Study of proline-directed protein kinases involved in phosphorylation of the heavy neurofilament subunit

Neurofilaments and Neurofilament Proteins in Health and Disease

SUMMARYNeurofilaments (NFs) are unique among tissue-specific classes of intermediate filaments (IFs) in being heteropolymers composed of four subunits (NF-L [neurofilament light]; NF-M [neurofilament middle]; NF-H [neurofilament heavy]; and α-internexin or peripherin), each having different domain structures and functions. Here, we review how NFs provide structural support for the highly asymmetric geometries of neurons and, especially, for the marked radial expansion of myelinated axons crucial for effective nerve conduction velocity. NFs in axons extensively cross-bridge and interconnect with other non-IF components of the cytoskeleton, including microtubules, actin filaments, and other fibrous cytoskeletal elements, to establish a regionally specialized network that undergoes exceptionally slow local turnover and serves as a docking platform to organize other organelles and proteins. We also discuss how a small pool of oligomeric and short filamentous precursors in the slow phase of axonal transport maintains this network. A complex pattern of phosphorylation and dephosphorylation events on each subunit modulates filament assembly, turnover, and organization within the axonal cytoskeleton. Multiple factors, and especially turnover rate, determine the size of the network, which can vary substantially along the axon. NF gene mutations cause several neuroaxonal disorders characterized by disrupted subunit assembly and NF aggregation. Additional NF alterations are associated with varied neuropsychiatric disorders. New evidence that subunits of NFs exist within postsynaptic terminal boutons and influence neurotransmission suggests how NF proteins might contribute to normal synaptic function and neuropsychiatric disease states.

Review : c-Jun and the c-Jun Amino-Terminal Kinases: Bipotential Components of the Neuronal Stress Response

Expression of the inducible transcription factor c-Jun in neurons is a common finding after neuronal injury or 'stress,' such as ischemia, excitotoxicity, axon transection, UV irradiation, stimulation by cytokines, or production of such lipid messengers as ceramide. The neuronal 'stress response' displays striking similarities to the stress response of other cell types such as lymphocytes or tumor cells and is characterized by the activation of programs that lead to apoptosis or survival. It is accepted knowledge that c-Jun can act as neuronal 'killer' under in vitro conditions (with the death inducing ligand fas-ligand as novel AP-1 controlled target gene), but there is also growing evidence that c-Jun is linked to neuronal repair or survival. The control of this dichotomous function of c-Jun is not fully understood. Similar to the expression of c-Jun, the transcriptional activation of c-Jun by amino-terminal phosphorylation and the activation of the catalyzing c- Jun amino-terminal kinases (JNK), also called stress activated protein kinases, can also be linked to both neuronal survival and apoptosis. We suggest a model for the control of gene transcription after neuronal stress with activation of JNK and phosphorylation of c-Jun as transcriptional prerequisites, and with asso ciated partners as transcriptional effectors, e.g., by the expression and/or suppression of other transcription factors as activating transcription factor 2 (ATF-2), c-Fos, or JunD. This scenario is complicated by the observation that activity of JNK does not lead automatically to c-Jun phosphorylation. This review summa rizes the role of c-Jun and JNK as down-stream mediators of neuronal stressors and places the function of these molecules in the context of other stressful stimuli and intraneuronal responses. NEUROSCIENTIST 5:147-154, 1999

NMDA Receptors and Oxidative Stress Induced by the Major Metabolites Accumulating in HMG Lyase Deficiency Mediate Hypophosphorylation of Cytoskeletal Proteins in Brain From Adolescent Rats: Potential Mechanisms Contributing to the Neuropathology of This Disease

Neurological symptoms and cerebral abnormalities are commonly observed in patients with 3-hydroxy-3-methylglutaryl-CoA lyase (HMG lyase) deficiency, which is biochemically characterized by predominant tissue accumulation of 3-hydroxy-3-methylglutaric (HMG), 3-methylglutaric (MGA), and 3-methylglutaconic (MGT) acids. Since the pathogenesis of this disease is poorly known, the present study evaluated the effects of these compounds on the cytoskeleton phosphorylating system in rat brain. HMG, MGA, and MGT caused hypophosphorylation of glial fibrillary acidic protein (GFAP) and of the neurofilament subunits NFL, NFM, and NFH. HMG-induced hypophosphorylation was mediated by inhibiting the cAMP-dependent protein kinase (PKA) on Ser55 residue of NFL and c-Jun kinase (JNK) by acting on KSP repeats of NFM and NFH subunits. We also evidenced that the subunit NR2B of NMDA receptor and Ca(2+) was involved in HMG-elicited hypophosphorylation of cytoskeletal proteins. Furthermore, the antioxidants L-NAME and TROLOX fully prevented both the hypophosphorylation and the inhibition of PKA and JNK caused by HMG, suggesting that oxidative damage may underlie these effects. These findings indicate that the main metabolites accumulating in HMG lyase deficiency provoke hypophosphorylation of cytoskeleton neural proteins with the involvement of NMDA receptors, Ca(2+), and reactive species. It is presumed that these alterations may contribute to the neuropathology of this disease.

Divergent and convergent roles for kinases and phosphatases in neurofilament dynamics

C-terminal neurofilament phosphorylation mediates cation-dependent self-association leading to neurofilament incorporation into the stationary axonal cytoskeleton. Multiple kinases phosphorylate the C-terminal domains of the heavy neurofilament subunit (NF-H), including cyclin-dependent protein kinase 5 (CDK5), mitogen-activated protein kinases (MAPKs), casein kinase 1 and 2 (CK1 and CK2) and glycogen synthase kinase 3β (GSK3β). The respective contributions of these kinases have been confounded because they phosphorylate multiple substrates in addition to neurofilaments and display extensive interaction. Herein, differentiated NB2a/d1 cells were transfected with constructs expressing GFP-tagged NF-H, isolated NF-H sidearms and NF-H lacking the distal-most 187 amino acids. Cultures were treated with roscovitine, PD98059, Li(+), D4476, tetrabromobenzotriazole and calyculin, which are active against CDK5, MKK1 (also known as MAP2K1), GSK3β, CK1, CK2 and protein phosphatase 1 (PP1), respectively. Sequential phosphorylation by CDK5 and GSK3β mediated the neurofilament-neurofilament associations. The MAPK pathway (i.e. MKK1 to ERK1/2) was found to downregulate GSK3β, and CK1 activated PP1, both of which promoted axonal transport and restricted neurofilament-neurofilament associations to axonal neurites. The MAPK pathway and CDK5, but not CK1 and GSK3β, inhibited neurofilament proteolysis. These findings indicate that phosphorylation of neurofilaments by the proline-directed MAPK pathway and CDK5 counterbalance the impact of phosphorylation of neurofilaments by the non-proline-directed CK1 and GSK3β.

Signaling mechanisms and disrupted cytoskeleton in the diphenyl ditelluride neurotoxicity

Evidence from our group supports that diphenyl ditelluride (PhTe)₂ neurotoxicity depends on modulation of signaling pathways initiated at the plasma membrane. The (PhTe)₂-evoked signal is transduced downstream of voltage-dependent Ca²⁺ channels (VDCC), N-methyl-D-aspartate receptors (NMDA), or metabotropic glutamate receptors activation via different kinase pathways (protein kinase A, phospholipase C/protein kinase C, mitogen-activated protein kinases (MAPKs), and Akt signaling pathway). Among the most relevant cues of misregulated signaling mechanisms evoked by (PhTe)₂ is the cytoskeleton of neural cells. The in vivo and in vitro exposure to (PhTe)₂ induce hyperphosphorylation/hypophosphorylation of neuronal and glial intermediate filament (IF) proteins (neurofilaments and glial fibrillary acidic protein, resp.) in different brain structures of young rats. Phosphorylation of IFs at specific sites modulates their association/disassociation and interferes with important physiological roles, such as axonal transport. Disrupted cytoskeleton is a crucial marker of neurodegeneration and is associated with reactive astrogliosis and apoptotic cell death. This review focuses the current knowledge and important results on the mechanisms of (PhTe)₂ neurotoxicity with special emphasis on the cytoskeletal proteins and their differential regulation by kinases/phosphatases and Ca²⁺-mediated mechanisms in developmental rat brain. We propose that the disrupted cytoskeletal homeostasis could support brain damage provoked by this neurotoxicant.

