Detection of the ETV6-NTRK3 chimeric RNA of infantile fibrosarcoma/cellular congenital mesoblastic nephroma in paraffin-embedded tissue: application to challenging pediatric renal stromal tumors

We report the development of a reverse transcriptase polymerase chain reaction assay that reliably detects the ETV6-NTRK3 chimeric RNA characteristic of infantile fibrosarcoma and the cellular variant of congenital mesoblastic nephroma (CMN) in formalin-fixed, paraffin-embedded tissue blocks. The 188 base pair polymerase chain reaction fusion product was detected in 11 of 12 cases of cellular CMN from which a larger sized control RNA band could be amplified, and even in 7 of 8 cases in which the control band was not detectable. A variety of other tumors that are in the histologic differential diagnosis of cellular CMN yielded negative results, including four classic CMNs, four rhabdoid tumors of the kidney, and four clear cell sarcomas of the kidney, confirming the assay's specificity. We further demonstrate the assay's utility by illustrating two cases of molecularly confirmed cellular CMN that mimicked rhabdoid tumor and clear cell sarcoma of the kidney. In contrast to previous reports, five mixed CMNs that had both classic and cellular areas all lacked the ETV6-NTRK3 fusion transcript. These results suggest that cases morphologically defined as mixed CMN may represent a mixed group of genetically distinct entities.

Role of neurotropins in rat embryonic testis morphogenesis (cord formation)

The process of seminiferous cord formation is the first morphological event that differentiates a testis from an ovary and indicates male sex determination. Cord formation occurs by embryonic Day 14 (Day 0 = plug date; E14) in the rat. A series of experiments were conducted to determine if neurotropins and their receptors are important for the process of rat embryonic cord formation. The expression of low affinity neurotropin receptor (p75/LNGFR) was determined by immunohistochemistry on sections of both testis and ovary from E13 through birth (Day 0, P0) with an antibody to p75/LNGFR. The staining for p75/LNGFR was present in the mesonephros of E13 gonads and in a sex-specific manner appeared around developing cords at E14 in the embryonic testis. At birth, staining for p75/LNGFR was localized to a single layer of cells (i.e., peritubular cells) that surrounded the seminiferous cords. The genes for both neurotropin 3 (NT3) and for corresponding high affinity neurotropin trkC receptor were found to be expressed in the E14 rat testis, as well as other neurotropins and receptors. Immunocytochemical analysis of E14 rat testis demonstrated that NT3 was localized to the Sertoli cells and trkC was present in individual cells of the interstitium at E16 and in selected preperitubular cells at E18. Previously, the peritubular cells adjacent to the cords were demonstrated to be derived from migrating mesonephros cells around the time of cord formation. To determine if neurotropins were involved in cord formation, the actions of neurotropins were inhibited. A high affinity neurotropin receptor (trk)-specific kinase inhibitor, K252a, was used to treat organ cultures of testes from E13 rats prior to cord formation. Treatment of E13 testis organ cultures with K252a completely inhibited cord formation. K252a-treated organ cultures of E14 testis that contained cords did not alter cord morphology. A second experiment to inhibit neurotropin actions utilized a specific antagonist trk-IgG chimeric fusion protein and E13 testis organ cultures. The trk-IgG molecules dimerize with endogenous trk receptors and inhibit receptor signaling and activation of ligand function. Forty percent of E13 testis organ cultures treated with trkC-IgG had significantly reduced cord formation. TrkA-IgG had no effect on initiation of cords; however, in fifty percent of the treated organs, a "swollen" appearance of the cord structures was observed. Experiments using trkB-IgG chimeric protein on E13 organ cultures had no effect on cord formation or cord morphology. The testes from trkC and NT3 knockout mice were examined to determine if there were any morphological differences in the testis. NT3 knockouts appeared to have normal cord morphology in E15 and E17 testis. TrkC knockout mice also had normal cord morphology in E14 and P0 testis. Both NT3 and trkC knockout-mice testis had less interstitial area than wild-type controls. In addition, the trkC knockout mice have an increased number of cells expressing p75LNGFR within the cords when compared to controls or NT3 knockout mice. Combined observations suggest compensation between the different neurotropin ligands, receptors, and/or possibly different growth factors for this critical biological process. In summary, results suggest a novel nonneuronal role for neurotropins in the process of cord formation during embryonic rat testis development. The hypothesis developed is that neurotropins are involved in the progression of male sex differentiation and are critical for the induction of embryonic testis cord formation.

