Microtubule-associated protein 1B (MAP1B) is present in glial cells phosphorylated different than in neurones

Neurogranin Expression Is Regulated by Synaptic Activity and Promotes Synaptogenesis in Cultured Hippocampal Neurons

Neurogranin (Ng) is a calmodulin (CaM)-binding protein that is phosphorylated by protein kinase C (PKC) and is highly enriched in the dendrites and spines of telencephalic neurons. It is proposed to be involved in regulating CaM availability in the post-synaptic environment to modulate the efficiency of excitatory synaptic transmission. There is a close relationship between Ng and cognitive performance; its expression peaks in the forebrain coinciding with maximum synaptogenic activity, and it is reduced in several conditions of impaired cognition. We studied the expression of Ng in cultured hippocampal neurons and found that both protein and mRNA levels were about 10% of that found in the adult hippocampus. Long-term blockade of NMDA receptors substantially decreased Ng expression. On the other hand, treatments that enhanced synaptic activity such as long-term bicuculline treatment or co-culture with glial cells or cholesterol increased Ng expression. Chemical long-term potentiation (cLTP) induced an initial drop of Ng, with a minimum after 15 min followed by a slow recovery during the next 2-4 h. This effect was most evident in the synaptosome-enriched fraction, thus suggesting local synthesis in dendrites. Lentiviral expression of Ng led to increased density of both excitatory and inhibitory synapses in the second and third weeks of culture. These results indicate that Ng expression is regulated by synaptic activity and that Ng promotes the synaptogenesis process. Given its relationship with cognitive function, we propose targeting of Ng expression as a promising strategy to prevent or alleviate the cognitive deficits associated with aging and neuropathological conditions.

PiT2 regulates neuronal outgrowth through interaction with microtubule-associated protein 1B

PiT2 is a member of the inorganic phosphate transporter family, and is extensively expressed in the nervous system. It was found that loop7 domain of PiT2 is not required for retroviral recognition and transport function. The exact functions of loop7 remain poorly understood. Here we show that loop7 of PiT2 is necessary for the transport of PiT2 protein to the cell surface. Further, loop7 is also related to the outgrowth of neurite in Neuro2A cells interacts with the light chain 1 of microtubule-associated protein 1B (MAP1B). PiT2 with mutated MAP1B binding sites affect neurite outgrowth whereas Pi transport function deficient mutants of PiT2 do not. We also show that Drosophila dPiT interacts with microtubule-associated protein Futsch, and dPiT is crucial for the normal development of neuromuscular junctions (NMJs). These results indicate that PiT2 might participate in the regulation of neuronal outgrowth by interacting with MAP1B and independently of its Pi transport function in the nervous system.

The MAP1B case: an old MAP that is new again

The functions of microtubule-associated protein 1B (MAP1B) have historically been linked to the development of the nervous system, based on its very early expression in neurons and glial cells. Moreover, mice in which MAP1B is genetically inactivated have been used extensively to show its role in axonal elongation, neuronal migration, and axonal guidance. In the last few years, it has become apparent that MAP1B has other cellular and molecular functions that are not related to its microtubule-stabilizing properties in the embryonic and adult brain. In this review, we present a systematic review of the canonical and novel functions of MAP1B and propose that, in addition to regulating the polymerization of microtubule and actin microfilaments, MAP1B also acts as a signaling protein involved in normal physiology and pathological conditions in the nervous system.

Cellular and molecular characterization of multipolar Map5-expressing cells: a subset of newly generated, stage-specific parenchymal cells in the mammalian central nervous system

Although extremely interesting in adult neuro-glio-genesis and promising as an endogenous source for repair, parenchymal progenitors remain largely obscure in their identity and physiology, due to a scarce availability of stage-specific markers. What appears difficult is the distinction between real cell populations and various differentiation stages of the same population. Here we focused on a subset of multipolar, polydendrocyte-like cells (mMap5 cells) expressing the microtubule associated protein 5 (Map5), which is known to be present in most neurons. We characterized the morphology, phenotype, regional distribution, proliferative dynamics, and stage-specific marker expression of these cells in the rabbit and mouse CNS, also assessing their existence in other mammalian species. mMap5 cells were never found to co-express the Ng2 antigen. They appear to be a population of glial cells sharing features but also differences with Ng2+progenitor cells. We show that mMap5 cells are newly generated, postmitotic parenchymal elements of the oligodendroglial lineage, thus being a stage-specific population of polydendrocytes. Finally, we report that the number of mMap5 cells, although reduced within the brain of adult/old animals, can increase in neurodegenerative and traumatic conditions.

