Histochemistry of proteases in ependyma, choroid plexus and leptomeninges

CD26/DPP4 - a potential biomarker and target for cancer therapy

CD26/dipeptidyl peptidase (DPP)4 is a membrane-bound protein found in many cell types of the body, and a soluble form is present in body fluids. There is longstanding evidence that various primary tumors and also metastases express CD26/DPP4 to a variable extent. By cleaving dipeptides from peptides with a proline or alanine in the penultimate position at the N-terminus, it regulates the activity of incretin hormones, chemokines and many other peptides. Due to these effects and interactions with other molecules, a tumor promoting or suppressing role can be attributed to CD26/DPP4. In this review, we discuss the existing evidence on the expression of soluble or membrane-bound CD26/DPP4 in malignant diseases, along with the most recent findings on CD26/DPP4 as a therapeutic target in specific malignancies. The expression and possible involvement of the related DPP8 and DPP9 in cancer are also reviewed. A higher expression of CD26/DPP4 is found in a wide variety of tumor entities, however more research on CD26/DPP4 in the tumor microenvironment is needed to fully explore its use as a tumor biomarker. Circulating soluble CD26/DPP4 has also been studied as a cancer biomarker, however, the observed decrease in most cancer patients does not seem to be cancer specific. Encouraging results from experimental work and a recently reported first phase clinical trial targeting CD26/DPP4 in mesothelioma, renal and urological tumors pave the way for follow-up clinical studies, also in other tumor entities, possibly leading to the development of more effective complementary therapies against cancer.

Glial cell type-specific changes in spinal dipeptidyl peptidase 4 expression and effects of its inhibitors in inflammatory and neuropatic pain

Altered pain sensations such as hyperalgesia and allodynia are characteristic features of various pain states, and remain difficult to treat. We have shown previously that spinal application of dipeptidyl peptidase 4 (DPP4) inhibitors induces strong antihyperalgesic effect during inflammatory pain. In this study we observed low level of DPP4 mRNA in the rat spinal dorsal horn in physiological conditions, which did not change significantly either in carrageenan-induced inflammatory or partial nerve ligation-generated neuropathic states. In naïve animals, microglia and astrocytes expressed DPP4 protein with one and two orders of magnitude higher than neurons, respectively. DPP4 significantly increased in astrocytes during inflammation and in microglia in neuropathy. Intrathecal application of two DPP4 inhibitors tripeptide isoleucin-prolin-isoleucin (IPI) and the antidiabetic drug vildagliptin resulted in robust opioid-dependent antihyperalgesic effect during inflammation, and milder but significant opioid-independent antihyperalgesic action in the neuropathic model. The opioid-mediated antihyperalgesic effect of IPI was exclusively related to mu-opioid receptors, while vildagliptin affected mainly delta-receptor activity, although mu- and kappa-receptors were also involved. None of the inhibitors influenced allodynia. Our results suggest pathology and glia-type specific changes of DPP4 activity in the spinal cord, which contribute to the development and maintenance of hyperalgesia and interact with endogenous opioid systems.

Concerted action of dipeptidyl peptidase IV and glutaminyl cyclase results in formation of pyroglutamate-modified amyloid peptides in vitro

Compelling evidence suggests a crucial role of amyloid beta peptides (Aβ(1-40/42)) in the etiology of Alzheimer's disease (AD). The N-terminal truncation of Aβ(1-40/42) and their modification, e.g. by glutaminyl cyclase (QC), is expected to enhance the amyloid toxicity. In this work, the MALDI-TOF mass spectrometry application proved N-terminal cleavage of Aβ(1-40/42) by purified dipeptidyl peptidase IV (DPPIV) in vitro observed earlier. The subsequent transformation of resulted Aβ(3-40/42) to pE-Aβ(3-40/42) in QC catalyzed glutamate cyclization was manifested. Hence, consecutive conversion of Aβ(1-40/42) by DPPIV and QC can be assumed as a potential mechanism of formation of non-degrading pyroglutamated pE-Aβ(3-40/42), which might accumulate and contribute to AD progression. The in vitro acceleration of Aβ(1-40) aggregation in the simultaneous presence of DPPIV and QC was shown also.

