Trivalent nanobody-based ligands mediate powerful activation of GPVI, CLEC-2, and PEAR1 in human platelets whereas FcγRIIA requires a tetravalent ligand.

BACKGROUND: Clustering of the receptors glycoprotein receptor VI (GPVI), C-type lectin-like receptor 2 (CLEC-2), low-affinity immunoglobulin γ Fc region receptor II-a (FcγRIIA), and platelet endothelial aggregation receptor 1 (PEAR1) leads to powerful activation of platelets through phosphorylation of tyrosine in their cytosolic tails and initiation of downstream signaling cascades. GPVI, CLEC-2, and FcγRIIA signal through YxxL motifs that activate Syk. PEAR1 signals through a YxxM motif that activates phosphoinositide 3-kinase. Current ligands for these receptors have an undefined valency and show significant batch variation and, for some, uncertain specificity. OBJECTIVES: We have raised nanobodies against each of these receptors and multimerized them to identify the minimum number of epitopes to achieve robust activation of human platelets. METHODS: Divalent and trivalent nanobodies were generated using a flexible glycine-serine linker. Tetravalent nanobodies utilize a mouse Fc domain (IgG2a, which does not bind to FcγRIIA) to dimerize the divalent nanobody. Ligand affinity measurements were determined by surface plasmon resonance. Platelet aggregation, adenosine triphosphate secretion, and protein phosphorylation were analyzed using standardized methods. RESULTS: Multimerization of the nanobodies led to a stepwise increase in affinity with divalent and higher-order nanobody oligomers having sub-nanomolar affinity. The trivalent nanobodies to GPVI, CLEC-2, and PEAR1 stimulated powerful and robust platelet aggregation, secretion, and protein phosphorylation at low nanomolar concentrations. A tetravalent nanobody was required to activate FcγRIIA with the concentration-response relationship showing a greater variability and reduced sensitivity compared with the other nanobody-based ligands, despite a sub-nanomolar binding affinity. CONCLUSION: The multivalent nanobodies represent a series of standardized, potent agonists for platelet glycoprotein receptors. They have applications as research tools and in clinical assays.

Validation of agent-based models of surface receptor oligomerisation.

Receptor dimerisation and higher order oligomerisation regulates signalling by a wide variety of transmembrane receptors. We discuss how agent-based modelling (ABM) combined with advanced microscopy and structural studies can provide new insights into the regulation of clustering, including spatial considerations, revealing novel targets for therapeutic intervention.

Divalent nanobodies to platelet CLEC-2 can serve as agonists or antagonists.

CLEC-2 is a target for a new class of antiplatelet agent. Clustering of CLEC-2 leads to phosphorylation of a cytosolic YxxL and binding of the tandem SH2 domains in Syk, crosslinking two receptors. We have raised 48 nanobodies to CLEC-2 and crosslinked the most potent of these to generate divalent and tetravalent nanobody ligands. Fluorescence correlation spectroscopy (FCS) was used to show that the multivalent nanobodies cluster CLEC-2 in the membrane and that clustering is reduced by inhibition of Syk. Strikingly, the tetravalent nanobody stimulated aggregation of human platelets, whereas the divalent nanobody was an antagonist. In contrast, in human CLEC-2 knock-in mouse platelets, the divalent nanobody stimulated aggregation. Mouse platelets express a higher level of CLEC-2 than human platelets. In line with this, the divalent nanobody was an agonist in high-expressing transfected DT40 cells and an antagonist in low-expressing cells. FCS, stepwise photobleaching and non-detergent membrane extraction show that CLEC-2 is a mixture of monomers and dimers, with the degree of dimerisation increasing with expression thereby favouring crosslinking of CLEC-2 dimers. These results identify ligand valency, receptor expression/dimerisation and Syk as variables that govern activation of CLEC-2 and suggest that divalent ligands should be considered as partial agonists.

Characterizing the binding of glycoprotein VI with nanobody 35 reveals a novel monomeric structure of glycoprotein VI where the conformation of D1+D2 is independent of dimerization.

