Gelsolin inhibition of fast axonal transport indicates a requirement for actin microfilaments

Synaptic vesicle traffic is supported by transient actin filaments and regulated by PKA and NO

Synaptic vesicles (SVs) can be pooled across multiple synapses, prompting questions about their dynamic allocation for neurotransmission and plasticity. We find that the axonal traffic of recycling vesicles is not supported by ubiquitous microtubule-based motility but relies on actin instead. Vesicles freed from synaptic clusters undergo ~1 µm bouts of active transport, initiated by nearby elongation of actin filaments. Long distance translocation arises when successive bouts of active transport were linked by periods of free diffusion. The availability of SVs for active transport can be promptly increased by protein kinase A, a key player in neuromodulation. Vesicle motion is in turn impeded by shutting off axonal actin polymerization, mediated by nitric oxide-cyclic GMP signaling leading to inhibition of RhoA. These findings provide a potential framework for coordinating post-and pre-synaptic strength, using retrograde regulation of axonal actin dynamics to mobilize and recruit presynaptic SV resources.

Role of Mitochondria-Cytoskeleton Interactions in the Regulation of Mitochondrial Structure and Function in Cancer Stem Cells

Despite the promise of cancer medicine, major challenges currently confronting the treatment of cancer patients include chemoresistance and recurrence. The existence of subpopulations of cancer cells, known as cancer stem cells (CSCs), contributes to the failure of cancer therapies and is associated with poor clinical outcomes. Of note, one of the recently characterized features of CSCs is augmented mitochondrial function. The cytoskeleton network is essential in regulating mitochondrial morphology and rearrangement, which are inextricably linked to its functions, such as oxidative phosphorylation (OXPHOS). The interaction between the cytoskeleton and mitochondria can enable CSCs to adapt to challenging conditions, such as a lack of energy sources, and to maintain their stemness. Cytoskeleton-mediated mitochondrial trafficking and relocating to the high energy requirement region are crucial steps in epithelial-to-mesenchymal transition (EMT). In addition, the cytoskeleton itself interplays with and blocks the voltage-dependent anion channel (VDAC) to directly regulate bioenergetics. In this review, we describe the regulation of cellular bioenergetics in CSCs, focusing on the cytoskeleton-mediated dynamic control of mitochondrial structure and function.

The Ca2+ sensor protein swiprosin-1/EFhd2 is present in neurites and involved in kinesin-mediated transport in neurons

Swiprosin-1/EFhd2 (EFhd2) is a cytoskeletal Ca²⁺ sensor protein strongly expressed in the brain. It has been shown to interact with mutant tau, which can promote neurodegeneration, but nothing is known about the physiological function of EFhd2 in the nervous system. To elucidate this question, we analyzed EFhd2^−/−/lacZ reporter mice and showed that lacZ was strongly expressed in the cortex, the dentate gyrus, the CA1 and CA2 regions of the hippocampus, the thalamus, and the olfactory bulb. Immunohistochemistry and western blotting confirmed this pattern and revealed expression of EFhd2 during neuronal maturation. In cortical neurons, EFhd2 was detected in neurites marked by MAP2 and co-localized with pre- and post-synaptic markers. Approximately one third of EFhd2 associated with a biochemically isolated synaptosome preparation. There, EFhd2 was mostly confined to the cytosolic and plasma membrane fractions. Both synaptic endocytosis and exocytosis in primary hippocampal EFhd2^−/− neurons were unaltered but transport of synaptophysin-GFP containing vesicles was enhanced in EFhd2^−/− primary hippocampal neurons, and notably, EFhd2 inhibited kinesin mediated microtubule gliding. Therefore, we found that EFhd2 is a neuronal protein that interferes with kinesin-mediated transport.

Hereditary gelsolin amyloidosis

Hereditary gelsolin amyloidosis (HGA) is an autosomally dominantly inherited form of systemic amyloidosis, characterized mainly by cranial and sensory peripheral neuropathy, corneal lattice dystrophy, and cutis laxa. HGA, originally reported from Finland and now increasingly from other countries in Europe, North and South America, and Asia, may still be underdiagnosed worldwide. It is the first and so-far only known disorder caused by a gelsolin gene defect, namely a G654A or G654T mutation. Gelsolin is a principal actin-modulating protein, implicated in multiple biological processes, also in the nervous system, e.g. axonal transport, myelination, neurite outgrowth, and neuroprotection. The gelsolin gene defect causes expression of variant gelsolin, followed by systemic deposition of gelsolin amyloid (AGel) in HGA patients and even other consequences on the metabolism and function of gelsolin. In HGA, specific therapy is not yet available but correct diagnosis enables adequate symptomatic treatment which decisively improves the quality of life in these patients. A transgenic murine model of HGA expressing AGel is available, in anticipation of new treatment options targeted toward this slowly progressive but devastating amyloidosis. Present and future lessons learned from HGA may be applicable even in diagnosis and treatment of other hereditary and sporadic amyloidoses.

The axonal transport of mitochondria

Vigorous transport of cytoplasmic components along axons over substantial distances is crucial for the maintenance of neuron structure and function. The transport of mitochondria, which serves to distribute mitochondrial functions in a dynamic and non-uniform fashion, has attracted special interest in recent years following the discovery of functional connections among microtubules, motor proteins and mitochondria, and their influences on neurodegenerative diseases. Although the motor proteins that drive mitochondrial movement are now well characterized, the mechanisms by which anterograde and retrograde movement are coordinated with one another and with stationary axonal mitochondria are not yet understood. In this Commentary, we review why mitochondria move and how they move, focusing particularly on recent studies of transport regulation, which implicate control of motor activity by specific cell-signaling pathways, regulation of motor access to transport tracks and static microtubule-mitochondrion linkers. A detailed mechanism for modulating anterograde mitochondrial transport has been identified that involves Miro, a mitochondrial Ca(2+)-binding GTPase, which with associated proteins, can bind and control kinesin-1. Elements of the Miro complex also have important roles in mitochondrial fission-fusion dynamics, highlighting questions about the interdependence of biogenesis, transport, dynamics, maintenance and degradation.

Molecular signaling how do axons die?

Axons depend critically on axonal transport both for supplying materials and for communicating with cell bodies. This chapter looks at each activity, asking what aspects are essential for axon survival. Axonal transport declines in neurodegenerative disorders, such as Alzheimer's disease, amyotrophic lateral sclerosis, and multiple sclerosis, and in normal ageing, but whether all cargoes are equally affected and what limits axon survival remains unclear. Cargoes can be differentially blocked in some disorders, either individually or in groups. Each missing protein cargo results in localized loss-of-function that can be partially modeled by disrupting the corresponding gene, sometimes with surprising results. The axonal response to losing specific proteins also depends on the rates of protein turnover and on whether the protein can be locally synthesized. Among cargoes with important axonal roles are components of the PI3 kinase, Mek/Erk, and Jnk signaling pathways, which help to communicate with cell bodies and to regulate axonal transport itself. Bidirectional trafficking of Bdnf, NT-3, and other neurotrophic factors contribute to intra- and intercellular signaling, affecting the axon's cellular environment and survival. Finally, several adhesion molecules and gangliosides are key determinants of axon survival, probably by mediating axon-glia interactions. Thus, failure of long-distance intracellular transport can deprive axons of one, few, or many cargoes. This can lead to axon degeneration either directly, through the absence of essential axonal proteins, or indirectly, through failures in communication with cell bodies and nonneuronal cells.

