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M Knipper

Publications and source records attributed to M Knipper.

At least 19 recordsLinked to original sources
PubMed2006

Two classes of outer hair cells along the tonotopic axis of the cochlea.

The molecular basis of high versus low frequency hearing loss and the differences in the sensitivity of outer hair cells depending on their cochlear localization are currently not understood. Here we demonstrate the existence of two different outer hair cell phenotypes along the cochlear axis. Outer hair cells in low frequency regions exhibit early sensitivity for loss of Ca(v)1.3 (alpha1 subunit 1.3 forming the class D L-type voltage-gated Ca(2+) channel), while high frequency regions display a progressive susceptibility for loss of the Ca(2+)-activated large conductance K(+) (BK) channel. Despite deafness, young Ca(v)1.3-deficient mice displayed distortion-product otoacoustic emissions (DPOAEs), indicating functional outer hair cells in the higher frequency range of the cochlea. Considering that DPOAEs are also found in the human deafness syndrome DFNB9 caused by mutations in the synaptic vesicle protein otoferlin, we tested the expression of otoferlin in outer hair cells. Surprisingly, otoferlin showed a distinct tonotopic expression pattern at both the mRNA and protein level. Otoferlin-expressing, Ca(v)1.3 deletion-sensitive outer hair cells in the low frequency range could be clearly separated from otoferlin-negative, BK deletion-sensitive outer hair cells in the high frequency range. In addition, BK deletion led to a higher noise vulnerability in low frequency regions, which are normally unaffected by the BK deletion alone, suggesting that BK currents are involved in survival mechanisms of outer hair cells under noise conditions. Our findings propose new mechanisms and candidate genes for explaining high and low frequency hearing loss.

Acoustic Stimulation↗
PubMed2003

Localization of the calcium channel subunits Cav1.2 (alpha1C) and Cav2.3 (alpha1E) in the mouse organ of Corti.

Voltage-activated Ca2+ channels play an important role in synaptic transmission, signal processing and development. The immunohistochemical localization of Cav1.2 (alpha1C) and Cav2.3 (alpha1E) Ca2+ channels was studied in the developing and adult mouse organ of Corti using subunit-specific antibodies and fluorescent secondary antibodies with cochlear cryosections. Cav1.2 immunoreactivity has been detected from postnatal day 14 (P14) onwards at the synapses between cholinergic medial efferents and outer hair cells as revealed by co-staining with anti-synaptophysin and anti-choline acetyltransferase. Most likely the Cav1.2 immunoreactivity was located presynaptically at the site of contact of the efferent bouton with the outer hair cell which suggests a role for class C L-type Ca2+ channels in synaptic transmission of the medial efferent system. The localization of the second Ca2+ channel tested, Cav2.3, showed a pronounced change during cochlear development. From P2 until P10, Cav2.3 immunoreactivity was found in the outer spiral bundle followed by the inner spiral bundle, efferent endings and by medial efferent fibers. Around P14, Cav2.3 immunoreactivity disappeared from these structures and from P19 onwards it was observed in the basal poles of the outer hair cell membranes.

Animals↗
PubMed2001

Thyroid hormone-deficient period prior to the onset of hearing is associated with reduced levels of beta-tectorin protein in the tectorial membrane: implication for hearing loss.

The genes for alpha- and beta-tectorin encode the major non-collagenous proteins of the tectorial membrane. Recently, a targeted deletion of the mouse alpha-tectorin gene was found to cause loss of cochlear sensitivity (). Here we describe that mRNA levels for beta-tectorin, but not alpha-tectorin, are significantly reduced in the cochlear epithelium under constant hypothyroid conditions and that levels of beta-tectorin protein in the tectorial membrane are lower. A delay in the onset of thyroid hormone supply prior to onset of hearing, recently described to result in permanent hearing defects and loss of active cochlear mechanics (), can also lead to permanently reduced beta-tectorin protein levels in the tectorial membrane. beta-Tectorin protein levels remain low in the tectorial membrane up to one year after the onset of thyroid hormone supply has been delayed until postnatal day 8 or later and are associated with an abnormally structured tectorial membrane and the loss of active cochlear function. These data indicate that a simple delay in thyroid hormone supply during a critical period of development can lead to low beta-tectorin levels in the tectorial membrane and suggest for the first time that beta-tectorin may be required for development of normal hearing.

Animals↗
PubMed2000

Molecular characterization of anion exchangers in the cochlea.