BACKGROUND: The platelet-signaling receptor glycoprotein VI (GPVI) is a promising antithrombotic target. We have previously raised a series of high-affinity nanobodies (Nbs) against GPVI and identified Nb2, Nb21, and Nb35 as potent GPVI inhibitors. The Nb2 binding site has been mapped to the D1 domain, which is directly adjacent to the CRP binding site. Ligand-binding complementary determining region 3 has only 15% conservation between all 3 Nbs. OBJECTIVES: To map the binding sites of Nb21 and Nb35 on GPVI. METHODS: We determined the X-ray crystal structure of the D1 and D2 extracellular domains of the GPVI-Nb35 complex. We then looked at the effects of various GPVI mutations on the ability of Nbs to inhibit collagen binding and GPVI signaling using surface binding assays and transfected cell lines. RESULTS: The crystal structure of GPVI bound to Nb35 was solved. GPVI was present as a monomer, and the D1+D2 conformation was comparable to that in the dimeric structure. Arg46, Tyr47, and Ala57 are common residues on GPVI targeted by both Nb2 and Nb35. Mutating Arg46 to an Ala abrogated the ability of Nb2, Nb21, and Nb35 to inhibit collagen-induced GPVI signaling and blocked the binding of all 3 Nbs. In addition, Arg60 was found to reduce Nb21 inhibition but not the inhibition Nb2 or Nb35. CONCLUSIONS: These findings reveal key residues involved in the high-affinity binding of GPVI inhibitors and negate the idea that GPVI dimerization induces a conformational change required for ligand binding.

Targeting platelet GPVI with glenzocimab: a novel mechanism for inhibition.

Platelet glycoprotein VI (GPVI) is attracting interest as a potential target for the development of new antiplatelet molecules with a low bleeding risk. GPVI binding to vascular collagen initiates thrombus formation and GPVI interactions with fibrin promote the growth and stability of the thrombus. In this study, we show that glenzocimab, a clinical stage humanized antibody fragment (Fab) with a high affinity for GPVI, blocks the binding of both ligands through a combination of steric hindrance and structural change. A cocrystal of glenzocimab with an extracellular domain of monomeric GPVI was obtained and its structure determined to a resolution of 1.9 Å. The data revealed that (1) glenzocimab binds to the D2 domain of GPVI, GPVI dimerization was not observed in the crystal structure because glenzocimab prevented D2 homotypic interactions and the formation of dimers that have a high affinity for collagen and fibrin; and (2) the light variable domain of the GPVI-bound Fab causes steric hindrance that is predicted to prevent the collagen-related peptide (CRP)/collagen fibers from extending out of their binding site and preclude GPVI clustering and downstream signaling. Glenzocimab did not bind to a truncated GPVI missing loop residues 129 to 136, thus validating the epitope identified in the crystal structure. Overall, these findings demonstrate that the binding of glenzocimab to the D2 domain of GPVI induces steric hindrance and structural modifications that drive the inhibition of GPVI interactions with its major ligands.

Experimental validation of computerised models of clustering of platelet glycoprotein receptors that signal via tandem SH2 domain proteins.

The clustering of platelet glycoprotein receptors with cytosolic YxxL and YxxM motifs, including GPVI, CLEC-2 and PEAR1, triggers activation via phosphorylation of the conserved tyrosine residues and recruitment of the tandem SH2 (Src homology 2) domain effector proteins, Syk and PI 3-kinase. We have modelled the clustering of these receptors with monovalent, divalent and tetravalent soluble ligands and with transmembrane ligands based on the law of mass action using ordinary differential equations and agent-based modelling. The models were experimentally evaluated in platelets and transfected cell lines using monovalent and multivalent ligands, including novel nanobody-based divalent and tetravalent ligands, by fluorescence correlation spectroscopy. Ligand valency, receptor number, receptor dimerisation, receptor phosphorylation and a cytosolic tandem SH2 domain protein act in synergy to drive receptor clustering. Threshold concentrations of a CLEC-2-blocking antibody and Syk inhibitor act in synergy to block platelet aggregation. This offers a strategy for countering the effect of avidity of multivalent ligands and in limiting off-target effects.

S100A8/A9 drives the formation of procoagulant platelets through GPIbα.

