Effects of protein kinase inhibitors on regeneration in vitro of adult frog sciatic sensory axons

The Rise of Molecules Able To Regenerate the Central Nervous System

Injury to the adult central nervous system (CNS) usually leads to permanent deficits of cognitive, sensory, and/or motor functions. The failure of axonal regeneration in the damaged CNS limits functional recovery. The lack of information concerning the biological mechanism of axonal regeneration and its complexity has delayed the process of drug discovery for many years compared to other drug classes. Starting in the early 2000s, the ability of many molecules to stimulate axonal regrowth was evaluated through automated screening techniques; many hits and some new mechanisms involved in axonal regeneration were identified. In this Perspective, we discuss the rise of the CNS regenerative drugs, the main biological techniques used to test these drug candidates, some of the most important screens performed so far, and the main challenges following the identification of a drug that is able to induce axonal regeneration in vivo.

Protein kinase C isoforms at the neuromuscular junction: localization and specific roles in neurotransmission and development

The protein kinase C family (PKC) regulates a variety of neural functions including neurotransmitter release. The selective activation of a wide range of PKC isoforms in different cells and domains is likely to contribute to the functional diversity of PKC phosphorylating activity. In this review, we describe the isoform localization, phosphorylation function, regulation and signalling of the PKC family at the neuromuscular junction. Data show the involvement of the PKC family in several important functions at the neuromuscular junction and in particular in the maturation of the synapse and the modulation of neurotransmission in the adult.

Large-scale in vivo femtosecond laser neurosurgery screen reveals small-molecule enhancer of regeneration

Discovery of molecular mechanisms and chemical compounds that enhance neuronal regeneration can lead to development of therapeutics to combat nervous system injuries and neurodegenerative diseases. By combining high-throughput microfluidics and femtosecond laser microsurgery, we demonstrate for the first time large-scale in vivo screens for identification of compounds that affect neurite regeneration. We performed thousands of microsurgeries at single-axon precision in the nematode Caenorhabditis elegans at a rate of 20 seconds per animal. Following surgeries, we exposed the animals to a hand-curated library of approximately one hundred small molecules and identified chemicals that significantly alter neurite regeneration. In particular, we found that the PKC kinase inhibitor staurosporine strongly modulates regeneration in a concentration- and neuronal type-specific manner. Two structurally unrelated PKC inhibitors produce similar effects. We further show that regeneration is significantly enhanced by the PKC activator prostratin.

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.

Synaptic activity-related classical protein kinase C isoform localization in the adult rat neuromuscular synapse

Protein kinase C (PKC) is essential for signal transduction in a variety of cells, including neurons and myocytes, and is involved in both acetylcholine release and muscle fiber contraction. Here, we demonstrate that the increases in synaptic activity by nerve stimulation couple PKC to transmitter release in the rat neuromuscular junction and increase the level of alpha, betaI, and betaII isoforms in the membrane when muscle contraction follows the stimulation. The phosphorylation activity of these classical PKCs also increases. It seems that the muscle has to contract in order to maintain or increase classical PKCs in the membrane. We use immunohistochemistry to show that PKCalpha and PKCbetaI were located in the nerve terminals, whereas PKCalpha and PKCbetaII were located in the postsynaptic and the Schwann cells. Stimulation and contraction do not change these cellular distributions, but our results show that the localization of classical PKC isoforms in the membrane is affected by synaptic activity.

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.

Staurosporine but not chelerythrine inhibits regeneration in hippocampal organotypic cultures

A mechanical lesion in hippocampal organotypic cultures is followed by a recovery process involving scar formation, sprouting of fibres and formation of new functional synapses. Here we tested the effect of staurosporine and chelerythrine, two protein kinase C (PKC) inhibitors, on this lesion-induced neurite outgrowth of Shaffer collaterals. At a concentration of 1 microM, staurosporine delayed functional recovery assessed by measuring synaptic field potentials across the lesion, without altering synaptic transmission on nonlesioned cultures. Immunostaining carried out by using antibodies directed against neurofilament proteins showed that there was a marked reduction in the number of regenerating fibres crossing the lesion. In contrast to this, chelerythrine (50 microM) did not prevent functional recovery, although it affected synaptic transmission and plasticity at this concentration. We conclude that the inhibition of sprouting produced by staurosporine is independent of its blockade of PKC-mediated phosphorylation mechanisms.

Protein kinase C isozyme expression in sciatic nerves and spinal cords of experimentally diabetic rats

Changes in the expression and activation of protein kinase C (PKC) have been implicated in the pathogenesis of diabetic neuropathy. Recent studies in liver, retina, and cardiovascular tissues from experimentally diabetic rats have demonstrated that diabetes has a selective effect on the expression and subcellular distribution of isozymes of PKC. In the light of this evidence, we investigated the expression of the PKC isozymes alpha, betaI, betaII, and gamma in sciatic nerves, spinal cords, and in the L4,5 dorsal root ganglia from streptozotocin-induced diabetic rats. Six weeks of diabetes had differential effects on the expression and distribution of PKC isozymes in sciatic nerves and spinal cords. In the sciatic nerves there was an apparent translocation of the alpha isoform from the cytosolic to the particulate fractions, the betaII isoform was reduced in the cytosolic fraction, and the betaI and gamma isoforms were unaffected. The changes in the isozyme immunoreactivities in the nerves were not a direct result of changes in either spinal cord or dorsal root ganglia alone, suggesting that diabetes has different effects on motor and sensory fibres and/or on Schwann cells. In nerves that had been crushed 14 days previously there was an increase in total PKC alpha immunoreactivity. This increase was potentiated in diabetic rats. On the other hand, PKC betaII immunoreactivity in crushed nerves was unaffected by diabetes. The data are consistent with diabetes-induced changes in expression of PKC betaII contributing to nerve damage, and changes in PKC alpha being a consequence of it.