Amyloid-beta peptide oligomers disrupt axonal transport through an NMDA receptor-dependent mechanism that is mediated by glycogen synthase kinase 3beta in primary cultured hippocampal neurons

Disruption of axonal transport is a hallmark of several neurodegenerative diseases, including Alzheimer's disease (AD). Even though defective transport is considered an early pathologic event, the mechanisms by which neurodegenerative insults impact transport are poorly understood. We show that soluble oligomers of the amyloid-beta peptide (AbetaOs), increasingly recognized as the proximal neurotoxins in AD pathology, induce disruption of organelle transport in primary hippocampal neurons in culture. Live imaging of fluorescent protein-tagged organelles revealed a marked decrease in axonal trafficking of dense-core vesicles and mitochondria in the presence of 0.5 microm AbetaOs. NMDA receptor (NMDAR) antagonists, including d-AP5, MK-801, and memantine, prevented the disruption of trafficking, thereby identifying signals for AbetaO action at the cell membrane. Significantly, both pharmacological inhibition of glycogen synthase kinase-3beta (GSK-3beta) and transfection of neurons with a kinase-dead form of GSK-3beta prevented the transport defect. Finally, we demonstrate by biochemical and immunocytochemical means that AbetaOs do not affect microtubule stability, indicating that disruption of transport involves a more subtle mechanism than microtubule destabilization, likely the dysregulation of intracellular signaling cascades. Results demonstrate that AbetaOs negatively impact axonal transport by a mechanism that is initiated by NMDARs and mediated by GSK-3beta and establish a new connection between toxic Abeta oligomers and AD pathology.

A perspective on neuronal cell death signaling and neurodegeneration

Although neuronal cell death through apoptotic pathways represents a common feature of dysferopathies, the canonical apoptotic changes familiar from nonneuronal cells are late events. Loss of neuronal function occurs at a much early time, when synaptic-based neuronal connectivity fails. In this context, apoptotic pathways may normally serve a cleanup role, rather than a pathogenic one. Reframing the consideration of cell death in the nervous system to include the early stages of axonal degeneration provides a better understanding of the roles played by various apoptotic signaling pathways in neurodegenerative diseases. Focusing on disease-specific mechanisms that initiate the sequence that eventually leads to neuronal loss should facilitate development of therapies that preserve neuronal function and neuronal numbers.

Accelerators, Brakes, and Gears of Actin Dynamics in Dendritic Spines

Dendritic spines are actin-rich structures that accommodate the postsynaptic sites of most excitatory synapses in the brain. Although dendritic spines form and mature as synaptic connections develop, they remain plastic even in the adult brain, where they can rapidly grow, change, or collapse in response to normal physiological changes in synaptic activity that underlie learning and memory. Pathological stimuli can adversely affect dendritic spine shape and number, and this is seen in neurodegenerative disorders and some forms of mental retardation and autism as well. Many of the molecular signals that control these changes in dendritic spines act through the regulation of filamentous actin (F-actin), some through direct interaction with actin, and others via downstream effectors. For example, cortactin, cofilin, and gelsolin are actin-binding proteins that directly regulate actin dynamics in dendritic spines. Activities of these proteins are precisely regulated by intracellular signaling events that control their phosphorylation state and localization. In this review, we discuss how actin-regulating proteins maintain the balance between F-actin assembly and disassembly that is needed to stabilize mature dendritic spines, and how changes in their activities may lead to rapid remodeling of dendritic spines.

The mechanism of Ca2+ -dependent regulation of kinesin-mediated mitochondrial motility

Mitochondria are mobile organelles and cells regulate mitochondrial movement in order to meet the changing energy needs of each cellular region. Ca(2+) signaling, which halts both anterograde and retrograde mitochondrial motion, serves as one regulatory input. Anterograde mitochondrial movement is generated by kinesin-1, which interacts with the mitochondrial protein Miro through an adaptor protein, milton. We show that kinesin is present on all axonal mitochondria, including those that are stationary or moving retrograde. We also show that the EF-hand motifs of Miro mediate Ca(2+)-dependent arrest of mitochondria and elucidate the regulatory mechanism. Rather than dissociating kinesin-1 from mitochondria, Ca(2+)-binding permits Miro to interact directly with the motor domain of kinesin-1, preventing motor/microtubule interactions. Thus, kinesin-1 switches from an active state in which it is bound to Miro only via milton, to an inactive state in which direct binding to Miro prevents its interaction with microtubules. Disrupting Ca(2+)-dependent regulation diminishes neuronal resistance to excitotoxicity.

Severe ataxia with neuropathy in hereditary gelsolin amyloidosis: a case report

Hereditary gelsolin amyloidosis (AGel amyloidosis) is a systemic disorder caused by a G654A or G654T gelsolin mutation, reported from Europe, North America, and Japan. Principal clinical signs are corneal lattice dystrophy, cutis laxa and cranial neuropathy, often deleterious at advanced age. Peripheral neuropathy, if present, is usually mild. We report a 78-year-old male Finnish patient who presented with ataxia and mainly sensory peripheral polyneuropathy (PNP) signs, causing severe disability and ambulation loss. Electrophysiological studies showed severe generalized chronic mainly axonal sensorimotor PNP with facial paralysis. In magnetic resonance imaging proximal lower limb and axial muscle atrophy with fatty degeneration as well as moderate spinal cord atrophy were seen. A G654A gelsolin mutation was demonstrated but no other possible causes of his disability were found. At age 79 years he became bedridden and died of pulmonary embolism. Neuropathological examination revealed marked gelsolin amyloid deposition at vascular and connective tissue sites along the entire length of the peripheral nerves extending to the spinal nerve roots, associated with severe degeneration of nerve fibers and posterior columns. Our report shows that advanced AGel amyloidosis due to degeneration of central and distal sensory nerve projections results in deleterious ataxia with fatal outcome. Severe posterior column atrophy may reflect radicular AGel deposition, although even altered gelsolin-actin interactions in neural cells possibly contribute to neurodegeneration with successive ataxia in carriers of a G654A gelsolin mutation.

