The ultrastructure of the neuronal growth cone: new insights from subcellular fractionation and rapid freezing studies

Molecular basis of the functions of the mammalian neuronal growth cone revealed using new methods

The neuronal growth cone is a highly motile, specialized structure for extending neuronal processes. This structure is essential for nerve growth, axon pathfinding, and accurate synaptogenesis. Growth cones are important not only during development but also for plasticity-dependent synaptogenesis and neuronal circuit rearrangement following neural injury in the mature brain. However, the molecular details of mammalian growth cone function are poorly understood. This review examines molecular findings on the function of the growth cone as a result of the introduction of novel methods such superresolution microscopy and (phospho)proteomics. These results increase the scope of our understating of the molecular mechanisms of growth cone behavior in the mammalian brain.

Proteomic identification of the molecular basis of mammalian CNS growth cones

The growth cone, which is a unique structure with high motility that forms at the tips of extending axons and dendrites, is crucial to neuronal network formation. Axonal growth of the mammalian CNS is most likely achieved by the complicated coordination of cytoskeletal rearrangement and vesicular trafficking via many proteins. Before recent advances, no methods to identify numerous proteins existed; however, proteomics revolutionarily resolved such problems. In this review, I summarize the profiles of the mammalian growth cone proteins revealed by proteomics as the molecular basis of the growth cone functions, with molecular mapping. These results should be used as a basis for understanding the mechanisms of the complex mammalian CNS developmental process.

Single vesicle imaging indicates distinct modes of rapid membrane retrieval during nerve growth

Background

During nerve growth, cytoplasmic vesicles add new membrane preferentially to the growth cone located at the distal tip of extending axons. Growth cone membrane is also retrieved locally, and asymmetric retrieval facilitates membrane remodeling during growth cone repulsion by a chemorepellent gradient. Moreover, growth inhibitory factors can stimulate bulk membrane retrieval and induce growth cone collapse. Despite these functional insights, the processes mediating local membrane remodeling during axon extension remain poorly defined.

Results

To investigate the spatial and temporal dynamics of membrane retrieval in actively extending growth cones, we have used a transient labeling and optical recording method that can resolve single vesicle events. Live-cell confocal imaging revealed rapid membrane retrieval by distinct endocytic modes based on spatial distribution in Xenopus spinal neuron growth cones. These modes include endocytic "hot-spots" triggered at the base of filopodia, at the lateral margins of lamellipodia, and along dorsal ridges of the growth cone. Additionally, waves of endocytosis were induced when individual filopodia detached from the substrate and fused with the growth cone dorsal surface or with other filopodia. Vesicle formation at sites of membrane remodeling by self-contact required F-actin polymerization. Moreover, bulk membrane retrieval by macroendocytosis correlated positively with the substrate-dependent rate of axon extension and required the function of Rho-family GTPases.

Conclusions

This study provides insight into the dynamic membrane remodeling processes essential for nerve growth by identifying several distinct modes of rapid membrane retrieval in the growth cone during axon extension. We found that endocytic membrane retrieval is intensified at specific subdomains and may drive the dynamic membrane ruffling and re-absorption of filopodia and lamellipodia in actively extending growth cones. The findings offer a platform for determining the molecular mechanisms of distinct endocytic processes that may remodel the surface distribution of receptors, ion channels and other membrane-associated proteins locally to drive growth cone extension and chemotactic guidance.

