Stiff microtubules and neuronal morphology

Loukoumasomes Are Distinct Subcellular Structures from Rods and Rings and Are Structurally Associated with MAP2 and the Nuclear Envelope in Retinal Cells

“Rods and rings” (RR) and loukoumasomes are similarly shaped, subcellular macromolecular structures with as yet unknown function. RR, so named because of their shape, are formed in response to inhibition in the GTP or CTP synthetic pathways and are highly enriched in the two key enzymes of the nucleotide synthetic pathway. Loukoumasomes also occur as linear and toroidal bodies and were initially inferred to be the same as RR, largely due to their shared shape and size and the fact that it was unclear if they shared the same subcomponents. In human retinoblastoma tissue and cells we have observed toroidal, perinuclear, macromolecular structures of similar size and antigenicity to those previously reported in neurons (neuronal-loukoumasomes). To further characterize the subcomponents of the retinal-loukoumasomes, confocal analysis following immunocytochemical staining for alpha-tubulin, beta-III tubulin and detyrosinated tubulin was performed. These studies indicate that retinal-loukoumasomes are enriched for beta-III tubulin and other tubulins associated with microtubules. Immunofluorescence together with the in situ proximity ligation assay (PLA), confirmed that beta-III tubulin colocalized with detyrosinated tubulin within loukoumasomes. Our results indicate that these tissues contain only loukoumasomes because these macromolecular structures are immunoreactive with an anti-tubulin antibody but are not recognized by the prototype anti-RR/inosine monophosphate dehydrogenase (IMPDH) antibody (It2006). To further compare the RR and retinal-loukoumasomes, retinoblastoma cells were exposed to the IMPDH-inhibitor ribavirin, a drug known to induce the formation of RR. In contrast to RR, the production of retinal-loukoumasomes was unaffected. Coimmunostaining of Y79 cells for beta-III tubulin and IMPDH indicate that these cells, when treated with ribavirin, can contain both retinal-loukoumasomes and RR and that these structures are antigenically distinct. Subcellular fractionation studies indicate that ribavirin increased the RR subcomponent, IMPDH, in the nuclear fraction of Y79 cells from 21.3 ± 5.8% (0 mM ribavirin) to 122.8 ± 7.9% (1 mM ribavirin) while the subcellular localization of the retinal-loukoumasome subcomponent tubulin went unaltered. Further characterization of retinal-loukoumasomes in retinoblastoma cells reveals that they are intimately associated with lamin folds within the nuclear envelope. Using immunofluorescence and the in situ PLA in this cell type, we have observed colocalization of beta-III tubulin with MAP2. As MAP2 is a microtubule-associated protein implicated in microtubule crosslinking, this supports a role for microtubule crosslinkers in the formation of retinal-loukoumasomes. Together, these results suggest that loukoumasomes and RR are distinct subcellular macromolecular structures, formed by different cellular processes and that there are other loukoumasome-like structures within retinal tissues and cells.

Tau-mediated synaptic damage in Alzheimer's disease

Synapses are the principal sites for chemical communication between neurons and are essential for performing the dynamic functions of the brain. In Alzheimer’s disease and related tauopathies, synapses are exposed to disease modified protein tau, which may cause the loss of synaptic contacts that culminate in dementia. In recent decades, structural, transcriptomic and proteomic studies suggest that Alzheimer’s disease represents a synaptic disorder. Tau neurofibrillary pathology and synaptic loss correlate well with cognitive impairment in these disorders. Moreover, regional distribution and the load of neurofibrillary lesions parallel the distribution of the synaptic loss. Several transgenic models of tauopathy expressing various forms of tau protein exhibit structural synaptic deficits. The pathological tau proteins cause the dysregulation of synaptic proteome and lead to the functional abnormalities of synaptic transmission. A large body of evidence suggests that tau protein plays a key role in the synaptic impairment of human tauopathies.

Neurodegeneration and microtubule dynamics: death by a thousand cuts

Microtubules form important cytoskeletal structures that play a role in establishing and maintaining neuronal polarity, regulating neuronal morphology, transporting cargo, and scaffolding signaling molecules to form signaling hubs. Within a neuronal cell, microtubules are found to have variable lengths and can be both stable and dynamic. Microtubule associated proteins, post-translational modifications of tubulin subunits, microtubule severing enzymes, and signaling molecules are all known to influence both stable and dynamic pools of microtubules. Microtubule dynamics, the process of interconversion between stable and dynamic pools, and the proportions of these two pools have the potential to influence a wide variety of cellular processes. Reduced microtubule stability has been observed in several neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), Amyotrophic Lateral Sclerosis (ALS), and tauopathies like Progressive Supranuclear Palsy. Hyperstable microtubules, as seen in Hereditary Spastic Paraplegia (HSP), also lead to neurodegeneration. Therefore, the ratio of stable and dynamic microtubules is likely to be important for neuronal function and perturbation in microtubule dynamics might contribute to disease progression.

Physiological and pathological phosphorylation of tau by Cdk5

Hyperphosphorylation of microtubule-associated protein tau is one of the major pathological events in Alzheimer’s disease (AD) and other related neurodegenerative diseases, including frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). Mutations in the tau gene MAPT are a cause of FTDP-17, and the mutated tau proteins are hyperphosphorylated in patient brains. Thus, it is important to determine the molecular mechanism of hyperphosphorylation of tau to understand the pathology of these diseases collectively called tauopathy. Tau is phosphorylated at many sites via several protein kinases, and a characteristic is phosphorylation at Ser/Thr residues in Ser/Thr-Pro sequences, which are targeted by proline-directed protein kinases such as ERK, GSK3β, and Cdk5. Among these kinases, Cdk5 is particularly interesting because it could be abnormally activated in AD. Cdk5 is a member of the cyclin-dependent kinases (Cdks), but in contrast to the major Cdks, which promote cell cycle progression in proliferating cells, Cdk5 is activated in post-mitotic neurons via the neuron-specific activator p35. Cdk5-p35 plays a critical role in brain development and physiological synaptic activity. In contrast, in disease brains, Cdk5 is thought to be hyperactivated by p25, which is the N-terminal truncated form of p35 and is generated by cleavage with calpain. Several reports have indicated that tau is hyperphosphorylated by Cdk5-p25. However, normal and abnormal phosphorylation of tau by Cdk5 is still not completely understood. In this article, we summarize the physiological and pathological phosphorylation of tau via Cdk5.

Developmental but not adult cannabinoid treatments persistently alter axonal and dendritic morphology within brain regions important for zebra finch vocal learning

Prior work shows developmental cannabinoid exposure alters zebra finch vocal development in a manner associated with altered CNS physiology, including changes in patterns of CB1 receptor immunoreactivity, endocannabinoid concentrations and dendritic spine densities. These results raise questions about the selectivity of developmental cannabinoid effects: are they a consequence of a generalized developmental disruption, or are effects produced through more selective and distinct interactions with biochemical pathways that control receptor, endogenous ligand and dendritic spine dynamics? To begin to address this question we have examined effects of developmental cannabinoid exposure on the pattern and density of expression of proteins critical to dendritic (MAP2) and axonal (Nf-200) structure to determine the extent to which dendritic vs. axonal neuronal morphology may be altered. Results demonstrate developmental, but not adult cannabinoid treatments produce generalized changes in expression of both dendritic and axonal cytoskeletal proteins within brain regions and cells known to express CB1 cannabinoid receptors. Results clearly demonstrate that cannabinoid exposure during a period of sensorimotor development, but not adulthood, produce profound effects upon both dendritic and axonal morphology that persist through at least early adulthood. These findings suggest an ability of exogenous cannabinoids to alter general processes responsible for normal brain development. Results also further implicate the importance of endocannabinoid signaling to peri-pubertal periods of adolescence, and underscore potential consequences of cannabinoid abuse during periods of late-postnatal CNS development.

