Site-directed mutagenesis of predicted active site residues in glutamate carboxypeptidase II

Two Sides of Quantum-Based Modeling of Enzyme-Catalyzed Reactions: Mechanistic and Electronic Structure Aspects of the Hydrolysis by Glutamate Carboxypeptidase

We report the results of a computational study of the hydrolysis reaction mechanism of N-acetyl-l-aspartyl-l-glutamate (NAAG) catalyzed by glutamate carboxypeptidase II. Analysis of both mechanistic and electronic structure aspects of this multistep reaction is in the focus of this work. In these simulations, model systems are constructed using the relevant crystal structure of the mutated inactive enzyme. After selection of reaction coordinates, the Gibbs energy profiles of elementary steps of the reaction are computed using molecular dynamics simulations with ab initio type QM/MM potentials (QM/MM MD). Energies and forces in the large QM subsystem are estimated in the DFT(PBE0-D3/6-31G**) approximation. The established mechanism includes four elementary steps with the activation energy barriers not exceeding 7 kcal/mol. The models explain the role of point mutations in the enzyme observed in the experimental kinetic studies; namely, the Tyr552Ile substitution disturbs the "oxyanion hole", and the Glu424Gln replacement increases the distance of the nucleophilic attack. Both issues diminish the substrate activation in the enzyme active site. To quantify the substrate activation, we apply the QTAIM-based approaches and the NBO analysis of dynamic features of the corresponding enzyme-substrate complexes. Analysis of the 2D Laplacian of electron density maps allows one to define structures with the electron density deconcentration on the substrate carbon atom, i.e., at the electrophilic site of reactants. The similar electronic structure element in the NBO approach is a lone vacancy on the carbonyl carbon atom in the reactive species. The electronic structure patterns revealed in the NBO and QTAIM-based analyses consistently clarify the reactivity issues in this system.

Mouse glutamate carboxypeptidase II (GCPII) has a similar enzyme activity and inhibition profile but a different tissue distribution to human GCPII

Glutamate carboxypeptidase II (GCPII), also known as prostate-specific membrane antigen (PSMA) or folate hydrolase, is a metallopeptidase expressed predominantly in the human brain and prostate. GCPII expression is considerably increased in prostate carcinoma, and the enzyme also participates in glutamate excitotoxicity in the brain. Therefore, GCPII represents an important diagnostic marker of prostate cancer progression and a putative target for the treatment of both prostate cancer and neuronal disorders associated with glutamate excitotoxicity. For the development of novel therapeutics, mouse models are widely used. However, although mouse GCPII activity has been characterized, a detailed comparison of the enzymatic activity and tissue distribution of the mouse and human GCPII orthologs remains lacking. In this study, we prepared extracellular mouse GCPII and compared it with human GCPII. We found that mouse GCPII possesses lower catalytic efficiency but similar substrate specificity compared with the human protein. Using a panel of GCPII inhibitors, we discovered that inhibition constants are generally similar for mouse and human GCPII. Furthermore, we observed highest expression of GCPII protein in the mouse kidney, brain, and salivary glands. Importantly, we did not detect GCPII in the mouse prostate. Our data suggest that the differences in enzymatic activity and inhibition profile are rather small; therefore, mouse GCPII can approximate human GCPII in drug development and testing. On the other hand, significant differences in GCPII tissue expression must be taken into account when developing novel GCPII-based anticancer and therapeutic methods, including targeted anticancer drug delivery systems, and when using mice as a model organism.

Chemically synthesized molecules with the targeting and effector functions of antibodies

This article reports the design, synthesis, and evaluation of a novel class of molecules of intermediate size (approximately 7000 Da), which possess both the targeting and effector functions of antibodies. These compounds—called synthetic antibody mimics targeting prostate cancer (SyAM-Ps)—bind simultaneously to prostate-specific membrane antigen and Fc gamma receptor I, thus eliciting highly selective cancer cell phagocytosis. SyAMs have the potential to combine the advantages of both small-molecule and biologic therapies, and may address many drawbacks associated with available treatments for cancer and other diseases.

Glutamate carboxypeptidase II in diagnosis and treatment of neurologic disorders and prostate cancer

Glutamate carboxypeptidase II (GCPII) is a membrane-bound binuclear zinc metallopeptidase with the highest expression levels found in the nervous and prostatic tissue. Throughout the nervous system, glia-bound GCPII is intimately involved in the neuron-neuron and neuron-glia signaling via the hydrolysis of N-acetylaspartylglutamate (NAAG), the most abundant mammalian peptidic neurotransmitter. The inhibition of the GCPII-controlled NAAG catabolism has been shown to attenuate neurotoxicity associated with enhanced glutamate transmission and GCPII-specific inhibitors demonstrate efficacy in multiple preclinical models including traumatic brain injury, stroke, neuropathic and inflammatory pain, amyotrophic lateral sclerosis, and schizophrenia. The second major area of pharmacological interventions targeting GCPII focuses on prostate carcinoma; GCPII expression levels are highly increased in androgen-independent and metastatic disease. Consequently, the enzyme serves as a potential target for imaging and therapy. This review offers a summary of GCPII structure, physiological functions in healthy tissues, and its association with various pathologies. The review also outlines the development of GCPII-specific small-molecule compounds and their use in preclinical and clinical settings.