Protein kinase C and mouse sciatic nerve regeneration

We have studied the role of protein kinase C (PKC) in peripheral nerve regeneration by using the cultured adult mouse sciatic nerve, which displays regrowth of sensory axons under serum-free conditions. By the use of immunohistochemistry we show that one of the isoforms of PKC, PKC beta, is present in the nerve cell bodies of normal nerves and is upregulated after injury. In spite of this, the specific PKC inhibitor chelerythrine at 5 microM, a concentration well above its IC50 value for PKC, failed to reduce the outgrowth distance of new axons. This was not due to impermeability of the drug, since the same concentration caused a clear reduction of the injury-induced proliferation of Schwann cells in the crush region. Likewise, HA-1004, an inhibitor of cyclic nucleotide-dependent protein kinases, also lacked effect on outgrowth when used on its own, even at very high concentrations (100 microM). In contrast, outgrowth was significantly reduced when 5 microM chelerythrine and 5 microM HA-1004 were used in combination. In conclusion, the present results suggest that PKC-activity is important but not indispensable for the regeneration process. Successful completion of the latter could be achieved by several, perhaps redundant, phosphorylation systems.

Increased levels of mitogen activated protein kinase (MAP-K) detected in the injured adult mouse sciatic nerve

Adult mouse sciatic nerves (SNs) with attached dorsal root ganglia (DRG) were analysed for the presence of mitogen activated protein kinase (MAP-K) during normal and regenerative conditions. By immunohistochemistry, MAP-K was found to be present in the normal nerve at low levels in both Schwann cells and DRG nerve cell bodies, with a profoundly increased expression during regeneration. In axonal outgrowth assays, treatment with 2 mM 2-aminopurine (2-AP), a MAP-K antagonist, inhibited the regeneration of axons from the SN as well as from the cultured superior cervical ganglia. The reduced outgrowth was probably not due to toxic effects of the drug since the ganglionic protein synthesis was not inhibited. It is possible that 2-AP interferes with regeneration-related events by inhibition of MAP-K.

Increased cyclic AMP in in vitro regenerating frog sciatic nerves inhibits Schwann cell proliferation but has no effect on axonal outgrowth

In the present study the role of cAMP for axonal outgrowth and Schwann cell proliferation was studied using the cultured frog sciatic nerve. An intrinsic rise in nerve and ganglionic cAMP could be measured as a response to nerve injury, both in vitro and in vivo. Treatment with 0.1-1.0 microM forskolin, an activator of the cAMP-generating enzyme adenylyl cyclase, increased the cAMP content up to 13-fold, but was yet without effect on axonal outgrowth during an 8-day culturing period. HA-1004, an inhibitor of cAMP-dependent protein kinase, also lacked effect on the regeneration. In contrast, the proliferation of Schwann cells, measured as [3H]thymidine incorporation, was inhibited to about 70% of control by forskolin, whereas HA-1004 stimulated proliferation to approximately 130% of control. The results suggest that cAMP is involved in the injury-induced proliferation of Schwann cells of an adult peripheral nerve but that it lacks a central role in the regeneration of sensory axons of such nerves.

Detection of a purine analogue-sensitive kinase in frog sciatic nerves--possible involvement in nerve regeneration

This study reports the existence of a purine analogue-sensitive protein kinase in adult frog sciatic nerves. Cell-free supernatants of homogenized regenerating sciatic nerves were found to contain a phosphoprotein (MW 90 kDa, referred to as PP90), that was phosphorylated to a much higher degree than in normal, uninjured nerves. The spatial and temporal characteristics of PP90 phosphorylation suggested a relationship with the injury-induced proliferation of support cells of the regenerating nerve, i.e. its appearance and increment over time correlated with that of [3H]thymidine incorporation in the nerve. PP90 was phosphorylated under conditions that excluded enzyme activities due to Ca2+/calmodulin kinases, cyclic nucleotide-dependent kinases or protein kinase C. On the other hand, the phosphorylation could be selectively inhibited by the purine analogues adenosine, 2-aminopurine and 6-thioguanine (6-TG). The latter was the most potent and gave complete inhibition at 50 microM. Addition of histone H1 to the cell-free assay stimulated the phosphorylation of several proteins in both normal and regenerating nerves. The stimulation could be blocked by 6-TG, indicating the presence of a purine-sensitive kinase also in uninjured nerves. Separate experiments showed that in vitro regeneration of the frog sciatic sensory axons, as well as the proliferation of the support cells, was inhibited by 100 microM 6-TG.