Biomedical subjects

B Malitschek

Publications and source records attributed to B Malitschek.

At least 19 recordsLinked to original sources

PubMed2001

Subcellular localization of GABA(B) receptor subunits in rat visual cortex.

Although studies in the visual cortex have found gamma-aminobutyric acid B (GABA(B)) receptor-mediated pre- and postsynaptic inhibitory effects on neurons, the subcellular localization of GABA(B) receptors in different types of cortical neurons and synapses has not been shown directly. To provide this information, we have used antibodies against the GABA(B) receptor (R)1a/b and GABA(B)R2 subunits and have studied the localization of immunoreactivities in rat visual cortex. Light microscopic analyses have shown that both subunits are expressed in cell bodies and dendrites of 65-92% of corticocortically projecting pyramidal neurons and in 92-100% of parvalbumin (PV)-, calretinin (CR)-, and somatostatin (SOM)-containing GABAergic neurons. Electron microscopic analyses of immunoperoxidase- and immunogold-labeled tissue revealed staining in the nucleus, cytoplasm and cell surface membranes with both antibodies. Colocalization of both subunits was observed in all of these structures. GABA(B)R1a/b and GABA(B)R2 were concentrated in excitatory and inhibitory synapses and in extrasynaptic membranes. In GABAergic synapses, GABA(B)R1a/b and GABA(B)R2 were more strongly expressed postsynaptically on pyramidal and nonpyramidal cells than presynaptically. In type 1 synapses GABA(B)R1a/b and GABA(B)R2 was found in pre- and postsynaptic membranes. The nuclear localization of GABA(B)R1 and GABA(B)R2 subunits suggests a novel role for neurotransmitter receptors in controlling gene expression. The synaptic colocalization of GABA(B)R1 and GABA(B)R2 indicates that subunits form heteromeric assemblies of the functional receptor in inhibitory and excitatory synapses. Subunit coexpression in GABAergic synapses that include PV-containing and PV-deficient terminals suggests that pre- and postsynaptic GABA(B) receptor activation is provided by several different types of interneurons. The coexpression of both subunits in excitatory synapses suggests a role for GABA(B) receptors in the regulation of glutamate release and raises the question how these receptors are activated in the absence of pre-or postsynaptic GABAergic synaptic inputs to excitatory synapses.

Animals↗

PubMed2001

Ontogenic expression of anterior pituitary GABA(B) receptor subunits.

gamma-Aminobutyric acid (GABA) is involved in the neuroendocrine control of hypophyseal secretion, acting both in the central nervous system and directly at the pituitary. We have characterized the properties of anterior pituitary GABA(B) receptors. In this work the ontogeny of rat anterior pituitary GABA(B) receptors and the pattern of subunit expression in rats of both sexes were determined. Western blot analysis showed a temporal decrease in GABA(B) subunits GABA(B(1a)) and GABA(B(1b)) expression in female anterior pituitary membranes from day 4 to adulthood, with GABA(B(1a)) being significantly more abundant than GABA(B(1b)) at early stages of development; the GABA(B(2)) subunit was barely detectable. In the male, GABA(B(1a)) followed a similar pattern and appeared to be significantly less abundant than in 4- and 12-day-old females; GABA(B(1b)) and GABA(B(2)) expression in the male was barely detectable. Scatchard plot analysis showed a temporal decrease in binding sites in female anterior pituitary membranes, in agreement with the western blot results. The number of binding sites was significantly higher in female than in male 4-day-old membranes. Dissociation constant values were similar for both sexes at all ages studied. This study reports for the first time the ontogeny of anterior pituitary GABA(B) receptors, showing a particular developmental pattern of subunit expression and a clear sexual dimorphism.

