Differences in the regulation of microtubule dynamics by microtubule-associated proteins MAP1B and MAP2

Tubulin Complexity in Cancer and Metastasis

Tubulin plays a fundamental role in cellular function and as the subject for microtubule-active agents in the treatment of ovarian cancer. Microtubule-binding proteins (e.g., tau, MAP1/2/4, EB1, CLIP, TOG, survivin, stathmin) and posttranslational modifications (e.g., tyrosination, deglutamylation, acetylation, glycation, phosphorylation, polyamination) further diversify tubulin functionality and may permit additional opportunities to understand microtubule behavior in disease and to develop microtubule-modifying approaches to combat ovarian cancer. Tubulin-based structures that project from suspended ovarian cancer cells known as microtentacles may contribute to metastatic potential of ovarian cancer cells and could represent an exciting novel therapeutic target.

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

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

Cytoskeletal regulation guides neuronal trafficking to effectively supply the synapse

The development and proper function of the brain requires the formation of highly complex neuronal circuitry. These circuits are shaped from synaptic connections between neurons and must be maintained over a lifetime. The formation and continued maintenance of synapses requires accurate trafficking of presynaptic and postsynaptic components along the axon and dendrite, respectively, necessitating deliberate and specialized delivery strategies to replenish essential synaptic components. Maintenance of synaptic transmission also requires readily accessible energy stores, produced in part by localized mitochondria, that are tightly regulated with activity level. In this review, we focus on recent developments in our understanding of the cytoskeletal environment of axons and dendrites, examining how local regulation of cytoskeletal dynamics and organelle trafficking promotes synapse-specific delivery and plasticity. These new insights shed light on the complex and coordinated role that cytoskeletal elements play in establishing and maintaining neuronal circuitry.

Microtubule Dysfunction: A Common Feature of Neurodegenerative Diseases

Neurons are particularly susceptible to microtubule (MT) defects and deregulation of the MT cytoskeleton is considered to be a common insult during the pathogenesis of neurodegenerative disorders. Evidence that dysfunctions in the MT system have a direct role in neurodegeneration comes from findings that several forms of neurodegenerative diseases are associated with changes in genes encoding tubulins, the structural units of MTs, MT-associated proteins (MAPs), or additional factors such as MT modifying enzymes which modulating tubulin post-translational modifications (PTMs) regulate MT functions and dynamics. Efforts to use MT-targeting therapeutic agents for the treatment of neurodegenerative diseases are underway. Many of these agents have provided several benefits when tested on both in vitro and in vivo neurodegenerative model systems. Currently, the most frequently addressed therapeutic interventions include drugs that modulate MT stability or that target tubulin PTMs, such as tubulin acetylation. The purpose of this review is to provide an update on the relevance of MT dysfunctions to the process of neurodegeneration and briefly discuss advances in the use of MT-targeting drugs for the treatment of neurodegenerative disorders.

Microtubules: Evolving roles and critical cellular interactions

Microtubules are cytoskeletal elements known as drivers of directed cell migration, vesicle and organelle trafficking, and mitosis. In this review, we discuss new research in the lens that has shed light into further roles for stable microtubules in the process of development and morphogenesis. In the lens, as well as other systems, distinct roles for characteristically dynamic microtubules and stabilized populations are coming to light. Understanding the mechanisms of microtubule stabilization and the associated microtubule post-translational modifications is an evolving field of study. Appropriate cellular homeostasis relies on not only one cytoskeletal element, but also rather an interaction between cytoskeletal proteins as well as other cellular regulators. Microtubules are key integrators with actin and intermediate filaments, as well as cell–cell junctional proteins and other cellular regulators including myosin and RhoGTPases to maintain this balance.

Impact statement

The role of microtubules in cellular functioning is constantly expanding. In this review, we examine new and exciting fields of discovery for microtubule’s involvement in morphogenesis, highlight our evolving understanding of differential roles for stabilized versus dynamic subpopulations, and further understanding of microtubules as a cellular integrator.

ReMAPping the microtubule landscape: How phosphorylation dictates the activities of microtubule-associated proteins

Classical microtubule-associated proteins (MAPs) were originally identified based on their co-purification with microtubules assembled from mammalian brain lysate. They have since been found to perform a range of functions involved in regulating the dynamics of the microtubule cytoskeleton. Most of these MAPs play integral roles in microtubule organization during neuronal development, microtubule remodeling during neuronal activity, and microtubule stabilization during neuronal maintenance. As a result, mutations in MAPs contribute to neurodevelopmental disorders, psychiatric conditions, and neurodegenerative diseases. MAPs are post-translationally regulated by phosphorylation depending on developmental time point and cellular context. Phosphorylation can affect the microtubule affinity, cellular localization, or overall function of a particular MAP and can thus have profound implications for neuronal health. Here we review MAP1, MAP2, MAP4, MAP6, MAP7, MAP9, tau, and DCX, and how each is regulated by phosphorylation in neuronal physiology and disease. Developmental Dynamics 247:138-155, 2018. © 2017 Wiley Periodicals, Inc.

Neuronal polarization: From spatiotemporal signaling to cytoskeletal dynamics

Neuronal polarization establishes distinct molecular structures to generate a single axon and multiple dendrites. Studies over the past years indicate that this efficient separation is brought about by a network of feedback loops. Axonal growth seems to play a major role in fueling those feedback loops and thereby stabilizing neuronal polarity. Indeed, various effectors involved in feedback loops are pivotal for axonal growth by ultimately acting on the actin and microtubule cytoskeleton. These effectors have key roles in interconnecting actin and microtubule dynamics - a mechanism crucial to commanding the growth of axons. We propose a model connecting signaling with cytoskeletal dynamics and neurite growth to better describe the underlying processes involved in neuronal polarization. We will discuss the current views on feedback loops and highlight the current limits of our understanding.

