GCPII variants, paralogs and orthologs

Discovery of Glutamate Carboxypeptidase III Ligands to Compete the Uptake of [177Lu]Lu-PSMA-617 in Healthy Organs

Prostate-specific membrane antigen (PSMA)-targeted radio ligand therapeutics (RLTs), such as [177Lu]Lu-PSMA-617 (Pluvicto), have been shown to accumulate in salivary glands and kidneys, potentially leading to undesired side effects. As unwanted accumulation in normal organs may derive from the cross-reactivity of PSMA ligands to glutamate carboxypeptidase III (GCPIII), it may be convenient to block this interaction with GCPIII-selective ligands. Parallel screening of a DNA-encoded chemical library (DEL) against GCPIII and PSMA allowed the identification of GCPIII binders. Structure-activity relationship (SAR) studies resulted in the identification of nanomolar GCPIII ligands with up to 1000-fold selectivity over PSMA. We studied the ability of GCPIII ligands to counteract the binding of [177Lu]Lu-PSMA-617 to human salivary glands by autoradiography and could demonstrate a partial radioprotection.

Uncovering the essential roles of glutamate carboxypeptidase 2 orthologs in Caenorhabditis elegans

Human glutamate carboxypeptidase 2 (GCP2) from the M28B metalloprotease group is an important target for therapy in neurological disorders and an established tumor marker. However, its physiological functions remain unclear. To better understand general roles, we used the model organism Caenorhabditis elegans to genetically manipulate its three existing orthologous genes and evaluate the impact on worm physiology. The results of gene knockout studies showed that C. elegans GCP2 orthologs affect the pharyngeal physiology, reproduction, and structural integrity of the organism. Promoter-driven GFP expression revealed distinct localization for each of the three gene paralogs, with gcp-2.1 being most abundant in muscles, intestine, and pharyngeal interneurons, gcp-2.2 restricted to the phasmid neurons, and gcp-2.3 located in the excretory cell. The present study provides new insight into the unique phenotypic effects of GCP2 gene knockouts in C. elegans, and the specific tissue localizations. We believe that elucidation of particular roles in a non-mammalian organism can help to explain important questions linked to physiology of this protease group and in extension to human GCP2 involvement in pathophysiological processes.

Molecular Design of 68Ga- and 89Zr-Labeled Anticalin Radioligands for PET-Imaging of PSMA-Positive Tumors

Anticalin proteins directed against the prostate-specific membrane antigen (PSMA), optionally having tailored plasma half-life using PASylation technology, show promise as radioligands for PET-imaging of xenograft tumors in mice. To investigate their suitability, the short-circulating unmodified Anticalin was labeled with ⁶⁸Ga (τ_1/2 = 68 min), using the NODAGA chelator, whereas the half-life extended PASylated Anticalin was labeled with ⁸⁹Zr (τ_1/2 = 78 h), using either the linear chelator deferoxamine (Dfo) or a cyclic derivative, fusarinine C (FsC). Different PSMA targeting Anticalin versions (optionally carrying the PASylation sequence) were produced carrying a single exposed N- or C-terminal Cys residue and site-specifically conjugated with the different radiochelators via maleimide chemistry. These protein conjugates were labeled with radioisotopes having distinct physical half-lives and, subsequently, applied for PET-imaging of subcutaneous LNCaP xenograft tumors in CB17 SCID mice. Uptake of the protein tracers into tumor versus healthy tissues was assessed by segmentation of PET data as well as biodistribution analyses. PET-imaging with both the ⁶⁸Ga-labeled plain Anticalin and the ⁸⁹Zr-labeled PASylated Anticalin allowed clear delineation of the xenograft tumor. The radioligand A3A5.1-PAS(200)-FsC·⁸⁹Zr, having an extended plasma half-life, led to a higher tumor uptake 24 h p.i. compared to the ⁶⁸Ga·NODAGA-Anticalin imaged 60 min p.i. (2.5% ID/g vs 1.2% ID/g). Pronounced demetallation was observed for the ⁸⁹Zr·Dfo-labeled PASylated Anticalin, which was ∼50% lower in the case of the cyclic radiochelator FsC (p < 0.0001). Adjusting the plasma half-life of Anticalin radioligands using PASylation technology is a viable approach to increase radioisotope accumulation within the tumor. Furthermore, ⁸⁹Zr-ImmunoPET-imaging using the FsC radiochelator is superior to that using Dfo. Our strategy for the half-life adjustment of a tumor-targeting Anticalin to match the physical half-life of the applied radioisotope illustrates the potential of small binding proteins as an alternative to antibodies for PET-imaging.

