Regulated association of microtubule-associated protein 2 (MAP2) with Src and Grb2: evidence for MAP2 as a scaffolding protein

Deciphering the alteration of MAP2 interactome caused by a schizophrenia-associated phosphorylation

Microtubule-associated protein 2 (MAP2) is a crucial regulator of dendritic structure and neuronal function, orchestrating diverse protein interactions within the microtubule network. We have shown MAP2 is hyperphosphorylated at serine 1782 (S1782) in schizophrenia and phosphomimetic mutation of S1782 in mice (MAP2S1782E) is sufficient to impair dendritic architecture. We sought to determine how this hyperphosphorylation affects the MAP2 interactome to provide insights into the disorder's mechanisms. We investigated the MAP2 interactome using co-immunoprecipitation and mass spectrometry in MAP2S1782E and MAP2WT mice. We found that S1782E MAP2 led to a substantial disruption of protein-protein interactions relative to WT MAP2. Reduced interactions with PDZ domain-containing proteins, calmodulin-binding proteins, ribosome proteins, and kinesin proteins may all contribute to dendritic impairments induced by S1782E, and may be linked to schizophrenia pathogenesis. Interestingly, novel gain-of-function interactions with PPM1L and KLHL8 nominated these as regulators of phosphoS1782 MAP2 abundance and potential therapeutic targets in schizophrenia.

Structural basis of binding the unique N-terminal domain of microtubule-associated protein 2c to proteins regulating kinases of signaling pathways

Isoforms of microtubule-associated protein 2 (MAP2) differ from their homolog Tau in the sequence and interactions of the N-terminal region. Binding of the N-terminal region of MAP2c (N-MAP2c) to the dimerization/docking domains of the regulatory subunit RIIα of cAMP-dependent protein kinase (RIIDD2) and to the Src-homology domain 2 (SH2) of growth factor receptor-bound protein 2 (Grb2) have been described long time ago. However, the structural features of the complexes remained unknown due to the disordered nature of MAP2. Here, we provide structural description of the complexes. We have solved solution structure of N-MAP2c in complex with RIIDD2, confirming formation of an amphiphilic α-helix of MAP2c upon binding, defining orientation of the α-helix in the complex and showing that its binding register differs from previous predictions. Using chemical shift mapping, we characterized the binding interface of SH2-Grb2 and rat MAP2c phosphorylated by the tyrosine kinase Fyn in their complex and proposed a model explaining differences between SH2-Grb2 complexes with rat MAP2c and phosphopeptides with a Grb2-specific sequence. The results provide the structural basis of a potential role of MAP2 in regulating cAMP-dependent phosphorylation cascade via interactions with RIIDD2 and Ras signaling pathway via interactions with SH2-Grb2.

The atypical antidepressant tianeptine confers neuroprotection against oxygen-glucose deprivation

Proregenerative and neuroprotective effects of antidepressants are an important topic of inquiry in neuropsychiatric research. Oxygen-glucose deprivation (OGD) mimics key aspects of ischemic injury in vitro. Here, we studied the effects of 24-h pretreatment with serotonin (5-HT), citalopram (CIT), fluoxetine (FLU), and tianeptine (TIA) on primary mouse cortical neurons subjected to transient OGD. 5-HT (50 μM) significantly enhanced neuron viability as measured by MTT assay and reduced cell death and LDH release. CIT (10 μM) and FLU (1 μM) did not increase the effects of 5-HT and neither antidepressant conferred neuroprotection in the absence of supplemental 5-HT in serum-free cell culture medium. By contrast, pre-treatment with TIA (10 μM) resulted in robust neuroprotection, even in the absence of 5-HT. Furthermore, TIA inhibited mRNA transcription of candidate genes related to cell death and hypoxia and attenuated lipid peroxidation, a hallmark of neuronal injury. Finally, deep RNA sequencing of primary neurons subjected to OGD demonstrated that OGD induces many pathways relating to cell survival, the inflammation-immune response, synaptic dysregulation and apoptosis, and that TIA pretreatment counteracted these effects of OGD. In conclusion, this study highlights the comparative strength of the 5-HT independent neuroprotective effects of TIA and identifies the molecular pathways involved.

The neuroprotective mechanisms of naringenin: Inhibition of apoptosis through the PI3K/AKT pathway after hypoxic-ischemic brain damage

ETHNOPHARMACOLOGICAL RELEVANCE

Naringenin (NGN) is a widely distributed flavonoid with potent antioxidant and neuroprotective properties. Neuroprotective agents play a crucial role in the treatment of hypoxic-ischemic encephalopathy (HIE). It has shown potential therapeutic effects for neurological disorders. However, its efficacy on HIE is yet to be investigated.

AIM OF THE STUDY

This study aims to investigate the potential neuroprotective effect of naringenin and its underlying molecular mechanisms in reducing oxidative stress, apoptosis, and improving brain outcomes following HIE. Additionally, the study aims to identify the potential targets, mechanisms, and functions of naringenin using network pharmacology analysis.

MATERIALS AND METHODS

Neonatal mice were exposed to the hypoxic-ischemic brain damage (HIBD) model to determine brain water content, and brain tissue was subjected to hematoxylin and eosin (HE), immunohistochemistry (IHC), terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), and Nissl staining to investigate its neuroprotective effects. Furthermore, the neonatal mouse primary neuron oxygen-glucose deprivation (OGD) model to measure reactive oxygen species (ROS) production in vitro. The protein levels were characterized by Western Blot, and mRNA levels were evaluated by a real-time quantitative PCR detecting system (qPCR). Transmission electron microscopy (TEM) and mitochondrial fluorescent staining were used to observe mitochondrial morphology. Neuronal nuclei (NeuN) and microtubule-associated protein 2 (MAP2) were detected by Immunofluorescence (IF). Finally, network pharmacology was employed to determine the common target of naringenin and HIE. The core genes were obtained via protein-protein interaction networks (PPI) analysis and molecular docking was examined, and the mechanism of action was explored through Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. Additionally, small interfering RNA (siRNA) was constructed for verification.

RESULTS

Naringenin has a neuroprotective effect in HIBD by modulating Vegfa expression and activating the PI3K/AKT pathway to inhibit apoptosis. Furthermore, molecular docking results suggest that Vegfa is a potential binding target of naringenin, and silencing Vegfa partially reverses the pharmacological effects of NGN.

CONCLUSION

Our findings suggest that naringenin demonstrates potential clinical application for treating HIE as a novel neuroprotective agent.

More than a marker: potential pathogenic functions of MAP2

Microtubule-associated protein 2 (MAP2) is the predominant cytoskeletal regulator within neuronal dendrites, abundant and specific enough to serve as a robust somatodendritic marker. It influences microtubule dynamics and microtubule/actin interactions to control neurite outgrowth and synaptic functions, similarly to the closely related MAP Tau. Though pathology of Tau has been well appreciated in the context of neurodegenerative disorders, the consequences of pathologically dysregulated MAP2 have been little explored, despite alterations in its immunoreactivity, expression, splicing and/or stability being observed in a variety of neurodegenerative and neuropsychiatric disorders including Huntington’s disease, prion disease, schizophrenia, autism, major depression and bipolar disorder. Here we review the understood structure and functions of MAP2, including in neurite outgrowth, synaptic plasticity, and regulation of protein folding/transport. We also describe known and potential mechanisms by which MAP2 can be regulated via post-translational modification. Then, we assess existing evidence of its dysregulation in various brain disorders, including from immunohistochemical and (phospho) proteomic data. We propose pathways by which MAP2 pathology could contribute to endophenotypes which characterize these disorders, giving rise to the concept of a “MAP2opathy”—a series of disorders characterized by alterations in MAP2 function.