Microtubule-associated protein 1b is required for shaping the neural tube

BACKGROUND

Shaping of the neural tube, the precursor of the brain and spinal cord, involves narrowing and elongation of the neural tissue, concomitantly with other morphogenetic changes that contribue to this process. In zebrafish, medial displacement of neural cells (neural convergence or NC), which drives the infolding and narrowing of the neural ectoderm, is mediated by polarized migration and cell elongation towards the dorsal midline. Failure to undergo proper NC results in severe neural tube defects, yet the molecular underpinnings of this process remain poorly understood.

RESULTS

We investigated here the role of the microtubule (MT) cytoskeleton in mediating NC in zebrafish embryos using the MT destabilizing and hyperstabilizing drugs nocodazole and paclitaxel respectively. We found that MTs undergo major changes in organization and stability during neurulation and are required for the timely completion of NC by promoting cell elongation and polarity. We next examined the role of Microtubule-associated protein 1B (Map1b), previously shown to promote MT dynamicity in axons. map1b is expressed earlier than previously reported, in the developing neural tube and underlying mesoderm. Loss of Map1b function using morpholinos (MOs) or δMap1b (encoding a truncated Map1b protein product) resulted in delayed NC and duplication of the neural tube, a defect associated with impaired NC. We observed a loss of stable MTs in these embryos that is likely to contribute to the NC defect. Lastly, we found that Map1b mediates cell elongation in a cell autonomous manner and polarized protrusive activity, two cell behaviors that underlie NC and are MT-dependent.

CONCLUSIONS

Together, these data highlight the importance of MTs in the early morphogenetic movements that shape the neural tube and reveal a novel role for the MT regulator Map1b in mediating cell elongation and polarized cell movement in neural progenitor cells.

Coordinated interaction of Down syndrome cell adhesion molecule and deleted in colorectal cancer with dynamic TUBB3 mediates Netrin-1-induced axon branching

Modulation of actin and microtubule (MT) dynamics in neurons is implicated in guidance cue-dependent axon outgrowth, branching and pathfinding. Although the role of MTs in axon guidance has been well known, how extracellular guidance signals engage MT behavior in axon branching remains unclear. Previously, we have shown that TUBB3, the most dynamic β-tubulin isoform in neurons, directly binds to deleted in colorectal cancer (DCC) to regulate MT dynamics in Netrin-1-mediated axon guidance. Here, we report that TUBB3 directly interacted with another Netrin-1 receptor Down syndrome cell adhesion molecule (DSCAM) and Netrin-1 increased this interaction in primary neurons. MT dynamics were required for Netrin-1-promoted association of DSCAM with TUBB3. Knockdown of either DSCAM or DCC or addition of a function blocking anti-DCC antibody mutually blocked Netrin-1-induced interactions, suggesting that DSCAM interdependently coordinated with DCC in Netrin-1-induced binding to TUBB3. Both DSCAM and DCC were partially colocalized with TUBB3 in the axon branch and the axon branching point of primary neurons and Netrin-1 increased these colocalizations. Netrin-1 induced the interaction of endogenous DSCAM with polymerized TUBB3 in primary neurons and Src family kinases (SFKs) were required for regulating this binding. Knockdown of DSCAM only, DCC only or both was sufficient to block Netrin-1-induced axon branching of E15 mouse cortical neurons. Knocking down TUBB3 inhibited Netrin-1 induced axon branching as well. These results suggest that DSCAM collaborates with DCC to regulate MT dynamics via direct binding to dynamic TUBB3 in Netrin-1-induced axon branching.

Cytoskeletal and signaling mechanisms of neurite formation

The formation of a neurite, the basis for axons and dendrites, begins with the concerted accumulation and organization of actin and microtubules. Whereas much is known about the proteins that play a role in these processes, because they perform similar functions in axon branching and filopodia formation, much remains to be discovered concerning the interaction of these individual cytoskeletal regulators during neurite formation. Here, we review the literature regarding various models of filopodial formation and the way in which proteins that control actin organization and polymerization induce neurite formation. Although several different regulators of actin polymerization are involved in neurite initiation, redundancy occurs between these regulators, as the effects of the loss of a single regulator can be mitigated by the addition of neurite-promoting substrates and proteins. Similar to actin dynamics, both microtubule stabilizing and destabilizing proteins play a role in neurite initiation. Furthermore, interactions between the actin and microtubule cytoskeleton are required for neurite formation. Several lines of evidence indicate that the interactions between these two components of the cytoskeleton are needed for force generation and for the localization of microtubules at sites of nascent neurites. The general theme that emerges is the existence of several central regulatory pathways on which extracellular cues converge to control and organize both actin and microtubules to induce the formation of neurites.

Microtubule dynamics in axon guidance

Precise modulation of the cytoskeleton is involved in a variety of cellular processes including cell division, migration, polarity, and adhesion. In developing post-mitotic neurons, extracellular guidance cues not only trigger signaling cascades that act at a distance to indirectly regulate microtubule distribution, and assembly and disassembly in the growth cone, but also directly modulate microtubule stability and dynamics through coupling of guidance receptors with microtubules to control growth-cone turning. Microtubule-associated proteins including classical microtubule-associated proteins and microtubule plus-end tracking proteins are required for modulating microtubule dynamics to influence growth-cone steering. Multiple key signaling components, such as calcium, small GTPases, glycogen synthase kinase-3β, and c-Jun N-terminal kinase, link upstream signal cascades to microtubule stability and dynamics in the growth cone to control axon outgrowth and projection. Understanding the functions and regulation of microtubule dynamics in the growth cone provides new insights into the molecular mechanisms of axon guidance.

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.

Regulation of EB1/3 proteins by classical MAPs in neurons

Microtubules (MTs) are key cytoskeletal elements in developing and mature neurons. MT reorganization underlies the morphological changes that occur during neuronal development. Furthermore, MTs contribute to the maintenance of neuronal architecture, enable intracellular transport and act as scaffolds for signaling molecules. Thus, a fine-tuned regulation of MT dynamics and stability is crucial for the correct differentiation and functioning of neurons. Different types of proteins contribute to the regulation of the MT state, such as plus-end tracking proteins (+TIPs), which interact with the plus-ends of growing microtubules, and classical microtubule-associated proteins (MAPs), which bind along the microtubule lattice. Recent evidence indicates that MAPs interplay with End Binding Proteins (EBs), the core +TIPs, in neuronal cells. This might contribute to the orchestrated regulation of MT dynamics in neurons. In this mini-review article, we address recent research on the neuronal crosstalk between EBs and classical MAPs and speculate on its possible functional relevance.

