Neurofilaments can undergo axonal transport and cytoskeletal incorporation in a discontinuous manner

Expression of GSK3β, PICK1, NEFL, C4, NKCC1 and Synaptophysin in peripheral blood mononuclear cells of the first-episode schizophrenia patients

Schizophrenia (SZ) is a severe neurodevelopmental disease with unknown pathogenic mechanisms characterized with impaired cognitive function. The disturbed synaptic plasticity and synaptic loss have been widely reported in SZ. In this study, 41 first-episode schizophrenia (FES) patients and 44 healthy controls (HC) were recruited and the expression of six genes commonly relevant to synaptic functions was examined in the peripheral blood mononuclear cells (PBMCs). These genes were glycogen synthase kinase 3β (GSK3β), protein interacting with C-kinase 1 (PICK1), synaptophysin (SYP), neurofilament light (NEFL), complement component 4 (C4) and Na⁺-K^--2Cl^- cotransporter 1 (NKCC1). Real-time quantitative polymerase chain reaction (qPCR) was performed to determine the quantity of individual mRNA template. Compared to HC, the expression of PICK1 and NKCC1 genes in FES patients was relatively lower whereas the expression of NEFL was higher. No difference for the mRNA expression of GSK3β, SYP and C4 genes was detected between FES patients and HC, nor was the gender difference; Interestingly, the mRNA expression of PICK1 in female FES patients was significantly decreased compared to female HC, but not in males; and the NEFL gene was up-regulated in male FES patients but not in females. Our findings support an abnormal expression profile of synapse-related genes in the PBMCs of FES patients.

Assembly and turnover of neurofilaments in growing axonal neurites

Neurofilaments (NFs) are thought to provide stability to the axon. We examined NF dynamics within axonal neurites of NB2a/d1 neuroblastoma by transient transfection with green fluorescent protein-tagged NF-heavy (GFP-H) under the control of a tetracycline-inducible promoter. Immunofluorescent and biochemical analyses demonstrated that GFP-H expressed early during neurite outgrowth associated with a population of centrally-situated, highly-phosphorylated crosslinked NFs along the length of axonal neurites (‘bundled NFs’). By contrast, GFP-H expressed after considerable neurite outgrowth displayed markedly reduced association with bundled NFs and was instead more evenly distributed throughout the axon. This differential localization was maintained for up to 2 weeks in culture. Once considerable neurite outgrowth had progressed, GFP that had previously associated with the NF bundle during early expression was irreversibly depleted by photobleaching. Cessation of expression allowed monitoring of NF turnover. GFP-H associated bundled NFs underwent slower decay than GFP-H associated with surrounding, less-phosphorylated NFs. Notably, GFP associated with bundled NFs underwent similar decay rates within the core and edges of this bundle. These results are consistent with previous demonstration of a resident NF population within axonal neurites, but suggest that this population is more dynamic than previously considered.

Summary: Immunofluorescent and radiolabel analyses demonstrate that neurofilaments establish a resident population within growing axonal neurites that undergoes exchange with a surrounding, transporting pool.

Repeated Three-Hour Maternal Separation Induces Depression-Like Behavior and Affects the Expression of Hippocampal Plasticity-Related Proteins in C57BL/6N Mice

Adverse early life experiences can negatively affect behaviors later in life. Maternal separation (MS) has been extensively investigated in animal models in the adult phase of MS. The study aimed to explore the mechanism by which MS negatively affects C57BL/6N mice, especially the effects caused by MS in the early phase. Early life adversity especially can alter plasticity functions. To determine whether adverse early life experiences induce changes in plasticity in the brain hippocampus, we established an MS paradigm. In this research, the mice were treated with mild (15 min, MS15) or prolonged (180 min, MS180) maternal separation from postnatal day 2 to postnatal day 21. The mice underwent a forced swimming test, a tail suspension test, and an open field test, respectively. Afterward, the mice were sacrificed on postnatal day 31 to determine the effects of MS on early life stages. Results implied that MS induces depression-like behavior and the effects may be mediated partly by interfering with the hippocampal GSK-3β-CREB signaling pathway and by reducing the levels of some plasticity-related proteins.

