Kinesin superfamily protein 2A (KIF2A) functions in suppression of collateral branch extension

Molecular motors and mechanisms of directional transport in neurons

Intracellular transport is fundamental for neuronal morphogenesis, function and survival. Many proteins are selectively transported to either axons or dendrites. In addition, some specific mRNAs are transported to dendrites for local translation. Proteins of the kinesin superfamily participate in selective transport by using adaptor or scaffolding proteins to recognize and bind cargoes. The molecular components of RNA-transporting granules have been identified, and it is becoming clear how cargoes are directed to axons and dendrites by kinesin superfamily proteins. Here we discuss the molecular mechanisms of directional axonal and dendritic transport with specific emphasis on the role of motor proteins and their mechanisms of cargo recognition.

Microtubule-associated protein 1B controls directionality of growth cone migration and axonal branching in regeneration of adult dorsal root ganglia neurons

During development, microtubule-associated protein 1B (MAP1B) is one of the earliest MAPs, preferentially localized in axons and growth cones, and plays a role in axonal outgrowth. Although generally downregulated in the adult, we have shown that MAP1B is constitutively highly expressed in adult dorsal root ganglia (DRGs) and associated with central sprouting and peripheral regeneration of these neurons. Mutant mice with a complete MAP1B null allele that survive until adulthood exhibit a reduced myelin sheath diameter and conductance velocity of peripheral axons and lack of the corpus callosum. Here, to determine the function of MAP1B in axonal regeneration, we used cultures of adult DRG explants and/or dissociated neurons derived from this map1b-/- mouse line. Whereas the overall length of regenerating neurites lacking MAP1B was similar to wild-type controls, our analysis revealed two main defects. First, map1b-/- neurites exhibited significantly (twofold) higher terminal and collateral branching. Second, the turning capacity of growth cones (i.e., "choice" of a proper orientation) was impaired. In addition, lack of MAP1B may affect the post-translational modification of tubulin polymers: quantitative analysis showed a reduced amount of acetylated microtubules within growth cones, whereas the distribution of tyrosinated or detyrosinated microtubules was normal. Both growth cone turning and axonal branch formation are known to involve local regulation of the microtubule network. Our results demonstrate that MAP1B plays a role in these processes during plastic changes in the adult. In particular, the data suggest MAP1B implication in the locally coordinated assembly of cytoskeletal components required for branching and straight directional axon growth.

Kinesin superfamily proteins and their various functions and dynamics

Kinesin superfamily proteins (KIFs) are motor proteins that transport membranous organelles and macromolecules fundamental for cellular functions along microtubules. Their roles in transport in axons and dendrites have been studied extensively, but KIFs are also used in intracellular transport in general. Recent findings have revealed that in many cases, the specific interaction of cargoes and motors is mediated via adaptor/scaffolding proteins. Cargoes are sorted to precise destinations, such as axons or dendrites. KIFs also participate in polarized transport in epithelial cells as shown in the apical transport of annexin XIIIb-containing vesicles by KIFC3. KIFs play important roles in higher order neuronal activity; transgenic mice overexpressing KIF17, which transports N-methyl-d-asp (NMDA) receptors to dendrites, show enhanced memory and learning. KIFs also play significant roles in neuronal development and brain wiring: KIF2A suppresses elongation of axon collaterals by its unique microtubule-depolymerizing activity. X-ray crystallography has revealed the structural uniqueness of KIF2 underlying the microtubule-depolymerizing activity. In addition, single molecule biophysics and optical trapping have shown that the motility of monomeric KIF1A is caused by biased Brownian movement, and X-ray crystallography has shown how the conformational changes occur for KIF1A to move during ATP hydrolysis. These multiple approaches in analyzing KIF functions will illuminate many basic mechanisms underlying intracellular events and will be a very promising and fruitful area for future studies.

