Dynein is required for polarized dendritic transport and uniform microtubule orientation in axons

Branching out: mechanisms of dendritic arborization

Type-specific dendrite morphology is a hallmark of the neuron and has important functional implications in determining what signals a neuron receives and how these signals are integrated. During the past two decades, studies on dendritic arborization neurons in Drosophila melanogaster have started to identify mechanisms of dendrite morphogenesis that may have broad applicability to vertebrate species. Transcription factors, receptor-ligand interactions, various signalling pathways, local translational machinery, cytoskeletal elements, Golgi outposts and endosomes have been identified as contributors to the organization of dendrites of individual neurons and the placement of these dendrites in the neuronal circuitry. Further insight into these mechanisms will improve our understanding of how the nervous system functions and might help to identify the underlying causes of some neurological and neurodevelopmental disorders.

Prestin surface expression and activity are augmented by interaction with MAP1S, a microtubule-associated protein

Prestin is a member of the SLC26 family of anion transporters that is responsible for outer hair cell (OHC) electromotility. Measures of voltage-evoked charge density (Q(sp)) of prestin indicated that the protein is highly expressed in OHCs, with single cells expressing up to 10 million molecules within the lateral membrane. In contrast, charge density measures in transfected cells indicated that they express, at best, only a fifth as many proteins on their surface. We sought to determine whether associations with other OHC-specific proteins could account for this difference. Using a yeast two-hybrid technique, we found microtubule-associated protein 1S (MAP1S) bound to prestin. The interaction was limited to the STAS domain of prestin and the region connecting the heavy and light chain of MAP1S. Using reciprocal immunoprecipitation and Forster resonance energy transfer, we confirmed these interactions. Furthermore, co-expression of prestin with MAP1S resulted in a 2.7-fold increase in Q(sp) in single cells that was paralleled by a 2.8-fold increase in protein surface expression, indicating that the interactions are physiological. Quantitative PCR data showed gradients in the expression of prestin and MAP1S across the tonotopic axis that may partially contribute to a previously observed 6-fold increase in Q(sp) in high frequency hair cells. These data highlight the importance of protein partner effects on prestin.

Crossveinless-c, the Drosophila homolog of tumor suppressor DLC1, regulates directional elongation of dendritic branches via down-regulating Rho1 activity

Diverse neuronal subtypes develop distinctive morphologies of dendritic arbors that receive synaptic or sensory inputs. Dendritic arbors of many subtypes take on a polarized shape, and one underlying mechanism is unidirectionally biased elongation of dendritic branches. As reported herein, we found that Drosophila Crossveinless-c (Cv-c) was a key regulator for such directional growth. In the cv-c mutant, two subclass of multidendritic sensory neurons examined formed dorsally directed branches; however, dendritic branches had difficulty in growing along the anterior-posterior (A-P) body axis. Cv-c belongs to the family of Rho GTPase-activating proteins (RhoGAPs) and is the homolog of human tumor suppressor DLC1. The RhoGAP activity of Cv-c was required cell-autonomously for the A-P-oriented growth, and Cv-c elevated the GTPase activity of Rho1 and Cdc42 in a cell-free assay. Our analysis of genetic interactions suggested that Rho1 was the target of Cv-c in vivo. All of our results suggest that Cv-c contributes to sprouting and subsequent growth of the A-P-oriented branches through negative regulation of Rho1. We discuss a role of Cv-c in dendritic growth in response to environmental cues.

Mixed microtubules steer dynein-driven cargo transport into dendrites

BACKGROUND

To establish and maintain their polarized morphology, neurons employ active transport driven by molecular motors to sort cargo between axons and dendrites. However, the basic traffic rules governing polarized transport on neuronal microtubule arrays are unclear.

RESULTS

Here we show that the microtubule minus-end-directed motor dynein is required for the polarized targeting of dendrite-specific cargo, such as AMPA receptors. To directly examine how dynein motors contribute to polarized dendritic transport, we established a trafficking assay in hippocampal neurons to selectively probe specific motor protein activity. This revealed that, unlike kinesins, dynein motors drive cargo selectively into dendrites, governed by their mixed microtubule array. Moreover, axon-specific cargos, such as presynaptic vesicle protein synaptophysin, are redirected to dendrites by coupling to dynein motors. Quantitative modeling demonstrated that bidirectional dynein-driven transport on mixed microtubules provides an efficient mechanism to establish a stable density of continuously renewing vesicles in dendrites.

