Biomedical subjects

G J M Martens

Publications and source records attributed to G J M Martens.

7 recordsLinked to original sources

PubMed2006

Ontogenic reduction of Aph-1b mRNA and gamma-secretase activity in rats with a complex neurodevelopmental phenotype.

Selectively bred apomorphine susceptible (APO-SUS) rats display a complex behavioral phenotype remarkably similar to that of human neurodevelopmental disorders, such as schizophrenia. We recently found that the APO-SUS rats have only one or two Aph-1b gene copies (I/I and II/II rats, respectively), whereas their phenotypic counterpart has three copies (III/III). Aph-1b is a component of the gamma-secretase enzyme complex that is involved in multiple (neuro)developmental signaling pathways. Nevertheless, surprisingly little is known about gamma-secretase expression during development. Here, we performed a longitudinal quantitative PCR study in embryos and the hippocampus of I/I, II/II and III/III rats, and found gene-dosage dependent differences in Aph-1b, but not Aph-1a, mRNA expression throughout pre- and post-natal development. On the basis of the developmental mRNA profiles, we assigned relative activities to the various Aph-1a and -1b gene promoters. Furthermore, in the three rat lines, we observed both tissue-specific and temporal alterations in gamma-secretase cleavage activity towards one of its best-known substrates, the amyloid-beta precursor protein APP. We conclude that the low levels of Aph-1b mRNA and gamma-secretase activity observed in the I/I and II/II rats during the entire developmental period may well underlie their complex phenotype.

Animals↗

PubMed2005

A fast method to study the secretory activity of neuroendocrine cells at the ultrastructural level.

Cryo field emission scanning electron microscopy (cryo-FE-SEM) is a versatile technique that allows the investigation of the three-dimensional organization of cells at the ultrastructural level over a wide range of magnifications. Unfortunately, cryopreparation of the specimens for this technique remains cumbersome, in particular because ice crystal formation must be prevented during freezing. Here we report that a light prefixation with glutaraldehyde and incubation in glycerol as cryoprotectant or a high-pressure freezing approach are both excellent procedures for cryopreparation of animal cells to be used in combination with cryo-FE-SEM. Using the proopiomelanocortin-producing intermediate pituitary melanotrope cells of Xenopus laevis as a physiologically inducible neuroendocrine system, we compared the ultrastructural characteristics of inactive and hyperactive neuroendocrine cells. The overall quality of the ultrastructural images was comparable for the two cryopreparation procedures, although some fine structures were better conserved using high-pressure freezing. Melanotrope cells in a secretory inactive state contained numerous storage granules and a poorly developed endoplasmic reticulum (ER), while large amounts of rough ER were present in hyperactive cells. Thus, the cryo-FE-SEM approach described here allows a fast ultrastructural study on the secretory activity of neuroendocrine cells.

Animals↗

PubMed2004

Isolation and characterization of the Xenopus laevis orthologs of the human papillary renal cell carcinoma-associated genes PRCC and MAD2L2 (MAD2B).

Recently we found that the human papillary renal cell carcinoma-associated protein PRCC interacts with the cell cycle control protein Mad2B, and translocates this protein to the nucleus where it exerts its mitotic checkpoint function. Here we have successfully isolated Xenopus laevis Mad2B and PRCC cDNAs. The full-length xMad2B cDNA encodes a 211 amino acid protein that is highly homologous to human Mad2B, thus pointing to an important function for this protein in higher eukaryotes. The full-length xPRCC cDNA encodes a 544 amino acid protein. Remarkably, this protein contains an amino-terminal region distinct from that in mouse and human, whereas the C-terminal region is highly conserved. Northern blot and RT-PCR analyses revealed a relatively low expression of both xMad2B and xPRCC in most tissues examined. However, an abundant expression was observed in testis and oocyte, indicating a role in meiotic division processes. Coimmunoprecipitation and immunofluorescence analyses revealed that, despite its distinct amino terminus, the xPRCC-protein is still capable of interacting with xMad2B and of shuttling this protein to the nucleus. Therefore, the well-established animal model Xenopus laevis can be used as a powerful system to study in detail the role of xPRCC and xMad2B in the intricate processes of cell cycle control.

Adaptor Proteins, Signal Transducing↗

PubMed2003

NPY-mRNA expressions in the nucleus accumbens, caudate putamen and cerebral cortex of apomorphine-susceptible and apomorphine-unsusceptible rats.

Using the apomorphine-induced stereotyped gnawing response as a selection criterion, two distinct groups of rats can be distinguished, apomorphine-susceptible (APO-SUS) and apomorphine-unsusceptible (APO-UNSUS) rats. These two lines differ in several components of both striatal and extrastriatal areas. This study deals with the expression of neuropeptide Y (NPY)mRNA-expressing neurons in the nucleus accumbens, caudate putamen and cerebral cortex of both rat lines, using non-radioactive in situ hybridisation. The morphology of the neurons in the three regions is similar, viz. oblong, rectangular or triangular, with two or three processes. The neurons are homogeneously distributed in all regions, and in the nucleus accumbens they are particularly numerous ventrally to the anterior commissure. Using automated image analysis, the mean numerical density of NPYmRNA-positive neurons per brain region and the mean NPYmRNA expression level per neuron per brain region were determined. No differences appear in the numerical densities of NPYmRNA-containing neurons in the nucleus accumbens, caudate putamen and cortex between APO-SUS and APO-UNSUS rats. However, distinct differences between the rat lines are present in the level of NPYmRNA expression per neuron in the nucleus accumbens and in the caudate putamen, showing that NPY contributes to the differential neurochemical make-up of these rat lines that is responsible for their obvious differences in behaviour, physiology and immune competence.

