Protein kinase C and mouse sciatic nerve regeneration

Riluzole, a neuroprotective agent, preserves erectile function following bilateral cavernous nerve injury in male rats

Neurogenic erectile dysfunction is a highly prevalent complication in men undergoing radical prostatectomy. The underlying mechanisms remain incompletely defined and the effective therapy has been limited. This study aimed to evaluate the protective effect of riluzole and the role of PKC β and excitatory amino acid transporters (EAATs) mediating this effect in a rat model of bilateral cavernous injury (BCNI). A total of 48 male Sprague-Dawley rats were divided into sham, BCNI (at 7, 15 days post-injury) and BCNI treated with riluzole (8 mg/kg/day) groups. Erectile function was measured as maximum intracavernosal pressure (mICP)/mean arterial pressure (MAP) and total ICP/MAP. Changes in protein expressions of phospho (p)-PKC β IIser660 and EAATs were analysed in penis and major pelvic ganglion with western blotting. BCNI decreased erectile function at 7 and 15 days post-injury (mICP/MAP at 4 V: 0.45 ± 0.06 vs 0.84 ± 0.07; 0.34 ± 0.04 vs 0.77 ± 0.04 respectively; p < 0.001) whereas riluzole treatment (for 15 days) preserved erectile function (mICP/MAP at 4 V: 0.62 ± 0.03 vs 0.34 ± 0.04; p < 0.01). The decline in the expression of p-PKC β IIser660 was observed in penis at 7 and 15 days post-injury (p = 0.0003, p = 0.0033), which was prevented by riluzole treatment for 15 days (p = 0.0464). While expressions of EAAT-1 and EAAT-2 decreased in major pelvic ganglion following BCNI (p = 0.0428, p = 0.002), riluzole treatment for 15 days prevented the decrease only in EAAT-2 expression (p = 0.0456). Riluzole improved erectile function via possibly interacting with PKC β II and glutamatergic pathways, as a potential therapeutic candidate for erectile dysfunction.

A small molecule screen identifies in vivo modulators of peripheral nerve regeneration in zebrafish

Adult vertebrates have retained the ability to regenerate peripheral nerves after injury, although regeneration is frequently incomplete, often leading to functional impairments. Small molecule screens using whole organisms have high potential to identify biologically relevant targets, yet currently available assays for in vivo peripheral nerve regeneration are either very laborious and/or require complex technology. Here we take advantage of the optical transparency of larval zebrafish to develop a simple and fast pectoral fin removal assay that measures peripheral nerve regeneration in vivo. Twenty-four hours after fin amputation we observe robust and stereotyped nerve regrowth at the fin base. Similar to laser mediated nerve transection, nerve regrowth after fin amputation requires Schwann cells and FGF signaling, confirming that the fin amputation assay identifies pathways relevant for peripheral nerve regeneration. From a library of small molecules with known targets, we identified 21 compounds that impair peripheral nerve regeneration. Several of these compounds target known regulators of nerve regeneration, further validating the fin removal assay. Twelve of the identified compounds affect targets not previously known to control peripheral nerve regeneration. Using a laser-mediated nerve transection assay we tested ten of those compounds and confirmed six of these compounds to impair peripheral nerve regeneration: an EGFR inhibitor, a glucocorticoid, prostaglandin D2, a retinoic acid agonist, an inhibitor of calcium channels and a topoisomerase I inhibitor. Thus, we established a technically simple assay to rapidly identify valuable entry points into pathways critical for vertebrate peripheral nerve regeneration.

Early spatiotemporal progress of myelinated nerve fiber regenerating through biological chitin conduit after injury

Chitin conduits to bridge nerve injuries with a small gap (2 mm) give us a biological window in which the peripheral nerve regeneration process can be observed. In this study, the regeneration process was observed on different intervals postoperatively. Histological analysis revealed that the early regeneration process occurred in three phase: degeneration and matrix phase, axonal and Schwann migration phase, myelination and maturation phase. Schwann cells grew into the lumen from both the proximal and distal nerve segments. Axonal regrowth progressed at an average rate of 1.4 mm/d. Regenerating axonal fibers were myelinated by Schwann cells from both sides of the conduit.

Protein kinase C regulates neurite outgrowth in spinal cord neurons

OBJECTIVE

The functional roles of protein kinase C (PKC) in the neurite outgrowth and nerve regeneration remain controversial. The present study was aimed to investigate the role of PKC in neurite outgrowth, by studying their regulatory effects on neurite elongation in spinal cord neurons in vitro.