MAP1B regulates microtubule dynamics by sequestering EB1/3 in the cytosol of developing neuronal cells

MAP1B, a structural microtubule (MT)-associated protein highly expressed in developing neurons, plays a key role in neurite and axon extension. However, not all molecular mechanisms by which MAP1B controls MT dynamics during these processes have been revealed. Here, we show that MAP1B interacts directly with EB1 and EB3 (EBs), two core 'microtubule plus-end tracking proteins' (+TIPs), and sequesters them in the cytosol of developing neuronal cells. MAP1B overexpression reduces EBs binding to plus-ends, whereas MAP1B downregulation increases binding of EBs to MTs. These alterations in EBs behaviour lead to changes in MT dynamics, in particular overstabilization and looping, in growth cones of MAP1B-deficient neurons. This contributes to growth cone remodelling and a delay in axon outgrowth. Together, our findings define a new and crucial role of MAP1B as a direct regulator of EBs function and MT dynamics during neurite and axon extension. Our data provide a new layer of MT regulation: a classical MAP, which binds to the MT lattice and not to the end, controls effective concentration of core +TIPs thereby regulating MTs at their plus-ends.

Wnt transmembrane signaling and long-term spatial memory

Transmembrane signaling mechanisms are critical for regulating the plasticity of neuronal connections underlying the establishment of long-lasting memory (e.g., Linden and Routtenberg (1989) Brain Res Rev 14:279-296; Sossin (1996) Trends Neurosci 19:215-218; Mayr and Montminy (2001) Nat Rev Mol Cell Biol 2:599-609; Chen et al. (2011) Nature 469:491-497). One signaling mechanism that has received surprisingly little attention in this regard is the well-known Wnt transmembrane signaling pathway even though this pathway in the adult plays a significant role, for example, in postsynaptic dendritic spine morphogenesis and presynaptic terminal neurotransmitter release (Inestrosa and Arenas (2010) Nat Rev Neurosci 11:77-86). The present report now provides the first evidence of Wnt signaling in spatial information storage processes. Importantly, this Wnt participation is specific and selective. Thus, spatial, but not cued, learning in a water maze selectively elevates the levels in hippocampus of Wnt 7 and Wnt 5a, but not the Wnt 3 isoform, indicating behavioral selectivity and isoform specificity. Wnt 7 elevation is subfield-specific: granule cells show an increase with no detectable change in CA3 neurons. Wnt 7 elevation is temporally specific: increased Wnt signaling is not observed during training, but is seen 7 days and, unexpectedly, 30 days later. If the Wnt elevation after learning is activity-dependent, then it may be possible to model this effect in primary hippocampal neurons in culture. Here, we evaluate the consequence of potassium or glutamate depolarization on Wnt signaling. This represents, to our knowledge, the first demonstration of an activation-dependent elevation of Wnt levels and surprisingly an increased number of Wnt-stained puncta in neurites suggestive of trafficking from the cell body to neuronal processes, probably dendrites. It is proposed that Wnt signaling pathways regulate long-term information storage in a behavioral-, cellular-, and isoform-specific manner.

Leucine-rich repeat kinase 2 (LRRK2) cellular biology: a review of recent advances in identifying physiological substrates and cellular functions

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common forms of inheritable Parkinson's disease and likely play a role in sporadic disease as well. LRRK2 is a large multidomain protein containing two key groups, a Ras-like GTP binding domain and a serine, threonine kinase domain. Mutations in the LRRK2 gene that associate with Parkinson's disease reside primarily within the two functional domains of the protein, suggesting that LRRK2 function is critical to the pathogenesis of the disease. The most common LRRK2 mutation increases kinase activity, making LRRK2 kinase inhibition an attractive target for small molecule drug development. However, the physiological function of LRRK2 kinase as well as its endogenous protein substrates remains poorly understood and has hindered drug development efforts. Recent advances in LRRK2 biology have revealed several potential cellular roles, interacting proteins, and putative physiological substrates. Together, a picture emerges of a complex multifunctional protein that exists in multiple cellular compartments. Through unclear mechanisms, LRRK2 kinase regulates cytoskeleton architecture through control of protein translation, phosphorylation of cytoskeletal proteins, and response to cellular stressors. This article will briefly cover some interesting recent studies in LRRK2 cellular biology and highlight emerging cellular models of LRRK2 kinase function.

Mechanics of microtubules

Microtubules are rigid cytoskeletal filaments, and their mechanics affect cell morphology and cellular processes. For instance, microtubules for the support structures for extended morphologies, such as axons and cilia. Further, microtubules act as tension rods to pull apart chromosomes during cellular division. Unlike other cytoskeletal filaments (e.g., actin) that work as large networks, microtubules work individually or in small groups, so their individual mechanical properties are quite important to their cellular function. In this review, we explore the past work on the mechanics of individual microtubules, which have been studied for over a quarter of a century. We also present some prospective on future endeavors to determine the molecular mechanisms that control microtubule rigidity.

Agrin induced morphological and structural changes in growth cones of cultured hippocampal neurons

The role of agrin in synaptogenesis has been extensively studied. On the other hand, little is known about the function of this extracellular matrix protein during developmental processes that precede the formation of synapses. Recently, agrin was shown to regulate the rate of neurite elongation and the behavior of growth cones in hippocampal and spinal neurons, respectively. However, the molecular mechanisms underlying these effects have not been completely elucidated. In the present study, we analyzed the morphological and molecular changes induced by agrin in growth cones of hippocampal neurons that developed in culture. Morphometric analysis showed a significant enlargement of growth cones of hippocampal neurons cultured in the presence of agrin. These agrin-induced growth cone changes were accompanied by the formation of loops of microtubules highly enriched in acetylated tubulin and an increase in the content of the microtubule-associated protein (MAP)1B. Together, these data provide further insights into the potential molecular mechanisms underlying the effects of agrin on neurite outgrowth in rat central neurons.