Animals↗

PubMed2003

Phlorizin treatment prevents the decrease in plasma insulin levels but not the progressive histopathological changes in the pancreatic islets during aging of Zucker diabetic fatty rats.

A hallmark of Type 2 diabetes mellitus (T2DM) is chronic hyperglycemia, which is thought to play a role in pancreatic beta-cell failure. Here we investigated whether treatment of Zucker diabetic fatty (ZDF) rats, an animal model for T2DM, with the renal glucose transport inhibitor phlorizin could prevent alterations in the pancreatic islets. ZDF rats were treated with phlorizin or vehicle for 13 weeks starting with 6-week-old rats and before the onset of hyperglycemia. During the treatment, blood glucose levels in sham-treated ZDF rats increased rapidly from 7.7 +/- 0.3 to 24.8 +/- 0.6 mmol/l, whereas those in phlorizin-treated ZDF rats increased only slightly, but significantly, from 7.0 +/- 0.2 to 8.9 +/- 0.6 mmol/l. Phlorizin prevented the decrease in plasma insulin levels and caused a higher increase in body weight of the ZDF rats. Compared to 6-week-old untreated ZDF rats, in 19-week-old sham- and phlorizin-treated ZDF rats similar changes were found in islet architecture (more irregular boundaries and a disrupted mantle of peripheral islet cells) and in the mitochondria at the ultrastructural level (swelling of the matrix and disruption of the cristae). Using reverse transcriptase-polymerase chain reaction, no differences in mRNA expression levels were found for insulin, islet amyloid polypeptide (IAPP), and the prohormone convertase (PC) 1 and PC2 between 6-week-old untreated ZDF rats and 19-week-old sham- and phlorizin-treated ZDF rats. However, immunohistochemistry revealed similar decreases in insulin and IAPP protein expression in 19-week-old sham- and phlorizin-treated ZDF rats compared to those in 6-week-old untreated ZDF rats. These observations indicate that during aging of ZDF rats phlorizin treatment does not prevent the decreases in insulin and IAPP protein expression and the progressive histopathological changes in the pancreatic islets. Therefore, it is highly unlikely that these changes are caused by chronic hyperglycemia.

Aging↗

PubMed2002

Automated nanoflow liquid chromatography-tandem mass spectrometry for a differential display proteomic study on Xenopus laevis neuroendocrine cells.

Many proteomic projects based on a comparison of protein profiles displayed on two-dimensional polyacrylamide gel electrophoresis rely on the identification of these proteins using peptide mass fingerprinting on a matrix-assisted laser desorption/ionization mass spectrometer after tryptic digestion. However, this approach is limited to an organism of which genomic information is largely available, i.e. when the total genome sequence is known. For other organisms, mass spectrometric sequence analysis is necessary for protein identification. We established a nano-LC-MS-MS system based on a quadrupole time-of-flight mass spectrometer, which allows automated sequence analysis of tryptic digestion mixtures from single gel spots. This system is applied in a differential-display proteomic study to identify differentially expressed proteins in the neuroendocrine cells of the neurointermediate pituitary of black- and white-background adapted Xenopus laevis.

Amino Acid Sequence↗

PubMed2002

Demonstration of postsynaptic receptor plasticity in an amphibian neuroendocrine interface.

Pituitary pars intermedia melanotrope cells are often used as a model to study mechanisms of neuroendocrine integration. In the amphibian Xenopus laevis, the synthesis and release of alpha-melanophore-stimulating hormone (alpha-MSH) from these cells is a dynamic process dependent upon the colour of background. In animals on a black background, there is a higher level of synthesis and secretion of alpha-MSH than in animals on a white background, and, consequently, there is skin darkening in animals on a black background. The melanotropes are innervated by hypothalamic neurones that produce neuropeptide Y (NPY), a peptide that inhibits alpha-MSH secretion via the NPY Y1 receptor. The inhibitory neurones have a higher expression of NPY in animals adapted to a white background and both the size and the number of inhibitory synapses on the melanotrope cells are enhanced. The purpose of the present study was to determine if this presynaptic plasticity displayed by the inhibitory neurones is reciprocated by postsynaptic plasticity (i.e. if there is an enhanced expression of the Y1 receptor in melanotropes of animals adapted to a white background). For this purpose quantitative real-time reverse transcriptase-polymerase chain reaction was used to determine the level of Y1 receptor mRNA in melanotropes of animals undergoing the process of background adaptation. The results showed that there is a higher Y1 receptor mRNA expression in melanotropes of white-adapted animals. We conclude that the inhibitory neuroendocrine interface in the Xenopus pars intermedia displays postsynaptic plasticity in response to changes of background colour. To our knowledge, this is the first demonstration of a physiological environmental change leading to changes in postsynaptic receptor expression in a fully identified vertebrate neuroendocrine reflex.

Animals↗