Middle East Respiratory Syndrome Coronavirus Causes Multiple Organ Damage and Lethal Disease in Mice Transgenic for Human Dipeptidyl Peptidase 4

Middle East respiratory syndrome coronavirus (MERS-CoV) causes life-threatening disease. Dipeptidyl peptidase 4 (DPP4) is the receptor for cell binding and entry. There is a need for small-animal models of MERS, but mice are not susceptible to MERS because murine dpp4 does not serve as a receptor. We developed transgenic mice expressing human DPP4 (hDPP4) under the control of the surfactant protein C promoter or cytokeratin 18 promoter that are susceptible to infection with MERS-CoV. Notably, mice expressing hDPP4 with the cytokeratin 18 promoter developed progressive, uniformly fatal disease following intranasal inoculation. High virus titers were present in lung and brain tissues 2 and 6 days after infection, respectively. MERS-CoV–infected lungs revealed mononuclear cell infiltration, alveolar edema, and microvascular thrombosis, with airways generally unaffected. Brain disease was observed, with the greatest involvement noted in the thalamus and brain stem. Animals immunized with a vaccine candidate were uniformly protected from lethal infection. These new mouse models of MERS-CoV should be useful for investigation of early disease mechanisms and therapeutic interventions.

DPP4-deficient congenic rats display blunted stress, improved fear extinction and increased central NPY

BACKGROUND

Inhibitors of dipeptidyl peptidase 4 (DPP4, CD26) are used for the treatment of type 2 diabetic patients and better glucose tolerance has been confirmed in functionally DPP4-deficient congenic rats (DPP4mut), along with immunological alterations and, interestingly, a stress-resilient phenotype. All these findings are in agreement with the "moonlighting" properties of DPP4, whose proteolytic action is responsible for the inactivation of a number of regulatory peptides including, but not limited to, neuropeptide Y (NPY). Among all candidate substrates, DPP4 displays highest affinity for NPY, an endogenous anxiolytic neurotransmitter that is suggested as a candidate biomarker in post-traumatic stress disorder (PTSD) and depression.

METHODS AND RESULTS

Central and peripheral NPY levels were measured by ELISA in DPP4mut and DAwt rats revealing a significantly higher concentration of the peptide in the CSF of DPP4mut animals. This finding positively correlated with the blunted stress phenotype measured on an analgesia-meter. Additionally, when a classical fear-conditioning paradigm was investigated, short-term fear extinction was significantly potentiated in DPP4mut rats as compared to wt controls.

CONCLUSIONS

Our findings indicate a positive correlation between reduced stress-responsiveness and increased central NPY, in DPP4mut rats. Most interestingly, the behavioral phenotype extends to facilitation of fear extinction. These observations raise further interest in DPP4-modulating drugs for the potential effect on NPY metabolism, as a therapeutic tool for psychiatric conditions such as anxiety disorders and PTSD.

Regulation of hindbrain Pyy expression by acute food deprivation, prolonged caloric restriction, and weight loss surgery in mice

PYY is a gut-derived putative satiety signal released in response to nutrient ingestion and is implicated in the regulation of energy homeostasis. Pyy-expressing neurons have been identified in the hindbrain of river lamprey, rodents, and primates. Despite this high evolutionary conservation, little is known about central PYY neurons. Using in situ hybridization, PYY-Cre;ROSA-EYFP mice, and immunohistochemistry, we identified PYY cell bodies in the gigantocellular reticular nucleus region of the hindbrain. PYY projections were present in the dorsal vagal complex and hypoglossal nucleus. In the hindbrain, Pyy mRNA was present at E9.5, and expression peaked at P2 and then decreased significantly by 70% at adulthood. We found that, in contrast to the circulation, PYY-(1-36) is the predominant isoform in mouse brainstem extracts in the ad libitum-fed state. However, following a 24-h fast, the relative amounts of PYY-(1-36) and PYY-(3-36) isoforms were similar. Interestingly, central Pyy expression showed nutritional regulation and decreased significantly by acute starvation, prolonged caloric restriction, and bariatric surgery (enterogastroanastomosis). Central Pyy expression correlated with body weight loss and circulating leptin and PYY concentrations. Central regulation of energy metabolism is not limited to the hypothalamus but also includes the midbrain and the brainstem. Our findings suggest a role for hindbrain PYY in the regulation of energy homeostasis and provide a starting point for further research on gigantocellular reticular nucleus PYY neurons, which will increase our understanding of the brain stem pathways in the integrated control of appetite and energy metabolism.