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.

Ginseng Total Saponins Reverse Corticosterone-Induced Changes in Depression-Like Behavior and Hippocampal Plasticity-Related Proteins by Interfering with GSK-3 β -CREB Signaling Pathway

This study aimed to explore the antidepressant mechanisms of ginseng total saponins (GTS) in the corticosterone-induced mouse depression model. In Experiment 1, GTS (50, 25, and 12.5 mg kg(-1) d(-1), intragastrically) were given for 3 weeks. In Experiment 2, the same doses of GTS were administrated after each corticosterone (20 mg kg(-1) d(-1), subcutaneously) injection for 22 days. In both experiments, mice underwent a forced swimming test and a tail suspension test on day 20 and day 21, respectively, and were sacrificed on day 22. Results of Experiment 1 revealed that GTS (50 and 25 mg kg(-1) d(-1)) exhibited antidepressant activity and not statistically altered hippocampal protein levels of brain-derived neurotrophic factor (BDNF) and neurofilament light chain (NF-L). Results of Experiment 2 showed that GTS (50 and 25 mg kg(-1) d(-1)) ameliorated depression-like behavior without normalizing hypercortisolism. The GTS treatments reversed the corticosterone-induced changes in mRNA levels of BDNF and NF-L, and protein levels of BDNF NF-L, phosphor-cAMP response element-binding protein (Ser133), and phosphor-glycogen synthase kinase-3 β (Ser9) in the hippocampus. These findings imply that the effect of GTS on corticosterone-induced depression-like behavior may be mediated partly through interfering with hippocampal GSK-3 β -CREB signaling pathway and reversing decrease of some plasticity-related proteins.

Charcot-Marie-Tooth causing HSPB1 mutations increase Cdk5-mediated phosphorylation of neurofilaments

Mutations in the small heat shock protein HSPB1 (HSP27) are a cause of axonal Charcot-Marie-Tooth neuropathy (CMT2F) and distal hereditary motor neuropathy. To better understand the effect of mutations in HSPB1 on the neuronal cytoskeleton, we stably transduced neuronal cells with wild-type and mutant HSPB1 and investigated axonal transport of neurofilaments (NFs). We observed that mutant HSPB1 affected the binding of NFs to the anterograde motor protein kinesin, reducing anterograde transport of NFs. These deficits were associated with an increased phosphorylation of NFs and cyclin-dependent kinase Cdk5. As Cdk5 mediates NF phosphorylation, inhibition of Cdk5/p35 restored NF phosphorylation level, as well as NF binding to kinesin in mutant HSPB1 neuronal cells. Altogether, we demonstrate that HSPB1 mutations induce hyperphosphorylation of NFs through Cdk5 and reduce anterograde transport of NFs.

Interference with kinesin-based anterograde neurofilament axonal transport increases neurofilament-neurofilament bundling

Neurofilaments (NFs) associate with each other and with other cytoskeletal elements to form a lattice that supports the mature axon. Phosphorylation contributes to formation of this stationary population of NFs by fostering cation-dependent interactions among NF sidearms. Association of NFs with the stationary phase indirectly competes with NF axonal transport by withdrawing NFs from kinesin-dependent motility along microtubules. We therefore hypothesized that inhibition of anterograde NF transport may increase incorporation into the stationary phase. To test this hypothesis, we treated differentiated NB2a/d1 cells expressing GFP-tagged NF subunits with monastrol, a specific inhibitor of kinesin-5. Monastrol significantly inhibited anterograde axonal transport of NF-H but not NF-M, and increased the incorporation of newly-transported NF subunits into axonal NF bundles. These findings support the notion that NF transport and bundling exert opposing forces on axonal NF dynamics, and that inhibition of anterograde transport of NFs can increase their incorporation into the stationary phase.

