Biomedical subjects

M Lenoir

Publications and source records attributed to M Lenoir.

At least 37 records · Page 2Linked to original sources

PubMed1998

Characterization of atypical cells in the juvenile rat organ of corti after aminoglycoside ototoxicity.

Hair cell regeneration is well documented in the inner ear sensory epithelia of lower vertebrates and birds and may occur in the vestibular organs of mammals. By contrast, hair cell loss in the mature mammalian cochlea is considered irreversible. However, recent reports have suggested that an attempt at hair cell regeneration could occur in vivo in aminoglycoside-lesioned cochleas from neonatal rats. After amikacin treatment, atypical cells with apical specialization reminiscent of early differentiating stereocilia are transiently present at the apex of the intoxicated cochleas but fail to differentiate as hair cells in later stages. In the present study, we used electronic microscopy, histochemistry, and confocal microscopy to investigate the cellular rearrangements in the amikacin-lesioned organ of Corti of rat pups. In addition, we used 5-bromo-2'-deoxyuridine immunocytochemistry to determine whether mitotic processes are involved in the formation of the atypical cells. The morphologic and molecular data suggest that atypical cells are not recovering hair cells, but share characteristics of immature hair cells and supporting cells. Proliferative cells were absent from the region occupied by atypical cells, suggesting that the latter did not arise through mitotic processes. Altogether, the present results support the hypothesis that atypical cells arise through direct transformation of some of the supporting cells that reorganize during hair cell degeneration.

Amikacin↗

PubMed1998

Amikacin intoxication induces apoptosis and cell proliferation in rat organ of Corti.

Recently, an attempt at cochlear hair cell neodifferentiation has been reported in amikacin-treated rats. In the present study, we aimed to ascertain whether hair cell losses are mediated by apoptosis and whether cell proliferation occurs in damaged intoxicated cochleas. The results show that apoptosis is responsible for hair cell losses and that cell proliferation occurs in the region of the outer spiral sulcus but not in the region of Deiters cells and pre-existing hair cells. We suggest that cell proliferation maintains a certain homeostasis in the number of non-sensory cells and participates in epithelial scar formation. Neodifferentiated cells therefore probably arise from direct transdifferentiation, which could be triggered by phagocytosis of apoptotic bodies.

Amikacin↗

PubMed1997

Cloning of the genes encoding two murine and human cochlear unconventional type I myosins.

Several lines of evidence indicate a crucial role for unconventional myosins in the function of the sensory hair cells of the inner ear. We report here the characterization of the cDNAs encoding two unconventional type I myosins from a mouse cochlear cDNA library. The first cDNA encodes a putative protein named Myo1c, which is likely to be the murine orthologue of the bullfrog myosin I beta and which may be involved in the gating of the mechanotransduction channel of the sensory hair cells. This myosin belongs to the group of short-tailed myosins I, with its tail ending shortly after a polybasic, TH-1-like domain. The second cDNA encodes a novel type I myosin Myo1f which displays three regions: a head domain with the conserved ATP- and actin-binding sites, a neck domain with a single IQ motif, and a tail domain with the tripartite structure initially described in protozoan myosins I. The tail of Myo1f includes (1) a TH-1 region rich in basic residues, which may interact with anionic membrane phospholipids; (2) a TH-2 proline-rich region, expected to contain an ATP-insensitive actin-binding site; and (3) a SH-3 domain found in a variety of cytoskeletal and signaling proteins. Northern blot analysis indicated that the genes encoding Myo1c and Myo1f display a widespread tissue expression in the adult mouse. Myo1c and Myo1f were mapped by in situ hybridization to the chromosomal regions 11D-11E and 17B-17C, respectively. The human orthologuous genes MYO1C and MYO1F were also characterized, and mapped to the human chromosomal regions 17p18 and 19p13.2-19p13.3, respectively.

Amino Acid Sequence↗

PubMed1997

Development of the organ of Corti in horseshoe bats: scanning and transmission electron microscopy.