Heterogeneity of intrinsically photosensitive retinal ganglion cells in the mouse revealed by molecular phenotyping

Intrinsically photosensitive retinal ganglion cell (ipRGC) types can be distinguished by their dendritic tree stratification and intensity of melanopsin staining. We identified heavily stained melanopsin-positive M1 cells branching in the outermost part of the inner plexiform layer (IPL) and weakly melanopsin-positive M2 cells branching in the innermost layer of the IPL. A third type can be distinguished by the displacement of the soma to the inner nuclear layer and has morphological similarities with either M1 cells or M2 cells, and is termed here displaced or M-d cells. The aim of the present study was to examine the phenotypic traits of ipRGC types. Using whole retinae from adult mice, we performed immunohistochemistry using melanopsin immunostaining and a number of antibodies directed against proteins typically expressed in retinal ganglion cells. The majority of M1 and M2 ipRGCs expressed Isl-1, microtubule associated protein-2 (MAP2), γ-synuclein, and NeuN, whereas Brn3 transcription factor and the different neurofilaments (NF68, NF160, NF200) were able to discriminate between ipRGC subtypes. Brn3 was expressed preferentially in M2 cells and in a small subpopulation of weakly melanopsin-positive M-d cells with similarities to M2 cells. All three neurofilaments were primarily expressed in large M2 cells with similarities to the recently described alpha-like M4 cells, but not in M1 cells. Expression of NF68 and NF160 was also observed in a few large M-d ipRGCs. These findings show that ipRGCs are not a phenotypically homogenous population and that specific neuronal markers (Brn3 and neurofilament) can partly distinguish between different ipRGC subtypes.

Topographic regulation of neuronal intermediate filaments by phosphorylation, role of peptidyl-prolyl isomerase 1: significance in neurodegeneration

The neuronal cytoskeleton is tightly regulated by phosphorylation and dephosphorylation reactions mediated by numerous associated kinases, phosphatases and their regulators. Defects in the relative kinase and phosphatase activities and/or deregulation of compartment-specific phosphorylation result in neurodegenerative disorders. The largest family of cytoskeletal proteins in mammalian cells is the superfamily of intermediate filaments (IFs). The neurofilament (NF) proteins are the major IFs. Aggregated forms of hyperphosphorylated tau and phosphorylated NFs are found in pathological cell body accumulations in the central nervous system of patients suffering from Alzheimer's disease, Parkinson's disease, and Amyotrophic Lateral Sclerosis. The precise mechanisms for this compartment-specific phosphorylation of cytoskeletal proteins are not completely understood. In this review, we focus on the mechanisms of neurofilament phosphorylation in normal physiology and neurodegenerative diseases. We also address the recent breakthroughs in our understanding the role of different kinases and phosphatases involved in regulating the phosphorylation status of the NFs. In addition, special emphasis has been given to describe the role of phosphatases and Pin1 in phosphorylation of NFs.

Heterogeneity in the properties of NEFL mutants causing Charcot-Marie-Tooth disease results in differential effects on neurofilament assembly and susceptibility to intervention by the chaperone-inducer, celastrol

Aberrant aggregation of neurofilament proteins is a common feature of neurodegenerative diseases. For example, neurofilament light protein (NEFL) mutants causing Charcot-Marie-Tooth disease induce misassembly of neurofilaments. This study demonstrated that mutations in different functional domains of NEFL have different effects on filament assembly and susceptibility to interventions to restore function. The mouse NEFL mutants, NEFL(Q333P) and NEFL(P8R), exhibited different assembly properties in SW13-cells, cells lacking endogenous intermediate filaments, indicating different consequences of these mutations on the biochemical properties of NEFL. The p.Q333P mutation caused reversible misfolding of the protein. NEFL(Q333P) could be refolded and form coil-coiled dimers, in vitro using chaotropic agent, and in cultured cells by induction of HSPA1 and HSPB1. Celastrol, an inducer of chaperone proteins, induced HSPA1 expression in motor neurons and prevented the formation of neurofilament inclusions and mitochondrial shortening induced by expression of NEFL(Q333P), but not in sensory neurons. Conversely, celastrol had a protective effect against the toxicity of NEFL(P8R), a mutant which is sensitive to HSBP1 but not HSPA1 chaperoning, only in large-sized sensory neurons, not in motor neurons. Importantly, sensory and motor neurons do not respond identically to celastrol and different chaperones are upregulated by the same treatment. Thus, effective therapy of CMT not only depends on the identity of the mutated gene, but the consequences of the specific mutation on the properties of the protein and the neuronal population targeted.

Activation of c-Jun N-terminal kinase (JNK) during mitosis in retinal progenitor cells

Most studies of c-Jun N-terminal Kinase (JNK) activation in retinal tissue were done in the context of neurodegeneration. In this study, we investigated the behavior of JNK during mitosis of progenitor cells in the retina of newborn rats. Retinal explants from newborn rats were kept in vitro for 3 hours and under distinct treatments. Sections of retinal explants or freshly fixed retinal tissue were used to detect JNK phosphorylation by immunohistochemistry, and were examined through both fluorescence and confocal microscopy. Mitotic cells were identified by chromatin morphology, histone-H3 phosphorylation, and location in the retinal tissue. The subcellular localization of proteins was analyzed by double staining with both a DNA marker and an antibody to each protein. Phosphorylation of JNK was also examined by western blot. The results showed that in the retina of newborn rats (P1), JNK is phosphorylated during mitosis of progenitor cells, mainly during the early stages of mitosis. JNK1 and/or JNK2 were preferentially phosphorylated in mitotic cells. Inhibition of JNK induced cell cycle arrest, specifically in mitosis. Treatment with the JNK inhibitor decreased the number of cells in anaphase, but did not alter the number of cells in either prophase/prometaphase or metaphase. Moreover, cells with aberrant chromatin morphology were found after treatment with the JNK inhibitor. The data show, for the first time, that JNK is activated in mitotic progenitor cells of developing retinal tissue, suggesting a new role of JNK in the control of progenitor cell proliferation in the retina.

Post-translational modification of Cu/Zn superoxide dismutase under anaerobic conditions

In eukaryotic organisms, the largely cytosolic copper- and zinc-containing superoxide dismutase (Cu/Zn SOD) enzyme represents a key defense against reactive oxygen toxicity. Although much is known about the biology of this enzyme under aerobic conditions, less is understood regarding the effects of low oxygen levels on Cu/Zn SOD enzymes from diverse organisms. We show here that like bakers' yeast (Saccharomyces cerevisiae), adaptation of the multicellular Caenorhabditis elegans to growth at low oxygen levels involves strong downregulation of its Cu/Zn SOD. Much of this regulation occurs at the post-translational level where CCS-independent activation of Cu/Zn SOD is inhibited. Hypoxia inactivates the endogenous Cu/Zn SOD of C. elegans Cu/Zn SOD as well as a P144 mutant of S. cerevisiae Cu/Zn SOD (herein denoted Sod1p) that is independent of CCS. In our studies of S. cerevisiae Sod1p, we noted a post-translational modification to the inactive enzyme during hypoxia. Analysis of this modification by mass spectrometry revealed phosphorylation at serine 38. Serine 38 represents a putative proline-directed kinase target site located on a solvent-exposed loop that is positioned at one end of the Sod1p β-barrel, a region immediately adjacent to residues previously shown to influence CCS-dependent activation. Although phosphorylation of serine 38 is minimal when the Sod1p is abundantly active (e.g., high oxygen level), up to 50% of Sod1p can be phosphorylated when CCS activation of the enzyme is blocked, e.g., by hypoxia or low-copper conditions. Serine 38 phosphorylation can be a marker for inactive pools of Sod1p.