Neurofilament phosphorylation regulates axonal transport by an indirect mechanism: a merging of opposing hypotheses

Neurofilaments (NFs) are among the most abundant constituents of the axonal cytoskeleton. NFs consist of four subunits, termed NF-H, NF-M and NF-L, corresponding to heavy, medium and light in reference to their molecular mass and α-internexin. Phosphorylation of the C-terminal "sidearms" of NF-H and NF-M regulates the ability of NFs to form a cytoskeletal lattice that supports the mature axon. C-terminal phosphorylation events have classically been considered to regulate NF axonal transport. By contrast, studies demonstrating that NF axonal transport was not accelerated following sidearm deletion provided evidence that phosphorylation does not regulate NF transport. Herein, we demonstrate how comparison of transport and distribution of differentially phosphorylated NFs along axons identify common ground between these hypotheses and may resolve this controversy.

C-terminal neurofilament phosphorylation fosters neurofilament-neurofilament associations that compete with axonal transport

Neurofilaments (NFs) associate with each other and with other cytoskeletal elements to form a lattice that supports the mature axon. Phosphorylation contributes to formation of this structure by fostering cation-dependent interactions among NF sidearms. By inducing NF bundling, phosphorylation impedes their axonal transport. To examine the impact of the known NF kinase cdk5 on these phenomena, transfected cells with constructs expressing GFP-tagged NF-H sidearms (lacking the rod domain to preclude assembly) with and without site-directed mutagenesis of 7 cdk5 consensus sites, and monitored the impact on NF transport and association with the axonal NF bundle. These mutations did not alter transport but pseudo-phosphorylated mutants displayed a greater association with axonal NF bundles. By contrast, these same mutations in full-length NF-H altered NF transport as well as bundling. Since isolated sidearms cannot assemble, they can only interact with NFs via a single sidearm-sidearm interaction, while assembled NFs can form multiple such interactions. These finding suggest that individual sidearm-sidearm interactions are dynamic and do not persist long enough to slow NF transport, and that bundle formation and maintenance depends upon both the long half-life of NF polymers and the establishment of multiple phosphorylation-dependent sidearm-mediated interactions among NFs.

Neurofilament cross-bridging competes with kinesin-dependent association of neurofilaments with microtubules

The phosphorylation of neurofilaments (NFs) has long been considered to regulate their axonal transport rate and in doing so to provide stability to mature axons. Axons contain a centrally situated ;bundle' of closely opposed phospho-NFs that display a high degree of NF-NF associations and phospho-epitopes, surrounded by less phosphorylated ;individual' NFs that are often associated with kinesin and microtubules (MTs). Bundled NFs transport substantially slower than the surrounding individual NFs and might represent a resident population that stabilizes axons and undergoes replacement by individual NFs. To examine this possibility, fractions enriched in bundled NFs and individual NFs were generated from mice and NB2a/d1 cells by sedimentation of cytoskeletons over a sucrose cushion. More kinesin was recovered within individual versus bundled NF fractions. Individual but not bundled NFs aligned with purified MTs under cell-free conditions. The percentage of NFs that aligned with MTs was increased by the addition of kinesin, and inhibited by anti-kinesin antibodies. Bundles dissociated following incubation with EGTA or alkaline phosphatase, generating individual NFs that retained or were depleted of phospho-epitopes, respectively. These dissociated NFs aligned with MTs at a level identical to those originally isolated as individual NFs regardless of phosphorylation state. EGTA-mediated dissociation of bundles was prevented and reversed by excess Ca(2+), whereas individual NFs did not associate in the presence of excess Ca(2+). These findings confirm that bundling competes with NF-MT association, and provide a mechanism by which C-terminal NF phosphorylation might indirectly contribute to the observed slowing in axonal transport of phospho-NFs.