METHODS

The anterior-horn neurons of spinal cord from embryonic day 14 (E14) Sprague-Dawley (SD) rats were dissociated, purified and cultured in the serum-containing medium. The ratio of membrane-PKC (mPKC) activity to cytoplasm-PKC (cPKC) activity (m/c-PKC) was studied at different time points during culture.

RESULTS

Between 3-11 d of culture, the change of m/c-PKC activity ratio and PKC-betaII expression in the neurite were both significantly correlated with neurite outgrowth (r=0.95, P< 0.01; r=0.73, P< 0.01, respectively). Moreover, PMA, an activator of PKC, induced a dramatic elevation in the m/c-PKC activity ratio, accompanied with the increase in neurite length (r=0.99, P< 0.01). In contrast, GF 109203X, an inhibitor of PKC, significantly inhibited neurite elongation, which could not be reversed by PMA.

CONCLUSION

PKC activity may be important in regulating neurite outgrowth in spinal cord neurons, and betaII isoform of PKC probably plays a major role in this process.

Peripheral sensory axon growth: from receptor binding to cellular signaling

Regeneration following axonal injury of the adult peripheral sensory nervous system is heavily influenced by factors located in a neuron's extracellular environment. These factors include neurotrophins, such as Nerve Growth Factor (NGF) and the extracellular matrix, such as laminin. The presence of these molecules in the peripheral nervous system (PNS) is a major contributing factor for the dichotomy between regenerative capacities of central vs. peripheral neurons. Although PNS neurons are capable of spontaneous regeneration, this response is critically dependent on many different factors including the type, location and severity of the injury. In this article, we will focus on the plasticity of adult dorsal root ganglion (DRG) sensory neurons and how trophic factors and the extracellular environment stimulate the activation of intracellular signaling cascades that promote axonal growth in adult dorsal root ganglion neurons.

Laminin and growth factor receptor activation stimulates differential growth responses in subpopulations of adult DRG neurons

Neurons in the adult rat dorsal root ganglion (DRG) can be classified into at least three separate subpopulations based on morphologic and phenotypic differences. In this study we have focused on the growth response of these specific subpopulations in vitro with respect to laminin (LN) and growth factor receptor activation. Using a cell selection approach we show that LN-induced neurite growth occurs in the absence of added trophic factors only in heavy-chain neurofilament-positive and calcitonin gene-related peptide-positive DRG neurons [nerve growth factor (NGF)-responsive population]. In contrast, LN alone is not sufficient to stimulate significant neurite growth from lectin Griffonia simplicifolia IB4-positive neurons (IB4+ve), although it is still required to elicit a growth response from these cells in the presence of glial-derived neurotrophic factor (GDNF, e.g. neurite growth occurred only when cells were plated on LN in the presence of GDNF). By using chemical inhibitors we demonstrate that only the phosphatidylinositol 3 kinase (PI 3-K)/Akt pathway is required for neurite growth from the NGF-responsive cell population. However, both the PI 3-K/Akt and MEK/mitogen-activated protein kinase signaling pathways are required for neurite growth from the IB4+ve cell population. Thus, we have identified specific signaling events and environmental requirements associated with neurite growth for different subpopulations of adult DRG neurons, pointing to potential therapeutic targets while identifying an inability for any one treatment alone to repair peripheral nerve damage.

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 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.

Mitogen-activated protein kinase inhibition reveals differences in signalling pathways activated by neurotrophin-3 and other growth-stimulating conditions of adult mouse dorsal root ganglia neurons

PD98059 blocks mitogen-activated protein kinase (MAPK) by inhibiting its activator, MAP kinase kinase (MEK). We have previously found that PD98059 only transiently inhibits spontaneous axonal outgrowth from adult mouse dorsal root ganglia (DRG) explants, whereas it causes sustained inhibition of nerve growth factor (NGF)-stimulated growth. Surprisingly, the present results showed that outgrowth stimulation by neurotrophin-3 (NT-3), interacting with another neuronal subgroup, was markedly enhanced by PD98059 and also by U0126, another inhibitor of MAPK activation. In contrast, the effects of glial cell line-derived neurotrophic factor (GDNF), which stimulates still another subgroup of DRG neurons, was opposed by PD98059. Axonal outgrowth in vitro can also be strongly increased by a prior axotomy in vivo. The increased outgrowth in preaxotomized explants was effectively inhibited by the presence of PD98059. Immunocytochemistry based on whole-mount labelling revealed the presence of neuronal MAPK, which was found to be activated by NGF, NT-3, and GDNF in separate axonal populations and by a prior axotomy in a majority of growing axons. The results suggest that there are important differences in the NGF and NT-3 signalling pathways, which may involve positive and negative control mechanisms by MAPK activation, respectively. Other findings indicate that GDNF exerts its growth effects by activation of MAPK and that expression of the conditioning effect in vitro in preaxotomized preparations also requires activation of MAPK.