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.

The MAP1 family of microtubule-associated proteins

MAP1-family proteins are classical microtubule-associated proteins (MAPs) that bind along the microtubule lattice and stabilize microtubules.

MAP1-family proteins are classical microtubule-associated proteins (MAPs) that bind along the microtubule lattice. The founding members, MAP1A and MAP1B, are predominantly expressed in neurons, where they are thought to be important in the formation and development of axons and dendrites. Mammalian genomes usually contain three family members, MAP1A, MAP1B and a shorter, more recently identified gene called MAP1S. By contrast, only one family member, Futsch, is found in Drosophila. After their initial expression, the MAP1A and MAP1B polypeptides are cleaved into light and heavy chains, which are then assembled into mature complexes together with the separately encoded light chain 3 subunit (LC3). Both MAP1A and MAP1B are well known for their microtubule-stabilizing activity, but MAP1 proteins can also interact with other cellular components, including filamentous actin and signaling proteins. Furthermore, the activity of MAP1A and MAP1B is controlled by upstream signaling mechanisms, including the MAP kinase and glycogen synthase kinase-3 β pathways.

Tobacco mosaic virus movement protein functions as a structural microtubule-associated protein

The cell-to-cell spread of Tobacco mosaic virus infection depends on virus-encoded movement protein (MP), which is believed to form a ribonucleoprotein complex with viral RNA (vRNA) and to participate in the intercellular spread of infectious particles through plasmodesmata. Previous studies in our laboratory have provided evidence that the vRNA movement process is correlated with the ability of the MP to interact with microtubules, although the exact role of this interaction during infection is not known. Here, we have used a variety of in vivo and in vitro assays to determine that the MP functions as a genuine microtubule-associated protein that binds microtubules directly and modulates microtubule stability. We demonstrate that, unlike MP in whole-cell extract, microtubule-associated MP is not ubiquitinated, which strongly argues against the hypothesis that microtubules target the MP for degradation. In addition, we found that MP interferes with kinesin motor activity in vitro, suggesting that microtubule-associated MP may interfere with kinesin-driven transport processes during infection.

Cells positive for microtubule-associated protein 1B (MAP 1B) are present along rat and human efferent ductules and epididymis

Microtubule-associated protein 1B (MAP 1B) is a neuronal cytoskeleton marker with predominant expression in the developing nervous system. The present study provides evidence for the expression of this cytoskeleton protein in non-neuronal and neuronal cells along rat and human efferent ductules and epididymis (initial segment, caput, and cauda). Reverse transcription/polymerase chain reaction and Western blot analysis were used to confirm the presence of MAP 1B (mRNA and protein) in rat tissues. Immunohistochemical studies revealed MAP-1B-positive staining in columnar ciliated cells present in efferent ductules and in narrow cells located in the initial segment, in both rat and human. MAP-1B-positive basal cells, located underneath the columnar cells, were only identified in the initial segment and caput epididymidis of the rat. Qualitative analysis of tissues from 40-day-old and 120-day-old rats indicated that the number of MAP-1B-positive ciliated, narrow, and basal cells per tubule increased with sexual maturation. These immunoreactive cells did not stain for dopamine beta-hydroxylase or acetylcholinesterase, indicating that they were not adrenergic or cholinergic in nature. Immunohistochemical studies also revealed the presence of MAP-1B-positive staining in interstitial nerve fibers in caput and cauda epididymidis from both rat and human. Thus, the expression of MAP 1B is not confined to a specific cell type in rat and human efferent ductules and epididymis. The functional significance of this cytoskeleton protein in tissues from the male reproductive tract requires further investigation.

The control of microtubule stability in vitro and in transfected cells by MAP1B and SCG10

In neurons, the regulation of microtubules plays an important role for neurite outgrowth, axonal elongation, and growth cone steering. SCG10 family proteins are the only known neuronal proteins that have a strong destabilizing effect, are highly enriched in growth cones and are thought to play an important role during axonal elongation. MAP1B, a microtubule-stabilizing protein, is found in growth cones as well, therefore it was important to test their effect on microtubules in the presence of both proteins. We used recombinant proteins in microtubule assembly assays and in transfected COS-7 cells to analyze their combined effects in vitro and in living cells, respectively. Individually, both proteins showed their expected activities in microtubule stabilization and destruction respectively. In MAP1B/SCG10 double-transfected cells, MAP1B could not protect microtubules from SCG10-induced disassembly in most cells, in particular not in cells that contained high levels of SCG10. This suggests that SCG10 is more potent to destabilize microtubules than MAP1B to rescue them. In microtubule assembly assays, MAP1B promoted microtubule formation at a ratio of 1 MAP1B per 70 tubulin dimers while a ratio of 1 SCG10 per two tubulin dimers was needed to destroy microtubules. In addition to its known binding to tubulin dimers, SCG10 binds also to purified microtubules in growth cones of dorsal root ganglion neurons in culture. In conclusion, neuronal microtubules are regulated by antagonistic effects of MAP1B and SCG10 and a fine tuning of the balance of these proteins may be critical for the regulation of microtubule dynamics in growth cones.

Acute inactivation of MAP1b in growing sympathetic neurons destabilizes axonal microtubules

Microtubule-associated-protein 1b (MAP1b) is abundant in neurons actively extending axons. MAP1b is present on microtubules throughout growing axons, but is preferentially concentrated on microtubule polymer in the distal axon and growth cone. Although MAP1b has been implicated in axon growth and pathfinding, its specific functions are not well understood. Biochemical and transfection studies suggest that MAP1b has microtubule-stabilizing activity, but recent studies with neurons genetically deficient in MAP1b have not confirmed this. We have explored MAP1b functions in growing sympathetic neurons using an acute inactivation approach. Neurons without axons were injected with polyclonal MAP1b antibodies and then stimulated to extend axons. Injected cells were compared to controls in terms of axon growth behavior and several properties of axonal microtubules. The injected antibodies rapidly and quantitatively sequestered MAP1b in the cell body, making it unavailable to perform its normal functions. This immunodepletion of MAP1b had no statistically significant effect on axon growth, the amount of microtubule polymer in the axon, and the relative tyrosinated tubulin content of this polymer, and this was true in sympathetic neurons from rat, wild type mice, and tau knockout mice. Thus, robust axon growth can occur in the absence of MAP1b alone or both MAP1b and tau. However, immunodepletion of MAP1b significantly increased the sensitivity of microtubules in the distal axon and growth cone to nocodazole-induced depolymerization. These results indicate that MAP1b has microtubule-stabilizing activity in growing axons. This stabilizing activity may be required for some axonal functions, but it is not necessary for axon growth.

