Real-time imaging of the dynamics of secretory granules in growth cones

Axonal Transport of Lysosomes Is Unaffected in Glucocerebrosidase-Inhibited iPSC-Derived Forebrain Neurons

Lysosomes are acidic organelles that traffic throughout neurons delivering catabolic enzymes to distal regions of the cell and maintaining degradative demands. Loss of function mutations in the gene GBA encoding the lysosomal enzyme glucocerebrosidase (GCase) cause the lysosomal storage disorder Gaucher's disease (GD) and are the most common genetic risk factor for synucleinopathies like Parkinson's disease (PD) and dementia with Lewy bodies (DLB). GCase degrades the membrane lipid glucosylceramide (GlcCer) and mutations in GBA, or inhibiting its activity, results in the accumulation of GlcCer and disturbs the composition of the lysosomal membrane. The lysosomal membrane serves as the platform to which intracellular trafficking complexes are recruited and activated. Here, we investigated whether lysosomal trafficking in axons was altered by inhibition of GCase with the pharmacological agent Conduritol B Epoxide (CBE). Using live cell imaging in human male induced pluripotent human stem cell (iPSC)-derived forebrain neurons, we demonstrated that lysosomal transport was similar in both control and CBE-treated neurons. Furthermore, we tested whether lysosomal rupture, a process implicated in various neurodegenerative disorders, was affected by inhibition of GCase. Using L-leucyl-L-leucine methyl ester (LLoME) to induce lysosomal membrane damage and immunocytochemical staining for markers of lysosomal rupture, we found no difference in susceptibility to rupture between control and CBE-treated neurons. These results suggest the loss of GCase activity does not contribute to neurodegenerative disease by disrupting either lysosomal transport or rupture.

Intranasal Paclitaxel Alters Alzheimer's Disease Phenotypic Features in 3xTg-AD Mice

BACKGROUND

Microtubule stabilizing drugs, commonly used as anti-cancer therapeutics, have been proposed for treatment of Alzheimer's disease (AD); however, many do not cross the blood-brain barrier.

OBJECTIVE

This research investigated if paclitaxel (PTX) delivered via the intranasal (IN) route could alter the phenotypic progression of AD in 3xTg-AD mice.

METHODS

We administered intranasal PTX in 3XTg-AD mice (3xTg-AD n = 15, 10 weeks and n = 10, 44 weeks, PTX: 0.6 mg/kg or 0.9%saline (SAL)) at 2-week intervals. After treatment, 3XTg-AD mice underwent manganese-enhanced magnetic resonance imaging to measure in vivo axonal transport. In a separate 3XTg-AD cohort, PTX-treated mice were tested in a radial water tread maze at 52 weeks of age after four treatments, and at 72 weeks of age, anxiety was assessed by an elevated-plus maze after 14 total treatments.

RESULTS

PTX increased axonal transport rates in treated 3XTg-AD compared to controls (p≤0.003). Further investigation using an in vitro neuron model of Aβ-induced axonal transport disruption confirmed PTX prevented axonal transport deficits. Confocal microscopy after treatment found fewer phospho-tau containing neurons (5.25±3.8 versus 8.33±2.5, p < 0.04) in the CA1, altered microglia, and reduced reactive astrocytes. PTX improved performance of 3xTg-AD on the water tread maze compared to controls and not significantly different from WT (Day 5, 143.8±43 versus 91.5±77s and Day 12, 138.3±52 versus 107.7±75s for SAL versus PTX). Elevated plus maze revealed that PTX-treated 3xTg-AD mice spent more time exploring open arms (Open arm 129.1±80 versus 20.9±31s for PTX versus SAL, p≤0.05).

CONCLUSION

Taken collectively, these findings indicate that intranasal-administered microtubule-stabilizing drugs may offer a potential therapeutic option for treating AD.