The light chains of microtubule-associated proteins MAP1A and MAP1B interact with α1-syntrophin in the central and peripheral nervous system

Microtubule-associated proteins of the MAP1 family (MAP1A, MAP1B, and MAP1S) share, among other features, a highly conserved COOH-terminal domain approximately 125 amino acids in length. We conducted a yeast 2-hybrid screen to search for proteins interacting with this domain and identified α1-syntrophin, a member of a multigene family of adapter proteins involved in signal transduction. We further demonstrate that the interaction between the conserved COOH-terminal 125-amino acid domain (which is located in the light chains of MAP1A, MAP1B, and MAP1S) and α1-syntrophin is direct and occurs through the pleckstrin homology domain 2 (PH2) and the postsynaptic density protein 95/disk large/zonula occludens-1 protein homology domain (PDZ) of α1-syntrophin. We confirmed the interaction of MAP1B and α1-syntrophin by co-localization of the two proteins in transfected cells and by co-immunoprecipitation experiments from mouse brain. In addition, we show that MAP1B and α1-syntrophin partially co-localize in Schwann cells of the murine sciatic nerve during postnatal development and in the adult. However, intracellular localization of α1-syntrophin and other Schwann cell proteins such as ezrin and dystrophin-related protein 2 (DRP2) and the localization of the axonal node of Ranvier-associated protein Caspr1/paranodin were not affected in MAP1B null mice. Our findings add to a growing body of evidence that classical MAPs are likely to be involved in signal transduction not only by directly modulating microtubule function, but also through their interaction with signal transduction proteins.

Microtubule-associated protein 1B (MAP1B) is required for dendritic spine development and synaptic maturation

Microtubule-associated protein 1B (MAP1B) is prominently expressed during early stages of neuronal development, and it has been implicated in axonal growth and guidance. MAP1B expression is also found in the adult brain in areas of significant synaptic plasticity. Here, we demonstrate that MAP1B is present in dendritic spines, and we describe a decrease in the density of mature dendritic spines in neurons of MAP1B-deficient mice that was accompanied by an increase in the number of immature filopodia-like protrusions. Although these neurons exhibited normal passive membrane properties and action potential firing, AMPA receptor-mediated synaptic currents were significantly diminished. Moreover, we observed a significant decrease in Rac1 activity and an increase in RhoA activity in the post-synaptic densities of adult MAP1B(+/-) mice when compared with wild type controls. MAP1B(+/-) fractions also exhibited a decrease in phosphorylated cofilin. Taken together, these results indicate a new and important role for MAP1B in the formation and maturation of dendritic spines, possibly through the regulation of the actin cytoskeleton. This activity of MAP1B could contribute to the regulation of synaptic activity and plasticity in the adult brain.

Development and application of a phosphoproteomic method using electrostatic repulsion-hydrophilic interaction chromatography (ERLIC), IMAC, and LC-MS/MS analysis to study Marek's Disease Virus infection

Marek's Disease (MD) is an avian neoplastic disease caused by Marek's Disease Virus (MDV). The mechanism of virus transition between the lytic and latent cycle is still being investigated; however, post-translational modifications, especially phosphorylation, have been thought to play an important role. Previously, our group has used strong cation exchange chromatography in conjunction with reversed-phase liquid chromatography-tandem mass spectrometry (LC-MS/MS) to study the changes in global proteomic expression upon MDV infection (Ramaroson , M. F.; Ruby, J.; Goshe, M. B.; Liu , H.-C. S. J. Proteome Res. 2008, 7, 4346-4358). Here, we extend our study by developing an effective separation and enrichment approach to investigate the changes occurring in the phosphoproteome using electrostatic repulsion-hydrophilic interaction chromatography (ERLIC) to fractionate peptides from chicken embryo fibroblast (CEF) digests and incorporating a subsequent IMAC enrichment step to selectively target phosphorylated peptides for LC-MS/MS analysis. To monitor the multidimensional separation between mock- and MDV-infected CEF samples, a casein phosphopeptide mixture was used as an internal standard. With LC-MS/MS analysis alone, no CEF phosphopeptides were detected, while with ERLIC fractionation only 1.2% of all identified peptides were phosphorylated. However, the incorporation of IMAC enrichment with ERLIC fractionation provided a 50-fold increase in the percentage of identified phosphopeptides. Overall, a total of 581 unique phosphopeptides were identified (p < 0.05) with those of the MDV-infected CEF sample containing nearly twice as many as the mock-infected control of which 11% were unique to MDV proteins. The changes in the phosphoproteome are discussed including the role that microtubule-associated proteins may play in MDV infection mechanisms.