CONCLUSIONS

These results demonstrate a powerful approach to study specific motor protein activity inside living cells and imply a key role for dynein in dendritic transport. We propose that dynein establishes the initial sorting of dendritic cargo and additional motor proteins assist in subsequent delivery.

Paclitaxel induces axonal microtubules polar reconfiguration and impaired organelle transport: implications for the pathogenesis of paclitaxel-induced polyneuropathy

In differentiated axons almost all microtubules (MTs) uniformly point their plus ends towards the axonal tip. The uniform polar pattern provides the structural substrate for efficient organelle transport along axons. It is generally believed that the mass and pattern of MTs polar orientation remain unchanged in differentiated neurons. Here we examined long-term effects of the MTs stabilizing reagent paclitaxel (taxol) over MTs polar orientation and organelle transport in cultured Aplysia neurons. Unexpectedly, we found that rather than stabilizing the MTs, paclitaxel leads to their massive polar reconfiguration, accompanied by impaired organelle transport. Washout of paclitaxel does not lead to recovery of the polar orientation indicating that the new pattern is self-maintained. Taken together the data suggest that MTs in differentiated neurons maintain the potential to be reconfigured. Such reconfiguration may serve physiological functions or lead to degeneration. In addition, our observations offer a novel mechanism that could account for the development of peripheral neuropathy in patients receiving paclitaxel as an antitumor drug.

Global up-regulation of microtubule dynamics and polarity reversal during regeneration of an axon from a dendrite

Axon regeneration is crucial for recovery after trauma to the nervous system. For neurons to recover from complete axon removal they must respecify a dendrite as an axon: a complete reversal of polarity. We show that Drosophila neurons in vivo can convert a dendrite to a regenerating axon and that this process involves rebuilding the entire neuronal microtubule cytoskeleton. Two major microtubule rearrangements are specifically induced by axon and not dendrite removal: 1) 10-fold up-regulation of the number of growing microtubules and 2) microtubule polarity reversal. After one dendrite reverses its microtubules, it initiates tip growth and takes on morphological and molecular characteristics of an axon. Only neurons with a single dendrite that reverses polarity are able to initiate tip growth, and normal microtubule plus-end dynamics are required to initiate this growth. In addition, we find that JNK signaling is required for both the up-regulation of microtubule dynamics and microtubule polarity reversal initiated by axon injury. We conclude that regulation of microtubule dynamics and polarity in response to JNK signaling is key to initiating regeneration of an axon from a dendrite.

A modifier screen in the Drosophila eye reveals that aPKC interacts with Glued during central synapse formation

BACKGROUND

The Glued gene of Drosophila melanogaster encodes the homologue of the vertebrate p150Glued subunit of dynactin. The Glued1 mutation compromises the dynein-dynactin retrograde motor complex and causes disruptions to the adult eye and the CNS, including sensory neurons and the formation of the giant fiber system neural circuit.

RESULTS

We performed a 2-stage genetic screen to identify mutations that modified phenotypes caused by over-expression of a dominant-negative Glued protein. We screened over 34,000 flies and isolated 41 mutations that enhanced or suppressed an eye phenotype. Of these, 12 were assayed for interactions in the giant fiber system by which they altered a giant fiber morphological phenotype and/or altered synaptic function between the giant fiber and the tergotrochanteral muscle motorneuron. Six showed interactions including a new allele of atypical protein kinase C (aPKC). We show that this cell polarity regulator interacts with Glued during central synapse formation. We have mapped the five other interacting mutations to discrete chromosomal regions.

CONCLUSION

Our results show that an efficient way to screen for genes involved in central synapse formation is to use a two-step strategy in which a screen for altered eye morphology precedes the analysis of central synaptogenesis. This has highlighted a role for aPKC in the formation of an identified central synapse.