Anion exchange proteins (AE) in the inner ear have been the focus of attention for some time. They have been suggested to play a role as anion exchangers for the regulation of endolymphatic pH or as anion exchangers and anchor proteins for the maintenance of the shape and turgor of outer hair cells, and they also have been discussed as a candidate protein for motile hair cell responses that follow high-frequency stimulation. The existence of anion exchangers in hair cells and the specific isoforms which are expressed in hair cells and the organ of Corti is controversial. Using a polyclonal antibody to AE1 (AB 1992, Chemicon), we immunoprecipitated a 100 kDa AE polypeptide in isolated outer hair cells which, due to its glycosylation, is comprised of AE2 than AE1 isoforms. We confirmed AE2 expression in outer hair cells with the help of subtype-specific monoclonal and polyclonal antibodies to AE, AE subtype-specific primers and AE subtype-specific cDNA and found glycosylated truncated as well as full-length AE2 isoforms. No AE1 or AE3 subtypes were noted in outer hair cells. In contrast, AE2 and AE3 but not AE1 subtypes were seen in supporting cells of the organ of Corti. Their expression preceded the development of cochlear function, coincident with the establishment of the endocochlear potential and the differentiation of supporting cells. While most developmental processes in the inner ear usually begin in the basal cochlear turn, the AE2 expression in outer hair cells (but not that of AE2 and AE3 in supporting cells) progressed from the apical to the basal cochlear turn, reminiscent of the maturation of frequency-dependency. Irrespective of their presumed individual role as either anion exchanger, anchor protein or motility protein, the differential expression and developmental profile of these proteins suggest a most important role of anion exchange proteins in the development of normal hearing. These findings may also provide novel insights into AE function in general.

Animals↗
PubMed2000

Neurotrophin 3 potentiates glutamatergic responses of IHC afferents in the cochlea in vivo.

Neurotrophins have traditionally been regarded as slow-acting signals essential for neuronal survival and differentiation. Recent studies with neuronal slices, cultures and nerve ending preparations have shown that neurotrophins generate acute changes in nerve activity. Among the secondary sensory cells are the inner hair cells (IHC) and taste buds, cells which express the neurotrophic factors necessary for the survival of their innervating neurons. If in these cells neurotrophins acutely affect the nerve activity of their afferent neurons, as in the central nervous system (CNS), this may have important functional implications for the corresponding sensory transduction processes. The neurotrophin NT-3 has been reported to be expressed in IHCs. We chose an in vivo application system for the microiontophoretic supply of NT-3 in the subsynaptic region of the IHC. The effect of NT-3 on spontaneous and evoked afferent cochlear nerve activities in adult guinea pig inner ear was studied. We observed that NT-3 rapidly increases the spontaneous and glutamate-evoked firing rate of IHC afferents. Moreover, firing induced by both N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) were specifically enhanced during the presence of NT-3, a process which was selectively blocked by the tyrosine kinase receptor inhibitor K252a. Because we localized NT-3 mRNA not only in IHCs but also in the spiral ganglion, we propose that similar to other sensory systems, afferent and autocrine neurotrophin activities may be responsible for survival of cochlear neurons. In addition, NT-3 in IHCs may operate as a signal-dependent, intrinsic neuromodulator and/or neuroprotector.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed2000

Thyroid hormone deficiency before the onset of hearing causes irreversible damage to peripheral and central auditory systems.

Both a genetic or acquired neonatal thyroid hormone (TH) deficiency may result in a profound mental disability that is often accompanied by deafness. The existence of various TH-sensitive periods during inner ear development and general success of delayed, corrective TH treatment was investigated by treating pregnant and lactating rats with the goitrogen methimazole (MMI). We observed that for the establishment of normal hearing ability, maternal TH, before fetal thyroid gland function on estrus days 17-18, is obviously not required. Within a crucial time between the onset of fetal thyroid gland function and the onset of hearing at postnatal day 12 (P12), any postponement in the rise of TH-plasma levels, as can be brought about by treating lactating mothers with MMI, leads to permanent hearing defects of the adult offspring. The severity of hearing defects that were measured in 3- to 9-mo-old offspring could be increased with each additional day of TH deficiency during this critical period. Unexpectedly, the active cochlear process, assayed by distortion product otoacoustic emissions (DPOAE) measurements, and speed of auditory brain stem responses, which both until now were not thought to be controlled by TH, proved to be TH-dependent processes that were damaged by a delay of TH supply within this critical time. In contrast, no significant differences in the gross morphology and innervation of the organ of Corti or myelin gene expression in the auditory system, detected as myelin basic protein (MBP) and proteolipid protein (PLP) mRNA using Northern blot approach, were observed when TH supply was delayed for few days. These classical TH-dependent processes, however, were damaged when TH supply was delayed for several weeks. These surprising results may suggest the existence of different TH-dependent processes in the auditory system: those that respond to corrective TH supply (e.g., innervation and morphogenesis of the organ of Corti) and those that do not, but require T3 activity during a very tight time window (e.g. , active cochlear process, central processes).