Declining phosphatases underlie aging-related hyperphosphorylation of neurofilaments

Cytoskeletal protein phosphorylation is frequently altered in neuropathologic states but little is known about changes during normal aging. Here we report that declining protein phosphatase activity, rather than activation of kinases, underlies aging-related neurofilament hyperphosphorylation. Purified PP2A or PP2B dephosphorylated the heavy neurofilament (NFH) subunit or its extensively phorphorylated carboxyl-terminal domain in vitro. In cultured primary hippocampal neurons, inhibiting either phosphatase induced NFH phosphorylation without activating known neurofilament kinases. Neurofilament phosphorylation in the mouse CNS, as reflected by levels of the RT-97 phosphoepitope associated with late axon maturation, more than doubled during the 12-month period after NFH expression plateaued at p21. This was accompanied by declines in levels and activity of PP2A but not PP2B, and no rise in activities of neurofilament kinases (Erk1,2, cdk5 and JNK1,2). Inhibiting PP2A in mice in vivo restored brain RT-97 to levels seen in young mice. Declining PP2A activity, therefore, can account for rising neurofilament phosphorylation in maturing brain, potentially compounding similar changes associated with adult-onset neurodegenerative diseases.

Dynamic changes of neuroskeletal proteins in DRGs underlie impaired axonal maturation and progressive axonal degeneration in type 1 diabetes

We investigated mechanisms underlying progressive axonal dysfunction and structural deficits in type 1 BB/Wor-rats from 1 week to 10 month diabetes duration. Motor and sensory conduction velocities were decreased after 4 and 6 weeks of diabetes and declined further over the remaining 9 months. Myelinated sural nerve fibers showed progressive deficits in fiber numbers and sizes. Structural deficits in unmyelinated axonal size were evident at 2 month and deficits in number were present at 4 mo. These changes were preceded by decreased availability of insulin, C-peptide and IGF-1 and decreased expression of neurofilaments and β-III-tubulin. Upregulation of phosphorylating stress kinases like Cdk5, p-GSK-3β, and p42/44 resulted in increased phosphorylation of neurofilaments. Increasing activity of p-GSK-3β correlated with increasing phosphorylation of NFH, whereas decreasing Cdk5 correlated with diminishing phosphorylation of NFM. The data suggest that impaired neurotrophic support results in sequentially impaired synthesis and postranslational modifications of neuroskeletal proteins, resulting in progressive deficits in axonal function, maturation and size.

Neurofilament tail phosphorylation: identity of the RT-97 phosphoepitope and regulation in neurons by cross-talk among proline-directed kinases

As axons myelinate, establish a stable neurofilament network, and expand in caliber, neurofilament proteins are extensively phosphorylated along their C-terminal tails, which is recognized by the monoclonal antibody, RT-97. Here, we demonstrate in vivo that RT-97 immunoreactivity (IR) is generated by phosphorylation at KSPXK or KSPXXXK motifs and requires flanking lysines at specific positions. extracellular signal regulated kinase 1,2 (ERK1,2) and pERK1,2 levels increase in parallel with phosphorylation at the RT-97 epitope during early postnatal brain development. Purified ERK1,2 generated RT-97 on both KSP motifs on recombinant NF-H tail domain proteins, while cdk5 phosphorylated only KSPXK motifs. RT-97 epitope generation in primary hippocampal neurons was regulated by extensive cross-talk among ERK1,2, c-Jun N-terminal kinase 1,2 (JNK1,2) and cdk5. Inhibition of both ERK1,2 and JNK1,2 completely blocked RT-97 generation. Cdk5 influenced RT-97 generation indirectly by modulating JNK activation. In mice, cdk5 gene deletion did not significantly alter RT-97 IR or ERK1,2 and JNK activation. In mice lacking the cdk5 activator P35, the partial suppression of cdk5 activity increased RT-97 IR by activating ERK1,2. Thus, cdk5 influences RT-97 epitope generation partly by modulating ERKs and JNKs, which are the two principal kinases regulating neurofilament phosphorylation. The regulation of a single target by multiple protein kinases underscores the importance of monitoring other relevant kinases when the activity of a particular one is blocked.

Adult human dental pulp stem cells differentiate toward functionally active neurons under appropriate environmental cues

Human adult dental pulp stem cells (DPSCs) reside within the perivascular niche of dental pulp and are thought to originate from migrating cranial neural crest (CNC) cells. During embryonic development, CNC cells differentiate into a wide variety of cell types, including neurons of the peripheral nervous system. Previously, we have demonstrated that DPSCs derived from adult human third molar teeth differentiate into cell types reminiscent of CNC embryonic ontology. We hypothesized that DPSCs exposed to the appropriate environmental cues would differentiate into functionally active neurons. The data demonstrated that ex vivo-expanded human adult DPSCs responded to neuronal inductive conditions both in vitro and in vivo. Human adult DPSCs, but not human foreskin fibroblasts (HFFs), acquired a neuronal morphology, and expressed neuronal-specific markers at both the gene and protein levels. Culture-expanded DPSCs also exhibited the capacity to produce a sodium current consistent with functional neuronal cells when exposed to neuronal inductive media. Furthermore, the response of human DPSCs and HFFs to endogenous neuronal environmental cues was determined in vivo using an avian xenotransplantation assay. DPSCs expressed neuronal markers and acquired a neuronal morphology following transplantation into the mesencephalon of embryonic day-2 chicken embryo, whereas HFFs maintained a thin spindle fibroblastic morphology. We propose that adult human DPSCs provide a readily accessible source of exogenous stem/precursor cells that have the potential for use in cell-therapeutic paradigms to treat neurological disease.

Large scale hippocampal cellular distress may explain the behavioral consequences of repetitive traumatic experiences--a proteomic approach

Early life traumatic experiences are associated with psychopathology in adulthood. This may be due in part to the effects of trauma on hippocampal development and protein expression. The purpose of the study was to investigate the effects of early life trauma and adult re-stress on ventral hippocampal protein expression. Adolescent rats (n = 19) were subjected to a triple stressor on post-natal day 28 followed 7 days later by the first re-stress session and 25 days later (post-natal day 60 = adulthood) by the second re-stress session. Ventral hippocampi were collected on post-natal day 68 for protein expression determinations using protein arrays and 2D-gel electrophoresis with liquid chromatography tandem mass spectrometry. Compared to controls, traumatized animals showed an increase in Ca(2+) homeostatic proteins, dysregulated signaling pathways and energy metabolism enzymes, cytoskeletal protein changes, a decrease in neuroplasticity regulators, energy metabolism enzymes and an increase in apoptotic initiator proteins. These results indicate the extensive impact of trauma on adult brain development and behavior.

c-Jun N-terminal kinase regulates mitochondrial bioenergetics by modulating pyruvate dehydrogenase activity in primary cortical neurons

This study examines the role of c-jun N-terminal kinase (JNK) in mitochondrial signaling and bioenergetics in primary cortical neurons and isolated rat brain mitochondria. Exposure of neurons to either anisomycin (an activator of JNK/p38 mitogen-activated protein kinases) or H2O2 resulted in activation (phosphorylation) of JNK (mostly p46(JNK1)) and its translocation to mitochondria. Experiments with mitochondria isolated from either rat brain or primary cortical neurons and incubated with proteinase K revealed that phosphorylated JNK was associated with the outer mitochondrial membrane; this association resulted in the phosphorylation of the E(1alpha) subunit of pyruvate dehydrogenase, a key enzyme that catalyzes the oxidative decarboxylation of pyruvate and that links two major metabolic pathways: glycolysis and the tricarboxylic acid cycle. JNK-mediated phosphorylation of pyruvate dehydrogenase was not observed in experiments carried out with mitoplasts, thus suggesting the requirement of intact, functional mitochondria for this effect. JNK-mediated phosphorylation of pyruvate dehydrogenase was associated with a decline in its activity and, consequently, a shift to anaerobic pyruvate metabolism: the latter was confirmed by increased accumulation of lactic acid and decreased overall energy production (ATP levels). Pyruvate dehydrogenase appears to be a specific phosphorylation target for JNK, for other kinases, such as protein kinase A and protein kinase C did not elicit pyruvate dehydrogenase phosphorylation and did not decrease the activity of the complex. These results suggest that JNK mediates a signaling pathway that regulates metabolic functions in mitochondria as part of a network that coordinates cytosolic and mitochondrial processes relevant for cell function.