Microtubule-associated protein 1B, a growth-associated and phosphorylated scaffold protein

Microtubule-associated protein 1B, MAP1B, is one of the major growth associated and cytoskeletal proteins in neuronal and glial cells. It is present as a full length protein or may be fragmented into a heavy chain and a light chain. It is essential to stabilize microtubules during the elongation of dendrites and neurites and is involved in the dynamics of morphological structures such as microtubules, microfilaments and growth cones. MAP1B function is modulated by phosphorylation and influences microtubule stability, microfilaments and growth cone motility. Considering its large size, several interactions with a variety of other proteins have been reported and there is increasing evidence that MAP1B plays a crucial role in the stability of the cytoskeleton and may have other cellular functions. Here we review molecular and functional aspects of this protein, evoke its role as a scaffold protein and have a look at several pathologies where the protein may be involved.

Expression of protein kinase CK2 in astroglial cells of normal and neovascularized retina

We previously documented protein kinase CK2 involvement in retinal neovascularization. Here we describe retinal CK2 expression and combined effects of CK2 inhibitors with the somatostatin analog octreotide in a mouse model of oxygen-induced retinopathy (OIR). CK2 expression in human and rodent retinas with and without retinopathy and in astrocytic and endothelial cultures was examined by immunohistochemistry, Western blotting, and reverse transcriptase-polymerase chain reaction. A combination of CK2 inhibitors, emodin or 4,5,6,7-tetrabromobenzotriazole, with octreotide was injected intraperitoneally from postnatal (P) day P11 to P17 to block mouse OIR. All CK2 subunits (alpha, alpha', beta) were expressed in retina, and a novel CK2alpha splice variant was detected by reverse transcriptase-polymerase chain reaction. CK2 antibodies primarily reacted with retinal astrocytes, and staining was increased around new intraretinal vessels in mouse OIR and rat retinopathy of prematurity, whereas preretinal vessels were negative. Cultured astrocytes showed increased perinuclear CK2 staining compared to endothelial cells. In the OIR model, CK2 mRNA expression increased modestly on P13 but not on P17. Octreotide combined with emodin or 4,5,6,7-tetrabromobenzotriazole blocked mouse retinal neovascularization more efficiently than either compound alone. Based on its retinal localization, CK2 may be considered a new immunohistochemical astrocytic marker, and combination of CK2 inhibitors and octreotide may be a promising future treatment for proliferative retinopathies.

The fragile X protein controls microtubule-associated protein 1B translation and microtubule stability in brain neuron development

The fragile X mental retardation protein (FMRP) is a selective RNA-binding protein implicated in regulating translation of its mRNA ligands. The absence of FMRP results in fragile X syndrome, one of the leading causes of inherited mental retardation. Delayed dendritic spine maturation was found in fragile X mental retardation patients as well as in Fmr1 knockout (KO) mice, indicating the functional requirement of FMRP in synaptic development. However, the biochemical link between FMRP deficiency and the neuronal impairment during brain development has not been defined. How FMRP governs normal synapse development in the brain remains elusive. We report here that the developmentally programmed FMRP expression represses the translation of microtubule associated protein 1B (MAP1B) and is required for the accelerated decline of MAP1B during active synaptogenesis in neonatal brain development. The lack of FMRP results in misregulated MAP1B translation and delayed MAP1B decline in the Fmr1 KO brain. Furthermore, the aberrantly elevated MAP1B protein expression leads to abnormally increased microtubule stability in Fmr1 KO neurons. Together, these results indicate that FMRP plays critical roles in controlling cytoskeleton organization during neuronal development, and the abnormal microtubule dynamics is a conceivable underlying factor for the pathogenesis of fragile X mental retardation.

Expression of MAP1B protein and its phosphorylated form MAP1B-P in the CNS of a continuously growing fish, the rainbow trout

Microtubule-associated protein-1B (MAP1B), and particularly its phosphorylated isoform MAP1B-P, play an important role in axonal outgrowth during development of the mammalian nervous system and have also been shown to be associated with axonal plasticity in the adult. Here, we used antibodies and mRNA probes directed against mammalian MAP1B to extend our analysis to fish species, trout (Oncorhynchus mykiss), at different stages of development. The specificity of the cross-reaction of our anti-total-MAP1B/MAP1B-P antibodies was confirmed by Western blotting. Trout MAP1B-like proteins exhibited about the same apparent molecular weight (320 kDa) as rat-MAP1B. Immunohistochemistry and in situ hybridization analysis performed on hindbrain and spinal cord revealed the presence of MAP1B in neurons and some glial subpopulations. Primary sensory neurons and motoneurons maintain high levels of MAP1B expression from early stages throughout adulthood, as has been shown for mammals. Unlike mammals, however, MAP1B and axon-specific MAP1B-P continue to be strongly expressed by hindbrain neurons projecting into spinal cord, with the important exception of Mauthner cells. MAP1B/MAP1B-P immunostaining were also detected elsewhere within the brain, including axons of the retino-tectal projection. This obvious difference between adult fish and mammals is likely to reflect the capacity of fish for continued growth and regeneration. Our results suggest that MAP1B/MAP1B-P expression is generally maintained in neurons known to regenerate after axotomy. The regenerative potential of the adult nervous system may in fact depend on continued expression of neuron-intrinsic growth related proteins, a feature of MAP1B that appears phylogenetically conserved.