The late prenatal and early postnatal development of the organ of Corti were studied in the horseshoe bat (Rhinolophus rouxi) by using scanning and transmission electron microscopy. Arrangements and dimensions of stereocilia bundles, together with their contacts with the tectorial membrane, were found to be adult-like shortly before birth, and thus before the biological onset of hearing (3-5 days after birth). During the first postnatal week, there were baso-apical gradients in disappearing kinocilia on inner hair cells (IHC), microvillis of supporting cells, and marginal pillars. The lower basal cochlear turn was mature with respect to these regressing structures at 3 days after birth, the apical turn at 10 days after birth. At birth, cytodifferentiation was found to be completed, and the tunnel of Corti and innermost spaces of Nuel had opened. The ultrastructure of IHCs was not markedly different from that at later ages. In outer hair cells (OHC), the adult-like regular arrangement of a single layer of subsurface cisternae and pillars was seen as soon as protrusions of supporting cells had withdrawn from the lateral wall of OHCs (basal turn at birth and throughout the cochlea 2 days after birth). Numerous efferent endings contacted the somata of IHCs up to the second postnatal week. Since the medial olivocochlear system is absent in horseshoe bats, the adult-like innervation pattern of OHCs was established at the biological onset of hearing. During the first 2 postnatal weeks, the cytoskeleton of pillar and Deiters cells, and the specialized Deiters cups developed. The organ of Corti appeared adult-like at 14 days, apart from the persistence of a reduced tympanic cover layer attached to the basilar membrane. Morphological data support physiological findings that the first broadly tuned auditory responses arise from the basal turn. The distinct low to high frequency gradient in development of sensitivity during the first 2 postnatal weeks of the horseshoe bat was not, however, matched by morphological gradients, and it would appear that the development of the cytoskeleton of supporting cells contributed to the establishment of tuning in the auditory fovea. Adult-like morphology of the organ of Corti coincided with the emergence of sharply tuned responses from the auditory fovea, but there was no clear-cut correlate for the shift in tuned foveal frequency representation that occurred during the following 3 weeks.

Animals↗

PubMed1997

Identification of preferentially expressed cochlear genes by systematic sequencing of a rat cochlea cDNA library.

107 expressed sequence tags (ESTs) from a rat cochlea cDNA library were identified by systematic sequencing coupled to database selection and RT-PCR analysis of novel sequences. This approach led us to select a clone, pCO8, showing no significant homology with any database sequence, that corresponds to a mRNA whose expression is restricted to the cochlea, except for traces detected in brain. Additional clones with novel sequences enriched in the cochlea were also found. ESTs bearing significant homologies with database sequences (63 out of 107) were classified according to the putatively encoded protein. They include tissue-specific genes not previously described in the cochlea as well as known genes from other species. We performed in situ hybridization in cochlear tissues to localize the pCO8 mRNA and that of clone pCO6 which is 100% homologous to the delayed rectifier potassium channel drk1. We found that both mRNAs were exclusively expressed in the cellular body of the primary auditory neurons from the spiral ganglion of the cochlea. These results indicate that this approach is an efficient way to identify novel genes that could be of importance in cochlear function.

Animals↗

PubMed1997

Does the organ of Corti attempt to differentiate new hair cells after antibiotic intoxication in rat pups?