Divergent effects of the MEKK-1/JNK pathway on NB2a/d1 differentiation: Some activity is required for outgrowth and stabilization of neurites but overactivation inhibits both phenomena

c-Jun N-terminal kinase (JNK), along with its upstream activator MEKK-1, is typically thought of as a stress-activated kinase that mediates apoptosis. However, additional studies indicate that the MEKK-1/JNK pathway mediates critical aspects of neuronal survival and differentiation. Herein, we demonstrate that transfection of differentiated NB2a/d1 cells with a construct expression constitutively activated (ca) MEKK-1 increases levels of phospho-dependent neurofilament (NF) immunoreactivity within perikarya, while expression of a dominant-negative (dn) form of MEKK-1 decreases it. Steady-state levels of perikaryal phospho-NF immunoreactivity are reduced and the increase resulting from expression of caMEKK-1 is prevented, by the JNK inhibitor SP600125, suggesting that JNK is a major downstream effector of MEKK-1 on NF phosphorylation. Unexpectedly, both caMEKK-1 and dnMEKK-1 inhibited neuritogenesis as well as translocation of NFs into newly elaborated neurites. The JNK inhibitor SP600125 also inhibited NF transport in a dose-dependent manner. caMEKK-1 also prevented the increase in NF transport otherwise mediated by MAP kinase. Finally, both caMEKK-1 and dnMEKK-1 prevented initial neuritogenesis. These findings indicate that the MEKK-1/JNK pathway regulates critical aspects of initial outgrowth, and subsequent stabilization of axonal neurites.

Squid (Loligo pealei) giant fiber system: a model for studying neurodegeneration and dementia?

In many neurodegenerative disorders that lead to memory loss and dementia, the brain pathology responsible for neuronal loss is marked by accumulations of proteins in the form of extracellular plaques and intracellular filamentous tangles, containing hyperphosphorylated cytoskeletal proteins. These are assumed to arise as a consequence of deregulation of a normal pattern of topographic phosphorylation-that is, an abnormal shift of cytoskeletal protein phosphorylation from the normal axonal compartment to cell bodies. Although decades of studies have been directed to this problem, biochemical approaches in mammalian systems are limited: neurons are too small to permit separation of cell body and axon compartments. Since the pioneering studies of Hodgkin and Huxley on the giant fiber system of the squid, however, the stellate ganglion and its giant axons have been the focus of a large literature on the physiology and biochemistry of neuron function. This review concentrates on a host of studies in our laboratory and others on the factors regulating compartment-specific patterns of cytoskeletal protein phosphorylation (primarily neurofilaments) in an effort to establish a normal baseline of information for further studies on neurodegeneration. On the basis of these data, a model of topographic regulation is proposed that offers several possibilities for further studies on potential sites of deregulation that may lead to pathologies resembling those seen in mammalian and human brains showing neurodegeneration, dementia, and neuronal cell death.

Role of extracellular-regulated kinase and c-jun NH2-terminal kinase in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurofilament phosphorylation

The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) causes selective degeneration of dopaminergic neurons in which the c-Jun NH2-terminal kinase (JNK) signalling cascade has been implicated. We have employed a differentiated mouse neuroblastoma N2a cell model to investigate the involvement of JNK and extracellular-regulated kinase (ERK) in MPTP-mediated toxicity and their role in neurofilament heavy chain (NF-H) phosphorylation. Acute treatment with a cytotoxic MPTP concentration (5 mM) caused rapid and sustained JNK phosphorylation and ERK dephosphorylation, accompanied by cell death. In contrast, subcytotoxic concentrations of 10 microM MPTP resulted in lower, transient JNK activation in the presence of sustained ERK activity. This resulted in an aberrant increase in a phosphorylation-dependent NF-H epitope, perikaryal accumulation of NF-H, and loss of axon-like processes, prior to cell death. Inhibition of MEK kinase, using PD98059, showed that MEK 1/2 or the downstream kinase, ERK, is required for N2a cell differentiation, NF-H phosphorylation and survival. Indeed, MPTP-induced cell death was exacerbated by the presence of PD98059. However, in the presence of MPTP, reducing JNK activity by using an upstream specific mixed-lineage kinase inhibitor (CEP-11004) significantly attenuated aberrant NF-H phosphorylation and perikaryal NF-H accumulation and maintained axon-like processes, in addition to attenuating cell death. This study reports a switch in the predominant kinase involved in NF phosphorylation in a neuronal cell model and may have implications for the formation of inclusions. Our studies provide further evidence that modulation of the JNK pathway could have a role in alleviating neuronal cell death.

MEKK1 controls neurite regrowth after experimental injury by balancing ERK1/2 and JNK2 signaling

After injury, peripheral neuronal cells initiate complex signaling cascades to promote survival and regeneration. In the present study, we have identified the mitogen-activated protein kinase (MAPK) isoforms which are necessary for nerve growth factor (NGF)-induced neurite regrowth after injury of differentiated PC12 cells. Extracellular signal-regulated kinases 1 and 2 (ERK1/2) and the usually pro-apoptotic c-Jun N-terminal kinase 2 (JNK2) are crucial for neurite regrowth, while p38 plays no role in this context. Surprisingly, the MEK1 inhibitors PD 98059 and U 0126 blocked both ERK1/2 and JNK phosphorylation, indicating a novel form of balancing MAPK cascade cross-talk. Results from RNAi experiments excluded direct ERK/JNK interactions. We identified the upstream kinase MEKK1 as an activator of both the ERK1/2 and JNK2 pathways, whereby the ERK1/2 kinase MEK1 and the JNK kinase MKK7 bind to MEKK1 in a competing fashion. Our findings suggest an important role of JNK2 and MAPK pathway cross-talk in neurite regeneration.

Allelic variants of the canine heavy neurofilament (NFH) subunit and extensive phosphorylation in dogs with motor neuron disease

Aberrant accumulation of extensively phosphorylated heavy (high molecular weight) neurofilament (NFH) and neurodegeneration are features of hereditary canine spinal muscular atrophy (HCSMA), an animal model of human motor neuron disease. In this study, the canine NFH gene was mapped, cloned, and sequenced, and electrospray/mass spectrometry was used to evaluate the phosphorylation state of NFH protein from normal dogs and dogs with HCSMA. The canine NFH gene was localized to a region on canine chromosome 26 that corresponds to human NFH on chromosome 22q. The predicted length of the canine NFH protein is 1135 amino acids, and it shares an 80.3% identity with human NFH and >74.6% with murine NFH proteins. Direct sequencing of NFH cDNA from HCSMA dogs revealed no mutations, although cDNA sequence and restriction fragment length polymorphism (RFLP) analysis indicates that there are at least three canine NFH alleles, differing in the position and number (61 or 62) of Lys-Ser-Proline (KSP) motifs. The two longest alleles (L1 and L2), each with 62 KSP repeats, contain an additional 24-base insert and were observed in both normal and HCSMA dogs. However, the shorter allele (the C allele), with 61 KSP sites and lacking the 24-base insertion, was absent in dogs with HCSMA. Mass spectrometry data indicated that almost all of the NFH KSP phosphorylation sites were occupied. No new or extra sites were identified in native NFH purified from the HCSMA dogs. The predominance of the two longest NFH alleles and the additional KSP phosphorylation sites they confer probably account for the presence of extensively phosphorylated NFs detected immunohistochemically in dogs with HCSMA.

Impaired long-term potentiation in c-Jun N-terminal kinase 2-deficient mice

c-Jun N-terminal kinases (JNKs) are thought to be involved in regulating synaptic plasticity. We therefore investigated the specific role of JNK2 in modulating long-term potentiation (LTP) in hippocampus during development, using JNK2-deficient mice. The morphological structure and the numbers of both NeuN, a specific neuronal marker, and GABA-positive neurons in the hippocampal areas were similar in wild-type and Jnk2(-/-) mice. Western blot analysis revealed that JNK2 expression was higher and stable at 1 and 3 months of age, but JNK1 levels were lower at 1 month of age and almost undetectable in 3-month-old wild-type mice. In contrast to wild-type mice, there was a significant increase in JNK1 expression in JNK2 mutant mice, especially at 1 month of age. Electrophysiological studies demonstrated that LTP was impaired in both the CA1 and CA3 regions in 1-month-old, but not in adult, Jnk2(-/-) mice, probably owing to decreased presynaptic neurotransmitter release. Moreover, late-phase LTP, but not early-phase LTP, was impaired in the Jnk2(-/-) adult mice, suggesting that JNK2 plays a role in transforming early LTP to late LTP. Together, the data highlight the specific role of JNK2 in hippocampal synaptic plasticity during development.