Specific phosphorylation of microtubule-associated protein 2c by extracellular signal-regulated kinase reduces interactions at its Pro-rich regions

Microtubule-associated protein 2 (MAP2) is an important neuronal target of extracellular signal-regulated kinase 2 (ERK2) involved in Raf signaling pathways, but mechanistic details of MAP2 phosphorylation are unclear. Here, we used NMR spectroscopy to quantitatively describe the kinetics of phosphorylation of individual serines and threonines in the embryonic MAP2 variant MAP2c. We carried out real-time monitoring of phosphorylation to discover major phosphorylation sites that were not identified in previous studies relying on specific antibodies. Our comparison with phosphorylation of MAP2c by a model cyclin-dependent kinase CDK2 and with phosphorylation of the MAP2c homolog Tau revealed differences in phosphorylation profiles that explain specificity of regulation of biological functions of MAP2c and Tau. To probe the molecular basis of the regulatory effect of ERK2, we investigated the interactions of phosphorylated and unphosphorylated MAP2c by NMR with single-residue resolution. As ERK2 phosphorylates mostly outside the regions binding microtubules, we studied the binding of proteins other than tubulin, namely regulatory subunit RIIα of cAMP-dependent protein kinase (PKA), adaptor protein Grb2, Src homology domain 3 of tyrosine kinases Fyn and Abl, and ERK2 itself. We found ERK2 phosphorylation interfered mostly with binding to proline-rich regions of MAP2c. Furthermore, our NMR experiments in SH-SY5Y neuroblastoma cell lysates showed that the kinetics of dephosphorylation are compatible with in-cell NMR studies and that residues targeted by ERK2 and PKA are efficiently phosphorylated in the cell lysates. Taken together, our results provide a deeper characterization of MAP2c phosphorylation and its effects on interactions with other proteins.

Human Cytomegalovirus Immediate Early Protein 2 Protein Causes Cognitive Disorder by Damaging Synaptic Plasticity in Human Cytomegalovirus-UL122-Tg Mice

Human cytomegalovirus (HCMV) infection is very common in the human population all around the world. Although the majority of HCMV infections are asymptomatic, they can cause neurologic deficits. Previous studies have shown that immediate early protein 2 (IE2, also known as UL122) of HCMV is related with the cognitive disorder mechanism. Due to species isolation, a HCMV-infected animal model could not be established which meant a study into the long-term effects of IE2 on neural development could not be carried out. By establishing HCMV-UL122-Tg mice (UL122 mice), we explored the cognitive behavior and complexity of neuron changes in this transgenic UL122 mice that could consistently express IE2 protein at different ages (confirmed in both 6- and 12-month-old UL122 mice). In the Morris water maze, cognitive impairment was more pronounced in 12-month-old UL122 mice than in 6-month-old ones. At the same time, a decrease of the density of dendritic spines and branches in the hippocampal neurons of 12-month-old mice was observed. Moreover, long-term potentiation was showed to be impaired in 12-month-old UL122 mice. The expressions of several synaptic plasticity-regulated molecules were reduced in 12-month-old UL122 mice, including scaffold proteins postsynaptic density protein 95 (PSD95) and microtubule-associated protein 2 (MAP2). Binding the expression of IE2 was increased in 12-month-old mice compared with 6-month-old mice, and results of statistical analysis suggested that the cognitive damage was not caused by natural animal aging, which might exclude the effect of natural aging on cognitive impairment. All these results suggested that IE2 acted as a pathogenic regulator in damaging synaptic plasticity by downregulating the expression of plasticity-related proteins (PRPs), and this damage increased with aging.

MAP2 is differentially phosphorylated in schizophrenia, altering its function

Schizophrenia (Sz) is a highly polygenic disorder, with common, rare, and structural variants each contributing only a small fraction of overall disease risk. Thus, there is a need to identify downstream points of convergence that can be targeted with therapeutics. Reduction of microtubule-associated protein 2 (MAP2) immunoreactivity (MAP2-IR) is present in individuals with Sz, despite no change in MAP2 protein levels. MAP2 is phosphorylated downstream of multiple receptors and kinases identified as Sz risk genes, altering its immunoreactivity and function. Using an unbiased phosphoproteomics approach, we quantified 18 MAP2 phosphopeptides, 9 of which were significantly altered in Sz subjects. Network analysis grouped MAP2 phosphopeptides into three modules, each with a distinct relationship to dendritic spine loss, synaptic protein levels, and clinical function in Sz subjects. We then investigated the most hyperphosphorylated site in Sz, phosphoserine1782 (pS1782). Computational modeling predicted phosphorylation of S1782 reduces binding of MAP2 to microtubules, which was confirmed experimentally. We generated a transgenic mouse containing a phosphomimetic mutation at S1782 (S1782E) and found reductions in basilar dendritic length and complexity along with reduced spine density. Because only a limited number of MAP2 interacting proteins have been previously identified, we combined co-immunoprecipitation with mass spectrometry to characterize the MAP2 interactome in mouse brain. The MAP2 interactome was enriched for proteins involved in protein translation. These associations were shown to be functional as overexpression of wild type and phosphomimetic MAP2 reduced protein synthesis in vitro. Finally, we found that Sz subjects with low MAP2-IR had reductions in the levels of synaptic proteins relative to nonpsychiatric control (NPC) subjects and to Sz subjects with normal and MAP2-IR, and this same pattern was recapitulated in S1782E mice. These findings suggest a new conceptual framework for Sz-that a large proportion of individuals have a "MAP2opathy"-in which MAP function is altered by phosphorylation, leading to impairments of neuronal structure, synaptic protein synthesis, and function.

Beyond Neuronal Microtubule Stabilization: MAP6 and CRMPS, Two Converging Stories

The development and function of the central nervous system rely on the microtubule (MT) and actin cytoskeletons and their respective effectors. Although the structural role of the cytoskeleton has long been acknowledged in neuronal morphology and activity, it was recently recognized to play the role of a signaling platform. Following this recognition, research into Microtubule Associated Proteins (MAPs) diversified. Indeed, historically, structural MAPs—including MAP1B, MAP2, Tau, and MAP6 (also known as STOP);—were identified and described as MT-binding and -stabilizing proteins. Extensive data obtained over the last 20 years indicated that these structural MAPs could also contribute to a variety of other molecular roles. Among multi-role MAPs, MAP6 provides a striking example illustrating the diverse molecular and cellular properties of MAPs and showing how their functional versatility contributes to the central nervous system. In this review, in addition to MAP6’s effect on microtubules, we describe its impact on the actin cytoskeleton, on neuroreceptor homeostasis, and its involvement in signaling pathways governing neuron development and maturation. We also discuss its roles in synaptic plasticity, brain connectivity, and cognitive abilities, as well as the potential relationships between the integrated brain functions of MAP6 and its molecular activities. In parallel, the Collapsin Response Mediator Proteins (CRMPs) are presented as examples of how other proteins, not initially identified as MAPs, fall into the broader MAP family. These proteins bind MTs as well as exhibiting molecular and cellular properties very similar to MAP6. Finally, we briefly summarize the multiple similarities between other classical structural MAPs and MAP6 or CRMPs.In summary, this review revisits the molecular properties and the cellular and neuronal roles of the classical MAPs, broadening our definition of what constitutes a MAP.

Plectin dysfunction in neurons leads to tau accumulation on microtubules affecting neuritogenesis, organelle trafficking, pain sensitivity and memory

AIMS

Plectin, a universally expressed multi-functional cytolinker protein, is crucial for intermediate filament networking, including crosstalk with actomyosin and microtubules. In addition to its involvement in a number of diseases affecting skin, skeletal muscle, heart, and other stress-exposed tissues, indications for a neuropathological role of plectin have emerged. Having identified P1c as the major isoform expressed in neural tissues in previous studies, our aim for the present work was to investigate whether, and by which mechanism(s), the targeted deletion of this isoform affects neuritogenesis and proper nerve cell functioning.

METHODS

For ex vivo phenotyping, we used dorsal root ganglion and hippocampal neurons derived from isoform P1c-deficient and plectin-null mice, complemented by in vitro experiments using purified proteins and cell fractions. To assess the physiological significance of the phenotypic alterations observed in P1c-deficient neurons, P1c-deficient and wild-type littermate mice were subjected to standard behavioural tests.

RESULTS

We demonstrate that P1c affects axonal microtubule dynamics by isoform-specific interaction with tubulin. P1c deficiency in neurons leads to altered dynamics of microtubules and excessive association with tau protein, affecting neuritogenesis, neurite branching, growth cone morphology, and translocation and directionality of movement of vesicles and mitochondria. On the organismal level, we found P1c deficiency manifesting as impaired pain sensitivity, diminished learning capabilities and reduced long-term memory of mice.

CONCLUSIONS

Revealing a regulatory role of plectin scaffolds in microtubule-dependent nerve cell functions, our results have potential implications for cytoskeleton-related neuropathies.

