Neurotrophin and Trk neurotrophin receptors in the inner ear of Salmo salar and Salmo trutta

Neurotrophins (NTs) and their signal transducing Trk receptors play a critical role in the development and maintenance of specific neuronal populations in the nervous system of higher vertebrates. They are responsible for the innervation of the inner ear cochlear and vestibular sensory epithelia. Neurotrophins and Trks are also present in teleosts but their distribution in the inner ear is unknown. Thus, in the present study, we used Western-blot analysis and immunohistochemistry to investigate the expression and cell localization of both NTs and Trk receptors in the inner ear of alevins of Salmo salar and Salmo trutta. Western-blot analysis revealed the occurrence of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), but not nerve growth factor (NGF), as well as all three Trk receptors, i.e. TrkA, TrkB and TrkC, the estimated molecular weights of which were similar to those expected for mammals. Specific immunoreactivity for neurotrophins was detected mainly in the sensory epithelia. In particular, BDNF immunoreactivity was found in the maculae of the utricle and saccule, whereas NT-3 immunoreactivity was present in the sensory epithelium of the cristae ampullaris. As a rule the sensory epithelia of the inner ear lacked immunoreactivity for Trks, thus excluding possible mechanisms of autocrinia and/or paracrinia. By contrast, overlapping subpopulations of neurons in the statoacoustic ganglion expressed TrkA (about 15%), TrkB (about 65%) and TrkC (about 45%). The present results demonstrate that, as in mammals and birds, the inner ear of teleosts expresses the components of the neurotrophin-Trk system, but their roles remain to be elucidated.

Reduction of glial fibrillary acidic protein-immunoreactive astrocytes in some brain areas of old hairless rhino-j mice (hr-rh-j)

Mutations in the hairless (hr) gene of mice result in hair follicle and other epithelial defects. The hr gene is expressed at high levels in the brain where it probably participates in the survival and maintenance of some neuronal populations, but whether it also supports glial populations of the central nervous system has been not investigated. To clarify this, quantitative immunohistochemistry for astrocytes (glial fibrillary acidic protein (GFAP)) and microglial cells (CD11b macrophage antigen) was used in the brain of a mutant mouse strain, the hairless (hr-rh-j) type, which carries the homozygous hr gene rhino mutation. The glial cell density was assessed in the cerebral cortex, hippocampus, striatum, hypothalamus and cerebellum of young (3 months) and old (9 months) hr-rh-j mice. No significant differences were found between young wild-type and hr-rh-j mice. The density of GFAP immunoreactive astrocytes normally increased as a function of age, but in older hr-rh-j mice there was a severe reduction (P<0.01) in the striatum, hypothalamus, and hippocampus. Conversely, the microglial cells were insensible to aging or to hr-rh-j mutation. These results suggest that the hr gene is involved in the maintenance of the GFAP immunoreactive cells in some cerebral areas. Nevertheless, because these animals do not show any neurological signs, the functional significance of the present findings remains to be established.

Genetic patterning of embryonic inner ear development

The commitment of the otocyst to form balance (pars superior) and hearing (pars inferior) receptors is under the control of patterning genes. Intersecting patterns of gene expression has been proposed to explain the regionalization of inner ear sensory receptor development. Insights into the roles of patterning genes is being acquired from the analysis of inner ear development of null mutation mice. An example of the consequences of the loss of expression of a single patterning gene is the effect of a null mutation of paired-box gene 2 (Pax2) on formation of the cochlea. Pax2 transcripts are expressed in the pars inferior of the otocyst and null mutant inner ears show agenesis of the cochlea. This is an example of the profound effect that loss of a single patterning gene can have on inner ear development. However, more typically there is redundancy of gene action during inner ear development as exemplified by the overlapping pattern of expression of two closely related homeobox-containing genes (i.e. Hmx2 and Hmx3) in the pars superior of the otocyst. Hmx2 and Hmx3 genes are both expressed early in otic development with Hmx3 transcripts present in the placode. However, null mutation of the Hmx3 patterning gene produced only a limited vestibular defect, and did not result in the agenesis of the vestibule. This result suggests that there is redundancy in genes that have similar patterns of regional expression in the otic anlagen.

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.

Changes in the cerebellar cortex of hairless Rhino-J mice (hr-rh-j)

A mutation in the hr gene is responsible for typical epithelium phenotype in hairless mice. As this gene is expressed at high levels not only in the skin but also in the brain, the aim of the study was to clarify its role in the central nervous system. We have analyzed by morphological and immunocytochemical methods (calbindin D-28k, phosphorylated and 200 kDa neurofilament protein) the cerebellum of a mutated mouse strain, the hairless (hr-rh-j) type carrying the homozygous hr gene rhino mutation. The cerebellar cortex was studied in young (3 months) and adult (9 months) wild type and mutated mice. No major structural change was found in any of the groups and neuronal density or neuronal arrangement were similar in mutated animals to their age-matched controls. Nevertheless there were changes in shape and size of the Purkinje neurons in the old mutated animals respect to their normal littermates, while the molecular and the granule cell layers were apparently invariable. Calbindin (CB) immunohistochemistry revealed a significant decrease in the expression of this protein in the Purkinje cells of the aged mutated mice. Immunohistochemistry for a neurofilament protein (NFP) showed a reduction of staining in all the cerebellar cortex layers in the older animals, which was much more evident in the (hr-rh-j) mutated mice. These results suggest that hr gene is involved in the structural maintenance of the mature cerebellar cortex, rather than in the development. Our findings may also be consistent with an accelerated aging of the central nervous system in rh-rh-j mice.