Cdk5 inhibits anterograde axonal transport of neurofilaments but not that of tau by inhibition of mitogen-activated protein kinase activity

Cyclin-dependent kinase 5 (cdk5) inhibits neurofilament (NF) anterograde axonal transport while p42/44 mitogen-activated protein kinase (MAPk) promotes it. Since cdk5 is known to inhibit MAP kinase activity, we examined whether or not cdk5 inhibits anterograde NF transport via inhibition of MAPk activity. To accomplish this, we manipulated the activity of these kinases in differentiated NB2a/d1 cells, and monitored anterograde axonal transport of green fluorescent protein-conjugated-NF-M (GFP-M) and cyan fluorescent protein-conjugated (CFP)-tau. The cdk5 inhibitor roscovitine increased anterograde axonal transport of GFP-M and CFP-tau; transfection with cdk5/p25 inhibited transport of both. Inhibition of MAPk activity by PD98059 or expression of dominant-negative MAPk inhibited anterograde GFP-M transport, while expression of constitutively active MAPk enhanced it; these treatments did not affect CFP-tau transport. PD98059 prevented roscovitine-mediated enhancement of GFP-M transport, but did not prevent enhancement of CFP-tau transport. Co-transfection with constitutively activated MAPk prevented the inhibition of GFP-M transport that normally accompanied transfection with cdk5/p25, but did not prevent inhibition of tau transport by cdk5/p25. Finally, the extent of inhibition of GFP-M axonal transport by PD98059 was not additive to that derived from transfection with cdk5/p35, and the increase in NF transport that accompanies roscovitine treatment was not additive to that derived from transfection with constitutively activated MAPk, suggesting that the influence of these kinases on NF transport was within the same, rather than distinct, pathways. These findings suggest that axonal transport of tau and NFs is under the control of distinct kinase cascades, and that cdk5 inhibits NF transport at least in part by inhibiting MAPk.

UV-induced apoptosis in SH-SY5Y cells: Contribution to apoptosis by JNK signaling and cytochrome c

Activation of the c-Jun N-terminal kinase (JNK) pathway is suggested to be required for neuronal apoptosis. We investigated the role of JNK on phosphorylation of c-Jun, Bcl-2, and apoptotic translocation of cytochrome c (cyt c) in UV-induced apoptosis in human neuroblastoma SH-SY5Y cells. We confirm that UV irradiation induces both apoptosis and necrosis in SH-SY5Y cells and that phosphorylation of JNK at Thr183/Tyr185 in SH-SY5Y cells treated with UV is an early event preceding apoptosis. We also demonstrate that phosphorylation of c-Jun at Ser63 is an early event coinciding with JNK activation, and that the phosphorylation of c-Jun is partially prevented by the JNK inhibitor SP600125. Despite the use of SP600125, the amount of cyt c released into the cytoplasm is not diminished and SP600125 is also unable to decrease the extent of UV-induced apoptosis. These data support the hypothesis that in this system, UV-induced apoptosis is not dependent exclusively on JNK activation. Possible involvement of cyclin-dependent kinases (CDKs) in c-Jun phosphorylation at Ser63 was excluded by pretreating UV-irradiated SH-SY5Y cells with the CDK1/2/5 inhibitor roscovitine.

INGAP peptide improves nerve function and enhances regeneration in streptozotocin-induced diabetic C57BL/6 mice

INGAP peptide comprises the core active sequence of Islet Neogenesis Associated Protein (INGAP), a pancreatic cytokine that can induce new islet formation and restore euglycemia in diabetic rodents. The ability of INGAP peptide in vitro to enhance nerve growth from sensory ganglia suggests its potential utility in peripheral nerve disorders. In this study, INGAP peptide was administered alone or in combination with insulin to streptozotocin-induced diabetic mice exhibiting signs of peripheral neuropathy. Following a 2-wk treatment period, thermal hypoalgesia in diabetic mice was significantly improved in groups that received INGAP peptide, without development of hyperalgesia. Explanted dorsal root ganglia (DRG) from these groups showed enhanced nerve outgrowth and evidence of increased mitochondrial activity. Western blotting experiments revealed attenuation of neurofilament hyperphosphorylation, up-regulation of beta-tubulin and actin, and increased phosphorylation of the transcription factor STAT3 in DRG. These findings suggest that INGAP peptide can activate some of the signaling pathways implicated in nerve regeneration in sensory ganglia, thereby providing a means of improvement of nociceptive dysfunction in the peripheral nervous system.

Mitogen-activated protein kinase regulates neurofilament axonal transport

Mitogen-activated protein kinase (MAP) kinase plays a pivotal role in the development of the nervous system by mediating both neurogenesis and neuronal differentiation. Here we examined whether p42/44 MAP kinase plays a role in axonal transport and the organization of neurofilaments (NFs) in axonal neurites. Dominant-negative p42/44 MAP kinase, anti-MAP kinase antisense oligonucleotides and the MAP kinase inhibitor PD98059 all reduced NF phospho-epitopes and inhibited anterograde NF axonal transport of GFP-tagged NF subunits in differentiated NB2a/d1 neuroblastoma cells. Expression of constitutively active MAP kinase and intracellular delivery of active enzyme increased NF phospho-epitopes and increased NF axonal transport. Longer treatment with PD98059 shifted NF transport from anterograde to retrograde. PD98059 did not inhibit overall axonal transport nor compromise overall axonal architecture or composition. The p38 MAP kinase inhibitor SB202190 did not inhibit NF transport whereas the kinase inhibitor olomoucine inhibited both NF and mitochondrial transport. Axonal transport of NFs containing NF-H whose C-terminal region was mutated to mimic extensive phosphorylation was substantially less affected by PD98059 compared to a wild-type construct. These data suggest that p42/44 MAP kinase regulates NF anterograde transport by NF C-terminal phosphorylation. MAP kinase may therefore stabilize developing axons by promoting the accumulation of NFs within growing axonal neurites.

Activation Mechanism of c-Jun Amino-terminal Kinase in the Course of Neural Differentiation of P19 Embryonic Carcinoma Cells

P19 embryonic carcinoma cells, a model system for studying early development and differentiation, can differentiate into neurons and primitive endoderm-like cells depending on the culture conditions. We have previously reported that the activation of c-Jun amino-terminal kinase (JNK) is required for the retinoic acid-induced neural differentiation of P19 cells. However, the signaling pathway(s) responsible for the activation of JNK has not been known. In this study, we demonstrated that activities of MAPK kinase 4 (MKK4) and TAK1, one of the upstream kinases of MKK4, were enhanced in the neurally differentiating cells. Inhibition of the neural differentiation by an overexpression of protein phosphatase 2Cepsilon, an inactivator of TAK1, suggested a critical role of the TAK1 signaling pathway during the differentiation. Confocal microscopic analysis indicated that TAK1, phospho-MKK4, and phospho-JNK were colocalized with tubulin in the neurites and localized also in the nuclei of the differentiating cells. In contrast, two TAK1-binding proteins, TAB1 and TAB2, which are involved in the activation of TAK1, were localized in the neurites and the nuclei of the differentiating cells, respectively. These results suggest that two distinct TAK1-MKK4-JNK signaling pathways are independently activated at the different intracellular locations and may participate in the regulation of the neural differentiation of P19 cells.