Reparative Effects of Stem Cell Factor and Granulocyte Colony-Stimulating Factor in Aged APP/PS1 Mice

Alzheimer's disease (AD), characterized by the accumulation of β-amyloid (Aβ) plaques and tau neurofibrillary tangles in the brain, neuroinflammation and neurodegeneration, is the most common form of neurodegenerative disease among the elderly. No effective treatment is available now in restricting the pathological progression of AD. The aim of this study is to determine the therapeutic efficacy of stem cell factor (SCF) and granulocyte colony-stimulating factor (G-CSF) (SCF+G-CSF) in aged APPswe/PS1dE9 (APP/PS1) mice. SCF+G-CSF was subcutaneously injected for 12 days to 25-month-old male APP/PS1 mice. We observed that SCF+G-CSF treatment reduced the Aβ plaques in both the cortex and hippocampus. SCF+G-CSF treatment increased the association of TREM2+/Iba1+ cells with Aβ plaques and enhanced Aβ uptake by Iba1+ and CD68+cells in the brains of aged APP/PS1 mice. Importantly, cerebral expression area of P2RY12+and TMEM119+ homeostatic microglia and the branches of P2RY12+ homeostatic microglia were increased in the SCF+G-CSF-treated aged APP/PS1 mice. SCF+G-CSF treatment also decreased NOS-2 and increased IL-4 in the brains of aged APP/PS1 mice. Moreover, the loss of MAP2+dendrites and PSD-95+post-synapses and the accumulation of aggregated tau in the brains of aged APP/PS1 mice were ameliorated by SCF+G-CSF treatment. Furthermore, the density of P2RY12+ microglia was negatively correlated with Aβ deposits, but positively correlated with the densities of MAP2+ dendrites and PSD-95+ puncta in the brains of aged APP/PS1 mice. These findings reveal the therapeutic potential of SCF+G-CSF treatment in ameliorating AD pathology at the late stage.

Microtubule-Associated Proteins: Structuring the Cytoskeleton

Microtubule-associated proteins (MAPs) were initially discovered as proteins that bind to and stabilize microtubules. Today, an ever-growing number of MAPs reveals a more complex picture of these proteins as organizers of the microtubule cytoskeleton that have a large variety of functions. MAPs enable microtubules to participate in a plethora of cellular processes such as the assembly of mitotic and meiotic spindles, neuronal development, and the formation of the ciliary axoneme. Although some subgroups of MAPs have been exhaustively characterized, a strikingly large number of MAPs remain barely characterized other than their interactions with microtubules. We provide a comprehensive view on the currently known MAPs in mammals. We discuss their molecular mechanisms and functions, as well as their physiological role and links to pathologies.

Mining the Nav1.7 interactome: Opportunities for chronic pain therapeutics

The peripherally expressed voltage-gated sodium Na_V1.7 (gene SCN9A) channel boosts small stimuli to initiate firing of pain-signaling dorsal root ganglia (DRG) neurons and facilitates neurotransmitter release at the first synapse within the spinal cord. Mutations in SCN9A produce distinct human pain syndromes. Widely acknowledged as a "gatekeeper" of pain, Na_V1.7 has been the focus of intense investigation but, to date, no Na_V1.7-selective drugs have reached the clinic. Elegant crystallographic studies have demonstrated the potential of designing highly potent and selective Na_V1.7 compounds but their therapeutic value remains untested. Transcriptional silencing of Na_V1.7 by a naturally expressed antisense transcript has been reported in rodents and humans but whether this represents a viable opportunity for designing Na_V1.7 therapeutics is currently unknown. The demonstration that loss of Na_V1.7 function is associated with upregulation of endogenous opioids and potentiation of mu- and delta-opioid receptor activities, suggests that targeting only Na_V1.7 may be insufficient for analgesia. However, the link between opioid-dependent analgesic mechanisms and function of sodium channels and intracellular sodium-dependent signaling remains controversial. Thus, additional new targets - regulators, modulators - are needed. In this context, we mine the literature for the known interactome of Na_V1.7 with a focus on protein interactors that affect the channel's trafficking or link it to opioid signaling. As a case study, we present antinociceptive evidence of allosteric regulation of Na_V1.7 by the cytosolic collapsin response mediator protein 2 (CRMP2). Throughout discussions of these possible new targets, we offer thoughts on the therapeutic implications of modulating Na_V1.7 function in chronic pain.

Proteomics reveals ablation of PlGF increases antioxidant and neuroprotective proteins in the diabetic mouse retina

Placental growth factor (PlGF or PGF), a member of the vascular endothelial growth factor (VEGF) sub-family, plays a crucial role in pathological angiogenesis and inflammation. However, the underlying molecular mechanisms that PlGF mediates regarding the complications of non-proliferative diabetic retinopathy (DR) remain elusive. Using an LC-MS/MS-based label-free quantification proteomic approach we characterized the alterations in protein expression caused by PlGF ablation in the retinas obtained from C57BL6, Akita, PlGF-/- and Akita.PlGF-/- mice. After extraction and enzymatic digestion with Trypsin/LysC, the retinal proteins were analyzed by Q-Exactive hybrid Quadrupole-Orbitrap mass spectrometry. Differentially expressed proteins (DEPs) were identified in four comparisons based on Z-score normalization and reproducibility by Pearson's correlation coefficient. The gene ontology (GO), functional pathways, and protein-protein network interaction analysis suggested that several proteins involved in insulin resistance pathways (Gnb1, Gnb2, Gnb4, Gnai2, Gnao1, Snap2, and Gngt1) were significantly down-regulated in PlGF ablated Akita diabetic mice (Akita.PlGF-/- vs. Akita) but up-regulated in Akita vs. C57 and PlGF-/- vs. C57 conditions. Two proteins involved in the antioxidant activity and neural protection pathways, Prdx6 and Map2 respectively, were up-regulated in the Akita.PlGF-/- vs. Akita condition. Overall, we predict that down-regulation of proteins essential for insulin resistance, together with the up-regulation of antioxidant and neuroprotection proteins highlight and epitomize the potential mechanisms important for future anti-PlGF therapies in the treatment of DR.

Amygdala stimulation promotes recovery of behavioral performance in a spatial memory task and increases GAP-43 and MAP-2 in the hippocampus and prefrontal cortex of male rats

The relationships between affective and cognitive processes are an important issue of present neuroscience. The amygdala, the hippocampus and the prefrontal cortex appear as main players in these mechanisms. We have shown that post-training electrical stimulation of the basolateral amygdala (BLA) speeds the acquisition of a motor skill, and produces a recovery in behavioral performance related to spatial memory in fimbria-fornix (FF) lesioned animals. BLA electrical stimulation rises bdnf RNA expression, BDNF protein levels, and arc RNA expression in the hippocampus. In the present paper we have measured the levels of one presynaptic protein (GAP-43) and one postsynaptic protein (MAP-2) both involved in synaptogenesis to assess whether structural neuroplastic mechanisms are involved in the memory enhancing effects of BLA stimulation. A single train of BLA stimulation produced in healthy animals an increase in the levels of GAP-43 and MAP-2 that lasted days in the hippocampus and the prefrontal cortex. In FF-lesioned rats, daily post-training stimulation of the BLA ameliorates the memory deficit of the animals and induces an increase in the level of both proteins. These results support the hypothesis that the effects of amygdala stimulation on memory recovery are sustained by an enhanced formation of new synapses.

miR-484/MAP2/c-Myc-positive regulatory loop in glioma promotes tumor-initiating properties through ERK1/2 signaling

Glioma is the most common intracranial malignant tumor. Cancer stem cells (CSCs) are resistant to chemotherapy and radiotherapy, and are closely related to cancer metastasis and recurrence. In this study, we aimed to explore the effect of miR-484 on glioma stemness and the underlying mechanism involved. miR-484 enhanced glioma tumor-initiating properties in vitro and in vivo. Moreover, miR-484 was shown to directly target MAP2, resulting in activation of ERK1/2 signaling and promotion of stemness in glioma. The ERK1/2 signaling facilitated the formation of a miR-484/MAP2/c-Myc positive feedback loop in glioma. High miR-484 expression predicted a poor prognosis for glioma patients, and high MAP2 expression predicted a good prognosis for glioma patients. Low miR-484 expression and high MAP2 expression was associated with the best prognosis in glioma. Our study suggests that miR-484 and MAP2 can be utilized as predictors for the clinical diagnosis and prognosis of glioma, and miR-484 and MAP2 are potential targets for the treatment of glioma.

