MAP2: a sensitive cross-linker and adjustable spacer in dendritic architecture

Ubiquitinome Analysis Uncovers Alterations in Synaptic Proteins and Glucose Metabolism Enzymes in the Hippocampi of Adolescent Mice Following Cold Exposure

Cold exposure exerts negative effects on hippocampal nerve development in adolescent mice, but the underlying mechanisms are not fully understood. Given that ubiquitination is essential for neurodevelopmental processes, we attempted to investigate the effects of cold exposure on the hippocampus from the perspective of ubiquitination. By conducting a ubiquitinome analysis, we found that cold exposure caused changes in the ubiquitination levels of a variety of synaptic-associated proteins. We validated changes in postsynaptic density-95 (PSD-95) ubiquitination levels by immunoprecipitation, revealing reductions in both the K48 and K63 polyubiquitination levels of PSD-95. Golgi staining further demonstrated that cold exposure decreased the dendritic-spine density in the CA1 and CA3 regions of the hippocampus. Additionally, bioinformatics analysis revealed that differentially ubiquitinated proteins were enriched in the glycolytic, hypoxia-inducible factor-1 (HIF-1), and 5'-monophosphate (AMP)-activated protein kinase (AMPK) pathways. Protein expression analysis confirmed that cold exposure activated the mammalian target of rapamycin (mTOR)/HIF-1α pathway. We also observed suppression of pyruvate kinase M2 (PKM2) protein levels and the pyruvate kinase (PK) activity induced by cold exposure. Regarding oxidative phosphorylation, a dramatic decrease in mitochondrial respiratory-complex I activity was observed, along with reduced gene expression of the key subunits NADH: ubiquinone oxidoreductase core subunit V1 (Ndufv1) and Ndufv2. In summary, cold exposure negatively affects hippocampal neurodevelopment and causes abnormalities in energy homeostasis within the hippocampus.

Alterations in Cerebellar Microtubule Cytoskeletal Network in a ValproicAcid-Induced Rat Model of Autism Spectrum Disorders

Autism spectrum disorders (ASD) are neurodevelopmental diseases characterised by deficits in social communication, restricted interests, and repetitive behaviours. The growing body of evidence points to a role for cerebellar changes in ASD pathology. Some of the findings suggest that not only motor problems but also social deficits, repetitive behaviours, and mental inflexibility associated with ASD are connected with damage to the cerebellum. However, the understanding of this brain structure's functions in ASD pathology needs future investigations. Therefore, in this study, we generated a rodent model of ASD through a single prenatal administration of valproic acid (VPA) into pregnant rats, followed by cerebellar morphological studies of the offspring, focusing on the alterations of key cytoskeletal elements. The expression (Western blot) of α/β-tubulin and the major neuronal MT-associated proteins (MAP) such as MAP-Tau and MAP1B, MAP2, MAP6 (STOP) along with actin-crosslinking αII-spectrin and neurofilament light polypeptide (NF-L) was investigated. We found that maternal exposure to VPA induces a significant decrease in the protein levels of α/β-tubulin, MAP-Tau, MAP1B, MAP2, and αII-spectrin. Moreover, excessive MAP-Tau phosphorylation at (Ser396) along with key Tau-kinases activation was indicated. Immunohistochemical staining showed chromatolysis in the cerebellum of autistic-like rats and loss of Purkinje cells shedding light on one of the possible molecular mechanisms underpinning neuroplasticity alterations in the ASD brain.

Electrical and Morphological Properties of Developing Motoneurons in Postnatal Mice and Early Abnormalities in SOD1 Transgenic Mice

In this chapter, we review electrical and morphological properties of lumbar motoneurons during postnatal development in wild-type (WT) and transgenic superoxide dismutase 1 (SOD1) mice, models of amyotrophic lateral sclerosis. First we showed that sensorimotor reflexes do not develop normally in transgenic SOD1^G85R pups. Fictive locomotor activity recorded in in vitro whole brainstem/spinal cord preparations was not induced in these transgenic SOD1^G85R mice using NMDA and 5HT in contrast to WT mice. Further, abnormal electrical properties were detected as early as the second postnatal week in lumbar motoneurons of SOD1 mice while they develop clinical symptoms several months after birth. We compared two different strains of mice (G85R and G93A) at the same postnatal period using intracellular recordings and patch clamp recordings of WT and SOD1 motoneurons. We defined three types of motoneurons according to their discharge firing pattern (transient, sustained and delayed onset firing) when motor units are not yet mature. The delayed-onset firing motoneurons had the higher rheobase compared to the transient and sustained firing groups in the WT mice. We demonstrated hypoexcitability in the delayed onset-firing motoneurons of SOD1 mice. Intracellular staining of motoneurons revealed dendritic overbranching in SOD1 lumbar motoneurons that was more pronounced in the sustained firing motoneurons. We suggested that motoneuronal hypoexcitability is an early pathological sign affecting a subset of lumbar motoneurons in the spinal cord of SOD1 mice.

Application of Machine Learning and Weighted Gene Co-expression Network Algorithm to Explore the Hub Genes in the Aging Brain

Aging is a major risk factor contributing to neurodegeneration and dementia. However, it remains unclarified how aging promotes these diseases. Here, we use machine learning and weighted gene co-expression network (WGCNA) to explore the relationship between aging and gene expression in the human frontal cortex and reveal potential biomarkers and therapeutic targets of neurodegeneration and dementia related to aging. The transcriptional profiling data of the human frontal cortex from individuals ranging from 26 to 106 years old was obtained from the GEO database in NCBI. Self-Organizing Feature Map (SOM) was conducted to find the clusters in which gene expressions downregulate with aging. For WGCNA analysis, first, co-expressed genes were clustered into different modules, and modules of interest were identified through calculating the correlation coefficient between the module and phenotypic trait (age). Next, the overlapping genes between differentially expressed genes (DEG, between young and aged group) and genes in the module of interest were discovered. Random Forest classifier was performed to obtain the most significant genes in the overlapping genes. The disclosed significant genes were further identified through network analysis. Through WGCNA analysis, the greenyellow module is found to be highly negatively correlated with age, and functions mainly in long-term potentiation and calcium signaling pathways. Through step-by-step filtering of the module genes by overlapping with downregulated DEGs in aged group and Random Forest classifier analysis, we found that MAPT, KLHDC3, RAP2A, RAP2B, ELAVL2, and SYN1 were co-expressed and highly correlated with aging.