Mitochondrial transport in processes of cortical neurons is independent of intracellular calcium

Mitochondria show extensive movement along neuronal processes, but the mechanisms and function of this movement are not clearly understood. We have used high-resolution confocal microscopy to simultaneously monitor movement of mitochondria and changes in intracellular [Ca2+] ([Ca2+]i) in rat cortical neurons. A significant percentage (27%) of the total mitochondria in cortical neuronal processes showed movement over distances of >2 μM. The average velocity was 0.52 μm/s. The velocity, direction, and pattern of mitochondrial movement were not affected by transient increases in [Ca2+]i associated with spontaneous firing of action potentials. Stimulation of Ca2+ transients with forskolin (10 μM) or bicuculline (10 μM), or sustained elevations of [Ca2+]i evoked by glutamate (10 μM) also had no effect on mitochondrial transit. Neither removal of extracellular Ca2+, depletion of intracellular Ca2+ stores with thapsigargin, or inhibition of synaptic activity with TTX (1 μM) or a cocktail of CNQX (10 μM) and MK801 (10 μM) affected mitochondrial movement. These results indicate that movement of mitochondria along processes is a fundamental activity in neurons that occurs independently of physiological changes in [Ca2+]i associated with action potential firing, synaptic activity, or release of Ca2+ from intracellular stores.

Trafficking of macromolecules and organelles in culturedDystonia musculorumsensory neurons is normal

Dystonia musculorum (dt) mice suffer from a recessive neuropathy characterized by the progressive loss of sensory axons. The gene responsible for this disorder, dystonin/Bpag1, encodes several alternatively spliced forms of a cytoskeletal linker protein. Neural isoforms of dystonin/Bpag1 are predicted to link actin filaments to microtubules. Consistent with this, previous observations have demonstrated that the cytoskeleton within sensory neurites of dt mice is perturbed. Also, recent results have indicated that a neural isoform of dystonin/Bpag1 interacts with the dynein motor complex. Because microtubule organization and dynein motor function are essential for trafficking, we hypothesized that this process would be perturbed in dt sensory neurons. Here, we demonstrate that cultured primary dorsal root ganglion (DRG) neurons express dystonin/Bpag1 and that loss of this expression causes an increase in apoptosis and a decrease in average neurite length. In contrast, detailed examination showed that the organization of microtubules is indistinguishable in DRG neuronal cultures from neonatal dt and wild-type mice. In addition, the steady-state distribution of several molecules and organelles is unchanged in these cultures. Furthermore, the speeds of mitochondrial movement in both anterograde and retrograde directions were comparable in dt and wild-type sensory neurons cultured from neonatal mice. Thus, dystonin/Bpag1 is not essential for microtubule network assembly since the microtubule network is intact in short-term cultures of sensory neurons from neonatal mice lacking this protein. In addition, dystonin/Bpag1 is not an essential part of the dynein motor complex for mitochondrial transport since mitochondrial trafficking is normal in cultured sensory neurons from dt mice.

Transport of neurofilaments in growing axons requires microtubules but not actin filaments

Neurofilament (NF) polymers are conveyed from cell body to axon tip by slow axonal transport, and disruption of this process is implicated in several neuronal pathologies. This movement occurs in both anterograde and retrograde directions and is characterized by relatively rapid but brief movements of neurofilaments, interrupted by prolonged pauses. The present studies combine pharmacologic treatments that target actin filaments or microtubules with imaging of NF polymer transport in living axons to examine the dependence of neurofilament transport on these cytoskeletal systems. The heavy NF subunit tagged with green fluorescent protein was expressed in cultured sympathetic neurons to visualize NF transport. Depletion of axonal actin filaments by treatment with 5 microM latrunculin for 6 hr had no detectable effect on directionality or transport rate of NFs, but frequency of movement events was reduced from 1/3.1 min of imaging time to 1/4.9 min. Depolymerization of axonal microtubules using either 5 microM vinblastine for 3 hr or 5 microg/ml nocodazole for 4-6 hr profoundly suppressed neurofilament transport. In 92% of treated neurons, NF transport was undetected. These observations indicate that actin filaments are not required for neurofilament transport, although they may have subtle effects on neurofilament movements. In contrast, axonal transport of NFs requires microtubules, suggesting that anterograde and retrograde NF transport is powered by microtubule-based motors.

Myosin Va and microtubule-based motors are required for fast axonal retrograde transport of tetanus toxin in motor neurons

Using a novel assay based on the sorting and transport of a fluorescent fragment of tetanus toxin, we have investigated the cytoskeletal and motor requirements of axonal retrograde transport in living mammalian motor neurons. This essential process ensures the movement of neurotrophins and organelles from the periphery to the cell body and is crucial for neuronal survival. Unlike what is observed in sympathetic neurons, fast retrograde transport in motor neurons requires not only intact microtubules, but also actin microfilaments. Here, we show that the movement of tetanus toxin-containing carriers relies on the nonredundant activities of dynein as well as kinesin family members. Quantitative kinetic analysis indicates a role for dynein as the main motor of these carriers. Moreover, this approach suggests the involvement of myosin(s) in retrograde movement. Immunofluorescence screening with isoform-specific myosin antibodies reveals colocalization of tetanus toxin-containing retrograde carriers with myosin Va. Motor neurons from homozygous myosin Va null mice showed slower retrograde transport compared with wild-type cells, establishing a unique role for myosin Va in this process. On the basis of our findings, we propose that coordination of myosin Va and microtubule-dependent motors is required for fast axonal retrograde transport in motor neurons.

Glutamate and amyloid beta-protein rapidly inhibit fast axonal transport in cultured rat hippocampal neurons by different mechanisms

Impairment of axonal transport leads to neurodegeneration and synapse loss. Glutamate and amyloid beta-protein (Abeta) have critical roles in the pathogenesis of Alzheimer's disease (AD). Here we show that both agents rapidly inhibit fast axonal transport in cultured rat hippocampal neurons. The effect of glutamate (100 microm), but not of Abeta25-35 (20 microm), was reversible, was mimicked by NMDA or AMPA, and was blocked by NMDA and AMPA antagonists and by removal of extracellular Ca2+. The effect of Abeta25-35 was progressive and irreversible, was prevented by the actin-depolymerizing agent latrunculin B, and was mimicked by the actin-polymerizing agent jasplakinolide. Abeta25-35 induced intracellular actin aggregation, which was prevented by latrunculin B. Abeta31-35 but not Abeta15-20 exerted effects similar to those of Abeta25-35. Full-length Abeta1-42 incubated for 7 d, which specifically contained 30-100 kDa molecular weight assemblies, also caused an inhibition of axonal transport associated with intracellular actin aggregation, whereas freshly dissolved Abeta1-40, incubated Abeta1-40, and fresh Abeta1-42 had no effect. These results suggest that glutamate inhibits axonal transport via activation of NMDA and AMPA receptors and Ca2+ influx, whereas Abeta exerts its inhibitory effect via actin polymerization and aggregation. The ability of Abeta to inhibit axonal transport seems to require active amino acid residues, which is probably present in the 31-35 sequence. Full-length Abeta may be effective when it represents a structure in which these active residues can access the cell membrane. Our results may provide insight into the early pathogenetic mechanisms of AD.