Clinical neuropathology practice guide 5-2013: markers of neuronal maturation

This review surveys immunocytochemical and histochemical markers of neuronal lineage for application to tissue sections of fetal and neonatal brain. They determine maturation of individual nerve cells as the tissue progresses to mature architecture. From a developmental perspective, neuronal markers are all about timing. These diverse cellular labels may be classified in two ways: 1) time of onset of expression (early; intermediate; late); 2) labeling of subcellular structures or metabolic functions (nucleoproteins; synaptic vesicle proteins; enolases; cytoskeletal elements; calcium-binding; nucleic acids; mitochondria). Apart from these positive markers of maturation, other negative markers are expressed in primitive neuroepithelial cells and early stages of neuroblast maturation, but no longer are demonstrated after initial stages of maturation. These examinations are relevant for studies of normal neuroembryology at the cellular level. In fetal and perinatal neuropathology they provide control criteria for application to malformations of the brain, inborn metabolic disorders and acquired fetal insults in which neuroblastic maturation may be altered. Disorders, in which cells differentiate abnormally, as in tuberous sclerosis and hemimegalencephaly, pose another yet aspect of mixed cellular lineage. The measurement in living patients, especially neonates, of serum and CSF levels of enolases, chromogranins and S-100 proteins as biomarkers of brain damage may potentially be correlated with their corresponding tissue markers at autopsy in infants who do not survive. The neuropathological markers here described can be performed in ordinary hospital laboratories, not just research facilities, and offer another dimension of diagnostic precision in interpreting abnormally developed fetal and postnatal brains.

Experimental inhibition of fibrillogenesis and neurotoxicity by amyloid-beta (Aβ) and other disease-related peptides/proteins by plant extracts and herbal compounds

Amyloid-β (Aβ) fibrillogenesis and associated cyto/neurotoxicity are major pathological events and hallmarks in diseases such as Alzheimer's disease (AD). The understanding of Aβ molecular pathogenesis is currently a pharmacological target for rational drug design and discovery based on reduction of Aβ generation, inhibition of Aβ fibrillogenesis and aggregation, enhancement of Aβ clearance and amelioration of associated cytotoxicity. Molecular mechanisms for other amyloidoses, such as transthyretin amyloidosis, AL-amyloidosis, as well as α-synuclein and prion protein are also pharmacological targets for current drug therapy, design and discovery. We report on natural herbal compounds and extracts that are capable binding to and inhibiting different targets associated with AD and other amyloid-associated diseases, providing a basis for future therapeutic strategies. Many herbal compounds, including curcumin, galantamine, quercetin and other polyphenols, are under active investigation and hold considerable potential for future prophylactic and therapeutic treatment against AD and other neurodegenerative diseases, as well as systemic amyloid diseases. A common emerging theme throughout many studies is the anti-oxidant and anti-inflammatory properties of the compounds or herbal extracts under investigation, within the context of the inhibition of cyto/neurotoxicity and anti-amyloid activity.

Dendrite plasticity in the lateral geniculate nucleus in primate glaucoma

Neural degeneration in glaucoma involves retinal ganglion cells and neurons of their major target, the lateral geniculate nucleus (LGN). Dendrites of relay LGN neurons projecting to the visual cortex were studied by immunocytochemical and quantitative Sholl analysis in combination with confocal microscopy and 3D-morphometry. In non-human adult primate glaucoma, relay LGN neurons showed reduced dendrite complexity and length, and these changes were modified by NMDA receptor blockade. Dendrite plasticity of LGN relay neurons in adult primate glaucoma has implications for potential disease modification by treatment interventions.

Tubulin post-translational modifications: encoding functions on the neuronal microtubule cytoskeleton

In the past decades, a range of post-translational modifications has been discovered on tubulins, the major constituents of microtubules. Pioneering studies have described the occurrence and dynamics of these modifications and provided first insights into their potential functions in regulating the microtubule cytoskeleton. By contrast, several tubulin-modifying enzymes were only discovered in the last few years, and studies on molecular mechanisms and cellular functions of tubulin modifications are just beginning to emerge. This review highlights the roles of tubulin modifications in neurons. Recent studies are also discussed in relation to how the combinatorial use of tubulin modifications could generate a dynamic microtubule code, and how such a code might regulate basic as well as higher-order neuronal functions.

Diabetes induces changes in ILK, PINCH and components of related pathways in the spinal cord of rats

Recent work suggests that diabetes affects processing of peripheral, spinal and supraspinal signals in the spinal cord. However, there is little evidence for spinal cord lesions that would account for alterations in behavioral responses induced by experimental diabetes. Therefore, we assessed the expression of proteins that might affect neuronal cytoskeletal stability and thus promote dendritic and synaptic reorganization in diabetic rats. Expression of ILK, PINCH, PI3K, GSK-3beta, tau, MAP2, synaptophysin and drebrin in the lumbar spinal cord of non-diabetic and streptozotocin-diabetic rats was assessed by Western-blot analysis and immunocytochemistry after 8 and 20weeks of diabetes. The impact of diabetes on the proteins studied was duration-dependent with changes observed after 20 but not 8weeks of diabetes. ILK and PINCH proteins levels were significantly decreased and both colocalized to neurons and oligodendrocytes. PI3K protein levels were also significantly decreased, while GSK-3beta activity tended to be increased. Phosphorylation of tau and MAP2A/B protein expression were significantly increased, and expression of synaptophysin and drebrin were reduced in diabetic rats. Decreased ILK and PINCH as well as alterations of components of related signaling pathways are associated with tau hyperphosphorylation, MAP2 overexpression and reduction of synaptic proteins in the spinal cord of diabetic rats, suggesting that ILK and PINCH contribute to stabilization of axonal and dendritic structures. However, these changes are not likely the cause of altered behavioral responses in diabetic rats that occur after short-term diabetes, but may contribute to structural changes occurring in long-term diabetes.

Critical roles for microtubules in axonal development and disease

Axons are occupied by dense arrays of cytoskeletal elements called microtubules, which are critical for generating and maintaining the architecture of the axon, and for acting as railways for the transport of organelles in both directions within the axon. Microtubules are organized and regulated by molecules that affect their assembly and disassembly, their stabilization, their association with other cytoskeletal elements, and their alignment and bundling with one another. Recent studies have accentuated the role of molecular motor proteins and microtubule-severing proteins in the establishment and maintenance of the axonal microtubule array. The growing body of knowledge on the proteins and mechanisms that regulate axonal microtubules has fostered a better understanding of how many debilitating diseases cause axons to degenerate. The purpose of this chapter is to provide an update on current knowledge of axonal microtubules and the proteins that regulate them, and to reflect on cutting-edge findings linking these proteins and mechanisms to diseases that afflict the human population.