Structural characterization of Zn(II)-, Co(II)-, and Mn(II)-loaded forms of the argE-encoded N-acetyl-L-ornithine deacetylase from Escherichia coli

The Zn, Co, and Mn K-edge extended X-ray absorption fine structure (EXAFS) spectra of the N-acetyl-l-ornithine deacetylase (ArgE) from Escherichia coli, loaded with one or two equivalents of divalent metal ions (i.e., [Zn(II)_(ArgE)], [Zn(II)Zn(II)(ArgE)], [Co(II)_(ArgE)], [Co(II)Co(II)(ArgE)], [Mn(II)_(ArgE)], and [Mn(II)Mn(II)(ArgE)]), were recorded. The Fourier transformed data (FT) for [Zn(II)_(ArgE)], [Zn(II)Zn(II)(ArgE)], [Co(II)_(ArgE)] and [Co(II)Co(II)(ArgE)] are dominated by a peak at 2.05Å, that can be fit assuming five or six light atom (N,O) scatterers. Inclusion of multiple-scattering contributions from the outer-shell atoms of a histidine-imidazole ring resulted in reasonable Debye-Waller factors for these contributions and a slight reduction in the goodness-of-fit value (f'). Furthermore, the data best fit a model that included a M-M vector at 3.3 and 3.4Å for Zn(II) and Co(II), respectively, suggesting the formation of a dinuclear site. Multiple scattering contributions from the outer-shell atoms of a histidine-imidazole rings are observed at ~3 and 4Å for Zn(II)- and Co(II)-loaded ArgE suggesting at least one histidine ligand at each metal binding site. Likewise, EXAFS data for Mn(II)-loaded ArgE are dominated by a peak at 2.19Å that was best fit assuming six light atom (N,O) scatterers. Due to poor signal to noise ratios for the Mn EXAFS spectra, no Mn-Mn vector could be modeled. Peak intensities for [M(II)_(ArgE)] vs. [M(II)M(II)(ArgE)] suggest the Zn(II), Co(II), and Mn(II) bind to ArgE in a cooperative manner. Since no structural data has been reported for any ArgE enzyme, the EXAFS data reported herein represent the first structural glimpse for ArgE enzymes. These data also provide a structural foundation for the future design of small molecules that function as inhibitors of ArgE and may potentially function as a new class of antibiotics.

Diagnostic and therapeutic imaging for cancer: therapeutic considerations and future directions

As cancer treatment cost soar and the mantra for "personalized medicine" grows louder, we will increasingly be searching for solutions to these diametrically opposed forces. In this review we highlight several exciting novel imaging strategies including MRI, CT, PET SPECT, sentinel node, and ultrasound imaging that hold great promise for improving outcomes through detection of lymph node involvement. We provide clinical data that demonstrate how these evolving strategies have the potential to transform treatment paradigms.

Baculoviral expression and characterization of human recombinant PGCP in the form of an active mature dimer and an inactive precursor protein

The human-blood plasma glutamate carboxypeptidase (PGCP) is a proteinase that acts on the unsubstituted N- and C-termini of dipeptides. It has been suggested that this PGCP is involved in the release of thyroxine. Furthermore, research has suggested that its activity is up-regulated in hepatitis-C-virus-infected patients with hepatocellular carcinoma. In this study expressed human PGCP in the baculovirus expression system was produced by a Sf9 insect cell line with aim to prepare sufficient amounts of active recombinant enzyme for a subsequent biological characterization. Recombinant PGCP was expressed and secreted into the medium in the form of an inactive proenzyme. It was gradually converted into an active form in the medium after three days, with the highest expression of the active form on day six. The protein was sequentially purified by a combination of various liquid chromatographies, such as hydroxyapatite, ion exchange, and gel chromatography, and as final step with affinity chromatography on Phe-Leu-Sepharose. The human PGCP was purified as an active enzyme in the dimer form and as inactive precursor protein. The dipeptidase activity was confirmed by measuring the hydrolysis of the Ser-Met dipeptide at a slightly acidic pH.

Phenotypic characterization of mice heterozygous for a null mutation of glutamate carboxypeptidase II

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. Disturbed glutamate signaling resulting in hypofunction of N-methyl-D-aspartate receptors (NMDAR) has been implicated in the pathophysiology of schizophrenia. Glutamate Carboxypeptidase II (GCP II) hydrolyzes N-acetyl-alpha L-aspartyl-L-glutamate (NAAG) into glutamate and N-acetyl-aspartate. NAAG is a neuropeptide that is an NMDAR antagonist as well as an agonist for the metabotropic glutamate receptor-3 (mGluR3), which inhibits glutamate release. The aggregate effect of NAAG is thus to attenuate NMDAR activation. To manipulate the expression of GCP II, LoxP sites were inserted flanking exons 1 and 2, which were excised by crossing with a Cre-expressing mouse. The mice heterozygous for this deletion showed a 50% reduction in the expression level of protein and functional activity of GCP II in brain samples. Heterozygous mutant crosses did not yield any homozygous null animals at birth or as embryos (N > 200 live births and fetuses). These data are consistent with the previous report that GCP II homozygous mutant mice generated by removing exons 9 and 10 of GCP II gene were embryonically lethal and confirm our hypothesis that GCP II plays an essential role early in embryonic development. Heterozygous mice, however, developed normally to adulthood and exhibited increased locomotor activity, reduced social interaction, and a subtle cognitive deficit in working memory.