Development of the corticospinal tract in the mouse spinal cord: A quantitative ultrastructural analysis

The growth of corticospinal tract (CST) axons was studied quantitatively at the 7th cervical (C7) and the 4th lumbar (L4) spinal segments in the balb/cByJ mice at the ages of postnatal day (P) 0, 2, 4, 6, 8, 10, 14, and 28. The cross-sectional area of the CST increased progressively with time. Unmyelinated axons, the most prominent CST element during early development, reached maximum at C7 and L4 on P14. Two phases of increase in the number of unmyelinated axons were observed at C7, while only one surge of axonal outgrowth was found at the L4 level. Pro-myelinated axons, defined as axons surrounded by only one layer of oligodendrocytic process, were first seen at P2 and P4 in the C7 and the L4 level, respectively, followed by a dramatic increase in the number of myelinated axons from P14 onwards at both spinal levels. Myelination of the CST axons occurred topographically in a dorsal-to-ventral pattern. The number of growth cones increased rapidly at the C7 level to reach its maximum at P4, while those at L4 increased steadily to the peak at P10. Growth cones with synapse-like junctions were occasionally observed in the growing CST. Degenerating axons and growth cones partly accounted for the massive axon loss at both spinal segments during CST development. Overall, the mouse CST elements changed dynamically in numbers during postnatal development, suggesting a vigorous growing and pruning activity in the tract. The mouse CST also showed a similar growth pattern to that of the rat CST.

Regenerating crayfish motor axons assimilate glial cells and sprout in cultured explants

Phasic and tonic motor nerves originating from crayfish abdominal ganglia, in 2-3-day-old cultured explants, display at their transected distal ends growth zones from which axonal sprouts arise. The subcellular morphology of this regenerative response was examined with thin serial-section electron microscopy and reveals two major remodeling features. First, the external sprouts that exit the nerve are a very small part of a much more massive sprouting response by individual axons comprising several orders of internal sprouts confined to the nerve. Both internal and external sprouts have a simple construction: a cytoskeleton of microtubules and populations of mitochondria, clear synaptic vesicles, membranous sacs, and extrasynaptic active zone dense bars, features reminiscent of motor nerve terminals. Close intermingling of the sprouts of several axons give rise to a neuropil-like arbor within the nerve. Thus, extensive sprouting is an intrinsic response of crayfish motor axons to transection. Second, an equally dramatic remodeling feature is the appearance of nuclei, which resemble those of adjacent glial cells, within the motor axons. These nuclei often appear where the adjoining membranes of the axon and glial cell are disrupted and where free-standing lengths of the double membrane are present. These images signify a breakdown of the dividing membranes and assimilation of the glial cell by the axon, the nucleus being the most visible sign of such assimilation. Thus, crayfish motor axons respond to transection by assimilating glial cells that may provide regulatory and trophic support for the sprouting response.

Observation of the dynamics of live cardiomyocytes through a free-running scanning near-field optical microscopy setup

We report the observation of live-cell dynamics by noncontact scanning near-field optical microscopy (SNOM) modified to work with living biological samples that are fully immersed in liquid. We did not use the SNOM setup in strictly near-field conditions (we used 1-mum constant-height mode); however, we could examine the dynamics of rhythmically beating cardiac myocytes in culture with extremely high vertical sensitivity below the nanometric range. We could halt scans at any point to record localized contraction profiles of the cell membrane. We show that the contractions of the organisms changed shape dramatically within adjacent areas. We believe that the spatial dependency of the contractions arises because of the measurement system's ability to resolve the behavior of individual submembrane actin bundles. Our results, combining imaging and real-time recording in localized areas, reveal a new, to our knowledge, noninvasive method for using SNOM setups for studying the dynamics of live biological samples.

Enhanced axonal regeneration following combined demyelination plus schwann cell transplantation therapy in the injured adult spinal cord

