Actin waves transport RanGTP to the neurite tip to regulate non-centrosomal microtubules in neurons

Direct observation of importin α family member KPNA1 in axonal transport with or without a schizophrenia-related mutation

Karyopherin α1 (KPNA1)/(human importin α5; mouse importin α1) facilitates cargo transport into the nucleus by forming a complex with a nuclear localization sequence containing cargo and importin β1 (IPOB1). The elevated KPNA1 expression in neurons and the correlation between mutations and psychiatric disorders suggest its broader significance beyond nucleocytoplasmic transport. Although KPNA1 is localized in the neurites of neurons, its role in axonal transport mechanisms remains unclear, and data on the connection between psychiatric disorders and signaling at the periphery of neurons remain limited. To address this knowledge gap, we investigated the dynamics of KPNA1 and related factors within axons. Our results showed that many of the axonal KPNA1 did not form a complex with IPOB1 in noninjured steady-state neurons. Axonal KPNA1 exhibited relatively stationary mobility and some showed bidirectional motility with fluctuating motion. KPNA1 partly comigrated with endosome/lysosome-associated factors, suggesting the presence of novel mechanisms underlie axonal transport and nucleocytoplasmic shuttling involving KPNA1 and IPOB1. Mutated KPNA1, which has been shown to be associated with psychiatric disorders (KPNA1E448X), was predominantly localized to the nucleus and lost from the axon. Incorporating a nuclear export signal (KPNA1E448X-NES) enhanced its subcellular localization and dynamics in the axon. Our findings demonstrate that KPNA1 functions not only as a shuttle between the cytoplasm and nucleus but also as a transporter in neuronal axons, relying on the endosomes for movement away from the nucleus with relatively slow net motions. Furthermore, a mutation in the Kpna1 gene can affect the dynamics of axonal transport. The insights from these mutations provide valuable knowledge for expanding our understanding of psychiatric disorders and facilitate the development of novel treatment strategies.

B-type Plexins Regulate Mitosis via RanGTPase

Aberrant mitosis can result in aneuploidy and cancer. The small GTPase, Ras-related nuclear protein (Ran), is a key regulator of mitosis. B-type plexins regulate Ran activity by acting as RanGTPase-activating proteins and have been implicated in cancer progression. However, whether B-type plexins have a role in mitosis has not so far been investigated. We show here that Plexin B1 functions in the control of mitosis. Depletion of Plexin B1 affects mitotic spindle assembly, significantly delaying anaphase. This leads to mitotic catastrophe in some cells and prolonged application of the spindle assembly checkpoint. Plexin B1 depletion also promoted acentrosomal microtubule nucleation and defects in spindle pole refocusing and increased the number of cells with multipolar or aberrant mitotic spindles. An increase in lagging chromosomes or chromosomal bridges at anaphase was also found upon Plexin B1 depletion. Plexin B1 localizes to the mitotic spindle in dividing cells. The mitotic defects observed upon Plexin B1 depletion were rescued by an RCC1 inhibitor, indicating that Plexin B1 signals, via Ran, to affect mitosis. These errors in mitosis generated multinucleate cells and nuclei of altered morphology and abnormal karyotype. Furthermore, semaphorin 4D treatment increased the percentage of cells with micronuclei, precursors of chromothripsis. Implications: Defects in B-type plexins may contribute to the well-established role of plexins in cancer progression by inducing chromosomal instability.

Competition and synergy of Arp2/3 and formins in nucleating actin waves

Actin assembly and dynamics are crucial for maintaining cell structure and changing physiological states. The broad impact of actin on various cellular processes makes it challenging to dissect the specific role of actin regulatory proteins. Using actin waves that propagate on the cortex of mast cells as a model, we discovered that formins (FMNL1 and mDia3) are recruited before the Arp2/3 complex in actin waves. GTPase Cdc42 interactions drive FMNL1 oscillations, with active Cdc42 and the constitutively active mutant of FMNL1 capable of forming waves on the plasma membrane independently of actin waves. Additionally, the delayed recruitment of Arp2/3 antagonizes FMNL1 and active Cdc42. This antagonism is not due to competition for monomeric actin but rather for their common upstream regulator, active Cdc42, whose levels are negatively regulated by Arp2/3 via SHIP1 recruitment. Collectively, our study highlights the complex feedback loops in the dynamic control of the actin cytoskeletal network.