Reaction mechanism of glutamate carboxypeptidase II revealed by mutagenesis, X-ray crystallography, and computational methods

Glutamate carboxypeptidase II (GCPII, EC 3.4.17.21) is a zinc-dependent exopeptidase and an important therapeutic target for neurodegeneration and prostate cancer. The hydrolysis of N-acetyl-l-aspartyl-l-glutamate (N-Ac-Asp-Glu), the natural dipeptidic substrate of the GCPII, is intimately involved in cellular signaling within the mammalian nervous system, but the exact mechanism of this reaction has not yet been determined. To investigate peptide hydrolysis by GCPII in detail, we constructed a mutant of human GCPII [GCPII(E424A)], in which Glu424, a putative proton shuttle residue, is substituted with alanine. Kinetic analysis of GCPII(E424A) using N-Ac-Asp-Glu as substrate revealed a complete loss of catalytic activity, suggesting the direct involvement of Glu424 in peptide hydrolysis. Additionally, we determined the crystal structure of GCPII(E424A) in complex with N-Ac-Asp-Glu at 1.70 A resolution. The presence of the intact substrate in the GCPII(E424A) binding cavity substantiates our kinetic data and allows a detailed analysis of GCPII/N-Ac-Asp-Glu interactions. The experimental data are complemented by the combined quantum mechanics/molecular mechanics calculations (QM/MM) which enabled us to characterize the transition states, including the associated reaction barriers, and provided detailed information concerning the GCPII reaction mechanism. The best estimate of the reaction barrier was calculated to be DeltaG(++) approximately 22(+/-5) kcal x mol(-1), which is in a good agreement with the experimentally observed reaction rate constant (k(cat) approximately 1 s(-1)). Combined together, our results provide a detailed and consistent picture of the reaction mechanism of this highly interesting enzyme at the atomic level.

A series of halogenated heterodimeric inhibitors of prostate specific membrane antigen (PSMA) as radiolabeled probes for targeting prostate cancer

Prostate specific membrane antigen (PSMA) is a validated molecular marker for prostate cancer. A series of glutamate-urea (Glu-urea-X) heterodimeric inhibitors of PSMA were designed and synthesized where X = epsilon-N-(o-I, m-I, p-I, p-Br, o-Cl, m-Cl, p-Cl, p-F, H)-benzyl-Lys and epsilon-(p-I, p-Br, p-Cl, p-F, H)-phenylureido-Lys. The affinities for PSMA were determined by screening in a competitive binding assay. PSMA binding of the benzyllysine series was significantly affected by the nature of the halogen substituent (IC(50) values, Cl < I = Br << F = H) and the ring position of the halogen atom (IC(50) values, p-I < o-I << m-I). The halogen atom had little affect on the binding affinity in the para substituted phenylureido-Lys series. Two lead iodine compounds were radiolabeled with (123)I and (131)I and demonstrated specific PSMA binding on human prostate cancer cells, warranting evaluation as radioligands for the detection, staging, and monitoring of prostate cancer.

Progress in the discovery and development of glutamate carboxypeptidase II inhibitors

During the past 10 years, substantial progress has been made in the discovery and development of small molecule glutamate carboxypeptidase II (GCP II) inhibitors. These inhibitors have provided the necessary tools to investigate the physiological role of GCP II as well as the potential therapeutic benefits of its inhibition in neurological disorders of glutamatergic dysregulation. This review article details key GCP II inhibitors discovered in the last decade and important findings from preclinical and clinical studies.

Mapping of the active site of glutamate carboxypeptidase II by site-directed mutagenesis

Human glutamate carboxypeptidase II [GCPII (EC 3.4.17.21)] is recognized as a promising pharmacological target for the treatment and imaging of various pathologies, including neurological disorders and prostate cancer. Recently reported crystal structures of GCPII provide structural insight into the organization of the substrate binding cavity and highlight residues implicated in substrate/inhibitor binding in the S1' site of the enzyme. To complement and extend the structural studies, we constructed a model of GCPII in complex with its substrate, N-acetyl-l-aspartyl-l-glutamate, which enabled us to predict additional amino acid residues interacting with the bound substrate, and used site-directed mutagenesis to assess the contribution of individual residues for substrate/inhibitor binding and enzymatic activity of GCPII. We prepared and characterized 12 GCPII mutants targeting the amino acids in the vicinity of substrate/inhibitor binding pockets. The experimental results, together with the molecular modeling, suggest that the amino acid residues delineating the S1' pocket of the enzyme (namely Arg210) contribute primarily to the high affinity binding of GCPII substrates/inhibitors, whereas the residues forming the S1 pocket might be more important for the 'fine-tuning' of GCPII substrate specificity.

Catalytic mechanism of SGAP, a double-zinc aminopeptidase from Streptomyces griseus

The catalytic mechanism underlying the aminopeptidase from Streptomyces griseus (SGAP) was investigated. pH-dependent activity profiles revealed the enthalpy of ionization for the hydrolysis of leucine-para-nitroanilide by SGAP. The value obtained (30 +/- 5 kJ.mol(-1)) is typical of a zinc-bound water molecule, suggesting that the zinc-bound water/hydroxide molecule acts as the reaction nucleophile. Fluoride was found to act as a pure noncompetitive inhibitor of SGAP at pH values of 5.9-8 with a K(i) of 11.4 mM at pH 8.0, indicating that the fluoride ion interacts equally with the free enzyme as with the enzyme-substrate complex. pH-dependent pK(i) experiments resulted in a pK(a) value of 7.0, suggesting a single deprotonation step of the catalytic water molecule to an hydroxide ion. The number of proton transfers during the catalytic pathway was determined by monitoring the solvent isotope effect on SGAP and its general acid-base mutant SGAP(E131D) at different pHs. The results indicate that a single proton transfer is involved in catalysis at pH 8.0, whereas two proton transfers are implicated at pH 6.5. The role of Glu131 in binding and catalysis was assessed by determining the catalytic constants (K(m), k(cat)) over a temperature range of 293-329 degrees K for both SGAP and the E131D mutant. For the binding step, the measured and calculated thermodynamic parameters for the reaction (free energy, enthalpy and entropy) for both SGAP and the E131D mutant were similar. By contrast, the E131D point mutation resulted in a four orders of magnitude decrease in k(cat), corresponding to an increase of 9 kJ.mol(-1) in the activation energy for the E131D mutant, emphasizing the crucial role of Glu131 in catalysis.