Intracellular membrane trafficking in bone resorbing osteoclasts

There is ample evidence now that the two major events in bone resorption, namely dissolution of hydroxyapatite and degradation of the organic matrix, are performed by osteoclasts. The resorption cycle involves several specific cellular activities, where intracellular vesicular trafficking plays a crucial role. Although details of these processes started to open up only recently, it is clear that vesicular trafficking is needed in several specific steps of osteoclast functioning. Several plasma membrane domains are formed during the polarization of the resorbing cells. Multinucleated osteoclasts create a tight sealing to the extracellular matrix as a first indicator of their resorption activity. Initial steps of the sealing zone formation are alpha(v)beta(3)-integrin mediated, but the final molecular interaction(s) between the plasma membrane and mineralized bone matrix is still unknown. A large number of acidic intracellular vesicles then fuse with the bone-facing plasma membrane to form a ruffled border membrane, which is the actual resorbing organelle. The formation of a ruffled border is regulated by a small GTP-binding protein, rab7, which indicates the late endosomal character of the ruffled border membrane. Details of specific membrane transport processes in the osteoclasts, e.g., the formation of the sealing zone and transcytosis of bone degradation products from the resorption lacuna to the functional secretory domain remain to be clarified. It is tempting to speculate that specific features of vesicular trafficking may offer several potential new targets for drug therapy of bone diseases.

Osteoclast ruffled border has distinct subdomains for secretion and degraded matrix uptake

Subosteoclastic bone resorption is a result of HCl and proteinase secretion through a late endosome-like bone facing membrane domain called ruffled border. As bone matrix is degraded, it enters osteoclasts' transcytotic vesicles for further processing and is then finally exocytosed to the intercellular space. The present study clarifies the spatial relationship between these vesicle fusion and matrix uptake processes at the ruffled border. Our results show the presence of vacuolar H+-ATPase, small GTPase rab7 as well as dense aggregates of F-actin at the peripheral ruffled border, where basolaterally endocytosed transferrin and cathepsin K are delivered. On the contrary, rhodamine-labeled bone matrix enters transcytotic vesicles at the central ruffled border, where the vesicle budding proteins such as clathrin, AP-2 and dynamin II are also localized. We present a model for the mechanism of ruffled border turnover and suggest that, due to its late endosomal characteristics, the ruffled border serves as a valuable model for studying the dynamic organization of other endosomal compartments as well.

Neuromuscular pathology in hereditary gelsolin amyloidosis

Hereditary gelsolin amyloidosis (AGel amyloidosis) is a systemic disorder reported worldwide in kindreds with a G654A or G654T gelsolin gene mutation. The clinically characteristic peripheral nerve involvement has been poorly characterized morphologically, and its pathogenesis remains unknown. We studied peripheral nerve and skeletal muscle biopsy or autopsy specimens of 35 patients with a G654A gelsolin gene mutation. Histological, immunohistochemical, and electron microscopic studies showed consistent deposition of gelsolin amyloid (AGel), particularly in the vascular walls and perineurial sheaths. Nerve roots were more severely affected than distal nerves. The amyloid deposits also displayed variable immunoreactivity for apolipoprotein E, amyloid P component, cystatin C, and alpha-smooth muscle actin. Sural nerve morphometry showed preferential age-related large myelinated nerve fiber loss and reduction of myelin sheath cross-sectional area. There was evidence of denervation atrophy and fiber type grouping in skeletal muscle. Our study shows that marked proximal nerve involvement with AGel angiopathy is an essential feature of AGel amyloidosis. The preferential large fiber loss, not generally seen in amyloid neuropathy, may be caused by ischemia due to AGel angiopathy. Deficient actin modulation by variant gelsolin in neurons and Schwann cells, however, may alter axonal transport and myelination and contribute to AGel polyneuropathy.

Characterization of the phosphorylation sites of the squid (Loligo pealei) high-molecular-weight neurofilament protein from giant axon axoplasm

Axonal caliber in vertebrates is attributed, in part, to the extensive phosphorylation of NFM and NFH C-terminal tail domain KSP repeats by proline-directed kinases. The squid giant axon, primarily involved in rapid impulse conduction during jet propulsion motility, is enriched in squid-specific neurofilaments, particularly the highly phosphorylated NF-220. Of the 228 serine-threonine candidate phosphate acceptor sites in the NF-220 tail domain (residues 401-1220), 82 are found in numerous repeats of three different motifs SAR/K, SEK/R, K/RSP, with 62 of these tightly clustered in the C-terminal repeat segment (residues 840-1160). Characterization of the in vivo NF-220 phosphorylated sites should provide clues as to the relevant kinases. To characterize these sites, proteolytic digests of NF-220 were analyzed by a combination of HPLC, electrospray tandem mass spectrometry and database searching. A total of 53 phosphorylation sites were characterized, with 47 clustered in the C-terminal repeat segment (residues 840-1160), representing 76% (47/62) of the total acceptor sites in the region. As in mammalian NFH, approximately 64% of the K/RSP sites (14/22) in this region were found to be phosphorylated implicating proline-directed kinases. Significantly, 78% of serines (31/40) in the KAES*EK and EKS*ARSP motifs were also phosphorylated suggesting that non proline-directed kinases such as CKI may also be involved. This is consistent with previous studies showing that CKI is the principal kinase associated with axoplasmic NF preparations. It also suggests that phosphorylation of large macromolecules with multiple phospho-sites requires sequential phosphorylation by several kinases.

Association of actin filaments with axonal microtubule tracts

Axoplasmic organelles move on actin as well as microtubules in vitro and axons contain a large amount of actin, but little is known about the organization and distribution of actin filaments within the axon. Here we undertake to define the relationship of the microtubule bundles typically found in axons to actin filaments by applying three microscopic techniques: laser-scanning confocal microscopy of immuno-labeled squid axoplasm; electronmicroscopy of conventionally prepared thin sections; and electronmicroscopy of touch preparations-a thin layer of axoplasm transferred to a specimen grid and negatively stained. Light microscopy shows that longitudinal actin filaments are abundant and usually coincide with longitudinal microtubule bundles. Electron microscopy shows that microfilaments are interwoven with the longitudinal bundles of microtubules. These bundles maintain their integrity when neurofilaments are extracted. Some, though not all microfilaments decorate with the S1 fragment of myosin, and some also act as nucleation sites for polymerization of exogenous actin, and hence are definitively identified as actin filaments. These actin filaments range in minimum length from 0.5 to 1.5 microm with some at least as long as 3.5 microm. We conclude that the microtubule-based tracks for fast organelle transport also include actin filaments. These actin filaments are sufficiently long and abundant to be ancillary or supportive of fast transport along microtubules within bundles, or to extend transport outside of the bundle. These actin filaments could also be essential for maintaining the structural integrity of the microtubule bundles.

