Plasma membrane recycling and flow in growing neurites

Down Syndrome Fetal Fibroblasts Display Alterations of Endosomal Trafficking Possibly due to SYNJ1 Overexpression

Endosomal trafficking is essential for cellular homeostasis. At the crossroads of distinct intracellular pathways, the endolysosomal system is crucial to maintain critical functions and adapt to the environment. Alterations of endosomal compartments were observed in cells from adult individuals with Down syndrome (DS), suggesting that the dysfunction of the endosomal pathway may contribute to the pathogenesis of DS. However, the nature and the degree of impairment, as well as the timing of onset, remain elusive. Here, by applying imaging and biochemical approaches, we demonstrate that the structure and dynamics of early endosomes are altered in DS cells. Furthermore, we found that recycling trafficking is markedly compromised in these cells. Remarkably, our results in 18–20 week-old human fetal fibroblasts indicate that alterations in the endolysosomal pathway are already present early in development. In addition, we show that overexpression of the polyphosphoinositide phosphatase synaptojanin 1 (Synj1) recapitulates the alterations observed in DS cells, suggesting a role for this lipid phosphatase in the pathogenesis of DS, likely already early in disease development. Overall, these data strengthen the link between the endolysosomal pathway and DS, highlighting a dangerous liaison among Synj1, endosomal trafficking and DS.

Mitochondrial and Peroxisomal Alterations Contribute to Energy Dysmetabolism in Riboflavin Transporter Deficiency

Riboflavin transporter deficiency (RTD) is a childhood-onset neurodegenerative disorder characterized by progressive pontobulbar palsy, sensory and motor neuron degeneration, sensorineural hearing loss, and optic atrophy. As riboflavin (RF) is the precursor of FAD and FMN, we hypothesize that both mitochondrial and peroxisomal energy metabolism pathways involving flavoproteins could be directly affected in RTD, thus impacting cellular redox status. In the present work, we used induced pluripotent stem cells (iPSCs) from RTD patients to investigate morphofunctional features, focusing on mitochondrial and peroxisomal compartments. Using this model, we document the following RTD-associated alterations: (i) abnormal colony-forming ability and loss of cell-cell contacts, revealed by light, electron, and confocal microscopy, using tight junction marker ZO-1; (ii) mitochondrial ultrastructural abnormalities, involving shape, number, and intracellular distribution of the organelles, as assessed by focused ion beam/scanning electron microscopy (FIB/SEM); (iii) redox imbalance, with high levels of superoxide anion, as assessed by MitoSOX assay accompanied by abnormal mitochondrial polarization state, evaluated by JC-1 staining; (iv) altered immunofluorescence expression of antioxidant systems, namely, glutathione, superoxide dismutase 1 and 2, and catalase, as assessed by quantitatively evaluated confocal microscopy; and (v) peroxisomal downregulation, as demonstrated by levels and distribution of fatty acyl β-oxidation enzymes. RF supplementation results in amelioration of cell phenotype and rescue of redox status, which was associated to improved ultrastructural features of mitochondria, thus strongly supporting patient treatment with RF, to restore mitochondrial- and peroxisomal-related aspects of energy dysmetabolism and oxidative stress in RTD syndrome.

ESCRT-II controls retinal axon growth by regulating DCC receptor levels and local protein synthesis

Endocytosis and local protein synthesis (LPS) act coordinately to mediate the chemotropic responses of axons, but the link between these two processes is poorly understood. The endosomal sorting complex required for transport (ESCRT) is a key regulator of cargo sorting in the endocytic pathway, and here we have investigated the role of ESCRT-II, a critical ESCRT component, in Xenopus retinal ganglion cell (RGC) axons. We show that ESCRT-II is present in RGC axonal growth cones (GCs) where it co-localizes with endocytic vesicle GTPases and, unexpectedly, with the Netrin-1 receptor, deleted in colorectal cancer (DCC). ESCRT-II knockdown (KD) decreases endocytosis and, strikingly, reduces DCC in GCs and leads to axon growth and guidance defects. ESCRT-II-depleted axons fail to turn in response to a Netrin-1 gradient in vitro and many axons fail to exit the eye in vivo. These defects, similar to Netrin-1/DCC loss-of-function phenotypes, can be rescued in whole (in vitro) or in part (in vivo) by expressing DCC. In addition, ESCRT-II KD impairs LPS in GCs and live imaging reveals that ESCRT-II transports mRNAs in axons. Collectively, our results show that the ESCRT-II-mediated endocytic pathway regulates both DCC and LPS in the axonal compartment and suggest that ESCRT-II aids gradient sensing in GCs by coupling endocytosis to LPS.

