The effects of laminin-1 on spiral ganglion neurons are dependent on the MEK/ERK signaling pathway and are partially independent of Ras

Canonical MAPK signaling in auditory neuropathy

Auditory neuropathy (AN) is an under-recognized form of hearing loss characterized by lesions in inner hair cells (IHCs), ribbon synapses and spiral ganglion neurons (SGNs). The lack of a targeted therapy for AN has increased the need for a better understanding of the pathogenic mechanism of AN. As mitogen-activated protein kinase (MAPK) signaling is ubiquitous in many biological processes, its alteration may facilitate the pathogenesis of multiple sites in AN. Here, we summaries the characteristics of AN under different molecular bases and first explore the mechanism of MAPK at different lesion sites. Alterations of extracellular signal-regulated kinase (ERK)/MAPK occur in IHCs and SGNs, whereas modulations of p38 and c-Jun NH2-terminal kinase (JNK) were found in ribbon synapses and SGNs. In conclusion, inductive MAPK alterations in the pathogenesis and development of AN are likely to represent a potential therapeutic target to guide the development of treatments.

Interaction of micropatterned topographical and biochemical cues to direct neurite growth from spiral ganglion neurons

Functional outcomes with neural prosthetic devices, such as cochlear implants, are limited in part due to physical separation between the stimulating elements and the neurons they stimulate. One strategy to close this gap aims to precisely guide neurite regeneration to position the neurites in closer proximity to electrode arrays. Here, we explore the ability of micropatterned biochemical and topographic guidance cues, singly and in combination, to direct the growth of spiral ganglion neuron (SGN) neurites, the neurons targeted by cochlear implants. Photopolymerization of methacrylate monomers was used to form unidirectional topographical features of ridges and grooves in addition to multidirectional patterns with 90^o angle turns. Microcontact printing was also used to create similar uni- and multi-directional patterns of peptides on polymer surfaces. Biochemical cues included peptides that facilitate (laminin, LN) or repel (EphA4-Fc) neurite growth. On flat surfaces, SGN neurites preferentially grew on LN-coated stripes and avoided EphA4-Fc-coated stripes. LN or EphA4-Fc was selectively adsorbed onto the ridges or grooves to test the neurite response to a combination of topographical and biochemical cues. Coating the ridges with EphA4-Fc and grooves with LN lead to enhanced SGN alignment to topographical patterns. Conversely, EphA4-Fc coating on the grooves or LN coating on the ridges tended to disrupt alignment to topographical patterns. SGN neurites respond to combinations of topographical and biochemical cues and surface patterning that leverages both cues enhance guided neurite growth.

Photopolymerized micropatterns with high feature frequencies overcome chemorepulsive borders to direct neurite growth

Developing and regenerating neurites respond to a variety of biophysical and biochemical cues in their micro-environment to reach target cells and establish appropriate synapses. Defining the hierarchal relationship of both types of cues to direct neurite growth carries broad significance for neural development, regeneration, and, in particular, engineering of neural prostheses that improve tissue integration with native neural networks. In this work, chemorepulsive biochemical borders are established on substrates with a range of surface microfeatures to determine the potential of physical cues to overcome conflicting biochemical cues. Physical micropatterns are fabricated using photomasking techniques to spatially control photoinitiation events of the polymerization. Temporal control of the reaction allows for generation of microfeatures with the same amplitude across a range of feature frequencies or periodicities. The micropatterned substrates are then modified with repulsive chemical borders between laminin and either EphA4-Fc or tenascin C that compete with the surface microfeatures to direct neurite growth. Behaviour of neurites from spiral ganglion and trigeminal neurons is characterized at biochemical borders as cross, turn, stop, or repel events. Both the chemical borders and physical patterns significantly influence neurite pathfinding. On unpatterned surfaces, most neurites that originate on laminin are deterred by the border with tenascin C or EphA4-Fc. Importantly, substrates with frequent micropattern features overcome the influence of the chemorepulsive border to dominate neurite trajectory. Designing prosthesis interfaces with appropriate surface features may allow for spatially organized neurite outgrowth in vivo even in the presence of conflicting biochemical cues in native target tissues.