Novel role of SARM1 mediated axonal degeneration in the pathogenesis of rabies

Neurotropic viral infections continue to pose a serious threat to human and animal wellbeing. Host responses combatting the invading virus in these infections often cause irreversible damage to the nervous system, resulting in poor prognosis. Rabies is the most lethal neurotropic virus, which specifically infects neurons and spreads through the host nervous system by retrograde axonal transport. The key pathogenic mechanisms associated with rabies infection and axonal transmission in neurons remains unclear. Here we studied the pathogenesis of different field isolates of lyssavirus including rabies using ex-vivo model systems generated with mouse primary neurons derived from the peripheral and central nervous systems. In this study, we show that neurons activate selective and compartmentalized degeneration of their axons and dendrites in response to infection with different field strains of lyssavirus. We further show that this axonal degeneration is mediated by the loss of NAD and calpain-mediated digestion of key structural proteins such as MAP2 and neurofilament. We then analysed the role of SARM1 gene in rabies infection, which has been shown to mediate axonal self-destruction during injury. We show that SARM1 is required for the accelerated execution of rabies induced axonal degeneration and the deletion of SARM1 gene significantly delayed axonal degeneration in rabies infected neurons. Using a microfluidic-based ex-vivo neuronal model, we show that SARM1-mediated axonal degeneration impedes the spread of rabies virus among interconnected neurons. However, this neuronal defense mechanism also results in the pathological loss of axons and dendrites. This study therefore identifies a potential host-directed mechanism behind neurological dysfunction in rabies infection. This study also implicates a novel role of SARM1 mediated axonal degeneration in neurotropic viral infection.

Lyssaviruses including rabies, still causes devastating loss of human life every year and many victims are children under the age of 15. Rabies infection causes severe neurological dysfunction in the host resulting in paralysis, cognitive deficits and behavioural abnormalities. The mechanism of how rabies infection induces neurological dysfunction in the host remains unclear. This is because unlike other microbial infections, rabies infection rarely causes neuronal cell death and loss of neurons in the host nervous system. In this study, we show that neurons activate specific axonal self-destruction mechanism during rabies infection to prevent the spread of virus. However, this neuronal self-defense mechanism results in the loss of axons and dendrites, the structural components essential for the functioning of neurons. We further show that axonal degeneration in rabies infection is mediated by SARM1 gene, which has been previously shown to mediate defensive self-destruction of axons and dendrites in the event of neuronal injury. In summary, this study identifies a novel molecular mechanism behind neuronal dysfunction in rabies infection. This study also describes a novel intrinsic anti-viral defence mechanism in neurons, which could influence the pathogenesis of neurotropic viral infections.

Wnt7b through Frizzled-7 receptor promotes dendrite development by coactivation of CaMKII and JNK

The formation of complex dendritic arbors is crucial for the assembly of functional networks as abnormal dendrite formation underlies several neurodevelopmental and psychiatric disorders. Many extracellular factors have been postulated as regulators of dendritic growth. Wnt proteins play a critical role in neuronal development and circuit formation. We previously demonstrated that Wnt7b acts through the scaffold protein Dishevelled (Dvl) to modulate dendrite arborization by activating a Wnt non-canonical signalling pathway. Here, we identify the seven-transmembrane Frizzled-7 (Fz7) as the receptor for Wnt7b-mediated dendrite growth and complexity. Importantly, Fz7 is developmentally regulated in the intact hippocampus localised along neurites and at dendritic growth cones, suggesting a role in dendrite formation and maturation. Fz7 loss of function studies demonstrated that Wnt7b requires Fz7 to promote dendritic arborisation. Moreover, in vivo Fz7 loss of function results in dendritic defects in the intact mouse hippocampus. Furthermore, our findings revealed that Wnt7b and Fz7 induce the phosphorylation of CaMKII and JNK, which are required for dendritic development. Here we demonstrate that Wnt7b-Fz7 signals through two Wnt non-canonical pathways to modulate dendritic growth and complexity.

Modulation of Synaptic Plasticity by Exercise Training as a Basis for Ischemic Stroke Rehabilitation

In recent years, rehabilitation of ischemic stroke draws more and more attention in the world, and has been linked to changes of synaptic plasticity. Exercise training improves motor function of ischemia as well as cognition which is associated with formation of learning and memory. The molecular basis of learning and memory might be synaptic plasticity. Research has therefore been conducted in an attempt to relate effects of exercise training to neuroprotection and neurogenesis adjacent to the ischemic injury brain. The present paper reviews the current literature addressing this question and discusses the possible mechanisms involved in modulation of synaptic plasticity by exercise training. This review shows the pathological process of synaptic dysfunction in ischemic roughly and then discusses the effects of exercise training on scaffold proteins and regulatory protein expression. The expression of scaffold proteins generally increased after training, but the effects on regulatory proteins were mixed. Moreover, the compositions of postsynaptic receptors were changed and the strength of synaptic transmission was enhanced after training. Finally, the recovery of cognition is critically associated with synaptic remodeling in an injured brain, and the remodeling occurs through a number of local regulations including mRNA translation, remodeling of cytoskeleton, and receptor trafficking into and out of the synapse. We do provide a comprehensive knowledge of synaptic plasticity enhancement obtained by exercise training in this review.

MicroRNA-195 protects against dementia induced by chronic brain hypoperfusion via its anti-amyloidogenic effect in rats

Previous studies have demonstrated that chronic brain hypoperfusion (CBH) causes Aβ aggregation by upregulating expression of amyloid precursor protein (APP) and β-site APP cleaving enzyme 1 (BACE1) protein, which is accompanied by cognitive impairment, but the mechanisms are not fully understood. In this study, we evaluated the effect of microRNA on memory impairment in rats induced by CBH. We show here that CBH generated by bilateral common carotid artery occlusion (2VO) significantly decreased the learning and memory ability in rats, as assessed by Morris water maze, and upregulated expression of APP and BACE1 proteins in the hippocampus and cortex of rats, as evaluated by Western blot and immunofluorescence. In reciprocal, qRT-PCR analysis showed that microRNA-195 (miR-195) was downregulated in both the hippocampus and cortex of rats following CBH, and in the plasma of dementia patients. APP and BACE1 proteins were downregulated by miR-195 overexpression, upregulated by miR-195 inhibition, and unchanged by binding-site mutation or miR-masks, indicating that APP and BACE1 are two potential targets for miR-195. Knockdown of endogenous miR-195 by lentiviral vector-mediated overexpression of its antisense molecule (lenti-pre-AMO-miR-195) elicited dementia in rats, whereas overexpression of miR-195 using lenti-pre-miR-195 reduced dementia vulnerability triggered by 2VO. Additionally, chromatin immunoprecipitation analysis showed that NFκB was bound to the promoter region of miR-195 and inhibited its expression. We conclude that miR-195 may play a key role in determining dementia susceptibility in 2VO rats by regulating APP and BACE1 expression at the post-transcriptional level, and exogenous complement of miR-195 may be a potentially valuable anti-dementia approach.

Dysfunction of microtubule-associated proteins of MAP2/tau family in Prion disease

The aggregation of PrP (Sc) is thought to be crucial for the neuropathology of prion diseases. A growing body of evidence demonstrates that the perturbation of the microtubule network contributes to PrP (Sc) -mediated neurodegeneration. Microtubules are a component of the cytoskeleton and play a central role in organelle transport, axonal elongation and cellular architecture in neurons. The polymerization, stabilization, arrangement of microtubules can be modulated by interactions with a series of microtubule-associated proteins (MAPs). Recent studies have proposed the abnormal alterations of two major microtubule-associated proteins, tau and MAP2, in the brain tissues of naturally occurred and experimental human and animal prion diseases. Increased total tau protein and hyperphosphorylation of tau at multiple residues are observed at the terminal stage of prion disease. The abnormal aggregation of tau protein disturbs its binding ability to microtubules and affects the microtubule dynamic. Significantly downregulated MAP2 is detected in the brain tissues of scrapie-infected hamsters and PrP106-126 treated cells, which corresponds well with the remarkably low levels of tubulin. In conclusion, dysfunction of MAP2/tau family leads to disruption of microtubule structure and impairment of axonal transport, and eventually triggers apoptosis in neurons, which becomes an essential pathway for prion to induce the neuropathology.