S100A8/A9, also known as "calprotectin" or "MRP8/14," is an alarmin primarily secreted by activated myeloid cells with antimicrobial, proinflammatory, and prothrombotic properties. Increased plasma levels of S100A8/A9 in thrombo-inflammatory diseases are associated with thrombotic complications. We assessed the presence of S100A8/A9 in the plasma and lung autopsies from patients with COVID-19 and investigated the molecular mechanism by which S100A8/A9 affects platelet function and thrombosis. S100A8/A9 plasma levels were increased in patients with COVID-19 and sustained high levels during hospitalization correlated with poor outcomes. Heterodimeric S100A8/A9 was mainly detected in neutrophils and deposited on the vessel wall in COVID-19 lung autopsies. Immobilization of S100A8/A9 with collagen accelerated the formation of a fibrin-rich network after perfusion of recalcified blood at venous shear. In vitro, platelets adhered and partially spread on S100A8/A9, leading to the formation of distinct populations of either P-selectin or phosphatidylserine (PS)-positive platelets. By using washed platelets, soluble S100A8/A9 induced PS exposure but failed to induce platelet aggregation, despite GPIIb/IIIa activation and alpha-granule secretion. We identified GPIbα as the receptor for S100A8/A9 on platelets inducing the formation of procoagulant platelets with a supporting role for CD36. The effect of S100A8/A9 on platelets was abolished by recombinant GPIbα ectodomain, platelets from a patient with Bernard-Soulier syndrome with GPIb-IX-V deficiency, and platelets from mice deficient in the extracellular domain of GPIbα. We identified the S100A8/A9-GPIbα axis as a novel targetable prothrombotic pathway inducing procoagulant platelets and fibrin formation, in particular in diseases associated with high levels of S100A8/A9, such as COVID-19.

Hospital Access Block: A Scoping Review.

INTRODUCTION: The overarching objective of this scoping review was to explore the breadth of health care literature in attempts to identify current strategies that hospitals adopt to improve patient bed flow, reduce access and exit block while optimizing patient care. METHODS: PubMed, CINAHL, Embase, Proquest, and Cochrane electronic library databases supported literature search in March 2021. Scholarly articles that met the 3 eligibility criteria-access block causes, effects, and solutions-were considered. Joanna Briggs Institute Guidelines supported first- and second-level literary screening processes. RESULTS: The synthesis included 43 references. Most initiatives addressed access (n = 15), followed by care (n = 16) and then community (n = 9), with a further 3 articles providing commentary across all 3 domains (n = 3). Evidence supported Lean principles in both emergency department and inpatient sector. Lean principles addressing access included physician-led ED triage models, point-of-care testing, overcapacity protocols, mental health team collocation models, and fast-track services. Inpatient care Lean concepts validated gains in multidisciplinary rounds, appropriate allocation of allied health services with a 7-days-a-week model, staggering of elective surgeries, journey boards usage, transit lounges, and lateral transfers. Most literature addressing the backend was narrative in nature, theorized, and advocating for solutions and policy reform. DISCUSSION: This study addressed aims and identified current strategies that hospitals adopt to tackle access block while guaranteeing patient care. Government-supported research to map out evidence-based models of care that address exit block and demonstrate efficiencies is required to optimize access to care in the community.

Katacine Is a New Ligand of CLEC-2 that Acts as a Platelet Agonist.

BACKGROUND: CLEC-2 is a platelet receptor with an important role in thromboinflammation but a minor role in hemostasis. Two endogenous ligands of CLEC-2 have been identified, the transmembrane protein podoplanin and iron-containing porphyrin hemin, which is formed following hemolysis from red blood cells. Other exogenous ligands such as rhodocytin have contributed to our understanding of the role of CLEC-2. OBJECTIVES: To identify novel CLEC-2 small-molecule ligands to aid therapeutic targeting of CLEC-2. METHODS: ALPHA screen technology has been used for the development of a high-throughput screening (HTS) assay recapitulating the podoplanin-CLEC-2 interaction. Light transmission aggregometry was used to evaluate platelet aggregation. Immunoprecipitation and western blot were used to evaluate direct phosphorylation of CLEC-2 and downstream protein phosphorylation. Autodock vina software was used to predict the molecular binding site of katacine and mass spectrometry to determine the polymeric nature of the ligand. RESULTS AND CONCLUSION: We developed a CLEC-2-podoplanin interaction assay in a HTS format and screened 5,016 compounds from a European Union-open screen library. We identified katacine, a mixture of polymers of proanthocyanidins, as a novel ligand for CLEC-2 and showed that it induces platelet aggregation and CLEC-2 phosphorylation via Syk and Src kinases. Platelet aggregation induced by katacine is inhibited by the anti-CLEC-2 monoclonal antibody fragment AYP1 F(ab)'2. Katacine is a novel nonprotein ligand of CLEC-2 that could contribute to a better understanding of CLEC-2 activation in human platelets.