A role for cyclin-dependent kinase(s) in the modulation of fast anterograde axonal transport: effects defined by olomoucine and the APC tumor suppressor protein

Proteins that interact with both cytoskeletal and membrane components are candidates to modulate membrane trafficking. The tumor suppressor proteins neurofibromin (NF1) and adenomatous polyposis coli (APC) both bind to microtubules and interact with membrane-associated proteins. The effects of recombinant NF1 and APC fragments on vesicle motility were evaluated by measuring fast axonal transport along microtubules in axoplasm from squid giant axons. APC4 (amino acids 1034-2844) reduced only anterograde movements, whereas APC2 (aa 1034-2130) or APC3 (aa 2130-2844) reduced both anterograde and retrograde transport. NF1 had no effect on organelle movement in either direction. Because APC contains multiple cyclin-dependent kinase (CDK) consensus phosphorylation motifs, the kinase inhibitor olomoucine was examined. At concentrations in which olomoucine is specific for cyclin-dependent kinases (5 microM), it reduced only anterograde transport, whereas anterograde and retrograde movement were both affected at concentrations at which other kinases are inhibited as well (50 microM). Both anterograde and retrograde transport also were inhibited by histone H1 and KSPXK peptides, substrates for proline-directed kinases, including CDKs. Our data suggest that CDK-like axonal kinases modulate fast anterograde transport and that other axonal kinases may be involved in modulating retrograde transport. The specific effect of APC4 on anterograde transport suggests a model in which the binding of APC to microtubules may limit the activity of axonal CDK kinase or kinases in restricted domains, thereby affecting organelle transport.

A class VI unconventional myosin is associated with a homologue of a microtubule-binding protein, cytoplasmic linker protein-170, in neurons and at the posterior pole of Drosophila embryos

Coordination of cellular organization requires the interaction of the cytoskeletal filament systems. Recently, several lines of investigation have suggested that transport of cellular components along both microtubules and actin filaments is important for cellular organization and function. We report here on molecules that may mediate coordination between the actin and microtubule cytoskeletons. We have identified a 195-kD protein that coimmunoprecipitates with a class VI myosin, Drosophila 95F unconventional myosin. Cloning and sequencing of the gene encoding the 195-kD protein reveals that it is the first homologue identified of cytoplasmic linker protein (CLIP)-170, a protein that links endocytic vesicles to microtubules. We have named this protein D-CLIP-190 (the predicted molecular mass is 189 kD) based on its similarity to CLIP-170 and its ability to cosediment with microtubules. The similarity between D-CLIP-190 and CLIP-170 extends throughout the length of the proteins, and they have a number of predicted sequence and structural features in common. 95F myosin and D-CLIP-190 are coexpressed in a number of tissues during embryogenesis in Drosophila. In the axonal processes of neurons, they are colocalized in the same particulate structures, which resemble vesicles. They are also colocalized at the posterior pole of the early embryo, and this localization is dependent on the actin cytoskeleton. The association of a myosin and a homologue of a microtubule-binding protein in the nervous system and at the posterior pole, where both microtubule and actin-dependent processes are known to be important, leads us to speculate that these two proteins may functionally link the actin and microtubule cytoskeletons.

The role of axonal cytoskeleton in diabetic neuropathy

The neuropathy associated with diabetes includes well documented impairment of axonal transport, a reduction in axon calibre and a reduced capacity for nerve regeneration. All of those aspects of nerve function rely on the integrity of the axonal cytoskeleton. Alterations in the axonal cytoskeleton in experimental diabetes include an insulin-dependent non-enzymatic glycation of actin that is reflected in increased glycation of platelet actin in the clinical situation. There is a reduced synthesis of mRNA for the isoforms of tubulin that are associated with nerve growth and regeneration and an elevated non-enzymatic glycation of peripheral nerve tubulin in both diabetic patients and diabetic animals. mRNAs for neurofilament proteins are selectively reduced in the diabetic rat and post-translational modification of at least one of the neurofilament proteins is altered. There is some evidence that altered expression of isoforms of protein kinases may contribute to these changes.

Surface dynamics in living acinar cells imaged by atomic force microscopy: identification of plasma membrane structures involved in exocytosis

The dynamics at the plasma membrane resulting from secretory vesicle docking and fusion and compensatory endocytosis has been difficult to observe in living cells primarily due to limited resolution at the light microscopic level. Using the atomic force microscope, we have been able to image and record changes in plasma membrane structure at ultrahigh resolution after stimulation of secretion from isolated pancreatic acinar cells. "Pits" measuring 500-2000 nm and containing 3-20 depressions measuring 100-180 nm in diameter were observed only at the apical region of acinar cells. The time course of an increase and decrease in "depression" size correlated with an increase and decrease of amylase secretion from live acinar cells. Depression dynamics and amylase release were found to be regulated in part by actin. No structural changes were identified at the basolateral region of these cells. Our results suggest depressions to be the fusion pores identified earlier in mast cells by freeze-fracture electron microscopy and by electrophysiological measurements. The atomic force microscope has enabled us to observe plasma membrane dynamics of the exocytic process in living cells in real time.

Actin-based motility of isolated axoplasmic organelles

We previously showed that axoplasmic organelles from the squid giant axon move toward the barbed ends of actin filaments and that KI-washed organelles separated from soluble proteins by sucrose density fractionation retain a 235-kDa putative myosin. Here, we examine the myosin-like activities of KI-washed organelles after sucrose density fractionation to address the question whether the myosin on these organelles is functional. By electron microscopy KI-washed organelles bound to actin filaments in the absence of ATP but not in its presence. Analysis of organelle-dependent ATPase activity over time and with varying amounts of organelles revealed a basal activity of 350 (range: 315-384) nmoles Pi/mg/min and an actin-activated activity of 774 (range: 560-988) nmoles/mg/min, a higher specific activity than for the other fractions. By video microscopy washed organelles moved in only one direction on actin filaments with a net velocity of 1.11 +/- .03 microns/s and an instantaneous velocity of 1.63 +/- 0.29 microns/s. By immunogold electronmicroscopy, 7% of KI-washed organelles were decorated with an anti-myosin antibody as compared to 0.5% with non-immune serum. Thus, some axoplasmic organelles have a tightly associated myosin-like activity.

Axonal transport of mitochondria along microtubules and F-actin in living vertebrate neurons

A large body of evidence indicates that microtubules (MTs) conduct organelle transport in axons, but recent studies on extruded squid axoplasm have suggested that actin microfilaments (MFs) may also play a role in this process. To investigate the separate contributions to transport of each class of cytoskeletal element in intact vertebrate axons, we have monitored mitochondrial movements in chick sympathetic neurons experimentally manipulated to eliminate MTs, MFs, or both. First, we grew neurons in the continuous presence of: (a) cytochalasin E to create neurites which had never contained MFs; or (b) nocodazole or vinblastine to produce neurites which had never contained MTs. Mitochondria moved bidirectionally at normal velocities along the length of neurites which contained MTs and lacked MFs, but did not even enter neurites grown without MTs but containing MFs. In a second approach, we treated established neuronal cultures with cytoskeletal drugs to disrupt either MTs or MFs in axons already containing mitochondria. In cytochalasin-treated cells, which retained MTs but lacked MFs, average mitochondrial velocity increased in both directions, but net directional transport decreased. In vinblastine-treated cells, which lacked MTs but retained essentially normal levels of MFs, mitochondria continued to move bidirectionally but the average mitochondrial velocity and excursion length were reduced for both directions of movement, and the mitochondria spent threefold as much time moving in the retrograde as in the anterograde direction, resulting in net retrograde transport. Treatment of established cultures with both drugs produced neurites lacking MTs and MFs but still rich in neurofilaments; these showed a striking absence of any mitochondrial motility. These data indicate that axonal organelle transport can occur along both MTs and MFs in vivo, but with different velocities and net transport properties.