Mitogen activated protein kinase inhibition by PD98059 blocks nerve growth factor stimulated axonal outgrowth from adult mouse dorsal root ganglia in vitro

Nerve growth factor stimulated axonal outgrowth from explanted mouse dorsal root ganglia is dependent on mitogen activated protein kinase. PD98059 ([2-(2'amino-3'-methoxyphenyl)-oxanaphthalen-4-one]) blocks mitogen activated protein kinase by inhibiting its immediate upstream activator, mitogen activated protein kinase kinase (also known as MEK). Here we used PD98059 to study the role of mitogen activated protein kinase in the axonal outgrowth of adult dorsal root ganglia explants. Whereas PD98059 at 50 microM left spontaneous axonal outgrowth unaffected, it markedly inhibited nerve growth factor stimulated axon growth when assessed after two days in culture. A mitogen activated protein kinase assay and immunoblotting using antibodies discriminating between activated and inactivated kinase, both confirmed that PD98059 reduced the amount of activated enzyme in nerve growth factor stimulated preparations, while the total amounts of the kinase remained unchanged. Immunohistochemistry revealed the presence of neuronal mitogen activated protein kinase kinase and mitogen activated protein kinase itself. The latter enzyme was found to be activated in the growing axons, as seen by whole-mount labelling. At the ganglionic level activated mitogen activated protein kinase was preferentially detected in satellite cells. The results show that nerve growth factor stimulated axonal outgrowth in vitro from adult mouse dorsal root ganglia utilizes the mitogen activated protein kinase pathway.

Axonal outgrowth from adult mouse nodose ganglia in vitro is stimulated by neurotrophin-4 in a Trk receptor and mitogen-activated protein kinase-dependent way

The actions of neurotrophic factors on sensory neurons of the adult nodose ganglion were studied in vitro. The ganglia were explanted in an extracellular matrix-based gel that permitted observation of the growing axons. Neurotrophin-4 (NT-4) was a very efficient stimulator of outgrowth of axons from the nodose ganglion and had almost doubled the outgrowth length when this was analyzed after 2 days in culture. Brain-derived neurotrophic factor also stimulated outgrowth, but to a lesser degree, whereas NT-3 gave only weak stimulatory tendencies. Nerve growth factor and glial cell line-derived neurotrophic factor both lacked stimulatory effects. NT-4 is known to act via TrkB receptors, and the presence of these on growing nodose neurons was demonstrated immunohistochemically. In line with a Trk-mediated growth effect, the NT-4 stimulation was abolished by K252a, a selective inhibitor of neurotrophin receptor-associated tyrosine kinase activity. K252a had no effect on the unstimulated preparation. NT-4 treatment led to activation of the mitogen-activated protein kinase and inhibition of the latter pathway by PD98059 significantly reduced the NT-4 stimulated outgrowth, whereas the drug had no effect on the unstimulated growth. In conclusion, the data suggest that NT-4 can serve as a powerful growth factor for neurons of adult nodose ganglia and that the growth stimulation involves TrkB- and mitogen-activated protein kinase.

Facial motor neuron regeneration induces a unique spatial and temporal pattern of myristoylated alanine-rich C kinase substrate expression