S100alpha and S100beta proteins in human cutaneous sensory corpuscles: effects of nerve and spinal cord injury

S100 protein in the vertebrate peripheral nervous system consists of homo- or heterodimers of S100alpha and S100beta proteins, the first predominating in neurons and the second in glial cells. Recently, however, occurrence of S100beta protein in neurons has been reported. The expression of S100 protein by Schwann cells, as well as their derivatives in sensory corpuscles, depends on the sensory axon (i.e., the Schwann cell-axon contact). The present study analyzed the distribution of S100alpha and S100beta proteins in human cutaneous sensory corpuscles and the effects of peripheral or central sensory axon severance in the expression of these proteins. Simple or double immunohistochemistry was carried out using a panel of antibodies against S100alpha, S100beta or S100alpha+beta proteins, and the sections were examined by light or laser confocal scanning microscopy. Skin samples were obtained from normal subjects and patients with spinal cord injury, nerve entrapment, and nerve sections plus graft. The lamellar cells of Meissner corpuscles as well as the inner-core lamellae of the Pacinian corpuscles displayed strong immunoreactivity (IR) for all antigens examined, the most intense labeling being obtained for S100beta protein. The pattern of immunostaining was unchanged after spinal cord injury, whereas the number of stained corpuscles as well as the intensity of IR for each antigen decreased in cutaneous sensory corpuscles after nerve injury, both entrapment and section plus graft. No evidence was found of axonal labeling. The present results provide evidence that Schwann-related cells in human cutaneous sensory corpuscles contain both S100alpha and S100beta and that the expression of these proteins is dependent on the functional and structural integrity of sensory fibers.

p75 and TrkA neurotrophin receptors in human skin after spinal cord and peripheral nerve injury, with special reference to sensory corpuscles

Human skin, including nerves and sensory corpuscles, displays immunoreactivity (IR) for low- (p75) and high-affinity (TrkA-like) receptors for nerve growth factor (NGF), the best characterized member of the family of neurotrophins. This study was designed to analyze the changes induced by spinal cord and peripheral nerve injuries in the expression of neurotrophin receptors in digital skin, with special reference to nerves and sensory corpuscles. Skin biopsy samples were obtained from 1) the hand and toes of normal subjects, 2) below the level of the lesion of patients with spinal cord injury affecting dorsal and lateral funiculi, 3) the cutaneous territory of entrapped peripheral nerves (median and ulnar nerves), and 4) the cutaneous territory of sectioned and grafted nerves (median nerve). The pieces were formalin-fixed and paraffin-embedded, cut in serial sections, and processed for immunohistochemistry using antibodies against human p75 and TrkA proteins. The percentage of sensory corpuscles displaying IR for p75 and TrkA-like, as well as the intensity of IR developed within them, was assessed using quantitative image analysis. Spinal cord severance causes a decrease in p75 IR in Meissner and Pacinian corpuscles, whereas TrkA-like IR did not vary. In other nonnervous tissues (i.e., epidermis, sweat glands), both p75 and TrkA-like IR was diminished or even absent. Similar but more severe changes were encountered in the skin from the territory of entrapped nerves. Finally, in subjects with sectioned-grafted nerves, p75 IR was found close to controls in nerves, reduced in Meissner corpuscles, and absent in the inner core of the Pacinian ones; TrkA-like IR was in the perineurium, a small percentage of Meissner corpuscles (about 7%), and the outer core and capsule of the Pacinan corpuscles. In the nonnervous tissues, p75 IR was practically absent, whereas TrkA-like IR did not change. No changes in the expression of neurotrophin receptors were observed in Merkel cells of the different groups. Present results show the following: 1) expression of nerve p75 IR in human cutaneous sensory corpuscles is sensitive to central deafferentation, to blockade or difficulty in axonal transport, and to disruption of axonal continuity independently of possible restoration of axonal integrity due to grafts; 2) expression of TrkA-like IR in nerves and sensory corpuscles is sensitive only to nerve transection; 3) the corpuscular Schwann-related cells are the only cells involved in the above modifications, the perineurial cells remaining unchanged; 4) the expression of p75 and TrkA-like IR by Merkel cells is independent of normal innervation; 5) an adequate innervation of the skin seems to be necessary for the expression of p75 but not TrkA-like in nonneuronal cells, especially in the epidermis. A role for NGF in the maintenance of epidermis integrity is discussed.

Survival of inner ear sensory neurons in trk mutant mice

Analysis of trkB-/-; trkC-/- double mutant mice revealed that peripheral and central inner ear sensory neurons are affected in these mice. However, a substantial amount of cochlear and vestibular neurons survive, possibly due to maintenance or upregulation of TrkA expression. To clarify the function of the TrkA receptor during development of the cochlear and vestibular ganglion we analysed trkA-/- mice and the expression of this receptor in inner ear sensory neurons of trkB-/-; trkC-/- animals. TrkA homozygous mutant mice showed normal numbers of neurons and no TrkA expression was detected in neurons of trkB-/-; trkC-/- double mutant mice. We conclude that TrkA is not essential for inner ear development and that in the absence of any of the known catalytic Trk receptors peripheral inner ear sensory neurons are prone to undergo cell death or must use a different signaling mechanism to survive.

Differential expression of microtubule associated protein MAP-2 in developing cochleovestibular neurons and its modulation by neurotrophin-3

Microtubule associated proteins (MAPs) are essential cytoskeletal proteins in developing neurons. The present study was undertaken to analyze the expression of MAP2 and its isoforms (a,b,c) during the embryonal and early post-hatching development of chicken cochleovestibular ganglion (CVG) neurons. Moreover, we have investigated MAP2 expression in primary cultures of CVG neurons, and whether it is regulated by neurotrophin-3 (NT3). The expression of MAP2 immunoreactivity (IR) was studied using both Western blot and immunohistochemistry on tissue sections and primary cultures. In vivo MAP2c was expressed from incubation day 4 (E4) to E10, and MAP2b was found in all embryonal stages studied and at post-hatching day 10 (P10), whereas MAP2a was restricted to the post-hatching periods. The cellular localization of IR was in the neuronal perikarya and their peripheral processes (dendrites) but not in axons. Primary cultures matched the in vivo pattern of MAP2 expression, and IR was localized in neuronal cell bodies and the initial segment of the neuronal processes. Exogenous NT3 regulated the expression of MAP2 isoforms in a dose dependent manner. At the survival dose of 0.5 ng/ml NT3, the main MAP2 expression was MAP2c. Conversely, at the neuritogenic dose of 5 ng/ml NT3 increased MAP2b and MAP2a expression, but not MAP2c. The present results demonstrate that MAP2 isoforms are developmentally regulated, thus suggesting that each isoform is specifically involved in CVG neuron maturation. Furthermore, we provide evidence of MAP2 regulation in culture by the neurotrophic factor NT3.