Neurofilament dynamics: a tug of war by microtubule motors

Structural support for axons, which can consist of volumes thousands of times larger than the neuronal perikaryon, is provided in part by neurofilaments (NFs), the major fibrous constituent of the axonal cytoskeleton. Most NFs undergo anterograde transport (towards the synapse or growth cone), while a few undergo retrograde transport (back towards the perikaryon). Some NFs exhibit an extended residence time along axons, which allows NFs to provide structural support to the axon yet minimizes NF turnover, which would otherwise impart a prohibitive metabolic burden upon the neuron. Herein, we explore known and hypothesized roles for microtubule motors in transport and distribution of NFs along axons. We present evidence that those NFs that display extended residence along axons are critically dependent upon surrounding microtubules, and that simultaneous interaction with multiple microtubule motors provides the architectural force regulating their distribution.

Organizational dynamics, functions, and pathobiological dysfunctions of neurofilaments

Neurofilament phosphorylation has long been considered to regulate their axonal transport rate, and in doing so it provides stability to mature axons. We evaluate the collective evidence to date regarding how neurofilament C-terminal phosphorylation may regulate axonal transport. We present a few suggestions for further experimentation in this area, and expand upon previous models for axonal NF dynamics. We present evidence that the NFs that display extended residence along axons are critically dependent upon the surrounding microtubules, and that simultaneous interaction with multiple microtubule motors provides the architectural force that regulates their distribution. Finally, we address how C-terminal phosphorylation is regionally and temporally regulated by a balance of kinase and phosphatase activities, and how misregulation of this balance might contribute to motor neuron disease.

Regulation of neurofilament dynamics by phosphorylation

Neurofilament (NF) phosphorylation has long been considered to regulate axonal transport rate and in doing so to provide stability to mature axons. Studies utilizing mice in which the C-terminal region of NF subunits (which contains the vast majority of phosphorylation sites) has been deleted has prompted an ongoing challenge to this hypothesis. We evaluate the collective evidence to date for and against a role for NF C-terminal phosphorylation in regulation of axonal transport and in providing structural support for axons, including some novel studies from our laboratory. We present a few suggestions for further experimentation in this area, and expand upon previous models for axonal NF dynamics. Finally, we address how C-terminal phosphorylation is regionally and temporally regulated by a balance of kinase and phosphatase activities, and how misregulation of this balance can contribute to motor neuron disease.

AS601245, a c-Jun NH2-terminal kinase (JNK) inhibitor, reduces axon/dendrite damage and cognitive deficits after global cerebral ischaemia in gerbils

BACKGROUND AND PURPOSE

Based on their proven ability, in animal models of stroke, to reduce damage to brain grey matter, many drugs have been tested in clinical trials but without success. Failure to save axons from injury and to protect functional outcome has been proposed as the major reason for this lack of success. We have previously demonstrated in two rodent models of cerebral ischaemia, that AS601245 (1,3-benzothiazol-2-yl (2-([2-(3-pyridinyl) ethyl] amino)-4 pyrimidinyl) acetonitrile), an inhibitor of the c-Jun NH(2)-terminal kinase (JNK), has neuroprotective properties. The aim of the present study was to further investigate if AS601245 in addition to its ability to protect neurons also could protect neurites and preserve memory after cerebral ischaemia, in gerbils.

EXPERIMENTAL APPROACH

Using immunohistochemical techniques and a behavioural test, we studied the effect of the compound AS601245 on neurodegeneration and cognitive deficits after global cerebral ischaemia in gerbils.

KEY RESULTS

At a dose of 80 mg kg(-1), i.p., AS601245 reduced damage to neurites by 67% (P<0.001 versus controls) and activation of astrocytes by 84% (P<0.001 versus controls). In addition, AS601245 (80 mg kg(-1), i.p.) prevented ischaemia-induced impairment of memory in the inhibitory avoidance task model.