In the adult mammalian cochlea, post-injury hair cell losses are considered to be irreversible. Recent studies in cochlear explants of embryonic rodents show that the organ of Corti can replace lost hair cells after injury. We have investigated this topic in vivo during the period of cochlear development. Rat pups were treated with a daily subcutaneous injection of 500 mg/kg amikacin for eight consecutive days between postnatal day 9 (PND 9) and PND 16. During this period the organ of Corti is not fully mature, but hair cells are hyper-sensitive to aminoglycoside antibiotics. Scanning and transmission electron microscopy was used to evaluate morphological changes in the organs of Corti during the treatment and at different post-treatment periods, up until PND 90. A massive loss in outer and inner hair cells was observed at least as early as PND 14. A prominent feature in the apical part of cochleas at PND 21 and 35 was the transient presence of small atypical cells in the region of pre-existing outer hair cells. These atypical cells had tufts of microvilli reminiscent of nascent stereociliary bundles. A second striking observation was the replacement of degenerating inner hair cells by pear-shaped supporting cells throughout the cochlea. These cells were covered with long microvilli, and their basal pole was contacted by both afferent and efferent fibers, as in the early stages of inner hair cell maturation. At PND 55 and 90, these features were not clearly observed due to further cytological changes in the organ of Corti. It is possible that an attempt at hair cell neodifferentiation could occur in vivo after an amikacin treatment in the rat during the period of cochlear hyper-sensitivity to antibiotic.

Amikacin↗

PubMed1996

Electrical stimulation activates two different sites within the guinea pig cochlea.

This study investigates whether auditory brainstem responses (ABRs) can be used to assess the functioning of electrically stimulated cochleas. Electrically evoked auditory brainstem responses (EABRs) were recorded in guinea pigs with normal hearing and guinea pigs deafened by amikacin, a powerful ototoxic antibiotic, combined with diuretic aminooxyacetic acid (AOAA). Two different types of EABRs were observed in normal animals, depending on the electrical pulse intensity applied to the round window: long-latency brainstem responses were evoked by low stimulation intensities, short-latency brainstem responses by high intensities. The absence of effect of strychnine applied intracochlearly ruled out the possibility of medial efferents being involved in these responses. Conversely, an intracochlear application of tetrodotoxin (TTX), an Na(+)-channel blocker, resulted in the disappearance of both types of responses, attesting that the sites activated by the electrical stimulation were located within the cochlea. In AOAA/ amikacin poisoned cochleas, in which most of the hair cells were missing with apparently normal ganglion neurons, the long-latency brainstem responses evoked by low intensities were completely lacking. These findings suggest that low currents applied to the round window of the guinea pig cochlea primarily activate the hair cells, the neurons being directly excited at higher intensities.

Acoustic Stimulation↗

PubMed1996

Cholinergic responses in developing outer hair cells of the rat cochlea.

Acetylcholine-evoked currents were investigated using the conventional whole-cell patch-clamp recording technique in developing outer hair cells (OHCs). The cells were isolated from the rat cochlea at different stages of postnatal development ranging from day 4 (P4) to P30. Acetylcholine-evoked currents could be recorded at P6 and P8. At this developmental stage, the majority of OHCs displayed inward nicotinic-like currents near the resting membrane potential. These cholinergic currents zeroed near 0 mV, as expected for a non-selective cation current, and could be reversibly blocked by d-tubocurarine. At P12 and adult stage, the cholinergic response of OHCs switched to an outward current reversing near EK and displaying a bell shape peaking between -40 and -30 mV. This change in polarity of the acetylcholine response during postnatal development might be explained by progressive functional coupling between acetylcholine ionotropic receptors permeable to Ca2+ and nearby Ca(2+)-activated K+ channels at the synaptic pole of OHCs.

Acetylcholine↗

PubMed1996

Distribution of actin and tubulin in outer hair cells isolated from developing rat cochlea: a quantitative study.

In the organ of Corti, outer hair cells (OHCs) are sensory effectors responsible for the high sensitivity and sharp tuning of the cochlea. Whilst the distribution and organization of actin and tubulin in adult OHCs have been extensively studied, less is known about developing OHCs. In this study we use a quantitative cytometric approach on rat isolated OHCs to measure the distribution of these cytoskeletal proteins from the first stages of development (postnatal day 5) to the adult stage. We report a general decrease in both actin and tubulin concentrations during OHC maturation. Actin first decreases in the apical domain, and then in the medio-basal domain. In the apical domain, this could be related to the physiological reduction in the number of stereocilia occurring during ciliogenesis. In the medio-basal domain, the decrease, accompanied by a redistribution of actin toward the lateral wall, is possibly related to the general reorganization of cytoplasmic organelles, to the maturation of the cortical lattice, and to cell growth. Tubulin concentration decreases regularly in both the apical and the medio-basal domains. This developmental change in tubulin concentration could be due to the regression of the kinocilium that occurs by the end of ciliogenesis, and, as argued for actin, to the general reorganization of cytoplasmic organelles, and to cell growth.