Cell signaling and neuronal death

The past few decades have revealed that cell death can be precisely programmed with two principal forms, apoptosis and necrosis. Besides pathophysiological alterations, physiologic processes, such as the pruning of neurons during normal development and the involution of the thymus, involve apoptosis. This review focuses on the role of inter- and intracellular signaling systems in cell death, especially in the nervous system. Among neurotransmitters, glutamate and nitric oxide have been most extensively characterized and contribute to cell death in excitotoxic damage, especially in stroke and possibly in neurodegenerative diseases. Within cells, calcium, the most prominent of all intracellular messengers, mediates diverse forms of cell death with actions modulated by many proteins, including IP3 receptors, calcineurin, calpain, and cytochrome c.

Molecular Evolution of the Transferrin Receptor/Glutamate Carboxypeptidase II Family

The transferrin receptor family is represented by at least seven different homologous proteins in primates. Transferrin receptor (TfR1) is a type II membrane glycoprotein that, as a cell surface homodimer, binds iron-loaded transferrin as part of the process of iron transfer and uptake. Other family members include transferrin receptor 2 (TfR2), glutamate carboxypeptidase II (GCP2 or PSMA), N-acetylated alpha-linked acidic dipeptidase-like protein (NLDL), N-acetylated alpha-linked acidic dipeptidase 2 (NAALAD2), and prostate-specific membrane antigen-like protein (PMSAL/GCPIII). We compared 86 different sequences from 24 different species, from mammals to fungi. Through this comparison, we have identified several highly conserved residues specific to each family not previously associated with clinical mutations. The evolutionary history of the TfR/GCP2 family shows repeated episodes of duplications consistent with recent theories that nondispensable, slowly evolving genes are more likely to form multiple gene families.

Gamma-glutamyl hydrolase and drug resistance

Gamma-glutamyl hydrolase (GGH) is a lysosomal enzyme involved in the metabolism of folates and anti-folates. It acts as an endo- and/or exo-peptidase to cleave gamma-polyglutamate chains that are attached to folates and anti-folates after they enter a mammalian cell. Whereas the addition of multiple glutamates is necessary to enable the cell to retain folates and anti-folates, hydrolysis of the polyglutamate tails by GGH has the opposite effect of making (anti)-folates exportable again. Thus, GGH plays an important role in the cellular homeostasis of folate. Furthermore, high levels of GGH have been associated with cellular resistance to anti-folates, in particular methotrexate. Consequently, GGH also has pharmacological importance. In addition to the intracellular GGH, carboxypeptidase II (also called intestinal folate conjugase, prostate specific membrane antigen or N-acetyl-alpha-linked acidic dipeptidase) is another enzyme with gamma-glutamyl hydrolase activity; it resides, however, in the cellular membrane. Although genetically and biochemically distinct, this enzyme too appears to play a major role in folate homeostasis, by cleaving polyglutamates from extracellular folate-polyglutamates, so that they can be imported into the cell. Finally, there have been reports suggesting that gamma-glutamyl hydrolase plays a role as a tumor marker in breast and lung cancer.

Structure of glutamate carboxypeptidase II, a drug target in neuronal damage and prostate cancer

Membrane-bound glutamate carboxypeptidase II (GCPII) is a zinc metalloenzyme that catalyzes the hydrolysis of the neurotransmitter N-acetyl-L-aspartyl-L-glutamate (NAAG) to N-acetyl-L-aspartate and L-glutamate (which is itself a neurotransmitter). Potent and selective GCPII inhibitors have been shown to decrease brain glutamate and provide neuroprotection in preclinical models of stroke, amyotrophic lateral sclerosis, and neuropathic pain. Here, we report crystal structures of the extracellular part of GCPII in complex with both potent and weak inhibitors and with glutamate, the product of the enzyme's hydrolysis reaction, at 2.0, 2.4, and 2.2 A resolution, respectively. GCPII folds into three domains: protease-like, apical, and C-terminal. All three participate in substrate binding, with two of them directly involved in C-terminal glutamate recognition. One of the carbohydrate moieties of the enzyme is essential for homodimer formation of GCPII. The three-dimensional structures presented here reveal an induced-fit substrate-binding mode of this key enzyme and provide essential information for the design of GCPII inhibitors useful in the treatment of neuronal diseases and prostate cancer.

Transferrin receptor-like proteins control the degradation of a yeast metal transporter

Plasma membrane transporters are often downregulated by their substrates. The yeast manganese transporter Smf1 is subject to two levels of regulation: heavy metals induce its sequestration within the cell, and also its ubiquitination and degradation in the vacuole. Degradation requires Bsd2, a membrane protein with a PPxY motif that recruits the ubiquitin ligase Rsp5, and which has a role in the quality control of membrane proteins, that expose hydrophilic residues to the lipid bilayer. We show that degradation of Smf1 requires in addition one of a pair of related yeast proteins, Tre1 and Tre2, that also contain PPxY motifs. Tre1 can partially inhibit manganese uptake without Bsd2, but requires Bsd2 to induce Smf1 degradation. It has a relatively hydrophilic transmembrane domain and binds to Bsd2. We propose that the Tre proteins specifically link Smf1 to the Bsd2-dependent quality control system. Their luminal domains are related to the transferrin receptor, but these are dispensable for Smf1 regulation. Tre proteins and the transferrin receptors appear to have evolved independently from the same family of membrane-associated proteases.