Neuron and microglia/macrophage-derived FGF10 activate neuronal FGFR2/PI3K/Akt signaling and inhibit microglia/macrophages TLR4/NF-κB-dependent neuroinflammation to improve functional recovery after spinal cord injury

Therapeutics used to treat central nervous system (CNS) injury were designed to repair neurites and inhibit cell apoptosis. Previous studies have shown that neuron-derived FGF10 exerts potential neuroprotective effects after cerebral ischemia injury. However, little is known about the role of endogenous FGF10 in the recovery process after spinal cord injury (SCI). In this study, we found that FGF10 is mainly produced by neuron and microglia/macrophages, and its expression is increased after SCI. Exogenous treatment of FGF10 improved functional recovery after injury by reducing apoptosis, as well as repairing neurites via FGFR2/PI3K/Akt pathway. On another hand, inhibiting the PI3K/Akt pathway with LY294002 partially reversed the therapeutic effects of FGF10. In addition, small interfering RNA knockdown of FGFR2 suppressed PI3K/Akt pathway activation by FGF10 and abolished its anti-apoptotic and neurite repair effects in vitro. Furthermore, FGF10 treatment inhibited the activation and proliferation of microglia/macrophages through regulation of TLR4/NF-κB pathway, and attenuated the release of pro-inflammatory cytokines after SCI. Thus, the increased expression of FGF10 after acute SCI is an endogenous self-protective response, suggesting that FGF10 could be a potential treatment for CNS injury.

MAP2 IHC detection: a marker of antigenicity in CNS tissues

Immunohistochemistry (IHC) is used to detect antibody-specific antigens in tissues; the results depend on the ability of the primary antibodies to bind to their antigens. Therefore, results depend on the quality of preservation of the specimen. Many investigators have overcome the deleterious effects of over-fixation on the binding of primary antibodies to specimen antigens using IHC, but if the specimen is under-fixed or fixation is delayed, false negative results could be obtained despite certified laboratory practices. Microtubule-associated protein 2 (MAP2) is an abundant microtubule-associate protein that participates in the outgrowth of neuronal processes and synaptic plasticity; it is localized primarily in cell bodies and dendrites of neurons. MAP2 immunolabeling has been reported to be absent in areas of the entorhinal cortex and hippocampus of Alzheimer's disease brains that were co-localized with the dense-core type of amyloid plaques. It was hypothesized that the lack of MAP2 immunolabeling in these structures was due to the degradation of the MAP2 antigen by the neuronal proteases that were released as the neurons lysed leading to the formation of these plaques. Because MAP2 is sensitive to proteolysis, we hypothesized that changes in MAP2 immunolabeling may be correlated with the degree of fixation of central nervous system (CNS) tissues. We detected normal MAP2 immunolabeling in fixed rat brain tissues, but MAP2 immunolabeling was decreased or lost in unfixed and delayed-fixed rat brain tissues. By contrast, two ubiquitous CNS-specific markers, myelin basic protein and glial fibrillary acidic protein, were unaffected by the degree of fixation in the same tissues. Our observations suggest that preservation of various CNS-specific antigens differs with the degree of fixation and that the lack of MAP2 immunolabeling in the rat brain may indicate inadequate tissue fixation. We recommend applying MAP2 IHC for all CNS tissues as a pre-screen to assess the quality of the tissue preservation and to avoid potentially false negative IHC results.

Cellular levels of Grb2 and cytoskeleton stability are correlated in a neurodegenerative scenario

Alzheimer's disease (AD) manifests as neuronal loss. On the premise of Grb2 overexpression in AD mouse brain and brain tissues of AD patients, our study primarily focuses on the stability of cytoskeletal proteins in the context of degenerative AD-like conditions. Two predominant molecular features of AD, extracellular accumulation of β-amyloid oligomers and intracellular elevation of amyloid precursor protein intracellular domain levels, have been used to closely inspect the series of signalling events. In their presence, multiple signalling pathways involving ROCK and PAK1 proteins lead to disassembly of the cytoskeleton, and Grb2 partially counterbalances the cytoskeletal loss. Increased Grb2-NOX4 interactions play a preventive role against cytoskeletal disassembly, in turn blocking the activity of nitrogen oxides and decreasing the expression of slingshot homolog 1 (SSH-1) protein, a potent inducer of cytoskeleton disassembly. This study unravels a unique role of Grb2 in protecting the cytoskeletal architecture in AD-like conditions and presents a potential new strategy for controlling neurodegeneration.

Neural Transdifferentiation: MAPTau Gene Expression in Breast Cancer Cells

BACKGROUND

In tumor cells, aberrant differentiation programs have been described. Several neuronal proteins have been found associated with morphological neuronal-glial changes in breast cancer (BCa). These neuronal proteins have been related to mechanisms that are involved in carcinogenesis; however, this regulation is not well understood. Microtubule-associated protein-tau (MAP-Tau) has been describing in BCa but not its variants. This finding could partly explain the neuronal-glial morphology of BCa cells. Our aim was to determine mRNA expression of MAP-tau variants 2, 4 and 6 in breast cancer cell lines.

MATERIALS AND METHODS

Cultured cell lines MCF-10A, MDA-MB-231, SKBR3 and T47D were observed under phase-contrast microscopy for neural morphology and analyzed for gene expression of MAP-Tau transcript variants 2, 4 and 6 by real-time PCR.

RESULTS

Regarding morphology like neural/glial cells, T47D line shown more cells with these features than MDA-MB-231 and SKBR. In another hand, we found much greater mRNA expression of MAP-Tau transcript variants 2, and to a lesser extent 4 and 6, in T47D cells than the other lines. In conclusion, regulation of MAP- Tau could bring about changes in cytoskeleton, cell morphology and motility; these findings cast further light on neuronal transdifferentiation in BCa.

Microtubule-associated protein 6 mediates neuronal connectivity through Semaphorin 3E-dependent signalling for axonal growth

Structural microtubule associated proteins (MAPs) stabilize microtubules, a property that was thought to be essential for development, maintenance and function of neuronal circuits. However, deletion of the structural MAPs in mice does not lead to major neurodevelopment defects. Here we demonstrate a role for MAP6 in brain wiring that is independent of microtubule binding. We find that MAP6 deletion disrupts brain connectivity and is associated with a lack of post-commissural fornix fibres. MAP6 contributes to fornix development by regulating axonal elongation induced by Semaphorin 3E. We show that MAP6 acts downstream of receptor activation through a mechanism that requires a proline-rich domain distinct from its microtubule-stabilizing domains. We also show that MAP6 directly binds to SH3 domain proteins known to be involved in neurite extension and semaphorin function. We conclude that MAP6 is critical to interface guidance molecules with intracellular signalling effectors during the development of cerebral axon tracts.

Loss of the structural microtubule-associated protein 6 (MAP6) leads to neuronal differentiation defects that are independent of MAP6's microtubule-binding properties. Here the authors establish a functional link between MAP6 and Semaphorin 3E signalling for proper formation of the fornix of the brain.

Caloric restriction mimetic 2-deoxyglucose maintains cytoarchitecture and reduces tau phosphorylation in primary culture of mouse hippocampal pyramidal neurons

Typical form of neurons is crucially important for their functions. This is maintained by microtubules and associated proteins like tau. Hyperphosphorylation of tau is a major concern in neurodegenerative diseases. Glycogen synthase kinase3β (GSK3β) and cyclin-dependent protein kinase 5 (Cdk5) are the enzymes that govern tau phosphorylation. Currently, efforts are being made to target GSK3β and Cdk5 as possible therapeutic avenues to control tau phosphorylation and treat neurodegenerative diseases related to taupathies. In a number of studies, caloric restriction mimetic 2-deoxyglucose (C6H12O5) was found to be beneficial in improving the brain functions. However, no reports are available on the effect of 2-deoxyglucose 2-DG on tau phosphorylation. In the present study, hippocampal pyramidal neurons from E17 mouse embryos were isolated and cultured on poly-L-lysine-coated coverslips. Neurons from the experimental group were treated with 10 mM 2-deoxyglucose. The treatment of 2-DG resulted in healthier neuronal morphology in terms of significantly lower number of cytoplasmic vacuoles, little or no membrane blebbings, maintained axon hillock and intact neurites. There were decreased immunofluorescence signals for GSK3β, pTau at Ser262, Cdk5 and pTau at Ser235 suggesting decreased tau phosphorylation, which was further confirmed by Western blotting. The results indicate the beneficial effects of 2-DG in controlling the tau phosphorylation and maintaining the healthy neuronal cytoarchitecture.

