Three-Step Model for Polarized Sorting of KIF17 into Dendrites

Kinesin and dynein motors drive bidirectional cargo transport along microtubules and have a critical role in polarized cargo trafficking in neurons [1, 2]. The kinesin-2 family protein KIF17 is a dendrite-specific motor protein and has been shown to interact with several dendritic cargoes [3-7]. However, the mechanism underlying the dendritic targeting of KIF17 remains poorly understood [8-11]. Using live-cell imaging combined with inducible trafficking assays to directly probe KIF17 motor activity in living neurons, we found that the polarized sorting of KIF17 to dendrites is regulated in multiple steps. First, cargo binding of KIF17 relieves autoinhibition and initiates microtubule-based cargo transport. Second, KIF17 does not autonomously target dendrites, but enters the axon where the actin cytoskeleton at the axon initial segment (AIS) prevents KIF17 vesicles from moving further into the axon. Third, dynein-based motor activity is able to redirect KIF17-coupled cargoes into dendrites. We propose a three-step model for polarized targeting of KIF17, in which the collective function of multiple motor teams is required for proper dendritic sorting

Three-Step Model for Polarized Sorting of KIF17 into Dendrites

Kinesin and dynein motors drive bidirectional cargo transport along microtubules and have a critical role in polarized cargo trafficking in neurons [1, 2]. The kinesin-2 family protein KIF17 is a dendrite-specific motor protein and has been shown to interact with several dendritic cargoes [3-7]. However, the mechanism underlying the dendritic targeting of KIF17 remains poorly understood [8-11]. Using live-cell imaging combined with inducible trafficking assays to directly probe KIF17 motor activity in living neurons, we found that the polarized sorting of KIF17 to dendrites is regulated in multiple steps. First, cargo binding of KIF17 relieves autoinhibition and initiates microtubule-based cargo transport. Second, KIF17 does not autonomously target dendrites, but enters the axon where the actin cytoskeleton at the axon initial segment (AIS) prevents KIF17 vesicles from moving further into the axon. Third, dynein-based motor activity is able to redirect KIF17-coupled cargoes into dendrites. We propose a three-step model for polarized targeting of KIF17, in which the collective function of multiple motor teams is required for proper dendritic sorting

A role for Bicaudal-D2 in radial cerebellar granule cell migration

Bicaudal-D (BICD) belongs to an evolutionary conserved family of dynein adaptor proteins. It was first described in Drosophila as an essential factor in fly oogenesis and embryogenesis. Missense mutations in a human BICD homologue, BICD2, have been linked to a dominant mild early onset form of spinal muscular atrophy. Here we further examine the in vivo function of BICD2 in Bicd2 knockout mice. BICD2-deficient mice develop disrupted laminar organization of cerebral cortex and the cerebellum, pointing to impaired radial neuronal migration. Using astrocyte and granule cell specific inactivation of BICD2, we show that the cerebellar migration defect is entirely dependent upon BICD2 expression in Bergmann glia cells. Proteomics analysis reveals that Bicd2 mutant mice have an altered composition of extracellular matrix proteins produced by glia cells. These findings demonstrate an essential non-cell-autonomous role of BICD2 in neuronal cell migration, which might be connected to cargo trafficking pathways in glia cells

Kinesin-Binding Protein Controls Microtubule Dynamics and Cargo Trafficking by Regulating Kinesin Motor Activity

Kinesin motor proteins play a fundamental role for normal neuronal development by controlling intracellular cargo transport and microtubule (MT) cytoskeleton organization. Regulating kinesin activity is important to ensure their proper functioning, and their misregulation often leads to severe human neurological disorders. Homozygous nonsense mutations in kinesin-binding protein (KBP)/KIAA1279 cause the neurological disorder Goldberg-Shprintzen syndrome (GOSHS), which is characterized by intellectual disability, microcephaly, and axonal neuropathy. Here, we show that KBP regulates kinesin activity by interacting with the motor domains of a specific subset of kinesins to prevent their association with the MT cytoskeleton. The KBP-interacting kinesins include cargo-transporting motors such as kinesin-3/KIF1A and MT-depolymerizing motor kinesin-8/KIF18A. We found that KBP blocks KIF1A/UNC-104-mediated synaptic vesicle transport in cultured hippocampal neurons and in C. elegans PVD sensory neurons. In contrast, depletion of KBP results in the accumulation of KIF1A motors and synaptic vesicles in the axonal growth cone. We also show that KBP regulates neuronal MT dynamics by controlling KIF18A activity. Our data suggest that KBP functions as a kinesin inhibitor that modulates MT-based cargo motility and depolymerizing activity of a subset of kinesin motors. We propose that misregulation of KBP-controlled kinesin motors may represent the underlying molecular mechanism that contributes to the neuropathological defects observed in GOSHS patients