Calcium currents in dissociated cochlear neurons from the chick embryo and their modification by neurotrophin-3

Calcium entry through voltage-dependent channels play a critical role in neuronal development. Using patch-clamp techniques we have identified the components of the macroscopic Ca2+ current in acutely-isolated chick cochlear ganglion neurons and analysed their functional expression throughout embryonic development. With Ba2+ as a charge carrier, the currents exhibited two main components, both with a high activation threshold but differing in their inactivation kinetics. One component showed inactivation with a time constant around 100 ms (transient) whereas the other hardly inactivated (sustained). The currents were sensitive to omega-Conotoxin GVIA and dihydropyridines, blocked by 20 microM Cd2+, but unaffected by omega-Agatoxin IVA. In a few cases, only with Ca2+ as a charge carrier, an additional component with low activation threshold and fast inactivation (time constant of 20 ms), was observed. Currents were first detected at day 7 of embryonic development. Current density (amplitude/cell capacitance) increased through embryonic day 9, when early synaptic contacts are established, and decreased thereafter to lower steady values. The effect of neurotrophin-3, a neurotrophic factor required for survival and differentiation of cochlear ganglion neurons, was also examined. Neurons isolated at embryonic day 7 or day 11 and maintained two days in culture with 2 ng/ml neurotrophin-3 showed a substantial increase in Ca2+ current density, particularly in the transient component. These findings indicate that the expression of neuronal Ca2+ channels is predominant at the time of synapse formation between transducing hair cells and their primary afferents. Besides its effects on survival and neuritogenesis, neurotrophin-3 enhances the expression of Ca2+ channels in cultured neurons. Taken together these results suggest that the functional expression of Ca2+ channels is regulated during embryonic development of cochlear neurons by the release of neurotrophin-3 from the differentiating sensory epithelium of the cochlea.

Sensory epithelium of the vomeronasal organ express TrkA-like and epidermal growth factor receptor in adulthood. An immunohistochemical study in the horse

BACKGROUND

The medial wall of the vomeronasal organ (VNO) is lined with a sensory epithelium that is closely related to the olfactory epithelium, which is developed from the olfactory placode. It undergoes continuous replacement during its life span. In other sensory epithelia, cell proliferation is under the control of some trophic factors. Whether these proteins are involved in the continuous turnover of the VNO epithelium is unknown. This study approaches this topic by analyzing the occurrence of signal-transducing receptor proteins for neurotrophins (Trk proteins) and epidermal growth factor (EGFr).

METHODS

VNO samples were obtained from adult horses (n = 9) and processed for Western blot or immunohistochemical detection of TrkA, TrkB, TrkC, and EGFr. For immunohistochemistry, both frozen and formalin-fixed, paraffin-embedded sections were used. Antibodies against Trk proteins were polyclonal antibodies that map within the intracytoplasmic domain. Antibodies against EGFr were monoclonal antibodies that map within the external (clone EGFR1) or the cytoplasmic (clone F4) domains.

RESULTS

TrkA-like, but not TrkB- or TrkC-like, protein was detected in the VNO. By using immunoblotting, protein bands of TrkA-like protein with estimated molecular weights of 43-45, 55, and 60 kDa were found. In agreement with these findings, the sensory epithelium lining the VNO displayed strong TrkA-like immunoreactivity. On the other hand, regular protein bands with estimated molecular weights of 100 and 170 kDa, corresponding with immature and full-length EGFr, respectively, were found with the clone F4, whereas the clone EGFR1 was ineffective in detecting EGFr with Western blot analysis. Positive EGFr immunolabelling was observed regularly in the supranuclear pole of the sensory epithelial cells, and the pattern was identical with both antibodies used.

CONCLUSIONS

The present results provide evidence for the occurrence of EGFr in the VNO of the adult horse, suggesting a role for their ligands (EGF and transforming growth factor-alpha) in this organ, probably in continuous cell replacement, during the adult life span. However, although immunoreactivity for TrkA-like protein was regularly observed, because the full-length protein was not found, whether or not its putative ligands (nerve growth factor and neurotrophin-3) act on these cells remains to be demonstrated.

Expression of the cytoskeletal protein MAP5 and its regulation by neurotrophin 3 (NT3) in the inner ear sensory neurons

Microtubule-associated proteins (MAPs) are essential cytoskeletal components during development for neurogenesis and neuronal plasticity. Inner ear innervation is accomplished by cochleovestibular ganglion (CVG) neurons in a highly specific, well-defined pattern, which is regulated by neurotrophic factors belonging to the neurotrophin family. The inner ear offers a suitable model for studying the expression of MAPs and assessing their role in neurotrophin-induced effects that are required for neuron-target innervation. The present study was undertaken to analyze the expression and localization of MAP5 isoforms during development of CVG neurons in vivo an in vitro; as well as the regulation of MAP5 by neurotrophin-3 (NT3) in cell culture. MAP5 expression in the inner ear of chick embryos and postnatal specimens was monitored using immunoblots and immunohistochemistry on frozen sections. MAP5 was highly expressed during the early stages of CVG development, at embryonic day (E)4, being located in both neuronal perikarya and neurites. Expression was maintained during the neurite outgrowth phase (E6-E12), when strong MAP5 immunostaining was observed at the same cellular locations. MAP5 expression decreased suddenly at E14, after the establishment of specific connections between the CVG neurons and their targets, the sensory epithelium of the inner ear. In cultured CVG neurons addition of NT3 led to increased MAP5 expression and produced neurite outgrowth. Both effects are differentially regulated in parallel by low (0.5 ng/ml) and high (5 ng/ml) NT3 concentrations. Present results suggest that MAP5 may be involved in neurotrophin-induced microtubule bundling during neurite outgrowth of auditory neurons.