Role of cyclin-dependent kinase 5 and its activator P35 in local axon and growth cone stabilization

Axons elongate and perform steering reactions with their growth cones constantly undergoing local collapse and stabilization. Our previous studies have shown that a type-1 phosphorylated form of microtubule-associated protein 1B, recognized by monoclonal antibody 1E11 (mab1E11), is present in stable regions and absent from unstable regions of turning growth cones of retinal ganglion cells. In contrast, the total population of microtubule-associated protein 1B is present in the entire growth cone. Here we demonstrate that inhibition of cyclin-dependent kinase 5 (Cdk5) results in loss of mab1E11 binding whereas inhibition of glycogen synthase kinase 3 has no such effect, revealing that mab1E11 recognizes a Cdk5 phosphorylation site on type-1 phosphorylated form of microtubule-associated protein 1B. We moreover show that kinase Cdk5 as well as its activator P35 is present in retinal ganglion cells in the early developing chick embryo retina and enriched in their extending axons. Cdk5 and P35 are concentrated in the youngest, distal axon region and the growth cone as also seen for Cdk5-phosphorylated type-1 phosphorylated form of microtubule-associated protein 1B. Inhibition of Cdk5 by antibodies or inhibitor Roscovitine results in growth cone collapse and axon retraction and prevents substantial axon outgrowth. In contrast, glycogen synthase kinase 3 inhibition causes only a transient axon retraction which is soon recovered and allows for axon formation. In growth cones induced to turn at substrate borders, where stable and instable parts of the growth cone are clearly defined, Cdk5 is present in the entire growth cone. P35, in contrast, is restricted to the stable parts of the growth cone, which do not collapse but instead transform into new distal axon. The local presence of Cdk5-phosphorylated type-1 phosphorylated form of microtubule-associated protein 1B in stabilized growth cone areas can be therefore attributed to the local activation of Cdk5 by P35 in these regions. Together our data demonstrate a crucial role of Cdk5 and its activator P35 in elongation and maintenance of axons as well as for stability and steering of their growth cones.

Microtubule-associated protein 1B function during normal development, regeneration, and pathological conditions in the nervous system

Microtubule-associated protein 1B is the first MAP to be expressed during the development of the nervous system. Several different approaches have revealed that MAP1B function is associated with microtubule and actin microfilament polymerization and dynamics. In recent years, the generation of molecular models to inactivate MAP1B function in invertebrates and mammals has sparked some controversy about the real role of MAP1B. Despite discrepancies between some studies, it is clear that MAP1B plays a principal role in the development of the nervous system. In this article, we summarize the evidence for MAP1B function in a wide variety of cellular processes implicated in the proper construction of the nervous system. We also discuss the role of MAP1B in pathological processes.

Hmob3 brain-specific sequence is a part of phylogenetically conserved human MAP1B gene 3'-untranslated region

Using in vitro and in silico approaches, we demonstrate that the Hmob3 brain-specific sequence is a part of the extended human MAP1B gene 3'UTR encoded by the portion of the exon 7. Previously we had isolated the 1.4 kb Hmob3 clone (Acc. No Y09836) from the human medulla oblongata cDNA library. In the present paper, we report the RT-PCR detection of extended transcripts containing the Hmob3 sequence as well as a fragment of MAP1B mRNA in the human brain mRNA samples. The existence of such transcripts confirms that Hmob3 is transcribed from the MAP1B gene. Thus we report a previously unknown region of the human MAP1B gene which encodes the MAP1B mRNA 3'UTR. The length of the 3'UTR was estimated of 4330 nucleotides. Comparative sequence analysis shows high phylogenetic conservation of MAP1B 3'UTR in mammals. Using Hmob3 as a probe in the Northern blot analysis, we have revealed two different types of transcripts for brain and kidney mRNA samples; in the skeletal muscle sample we detected both transcripts.

Microtubule-associated protein 1B is involved in the initial stages of axonogenesis in peripheral nervous system cultured neurons

Neuronal process extension is dependent on the reorganisation of the cytoskeleton, in particular microtubules and microfilaments, and one of the ways in which microtubules are regulated is by a group of microtubule-associated proteins called MAPs. MAP1B, the first MAP to be expressed in developing neurons, has been shown to play an important role during axonogenesis. Previously, we have shown that a phosphorylated isoform of MAP1B is involved in maintaining growth cone microtubules in a dynamically unstable state. In order to further investigate the role of MAP1B during axonogenesis we have cultured dorsal root ganglion (DRG) neurons from a MAP1B deficient mutant mouse. These mice express only trace amounts of MAP1B, have defects in the development of their nervous system and die perinatally. Cultured DRG neurons from MAP1B deficient mice show a reduction in axon elongation and an increase in growth cone area. The reduction in axon elongation is most likely to occur due to an inhibition in the early stages of axonogenesis. Using time-lapse video we have verified that during the first 2 h after plating, MAP1B deficient neurones extend their axons with an average speed that is half the speed of control neurones. These results support the participation of MAP1B during the initial stages of axonogenesis.

Regulation of microtubule-associated proteins

Microtubule-associated proteins (MAPs) function to regulate the assembly dynamics and organization of microtubule polymers. Upstream regulation of MAP activities is the major mechanism used by cells to modify and control microtubule assembly and organization. This review summarizes the functional activities of MAPs found in animal cells and discusses how these MAPs are regulated. Mechanisms controlling gene expression, isoform-specific expression, protein localization, phosphorylation, and degradation are discussed. Additional regulatory mechanisms include synergy or competition between MAPs and the activities of cofactors or binding partners. For each MAP it is likely that regulation in vivo reflects a composite of multiple regulatory mechanisms.