GSK3β Impairs KIF1A Transport in a Cellular Model of Alzheimer's Disease but Does Not Regulate Motor Motility at S402

Impairment of axonal transport is an early pathologic event that precedes neurotoxicity in Alzheimer’s disease (AD). Soluble amyloid-β oligomers (AβOs), a causative agent of AD, activate intracellular signaling cascades that trigger phosphorylation of many target proteins, including tau, resulting in microtubule destabilization and transport impairment. Here, we investigated how KIF1A, a kinesin-3 family motor protein required for the transport of neurotrophic factors, is impaired in mouse hippocampal neurons treated with AβOs. By live cell imaging, we observed that AβOs inhibit transport of KIF1A-GFP similarly in wild-type and tau knock-out neurons, indicating that tau is not required for this effect. Pharmacological inhibition of glycogen synthase kinase 3β (GSK3β), a kinase overactivated in AD, prevented the transport defects. By mass spectrometry on KIF1A immunoprecipitated from transgenic AD mouse brain, we detected phosphorylation at S402, which conforms to a highly conserved GSK3β consensus site. We confirmed that this site is phosphorylated by GSK3β in vitro. Finally, we tested whether a phosphomimic of S402 could modulate KIF1A motility in control and AβO-treated mouse neurons and in a Golgi dispersion assay devoid of endogenous KIF1A. In both systems, transport driven by mutant motors was similar to that of WT motors. In conclusion, GSK3β impairs KIF1A transport but does not regulate motor motility at S402. Further studies are required to determine the specific phosphorylation sites on KIF1A that regulate its cargo binding and/or motility in physiological and disease states.

Stochastic Subcellular Organization of Dense-Core Vesicles Revealed by Point Pattern Analysis

Dense-core vesicles (DCVs) are regulated secretory organelles found in many types of neurons. In neurons of the hippocampus, their cargo includes proteins that mediate several pivotal processes, including differentiation and synaptic plasticity. Motivated by interest in DCV distribution and its impact on cargo action, we have used fluorescence microscopy and statistical analysis to develop a quantitative model of the subcellular organization of DCVs in hippocampal neurons that are spontaneously active (their most prevalent state). We also have tested the functionally motivated hypothesis that these organelles are synaptically enriched. Variance-to-mean ratio, frequency distribution, and Moran's autocorrelation analyses reveal that DCV distribution along shafts, and within synapses, follows Poisson statistics, establishing that stochastically dictated organization sustains cargo function. Occupancy in boutons exceeds that at nearby extrasynaptic axonal sites by approximately threefold, revealing significant local presynaptic enrichment. Widespread stochastic organization is consistent with the emerging functional importance of synaptically and extrasynaptically localized DCVs. Presynaptic enrichment is consistent with the established importance of protecting presynaptic sites from depletion of DCV cargo. These results enhance understanding of the link between DCV organization and mechanisms of cargo action, and they reinforce the emerging theme that randomness is a prevalent aspect of synaptic organization and composition.

Dendritic and axonal mechanisms of Ca2+ elevation impair BDNF transport in Aβ oligomer-treated hippocampal neurons

Intracellular Ca²⁺ dysregulation and transport disruption precede cell death in Alzheimer's disease. Mechanisms of AβO-induced Ca²⁺ elevation are identified that regulate the onset, severity, and spatiotemporal progression of BDNF transport defects. The results challenge dogmatic views on mechanisms of AβO toxicity and subcellular sites of action.

Disruption of fast axonal transport (FAT) and intracellular Ca²⁺ dysregulation are early pathological events in Alzheimer's disease (AD). Amyloid-β oligomers (AβOs), a causative agent of AD, impair transport of BDNF independent of tau by nonexcitotoxic activation of calcineurin (CaN). Ca²⁺-dependent mechanisms that regulate the onset, severity, and spatiotemporal progression of BDNF transport defects from dendritic and axonal AβO binding sites are unknown. Here we show that BDNF transport defects in dendrites and axons are induced simultaneously but exhibit different rates of decline. The spatiotemporal progression of FAT impairment correlates with Ca²⁺ elevation and CaN activation first in dendrites and subsequently in axons. Although many axonal pathologies have been described in AD, studies have primarily focused only on the dendritic effects of AβOs despite compelling reports of presynaptic AβOs in AD models and patients. Indeed, we observe that dendritic CaN activation converges on Ca²⁺ influx through axonal voltage-gated Ca²⁺ channels to impair FAT. Finally, FAT defects are prevented by dantrolene, a clinical compound that reduces Ca²⁺ release from the ER. This work establishes a novel role for Ca²⁺ dysregulation in BDNF transport disruption and tau-independent Aβ toxicity in early AD.