Novel observations on the origin of ependymal cells in the ventricular zone of the rat spinal cord

Despite extensive investigations of gliogenesis, the time of origin of ependymal cells in the spinal cord has not yet been fully elucidated. Using a single dose of 5-bromo-2-deoxyuridine combined with various survival times we monitored: mitotic activity (short survival time), the presence of newly formed cells in the ventricular zone (intermediate survival time) and the formation of ependymal cells (long survival time) during the late embryonic and early postnatal development in the ventricular zone of the spinal cord of rats. In the period of study it was found that the ependymal cells populated this region in two waves. Most of the ependymal cells originated around embryonic day 18 and then between postnatal days 8 and 15. In addition, it was observed that in the ventricular zone of the spinal cord, proliferation and production of ependymal cells continues at the slower rate at least until day 36 of postnatal development. Elucidation of the relationship between progenitors in the embryonic ventricular zone and the relative quiescent ependymal lining of the central canal in adulthood could be important in the search for the adult neural stem cell niche.

Delivery of peptide and protein drugs over the blood-brain barrier

Peptide and protein (P/P) drugs have been identified as showing great promises for the treatment of various neurodegenerative diseases. A major challenge in this regard, however, is the delivery of P/P drugs over the blood-brain barrier (BBB). Intense research over the last 25 years has enabled a better understanding of the cellular and molecular transport mechanisms at the BBB, and several strategies for enhanced P/P drug delivery over the BBB have been developed and tested in preclinical and clinical-experimental research. Among them, technology-based approaches (comprising functionalized nanocarriers and liposomes) and pharmacological strategies (such as the use of carrier systems and chimeric peptide technology) appear to be the most promising ones. This review combines a comprehensive overview on the current understanding of the transport mechanisms at the BBB with promising selected strategies published so far that can be applied to facilitate enhanced P/P drug delivery over the BBB.

Dipeptidyl-peptidase IV (CD26)--role in the inactivation of regulatory peptides

Dipeptidyl-peptidase IV (DPP IV/CD26) has a dual function as a regulatory protease and as a binding protein. Its role in the inactivation of bioactive peptides was recognized 20 years ago due to its unique ability to liberate Xaa-Pro or Xaa-Ala dipeptides from the N-terminus of regulatory peptides, but further examples are now emerging from in vitro and vivo experiments. Despite the minimal N-terminal truncation by DPP IV, many mammalian regulatory peptides are inactivated--either totally or only differentially--for certain receptor subtypes. Important DPP IV substrates include neuropeptides like neuropeptide Y or endomorphin, circulating peptide hormones like peptide YY, growth hormone-releasing hormone, glucagon-like peptides(GLP)-1 and -2, gastric inhibitory polypeptide as well as paracrine chemokines like RANTES (regulated on activation normal T cell expressed and secreted), stromal cell-derived factor, eotaxin and macrophage-derived chemokine. Based on these findings the potential clinical uses of selective DPP IV inhibitors or DPP IV-resistant analogues, especially for the insulinotropic hormone GLP-1, have been tested to enhance insulin secretion and to improve glucose tolerance in diabetic animals. Thus, DPP IV appears to be a major physiological regulator for some regulatory peptides, neuropeptides, circulating hormones and chemokines.

Immunohistochemical study of localization of gamma-glutamyl transpeptidase in the rat brain

Gamma-glutamyl transpeptidase (gamma-GTP) is a membrane-bound enzyme which is known to play a crucial role in active transport of amino acids across membrane barriers. We prepared a monoclonal antibody recognizing specifically rat gamma-GTP and investigated localization of the enzyme in the rat brain by immunohistochemistry with this antibody. The antigen was localized on the ependyma, epithelia of the choroid plexus and microvessels. More precise localization of gamma-GTP was examined with immuno-electron microscopy. The antigen was recognized on the microvilli and cilia of the ependymal cells, microvilli of the choroid epithelial cells and luminal membranes of the vascular endothelial cells.

The ependyma: a protective barrier between brain and cerebrospinal fluid

This review summarizes the current scientific literature concerning the ependymal lining of the cerebral ventricles of the brain with an emphasis on selective barrier function and protective roles for the common ependymal cell. Topics covered include the development, morphology, protein and enzyme expression including reactive changes, and pathology. Some cells lining the neural tube are committed at an early stage to becoming ependymal cells. They serve a secretory function and perhaps act as a cellular/axonal guidance system, particularly during fetal development. In the mature mammalian brain ependymal cells possess the structural and enzymatic characteristics necessary for scavenging and detoxifying a wide variety of substances in the CSF, thus forming a metabolic barrier at the brain-CSF interface.