HSV-1 vector mediated transfer of BDNF into cerebellar granule cells

Cerebellar granule cells offer a useful model system to study the effects of neurotrophins during development. We have used a defective herpes simplex virus type 1 (HSV-1) vector containing brain-derived neurotrophic factor (BDNF) to express this neurotrophin in aggregate cultures of granule cells. Viral infection led to easily detectable BDNF expression and neurite outgrowth of granule cells, expressing the high affinity receptor TrkB. Neurite elongation mediated by the HSV-1 vector producing BDNF was similar to that found after exposure to purified BDNF. This study demonstrates the efficacy of HSV-1 vectors for delivery and expression of neurotrophins in cerebellar granule cells. The biological responses measured indicate the effectiveness of HSV-1 vectors as potential therapeutic tools.

Differential effects of combined trk receptor mutations on dorsal root ganglion and inner ear sensory neurons

We have generated double mutant mice deficient in pairs of two different Trk receptors and have analysed the effects on survival and differentiation of dorsal root ganglion (DRG), inner ear cochlear and vestibular sensory neurons. In most combinations of mutant trk alleles, the defects observed in double compared to single mutant mice were additive. However, double homozygous trkA−/−;trkB−/− DRG and trkB−/−;trkC−/− vestibular neurons showed the same degree of survival as single trkA−/− and trkB−/− mice, respectively, suggesting that those neurons required both Trk signaling pathways for survival. In situ hybridisation analysis of DRG neurons of double mutant mice revealed differential expression of excitatory neuropeptides. Whereas calcitonin-gene-related peptide expression correlated with the trkA phenotype, substance P expression was detected in all combinations of double mutant mice. In the inner ear, TrkB- and TrkC-dependent neurons were shown to at least partially depend on each other for survival, most likely indirectly due to abnormal development of their common targets. This effect was not observed in DRGs, where neurons depending on different Trk receptors generally innervate different targets.

Developing inner ear sensory neurons require TrkB and TrkC receptors for innervation of their peripheral targets

The trkB and trkC genes are expressed during the formation of the vestibular and auditory system. To elucidate the function of trkB and trkC during this process, we have analysed mice carrying a germline mutation in the tyrosine kinase catalytic domain of these genes. Neuroanatomical analysis of homozygous mutant mice revealed neuronal deficiencies in the vestibular and cochlear ganglia. In trkB (−/−) animals vestibular neurons and a subset of cochlear neurons responsible for the innervation of outer hair cells were drastically reduced. The peripheral targets of the respective neurons showed severe innervation defects. A comparative analysis of ganglia from trkC (−/−) mutants revealed a moderate reduction of vestibular neurons and a specific loss of cochlear neurons innervating inner hair cells. No nerve fibres were detected in the sensory epithelium containing inner hair cells. A developmental study of trkB (−/−) and trkC (−/−) mice showed that some vestibular and cochlear fibres initially reached their peripheral targets but failed to maintain innervation and degenerated. TrkB and TrkC receptors are therefore required for the survival of specific neuronal populations and the maintenance of target innervation in the peripheral sensory system of the inner ear.

Insulin-like growth factor-I regulates cell proliferation in the developing inner ear, activating glycosyl-phosphatidylinositol hydrolysis and Fos expression

The role of insulin-like growth factors (IGF) was investigated during the early development of the inner ear. IGF-I stimulated growth of otic vesicles that were isolated and cultured in vitro. IGF-I induced DNA synthesis, increased cell number, and mitotic rate in a dose-dependent manner at concentrations between 0.1-10 nM. IGF-II also induced growth but with a lower potency, whereas insulin had no effect. In the presence of IGF-I, otic vesicles developed from stage 18 to stage 21 in 24-h cultures, mimicking the normal mitotic pattern and morphogenesis in vivo. IGF-I also stimulated growth in the cochleovestibular ganglion. Binding of 125I-IGF-I to specific receptors occurred with high affinity. An autoradiographic study of sections from otic vesicles showed radiolabeled IGF-I in the epithelium. Immunoreactivity to IGF-I was detected in the otic vesicle and in the cochleovestibular ganglion. Intracellular signaling mechanisms of IGF were explored by studying the turnover of glycosylated phosphatidylinositols and the expression of Fos oncoprotein. IGF-I rapidly increased Fos levels in cultured otic vesicles. Furthermore, antisense oligonucleotides complementary to c-fos were able to inhibit IGF-I-induced growth. Both IGF-I-induced cell proliferation and Fos expression were blocked by an antiinositol phosphoglycan (alpha-IPG) antibody. This work suggests that IGF-I may be a candidate to regulate proliferative growth of the otic primordium during normal development and that this action requires the sequential modulation of glycosyl-phosphatidylinositol turnover and Fos expression.