Amyloid-β oligomers induce tau-independent disruption of BDNF axonal transport via calcineurin activation in cultured hippocampal neurons

The role of tau in axonal transport disruption during early-stage Alzheimer disease is controversial. The amyloid-β oligomers markedly impair BDNF transport in primary wild-type and tau-knockout neurons. This occurs by nonexcitotoxic activation of calcineurin, and inhibition of calcineurin rescues transport defects independent of tau.

Disruption of fast axonal transport (FAT) is an early pathological event in Alzheimer's disease (AD). Soluble amyloid-β oligomers (AβOs), increasingly recognized as proximal neurotoxins in AD, impair organelle transport in cultured neurons and transgenic mouse models. AβOs also stimulate hyperphosphorylation of the axonal microtubule-associated protein, tau. However, the role of tau in FAT disruption is controversial. Here we show that AβOs reduce vesicular transport of brain-derived neurotrophic factor (BDNF) in hippocampal neurons from both wild-type and tau-knockout mice, indicating that tau is not required for transport disruption. FAT inhibition is not accompanied by microtubule destabilization or neuronal death. Significantly, inhibition of calcineurin (CaN), a calcium-dependent phosphatase implicated in AD pathogenesis, rescues BDNF transport. Moreover, inhibition of protein phosphatase 1 and glycogen synthase kinase 3β, downstream targets of CaN, prevents BDNF transport defects induced by AβOs. We further show that AβOs induce CaN activation through nonexcitotoxic calcium signaling. Results implicate CaN in FAT regulation and demonstrate that tau is not required for AβO-induced BDNF transport disruption.

The plasminogen activation system and the regulation of catecholaminergic function

The local environment of neurosecretory cells contains the major components of the plasminogen activation system, including the plasminogen activators, tissue plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA), as well as binding sites for t-PA, the receptor for u-PA (uPAR), and also the plasminogen activator inhibitor, PAI-1. Furthermore, these cells express specific binding sites for plasminogen, which is available in the circulation and in interstitial fluid. Colocalization of plasminogen and its activators on cell surfaces provides a mechanism for promoting local plasminogen activation. Plasmin is retained on the cell surface where it is protected from its inhibitor, α₂-antiplasmin. In neurosecretory cells, localized plasmin activity provides a mechanism for extracellular processing of secreted hormones. Neurotransmitter release from catecholaminergic cells is negatively regulated by cleavage products formed by plasmin-mediated proteolysis. Recently, we have identified a major plasminogen receptor, Plg-R_KT. We have found that Plg-R_KT is highly expressed in chromaffin cells of the adrenal medulla as well as in other catecholaminergic cells and tissues. Plg-R_KT-dependent plasminogen activation plays a key role in regulating catecholaminergic neurosecretory cell function.

Hindered submicron mobility and long-term storage of presynaptic dense-core granules revealed by single-particle tracking

Dense-core granules (DCGs) are organelles found in neuroendocrine cells and neurons that house, transport, and release a number of important peptides and proteins. In neurons, DCG cargo can include the secreted neuromodulatory proteins tissue plasminogen activator (tPA) and/or brain-derived neurotrophic factor (BDNF), which play a key role in modulating synaptic efficacy in the hippocampus. This function has spurred interest in DCGs that localize to synaptic contacts between hippocampal neurons, and several studies recently have established that DCGs localize to, and undergo regulated exocytosis from, postsynaptic sites. To complement this work, we have studied presynaptically localized DCGs in hippocampal neurons, which are much more poorly understood than their postsynaptic analogs. Moreover, to enhance relevance, we visualized DCGs via fluorescence labeling of exogenous and endogenous tPA and BDNF. Using single-particle tracking, we determined trajectories of more than 150 presynaptically localized DCGs. These trajectories reveal that mobility of DCGs in presynaptic boutons is highly hindered and that storage is long-lived. We also computed mean-squared displacement curves, which can be used to elucidate mechanisms of transport. Over shorter time windows, most curves are linear, demonstrating that DCG transport in boutons is driven predominantly by diffusion. The remaining curves plateau with time, consistent with motion constrained by a submicron-sized corral. These results have relevance to recent models of presynaptic organization and to recent hypotheses about DCG cargo function. The results also provide estimates for transit times to the presynaptic plasma membrane that are consistent with measured times for onset of neurotrophin release from synaptically localized DCGs.