Axonal growth is sensitive to the levels of katanin, a protein that severs microtubules

Katanin is a heterodimeric enzyme that severs microtubules from the centrosome so that they can move into the axon. Katanin is broadly distributed in the neuron, and therefore presumably also severs microtubules elsewhere. Such severing would generate multiple short microtubules from longer microtubules, resulting in more microtubule ends available for assembly and interaction with other structures. In addition, shorter microtubules are thought to move more rapidly and undergo organizational changes more readily than longer microtubules. In dividing cells, the levels of P60-katanin (the subunit with severing properties) increase as the cell transitions from interphase to mitosis. This suggests that katanin is regulated in part by its absolute levels, given that katanin activity is high during mitosis. In the rodent brain, neurons vary significantly in katanin levels, depending on their developmental stage. Levels are high during rapid phases of axonal growth but diminish as axons reach their targets. Similarly, in neuronal cultures, katanin levels are high when axons are allowed to grow avidly but drop when the axons are presented with target cells that cause them to stop growing. Expression of a dominant-negative P60-katanin construct in cultured neurons inhibits microtubule severing and is deleterious to axonal growth. Overexpression of wild-type P60-katanin results in excess microtubule severing and is also deleterious to axonal growth, but this only occurs in some neurons. Other neurons are relatively unaffected by overexpression. Collectively, these observations indicate that axonal growth is sensitive to the levels of P60-katanin, but that other factors contribute to modulating this sensitivity.

CORTICAL NEURONAL MIGRATION MUTANTS SUGGEST SEPARATE BUT INTERSECTING PATHWAYS

During brain development, neurons migrate great distances from proliferative zones to generate the cortical gray matter. A series of studies has identified genes that are critical for migration and targeting of neurons to specific brain regions. These genes encode three basic groups of proteins and produce three distinct phenotypes. The first group encodes cytoskeletal molecules and produces graded and dosage-dependent effects, with a significant amount of functional redundancy. This group also appears to play important roles during the initiation and ongoing progression of neuronal movement. The second group encodes signaling molecules for which homozygous mutations lead to an inverted cortex. In addition, this group is responsible for movement of neurons through anatomic boundaries to specific cortical layers. The third group encodes enzymatic regulators of glycosylation and appears to delineate where neuronal migration will arrest. There is significant cross-talk among these different groups of molecules, suggesting possible points of pathway convergence.

Molecular motors in neuronal development, intracellular transport and diseases

Molecular motors such as kinesin superfamily proteins (KIFs), dynein superfamily proteins and myosin superfamily proteins have diverse and fundamental roles in many cellular processes, including neuronal development and the pathogenesis of neuronal diseases. During neuronal development, KIFs take significant roles in the regulation of axon-collateral branch extension, which is essential for brain wiring. Cytoplasmic dynein together with LIS1 takes pivotal roles in neocortical layer formation. In axons, anterograde transport is mediated by KIFs, whereas retrograde transport is mediated mainly by cytoplasmic dynein, and dysfunction of motors results in neurodegenerative diseases. In dendrites, the transport of NMDA and AMPA receptors is mediated by KIFs, and the motor has been shown to play a significant part in establishing learning and memory.

Kin I kinesins: insights into the mechanism of depolymerization

Kin I kinesins are members of the diverse kinesin superfamily of molecular motors. Whereas most kinesins use ATP to move along microtubules, Kin I kinesins depolymerize microtubules rather than walk along them. Functionally, this distinct subfamily of kinesins is important in regulating cellular microtubule dynamics and plays a crucial role in spindle assembly and chromosome segregation. The molecular mechanism of Kin I-induced microtubule destabilization is as yet unclear. It is generally believed that Kin Is induce a structural change on the microtubule that leads to microtubule destabilization. Recently, much progress has been made towards understanding how Kin Is may cause this structural change, and how ATPase activity is employed in the catalytic cycle.