Basic fibroblast growth factor elicits formation of interstitial axonal branches via enhanced severing of microtubules

This article demonstrates that the augmentation of axonal branching induced by bFGF is explicable on the basis of an enhancement of microtubule-severing, and that three different proteins related to microtubule-severing are affected by treatment of neurons with bFGF.

The formation of interstitial axonal branches involves the severing of microtubules at sites where new branches form. Here we wished to ascertain whether basic fibroblast growth factor (bFGF) enhances axonal branching through alterations in proteins involved in the severing of microtubules. We found that treatment of cultured hippocampal neurons with bFGF heightens expression of both katanin and spastin, which are proteins that sever microtubules in the axon. In addition, treatment with bFGF enhances phosphorylation of tau at sites expected to cause it to dissociate from microtubules. This is important because tau regulates the access of katanin to the microtubule. In live-cell imaging experiments, axons of neurons treated with bFGF displayed greater numbers of dynamic free ends of microtubules, as well as greater numbers of short mobile microtubules. Entirely similar enhancement of axonal branching, short microtubule transport, and frequency of microtubule ends was observed when spastin was overexpressed in the neurons. Depletion of either katanin or spastin with siRNA diminished but did not eliminate the enhancement in branching elicited by bFGF. Collectively, these results indicate that bFGF enhances axonal branch formation by augmenting the severing of microtubules through both a spastin-based mode and a katanin-based mode.

Multidendritic sensory neurons in the adult Drosophila abdomen: origins, dendritic morphology, and segment- and age-dependent programmed cell death

Background

For the establishment of functional neural circuits that support a wide range of animal behaviors, initial circuits formed in early development have to be reorganized. One way to achieve this is local remodeling of the circuitry hardwiring. To genetically investigate the underlying mechanisms of this remodeling, one model system employs a major group of Drosophila multidendritic sensory neurons - the dendritic arborization (da) neurons - which exhibit dramatic dendritic pruning and subsequent growth during metamorphosis. The 15 da neurons are identified in each larval abdominal hemisegment and are classified into four categories - classes I to IV - in order of increasing size of their receptive fields and/or arbor complexity at the mature larval stage. Our knowledge regarding the anatomy and developmental basis of adult da neurons is still fragmentary.

Results

We identified multidendritic neurons in the adult Drosophila abdomen, visualized the dendritic arbors of the individual neurons, and traced the origins of those cells back to the larval stage. There were six da neurons in abdominal hemisegment 3 or 4 (A3/4) of the pharate adult and the adult just after eclosion, five of which were persistent larval da neurons. We quantitatively analyzed dendritic arbors of three of the six adult neurons and examined expression in the pharate adult of key transcription factors that result in the larval class-selective dendritic morphologies. The 'baseline design' of A3/4 in the adult was further modified in a segment-dependent and age-dependent manner. One of our notable findings is that a larval class I neuron, ddaE, completed dendritic remodeling in A2 to A4 and then underwent caspase-dependent cell death within 1 week after eclosion, while homologous neurons in A5 and in more posterior segments degenerated at pupal stages. Another finding is that the dendritic arbor of a class IV neuron, v'ada, was immediately reshaped during post-eclosion growth. It exhibited prominent radial-to-lattice transformation in 1-day-old adults, and the resultant lattice-shaped arbor persisted throughout adult life.

Conclusion

Our study provides the basis on which we can investigate the genetic programs controlling dendritic remodeling and programmed cell death of adult neurons, and the life-long maintenance of dendritic arbors.

Rab5 isoforms differentially regulate the trafficking and degradation of epidermal growth factor receptors