Developmental changes in nerve growth factor (NGF) binding and NGF receptor proteins trkA and p75 in the facial nerve

This study characterizes the temporal-spatial distribution of nerve growth factor (NGF) low (p75) and high-affinity (trkA) receptors in the facial nerve and geniculate ganglion (GG) of developing quail embryos (E-3 to E-14). We used 125I-labeled NGF (125I-NGF) to study binding dynamics in a temporal series of isolated primordia and an autoradiographic series of staged specimens to characterize the occurrence and distribution of NGF receptors in this cranial nerve and its ganglion. In addition, expression of trkA and p75 protein-like immunoreactivity in the facial nerve and GG was studied by Western blot, in order to distinguish between high- and low-affinity NGF receptors respectively. The quantitative study of binding show that isolated facial primordia ranging from E-3 to E-14 exhibit different levels of specific binding. High initial binding levels were observed on E-3 specimens, then an initial decrease on day 4 (E-4) followed by a steady increase from days E-4 to E-7. Maximum 125I-NGF binding was achieved on E-7, followed by a steady decline in binding on days 8 (E-8) and 9 (E-9), reaching near background levels on day 10 (E-10) of development and until the oldest stage assayed (E-14). Most of the cells bearing NGF receptors appeared to be non-neuronal crest-derived cells, but some placode-derived neurons and motor fibers of the VIIth cranial nerve transiently expressed the ability to bind 125I-NGF. The temporal pattern of p75 expression matches the pattern of quantitative binding of NGF, while the trkA expression is restricted to a few stages mainly E7 and E9, implying that most of the binding detected is via low-affinity receptors, except for a proportion of high-affinity receptors present at stages of maximum binding. This temporal pattern of NGF binding sites suggests that cells within the VIIth cranial nerve are responsive to and/or dependent upon NGF in vivo, so NGF may play a biological role during normal development of the facial nerve. In view of the developmental events that parallel the occurrence and type of NGF binding sites, we suggest that this role may be to modulate from earlier chemotaxis and cell proliferation to much later events, such as neuronal differentiation and neuron-glia interactions. The significance of these findings in regeneration during adult life remain to be investigated.

Immunohistochemical localization of calcium-binding proteins in the human cutaneous sensory corpuscles

The localization of the calcium-binding proteins (CaBP) calbindin-D28k (CB), parvalbumin (PV) and S-100 protein (S100P) in the human cutaneous sensory corpuscles was studied by immunohistochemical procedure using monoclonal antibodies. Occurrence of CB, PV and S100P immunoreactivity (IR) was observed in the lamellar cells of Meissner's corpuscles. In the pacinian corpuscles, S100P IR was restricted to the inner-core cells whereas CB and PV IR were found labelling the axon, inner core, outer core and capsule. At the light-microscope level of resolution, the presence of IR in the axon of Meissner's corpuscles cannot be ensured. Since calcium ions (Ca2+) seem to participate in the mechanoreceptor electrogenesis, present results suggest that CaBP could be involved in buffering and/or transport of Ca2+ within the specialized cells surrounding the axon tips of sensory corpuscles, thus, maintaining the periaxonal microenvironment.

Pattern of trkB protein-like immunoreactivity in vivo and the in vitro effects of brain-derived neurotrophic factor (BDNF) on developing cochlear and vestibular neurons

The cochleo-vestibular ganglion (CVG) contains the neurons connecting the sensory epithelia of the inner ear to the cochlear and vestibular nuclei in the medulla. Expression of trkB protein-like immunoreactivity was studied in the developing CVG, using both Western blot and immunocytochemistry on tissue sections. Specific immunoreactivity was observed in the CVG from the 12th gestation day (gd) to the first postnatal week, reflecting the presence of high-affinity receptors for brain-derived neurotrophic factor (BDNF), a member of the NGF family of neurotrophins. Whole explants and dissociated cell cultures of cochlear (CG) and vestibular ganglion (VG) from mouse embryos and postnatal specimens were grown in neurotrophin-free medium to assay changes in neurite outgrowth and neuronal survival in response to the addition of physiological concentrations (0-5 ng/ml) of BDNF. Exogenous BDNF (2 ng/ml) promoted neurite outgrowth and neuronal survival in explants of both CG and VG, and the effects were stage-dependent. The onset of the response to BDNF occurred at gd 11-12. The response then reached a maximum between 14 and 18 gd and subsequently decreased, although it remained significantly present during the first postnatal week. BDNF-induced response was no longer observed in the mature cochlear and vestibular ganglion (after 30 postnatal days). The effects of BDNF on neuronal differentiation and survival were dose-dependent, starting at 0.5 ng/ml, with saturation at 2 ng/ml and half-maximal effect occurring between 1 and 1.5 ng/ml. On the basis of our results, we propose that BDNF may be physiologically involved in the control of both neuronal differentiation, and central and peripheral target-dependent neuronal death, in the CVG of embryos and early postnatal mice. BDNF may act alone or in cooperation with other neurotrophins to establish the afferent innervation of the inner ear sensory epithelium.

Brain-derived neurotrophic factor and neurotrophin-3 support the survival and neuritogenesis response of developing cochleovestibular ganglion neurons

The effects of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) on the differentiation of avian cochleovestibular ganglion and their possible association with the hydrolysis of glycosyl-phosphatidylinositol (GPI) were studied. BDNF and NT-3 (2 ng/ml) promoted neurite outgrowth in explants of both cochlear and vestibular ganglia. This effect on neuritogenesis was stage-dependent, reaching a maximum at E7 for NT-3 and at E9 for BDNF. The magnitude of the response of the vestibular ganglion to BDNF was always smaller than that of the cochlear ganglion of an equivalent stage. BDNF and NT-3 stimulation of neuronal survival and neurite extension was also demonstrated in dissociated neuronal cell cultures. The effect was concentration-dependent with saturation of the response occurring at 4 ng/ml for BDNF and at 2 ng/ml for NT-3, the half-maximal effect occurring at 2 and 1 ng/ml, respectively, for the most sensitive stages of the chick cochlear ganglion. Inositol phosphoglycan (IPG) did not mimic the effects of BDNF or NT-3 on neuronal survival and neurite outgrowth, nor was it able to potentiate their responses. Antibodies raised against IPG did not block the effects of these neurotrophins. The results suggest that BDNF and NT-3 may act in cooperation to establish the innervation pattern of the inner ear. Unlike their early proliferative effects, neurotrophic effects are uncoupled from the GPI/IPG signal transduction system.