Evidence that GPVI is Expressed as a Mixture of Monomers and Dimers, and that the D2 Domain is not Essential for GPVI Activation.

Collagen has been proposed to bind to a unique epitope in dimeric glycoprotein VI (GPVI) and the number of GPVI dimers has been reported to increase upon platelet activation. However, in contrast, the crystal structure of GPVI in complex with collagen-related peptide (CRP) showed binding distinct from the site of dimerization. Further fibrinogen has been reported to bind to monomeric but not dimeric GPVI. In the present study, we have used the advanced fluorescence microscopy techniques of single-molecule microscopy, fluorescence correlation spectroscopy (FCS) and bioluminescence resonance energy transfer (BRET), and mutagenesis studies in a transfected cell line model to show that GPVI is expressed as a mixture of monomers and dimers and that dimerization through the D2 domain is not critical for activation. As many of these techniques cannot be applied to platelets to resolve this issue, due to the high density of GPVI and its anucleate nature, we used Förster resonance energy transfer (FRET) to show that endogenous GPVI is at least partially expressed as a dimer on resting and activated platelet membranes. We propose that GPVI may be expressed as a monomer on the cell surface and it forms dimers in the membrane through diffusion, giving rise to a mixture of monomers and dimers. We speculate that the formation of dimers facilitates ligand binding through avidity.

Structure-function relationship of the platelet glycoprotein VI (GPVI) receptor: does it matter if it is a dimer or monomer?

GPVI is a critical signaling receptor responsible for collagen-induced platelet activation and a promising anti-thrombotic target in conditions such as coronary artery thrombosis, ischemic stroke, and atherothrombosis. This is due to the ability to block GPVI while having minimal effects on hemostasis, making it a more attractive target over current dual-antiplatelet therapy (DAPT) with acetyl salicylic acid and P2Y12 inhibitors where bleeding can be a problem. Our current understanding of how the structure of GPVI relates to function is inadequate and recent studies contradict each other. In this article, we summarize the structure-function relationships underlying the activation of GPVI by its major ligands, including collagen, fibrin(ogen), snake venom toxins and charged exogenous ligands such as diesel exhaust particles. We argue that contrary to popular belief dimerization of GPVI is not required for binding to collagen but serves to facilitate binding through increased avidity, and that GPVI is expressed as a mixture of monomers and dimers on resting platelets, with binding of multivalent ligands inducing higher order clustering.

Structural characterization of a novel GPVI-nanobody complex reveals a biologically active domain-swapped GPVI dimer.

Glycoprotein VI (GPVI) is the major signaling receptor for collagen on platelets. We have raised 54 nanobodies (Nb), grouped into 33 structural classes based on their complementary determining region 3 loops, against recombinant GPVI-Fc (dimeric GPVI) and have characterized their ability to bind recombinant GPVI, resting and activated platelets, and to inhibit platelet activation by collagen. Nbs from 6 different binding classes showed the strongest binding to recombinant GPVI-Fc, suggesting that there was not a single dominant class. The most potent 3, Nb2, 21, and 35, inhibited collagen-induced platelet aggregation with nanomolar half maximal inhibitory concentration (IC50) values and inhibited platelet aggregation under flow. The binding KD of the most potent Nb, Nb2, against recombinant monomeric and dimeric GPVI was 0.6 and 0.7 nM, respectively. The crystal structure of monomeric GPVI in complex with Nb2 revealed a binding epitope adjacent to the collagen-related peptide (CRP) binding groove within the D1 domain. In addition, a novel conformation of GPVI involving a domain swap between the D2 domains was observed. The domain swap is facilitated by the outward extension of the C-C' loop, which forms the domain swap hinge. The functional significance of this conformation was tested by truncating the hinge region so that the domain swap cannot occur. Nb2 was still able to displace collagen and CRP binding to the mutant, but signaling was abolished in a cell-based NFAT reporter assay. This demonstrates that the C-C' loop region is important for GPVI signaling but not ligand binding and suggests the domain-swapped structure may represent an active GPVI conformation.