Cold stability of intrinsically disordered proteins

Contrary to globular proteins, intrinsically disordered proteins (IDPs) lack a folded structure and they do not lose solubility at elevated temperatures. Although this should also be true at low temperatures, cold stability of IDPs has not been addressed in any scientific work so far. As direct characterization of cold-denaturation is difficult, we approached the problem through a freezing-induced loss-of-function model of globular-disordered functional protein pairs (m-calpain-calpastatin, tubulin-Map2c, Hsp90-ERD14). Our results affirm that in contrast with globular proteins IDPs are resistant to cold treatment. The theoretical and functional aspects of this observation are discussed.

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.

The ubiquitin ligase Phr1 regulates axon outgrowth through modulation of microtubule dynamics

To discover new genes involved in axon navigation, we conducted a forward genetic screen for recessive alleles affecting motor neuron pathfinding in GFP reporter mice mutagenized with ENU. In Magellan mutant embryos, motor axons were error prone and wandered inefficiently at choice points within embryos, but paradoxically responded to guidance cues with normal sensitivity in vitro. We mapped the Magellan mutation to the Phr1 gene encoding a large multidomain E3 ubiquitin ligase. Phr1 is associated with the microtubule cytoskeleton within neurons and selectively localizes to axons but is excluded from growth cones. Motor and sensory neurons from Magellan mutants display abnormal morphologies due to a breakdown in the polarized distribution of components that segregate between axons and growth cones. The Magellan phenotype can be reversed by stabilizing microtubules with taxol or inhibiting p38MAPK activity. Thus, efficacious pathfinding requires Phr1 activity for coordinating the cytoskeletal organization that distinguishes axons from growth cones.

Motor discoordination of transgenic mice overexpressing a microtubule destabilizer, stathmin, specifically in Purkinje cells

The proper regulation of microtubule (MT) structure is important for dendritic and neural circuit development. However, the relationship between the regulation of the MTs in dendrites and the formation of neural function is still unclear. Stathmin is a MT destabilizer, and we have previously reported that the expression and the activity of stathmin is downregulated during cerebellar Purkinje cell (PC) development. In this study, we generated transgenic mice that specifically overexpress the constitutively active form of stathmin in the PCs. These mutant mice did not show any obvious morphological or excitatory transmission abnormalities in the cerebellum. In contrast, we observed a decline in the expression of MAP2 and KIF5 signal in the PC dendrites and a discoordination of motor function in the mutant mice, although they displayed normal general behavior. These data indicate that the overexpression of stathmin disrupts dendritic MT organization, motor protein distribution, and neural function in PCs.

Molecular heterogeneity of developing retinal ganglion and amacrine cells revealed through single cell gene expression profiling

During development of the central nervous system (CNS), cycling uncommitted progenitor cells give rise to a variety of distinct neuronal and glial cell types. As these different cell types are born they progress from newly specified cells to fully differentiated neurons and glia. In order to define the developmental processes of individual cell types, single cell expression profiling was carried out on developing ganglion and amacrine cells of the murine retina. Individual cells from multiple developmental stages were isolated and profiled on Affymetrix oligonucleotide arrays. Two-color fluorescent in situ hybridization on dissociated retinas was used to verify and extend the microarray results by allowing quantitative measurements of a large number of cells coexpressing two genes. Together, these experiments have yielded an expanded view of the processes underway in developing retinal ganglion and amacrine cells, as well as several hundred new marker genes for these cell types. In addition, this study has allowed for the definition of some of the molecular heterogeneity both between developing ganglion and amacrine cells and among subclasses of each cell type.

Toluene inhalation induced neuronal damage in the spinal cord and changes of neurotrophic factors in rat

We investigated the effects of toluene inhalation on neurons and neurotrophic factors in the spinal cord and the relationship between them. Male Wistar rats were exposed to toluene (1500ppm for 4h per day) for 7 days. To observe damage of the neurons in spinal cord with the toluene, expression of microtubule associated protein 2 (MAP2) and 70kDa heat shock protein (HSP70) in spinal cord were performed by immunohistochemistry. MAP2 was degraded and HSP70-immunoreactivity was enhanced in nerve cell bodies of the gray matter in toluene inhalation group. Immunoreactivity of glial fibrillary acidic protein (GFAP), a marker of astrocytes, was enhanced in the toluene-treated group. Furthermore, glial cell line-derived neurotrophic factor (GDNF)- and brain-derived neurotrophic factor (BDNF)-immunoreactivity in spinal cord were slightly decreased in the treated group. In addition, the concentrations of GDNF and BDNF in the spinal cord were determined using enzyme linked immunosorbent assay (ELISA). Concentration of GDNF was reduced significantly by toluene exposure. BDNF also reduced, but not significantly. The toluene inhalation caused the damage of the neuron in the spinal cord, which was accompanied by the decrease in the neurotrophic factors, such as BDNF and GDNF.

The microtubule destabilizer stathmin mediates the development of dendritic arbors in neuronal cells

The regulation of microtubule dynamics is important for the appropriate arborization of neuronal dendrites during development, which in turn is critical for the formation of functional neural networks. Here we show that stathmin, a microtubule destabilizing factor, is downregulated at both the expression and activity levels during cerebellar development, and this down-regulation contributes to dendritic arborization. Stathmin overexpression drastically limited the dendritic growth of cultured Purkinje cells. The stathmin activity was suppressed by neural activity and CaMKII-dependent phosphorylation at Ser16, which led to dendritic arborization. Stathmin phosphorylation at Ser16 was mediated by the activation of voltage-gated calcium channels and metabotropic glutamate receptor 1. Although overexpression of SCG10, a member of the stathmin family, also limited the dendritic arborization, SCG10 did not mediate the CaMKII regulation of dendritic development. These results suggest that calcium elevation activates CaMKII, which in turn phosphorylates stathmin at Ser16 to stabilize dendritic microtubules. siRNA knockdown of endogenous stathmin significantly reduced dendritic growth in Purkinje cells. Thus, these data suggest that proper regulation of stathmin activity is a key factor for controlling the dendritic microtubule dynamics that are important for neuronal development.

The effects of p-chloroamphetamine, methamphetamine and 3,4-methylenedioxymethamphetamine (ecstasy) on the gene expression of cytoskeletal proteins in the rat brain

Repeated administration of beta-phenylalkylamines is known to produce neuronal changes in the central and peripheral nervous systems of mammals. It is suggested that various components of the cytoskeleton undergo profound alterations after amphetamine use and misuse, contributing to behavioral changes and neurotoxicity. Here we studied the expression of microtubule-associated protein 2 (MAP2) and beta-actin after repeated intraperitoneal applications with equimolar doses of p-chloroamphetamine (PCA), methamphetamine (METH) and 3,4-methylenedioxymethamphetamine (MDMA) in the brain of male Wistar rats. Effective (molecular) pharmacological doses (ED) were derived and used for the calculation of (molecular) pharmacological indices (PI). Besides clear but different dose-response curves on the toxicity of the drugs, in situ hybridization and Western blot analysis revealed that repeated administration of these compounds resulted in different substance- and dose-dependent changes in MAP2 gene expression, e.g. in the frontoparietal somatosensoric cortex. In contrast, the expression of beta-actin was not influenced by any of the compounds at the dose levels tested. Lethal doses were determined with 2.1 (PCA), >5.1 (METH) and 8.4 mg/kg/day (MDMA). Linear and non-linear repeat-dose lethality was observed for MDMA and PCA, respectively, whereas METH was non-lethal in the dose range used. Values for ED(MAP2) were 0.3, 0.52 and >16.8 mg/kg/day, and therefore those for PI(MAP2) were 20, 4, and 0.5 for METH, PCA and MDMA, respectively. Although the results on mortality did not reflect changes in MAP2 gene expression, they suggest a remarkable difference for those amphetamines without substituents or with a halogen atom at the paraposition of the benzene ring, such as METH or PCA, when compared with MDMA-like substances.