Microtubule-associated protein 1A (MAP1A) and MAP1B: light chains determine distinct functional properties

The microtubule-associated proteins 1A (MAP1A) and 1B (MAP1B) are distantly related protein complexes consisting of heavy and light chains and are thought to play a role in regulating the neuronal cytoskeleton, MAP1B during neuritogenesis and MAP1A in mature neurons. To elucidate functional differences between MAP1B and MAP1A and to determine the role of the light chain in the MAP1A protein complex, we chose to investigate the functional properties of the light chain of MAP1A (LC2) and compare them with the light chain of MAP1B (LC1). We found that LC2 binds to microtubules in vivo and in vitro and induces rapid polymerization of tubulin. A microtubule-binding domain in its NH(2) terminus was found to be necessary and sufficient for these activities. The analysis of LC1 revealed that it too bound to microtubules and induced tubulin polymerization via a crucial but structurally unrelated NH(2)-terminal domain. The two light chains differed, however, in their effects on microtubule bundling and stability in vivo. Furthermore, we identified actin filament binding domains located at the COOH terminus of LC2 and LC1 and obtained evidence that binding to actin filaments is attributable to direct interaction with actin. Our findings establish LC2 as a crucial determinant of MAP1A function, reveal LC2 as a potential linker of neuronal microtubules and microfilaments, and suggest that the postnatal substitution of MAP1B by MAP1A leads to expression of a protein with an overlapping but distinct set of functions.

Functional differences of tau isoforms containing 3 or 4 C-terminal repeat regions and the influence of oxidative stress

We report functional differences between tau isoforms with 3 or 4 C-terminal repeats and a difference in susceptibility to oxidative conditions, with respect to the regulation of microtubule dynamics in vitro and tau-microtubule binding in cultured cells. In the presence of dithiothreitol in vitro, a 3-repeat tau isoform promotes microtubule nucleation, reduces the tubulin critical concentration for microtubule assembly, and suppresses dynamic instability. Under non-reducing conditions, threshold concentrations of 3-repeat tau and tubulin exist below which this isoform still promotes microtubule nucleation and assembly but fails to reduce the tubulin critical concentration or suppress dynamic instability; above these threshold concentrations, amorphous aggregates of 3-repeat tau and tubulin can be produced at the expense of microtubule formation. A 4-repeat tau isoform is less sensitive to the oxidative potential of the environment, behaving under oxidative conditions similarly to the 3-repeat isoform under reducing conditions. Under conditions of oxidative stress, in Chinese hamster ovary cells stably expressing either 3- or 4-repeat tau, 3-repeat tau disassociates from microtubules more readily than the 4-repeat isoform, and tau-containing high molecular weight aggregates are preferentially observed in lysates from the Chinese hamster ovary cells expressing 3-repeat tau, indicating greater susceptibility of 3-repeat tau to oxidative conditions, compared with 4-repeat tau in vivo.

Evidence for the role of MAP1B in axon formation

Cultured neurons obtained from a hypomorphous MAP1B mutant mouse line display a selective and significant inhibition of axon formation that reflects a delay in axon outgrowth and a reduced rate of elongation. This phenomenon is paralleled by decreased microtubule formation and dynamics, which is dramatic at the distal axonal segment, as well as in growth cones, where the more recently assembled microtubule polymer normally predominates. These neurons also have aberrant growth cone formation and increased actin-based protrusive activity. Taken together, this study provides direct evidence showing that by promoting microtubule dynamics and regulating cytoskeletal organization MAP1B has a crucial role in axon formation.

Multiple isoforms of the high molecular weight microtubule associated protein XMAP215 are expressed during development in Xenopus

We have cloned and sequenced cDNAs encoding two isoforms of XMAP215, a high molecular weight microtubule-associated protein identified in Xenopus eggs. XMAP215 is approximately 80% identical in amino acid sequence to the product of ch-TOG, a cDNA that is over expressed in certain human tumors [Charrasse et al., 1995: Eur J Biochem 234:406-413]. Northern and Western blots demonstrated that XMAP215 is expressed throughout development, from oogenesis to tadpole. We identified two XMAP215 transcripts differing only in the presence of a 108-bp sequence encoding a 36 amino acid insert. RT-PCR revealed that the transcripts encoding these two isoforms are expressed at distinct times during development: a transcript containing the insert (encoding XMAP215(M)) is expressed during oogenesis and is present through gastrulation. The second transcript (encoding XMAP215(Z)) lacks the 108-bp insert and is expressed from gastrulation onward. In situ hybridization demonstrated that XMAP215 transcripts are localized to the ectoderm of early embryos and in the developing nervous system during later development. These results suggest that XMAP215 plays important roles in at least two phases of development: (1) regulating the assembly of MTs during the rapid cell divisions after fertilization, and (2) regulating MT assembly during the development of the nervous system.

Perinatal lethality of microtubule-associated protein 1B-deficient mice expressing alternative isoforms of the protein at low levels

Microtubule-associated protein 1B (MAP1B) has been implicated in axogenesis in cultured cells. To gain insight into the functions that MAP1B plays in vivo, we analyzed a strain of Map1B mutant mice generated by a gene trapping approach. Homozygous mice die on the first day after birth, probably due to a severe abnormal development of the nervous system. They present alterations in the structure of several brain regions. The normal Map1B gene yields different protein isoforms from alternatively spliced transcripts. The smaller isoforms were present in wild type, hetero-, and homozygous mice, but their expression was higher in the mutants than in the wild-type. Moreover, trace amounts of MAP1B protein were also observed in Map1B homozygous mutants, indicating an alternative splicing around the gene trap insertion. Thus, the Map1B gene trapped mutation reported in this work did not generated a null mutant, but a mouse with a drastic deficiency in MAP1B expression. Analyses of these mice indicate the presence of several neural defects and suggest the participation of MAP1B in neuronal migration.