The KinI kinesin Kif2a is required for bipolar spindle assembly through a functional relationship with MCAK

Although the microtubule-depolymerizing KinI motor Kif2a is abundantly expressed in neuronal cells, we now show it localizes to centrosomes and spindle poles during mitosis in cultured cells. RNAi-induced knockdown of Kif2a expression inhibited cell cycle progression because cells assembled monopolar spindles. Bipolar spindle assembly was restored in cells lacking Kif2a by treatments that altered microtubule assembly (nocodazole), eliminated kinetochore–microtubule attachment (loss of Nuf2), or stabilized microtubule plus ends at kinetochores (loss of MCAK). Thus, two KinI motors, MCAK and Kif2a, play distinct roles in mitosis, and MCAK activity at kinetochores must be balanced by Kif2a activity at poles for spindle bipolarity. These treatments failed to restore bipolarity to cells lacking the activity of the kinesin Eg5. Thus, two independent pathways contribute to spindle bipolarity, with the Eg5-dependent pathway using motor force to drive spindle bipolarity and the Kif2a-dependent pathway relying on microtubule polymer dynamics to generate force for spindle bipolarity.

Netrin-1 and semaphorin 3A promote or inhibit cortical axon branching, respectively, by reorganization of the cytoskeleton

In many CNS pathways, target innervation occurs by axon branching rather than extension of the primary growth cone into targets. To investigate mechanisms of branch formation, we studied the effects of attractive and inhibitory guidance cues on cortical axon branching. We found that netrin-1, which attracts cortical axons, and FGF-2 increased branching by >50%, whereas semaphorin 3A (Sema3A), which repels cortical axons, inhibited branching by 50%. Importantly, none of the factors affected axon length significantly. The increase in branching by FGF-2 and the inhibition of branching by Sema3A were mediated by opposing effects on the growth cone (expansion vs collapse) and on the cytoskeleton. FGF-2 increased actin polymerization and formation of microtubule loops in growth cones over many hours, whereas Sema3A depolymerized actin filaments, attenuated microtubule dynamics, and collapsed microtubule arrays within minutes. Netrin-1 promoted rapid axon branching, often without involving the growth cone. Branches formed de novo on the axon shaft within 30 min after local application of netrin-1, which induced rapid accumulation of actin filaments in filopodia. Importantly, increased actin polymerization and microtubule dynamics were necessary for axon branching to occur. Taken together, these results show that guidance factors influence the organization and dynamics of the cytoskeleton at the growth cone and the axon shaft to promote or inhibit axon branching. Independent of axon outgrowth, axon branching in response to guidance cues can occur over different time courses by different cellular mechanisms.

A common mechanism for microtubule destabilizers-M type kinesins stabilize curling of the protofilament using the class-specific neck and loops

Unlike other kinesins, middle motor domain-type kinesins depolymerize the microtubule from its ends. To elucidate its mechanism, we solved the X-ray crystallographic structure of KIF2C, a murine member of this family. Three major class-specific features were identified. The class-specific N-terminal neck adopts a long and rigid helical structure extending out vertically into the interprotofilament groove. This structure explains its dual roles in targeting to the end of the microtubule and in destabilization of the lateral interaction of the protofilament. The loop L2 forms a unique finger-like structure, long and rigid enough to reach the next tubulin subunit to stabilize the peeling of the protofilament. The open conformation of the switch I loop could be reversed by the shift of the microtubule binding L8 loop, suggesting its role as the sensor to trigger ATP hydrolysis. Mutational analysis supports these structural implications.

