Protein gene product 9.5 in the developing cochlea of the rat: cellular distribution and relation to the cochlear cytoskeleton

Modifier variant of METTL13 suppresses human GAB1-associated profound deafness

A modifier variant can abrogate the risk of a monogenic disorder. DFNM1 is a locus on chromosome 1 encoding a dominant suppressor of human DFNB26 recessive, profound deafness. Here, we report that DFNB26 is associated with a substitution (p.Gly116Glu) in the pleckstrin homology domain of GRB2-associated binding protein 1 (GAB1), an essential scaffold in the MET proto-oncogene, receptor tyrosine kinase/HGF (MET/HGF) pathway. A dominant substitution (p.Arg544Gln) of METTL13, encoding a predicted methyltransferase, is the DFNM1 suppressor of GAB1-associated deafness. In zebrafish, human METTL13 mRNA harboring the modifier allele rescued the GAB1-associated morphant phenotype. In mice, GAB1 and METTL13 colocalized in auditory sensory neurons, and METTL13 coimmunoprecipitated with GAB1 and SPRY2, indicating at least a tripartite complex. Expression of MET-signaling genes in human lymphoblastoid cells of individuals homozygous for p.Gly116Glu GAB1 revealed dysregulation of HGF, MET, SHP2, and SPRY2, all of which have reported variants associated with deafness. However, SPRY2 was not dysregulated in normal-hearing humans homozygous for both the GAB1 DFNB26 deafness variant and the dominant METTL13 deafness suppressor, indicating a plausible mechanism of suppression. Identification of METTL13-based modification of MET signaling offers a potential therapeutic strategy for a wide range of associated hearing disorders. Furthermore, MET signaling is essential for diverse functions in many tissues including the inner ear. Therefore, identification of the modifier of MET signaling is likely to have broad clinical implications.

Expression of an inwardly rectifying K(+) channel, Kir4.1, in satellite cells of rat cochlear ganglia

Satellite cells are glial cells wrapped around somata of sensory and autonomic ganglion neurons. Neither their functional roles nor electrical properties have been fully clarified so far. Using immunohistochemistry, we found that inwardly rectifying K(+) channel subunit Kir4.1 (also called Kir1.2 or K(AB)-2) was expressed prominently in the satellite cells of cochlear ganglia. The Kir4.1 immunoreactivity was localized specifically at the myelin sheaths of satellite cells wrapping the somata of the ganglion neurons. Developmental expression of Kir4.1 in satellite cells paralleled development of the action potential in the auditory nerve. These results suggest that this channel in satellite cells may be responsible for the regulation of K(+) extruded from the ganglion neurons during excitation.

Innervation of blood vessels in the rat incisor pulp: a scanning electron microscopic and immunoelectron microscopic study

Although two types of nerve endings have been proposed to innervate blood vessels in the dental pulp, the precise innervation pattern is not well understood. This is mainly due to the lack of information regarding the positional relationships of nerve fibers with blood vessels at the electron microscopic level. The rat incisor pulp was investigated by scanning electron microscopy (SEM) after connective tissue digestion and by transmission electron microscopy after immunohistochemical localization with polyclonal anti-PGP 9.5 antibody. SEM specimens revealed that unmyelinated nerve fibers passed through the tunica adventitia of the blood vessel in the center of the pulp and that these fibers then entered the tunica media of the smaller arterioles. The nerve fibers divided into many collaterals, and these terminated on the surface of smooth muscle cells and endothelial cells. The fibers extended toward smaller vessels. The terminal fibers then reached the subodontoblastic capillary plexus and terminated on pericytes, while the odontoblastic capillary plexus had no direct innervation. In the pulpal venules, nerve fibers were located adjacent to the pericytes and endothelial cells, and they extended toward postcapillary venules. The same results were confirmed by immunoelectron microscopy. The present study demonstrated that in the rat incisor pulp the microvasculature as well as larger vessels were directly innervated by free nerve endings, suggesting that the local regulation of blood flow could take place not only at larger vessels but also at the level of the microvasculature in this tissue.

UFD1L, a developmentally expressed ubiquitination gene, is deleted in CATCH 22 syndrome

The CATCH 22 acronym outlines the main clinical features of 22q11.2 deletions (cardiac defects, abnormal facies, thymic hypoplasia, cleft palate and hypocalcemia), usually found in DiGeorge (DGS) and velo-cardio-facial (VCFS) syndromes. Hemizygosity of this region may also be the cause of over 100 different clinical signs. The CATCH 22 locus maps within a 1.5 Mb region, which encompasses several genes. However, no single defect in 22q11.2 hemizygous patients can be ascribed to any gene so far isolated from the critical region of deletion. We have identified a gene in the CATCH 22 critical region, whose functional features and tissue-specific expression suggest a distinct role in embryogenesis. This gene, UFD1L, encodes the human homolog of the yeast ubiquitin fusion degradation 1 protein (UFD1p), involved in the degradation of ubiquitin fusion proteins. Cloning and characterization of the murine homolog (Ufd1l) showed it to be expressed during embryogenesis in the eyes and in the linear ear primordia. These data suggest that the proteolytic pathway that recognizes ubiquitin fusion proteins for degradation is conserved in vertebrates and that the UFD1L gene hemizygosity is the cause of some of the CATCH 22-associated developmental defects.