Expression of the dominant-negative tail of myosin Va enhances exocytosis of large dense core vesicles in neurons

Regulated exocytosis of secretory vesicles is a fundamental process in neurotransmission and the release of hormones and growth factors. The F-actin-binding motor protein myosin Va was recently shown to be involved in exocytosis of peptide-containing large dense core vesicles of neuroendocrine cells. It has not previously been discussed whether it plays a similar role in neurons. We performed live-cell imaging of cultured hippocampal neurons to measure the exocytosis of large dense core vesicles containing fluorescently labelled neuropeptide Y. To address the role of myosin Va in this process, neurons were transfected with the dominant-negative tail domain of myosin Va (myosinVa-tail). Under control conditions, about 0.75% of the labelled large dense core vesicles underwent exocytosis during 5 min of stimulation. This value was doubled to 1.80% of the vesicles when myosinVa-tail was expressed. Depolymerization of F-actin using latrunculin B resulted in a similar increase in exocytosis in both control and myosinVa-tail expressing cells. Interestingly, the increase in exocytosis caused by myosinVa-tail expression was completely abolished in the presence of KN-62, an inhibitor of calcium-calmodulin-dependent kinase II. We suggest that myosinVa-tail causes the liberation of large dense core vesicles from the actin cytoskeleton, leading to an increase in exocytosis in the cultured hippocampal neurons.

Efficient copackaging and cotransport yields postsynaptic colocalization of neuromodulators associated with synaptic plasticity

Recent data suggest that tissue plasminogen activator (tPA) influences long-term plasticity at hippocampal synapses by converting plasminogen into plasmin, which then generates mature brain-derived neurotrophic factor (mBDNF) from its precursor, proBDNF. Motivated by this hypothesis, we used fluorescent chimeras, expressed in hippocampal neurons, to elucidate (1) mechanisms underlying plasminogen secretion from hippocampal neurons, (2) if tPA, plasminogen, and proBDNF are copackaged and cotransported in hippocampal neurons, especially within dendritic spines, and (3) mechanisms mediating the transport of these neuromodulators to sites of release. We find that plasminogen chimeras traffic through the regulated secretory pathway of hippocampal neurons in dense-core granules (DCGs) and that tPA, plasminogen, and proBDNF chimeras are extensively copackaged in DCGs throughout hippocampal neurons. We also find that 80% of spines that contain DCGs contain chimeras of these neuromodulators in the same DCG. Finally, we demonstrate, for the first time, that neuromodulators undergo cotransport along dendrites in rapidly mobile DCGs, indicating that neuromodulators can be efficiently recruited into active spines. These results support the hypothesis that tPA mediates synaptic activation of BDNF by demonstrating that tPA, plasminogen, and proBDNF colocalize in DCGs in spines, where these neuromodulators can undergo activity-dependent release and then interact and/or mediate changes that influence synaptic efficacy. The results also raise the possibility that frequency-dependent changes in extents of neuromodulator release from DCGs influence the direction of plasticity at hippocampal synapses by altering the relative proportions of two proteins, mBDNF and proBDNF, that exert opposing effects on synaptic efficacy.

Different microtubule motors move early and late endocytic compartments

Important progress has been made during the past decade in the identification of molecular motors required in the distribution of early and late endosomes and the proper trafficking along the endocytic pathway. There is little direct evidence, however, that these motors drive movement of the endosomes. To evaluate the contributions of kinesin-1, dynein and kinesin-2 to the movement of early and late endosomes along microtubules, we made use of a cytosol-free motility assay using magnetically isolated early and late endosomes as well as biochemical analyses and live-cell imaging. By making use of specific antibodies, we confirmed that kinesin-1 and dynein move early endosomes and we found that kinesin-2 moves both early and late endosomes in the cell-free assay. Unexpectedly, dynein did not move late endosomes in the cell-free assay. We provide evidence from disruption of dynein function and latrunculin A treatment, suggesting that dynein regulates late endosome movement indirectly, possibly through a mechanism involving the actin cytoskeleton. These data provide new insights into the complex regulation of endosomes' motility and suggest that dynein is not the major motor required to move late endosomes toward the minus end of microtubules.