Ligand-mediated endocytosis is an intricate regulatory mechanism for epidermal growth factor receptor (EGFR) signal transduction. Coordinated trafficking of EGFR ensures its temporal and spatial communication with downstream signaling effectors. We focused our work on Rab5, a monomeric GTPase shown to participate in early stages of the endocytic pathway. Rab5 has three isoforms (A, B, and C) sharing more than 90% of sequence identity. We individually ablated endogenous isoforms in HeLa cells with short interfering RNAs and examined the loss-of-function phenotypes. We found that suppression of Rab5A or 5B hampered the degradation of EGFR, whereas Rab5C depletion had very little effect. The differential delay of EGFR degradation also corresponds with retarded progression of EGFR from early to late endosomes. We investigated the activators/effectors of Rab5A that can potentially separate its potency in EGFR degradation from other isoforms and found that Rin1, a Rab5 exchange factor, preferably associated with Rab5A. Moreover, Rab5A activation is sensitive to EGF stimulation, and suppression of Rin1 diminished this sensitivity. Based on our results together with previous work showing that Rin1 interacts with signal transducing adapter molecule to facilitate the degradation of EGFR (Kong, C., Su, X., Chen, P. I., and Stahl, P. D. (2007) J. Biol. Chem. 282, 15294-15301), we hypothesize that the selective association of Rab5A and Rin1 contributes to the dominance of Rab5A in EGFR trafficking, whereas the other isoforms may have major functions unrelated to the EGFR degradation pathway.

Relocalization of a microtubule-anchoring protein, ninein, from the centrosome to dendrites during differentiation of mouse neurons

Microtubules in typical cells form radial arrays with their plus-ends pointing toward the cell periphery. In contrast, microtubules in dendrites of neurons are free from centrosomes and have a unique arrangement in which about half have a polarity with a minus-end distal orientation. Mechanisms for generation and maintenance of the microtubule arrangement in dendrites are not well understood. Here, we examined dendritic localization of a centrosomal protein, ninein, which has microtubule-anchoring and stabilizing functions. Immunohistochemical analysis of developing mouse cerebral and cerebellar cortices showed that ninein is localized at the centrosome in undifferentiated neural precursors. In contrast, ninein was barely detected in migrating neurons, such as those in the intermediate layer of the cerebral cortex and the internal granular layer of the cerebellar cortex. High expression was observed in thick dendrite-bearing neurons such as pyramidal neurons of the cerebral cortex and Purkinje neurons in the cerebellar cortex. Ninein was not detected at the centrosome of these cells, but was diffusely present in cell soma and dendrites. In cultured cortical neurons, ninein formed granular structures in soma and dendrites, being not associated with gamma-tubulin. About 60% of these structures showed resistance to detergent and association with microtubules. Our observations suggest that the minus-ends of microtubules may be anchored and stabilized by centrosomal proteins localized in dendrites.

The microtubule network and neuronal morphogenesis: Dynamic and coordinated orchestration through multiple players

Nervous system function and plasticity rely on the complex architecture of neuronal networks elaborated during development, when neurons acquire their specific and complex shape. During neuronal morphogenesis, the formation and outgrowth of functionally and structurally distinct axons and dendrites require a coordinated and dynamic reorganization of the microtubule cytoskeleton involving numerous regulators. While most of these factors act directly on microtubules to stabilize them or promote their assembly, depolymerization or fragmentation, others are now emerging as essential regulators of neuronal differentiation by controlling tubulin availability and modulating microtubule dynamics. In this review, we recapitulate how the microtubule network is actively regulated during the successive phases of neuronal morphogenesis, and what are the specific roles of the various microtubule-regulating proteins in that process. We then describe the specific signaling pathways and inter-regulations that coordinate the different activities of these proteins to sustain neuronal development in response to environmental cues.

Dendrites differ from axons in patterns of microtubule stability and polymerization during development

Background

Dendrites differ from axons in patterns of growth and development, as well as in morphology. Given that microtubules are key structural elements in cells, we assessed patterns of microtubule stability and polymerization during hippocampal neuron development in vitro to determine if these aspects of microtubule organization could distinguish axons from dendrites.

Results

Quantitative ratiometric immunocytochemistry identified significant differences in microtubule stability between axons and dendrites. Most notably, regardless of developmental stage, there were high levels of dynamic microtubules throughout the dendritic arbor, whereas dynamic microtubules were predominantly concentrated in the distal end of axons. Analysis of microtubule polymerization using green fluorescent protein-tagged EB1 showed both developmental and regional differences in microtubule polymerization between axons and dendrites. Early in development (for example, 1 to 2 days in vitro), polymerization events were distributed equally in both the anterograde and retrograde directions throughout the length of both axons and dendrites. As development progressed, however, polymerization became biased, with a greater number of polymerization events in distal than in proximal and middle regions. While polymerization occurred almost exclusively in the anterograde direction for axons, both anterograde and retrograde polymerization was observed in dendrites. This is in agreement with predicted differences in microtubule polarity within these compartments, although fewer retrograde events were observed in dendrites than expected.