NAAG peptidase inhibitors and their potential for diagnosis and therapy

Modulation of N-acetyl-L-aspartyl-L-glutamate peptidase activity with small-molecule inhibitors holds promise for a wide variety of diseases that involve glutamatergic transmission, and has implications for the diagnosis and therapy of cancer. This new class of compounds, of which at least one has entered clinical trials and proven to be well tolerated, has demonstrated efficacy in experimental models of pain, schizophrenia, amyotrophic lateral sclerosis, traumatic brain injury and, when appropriately functionalized, can image prostate cancer. Further investigation of these promising drug candidates will be needed to bring them to the marketplace. The recent publication of the X-ray crystal structure for the enzymatic target of these compounds should facilitate the development of other new agents with enhanced activity that could improve both the diagnosis and treatment of neurological disorders.

Induction of autoantibodies to syngeneic prostate-specific membrane antigen by xenogeneic vaccination

Prostate-specific membrane antigen (PSMA) is a prototypical differentiation antigen expressed on normal and neoplastic prostate epithelial cells, and on the neovasculature of many solid tumors. Monoclonal antibodies specific for PSMA are in development as therapeutic agents. Methodologies to actively immunize against PSMA may be limited by immunologic ignorance and/or tolerance that restrict the response to self-antigens. Our studies have previously shown that xenogeneic immunization with DNA vaccines encoding melanosomal differentiation antigens induces immunity in a mouse melanoma model. Here we apply this approach to PSMA to establish proof of principle in a mouse model. Immunization with xenogeneic human PSMA protein or DNA induced antibodies to both human and mouse PSMA in mice. Monoclonal antibodies specific for mouse PSMA were generated to analyze antibody isotypes and specificity for native and denatured PSMA at the clonal level. Most antibodies recognized denatured PSMA, but C57BL/6 mice immunized with xenogeneic PSMA DNA followed by a final boost with xenogeneic PSMA protein yielded autoantibodies that reacted with native folded mouse PSMA. Monoclonal antibodies were used to confirm the expression of PSMA protein in normal mouse kidney. These results establish the basis for clinical trials to test PSMA DNA vaccines in patients with solid tumors that either express PSMA directly or that depend on normal endothelial cells expressing PSMA for their continued growth.

Crystal structure of prostate-specific membrane antigen, a tumor marker and peptidase

Prostate-specific membrane antigen (PSMA) is highly expressed in prostate cancer cells and nonprostatic solid tumor neovasculature and is a target for anticancer imaging and therapeutic agents. PSMA acts as a glutamate carboxypeptidase (GCPII) on small molecule substrates, including folate, the anticancer drug methotrexate, and the neuropeptide N-acetyl-l-aspartyl-l-glutamate. Here we present the 3.5-A crystal structure of the PSMA ectodomain, which reveals a homodimer with structural similarity to transferrin receptor, a receptor for iron-loaded transferrin that lacks protease activity. Unlike transferrin receptor, the protease domain of PSMA contains a binuclear zinc site, catalytic residues, and a proposed substrate-binding arginine patch. Elucidation of the PSMA structure combined with docking studies and a proposed catalytic mechanism provides insight into the recognition of inhibitors and the natural substrate N-acetyl-l-aspartyl-l-glutamate. The PSMA structure will facilitate development of chemotherapeutics, cancer-imaging agents, and agents for treatment of neurological disorders.

N-Methyl-D-aspartate receptors as a target for improved antipsychotic agents: novel insights and clinical perspectives

RATIONALE

Activation of "co-agonist" N-methyl-D-aspartate (NMDA) and Glycine(B) sites is mandatory for the operation of NMDA receptors, which play an important role in the control of mood, cognition and motor function.

OBJECTIVES

This article outlines the complex regulation of activity at Glycine(B)/NMDA receptors by multiple classes of endogenous ligand. It also summarizes the evidence that a hypoactivity of Glycine(B)/NMDA receptors contributes to the pathogenesis of psychotic states, and that drugs which enhance activity at these sites may possess antipsychotic properties.

RESULTS

Polymorphisms in several genes known to interact with NMDA receptors are related to an altered risk for schizophrenia, and psychotic patients display changes in levels of mRNA encoding NMDA receptors, including the NR1 subunit on which Glycine(B) sites are located. Schizophrenia is also associated with an overall decrease in activity of endogenous agonists at Glycine(B)/NMDA sites, whereas levels of endogenous antagonists are elevated. NMDA receptor "open channel blockers," such as phencyclidine, are psychotomimetic in man and in rodents, and antipsychotic agents attenuate certain of their effects. Moreover, mice with genetically invalidated Glycine(B)/NMDA receptors reveal similar changes in behaviour. Finally, in initial clinical studies, Glycine(B) agonists and inhibitors of glycine reuptake have been found to potentiate the ability of "conventional" antipsychotics to improve negative and, albeit modestly, cognitive and positive symptoms. In contrast, therapeutic effects of clozapine are not reinforced, likely since clozapine itself enhances activity at NMDA receptors.

CONCLUSIONS

Reduced activity at NMDA receptors is implicated in the aetiology of schizophrenia. Correspondingly, drugs that (directly or indirectly) increase activity at Glycine(B) sites may be of use as adjuncts to other classes of antipsychotic agent. However, there is an urgent need for broader clinical evaluation of this possibility, and, to date, there is no evidence that stimulation of Glycine(B) sites alone improves psychotic states.