Animals↗

PubMed2000

gamma-aminobutyric acid type B receptors are expressed and functional in mammalian cardiomyocytes.

gamma-Hydroxybutyrate (GHB), an anesthetic adjuvant analog of gamma-aminobutyrate (GABA), depresses cell excitability in hippocampal neurons by inducing hyperpolarization through the activation of a prominent inwardly rectifying K(+) (Kir3) conductance. These GABA type B (GABA(B))-like effects are clearly shown at high concentrations of GHB corresponding to blood levels usually reached during anesthesia and are mimicked by the GABA(B) agonist baclofen. Recent studies of native GABA(B) receptors (GABA(B)Rs) have favored the concept that GHB is also a selective agonist. Furthermore, cloning has demonstrated that GABA(B)Rs assemble heteromeric complexes from the GABA(B)R1 and GABA(B)R2 subtypes and that these assemblies are activated by GHB. The surprisingly high tissue content, together with anti-ischemic and protective effects of GHB in the heart, raises the question of a possible influence of GABA(B) agonists on excitable cardiac cells. In the present study, we provide electrophysiological evidence that GHB activates an inwardly rectifying K(+) current in rat ventricular myocytes. This effect is mimicked by baclofen, reversibly inhibited by GABA(B) antagonists, and prevented by pertussis toxin pretreatment. Both GABA(B)R1 and GABA(B)R2 are detected in cardiomyocytes by Western blotting and are shown to coimmunoprecipitate. Laser scanning confocal microscopy discloses an even distribution of the two receptors in the sarcolemma and along the transverse tubular system. Hence, we conclude that GABA(B)Rs are distributed not only in neuronal tissues but also in the heart, where they can be activated and induce electrophysiological alterations through G-protein-coupled inward rectifier potassium channels.

Adjuvants, Anesthesia↗

PubMed2000

Ca(2+) requirement for high-affinity gamma-aminobutyric acid (GABA) binding at GABA(B) receptors: involvement of serine 269 of the GABA(B)R1 subunit.

The gamma-aminobutyric acid (GABA) receptor type B (GABA(B)R) is constituted of at least two homologous proteins, GABA(B)R1 and GABA(B)R2. These proteins share sequence and structural similarity with metabotropic glutamate and Ca(2+)-sensing receptors, both of which are sensitive to Ca(2+). Using rat brain membranes, we report here that the affinity of GABA and 3-aminopropylphosphinic acid for the GABA(B)R receptor is decreased by a factor >10 in the absence of Ca(2+). Such a large effect of Ca(2+) is not observed with baclofen or the antagonists CGP64213 and CGP56999A. In contrast to baclofen, the potency of GABA in stimulating GTPgammaS binding in rat brain membranes is also decreased by a factor >10 upon Ca(2+) removal. The potency for Ca(2+) in regulating GABA affinity was 37 microM. In cells expressing GABA(B)R1, the potency of GABA, but not of baclofen, in displacing bound (125)I-CGP64213 was similarly decreased in the absence of Ca(2+). To identify residues that are responsible for the Ca(2+) effect, the pharmacological profile and the Ca(2+) sensitivity of a series of GABA(B)R1 mutants were examined. The mutation of Ser269 into Ala was found to decrease the affinity of GABA, but not of baclofen, and the GABA affinity was found not to be affected upon Ca(2+) removal. Finally, the effect of Ca(2+) on the GABA(B) receptor function is no longer observed in cells coexpressing this GABA(B)R1-S269A mutant and the wild-type GABA(B)R2. Taken together, these results show that Ser269, which is conserved in the GABA(B)R1 protein from Caenorhabditis elegans to mammals, is critical for the Ca(2+)-effect on the heteromeric GABA(B) receptor.

Amino Acid Sequence↗

PubMed2000

Up-regulation of the metabotropic glutamate receptor mGluR4 in hippocampal neurons with reduced seizure vulnerability.