Humanized mouse models for HIV-1 infection of the CNS

Since the onset of the HIV epidemic, there has been a shift from a deadly diagnosis to the management of a chronic disease. This shift is the result of the development of highly effective drugs that are able to suppress viral replication for years. The availability of these regimens has also shifted the neurocognitive pathology associated with infection from potentially devastating to a much milder phenotype. As the disease outcome has changed significantly with the availability of antiretroviral therapy, there is an opportunity to re-evaluate the currently available models to address the neurocognitive pathology seen in suppressed patients. In the following, we seek to summarize the current literature on humanized mouse models and their utility in understanding how HIV infection leads to changes in the central nervous system (CNS). Also, we identify some of the unanswered questions regarding HIV infection of the CNS as well as the opportunities and limitations of currently existing models to address those questions. Finally, our conclusions indicate that the earlier humanized models used to study HIV infection in the CNS provided an excellent foundation for the type of work currently being performed using novel humanized mouse models. We also indicate the potential of some humanized mouse models that have not been used as of this time for the analysis of HIV infection in the brain.

Differentiation of dental pulp stem cells into neuron-like cells in serum-free medium

Dental pulp tissue contains dental pulp stem cells (DPSCs). Dental pulp cells (also known as dental pulp-derived mesenchymal stem cells) are capable of differentiating into multilineage cells including neuron-like cells. The aim of this study was to examine the capability of DPSCs to differentiate into neuron-like cells without using any reagents or growth factors. DPSCs were isolated from teeth extracted from 6- to 8-week-old mice and maintained in complete medium. The cells from the fourth passage were induced to differentiate by culturing in medium without serum or growth factors. RT-PCR molecular analysis showed characteristics of Cd146(+) , Cd166(+) , and Cd31(-) in DPSCs, indicating that these cells are mesenchymal stem cells rather than hematopoietic stem cells. After 5 days of neuronal differentiation, the cells showed neuron-like morphological changes and expressed MAP2 protein. The activation of Nestin was observed at low level prior to differentiation and increased after 5 days of culture in differentiation medium, whereas Tub3 was activated only after 5 days of neuronal differentiation. The proliferation of the differentiated cells decreased in comparison to that of the control cells. Dental pulp stem cells are induced to differentiate into neuron-like cells when cultured in serum- and growth factor-free medium.

Efficient protocol for backbone and side-chain assignments of large, intrinsically disordered proteins: transient secondary structure analysis of 49.2 kDa microtubule associated protein 2c

Microtubule-associated proteins (MAPs) are abundantly present in axons and dendrites, and have been shown to play crucial role during the neuronal morphogenesis. The period of main dendritic outgrowth and synaptogenesis coincides with high expression levels of one of MAPs, the MAP2c, in rats. The MAP2c is a 49.2 kDa intrinsically disordered protein. To achieve an atomic resolution characterization of such a large protein, we have developed a protocol based on the acquisition of two five-dimensional (13)C-directly detected NMR experiments. Our previously published 5D CACONCACO experiment (Nováček et al. in J Biomol NMR 50(1):1-11, 2011) provides the sequential assignment of the backbone resonances, which is not interrupted by the presence of the proline residues in the amino acid sequence. A novel 5D HC(CC-TOCSY)CACON experiment facilitates the assignment of the aliphatic side chain resonances. To streamline the data analysis, we have developed a semi-automated procedure for signal assignments. The obtained data provides the first atomic resolution insight into the conformational state of MAP2c and constitutes a model for further functional studies of MAPs.

Involvement of α2-antiplasmin in dendritic growth of hippocampal neurons

The α2-Antiplasmin (α2AP) protein is known as a principal physiological inhibitor of plasmin, but we previously demonstrated that it acts as a regulatory factor for cellular functions independent of plasmin. α2AP is highly expressed in the hippocampus, suggesting a potential role for α2AP in hippocampal neuronal functions. However, the role for α2AP was unclear. This study is the first to investigate the involvement of α2AP in the dendritic growth of hippocampal neurons. The expression of microtubule-associated protein 2, which contributes to neurite initiation and neuronal growth, was lower in the neurons from α2AP⁻/⁻ mice than in the neurons from α2AP⁺/⁺ mice. Exogenous treatment with α2AP enhanced the microtubule-associated protein 2 expression, dendritic growth and filopodia formation in the neurons. This study also elucidated the mechanism underlying the α2AP-induced dendritic growth. Aprotinin, another plasmin inhibitor, had little effect on the dendritic growth of neurons, and α2AP induced its expression in the neurons from plaminogen⁻/⁻ mice. The activation of p38 MAPK was involved in the α2AP-induced dendritic growth. Therefore, our findings suggest that α2AP induces dendritic growth in hippocampal neurons through p38 MAPK activation, independent of plasmin, providing new insights into the role of α2AP in the CNS.

Intermediate filament-associated cytolinker plectin 1c destabilizes microtubules in keratinocytes

The transition of microtubules (MTs) from an assembled to a disassembled state plays an essential role in several cellular functions. While MT dynamics are often linked to those of actin filaments, little is known about whether intermediate filaments (IFs) have an influence on MT dynamics. We show here that plectin 1c (P1c), one of the multiple isoforms of the IF-associated cytolinker protein plectin, acts as an MT destabilizer. We found that MTs in P1c-deficient (P1c(-/-)) keratinocytes are more resistant toward nocodazole-induced disassembly and display increased acetylation. In addition, live imaging of MTs in P1c(-/-), as well as in plectin-null, cells revealed decreased MT dynamics. Increased MT stability due to P1c deficiency led to changes in cell shape, increased velocity but loss of directionality of migration, smaller-sized focal adhesions, higher glucose uptake, and mitotic spindle aberrations combined with reduced growth rates of cells. On the basis of ex vivo and in vitro experimental approaches, we suggest a mechanism for MT destabilization in which isoform-specific binding of P1c to MTs antagonizes the MT-stabilizing and assembly-promoting function of MT-associated proteins through an inhibitory function exerted by plectin's SH3 domain. Our results open new perspectives on cytolinker-coordinated IF-MT interaction and its physiological significance.

L1CAM increases MAP2 expression via the MAPK pathway to promote neurite outgrowth

The neural cell adhesion molecule L1 (L1CAM) promotes neurite outgrowth via mechanisms that are not completely understood, but are known to involve the cytoskeleton. Here, we show that L1 binds directly to the microtubule associated protein 2c (MAP2c). This isoform of MAP2 is predominantly expressed in developing neurons. We found that the mRNA and protein levels of MAP2c, but not of MAP2a/b, are reduced in brains of young adult L1-deficient transgenic mice. We show via ELISA, that MAP2c, but not MAP2a/b, binds directly to the intracellular domain of L1. Remarkably, all these MAP2 isoforms co-immunoprecipitate with L1, suggesting that MAP2a/b associates with L1 via intermediate binding partners. The expression levels of MAP2a/b/c correlate with those of L1 in different brain regions of early postnatal mice, while expression levels of heat shock cognate protein 70 (Hsc70) or actin do not. L1 enhances the expression of MAP2a/b/c in cultured hippocampal neurons depending on activation of the mitogen-activated protein kinase (MAPK) pathway. Deficiency in both L1 and MAP2a/b/c expression results in reduced neurite outgrowth in vitro. We propose that the L1-triggered increase in MAP2a/b/c expression is required to promote neurite outgrowth.