CONCLUSIONS AND IMPLICATIONS

The present results suggest that AS601245 reduced damage to neurites and decreased astrogliosis following global ischaemia and also improved long-term memory, supporting JNK inhibition as a promising therapeutic strategy for ischaemic insults to the CNS.

Inhibition of dynein but not kinesin induces aberrant focal accumulation of neurofilaments within axonal neurites

Studies from several laboratories indicate that the microtubule motors kinesin and dynein respectively participate in anterograde and retrograde axonal transport of neurofilaments. Inhibition of dynein function by transfection with a construct expressing dynamitin or intracellular delivery of anti-dynein antibodies accelerates anterograde transport, which has been interpreted to indicate that the opposing action of both motors mediates the normal distribution of neurofilaments along axons. Herein, we demonstrate that, while expression of relatively low levels of exogenous dynamitin indeed accelerated anterograde neurofilament transport along axonal neurites in culture, expression of progressively increasing levels of dynamitin induced focal accumulation of neurofilaments within axonal neurites and eventually caused neurite retraction. Inhibition of kinesin inhibited anterograde transport, but did not induce similar focal accumulations. These findings are consistent with studies indicating that perturbations in dynein activity can contribute to the aberrant accumulations of neurofilaments that accompany ALS/motor neuron disease.

Softness, strength and self-repair in intermediate filament networks

One cellular function of intermediate filaments is to provide cells with compliance to small deformations while strengthening them when large stresses are applied. How IFs accomplish this mechanical role is revealed by recent studies of the elastic properties of single IF protein polymers and by viscoelastic characterization of the networks they form. IFs are unique among cytoskeletal filaments in withstanding large deformations. Single filaments can stretch to more than 3 times their initial length before breaking, and gels of IF withstand strains greater than 100% without damage. Even after mechanical disruption of gels formed by crossbridged neurofilaments, the elastic modulus of these gels rapidly recovers under conditions where gels formed by actin filaments are irreversibly ruptured. The polyelectrolyte properties of IFs may enable crossbridging by multivalent counterions, but identifying the mechanisms by which IFs link into bundles and networks in vivo remains a challenge.

Dynein mediates retrograde neurofilament transport within axons and anterograde delivery of NFs from perikarya into axons: Regulation by multiple phosphorylation events

We examined the respective roles of dynein and kinesin in axonal transport of neurofilaments (NFs). Differentiated NB2a/d1 cells were transfected with green fluorescent protein-NF-M (GFP-M) and dynein function was inhibited by co-transfection with a construct expressing myc-tagged dynamitin, or by intracellular delivery of purified dynamitin and two antibodies against dynein's cargo domain. Monitoring of the bulk distribution of GFP signal within axonal neurites, recovery of GFP signal within photobleached regions, and real-time monitoring of individual NFs/punctate structures each revealed that pertubation of dynein function inhibited retrograde transport and accelerated anterograde, confirming that dynein mediated retrograde axonal transport, while intracellular delivery of two anti-kinesin antibodies selectively inhibited NF anterograde transport. In addition, dynamitin overexpression inhibited the initial translocation of newly-expressed NFs out of perikarya and into neurites, indicating that dynein participated in the initial anterograde delivery of NFs into neurites. Delivery of NFs to the axon hillock inner plasma membrane surface, and their subsequent translocation into neurites, was also prevented by vinblastine-mediated inhibition of microtubule assembly. These data collectively suggest that some NFs enter axons as cargo of microtubues that are themselves undergoing transport into axons via dynein-mediated interactions with the actin cortex and/or larger microtubules. C-terminal NF phosphorylation regulates motor association, since anti-dynein selectively coprecipitated extensively phosphorylated NFs, while anti-kinesin selectively coprecipitated less phosphorylated NFs. In addition, however, the MAP kinase inhibitor PD98059 also inhibited transport of a constitutively-phosphorylated NF construct, indicating that one or more additional, non-NF phosphorylation events also regulated NF association with dynein or kinesin.