Glia-to-axon communication: enrichment of glial proteins transferred to the squid giant axon

The transfer of newly synthesized proteins from the glial sheath into the axon is a well-documented process for the squid giant axon. In this study, we used a novel approach to separate the transferred glial proteins (TGPs) from the endogenous axoplasmic proteins of the squid giant axon. Axoplasm, containing radiolabelled TGPs, was extruded as a cylinder and immersed in an intracellular buffer. After 1-30 min, the TGPs were enriched in the intracellular buffer, because they were eluted from the axoplasm into the intracellular buffer much faster than the endogenous axoplasmic proteins. Most of the TGPs enriched in the intracellular buffer did not pellet when centrifuged at 24,000 g for 20 min and were susceptible to protease digestion without the addition of Triton X-100. Additionally, transmission electron microscopic autoradiography of intact axons, containing radiolabelled TGPs, suggested that most TGPs were not associated with vesicular organelles within the axon. We conclude that most of the TGPs are not contained within vesicles in the axoplasm of the squid giant axon, as would be expected if the mechanism of glia-to-axon transfer were conventional exocytosis-endocytosis or microphagocytosis.

Actin filament disassembly is a sufficient final trigger for exocytosis in nonexcitable cells

Although the actin cytoskeleton has been implicated in vesicle trafficking, docking and fusion, its site of action and relation to the Ca(2+)-mediated activation of the docking and fusion machinery have not been elucidated. In this study, we examined the role of actin filaments in regulated exocytosis by introducing highly specific actin monomer-binding proteins, the beta-thymosins or a gelsolin fragment, into streptolysin O-permeabilized pancreatic acinar cells. These proteins had stimulatory and inhibitory effects. Low concentrations elicited rapid and robust exocytosis with a profile comparable to the initial phase of regulated exocytosis, but without raising [Ca2+], and even when [Ca2+] was clamped at low levels by EGTA. No additional cofactors were required. Direct visualization and quantitation of actin filaments showed that beta-thymosin, like agonists, induced actin depolymerization at the apical membrane where exocytosis occurs. Blocking actin depolymerization by phalloidin or neutralizing beta-thymosin by complexing with exogenous actin prevented exocytosis. These findings show that the cortical actin network acts as a dominant negative clamp which blocks constitutive exocytosis. In addition, actin filaments also have a positive role. High concentrations of the actin depolymerizing proteins inhibited all phases of exocytosis. The inhibition overrides stimulation by agonists and all downstream effectors tested, suggesting that exocytosis cannot occur without a minimal actin cytoskeletal structure.

Polylysine cross-links axoplasmic neurofilaments into tight bundles

We have used axoplasm from the squid giant axon to investigate the effects of anionic and cationic polypeptides on the mobility and organization of axonal neurofilaments (NFs). Intact cylinders of axoplasm were extruded from squid giant axons into an excess volume of artificial axoplasm solution. In a previous study on the mobility of NFs in extruded axoplasm, we showed that these polymers disperse freely and diffusively into the surrounding solution, thereby expanding the axoplasmic cross-sectional area [Brown and Lasek, 1993: Cell Motil. Cytoskeleton 26:313-324]. In the present study, we found that 83nm-long ("long-chain") polylysine, a synthetic multivalent cationic protein, inhibited the radial expansion of isolated axoplasm and condensed the axoplasm, thereby reducing the cross-sectional area. Equivalent concentrations of a 7nm-long ("short-chain") polylysine did not inhibit the expansion of axoplasm by long-chain polylysine was dependent on the polylysine concentration; condensation of axoplasm was observed at concentrations of 0.01 mg/ml (0.27 microM) or greater. Electron microscopy of the condensed axoplasm showed that the NFs were aligned side-by-side and in parallel in closely-packed bundles. Equivalent concentrations of 91 nm-long ("long-chain") polyglutamate, a synthetic multivalent anionic protein, partially inhibited the expansion of axoplasm but did not cause the NFs to bundle and did not cause the axoplasm to condense. These studies indicate that cationic proteins bind tightly to the highly charged anionic surfaces of NFs and can link them together into compact bundles in a charge-dependent and length-dependent manner. The tightly packed organization of these cross-linked NFs differs from the normal loose organization of NFs in healthy axons. However, tightly bundled NFs are sometimes found in certain neuropathologies, such as giant axonal neuropathy.

Movement of axoplasmic organelles on actin filaments from skeletal muscle

It was recently shown that, in addition to the well-established microtubule-dependent mechanism, fast transport of organelles in squid giant axons also occurs in the presence of actin filaments [Kuznetsov et al., 1992, Nature 356:722-725]. The objectives of this study were to obtain direct evidence of axoplasmic organelle movement on actin filaments and to demonstrate that these organelles are able to move on skeletal muscle actin filaments. Organelles and actin filaments were visualized by video-enhanced contrast differential interference contrast (AVEC-DIC) microscopy and by video intensified fluorescence microscopy. Actin filaments, prepared by polymerization of monomeric actin purified from rabbit skeletal muscle, were stabilized with rhodamine-phalloidin and adsorbed to cover slips. When axoplasm was extruded on these cover slips in the buffer containing cytochalasin B that prevents the formation of endogenous axonal actin filaments, organelles were observed to move at the fast transport rate. Also, axoplasmic organelles were observed to move on bundles of actin filaments that were of sufficient thickness to be detected directly by AVEC-DIC microscopy. The range of average velocities of movement on the muscle actin filaments was not statistically different from that on axonal filaments. The level of motile activity (number of organelles moving/min/field) on the exogenous filaments was less than on endogenous filaments probably due to the entanglement of filaments on the cover slip surface. We also found that calmodulin (CaM) increased the level of motile activity of organelles on actin filaments. In addition, CaM stimulated the movement of elongated membranous organelles that appeared to be tubular elements of smooth endoplasmic reticulum or extensions of prelysosomes. These studies provide the first direct evidence that organelles from higher animal cells such as neurons move on biochemically defined actin filaments.

The actin-binding protein comitin (p24) is a component of the Golgi apparatus

Comitin (p24) was first identified in Dictyostelium discoideum as a membrane-associated protein which binds in gel overlay assays to G and F actin. To analyze its actin-binding properties we used purified, bacterially expressed comitin and found that it binds to F actin in spin down experiments and increases the viscosity of F actin solutions even under high-salt conditions. Immunofluorescence studies, cell fractionation experiments and EM studies of vesicles precipitated with comitin-specific monoclonal antibodies showed that comitin was present in D. discoideum on: (a) a perinuclear structure with tubular or fibrillary extensions; and (b) on vesicles distributed throughout the cell. In immunofluorescence experiments using comitin antibodies NIH 3T3 fibroblasts showed a similar staining pattern as D. discoideum cells. Using bona fide Golgi markers the perinuclear structure was identified as the Golgi apparatus. The results were supported by an electron microscopic study using cryosections. Based on these data we propose that also in Dictyostelium the stained perinuclear structure is the Golgi apparatus. In vivo the perinuclear structure was found to be attached to the actin and the microtubule network. Alteration of the actin network or depolymerization of the microtubules led to its dispersal into vesicles distributed throughout the cell. These results suggest that the Golgi apparatus in D. discoideum is connected to the actin network by comitin. This protein seems also to be present in mammalian cells.