Adenosine and Forskolin Inhibit Platelet Aggregation by Collagen but not the Proximal Signalling Events.

BACKGROUND: The G protein-coupled receptor, adenosine A2A, signals through the stimulatory G protein, Gs, in platelets leading to activation of adenylyl cyclase and elevation of cyclic adenosine monophosphate (cAMP) and inhibition of platelet activation. OBJECTIVE: This article investigates the effect of A2A receptor activation on signalling by the collagen receptor glycoprotein (GP) VI in platelets. METHODS: Washed human platelets were stimulated by collagen or the GPVI-specific agonist collagen-related peptide (CRP) in the presence of the adenosine receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA) or the adenylyl cyclase activator, forskolin and analysed for aggregation, adenosine triphosphate secretion, protein phosphorylation, spreading, Ca2+ mobilisation, GPVI receptor clustering, cAMP, thromboxane B2 (TxB2) and P-selectin exposure. RESULTS: NECA, a bioactive adenosine analogue, partially inhibits aggregation and secretion to collagen or CRP in the absence or presence of the P2Y12 receptor antagonist, cangrelor and the cyclooxygenase inhibitor, indomethacin. The inhibitory effect in the presence of the three inhibitors is largely overcome at higher concentrations of collagen but not CRP. Neither NECA nor forskolin altered clustering of GPVI, elevation of Ca2+ or spreading of platelets on a collagen surface. Further, neither NECA nor forskolin, altered collagen-induced tyrosine phosphorylation of Syk, LAT nor PLCγ2. However, NECA and forskolin inhibited platelet activation by the TxA2 mimetic, U46619, but not the combination of adenosine diphosphate and collagen. CONCLUSION: NECA and forskolin have no effect on the proximal signalling events by collagen. They inhibit platelet activation in a response-specific manner in part through inhibition of the feedback action of TxA2.

Integration of "omics" Data and Phenotypic Data Within a Unified Extensible Multimodal Framework.

Analysis of "omics" data is often a long and segmented process, encompassing multiple stages from initial data collection to processing, quality control and visualization. The cross-modal nature of recent genomic analyses renders this process challenging to both automate and standardize; consequently, users often resort to manual interventions that compromise data reliability and reproducibility. This in turn can produce multiple versions of datasets across storage systems. As a result, scientists can lose significant time and resources trying to execute and monitor their analytical workflows and encounter difficulties sharing versioned data. In 2015, the Ludmer Centre for Neuroinformatics and Mental Health at McGill University brought together expertise from the Douglas Mental Health University Institute, the Lady Davis Institute and the Montreal Neurological Institute (MNI) to form a genetics/epigenetics working group. The objectives of this working group are to: (i) design an automated and seamless process for (epi)genetic data that consolidates heterogeneous datasets into the LORIS open-source data platform; (ii) streamline data analysis; (iii) integrate results with provenance information; and (iv) facilitate structured and versioned sharing of pipelines for optimized reproducibility using high-performance computing (HPC) environments via the CBRAIN processing portal. This article outlines the resulting generalizable "omics" framework and its benefits, specifically, the ability to: (i) integrate multiple types of biological and multi-modal datasets (imaging, clinical, demographics and behavioral); (ii) automate the process of launching analysis pipelines on HPC platforms; (iii) remove the bioinformatic barriers that are inherent to this process; (iv) ensure standardization and transparent sharing of processing pipelines to improve computational consistency; (v) store results in a queryable web interface; (vi) offer visualization tools to better view the data; and (vii) provide the mechanisms to ensure usability and reproducibility. This framework for workflows facilitates brain research discovery by reducing human error through automation of analysis pipelines and seamless linking of multimodal data, allowing investigators to focus on research instead of data handling.