The expression pattern and inhibitory influence of Tenascin-C on the growth of spiral ganglion neurons suggest a regulatory role as boundary formation molecule in the postnatal mouse inner ear

Sensorineural hearing loss, as a consequence of acoustic trauma, aging, genetic defects or ototoxic drugs, is highly associated with irreversible damage of cochlear hair cells (HCs) and secondary degeneration of spiral ganglion (SG) cells. Cochlear implants (CIs), which bypass the lost HC function by direct electrical stimulation of the remaining auditory neurons, offer an effective therapy option. Several studies imply that components of the extracellular matrix (ECM) have a great impact on the adhesion and growth of spiral ganglion neurons (SGNs) during development. Based on these findings, ECM proteins might act as bioactive CI substrates to optimize the electrode-nerve interface and to improve efficacy of these implants. In the present study, we focused on the ECM glycoproteins Tenascin-C (TN-C), Laminin (LN), and Fibronectin (FN), which show a prominent expression along the growth route of SGNs and the niche around HCs during murine postnatal development in vivo. We compared their influence on adhesion, neurite length, and neurite number of SGNs in vitro. Moreover, we studied the expression of the chondroitin sulfate proteoglycan (CSPG) dermatan sulfate-dependent proteoglycan-1 (DSD-1-PG), an interaction partner of TN-C. In sum, our in vitro data suggest that TN-C acts as an anti-adhesive and inhibitory factor for the growth of SGNs. The DSD-1 carbohydrate epitope is specifically localized to HC stereocilia and SG fibers. Interestingly, TN-C and the DSD-1-PG exhibit a mutually exclusive expression pattern, with the exception of a very restricted region beneath the habenula perforata, where SG neurites grow through the basilar membrane (BM) toward the HCs. The complementary expression of TN-C, LN, FN, and the DSD-1 epitope suggests that TN-C may act as an important boundary formation molecule in the developing postnatal mouse inner ear, which makes it a promising candidate to regulate neurite outgrowth in the light of CIs.

Novel High Content Screen Detects Compounds That Promote Neurite Regeneration from Cochlear Spiral Ganglion Neurons

The bipolar spiral ganglion neurons (SGN) carry sound information from cochlear hair cells to the brain. After noise, antibiotic or toxic insult to the cochlea, damage to SGN and/or hair cells causes hearing impairment. Damage ranges from fiber and synapse degeneration to dysfunction and loss of cells. New interventions to regenerate peripheral nerve fibers could help reestablish transfer of auditory information from surviving or regenerated hair cells or improve results from cochlear implants, but the biochemical mechanisms to target are largely unknown. Presently, no drugs exist that are FDA approved to stimulate the regeneration of SGN nerve fibers. We designed an original phenotypic assay to screen 440 compounds of the NIH Clinical Collection directly on dissociated mouse spiral ganglia. The assay detected one compound, cerivastatin, that increased the length of regenerating neurites. The effect, mimicked by other statins at different optimal concentrations, was blocked by geranylgeraniol. These results demonstrate the utility of screening small compound libraries on mixed cultures of dissociated primary ganglia. The success of this screen narrows down a moderately sized library to a single compound which can be elevated to in-depth in vivo studies, and highlights a potential new molecular pathway for targeting of hearing loss drugs.