Chronic ocular hypertension induces dendrite pathology in the lateral geniculate nucleus of the brain

In glaucoma, there is atrophy and loss of retinal ganglion cells (RGC), in addition to atrophy and loss of target neurons in the lateral geniculate nucleus (LGN) of the brain. To investigate possible changes to the dendrites of LGN neurons in glaucoma, a selective marker for dendrites called microtubule-associated protein-2 (MAP2) was used. The LGNs from five monkeys with varying degrees of optic nerve fiber loss were compared to those from five normal control monkeys. Dendrites in magno- and parvocellular layers connected to the glaucomatous eye were evaluated. In controls, long MAP2-positive dendrites with multiple fine branches were seen. However, chronic ocular hypertension induced striking disruption of dendrites with a thickened and shortened appearance. Dendrite field area was significantly reduced in the glaucoma group compared to controls. Sholl analysis revealed reduced dendrite complexity by 47% and 41% in magnocellular layer 1 and parvocellular layer 6, respectively in the glaucoma group compared to controls. The striking dendrite changes in the LGN following chronically elevated intraocular pressure may be relevant to early visual dysfunction in glaucoma.

Hyperphosphorylation at serine 199/202 of tau factor in the gerbil hippocampus after transient forebrain ischemia

We examined the phosphorylation state of tau factor in hippocampal delayed neuronal death (DND) after transient forebrain ischemia. A transient phosphorylation increase at serine 199/202 but not serine 396 of tau factor after transient ischemia was clearly observed. Intraventricular injections of olomoucine and U-0126 (CDK5 and MAP kinase inhibitors, respectively) inhibited hyperphosphorylation. In contrast, wortmannin (PI3 kinase inhibitor) increased phosphorylation at serine 199/202 and corresponded with an increase in GSK3 phosphorylation. Our findings suggest that CDK5, MAP kinase, and GSK3 phosphorylate these sites after ischemia. We prepared recombinant normal human tau (N-Tau40) with TAT-HA protein and dephosphorylated-form human Tau-40 (D-tau40) in which 199/202 serines were changed to alanine by site-directed mutagenesis. Intraventricularly injected D-tau40 protected somewhat against DND while N-Tau40 did not. These data suggest that hyperphosphorylation at serine 199/202 of tau factor is induced by MAP kinase, CDK5, and GSK3, and contributes to ischemic neuronal injury.

Constitutively active cytoplasmic c-Jun N-terminal kinase 1 is a dominant regulator of dendritic architecture: role of microtubule-associated protein 2 as an effector

Normal functioning of the nervous system requires precise regulation of dendritic shape and synaptic connectivity. Here, we report a severe impairment of dendritic structures in the cerebellum and motor cortex of c-Jun N-terminal kinase 1 (JNK1)-deficient mice. Using an unbiased screen for candidate mediators, we identify the dendrite-specific high-molecular-weight microtubule-associated protein 2 (MAP2) as a JNK substrate in the brain. We subsequently show that MAP2 is phosphorylated by JNK in intact cells and that MAP2 proline-rich domain phosphorylation is decreased in JNK1-/- brain. We developed compartment-targeted JNK inhibitors to define whether a functional relationship exists between the physiologically active, cytosolic pool of JNK and dendritic architecture. Using these, we demonstrate that cytosolic, but not nuclear, JNK determines dendritic length and arbor complexity in cultured neurons. Moreover, we confirm that MAP2-dependent process elongation is enhanced after activation of JNK. Using JNK1-/- neurons, we reveal a dominant role for JNK1 over ERK in regulating dendritic arborization, whereas ERK only regulates dendrite shape under conditions in which JNK activity is low (JNK1-/- neurons). These results reveal a novel antagonism between JNK and ERK, potentially providing a mechanism for fine-tuning the dendritic arbor. Together, these data suggest that JNK phosphorylation of MAP2 plays an important role in defining dendritic architecture in the brain.

Transient differentiation of adult human bone marrow cells into neuron-like cells in culture: development of morphological and biochemical traits is mediated by different molecular mechanisms

Studies on rodent bone marrow stromal cells (MSCs) have revealed a capacity, for at least a portion of cells, to express neuron-like traits after differentiation in culture. Little, however, is known about the ability of human MSCs in this regard. We show here that incubation with certain differentiation cocktails, particularly those that include reagents that increase cellular cAMP levels, produces a rapid (1-4 h) and transient (24-48 h) transformation of nearly all hMSCs into neuron-like cells displaying a complex network of processes using phase or scanning electron microscopic optics. In addition, differentiated human (h) MSCs express increased quantities of neuron-[beta-tubulin III, neurofilament (NF), neuronal-specific enolase (NSE)] and glial- [glial fibrillary acidic protein (GFAP)] specific proteins and mRNAs, which are also expressed in low levels in undifferentiated MSCs. In contrast, the mesenchymal marker, fibronectin, which is highly expressed in the undifferentiated state, is reduced following differentiation. These biochemical changes, but not the acquisition of a neuron-like appearance, are partially inhibited by incubation of hMSCs with protein (cycloheximide) and mRNA (actinomycin D) synthesis inhibitors with differentiating reagents. Only incubation with 100 ng/ml colchicine, which disrupts the microtubular cytoskeleton, prevents the conversion of hMSCs into neuron- like cells. These results demonstrate that hMSCs acquire the morphological appearance and the biochemical makeup typical of neurons by independently regulated mechanisms.

Functional alterations in immature cultured rat hippocampal neurons after sustained exposure to static magnetic fields

In cultured rat hippocampal neurons, gradual increases were seen in the expression of microtubule-associated protein-2 (MAP-2), neuronal nuclei (NeuN) and growth-associated protein-43 (GAP-43), in proportion to increased duration, up to 9 days in vitro (DIV). Sustained exposure to static magnetic fields at 100 mT for up to 9 DIV significantly decreased expression of MAP-2 and NeuN in cultured rat hippocampal neurons without markedly affecting GAP-43 expression. Although a significant increase was seen in the expression of glial fibrillary acidic protein (GFAP) in hippocampal neuronal preparations cultured for 6-9 DIV under sustained magnetism, GFAP and proliferating cell nuclear antigen expression were not affected markedly in cultured astrocytes prepared from rat hippocampus and neocortex, irrespective of cellular maturity. No significant alteration was seen in cell survivability of hippocampal neurons or astrocytes cultured under sustained magnetism. In hippocampal neurons cultured for 3 DIV under sustained magnetism, marked mRNA expression was seen for N-methyl-D-aspartate (NMDA) receptor subunits, NR1, NR2A-2C, NR2D, and NR3A. In addition, significant potentiation of the ability of NMDA to increase intracellular free Ca(2+) ions was observed. Differential display analysis revealed a significant decrease in mRNA expression for the transcription factor ALF1 in response to sustained magnetism for 3 DIV. These results suggest that sustained exposure to static magnetic fields may affect cellular functionality and maturity in immature cultured rat hippocampal neurons through modulation of expression of particular NMDA receptor subunits.