γ-Diketone neuropathy: axon atrophy and the role of cytoskeletal protein adduction

Multifocal giant neurofilamentous axonal swellings and secondary distal degeneration have been historically considered the hallmark features of gamma-diketone neuropathy. Accordingly, research conducted over the past 25 years has been directed toward discerning mechanisms of axonal swelling. However, this neuropathological convention has been challenged by recent observations that swollen axons were an exclusive product of long-term 2.5-hexanedione (HD) intoxication at lower daily dose-rates (e.g., 175 mg/kg/day); that is, higher HD dose-rates (e.g., 400 mg/kg/day) produced neurological deficits in the absence of axonal swellings. The observation that neurological toxicity can be expressed without axonal swelling suggests that this lesion is not an important pathophysiological event. Instead, several research groups have now shown that axon atrophy is prevalent in nervous tissues of laboratory animals intoxicated over a wide range of HD dose-rates. The well-documented nerve conduction defects associated with axon atrophy, in conjunction with the temporal correspondence between this lesion and the onset of neurological deficits, strongly suggest that atrophy has pathophysiological significance. In this commentary, we present evidence that supports a pathognomonic role for axon atrophy in gamma-diketone neuropathy and suggests that the functional consequences of this lesion mediate the corresponding neurological toxicity. Previous research has demonstrated that HD interacts with proteins via formation of pyrrole adducts. We therefore discuss the possibility that this chemical process is essential to the mechanism of atrophy. Evidence presented in this review suggests that "distal axonopathy" is an inaccurate classification and future nosological schemes should be based on the apparent primacy of axon atrophy.

p38α stress-activated protein kinase phosphorylates neurofilaments and is associated with neurofilament pathology in amyotrophic lateral sclerosis

Neurofilament middle and heavy chains (NFM and NFH) are heavily phosphorylated on their carboxy-terminal side-arm domains in axons. The mechanisms that regulate this phosphorylation are complex. Here, we demonstrate that p38alpha, a member of the stress-activated protein kinase family, will phosphorylate NFM and NFH on their side-arm domains. Aberrant accumulations of neurofilaments containing phosphorylated NFM and NFH side-arms are a pathological feature of amyotrophic lateral sclerosis (ALS) and we also demonstrate that p38alpha and active forms of p38 family kinases are associated with these accumulations. This is the case for sporadic and familial forms of ALS and also in a transgenic mouse model of ALS caused by expression of mutant superoxide dismutase-1 (SOD1). Thus, p38 kinases may contribute to the aberrant phosphorylation of NFM and NFH side-arms in ALS.

Neuronal cyclin-dependent kinase 5: role in nervous system function and its specific inhibition by the Cdk5 inhibitory peptide

Cyclin-dependent kinase 5 (Cdk5) is a member of the cyclin-dependent kinase family that is involved in the regulation of the cell cycle. As their name suggests, the Cdks require association with activator proteins called cyclins for their activity. Cdk5, however, is unique to this family of proline-directed serine/threonine kinases on two accounts. Firstly, Cdk5 has not been found to function in the cell cycle and, although expressed in a number of tissues, its activity is restricted to the nervous system. Secondly, unlike the other members of the Cdk family, Cdk5 is not activated by association with a cyclin, although it can bind them. Instead, Cdk5 is activated by the activator proteins p35 and p39 that are structurally distinct from cyclins and have, for the most part, a neuronal-specific expression pattern. In the past decade of research on Cdk5, it is now established that Cdk5 activity is critical for the proper formation and function of the brain. Moreover, its role as a central kinase, phosphorylating its substrates in its 'cross-talk' control of other kinase and signal transduction pathways, has also been determined. In addition to the normal physiological role of Cdk5, the kinase has been implicated in certain neurodegenerative disorders. For example, Cdk5 associates with the proteolytic, more active p25 fragment that is derived through the cleavage of p35. In turn, the p25/Cdk5 complex aberrantly phosphorylates its substrates tau and neurofilaments, which has been implicated in the pathogenesis of these disorders. Here, we attempt to review the past decade of research on Cdk5 from our laboratory and others, on the roles of Cdk5 in nervous system function. Additionally, our research has recently uncovered a possible therapeutic avenue of research, focusing on inhibition of aberrant Cdk5 hyperactivity which may well be used to treat the symptoms of a number of neurodegenerative diseases. The elucidation of a specific inhibitor of p25/Cdk5, termed CIP, also inhibits p25/Cdk5-mediated tau phosphorylation. This may well provide us with avenues of research focusing on the inhibition of pathologically damaging p25/Cdk5 species.

Decreased cyclin-dependent kinase 5 (cdk5) activity is accompanied by redistribution of cdk5 and cytoskeletal proteins and increased cytoskeletal protein phosphorylation in p35 null mice

Cdk5/p35 has been implicated in cytoskeletal protein phosphorylation in normal brain and in many human neurodegenerative disorders. Yet, mouse models of cdk5/p35 hyperactivity have not yielded corresponding changes in cytoskeletal protein phosphorylation. To elucidate the relationship between p35, cdk5, and the neuronal cytoskeleton, we deleted the p35 gene in mice having a pure C57BL/6 background. We found that p35 deficiency leads to a 38% reduction of cdk5 activity in adult brain. In addition, loss of p35 causes an anterograde redistribution of cdk5 toward peripheral neuronal processes. The unusual presence of nonphosphorylated neurofilament (NF) in aberrant axon fascicles and the relocation of tau and MAP2B from cell bodies and proximal neuronal processes to more distal sites of the neuropil in p35-/- mouse brain implicate p35 in neuronal trafficking, particularly in dynein-driven retrograde transport. In many axons of normal brain, cdk5 fails to colocalize with phosphorylated cytoskeletal protein epitopes. This observation, together with an unexpected increase of NF, tau, and MAP2B phosphoepitopes accompanying the decreased cdk5 activity in p35-/- mice, supports the idea that cdk5 does not phosphorylate cytoskeletal proteins directly. Rather, in structures where cdk5 does colocalize with phosphorylated cytoskeletal protein epitopes, it may function as a negative regulator of other proline-directed kinases that directly phosphorylate the proteins. Evidence for increased glycogen synthase kinase 3beta (GSK3beta) activity in p35-/- mice suggests that GSK3beta may be one such kinase regulated by cdk5. Our studies illustrate that p35 regulates the subcellular distribution of cdk5 and cytoskeletal proteins in neurons and that cdk5 has a hierarchical role in regulating the phosphorylation and function of cytoskeletal proteins.

Phosphorylation motifs regulating the stability and function of myocyte enhancer factor 2A

The phosphorylation status of the myocyte enhancer factor 2 (MEF2) transcriptional regulator is a critical determinant of its tissue-specific functions. However, due to the complexity of its phosphorylation pattern in vivo, a systematic inventory of MEF2A phosphorylation sites in mammalian cells has been difficult to obtain. We employed modern affinity purification techniques, combined with mass spectrometry, to identify several novel MEF2 phosphoacceptor sites. These include an evolutionarily conserved KSP motif, which we show is important in regulating the stability and function of MEF2A. Also, an indirect pathway in which a protein kinase casein kinase 2 phosphoacceptor site is phosphorylated by activation of p38 MAPK signaling was documented. Together, these findings identify several novel aspects of MEF2 regulation that may prove important in the control of gene expression in neuronal and muscle cells.

Diabetic neuropathy and nerve regeneration

Diabetic neuropathy is the most common peripheral neuropathy in western countries. Although every effort has been made to clarify the pathogenic mechanism of diabetic neuropathy, thereby devising its ideal therapeutic drugs, neither convinced hypotheses nor unequivocally effective drugs have been established. In view of the pathologic basis for the treatment of diabetic neuropathy, it is important to enhance nerve regeneration as well as prevent nerve degeneration. Nerve regeneration or sprouting in diabetes may occur not only in the nerve trunk but also in the dermis and around dorsal root ganglion neurons, thereby being implicated in the generation of pain sensation. Thus, inadequate nerve regeneration unequivocally contributes to the pathophysiologic mechanism of diabetic neuropathy. In this context, the research on nerve regeneration in diabetes should be more accelerated. Indeed, nerve regenerative capacity has been shown to be decreased in diabetic patients as well as in diabetic animals. Disturbed nerve regeneration in diabetes has been ascribed at least in part to all or some of decreased levels of neurotrophic factors, decreased expression of their receptors, altered cellular signal pathways and/or abnormal expression of cell adhesion molecules, although the mechanisms of their changes remain almost unclear. In addition to their steady-state changes in diabetes, nerve injury induces injury-specific changes in individual neurotrophic factors, their receptors and their intracellular signal pathways, which are closely linked with altered neuronal function, varying from neuronal survival and neurite extension/nerve regeneration to apoptosis. Although it is essential to clarify those changes for understanding the mechanism of disturbed nerve regeneration in diabetes, very few data are now available. Rationally accepted replacement therapy with neurotrophic factors has not provided any success in treating diabetic neuropathy. Aside from adverse effects of those factors, more rigorous consideration for their delivery system may be needed for any possible success. Although conventional therapeutic drugs like aldose reductase (AR) inhibitors and vasodilators have been shown to enhance nerve regeneration, their efficacy should be strictly evaluated with respect to nerve regenerative capacity. For this purpose, especially clinically, skin biopsy, by which cutaneous nerve pathology including nerve regeneration can be morphometrically evaluated, might be a safe and useful examination.