We have treated spinal cord injured rats with demyelination plus Schwann cell transplantation and assessed neurite outgrowth in a quantifiable model of axonal regeneration. Axonal injuries of differing severity were induced in the dorsal funiculus of adult rats using a micromanipulator-controlled Scouten knife. Demyelinated regions were produced so as to overlap with the injury site by the injection of galactocerebroside antibodies plus complement one segment cranial to the axonal injury site. Schwann cells were isolated from the sciatic nerve, expanded in vitro, and transplanted into the injury site 1 day later. Animals were killed after an additional 7 days. Schwann cells were evenly distributed throughout the region of demyelination, which extended 6-7 mm cranial to the axonal injury site. The severity of axonal injury was quantified by counting degenerate axons in transverse resin sections. The degree of axonal regeneration was assessed by an electron microscopic analysis of growth cone frequency and distribution relative to the site of axonal injury. Quantification of growth cones at a distance from the site of axonal injury indicated a strong linear relationship (P < 0.001) between the number of growth cones and the number of severed axons; the ratio of growth cones to severed axons was increased by 26.5% in demyelinated plus transplanted animals compared to demyelinated animals without a transplant. Furthermore, only the demyelinated plus transplanted animals contained growth cones associated with myelin in white matter immediately outside of the region of complete demyelination. Growth cones were absent in transplanted-only animals at a distance from the site of axonal injury. These findings indicate that combined demyelination plus Schwann cell transplantation therapy enhances axonal regeneration following injury and suggests that growth cones are able to overcome myelin-associated inhibitors of neurite outgrowth in the presence of trophic support.

A quantifiable model of axonal regeneration in the demyelinated adult rat spinal cord

Strategies to increase the extent of axonal regeneration in the adult CNS must address an array of intrinsic and environmental factors which influence neuritic outgrowth. In order to develop an in vivo model of axonal regeneration in which potential therapies may be assessed, we have quantified growth cones within demyelinated regions in the dorsal funiculus of the spinal cord, following a discrete axotomy. Demyelinated lesions were produced by the intraspinal injection of galactocerebroside antibodies plus serum complement proteins. Axonal integrity was not compromised by the demyelination protocol. Axonal injury was induced at the caudal extent of the demyelinated region using a micromanipulator-controlled Scouten knife. The severity of axonal injury was varied in different animals at the time of surgery and was quantified 8 days later by counting degenerate axons in transverse 1-microm resin sections. Evidence of axonal regeneration within these animals was assessed by an electron microscopic analysis of growth cone frequency and position relative to the site of axotomy. Growth cones were identified within the region of demyelination only; no growth cones were identified within the dorsal column white matter adjacent to the demyelinated region, or rostral or caudal to the region of demyelination, or in animals with an injury but no demyelination. Quantification of growth cones within regions of demyelination indicated a strong linear relationship (P < 0.001) between the number of growth cones and the number of axons severed. These findings indicate that demyelination facilitates axonal regeneration in the adult rat CNS and illustrate a quantifiable method of assessing axonal regeneration.

Actin depolymerisation induces process formation on MAP2-transfected non-neuronal cells

We have previously shown that microtubules in nonneuronal cells form long, stable bundles after transfection with the embryonic neuronal microtubule-associated protein MAP2c. In this study, we found that treating MAP2c-transfected cells with the actin depolymerising drug cytochalasin B led to the outgrowth of microtubule-containing processes from the cell surface. This effect was specific to MAP2c and did not occur in untransfected cells whose microtubules had been stabilised by treatment with taxol. The outgrowth and retraction of these processes during repeated cycles of cytochalasin addition and removal was followed by video time-lapse microscopy and was suggestive of a physical interaction between compressive forces exerted by the MAP2c-stabilised microtubule bundles and tensile forces originating in the cortical actin network. We suggest that MAP2c confers three properties on cellular microtubules that are essential for process outgrowth: stability, bundling and stiffness. The latter probably arises from the linking together of neighbouring tubulin subunits by three closely spaced tubulin-binding motifs in the MAP2 molecule that limits their motion relative to one another and thus reduces the flexibility of the polymer. Similar multimeric tubulin-binding domains in other proteins of the MAP2 class, including tau in axons and MAP4 in glial cells, may play the same role in the development and support of asymmetric cell morphology. Axial bundles of microtubules are found in growing neurites but not in growth cones, suggesting that the regulated expression of these MAP-induced properties makes an important contribution to the establishment of a stable process behind the advancing growth cone.