Inhibition of MMP-2 but not MMP-9 influences inner ear spiral ganglion neurons in vitro

Matrix metalloproteinases (MMPs) play an important role in modeling of the extracellular matrix. There is increasing evidence that these proteases are important in neurite elongation and axonal guidance during development in the central nervous system and retina. Moreover, they are also expressed after acute injury and can be the key mediators of pathogenesis. However, the role of MMPs in the inner ear is largely unknown. Our group recently demonstrated that general inhibition of MMPs resulted in auditory hair cell loss in vitro. In the present study, we investigated the role of MMPs in inner ear spiral ganglion neuron (SGN) survival, neuritogenesis and neurite extension by blocking MMPs known to be involved in axonal guidance, neurite elongation, and apoptosis in other neuronal systems. Spiral ganglion (SG) explants from 5-day-old Wistar rats were treated with different concentrations of the general MMP inhibitor GM6001, a specific MMP-2 inhibitor, and a specific MMP-9 inhibitor, in vitro. The general inhibitor of MMPs and the specific inhibition of MMP-2 significantly reduced both the number of neurites that extended from SG explants, as well as the length of individual neurites. However, neither the general inhibitor of MMPs nor the specific inhibition of MMP-2 influenced SGN survival. Inhibition of MMP-9 had no influence on SGNs. The data suggest that MMPs, and more specifically MMP-2, influence the growth of developing afferent neurites in the mammalian inner ear in vivo.

Lithium alters the morphology of neurites regenerating from cultured adult spiral ganglion neurons

The small-molecule drug lithium (as a monovalent ion) promotes neurite regeneration and functional recovery, is easy to administer, and is approved for human use to treat bipolar disorder. Lithium exerts its neuritogenic effect mainly by inhibiting glycogen synthase kinase 3, a constitutively-active serine/threonine kinase that is regulated by neurotrophin and "wingless-related MMTV integration site" (Wnt) signaling. In spiral ganglion neurons of the cochlea, the effects of lithium and the function of glycogen synthase kinase 3 have not been investigated. We, therefore, set out to test whether lithium modulates neuritogenesis from adult spiral ganglion neurons. Primary cultures of dissociated spiral ganglion neurons from adult mice were exposed to lithium at concentrations between 0 and 12.5 mM. The resulting neurite morphology and growth-cone appearance were measured in detail by using immunofluorescence microscopy and image analysis. We found that lithium altered the morphology of regenerating neurites and their growth cones in a differential, concentration-dependent fashion. Low concentrations of 0.5-2.5 mM (around the half-maximal inhibitory concentration for glycogen synthase kinase 3 and the recommended therapeutic serum concentration for bipolar disorder) enhanced neurite sprouting and branching. A high concentration of 12.5 mM, in contrast, slowed elongation. As the lithium concentration rose from low to high, the microtubules became increasingly disarranged and the growth cones more arborized. Our results demonstrate that lithium selectively stimulates phases of neuritogenesis that are driven by microtubule reorganization. In contrast, most other drugs that have previously been tested on spiral ganglion neurons are reported to inhibit neurite outgrowth or affect only elongation. Lithium sensitivity is a necessary, but not sufficient condition for the involvement of glycogen synthase kinase 3. Our results are, therefore, consistent with, but do not prove lithium inhibiting glycogen synthase kinase 3 activity in spiral ganglion neurons. Experiments with additional drugs and molecular-genetic tools will be necessary to test whether glycogen synthase kinase 3 regulates neurite regeneration from spiral ganglion neurons, possibly by integrating neurotrophin and Wnt signals at the growth cone.

Structure and development of cochlear afferent innervation in mammals

In mammals, sensorineural deafness results from damage to the auditory receptors of the inner ear, the nerve pathways to the brain or the cortical area that receives sound information. In this review, we first focused on the cellular and molecular events taking part to spiral ganglion axon growth, extension to the organ of Corti, and refinement. In the second half, we considered the functional maturation of synaptic contacts between sensory hair cells and their afferent projections. A better understanding of all these processes could open insights into novel therapeutic strategies aimed to re-establish primary connections from sound transducers to the ascending auditory nerve pathways.

