Glutamate carboxypeptidase II


TAH molecule, also known as N-acetyl-L-aspartyl-L-glutamate peptidase I, NAAG peptidase, or prostate-specific membrane antigen is an enzyme that in humans is encoded by the FOLH1 gene. Human GCPII contains 750 amino acids and weighs approximately 84 kDa.
GCPII is a zinc metalloenzyme that resides in membranes. Most of the enzyme resides in the extracellular space. GCPII is a class II membrane glycoprotein. It catalyzes the hydrolysis of N-acetylaspartylglutamate to glutamate and N-acetylaspartate according to the reaction scheme to the right.
Neuroscientists primarily use the term NAALADase in their studies, while those studying folate metabolism use folate hydrolase, and those studying prostate cancer or oncology, PSMA. All refer to the same protein glutamate carboxypeptidase II.

Discovery

GCPII is mainly expressed in four tissues of the body, including prostate epithelium, the proximal tubules of the kidney, the jejunal brush border of the small intestine and ganglia of the nervous system.
Indeed, the initial cloning of the cDNA encoding the gene expressing PSMA was accomplished with RNA from a prostate tumor cell line, LNCaP. PSMA was first detected in the LNCaP cell line using the murine monoclonal antibody 7E11-C5.3, generated from murine spleen cells treated with LNCaP cell membranes. However, 7E11-C5.3 exclusively targets an intracellular epitope of PSMA, thus only binding to dead or dying cells. PSMA shares homology with the transferrin receptor and undergoes endocytosis but the ligand for inducing internalization has not been identified. It was found that PSMA was the same as the membrane protein in the small intestine responsible for removal of gamma-linked glutamates from polygammaglutamate folate. This enables the freeing of folic acid, which then can be transported into the body for use as a vitamin. This resulted in the cloned genomic designation of PSMA as FOLH1 for folate hydrolase.

Structure

The three domains of the extracellular portion of GCPII—the protease, apical and C-terminal domains—collaborate in substrate recognition. The protease domain is a central seven-stranded mixed β-sheet. The β-sheet is flanked by 10 α-helices. The apical domain is located between the first and the second strands of the central β-sheet of the protease domain. The C-terminal domain is an Up-Down-Up-Down four-helix bundle. The apical, protease and C-terminal domains create a pocket that facilitates substrate binding.
The central pocket is approximately 2 nanometers in depth and opens from the extracellular space to the active site. This active site contains two zinc ions. During inhibition, each acts as a ligand to an oxygen in 2-PMPA or phosphate. There is also one calcium ion coordinated in GCPII, far from the active site. It has been proposed that calcium holds together the protease and apical domains. In addition, human GCPII has ten sites of potential glycosylation, and many of these sites affect the ability of GCPII to hydrolyze NAAG.
The human FOLH1 gene is positioned at the 11p11.12 locus of chromosome 11. The gene is 4,110 base pairs in length and composed of 22 exons. The encoded protein is a member of the M28 peptidase family. Orthologs of the human FOLH1 gene have also been identified in other mammals, including the 7 D3; 7 48.51 cM locus in mice. The FOLH1 gene has multiple potential start sites and splice forms, giving rise to differences in membrane protein structure, localization, and carboxypeptidase activity based on the parent tissue.

Enzyme kinetics

The hydrolysis of NAAG by GCPII obeys Michaelis–Menten kinetics. Hlouchová et al. determined the Michaelis constant for NAAG to be 1.2*10−6 ± 0.5*10−6 M and the turnover number to be 1.1 ± 0.2 s−1.

Role in cancer

Human PSMA is highly expressed in the prostate, roughly a hundred times greater than in most other tissues. In some prostate cancers, PSMA is the second-most upregulated gene product, with an 8- to 12-fold increase over levels in noncancerous prostate cells. Because of this high expression, PSMA is being developed as potential biomarker for therapy and imaging of some cancers. In human prostate cancer, the higher expressing tumors are associated with quicker time to progression and a greater percentage of patients suffering relapse. In vitro studies using prostate and breast cancer cell lines with decreased PSMA levels showed a significant decrease in the proliferation, migration, invasion, adhesion and survival of the cells.