Animals↗
PubMed1999

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

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

Animals↗
PubMed1999

A changing pattern of brain-derived neurotrophic factor expression correlates with the rearrangement of fibers during cochlear development of rats and mice.

The reorganization of specific neuronal connections is a typical feature of the developing nervous system. It is assumed that the refinement of connections in sensory systems requires spontaneous activity before the onset of cochlear function and selective sensory experience during the ensuing period. The mechanism of refinement through sensory experience is currently postulated as being based on the selective reinforcement of active projections by neurotrophins. We studied a presumed role of neurotrophins for rearrangement of afferent and efferent fibers before the onset of sensory function in the precisely innervated auditory end organ, the cochlea. We observed a spatiotemporal change in the localization of brain-derived neurotrophic factor (BDNF) protein and mRNA, which correlated with the reorganization of fibers. Thus, BDNF decreased in target hair cells during fiber retraction and was subsequently upregulated in neurons, target hair cells, and adjacent supporting cells concomitant with the formation of new synaptic contacts. Analysis of the innervation pattern in BDNF gene-deleted mice by immunohistochemistry and confocal microscopy revealed a failure in the rearrangement of fibers and a BDNF dependency of distinct neuronal projections that reorganize in control animals. Our data suggest that, before the onset of auditory function, a spatiotemporal change in BDNF expression in sensory, epithelial, and neuronal cells may guide the initial steps of refinement of the innervation pattern.

Afferent Pathways↗
PubMed1999

Distinct thyroid hormone-dependent expression of TrKB and p75NGFR in nonneuronal cells during the critical TH-dependent period of the cochlea.

Analyzing the thyroid hormone (TH)-dependent period of the inner ear, we observed that the presence of triiodothyronine (T3) between postnatal day 3 (P3) and P12 is sufficient for functional maturation of the auditory system. Within this short time period, an unusual transient TH-dependent expression of nonneuronal neurotrophin receptors (NT-R) trkB and p75(NGFR) was observed in correlation with neuronal and morphogenetic processes. The availability of thyroid hormone was revealed to be invariably correlated with (a) a transient expression of full-length trkB in TRalpha1-, TRalpha2- and TRbeta1-expressing hair cells concomitant to the segregation of afferent fibers and the synaptogenesis of efferent fibers; and (b) a transient expression of p75(NGFR) in TRalpha1- and TRbeta1-expressing great epithelia ridge cells in direct spatiotemporal correlation with the appearance of apoptotic cells and morphogenetic maturation of the organ. For the first time, these data suggest a TH dependency of the expression of neurotrophin receptors in nonneuronal cells. A potential role of these peculiar neurotrophin receptor expression for the conversion of the biological function of TH on innervation patterning and morphogenesis during the critical TH-dependent period of the inner ear may be considered.

Animals↗
PubMed1998

Thyroid hormone affects Schwann cell and oligodendrocyte gene expression at the glial transition zone of the VIIIth nerve prior to cochlea function.

