Phenylalanine hydroxylase variants interact with the co-chaperone DNAJC12.

DNAJC12, a type III member of the HSP40/DNAJ family, has been identified as the specific co-chaperone of phenylalanine hydroxylase (PAH) and the other aromatic amino acid hydroxylases. DNAJ proteins work together with molecular chaperones of the HSP70 family to assist in proper folding and maintenance of intracellular stability of their clients. Autosomal recessive mutations in DNAJC12 were found to reduce PAH levels, leading to hyperphenylalaninemia (HPA) in patients without mutations in PAH. In this work, we investigated the interaction of normal wild-type DNAJC12 with mutant PAH in cells expressing several PAH variants associated with HPA in humans, as well as in the Enu1/1 mouse model, homozygous for the V106A-Pah variant, which leads to severe protein instability, accelerated PAH degradation and mild HPA. We found that mutant PAH exhibits increased ubiquitination, instability, and aggregation compared with normal PAH. In mouse liver lysates, we showed that DNAJC12 interacts with monoubiquitin-tagged PAH. This form represented a major fraction of PAH in the Enu1/1 but was also present in liver of wild-type PAH mice. Our results support a role of DNAJC12 in the processing of misfolded ubiquitinated PAH by the ubiquitin-dependent proteasome/autophagy systems and add to the evidence that the DNAJ proteins are important players both for proper folding and degradation of their clients.

Phosphorylation at serine 31 targets tyrosine hydroxylase to vesicles for transport along microtubules.

Tyrosine hydroxylase (TH) catalyzes the conversion of l-tyrosine into l-DOPA, which is the rate-limiting step in the synthesis of catecholamines, such as dopamine, in dopaminergergic neurons. Low dopamine levels and death of the dopaminergic neurons are hallmarks of Parkinson's disease (PD), where α-synuclein is also a key player. TH is highly regulated, notably by phosphorylation of several Ser/Thr residues in the N-terminal tail. However, the functional role of TH phosphorylation at the Ser-31 site (THSer(P)-31) remains unclear. Here, we report that THSer(P)-31 co-distributes with the Golgi complex and synaptic-like vesicles in rat and human dopaminergic cells. We also found that the TH microsomal fraction content decreases after inhibition of cyclin-dependent kinase 5 (Cdk5) and ERK1/2. The cellular distribution of an overexpressed phospho-null mutant, TH1-S31A, was restricted to the soma of neuroblastoma cells, with decreased association with the microsomal fraction, whereas a phospho-mimic mutant, TH1-S31E, was distributed throughout the soma and neurites. TH1-S31E associated with vesicular monoamine transporter 2 (VMAT2) and α-synuclein in neuroblastoma cells, and endogenous THSer(P)-31 was detected in VMAT2- and α-synuclein-immunoprecipitated mouse brain samples. Microtubule disruption or co-transfection with α-synuclein A53T, a PD-associated mutation, caused TH1-S31E accumulation in the cell soma. Our results indicate that Ser-31 phosphorylation may regulate TH subcellular localization by enabling its transport along microtubules, notably toward the projection terminals. These findings disclose a new mechanism of TH regulation by phosphorylation and reveal its interaction with key players in PD, opening up new research avenues for better understanding dopamine synthesis in physiological and pathological states.

Stabilization of Human Tyrosine Hydroxylase in Maltodextrin Nanoparticles for Delivery to Neuronal Cells and Tissue.

Enzyme replacement therapy (ERT) is a therapeutic approach envisioned decades ago for the correction of genetic disorders, but ERT has been less successful for the correction of disorders with neurological manifestations. In this work, we have tested the functionality of nanoparticles (NP) composed of maltodextrin with a lipid core to bind and stabilize tyrosine hydroxylase (TH). This is a complex and unstable brain enzyme that catalyzes the rate-limiting step in the synthesis of dopamine and other catecholamine neurotransmitters. We have characterized these TH-loaded NPs to evaluate their potential for ERT in diseases associated with TH dysfunction. Our results show that TH can be loaded into the lipid core maltodextrin NPs with high efficiency, and both stability and activity are maintained through loading and are preserved during storage. Binding to NPs also favored the uptake of TH to neuronal cells, both in cell culture and in the brain. The internalized NP-bound TH was active as we measured an increase in intracellular L-Dopa synthesis following NP uptake. Our approach seems promising for the use of catalytically active NPs in ERT to treat neurodegenerative and neuropsychiatric disorders characterized by dopamine deficiency, notably Parkinson's disease.

Discovery of compounds that protect tyrosine hydroxylase activity through different mechanisms.