Selective hippocampal cell loss and altered neurotransmitter receptor expression have been proposed as pathogenic mechanisms in the development of chronic mesial temporal lobe epilepsy (TLE). Studies in animal models point to metabotropic glutamate receptors (mGluRs) as modulators of hippocampal epileptogenesis. In addition, mGluRs may constitute specific targets for the development of novel anticonvulsive drugs. As mGluR4 represents an inhibitory class III mGluR associated with the reduction of intracellular cyclic AMP levels and calcium influx, we have analyzed the regional and cellular expression of mGluR4 in surgical hippocampal specimens obtained from patients with TLE by using immunohistochemistry and in situ hybridization. Although the hippocampi of control specimens (n = 11) were almost devoid of mGluR4 immunolabeling, all TLE specimens (n = 35) showed a striking up-regulation of mGluR4 immunoreactivity, in particular within the dentate gyrus. Immunoelectron microscopy localized the receptor protein to the periphery of presynaptic and postsynaptic membranes. In situ hybridization revealed increased transcript levels of mGluR4 in dentate granule cells and residual CA4 neurons of TLE specimens compared with controls. Our results suggest a potential role of mGluR4 in counteracting excitatory hippocampal activity and in modulating seizure-associated vulnerability of hippocampal neurons. These data may also provide a basis for pharmacological studies of mGluR4 agonists as potential novel drugs in the treatment of TLE.

Adult↗

PubMed1999

Mutagenesis and modeling of the GABAB receptor extracellular domain support a venus flytrap mechanism for ligand binding.

The gamma-aminobutyric acid type B (GABAB) receptor is distantly related to the metabotropic glutamate receptor-like family of G-protein-coupled receptors (family 3). Sequence comparison revealed that, like metabotropic glutamate receptors, the extracellular domain of the two GABAB receptor splice variants possesses an identical region homologous to the bacterial periplasmic leucine-binding protein (LBP), but lacks the cysteine-rich region common to all other family 3 receptors. A three-dimensional model of the LBP-like domain of the GABAB receptor was constructed based on the known structure of LBP. This model predicts that four of the five cysteine residues found in this GABAB receptor domain are important for its correct folding. This conclusion is supported by analysis of mutations of these Cys residues and a decrease in the thermostability of the binding site after dithiothreitol treatment. Additionally, Ser-246 was found to be critical for CGP64213 binding. Interestingly, this residue aligns with Ser-79 of LBP, which forms a hydrogen bond with the ligand. The mutation of Ser-269 was found to differently affect the affinity of various ligands, indicating that this residue is involved in the selectivity of recognition of GABAB receptor ligands. Finally, the mutation of two residues, Ser-247 and Gln-312, was found to increase the affinity for agonists and to decrease the affinity for antagonists. Such an effect of point mutations can be explained by the Venus flytrap model for receptor activation. This model proposes that the initial step in the activation of the receptor by agonist results from the closure of the two lobes of the binding domain.

Amino Acid Sequence↗

PubMed1999

mGluR7-like receptor and GABA(B) receptor activation enhance neurotoxic effects of N-methyl-D-aspartate in cultured mouse striatal GABAergic neurones.

Presynaptic metabotropic glutamate receptors (mGluRs) of group III constitute possible targets for putative neuroprotective drugs acting against glutamate excitotoxic insults. Indeed, in glutamatergic cerebellar granule neurones in culture, high concentrations of L-2-amino-4-phosphonobutyrate (L-AP4, above 0.3 mM, thus activating mGluR7) inhibit NMDA-induced cell death. In contrast, in striatal cultures which are enriched in GABAergic neurones, we show that high concentrations of L-AP4 increased neuronal death in control as well as in NMDA-stimulated cultures. Moreover, similar results were obtained with the GABA(B)R agonist. baclofen. Both the neuroprotective effects in cerebellar granule cells and the neurotoxic effects in striatal neurones were mediated via Gi-Go-coupled mGluRs, suggesting that these effects were probably mediated by mGluR7a or b and GABA(B)R expressed in these neurones. In striatal neurones, we found that L-AP4 and baclofen inhibited both basal and NMDA-stimulated GABA release. These inhibitions of GABA release may be responsible for the increase in basal and NMDA-stimulated neuronal death. Indeed, blockade of GABA(A) receptors with bicuculline increased neuronal death of control and NMDA-treated striatal cultures. Taken together, these results suggest that L-AP4 and baclofen, via mGluR7 and GABA(B)R, reduced the neuroprotective effect of GABA present in striatal cultures acting via GABA(A) receptors. Although caution must be taken when extrapolating from in vitro to in vivo situations, the present experiments and the recent observations that mGluR7 and GABA(B)R are expressed in heterologous synapses, should be taken into consideration when evaluating the neuroprotective action of future mGluR7 specific agonists or GABA(B)R specific antagonists.