Endothelin and the neurovascular unit in pediatric traumatic brain injury

OBJECTIVE

This study characterized the association between endothelin-1, cerebral hemodynamics, and histopathology after fluid percussion brain injury in the newborn pig.

METHODS

Lateral fluid percussion injury was induced in newborn pigs equipped with a closed cranial window. Cerebral blood flow was determined with radiolabeled microspheres and cerebrospinal fluid endothelin-1 was measured by radioimmunoassay.

RESULTS

Cerebrospinal fluid endothelin-1 was increased from 26±4 to 296±37 pg/ml (∼10(-10) M) at 8 hours following fluid percussion injury. Post-injury treatment (30 minutes) with the endothelin-1 antagonist BQ-123 (1 mg/kg, intravenous) blocked pial artery vasoconstriction to topical endothelin-1 (∼10(-10) M) and blunted fluid percussion injury-induced reductions in cerebral blood flow at 8 hours post-insult (56±6 and 26±4 ml/minute versus 57±6 and 40± ml/minute; 100 g for cerebral blood flow before injury and 8 hours post-fluid percussion injury in vehicle and BQ-123 post-treated animals, respectively). Fluid percussion injury resulted in neuronal cell loss and decreased microtubule associated protein 2 immunoreactivity in the parietal cortex, which were blunted by BQ-123.

DISCUSSION

These data indicate that fluid percussion injury-induced changes in cerebral hemodynamics are associated with neuronal damage and that endothelin-1 contributes to fluid percussion injury-induced histopathologic changes.

Vav3-deficient mice exhibit a transient delay in cerebellar development

Vav3 is a guanosine diphosphate/guanosine triphosphate exchange factor for Rho/Rac GTPases that has been involved in functions related to the hematopoietic system, bone formation, cardiovascular regulation, angiogenesis, and axon guidance. We report here that Vav3 is expressed at high levels in Purkinje and granule cells, suggesting additional roles for this protein in the cerebellum. Consistent with this hypothesis, we demonstrate using Vav3-deficient mice that this protein contributes to Purkinje cell dendritogenesis, the survival of granule cells of the internal granular layer, the timely migration of granule cells of the external granular layer, and to the formation of the cerebellar intercrural fissure. With the exception of the latter defect, the dysfunctions found in Vav3(-/-) mice only occur at well-defined postnatal developmental stages and disappear, or become ameliorated, in older animals. Vav2-deficient mice do not show any of those defects. Using primary neuronal cultures, we show that Vav3 is important for dendrite branching, but not for primary dendritogenesis, in Purkinje and granule cells. Vav3 function in the cerebellum is functionally relevant, because Vav3(-/-) mice show marked motor coordination and gaiting deficiencies in the postnatal period. These results indicate that Vav3 function contributes to the timely developmental progression of the cerebellum.

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.

A possible mechanism for controlling processive transport by microtubule-associated proteins

Molecular mechanisms of axonal transport have been evaluated by several investigators. It seems that microtubules (MTs) act as a track for the transport and microtubule-associated proteins (MAPs) seem to play as a regulating factor in it. In order to transport MTs must move in the radial direction to make room for a vesicle and when the cargo passes, return to the previous position for the maintenance of neuronal structure. An inhibitor factor against the radial movement is the steric constraints resulted from presence of MAPs. In fact, inter-microtubular spaces (IMS) in the neuronal processes are resulted from the space-making role of the MAPs. Since the IMS must be locally altered to make enough room for a vesicle, it seems relevant to imagine some mechanisms that control the steric constraints for an efficient vesicular transport. Here we juxtapose the older findings and the recent ones to investigate the possible effects of MAPs on the processive transport.

Very-KIND, a KIND domain containing RasGEF, controls dendrite growth by linking Ras small GTPases and MAP2

The regulation of cytoskeletal components in the dendritic shaft core is critical for dendrite elongation and branching. Here, we report that a brain-specific Ras guanine nucleotide exchange factor (RasGEF) carrying two kinase non-catalytic C-lobe domains (KINDs), very-KIND (v-KIND), regulates microtubule-associated protein 2 (MAP2). v-KIND is expressed in developing mouse brain, predominantly in the cerebellar granule cells. v-KIND not only activates Ras small GTPases via the C-terminal RasGEF domain, but also specifically binds to MAP2 via the second KIND domain (KIND2), leading to threonine phosphorylation of MAP2. v-KIND overexpression suppresses dendritic extension and branching of hippocampal neurons and cerebellar granule cells, whereas knockdown of endogenous v-KIND expression promotes dendrite growth. These findings suggest that v-KIND mediates a signaling pathway that links Ras and MAP2 to control dendrite growth.

Pre-treatment and post-treatment with α-tocopherol attenuates hippocampal neuronal damage in experimental cerebral hypoperfusion

Alpha-tocopherol, a potent antioxidant, has been widely investigated as a dietary supplement with which to reduce the risk of atherosclerosis, and has recently been considered as a potential supplement to moderate oxidative neuronal damage in Alzheimer's disease patients. Since alpha-tocopherol appears beneficial in vascular and neurodegenerative disorders, we set out to identify its neuroprotective action in a rat model of chronic cerebral hypoperfusion-induced brain injury. The bilateral common carotid arteries of male Wistar rats were permanently occluded (2VO). Sham-operated animals served as controls. Half of the animals were pre- or post-treated repeatedly with alpha-tocopherol (5x100 mg/kg daily, i.p.), the other half receiving only soybean oil, the alpha-tocopherol vehicle. One week after the onset of 2VO, the spatial learning capacity of the animals was assessed in the Morris water maze. After testing, hippocampal slices were stained with cresyl violet in order to examine the pyramidal cell layer integrity. The density of microtubule-associated protein-2 (MAP-2)-positive dendrites and the OX-42-labeled microglial activation level were determined immunocytochemically. Finally, alpha-tocopherol was determined in the peripheral tissues, blood and brain. Alpha-tocopherol moderated the 2VO-induced learning impairment. The various forms of alpha-tocopherol treatment, and particularly the post-treatment, prevented the 2VO-induced pyramidal cell death and the activation of microglia in the hippocampus CA1 region, and the degeneration of MAP-2-positive dendrites in the CA3 region. The alpha-tocopherol concentration was elevated in the peripheral tissues and the blood, but not in the brain. The data indicate that alpha-tocopherol, particularly when administered as post-treatment, is neuroprotective in chronic cerebral hypoperfusion.