Microtubule-associated protein 2 phosphorylation is decreased in the human epileptic temporal lobe cortex

Microtubule-associated protein 2 (MAP2) is an abundant component of the neuronal cytoskeleton whose function is related to the outgrowth and stability of neuronal processes, to synaptic plasticity and neuronal cell death. We have sought to study whether abnormal patterns of neuronal activity which are characteristic of epileptic patients are associated to alterations of MAP2 phosphorylation. An antibody (305) that selectively recognizes a phosphorylated epitope in a proline-rich region of the MAP2 molecule has been used to analyze neocortical biopsy samples from temporal lobe epileptic patients, whose electrocorticogram activity had been previously monitored. Immunoblot analysis showed that samples with greater spiking activity displayed significantly diminished MAP2 phosphorylation. Immunocytochemical analysis revealed the occurrence of discrete areas in the neocortex with highly decreased or no immunostaining for antibody 305, which showed a clear, although non-significant, tendency to appear more frequently in areas with greater spiking activity. To further support an association between epileptiform activity and MAP2 dephosphorylation an experimental model of epileptiform activity in cultures of rat hippocampal neurons was used. Neurons were cultured during 15 days in the presence of kynurenic acid, an antagonist of glutamate receptors. At this time, kynurenic acid was removed from the culture medium and neurons developed seizure-like activity. Using antibody 305, we found a decrease of MAP2 phosphorylation that was already visible after 15 min of kynurenic acid withdrawal. We therefore propose that MAP2 phosphorylation is decreased in the neocortex of epileptic patients and that this decrease is a likely consequence of seizure activity. Also, MAP2 dephosphorylation may lead to alterations of the neuronal cytoskeleton and eventually to neuronal damage and loss, which is typical of epileptic patients.

bFGF stimulates GAP-43 phosphorylation at ser41 and modifies its intracellular localization in cultured hippocampal neurons

Cultured hippocampal neurons have been used to study GAP-43 phosphorylation and subcellular distribution. By immunofluorescence, GAP-43 was found associated with adherent membrane patches that remained attached to the substratum after in situ permeabilization with Nonidet-NP40. This association increases during neuronal development and is stabilized by the actin cytoskeleton. Basic fibroblast growth factor (bFGF) promotes GAP-43 translocation from the cytosol to adherent membrane patches and, at the same time, stimulates GAP-43 phosphorylation, mainly at the protein kinase C (PKC) site (Ser41). Inhibition of PKC prevented bFGF-stimulated GAP-43 phosphorylation and translocation, while activation by phorbol esters mimicked bFGF effects, suggesting that phosphorylation at Ser41 regulates GAP-43 subcellular localization. Using biochemical fractionation and phosphorylation analysis, it was found that Ser41 phosphorylation was highest in cytoskeleton-associated GAP-43 and lowest in membrane-associated GAP-43. It is proposed that GAP-43 is continuously cycling between intracellular compartments depending on its phosphorylation state and could be taking part in initial adhesive complexes assembled during growth cone advance.

Regulation of the expression and phosphorylation of microtubule-associated protein 1B during regeneration of adult dorsal root ganglion neurons