Antipsychotic polypharmacy and augmentation strategies prior to clozapine initiation: a historical cohort study of 310 adults with treatment-resistant schizophrenic disorders.

RATIONALE: Antipsychotic polypharmacy (APP) is commonly used in schizophrenia despite a lack of robust evidence for efficacy, as well as evidence of increased rates of adverse drug reactions and mortality. OBJECTIVES: We sought to examine APP and the use of other adjunctive medications in patients with treatment-resistant schizophrenic disorders (ICD-10 diagnoses F20-F29) immediately prior to clozapine initiation, and to investigate clinical and sociodemographic factors associated with APP use in this setting. METHODS: Analysis of case notes from 310 patients receiving their first course of clozapine at the South London and Maudsley NHS Trust (SLaM) was undertaken using the Clinical Record Interactive Search (CRIS) case register. Medication taken immediately prior to clozapine initiation was recorded, and global clinical severity was assessed at time points throughout the year prior to medication assessment using the Clinical Global Impression - Severity scale (CGI-S). Logistic regression was used to investigate factors associated with APP. RESULTS: The point prevalence of APP prior to clozapine initiation was 13.6% (n=42), with 32.6% of subjects prescribed adjuvant psychotropic medications. APP was associated with increasing number of adjuvant medications (odds ratio (OR) 1.97, 95% confidence interval (CI) 1.27-3.06), concurrent depot antipsychotic prescription (OR 2.64, CI 1.24-5.62), concurrent antidepressant prescription (OR 4.40, CI 1.82-10.63) and a CGI-S over the previous year within the two middle quartiles (Quartile 2 vs 1 OR 6.19, CI 1.81-21.10; Quartile 3 vs 1 OR 4.45, CI 1.29-15.37; Quartile 4 vs 1 OR 1.88, CI 0.45-7.13). CONCLUSIONS: APP and augmentation of antipsychotics with antidepressants, mood stabilizers and benzodiazepines are being employed in treatment-resistant schizophrenia prior to clozapine. The conservative APP rate observed may have been influenced by an initiative within SLaM that reduced APP rates during the study window. Efforts to reduce the use of poorly evidenced prescribing should focus on adjuvant medications as well as APP.

Somatic mitochondrial DNA mutations do not increase neuronal vulnerability to MPTP in young POLG mutator mice.

Mitochondrial DNA (mtDNA) mutations are hypothesized to play a pathogenic role in aging and age-related neurodegenerative diseases such as Parkinson's disease (PD). In support of this, high levels of somatic mtDNA mutations in "POLG mutator" mice carrying a proofreading-deficient form of mtDNA polymerase ã (Polg(D257A)) lead to a premature aging phenotype. However, the relevance of this finding to the normal aging process has been questioned as the number of mutations is greater even in young POLG mutator mice, which shows no overt phenotype, than levels achieved during normal aging in mice. Vulnerability of dopaminergic neurons to 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) increases with age, and we hypothesized that this may result in part from the accumulation with age of somatic mtDNA mutations. If correct, then levels of mutations in young (2­3 month old) POLG mutator mice should be sufficient to increase vulnerability to MPTP. In contrast, we find that susceptibility to MPTP in both heterozygous and homozygous POLG mutator mice at this young age is not different from that of wild type littermate controls as measured by levels of tyrosine hydroxylase positive (TH+) striatal terminals, striatal dopamine and its metabolites, a marker of oxidative damage, or stereological counts of TH+ and total substantia nigra neurons. These unexpected results do not support the hypothesis that somatic mtDNA mutations contribute to the age-related vulnerability of dopaminergic neurons to MPTP. It remains possible that somatic mtDNA mutations influence vulnerability to other stressors, or require additional time for the deleterious consequences to manifest. Furthermore, the impact of the higher levels of mutations present at older ages in these mice was not assessed in our study, although a prior study also failed to detect an increase in vulnerability to MPTP in older mice. With these caveats, the current data do not provide evidence for a role of somatic mtDNA mutations in determining the vulnerability to MPTP.

Rapamycin drives selection against a pathogenic heteroplasmic mitochondrial DNA mutation.

