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 II populations 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 nuclear factor-κB (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.
Abstract 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 betaIII-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 betaIII-tubulin was still found in projections of all cochlear neurons, only the type I population had betaIII-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 betaIII-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 betaIII-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.
In the current study, we have investigated the ability of substance P (SP) to protect 3‐day‐old (P3) rat spiral ganglion neurons (SGNs) from trophic factor deprivation (TFD)‐induced cell death. The ...presence of SP high affinity neurokinin‐1 receptor (NK1) transcripts was detected in the spiral ganglion and the NK1 protein localized to SGNs both ex vivo and in vitro. Treatment with SP increased cytoplasmic Ca2+ in SGNs, further arguing for the presence of functional NK1 on these neurons. Both SP and the agonist Sar9,Met(O2)11‐SP significantly decreased SGN cell death induced by TFD, with no effect on neurite outgrowth. The survival promoting effect of SP was blocked by the NK1 antagonist, WIN51708. Both pan‐caspase inhibitor BOC‐D‐FMK and SP treatments markedly reduced activation of caspases and DNA fragmentation in trophic factor deprived‐neurons. The neuroprotective action of SP was antagonised by specific inhibitors of second messengers, including 1.2‐bis‐(O‐aminophenoxy)‐ethane‐N,N,N′,N′‐tetraacetic acid (BAPTA‐AM) to chelate cytosolic Ca2+, the protein kinase C (PKC) inhibitors bisindolylmaleimide I, Gö6976 and LY333531 and the MAPK/ERK inhibitor U0126. In contrast, nifedipine, a specific inhibitor of l‐type Ca2+ channel, and LY294002, a phosphatidylinositol‐3‐OH kinase (PI3K) inhibitor, had no effect on SP trophic support of SGNs. Moreover, activation of endogenous PKC by 4β‐phorbol 12‐myristate 13‐acetate (PMA) also reduced the loss of trophic factor‐deprived SGNs. Thus, NK1 expressed by SGNs transmit a survival‐promoting regulatory signal during TFD‐induced SGN cell death via pathways involving PKC activation, Ca2+ signalling and MAPK/ERK activation, which can be accounted for by an inhibition of caspase activation.
This review covers the general roles of members of the cysteine protease family of caspases in the process of apoptosis (programmed cell death) looking at their participation in both the "extrinsic" ...cell death receptor and the "intrinsic" mitochondrial cell death pathways. It defines the difference between initiator and effector caspases and shows the progression of caspase activations that ends up in the apoptotic cell death and elimination of a damaged cell. The review then presents what is currently know about the participation of caspases in the programmed cell death of inner ear sensory cells during the process of normal development and maturation of the inner ear and their importance in this process as illustrated by the results of caspase-3 gene knockout experiments. The participation of specific caspases and the sequence of their activation in the elimination (apoptosis) of damaged sensory cells from adult inner ears after an injury that generates oxidative stress are reviewed. Both the possibility and the potential efficacy of caspase inhibition with a broad-spectrum pancaspase inhibitor as an interventional therapy to treat and rescue oxidative stress-damaged inner ear sensory cells from apoptosis are presented and discussed.
Galanin receptor1 (GalR1) transcript levels are elevated in the rat ventral periaqueductal gray (vPAG) after chronic mild stress (CMS) and are related to depression-like behavior. To explore the ...mechanisms underlying the elevated GalR1 expression, we carried out molecular biological experiments in vitro and in animal behavioral experiments in vivo. It was found that a restricted upstream region of the
gene, from -250 to -220, harbors an E-box and plays a negative role in the
promoter activity. The transcription factor Scratch2 bound to the E-box to down-regulate
promoter activity and lower expression levels of the
gene. The expression of Scratch2 was significantly decreased in the vPAG of CMS rats. Importantly, local knockdown of Scratch2 in the vPAG caused elevated expression of GalR1 in the same region, as well as depression-like behaviors. RNAscope analysis revealed that GalR1 mRNA is expressed together with Scratch2 in both GABA and glutamate neurons. Taking these data together, our study further supports the involvement of GalR1 in mood control and suggests a role for Scratch2 as a regulator of depression-like behavior by repressing the
gene in the vPAG.
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 PKCbetaI 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 PKCbetaI. 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 PKCbetaI activation represents a key factor for the protection of the integrity of neural elements in the cochlea.
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.
Type I spiral ganglion neurons (I-SGNs) of the mammalian cochlea convey all acoustic information from the sensory hair cells to second order neurons in the brainstem. Despite evidence supporting ...physiological diversity of I-SGNs and of its importance for encoding the various features of sounds, knowledge of their molecular diversity is only emerging. In this review, we outline the recent efforts in the identification of mammalian I-SGN types and summarize how genetic and anatomical features of each individual neuron type relate to functional aspects that characterize sound information processing in the primary auditory afferent system.
Hypothyroidism due to THRA1 (gene coding for thyroid hormone receptor α1) mutation-mediated Resistance to Thyroid Hormone (RTH) has been recently reported in human and is associated with memory ...deficits similar to those found in a mouse model for Thra1 mutation mediated RTH (Thra1(+/m) mice). Here, we show that a short-term treatment of Thra1(+/m) mice with GABAA receptor antagonist pentylenetetrazol (PTZ) completely and durably rescues their memory performance. In the CA1 region of the hippocampus, improvement of memory is associated with increased in long-term potentiation (LTP) and an augmentation of density of dendritic spines (DDS) onto the apical dendrites of pyramidal cells reflecting an increase in the local excitatory drive. Unbiased gene profiling analysis of hippocampi of treated Thra1(+/+) and Thra1(+/m) mice were performed two weeks and three months post treatment and identified co-expression modules that include differentially expressed genes related with and predicting higher memory, LTP and DDS in the hippocampi of PTZ-treated animals. We observed that PTZ treatment changed similar sets of genes in both Thra1(+/+) and Thra1(+/m) mice, which are known to be involved in memory consolidation and neurotransmission dynamics and could participate in the persistent effects of PTZ on memory recovery.
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.
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