Novel Role of Prostate-Specific Membrane Antigen in Suppressing Prostate Cancer Invasiveness

Prostate-specific membrane antigen (PSMA), a type II transmembrane glycoprotein, is overexpressed in prostate cancer. PSMA is a unique cell surface marker, negatively regulated by androgen and extensively used for imaging of hormone refractory carcinomas and metastatic foci. PSMA is a carboxypeptidase with two important enzymatic functions, namely, folate hydrolase and NAALADase. PSMA also exhibits an endocytic function, in which it spontaneously recycles through endocytic vesicles. PSMA is overexpressed at various stages of prostate cancer, including androgen-sensitive and -independent disease, increased in expression with early relapse after therapy. We have used in vitro invasion assays to explore the possible role of PSMA in the metastasis of prostate cancer cells. Androgen-dependent prostate cancer lines, which express PSMA endogenously (e.g., LNCaP, MDA PCa2b, and CWR22Rv1) are less invasive compared with androgen-independent PC3 or DU145 cells, neither of which expresses PSMA. Ectopic expression of PSMA in PC3 cells reduced the invasiveness of these cells, suggesting that this reduction in the invasion capability of PSMA-expressing cells is due to PSMA expression and not to intrinsic properties of different prostate cancer cell lines. Furthermore, knockdown of PSMA expression increased invasiveness of LNCaP cells by 5-fold. Finally, expression of PSMA mutants lacking carboxypeptidase activity reduced the impact of PSMA expression on invasiveness. Thus, it seems that the enzymatic activity is associated with the effect of PSMA on invasiveness.

Identification of the catalytic residues in the double-zinc aminopeptidase from Streptomyces griseus

The aminopeptidase from Streptomyces griseus (SGAP) has been cloned and expressed in Escherichia coli. By growing the cells in the presence of 1 M sorbitol at 18 degrees C, the protein was obtained in a soluble and active form. The amino acid sequence of the recombinant SGAP contained four amino acids differing from the previously published sequence. Re-sequencing of the native protein indicated that asparagines 70 and 184 are in fact aspartic acids as in the recombinant protein. Based on the crystal structure of SGAP, Glu131 and Tyr246 were proposed to be the catalytic residues. Replacements of Glu131 resulted in loss of activity of 4-5 orders of magnitude, consistent with Glu131 acting as the general base residue. Mutations in Tyr246 resulted in about 100-fold reduction of activity, suggesting that this residue is involved in the stabilization of the transition state intermediate.

Amino acids at the N- and C-termini of human glutamate carboxypeptidase II are required for enzymatic activity and proper folding

Human glutamate carboxypeptidase II (GCPII) is a co-catalytic metallopeptidase and its putative catalytic domain is homologous to the aminopeptidases from Vibrio proteolyticus and Streptomyces griseus. In humans, the enzyme is expressed predominantly in the nervous system and the prostate. The prostate form, termed prostate-specific membrane antigen, is overexpressed in prostate cancer and is used as a diagnostic marker of the disease. Inhibition of the form of GCPII expressed in the central nervous system has been shown to protect against ischemic injury in experimental animal models. Human GCPII consists of 750 amino acids, and six individual domains were predicted to constitute the protein structure. Here, we report the analysis of the contribution of these putative domains to the structure/function of recombinant human GCPII. We cloned 13 mutants of human GCPII that are truncated or extended at one or both the N- and C-termini of the GCPII sequence. The clones were used to generate stably transfected Drosophila Schneider's cells, and the expression and carboxypeptidase activities of the individual protein products were determined. The extreme C-terminal region of human GCPII was found to be critical for the hydrolytic activity of the enzyme. The deletion of as few as 15 amino acids from the C-terminus was shown to completely abolish the enzymatic activity of GCPII. Furthermore, the GCPII carboxypeptidase activity was abrogated upon removal of more than 60 amino acid residues from the N-terminus of the protein. Overall, these results clearly show that amino acid segments at the N- and C-termini of the ectodomain of GCPII are essential for its carboxypeptidase activity and/or proper folding.

The homodimer of prostate-specific membrane antigen is a functional target for cancer therapy

Prostate-specific membrane antigen (PSMA) is a type 2 integral membrane glycoprotein that serves as an attractive target for cancer immunotherapy by virtue of its abundant and restricted expression on the surface of prostate carcinomas and the neovasculature of most other solid tumors. However, relatively little is known about the molecular structure of this target. Here, we report that PSMA is expressed on tumor cells as a noncovalent homodimer. A truncated PSMA protein, lacking transmembrane and cytoplasmic domains, also formed homodimers, indicating that the extracellular domain is sufficient for dimerization. PSMA dimers but not monomers displayed a native conformation and possessed high-level carboxypeptidase activity. A unique dimer-specific epitope was identified by using one of a panel of novel mAbs. When used to immunize animals, dimer but not monomer elicited antibodies that efficiently recognized PSMA-expressing tumor cells. These findings on PSMA structure and biology may have important implications for active and passive immunotherapy of prostate and other cancers.