Actins↗

PubMed1996

Morphological indications of hair cell neodifferentiation in the organ of Corti of amikacin treated rat pups.

Postnatal rats were treated with amikacin during the period of cochlear supra-normal sensitivity to ototoxic antibiotics, i.e. from day 9 to day 16 when the organ of Corti is achieving its maturation. Scanning and transmission electron microscopies were used to evaluate morphological changes in the organs of Corti at different post-treatment periods, up until postnatal day 90. A massive loss in outer (OHCs) and inner (IHCs) hair cells was observed from as early as day 17. A prominent feature in the apical part of cochleas at days 21 and 35 was the transient presence of small atypical cells in the region of pre-existing OHCs. These atypical cells did not have the appearance of degenerating OHCs, but had tuffs of microvilli reminiscent of nascent stereociliary bundles. A second striking observation was the presence of pear-shaped cells in the place normally occupied by IHCs. These cells had neither cuticular plate nor stereocilia, but their basal pole was contacted by both afferent and efferent fibers, as in the early stages of IHC maturation. Such efferent contacts are very rare in the adult cochlea. These findings, not previously reported in antibiotic poisoned cochleas of adult animals, suggest that an attempt at sensory cell neodifferentiation occurs in vivo in the developing organ of Corti intoxicated with aminoglycoside antibiotics.

Amikacin↗

PubMed1995

Structural and ultrastructural aspects of isolated immature cochlear outer hair cells maintained in short-term culture.

Immature outer hair cells (OHCs), isolated from developing rat cochlea without using proteolytic enzymes, were maintained in short-term culture in a clot of coagulated plasma. Cell viability was assessed by a laser scanning image cytometer, using double-fluorescent labeling. Light and transmission electron microscopy was used to study the morphology of isolated cells. Ten to 60 healthy OHCs were obtained from one cochlea, either as single isolated cells or clusters containing 2-10 cells from the same row. Although dead cells were observed only 1 h after dissociation, there were still viable cells after 6 h. Isolated OHCs were not perfectly cylindrical, due to the immaturity of their cortical structures. One hour after dissociation the ultrastructural organization of the isolated cells was generally well preserved, but this was followed by dilatation of the Golgi apparatus and endoplasmic reticulum. Specific changes in isolated OHCs were also observed at the subsurface cisternae and cuticular plate. Although degenerating OHCs generally showed a classic pattern of necrosis, certain morphological features reminiscent of apoptosis were also observed. This study emphasises the difficulty involved in investigating isolated immature OHCs in vitro and provides a basis for future research into the physiological requirements of isolated immature OHCs.

Animals↗

PubMed1992

Ultrastructure of the horseshoe bat's organ of Corti. I. Scanning electron microscopy.

