction and cell injury via phosphorylation of PP2A at Tyr307 and inhibition of its activity. Concomitant overexpression of PINK1 reversed this effect and restored the activity. The level of phospho-activated Src was increased in cells overexpressing α-Syn, which was reversed by co-expressing PINK1, suggesting that the latter suppressed α-Syn-induced PP2A inactivation by inhibiting Src activity. Calmodulin/Src complex formation was also enhanced in α-Syn-overexpressing cells, which was reversed by co-expression of PINK1 as a result of reduced mitochondrial Ca²⁺ releasing. Interestingly, the protective effects of PINK1 in α-Syn induced models were abolished by treatment with the PP2A inhibitor okadaic acid, indicating that PP2A is a target of PINK1. These findings indicate that PINK1 protects against α-Syn-induced neurodegeneration by promoting the dissociation of the calmodulin/Src complex and inhibiting Src, thereby enhancing PP2A activity. This was supported by the observation that PP2A activity was decreased in PD patients, which was negatively correlated with Hoehn and Yahr scores. Our results provide novel insight into the mechanisms underlying neurodegeneration in PD as well as possible avenues for therapeutic intervention in the treatment of this disease.

Age-dependent alpha-synuclein accumulation is correlated with elevation of mitochondrial TRPC3 in the brains of monkeys and mice

Aberrant α-synuclein (α-syn) accumulation has been shown to impair mitochondrial function by reducing mitochondrial membrane potential (MMP). However, the underlying mechanisms remain elusive. Transient receptor potential canonical (TRPC) channels are a diverse group of non-selective Ca²⁺ channels, among which TRPC3 is the only one that is localized in mitochondria and plays a role in maintaining the normal MMP. This raises a possibility that altered TRPC3 expression may play a role in the mitochondrial dysfunction induced by α-syn accumulation. To demonstrate this possibility, we first examined the expressions of mitochondrial TRPC3 in the brains of aging monkeys and α-syn transgenic and wild-type mice. We showed that α-syn levels increased in mitochondria in an age-dependent manner that was positively correlated to an elevation of mitochondrial TRPC3. This correlation was more prominent in the striatum than in the cerebellum, possibly due to the greater age-dependent α-syn accumulation in the striatum than in the cerebellum. We then used primary neurons overexpressing α-syn to investigate the effect of the α-syn-induced elevation of mitochondrial TRPC3 on the MMP and apoptotic cell death. We found that neurons with overexpressed α-syn had increased mitochondrial TRPC3 and decreased MMP, which were accompanied by increased number of apoptotic neurons. Suppressing TRPC3 expression partially reversed the reduction of MMP and alleviated the apoptotic cell death, indicating that the mitochondrial TRPC3 may play a role in the mitochondrial dysfunction in neurons with α-syn accumulation that may occur in not only the aged brain but also the brain with PD.