Characterization of glutamate carboxypeptidase 2 orthologs in trematodes

BACKGROUND

Glutamate carboxypeptidase 2 (GCP2) belongs to the M28B metalloprotease subfamily encompassing a variety of zinc-dependent exopeptidases that can be found in many eukaryotes, including unicellular organisms. Limited information exists on the physiological functions of GCP2 orthologs in mammalian tissues outside of the brain and intestine, and such data are completely absent for non-mammalian species. Here, we investigate GCP2 orthologs found in trematodes, not only as putative instrumental molecules for defining their basal function(s) but also as drug targets.

METHODS

Identified genes encoding M28B proteases Schistosoma mansoni and Fasciola hepatica genomes were analyzed and annotated. Homology modeling was used to create three-dimensional models of SmM28B and FhM28B proteins using published X-ray structures as the template. For S. mansoni, RT-qPCR was used to evaluate gene expression profiles, and, by RNAi, we exploited the possible impact of knockdown on the viability of worms. Enzymes from both parasite species were cloned for recombinant expression. Polyclonal antibodies raised against purified recombinant enzymes and RNA probes were used for localization studies in both parasite species.

RESULTS

Single genes encoding M28B metalloproteases were identified in the genomes of S. mansoni and F. hepatica. Homology models revealed the conserved three-dimensional fold as well as the organization of the di-zinc active site. Putative peptidase activities of purified recombinant proteins were assayed using peptidic libraries, yet no specific substrate was identified, pointing towards the likely stringent substrate specificity of the enzymes. The orthologs were found to be localized in reproductive, digestive, nervous, and sensory organs as well as parenchymal cells. Knockdown of gene expression by RNAi silencing revealed that the genes studied were non-essential for trematode survival under laboratory conditions, reflecting similar findings for GCP2 KO mice.

CONCLUSIONS

Our study offers the first insight to our knowledge into M28B protease orthologs found in trematodes. Conservation of their three-dimensional structure, as well as tissue expression pattern, suggests that trematode GCP2 orthologs may have functions similar to their mammalian counterparts and can thus serve as valuable models for future studies aimed at clarifying the physiological role(s) of GCP2 and related subfamily proteases.

Looking for Drugs in All the Wrong Places: Use of GCPII Inhibitors Outside the Brain

In tribute to our friend and colleague Michael Robinson, we review his involvement in the identification, characterization and localization of the metallopeptidase glutamate carboxypeptidase II (GCPII), originally called NAALADase. While Mike was characterizing NAALADase in the brain, the protein was independently identified by other laboratories in human prostate where it was termed prostate specific membrane antigen (PSMA) and in the intestines where it was named Folate Hydrolase 1 (FOLH1). It was almost a decade to establish that NAALADase, PSMA, and FOLH1 are encoded by the same gene. The enzyme has emerged as a therapeutic target outside of the brain, with the most notable progress made in the treatment of prostate cancer and inflammatory bowel disease (IBD). PSMA-PET imaging with high affinity ligands is proving useful for the clinical diagnosis and staging of prostate cancer. A molecular radiotherapy based on similar ligands is in trials for metastatic castration-resistant prostate cancer. New PSMA inhibitor prodrugs that preferentially block kidney and salivary gland versus prostate tumor enzyme may improve the clinical safety of this radiotherapy. The wide clinical use of PSMA-PET imaging in prostate cancer has coincidentally led to clinical documentation of GCPII upregulation in a wide variety of tumors and inflammatory diseases, likely associated with angiogenesis. In IBD, expression of the FOLH1 gene that codes for GCPII is strongly upregulated, as is the enzymatic activity in diseased patient biopsies. In animal models of IBD, GCPII inhibitors show substantial efficacy, suggesting potential theranostic use of GCPII ligands for IBD.