The organ of Corti of the echolocating horseshoe bat (Rhinolophus rouxi) was investigated with scanning electron microscopy in order to provide a comparison with non-echolocating mammals. Throughout the cochlea of horseshoe bats, each outer hair cell (OHC) possesses three rows of stereocilia and there are no morphological distinctions among the different rows of OHCs. However, there are morphological differences between different regions along the cochlea. In the lower and upper basal turn, the receptor surfaces of OHCs are characterized by extremely wide W-shaped stereocilia bundles and wingshaped cuticular plates. The cuticular plates of OHCs of the middle and outermost rows are arranged parallel to each other. Stereocilia length is only 0.8 microns and there is an exaggerated angle of inclination of the shortest row of stereocilia towards the next taller one. Stereocilia arrangements in the apex of the horseshoe bat's cochlea closely resembles those observed in the midbasal region of the rat cochlea. Inner hair cells (IHC) in the lower basal turn appear specialized. They possess only two rows of stereocilia and only 7-8 stereocilia per row. Their cuticular plates are small and oval and widely separated from one another in the longitudinal direction. IHCs at all other locations possess three and up to four rows of stereocilia and 17-20 stereocilia per row. Their cuticular plates are elongated and closely spaced. The transition from specialized to typical mammalian morphology occurs abruptly (over a distance of about 100-150 microns) at the border between the lower and the upper basal turn. This transition is not accompanied by a change in OHC morphology. In the subsurface of the tectorial membrane, throughout the cochlea, there are distinct imprints of the tallest row of stereocilia of all three rows of OHCs and of the IHCs. Data are discussed in relation to specialized aspects of the cochlear frequency map in horseshoe bats and as possible micromechanical adaptations to ultra-high frequency hearing.

Animals↗

PubMed1992

Ultrastructure of the horseshoe bat's organ of Corti. II. Transmission electron microscopy.

The fine structure of the organ of Corti was investigated in the echolocating horseshoe bat (Rhinolophus rouxi) by transmission electron microscopy. Particular emphasis was placed on the receptor cells and their supporting cells. The receptor cells, inner hair cells (IHC) and outer hair cells (OHC), possess the typical mammalian shape, but OHCs are extremely short (length: 12-15 microns in the basal turn and up to 28-30 microns in the apical turn). The afferent innervation of both types of receptor cells and the efferent innervation of the IHC system conform to the general mammalian scheme; however, confirming earlier reports, an efferent innervation to the OHCs is absent. Throughout the cochlea, IHCs and OHCs possess a single layer of subsurface cisternae. Above the level of the nucleus of the OHCs, the arrangements of the subsurface cisternae and their connection to the lateral cell membrane via pillars are highly regular, whereas in IHCs, the cisternae are of irregular shape and the pillar system is much less distinct. In the basal turn of the cochlea, the attachment sites of the OHCs to the supporting cells possess specialized features: (a) in the reticular lamina, the contact sites of the cuticular plates of OHCs with the outer pillar cells and the Deiters cell phalanges are of exaggerated length, and (b) the cup formation of the Deiters cell body, which houses the bottom of the OHC, has a specialized shape and is packed with electron-dense material and microtubules. The results are discussed in relation to cochlear ultrastructure in other mammals and in the context of active processes in cochlear mechanics.

Afferent Pathways↗

PubMed1992

[Two thousand years of historical study on the words atheroma, atheromatosis, atherosclerosis, arteriosclerosis].

Renowned authors, when studying arterial diseases, use indifferently the words atheroma, atheromatosis, atherosclerosis or arteriosclerosis. The historical record of these words permits the justification of a selective choice. The word atheroma was created by Celsius, two thousand years ago; he have it the meaning of fatty tumor, which was kept till the middle of the nineteenth century. The word atheromatosis appeared on 1815, since J. Hodgson defined so the fatty arterial degeneration. Marchand, from Leipzig, proposed on 1904 the word atherosclerosis which has always been with us. The term arteriosclerosis was well defined on 1833 by Lobstein, from Strasbourg. The authors propose to reserve the word atherosclerosis to the arterial disease described by the WHO, to keep the words atheroma and atheromatosis to anatomical lesions well-defined. Arteriosclerosis is a disease which invades an extensive arterial network, with a particular anatomical image.

Arteriosclerosis↗

PubMed1991

The sensory epithelium and its innervation in the mole rat cochlea.