Enzyme histochemical studies of membrane proteases in rat subfornical organ

Localization of membrane proteases glutamyl aminopeptidase (EAP), microsomal alanyl aminopeptidase (mAAP), dipeptidyl peptidase IV (DPP IV) and gamma-glutamyl transpeptidase (gamma-GTP) were studied in vessels of the rat subfornical organ (SFO), ependyma which cover the surface of the SFO, and adjacent brain structures. Results of enzyme histochemical reactions showed strong activity for EAP, mAAP, and gamma-GTP, but absence of DPP IV in microvessels of SFO. The ependyma which cover the SFO was positive for gamma-GTP, but negative for other studied proteases. Our results showed that the spectrum of enzymes in the majority of the vessels of SFO is similar to that of the microvessels of the adjacent brain tissue which were positive for EAP, mAAP, and gamma-GTP, but negative for DPP IV. The relative intensity of the enzyme reactions in vessels varied from central to lateral locations in the SFO and the adjacent brain tissue. There was also a difference in the relative reaction intensity from one enzyme to the other. The presence and heterogeneous distribution of the enzymes are consistent with the hypothesis that membrane proteases of the microvascular endothelium constitute an enzyme-barrier between blood and parenchyma of the SFO and between blood and brain tissue, and may be involved in metabolism or modulation of various peptides when they contact the plasma membrane of the endothelial cells of the vessels.

Membrane-bound proteases of the gerbil subfornical organ and choroid plexus: an enzyme histochemical study

Using enzyme-histochemical methods, the membrane-bound peptidases, gamma-glutamyl transpeptidase (gamma-GTP), microsomal alanyl aminopeptidase (mAAP), glutamyl aminopeptidase (EAP), and dipeptidyl peptidase IV (DPP IV), were studied in microvessels of the gerbil subfornical organ (SFO), choroid plexus adjacent to the SFO, and the ependyma of brain ventricle walls in the vicinity of the SFO. Vessels and microvessels of gerbil SFO and choroid plexus were positive for gamma-GTP, mAAP, and EAP, but negative for DPP IV. Blood-brain barrier (BBB) microvessels in the surrounding brain tissue also showed positive reactions for gamma-GTP, mAAP, and EAP but a negative reaction for DPP IV. Both epithelial cells of the choroid plexus and ependymal cells of the ventricle walls were negative for all four studied enzymes. It is suggested that blood-borne peptide hormones which can be substrates for these membrane-bound proteases can be modulated by gamma-GTP, mAAP, and EAP, but not by DPP IV, when they come in contact with the plasma membrane of the endothelial cells of the vessels in gerbil SFO, choroid plexus, and surrounding brain tissue.

Expression of neuropeptide processing enzymes and neurosecretory proteins in ependyma and choroid plexus epithelium

Recent studies suggest that brain ependyma and choroid plexus produce neuropeptide processing enzymes. To facilitate the understanding of these cells and their ability to produce biologically active peptides, we developed cultures of defined cell type. Ependymal cells were characterized by morphological criteria, and choroid plexus epithelial cell lines were characterized by the presence of the mRNA for IGF-II and transthyretin, a thyroxine binding protein produced in liver and choroid plexus. The ependymal cells and the choroid plexus epithelial cell lines were then examined for the presence of mRNAs for various neuropeptide processing enzymes. Northern blot analysis revealed high levels of furin, carboxypeptidase E, and peptidyl glycine alpha-amidating monooxygenase mRNAs, with levels in ependymal cells comparable to those in brain or pituitary. Carboxypeptidase E activity was detected in medium from cultured ependymal cells; this activity was identified as carboxypeptidase E based on the acidic pH optimum and sensitivity to various inhibitors. The mRNAs for other neuropeptide processing enzymes, such as prohormone convertases 1 and 2, were not detected on Northern blots of RNA from ependyma or choroid plexus epithelium. Since ependyma and choroid plexus epithelium express a subset of processing enzymes, we suggest that these cells have the capacity to produce biologically active peptides. Initial screening by reverse transcriptase-polymerase chain reaction assays has demonstrated the presence of mRNA for the neurosecretory proteins chromogranin B and secretogranin II in both ependyma and choroid plexus epithelium.