Early embryonic death of glutamate carboxypeptidase II (NAALADase) homozygous mutants

Glutamate carboxypeptidase II (EC 3.4.17.21) catalyzes the hydrolysis (Km = 0.2 microM) of the neuropeptide N-acetylaspartylglutamate to yield N-acetylaspartate and glutamate and also serves as a high-affinity folate hydrolase in the gut, cleaving the polyglutamate chain to permit the absorption of folate. N-acetylaspartylglutamate is an agonist at the mGluR3 metabotropic receptor and a source of extracellular glutamate through hydrolysis by glutamate carboxypeptidase II. Given the important role of glutamate in brain development and function, we were interested in the effects of a null mutation of glutamate carboxypeptidase II that would potentiate the effects of N-acetylaspartylglutamate. The PGK-Neomycin cassette was inserted to delete exons 9 and 10, which we previously demonstrated encode for the zinc ligand domain essential for enzyme activity. Successful germline transmission was obtained from chimeras derived from embryonic stem cells with the targeted mutation of glutamate carboxypeptidase II. Homozygous null mutants did not survive beyond embryonic day 8. Folate supplementation of the heterozygous mothers did not rescue the homozygous embryos. Mice heterozygous for the null mutation appeared grossly normal and expressed both mutated and wild-type mRNA but the activity of glutamate carboxypeptidase II is comparable to the wild-type mice. The results indicate that the expression of glutamate carboxypeptidase II is upregulated when one allele is inactivated and that its activity is essential for early embryogenesis.

A novel cytoplasmic tail MXXXL motif mediates the internalization of prostate-specific membrane antigen

Prostate-specific membrane antigen (PSMA) is a transmembrane protein expressed at high levels in prostate cancer and in tumor-associated neovasculature. In this study, we report that PSMA is internalized via a clathrin-dependent endocytic mechanism and that internalization of PSMA is mediated by the five N-terminal amino acids (MWNLL) present in its cytoplasmic tail. Deletion of the cytoplasmic tail abolished PSMA internalization. Mutagenesis of N-terminal amino acid residues at position 2, 3, or 4 to alanine did not affect internalization of PSMA, whereas mutation of amino acid residues 1 or 5 to alanine strongly inhibited internalization. Using a chimeric protein composed of Tac antigen, the alpha-chain of interleukin 2-receptor, fused to the first five amino acids of PSMA (Tac-MWNLL), we found that this sequence is sufficient for PSMA internalization. In addition, inclusion of additional alanines into the MWNLL sequence either in the Tac chimera or the full-length PSMA strongly inhibited internalization. From these results, we suggest that a novel MXXXL motif in the cytoplasmic tail mediates PSMA internalization. We also show that dominant negative micro2 of the adaptor protein (AP)-2 complex strongly inhibits the internalization of PSMA, indicating that AP-2 is involved in the internalization of PSMA mediated by the MXXXL motif.

Effect of carbohydrate moieties on the folate hydrolysis activity of the prostate specific membrane antigen

BACKGROUND

Prostate specific membrane antigen or PSMA has been recognized as one of the important cellular markers for prostate cancer, the expression of which is enhanced many fold in prostate cancer and other tumor neovasculature. PSMA is a type II membrane glycoprotein with a short cytoplasmic N-terminal region, a transmembrane domain, and a 701 amino acid extracellular portion with 10 potential N-linked glycosylation sites. PSMA is a folate hydrolase, which cleaves terminal glutamates from poly- and gamma-glutamated folates; and NAALADase, which hydrolyses alpha-glutamate-linked dipeptide, N-acetyl-aspartyl-glutamate (NAAG) and is a glutamate carboxypeptidase.

METHODS

In our study we have used various enzymes or site directed mutagenesis to remove sugar molecules from PSMA protein and studied its folate hydrolase function. We have performed a biochemical characterization of N-linked glycosylation of the various mutant proteins.

RESULTS

PSMA protein expressed in different prostate cancer cell lines is differentially glycosylated. Removal of sugar residues either enzymatically or by mutagenesis abolishes the enzyme activity of PSMA protein completely.

CONCLUSION

N-linked carbohydrate structures are important for the folate hydrolase function of the protein. Removal of sugars partially or completely causes PSMA to be enzymatically inactive, improperly folded, resulting in increased rate of degradation.

Recombinant glutamate carboxypeptidase II (prostate specific membrane antigen--PSMA)--cellular localization and bioactivity analyses

Glutamate Carboxypeptidase II (also known as Prostate Specific Membrane Antigen-PSMA) is an important marker in the diagnosis of prostate cancer, however, relatively little is known about its biochemical and structure-function characteristics. We have expressed mutant forms of PSMA and have started to address the roles of three putative domains of PSMA in its cellular localization and peptidase activity. Three mutants, a full-length recombinant PSMA (rPSMA-FL), one expressing only the proposed extracellular domain of PSMA (rPSMA-ECD) and one form omitting the proposed transmembrane domain (rPSMA-deltaTMD) have been produced in human cells via a mammalian expression vector system. We show that rPSMA-FL is associated with the cell surface membrane; so too is rPSMA-deltaTMD even though it lacks the proposed transmembrane domain, whereas rPSMA-ECD has a cytosolic localization. Only rPSMA-FL retains functional hydrolytic activity and is similarly glycosylated to PSMA found in the cultured prostate cancer cell line LNCaP.

Regulation of glutamate carboxypeptidase II function in corticolimbic regions of rat brain by phencyclidine, haloperidol, and clozapine

Mounting evidence indicates that hypofunction of NMDA glutamate receptors causes or contributes to the full symptomatology of schizophrenia. N-acetyl-aspartyl-glutamate (NAAG), an endogenous neuropeptide, blocks NMDA receptors and inhibits glutamate release by activating metabotropic mGluR3 receptors. NAAG is catabolized to glutamate and N-acetyl-aspartate by the astrocytic enzyme glutamate carboxypeptidase II (GCP II). Changes in GCP II activity may be critically linked to changes in glutamatergic neurotransmission especially at NMDA receptors. We examined whether GCP II function is altered by treatment with the noncompetitive antagonist and psychotomimetic drug phencyclidine (PCP) and with the neuroleptics haloperidol (HAL) and clozapine (CLOZ), in corticolimbic brain regions of the adult rat. Chronic exposure to PCP produced significant increases in GCP II protein expression and activity in the prefrontal cortex (PFC) and hippocampus (HIPP). This effect may be explained by a compensatory response to persistent blockade of NMDA receptors. In addition, chronic treatment with neuroleptics upregulated GCP II activity, but not protein expression, in the PFC. In contrast, GCP II activity was decreased after acute exposure to HAL or CLOZ and was not changed after acute PCP treatment. These findings provide support for a role of GCP II function in the control of glutamatergic neurotransmission and suggest that some of the therapeutic actions of neuroleptic drugs may be mediated through their effects on GCP II activity. These results demonstrate that psychotomimetic and neuroleptic drugs modulate GCP II function in brain regions that are widely involved in the neuropathology of schizophrenia.