The protein phosphatase PP2A/Bα binds to the microtubule-associated proteins Tau and MAP2 at a motif also recognized by the kinase Fyn: implications for tauopathies

The predominant brain microtubule-associated proteins MAP2 and tau play a critical role in microtubule cytoskeletal organization and function. We have previously reported that PP2A/Bα, a major protein phosphatase 2A (PP2A) holoenzyme, binds to and dephosphorylates tau, and regulates microtubule stability. Here, we provide evidence that MAP2 co-purifies with and is dephosphorylated by endogenous PP2A/Bα in bovine gray matter. It co-localizes with PP2A/Bα in immature and mature human neuronal cell bodies. PP2A co-immunoprecipitates with and directly interacts with MAP2. Using in vitro binding assays, we show that PP2A/Bα binds to MAP2c isoforms through a region encompassing the microtubule-binding domain and upstream proline-rich region. Tau and MAP2 compete for binding to and dephosphorylation by PP2A/Bα. Remarkably, the protein-tyrosine kinase Fyn, which binds to the proline-rich RTPPKSP motif conserved in both MAP2 and tau, inhibits the interaction of PP2A/Bα with either tau or MAP2c. The corresponding synthetic RTPPKSP peptide, but not the phosphorylated RpTPPKSP version, competes with Tau and MAP2c for binding to PP2A/Bα. Significantly, down-regulation of PP2A/Bα and deregulation of Fyn-Tau protein interactions have been linked to enhanced tau phosphorylation in Alzheimer disease. Together, our results suggest that PP2A/Bα is part of segregated MAP2 and tau signaling scaffolds that can coordinate the action of key kinases and phosphatases involved in modulating neuronal plasticity. Deregulation of these compartmentalized multifunctional protein complexes is likely to contribute to tau deregulation, microtubule disruption, and altered signaling in tauopathies.

NMDA receptor activation suppresses microtubule growth and spine entry

Dynamic microtubules are important to maintain neuronal morphology and function, but whether neuronal activity affects the organization of dynamic microtubules is unknown. Here, we show that a protocol to induce NMDA-dependent long-term depression (LTD) rapidly attenuates microtubule dynamics in primary rat hippocampal neurons, removing the microtubule-binding protein EB3 from the growing microtubule plus-ends in dendrites. This effect requires the entry of calcium and is mediated by activation of NR2B-containing NMDA-type glutamate receptor. The rapid NMDA effect is followed by a second, more prolonged response, during which EB3 accumulates along MAP2-positive microtubule bundles in the dendritic shaft. MAP2 is both required and sufficient for this activity-dependent redistribution of EB3. Importantly, NMDA receptor activation suppresses microtubule entry in dendritic spines, whereas overexpression of EB3-GFP prevents NMDA-induced spine shrinkage. These results suggest that short-lasting and long-lasting changes in dendritic microtubule dynamics are important determinants for NMDA-induced LTD.

Src family kinases and paclitaxel sensitivity

Src-family Kinases (SFKs) participate in the regulation of proliferation, differentiation, apoptosis, autophagy, adhesion, migration, invasion and angiogenesis in normal and cancer cells. Abnormal expression of SFKs has been documented in cancers that arise in breast, colon, ovary, melanocyte, gastric mucosa, head and neck, pancreas, lung, and brain. Targeting SFKs in cancer cells has been shown to be a promising therapeutic strategy in solid tumors, particularly in ovarian, colon and breast cancers. Paclitaxel is one of most widely used chemotherapeutic agents for the management of ovarian, breast, lung and head/neck cancers. As a microtubule-stabilizing agent, paclitaxel possesses both mitosis-dependent and mitosis-independent activities against cancer cells. A variety of mechanisms such as deregulation of P-glycoprotein, alteration of tubulin isotypes, alteration of microtubule-regulatory proteins, deregulation of apoptotic signaling pathways, mutation of tubulins and overexpression of copper transporters have been implicated in the development of primary or secondary resistance to paclitaxel. By affecting cancer cell survival, proliferation, autophagy, microtubule stability, motility, and/or angiogenesis, SFKs interact with mechanisms that regulate paclitaxel sensitivity. Inhibition of SFKs can potentiate the anti-tumor activity of paclitaxel by enhancing apoptosis, autophagy and microtubule stability. Based on pre-clinical observations, administration of SFK inhibitors in combination with paclitaxel could improve treatment for ovarian, breast, lung and head/neck cancers. Identification and validation of predictive biomarkers could also permit personalization of the therapy.

High-performance capillary electrophoresis for determining HIV-1 Tat protein in neurons

The HIV-1 protein, Tat has been implicated in AIDS pathogenesis however, the amount of circulating Tat is believed to be very low and its quantification has been difficult. We performed the quantification of Tat released from infected cells and taken up by neurons using high performance capillary electrophoresis. This is the first report to successfully measure the amount of Tat in neurons and places Tat as a key player involved in HIV-associated neurocognitive disorders.

Networking with AKAPs: context-dependent regulation of anchored enzymes

A-Kinase Anchoring Proteins (AKAPs) orchestrate and synchronize cellular events by tethering the cAMP-dependent protein kinase (PKA) and other signaling enzymes to organelles and membranes. The control of kinases and phosphatases that are held in proximity to activators, effectors, and substrates favors the rapid dissemination of information from one cellular location to the next. This article charts the inception of the PKA-anchoring hypothesis, the characterization of AKAPs and their nomenclature, and the physiological roles of context-specific AKAP signaling complexes.

The antiproliferative effects of progestins in T47D breast cancer cells are tempered by progestin induction of the ETS transcription factor Elf5

Prolactin and progesterone act together to regulate mammary alveolar development, and both hormones have been implicated in breast cancer initiation and progression. Here we show that Elf5, a prolactin-induced ETS transcription factor that specifies the mammary secretory cell lineage, is also induced by progestins in breast cancer cells via a direct mechanism. To define the transcriptional response to progestin elicited via Elf5, we made an inducible Elf5 short hairpin-RNA knock-down model in T47D breast cancer cells and used it to prevent the progestin-induction of Elf5. Functional analysis of Affymetrix gene expression data using Gene Ontologies and Gene Set Enrichment Analysis showed enhancement of the progestin effects on cell cycle gene expression. Cell proliferation assays showed a more efficacious progestin-induced growth arrest when Elf5 was kept at baseline levels. These results showed that progestin induction of Elf5 expression tempered the antiproliferative effects of progestins in T47D cells, providing a further mechanistic link between prolactin and progestin in the regulation of mammary cell phenotype.

Chronic restraint stress alters the expression and distribution of phosphorylated tau and MAP2 in cortex and hippocampus of rat brain

Microtubule-associated proteins (MAPs) play a critical role in maintaining normal cytoskeletal architecture and functions. In the present study, we aim to explore the effects of the emotional stressor, chronic restraint stress, on the expression levels and localization of tau and MAP2. We found that after chronic restraint stress, soluble hyperphosphorylated tau was greatly increased, whereas MAP2 was decreased. Moreover, immunohistochemistry analysis demonstrated that phosphorylated tau and MAP2 displayed the similar subcellular distribution pattern after chronic restraint stress. Robust hyperphosphorylated tau immunolabeling was observed both in cortex and hippocampus of stressed animals and mainly located to perikaryal/dendritic elements. After stress, the MAP2 was mainly distributed in the perikaryal compartments, discontinuous dendrites and neuropil. Moreover, the distribution pattern of MAP2 in hippocampus significantly changed. Immunofluorescence double labeling indicated that hyperphosphorylated tau increased in the regions where there displayed an decrease of MAP2. These results suggest that the involvement of repeated restraint stress may not only induce phosphorylation state of tau and distribution of phosphorylated tau, but also alter the content and neuronal distribution of MAP2. Tau and MAP2 are most important MAPs for neuronal cells, the subcellular distribution change of them might be link to functional change of neurons after emotional stress.

Mechanisms of protein kinase A anchoring

The second messenger cyclic adenosine monophosphate (cAMP), which is produced by adenylyl cyclases following stimulation of G-protein-coupled receptors, exerts its effect mainly through the cAMP-dependent serine/threonine protein kinase A (PKA). Due to the ubiquitous nature of the cAMP/PKA system, PKA signaling pathways underlie strict spatial and temporal control to achieve specificity. A-kinase anchoring proteins (AKAPs) bind to the regulatory subunit dimer of the tetrameric PKA holoenzyme and thereby target PKA to defined cellular compartments in the vicinity of its substrates. AKAPs promote the termination of cAMP signals by recruiting phosphodiesterases and protein phosphatases, and the integration of signaling pathways by binding additional signaling proteins. AKAPs are a heterogeneous family of proteins that only display similarity within their PKA-binding domains, amphipathic helixes docking into a hydrophobic groove formed by the PKA regulatory subunit dimer. This review summarizes the current state of information on compartmentalized cAMP/PKA signaling with a major focus on structural aspects, evolution, diversity, and (patho)physiological functions of AKAPs and intends to outline newly emerging directions of the field, such as the elucidation of AKAP mutations and alterations of AKAP expression in human diseases, and the validation of AKAP-dependent protein-protein interactions as new drug targets. In addition, alternative PKA anchoring mechanisms employed by noncanonical AKAPs and PKA catalytic subunit-interacting proteins are illustrated.