Laminin-1 induces neurite outgrowth in human mesenchymal stem cells in serum/differentiation factors-free conditions through activation of FAK-MEK/ERK signaling pathways

Mesenchymal stem cells (MSCs) can be differentiated into cell types derived from all three germ layers by manipulating culture conditions in vitro. A multitude of growth and differentiation factors have been employed for driving MSCs towards a neuronal phenotype. In the present study, we investigated the potential of extracellular matrix (ECM) proteins-fibronectin, collagen-1, collagen-IV, laminin-1, and laminin-10/11, to induce a neuronal phenotype in bone marrow derived human MSCs in the absence of growth factors/differentiating agents. All of the ECM proteins tested were found to support adhesion of MSCs to different extents. However, direct interaction only with laminin-1 triggered sprouting of neurite-like processes. Cells plated on laminin-1 exhibited neurite out growth as early as 3h, and by 24h, the cells developed elaborate neurites with contracted cell bodies and neuronal-like morphology. Function-blocking antibodies directed against alpha6 and beta1 integrin subunits inhibited neurite formation on laminin-1 which confirmed the involvement of integrin alpha6beta1 in neurite outgrowth. Mechanistic studies revealed that cell adhesion to laminin-1 activated focal adhesion kinase (FAK), and mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (MEK/ERK) signaling pathways. Abrogation of FAK phosphorylation by herbimycin-A inhibited neurite formation and also decreased activities of MEK and ERK. Pharmacological inhibitors of MEK (U0126) and ERK (PD98059) also blocked neurite outgrowth in cells plated on laminin-1. Our study demonstrates the involvement of integrin alpha6beta1 and FAK-MEK/ERK signaling pathways in laminin-1-induced neurite outgrowth in MSCs in the absence of serum and differentiation factors.

Hearing development and spiral ganglion neurite growth in VASP deficient mice

Vasodilator-stimulated phosphoprotein (VASP) has been found to be involved in intracellular signalling pathways and to play an important role in the actin associated organization and formation of the cytoskeleton. Since differential VASP expression was noted in inner ear tissues, the present study was performed to investigate the hearing development in VASP deficient mice. Hearing development in VASP-/- mice and wild type animals was investigated by auditory brain stem (ABR) measurements. In addition, inner ear tissues of wild type animals were tested for VASP expression using PCR, Western blot analysis, in situ hybridisation, and immunohistochemistry. To compare spiral ganglion (SG) neurite growth, SG explants from VASP-/- and wild type mice were analyzed under cell culture conditions. The electroacoustical results of the present study indicate that VASP deficient mice present with a later onset of hearing during postnatal development compared to wild type animals. Transient VASP expression was detected in neonatal SG of wild type mice. Tissue culture experiments with SG explants from VASP-/- animals revealed significant alterations in SG neurite extension compared to wild types. The present findings suggest a role for VASP during neonatal development of the mammalian cochlea and allow speculation on a possible delayed innervation of cochlear hair cells due to changes in SG neurite growth in VASP-deficient mice. Temporary VASP deficits in the neonatal inner ear may be compensated by related proteins like MENA leading to a delayed but complete development of hearing function in VASP-/- animals.

Interaction of Cochlear Nucleus Explants With Semiconductor Materials

OBJECTIVE/HYPOTHESIS

Implantable hearing devices such as cochlear implants and auditory brainstem implants deliver auditory information through electrical stimulation of auditory neurons. The combination of microelectronic electrodes with auditory nerve cells may lead to further improvement of the hearing quality with these devices. Whereas several kinds of neurons are known to grow on semiconductor substrates, interactions of cochlear nucleus (CN) neurons with such materials have yet to be described.

