Distribution of tubulin and actin in neurites and growth cones of differentiating nerve cells

The Axonal Actin Filament Cytoskeleton: Structure, Function, and Relevance to Injury and Degeneration

Early investigations of the neuronal actin filament cytoskeleton gave rise to the notion that, although growth cones exhibit high levels of actin filaments, the axon shaft exhibits low levels of actin filaments. With the development of new tools and imaging techniques, the axonal actin filament cytoskeleton has undergone a renaissance and is now an active field of research. This article reviews the current state of knowledge about the actin cytoskeleton of the axon shaft. The best understood forms of actin filament organization along axons are axonal actin patches and a submembranous system of rings that endow the axon with protrusive competency and structural integrity, respectively. Additional forms of actin filament organization along the axon have also been described and their roles are being elucidated. Extracellular signals regulate the axonal actin filament cytoskeleton and our understanding of the signaling mechanisms involved is being elaborated. Finally, recent years have seen advances in our perspective on how the axonal actin cytoskeleton is impacted by, and contributes to, axon injury and degeneration. The work to date has opened new venues and future research will undoubtedly continue to provide a richer understanding of the axonal actin filament cytoskeleton.

The functional architecture of axonal actin

The cytoskeleton builds and supports the complex architecture of neurons. It orchestrates the specification, growth, and compartmentation of the axon: axon initial segment, axonal shaft, presynapses. The cytoskeleton must then maintain this intricate architecture for the whole life of its host, but also drive its adaptation to new network demands and changing physiological conditions. Microtubules are readily visible inside axon shafts by electron microscopy, whereas axonal actin study has long been focused on dynamic structures of the axon such as growth cones. Super-resolution microscopy and live-cell imaging have recently revealed new actin-based structures in mature axons: rings, hotspots and trails. This has caused renewed interest for axonal actin, with efforts underway to understand the precise organization and cellular functions of these assemblies. Actin is also present in presynapses, where its arrangement is still poorly defined, and its functions vigorously debated. Here we review the organization of axonal actin, focusing on recent advances and current questions in this rejuvenated field.

Sonic Hedgehog Guides Axons via Zipcode Binding Protein 1-Mediated Local Translation

Sonic hedgehog (Shh) attracts spinal cord commissural axons toward the floorplate. How Shh elicits changes in the growth cone cytoskeleton that drive growth cone turning is unknown. We find that the turning of rat commissural axons up a Shh gradient requires protein synthesis. In particular, Shh stimulation increases β-actin protein at the growth cone even when the cell bodies have been removed. Therefore, Shh induces the local translation of β-actin at the growth cone. We hypothesized that this requires zipcode binding protein 1 (ZBP1), an mRNA-binding protein that transports β-actin mRNA and releases it for local translation upon phosphorylation. We found that Shh stimulation increases phospho-ZBP1 levels in the growth cone. Disruption of ZBP1 phosphorylation in vitro abolished the turning of commissural axons toward a Shh gradient. Disruption of ZBP1 function in vivo in mouse and chick resulted in commissural axon guidance errors. Therefore, ZBP1 is required for Shh to guide commissural axons. This identifies ZBP1 as a new mediator of noncanonical Shh signaling in axon guidance.SIGNIFICANCE STATEMENT Sonic hedgehog (Shh) guides axons via a noncanonical signaling pathway that is distinct from the canonical Hedgehog signaling pathway that specifies cell fate and morphogenesis. Axon guidance is driven by changes in the growth cone in response to gradients of guidance molecules. Little is known about the molecular mechanism of how Shh orchestrates changes in the growth cone cytoskeleton that are required for growth cone turning. Here, we show that the guidance of axons by Shh requires protein synthesis. Zipcode binding protein 1 (ZBP1) is an mRNA-binding protein that regulates the local translation of proteins, including actin, in the growth cone. We demonstrate that ZBP1 is required for Shh-mediated axon guidance, identifying a new member of the noncanonical Shh signaling pathway.