Mild experimental brain injury differentially alters the expression of neurotrophin and neurotrophin receptor mRNAs in the hippocampus

The molecular events responsible for impairments in cognition following mild traumatic brain injury are poorly understood. Neurotrophins, such as brain-derived neurotrophic factor (BDNF), have been identified as having a role in learning and memory. We have previously demonstrated that following experimental brain trauma of moderate severity (2.0-2.1 atm), mRNA levels of BDNF and its high-affinity receptor, trkB, are increased bilaterally in the hippocampus for several hours, whereas NT-3 mRNA expression is decreased. In the present study, we used in situ hybridization to compare BDNF, trkB, NT-3, and trkC mRNA expression in rat hippocampus at 3 or 6 h after a lateral fluid percussion brain injury (FPI) of mild severity (1.0 atm) to sham-injured controls at equivalent time points. Mild FPI induced significant increases in hybridization levels for BDNF and trkB mRNAs, and a decrease in NT-3 mRNA in the hippocampus. However, in contrast to the bilateral effects of moderate experimental brain injury, the present changes with mild injury were restricted to the injured side. These findings demonstrate that even a mild traumatic brain injury differentially alters neurotrophin and neurotrophin receptor levels in the hippocampus. Such alterations may have important implications for neural plasticity and recovery of function in people who sustain a mild head injury.

Differential expression of trkB.T1 and trkB.T2, truncated trkC, and p75(NGFR) in the cochlea prior to hearing function

Prior to the onset of hearing, synchronous cellular, neuronal, and morphogenetic processes participate in the development of a functional cochlea. We have studied the expression of different splice forms of trkB and trkC as well as p75(NGFR) in rat and mouse cochlea within this critical developmental period, using in situ hybridization, PCR, Northern blotting, and immunohistochemical analyses. An antibody to full-length trkB receptors proved to detect full-length trkB receptors as well as truncated trkB.T2 but not trkB. T1 isoforms. Full-length trkB and trkC isoforms as well as trkB.T2 but not trkB.T1 receptors were noted in cochlear neurons. A transient expression of trkB.T1 and trkB.T2 was observed at the epithelial-mesenchymal border of the spiral ligament during this time. A sequential appearance of trkB.T1, the low-affinity neurotrophin receptor p75(NGFR), and trkB.T2 in epithelial cochlear cells correlated with the formation of the inner sulcus of the organ. A differential expression of presumptive trkB.T2 in hair and supporting cells was observed concomitant with the maturation of the distinct innervation pattern of these cells. A gradual shift from p75(NGFR) to truncated trkC receptors in Pillar cells occurred during the formation of the tunnel of Corti. A distinct expression of full-length trkC correlated with the time of differentiation of the stria vascularis. Finally, an expression of trkB.T1 and trkB.T2 in oligodendrocytes, full-length trkB and trkC in nerve fibers, and p75(NGFR) in Schwann cells was noted at the glial interface of the VIIIth nerve during the establishment of the glial transition zone. These various transitory neurotrophin receptor expression patterns, which were related to final maturation processes of the cochlea, may provide new insights into the as yet obscure role of neurotrophin receptors in nonneuronal tissue.

Diagnostic molecular pathology comes of age

Specific and consistent chromosomal translocations, identified in several lymphoreticular, neuroectodermal, and soft tissue neoplasms, generally result in the development of novel fusion transcripts.