MATERIALS AND METHODS

To investigate survival and growth behavior of CN neurons on different semiconductor materials. CN explants from postnatal day 10 Sprague-Dawley rats were cultured for 96 hours in Neurobasal medium on polished and unpolished silicon wafers (p-type Si [100] and p-type Si3N4[100]) as well as plastic surface. These surfaces had been coated with poly-L-lysine and laminin. Neuronal outgrowth was examined using image analysis software after immunohistologic staining for neurofilament. Neurite length and directional changes were quantified. Additionally, neurite morphology and adhesion to the semiconductor material was evaluated by scanning electron microscopy.

RESULTS

Although proper adhesion of CN explants was seen, no neurite growth could be detected on unpolished silicon wafers (Si and Si3N4). Compared with the other test conditions, polished, laminin-coated Si3N4 wafers showed best biocompatibility regarding neurite length and number per explant. CN explants developed a mean of eight neurons with an average length of 236 mum in 96 hours of culture on these wafers.

CONCLUSION

The results of this study demonstrate the general possibility of CN neuron growth in culture on semiconductors in vitro. The differences in neuron length and number per explant indicate that the growth of CN neurons is influenced by the semiconductor substrate as well as extracellular matrix proteins, with laminin-coated p-type Si3N4[100] being a preferable material for future hybrid experiments on auditory-neuron semiconductor chips.

Sustained ERK1/2 but not STAT1 or 3 activation is required for thanatophoric dysplasia phenotypes in PC12 cells

Mutations in fibroblast growth factor receptor 3 (FGFR3) cause the most common genetic form of short-limbed dwarfism, achondroplasia (ACH), as well as neonatal lethal forms, thanatophoric dysplasia (TD) I and II. The causative mutations induce graded levels of constitutive activation of the receptor that correspond to the severity of the disorder, resulting in premature entry into hypertrophic differentiation and reduced proliferation of chondrocytes in developing cartilage. Although FGFR3 promotes growth in most tissues, it is a negative regulator of endochondral bone growth. Several signaling pathways have been implicated in these skeletal disorders including the Ras/MEK/ERK pathway and the JAK/STAT, the latter in the most severe phenotypes, however their functional relevance remains incompletely understood. Using PC12 cell lines stably expressing inducible mutant receptors containing the TDII mutation, K650E, sustained activation of ERK1/2 and activation of STAT1 and STAT3, but not STAT5, is observed in the absence of ligand. This activation leads to neurite outgrowth, a phenotypic readout of constitutive receptor activity, and sustained ERK1/2 activity is required for this ligand-independent differentiation. To assess the functional relevance of STAT activation induced by the mutant receptor, STATs were specifically downregulated using RNA-interference. Silencing of STAT1 or 3 independently or in combination had no significant effect on ligand-independent neurite outgrowth, ERK1/2 activation or p21(WAF1/CIP1) protein levels. These results support a model in which sustained activation of ERK1/2 is a key regulator of the increased transition to hypertrophic differentiation of the growth plate, whereas activation of STATs 1 and 3 is not required.

Signal transduction pathways implicated in neural recognition molecule L1 triggered neuroprotection and neuritogenesis

The signal transduction pathways involved in adhesion molecule L1-triggered neuritogenesis and neuroprotection were investigated using the extracellular domain of mouse or human L1 in fusion with the Fc portion of human immunoglobulin G or L1 purified from mouse brain by affinity chromatography. Substrate L1-triggered neuritogenesis and neuroprotection depended on distinct but also overlapping signal transduction pathways and on the expression of L1 at the neuronal cell surface. PI3 kinase inhibitors, Src family kinase inhibitors as well as mitogen-activated protein kinase kinase inhibitors reduced both L1-triggered neuritogenesis and neuroprotection. In contrast, fibroblast growth factor receptor inhibitors, a protein kinase A inhibitor, and an inhibitor of cAMP-mediated signal transduction pathways, blocked neuritogenesis, but did not affect L1-triggered neuroprotection. Proteolytic cleavage of L1 or its interaction partners is necessary for both L1-mediated neuritogensis and neuroprotection. Furthermore, L1-triggered neuroprotection was found to be associated with increased phosphorylation of extracellular signal-regulated kinases 1/2, Akt and Bad, and inhibition of caspases. These observations suggest possibilities of differentially targeting signal transduction pathways for L1-dependent neuritogenesis and neuroprotection.