Pharmacological chaperones are small compounds that correct the folding of mutant proteins, and represent a promising therapeutic strategy for misfolding diseases. We have performed a screening of 10,000 compounds searching for pharmacological chaperones of tyrosine hydroxylase (TH), the tetrahydrobiopterin (BH4)-dependent enzyme that catalyzes the rate-limiting step in the synthesis of catecholamines. A large number of compounds bound to human TH, isoform 1 (hTH1), but only twelve significantly protected wild-type (hTH1-wt) and mutant TH-R233H (hTH1-p.R202H), associated to the rare neurological disorder TH deficiency (THD), from time-dependent loss of activity. Three of them (named compounds 2, 4 and 5) were subjected to detailed characterization of their functional and molecular effects. Whereas compounds 2 and 4 had a characteristic pharmacological chaperone (stabilizing) effect, compound 5 protected the activity in a higher extent than expected from the low conformational stabilization exerted on hTH1. Compounds 4 and 5 were weak competitive inhibitors with respect to the cofactor BH4 and, as seen by electron paramagnetic resonance, they induced small changes to the first coordination sphere of the catalytic iron. Molecular docking also indicated active-site location with coordination to the iron through a pyrimidine nitrogen atom. Interestingly, compound 5 increased TH activity in cells transiently transfected with either hTH1-wt or the THD associated mutants p.L205P, p.R202H and p.Q381K without affecting the steady-state TH protein levels. This work revealed different mechanisms for the action of pharmacological chaperones and identifies a subtype of compounds that preserve TH activity by weak binding to the catalytic iron. This article is part of a Special Issue entitled: Cofactor-dependent proteins: Evolution, chemical diversity and bio-applications.

Regulation of tyrosine hydroxylase is preserved across different homo- and heterodimeric 14-3-3 proteins.

Tyrosine hydroxylase (TH) is regulated by members of the 14-3-3 protein family. However, knowledge about the variation between 14-3-3 proteins in their regulation of TH is still limited. We examined the binding, effects on activation and dephosphorylation kinetics of tyrosine hydroxylase (TH) by abundant midbrain 14-3-3 proteins (ß, η, ζ, γ and ε) of different dimer composition. All 14-3-3 homodimers and their respective 14-3-3ε-heterodimers bound with similar high affinity (K d values of 1.4-3.8 nM) to serine19 phosphorylated human TH (TH-pS19). We similarly observed a consistent activation of bovine (3.3- to 4.4-fold) and human TH-pS19 (1.3-1.6 fold) across all the different 14-3-3 dimer species, with homodimeric 14-3-3γ being the strongest activator. Both hetero- and homodimers of 14-3-3 strongly inhibited dephosphorylation of TH-pS19, and we speculate if this is an important homeostatic mechanism of 14-3-3 target-protein regulation in vivo. We conclude that TH is a robust interaction partner of different 14-3-3 dimer types with moderate variability between the 14-3-3 dimers on their regulation of TH.

Phosphorylation dependence and stoichiometry of the complex formed by tyrosine hydroxylase and 14-3-3γ.

Phosphorylated tyrosine hydroxylase (TH) can form complexes with 14-3-3 proteins, resulting in enzyme activation and stabilization. Although TH was among the first binding partners identified for these ubiquitous regulatory proteins, the binding stoichiometry and the activation mechanism remain unknown. To address this, we performed native mass spectrometry analyses of human TH (nonphosphorylated or phosphorylated on Ser19 (TH-pS19), Ser40 (TH-pS40), or Ser19 and Ser40 (TH-pS19pS40)) alone and together with 14-3-3γ. Tetrameric TH-pS19 (224 kDa) bound 14-3-3γ (58.3 kDa) with high affinity (Kd = 3.2 nM), generating complexes containing either one (282.4 kDa) or two (340.8 kDa) dimers of 14-3-3. Electron microscopy also revealed one major population of an asymmetric complex, consistent with one TH tetramer and one 14-3-3 dimer, and a minor population of a symmetric complex of one TH tetramer with two 14-3-3 dimers. Lower phosphorylation stoichiometries (0.15-0.54 phosphate/monomer) produced moderate changes in binding kinetics, but native MS detected much less of the symmetric TH:14-3-3γ complex. Interestingly, dephosphorylation of [(32)P]-TH-pS19 was mono-exponential for low phosphorylation stoichiometries (0.18-0.52), and addition of phosphatase accelerated the dissociation of the TH-pS19:14-3-3γ complex 3- to 4-fold. All together this is consistent with a model in which the pS19 residues in the TH tetramer contribute differently in the association to 14-3-3γ. Complex formation between TH-pS40 and 14-3-3γ was not detected via native MS, and surface plasmon resonance showed that the interaction was very weak. Furthermore, TH-pS19pS40 behaved similarly to TH-pS19 in terms of binding stoichiometry and affinity (Kd = 2.1 nM). However, we found that 14-3-3γ inhibited the phosphorylation rate of TH-pS19 by PKA (3.5-fold) on Ser40. We therefore conclude that Ser40 does not significantly contribute to the binding of 14-3-3γ, and rather has reduced accessibility in the TH:14-3-3γ complex. This adds to our understanding of the fine-tuned physiological regulation of TH, including hierarchical phosphorylation at multiple sites.