Neurotrophins and Trk receptors in primitive neuroectodermal tumor cell lines

OBJECTIVE

Primitive neuroectodermal tumors (PNETs) are thought to be derived from early central nervous system precursors. Therefore, we hypothesized that the neurotrophins (nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3) and their receptors (TrkA, TrkB, and TrkC), which are involved in the proliferation, differentiation, and survival of neuronal cells, might be important in regulating tumor growth.

METHODS

Using ribonucleic acid (RNA) blotting and reverse transcription-polymerase chain reaction analysis, we investigated the expression of these ligands and their receptors in six PNET cell lines (Daoy, PFSK, D283 Med, UW288-1, CHP707m, and D341 Med). Neurotrophin protein levels were measured using enzyme-linked immunosorbent assay procedures. Receptor function was demonstrated by autophosphorylation. Induction of c-Fos expression and effects on cell proliferation were assessed after the addition of exogenous neurotrophin.

RESULTS

Three cell lines expressed messenger RNA for all neurotrophins, whereas the other three expressed two of the three neurotrophins. Neurotrophin protein levels were low. All cell lines expressed trkA messenger RNA. Five expressed the amino terminus of trkB, but three of these did not express the carboxyl terminus. All cell lines contained trkC messenger RNA, but the receptor was truncated in two cell lines. No cell line contained message for a receptor containing an insertion in the tyrosine kinase domain. The addition of neurotrophin to PNET cells resulted in phosphorylation of a protein that was immunoprecipitated with an anti-pan-Trk antibody. c-Fos expression and cell growth were increased by preincubation with neurotrophins, but only in the cell lines expressing the relevant full-length receptors.

CONCLUSION

The expression of neurotrophins and neurotrophin receptors by PNET cell lines is variable. The presence of activated Trk receptors in these cell lines may be required for rapid growth, via an autocrine loop mechanism. This will require further investigation.

Ontogenetic expression of trk neurotrophin receptors in the chick auditory system

Neurotrophins and their cognate receptors are critical to normal nervous system development. Trk receptors are high-affinity receptors for nerve-growth factor (trkA), brain-derived neurotrophic factor and neurotrophin-4/5 (trkB), and neurotrophin-3 (trkC). We examine the expression of these three neurotrophin tyrosine kinase receptors in the chick auditory system throughout most of development. Trks were localized in the auditory brainstem, the cochlear ganglion, and the basilar papilla of chicks from embryonic (E) day 5 to E21, by using antibodies and standard immunocytochemical methods. TrkB mRNA was localized in brainstem nuclei by in situ hybridization. TrkB and trkC are highly expressed in the embryonic auditory brainstem, and their patterns of expression are both spatially and temporally dynamic. During early brainstem development, trkB and trkC are localized in the neuronal cell bodies and in the surrounding neuropil of nucleus magnocellularis (NM) and nucleus laminaris (NL). During later development, trkC is expressed in the cell bodies of NM and NL, whereas trkB is expressed in the nerve calyces surrounding NM neurons and in the ventral, but not the dorsal, dendrites of NL. In the periphery, trkB and trkC are located in the cochlear ganglion neurons and in peripheral fibers innervating the basilar papilla and synapsing at the base of hair cells. The protracted expression of trks seen in our materials is consistent with the hypothesis that the neurotrophins/tyrosine kinase receptors play one or several roles in the development of auditory circuitry. In particular, the polarized expression of trkB in NL is coincident with refinement of NM terminal arborizations on NL.

Anatomical evidence supporting the potential for modulation by multiple neurotrophins in the majority of adult lumbar sensory neurons