Calpain and synaptic function

Proteolysis by calpain is a unique posttranslational modification that can change integrity, localization, and activity of endogenous proteins. Two ubiquitous calpains, mu-calpain and m-calpain, are highly expressed in the central nervous system, and calpain substrates such as membrane receptors, postsynaptic density proteins, kinases, and phosphatases are localized to the synaptic compartments of neurons. By selective cleavage of synaptically localized molecules, calpains may play pivotal roles in the regulation of synaptic processes not only in physiological states but also during various pathological conditions. Activation of calpains during sustained synaptic activity is crucial for Ca2+-dependent neuronal functions, such as neurotransmitter release, synaptic plasticity, vesicular trafficking, and structural stabilization. Overactivation of calpain following dysregulation of Ca2+ homeostasis can lead to neuronal damage in response to events such as epilepsy, stroke, and brain trauma. Calpain may also provide a neuroprotective effect from axotomy and some forms of glutamate receptor overactivation. This article focuses on recent findings on the role of calpain-mediated proteolytic processes in potentially regulating synaptic substrates in physiological and pathophysiological events in the nervous system.

Permanent, bilateral common carotid artery occlusion in the rat: a model for chronic cerebral hypoperfusion-related neurodegenerative diseases

Chronic cerebral hypoperfusion has been associated with cognitive decline in aging and Alzheimer's disease. Moreover, the pattern of cerebral blood flow in mild cognitive impairment has emerged as a predictive marker for the progression into Alzheimer's disease. The reconstruction of a pathological condition in animal models is a suitable approach to the unraveling of causal relationships. For this reason, permanent, bilateral occlusion of the common carotid arteries (2VO) in rats has been established as a procedure to investigate the effects of chronic cerebral hypoperfusion on cognitive dysfunction and neurodegenerative processes. Over the years, the 2VO model has generated a large amount of data, revealing the 2VO-related pattern of cerebral hypoperfusion and metabolic changes, learning and memory disturbances, failure of neuronal signaling, and the neuropathological changes in the hippocampus. In addition, the model has been introduced in research into ischemic white matter injury and ischemic eye disease. The present survey sets out to provide a comprehensive summary of the achievements made with the 2VO model, and a critical evaluation and integration of the various results, and to relate the experimental data to human diseases. The data that have accumulated from use of the 2VO model in the rat permit an understanding of the causative role played by cerebral hypoperfusion in neurodegenerative diseases. Thorough characterization of the model suggests that 2VO in the rat is suitable for the development of potentially neuroprotective strategies in neurodegenerative diseases.

Elevation of cytoskeletal protein breakdown in aged Wistar rat brain

Previous studies indicated there is an overall increase of proteolysis in aging rat brains. We monitored the potential degradation of cytoskeletal proteins in neuronal tissue taken from cerebral cortex and cerebellum of young (3 month) and aging (17, 21 and 23.5 month) Wistar rats. We found significant age-dependent proteolysis of cytoskeletal proteins (alphaII-spectrin and microtubule-associated protein MAP-2A/B) in the cerebral cortex and the cerebellum. The pattern of alphaII-spectrin breakdown shows a marked increase in 150- and 145-kDa fragments (SBDP150 and SBDP145, respectively), but we did not detect the caspase-3-mediated 120-kDa fragment (SBDP120) in aged rat brains, suggesting the involvement of the calpain proteases. The pattern of MAP-2A/B breakdown in aged rat brains mirrors that produced by in vitro calpain digestion of 3-month control rat brain MAP-2A/B. In aged rat brains, there is no significant increase in pro-caspase-3 processing; rather, there is a moderate reduction in pro-caspase-3 protein and caspase-3 hydrolytic activity in the cortex. These results point to selective susceptibility of cytoskeletal proteins to calpain-mediated degradation, but not caspase-3 in aging rat brains.

Microtubule-associated protein MAP1A, MAP1B, and MAP2 proteolysis during soluble amyloid beta-peptide-induced neuronal apoptosis. Synergistic involvement of calpain and caspase-3

A growing body of evidence supports the notion that soluble oligomeric forms of the amyloid beta-peptide (Abeta) may be the proximate effectors of neuronal injuries and death in the early stages of Alzheimer disease. However, the molecular mechanisms associated with neuronal apoptosis induced by soluble Abeta remain to be elucidated. We recently demonstrated the involvement of an early reactive oxygen species-dependent perturbation of the microtubule network (Sponne, I., Fifre, A., Drouet, B., Klein, C., Koziel, V., Pincon-Raymond, M., Olivier, J.-L., Chambaz, J., and Pillot, T. (2003) J. Biol. Chem. 278, 3437-3445). Because microtubule-associated proteins (MAPs) are responsible for the polymerization, stabilization, and dynamics of the microtubule network, we investigated whether MAPs might represent the intracellular targets that would enable us to explain the microtubule perturbation involved in soluble Abeta-mediated neuronal apoptosis. The data presented here show that soluble Abeta oligomers induce a time-dependent degradation of MAP1A, MAP1B, and MAP2 involving a perturbation of Ca2+ homeostasis with subsequent calpain activation that, on its own, is sufficient to induce the proteolysis of isoforms MAP2a, MAP2b, and MAP2c. In contrast, MAP1A and MAP1B sequential proteolysis results from the Abeta-mediated activation of caspase-3 and calpain. The prevention of MAP1A, MAP1B, and MAP2 proteolysis by antioxidants highlights the early reactive oxygen species generation in the perturbation of the microtubule network induced by soluble Abeta. These data clearly demonstrate the impact of cytoskeletal perturbations on soluble Abeta-mediated cell death and support the notion of microtubule-stabilizing agents as effective Alzheimer disease drugs.

Molecular mechanisms for organizing the neuronal cytoskeleton

Neurofilaments and microtubules are important components of the neuronal cytoskeleton. In axons or dendrites, these filaments are aligned in parallel arrays, and separated from one another by nonrandom distances. This distinctive organization has been attributed to cross bridges formed by NF side arms or microtubule-associated proteins. We recently proposed a polymer-brush-based mechanism for regulating interactions between neurofilaments and between microtubules. In this model, the side arms of neurofilaments and the projection domains of microtubule-associated proteins are highly unstructured and exert long-range repulsive forces that are largely entropic in origin; these forces then act to organize the cytoskeleton in axons and dendrites. Here, we review the biochemical, biophysical, genetic and cell biological data for the polymer-brush and cross-bridging models. We explore how the data traditionally used to support cross bridging may be reconciled with a polymer-brush mechanism and compare the implications of recent experimental insights into axonal transport and physiology for each model.