Differential involvement and heterogeneous phosphorylation of tau isoforms in progressive supranuclear palsy

We found previously that aggregated insoluble tau protein in progressive supranuclear palsy (PSP) brains exhibits a heterogeneous pattern that is not segregated by the type of clinical presentation. Here we have investigated tau isoform composition from 20 PSP cases and found marked variation between different brains. Cases were classified into three groups, each comprising essentially of (1) 1N4R; (2) 1N4R and 1N3R; or (3) 1N4R, 1N3R and 0N4R tau isoforms. There was also an absence of a simple relationship between isoform composition and the pattern of insoluble tau before dephosphorylation. We conclude that there is distinct molecular heterogeneity in the involvement of tau isoforms in the tau pathology in PSP.

Specific binding of dehydroepiandrosterone to the N terminus of the microtubule-associated protein MAP2

The effect of neurosteroids is mediated through their membrane or nuclear receptors. However, no dehydroepiandrosterone (DHEA)-specific receptors have been evidenced so far in the brain. In this paper, we showed by isothermal titration calorimetry that the DHEA specifically binds to the dendritic brain microtubule-associated protein MAP2C with an association constant of 2.7 x 10(7) m-1 and at a molar ratio of 1:1. By partial tryptic digestions and mass spectrometry analysis, we found that the binding involved the N-terminal region of MAP2C. Interestingly, MAP2C displays homologies with 17 beta-hydroxysteroid dehydrogenase 1, an enzyme required for estrogen synthesis. Based on these sequence homologies and on the x-ray structure of the DHEA-binding pocket of 17 beta-hydroxysteroid dehydrogenase 1, we modeled the complex of DHEA with MAP2C. The binding of DHEA to MAP2C involved specific hydrogen bonds that orient the steroid into the pocket. This work suggests that DHEA can directly influence brain plasticity via MAP2C binding. It opens interesting ways for understanding the role of DHEA in the brain.

Role for RFX transcription factors in non-neuronal cell-specific inactivation of the microtubule-associated protein MAP1A promoter

Microtubule-associated protein MAP1A is expressed abundantly in mature neurons and is necessary for maintenance of neuronal morphology and localization of some molecules in association with the microtubule-based cytoskeleton. Previous studies indicated that its complementary expression together with MAP1B during nervous system development is regulated at the transcriptional level and that the mouse Map1A gene is transcribed under the control of 5' and intronic promoters. In this study, we investigated the regulatory mechanisms that govern the neuronal cell-specific activation of the MAP1A 5' promoter. We found that two regulatory factor for X box (RFX) binding sites in exon1 of both the mouse and human genes are important for effective transcriptional repression observed only in non-neuronal cells by reporter assays. Among RFX transcription factor family members, RFX1 and 3 mainly interact with repressive elements in vitro. Cotransfection studies indicated that RFX1, which is expressed ubiquitously, down-regulated the MAP1A 5' promoter activity in non-neuronal cells. Unexpectedly, RFX3, which is abundantly expressed in neuronal cells, down-regulated the transactivity as well, when it was expressed in non-neuronal cells. Both RFX1 and 3 did not down-regulate the transactivity in neuronal cells. These results suggest that RFX1 and 3 are pivotal factors in down-regulation of the MAP1A 5' promoter in non-neuronal cells. The cell type-specific down-regulation, however, does not depend simply on which RFX interacts with the elements, but seems to depend on underlying profound mechanisms.

Myelin biogenesis: vesicle transport in oligodendrocytes

Intracellular trafficking of membranes plays an essential role in the biogenesis and maintenance of myelin. The requisite proteins and lipids are transported from their sites of synthesis to myelin via vesicles. Vesicle transport is tightly coordinated with synthesis of lipids and proteins. To maintain the structural and functional organization of oligodendrocytes it is essential synchronize the various pathways of vesicle transport and to coordinate vesicle transport with reorganization of cytoskeleton. The systems that regulate the targeting of protein to myelin by vesicle transport are now being described. Here we review the current knowledge of these systems including those involved in (a) protein folding, (b) protein sorting and formation of carrier vesicles, (c) vesicle transport along elements of the cytoskeleton, and (d) vesicle targeting/fusion.

MAP2 is required for dendrite elongation, PKA anchoring in dendrites, and proper PKA signal transduction

Microtubule-associated protein 2 (MAP2) is a major component of cross-bridges between microtubules in dendrites, and is known to stabilize microtubules. MAP2 also has a binding domain for the regulatory subunit II of cAMP-dependent protein kinase (PKA). We found that there is reduction in microtubule density in dendrites and a reduction of dendritic length in MAP2-deficient mice. Moreover, there is a significant reduction of various subunits of PKA in dendrites and total amounts of various PKA subunits in hippocampal tissue and cultured neurons. In MAP2-deficient cultured neurons, the induction rate of phosphorylated CREB after forskolin stimulation was much lower than in wild-type neurons. Therefore, MAP2 is an anchoring protein of PKA in dendrites, whose loss leads to reduced amount of dendritic and total PKA and reduced activation of CREB.

Thalamocortical synaptic connections: efficacy, modulation, inhibition and plasticity

The thalamic input to the neocortex is communicated by glutamatergic synapses. The properties and organization of these synapses determine the primary level of cortical processing. Similar to intracortical synapses, both AMPA and NMDA receptors in young and mature animals mediate thalamocortical transmission. Kainate receptors participate in thalamocortical transmission during early development. The shape of thalamocortical synaptic potentials is similar to the shape of intracortical potentials. On the other hand, thalamocortical synapses have on average a higher release probability than intracortical synapses, and a much higher number of release sites per axon. As a result, the transmission of each thalamocortical axon is significantly more reliable and efficient than most intracortical axons. Thalamic axons specifically innervate a subset of inhibitory cells, to create a strong and secure feed-forward inhibitory pathway. Thalamocortical connections display many forms of synaptic plasticity in the first postnatal week, but not afterwards. The implications of the functional organization of thalamocortical synapses for neocortical processing are discussed.

Developmental changes of tau protein and mRNA in cultured rat brain oligodendrocytes

Oligodendrocytes elaborate an extensive network of multibranched processes and flat membranous sheets. Microtubules (MT) participate in the elaboration and stabilization of myelin-forming processes and are essential for cellular sorting processes. Microtubule-associated proteins (MAPs) are involved in the regulation and stabilization of the dynamic MT network. It has been shown previously that oligodendrocytes express the MAP tau, a phosphoprotein most abundant in neurons of the CNS. In this article, we demonstrate for the first time that oligodendrocytes contain all six tau isoforms, and that tau mRNA and protein expression is developmentally regulated. Immunoblot analysis reveals that tau protein is more abundant, and mature isoforms are more prominent at later stages of development. During the first week of culture maturation, a marked decrease in phosphorylation is observable. Using an RT-PCR approach, we can show that oligodendrocytes express small amounts of exon 3 containing isoforms and that during culture maturation, tau mRNA splice products with 3 MT-binding domains (3R) decrease and mRNA with 4 MT-binding domains (4R) increase. In situ hybridization study demonstrates that tau mRNA is present in precursor cells and in mature oligodendrocytes. Tau mRNA is actively transported into the cellular processes, is specifically present in the primary and some of the secondary processes, enriched at the turning and branching points and the growing tips, and often appears as small patches. Hence, localized tau translation at specific sites in the cellular extensions might contribute to the regulation of MT stability during process formation, early axonal contact establishment, and myelination.