Microtubule-associated protein 1B is a major constituent of the neuronal cytoskeleton during the early stages of development. This protein and its phosphorylated isoform, microtubule-associated protein 1B-P, defined by the monoclonal antibody 1B-P [Boyne L. J. et al. (1995) J. Neurosci. Res. 40, 439-450], are present in growing axons and concentrated in the distal end near the growth cone. In most regions of the central nervous system, microtubule-associated protein 1B and microtubule-associated protein 1B-P are developmentally down-regulated. They remain, however, at relatively high levels in the adult peripheral nervous system, where microtubule-associated protein 1B-P is localized exclusively in axons. The aim of this study was to examine the levels of microtubule-associated protein 1B and its phosphorylated isoform during regenerative growth of peripheral axons. Following transection and re-apposition of the sciatic nerve at midthigh, the levels of total microtubule-associated protein 1B, microtubule-associated protein 1B-P and microtubule-associated protein 1B messenger RNA were analysed in dorsal root ganglion neurons and sciatic nerve axons using western blots and RNase protection assays. After the lesion, there was a small decrease in the levels of microtubule-associated protein 1B and its messenger RNA in dorsal root ganglion neurons. The proximal axonal stump showed a similar decrease in the levels of microtubule-associated protein 1B 30days after lesion and returned to normal 60-90days post-lesion. In the distal stump of the sciatic nerve, the levels of microtubule-associated protein 1B increased dramatically and rapidly between three and 14days, but the protein was localized mainly in activated Schwann cells and myelin-like structures, and not in axons [Ma D. et al. (1999) Brain Res. 823, 141-153]. With the regeneration of axons into the distal stump, an intense expression of microtubule-associated protein 1B was observed in these axons. Microtubule-associated protein 1B-P, however, disappeared from the degenerated distal axonal stump as early as three days post-operation, and was absent in the regenerating axons and in Schwann cells between three and 14days. The levels of microtubule-associated protein 1B-P recovered slowly and did not reach the normal levels even after 90days post-operation. In contrast to the response following transection, the levels of microtubule-associated protein 1B and microtubule-associated protein 1B-P were much less affected after nerve crush. We propose that the relatively high levels of microtubule-associated protein 1B and its messenger RNA in adult dorsal root ganglions support peripheral neuron regeneration. The presence of microtubule-associated protein 1B in the regenerating axons suggests that microtubule-associated protein 1B is involved in axonal growth during peripheral nerve regeneration. However, the phosphorylated microtubule-associated protein 1B-P isoform, associated with growing axons during development, is not present in the regenerating axons after transection, presumably because of changes in the activities of kinases and phosphatases associated with the injury. These observations underscore the difference between axonal development and regeneration and the importance of injury-related effects that occur locally.

Evidence for expression of some microtubule-associated protein 1B in neurons as a plasma membrane glycoprotein

Microtubule-associated protein (MAP) 1B is a high-molecular-weight cytoskeletal protein that is abundant in developing neuronal processes and appears to be necessary for axonal growth. Various biochemical and immunocytochemical results are reported, indicating that a significant fraction of MAP1B is expressed as an integral membrane glycoprotein in vesicles and the plasma membrane of neurons. MAP1B is present in microsomal fractions isolated from developing rat brain and fractionates across a sucrose gradient in a manner similar to synaptophysin, a well-known vesicular and plasma membrane protein. MAP1B is also in axolemma-enriched fractions (AEFs) isolated from myelinated axons of rat brain. MAP1B in AEFs and membrane fractions from cultured dorsal root ganglion neurons (DRGNs) remains membrane-associated following high-salt washes and contains sialic acid. Furthermore, MAP1B in intact DRGNs is readily degraded by extracellular trypsin and is labeled by the cell surface probe sulfosuccinimidobiotin. Immunocytochemical examination of DRGNs shows that MAP1B is concentrated in vesicle-rich varicosities along the length of axons. Myelinated peripheral nerves immunostained for MAP1B show an enrichment at the axonal plasma membrane. These observations demonstrate that some of the MAP1B in developing neurons is an integral plasma membrane glycoprotein.

Rho and Rho kinase mediate thrombin-stimulated vascular smooth muscle cell DNA synthesis and migration

Aberrant regulation of smooth muscle cell proliferation and migration is associated with the pathophysiology of vascular disorders such as hypertension, atherosclerosis, restenosis, and graft rejection. To elucidate molecular mechanisms that regulate proliferation and migration of vascular smooth muscle cells, we determined whether signaling through the small G protein Rho is involved in thrombin- and phenylephrine-stimulated proliferation and migration of rat aortic smooth muscle cells (RASMCs). Thrombin and the thrombin peptide SFLLRNP stimulated DNA synthesis of RASMCs as measured by [3H]thymidine incorporation. Both ligands also increased cell migration as measured by the Boyden chamber method. L-Phenylephrine failed to induce either of these responses but increased inositol phosphate accumulation and mitogen-activated protein kinase activation in these cells, which indicated that the cells were responsive to alpha1-adrenergic stimulation. The C3 exoenzyme, which ADP-ribosylates and inactivates Rho, fully inhibited both thrombin-stimulated proliferation and migration but had no effect on inositol phosphate accumulation. In addition, Y-27632, an inhibitor of the Rho effector p160ROCK/Rho kinase, decreased thrombin-stimulated DNA synthesis and migration. To directly examine Rho activation, Rho-[35S]GTPgammaS binding was measured. The addition of the thrombin peptide SFLLRNP, but not phenylephrine, to RASMC lysates resulted in a significant increase in Rho-[35S]GTPgammaS binding. Thrombin and SFLLRNP, but not phenylephrine, also increased membrane-associated Rho in intact RASMCs, consistent with selective activation of Rho by thrombin. These results indicate that thrombin activates Rho in RASMCs and establish Rho as a critical mediator of thrombin receptor effects on DNA synthesis and cell migration in these cells.