Origin of cells in contact with the growth cones of embryonal peripheral nerves and histochemical detection of nonspecific cholinesterase activity in quail-chick and chick-quail chimeras

The types of cells contacted by growth cones were examined in chick-quail and vice versa chimeras. Simultaneously, the presence of nonspecific cholinesterase (nChE) was monitored in developing peripheral nerves of these embryos. In all the chimeras studied, the growth cones were in contact with both Schwann cell progenitors and mesenchymal cells of the limb bud. This observation implies that the growth cones may obtained guidance cues directly from the mesenchymal cells of the limb bud. Schwann cell progenitors as well as mesenchymal cells in contact with growth cones were capable to produce nChE molecules. The reaction product indicating nChE activity was localized on the outer surface of the plasma membrane of both types of cells in contact with growth cones. However, mesenchymal cells forming the "primitive perineurium" were devoid of the reaction product on their plasma membrane. We conclude that with exception of classic adhesive molecules the process of axonal guidance can be mediated by nChE molecules.

Structure and organization of membrane organelles along distal microtubule segments in growth cones

Advance and stabilization of organelle-rich cytoplasm within the neuronal growth cone is coupled to axon elongation (Goldberg and Burmeister, 1986; Aletta and Greene, 1988), and this involves forward movement of organelles from the growth cone base along distinct tracks toward the leading edge. Membrane-bound organelles that advance first within the growth cone often make transient excursions toward the leading edge, and at the light microscope level these leading organelles appear to colocalize with distal microtubule (MT) segments (Dailey and Bridgman, 1989). We have used electron microscopy (EM) to identify the membranous organelles adjacent to distal MT segments, and to examine their structural interactions with MTs. In both glutaraldehyde-fixed and rapid frozen whole-mount growth cones, attenuated endoplasmic reticulum (ER)-like membrane elements were the most common organelle type located adjacent to distal MT segments. These ER-like membrane elements coursed roughly parallel to MTs and frequently terminated within an electron-dense bulb at the MT tip. Blind-ended membrane tubes, dense-core vesicles, clear vesicles, and vacuoles were also found adjacent to distal MT segments. Quantitative analyses of organelle-MT associations suggest that elements of the ER-like membrane system may frequently advance ahead of other membrane-bound organelles. Freeze-etch EM revealed crossbridging structures between MTs and membranous organelles, which is consistent with the idea that advance of leading membrane organelles into the growth cone periphery is mediated by microtubule-based motor transport mechanisms. The results suggest that distal microtubule segments serve as transport elements for advance of membrane organelles into more peripheral growth cone regions, and together MTs and ER-like membrane organelles may initiate the conversion of dynamic F-actin-rich cytoplasm to more stable organelle-rich cytoplasm (i.e., axoplasm).

Growth cones: the mechanism of neurite advance

Growth cones are the highly motile structures found at the tips of growing axons and dendrites (neurites), which extend from neurones, during the development of the nervous system. They function both as detectors and transducers of extrinsic guidance cues and as regions where the neurite assembly, advance cannot occur. Assembly of the neurite cytoskeleton in growing neurites chiefly involves microtubule assembly at the growth cone. Some of the factors that may influence microtubule assembly in growth cones are becoming apparent and include post-translational modification of tubulin itself and microtubule associated proteins, particularly tau and MAP1B.

GABAergic growth cones: release of endogenous gamma-aminobutyric acid precedes the expression of synaptic vesicle antigens