Intrinsic disorder in scaffold proteins: getting more from less

Regulation, recognition and cell signaling involve the coordinated actions of many players. Signaling scaffolds, with their ability to bring together proteins belonging to common and/or interlinked pathways, play crucial roles in orchestrating numerous events by coordinating specific interactions among signaling proteins. This review examines the roles of intrinsic disorder (ID) in signaling scaffold protein function. Several well-characterized scaffold proteins with structurally and functionally characterized ID regions are used here to illustrate the importance of ID for scaffolding function. These examples include scaffolds that are mostly disordered, only partially disordered or those in which the ID resides in a scaffold partner. Specific scaffolds discussed include RNase, voltage-activated potassium channels, axin, BRCA1, GSK-3beta, p53, Ste5, titin, Fus3, BRCA1, MAP2, D-AKAP2 and AKAP250. Among the mechanisms discussed are: molecular recognition features, fly-casting, ease of encounter complex formation, structural isolation of partners, modulation of interactions between bound partners, masking of intramolecular interaction sites, maximized interaction surface per residue, toleration of high evolutionary rates, binding site overlap, allosteric modification, palindromic binding, reduced constraints for alternative splicing, efficient regulation via posttranslational modification, efficient regulation via rapid degradation, protection of normally solvent-exposed sites, enhancing the plasticity of interaction and molecular crowding. We conclude that ID can enhance scaffold function by a diverse array of mechanisms. In other words, scaffold proteins utilize several ID-facilitated mechanisms to enhance function, and by doing so, get more functionality from less structure.

Microtubule-mediated Src tyrosine kinase trafficking in neuronal growth cones

Src family tyrosine kinases are important signaling enzymes in the neuronal growth cone, and they have been implicated in axon guidance; however, the detailed localization, trafficking, and cellular functions of Src kinases in live growth cones are unclear. Here, we cloned two novel Aplysia Src kinases, termed Src1 and Src2, and we show their association with both the plasma membrane and the microtubule cytoskeleton in the growth cone by live cell imaging, immunocytochemistry, and cell fractionation. Activated Src2 is enriched in filopodia tips. Interestingly, Src2-enhanced green fluorescent protein-positive endocytic vesicles and tubulovesicular structures undergo microtubule-mediated movements that are bidirectional in the central domain and mainly retrograde in the peripheral domain. To further test the role of microtubules in Src trafficking in the growth cone, microtubules were depleted with either nocodazole or vinblastine treatment, resulting in an increase in Src2 plasma membrane levels in all growth cone domains. Our data suggest that microtubules regulate the steady-state level of active Src at the plasma membrane by mediating retrograde recycling of endocytosed Src. Expression of constitutively active Src2 results in longer filopodia that protrude from smaller growth cones, implicating Src2 in controlling the size of filopodia and lamellipodia.

Grb2 signaling in cell motility and cancer

BACKGROUND

Metastasis is the primary cause of death in most human cancers, and understanding the molecular mechanisms underpinning this multistep process is fundamental to identifying novel molecular targets and developing more effective therapies.

OBJECTIVE/METHODS

Here we review the role of growth factor receptor-bound protein 2 (Grb2) in cancer and specifically in metastasis-related processes, and summarize the development of anticancer therapeutics selectively targeting this adapter protein.

RESULTS/CONCLUSION

Grb2 is a key molecule in intracellular signal transduction, linking activated cell surface receptors to downstream targets by binding to specific phosphotyrosine-containing and proline-rich sequence motifs. Grb2 signaling is critical for cell cycle progression and actin-based cell motility, and, consequently, more complex processes such as epithelial morphogenesis, angiogenesis and vasculogenesis. These functions make Grb2 a therapeutic target for strategies designed to prevent the spread of solid tumors through local invasion and metastasis.

Tau binding to microtubules does not directly affect microtubule-based vesicle motility

Tau protein is a major microtubule (MT)-associated brain protein enriched in axons. Multiple functional roles are proposed for tau protein, including MT stabilization, generation of cell processes, and targeting of phosphotransferases to MTs. Recently, experiments involving exogenous tau expression in cultured cells suggested a role for tau as a regulator of kinesin-1-based motility. Tau was proposed to inhibit attachment of kinesin-1 to MTs by competing for the kinesin-1 binding site. In this work, we evaluated effects of tau on fast axonal transport (FAT) by using vesicle motility assays in isolated squid axoplasm. Effects of recombinant tau constructs on both kinesin-1 and cytoplasmic dynein-dependent FAT rates were evaluated by video microscopy. Exogenous tau binding to endogenous squid MTs was evidenced by a dramatic change in individual MT morphologies. However, perfusion of tau at concentrations approximately 20-fold higher than physiological levels showed no effect on FAT. In contrast, perfusion of a cytoplasmic dynein-derived peptide that competes with kinesin-1 and cytoplasmic dynein binding to MTs in vitro rapidly inhibited FAT in both directions. Taken together, our results indicate that binding of tau to MTs does not directly affect kinesin-1- or cytoplasmic dynein-based motilities. In contrast, our results provide further evidence indicating that the functional binding sites for kinesin-1 and cytoplasmic dynein on MTs overlap.

Constitutive activation of neuronal Src causes aberrant dendritic morphogenesis in mouse cerebellar Purkinje cells

Src family tyrosine kinases are essential for neural development, but their in vivo functions remain elusive because of functional compensation among family members. To elucidate the roles of individual Src family members in vivo, we generated transgenic mice expressing the neuronal form of c-Src (n-Src), Fyn, and their constitutively active forms in cerebellar Purkinje cells using the L7 promoter. The expression of the constitutively active n-Src retarded the postnatal development of Purkinje cells and disrupted dendritic morphogenesis, whereas the wild-type n-Src had only moderate effects. Neither wild-type nor constitutively active Fyn over-expression significantly affected Purkinje-cell morphology. The aberrant Purkinje cells in n-Src transgenic mice retained multiple dendritic shafts extending in non-polarized directions and were located heterotopically in the molecular layer. Ultrastructural observation of the dendritic shafts revealed that the microtubules of n-Src transgenic mice were more densely and irregularly arranged, and had structural deformities. In primary culture, Purkinje cells from n-Src transgenic mice developed abnormally thick dendritic shafts and large growth-cone-like structures with poorly extended dendrites, which could be rescued by treatment with a selective inhibitor of Src family kinases, PP2. These results suggest that n-Src activity regulates the dendritic morphogenesis of Purkinje cells through affecting microtubule organization.

Phosphoproteomic analysis with a solid-phase capture-release-tag approach

A comprehensive study of global phosphorylation events in biological systems is critical. We report a chemistry-based capture-release-tag method for isolation of complex phospho-Ser/Thr-containing peptides by liquid beta-elimination combined with solid-phase Michael addition. The free thiol groups of 6-(mercapto-acetylamino)-hexanoic acid functionalized resin are used as immobilized Michael donors to capture dehydro-serine/threonine peptides. After an acid-mediated release step, phospho-peptides are labeled with a 6-(2-mercapto-acetylamine)-hexanoic amide tag at phosphorylated sites. We applied this method to analyze the phosphorylation status of microtubule-associated proteins. We find that a CDK5 substrate microtubule-associated protein 2 (MAP2) is phosphorylated on residues that are within a homologous region of Tau. The chemical method corroborates previous results and suggests that Tau and MAP2 may contain a CDK5 phosphorylation motif.