Pharmacological chaperones as a potential therapeutic option in methylmalonic aciduria cblB type.

Methylmalonic aciduria (MMA) cblB type is caused by mutations in the MMAB gene. This encodes the enzyme ATP:cob(I)alamin adenosyltransferase (ATR), which converts reduced cob(I)alamin to an active adenosylcobalamin cofactor. We recently reported the presence of destabilizing pathogenic mutations that retain some residual ATR activity. The aim of the present study was to seek pharmacological chaperones as a tailored therapy for stabilizing the ATR protein. High-throughput ligand screening of over 2000 compounds was performed; six were found to enhance the thermal stability of purified recombinant ATR. Further studies using a well-established bacterial system in which the recombinant ATR protein was expressed in the presence of these six compounds, showed them all to increase the stability of the wild-type ATR and the p.Ile96Thr mutant proteins. Compound V (N-{[(4-chlorophenyl)carbamothioyl]amino}-2-phenylacetamide) significantly increased this stability and did not act as an inhibitor of the purified protein. Importantly, compound V increased the activity of ATR in patient-derived fibroblasts harboring the destabilizing p.Ile96Thr mutation in a hemizygous state to within control range. When cobalamin was coadministrated with compound V, mutant ATR activity further improved. Oral administration of low doses of compound V to C57BL/6J mice for 12 days, led to increase in steady-state levels of ATR protein in liver and brain (disease-relevant organs). These results hold promise for the clinical use of pharmacological chaperones in MMA cblB type patients harboring chaperone-responsive mutations.

Different altered pattern expression of genes related to apoptosis in isolated methylmalonic aciduria cblB type and combined with homocystinuria cblC type.

An increased reactive oxygen species (ROS) production and apoptosis rate have been associated with several disorders involved in cobalamin metabolism, including isolated methylmalonic aciduria (MMA) cblB type and MMA combined with homocystinuria (MMAHC) cblC type. Given the relevance of p38 and JNK kinases in stress-response, their activation in fibroblasts from a spectrum of patients (mut, cblA, cblB, cblC and cblE) was analyzed revealing an increased expression of the phosphorylated-forms, specially in cblB and cblC cell lines that presented the highest ROS and apoptosis levels. To gain further insight into the molecular mechanisms responsible for the enhanced apoptotic process observed in cblB and cblC fibroblasts, we evaluated the expression pattern of 84 apoptosis-related genes by quantitative real-time PCR. An elevated number of pro-apoptotic genes were overexpressed in cblC cells showing a higher rate of apoptosis compared to cblB and control samples. Additionally, apoptosis appears to be mainly triggered through the extrinsic pathway in cblC, while the intrinsic pathway was primarily activated in cblB cells. The differences observed regarding the apoptosis rate and preferred pathway between cblB and cblC patients, who both built up methylmalonic acid, might be explained by the accumulated homocysteine in the cblC group. The loss of MMACHC function in cblC patients might be partially responsible for the oxidative stress and apoptosis processes observed in these cell lines. Our results suggest that ROS production may represent a genetic modifier of the phenotype and support the potential of using antioxidants as a novel therapeutic strategy to improve the severe neurological outcome of these rare diseases.

Functional and structural analysis of five mutations identified in methylmalonic aciduria cblB type.

ATP:cob(I)alamin adenosyltransferase (ATR, E.C.2.5.1.17) converts reduced cob(I)alamin to the adenosylcobalamin cofactor. Mutations in the MMAB gene encoding ATR are responsible for the cblB type methylmalonic aciduria. Here we report the functional analysis of five cblB mutations to determine the underlying molecular basis of the dysfunction. The transcriptional profile along with minigenes analysis revealed that c.584G>A, c.349-1G>C, and c.290G>A affect the splicing process. Wild-type ATR and the p.I96T (c.287T>C) and p.R191W (c.571C>T) mutant proteins were expressed in a prokaryote and a eukaryotic expression systems. The p.I96T protein was enzymatically active with a K(M) for ATP and K(D) for cob(I)alamin similar to wild-type enzyme, but exhibited a 40% reduction in specific activity. Both p.I96T and p.R191W mutant proteins are less stable than the wild-type protein, with increased stability when expressed under permissive folding conditions. Analysis of the oligomeric state of both mutants showed a structural defect for p.I96T and also a significant impact on the amount of recovered mutant protein that was more pronounced for p.R191W that, along with the structural analysis, suggest they might be misfolded. These results could serve as a basis for the implementation of pharmacological therapies aimed at increasing the residual activity of this type of mutations.