A novel PSMA/GCPII-deficient mouse model shows enlarged seminal vesicles upon aging

BACKGROUND

Prostate-specific membrane antigen (PSMA), also known as glutamate carboxypeptidase II (GCPII), is an important diagnostic and therapeutic target in prostate cancer. PSMA/GCPII is also expressed in many healthy tissues, but its function has only been established in the brain and small intestine. Several research groups have attempted to produce PSMA/GCPII-deficient mice to study the physiological role of PSMA/GCPII in detail. The outcomes of these studies differ dramatically, ranging from embryonic lethality to production of viable PSMA/GCPII-deficient mice without any obvious phenotype.

METHODS

We produced PSMA/GCPII-deficient mice (hereafter also referred as Folh1^-/- mice) by TALEN-mediated mutagenesis on a C57BL/6NCrl background. Using Western blot and an enzyme activity assay, we confirmed the absence of PSMA/GCPII in our Folh1^-/- mice. We performed anatomical and histopathological examination of selected tissues with a focus on urogenital system. We also examined the PSMA/GCPII expression profile within the mouse urogenital system using an enzyme activity assay and confirmed the presence of PSMA/GCPII in selected tissues by immunohistochemistry.

RESULTS

Our Folh1^-/- mice are viable, breed normally, and do not show any obvious phenotype. Nevertheless, aged Folh1^-/- mice of 69-72 weeks exhibit seminal vesicle dilation, which is caused by accumulation of luminal fluid. This phenotype was also observed in Folh1^+/- mice; the overall difference between our three cohorts (Folh1^-/- , Folh1^+/- , and Folh1^+/+ ) was highly significant (P < 0.002). Of all studied tissues of the mouse urogenital system, only the epididymis appeared to have a physiologically relevant level of PSMA/GCPII expression. Additional experiments demonstrated that PSMA/GCPII is also present in the human epididymis.

CONCLUSIONS

In this study, we provide the first evidence characterizing the reproductive tissue phenotype of PSMA/GCPII-deficient mice. These findings will help lay the groundwork for future studies to reveal PSMA/GCPII function in human reproduction.

Synthesis and Preclinical Characterization of the PSMA-Targeted Hybrid Tracer PSMA-I&F for Nuclear and Fluorescence Imaging of Prostate Cancer