Growth cone fractions isolated from neonatal [postnatal day 3 (P3)] rat forebrain contain GABAergic growth cones as demonstrated by immunofluorescence staining with monospecific antibodies to gamma-aminobutyric acid (GABA). HPLC analysis shows that GABAergic growth cones release this endogenous GABA when stimulated with high K+. Endogenous GABA release is Ca2(+)-independent and, in this respect, similar to that seen previously with [3H]GABA. Isolated growth cone fractions also exhibit a K(+)-stimulated, Ca2(+)-independent release of endogenous taurine. None of the other amino acids shown to be present in isolated growth cone fractions were released, including glutamate, aspartate, and glycine. A population of dissociated cerebral cortical neurones prepared from P1 rat forebrain were GABA-immunoreactive after 1 day in culture. The cell body, neurites, and growth cones of these neurones were all stained with GABA antibodies. At this time in culture, neurones did not stain with either of two antibodies to synaptic vesicle antigens, i.e., p65 and synaptophysin. Growth cones isolated from P3 rat forebrain were also not immunoreactive with these antibodies. After about 8 days in culture, when neurones had established extensive networks of long, varicose axons and elaborately branched dendrites, many neurones and their neurites were immunoreactive for GABA antibodies. At this time in culture, p65 and synaptophysin antibodies did stain neuronal cell bodies and particularly their varicose axons. Dendrites were not stained with synaptic vesicle antibodies. These results suggest that GABAergic neurones synthesize GABA during neurite outgrowth and that GABA is present in, and can be released from, the growth cones of these neurones. The presence of GABA in GABAergic growth cones is not associated with synaptic vesicles, which explains the Ca2+ independency of both endogenous and [3H]GABA release from these growth cones.

Biochemical pharmacology of isolated neuronal growth cones: implications for synaptogenesis

The neuronal growth cone is critical to the establishment of neuronal polarity through its motile, pathfinding and target recognition properties exhibited during synaptogenesis. Subcellular fractionation procedures yielding milligram quantities of isolated growth cones has allowed for biochemical and pharmacological investigation of intrinsic growth cone components that are likely to be involved in regulation of growth cone function in neuronal development. These 'mapping' studies of growth cone components are prerequisites to elucidating the mechanisms by which extracellular factors influence the motility, adhesion and directed growth of the growth cone. For example, neurotransmitters and polypeptide growth factors which have been shown in other systems to modulate growth cone behavior are presumed to act through receptors on the growth cone, inducing second-messenger molecule formation and consequent modification and regulation of proteins effecting the response(s) of the growth cone (i.e. proteins involved in motility, adhesion and membrane turnover). In a relatively short period of time, work with the isolated growth cone preparation has identified, in independent studies, many of the elements involved in this proposed scheme of events, including transmitter receptors, second-messenger cascades, and second-messenger post-translational modifications. An obvious future goal will be to analyze in more detail the intracellular events, and the relationships between them, in the growth cone and how they transmit extracellular signals into responses such as motility and adhesivity which underly the growth cone's synaptogenic properties. It is to be expected that much of this information will come forth from experimentation with the isolated growth cone preparation.

Direct visualisation of the soluble pool of tubulin in the neuronal growth cone: immunofluorescence studies following taxol polymerisation

Previously we have shown that the predominant form of soluble alpha-tubulin in the growth cone is C-terminally tyrosinated (Gordon-Weeks and Lang. Dev. Brain Res., 42 (1988) 156-160). Here we show that when growth cones are incubated in taxol, this soluble pool of alpha-tubulin is polymerised onto the ends of the neurite microtubules that enter the proximal part of the growth cone indicating that it is assembly-competent.

The alpha-tubulin of the growth cone is predominantly in the tyrosinated form

Growth cone cytoskeletons were prepared by detergent extraction of growth cones isolated from neonatal rat forebrain by the method of Gordon-Weeks and Lockerbie (Neuroscience, 13 (1984) 119-136). SDS-PAGE analysis of growth cone cytoskeletons revealed the presence of several major bands, identified by their mobility as actin (43 kDa Mr), myosin heavy chain (195 kDa Mr), spectrin (235 and 240 kDa Mr), and tubulin (51-54 kDa Mr). The identity of these proteins was confirmed by immunoblot analysis using specific antibodies to these proteins which further revealed that the predominant form of alpha-tubulin in the growth cone cytoskeleton and in the soluble pool of tubulin is tyrosinated at the C-terminal.