Brain-derived neurotrophic factor and neurotrophin-3 induce cell proliferation in the cochleovestibular ganglion through a glycosyl-phosphatidylinositol signaling system

We have investigated the role of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) in the regulation of cell proliferation in the early developing cochleovestibular ganglion (CVG). Ganglia were isolated from 72-hr chick embryos and cultured for 24 hr. Both BDNF and NT-3 had a powerful mitogenic effect, at doses of 1-5 ng/ml, consistent with an involvement of the high-affinity receptor. Evidence for the participation of the glycosyl-phosphatidylinositol (GPI)/inositol phosphoglycan (IPG) signaling system in the mediation of proliferative effects of BDNF and NT-3 is presented. Both of these neurotrophins elicited a fast and transient hydrolysis of labeled GPI, approximately 60% in 30 sec. The dose-response profile of GPI hydrolysis overlaps the neurotrophin-induced cell proliferation response profile. Anti-IPG antibodies were able to block the growth-promoting effects of BDNF and NT-3. Anti-IPG antibodies immunoprecipitated a CVG-endogenous IPG, induced upon BDNF treatment, which exhibited proliferative stimulating properties. Both BDNF and NT-3 are proposed as potential candidates for regulation of growth during CVG development, with this mitogenic effect being mediated by the GPI/IPG signaling system.

Transient outward currents in cochlear ganglion neurons of the chick embryo

Cochlear ganglion neurons were isolated from chick embryos and membrane currents recorded using the patch-clamp technique. Depolarizing voltage steps elicited transient outward currents whose inactivation was best fitted by a double-exponential function with time constants < 30 ms and > 100 ms. The fast inactivating transient outward current (Ito,f) had a threshold for activation of -61 +/- 5.5 mV; steady-state inactivation was voltage-dependent between -90 and -60 mV, with half-inactivation near -75 mV. The slowly inactivating outward current (Ito,s) showed an activation threshold of 34 +/- 4 mV. Half-inactivation was at -67 +/- 3 mV. Ito,f was blocked by 4-aminopyridine which did not affect Ito,s. The effect was concentration- and voltage-dependent. Tetraethylammonium had no effect on either fast or slow transient currents but reduced the amplitude of the non-inactivating outward current in a dose-dependent manner. Ito,f was strongly inhibited by removing Ca2+ from the extracellular bathing solution. Cobalt ions inhibited Ito,f in a dose-dependent manner between 2 and 20 mM. The inhibitory effect of Co2+ was voltage-dependent, displaying a bell-shaped inhibition curve as a function of membrane voltage, maximal inhibition occurring between -20 and 0 mV. Ca2+ removal did not affect Ito,s and partially reduced the amplitude of the steady-state current. These results provide kinetic and pharmacological evidence for the presence of two distinct transient outward currents in cochlear neurons. These currents may play a role in the first synaptic relay of sound transmission.

Development of Na(+)- and K(+)-currents in the cochlear ganglion of the chick embryo

The development of Na(+)- and K(+)-currents in the primary afferent neurons of the cochlear ganglion was studied using the patch-clamp technique. Cells were dissociated between days 6 and 17 of development and membrane currents recorded within the following 24 h. Outward currents were the first to appear between days 6 and 7 of embryonic development and their magnitude increased throughout development from 200 pA on day 7 to 900 pA on days 14-16. Threshold for activation decreased by 20 mV between days 8 and 14. Outward currents were absent when Cs+ replaced K+ in the pipette and were partially blocked by external tetraethylammonium. Outward currents contained at least three components: (i) a non-inactivating outward current, similar to the delayed-rectifier, predominating in mature neurons; (ii) a slowly inactivating current (tau about 200 ms), most evident in early and intermediate stages (days 7-10); and (iii) a rapidly inactivating outward current (tau about 20 ms) similar to the A-current (IA) described in other neurons, which was distinctly expressed in mature neurons. Sodium currents were identified as fast transient inward currents, sensitive to tetrodotoxin and extracellular Na(+)-removal. They appeared later than K(+)-currents and increased in size from about 100 pA between days 9-11 to 600 pA by days 13-16. The development of membrane currents in cochlear ganglion neurons corresponded to defined stages of the innervation pattern of the chick cochlea [Whitehead and Morest (1985) Neuroscience 14, 255-276]. These currents could be functionally related to the establishment of synaptic connections between transducing cells and primary afferent neurons.

Myb p75 oncoprotein is expressed in developing otic and epibranchial placodes

The c-myb proto-oncogene encodes a transcriptional regulatory protein which is highly conserved throughout evolution. Myb has been considered to be normally restricted to hematopoietic tissues, but there are indications that this might not always be the case. The present work shows the expression of a p75 Myb oncoprotein in the otic vesicle and epibranchial placodes of the early chick embryo. Expression was sequential and followed the same time course as the formation of placode-derived cranial ganglia. The results suggest a potential role for c-myb in regulation of placode development and neurogenesis.

The int-2 proto-oncogene is responsible for induction of the inner ear

The int-2 proto-oncogene encodes several products related to the fibroblast growth factor (FGF) family. FGFs have been associated with mesoderm induction in the amphibian embryo and int-2 has a distinct pattern of expression throughout development in vertebrates. But evidence for a function of int-2 in embryo-genesis has been lacking. In the mouse embryo, int-2 transcripts have been detected in the rhombencephalon at a developmental stage where classical experiments showed that the induction of the inner ear occurs. This raises the possibility that int-2 may constitute a signal for the induction of the otic vesicle, the primordium of the inner ear. We provide direct evidence for this view by showing that (1) the formation of the otic vesicle is inhibited by antisense oligonucleotides targeted to the secreted form of int-2, and by antibodies against int-2 oncoproteins, and (2) basic FGF (bFGF) can mimic the inductive signal in the absence of the rhombencephalon.