Biofunctionalized carbon nanotubes in neural regeneration: a mini-review

Carbon nanotubes (CNTs) have become an intriguing and promising biomaterial platform for the regeneration and functional recovery of damaged nerve tissues. The unique electrical, structural and mechanical properties, diversity of available surface chemistry and cell-penetrating ability of CNTs have made them useful implantable matrices or carriers for the delivery of therapeutic molecules. Although there are still challenges being faced in the clinical applications of CNTs mainly due to their toxicity, many studies to overcome this issue have been published. Modification of CNTs with chemical groups to ensure their dissolution in aqueous media is one possible solution. Functionalization of CNTs with biologically relevant and effective molecules (biofunctionalization) is also a promising strategy to provide better biocompatibility and selectivity for neural regeneration. Here, we review recent advances in the use of CNTs to promote neural regeneration.

Neuromodulation: selected approaches and challenges

The brain operates through complex interactions in the flow of information and signal processing within neural networks. The 'wiring' of such networks, being neuronal or glial, can physically and/or functionally go rogue in various pathological states. Neuromodulation, as a multidisciplinary venture, attempts to correct such faulty nets. In this review, selected approaches and challenges in neuromodulation are discussed. The use of water-dispersible carbon nanotubes has been proven effective in the modulation of neurite outgrowth in culture and in aiding regeneration after spinal cord injury in vivo. Studying neural circuits using computational biology and analytical engineering approaches brings to light geometrical mapping of dynamics within neural networks, much needed information for stimulation interventions in medical practice. Indeed, sophisticated desynchronization approaches used for brain stimulation have been successful in coaxing 'misfiring' neuronal circuits to resume productive firing patterns in various human disorders. Devices have been developed for the real-time measurement of various neurotransmitters as well as electrical activity in the human brain during electrical deep brain stimulation. Such devices can establish the dynamics of electrochemical changes in the brain during stimulation. With increasing application of nanomaterials in devices for electrical and chemical recording and stimulating in the brain, the era of cellular, and even intracellular, precision neuromodulation will soon be upon us.

Astrogliopathology: could nanotechnology restore aberrant calcium signalling and pathological astroglial remodelling?

Pathology of the brain is caused by the deficiency in tissue homeostasis. As the main homeostatic element of the mammalian nervous system is represented by astrocytes, these glial cells are involved in many, if not all, brain disorders. Diseased astrocytes undergo a variety of morphological and functional changes, including deregulation of calcium dynamics. To rectify undesirable changes in astrocytes and/or neurones that occur in disease, we postulate the future use of nanotechnology-based therapeutics. Carbon nanotubes emerged as one of the most promising advanced nanomaterials for use in neuroprosthesis. Recently, they have been used to affect morpho-functional characteristics of astrocytes. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Identification of a developmentally regulated pathway of membrane retrieval in neuronal growth cones

The growth-cone plasma membrane constantly reconfigures during axon navigation and upon target recognition. The identity and regulation of the membrane pathway(s) participating in remodeling of the growth-cone surface remain elusive. Here, we identify a constitutive, high-capacity plasma-membrane-recycling activity in the axonal growth cones, which is mediated by a novel bulk endocytic pathway that is mechanistically related to macropinocytosis. This pathway generates large compartments at sites of intense actin-based membrane ruffling through the actions of phosphatidylinositol 3-kinase, the small GTPase Rac1 and the pinocytic chaperone Pincher. At early developmental stages, bulk endocytosis is the primary endocytic pathway for rapid retrieval of the growth-cone plasma membrane. At later stages, during the onset of synaptogenesis, an intrinsic program of maturation leads to downregulation of basal bulk endocytosis and the emergence of depolarization-induced synaptic-vesicle exo-endocytosis. We propose that the control of bulk membrane retrieval contributes to the homeostatic regulation of the axonal plasma membrane and to growth-cone remodeling during axonal outgrowth. In addition, we suggest that the downregulation of bulk endocytosis during synaptogenesis might contribute to the preservation of synaptic-vesicle specificity.