The prostate-specific membrane antigen (PSMA)–targeted radiotracers ⁶⁸Ga/¹⁷⁷Lu-PSMA-I&T and ^99mTc-PSMA-I&S (for imaging and surgery) are currently successfully used for clinical PET imaging, radionuclide therapy, and radioguided surgery of metastatic prostate cancer. To additionally exploit the high sensitivity and spatial resolution of fluorescence imaging for improved surgical guidance, a PSMA-I&T–based hybrid tracer, PSMA-I&F (DOTAGA-k(Sulfo-Cy5)-y-nal-k-Sub-KuE), has been developed and evaluated. Methods: The in vitro PSMA-targeting efficiency of PSMA-I&F, the reference PSMA-I&T, and their corresponding ^natGa-/⁶⁸Ga- and ^natLu/¹⁷⁷Lu counterparts was determined in LNCaP cells via competitive binding assays (IC₅₀) and dual-tracer radioligand and fluorescence internalization studies. Biodistribution and small-animal PET imaging studies were performed in CB17 SCID and LNCaP xenograft–bearing SHO mice, respectively, and complemented by intraoperative far-red fluorescence imaging using a clinical laparoscope. Additionally, fully automated serial cryosectioning and fluorescence imaging of 1 tumor-bearing animal as well as PSMA immunohistochemistry and fluorescence microscopy of organ cryosections (tumor, kidney, spleen) were also performed. Results: Compared with the parent PSMA-I&T analogs, the PSMA affinities of PSMA-I&F and its ^natGa-/^natLu-complexes remained high and unaffected by dye conjugation (7.9 < IC₅₀ < 10.5 nM for all ligands). The same was observed for the internalization of ⁶⁸Ga- and ¹⁷⁷Lu-PSMA-I&F. In vivo, blood clearance of ⁶⁸Ga- and ¹⁷⁷Lu-PSMA-I&F was only slightly delayed by high plasma protein binding (94%–95%), and very low accumulation in nontarget organs was observed already at 1 h after injection. Dynamic PET imaging confirmed PSMA-specific (as demonstrated by coinjection of 2-PMPA) uptake into the LNCaP xenograft (4.5% ± 1.8 percentage injected dose per gram) and the kidneys (106% ± 23 percentage injected dose per gram). Tumor-to-background ratios of 2.1, 5.2, 9.6, and 9.6 for blood, liver, intestines, and muscle, respectively, at 1 h after injection led to excellent imaging contrast in ⁶⁸Ga-PSMA-I&F PET and in intraoperative fluorescence imaging. Furthermore, fluorescence imaging of tissue cryosections allowed high-resolution visualization of intraorgan PSMA-I&F distribution in vivo and its correlation with PSMA expression as determined by immunohistochemistry. Conclusion: Thus, with its high PSMA-targeting efficiency and favorable pharmacokinetic profile, ⁶⁸Ga/¹⁷⁷Lu-PSMA-I&F serves as an excellent proof-of-concept compound for the general feasibility of PSMA-I&T–based hybrid imaging. The PSMA-I&T scaffold represents a versatile PSMA-targeted lead structure, allowing relatively straightforward adaptation to the different structural requirements of dedicated nuclear or hybrid imaging agents.

Synthesis and preclinical evaluation of novel 18F-labeled Glu-urea-Glu-based PSMA inhibitors for prostate cancer imaging: a comparison with 18F-DCFPyl and 18F-PSMA-1007

Background

Due to its high and consistent expression in prostate cancer (PCa), the prostate-specific membrane antigen (PSMA) represents an ideal target for molecular imaging and targeted therapy using highly specific radiolabeled PSMA ligands. To address the continuously growing clinical demand for ¹⁸F-labeled PSMA-probes, we developed two novel Glu-urea-Glu-(EuE)-based inhibitors, EuE-k-¹⁸F-FBOA (1) and EuE-k-β-a-¹⁸F-FPyl (2), both with optimized linker structure and different ¹⁸F-labeled aromatic moieties. The inhibitors were evaluated in a comparative preclinical study with ¹⁸F-DCFPyl and ¹⁸F-PSMA-1007.

Results

Radiolabeling procedures allowed preparation of (1) and (2) with high radiochemical yields (67 ± 7 and 53 ± 7%, d.c.) and purity (> 98%). When compared with ¹⁸F-DCFPyl (IC₅₀ = 12.3 ± 1.2 nM) and ¹⁸F-PSMA-1007 (IC₅₀ = 4.2 ± 0.5 nM), both metabolically stable EuE-based ligands showed commensurable or higher PSMA affinity (IC₅₀ = 4.2 ± 0.4 nM (1), IC₅₀ = 1.1 ± 0.2 nM (2)). Moreover, 1.4- and 2.7-fold higher internalization rates were observed for (1) and (2), respectively, resulting in markedly enhanced tumor accumulation in LNCaP-tumor-bearing mice ((1) 12.7 ± 2.0% IA/g, (2) 13.0° ± 1.0% IA/g vs. 7.3 ± 1.0% IA/g (¹⁸F-DCFPyl), 7.1 ± 1.5% IA/g (¹⁸F-PSMA-1007), 1 h p.i.). In contrast to (1), (2) showed higher kidney accumulation and delayed clearance kinetics. Due to the high hydrophilicity of both compounds, almost no unspecific uptake in non-target tissue was observed. In contrast, due to the less hydrophilic character (logP = − 1.6) and high plasma protein binding (98%), ¹⁸F-PSMA-1007 showed uptake in non-target tissue and predominantly hepatobiliary excretion, whereas, ¹⁸F-DCFPyl exhibited pharmacokinetics quite similar to those obtained with (1) and (2).