The mole rat (Spalax ehrenbergi) burrows throughout its life in subterranean tunnels. Several structural and functional features which adapt the mole rat to its habitat have been discerned. The goal of this study was to elucidate the structural basis for adaptation of the auditory end-organ to an environment where low-frequency acoustical signals prevail. For this purpose, cochleae of adult mole rats were studied with light and electron microscopy. Inner hair cells throughout the cochlea, and outer hair cells in the basal (high-frequency) portion of the cochlea, were similar in structure to those seen in other mammals. In contrast, outer hair cells in the apical (low-frequency) portion displayed unique structural features. These features resembled the structure of inner hair cells or immature outer hair cells. The innervation of outer hair cells was most uncommon, in that classical medial efferent innervation was not found throughout the cochlear spiral.

Animals↗

PubMed1990

Carbachol-induced inositol phosphate formation during rat cochlea development.

The age related-intensity developmental pattern of the phosphoinositide breakdown, which leads to the formation of intracellular second messengers, was investigated in rat cochleas by measuring the accumulation of inositol phosphates induced by carbachol in the presence of LiCl. The accumulation of the phosphoinositide metabolites elicited by this muscarinic agonist is very low at post-natal day 1 and particularly large during the period between post-natal days 8 and 14 with a peak around day 12. In the 25-day-old rat cochlea, carbachol induced a 2-fold increase in inositol phosphates (IPs) accumulation, with respect to the basal control level. The apparent affinities of the carbachol-induced IPs responses are 49.6, 31.6 and 36.7 microM in cochleas of 12-, 16- and 25-day-old rats, respectively, thus suggesting that the specific developmental changes are rather due to a modification in the number of muscarinic cholinergic receptors than to alterations of the apparent affinity of carbachol for its receptors. This developmental pattern of carbachol-elicited IPs accumulation reveals a striking time coincidence with both the efferent synaptogenesis at the outer hair cells (OHCs) level and the period of increased sensitivity of OHCs to aminoglycoside toxicity. Phosphoinositide breakdown may, consequently, play a role in the maturation of OHCs and their efferent supply. In addition, the remaining IPs response measured at 25 post-natal days indicates that muscarinic agonist-mediated IPs metabolism also occurs in mature cochlea, and might be involved in the regulation of OHCs motility.

Aging↗

PubMed1990

A M3 muscarinic receptor coupled to inositol phosphate formation in the rat cochlea?

Various neuroactive substances, including excitatory and inhibitory amino acids, biogenic amines and neuropeptides, were tested for their ability to stimulate the inositol phosphate (IPs) cascade in the presence of lithium in the rat cochlea. Among them, only the muscarinic agonists (carbachol and oxotremorine M) were able to stimulate the IPs formation in 12-day-old rat cochleas. The carbachol-elicited IPs formation was inhibited by muscarinic antagonists with the following relative order of potency: atropine greater than 4-DAMP much greater than pirenzepine greater than methoctramine = AF-DX 116. This pharmacological profile suggests that the activation of the M3 muscarinic receptor subtype is responsible for the increase in IPs synthesis in the rat cochlea. However, an interaction with a m5 receptor subtype could not be completely excluded. The unusual link of only one receptor subtype with the phosphoinositide breakdown in the cochlea, as opposed to the usual existence of several receptors coupled to this transduction system in other organs such as the brain, suggest a unique role for muscarinic agonists in the cochlea.

Animals↗

PubMed1990

Ototoxic and nephrotoxic drugs inhibit agonist-induced inositol phosphate formation in rat brain synaptoneurosomes.

Neomycin (an aminoglycoside antibiotic), ethacrynate (a loop diuretic), cisplatin (an anticancer drug) and mercuric chloride are chemically unrelated drugs which present similar ototoxic and nephrotoxic properties. We have found that all these molecules inhibit inositol phosphate turnover induced by carbachol or glutamate in rat brain synaptoneurosomes. Since this second messenger system appears to be a key mechanism for cell functioning and even survival, our observations raise the possibility that the expression of the specific toxicity of these compounds may result from excessive inhibition of the phosphoinositide cascade.

Animals↗