The microtubule-associated protein MAP1B is involved in local stabilization of turning growth cones

For the development of the nervous system it is crucial that growth cones detect environmental information and react by altering their growth direction. The latter process is thought to depend on local stabilization of growth cone microtubules. We have obtained evidence of a role for the microtubule-associated protein MAP1B, in particular a mode 1 phosphoisoform of the molecule, P1-MAP1B, in this process. P1-MAP1B is tightly associated with the cytoskeleton and is present at highest concentrations in the distal axon and the growth cone of chick retinal ganglion cells. In growth cones turning at nonpermissive substrate borders, P1-MAP1B is restricted to regions which are stabilized. Unilateral neutralization of P1-MAP1B in one-half the growth cone by microscale chromophore-assisted laser inactivation changes growth cone motility, morphology, and growth direction. The results indicate a functional role for P1-MAP1B in local growth cone stabilization and thus growth cone steering.

Glycogen synthase kinase 3beta phosphorylation of microtubule-associated protein 1B regulates the stability of microtubules in growth cones

We have recently shown that glycogen synthase kinase 3beta (GSK3beta) phosphorylates the microtubule-associated protein (MAP) 1B in an in vitro kinase assay and in cultured cerebellar granule cells. Mapping studies identified a region of MAP1B high in serine-proline motifs that is phosphorylated by GSK3beta. Here we show that COS cells, transiently transfected with both MAP1B and GSK3beta, express high levels of the phosphorylated isoform of MAP1B (MAP1B-P) generated by GSK3beta. To investigate effects of MAP1B-P on microtubule dynamics, double transfected cells were labelled with antibodies to tyrosinated and detyrosinated tubulin markers for stable and unstable microtubules. This showed that high levels of MAP1B-P expression are associated with the loss of a population of detyrosinated microtubules in these cells. Transfection with MAP1B protected microtubules in COS cells against nocodazole depolymerisation, confirming previous studies. However, this protective effect was greatly reduced in cells containing high levels of MAP1B-P following transfection with both MAP1B and GSK3beta. Since we also found that MAP1B binds to tyrosinated, but not to detyrosinated, microtubules in transfected cells, we propose that MAP1B-P prevents tubulin detyrosination and subsequent conversion of unstable to stable microtubules and that this involves binding of MAP1B-P to unstable microtubules. The highest levels of MAP1B-P are found in neuronal growth cones and therefore our findings suggest that a primary role of MAP1B-P in growing axons may be to maintain growth cone microtubules in a dynamically unstable state, a known requirement of growth cone microtubules during pathfinding. To test this prediction, we reduced the levels of MAP1B-P in neuronal growth cones of dorsal root ganglion cells in culture by inhibiting GSK3beta with lithium. In confirmation of the proposed role of MAP1B-P in maintaining microtubule dynamics we found that lithium treatment dramatically increased the numbers of stable (detyrosinated) microtubules in the growth cones of these neurons.

Phosphorylation by CDK1 regulates XMAP215 function in vitro

XMAP215, a microtubule-associated protein isolated from Xenopus eggs, promotes microtubule assembly dynamics in an end-specific manner: addition of XMAP215 to purified porcine tubulin increases both elongation and shortening rates at microtubule plus ends, with minimal effects at minus ends. Previous results indicated that XMAP215 is phosphorylated during M phase, suggesting that its activity may be regulated by cell cycle phosphorylation. To test this hypothesis, we used video-enhanced DIC microscopy to examine the effects of XMAP215 phosphorylated by CDK1 on the assembly of purified tubulin. XMAP215 incubated with ATP at 30 degrees C for 10- 20 min in the absence of CDK1 exhibited a 4.1-fold increase in plus end elongation rate compared to purified tubulin. Elongation was promoted to a lesser degree (2.4-fold) by phosphorylated XMAP215. In contrast, XMAP215 phosphorylation did not alter the approximately 3-fold increase in shortening rate. XMAP215 binding to taxol microtubules was also not changed by phosphorylation. To further investigate mechanisms responsible for the faster microtubule shortening rate observed with XMAP215, we made microtubules with segments assembled prior to XMAP215 addition (proximal segments) and segments assembled in the presence of XMAP215 (distal segments). In 9 of 10 microtubules, the distal segment shortened faster (distal = 60.7 microm/min; proximal = 37.5 microm/min), suggesting that MTs assembled in the presence of XMAP215 have an altered lattice that results in subsequent faster shortening.

The growth-related, translationally controlled protein P23 has properties of a tubulin binding protein and associates transiently with microtubules during the cell cycle

The translationally controlled protein P23 was discovered by the early induction of its rate of synthesis after mitogenic stimulation of mouse fibroblasts. P23 is expressed in almost all mammalian tissues and it is highly conserved between animals, plants and yeast. Based on its amino acid sequence, P23 cannot be attributed to any known protein family, and its cellular function remains to be elucidated. Here, we present evidence that P23 has properties of a tubulin binding protein that associates with microtubules in a cell cycle-dependent manner. (1) P23 is a cytoplasmic protein that occurs in complexes of 100–150 kDa, and part of P23 can be immunoprecipitated from HeLa cell extracts with anti-tubulin antibodies. (2) In immunolocalisation experiments we find P23 associated with microtubules during G1, S, G2 and early M phase of the cell cycle. At metaphase, P23 is also bound to the mitotic spindle, and it is detached from the spindle during metaphase-anaphase transition. (3) A GST-P23 fusion protein interacts with alpha- and beta-tubulin, and recombinant P23 binds to taxol-stabilised microtubules in vitro. The tubulin binding domain of P23 was identified by mutational analysis; it shows similarity to part of the tubulin binding domain of the microtubule-associated protein MAP-1B. (4) Overexpression of P23 results in cell growth retardation and in alterations of cell morphology. Moreover, elevation of P23 levels leads to microtubule rearrangements and to an increase in microtubule mass and stability.

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.