Neurotrophins exert effects on sensory neurons through receptor tyrosine kinases (trks) and a common neurotrophin receptor (p75). Quantitative in situ hybridization studies were performed on serial sections to identify neurons expressing single or multiple neurotrophin trk receptor mRNA(s) in adult lumbar dorsal root ganglion (DRG) in order to examine the possibility of multi-neurotrophin modulation of phenotype via different trk receptors or various trk isoforms. Expression of mRNA encoding trkA, trkB, trkC, or p75 is restricted to select subpopulations representing approximately 41%, 33%, 43%, and 79% of DRG neurons, respectively. Colocalization studies reveal that approximately 10% of DRG neurons coexpress trkA and trkB mRNA; 19% coexpress trkA and trkC mRNA; and 18% coexpress trkB and trkC mRNA. Trilocalization of all three trk mRNAs is rare, with approximately 3-4% of neurons in this category. Overall incidence of expression of more than one full length trk mRNA occurs in approximately 40% of DRG neurons, whereas expression of individual trk mRNA is found in approximately 34%. Full length trk receptor mRNA is rarely detected without p75, implicating the latter in neuronal response to neurotrophins. Examination of two full-length isoforms of trkA reveal that they are coexpressed with relative levels of expression positively correlated. TrkC mRNAs corresponding to 14- or 39-amino acid insert isoforms colocalize with the non-insert trkC isoform, but the converse is not necessarily true. The data suggest that substantial subpopulations of adult sensory neurons may be modulated through interactions with multiple neurotrophins, the consequences of which are largely unknown.

Trks and p75 genes are differentially expressed in the inner ear of human embryos. What may Trks and p75 null mutant mice suggest on human development?

Recent work has shown the expression of Neurotrophins low (p75) and high affinity (Trk's A, B, and C) receptors in the developing inner ear sensory neurons of chick and mouse. Likewise the biological significance of such receptor expression was demonstrated by using both Trks and Neurotrophins null mutant mice. The present study was conducted to determine the expression of Trks and p75 proteins in the human inner ear throughout development. Hence to assess the potential role of Neurotrophins in the development of auditory and vestibular specific innervation in man. In other words, we intend to address the issue whether or not what null mutant mice for Trks and p75 have revealed on inner ear development may be relevant for human embryos. Fifty-two inner ears and their cochleovestibular ganglions (CVG) from human embryos and fetuses, ranging from 5 to 24 weeks of pregnancy were analyzed. Both Western blot and immunocytochemistry on frozen sections were used as complementary procedures. Quantitative Western blot studies revealed that Trk-B and C immunoreactivity (IR) appeared by embryonic week 5 in CVG neurons, increased at high levels between embryonic weeks 7 and 12, and later on, in 15 week-old specimens and older began to decrease to minimal levels. Trk-A IR was detected at just moderate levels during 5 and 7 weeks reflecting the presence of NGF high affinity receptors only at these earlier developmental ages. The p75 IR was detected at high degrees in the early stage of the 5th week and at abundant levels in all studied inner ears from the 7th to the 24th pregnancy week. These Western blot observations were corroborated by immunocytochemistry on frozen sections, which also revealed a major distribution of both p75 and Trks on neuronal bodies while p75 appears localized on supporting cells. Our findings reveal a tight correlation between p75 and Trks expression throughout human development and specific inner ear developmental events, such as target-dependent neuronal cell death and afferent hair cells innervation. That kind of association of p75 and Trks temporal pattern with distinctive steps in inner ear developmental schedule, is a feature shared between human embryos and other mammals, such as mouse. Based on the present results and considering them together with the reported phenotype of p75 and Trks null mutant mice, we hypothesize that p75 and Trk receptors, as well as, their binding Neurotrophins may be essential in human inner ear development. Accordingly, they may be required molecules for sensory epitheliums innervation and target-dependent neuronal cell death, during embryogenesis and even early postnatal life, in man.

Differential expression of trkC mRNA in the chicken embryo from gastrulation to development of secondary brain vesicles

In situ hybridization revealed that mRNA for the neurotrophin receptor trkC first appears in the chicken embryo at stage 4 anterior to Hensen's node. At stages 6 and 8, trkC mRNA was restricted to the neural plate. By stage 11, trkC mRNA was absent from much of the prosencephalon, the entire mesencephalon and rhombomeres 1 and 4. At stage 15, trkC mRNA expression was limited to rhombomeres 3 and 5, and, by stage 18, there was no apparent expression of trkC mRNA in the hindbrain.