Aurora B regulates MCAK at the mitotic centromere

Chromosome orientation and alignment within the mitotic spindle requires the Aurora B protein kinase and the mitotic centromere-associated kinesin (MCAK). Here, we report the regulation of MCAK by Aurora B. Aurora B inhibited MCAK's microtubule depolymerizing activity in vitro, and phospho-mimic (S/E) mutants of MCAK inhibited depolymerization in vivo. Expression of either MCAK (S/E) or MCAK (S/A) mutants increased the frequency of syntelic microtubule-kinetochore attachments and mono-oriented chromosomes. MCAK phosphorylation also regulates MCAK localization: the MCAK (S/E) mutant frequently localized to the inner centromere while the (S/A) mutant concentrated at kinetochores. We also detected two different binding sites for MCAK using FRAP analysis of the different MCAK mutants. Moreover, disruption of Aurora B function by expression of a kinase-dead mutant or RNAi prevented centromeric targeting of MCAK. These results link Aurora B activity to MCAK function, with Aurora B regulating MCAK's activity and its localization at the centromere and kinetochore.

Structure of a kinesin microtubule depolymerization machine

With their ability to depolymerize microtubules (MTs), KinI kinesins are the rogue members of the kinesin family. Here we present the 1.6 A crystal structure of a KinI motor core from Plasmodium falciparum, which is sufficient for depolymerization in vitro. Unlike all published kinesin structures to date, nucleotide is not present, and there are noticeable differences in loop regions L6 and L10 (the plus-end tip), L2 and L8 and in switch II (L11 and helix4); otherwise, the pKinI structure is very similar to previous kinesin structures. KinI-conserved amino acids were mutated to alanine, and studied for their effects on depolymerization and ATP hydrolysis. Notably, mutation of three residues in L2 appears to primarily affect depolymerization, rather than general MT binding or ATP hydrolysis. The results of this study confirm the suspected importance of loop 2 for KinI function, and provide evidence that KinI is specialized to hydrolyze ATP after initiating depolymerization.

Regulation of KinI kinesin ATPase activity by binding to the microtubule lattice

KinI kinesins are important in regulating the complex dynamics of the microtubule cytoskeleton. They are unusual in that they depolymerize, rather than move along microtubules. To determine the attributes of KinIs that distinguish them from translocating kinesins, we examined the ATPase activity, microtubule affinity, and three-dimensional microtubule-bound structure of a minimal KinI motor domain. Together, the kinetic, affinity, and structural data lead to the conclusion that on binding to the microtubule lattice, KinIs release ADP and enter a stable, low-affinity, regulated state, from which they do not readily progress through the ATPase cycle. This state may favor detachment, or diffusion of the KinI to its site of action, the microtubule ends. Unlike conventional translocating kinesins, which are microtubule lattice–stimulated ATPases, it seems that with KinIs, nucleotide-mediated modulation of tubulin affinity is only possible when it is coupled to protofilament deformation. This provides an elegant mechanistic basis for their unique depolymerizing activity.

Cytoskeletal dynamics and transport in growth cone motility and axon guidance

Recent studies indicate the actin and microtubule cytoskeletons are a final common target of many signaling cascades that influence the developing neuron. Regulation of polymer dynamics and transport are crucial for the proper growth cone motility. This review addresses how actin filaments, microtubules, and their associated proteins play crucial roles in growth cone motility, axon outgrowth, and guidance. We present a working model for cytoskeletal regulation of directed axon outgrowth. An important goal for the future will be to understand the coordinated response of the cytoskeleton to signaling cascades induced by guidance receptor activation.

Neuronal Polarity and Trafficking

Among the most morphologically complex cells, neurons are masters of membrane specialization. Nowhere is this more striking than in the division of cellular labor between the axon and the dendrites. In morphology, signaling properties, cytoskeletal organization, and physiological function, axons and dendrites (or more properly, the somatodendritic compartment) are radically different. Such polarization of neurons into domains specialized for either receiving (dendrites) or transmitting (axons) cellular signals provides the underpinning for all neural circuitry. The initial specification of axonal and dendritic identity occurs early in neuronal life, persists for decades, and is manifested by the presence of very different sets of cell surface proteins. Yet, how neuronal polarity is established, how distinct axonal and somatodendritic domains are maintained, and how integral membrane proteins are directed to dendrites or accumulate in axons remain enduring and formidable questions in neuronal cell biology.