Conclusion

Both immunocytochemical and live imaging analyses showed that newly formed microtubules predominated at the distal end of axons and dendrites, suggesting a common mechanism that incorporates increased microtubule polymerization at growing process tips. Dendrites had more immature, dynamic microtubules throughout the entire arbor than did axons, however. Identifying these differences in microtubule stability and polymerization is a necessary first step toward understanding how they are developmentally regulated, and may reveal novel mechanisms underlying neuron maturation and dendritic plasticity that extend beyond the initial specification of polarity.

Molecules and mechanisms of dendrite development in Drosophila

Neurons are one of the most morphologically diverse cell types, in large part owing to their intricate dendrite branching patterns. Dendrites are structures that are specialized to receive and process inputs in neurons, thus their specific morphologies reflect neural connectivity and influence information flow through circuits. Recent studies in Drosophila on the molecular basis of dendrite diversity, dendritic guidance, the cell biology of dendritic branch patterning and territory formation have identified numerous intrinsic and extrinsic cues that shape diverse features of dendrites. As we discuss in this review, many of the mechanisms that are being elucidated show conservation in diverse systems.

Nudel promotes axonal lysosome clearance and endo-lysosome formation via dynein-mediated transport

Axonal transport is critical for neuronal function and survival. Cytoplasmic dynein and its accessory complex dynactin form a microtubule minus end-directed motor in charge of retrograde transport. In this study, we show that Nudel, a dynein regulator, was highly expressed in dorsal root ganglion (DRG) neurons. Microinjection of anti-Nudel antibody into cultured DRG neurons abolished retrograde transport of membranous organelles in the axon and led to dispersions of Golgi cisternae in the soma. As a result, lysosomes, which are normally enriched in the soma, moved persistently into and thus accumulated in axons. Endo-lysosome formation was also markedly delayed. As anterograde motility of mitochondria was not inhibited, the antibody apparently did not abolish retrograde transport by destructing axonal microtubule tracks. Similar results were obtained by microinjecting N-terminal Nudel, anti-dynein antibody or a p150(Glued) mutant capable of abrogating the dynein-dynactin association. These results indicate a critical role of Nudel in dynein-mediated axonal transport. Moreover, the effects of dynein on endolysosome formation and regional sequestration of lysosomes may contribute to defects in the endocytic pathway seen in neurons of patients or animals with malfunction of dynein.

Microtubule assembly, organization and dynamics in axons and dendrites

During the past decade enormous advances have been made in our understanding of the basic molecular machinery that is involved in the development of neuronal polarity. Far from being mere structural elements, microtubules are emerging as key determinants of neuronal polarity. Here we review the current understanding of the regulation of microtubule assembly, organization and dynamics in axons and dendrites. These studies provide new insight into microtubules' function in neuronal development and their potential contribution to plasticity.

Molecular motors and the Golgi complex: staying put and moving through

The Golgi apparatus is a highly dynamic organelle through which nascent proteins released from the endoplasmic reticulum (ER) are trafficked. Proteins are post-translationally modified within the Golgi and subsequently packaged into carriers for transport to a variety of cellular destinations. This transit of proteins, as well as the maintenance of Golgi structure and position, is highly dependent upon the actin and microtubule cytoskeletons and their associated molecular motors. Here we review how motors contribute to the correct functioning of the Golgi in higher eukaryotes and discuss the secretory pathway as a model system for studying cooperation between motor proteins.

Polarity regulation in migrating neurons in the cortex

The formation of the cerebral cortex requires migration of billions of cells from their birth position to their final destination. A motile cell must have internal polarity in order to move in a specified direction. Locomotory polarity requires the coordinated polymerization of cytoskeletal elements such as microtubules and actin combined with regulated activities of the associated molecular motors. This review is focused on migrating neurons in the developing cerebral cortex, which need to attain internal polarity in order to reach their proper target. The position and dynamics of the centrosome plays an important function in this directed motility. We highlight recent interesting findings connecting polarity proteins with neuronal migration events regulated by the microtubule-associated molecular motor, cytoplasmic dynein.