Activity-dependent release of tissue plasminogen activator from the dendritic spines of hippocampal neurons revealed by live-cell imaging

Tissue plasminogen activator (tPA) has been implicated in a variety of important cellular functions, including learning-related synaptic plasticity and potentiating N-methyl-D-aspartate (NMDA) receptor-dependent signaling. These findings suggest that tPA may localize to, and undergo activity-dependent secretion from, synapses; however, conclusive data supporting these hypotheses have remained elusive. To elucidate these issues, we studied the distribution, dynamics, and depolarization-induced secretion of tPA in hippocampal neurons, using fluorescent chimeras of tPA. We found that tPA resides in dense-core granules (DCGs) that traffic to postsynaptic dendritic spines and that can remain in spines for extended periods. We also found that depolarization induced by high potassium levels elicits a slow, partial exocytotic release of tPA from DCGs in spines that is dependent on extracellular Ca(+2) concentrations. This slow, partial release demonstrates that exocytosis occurs via a mechanism, such as fuse-pinch-linger, that allows partial release and reuse of DCG cargo and suggests a mechanism that hippocampal neurons may rely upon to avoid depleting tPA at active synapses. Our results also demonstrate release of tPA at a site that facilitates interaction with NMDA-type glutamate receptors, and they provide direct confirmation of fundamental hypotheses about tPA localization and release that bear on its neuromodulatory functions, for example, in learning and memory.

Real-time dynamics of label-free single mast cell granules revealed by differential interference contrast microscopy

We demonstrate the capability of differential interference contrast (DIC) microscopy as a simple and useful tool for studying cellular events without fluorescence labeling. By coupling an advanced DIC microscope to a computer-controlled motorized vertical stage and a high-speed, high-resolution CCD camera, real-time three-dimensional monitoring is possible in a high-throughput manner. The performance among three modes of microscopy, bright-field, dark-field and DIC, in terms of horizontal resolving power and vertical sectioning was investigated. As a model, exocytosis of rat peritoneal mast cells was recorded on the subsecond time scale. Three-dimensional tracking of granules during degranulation was achieved and granule-granule fusion before plasma membrane fusion was recorded.

Mechanisms of transport and exocytosis of dense-core granules containing tissue plasminogen activator in developing hippocampal neurons

Dense-core granules (DCGs) are organelles found in specialized secretory cells, including neuroendocrine cells and neurons. Neuronal DCGs facilitate many critical processes, including the transport and secretion of proteins involved in learning, and yet their transport and exocytosis are poorly understood. We have used wide-field and total internal reflection fluorescence microscopy, in conjunction with transport theory, to visualize the transport and exocytosis of DCGs containing a tissue plasminogen activator-green fluorescent protein hybrid in cell bodies, neurites, and growth cones of developing hippocampal neurons and to quantify the roles that diffusion, directed motion, and immobility play in these processes. Our results demonstrate that shorter-ranged transport of DCGs near sites of exocytosis in hippocampal neurons and neuroendocrine cells differs markedly. Specifically, the immobile fraction of DCGs within growth cones and near the plasma membrane of hippocampal neurons is small and relatively unaltered by actin disruption, unlike in neuroendocrine cells. Moreover, transport of DCGs in these domains of hippocampal neurons is unusually heterogeneous, being significantly rapid and directed as well as slow and diffusive. Our results also demonstrate that exocytosis is preceded by substantial movement and heterogeneous transport; this movement may facilitate delivery of DCG cargo in hippocampal neurons, given the relatively low abundance of neuronal DCGs. In addition, the extensive mobility of DCGs in hippocampal neurons argues strongly against the hypothesis that cortical actin is a major barrier to membrane-proximal DCGs in these cells. Instead, our results suggest that extended release of DCG cargo from hippocampal neurons arises from heterogeneity in DCG mobility.