Extraction of protein profiles from primary neurons using active contour models and wavelets

The function of complex networks in the nervous system relies on the proper formation of neuronal contacts and their remodeling. To decipher the molecular mechanisms underlying these processes, it is essential to establish unbiased automated tools allowing the correlation of neurite morphology and the subcellular distribution of molecules by quantitative means. We developed NeuronAnalyzer2D, a plugin for ImageJ, which allows the extraction of neuronal cell morphologies from two dimensional high resolution images, and in particular their correlation with protein profiles determined by indirect immunostaining of primary neurons. The prominent feature of our approach is the ability to extract subcellular distributions of distinct biomolecules along neurites. To extract the complete areas of neurons, required for this analysis, we employ active contours with a new distance based energy. For locating the structural parts of neurons and various morphological parameters we adopt a wavelet based approach. The presented approach is able to extract distinctive profiles of several proteins and reports detailed morphology measurements on neurites. We compare the detected neurons from NeuronAnalyzer2D with those obtained by NeuriteTracer and Vaa3D-Neuron, two popular tools for automatic neurite tracing. The distinctive profiles extracted for several proteins, for example, of the mRNA binding protein ZBP1, and a comparative evaluation of the neuron segmentation results proves the high quality of the quantitative data and proves its practical utility for biomedical analyses.

Growth factor binding on heparin mimetic peptide nanofibers

Immobilization of growth factors in scaffolds is important for controlling their dose and bioactivity for regenerative medicine applications. Although numerous covalent and noncovalent immobilization strategies have been proposed, better growth factor loading and dose control inside the scaffold is necessary. Nature of the binding site on the growth factor interacting with scaffold is critical for preserving and achieving maximal growth factor functionality, which has been a relatively less emphasized issue in previous studies. We recently reported heparin mimetic peptide nanofibers, which mimic chemistry of heparan sulfates. Heparin mimetic nanofibers were shown to bind to vascular endothelial growth factor (VEGF) and direct endothelial cells to angiogenesis. Here, we further investigated interactions between heparin mimetic peptide nanofibers and growth factors. We tested bioactivity of the nanofiber bound growth factors in order to understand the potential use of these peptide nanofiber scaffolds as analogues of heparan sulfates. We observed that heparin mimetic peptide nanofibers demonstrate better binding profiles to VEGF, hepatocyte growth factor (HGF), and fibroblast growth factor-2 (FGF-2) than control peptide nanofibers. We also identified that the heparin-binding domain of VEGF is critical for its interaction with these nanofibers. However, the heparin-binding site is not indispensable for binding of all growth factors to nanofibers. We also showed that binding of growth factors to nanofibers does not cause any loss in bioactivity through in vitro cell culture assays with PC-12 cells. These results reveal that heparin mimetic peptide nanofibers can effectively mimic heparan sulfates in extracellular matrix and provide an optimal milieu for spatial presentation of important growth factors. These properties make peptide nanofiber scaffolds promising materials for regenerative medicine applications through efficient and precisely controlled growth factor delivery.

Netrin-1-induced local β-actin synthesis and growth cone guidance requires zipcode binding protein 1

Local β-actin synthesis in growth cones of developing axons plays an important role in growth cone steering; however, the mRNA binding proteins required for this process are unknown. Here we used Zbp1/Imp1(-/)(-) mice to test the hypothesis that zipcode binding protein 1 (ZBP1) is required for the regulation of β-actin mRNA transport and local translation underlying growth cone guidance. To address the biological function of ZBP1, we developed a novel in vitro turning assay with primary cortical neuron balls having axons >1 mm in length and demonstrate that growth cones of mammalian neurons exhibit protein synthesis-dependent attraction to either netrin-1 or brain-derived neurotrophic factor (BDNF). Interestingly, this attraction is lost in Zbp1-deficient neurons. Furthermore, BDNF-stimulated β-actin mRNA localization was attenuated in Zbp1-deficient neurons, which impaired enrichment of β-actin protein in the growth cone. Finally, using a photoconvertible translation reporter, we found that ZBP1 is necessary for netrin-1 stimulated local translation of β-actin mRNA in axonal growth cones. Together, these results suggest that netrin-1- and BDNF-induced growth cone attraction required ZBP1-mediated local translation of β-actin mRNA, and therefore ZBP1 regulates protein synthesis-dependent axon guidance. Thus, mRNA binding proteins regulating local translation can control spatiotemporal protein expression in response to guidance cues and directional cell motility.

Correlated Levels of mRNA and Soma Size in Single Identified Neurons: Evidence for Compartment-specific Regulation of Gene Expression

In addition to the overall complexity of transcriptional regulation, cells also must take into account the subcellular distribution of these gene products. This is particularly challenging for morphologically complex cells such as neurons. Yet the interaction between cellular morphology and gene expression is poorly understood. Here we provide some of the first evidence for a relationship between neuronal compartment size and maintenance of mRNA levels in neurons. We find that single-cell transcript levels of 18S rRNA, GAPDH, and EF1-alpha, all gene products with primary functions in the cell soma, are strongly correlated to soma size in multiple distinct neuronal types. Levels of mRNA for the K+ channel shal, which is localized exclusively to the soma, are negatively correlated with soma size, suggesting that gene expression does not simply track positively with compartment size. Conversely, levels of beta-actin and beta-tubulin mRNA, which are major cytoskeletal proteins of neuronal processes, do not correlate with soma size, but are strongly correlated with one another. Additionally, actin/tubulin expression levels correlate with voltage-gated ion channels that are uniquely localized to axons. These results suggest that steady-state transcript levels are differentially regulated based on the subcellular compartment within which a given gene product primarily acts.