All cranial nerves, as well as the VIIIth nerve which invades the cochlea, have a proximal end in which myelin is formed by Schwann cells and a distal end which is surrounded by oligodendrocytes. The question which arises in this context is whether peripheral and central parts of these nerves myelinate simultaneously or subsequently and whether the myelination of either of the parts occurs simultaneously at the onset of the cochlea function and under the control of neuronal activity. In the present paper, we examined the relative time course of the myelinogenesis of the distal part of the VIIIth nerve by analyzing the expression of peripheral protein P0, proteolipid protein and myelin basic protein. To our surprise, we observed that the expression of myelin markers in the peripheral and central part of the intradural part of the VIIIth nerve started simultaneously, from postnatal day 2 onwards, long before the onset of cochlea function. The expression rapidly achieved saturation levels on the approach to postnatal day 12, the day on which the cochlea function commenced. Because of its importance for the neuronal and morphological maturation of the cochlea during this time, an additional role of thyroid hormone in cochlear myelinogenesis was considered. Indeed, it transpires that this hormone ensures the rapid accomplishment of glial gene expression, not only in the central but also in the peripheral part of the cochlea. Furthermore, an analysis of the thyroid hormone receptors, TRaplha and TRbeta, indicates that TRbeta is necessary for myelinogenesis of the VIIIth nerve. Rapid thyroid hormone-dependent saturation of myelin marker gene expression in Schwann cells and oligodendrocytes of the VIIIth nerve may guarantee nerve conduction and synchronized impulse transmission at the onset of hearing. The thyroid hormone-dependent commencement of nerve conduction is discussed in connection with the patterning refinement of central auditory pathways and the acquisition of deafness.

Animals↗
PubMed1997

Transient expression of NMDA receptors during rearrangement of AMPA-receptor-expressing fibers in the developing inner ear.

A major reorganization of afferent and efferent nerve terminals, concomitant to significant neuronal cell loss and pruning of superfluous fibers, takes place during the development of the organ of Corti, prior to the onset of hearing. We examined the spatio/temporal distribution of subtype-specific AMPA- and N-methyl-d-aspartate (NMDA)-selective glutamate receptor proteins in postnatal inner ears from rats during this critical period. From the first postnatal day onwards, GluR2/3 receptor subtypes appeared in nerve endings of afferent fibers associated with inner and outer hair cells. During the following 2 weeks, GluR2/3 receptors were downregulated in exchange for GluR4 receptors. In parallel efferents projecting from the medial olivocochlear complex to the outer hair cells underwent synaptogenesis and efferents projecting from the lateral olivocochlear complex to the inner hair cells appeared to change contacts to the dendrites of afferents. Concomitant to these events, NMDA receptor subtypes NR1 and NR2A transiently appeared in hair cells as well as afferent and efferent fibers. Recently, we described a temporary expression of the neurotrophin receptor trkB in hair cells, coincident to the growth (GAP-43) and synaptogenesis (synaptophysin) of efferents. Here, we show that trkB was expressed together with NR1 receptors in hair cells in high spatio/temporal correlation with the rearrangement of afferents and efferents. Cochlea NMDA receptors may, therefore, be a part of the mechanism by which, in addition to neurotrophic activity, the mature phenotype of cochlea neurons is acquired through activity-dependent processes.

Animals↗
PubMed1997

A new twist in an old story: the role for crosstalk of neuronal and trophic activity.

A number of recent findings suggest a reciprocal interaction between neurotransmitters and neurotrophins functioning at the level of the synapse, which may be relevant not only for plasticity changes in the mature nervous system, but also for the development of synaptic connectivity and for survival or maturation of neurons prior to target contact. Thus, neurotrophin-induced attenuation of frequency-dependent depletion of releasable synaptic vesicle pools of neurotransmitter at synapses may participate in Hebbian and non-Hebbian forms of LTP, as a characteristic of mature synaptic contacts. Subsequent to nerve/target contact, neurotrophins also appear to mediate contact-induced enhancement of neurotransmitter release; this may participate in a developmental improvement of synapse efficacy, stabilization of synaptic contacts, and maturation of "conductive" functional synapses. Coincident with a transmitter-induced elevation of cytosolic Ca2+ levels within growth cones, a local neurotrophin-mediated increase in released neurotransmitter occurring subsequent to stabilization of a distinct synaptic contact may then participate in the refinement of synapses with retention of those neurites affected by neurotrophins and withdrawal of those neurites not affected by neurotrophins. Finally, prior to nerve/target contact, Ca2+ channel-generated spontaneous neuronal activity as well as co-expression of neurotrophins and their receptors may play a role in maturational changes.

Animals↗
PubMed1996

Expression of neurotrophin receptor trkB in rat cochlear hair cells at time of rearrangement of innervation.