Water soluble single-walled carbon nanotubes inhibit stimulated endocytosis in neurons

We report the use of chemically functionalized water soluble single-walled carbon nanotube (SWNT) graft copolymers to inhibit endocytosis. The graft copolymers were prepared by the functionalization of SWNTs with polyethylene glycol. When added to the culturing medium, these functionalized water soluble SWNTs were able to increase the length of various neuronal processes, neurites, as previously reported. Here we have determined that SWNTs are able to block stimulated membrane endocytosis in neurons, which could then explain the previously noted extended neurite length.

Mitochondrial membrane potential in axons increases with local nerve growth factor or semaphorin signaling

Neurons concentrate mitochondria at sites in the cell that have a high demand for ATP and/or calcium buffering. To accomplish this, mitochondrial transport and docking are thought to respond to intracellular signaling pathways. However, the cell might also concentrate mitochondrial function by locally modulating mitochondrial activity. We tested this hypothesis by measuring the membrane potential of individual mitochondria throughout the axons of chick sensory neurons using the dye tetramethylrhodamine methylester (TMRM). We found no difference in the TMRM mitochondrial-to-cytoplasmic fluorescence ratio (F(m)/F(c)) among three functionally distinct regions: axonal branch points, distal axons, and the remaining axon shaft. In addition, we found no difference in F(m)/F(c) among stationary, retrogradely moving, or anterogradely moving mitochondria. However, F(m)/F(c) was significantly higher in the lamellipodia of growth cones, and among a small fraction of mitochondria throughout the axon. To identify possible signals controlling membrane potential, we used beads covalently coupled to survival and guidance cues to provide a local stimulus along the axon shaft. NGF- or semaphorin 3A-coupled beads caused a significant increase in F(m)/F(c) in the immediately adjacent region of axon, and this was diminished in the presence of the PI3 (phosphatidylinositol-3) kinase inhibitor LY294002 [2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one] or the MAP (mitogen-activated protein) kinase inhibitor U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-amino-phenylthio]butadiene), demonstrating that signaling pathways downstream of both ligands affect the DeltaPsi(m) of mitochondria. In addition, general inhibition of receptor tyrosine kinase activity produced a profound global decrease in F(m)/F(c). Thus, two guidance molecules that exert different effects on growth cone motility both elicit local, receptor-mediated increases in membrane potential.

Pursuing a 'turning point' in growth cone research

Growth cones are highly motile structures found at the leading edge of developing and regenerating nerve processes. Their role in axonal pathfinding has been well established and many guidance cues that influence growth cone behavior have now been identified. Many studies are now providing insights into the transduction and integration of signals in the growth cone, though a full understanding of growth cone behavior still eludes us. This review focuses on recent studies adding to the growing body of literature on growth cone behavior, focusing particularly on the level of autonomy the growth cone possesses and the role of local protein synthesis.

Arp2/3 complex is important for filopodia formation, growth cone motility, and neuritogenesis in neuronal cells