Design and pharmacological activity of phosphinic acid based NAALADase inhibitors

A novel series of phosphinic acid based inhibitors of the neuropeptidase NAALADase are described in this work. This series of compounds is the most potent series of inhibitors of the enzyme described to date. In addition, we have shown that these compounds are protective in animal models of neurodegeneration. Compound 34 significantly prevented neurodegeneration in a middle cerebral artery occlusion model of cerebral ischemia. In addition, in the chronic constrictive model of neuropathic pain, compound 34 significantly attenuated the hypersensitivity observed with saline-treated animals. These data suggest that NAALADase inhibition may provide a new approach for the treatment of both neurodegenerative disorders and peripheral neuropathies.

Prostate-specific membrane antigen (PSMA) enzyme activity is elevated in prostate cancer cells

BACKGROUND

Prostate-specific membrane antigen (PSMA) is a glutamate carboxypeptidase that cleaves terminal carboxy glutamates from both the neuronal dipeptide N-acetylaspartylglutamate (NAAG) and gamma-linked folate polyglutamate. The prostate enzyme has activity in both the membrane and cytosolic fractions termed PSMA and PSMA', respectively.

METHODS

Using a NAAG hydrolytic radioenzymatic assay, we quantitated the enzymatic activity of PSMA and PSMA' in normal, benign prostatic hyperplasia (BPH), and prostate cancer (PC) tissues from radical prostatectomies. PSMA enzyme activity was evaluated in each tissue type and expressed per milligram protein and epithelial cell content.

RESULTS

PSMA and PSMA' enzyme activities were significantly elevated in prostate cancer when compared to normal prostate tissue and BPH. Ratios of PSMA to PSMA' were also decreased in BPH as compared to cancerous and normal tissue.

CONCLUSIONS

Prostate carcinogenesis is associated with an elevation in PSMA and PSMA' enzyme activity. In contrast, no such enhancement in PSMA activity is observed with benign neoplastic changes in BPH. Thus, the enhancement observed in prostate cancer is not simply related to a generalized prostatic hyperplasia, but is specific to its malignancy.

Suramin potently inhibits the enzymatic activity of PSM

BACKGROUND

The polysulfonated napthlyurea suramin has shown significant antitumor activity in patients with hormone-refractory metastatic prostate cancer. The mechanism by which suramin exerts this effect is unknown. In 1993, prostate-specific membrane antigen (PSM) was identified as a prostate biomarker that is elevated in hormone-refractory and metastatic prostate cancer. PSM is a glutamate exocarboxypeptidase capable of cleaving the terminal alpha-linked glutamate from the dipeptide N-acetyl-aspartyl-glutamate (NAAG) and the gamma-linked glutamates from folate polyglutamate.

METHODS

Using a NAAG hydrolytic radioenzymatic assay, we tested whether suramin had any effect on the enzymatic activity of PSM.

RESULTS

We demonstrate that suramin potently inhibits the enzymatic activity of PSM with a K(i) = 15 nM and 68 nM for the membrane-associated and soluble forms of PSM, respectively. In addition, we show that suramin inhibition of PSM enzyme activity displays the kinetics of a classic competitive inhibitor.

CONCLUSIONS

This is one of the most potent activities described for suramin to date and may represent a portion of its pharmacologic and/or toxicological mechanism of action.

Glutamate carboxypeptidase II is expressed by astrocytes in the adult rat nervous system

The enzyme glutamate carboxypeptidase II (GCP II) has been cloned from rat brain and human prostate. This enzyme, which catabolizes the neuropeptide N-acetylaspartylglutamate, has also been known as N-acetylated alpha-linked acidic dipeptidase (NAALADase), and is identical to the prostate-specific membrane antigen and to the jejunal folylpoly-gamma-glutamate carboxypeptidase. The goals of the present study were to elucidate the cell specificity and regional pattern of GCP II expression in the rat nervous system by using Northern blots and enzymatic assays of brain and subfractionated primary neuronal and glial cultures together with in situ hybridization histochemistry (ISHH) in sections of adult rat tissue. GCP II activity was assayed in astrocyte cultures (4.4 pmol/mg protein per minute), neuronal-glial cocultures (2.5 pmol/mg protein per minute) and neuron-enriched cultures (0.38 pmol/mg protein per minute), with the activity in each preparation correlating to its astrocytic content (r = 0.99). No activity was detected in cultured oligodendrocytes or microglia. Northern blots probed with a GCP II cDNA detected mRNAs exclusively in activity-positive cell preparations. ISHH results show that GCP II is expressed by virtually all astrocytes, by Bergmann glial cells in cerebellum, by Müller cells in retina and by the satellite cells in dorsal root ganglia. Astrocytes in select groups of nuclei (e.g., habenula, supraoptic nucleus, pontine nucleus) contained pronounced levels of GCP II message. The data of the present study suggest that GCP II is expressed in the adult rat nervous system exclusively in astrocytic glial cells.