Microtubule minus-end binding protein CAMSAP2 controls axon specification and dendrite development

In neurons, most microtubules are not associated with a central microtubule-organizing center (MTOC), and therefore, both the minus and plus-ends of these non-centrosomal microtubules are found throughout the cell. Microtubule plus-ends are well established as dynamic regulatory sites in numerous processes, but the role of microtubule minus-ends has remained poorly understood. Using live-cell imaging, high-resolution microscopy, and laser-based microsurgery techniques, we show that the CAMSAP/Nezha/Patronin family protein CAMSAP2 specifically localizes to non-centrosomal microtubule minus-ends and is required for proper microtubule organization in neurons. CAMSAP2 stabilizes non-centrosomal microtubules and is required for neuronal polarity, axon specification, and dendritic branch formation in vitro and in vivo. Furthermore, we found that non-centrosomal microtubules in dendrites are largely generated by γ-Tubulin-dependent nucleation. We propose a two-step model in which γ-Tubulin initiates the formation of non-centrosomal microtubules and CAMSAP2 stabilizes the free microtubule minus-ends in order to control neuronal polarity and development

CFEOM1-Associated Kinesin KIF21A Is a Cortical Microtubule Growth Inhibitor

Mechanisms controlling microtubule dynamics at the cell cortex play a crucial role in cell morphogenesis and neuronal development. Here, we identified kinesin-4 KIF21A as an inhibitor of microtubule growth at the cell cortex. In vitro, KIF21A suppresses microtubule growth and inhibits catastrophes. In cells, KIF21A restricts microtubule growth and participates in organizing microtubule arrays at the cell edge. KIF21A is recruited to the cortex by KANK1, which coclusters with liprin-alpha 1/beta 1 and the components of the LL5 beta-containing cortical microtubule attachment complexes. Mutations in KIF21A have been linked to congenital fibrosis of the extraocular muscles type 1 (CFEOM1), a dominant disorder associated with neurodevelopmental defects. CFEOM1-associated mutations relieve autoinhibition of the KIF21A motor, and this results in enhanced KIF21A accumulation in axonal growth cones, aberrant axon morphology, and reduced responsiveness to inhibitory cues. Our study provides mechanistic insight into cortical microtubule regulation and suggests that altered microtubule dynamics contribute to CFEOM1 pathogenesis

Neuron Specific Rab4 Effector GRASP-1 Coordinates Membrane Specialization and Maturation of Recycling Endosomes

The endosomal pathway in neuronal dendrites is essential for membrane receptor trafficking and proper synaptic function and plasticity. However, the molecular mechanisms that organize specific endocytic trafficking routes are poorly understood. Here, we identify GRIP-associated protein-1 (GRASP-1) as a neuron-specific effector of Rab4 and key component of the molecular machinery that coordinates recycling endosome maturation in dendrites. We show that GRASP-1 is necessary for AMPA receptor recycling, maintenance of spine morphology, and synaptic plasticity. At the molecular level, GRASP-1 segregates Rab4 from EEA1/Neep21/Rab5-positive early endosomal membranes and coordinates the coupling to Rab11-labelled recycling endosomes by interacting with the endosomal SNARE syntaxin 13. We propose that GRASP-1 connects early and late recycling endosomal compartments by forming a molecular bridge between Rab-specific membrane domains and the endosomal SNARE machinery. The data uncover a new mechanism to achieve specificity and directionality in neuronal membrane receptor trafficking