Characterization of two promoters that regulate alternative transcripts in the microtubule-associated protein (MAP) 1A gene

We cloned and characterized the mouse gene for microtubule-associated protein (MAP) 1A, an important protein for neuronal morphology and mitotic spindle formation. We also investigated the 5' untranslated region of the gene to characterize the promoter units. Two alternative transcripts different in the 5' region were identified by 5' RACE. Both transcripts were principally observed in the brain. Genomic cloning revealed that exons 1, 2, and 4 generate the 5' part of a long transcript, whereas exons 3 and 4 generate a short transcript. Putative 5' and intronic promoters flanking exons 1 and 3, respectively, are GC-rich and lack a canonical TATA box. DNase I footprinting from mouse cells revealed that several potential cis-elements were occupied by nuclear proteins. A reporter assay system in conjunction with a number of deletion and mutation constructs was used to test the two putative promoters. Both putative promoters showed transactivity and their function was dependent upon Sp1 sites. In addition, an NF-1 site, an HNF3B site, and an AP-1/ATF site were necessary for basal promoter activity of the intronic promoter. Our data provide insight into the regulatory mechanisms that govern the expression of the MAP1A gene.

Effects of FTDP-17 mutations on the in vitro phosphorylation of tau by glycogen synthase kinase 3beta identified by mass spectrometry demonstrate certain mutations exert long-range conformational changes

In vitro phosphorylation of recombinant wild-type 2N4R tau and FTDP-17 exonic mutant forms P301L, V337M and R406W by glycogen synthase kinase 3beta (GSK3beta) was examined by two dimensional phosphopeptide mapping analysis on thin layer cellulose plates. Comparison of these peptide maps with those generated from wild-type 1N4R tau isoform from which the phosphopeptide constituents and sites of phosphorylation had been determined previously, enabled us to monitor directly changes in phosphorylation of the individual tau proteins. No differences were found in the phosphorylation of wild-type, P301L or V337M tau by GSK3beta but the R406W mutant showed at least two clear differences from the other three tau proteins. The peptides, identified by mass spectrometry corresponding to phosphorylation at both threonine 231 and serine 235 (spot 3), serines 396, 400 and 404 (spot 6a) and serines 195 and 199 (spot 6b) were absent from the R406W peptide map. The findings imply that the R406W mutation in tau exerts long-range conformational effects on the structure of tau.

Organization and functional roles of the cytoskeleton in oligodendrocytes

Mature oligodendrocytes are characterized by their numerous cytoplasmic extensions and flat membranous sheets. These sheets contain an extensive cytoskeletal network of microtubules (MTs) that maintain the cellular morphology, are specifically important for cellular sorting, and provide the rails for organelle trafficking. Mitochondria are localized in the primary and secondary processes and follow the tracks of the MTs in the cytoplasmic extensions. Oligodendrocytes express microtubule associated proteins (MAPs), specifically MAP2 and tau, which might be involved in the regulation and stabilization of the dynamic MT network in the myelin-containing cellular processes. Tau and MAP2 heterogeneity increases during oligodendroglia maturation, and in mature oligodendrocytes tau mRNA with four MT binding domains are more prominent than in progenitor cells. Filamentous cell inclusions are a unifying mechanism underlying a variety of late-onset neurodegenerative disorders and have mainly been viewed as neuron-specific. Recent evidence indicated that glial changes occur in CNS degenerative diseases and seem to be a more common feature than previously thought. Glial fibrillary tangles (GFTs) in oligodendrocytes were observed in familial multiple system tauopathy, and glial cytoplasmic inclusions (GCIs) and oligodendroglia degeneration are the histological hallmark of multiple system atrophy (MSA). GCIs are associated with MTs and contain stress proteins and MAPs. Thus, neurons and glial cells share common cytoskeletal pathologies. During health and disease, MAPs might be important regulators of the structural stability and plasticity of the oligodendroglia cytoskeleton.

DCAMKL1 encodes a protein kinase with homology to doublecortin that regulates microtubule polymerization

Doublecortin (DCX) is a microtubule-associated protein required for neuronal migration to the cerebral cortex. DCAMKL1 consists of an N terminus that is 65% similar to DCX throughout the entire length of DCX, but also contains an additional 360 amino acid C-terminal domain encoding a putative Ca(2+)/calmodulin-dependent protein kinase. The homology to DCX suggested that DCAMKL1 may regulate microtubules, as well as mediate a phosphorylation-dependent signal transduction pathway. Here we show that DCAMKL1 is expressed throughout the CNS and PNS in migrating neuronal populations and overlaps in its expression with DCX and microtubules. Purified DCAMKL1 associates with microtubules and stimulates polymerization of purified tubulin and the formation of aster-like microtubule structures. Overexpressed DCAMKL1 leads to striking microtubule bundling in cell lines and cultured primary neural cells. Time-lapse imaging of cells transfected with a DCAMKL1-green fluorescent protein fusion protein shows that the microtubules associated with the protein remain dynamic. DCAMKL1 also encodes a functional kinase capable of phosphorylating myelin basic protein and itself. However, elimination of the kinase activity of DCAMKL1 has no detectable effect on its microtubule polymerization activity. Because DCAMKL1 is coexpressed with DCX, the two proteins form a potentially mutually regulatory network linking calcium signaling and microtubule dynamics.

Differential effects of apolipoprotein E isoforms on phosphorylation at specific sites on tau by glycogen synthase kinase-3 beta identified by nano-electrospray mass spectrometry

Previously published data have shown an allele-specific variation in the in vitro binding of apolipoprotein E (apoE) to tau, which prompted the hypothesis that apoE binding may protect tau from phosphorylation, apoE3 being more efficient than apoE4. We have, therefore, investigated the effects of apoE on tau phosphorylation in vitro by the proline-directed kinase, glycogen synthase kinase (GSK)-3 beta. The phosphopeptide maps of tau alone, of tau with apoE3 and of tau with apoE4 were very similar. When apoE2 was present a further four spots were evident. Additionally, of the 15 peptides phosphorylated in the presence or absence of apoE, subtle differences, some isoform-specific, in the relative amounts of phosphorylation were observed.

Intracellular distribution of myelin protein gene products is altered in oligodendrocytes of the taiep rat

Hypomyelination and subsequent demyelination of the taiep rat CNS are thought to result from the abnormal accumulation of microtubules (MTs) in oligodendrocytes that disrupts intracellular transport of components needed to form and maintain the myelin sheath. In this study, myelin gene expression was evaluated in mutant and age-matched controls to determine if MT abnormalities affect the distribution of myelin proteins and their mRNAs. Immunohistochemical analysis of taiep brains and spinal cords revealed a gradual decrease in levels of several myelin proteins including myelin basic protein (MBP), proteolipid protein (PLP), myelin-associated glycoprotein (MAG), and 2',3'-cyclic nucleotide 3'-phosphodiesterase. Accompanying early declines in MAG and PLP, accumulations of immunoreactive products were detected within oligodendrocytes, consistent with a defect in protein trafficking. Northern blot analysis indicated that diminishing protein levels could not be attributed to changes in transcriptional activity, except for MBP of which mRNA levels decreased with age. Cellular localization of MBP mRNA by in situ hybridization further revealed that transcripts were concentrated within oligodendrocyte cell bodies instead of uniformly distributed throughout processes. These results demonstrate that changes in expression and intracellular localization of myelin gene products are concurrent with increases in MT mass in taiep oligodendrocytes and support our hypothesis that cytoskeletal defects prevent the normal transport of elements required for the formation and maintenance of the myelin sheath.