Animals↗

PubMed1999

Alternative splicing generates a novel isoform of the rat metabotropic GABA(B)R1 receptor.

Here we present a novel isoform of the metabotropic G-protein-coupled receptor for gamma-aminobutyric acid (GABA). The isoform, termed GABA(B)R1c (R1c), differs from the recently identified R1a and R1b receptors by an in-frame insertion of 31 amino acids between the second extracellular loop and the fifth transmembrane region. Analysis of the rat GABA(B)R1 gene demonstrates that the insertion is the result of an alternative splicing event within a 567-bp intron between exons 16 and 17. In situ hybridization in the rat brain shows a wide distribution of R1c transcripts and an overlap with the R1a and R1b transcripts. The highest mRNA levels are found in cerebellar Purkinje cells, cerebral cortex, thalamus and hippocampal CA1 and CA3 regions. Western blots and immunodetection of recombinant epitope-tagged receptors as well as [125I]CGP71872 photoaffinity labelling of cell membranes demonstrate that R1c is correctly expressed, although at a lower level than the previously identified isoforms. When coexpressed with the newly characterized GABA(B)R2, R1c functionally couples to G-protein-activated Kir3.1/3.2 channels in Xenopus oocytes and to PLC-activating chimeric G(alpha)qo subunits in HEK-293 cells with a similar EC50 for agonists. These data suggest that the R1c isoform represents a functional GABA(B)R in the rat brain.

Alternative Splicing↗

PubMed1999

The N-terminal domain of gamma-aminobutyric Acid(B) receptors is sufficient to specify agonist and antagonist binding.

The recently identified gamma-aminobutyric acid type B receptors (GABA(B)Rs) share low sequence similarity with the metabotropic glutamate (mGlu) receptors. Like the mGlu receptors, the N-terminal extracellular domain (NTED) of GABA(B)Rs is proposed to be related to bacterial periplasmic binding proteins (PBPs). However, in contrast to the mGlu receptors, the GABA(B)Rs lack a cysteine-rich region that links the PBP-like domain to the first transmembrane domain. This cysteine-rich region is necessary for the PBP-like domain of mGlu receptors to bind glutamate. To delimit the ligand-binding domain of GABA(B)Rs, we constructed a series of chimeric GABA(B)R1/mGluR1 and truncated GABA(B)R1 receptor mutants. We provide evidence that despite the lack of a cysteine-rich region, the NTED of GABA(B)Rs contains all of the structural information that is necessary and sufficient for ligand binding. Moreover, a soluble protein corresponding to the NTED of GABA(B)Rs reproduces the binding pharmacology of wild-type receptors. This demonstrates that the ligand-binding domain of the GABA(B)Rs can correctly fold when dissociated from the transmembrane domains.

Animals↗

PubMed1998

GABA(B)-receptor subtypes assemble into functional heteromeric complexes.