Glycation of brain actin in experimental diabetes

Actin is a neuronal protein involved in axonal transport and nerve regeneration, both of which are known to be impaired in experimental diabetes. To determine if actin is subject to glycation, we rendered rats diabetic by injection of streptozotocin. Two or 6 weeks later brains were removed and a preparation of cytoskeletal proteins was analyzed by two-dimensional polyacrylamide gel electrophoresis. Brains from diabetic animals contained an extra polypeptide that migrated close to actin and reacted with monoclonal antibody C4 against actin. It was also found in a preparation of soluble synaptic proteins from diabetic rat brain, indicating that it was at least partly neuronal in origin. This polypeptide could be produced by incubation of cytoskeletal proteins from brains of nondiabetic rats with glucose-6-phosphate in vitro. The appearance of this glycated actin in diabetic animals was prevented by administration of insulin for a period of 6 weeks. We could not detect any effect of glycation in vitro on the ability of muscle G-actin to form F-actin filaments and its significance for the function of actin remains to be determined. The finding that glycation of platelet-derived actin from diabetic patients was significantly increased implies that the abnormality may also occur in clinical diabetes.

Myosin-like protein (M(r) 175,000) in Gregarina blaberae

A myosin-like protein (M(r) 175,000) was detected in the parasitic protozoan Gregarina blaberae, by both immunofluorescence and immunoblotting of one- and two-dimensional electrophoresis gels using anti-myosin antibodies. This protein was present in the trophozoite ghost but not in the cytoplasmic extract, nor in extract from the sexual stage, suggesting a protein-stage-dependent expression. The protein tightly bound to the cortical membranes was insoluble at low ionic strength, or in detergent solutions, but could be extracted from Gregarina ghosts by 6 M urea in high ionic strength solution (0.5 M NaCl) and in the presence of reducing agents (20 mM DTT). The protein was localized by indirect immunofluorescence in the cortex of the epimerite, in the fibrillar disc (the so-called septum) separating the proto- and the deutomerite segments, in the contractile ring or sphincter at the top of the protomerite, and as longitudinal lines underlying the G. blaberae epicyte folds. The presence of both actin-like and myosin-like proteins would be consistent with a role in gliding and other cell motility processes of this parasite.

G-protein effects on retrograde axonal transport

Movements of medium and large sized membranous organelles (0.5-1 microns in diameter) were visualized within segments of the crab walking leg nerve with Nomarski differential interference contrast optics and subjected to video contrast enhancement. Accessibility to the axoplasm was demonstrated by intra-axonal fluorescence following addition of rhodamine conjugated to 40 kDa dextran to the external medium. Perfusion of the axons with a 1 microM solution of the 20 kDa G-protein, cp20, but not control solutions, reduced the number of organelles moving in the retrograde direction per unit time, but not the number of organelles moving in the anterograde direction. Such alteration of organelle movement may contribute to memory-specific changes of neuronal morphology.

The regulation of bidirectional mitochondrial transport is coordinated with axonal outgrowth

Although small molecules such as ATP diffuse freely in the cytosol, many types of cells nonetheless position their mitochondria in regions of intense ATP consumption. We reasoned that in the highly elongated axonal processes of growing neurons in culture, the active growth cone would form a focus of ATP consumption so distant from the cell body as to require the positioning of mitochondria nearby via regulated axonal transport. To test this hypothesis, we quantified the distribution and transport behavior of mitochondria in live, aerobically respiring chick sympathetic neurons. We found that in the distal region of actively growing axons, the distribution of mitochondria was highly skewed toward the growth cone, with a sevenfold higher density in the region immediately adjacent to the growth cone than in the region 100 microns away. When axonal outgrowth was blocked by substratum-associated barriers or mild cytochalasin E treatment, the gradient of mitochondrial distribution collapsed as mitochondria exited retrogradely from the distal region, becoming uniformly distributed along the axon within one hour. Analysis of individual mitochondrial behaviors revealed that mitochondrial movement everywhere was bidirectional but balanced so that net transport was anterograde in growing axons and retrograde in blocked axons. This reversal in net transport derived from two separate modulations of mitochondrial movement. First, moving mitochondria underwent a transition to a persistently stationary state in the region of active growth cones that was reversed when growth cone activity was halted. Second, the fraction of time that mitochondria spent moving anterogradely was sharply reduced in non-growing axons. Together, these could account for the formation of gradients of mitochondria in growing axons and their dissipation when outgrowth was blocked. This regulated transport behavior was not dependent upon the ability of mitochondria to produce ATP. Our data indicate that mitochondria possess distinct motor activities for both directions of movement and that mitochondrial transport in axons is regulated by both recruitment between stationary and moving states, and direct regulation of the anterograde motor.

Motility of intracellular particles in rat fibroblasts is greatly enhanced by phorbol ester and by over-expression of normal p21N-ras

Particle motility in cultured rat fibroblasts was studied using video-enhanced differential interference contrast microscopy. The average velocity of large bright particles (apparent diameter about 0.5-0.7 micron) was measured in control cells and in cells treated with agents which affected targets related to signal transduction pathways. A Rat-2-derived fibroblast line transfected with a construct containing multiple copies of the N-ras proto-oncogene under the control of dexamethasone-sensitive promoter was used as a main experimental model. Dexamethasone treatment was shown to induce high levels of N-ras expression in these cells. This treatment greatly increased the average particle velocity. At the same time dexamethasone did not influence the particle motility in the non-transfected parent cells and in the cells transfected with a construct which did not contain N-ras. Phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), also induced an approximate eightfold increase in the particle rate after several hours of incubation, while sphingosine, an inhibitor of PKC, prevented this activation. Sphingosine alone reduced the particle motility after a 20 min incubation. The particle movements were inhibited also by colcemid. These data show that the activation of N-ras and PKC produced dramatic activation of microtubule-dependent particle motility. A possible role of this activation in signal-induced alterations of cell morphology is discussed.