The non-mitotic role of HMMR in regulating the localization of TPX2 and the dynamics of microtubules in neurons

A functional nervous system is built upon the proper morphogenesis of neurons to establish the intricate connection between them. The microtubule cytoskeleton is known to play various essential roles in this morphogenetic process. While many microtubule-associated proteins (MAPs) have been demonstrated to participate in neuronal morphogenesis, the function of many more remains to be determined. This study focuses on a MAP called HMMR in mice, which was originally identified as a hyaluronan binding protein and later found to possess microtubule and centrosome binding capacity. HMMR exhibits high abundance on neuronal microtubules and altering the level of HMMR significantly affects the morphology of neurons. Instead of confining to the centrosome(s) like cells in mitosis, HMMR localizes to microtubules along axons and dendrites. Furthermore, transiently expressing HMMR enhances the stability of neuronal microtubules and increases the formation frequency of growing microtubules along the neurites. HMMR regulates the microtubule localization of a non-centrosomal microtubule nucleator TPX2 along the neurite, offering an explanation for how HMMR contributes to the promotion of growing microtubules. This study sheds light on how cells utilize proteins involved in mitosis for non-mitotic functions.

Advances in Understanding the Molecular Mechanisms of Neuronal Polarity

The establishment and maintenance of neuronal polarity are important for neural development and function. Abnormal neuronal polarity establishment commonly leads to a variety of neurodevelopmental disorders. Over the past three decades, with the continuous development and improvement of biological research methods and techniques, we have made tremendous progress in the understanding of the molecular mechanisms of neuronal polarity establishment. The activity of positive and negative feedback signals and actin waves are both essential in this process. They drive the directional transport and aggregation of key molecules of neuronal polarity, promote the spatiotemporal regulation of ordered and coordinated interactions of actin filaments and microtubules, stimulate the specialization and growth of axons, and inhibit the formation of multiple axons. In this review, we focus on recent advances in these areas, in particular the important findings about neuronal polarity in two classical models, in vitro primary hippocampal/cortical neurons and in vivo cortical pyramidal neurons, and discuss our current understanding of neuronal polarity.​.

With the Permission of Microtubules: An Updated Overview on Microtubule Function During Axon Pathfinding

During the establishment of neural circuitry axons often need to cover long distances to reach remote targets. The stereotyped navigation of these axons defines the connectivity between brain regions and cellular subtypes. This chemotrophic guidance process mostly relies on the spatio-temporal expression patterns of extracellular proteins and the selective expression of their receptors in projection neurons. Axon guidance is stimulated by guidance proteins and implemented by neuronal traction forces at the growth cones, which engage local cytoskeleton regulators and cell adhesion proteins. Different layers of guidance signaling regulation, such as the cleavage and processing of receptors, the expression of co-receptors and a wide variety of intracellular cascades downstream of receptors activation, have been progressively unveiled. Also, in the last decades, the regulation of microtubule (MT) assembly, stability and interactions with the submembranous actin network in the growth cone have emerged as crucial effector mechanisms in axon pathfinding. In this review, we will delve into the intracellular signaling cascades downstream of guidance receptors that converge on the MT cytoskeleton of the growing axon. In particular, we will focus on the microtubule-associated proteins (MAPs) network responsible of MT dynamics in the axon and growth cone. Complementarily, we will discuss new evidences that connect defects in MT scaffold proteins, MAPs or MT-based motors and axon misrouting during brain development.

Distinct Microtubule Organizing Center Mechanisms Combine to Generate Neuron Polarity and Arbor Complexity

Dendrite and axon arbor wiring patterns determine the connectivity and computational characteristics of a neuron. The identities of these dendrite and axon arbors are created by differential polarization of their microtubule arrays, and their complexity and pattern are generated by the extension and organization of these arrays. We describe how several molecularly distinct microtubule organizing center (MTOC) mechanisms function during neuron differentiation to generate and arrange dendrite and axon microtubules. The temporal and spatial organization of these MTOCs generates, patterns, and diversifies arbor wiring.