Distribution of cytoskeletal proteins (neurofilaments, peripherin and MAP-tau) in the cochlea of the human fetus

We report here an immunohistochemical study of the distribution of intermediate filaments (neurofilament, peripherin) and a microtubule-associated protein, tau, in the human fetal cochlea at 27 weeks of gestation. Neurofilament immunoreactivity (160 and 200 KDa) was localized in afferent and efferent fibers of the cochlear innervation and restricted to a few small spiral ganglion neurons. Peripherin immunoreactivity was specifically distributed in some small ganglion neurons and in their central and peripheral extensions, particularly in fibers reaching the lower part of the outer hair cells. Double immuno-labelling studies with these neurofilaments and peripherin antibodies show that only small neuron cell bodies were stained. Morpholometrical data indicate that immunostained neurons could be related to the Type II neuron population in the spiral ganglion. Tau protein was localized in intraganglionic spiral bundle fibers and in fibers that reach the lower part of hair cells. These observations suggest that neurofilament and peripherin antibodies stain a particular population of human spiral ganglion neurons with Type II characteristics. Moreover, the specificity of peripherin labelling in Type II cells and their processes suggest that peripherin could be used as a probe for the developmental study of this system in the human cochlea. On the other hand, tau antibody appeared as a marker for efferent fibers during development and could give information on the ontogenesis of efferent innervation.

Protein gene product 9.5 in the developing and mature rat vomeronasal organ

Protein gene product 9.5 (PGP 9.5) was immunocytochemically identified in structures of the developing and mature rat vomeronasal organ (VNO). This study started with embryos at 17 days of gestation. By this stage, PGP 9.5 was immunolocalized within both the receptor cells of the neuroepithelium and cells of the receptor-free epithelium, located on the opposite side of the lumenal space. Nerve fibers surrounding the nascent vomeronasal vein also showed PGP 9.5 immunoreactivity. Labeling was retained in the neuroepithelium and nerve fibers around the vein during development and into adulthood. Within the pool of receptor cells of the adult neuroepithelium, labeling was observed within the cytoplasm of the somata and dendrites; some receptor cells also expressed nuclear labeling. The number of immunoreactive cells in the receptor-free epithelium appeared to increase postnatally but by adulthood immunoreactivity virtually was absent. These results suggest a role for PGP 9.5 in development and maturation of the VNO and a continuing role within the steadily renewing receptor cell population found in the mature neuroepithelium. The findings of this study also suggest that PGP 9.5 is localized within the nerve fibers surrounding the vomeronasal vein from early in development through adulthood.

Ontogenesis of type II spiral ganglion neurons during development: peripherin immunohistochemistry

In this study, we analysed the distribution of the intermediate filament peripherin in the developing cochlea of the rat. At gestational day 16, weak immunolabeling was observed in neuronal somas throughout the spiral ganglion. At gestational day 20, the peripherin labeling increased in intensity throughout the spiral ganglion. At gestational day 20, the peripherin labeling increased in intensity throughout the cochlea but became especially strong in some ganglion neurons of the basal turn. Homogeneous immunolabeling was observed throughout the spiral ganglion of the apical turn. Double immunofluorescence labeling of the prenatal cochlea with peripherin and neurofilament (NF) antibodies revealed colocalization on the same structures. By postnatal day 3, the peripherin labeling intensity had decreased in the majority of spiral ganglion neurons, but remained strong in some cells of the basal turn. Only a few neurons continued to be immunolabeled into adulthood that correspond to Type II spiral ganglion neurons expressing both NF protein and peripherin, two classes of intermediate filament proteins. In the organ of Corti, the first immunolabeling was observed on gestational day 20 as peripheral fibers reaching the receptor cells. Positive fibers were observed below both inner (IHCs) and outer (OHCs) hair cells. At birth and at postnatal day 3, peripherin immunolabeling was still observed below both IHCs and OHCs. By postnatal day 4, peripherin labeling became more dominant in fibers below OHCs, but some immunoreactivity was still present below IHCs. No immunoreactivity was present in the intraganglionic spiral bundle (IGSB) fibers containing the olivary complex efferent fibers before birth. A few days after birth some fibers of the IGSB started to be immunoreactive.

Immunohistochemical demonstration of protein gene product 9.5 (PGP 9.5) in the primary olfactory system of the rat

Immunohistochemical localization of protein gene product 9.5 (PGP 9.5) was examined in the primary olfactory system of the rat. Receptor cells were intensely immunopositive in the olfactory epithelium, vomeronasal organ and septal olfactory organ of Masara. The mitral cells and some of the short-axon cells of the main olfactory bulb (MOB) were also intensely immunopositive, while the tufted cells of the MOB and the mitral/tufted cells of the accessory olfactory bulb varied in immunoreactivity.