The spatio-temporal distribution of the high-affinity neurotrophin receptor trkB was monitored during postnatal development of the rat cochlea. In addition to expression in presumptive afferent type I collaterals, afferent type II fibers, and efferent fibers, trkB immunoreactivity also transiently appeared in the sensory hair cells themselves, from postnatal days 5-9 in the basal turn, and from postnatal days 9-13 in the apical turn. A comparison of trkB with p75(NGFR), which is expressed in afferent and efferent fibers, and GAP-43 and synaptophysin, which are expressed in efferent fibers, revealed a time/space correlation of trkB receptor expression in hair cells with the rearrangement of their innervation. Co-expression of the neurotrophin receptor and its ligand has been proposed to be functionally involved in regulating the survival of neurons independent of target-derived neurotrophin factor. Thus, the presence of trkB in target hair cells, suggests that auto/paracrine mechanisms play a role during this critical period of rearrangement of neural connections.

Animals↗
PubMed1995

Synaptophysin and GAP-43 proteins in efferent fibers of the inner ear during postnatal development.

A rearrangement of afferent and efferent fibers occurs in the postnatal development of the inner ear. Growth and synaptogenesis was explored during this critical period by immunohistochemically monitoring the expression of GAP-43 and synaptophysin. Both proteins were colocalized in efferent fibers beyond postnatal day 3 (pn3). Two distinct synaptophysin- and GAP-43-positive fibers innervated different parts of inner hair cells in the first and second postnatal weeks, respectively. GAP-43-positive efferents projecting to outer hair cells upregulated synaptophysin with base to apex gradient between postnatal day 5 and postnatal day 14. In efferents projecting to outer hair cells GAP-43 was downregulated about 6 days beyond synaptogenesis. In efferents projecting to inner hair cells, however, GAP-43 remained upregulated even beyond pn18, indicating continuous synapse replacement of this fiber type. Both proteins thus improved as excellent markers for growth and synaptogenesis of distinct postnatal efferent fibers.

Animals↗
PubMed1995

Immunological identification of candidate proteins involved in regulating active shape changes of outer hair cells.

By employing immunological methods, it has been demonstrated that myosin, myosin light chain (MLC) and myosin light chain kinase (MLCK) proteins in outer hair cells (OHC) are immunologically different from isoforms in platelets, smooth muscle and heart muscle, and are probably more related to isoforms found in red blood cells (RBC). Moreover, proteins related to band 3 protein (b3p) and protein 4.1 (p 4.1), ankyrin as well as fodrin and spectrin, but not glycophorin, have been identified in isolated OHCs. Both OHCs and RBC differ from other motile non-muscle cells in their lack of smooth muscle isoforms of actin, their common high levels of spectrin-, ankyrin- and band 3-like proteins, as well as the expression of the 80 kDa protein 4.1 isoform. The data support the notion that motility of OHC may be based upon regulation of the b3p/p 4.1/ankyrin complex, and thus may be reminiscent to the active shape changes in RBC.

Actins↗
PubMed1994

Short-term modulation of glutamatergic synapses in adult rat hippocampus by NGF.

Nerve growth factor promotes survival and differentiation of selected groups of neurones through long-term adaptive changes in cell function. While production of this neurotrophin in target cells in hippocampus is regulated in part by afferent glutamate neuronal input, we report now for the first time that nerve growth factor has a direct potentiating effect on spontaneous and depolarization-evoked release of glutamate from hippocampal nerve endings, accompanied by an increase in the excitatory postsynaptic potential of CA1 neurones. This correlated with increased incorporation of [32P]phosphate into synapsin I. Reciprocal positive feedback between production of trophic factors in target cells by neurotransmitter released from afferent neuronal inputs and the modulation of presynaptic neurotransmitter release by target-derived trophic factors provides a novel mechanism for short-term control over synaptic communication.

Animals↗
PubMed1994

Positive feedback between acetylcholine and the neurotrophins nerve growth factor and brain-derived neurotrophic factor in the rat hippocampus.

In the rat hippocampus, nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are synthesized by neurons in an activity-dependent manner. Glutamate receptor activation increases whereas GABAergic stimulation decreases NGF and BDNF mRNA levels. Here we demonstrate that NGF and BDNF mRNA and NGF protein are up-regulated in the rat hippocampus by the activation of muscarinic receptors. Conversely, NGF and BDNF enhance the release of acetylcholine (ACh) from rat hippocampal synaptosomes containing the nerve endings of the septal cholinergic neurons. NGF also rapidly increases the high-affinity choline transport into synaptosomes. The reciprocal regulation of ACh, NGF and BDNF in the hippocampus suggests a novel molecular framework by which the neurotrophins might influence synaptic plasticity.

Acetylcholine↗