B-type receptors for the neurotransmitter GABA (gamma-aminobutyric acid) inhibit neuronal activity through G-protein-coupled second-messenger systems, which regulate the release of neurotransmitters and the activity of ion channels and adenylyl cyclase. Physiological and biochemical studies show that there are differences in drug efficiencies at different GABA(B) receptors, so it is expected that GABA(B)-receptor (GABA(B)R) subtypes exist. Two GABA(B)-receptor splice variants have been cloned (GABA(B)R1a and GABA(B)R1b), but native GABA(B) receptors and recombinant receptors showed unexplained differences in agonist-binding potencies. Moreover, the activation of presumed effector ion channels in heterologous cells expressing the recombinant receptors proved difficult. Here we describe a new GABA(B) receptor subtype, GABA(B)R2, which does not bind available GABA(B) antagonists with measurable potency. GABA(B)R1a, GABA(B)R1b and GABA(B)R2 alone do not activate Kir3-type potassium channels efficiently, but co-expression of these receptors yields a robust coupling to activation of Kir3 channels. We provide evidence for the assembly of heteromeric GABA(B) receptors in vivo and show that GABA(B)R2 and GABA(B)R1a/b proteins immunoprecipitate and localize together at dendritic spines. The heteromeric receptor complexes exhibit a significant increase in agonist- and partial-agonist-binding potencies as compared with individual receptors and probably represent the predominant native GABA(B) receptor. Heteromeric assembly among G-protein-coupled receptors has not, to our knowledge, been described before.

Amino Acid Sequence↗

PubMed1998

Developmental changes of agonist affinity at GABABR1 receptor variants in rat brain.

Recently, two N-terminal splice variants of the metabotropic receptor for GABA (gamma-amino-butyric acid) were cloned. Here, we describe an antiserum that recognizes the two receptor variants. We demonstrate that these proteins are identical with GABAB receptors that are photoaffinity labeled with [125I]CGP71872 in rat brain. The C-terminal epitopes recognized by the antiserum are conserved in several vertebrate species but not in chicken. No hints for the existence of additional closely related receptor subtypes or variants are found in double-labeling experiments with antibody and photoaffinity ligand. Western blot analysis reveals widespread expression of the GABABR1 receptor proteins in rat brain with the highest level of expression at early postnatal stages. The binding affinity of the GABAB receptor agonist L-baclofen at native R1a and R1b variants is similar. In early postnatal development the affinity at R1a and R1b is 10-fold lower than in adult brain and gradually increases with aging.

Animals↗

PubMed1998

Presynaptic and postsynaptic localization of GABA(B) receptors in neurons of the rat retina.

The recently cloned GABA(B) receptors were localized in rat retina using specific antisera. Immunolabelling was detected in the inner and outer plexiform layers (IPL, OPL), and in a number of cells in the inner nuclear layer and the ganglion cell layer. Double-labelling experiments for GABA (gamma-aminobutyric acid) and GABA(B) receptors, respectively, demonstrated a co-localization in horizontal cells and amacrine cells. Electron microscopy showed that GABA(B) receptors of the OPL were localized presynaptically in horizontal cell processes invaginating into photoreceptor terminals. In the IPL, GABA(B) receptors were present presynaptically in amacrine cells, as well as postsynaptically in amacrine and ganglion cells. The postnatal development of GABA(B) receptors was also studied, and immunoreactivity was observed well before morphological and synaptic differentiation of retinal neurons. The present results suggest a presynaptic (autoreceptor) as well as postsynaptic role for GABA(B) receptors. In addition, the extrasynaptic localization of GABA(B) receptors could indicate a paracrine function of GABA in the retina.

Amino Acid Sequence↗

PubMed1996

Cellular localisation of metabotropic glutamate receptors in the mammalian optic nerve: a mechanism for axon-glia communication.

It has been proposed that neurotransmitter signalling can occur between axons and glia in the mammalian optic nerve in the absence of synaptic specialisations, and that this may be glutamate mediated. Here, the cellular distribution of five metabotropic glutamate receptors (mGluR's 1a, 1b, 1c, 2/3 and 5) have been assessed in the rat optic pathway using specific antibodies. Positive immunoreactivity is found for mGluR2/3 and 5. Both are found in axons, although only mGluR5 is present in the majority of these. Strong immunoreactivity for mGluR2/3 is found in cells in the optic pathway and thalamus. The cellular morphology and distribution is consistent with their being astrocytes. Examination of brain sections stained for mGluR2/3 is consistent with this notion, with many cells having end-feet processes terminating on blood vessels or the pial surface. The axonal immunoreactivity could represent the presence of these receptors on axons, but it is more probable that the receptor protein synthesised in the ganglion cell soma is being transported to the cell terminal in sufficient concentration to be revealed by immunohistochemistry. The reason for the axon-astrocyte signalling is unclear, and may be associated with metabolic coupling. In development, communication between axons and glia mediates a range of functions including pathway selection and myelination. It is probable that in the adult this form of signalling underpins a range of functions that have yet to be described.

