Molecular pathways involved in apoptotic cell death in the injured cochlea: Cues to novel therapeutic strategies

Most hearing loss results from lesions of the sensory cells and/or neurons of the auditory portion of the inner ear. To date, only the cochlear implantation offers long-term hearing-aid benefit, but still with limited performance and expensive cost. While the underlying causes of deafness are not clear, the death or hair cells and/or neurons and the loss of neuronal contacts are key pathological features. Pinpointing molecular events that control cell death in the cochlea is critical for the development of new strategies to prevent and treat deafness, whether in combination or not with cochlear implant therapy

Activation of protein kinase CbetaI constitutes a new neurotrophic pathway for deafferented spiral ganglion neurons

In mammals, degeneration of peripheral auditory neurons constitutes one of the main causes of sensorineural hearing loss. Unfortunately, to date, pharmacological interventions aimed at counteracting this condition have not presented complete effectiveness in protecting the integrity of cochlear neural elements. In this context, the protein kinase C (PKC) family of enzymes are important signalling molecules that play a role in preventing neurodegeneration after nervous system injury. The present study demonstrates, for the first time, that the PKC signalling pathway is directly neurotrophic to axotomised spiral ganglion neurons (SGNs). We found that PKC beta I was strictly expressed by postnatal and adult SGNs both in situ and in vitro. In cultures of SGNs, we observed that activators of PKC, such as phorbol esters and bryostatin 1, induced neuronal survival and neurite regrowth in a manner dependent on the activation of PKC beta I. The neuroprotective effects of PKC activators were suppressed by pre-treatment with LY294002 (a PI3K inhibitor) and with U0126 (a MEK inhibitor), indicating that PKC activators promote the survival and neurite outgrowth of SGNs by both PI3K/Akt and MEK/ERK-dependent mechanisms. In addition, whereas combining the neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3) was shown to provide only an additive effect on SGN survival, the interaction between PKC and neurotrophin signalling gave rise to a synergistic increase in SGN survival. Taken together, the data indicate that PKC beta I activation represents a key factor for the protection of the integrity of neural elements in the cochlea

New insights into peripherin expression in cochlear neurons

Peripherin is an intermediate filament protein that is expressed in peripheral and enteric neurons. In the cochlear nervous system, peripherin expression has been extensively used as a differentiation marker by preferentially labeling the type II neuronal population at adulthood, but yet without knowing its function. Since the expression of peripherin has been associated in time with the process of axonal extension and during regeneration of nerve fibers in other systems, it was of interest to determine whether peripherin expression in cochlear neurons was a static phenotypic trait or rather prone to modifications following nerve injury. In the present study, we first compared the expression pattern of peripherin and beta III-tubulin from late embryonic stages to the adult in rat cochlea. The staining for both proteins was seen before birth within all cochlear neurons. By birth, and for 2 or 3 days, peripherin expression was gradually restricted to the type II neuronal population and their projections. In contrast, from postnatal day (P) 10 onwards, while the expression of beta III-tubulin was still found in projections of all cochlear neurons, only the type I population had beta III-tubulin immunoreactivity in their cell bodies. We next investigated the expression of peripherin in axotomized cochlear neurons using an organotypic explant model. Peripherin expression was surprisingly re-expressed in a vast majority of neurons after axotomy. In parallel, the expression and localization of beta III-tubulin and peripherin in dissociated cultures of cochlear neurons were studied. Both proteins were distributed along the entire neuronal length but exhibited complementary distribution, especially within the projections. Moreover, peripherin immunoreactivity was still abundant in the growth cone, whereas that of beta III-tubulin was decreasing at this compartment. Our findings are consistent with a model in which peripherin plays an important structural role in cochlear neurons and their projections during both development and regenerative processes and which is compatible with the assumption that frequently developmentally regulated factors are reactivated during neuronal regeneration

Mechanisms of cell death in the injured auditory system: Otoprotective strategies

peer reviewedOxidative stress insults such as neurotrophin withdrawal, sound trauma, hypoxia/ischemia, ototoxic antibiotics, and chemotherapeutic agents have been shown to induce apoptosis of both auditory hair cells and neurons. In this paper, we review some components of the apoptotic pathways leading to the death of hair cells and auditory induced by growth factor withdrawal or cisplatin intoxication: (1) reactive oxygen species and free radicals are formed as by-products of several metabolic pathways and these molecules can themselves cause cell damage by reacting with cellular proteins; (2) activation of caspases, and (3) activation of calpain. These mechanisms have several different points at which inhibitors could be targeted to protect cells from programmed cell death, including the prevention of oxidative stress-induced apoptosis and the activation of caspases and calpains. Copyright (C) 2002 S. Karger AG, Basel

Differential expression and dynamic changes of murine NEDD9 in progenitor cells of diverse tissues.