RacGAP50C directs perinuclear gamma-tubulin localization to organize the uniform microtubule array required for Drosophila myotube extension

The microtubule (MT) cytoskeleton is reorganized during myogenesis as individual myoblasts fuse into multinucleated myotubes. Although this reorganization has long been observed in cell culture, these findings have not been validated during development, and proteins that regulate this process are largely unknown. We have identified a novel postmitotic function for the cytokinesis proteins RacGAP50C (Tumbleweed) and Pavarotti as essential regulators of MT organization during Drosophila myogenesis. We show that the localization of the MT nucleator gamma-tubulin changes from diffuse cytoplasmic staining in mononucleated myoblasts to discrete cytoplasmic puncta at the nuclear periphery in multinucleated myoblasts, and that this change in localization depends on RacGAP50C. RacGAP50C and gamma-tubulin colocalize at perinuclear sites in myotubes, and in RacGAP50C mutants gamma-tubulin remains dispersed throughout the cytoplasm. Furthermore, we show that the mislocalization of RacGAP50C in pavarotti mutants is sufficient to redistribute gamma-tubulin to the muscle fiber ends. Finally, myotubes in RacGAP50C mutants have MTs with non-uniform polarity, resulting in multiple guidance errors. Taken together, these findings provide strong evidence that the reorganization of the MT network that has been observed in vitro plays an important role in myotube extension and muscle patterning in vivo, and also identify two molecules crucial for this process.

Plasma membrane expansion: a neuron's Herculean task

The formation of axons and dendrites and maintenance of the neuron's vastly expanded surface require the continuous addition of new membrane. This is achieved by membrane synthesis through the secretory pathway followed by regulated vesicle fusion with the plasma membrane, typically in the distal neurite. However, it is far from simple: multiple distinct membrane carriers are used to target specific membrane domains, dendrites seem to operate semi-autonomously from the rest of the neuron, and exocytosis for membrane expansion is different from that for release of synaptic vesicles. Current knowledge of this process and its implications for neuronal development, function and repair are reviewed.

Dynein branches out

Individual neurons form specific elaborate dendritic structures that receive presynaptic information. The pattern of dendritic branching is regulated by the microtubule-associated motor protein dynein, which is responsible for the transport of essential endosomes and other organelles into the dendrites.

Regulation of dynactin through the differential expression of p150Glued isoforms

Cytoplasmic dynein and dynactin interact to drive microtubule-based transport in the cell. The p150Glued subunit of dynactin binds to dynein, and directly to microtubules. We have identified alternatively spliced isoforms of p150Glued that are expressed in a tissue-specific manner and which differ significantly in their affinity for microtubules. Live cell assays indicate that these alternatively spliced isoforms also differ significantly in their microtubule plus end-tracking activity, suggesting a mechanism by which the cell may regulate the dynamic localization of dynactin. To test the function of the microtubule-binding domain of p150Glued, we used RNAi to deplete the endogenous polypeptide from HeLa cells, followed by rescue with constructs encoding either the full-length polypeptide or an isoform lacking the microtubule-binding domain. Both constructs fully rescued defects in Golgi morphology induced by depletion of p150Glued, indicating that an independent microtubule-binding site in dynactin may not be required for dynactin-mediated trafficking in some mammalian cell types. In neurons, however, a mutation within the microtubule-binding domain of p150Glued results in motor neuron disease; here we investigate the effects of four other mutations in highly conserved domains of the polypeptide (M571T, R785W, R1101K, and T1249I) associated in genetic studies with Amyotrophic Lateral Sclerosis. Both biochemical and cellular assays reveal that these amino acid substitutions do not result in functional differences, suggesting that these sequence changes are either allelic variants or contributory risk factors rather than causative for motor neuron disease. Together, these studies provide further insight into the regulation of dynein-dynactin function in the cell.