Glycosyl-phosphatidylinositol/inositol phosphoglycan: a signaling system for the low-affinity nerve growth factor receptor

Nerve growth factor (NGF) exerts a variety of actions during embryonic development. At the early stages of inner ear development, NGF stimulates cell proliferation, an effect mediated through low-affinity receptors. We have studied the possibility that the glycosyl-phosphatidylinositol/inositol phosphoglycan (glycosyl-PtdIns/IPG) system is involved in transmitting this NGF signal. Endogenous glycosyl-PtdIns was characterized in extracts of cochleovestibular ganglia (CVGs) that incorporated [3H]glucosamine, [3H]galactose, [3H]myristic acid, and [3H]palmitic acid. Incubation of CVG with NGF produced a rapid and transient hydrolysis of glycosyl-PtdIns. Hydrolysis was complete at 100 ng/ml, and the half-maximal effect occurred at 25 ng/ml, overlapping with the concentration dependence of the mitogenic effect of NGF. An IPG was isolated from embryonic extracts. It had biological effects similar to those reported for the insulin-induced IPG in other tissues. It exerted a powerful mitogenic effect on CVG, comparable to that of NGF. Both the IPG- and NGF-induced cell proliferation were blocked by anti-IPG antibodies that recognized the endogenous IPG on a silica plate immunoassay. These results show that CVG possesses a fully active glycosyl-PtdIns/IPG signal transduction system and that the proliferative effects associated with NGF binding to low-affinity receptors require IPG generation.

Temporal pattern of nerve growth factor receptor expression in developing cochlear and vestibular ganglia in quail and mouse

We have previously demonstrated the presence of specific receptors for nerve growth factor (NGF) in cochleovestibular ganglia of 72 h (stage 19-20) quail embryos, with a greater density of NGF receptors in the cochlear portion of the ganglion. The present study was conducted to determine the temporal pattern of NGF receptor expression in cochlear and vestibular ganglia throughout development, and was conducted in two species, quail and mouse. As in the quail, specific binding of 125I-NGF was detected in cochleovestibular ganglia of mouse embryos from an embryonic age equivalent to 72 h quail embryos (embryonic day 11, E11), with a similar concentration of 125I-NGF binding in the cochlear portion. Quantitative studies revealed that 125I-NGF binding continued to increase, in both cochlear and vestibular ganglia, for several days of development, and then began to decrease to minimal levels. Maximal levels were achieved at E7 in the quail, and E14 to E16 in the mouse, while minimal levels were reached by E13 in the quail, and E18 in the mouse. The level of 125I-NGF binding in cochlear ganglia was two to three times higher than in vestibular ganglia; a finding corroborated by radioautographic studies. In both quail and mouse, NGF receptors were more heavily concentrated in the ventromedial portion of the cochlear ganglion, adjacent to the cochlear duct; an area containing both support cells and peripheral neuronal processes. In the vestibular ganglion, 125I-NGF binding was more homogeneous, although small areas containing high densities of silver grains were observed. The presence of NGF receptors in cochlear and vestibular ganglia suggests that these ganglia may be responsive to and/or dependent upon NGF during their development.

Inositol phospho-oligosaccharide stimulates cell proliferation in the early developing inner ear

The ability of an inositol phospho-oligosaccharide (POS) to mimic the mitogenic effects of nerve growth factor (NGF) and insulin on the early development of the inner ear was investigated. POS (10 microM) stimulated the incorporation of [3H]thymidine into the cochleovestibular ganglion by 3.9-fold. NGF (50 ng/ml) stimulation was 4.7-fold. POS and NGF showed no additivity. Cells induced to proliferate by POS overlapped with those expressing NGF receptors. POS, like insulin, potentiated the mitogenic effect of bombesin on the otic vesicle epithelium. DNA synthesis in the presence of bombesin (100 nM) plus POS (10 microM) was increased by 6.4-fold. POS stimulation was not additive with insulin. The results suggest that POS may play a role in growth factor regulation of cell proliferation during embryonic development.

Temporal pattern of nerve growth factor (NGF) binding in vivo and the in vitro effects of NGF on cultures of developing auditory and vestibular neurons

NGF binding patterns reflect the presence of receptors for this growth factor. High specific binding of 125I 2.5 S-NGF was observed for the 11 gestation day (gd) statoacoustic ganglion (SAG) with lower levels recorded for both 14 gd acoustic ganglion (AG) and vestibular ganglion (VG) samples. Fourteen day AG cells were more than twice as active for binding NGF when compared to VG samples of the same gestational age. Both whole ganglion explants and dissociated cell cultures were grown in chemically defined medium for short term culture to assay changes in neurite outgrowth and survival of neurons in response to the addition of exogenous 2.5 S-NGF. The most vigorous neurite outgrowth and neuronal survival responses were produced by 11 gd SAG samples treated with NGF. Acoustic ganglion specimens of both 11 gd and 14 gd embryos were much more responsive to the neurotrophic effects of NGF when compared to the responses of their VG counterparts. There was a correlation between NGF binding ability and in vitro responsiveness to exogenous NGF. We hypothesize based on the results of this study that NGF (and/or a member of the NGF family of growth factors) is involved in the control of developmentally regulated neuronal cell death of SAG neurons and may play a role in the innervation of developing inner ear sensory structures.

Retinoic acid modulation of the early development of the inner ear is associated with the control of c-fos expression

The effects of retinoic acid (RA) on the early development of the inner ear were studied in vitro using isolated chick embryo vesicles. Low concentrations of RA (1–50 nM) inhibited vesicular growth in stage 18 otic vesicles that were made quiescent and then reactivated by either serum or bombesin. Growth inhibition was concentration-dependent and was paralleled by a reduction in the rate of DNA synthesis as measured by [3H]thymidine incorporation. Half-inhibition occurred between 1 and 10 nM RA, and the full effect at 20 nM. Retinoic acid, in the presence of serum, induced the precocious differentiation of (1) secretory epithelium, the tegmentum vasculosum and endolymphatic sac and (2) early sensory and supporting epithelia. These structures were positioned in their corresponding normal presumptive areas. The overall direction of growth was reversed by RA and the ratio of the internal to the external vesicular surface area increased with RA concentration. The expression of the nuclear proto-oncogene c-fos in the developing otic vesicle was transient and stage-dependent. High levels of c-fos mRNA were positively correlated with cell proliferation. Incubation of growth-arrested otic vesicles with bombesin plus insulin at concentrations that induced cell proliferation produced a strong induction of c-fos. This mitogen-induced expression was suppressed by 25 nM RA. The results suggest (1) a role for retinoic acid in controlling the early development of the inner ear and (2) that this control is effected through the regulation of the proto-oncogene c-fos.