Animals↗

PubMed1996

Immunohistochemical distribution of metabotropic glutamate receptor subtypes mGluR1b, mGluR2/3, mGluR4a and mGluR5 in human hippocampus.

The metabotropic glutamate receptors (mGluRs) can be classified into three families based on amino acid sequence homology, signal transduction mechanisms and pharmacological properties. Generally, class I mGluRs mediate an excitation of neurons while activation of class II and III mGluRs results in a depression of synaptic transmission. In this study we have analyzed the expression pattern of mGluRs in human hippocampus using a panel of polyclonal antibodies specific for mGluR1b, mGluR2/3, mGluR4a, and mGluR5. Immunoreactivity for mGluR1b and mGluR5, i.e., the subtypes representing class I mGluRs, was found in all hippocampal neurons. The mGluR1b antiserum stained perikarya and proximal dendrites, whereas immunoreactivity for mGluR5 was also detectable in the distal dendritic compartments. Immunoreactivity for mGluR2/3, members of class II mGluRs, was present in all principle neurons in the dentate gyrus as well as in the CA4, CA3 and CA2 regions. Pyramidal cells of the CA1 region exhibited only weak labeling for mGluR2/3. Glial cells were also mGluR2/3-immunoreactive. The reaction obtained with an antiserum directed against mGluR4a, a member of class III mGluRs, was confined to the mossy fiber projection field in CA3 stratum lucidum. These data demonstrate differential expression of mGluR variants in the human hippocampus and may provide an important basis for future studies of mGluRs under various neuropathological conditions such as temporal lobe epilepsy, ischemia and neurodegenerative disorders.

Aged↗

PubMed1996

Group II and group III metabotropic glutamate receptors in the rat retina: distributions and developmental expression patterns.

We have studied the distributions of group II metabotropic glutamate receptors, mGluR2 and mGluR3, and a group III metabotropic glutamate receptor, mGluR4, in the adult rat retina and during postnatal development using receptor specific anti-peptide antisera. Of the three receptors examined, mGluR3 was not expressed in the retina. MGluR2 showed a distinct stratification pattern in the inner plexiform layer (IPL). Double-labelling immunocytochemistry revealed that mGluR2 was localized in the processes of cholinergic amacrine cells. MGluR4 was found throughout the entire IPL. At the subcellular level, both mGluR2 and mGluR4 were found to be localized exclusively in processes postsynaptic to bipolar cell synapses in the IPL. During postnatal development, labelling for mGluR2 was detected at around postnatal day five. MGluR4 was already present at postnatal day one, prior to the establishment of synaptic connections in the IPL. The differential expression patterns of individual metabotropic glutamate receptors in the adult and developing rat retina suggest distinct roles for these receptors in retinal synaptic circuitry.

Amino Acid Sequence↗

PubMed1996

Immortalized hypothalamic neurons express metabotropic glutamate receptors positively coupled to cyclic AMP formation.