Glutamate receptor subunit 3 is modified by site-specific limited proteolysis including cleavage by gamma-secretase

Ionotropic glutamate receptor (GluR) expression and function is regulated through multiple pre- and post-translational mechanisms. We find that limited proteolytic cleavage of GluR3 at two distinct sites generates stable GluR3 short forms that are glycosylated and found in association with other full-length GluRs in the mouse brain and cultured primary neurons. A combination of mutagenesis and transfection into HEK293 cells revealed cleavage by a gamma-secretase-like activity within the membrane-localized re-entry loop at or near the leucine-glycine pair (amino acids 585-586, GluR3sbeta) and a second site within a proline-rich PEST-like sequence in the first cytoplasmic loop (Asp570-Pro571, GluR3salpha). Generation of the prominent GluR3salpha form was effectively abolished in the mutant, GluR3D570A, but inhibitors of lysosomes, the proteasome, caspases, or calpains had no effect. The possible impact of cleavage on receptor function was suggested when the co-expression of the GluR3P571Stop mutant (creating GluR3salpha) co-assembled with other GluR subunits and decreased receptor function in Xenopus oocytes. In transiently transfected HEK293 cells, co-expression of GluR3salpha alters the relative association between GluR1 and GluR3 during assembly, and the presence of the novel C-terminal proline-rich domain of GluR3salpha imparts lateral membrane mobility to GluR complexes. These results suggest that limited proteolysis is another post-translational mechanism through which functional diversity and specialization between closely related GluR subunits is accomplished.

The phosphorylation state of threonine-220, a uniquely phosphatase-sensitive protein kinase A site in microtubule-associated protein MAP2c, regulates microtubule binding and stability

Phosphorylation of microtubule-associated protein 2 (MAP2) has a profound effect on microtubule stability and organization. In this work a consensus protein kinase A (PKA) phosphorylation site, T(220), of juvenile MAP2c is characterized. As confirmed by mass spectrometry, this site can be phosphorylated by PKA but shows less than average reactivity among the 3.5 +/- 0.5 phosphate residues incorporated into the protein. In contrast, T(220) is uniquely sensitive to dephosphorylation: three major Ser/Thr protein phosphatases, in the order of efficiency PP2B > PP2A(c) > PP1(c), remove this phosphate group first. MAP2c specifically dephosphorylated at this site binds and stabilizes microtubules stronger than either fully phosphorylated or nonphosphorylated MAP2c. Phosphorylation of this site also affects proteolytic sensitivity of MAP2c, which might represent a further level of control in this system. Thus, the phosphorylation state of T(220) may be a primary determinant of microtubule function.

A computational model of dendrite elongation and branching based on MAP2 phosphorylation

We introduce a new computational model of dendritic development in neurons. In contrast to previous models, our model explicitly includes cellular mechanisms involved in dendritic development. It is based on recent experimental data which indicates that the phosphorylation state of microtubule-associated protein 2 (MAP2) may play a key role in controlling dendritic elongation and branching (Audesirk et al., 1997). Dephosphorylated MAP2 favours elongation by promoting microtubule polymerization and bundling, whilst branching is more likely to occur when MAP2 is phosphorylated and microtubules are spaced apart. In the model, the rate of elongation and branching is directly determined by the ratio of phosphorylated to dephosphorylated MAP2. This is regulated by calmodulin-dependent protein kinase II (CaMKII) and calcineurin, which are both dependent on the intracellular calcium concentration. Results from computer simulations of the model suggest that the wide variety of branching patterns observed among different cell types may be generated by the same underlying mechanisms and that elongation and branching are not necessarily independent processes. The model predicts how the branching pattern will change following manipulations with calcium, CaMKII and MAP2 phosphorylation.

A brain derived peptide preparation reduces the translation dependent loss of a cytoskeletal protein in primary cultured chicken neurons

Neuronal cytoskeletal proteins like the microtubule associated protein 2 (MAP2) are objected to pathological proteolysis in case of Alzheimer's disease and brain ischemia. The neurotrophic peptidergic drug Cerebrolysin (EBEWE Arzneimittel, Austria, Europe) is produced by a standardized enzymatic break-down of lipid free porcine brain proteins. Cerebolysin protected MAP2 in primary neuronal cultures after a brief histotoxic hypoxia and in a rat model of acute brain ischemia. Furthermore the drug was shown to inhibit the proteases mu- and m-calpain dose dependently in several cell free protease activity assays. The question if the higher MAP2 levels are due to an alleviation of proteolysis, to a higher synthesis rate or both is addressed in the current investigation: Monitoring the MAP2 content of primary neuronal cell cultures over a period of eight days revealed MAP2 to reach a peak level on day six in vitro followed by a degradation phase. In other experiments the protein synthesis of Cerebrolysin treated and untreated cells was blocked with cycloheximide at that moment when all cells exhibited the same MAP2 content. After the following MAP2 degradation phase--i.e. after eight days in vitro--the MAP2 contents were determined by western blotting. Cerebrolysin treated cells contained more MAP2 than untreated controls proving that the drug protects MAP2 independently from de novo synthesis, although further work is in progress to investigate if the drug supplementary boosts this effect by increasing MAP2 synthesis.

Neuritogenic-neurotoxic effects of membrane-associated forms of amyloid precursor protein

The membrane-bound glycoprotein, amyloid precursor protein (APP), plays a central role in Alzheimer's disease (AD). The present paper investigates the neuritogenic-neurotoxic properties of this protein and relates them to possible aetiopathological mechanisms in AD. Marked differences in neuritic differentiation were detected when comparing untransfected tetraploid mouse neuroblastoma cells (or vector only cells) with transfected cells overexpressing APP(751). Transfected cells developed neurites quicker, and whereas all transfected cells differentiated, the degree of differentiation of untransfected cells was more variable. Fully differentiated transfected and untransfected cells had marked differences in neuritic morphology. Transfected cells had more neurites per cell, these being shorter and more branched than neurites on untransfected cells. The precocious differentiation of transfected cells was not maintained with neuritic process disintegration and cell death occurring from the seventh day onwards. Untransfected cells continued to extend their neuritic processes for up to five weeks. Membrane-associated forms of APP were responsible for these effects, rather than secreted APP or the beta/A4-peptide. Combined data from Western blot and immunocytochemical procedures showed a prominent accumulation of APP-C-terminal fragments in the perinuclear region, neuritic varicosities and growth cones of transfected cells, suggesting their involvement in the neuritogenic-neurotoxic process. Similar neuritogenic-neurotoxic mechanisms occurring in vivo, in association with compensatory synaptoplastic responses in the aged brain, may be part of the pathological process in AD.

Making sense of the multiple MAP-2 transcripts and their role in the neuron

Microtubule-associated protein-2 (MAP-2) is a family of heat-stable, phosphoproteins expressed predominantly in the cell body and dendrites of neurons. Three major MAP-2 isoforms, (MAP-2a, MAP-2b, MAP-2c) are differentially expressed during the development of the nervous system and have an important role in microtubule dynamics. Several MAP-2 cDNA clones that correspond to the major MAP-2 transcripts and additional, novel MAP-2 transcripts expressed in the CNS and PNS have been characterized. The transcripts result from the alternative splicing of a single MAP-2 gene consisting of 20 exons. Studies are now being directed toward understanding the role of the multiple MAP-2 forms that contain novel exons in the nervous system. The expression, localization, and possible functions of the newly identified spliced forms are the focus of this review.