Rab8 regulates the actin-based movement of melanosomes

Rab GTPases have been implicated in the regulation of specific microtubule- and actin-based motor proteins. We devised an in vitro motility assay reconstituting the movement of melanosomes on actin bundles in the presence of ATP to investigate the role of Rab proteins in the actin-dependent movement of melanosomes. Using this assay, we confirmed that Rab27 is required for the actin-dependent movement of melanosomes, and we showed that a second Rab protein, Rab8, also regulates this movement. Rab8 was partially associated with mature melanosomes. Expression of Rab8Q67L perturbed the cellular distribution and increased the frequency of microtubule-independent movement of melanosomes in vivo. Furthermore, anti-Rab8 antibodies decreased the number of melanosomes moving in vitro on actin bundles, whereas melanosomes isolated from cells expressing Rab8Q67L exhibited 70% more movements than wild-type melanosomes. Together, our observations suggest that Rab8 is involved in regulating the actin-dependent movement of melanosomes.

Uncoated endocytic vesicles require the unconventional myosin, Myo6, for rapid transport through actin barriers

After clathrin-mediated endocytosis, clathrin removal yields an uncoated vesicle population primed for fusion with the early endosome. Here we present the first characterization of uncoated vesicles and show that myo6, an unconventional myosin, functions to move these vesicles out of actin-rich regions found in epithelial cells. Time-lapse microscopy revealed that myo6-associated uncoated vesicles were motile and exhibited fusion and stretching events before endosome delivery, processes that were dependent on myo6 motor activity. In the absence of myo6 motor activity, uncoated vesicles remained trapped in the actin mesh, where they exhibited Brownian-like motion. Exit from the actin mesh occurred by a slow diffusion-based mechanism, delaying transferrin trafficking to the early endosome. Expression of a myo6 mutant that bound tightly to F-actin produced immobilized vesicles and blocked trafficking. Depolymerization of the actin cytoskeleton rescued this block and specifically accelerated transferrin delivery to the early endosome without affecting earlier steps in endocytosis. Therefore actin is a physical barrier impeding uncoated vesicle trafficking, and myo6 is recruited to move the vesicles through this barrier for fusion with the early endosome.

Visualization of vesicle transport along and between distinct pathways in neurites of living cells

Trafficking of secretory vesicles along neurites of PC12 cells was visualized by 2D and 3D real-time imaging using fluorescence microscopy. Vesicle motion along distinct pathways was directly seen. From an overlay of individual pathways, the underlying cytoskeletal filament could be imaged at a subwavelength resolution. Continuous vesicle transport was interrupted by periods of diffusive motion with concomitant pathway changes. Statistical analysis shows that such interruptions were distributed stochastically along the filament, indicating a limited processivity of motor proteins also in a cellular context. Periods of diffusive motion facilitated the interaction with actively transported vesicles. Frequent associations and dissociations of vesicles have been observed consistently, pointing to a functional relevance of vesicle cotransport.

Unexpected mobility variation among individual secretory vesicles produces an apparent refractory neuropeptide pool

Most stored neuropeptide cannot be released from nerve terminals suggesting the existence of a refractory pool of dense core vesicles (DCVs). Past fluorescence photobleaching recovery, single particle tracking and release experiments suggested that the refractory neuropeptide pool corresponds to a distinct immobile fraction of cytoplasmic DCVs. However, tracking of hundreds of individual green fluorescent protein-labeled neuropeptidergic vesicles by wide-field or evanescent-wave microscopy shows that a separate immobile fraction is not evident. Instead, the DCV diffusion coefficient (D) distribution is unusually broad and asymmetric. Furthermore, the distribution shifts with a release facilitator. This unexpected variation, which could reflect heterogeneity among vesicles or in their medium, is shown to generate the appearance of a regulated refractory neuropeptide pool.

Physical mobilization of secretory vesicles facilitates neuropeptide release by nerve growth factor-differentiated PC12 cells

It has been speculated that neurosecretion can be enhanced by increasing the motion, and hence, the availability of cytoplasmic secretory vesicles. However, facilitator-induced physical mobilization of secretory vesicles has not been observed directly in living cells, and recent experimental results call this hypothesis into question. Here, high resolution green fluorescent protein (GFP)-based measurements in nerve growth factor-differentiated PC12 cells are used to test whether altering dense core vesicle (DCV) motion affects neuropeptide release. Experiments with mycalolide B and jasplakinolide demonstrate that neuropeptidergic DCV motion at the ends of processes is proportional to F-actin. Furthermore, Ba2+ increases DCV mobility without detectably modifying F-actin. Finally, we show that altering DCV motion by changing F-actin or stimulating with Ba2+ proportionally changes sustained neuropeptide release. Therefore, increasing DCV mobility facilitates prolonged neuropeptide release.