Neurofilaments move apart freely when released from the circumferential constraint of the axonal plasma membrane

Squid giant axons were used to obtain axonal cytoskeletons that had been separated from the confines of their plasma membranes. To remove the plasma membrane, axoplasm was extruded from the giant axon directly into an artificial axoplasm solution (AAS). This procedure produces a smooth axoplasmic cylinder in which neurofilaments (NFs) are the most prevalent cytological elements. The NFs scatter light strongly and thus dark-field light microscopy can be used to quantify the volume occupied by these polymers. Measurements of the widths of the dark-field images of the axoplasmic cylinders showed that the cross-sectional area of the NF population increased by 60-110% (n = 8) between 1-100 min after plasma membrane removal, and then continued to increase more slowly for many hours. After 1,000 min, the cross-sectional area was 75-160% (n = 8) larger than at 1 min. These light microscopic measurements of axoplasm suggest that the NF population disperses to occupy a continuously increasing volume after removal of the plasma membrane and immersion in AAS. This inference was confirmed by quantitative ultrastructural studies of NFs in axoplasmic cross-sections, which demonstrated that the spacing between the NFs increased between 1-1,000 min after plasma membrane removal. Comparison of the NF density distribution after 1,000 min with a theoretical distribution calculated using the Poisson theorem indicated that the NFs dispersed randomly. These studies on NFs in isolated axoplasm suggest that ordinary thermal forces of Brownian motion are sufficient to move axonal NFs apart independently and thereby to disperse them. We propose that, in the intact axon, the dispersive movements of the NFs spread the NF cytoskeleton radially and expansively to fill out the cylindrical space contained by the axonal plasma membrane and its surrounding connective tissue elements.

Motors for fast axonal transport

In neurons and other animal cells, membrane-bound vesicles course rapidly along cytoskeletal filaments to reach their destinations. Based on a variety of in vivo studies it is becoming clear that the microtubule-based motor, kinesin (and its relatives), drive vesicle movements in axons. Surprisingly, some axonal membranes have the capacity to move on both microtubules and actin filaments.

Axoplasmic transport of horseradish peroxidase in single neurons of the dorsal root ganglion studied in vitro by microinjection

The dependence of anterograde axoplasmic transport on cytoskeletal components was investigated using microinjection of horseradish peroxidase (HRP) into the somata of chick dorsal root ganglion cells in vitro. Microinjected HRP was transported anterogradely in the neurites and their branches; this transport was disturbed by colchicine in a drug-dependent and time-dependent manner. Cytochalasin B, a drug that depolymerizes actin, did not inhibit the transport of HRP, despite the formation of local swellings in neurites. The microinjection of polyclonal antibodies directed against tubulin and monoclonal antibodies (mAbs) against 200-kDa neurofilaments disturbed the axoplasmic transport of co-injected HRP, which then exhibited an irregular and discontinuous distribution in the axonal branches. The transport of HRP became discontinuous after the injection of anti-tubulin antibodies and led to the formation of globular deposits of HRP. Polyclonal antibodies against actin and mAbs to 160-kDa and 68-kDa neurofilaments seemed to have no effect on the axoplasmic transport of co-injected HRP. Microinjection of antibodies against tubulin induced formation of perinuclear bundles consisting of cytoskeletal components. The transport of HRP thus appears to be regulated by an intact microtubular system and cross-linker components (200-kDa neurofilaments) of the cytoskeleton. Actin and most intermediate filament proteins do not seem to play an essential role in the transport of HRP.

Molecular motors in axonal transport. Cellular and molecular biology of kinesin

Neurons require a large amount of intracellular transport. Cytoplasmic polypeptides and membrane-bounded organelles move from the perikaryon, down the length of the axon, and to the synaptic terminals. This movement occurs at distinct rates and is termed axonal transport. Axonal transport is divided into the slow transport of cytoplasmic proteins including glycolytic enzymes and cytoskeletal structures and the fast transport of membrane-bounded organelles along linear arrays of microtubules. The polypeptide compositions of the rate classes of axonal transport have been well characterized, but the underlying molecular mechanisms of this movement are less clear. Progress has been particularly slow toward understanding force-generation in slow transport, but recent developments have provided insight into the molecular motors involved in fast axonal transport. Recent advances in the cellular and molecular biology of one fast axonal transport motor, kinesin, have provided a clearer understanding of organelle movement along microtubules. The availability of cellular and molecular probes for kinesin and other putative axonal transport motors have led to a reevaluation of our understanding of intracellular motility.

Posttranslational modifications of nerve cytoskeletal proteins in experimental diabetes

Axonal transport is known to be impaired in peripheral nerve of experimentally diabetic rats. As axonal transport is dependent on the integrity of the neuronal cytoskeleton, we have studied the way in which rat brain and nerve cytoskeletal proteins are altered in experimental diabetes. Rats were made diabetic by injection of streptozotocin (STZ). Up to six weeks later, sciatic nerves, spinal cords, and brains were removed and used to prepare neurofilaments, microtubules, and a crude preparation of cytoskeletal proteins. The extent of nonenzymatic glycation of brain microtubule proteins and peripheral nerve tubulin was assessed by incubation with 3H-sodium borohydride followed by separation on two-dimensional polyacrylamide gels and affinity chromatography of the separated proteins. There was no difference in the nonenzymatic glycation of brain microtubule proteins from two-week diabetic and nondiabetic rats. Nor was the assembly of microtubule proteins into microtubules affected by the diabetic state. On the other hand, there was a significant increase in nonenzymatic glycation of sciatic nerve tubulin after 2 weeks of diabetes. We also identified an altered electrophoretic mobility of brain actin from a cytoskeletal protein preparation from brain of 2 week and 6 week diabetic rats. An additional novel polypeptide was demonstrated with a slightly more acidic isoelectric point than actin that could be immunostained with anti-actin antibodies. The same polypeptide could be produced by incubation of purified actin with glucose in vitro, thus identifying it as a product of nonenzymatic glycation. These results are discussed in relation to data from a clinical study of diabetic patients in which we identified increased glycation of platelet actin. STZ-diabetes also led to an increase in the phosphorylation of spinal cord neurofilament proteins in vivo during 6 weeks of diabetes. This hyperphosphorylation along with a reduced activity of a neurofilament-associated protein kinase led to a reduced incorporation of 32P into purified neurofilament proteins when they were incubated with 32P-ATP in vitro. Our combined data show a number of posttranslation modifications of neuronal cytoskeletal proteins that may contribute to the altered axonal transport and subsequent nerve dysfunction in experimental diabetes.

Actin-dependent organelle movement in squid axoplasm

Studies of organelle movement in axoplasm extruded from the squid giant axon have led to the basic discoveries of microtubule-dependent organelle motility and the characterization of the microtubule-based motor proteins kinesin and cytoplasmic dynein. Rapid organelle movement in higher animal cells, especially in neurons, is considered to be microtubule-based. The role of actin filaments, which are also abundant in axonal cytoplasm, has remained unclear. The inhibition of organelle movement in axoplasm by actin-binding proteins such as DNase I, gelsolin and synapsin I has been attributed to their ability to disorganize the microtubule domains where most of the actin-filaments are located. Here we provide evidence of a new type of organelle movement in squid axoplasm which is independent of both microtubules and microtubule-based motors. This movement is ATP-dependent, unidirectional, actin-dependent, and probably generated by a myosin-like motor. These results demonstrate that an actomyosin-like mechanism can be directly involved in the generation of rapid organelle transport in nerve cells.