Effect of lead on cytoskeletal proteins expressed in E14 mesencephalic primary cultures

Several lines of evidence indicated that Pb exposure in vivo and in vitro altered neurite morphology in central and peripheral neurons. The present report shows that neurite length in mesencephalic primary cultures, consisting of neurons and glia, was decreased by Pb exposure when serum factors, presumably essential for glial functions, were absent in the culture medium. We studied whether a serum factor might control the mechanisms involved in the uptake and accumulation of Pb and its effect on cytoskeleton proteins. The total amount of Pb taken up in cell cultures was measured by atomic absorption spectroscopy and appeared to be down-regulated by a non-albumin-like serum component. In presence of serum, Pb exposure failed to alter cytoskeletal proteins. Instead, in serum-free neurobasal medium, Pb uptake failed to reach saturation within 6 h. Western blot analysis showed that the tau, 280 kDa MAP-2b, 70 kDa MAP-2c and GAP-43 protein bands were decreased 24 h after a 3 h exposure to 3 or 6 microM Pb in absence of serum. However, if cultures were maintained in serum-containing media after a 3 h Pb exposure without serum, the immunoblots did not differ from those of controls. It can be inferred that a serum factor prevents cytoskeletal protein alterations by Pb. In serum free medium, Pb that is primarily scavenged by the metallothionein I/II isoforms present in glial cells, may bind to thiol residues of proteins involved in either oxidative stress response or transcriptional regulation of cytoskeletal proteins.

Induction of growth cone formation by transient and localized increases of intracellular proteolytic activity

The formation of a growth cone at the tip of a transected axon is a crucial step in the subsequent regeneration of the amputated axon. During this process, the transected axon is transformed from a static segment into a motile growth cone. Despite the importance of this process for regeneration of the severed axon, little is known about the mechanisms underlying this transformation. Recent studies have suggested that Ca2+-activated proteinases underlay the morphological remodeling of neurons after injury. However, this hypothesis was never tested directly. Here we tested the ability of transient and localized increases in intracellular proteolytic activity to induce growth cone formation and neuritogenesis. Minute amounts of the proteinase trypsin were microinjected into intact axonal segments or somata of cultured Aplysia neurons, transiently elevating the intracellular protease concentration to 13-130 nM in the vicinity of the injection site. Such microinjections were followed by the formation of ectopic growth cones and irreversible neuritogenesis. Growth cones were not formed after external application of trypsin, microinjection of the carrier solution, or inactivated trypsin. Growth cone formation was not preceded by increases in free intracellular Ca2+ or changes in passive membrane properties, and was blocked by inhibitors of actin and tubulin polymerization. Trypsin-induced neuritogenesis was associated with ultrastructural alterations similar to those observed by us after axotomy. We conclude that local and transient elevations of cytoplasmic proteolytic activity can induce growth cone formation and neuritogenesis, and suggest that localized proteolytic activity plays a role in growth cone formation after axotomy.

Modulation of neurite branching by protein phosphorylation in cultured rat hippocampal neurons

The control of branching of axons and dendrites is poorly understood. It has been hypothesized that branching may be produced by changes in the cytoskeleton [F.J. Diez-Guerra, J. Avila, MAP2 phosphorylation parallels dendrite arborization in hippocampal neurones in culture, NeuroReport 4 (1993) 412-419; P. Friedrich, A. Aszodi, MAP2: a sensitive cross-linker and adjustable spacer in dendritic architecture, FEBS Lett. 295 (1991) 5-9]. The assembly and stability of microtubules, which are prominent cytoskeletal elements in both axons and dendrites, are regulated by microtubule-associated proteins, including tau (predominantly found in axons) and MAP2 (predominantly found in dendrites). The phosphorylation state of tau and MAP2 modulates their interactions with microtubules. In their low-phosphorylation states, tau and MAP2 bind to microtubules and increase microtubule assembly and/or stability. Increased phosphorylation decreases these effects. Diez-Guerra and Avila [F.J. Diez-Guerra, J. Avila, MAP2 phosphorylation parallels dendrite arborization in hippocampal neurones in culture, NeuroReport 4 (1993) 412-419] found that protein phosphorylation correlates with neurite branching in cultured rat hippocampal neurons, and hypothesized that increased protein phosphorylation stimulates neurite branching. To test this hypothesis, we cultured rat hippocampal neurons in the presence of specific modulators of serine-threonine protein kinases and phosphatases. Inhibitors of several protein kinases, which would be expected to decrease protein phosphorylation, reduced branching. KT5720, an inhibitor of cyclic AMP-dependent protein kinase, and KN62, an inhibitor of Ca(2+)-calmodulin-dependent protein kinases, inhibited branching of both axons and dendrites. Calphostin C and chelerythrine, inhibitors of protein kinase C, inhibited branching of axons but not dendrites. Treatments that would be expected to increase protein phosphorylation, including inhibitors of protein phosphatases (okadaic acid, cyclosporin A and FK506) and stimulators of PKA (SP-cAMPS) or PKC (phorbol 12-myristate 13-acetate), increased dendrite branching. Only FK506 and phorbol 12-myristate 13-acetate stimulated axon branching. A subset of these agents was tested to confirm their effects on protein phosphorylation in this preparation. Okadaic acid, FK506 and SP-cAMPS all increased protein phosphorylation; KT5720 and KN62 decreased protein phosphorylation. On Western blots, the position of MAP2c extracted from cultures exposed to okadaic acid was slightly shifted toward higher molecular weight, suggesting greater phosphorylation, while the position of MAP2c from cultures exposed to KT5720 and KN62 was slightly shifted toward lower molecular weight, suggesting less phosphorylation. We conclude that protein phosphorylation modulates both dendrite branching and axon branching, but with differences in sensitivity to phosphorylation and/or dephosphorylation by specific kinases and phosphatases.

Serum extravasation and cytoskeletal alterations following traumatic brain injury in rats. Comparison of lateral fluid percussion and cortical impact models

Disruption of the blood-brain barrier (BBB) and neuronal cytoskeletal damage were evaluated in two commonly used rat models of traumatic brain injury. Adult rats received a lateral cortical impact (CI) or lateral fluid percussion (FP) injury of mild or moderate severity or a sham procedure. Six hours after trauma, the brains were removed and analyzed with immunocytochemical techniques for alterations in the serum protein, IgG, and the cytoskeletal protein, microtubule-associated protein 2 (MAP2). Both models induced profound alterations in these proteins in the ipsilateral cortex and hippocampus compared to sham-injured controls. Following an injury of moderate severity, the CI injury resulted in greater IgG extravasation in the cortex and hippocampus than the FP injury. Conversely, after a mild injury, IgG extravasation in the hippocampus was greater for FP than CI. All of the animals in the CI group and most of the FP group showed a loss of MAP2 in the hippocampus. The specific subregions within the cortex and hippocampus that were affected by the injury varied between models, despite having identical impact sites. These data demonstrate that there are both similarities and differences between a CI and FP injury on vascular and neuronal cystoskeletal integrity, which should be considered when utilizing these animal models to study selected features of human head injury.