A role of Arp2/3 complex in lamellipodia is well established, whereas its roles in filopodia formation remain obscure. We addressed this question in neuronal cells, in which motility is heavily based on filopodia, and we found that Arp2/3 complex is involved in generation of both lamellipodia and filopodia in growth cones, and in neuritogenesis, the processes thought to occur largely in Arp2/3 complex-independent manner. Depletion of Arp2/3 complex in primary neurons and neuroblastoma cells by small interfering RNA significantly decreased the F-actin contents and inhibited lamellipodial protrusion and retrograde flow in growth cones, but also initiation and dynamics of filopodia. Using electron microscopy, immunochemistry, and gene expression, we demonstrated the presence of the Arp2/3 complex-dependent dendritic network of actin filaments in growth cones, and we showed that individual actin filaments in filopodia originated at Arp2/3 complex-dependent branch points in lamellipodia, thus providing a mechanistic explanation of Arp2/3 complex functions during filopodia formation. Additionally, Arp2/3 complex depletion led to formation of multiple neurites, erratic pattern of neurite extension, and excessive formation of stress fibers and focal adhesions. Consistent with this phenotype, RhoA activity was increased in Arp2/3 complex-depleted cells, indicating that besides nucleating actin filaments, Arp2/3 complex may influence cell motility by altering Rho GTPase signaling.

AP180 and CALM in the developing hippocampus: expression at the nascent synapse and localization to trafficking organelles

Genetic and biochemical evidence has established that clathrin assembly protein AP180 is required for the proper assembly of synaptic vesicles via clathrin-mediated endocytosis. The assembly protein CALM, the ubiquitously expressed homolog of AP180, also regulates the formation of clathrin-coated vesicles. In this study we found high expression levels of AP180 and CALM in hippocampal tissues as early as embryonic day 18, before the expression of synaptophysin. We also used immunoelectron microscopy to establish the distribution of AP180 and CALM in the developing hippocampal synapses. We found AP180 and CALM in synapses at all developmental stages and in nonsynaptic growing processes. In addition to localization on the plasma membrane and clathrin-coated vesicles that originated from the plasma membrane, we also report the presence of AP180 and CALM on other types of membrane structures. Our observations link AP180 and CALM to multiple vesicular organelles and raise the possibility that these proteins may play additional roles in developing neurons.

β-amyloid (25–35) enhances lipid metabolism and protein ubiquitination in cultured neurons

We investigated the effect of beta-amyloid (Abeta) (25-35), a cytotoxic fragment of Abeta peptide, on lipid metabolism and protein ubiquitination in cultured rat hippocampal neurons. After treatment with Abeta under conditions leading to apoptotis, as assessed by caspase activity assay, the total cell mass of lipids changed following a biphasic behavior, with an increase that reached a maximum after 16 hr of treatment, followed by a decrease. The increase at 16 hr was 15.3% in the case of phospholipids and 103.0% in the case of gangliosides and was due to enhanced biosynthesis as confirmed by increase of radioactivity incorporation (phospholipids +52.0%, gangliosides +193.1%) in cells fed with tritiated palmitic acid. No change with respect to cholesterol was observed. Strikingly, under these conditions, the ubiquitination state of cell proteins strongly increased. These effects were not observed with the (35-25) reverse sequence peptide. Similarly to Abeta, lactacystin treatment increased lipid synthesis and protein ubiquitination; only lactacystin, and not Abeta, induced a strong decrease of proteasome chimotrypsin activity. These results suggest that Abeta enhances protein ubiquitination, without inhibiting proteasomal activity, and lipid synthesis. These results may shed new light on the mechanisms of Abeta toxicity.

Signaling mechanisms underlying Slit2-induced collapse of Xenopus retinal growth cones

Slits mediate multiple axon guidance decisions, but the mechanisms underlying the responses of growth cones to these cues remain poorly defined. We show here that collapse induced by Slit2-conditioned medium (Slit2-CM) in Xenopus retinal growth cones requires local protein synthesis (PS) and endocytosis. Slit2-CM elicits rapid activation of translation regulators and MAP kinases in growth cones, and inhibition of MAPKs or disruption of heparan sulfate blocks Slit2-CM-induced PS and repulsion. Interestingly, Slit2-CM causes a fast PS-dependent decrease in cytoskeletal F-actin concomitant with a PS-dependent increase in the actin-depolymerizing protein cofilin. Our findings reveal an unexpected link between Slit2 and cofilin in growth cones and suggest that local translation of actin regulatory proteins contributes to repulsion.