Neurofilaments in diabetic neuropathy

This review discusses the role of abnormal neurofilament (NF) expression, processing, and structure as an etiological factor in diabetic neuropathy. Diabetic sensory and autonomic neuropathy in humans is associated with a spectrum of structural changes in peripheral nerve that includes axonal degeneration, paranodal demyelination, and loss of myelinated fibers-- the latter is probably the result of a dying-back of distal axons. NF filaments are composed of three subunit proteins, NFL, NFM, and NFH, and are major constituents of the axonal cylinder. It is clear that any abnormality in synthesis, delivery, or processing of these critical proteins could lead to severe impairments in axon structure and function. This article describes mechanisms of synthesis, phosphorylation, and delivery of NF and discusses how these processes may be abnormal in diabetics. The pathological alterations in the ganglion and preipheral nerve that occur in sensory and autonomic neuropath will be outlined and related to possible abnormal processing of NF. A major focus is the role or aberrant NF phosphorylation and its possible involvement in the imparied delivery of NF to the distal axon. Identification of stress-activated protein kinases (SAPKs) as NF kinases is discussed in detail and it is proposed that hyperglycemia-induced activation of SAPKs may be a primary etiological event in diabetic neuropathy.

The environmental toxin arsenite induces tau hyperphosphorylation

Abnormally hyperphosphorylated tau polymers known as paired helical filaments constitute one of the major characteristic lesions that lead to the demise of neurons in Alzheimer's disease. Here, we demonstrate that the environmental toxin arsenite causes a significant increase in the phosphorylation of several amino acid residues (Thr-181, Ser-202, Thr-205, Thr-231, Ser-262, Ser-356, Ser-396, and Ser-404) in tau, which are also hyperphosphorylated under pathological conditions. Complementary phosphopeptide mapping revealed a dramatic increase in the (32)P-labeling of many peptides in tau following arsenite treatment. Although arsenite activates extracellular-signal regulated kinases-1/-2 and stress-activated protein kinases, these enzymes did not contribute to the arsenite-increased phosphorylation, nor did they appear to normally modify tau in vivo. Tau phosphorylation induced by arsenite did not involve glycogen synthase kinase-3 or protein phosphatase-1 or -2, but the activity responsible for tau hyperphosphorylation could be inhibited with the protein kinase inhibitor roscovitine. The effects of arsenite on the phosphorylation of some tau mutations (DeltaKappa280, V337M, and R406W) associated with frontal-temporal dementia with parkinsonism linked to chromosome 17 was analyzed. The unchallenged and arsenite-induced phosphorylation of some mutant proteins, especially R406W, was altered at several phosphorylation sites, indicating that these mutations can significantly affect the structure of tau in vivo. Although the major kinase(s) involved in aberrant tau phosphorylation remains elusive, these results indicate that environmental factors, such as arsenite, may be involved in the cascade leading to deregulation of tau function associated with neurodegeneration.

Chronic estrogen deficiency leads to molecular aberrations related to neurodegenerative changes in follitropin receptor knockout female mice

The follitropin receptor knockout (FORKO) mouse undergoes ovarian failure, thereby providing an animal model to investigate the consequences of the depletion of circulating estrogen in females. The estrogen deficiency causes marked defects in the female reproductive system, obesity, and skeletal abnormalities. In light of estrogen's known pleiotropic effects in the nervous system, our study examined the effects of genetically induced estrogen-testosterone imbalance on this system in female FORKO mice. Circulating concentrations of 17-beta-estradiol (E2) in FORKO mice are significantly decreased (FORKO -/-: 1.13+/-0.34 pg/ml; wild-type +/+: 17.6+/-3.5 pg/ml, P<0.0001, n=32-41); in contrast, testosterone levels are increased (-/-: 37.7+/-2.3 pg/ml; wild-type +/+: 3.9+/-1.7 pg/ml, P<0.005, n=25-33). The focus was on the activities of key enzymes in the central cholinergic and peripheral nervous systems, on dorsal root ganglia (DRGs) capacity for neurite outgrowth, and on the phosphorylation state of structural neurofilament (NF) proteins. Choline acetyltransferase activity was decreased in several central cholinergic structures (striatum 50+/-3%, hippocampus 24+/-2%, cortex 12+/-3%) and in DRGs (11+/-6%). Moreover, we observed aberrations in the enzymatic activities of mitogen-activated protein kinases (extracellular-regulated kinase and c-Jun N-terminal kinase) in the hippocampus, DRGs, and sciatic nerves. Hippocampal and sensory ganglia samples from FORKO mice contained hyper-phosphorylated NFs. Finally, explanted ganglia of FORKO mice displayed decreased neurite outgrowth (20-50%) under non-treated conditions and when treated with E2 (10 nM). Our results demonstrate that genetic depletion of circulating estrogen leads to biochemical and morphological changes in central and peripheral neurons, and underlie the importance of estrogen in the normal development and functioning of the nervous system. In particular, the findings suggest that an early and persisting absence of the steroid leads to neurodegenerative changes and identify several key enzymes that may contribute to the process. This model provides a system to explore the consequences of circulating estrogen deprivation and other hormonal imbalances in the nervous system.

In vivo and in vitro phosphorylation at Ser-493 in the glutamate (E)-segment of neurofilament-H subunit by glycogen synthase kinase 3beta

Neurofilament (NF), a major neuronal intermediate filament, is composed of three subunits, NF-L, NF-M, and NF-H. All three subunits contain a well conserved glutamate (E)-rich region called "E-segment" in the N terminus of the tail region. Although the E-segments of NF-L and NF-M are phosphorylated by casein kinases, it has not been observed in NF-H. Using mass spectrometric analysis, we identified phosphorylation of the E-segment of NF-H, prepared from rat spinal cords, at Ser-493 and Ser-501 in the Ser-Pro sequences. The E-segment kinase was isolated from rat brain extract using column chromatography and identified as glycogen synthase kinase (GSK) 3beta. GSK3beta was shown to phosphorylate at Ser-493 in vitro by phosphopeptide mapping and site-directed mutagenesis, and in vivo in HEK293 cells using the phospho-Ser-493 antibody, but did not phosphorylate Ser-501. GSK3beta preferred Ser-493 to the KSP-repeated sequences for phosphorylation sites in the NF-H tail domain. Moreover, Ser-493 was a better phosphorylation site for GSK3beta than other proline-directed protein kinases, Cdk5/p35 and ERK. GSK3beta in the spinal cord extract was associated with NF cytoskeletons. Taken together, we concluded that Ser-493 in the E-segment of NF-H is phosphorylated by GSK3beta in rat spinal cords.

AMPA receptor-mediated toxicity in oligodendrocyte progenitors involves free radical generation and activation of JNK, calpain and caspase 3

The molecular mechanisms underlying AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate) receptor-mediated excitotoxicity were characterized in rat oligodendrocyte progenitor cultures. Activation of AMPA receptors, in the presence of cyclothiazide to selectively block desensitization, produced a massive Ca(2+) influx and cytotoxicity which were blocked by the antagonists CNQX and GYKI 52466. A role for free radical generation in oligodendrocyte progenitor cell death was deduced from three observations: (i) treatment with AMPA agonists decreased intracellular glutathione; (ii) depletion of intracellular glutathione with buthionine sulfoximine potentiated cell death; and (iii) the antioxidant N -acetylcysteine replenished intracellular glutathione and protected cultures from AMPA receptor-mediated toxicity. Cell death displayed some characteristics of apoptosis, including DNA fragmentation, chromatin condensation and activation of caspase-3 and c-Jun N-terminal kinase (JNK). A substrate of calpain and caspase-3, alpha-spectrin, was cleaved into characteristic products following treatment with AMPA agonists. In contrast, inhibition of either caspase-3 by DEVD-CHO or calpain by PD 150606 protected cells from excitotoxicity. Our results indicate that overactivation of AMPA receptors causes apoptosis in oligodendrocyte progenitors through mechanisms involving Ca(2+) influx, depletion of glutathione, and activation of JNK, calpain, and caspase-3.