The adult rat hippocampus contains primordial neural stem cells

Adult-derived hippocampal progenitors generate neurons, astrocytes, and oligodendrocytes in vitro and following grafting into the adult brain. Although these progenitors have a considerable capacity for in vitro self renewal, it is not known if each lineage is generated by separate committed precursors or by multipotent stem cells. By genetic marking, we have followed individual cells through the process of proliferative expansion, commitment, and differentiation. All three lineages are generated by single marked cells and the relative proportions of each lineage can be strongly influenced by environmental cues. Differentiation is accompanied by a characteristic progression of lineage-specific markers and can be potentiated by retinoic acid, elevated cyclic AMP, or neurotrophic factors. The ability to genetically mark and clone normal diploid hippocampal progenitors provides the first definitive evidence that multipotent neural stem cells exist outside of the adult striatal subventricular zone and supports the hypothesis that FGF-2-responsive neural stem cells may be broadly distributed in the adult brain.

Mutual protection of microtubule-associated protein 2 (MAP2) and cyclic AMP-dependent protein kinase II against mu-calpain

Phosphorylation by adenosine-3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA), but not by Ca(++)-calmodulin-dependent protein kinase II (CaMK II), was shown earlier to protect microtubule-associated protein 2 (MAP2) from cleavage by m-calpain (Johnson and Foley: J Neurosci Res 34: 642-647, 1993). We reinvestigated this phenomenon with the physiologically more relevant mu-calpain and found a qualitatively similar but quantitatively different picture. We further demonstrate that 1) protection is biphasically dependent on the degree of phosphorylation; 2) Ca(++)-phospholipid-dependent protein kinase (PKC) has about the same effect as PKA; 3) the effects of kinases A and C are not additive; and 4) stripping of native MAP2 from its phosphate content (17.8 +/- 2.3 mol/mol) enhances calpainolysis 3.6-fold. A reciprocal effect between kinase A and MAP2 was found: the RII, but not the RI, regulatory subunit of kinase A, which was shown to bind specifically to MAP2, is protected by MAP2 from mu-calpain attack. It is suggested that the specific anchoring of kinase A-II on MAP2 may serve not only kinase targeting in the dendrites, but also protection from calpainolytic degradation.

Effect of melatonin on beta-tubulin and MAP2 expression in NIE-115 cells

Physiological concentrations of the pineal hormone melatonin induce an increase of microtubules in neuroblastoma NIE-115 cells. This effect is due to an increase in the polymerization state of tubulin. Concomitantly, higher levels of soluble beta-tubulin are present in the treated cells. Unexpectedly, no significant changes in the levels of beta-tubulin or its mRNA occur in the presence of melatonin reflecting perhaps a strict control of its steady state in a physiological context. In contrast, higher amounts of microtubule-associated-protein 2 are found when the cells are exposed to melatonin. These findings support the idea that tubulin polymerization process is one of the targets of melatonin action. Furthermore, our results might explain the increase in the length and number of neurites present in these cells when they are treated with this hormone.

Characterization of microtubule-associated protein 1-associated protein kinases from rat brain

The microtubule-associated protein (MAP) 1 preparation, MAP1A and 1B, obtained from rat brain microtubules was associated with protein kinases that were insensitive to cAMP, cGMP, calcium, calcium/calmodulin and calcium/phosphatidylserine. The fractionation of highly purified MAP1 by phosphocellulose chromatography revealed that protein kinase activity to phosphorylate phosvitin was separated into three major peaks (MAP1 kinases A, B and C). MAP1 was recovered in the MAP1 kinase A fraction and phosphorylated by the contained kinase. MAP1 kinase A is a novel protein kinase that is remarkably activated by poly-L-lysine and poly-L-arginine, but very insensitive to heparin among the kinases. Photoaffinity labeling using [alpha-32P]8-azido ATP indicated that the 65 kDa polypeptide is identified as an ATP-binding protein on sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the highly purified MAP1 and MAP1 kinase A fractions. MAP1 kinases B and C may be identified as casein kinase I- and II-like kinases. The present results show that MAP1 is associated with at least three kinases and provide an insight for understanding thoroughly the MAP1-mediated microtubule functions.

Temporal response and effects of excitatory amino acid antagonism on microtubule-associated protein 2 immunoreactivity following experimental brain injury in rats

Alterations in microtubule-associated protein 2 (MAP2) immunoreactivity following lateral fluid-percussion (FP) brain injury were investigated in rats with survival times ranging between 10 min and 7 days. MAP2 immunoreactivity was profoundly diminished in the cortex and hippocampus ipsilateral to the site of injury by 10 min and remained diminished up to 7 days after injury. Nissl staining and silver impregnation histochemistry demonstrated a correlation between the loss of MAP2 and neuronal degeneration. The effect of excitatory amino acid receptor antagonism on MAP2 immunoreactivity was evaluated by administering kynurenate or buffer 15 min after FP injury. Administration of kynurenate significantly attenuated the loss of MAP2 observed in the cortex two weeks after injury when compared to buffer treated control animals (P < 0.02). We conclude that significant and prolonged cytoskeletal changes occur following lateral FP brain injury, and that these alterations can be attenuated by blocking excitatory amino acid receptors.

Removal of the projection domain of microtubule-associated protein 2 alters its interaction with tubulin

Microtubule-associated proteins (MAPs) can promote microtubule assembly in vitro. One of these MAPs (MAP2) consists of a short promoter domain which binds to the microtubule and promotes assembly and a long projection domain which projects out from the microtubule and may interact with other cytoskeletal elements. We have previously shown that MAP2 and another MAP, tau, differ in their interactions with tubulin in that tau, but not MAP2, promotes extensive aggregation of tubulin into spiral clusters in the presence of vinblastine and that microtuubles formed with MAP2 are more resistant than those formed with tau to the antimitotic drug maytansine [Luduena, R. F., et al. (1984), J. Biol. Chem. 259, 12890-12898; Fellous, A., et al. (1985), Cancer Res. 45, 5004-5010]. Here we have used chymotryptic digestion to remove the projection domain of MAP2 and examined the interaction of the digested MAP2 (ctMAP2) with tubulin in the presence of vinblastine and maytansine. We have found that ctMAP2 behaves very much like tau, but not like undigested MAP2, in the presence of vinblastine, in that ctMAP2 causes tubulin to polymerize into large clusters of spirals. In contrast, microtubule assembly in the presence of ctMAP2 is much more resistant to maytansine inhibition than is assembly in the presence of tau or undigested MAP2. Our results suggest that the projection domain of MAP2 may play a role in the interaction of tubulin with MAP2 during microtubule assembly.

Regulation of proteolytic activity in tissues

Degradation of tissue proteins is controlled by multiple means. These include regulation of the synthesis of proteinases, activation of the zymogen forms, the activity of the mature proteinase, and the degradation of these enzymes and the substrates. Mature proteinases can be controlled by pH, calcium ions, ATP, lipids and the formation of complexes with other proteinases, proteoglycans, and inhibitors.