Tau protein isoforms, phosphorylation and role in neurodegenerative disorders

Tau proteins belong to the family of microtubule-associated proteins. They are mainly expressed in neurons where they play an important role in the assembly of tubulin monomers into microtubules to constitute the neuronal microtubules network. Microtubules are involved in maintaining the cell shape and serve as tracks for axonal transport. Tau proteins also establish some links between microtubules and other cytoskeletal elements or proteins. Tau proteins are translated from a single gene located on chromosome 17. Their expression is developmentally regulated by an alternative splicing mechanism and six different isoforms exist in the human adult brain. Tau proteins are the major constituents of intraneuronal and glial fibrillar lesions described in Alzheimer's disease and numerous neurodegenerative disorders referred to as 'tauopathies'. Molecular analysis has revealed that an abnormal phosphorylation might be one of the important events in the process leading to their aggregation. Moreover, a specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution could characterize each of these disorders. Finally, a direct correlation has been established between the progressive involvement of the neocortical areas and the increasing severity of dementia, suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. The recent discovery of tau gene mutations in frontotemporal dementia with parkinsonism linked to chromosome 17 has reinforced the predominant role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies.

Tau-mediated process outgrowth is differentially altered by the expression of MAP2b and MAP2c in Sf9 cells

It is well documented that the MAPs, MAP2 and tau, play pivotal roles in neurite outgrowth. Several isoforms of MAP2 and tau are coexpressed in neurons, suggesting that the pattern of neurite outgrowth results from a functional equilibrium among these isoforms. In the present study, by coexpressing two of these MAPs at the same time in Sf9 cells, we demonstrated that tau-mediated process outgrowth is affected differently by MAP2b and MAP2c. MAP2b impairs tau ability to induce process outgrowth. Tau affects MAP2c capacity to induce the formation of multiple processes. There is evidence that actin microfilaments (F-actin) are involved in the elaboration of tau-mediated process outgrowth in Sf9 cells. We compared the effects of MAP2b and MAP2c with the effects of tau on F-actin distribution and stability in Sf9 cells. In MAP2b- and MAP2c-expressing cells with processes, F-actin was redistributed. However, in MAP2b-expressing cells without processes, the distribution of F-actin appears to be similar to the one in wild-type infected cells. Collectively, these results indicate that MAP2b could impair the ability of MAP2c and tau to redistribute F-actin in Sf9 cells, thereby decreasing their capacity to induce process formation. Furthermore, MAP2b and MAP2c patterns of process outgrowth were differentially modified by depolymerization of F-actin by cytochalasin D (CD). As previously reported for tau-expressing cells, the MAP2b-expressing cells developed a higher number of processes per cell and a higher number of cells presented processes in the presence of CD. However, the number of cells with multiple processes was lower in MAP2b-expressing cells than in tau-expressing cells treated with CD at 24 h postinfection. This suggests that MAP2b exerts an effect on F-actin stability at an earlier stage of infection than tau. MAP2c had also some stabilizing effects on F-actin at an early stage of infection, since the percentage of cells presenting one process was similar to the nontreated cells. Therefore, MAP2b seems to have less capacity than MAP2c to redistribute F-actin but, nonetheless, both of these MAP2 isoforms exert a stabilizing effect on F-actin at an early stage of infection. Finally, by modifying phosphorylation we showed that MAP2c capacity to induce multiple processes is related to protein phosphorylation in Sf9 cells. Therefore, the differential effect of MAP2c and MAP2b on process outgrowth seems also to depend on protein phosphorylation.

Overexpression of the 77-kD echinoderm microtubule-associated protein (EMAP), a WD-40 repeat protein, in baculovirus-infected Sf9 cells

The purpose of this study was to test whether any assembly-promoting microtubule-associated protein (MAP) would bundle microtubules and induce process formation in recombinant baculovirus-infected Sf9 cells, in particular, whether a non-neural MAP from a normally rounded cell would produce cellular asymmetries. To carry out these experiments, we constructed a recombinant baculovirus that expressed the full-length 77-kD EMAP, an abundant MAP that localizes to the mitotic spindle of cleavage-stage sea urchin embryos and to the interphase array of microtubules in adult coelomocytes. Expression of EMAP in Sf9 cells had no detectable effect on cellular morphology, microtubule organization, or stability. These results indicate that process formation in Sf9 cells is MAP specific.

GSK3beta-mediated phosphorylation of the microtubule-associated protein 2C (MAP2C) prevents microtubule bundling

A major determinant of neuronal morphology is the cytoskeleton. And one of the main regulatory mechanisms of cytoskeletal proteins is the modification of their phosphorylation state via changes in the relative activities of protein kinases and phosphatases in neurons. In particular, the microtubule-associated protein 2 (MAP2) family of proteins are abundant cytoskeletal components predominantly expressed in neurons and have been found to be substrates for most of protein kinases and phosphatases present in neurons, including glycogen-synthase kinase 3 (GSK3). It has been suggested that changes in GSK3-mediated MAP phosphorylation may modify MT stability and could control neuronal development. We have previously shown that MAP2 is phosphorylated in vitro and in situ by GSK3 at Thr1620 and Thr1623, located in the proline-rich region of MAP2 and recognized by antibody 305. However, the function of the phosphorylation of this site of MAP2 is still unknown. In this study, non-neuronal COS-1 cells have been co-transfected with cDNAs encoding MAP2C and either wild type or mutated GSK3beta to analyze possible effects on microtubule stability and on the association of MAP2 with microtubules. We have found that GSK3beta phosphorylates MAP2C in co-transfected cells. Moreover, this phosphorylation is inhibited by the specific GSK3 inhibitor lithium chloride. Additionally, the formation of microtubule bundles, which is observed after transfection with MAP2C, was decreased when MAP2C was co-transfected with GSK3beta wild type. Microtubule bundles were not observed in cells expressing MAP2C phosphorylated at the site recognized by antibody 305. The absence of microtubule bundles was reverted after treatment of MAP2C/GSK3beta wild type transfected cells with lithium chloride. Highly phosphorylated MAP2C species, which were phosphorylated at the site recognized by antibody 305, appeared in cells co-transfected with MAP2C and GSK3beta wild type. Interestingly, these MAP2C species were enriched in cytoskeleton-unbound protein preparations. These data suggests that GSK3-mediated phosphorylation of MAP2 may modify its binding to microtubules and regulate microtubule stability.

The oligodendroglia cytoskeleton in health and disease

Oligodendrocytes have a high rate of synthetic activity and produce vast amounts of myelin. The membrane production requires specific sorting and transport processes and structural support. In culture, oligodendrocytes extend flat membranous sheets containing an extensive cytoskeletal network of microtubules (MTs) and microfilaments (MFs). The microtubules participate in the elaboration and stabilization of the myelin-containing cellular processes and have an impact not only on the complex oligodendroglia architecture but also influence their functions. They participate in intracellular sorting processes and the translocation of myelin basic protein (MBP) mRNAs to the forming myelin sheath. The two major groups of neuronal microtubule-associated proteins (MAPs), MAP2 and tau are expressed in oligodendrocytes and might be involved in the regulation of MT stability and organization. Myelin-specific proteins, such as MBP and 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP), interact with the cytoskeleton. Glial changes occur in a variety of neurodegenerative diseases, and glial fibrillary tangles and glial cytoplasmic inclusions (GCls), containing abnormal microtubular structures which stain positively for stress proteins and microtubule-associated proteins, are found in oligodendrocytes of the affected brains. The role of MTs and their associated proteins in oligodendrocytes during normal development and pathological situations is specifically emphasized in this review.