We have previously shown that the myristoylated alanine-rich C kinase substrate, a primary protein kinase C substrate in brain that binds and cross-links filamentous actin, is enriched in neuronal growth cones and is developmentally regulated in brain. Here we examined myristoylated alanine-rich C kinase substrate expression in the facial motor nucleus during axonal regeneration following facial nerve axotomy or facial nerve resection lesions, which impede regeneration, or following motor neuron degeneration induced by the retrograde neurotoxin ricin. For comparative purposes, the protein kinase C substrates myristoylated alanine-rich C kinase substrate-like protein and growth-associated protein-43 were examined in parallel. Myristoylated alanine-rich C kinase substrate messenger RNA exhibited a robust increase in both neurons and non-neuronal cells in the facial motor nucleus beginning four days after axotomy, peaked at seven days (2.5-fold), and declined back to baseline levels by 40 days. Myristoylated alanine-rich C kinase substrate protein similarly exhibited a twofold elevation in the facial motor nucleus determined four and 14 days post-axotomy. Following nerve resection, myristoylated alanine-rich C kinase substrate messenger RNA levels increased at seven days and returned to baseline levels by 40 days. Unlike myristoylated alanine-rich C kinase substrate messenger RNA, myristoylated alanine-rich C kinase substrate-like messenger RNA levels did not increase in the facial motor nucleus at any time point following nerve axotomy or resection, whereas growth-associated protein-43 messenger RNA exhibited a rapid (one day) and prolonged (40 days) elevation in facial motor nucleus neurons following either nerve axotomy or resection. Ricin-induced degeneration of facial motor neurons elevated myristoylated alanine-rich C kinase substrate and myristoylated alanine-rich C kinase substrate-like messenger RNAs in both microglia (lectin-positive) and astrocytes (glial fibrillary acidic protein-positive).Collectively, these data demonstrate that myristoylated alanine-rich C kinase substrate exhibits a unique expression profile in the facial motor nucleus following facial nerve lesions, and it is proposed that myristoylated alanine-rich C kinase substrate may serve to mediate actin-membrane cytoskeletal plasticity in both neurons and glial cells in response to protein kinaseC-mediated signaling during nerve regeneration and degeneration.

Protein kinase C inhibition has only a transient growth arresting effect on in vitro regenerating mouse sensory neurons

Adult mice sensory ganglia were cultured in an extracellular matrix gel. Analyses of extending axons were made 48 h (long-term) or immediately (short-term) after addition of protein kinase inhibitors. Long- and short-term growth was insensitive to protein kinase A/G inhibition by HA-1004. Long-term protein kinase C inhibition by chelerythrine affected only certain, long axons. In the short-term virtually all axon growth was arrested, but largely recovered on the following day. When combined, the drugs inhibited all long- and short-term growth and largely prevented the recovery of the latter. The transient effect by chelerythrine, and the permanent inhibition after combination with HA-1004, suggests compensatory mechanisms, perhaps via other kinases.

Involvement of axonal phospholipase A2 activity in the outgrowth of adult mouse sensory axons in vitro

The effect on axonal outgrowth of inhibition of phospholipase A2 activity was studied in a recently developed in vitro model, where dorsal root ganglia with attached spinal roots and nerve stumps from young adult mice were cultured in an extracellular matrix material (Matrigel). The phospholipase A2 inhibitors 4-bromophenacyl bromide and oleyloxyethyl phosphorylcholine dose-dependently reduced axonal outgrowth from the sciatic nerve stump. A similar inhibitory effect was seen when only the cut nerve end was exposed to the inhibitors in a compartmental culture system. The local effect of phospholipase A2 inhibition was further investigated on axons established in culture, using time-lapse recording. Exposure to phospholipase A2 inhibitors caused the retraction of filopodia extensions and a reduction in growth cone motility within a few minutes. After removal of inhibition, normal growth cone motility and axonal growth were regained. Nerve cell bodies and axons, in contrast to Schwann cells, showed immunoreactivity after staining with an antiserum against secretory phospholipase A2, and elevated levels of the enzyme could be detected after culture for 24 h. The immunoreactive protein was of approximately 170,000 molecular weight (phospholipase A2-170) as determined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and immunoblotting. The localization of phospholipase A2-170 in axons growing into the Matrigel was also demonstrated by use of a whole-mount technique. The results of this study show the importance of continuous phospholipase A2 activity for growth cone motility and axonal outgrowth in the mammalian peripheral nerve, and suggest the involvement of an axonally localized enzyme.