Aminopeptidases in the circumventricular organs of the mouse brain: a histochemical study

The localization of four membrane-bound aminopeptidases--aminopeptidase A, aminopeptidase M, dipeptidylpeptidase IV, and gamma-glutamyl transpeptidase--known as characteristic enzymes of the blood-brain barrier was studied in the microvasculature of some circumventricular organs of the mouse brain (subfornical organ, area postrema, choroid plexus, and neurohypophysis). Enzyme activities were demonstrated histochemically in chloroform-acetone-pretreated cryostat sections applying an azo-coupling method. Reactions were evaluated using light microscopy and end-point microdensitometry. The results revealed differences in microvascular enzyme pattern between circumventricular organs and regions having a blood-brain barrier. Moreover, the cytochemical picture of the circumventricular organs themselves was not uniform. Dipeptidylpeptidase IV reaction showed a strongly reduced activity in the microvessels of all studied circumventricular organs. On the other hand, aminopeptidase M seemed to be present in both the leaky and the tight capillaries. Only a low activity of aminopeptidase A was found in parts of the choroid endothelium and the subfornical organ microvasculature. gamma-Glutamyl transpeptidase could neither be detected in the capillary part of the choroid plexus nor in the neurohypophysis. We are led to conclude that at least dipeptidylpeptidase IV might be involved in special mechanisms of the blood-brain barrier.

Biochemical study of prolactin binding sites in Xenopus laevis brain and choroid plexus

The occurrence of prolactin binding sites in some brain structures (telencephalon, ventral hypothalamus, myelencephalon, hypophysis, and choroid plexus) from Xenopus laevis (anuran amphibian) was studied by the in vitro biochemical technique. The higher binding values were obtained at the level of the choroid plexus and above all of the hypothalamus. On the bases of hormonal specificity and high affinity, these binding sites are very similar to those of prolactin receptors of classical target tissues as well as of those described by us in other structures from Xenopus. To our knowledge, the present results provide the first demonstration of the occurrence of prolactin specific binding sites in Xenopus laevis choroid plexus cells.

Purification of two dipeptidyl aminopeptidases II from rat brain and their action on proline-containing neuropeptides

From the soluble and membrane fractions of rat brain homogenate, two enzymes that liberate dipeptides of the type Xaa-Pro from chromogenic substrates were purified to homogeneity. The two isolated dipeptidyl peptidases had similar molecular and catalytic properties: For the native proteins, molecular weights of 110,000 were estimated; for the denatured proteins, the estimate was 52,500. Whereas the soluble peptidase yielded one band of pI 4.2 after analytical isoelectric focusing, two additional enzymatic active bands were detected between pI 4.2 and 4.3 for the membrane-associated form. As judged from identical patterns after neuraminidase treatment, both peptidases contained no sialic acid. A pH optimum of 5.5 was estimated for the hydrolysis of Gly-Pro- and Arg-Pro-nitroanilide. Substrates with alanine instead of proline in the penultimate position were hydrolyzed at comparable rates. Acidic amino acids in the ultimate N-terminal position of the substrates reduced the activities of the peptidases 100-fold as compared with corresponding substrates with unblocked neutral or, especially, basic termini. The action of the dipeptidyl peptidase on several peptides with N-terminal Xaa-Pro sequences was investigated. Tripeptides were rapidly hydrolyzed, but the activities considerably decreased with increasing chain length of the peptides. Although the tetrapeptide substance P 1-4 was still a good substrate, the activities detected for the sequential liberation of Xaa-Pro dipeptides from substance P itself or casomorphin were considerably lower. Longer peptides were not cleaved. The peptidases hydrolyzed Pro-Pro bonds, e.g., in bradykinin 1-3 or 1-5 fragments, but bradykinin itself was resistant. The enzymes were inhibited by serine protease inhibitors, like diisopropyl fluorophosphate or phenylmethylsulfonyl fluoride, and by high salt concentrations but not by the aminopeptidase inhibitors bacitracin and bestatin. Based on the molecular and catalytic properties, both enzymes can be classified as species of dipeptidyl peptidase II (EC 3.4.14.2) rather than IV (EC 3.4.14.5). However, some catalytic properties differentiate the brain enzyme from forms of dipeptidyl peptidase II of other sources.