Differential appearance of dynamin in constitutive and regulated exo-endocytosis: a single-cell multiplex RT-PCR study

Neurons in the central nervous system establish, via their axons and dendrites, an extended network that allows synaptic transmission. During developmental maturation and process outgrowth, membrane turnover is necessary for the enlargement and subsequent growth of axons and dendrites from the perikarya to the target cell (constitutive exocytosis/endocytosis). After targeting and synapse formation, small synaptic vesicles are needed for the quantal release of neurotransmitters from the presynaptic terminal with subsequent recycling by regulated exocytosis/endocytosis. An investigation of the onset of the appearance of mRNA and protein in dissociated cultures of neurons from mouse hippocampus or from chick retina has shown an early abundance of proteins involved in exocytosis, such as syntaxin 1, SNAP-25, and synaptotagmin 1, whereas dynamin 1, a protein necessary for clathrin-mediated endocytosis, can be detected only after neurons have established contacts with neighboring cells. The results reveal that constitutive membrane incorporation and regulated synaptic transmitter release is mediated by the same neuronal proteins. Moreover, the data exclude that dynamin 1 takes part in constitutive recycling before synapse formation, but dynamin 2 is present at this stage. Thus, dynamin 2 may be the constitutive counterpart of dynamin 1 in growing neurons. Synapse establishment is linked to an upregulation of dynamin 1 and thereby represents the beginning of the regulated recycling of membranes back into the presynaptic terminal.

The development of a long, coiled, optic nerve in the stalk-eyed fly Cyrtodiopsis whitei

In the stalk-eyed fly Cyrtodiopsis whitei (Diopsidae; Diptera), the relatively long optic nerve develops within the tight lumen of a very short eyestalk. Axonal growth is generally considered in terms of path finding, selective fasciculation, and towing. Physical forces that are necessary for axon lengthening are generated either by the growth cone or by the growth of surrounding tissues. Therefore, it is surprising to encounter a loosely coiled nerve apparently lacking any attachments that could allow for pull, or towing, of the nerve. In this study, we used histological sections and whole-mount preparations to confirm that the optic nerve of the stalk-eyed fly indeed elongates without the external application of tension to the nerve. Secondly, we examined the distribution of cytoskeletal elements and selected proteins that may be involved in axon extension. Staining against the vesicle fusion proteins SNAP-24 and SNAP-25 consistently results in stronger staining in the rapidly extending optic nerve than in a control nerve, suggesting a possible role of these proteins in the extension process. On a gross morphological level, SNAP-24/25 as well as the cytoskeletal elements actin and tubulin are uniformly distributed throughout the lengths of the growing nerve, suggesting that nerve elongation is distributed rather than localized. Finally, we identified glia as a possible source for tension within the nerve bundle. Glia proliferate rapidly in the optic nerve but not in the control nerve. Much work continues to focus on the growth of axons in culture, but this study is one of the few that considers the dynamics of nerve bundle extension as a whole.

Fate of constitutive endocytic vesicles formed in the growth cone: transport of vesicles from one growth cone to another in the same neuron

A developing neuron must have multiple paths of communication coordinating events among all its parts. One of these is the transport to the cell body of endocytic vesicles formed in growth cones. In order to observe this at single cell resolution, we developed a technique in which the fluorescent dye FM1-43 was applied to a single growth cone and newly formed constitutive endocytic vesicles were labeled. Using low light, time-lapse microscopy we were able to follow the movement of these vesicles throughout the neuron. The vast majority of the transported vesicles went to the cell body. However, many were observed to enter secondary neurites and to be transported to other growth cones. These new, more direct paths of transport that link the multiple growth cones of a neuron may play a role in several important developmental events involving interactions between the multiple neurites of a single neuron.