Phosphorylation of CREB in axon-induced Schwann cell proliferation

Axonal contact regulates Schwann cell (SC) proliferation during development. However, the intracellular signal transduction pathways involved in the axon-induced proliferation of SC have not been described. We have previously shown that SC proliferation induced by axolemma-enriched fractions (AEF) is accompanied by increased expression of cyclic AMP-responsive element binding protein, CREB. We now report the AEF and dorsal root ganglion neuritic-induced signal transduction pathway(s) which regulate the phosphorylation of CREB that correlate with the SC proliferative response. The phosphorylated form of CREB was significantly increased after 16 hr of axonal stimulation, continued to increase for 48 hr, and subsequently decreased as monitored by immunocytochemistry and Western blot analysis. Treatment with protein kinase A (PKA) inhibitor, H89, completely abolished both the CREB activation and SC proliferation. In contrast, treatment with protein kinase C (PKC) inhibitor (bisindolylmaleimide) inhibited AEF-induced SC proliferation, but did not immediately affect CREB phosphorylation. These data are consistent with the view that PKA and PKC pathways are essential for AEF-induced SC proliferation. Since PKC can influence SC proliferation without initially affecting CREB phosphorylation, PKC may regulate SC proliferation at pathways distal to the immediate CREB activation.

Taurine-stimulated outgrowth from the retina is impaired by protein kinase C activators and phosphatase inhibitors

Taurine increases neurite elongation of post-crush goldfish retinal explants, as well as the number of outgrowing isolated cells from goldfish and rat retina in culture. The trophic effect of taurine is related to an elevation in calcium flux rather than increased cell proliferation. Since taurine regulates phosphorylation in rat retina, we investigated if this process could be involved in the mechanism of taurine action on outgrowth. Control and taurine-supplemented post-crush goldfish retinal explants were cultured in the presence of protein kinase C activators or phosphatase inhibitors, and the length of neurites was measured after five days in culture. In some cases, there was an inhibition of the stimulatory effect of taurine without a modification in basal outgrowth. In others, outgrowth of control explants was also reduced. A certain level of protein phosphorylation seems to be critical for the trophic effect of taurine in the retina.

Use of explant cultures of peripheral nerves of adult vertebrates to study axonal regeneration in vitro

Explanted preparations of peripheral nerves with attached dorsal root ganglia of adult mammals and amphibia survive for several days in serum-free medium and can be used to study axonal regeneration in vitro. This review outlines the methods which we routinely use and how they may be applied to study different aspects of axonal regeneration. When the peripheral nerves are crushed in vitro, axons regenerate through the crush site into the distal stump within 1 day (mouse) or 3 days (frog). The outgrowth distance of the leading sensory axons can be determined with the use of a simple method based on axonal transport of labelled proteins. A compartmentalised system permits selective application of drugs and other agents to either ganglia or peripheral nerve containing the regenerating axons and has been used to study selected aspects of regeneration including influence of non-neuronal cells, retrograde signalling, axonal release of proteins during regeneration and the role of phospholipase A2 activity. Explanted preparations may also be cultured in a layer of extracellular matrix material (matrigel), in which spontaneous outgrowth of a large number of naked axons from the cut ends of nerves starts within 1 day and continues for several days. This provides an opportunity to study the direct effects of different agents on axonal elongation. Preparations cultured in collagen gels show sparse spontaneous axonal growth, but this can be increased by addition of certain growth factors. The phenotype of the regenerating axons can be studied using immunohistochemical methods.

In vitro phosphorylation of medial vestibular nucleus and prepositus hypoglossi proteins during behavioural recovery from unilateral vestibular deafferentation in the guinea pig

Unilateral removal of vestibular nerve input to the vestibular nuclei (e.g. by unilateral labyrinthectomy, UL) results in severe ocular motor and postural disorders which disappear over time (vestibular compensation). We investigated whether recovery of ocular motor function is temporally correlated with changes in protein phosphorylation in the medial vestibular nucleus (MVN) and prepositus hypoglossi (PH; MVN/PH) in vitro. Bilateral MVN/PH were dissected from 48 guinea pigs following decapitation at 10 h, 53 h or 2 weeks post-UL, or -sham operation and frozen. Tissue extracts were incubated with [gamma-32P]ATP +/- Ca2+ plus phorbol 12,13-dibutyrate and phosphatidylserine. UL resulted in a significant bilateral increase in the 32P-incorporation into a 65-85 kDa band (probably the myristoylated alanine-rich C kinase substrate, MARCKS) in compensated animals (53 h post-UL) under conditions which favoured the activation of protein kinase C. Under identical conditions, the labelling of a 42-49 kDa protein (P46) was increased significantly in the bilateral MVN/PH between either 10 h or 53 h and 2 weeks post-UL; there were no significant changes over time in sham controls. These results show that later stages of vestibular compensation are accompanied by changes in the phosphorylation of several likely protein kinase C substrates in the MVN/PH in vitro.