Neurturin has multiple neurotrophic effects on adult rat sacral parasympathetic ganglion neurons

Neurturin (NTN) is an important neurotrophic factor for parasympathetic neurons; however, no studies to date have investigated the signalling mechanisms downstream of GFRalpha2 and Ret activation underlying this neurotrophic support. This is particularly important for pelvic parasympathetic neurons, which are prone to injury during surgical procedures such as prostatectomy, and where there are no current therapies for axonal regeneration. To address this issue we have cultured dissociated adult rat pelvic ganglion neurons and also examined the structural changes in pelvic ganglion neurons after axotomy. Axotomised penile neurons deprived of target-derived support had smaller somata than intact neurons. Studies of cultured adult pelvic ganglion neurons also demonstrated that NTN stimulated soma growth. Further experiments showed that NTN reduced the up-regulation of tyrosine hydroxylase expression in cultured pelvic parasympathetic neurons. NTN stimulated the extension of neurites in cultured parasympathetic, but not sympathetic, pelvic ganglion neurons. Inhibition of phosphatidylinositol 3-kinase prevented initiation of neurite outgrowth, whereas inhibition of the mitogen-activated protein kinase and the Src family kinase pathways disrupted NTN-stimulated microtubule assembly. Surprisingly, NTN did not activate the transcription factor cAMP-response element binding protein (CREB), which is typically involved in neurotrophic signalling in sympathetic neurons. This is the first study to identify signalling pathways activated by NTN in adult parasympathetic neurons. Our results may lead to a better understanding of regenerative mechanisms in parasympathetic neurons, especially for those innervating urogenital organs. Our results also indicate that neurotrophic signalling in parasympathetic neurons is different from that in other types of peripheral neurons.

Reduced expression of kinase-associated phosphatase in cortical dendrites of MAP2-deficient mice

We previously demonstrated that cAMP-dependent protein kinase was reduced in the dendrites of MAP2-deficient mice. In this study, we compared the expression of various protein phosphatases (PPs) between wild-type and map2(-/-) dendrites. Kinase-associated phosphatase (KAP) was the only PP which showed difference between the two phenotypes: (1) the expression of KAP was reduced in map2(-/-) cortical dendrites, and (2) the amount of KAP bound to microtubules was reduced in map2(-/-) brains. We also demonstrated in cultured neuroblastoma cells that KAP is not only expressed in dividing cells, but also in the neurites of differentiated cells. Our findings propose that KAP, which has been reported to function in cell-cycle control, has an as yet uncovered role in regulating dendritic functions. We also propose MAP2-deficient mice as an ideal system for identifying protein phosphatases essential for dendritic functions.

Microtubule-associated protein 2, a marker of neuronal differentiation, induces mitotic defects, inhibits growth of melanoma cells, and predicts metastatic potential of cutaneous melanoma

Dynamic instability of microtubules is critical for mitotic spindle assembly and disassembly during cell division, especially in rapidly dividing tumor cells. Microtubule-associated proteins (MAPs) are a family of proteins that influence this property. We showed previously that MAP2, a neuron-specific protein that stabilizes microtubules in the dendrites of postmitotic neurons, is induced in primary cutaneous melanoma but is absent in metastatic melanomas. We proposed that induction of a microtubule-stabilizing protein in primary melanoma could disrupt the dynamic instability of microtubules, inhibit cell division and prevent or delay tumor progression. Here we show, by Kaplan-Meier survival and multivariate Cox regression analysis, that patients diagnosed with MAP2+ primary melanomas have significantly better metastatic disease-free survival than those with MAP2- disease. Investigation of the mechanisms that underlie the effect of MAP2 on melanoma progression showed that MAP2 expression in metastatic melanoma cell lines leads to microtubule stabilization, cell cycle arrest in G2-M phase and growth inhibition. Disruption of microtubule dynamics by MAP2 resulted in multipolar mitotic spindles, defects in cytokinesis and accumulation of cells with large nuclei, similar to those seen in vivo in MAP2+ primary melanomas cells. These data suggest that ectopic activation of a neuronal differentiation gene in melanoma during early tumor progression inhibits cell division and correlates with inhibition or delay of metastasis.

STAT5B-mediated Growth Hormone Signaling Is Organized by Highly Dynamic Microtubules in Hepatic Cells

In the last decade, the notion that microtubules are critical to the spatial organization of signal transduction and contribute to the transmission of signals to downstream targets has been proposed. Because the STAT5B transduction and transcription factor is the major STAT protein activated by growth hormone stimulation in hepatocytes and is a crossroads between many signaling pathways, we studied the involvement of microtubules in STAT5B-mediated growth hormone signaling pathway in the highly differentiated and polarized WIF-B hepatic cell line. We showed that depolymerization of the microtubule network impaired STAT5B translocation to the nucleus upon growth hormone treatment. A significant amount of STAT5B binds to microtubules, while STAT5A and STAT3 are exclusively compartmentalized in the cytosol. Moreover, taxol-induced stabilization of microtubules released STAT5B from its binding, and we show that STAT5B binds specifically to the highly dynamic microtubules and is absent of the stable microtubule subpopulation. The specific involvement of dynamic microtubule subpopulation in growth hormone signaling pathway was confirmed by the inhibition of growth hormone-induced STAT5B nuclear translocation after stabilization of microtubules or specific disruption of highly dynamic microtubules. Upon growth hormone treatment, MT-bound STAT5B was rapidly released from microtubules by a dynein-dependent transport to the nucleus. Altogether, our findings indicate that the labile microtubule subpopulation specifically and dynamically organizes STAT5B-mediated growth hormone signaling in hepatic cells.

The MAP2/Tau family of microtubule-associated proteins

Microtubule-associated proteins (MAPs) of the MAP2/Tau family include the vertebrate proteins MAP2, MAP4, and Tau and homologs in other animals. All three vertebrate members of the family have alternative splice forms; all isoforms share a conserved carboxy-terminal domain containing microtubule-binding repeats, and an amino-terminal projection domain of varying size. MAP2 and Tau are found in neurons, whereas MAP4 is present in many other tissues but is generally absent from neurons. Members of the family are best known for their microtubule-stabilizing activity and for proposed roles regulating microtubule networks in the axons and dendrites of neurons. Contrary to this simple, traditional view, accumulating evidence suggests a much broader range of functions, such as binding to filamentous (F) actin, recruitment of signaling proteins, and regulation of microtubule-mediated transport. Tau is also implicated in Alzheimer's disease and other dementias. The ability of MAP2 to interact with both microtubules and F-actin might be critical for neuromorphogenic processes, such as neurite initiation, during which networks of microtubules and F-actin are reorganized in a coordinated manner. Various upstream kinases and interacting proteins have been identified that regulate the microtubule-stabilizing activity of MAP2/Tau family proteins.

Fyn phosphorylates human MAP-2c on tyrosine 67

The Src homology 3 (SH3) domain of Fyn binds to a conserved PXXP motif on microtubule-associated protein-2. Co-transfections into COS7 cells and in vitro kinase assays performed with Fyn and wild-type, or mutant MAP-2c, determined that Fyn phosphorylated MAP-2c on tyrosine 67. The phosphorylation generated a consensus sequence for the binding of the SH2 domain of Grb2 (pYSN). Pull-down assays with SH2-Grb2 from human fetal brain homogenates, and co-immunoprecipitation of Grb2 and MAP-2 confirmed the interaction in vivo, and demonstrated that MAP-2c is tyrosine-phosphorylated in human fetal brain. Filter overlay assays confirmed that the SH2 domain of Grb2 binds to human MAP-2c following incubation with active Fyn. Enzyme-linked immunosorbent assays confirmed the interaction between the SH2 domain of Grb2 and a tyrosine-phosphorylated MAP-2 peptide spanning the pY(67)SN motif. Thus, MAP-2c can directly recruit multiple signaling proteins important for central nervous system development.

Gene expression profiles in the rat central nervous system induced by JP-8 jet fuel vapor exposure

Jet propulsion fuel-8 (JP-8) is the predominant fuel for military land vehicles and aircraft used in the US and NATO. Occupational exposure to jet fuel in military personnel has raised concern for the health risk associated with such exposure in the Department of Defense. Clinical studies of humans chronically exposed to jet fuel have suggested both neurotoxicity and neurobehavioral deficits. We utilized rat neurobiology U34 array to measure gene expression changes in whole brain tissue of rats exposed repeatedly to JP-8, under conditions that simulated possible occupational exposure (6 h/day for 91 days) to JP-8 vapor at 250, 500, and 1000 mg/m(3), respectively. Our studies revealed that the gene expression changes of exposure groups can be divided into two main categories according to their functions: (1). neurotransmitter signaling pathways; and (2). stress response. The implications of these gene expression changes are discussed.