Induction of microtubule-associated protein 1B expression in Schwann cells during nerve regeneration

Microtubule-associated protein 1B (MAP1B) is expressed at high levels during development of the nervous system and is localized primarily in neurons while specific phosphorylated isoforms of MAP1B are localized exclusively in growing axons. The levels of MAP1B are down regulated in most regions of the adult CNS, but remain high in neurons and axons of the PNS. This study demonstrates that the expression of MAP1B is induced in adult Schwann cells following sciatic nerve lesion and regeneration. High levels of both mRNA and the MAP1B protein were detected in Schwann cells associated with the axotomized distal stump. Expression of MAP1B was also observed in cultured primary Schwann cells from neonatal rats. The properties of the MAP1B protein in cultured Schwann cells were further characterized by Western blot analysis using specific antibodies that recognize the N-terminal, middle and C-terminal domains of MAP1B. All of these antibodies detected a protein of 320-340 kDa demonstrating that MAP1B expressed by Schwann cells is very similar, or identical, to MAP1B expressed by neurons. The phosphorylation of MAP1B in Schwann cells was also studied using monoclonal antibodies (mAb) that recognize specific phosphorylation epitopes. The results indicated that the expression of MAP1B in Schwann cells exhibited a differential phosphorylation state that was recognized by mAb 1B6 but not by other mAbs, including 1B-P, 150 and RT97, that recognize phosphorylated MAP1B in growing axons. We therefore conclude that MAP1B is expressed in Schwann cells during both development and axonal regeneration, suggesting that the developmental pattern of MAP1B in these cells is recapitulated in adult Schwann cells during the early stages of regeneration and remyelination of injured peripheral axons. The presence of MAP1B in Schwann cells may support morphological changes of these cells, particularly the formation of processes prior to their differentiation into myelin forming Schwann cells.

Two modes of microtubule-associated protein 1B phosphorylation are differentially regulated during peripheral nerve regeneration

Two major modes of MAP1B phosphorylation (I and II), respectively recognized by monoclonal antibodies 150 and 125, have been related to remodeling and formation of processes in the mature nervous system. To gain insight into the cytoskeletal modifications underlying peripheral nerve regeneration, the pattern of expression of both MAP1B phosphorylated modes was studied during this process. Sciatic nerves from adult Wistar rats were crushed and animals allowed to survive for 5, 7, 10 or 14 days. After those survival periods, damaged and undamaged sciatic nerves, dorsal root ganglia (DRG), and spinal cords, were subjected to immunohistochemistry and Western blot, using antibodies 150 and 125. At all survival periods analysed, MAP1B phosphorylated at mode I was concentrated at the distal region of regenerating nerves whereas mode II phosphorylation underwent an overall decrease in regenerating axons that was less evident in more proximal nerve regions. Very high levels of MAP1B phosphorylated at mode II were detected in the bodies of DRG neurons and in bodies and dendrites of spinal motor neurons. This phosphorylation mode was also encountered in some Schwann cells and oligodendroglia associated with more proximal regions of regenerating axons. In this study we conclude that MAP1B was differentially phosphorylated depending on the cell type, subcellular compartment and stage of the regenerative process and discuss the possible functional implications that differential expression of each MAP1B phosphorylation mode might have during nerve regeneration.

Evidence for the participation of the neuron-specific CDK5 activator P35 during laminin-enhanced axonal growth

Cultures of cerebellar macroneurons were used to study the pattern of expression, subcellular localization, and function of the neuronal cdk5 activator p35 during laminin-enhanced axonal growth. The results obtained indicate that laminin, an extracellular matrix molecule capable of selectively stimulating axonal extension and promoting MAP1B phosphorylation at a proline-directed protein kinase epitope, selectively stimulates p35 expression, increases its association with the subcortical cytoskeleton, and accelerates its redistribution to the axonal growth cones. Besides, suppression of p35, but not of a highly related isoform designated as p39, by antisense oligonucleotide treatment selectively reduces cdk5 activity, laminin-enhanced axonal elongation, and MAP1b phosphorylation. Taken collectively, the present results suggest that cdk5/p35 may serve as an important regulatory linker between environmental signals (e.g., laminin) and constituents of the intracellular machinery (e.g., MAP1B) involved in axonal elongation.