Mitochondrial DNA (mtDNA) mutations cause a variety of mitochondrial disorders for which effective treatments are lacking. Emerging data indicate that selective mitochondrial degradation through autophagy (mitophagy) plays a critical role in mitochondrial quality control. Inhibition of mammalian target of rapamycin (mTOR) kinase activity can activate mitophagy. To test the hypothesis that enhancing mitophagy would drive selection against dysfunctional mitochondria harboring higher levels of mutations, thereby decreasing mutation levels over time, we examined the impact of rapamycin on mutation levels in a human cytoplasmic hybrid (cybrid) cell line expressing a heteroplasmic mtDNA G11778A mutation, the most common cause of Leber's hereditary optic neuropathy. Inhibition of mTORC1/S6 kinase signaling by rapamycin induced colocalization of mitochondria with autophagosomes, and resulted in a striking progressive decrease in levels of the G11778A mutation and partial restoration of ATP levels. Rapamycin-induced upregulation of mitophagy was confirmed by electron microscopic evidence of increased autophagic vacuoles containing mitochondria-like organelles. The decreased mutational burden was not due to rapamycin-induced cell death or mtDNA depletion, as there was no significant difference in cytotoxicity/apoptosis or mtDNA copy number between rapamycin and vehicle-treated cells. These data demonstrate the potential for pharmacological inhibition of mTOR kinase activity to activate mitophagy as a strategy to drive selection against a heteroplasmic mtDNA G11778A mutation and raise the exciting possibility that rapamycin may have therapeutic potential for the treatment of mitochondrial disorders associated with heteroplasmic mtDNA mutations, although further studies are needed to determine if a similar strategy will be effective for other mutations and other cell types.

Behavioral and metabolic characterization of heterozygous and homozygous POLG mutator mice.

The mitochondrial DNA (mtDNA) polymerase γ (POLG) mutator mice provide the first experimental evidence that high levels of somatic mtDNA mutations can be functionally significant. Here we report that older homozygous, but not heterozygous, POLG mice show significant reductions in striatal dopaminergic terminals as well as deficits in motor function. However, resting oxygen consumption, heat production, mtDNA content and mitochondrial electron transport chain activities are significantly decreased at older ages in both homozygous and heterozygous mice. These results indicate that high levels of somatic mtDNA mutations can contribute to dopaminergic dysfunction and to behavioral and metabolic deficits.

Pgc-1α overexpression downregulates Pitx3 and increases susceptibility to MPTP toxicity associated with decreased Bdnf.

Multiple mechanisms likely contribute to neuronal death in Parkinson's disease (PD), including mitochondrial dysfunction and oxidative stress. Peroxisome proliferator-activated receptor gamma co-activator-1 alpha (PGC-1α) positively regulates the expression of genes required for mitochondrial biogenesis and the cell's antioxidant responses. Also, expression of PGC-1α-regulated genes is low in substantia nigra (SN) neurons in early PD. Thus upregulation of PGC-1α is a candidate neuroprotective strategy in PD. Here, an adeno-associated virus (AAV) was used to induce unilateral overexpression of Pgc-1α, or a control gene, in the SN of wild-type C57BL/6CR mice. Three weeks after AAV administration, mice were treated with saline or MPTP. Overexpression of Pgc-1α in the SN induced expression of target genes, but unexpectedly it also greatly reduced the expression of tyrosine hydroxylase (Th) and other markers of the dopaminergic phenotype with resultant severe loss of striatal dopamine. Reduced Th expression was associated with loss of Pitx3, a transcription factor that is critical for the development and maintenance of dopaminergic cells. Expression of the neurotrophic factor Bdnf, which also is regulated by Pitx3, similarly was reduced. Overexpression of Pgc-1α also led to increased sensitivity to MPTP-induced death of Th+ neurons. Pgc-1α overexpression alone, in the absence of MPTP treatment, did not lead to cell loss in the SN or to loss of dopaminergic terminals. These data demonstrate that overexpression of Pgc-1α results in dopamine depletion associated with lower levels of Pitx3 and enhances susceptibility to MPTP. These data may have ramifications for neuroprotective strategies targeting overexpression of PGC-1α in PD.