High expression of PKC-MAPK pathway mRNAs correlates with glomerular lesions in human diabetic nephropathy

BACKGROUND

Activation of protein kinase C (PKC) is a major signaling pathway for transforming growth factor (TGF)-beta to induce extracellular matrix (ECM) production in diabetic nephropathy (DN). PKC also activates mitogen-activated protein kinase (MAPK), which is called the PKC-MAPK pathway. The PKC-MAPK pathway is probably responsible for PKC-related abnormalities in diabetic glomeruli. To confirm the involvement of this pathway, we determined the localization and expression of mRNAs in glomeruli by in situ hybridization method.

METHODS

In the present study, we examined expression of PKCbeta1, MAPK/ERK kinase (MEK) 1, MEK2, extracellular signal-regulated protein kinase (ERK) 1, ERK2, and TGF-beta1 mRNAs using renal tissue samples from kidneys affected by DN (N= 21) and from normal human kidney (NHK; N= 6). We also performed an immunohistochemical study using anti-phosphorylated MEK1/2 (P-MEK) and ERK1/2 (P-ERK) antibodies. The glomerular severity of DN was classified into three groups according to mesangial expansion: D1 (N= 4), D2 (N= 13), and D3 (N= 4). We analyzed differences and correlations between variables.

RESULTS

In the glomeruli, the number of cells that stained for these mRNAs in DN was significantly higher than in NHK. The expression of PKC-MAPK pathway mRNAs tended to be inversely proportional to the degree of mesangial expansion. The P-MEK and P-ERK signal intensity were parallel to its mRNA expression pattern. Furthermore, there were significant correlations among the P-MEK, P-ERK signal intensity, PKCbeta1 mRNA expression.

CONCLUSION

Our results suggest that high expression of PKC-MAPK pathway mRNAs plays an important role in the development and/or progression of early tissue damage in DN.

Interaction of spiral ganglion neuron processes with alloplastic materials in vitro(1)

The cochlear implant (CI) involves the introduction of alloplastic materials into the cochlea. While current implants interact with cochlear neurons at a distance, direct interactions between spiral ganglion (SG) neurites and implants could be fostered by appropriate treatment with neurotrophic factors. The interactions of fibroblasts and osteoblasts with alloplastic materials have been well studied in vitro and in vivo. However, interactions of inner ear neurons with such alloplastic materials have yet to be described. To investigate survival and growth behavior of SG neurons on different materials, SG explants from post-natal day 5 rat SG were cultured for 72 h in the presence of neurotrophin-3 (10 ng/ml) on titanium, gold, stainless steel, platinum, silicone and plastic surfaces that had been coated with laminin and poly-L-lysine. Neurite outgrowth was investigated after immunohistological staining for neurofilament, by image analysis to determine neurite extension and directional changes. Neurite morphology and adhesion to the alloplastic material were also evaluated by scanning electron microscopy (SEM). On titanium, SG neurites reached the highest extent of outgrowth, with an average length of 662 microm and a mean of 31 neurites per explant, compared to 568 microm and 21 neurites on gold, 574 microm and 24 neurites on stainless steel, 509 microm and 16 neurites on platinum, 281 microm and 12 neurites on silicone and 483 microm and 31 neurites on plastic. SEM revealed details of adhesion of neurites and interaction with non-neuronal cells. The results of this study indicate that the growth of SG neurons in vitro is strongly influenced by alloplastic materials, with titanium exhibiting the highest degree of biocompatibility with respect to neurite extension. The knowledge of neurite interaction with different alloplastic materials is of clinical interest, as development in CI technology leads to closer contact of implanted electrodes with surviving inner ear neurons.