Genetic and cellular studies of oxidative stress in methylmalonic aciduria (MMA) cobalamin deficiency type C (cblC) with homocystinuria (MMACHC).

Methylmalonic aciduria (MMA) cobalamin deficiency type C (cblC) with homocystinuria (MMACHC) is the most frequent genetic disorder of vitamin B(12) metabolism. The aim of this work was to identify the mutational spectrum in a cohort of cblC-affected patients and the analysis of the cellular oxidative stress and apoptosis processes, in the presence or absence of vitamin B(12). The mutational spectrum includes nine previously described mutations: c.3G>A (p.M1L), c.217C>T (p.R73X), c.271dupA (p.R91KfsX14), c.331C>T (p.R111X), c.394C>T (p.R132X), c.457C>T (p.R153X), c.481C>T (p.R161X), c.565C>A (p.R189S), and c.615C>G (p.Y205X), and two novel changes, c.90G>A (p.W30X) and c.81+2T>G (IVS1+2T>G). The most frequent change was the known c.271dupA mutation, which accounts for 85% of the mutant alleles characterized in this cohort of patients. Owing to its high frequency, a real-time PCR and subsequent high-resolution melting (HRM) analysis for this mutation has been established for diagnostic purposes. All cell lines studied presented a significant increase of intracellular reactive oxygen species (ROS) content, and also a high rate of apoptosis, suggesting that elevated ROS levels might induce apoptosis in cblC patients. In addition, ROS levels decreased in hydroxocobalamin-incubated cells, indicating that cobalamin might either directly or indirectly act as a scavenger of ROS. ROS production might be considered as a phenotypic modifier in cblC patients, and cobalamin supplementation or additional antioxidant drugs might suppress apoptosis and prevent cellular damage in these patients.

Pharmacological Chaperones that Protect Tetrahydrobiopterin Dependent Aromatic Amino Acid Hydroxylases Through Different Mechanisms.

The aromatic amino acid hydroxylase (AAAH) enzyme family includes phenylalanine hydroxylase (PAH), tyrosine hydroxylase (TH) and the tryptophan hydroxylases (TPH1 and TPH2). All four members of the AAAH family require iron, dioxygen and the cofactor (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4) to hydroxylate their respective substrates. The AAAHs are involved in severe diseases; whereas polymorphisms and variants in the TPH genes are associated to neuropsychiatric disorders, mutations in PAH and TH are responsible for the autosomal recessive disorders phenylketonuria (PKU) and TH deficiency (THD), respectively. A large number of PKU and THD-causing mutations give rise to unstable, misfolded proteins. The degree of conformational instability correlates well with the severity of the patient phenotypes, underlying the relevance of searching for stabilizing compounds that may protect from loss of protein and activity in vivo. Supplementation with the cofactor BH4 exerts a multifactorial response in PAH, where one of the main mechanisms for the induced increase in PAH activity in BH4- responsive PKU patients appears to be a pharmacological chaperone effect. For TH the stabilizing effect of BH4 is less established. On the other hand, a number of compounds with pharmacological chaperone potential for PKU and THD mutants have been discovered. The stabilizing effect of these compounds has been established in vitro, in cells and in animal models. A recent study with TH has revealed different mechanisms for the action of pharmacological chaperones and identifies a subtype of compounds that preserve TH activity by weak binding to the catalytic iron. It is expected that synergistic combinations of different pharmacological chaperones could provide patient-tailored therapeutic options.

Tyrosine Hydroxylase Binding to Phospholipid Membranes Prompts Its Amyloid Aggregation and Compromises Bilayer Integrity.

Tyrosine hydroxylase (TH), a rate-limiting enzyme in the synthesis of catecholamine neurotransmitters and hormones, binds to negatively charged phospholipid membranes. Binding to both large and giant unilamellar vesicles causes membrane permeabilization, as observed by efflux and influx of fluorescence dyes. Whereas the initial protein-membrane interaction involves the N-terminal tail that constitutes an extension of the regulatory ACT-domain, prolonged membrane binding induces misfolding and self-oligomerization of TH over time as shown by circular dichroism and Thioflavin T fluorescence. The gradual amyloid-like aggregation likely occurs through cross-ß interactions involving aggregation-prone motives in the catalytic domains, consistent with the formation of chain and ring-like protofilaments observed by atomic force microscopy in monolayer-bound TH. PC12 cells treated with the neurotoxin 6-hydroxydopamine displayed increased TH levels in the mitochondrial fraction, while incubation of isolated mitochondria with TH led to a decrease in the mitochondrial membrane potential. Furthermore, cell-substrate impedance and viability assays showed that supplementing the culture media with TH compromises cell viability over time. Our results revealed that the disruptive effect of TH on cell membranes may be a cytotoxic and pathogenic factor if the regulation and intracellular stability of TH is compromised.