MAP2 expression in developing dendrites of human brainstem auditory neurons

Immunostaining of cytoskeletal elements has proved to be a useful technique for tracing ontogenetic development in the human central auditory system. In the present study, dendritic development in brainstem auditory nuclei (dorsal and ventral cochlear nuclei, medial and lateral superior olivary nuclei, and inferior colliculus) was studied using an antibody to a microtubule-associated protein, MAP2, a molecule which stabilizes dendritic processes by promoting assembly of microtubules. At 21-22 weeks of gestation, cells within the auditory nuclei first demonstrate cytoplasmic MAP2 immunoreactivity, but no dendritic structures have formed. Filamentous background staining at this stage may represent immunoreactivity in astrocytic processes. By the 24th fetal week, somata of auditory neurons are strongly immunostained and have developed short dendritic processes. During the perinatal period, dendrites extend up to 100-120 microm in length but are still sparsely branched and lack terminal formations. By the sixth postnatal month, neurons in all auditory nuclei have acquired dendritic arbors with a mature appearance. Thus MAP2 immunohistochemistry demonstrates that dendrogenesis in human brainstem auditory nuclei begins 16 weeks prior to term birth but does not reach the stage of mature dendritic morphology until several months into the postnatal period. This extended course of development implies a significant period of time during which neuronal activity could influence dendritic structure and function.

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.

The TOGp protein is a new human microtubule-associated protein homologous to the Xenopus XMAP215

We have recently identified a 6,449 bp cDNA, termed colonic, hepatic tumor over-expressed gene (ch-TOG), that is highly expressed in human tumors and brain. Its single open reading frame encodes a putative 218,000 Da polypeptide, TOGp. Antibodies generated against a bacterially expressed TOGp fragment specifically recognize a 218, 000 Da polypeptide in two human cell lines and in brain. Immunofluorescence microscopy using affinity-purified TOGp antibodies revealed that the distribution of TOGp was dependent upon the cell cycle. During interphase, TOGp was found concentrated in the perinuclear cytoplasm, where it co-localized with ER markers. In contrast anti-TOGp antibodies stained centrosomes and spindles in mitotic cells. TOGp co-sedimented with taxol-stabilized microtubules in vitro. Moreover, a TOGp enriched fraction promotes microtubule assembly both in solution and from nucleation centers. Finally, sequence comparison and immunologic cross-reaction suggest that TOGp is homologous to XMAP215, a previously described microtubule associated protein (MAP) from Xenopus eggs. These results suggest that TOGp is a MAP and that TOGp/XMAP215 may be necessary for microtubules rearrangements and spindle assembly in rapidly dividing cells.

Purification of a WD repeat protein, EMAP, that promotes microtubule dynamics through an inhibition of rescue

The major microtubule-associated protein in echinoderms is a 77-kDa, WD repeat protein, called EMAP. EMAP-related proteins have been identified in sea urchins, starfish, sanddollars, and humans. We describe the purification of sea urchin EMAP and demonstrate that EMAP binding to microtubules is saturable at a molar ratio of 1 mol of EMAP to 3 mol of tubulin dimer. Unlike MAP-2, MAP-4, or tau proteins, EMAP binding to microtubules is not lost by cleavage of tubulin with subtilisin. In addition to binding to the microtubule polymer, EMAP binds to tubulin dimers in a 1:1 molar ratio. The abundance of EMAP in the egg suggests that it could function to regulate microtubule assembly. To test this hypothesis, we examined the effects of EMAP on the dynamic instability of microtubules nucleated from axoneme fragments as monitored by video-enhanced differential interference contrast microscopy. Addition of 2.2 microM EMAP to 21 microM tubulin results in a slight increase in the elongation and shortening velocities at the microtubule plus ends but not at the minus ends. Significantly, EMAP inhibits the frequency of rescue 8-fold without producing a change in the frequency of catastrophe. These results indicate that EMAP, unlike brain microtubule-associated proteins, promotes microtubule dynamics.

Phosphorylation of tau by glycogen synthase kinase 3beta affects the ability of tau to promote microtubule self-assembly

To study the effects of phosphorylation by glycogen synthase kinase-3beta (GSK-3beta) on the ability of the microtubule-associated protein tau to promote microtubule self-assembly, tau isoform 1 (foetal tau) and three mutant forms of this tau isoform were investigated. The three mutant forms of tau had the following serine residues, known to be phosphorylated by GSK-3, replaced with alanine residues so as to preclude their phosphorylation: (1) Ser-199 and Ser-202 (Ser-199/202-->Ala), (2) Ser-235 (Ser-235-->Ala) and (3) Ser-396 and Ser-404 (Ser-396/404-->Ala). Wild-type tau and the mutant forms of tau were phosphorylated with GSK-3beta, and their ability to promote microtubule self-assembly was compared with the corresponding non-phosphorylated tau species. In the non-phosphorylated form, wild-type tau and all of the mutants affected the mean microtubule length and number concentrations of assembled microtubules in a manner consistant with enhanced microtubule nucleation. Phosphorylation of these tau species with GSK-3beta consistently reduced the ability of a given tau species to promote microtubule self-assembly, although the affinity of the tau for the microtubules was not greatly affected by phosphorylation since the tau species remained largely associated with the microtubules. This suggests that the regulation of microtubule assembly can be controlled by phosphorylation of tau at sites accessible to GSK-3beta by a mechanism that does not necessarily involve the dissociation of tau from the microtubules.

A 60-kDa plant microtubule-associated protein promotes the growth and stabilization of neurotubules in vitro

The search for microtubule-associated proteins (MAPs) in plants is relatively recent. In particular, the "classical MAPs," which stimulate the polymerization and stabilization of microtubules, have only been examined in heterogeneous fractions. As a first step in dissecting the role of individual MAPs, we have chromatographically purified a single 60-kDa protein from a carrot MAP fraction and analyzed its effects on tubulin assembly. MAP60 promoted the formation of long, morphologically regular brain microtubules in vitro, an effect inhibited by preincubation of the MAP with affinity-purified antibodies against this protein. MAP60 also increased the stability of microtubules to dilution and significantly enhanced cold stability to the normally cold-sensitive neurotubules. These in vitro properties are consistent with a role for MAP60 in regulating the turnover/assembly of dynamic plant microtubules in vivo.