[Tumor markers in liver metastases of colorectal carcinoma]

Colorectal cancer is a potentially curable tumour when diagnosed in the early stages. In order to improve the results obtained up to now, we propose application of a diagnostic program among patients who undergo curative resection for colorectal adenocarcinoma, which would consist of using a panel of tumor markers, in combination with endoscopic, histologic and ultrasonographic diagnostic methods. For this study we studied 105 patients, divided into two groups: A) Group 1: 30 control patients. B) Group 2: 75 patients diagnosed as having colorectal cancer. We performed the preoperative determination of a series of tumor markers (CEA, CA 19.9, GGT and PHI), endoscopy/biopsy and hepatic ultrasonography on these patients. Our results suggest that the design of the preoperative diagnostic program makes early detection of hepatic metastases possible. The tumor marker panel combination provided a visible increase in sensitivity for detecting hepatic metastases.

Mesoderm induction by the mesoderm of Xenopus neurulae

Combinations were made between explants of mesoderm from the archenteron roof of early Xenopus neurulae and explants of ectoderm from mid-blastulae. In each combination one component was labeled with the fluorescent lineage label RDA (rhodamine-dextran-amine). Frequent and large mesoderm inductions, consisting mainly of muscle, were found where the presomite plate was used as the inducer. Less frequent and smaller mesoderm inductions were found when notochord was used as the inducer. We conclude that induced mesoderm can itself be active as a mesoderm inducing tissue. If this capability is acquired in the blastula then it follows that mesoderm induction must propagate from cell to cell and its spread be antagonized by some other factor.

Nerve growth factor and serum differentially regulate development of the embryonic otic vesicle and cochleovestibular ganglion in vitro

The preceding paper (P. Bernd and J. Represa, 1989, Dev. Biol. 134) describes the characterization and localization of nerve growth factor (NGF) receptors in inner ear primordia, the otic vesicle (OV) and cochleovestibular ganglion (CVG), obtained from 72-hr (stage 19-20) quail embryos. The studies described in this paper investigated whether NGF serves as a mitogen, a survival factor, and/or a differentiation factor in this system. Explants of isolated OV and CVG were maintained for 24 hr in serum-free medium alone (M-199), M-199 containing serum, M-199 containing NGF, or M-199 containing both serum and NGF. [3H]Thymidine was also present for the entire culture period. Both OV and CVG incorporated greater amounts of [3H]thymidine in the presence of serum or NGF, and their combined effect was additive. NGF's effects were dose dependent, saturable, and specific (blocked by anti-NGF). NGF caused little or no morphological differentiation of OV and no increase in protein levels, in contrast to OV grown in the presence of serum. CVG had both cochlear and vestibular portions present in all cases, but the apparent size and protein content of CVG was increased in the presence of either serum or NGF. Effects of serum and NGF were completely, but reversibly, blocked by amiloride, suggesting that the Na+-H+ exchange system had been activated. In order to determine whether increases in [3H]thymidine incorporation were due to increased cell survival or perhaps to an increase in proliferation, explants were initially grown for a 24-hr period in serum-free medium, followed by reactivation for an additional 24 hr in medium containing serum and/or NGF. It is likely that cells requiring either serum or NGF for survival would die during a 24-hr period in their absence. Our results revealed that the level of [3H]thymidine incorporation in OV was the same after reactivation. In the case of CVG, only NGF treatment yielded similar results; [3H]thymidine incorporation was lower in CVG reactivated with serum. It appears, therefore, that serum has probable proliferative effects upon OV and CVG, as well as survival effects for CVG. NGF, however, does not appear to affect survival in either OV or CVG, so that increases in [3H]thymidine incorporation in response to NGF are most likely due to proliferative effects upon OV or CVG, at least at this embryonic stage.

Characterization and localization of nerve growth factor receptors in the embryonic otic vesicle and cochleovestibular ganglion

We have investigated the possibility that nerve growth factor (NGF) may play a role in the development of the inner ear. Primordia of the inner ear, the otic vesicle (OV) and cochleovestibular ganglion (CVG), were isolated from 72-hr (stage 19-20) quail embryos and examined for the presence of NGF receptors. Quantitative binding studies revealed that both OV and CVG exhibited specific 125I-NGF binding; levels of nonspecific binding were 6 to 26% of total binding. Scatchard analysis yielded a linear plot, indicating the presence of a single class of NGF receptor. The average binding constant (Kd) was 8.0 nM for OV and 8.6 nM for CVG, corresponding to the low affinity (site II) NGF receptor. Examination of light microscopic radioautographs indicated that most of the specific 125I-NGF binding was located in the ventromedial wall of the OV, with little or no binding in the lateral wall and endolymphatic primordia. These studies were corroborated by microdissection of OV, in which 70% of the radioactivity was found to be localized in the medial half of the OV. In CVG, specific 125I-NGF binding was more concentrated in the cochlear portion of the ganglion, with silver grains primarily over areas containing support cells and immature neurons. Quantitative binding studies with isolated cochlear and vestibular ganglia obtained from 144-hr (stage 29-30) quail embryos revealed that the cochlear ganglion exhibited three times more specific 125I-NGF binding than the vestibular ganglion. The presence of NGF receptors on OV and CVG suggests that these structures are responsive to and/or dependent upon NGF. The following paper (J. Represa and P. Bernd, 1989, Dev. Biol. 134) examines the question of whether NGF serves either as a mitogen, a survival factor, or a differentiation factor in this system.