Tau regulates the attachment/detachment but not the speed of motors in microtubule-dependent transport of single vesicles and organelles

We have performed a real time analysis of fluorescence-tagged vesicle and mitochondria movement in living CHO cells transfected with microtubule-associated protein tau or its microtubule-binding domain. tau does not alter the speed of moving vesicles, but it affects the frequencies of attachment and detachment to the microtubule tracks. Thus, tau decreases the run lengths both for plus-end and minus-end directed motion to an equal extent. Reversals from minus-end to plus-end directed movement of single vesicles are strongly reduced by tau, but reversals in the opposite direction (plus to minus) are not. Analogous effects are observed with the transport of mitochondria and even with that of vimentin intermediate filaments. The net effect is a directional bias in the minus-end direction of microtubules which leads to the retraction of mitochondria or vimentin IFs towards the cell center. The data suggest that tau can control intracellular trafficking by affecting the attachment and detachment cycle of the motors, in particular by reducing the attachment of kinesin to microtubules, whereas the movement itself is unaffected.

Redistribution of MAP2 immunoreactivity in the neurohypophysial astrocytes of adult rats during dehydration

The low-molecular-weight microtubule-associated protein-2 (LMW MAP2) is expressed in immature and developing brains, and decreases its content dramatically along with maturation of the central nervous system. In our previous studies, we demonstrated through western blots and dual-labeling immunohistochemistry that LMW MAP2 is expressed in the pituicytes, modified astrocytes of the neurohypophysis in adult rats. The present study aimed to examine changes in the MAP2 immunoreactivity within pituicyte in adult rats under various hydration states using quantitative morphometrical analysis to demonstrate in vivo shape conversion of the pituicyte morphology. In well-hydrated control rats, light microscopic observation revealed that MAP2-stained pituicytes ramified long and well-branched processes. At electron microscopic level, MAP2 immunoreactivity was found in the fine process and cell body of all pituicyte cytoplasm, but not in the axonal terminals containing neurosecretory vesicles. The quantitative analysis demonstrated that the cell size and perimeter of MAP2-stained pituicytes were significantly greater as compared with those of cells stained with glial fibrillary acidic protein (GFAP). When the rats were dehydrated with water deprivation or drinking of 2% saline solution, the process of MAP2-stained pituicytes was less branched due to retracting their cellular processes as compared with those of well-hydrated control and rehydrated rats. The quantitative analysis further demonstrated that water deprivation significantly reduced the cell size, perimeter and length of cellular processes of MAP2-stained pituicytes as compared with those of control. The present finding indicates that MAP2 staining is better method for investigating in vivo shape conversion of the pituicyte morphology than GFAP one. Moreover, the finding that hydration states significantly and reversibly alter in vivo pituicyte shape supports the hypothesis that the plastic shape conversion of pituicyte morphology is responsible for morphological plasticity in the neurohypophysis.

Microtubule-associated protein-2 in the hypothalamo-neurohypophysial system: low-molecular-weight microtubule-associated protein-2 in pituitary astrocytes

Microtubule-associated protein-2 is the most abundant microtubule-associated protein in the brain and is responsible for morphogenesis and maintenance of the nervous system. In the present experiments, we have examined the localization of microtubule-associated protein-2 in the hypothalamo-neurohypophysial system of the rat using western blots and immunohistochemistry. Two monoclonal antibodies against microtubule-associated protein-2, antibody C and AP20, were used: antibody C recognizes both the high- and low-molecular-weight isoforms of microtubule-associated protein-2; antibody AP20 specifically detects high-molecular-weight microtubule-associated protein-2 only. Western blot analysis revealed expression of high-molecular-weight microtubule-associated protein-2 in the whole brain, hippocampus and whole hypothalamus. While the supraoptic nucleus expressed only high-molecular-weight microtubule-associated protein-2, the adult posterior pituitary predominantly expressed low-molecular-weight microtubule-associated protein-2, which was also seen in the embryonic whole brain. Light microscopic immunohistochemistry revealed that both antibody C and AP20 intensely stained dendrites of the dendritic and somatic zones in the supraoptic nucleus. Double labeling with antibodies against microtubule-associated protein-2 and oxytocin (or vasopressin) demonstrated that microtubule-associated protein-2 was localized in dendrites of magnocellular neurons in the supraoptic nucleus. In the posterior pituitary, however, antibody C stained fine processes and cell bodies of astrocytes, which were identified by an antibody against glial fibrillary acidic protein. Antibody AP20 also stained fine processes of some astrocytes in the posterior pituitary, but the intensity of immunoreactivity with antibody AP20 was weaker than that with antibody C. This result suggests that microtubule-associated protein-2 in astrocytes of the posterior pituitary is predominantly of the low-molecular-weight type. Moreover, western blots revealed low-molecular-weight microtubule-associated protein-2 of the posterior pituitary at a molecular weight slightly higher than embryonically expressed low-molecular-weight microtubule-associated protein-2, indicating that low-molecular-weight microtubule-associated protein-2 in the posterior pituitary is possibly the isoform microtubule-associated protein-2d. The present results demonstrate that astrocytes in the posterior pituitary of adult rats still retain the ability to express the immature variant of microtubule-associated protein-2, low-molecular-weight microtubule-associated protein-2, and its expression is probably linked to structural plasticity.

Developmental regulation of alternatively spliced isoforms of mRNA encoding MAP2 and tau in rat brain oligodendrocytes during culture maturation

Oligodendrocytes are responsible for the formation and maintenance of the myelin sheaths in the central nervous system (CNS), and microtubules essentially participate in the elaboration and stabilization of myelin-containing cellular processes. We have shown before that the two major groups of neuronal microtubule-associated proteins (MAPs), MAP2 and tau, are expressed in the myelin forming cells of the CNS (Mueller et al. [1997] Cell Tissue Res. 288:239-249). Here we demonstrate for the first time that during culture maturation, changes in mRNA splicing and a shift from immature to mature MAP2 and tau mRNAs occur in oligodendrocytes. Similarly to neurons, a developmental shift from MAP2 isoforms with 3 microtubule (MT)-binding domains (3R) to the isoforms with 4 MT-binding domains (4R) is observable. MAP2c constitutes the major MAP2 isoform in oligodendrocytes. They contain tau mRNA splice products with both 3 and 4 MT-binding repeats (3R, 4R) with no amino terminal insert or with exon 2, and do not express isoforms containing exon 3. The shortest form tau 1 (3R; no inserts) representing the immature tau isoform is most prominently expressed in early progenitor cells and gradually decreases during culture maturation, while tau 5 (4R; with exon 2) appears later during in vitro differentiation. The product corresponding to tau 2 (3R; with exon 2) and tau 4 (4R; no inserts) remains approximately at the same level. Hence, the occurrence of MAPs in oligodendrocytes is developmentally regulated. While in progenitor cells, 3R- and 4R-MAP2c are expressed at approximately the same level, in mature oligodendrocytes after 12 days in vitro, the ratio of 4R- to 3R-MAP2c is nearly 2. In contrast, the ratio of 4R- to 3R-tau in progenitor cells is 1:3 and shifts to 1:1 after 12 days in culture.