Conclusion

Both ¹⁸F-labeled EuE-based PSMA ligands showed excellent in vitro and in vivo PSMA-targeting characteristics. The substantially higher tumor accumulation in mice compared to recently introduced ¹⁸F-PSMA-1007 and ¹⁸F-DCFPyl suggests their high value for preclinical studies investigating the effects on PSMA-expression. In contrast to (2), (1) seems to be more promising for further investigation, due to the more reliable ¹⁸F-labeling procedure, the faster clearance kinetics with comparable high tumor uptake, resulting therefore in better high-contrast microPET imaging as early as 1 h p.i.

Electronic supplementary material

The online version of this article (10.1186/s13550-018-0382-8) contains supplementary material, which is available to authorized users.

ALTERED MERISTEM PROGRAM1 suppresses ectopic stem cell niche formation in the shoot apical meristem in a largely cytokinin-independent manner

Plants are able to reiteratively form new organs in an environmentally adaptive manner during postembryonic development. Organ formation in plants is dependent on stem cell niches (SCNs), which are located in the so-called meristems. Meristems show a functional zonation along the apical-basal axis and the radial axis. Shoot apical meristems of higher plants are dome-like structures, which contain a central SCN that consists of an apical stem cell pool and an underlying organizing center. Organ primordia are formed in the circular peripheral zone (PZ) from stem cell descendants in which differentiation programs are activated. One mechanism to keep this radial symmetry integrated is that the existing SCN actively suppresses stem cell identity in the PZ. However, how this lateral inhibition system works at the molecular level is far from understood. Here, we show that a defect in the putative carboxypeptidase ALTERED MERISTEM PROGRAM1 (AMP1) causes the formation of extra SCNs in the presence of an intact primary shoot apical meristem, which at least partially contributes to the enhanced shoot meristem size and leaf initiation rate found in the mutant. This defect appears to be neither a specific consequence of the altered cytokinin levels in amp1 nor directly mediated by the WUSCHEL/CLAVATA feedback loop. De novo formation of supernumerary stem cell pools was further enhanced in plants mutated in both AMP1 and its paralog LIKE AMP1, indicating that they exhibit partially overlapping roles to suppress SCN respecification in the PZ.

Glutamate carboxypeptidase II is not an amyloid peptide-degrading enzyme

Glutamate carboxypeptidase II (GCPII) is an exopeptidase that catalyzes the hydrolysis of N-acetylated aspartate-glutamate (NAAG) to N-acetyl aspartate (NAA) and glutamate. Consequently, GCPII inhibition has been of interest for the treatment of central and peripheral nervous system diseases associated with excess glutamate. Recently, it was reported that GCPII can also serve as an endopeptidase cleaving amyloid β (Aβ) peptides and that its inhibition could increase the risk of Alzheimer's disease by increasing brain Aβ levels. This study aimed to corroborate and extend these new findings. We incubated Aβ peptides (20 μM) with human recombinant GCPII (300 ng/ml) and monitored the appearance of degradation products by mass spectrometry. Aβ peptides remained intact after 18 h incubation with GCPII. Under the same experimental conditions, Aβ1-40 (20 μM) was incubated with neprilysin (300 ng/ml), an endopeptidase known to hydrolyze Aβ1-40 and the expected cleavage products were observed. GCPII was confirmed active by catalyzing the complete hydrolysis of NAAG (100 μM). We also studied the hydrolysis of [(3)H]-NAAG (30 nM) catalyzed by GCPII (40 pM) in the presence of Aβ peptides (picomolar to micromolar range). The addition of Aβ peptides did not alter [(3)H]-NAAG hydrolysis. We conclude that GCPII is not an amyloid peptide-degrading enzyme.