Novel features of the light chain of microtubule-associated protein MAP1B: microtubule stabilization, self interaction, actin filament binding, and regulation by the heavy chain

Previous studies on the role of microtubule-associated protein 1B (MAP1B) in adapting microtubules for nerve cell-specific functions have examined the activity of the entire MAP1B protein complex consisting of heavy and light chains and revealed moderate effects on microtubule stability. Here we have analyzed the effects of the MAP1B light chain in the absence or presence of the heavy chain by immunofluorescence microscopy of transiently transfected cells. Distinct from all other MAPs, the MAP1B light chain-induced formation of stable but apparently flexible microtubules resistant to the effects of nocodazole and taxol. Light chain activity was inhibited by the heavy chain. In addition, the light chain was found to harbor an actin filament binding domain in its COOH terminus. By coimmunoprecipitation experiments using epitope-tagged fragments of MAP1B we showed that light chains can dimerize or oligomerize. Furthermore, we localized the domains for heavy chain-light chain interaction to regions containing sequences homologous to MAP1A. Our findings assign several crucial activities to the MAP1B light chain and suggest a new model for the mechanism of action of MAP1B in which the heavy chain might act as the regulatory subunit of the MAP1B complex to control light chain activity.

An alternatively spliced, 5'-truncated MAP1B isoform is expressed in the developing chick nervous system

Our laboratory has previously characterized a keratan sulfate proteoglycan, named claustrin, and shown by molecular cloning that claustrin and the mouse MAP1B protein share high homology, with claustrin representing a 5'-truncated fragment of MAP1B. In the present study, we examine further the relationship between claustrin and MAP1B, and also describe the isolation of a cDNA encoding the 3'-region of MAP1B, which shares 3'-untranslated sequence, but not coding sequence, with claustrin. We call this partial cDNA 3'-MAP1B-related clone (3'-MRC), since it is homologous to the 3'-region of the mouse MAP1B sequence. We show by Northern analysis that distinct mRNAs are recognized by the claustrin and 3'-MRC cDNAs, and by RT-PCR that mRNAs encoding these distinct MAP1B-related molecules are present in embryonic chick brain and cardiac and smooth muscle. Our data also suggest a higher level of expression of claustrin mRNA in astrocyte cultures, when compared to 3'-MRC. Our data therefore provide new evidence that alternatively spliced variants of MAP1B are expressed in brain, and that at least one of these variants encodes the claustrin proteoglycan.

Delayed development of nervous system in mice homozygous for disrupted microtubule-associated protein 1B (MAP1B) gene

Microtubule-associated protein 1B (MAP1B), one of the microtubule-associated proteins (MAPs), is a major component of the neuronal cytoskeleton. It is expressed at high levels in immature neurons during growth of their axons, which indicates that it plays a crucial role in neuronal morphogenesis and neurite extension. To better define the role of MAP1B in vivo, we have used gene targeting to disrupt the murine MAP1B gene. Heterozygotes of our MAP1B disruption exhibit no overt abnormalities in their development and behavior, while homozygotes showed a slightly decreased brain weight and delayed nervous system development. Our data indicate that while MAP1B is not essential for survival, it is essential for normal time course development of the murine nervous system. These conclusions are very different from those of a previous MAP1B gene-targeting study (Edelmann, W., M. Zervas, P. Costello, L. Roback, I. Fischer, A. Hammarback, N. Cowan, P. Davis, B. Wainer, and R. Kucherlapati. 1996. Proc. Natl. Acad. Sci. USA. 93: 1270-1275). In this previous effort, homozygotes died before reaching 8-d embryos, while heterozygotes showed severely abnormal phenotypes in their nervous systems. Because the gene targeting event in these mice produced a gene encoding a 571-amino acid truncated product of MAP1B, it seems likely that the phenotypes seen arise from the truncated MAP1B product acting in a dominant-negative fashion, rather than a loss of MAP1B function.