Distinct mode of microtubule-associated protein 2 expression in the neuroblastoma/glioma cell line 108CC15/NG108-15

The properties of microtubule-associated protein-2 (MAP-2) expression were examined in a transformed cell line, and compared to neurons from rodent brain where evidence supports both transcriptional and nontranscriptional regulation of MAP-2 synthesis. A monoclonal antibody that recognizes a common epitope in the adult (HMW MAP-2) and juvenile (MAP-2c) forms was used in an immunoblotting assay to assess the protein levels in actively dividing and differentiated neuroblastoma/glioma (108CC15, also designated NG108-15) cells. Multiply-phosphorylated MAP-2c was the predominant form in actively dividing cells, whereas HMW MAP-2 predominated in differentiated cells, which exhibited several other neuronal-like properties. A progressive increase in the levels of immunoreactive HMW MAP-2 was observed with increasing days of cell differentiation using dBcAMP as the inducing agent. However, the absolute levels of both HMW MAP-2 and MAP-2c in NG108-15 cells were significantly lower (at least 10-fold) than levels measured in rodent brain. To assess whether there are correspondingly lower levels of HMW MAP-2 and MAP-2c mRNAs in NG108-15 cells, relative to rodent brain, a highly sensitive RNA amplification assay (reverse transcription-polymerase chain reaction; RT-PCR) was developed. Oligonucleotide primers were designed to specify either HMW MAP-2 mRNA or MAP-2c mRNA, and whole tissue RNA extracted from adult and neonatal rodent brain was used to verify the reliability of the RT-PCR assay. Accordingly, PCR products of the predicted size, specificity, and abundance were obtained, with similar levels of HMW MAP-2 mRNA and proportionately higher levels of MAP-2c mRNA in neonatal brain, relative to adult brain. MAP-2c mRNA was the predominant transcript in actively dividing NG108-15 cells, and the amount of HMW MAP-2 mRNA gradually increased and became the predominant transcript in cells exposed to dBcAMP for 6-9 days. Thus, the observed changes in MAP-2-specific mRNAs during differentiation paralleled changes in expressed protein, suggesting that MAP-2 synthesis in NG108-15 cells is transcriptionally controlled. However, the levels of both MAP-2 mRNAs in NG108-15 cells were comparable to levels in rodent brain, despite the fact that MAP-2 protein levels are at least 10-fold lower in NG108-15 cells. These data suggest that the low levels of HMW MAP-2 and MAP-2c protein expression in NG108-15 cells are not due to correspondingly lower levels of MAP-2 mRNAs, and that transformed neuronal cell lines demonstrate a unique mode of MAP-2 regulation.

A-kinase anchoring proteins: a key to selective activation of cAMP-responsive events?

The cAMP-dependent protein kinase (PKA) regulates a variety of diverse biochemical events through the phosphorylation of target proteins. Because PKA is a multifunctional enzyme with a broad substrate specificity, its compartmentalization may be a key regulatory event in controlling which particular target substrates are phosphorylated. In recent years it has been demonstrated that differential localization of the type II holoenzyme is directed through interaction of the regulatory subunit (RII) with a family of A-Kinase Anchoring Proteins (AKAPs). In this report, we review evidence for PKA compartmentalization and discuss the structural and functional properties of AKAPs.

Acute anoxia-induced alterations in MAP2 immunoreactivity and neuronal morphology in rat hippocampus

Cerebral ischemia induces major neuronal morphological alterations. It is not clear, however, whether this is directly caused by O2 deprivation. To determine the effect of hypoxia on cytoskeletal structures and neuronal morphology, we performed experiments and examined anoxia-induced changes in microtubule-associated protein 2 (MAP2) and cell morphology in hippocampal slices in vitro. Anoxia (measured PO2 = 0 Torr) induced a marked loss in dendritic MAP2 immunoreactivity and cell swelling of hippocampal neurons by 2 h after O2 reinstitution. These changes were severe in CA1 and CA3 neurons and comparatively mild in dentate gyrus neurons. Quantitative analysis showed that 10 min of anoxia induced a 30% loss of MAP2-positive dendrites but this increased to 70% after 30 min of anoxia. A concurrent major increase in somata area of about 100% and 200% was observed in CA1 and CA3 neurons respectively. Somata area in the lower dentate gyrus, however, increased either insignificantly or by only 30% for the respective periods of anoxia. These results suggest that deprivation of O2 can by itself induce a major loss in dendritic MAP2 immunoreactivity and changes in cell morphology in hippocampal neurons. These alterations occur rapidly after hypoxia, and the severity of these changes is directly related to the duration of anoxia and brain region in the hippocampus.

The role of microtubule-associated protein 2 (MAP-2) in neuronal growth, plasticity, and degeneration

Microtubule associated protein 2 (MAP-2) historically has been perceived primarily as a static, structural protein, necessary along with other cytoskeletal proteins to maintain neuroarchitecture but somewhat removed from the "mainstream" of neuronal response mechanisms. Quite to the contrary, MAP-2 is exquisitely sensitive to many inputs and recent investigations have revealed dynamic functions for MAP-2 in the growth, differentiation, and plasticity of neurons, with key roles in neuronal responses to growth factors, neurotransmitters, synaptic activity, and neurotoxins. These discoveries indicate that modification and rearrangement of MAP-2 is an early obligatory step in many processes which modify neuronal function.

Microtubule organization by cross-linking and bundling proteins

To understand microtubule function the factors regulating their spatial organization and their interaction with cellular organelles, including other microtubules, must be elucidated. Many proteins are implicated in these organizational events and the known consequences of their actions within the cell are increasing. For example, the function of microtubule bundles at the surfaces of polarized cells has recently received attention, as has the action in cortical rotation of a transient arrangement of microtubules found beneath the vegetal surface of fertilized frog eggs. The in vivo association of microtubules during early Xenopus oogenesis has added interest as microtubules bundled in cell-free extracts are protected against the action of a severing protein found in this animal. A 52 kDa F-actin bundling protein purified from Physarum polycephalum organizes microtubules and causes the cobundling of microtubules and microfilaments. These observations, in concert with others that are presented, emphasize the diversity within the family of microtubule cross-linking proteins. The challenge is to determine which proteins are relevant from a physiological perspective, to ascertain their molecular mechanisms of action and to describe how they affect cytoplasmic organization and cell function. To realize this objective, the proteins which cross-link and bundle microtubules must be investigated by techniques which reveal different but related aspects of their properties. Cloning and sequencing of genes for cross-linking proteins, their subcellular localization especially as microtubule-related changes in cell morphology are occurring and the application of genetic studies are necessary. Study of the neural MAP provides the best example of just how powerful current experimental approaches are and at the same time shows their limits. The neural MAP have long been noted for their enhancement of tubulin assembly and microtubule stability. Their spatial distribution has been studied during the morphogenesis of neural cells. Sequencing of cloned genes has revealed the functional domains of neural MAP including carboxy-terminal microtubule-binding sites. Similarities to microtubule binding proteins from other cell types stimulate interest in the neural MAP and further suggest their importance in microtubule organization. For example, MAP4 enjoys a wide cellular distribution and has microtubule-binding sequences very similar to those in the neural MAP. Moreover, the nontubulin proteins of marginal bands are immunologically related to neural MAP, indicating shared structural/functional domains. Even with these findings the mechanism by which neural MAP cross-link microtubules remains uncertain. Indeed, some researchers express doubt that microtubule cross-linking is actually a function of neural MAP in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)