Myelin-associated glycoprotein modulates expression and phosphorylation of neuronal cytoskeletal elements and their associated kinases

Decreased phosphorylation of neurofilaments in mice lacking myelin-associated glycoprotein (MAG) was shown to be associated with decreased activities of extracellular-signal regulated kinases (ERK1/2) and cyclin-dependent kinase-5 (cdk5). These in vivo changes could be caused directly by the absence of a MAG-mediated signaling pathway or secondary to a general disruption of the Schwann cell-axon junction that prevents signaling by other molecules. Therefore, in vitro experimental paradigms of MAG interaction with neurons were used to determine if MAG directly influences expression and phosphorylation of cytoskeletal proteins and their associated kinases. COS-7 cells stably transfected with MAG or with empty vector were co-cultured with primary dorsal root ganglion (DRG) neurons. Total amounts of the middle molecular weight neurofilament subunit (NF-M), microtubule-associated protein 1B (MAP1B), MAP2, and tau were up-regulated significantly in DRG neurons in the presence of MAG. There was also increased expression of phosphorylated high molecular weight neurofilament subunit (NF-H), NF-M, and MAP1B. Additionally, in similar in vitro paradigms, total and phosphorylated NF-M were increased significantly in PC12 neurons co-cultured with MAG-expressing COS cells or treated with a soluble MAG Fc-chimera. The increased expression of phosphorylated cytoskeletal proteins in the presence of MAG in vitro was associated with increased activities of ERK 1/2 and cdk5. We propose that interaction of MAG with an axonal receptor(s) induces a signal transduction cascade that regulates expression of cytoskeletal proteins and their phosphorylation by these proline-directed protein kinases.

Overexpression of neurofilament H disrupts normal cell structure and function

Studying exogenously expressed tagged proteins in live cells has become a standard technique for evaluating protein distribution and function. Typically, expression levels of experimentally introduced proteins are not regulated, and high levels are often preferred to facilitate detection. However, overexpression of many proteins leads to mislocalization and pathologies. Therefore, for normative studies, moderate levels of expression may be more suitable. To understand better the dynamics of intermediate filament formation, transport, and stability in a healthy, living cell, we inserted neurofilament heavy chain (NFH)-green fluorescent protein (GFP) fusion constructs in adenoviral vectors with tetracycline (tet)-regulated promoters. This system allows for turning on or off the synthesis of NFH-GFP at a selected time, for a defined period, in a dose-dependent manner. We used this inducible system for live cell imaging of changes in filament structure and cell shape, motility, and transport associated with increasing NFH-GFP expression. Cells with low to intermediate levels of NFH-GFP were structurally and functionally similar to neighboring, nonexpressing cells. In contrast, overexpression led to pathological alterations in both filament organization and cell function.

Expression and purification of functional JNK2beta2: perspectives on high-level production of recombinant MAP kinases

The mitogen-activated protein (MAP) kinases are a group of serine/threonine kinases that mediate intracellular signal transduction in response to environmental stimuli including stress, growth factors, and various cytokines. Of this family, the c-Jun N-terminal kinases (JNKs) are members which, depending on cell type, have been shown to activate the transcription of genes involved in the inflammatory response, apoptosis, and hypertrophy. Here we report the use Baculovirus/Sf9 cells to produce milligram quantities of recombinant JNK2beta2 substrate which could be purified to >90% as judged by SDS-PAGE. In addition, we report a novel method for the site-specific biotinylation for this enzyme and demonstrate that the biotinylated product is an authentic substrate of the upstream kinases MKK4 and 7 and can phosphorylate a downstream target, ATF-2. We also show that the phosphorylated product can be captured efficiently on streptavidin-coated beads for use in scintillation proximity assays.

Signalling for survival and death in neurones: the role of stress-activated kinases, JNK and p38

The pathways involved in neuronal survival or death have been extensively studied mainly in cell lines. Recent evidence has suggested that activation of the stress activated pathways, jun N-terminal kinase (JNK) and p38 may play important roles in neuronal cell death or regeneration. In this review we will discuss these pahtways in detail. We will examine the evidence that these pathways are important in neuronal cell death. Finally we will review the evidence that inhibitors of these pathways have a neuroprotective effect both in vitro and in vivo.

Expression of Rho-family GTPases (Rac, cdc42, RhoA) and their association with p-21 activated kinase in adult rat peripheral nerve

To clarify the presence of the Rho family of small GTPases p21-activated kinase (pak) signaling pathway in the PNS, we have examined their expression, the association between the small GTPases and pak and the pak kinase activity in the PNS using immunoblot analysis, immunohistochemistry, co-immunoprecipitation study, and in vitro kinase assay. Immunoblot analysis showed the expression of Rac, cdc42, RhoA and pak in the dorsal root ganglion (DRG) and sciatic nerve. The localization of these proteins in the DRG neurons and axons and Schwann cells of the sciatic nerve was confirmed by immunohistochemistry. Co-immunoprecipitation studies indicated the in vivo associations of pak with Rac and cdc42, but not with RhoA, in both the DRG and sciatic nerve. The autophosphorylation of pak and phosphorylation of histone H4 by pak were also found in the DRG and sciatic nerve as well as in the CNS. These results suggest that the Rac/cdc42-pak signaling pathway exists and functions in the PNS and may mediate some intracellular signals.

Cyclin-dependent protein kinase 5 (Cdk5) and the regulation of neurofilament metabolism

Cyclin-dependent kinase 5 (Cdk5), a complex of Cdk5 and its activator p35 (Cdk5/p35), phosphorylates diverse substrates which have multifunctional roles in the nervous system. During development, it participates in neuronal differentiation, migration, axon outgrowth and synaptogenesis. Cdk5, acting together with other kinases, phosphorylates numerous KSPXK consensus motifs in diverse cytoskeletal protein target molecules, including neurofilaments, and microtubule associated proteins, tau and MAPs. Phosphorylation regulates the dynamic interactions of cytoskeletal proteins with one another during all aspects of neurogenesis and axon radial growth. In this review we shall focus on Cdk5 and its regulation as it modulates neurofilament metabolism in axon outgrowth, cytoskeletal stabilization and radial growth. We suggest that Cdk5/p35 forms compartmentalized macromolecular complexes of cytoskeletal substrates, other neuronal kinases, phosphatases and activators ('phosphorylation machines') which facilitate the dynamic molecular interactions that underlie these processes.

Cyclophosphamide attenuates the degenerative changes induced by CSF from patients with amyotrophic lateral sclerosis in the neonatal rat spinal cord

Our earlier studies have shown that cerebrospinal fluid (CSF) of patients with amyotrophic lateral sclerosis (ALS), when intrathecally injected into the neonatal rats, produces an aberrant phosphorylation of neurofilaments (NF) in the ventral horn neurons and reactive astrogliosis in the spinal cord. We wanted to investigate the effect of cyclophosphamide in the spinal cords of neonatal rats exposed to ALS-CSF. A single dose (5 microg in 5 microl saline) of cyclophosphamide was injected, 24 h after the administration of CSF samples from ALS and non-ALS neurological patients into the spinal subarachnoid space of 3-day-old rat pups. Rats were sacrificed after a period of 24 h, and stained with antibodies against the phosphorylated NF (SMI-31 antibody) and glial fibrillary acidic protein (GFAP). Cyclophosphamide treatment resulted in a 50% decrease in the number of SMI-31 stained neuronal soma in ventral horns of spinal cords of ALS-CSF exposed rats. This was accompanied by a decrease in the number of GFAP immunoreactive astrocytes. Furthermore, lactate dehydrogenase (LDH) activity was also decreased significantly, following cyclophosphamide treatment. These results suggest that cyclophosphamide could exert a neuroprotective effect against the neurotoxic action of factor(s) present in the ALS-CSF.