Differential expression of microtubule-associated protein 1B phosphorylated isoforms in the adult rat nervous system

Phosphorylated microtubule-associated protein 1B isoforms are thought to be involved in the plastic events taking place in neurons during development. However, little is known about their expression and possible role in the mature nervous system. To gain insight into the mechanisms underlying neuronal plasticity in the adult, we studied the pattern of expression of three microtubule-associated protein 1B isoforms in the entire adult rat nervous system. Accordingly, we performed western blots and immunohistochemistries using the antibodies 125, 150 and 531, which specifically recognize phosphorylated and unphosphorylated microtubule-associated protein 1B epitopes. Two electrophoretically distinct microtubule-associated protein 1B isoforms, slow-migrating and fast-migrating, were detected with the antibodies. The pattern of expression of these isoforms in the adult rat nervous system was region specific. Phosphorylated slow-migrating microtubule-associated protein 1B was expressed at all cellular compartments of primary sensory neurons in the central and peripheral nervous systems. In addition to primary sensory axons, slow-migrating microtubule-associated protein 1B was encountered at some other axons within the central nervous system. We discuss the correlation between slow-migrating microtubule-associated protein 1B axonal content and the regenerative potential of neurons. Phosphorylated fast-migrating microtubule-associated protein 1B was exclusively found in central nervous system dendrites where synaptic plasticity with morphological changes occurs in the adult. Unphosphorylated fast-migrating microtubule-associated protein 1B was the only isoform present in the bodies and dendrites of all motor neurons, and in peripheral and central nervous system glial cells of myelinated tracts with slow-migrating microtubule-associated protein 1B-containing axons. In summary, this report describes the pattern of expression of microtubule-associated protein 1B isoforms in the entire adult rat nervous system. In addition, it provides some information about the possible functional implications of phosphorylated microtubule-associated protein 1B isoforms in the adult.

Glutamate stimulates neurogranin phosphorylation in cultured rat hippocampal neurons

Neurogranin is a calmodulin-binding and a protein kinase C substrate, that is expressed in telencephalic regions of the rat brain and has been associated with signal transduction and long-term potentiation (LTP). We here report that neurogranin is present in cultured hippocampal neurones, although in amounts lower than those present in the adult hippocampus, and that is also phosphorylated 'in vivo'. Glutamate receptor activation rapidly and significantly increases neurogranin phosphorylation, which achieves maximal phosphate labeling after ionotropic receptor stimulation (kainate and N-methyl-D-aspartate) and more moderate one after metabotropic receptor activation. It is proposed that neurogranin phosphorylation responds to changes in intracellular free Ca2+ and, also, that an increase in neurogranin phosphorylation contributes to enhance and extend calmodulin action, and therefore participate in post-synaptic signal transduction and LTP.

Analysis of the expression, distribution and function of cyclin dependent kinase 5 (cdk5) in developing cerebellar macroneurons

Cultures of cerebellar macroneurons were used to study the expression, activity, subcellular localization, and function of cdk5 during neuronal morphogenesis. The results obtained indicate that in non-polarized neurons cdk5 is restricted to the cell body but as soon as polarity is established it becomes highly concentrated at the distal tip of growing axons where it associates with microtubules and the subcortical cytoskeleton. In addition, we show that laminin, an extracellular matrix molecule capable of stimulating axonal extension and promoting MAP1b phosphorylation (DiTella et al., 1996), accelerates the redistribution of cdk5 to the axonal tip and dramatically increases its activity. Finally, our results indicate that cdk5 suppression by antisense oligonucleotide treatment selectively reduces axonal elongation and decreases the phosphorylation status of MAP1b, as well as its binding to microtubules. Taken collectively, our observations suggest that cdk5 may serve as an important regulatory linker between environmental signals (e.g. laminin) and constituents of the intracellular machinery (e.g. MAP1b) involved in axonal formation.

MAP-1B/TAU functional redundancy during laminin-enhanced axonal growth

When cultured cerebellar macroneurons develop attached to a laminin-containing substrate or after the acute addition of laminin to the tissue culture medium, there is an acceleration in the rate and extent of axonal elongation. Furthermore, laminin is capable of inducing axonal formation and microtubule stabilization in neurons arrested at stage II of neuritic development by tau suppression (Caceres and Kosik, 1990; Caceres et al., 1991). Laminin-enhanced or induced axonal extension is paralleled by a selective and dramatic incorporation of phosphorylated MAP-1b into axonal microtubules. Axonal formation in neurons growing in the presence of laminin is prevented by treatment of the cultures with a mixture of MAP-1b and tau antisense oligonucleotides, but not by the single suppression of any one of these MAPs. However, suppression of MAP-1b, but not of tau, greatly reduces the increase in the rate and extent of axonal elongation induced by laminin. No such effects are elicited by MAP-1b antisense oligonucleotides in neurons growing in the absence of laminin, e.g. polylysine alone, where most of the MAP-1b present in the cells is dephosphorylated and not associated with the cytoskeleton. Taken collectively, these data suggest that, with regard to axonal elongation, MAP-1b and tau can be functionally substituted, and that extracellular matrix molecules, such as laminin, affect axonal extension by promoting the in vivo utilization of MAP-1b.