International audienceNEDD9 is a scaffolding protein in the integrin signaling pathway that is involved in cell adhesion dynamics. Little is known of the cellular localization of NEDD9 expression during embryonic development. In the present study, we have analyzed NEDD9 mRNA expression in the mouse and identified new relevant expression sites. In addition, we have characterized NEDD9 protein expression pattern for the first time in mammals. At E9.5-E10.5, high levels of Nedd9 and the neurogenic transcription factor neurogenin-2 (Ngn2) were found to largely overlap in two discrete domains of the trunk neural tube along its dorso-ventral axis, with Nedd9 extending to more ventral regions of the ventricular zone and Ngn2 differentially expressed in neuronally committed progenitors of the intermediate zone. At encephalic and trunk levels of the neural tube, NEDD9 was present in Sox2(+) progenitor cell populations mostly generating Ngn2(+) and/or Nurr1(+) cells. A sharp down-regulation of NEDD9 expression was found in cells upon lineage commitment, as observed in Nurr1(+) and Ngn2(+) mesencephalic dopaminergic and brainstem neuronal progenitors. In other tissues/organs, i.e. prospective heart, retina, olfactory epithelium, gonads, cartilage, gut and pituitary gland, NEDD9 was found to be co-expressed with Sox2, RXR alpha and/or Nurr1-like proteins, suggesting that NEDD9 expression is confined to early progenitors involved in diverse organogenesis and that it may depend on the repertoire and levels of retinoic acid co-receptors expressed by those cells

Striatal PSA-NCAM(+) precursor cells from the newborn rat express functional glycine receptors

Immunocytochemical analysis showed that ionotropic glycine receptors are expressed in neurogenic progenitors purified from the newborn rat striatum and expressing the polysialylated form of the neural cell adhesion molecule, both in vitro and in situ. To ascertain whether glycine receptors were functional in vitro, whole-cell patch-clamp recordings demonstrated that glycine triggers inward strychnine-sensitive currents in the majority of these cells. Moreover, we found that glycine receptors expressed by these neurogenic progenitors display intermediate electrophysiological characteristics between those of glycine receptors expressed by neural stem cells and by mature interneurons from the rat striatum. Altogether, the present data show that functional strychnine-sensitive glycine receptors are expressed in neurogenic progenitors purified from the newborn rat striatum

A role for the canonical nuclear factor-κB pathway in coupling neurotrophin-induced differential survival of developing spiral ganglion neurons

Neurotrophins are key players of neural development by controlling cell death programs. However, the signaling pathways that mediate their selective responses in different populations of neurons remain unclear. In the mammalian cochlea, sensory neurons differentiate perinatally into type I and type II population both expressing TrkB and TrkC, which bind respectively brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3). How these two neuronal populations respond differentially to these two neurotrophins remains unknown. Here, we report in rat the segregation of the NFκB subunit p65 specifically within the type II population postnatally. Using dissociated cultures of embryonic and postnatal spiral ganglion neurons, we observed a specific requirement of NFκB for BDNF- but not NT3-dependent neuronal survival during a particular postnatal time window that corresponds to a period of neuronal cell death and hair cell innervation refinement in the developing cochlea. Consistently, postnatal p65 knockout mice showed a specific decreased number in type II spiral ganglion neurons. Taken together, these results identify NFκB as a type II neuron-specific factor that participates in the selective survival effects of BDNF and NT3 signaling on developing spiral ganglion neurons

Human bone marrow harbors cells with neural crest-associated characteristics like human adipose and dermis tissues.

Adult neural crest stem-derived cells (NCSC) are of extraordinary high plasticity and promising candidates for use in regenerative medicine. Several locations such as skin, adipose tissue, dental pulp or bone marrow have been described in rodent, as sources of NCSC. However, very little information is available concerning their correspondence in human tissues, and more precisely for human bone marrow. The main objective of this study was therefore to characterize NCSC from adult human bone marrow. In this purpose, we compared human bone marrow stromal cells to human adipose tissue and dermis, already described for containing NCSC. We performed comparative analyses in terms of gene and protein expression as well as functional characterizations. It appeared that human bone marrow, similarly to adipose tissue and dermis, contains NESTIN+ / SOX9+ / TWIST+ / SLUG+ / P75NTR+ / BRN3A+/ MSI1+/ SNAIL1+ cells and were able to differentiate into melanocytes, Schwann cells and neurons. Moreover, when injected into chicken embryos, all those cells were able to migrate and follow endogenous neural crest migration pathways. Altogether, the phenotypic characterization and migration abilities strongly suggest the presence of neural crest-derived cells in human adult bone marrow

Role of ephrin-A5 forward signaling in the survival of post-natal auditory neurons

In mammals, degeneration of peripheral auditory neurons constitutes one of the prime causes of the sensorineural hearing loss. We believe that studying the developing auditory portion of the inner ear can provide important insights to mechanisms that would regulate regeneration after injury. In that context, our work has focused on a family of proteins, ephrins and Eph receptors, since they have been shown to have a dynamic influence in regulating developmental functions including axonal growth and guidance, synaptic formation and functions and neurogenesis. We have shown that ephrin-A5, EphA4 and EphA7 are expressed in the cell bodies of the spiral ganglion neurons, at different stages of the cochlear development. Ephrin-A5 also seems to be present in the hair cells of the organ of Corti. On the one hand, we demonstrated that activation of the signaling pathway by complexed ephrin-A5 in spiral ganglion explants reduced neuronal survival. On the other hand, an increase of neuronal survival was observed when blocking this mechanism of transduction by monomeric ephrin-A5. Taken together, these results suggest a role for ephrin/Eph signaling in regulating neuronal population generated in the spiral ganglion, especially during the post-natal period of the cochlear innervation