Imaging

PSMA is the target of several nuclear medicine imaging agents for prostate cancer. PSMA expression can be imaged with gallium-68 PSMA or fluorine-18 PSMA for positron emission tomography. This uses a radiolabelled small molecule that binds with high affinity to the extra-cellular domain of the PSMA receptor. Previously, an antibody targeting the intracellular domain was used, although detection rate was low.
In 2020, the results of a randomised phase 3 trial was published comparing Gallium-68 PSMA PET/CT to standard imaging. This 300 patient study conducted at 10 study sites demonstrated superior accuracy of PSMA PET/CT, higher significant change in management, less equivocal/uncertain imaging findings and lower radiation exposure. The study concludes that PSMA PET/CT is a suitable replacement for conventional imaging, providing superior accuracy, to the combined findings of CT and bone scanning. This new technology was approved by the FDA on Dec 1, 2020. A dual-modality small molecule that is positron-emitting and fluorescent targets PSMA and was tested in humans. The molecule found the location of primary and metastatic prostate cancer by PET, fluorescence-guided removal of cancer, and detects single cancer cells in tissue margins.
A Human-Derived, Genetic, Positron-emitting and Fluorescent reporter system uses a human protein, PSMA and non-immunogenic, and a small molecule that is positron-emitting and fluorescent for dual modality PET and fluorescence imaging of genome modified cells, e.g. cancer, CRISPR/Cas9, or CAR T-cells, in an entire mouse.

Therapy

PSMA can also be used as a target for treatment in unsealed source radiotherapy. Lutetium-177 is a beta emitter which can be combined with PSMA-targeting molecules to deliver treatment to prostate tumours. A prospective phase II study demonstrated a response in 64% of men. Common side effects include dry mouth, dry fatigue, nausea, dry eyes and thrombocytopenia. A follow-up randomized phase II trial, the ANZUP TheraP trial, compared Lu-177 PSMA-617 radionuclide therapy to cabazitaxel chemotherapy, demonstrating superior response rates, lower toxicity and better patient-reported outcomes with Lu-177 PSMA. The results of randomised trial VISION trial were positive with 40% reduction in mortality and 5 months increase in survival. phase III VISION trial.

Neurotransmitter degradation

For those studying neural based diseases, NAAG is one of the three most prevalent neurotransmitters found in the central nervous system and when it catalyzes the reaction to produce glutamate it is also producing another neurotransmitter. Glutamate is a common and abundant excitatory neurotransmitter in the central nervous system; however, if there is too much glutamate transmission, this can kill or at least damage neurons and has been implicated in many neurological diseases and disorders therefore the balance that NAAG peptidase contributes to is quite important.

Potential therapeutic applications

Function in the brain

GCPII has been shown to both indirectly and directly increase the concentration of glutamate in the extracellular space. GCPII directly cleaves NAAG into NAA and glutamate. NAAG has been shown, in high concentration, to indirectly inhibit the release of neurotransmitters, such as GABA and glutamate. It does this through interaction with and activation of presynaptic group II mGluRs. Thus, in the presence of NAAG peptidase, the concentration of NAAG is kept in check, and glutamate and GABA, among other neurotransmitters, are not inhibited.
Researchers have been able to show that effective and selective GCPII inhibitors are able to decrease the brain's levels of glutamate and even provide protection from apoptosis or degradation of brain neurons in many animal models of stroke, amyotrophic lateral sclerosis, and neuropathic pain. This inhibition of these NAAG peptidases, sometimes referred to as NPs, are thought to provide this protection from apoptosis or degradation of brain neurons by elevating the concentrations of NAAG within the synapse of neurons. NAAG then reduces the release of glutamate while stimulating the release of some trophic factors from the glia cells in the central nervous system, resulting in the protection from apoptosis or degradation of brain neurons. It is important to note, however, that these NP inhibitors do not seem to have any effect on normal glutamate function. The NP inhibition is able to improve the naturally occurring regulation instead of activating or inhibiting receptors that would disrupt this process. Research has also shown that small-molecule-based NP inhibitors are beneficial in animal models that are relevant to neurodegenerative diseases. Some specific applications of this research include neuropathic and inflammatory pain, traumatic brain injury, ischemic stroke, schizophrenia, diabetic neuropathy, amyotrophic lateral sclerosis, as well as drug addiction. Previous research has found that drugs that are able to reduce glutamate transmission can relieve the neuropathic pain, although the resultant side-effects have limited a great deal of their clinical applications. Therefore, it appears that, since GCPII is exclusively recruited for the purpose of providing a glutamate source in hyperglutamatergic and excitotoxic conditions, this could be an alternative to avert these side-effects. More research findings have shown that the hydrolysis of NAAG is disrupted in schizophrenia, and they have shown that specific anatomical regions of the brain may even show discrete abnormalities in the GCP II synthesis, so NPs may also be therapeutic for patients suffering with schizophrenia. One major hurdle with using many of the potent GCPII inhibitors that have been prepared to date are typically highly polar compounds, which causes problems because they do not then penetrate the blood–brain barrier easily.