We have characterized the expression pattern and pharmacological profile of activation of metabotropic glutamate receptors (mGluRs) in immortalized, gonadotropin releasing hormone (GnRH)-secreting GT1-7 cells, which represent a homogeneous cellular population of hypothalamic origin. These cells are known to respond to the mGluR agonist (1S,3R)-cyclopentanedicarboxylic acid (1S,3R-ACPD) with increased GnRH release. To establish which specific mGluR subtypes are expressed by GT1-7 cells, we used polyclonal antibodies raised against non-conserved regions of the carboxy-terminal domains of individual subtypes. The selectivity of these antibodies was tested in HEK 293 cells transiently transfected with each mGluR subtype. GT1-7 cells stained positively for the subtypes mGluR1a, -1b and -5 (belonging to group I mGluR2/3 (group II) and mGluR7 (group III). Agonists of group I mGluRs, including 1S,3R-ACPD, activated phosphoinositide hydrolysis in GT1-7 cells. This effect, however, was manifested only when cell density was low, and it disappeared when cells reached confluence. Stimulation of phosphoinositide hydrolysis could not therefore have been related to hormone secretion because 1S,3R-ACPD effectively released GnRH in confluent cultures. We then focused on group II and III mGluRs, which in transfected cells are negatively linked to adenylate cyclase activity. Unexpectedly, however, agonist which preferentially activate group II and III mGluRs increased both basal and forskolin-stimulated cAMP accumulation in GT1-7 cells. Stimulation of cAMP accumulation by mGluR agonists was not prevented by enzymatic depletion of endogenous adenosine, but was obliterated when cells were incubated with agonists of receptors positively coupled to adenylate cyclase, such as beta-adrenergic and prostaglandin E2 receptors. These results suggest that GT1-7 cells express a novel mGluR subtype positively coupled to adenylate cyclase, which shares the same transduction pathway of other classical receptors coupled with a Gs-type of GTP-binding protein.

Animals↗

PubMed1995

Spontaneous melanoma formation in nonhybrid Xiphophorus.

Melanoma in hybrids of Xiphophorus is due to the unrestricted activity of a cellular oncogene locus, Tu, encoding the growth factor receptor gene Xmrk. In nonhybrid parental fish, Tu is controlled by a tumor suppressor gene. Thus, its restricted activity leads there only to a nonmalignant, species- and population-specific macromelanophore spot pattern. Prompted by enigmatic reports on nonhybrid Xiphophorus with pigmentation abnormalities resembling melanoma, we have studied pigmentation in descendants of wild-caught fish and purebred laboratory stocks derived from wild populations. Whereas most stocks exhibiting macromelanophore patterns never developed pigmentation abnormalities, an exceptional situation for some nonhybrids was found. In X. variatus carrying the macromelanophore pattern "punctatus-2" and in X. cortezi with "spotted caudal," expressivity of the pigmentation gene ranges from a few black spots to extreme melanosis and eventually to malignant melanoma. In X. maculatus with the mutant pigmentation gene striped" carrying in addition the micromelanophore pattern "anal fin black" or "lower comet," testosterone-dependent melanoma develop originating from the corresponding micromelanophore pattern. The tumors are highly malignant and express a melanoma-associated antigen. Overexpression of the Xmrk oncogene appears as the underlying molecular mechanism for tumor induction. These findings clearly demonstrate that tumors can also develop in purebred wild-type fish. The classical model for formation of hereditary melanoma in Xiphophorus hybrids does not explain the development of melanoma in the absence of hybridization. However, their existence gives additional support to the reasoning that the Xmrk oncogene associated with the macromelanophore locus is potentially injurious.

Animals↗

PubMed1995

Melanoma formation in Xiphophorus: a model system for the role of receptor tyrosine kinases in tumorigenesis.

Cancer is one of the most frequent fatal human diseases. It is a genetic disease, and molecular analysis of the genes involved revealed that they belong to several distinct classes of molecules, one of which is the receptor tyrosine kinases. Neoplastic transformation is regarded as the result of a multistep process and, in most cases, it is hard to evaluate what the initial events in tumor formation are. What makes it difficult to approach this question is the paucity of animals models for tumorigenesis allowing investigation of the mechanisms leading to uncontrolled cell proliferation. Melanoma formation in Xiphophorus is one of these model systems. Here, overexpression and activation of a receptor tyrosine kinase causes neoplastic transformation of pigment cells. Xiphophorus provides all the advantages of a well-characterized genetic system. In addition, some crucial components of the transformation pathway have been identified at the molecular level. As a vertebrate, Xiphophorus might serve as a model system to aid understanding, in more general terms, of the mechanisms of tumorigenesis in human diseases.

Animals↗