Neuronal calcium sensor-1 binds to regulated secretory organelles and functions in basal and stimulated exocytosis in PC12 cells

Neuronal calcium sensor-1 (NCS-1) and its non-mammalian homologue,frequenin, have been implicated in a spectrum of cellular processes, including regulation of stimulated exocytosis of synaptic vesicles and secretory granules (SGs) in neurons and neuroendocrine cells and regulation of phosphatidylinositol 4-kinase beta activity in yeast. However, apart from these intriguing putative functions, NCS-1 and frequenin are relatively poorly understood. Here, the distribution, dynamics and function of NCS-1 were studied using PC12 cells that stably express NCS-1-EYFP (NCS-1 fused to enhanced yellow fluorescent protein) or that stably overexpress NCS-1. Fluorescence and electron microscopies show that NCS-1-EYFP is absent from SGs but is present on small clear organelles, some of which are just below the plasma membrane. Total internal reflection fluorescence microscopy shows that NCS-1-EYFP is associated with synaptic-like microvesicles (SLMVs) in growth cones. Overexpression studies show that NCS-1 enhances exocytosis of synaptotagmin-labeled regulated secretory organelles (RSOs) under basal conditions and during stimulation by UTP. Significantly, these studies implicate NCS-1 in the enhancement of both basal and stimulated phosphoinositide-dependent exocytosis of RSOs in PC12 cells, and they show that NCS-1 is distributed strategically to interact with putative targets on the plasma membrane and on SLMVs. These studies also reveal that SLMVs undergo both fast directed motion and highly hindered diffusive motion in growth cones, suggesting that cytoskeletal constituents can both facilitate and hinder SLMV motion. These results also reveal interesting similarities and differences between transport organelles in differentiated neuroendocrine cells and neurons.

Nerve growth factor-induced differentiation changes the cellular organization of regulated Peptide release by PC12 cells

PC12 cells, like endocrine chromaffin cells, undergo neuronal-like differentiation in response to nerve growth factor (NGF). Here we report that this phenotype conversion produces major changes in release of a green fluorescent protein-tagged neuropeptide-hormone. First, the spatial distribution of the releasable pool is altered; peptide release from untreated cells is supported predominantly by membrane-proximal vesicles, whereas a diffuse pool at the ends of processes is used by NGF-treated cells. Second, the time course of release evoked by photolysis of caged Ca(2+) is faster after differentiation. High-resolution measurements suggest that a slow step before membrane fusion dominates the kinetics of release in untreated cells. Finally, the effect of actin microfilament depolymerization on total release is altered by NGF treatment. This implies that the mechanism that limits the size of the releasable pool is altered by phenotype conversion. Therefore, the cellular organization of peptide release is plastic and changes in response to NGF. This flexibility may be used to generate cell-specific release properties.

Actin filaments and myosin I alpha cooperate with microtubules for the movement of lysosomes

An earlier report suggested that actin and myosin I alpha (MMIalpha), a myosin associated with endosomes and lysosomes, were involved in the delivery of internalized molecules to lysosomes. To determine whether actin and MMIalpha were involved in the movement of lysosomes, we analyzed by time-lapse video microscopy the dynamic of lysosomes in living mouse hepatoma cells (BWTG3 cells), producing green fluorescent protein actin or a nonfunctional domain of MMIalpha. In GFP-actin cells, lysosomes displayed a combination of rapid long-range directional movements dependent on microtubules, short random movements, and pauses, sometimes on actin filaments. We showed that the inhibition of the dynamics of actin filaments by cytochalasin D increased pauses of lysosomes on actin structures, while depolymerization of actin filaments using latrunculin A increased the mobility of lysosomes but impaired the directionality of their long-range movements. The production of a nonfunctional domain of MMIalpha impaired the intracellular distribution of lysosomes and the directionality of their long-range movements. Altogether, our observations indicate for the first time that both actin filaments and MMIalpha contribute to the movement of lysosomes in cooperation with microtubules and their associated molecular motors.