Trafficking of cholesterol from cell bodies to distal axons in Niemann Pick C1-deficient neurons

Niemann Pick type C (NPC) disease is a progressive neurodegenerative disorder. In cells lacking functional NPC1 protein, endocytosed cholesterol accumulates in late endosomes/lysosomes. We utilized primary neuronal cultures in which cell bodies and distal axons reside in separate compartments to investigate the requirement of NPC1 protein for transport of cholesterol from cell bodies to distal axons. We have recently observed that in NPC1-deficient neurons compared with wild-type neurons, cholesterol accumulates in cell bodies but is reduced in distal axons (Karten, B., Vance, D. E., Campenot, R. B., and Vance, J. E. (2002) J. Neurochem. 83, 1154-1163). We now show that NPC1 protein is expressed in both cell bodies and distal axons. In NPC1-deficient neurons, cholesterol delivered to cell bodies from low density lipoproteins (LDLs), high density lipoproteins, or cyclodextrin complexes was transported into axons in normal amounts, whereas transport of endogenously synthesized cholesterol was impaired. Inhibition of cholesterol synthesis with pravastatin in wild-type and NPC1-deficient neurons reduced axonal growth. However, LDLs restored a normal rate of growth to wild-type but not NPC1-deficient neurons treated with pravastatin. Thus, although LDL cholesterol is transported into axons of NPC1-deficient neurons, this source of cholesterol does not sustain normal axonal growth. Over the lifespan of NPC1-deficient neurons, these defects in cholesterol transport might be responsible for the observed altered distribution of cholesterol between cell bodies and axons and, consequently, might contribute to the neurological dysfunction in NPC disease.

Chemotropic responses of retinal growth cones mediated by rapid local protein synthesis and degradation

Growth cones contain mRNAs, translation machinery, and, as we report here, protein degradation machinery. We show that isolated retinal growth cones immediately lose their ability to turn in a chemotropic gradient of netrin-1 or Sema3A when translation is inhibited. Translation inhibition also prevents Sema3A-induced collapse, while LPA-induced collapse is not affected. Inhibition of proteasome function blocks responses to netrin-1 and LPA but does not affect Sema3A responses. We further demonstrate in isolated growth cones that netrin-1 and Sema3A activate translation initiation factors and stimulate a marked rise in protein synthesis within minutes, while netrin-1 and LPA elicit similar rises in ubiquitin-protein conjugates. These results suggest that guidance molecules steer axon growth by triggering rapid local changes in protein levels in growth cones.

Eye stalks or no eye stalks: a structural comparison of pupal development in the stalk-eyed fly Cyrtodiopsis and in Drosophila

After emergence from the puparium, stalk-eyed flies of the family Diopsidae rapidly expand their head capsule so that the eyes and optic lobes are displaced at the ends of stalks that extend from the central head. Because the expansion takes place in only 15 minutes, we are especially interested in ontogenetic modifications that may facilitate such a rapid and dramatic change. To examine the pupal development of the brain, we used Bodian staining in the stalk-eyed fly, Cyrtodiopsis whitei and compared it with development in the fruit fly, Drosophila melanogaster, which serves as a "typical" dipteran example without eye stalks. Early in pupal development, the neuropil organization of the two species is fairly similar. In both species, columns are present in the outer medulla and giant fibers are discernible in the lobula plate. In contrast to D. melanogaster, C. whitei shows a small, neck-like constriction between the optic lobes and the rest of the brain. By 20% of pupal development, the divergence is more apparent, and by 30%, the future eye stalk and optic nerve of C. whitei has started to form. During the remaining 70% of development, the initially thick optic nerve narrows, and becomes gradually elongated, eventually coiling and folding throughout the short eye stalk. Similarly, the cuticle of the surrounding region becomes constricted, slightly elongated, and gradually appears more and more densely corrugated, like an accordion bellows. However, except for the formation of the optic nerve, the dense aggregation of cuticle around it, and a shift in orientation of the neuropils, the developmental programs of the two species are remarkably similar. This suggests that only a few aspects of development have been modified during the course of evolution to generate the stalk-eyed phenotype. At eclosion, the imago of C. whitei goes through a pumping process to inflate the eye stalks to their full length. Measurements of the diameter of the optic nerve before and after the expansion reveal only a small decrease. We propose that the cuticular folding of the eye stalk as well as the coiling of the optic nerve prepare the pupa well for the rapid and dramatic eye-stalk inflation after eclosion.