The transcription factor, Pax6, is required for cell proliferation and differentiation in the developing cerebral cortex

The cerebral cortex develops from the dorsal telencephalon, at the anterior end of the neural tube. Neurons are generated by cell division at the inner surface of the telencephalic wall (in the ventricular zone) and migrate towards its outer surface, where they complete their differentiation. Recent studies have suggested that the transcription factor Pax6 is important for regulation of cell proliferation, migration and differentiation at various sites in the CNS. This gene is widely expressed from neural plate stage in the developing CNS, including the embryonic cerebral cortex, where it is required for radial glial cell development and neuronal migration. We report new findings indicating that, in the absence of Pax6, proliferative rates in the early embryonic cortex are increased and the differentiation of many cortical cells is defective. A major question concerns the degree to which cortical defects in the absence of Pax6 are a direct consequence of losing the gene function from defective cells themselves, rather than being secondary to abnormalities in other cells. Cortical defects in the absence of Pax6 become much more pronounced later in cortical development, and we propose that many result from a compounding of abnormalities in proliferation and differentiation that first appear at the onset of corticogenesis.

Expression of nerve growth factor, p75, and the high affinity neurotrophin receptors in the adult rat trigeminal system: evidence for multiple trophic support systems

We hypothesize that discrete trigeminal structures have the components required for autocrine regulation as well as redundant neurotrophin support systems. We examined the expression of nerve growth factor (NGF) and the low affinity (p75) and high affinity (trkA, trkB, and trkC) neurotrophin receptors in the trigeminal system of adult rats. Four sites were examined; the trigeminal ganglion, mesencephalic nucleus, principal sensory nucleus (PSN), and trigeminal motor nucleus. NGF was expressed by more than 60% of neurons in each area studied. NGF immunolabeling may have resulted from exogenous protein incorporated from the microenvironment or from NGF synthesized by the neuron per se. To resolve this issue, in situ hybridization for NGF mRNA was performed. The mRNA was expressed by 2/3 to 7/8 of neurons in trigeminal structures. Moreover, double-labeling studies showed that virtually every ganglion cell that was NGF-immunoreactive also expressed the NGF transcript. Neurotrophin receptors (p75 and trk isoforms) were expressed by more than 60% of the neurons in each trigeminal structure. The only exception was the PSN, where the receptors were expressed by fewer than half of the neurons. Taken together, these data imply that NGF must be elaborated by neurons that co-express both p75 and trkA. Therefore, each trigeminal structure has the machinery for autocrine/paracrine regulation, as well as the capacity for retrograde and/or anterograde trophic support. Furthermore, the co-expression of the specific trk isoforms indicates that trigeminal neurons are sensitive to more than one neurotrophin.

Brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4 bind to a single leucine-rich motif of TrkB

TrkB is a member of the Trk family of neurotrophin receptors. Its extracellular domain exhibits the same modular structure found in its homologs, TrkA and TrkC, consisting of an N-terminal LRM3 cassette and two immunoglobulin-like modules (Ig2 domain) adjacent to the membrane. The LRM3 cassette comprises two cysteine-rich clusters framing a tandem array of three leucine-rich motifs (LRMs). On the basis of the recent identification of a nerve growth factor (NGF) binding site within TrkA, the ability of the different structural entities within the extracellular domain of TrkB to bind the various neurotrophins was determined by using a recombinant receptor approach. Brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4) bound to the LRM3 cassette of TrkB, whereas NGF did not. These binding characteristics evidently reflect in vivo specificities. A more precise mapping of the region(s) responsible for binding BDNF, NT-3, and NT-4 identified the second leucine-rich motif of TrkB as a functional unit capable of binding all three neurotrophins. The affinities and kinetics that this short stretch of amino acids exhibited with respect to the different neurotrophins were clearly akin to those observed for cells ectopically expressing TrkB receptors. With 24 amino acids determining the affinities and kinetics of the interactions with three different partners, the leucine-rich motif is strongly established as one of the most potent and flexible protein--protein interaction motifs.