The effects of hypoxia on growth cones in the ovine fetal brain

OBJECTIVE

This study was designed to investigate the effects of hypoxia on neural process proliferation by studying its effects on growth cone tubulin and insulin-like growth factor (IGF)-I receptor content.

METHODS

Six fetal lambs were catheterized in the brachial artery and vein. Maternal oxygenation was reduced in steps from a fractional inspired oxygen concentration (FiO2) of 20% to 6% by addition of nitrogen to the inhaled gas mixture for a period of 4 h of reduced oxygen intake. Fetal arterial blood was sampled after the maternal FiO2 and oxygen were stable for >5 min at maternal FiO2 of 20% to 6%. Controls were obtained from normoxic fetuses whose ewes had similar surgery and were kept at an FiO2 of 20% throughout the experiment. Growth cones were isolated from the fetal cerebrum and cerebellum. alpha-tubulin and IGF-I receptors were quantified by immunoblotting. Tubulin and IGF-I receptor mRNA expressions were quantified by real-time polymerase chain reaction.

RESULTS

Maternal nitrogen breathing reduced fetal arterial pH from 7.32+/-0.06 to 6.99+/-0.02 (p<0.001). Hypoxia increased IGF-I receptors from 143+/-10 to 327+/-14 (p<0.001) and from 272+/-26 to 396+/-34 (p<0.001) fluorescence units/microg protein in the cerebrum and cerebellum, respectively. It also increased alpha-tubulin from 713+/-30 to 1873+/-126 (p<0.001) and from 780+/-34 to 2362+/-79 (p<0.001) fluorescence units/microg protein in the cerebrum and cerebellum, respectively. Expression of IGF-I receptor mRNA increased significantly in the hypoxic animals both in the cerebrum and the cerebellum, but there was no change in expression of alpha-tubulin mRNA.

CONCLUSIONS

This increase in IGF-I receptor expression and growth cone content may be an adaptive response to hypoxia to maintain neurite growth by facilitating binding of IGF-I. Hypoxia also increased the growth cone level of alpha-tubulin but did not increase its mRNA expression, which may indicate an inability to polymerize tubulin and build microtubules.

Perikaryal surface specializations of neurons in sensory ganglia

Slender projections, similar to microvilli, are the main specialization of the perikaryal surface of sensory ganglion neurons. The extent of these projections correlates closely with the volume of the corresponding nerve cell body. It is likely that the role of perikaryal projections of sensory ganglion neurons, which lack dendrites, is to maintain the surface-to-volume ratio of the nerve cell body above some critical level for adequate metabolic exchange. Satellite cells probably have the ability to promote, or provide a permissive environment for, the outgrowth of these projections. It is not yet known whether the effect of satellite cells is mediated by molecules associated with their plasma membrane or by diffusible factors. Furthermore, receptor molecules for numerous chemical agonists are located on the nerve cell body surface, but it is not known whether certain molecules are located exclusively on perikaryal projections or are also present on the smooth surface between these projections. Further study of the nerve cell body surface and of the influence that satellite cells exert on it will improve our understanding of the interactions between sensory ganglion neurons and satellite neuroglial cells.

Location of profilin at presynaptic sites in the cerebellar cortex; implication for the regulation of the actin-polymerization state during axonal elongation and synaptogenesis

Profilin is a 15 kDa protein that binds actin monomers and inhibits their polymerization in vitro. The actin-profilin complex can be rapidly dissociated in vitro by phosphatidylinositol-4,5-bis-phosphate, providing a mechanism for regulating actin assembly-disassembly cycles during cell motile events. We have used a polyclonal antibody to calf spleen profilin to analyse the developmental expression and cellular distribution of profilin in the rat cerebellum and cultured cortical neurons. Immature neurons contain large amount of profilin both in vivo and in vitro. Immunofluorescence showed it to be present in developing neurites and growth cones but not in the filopodia of cortical neurons in culture. Profilin immunoreactivity was intense in the parallel fibres, the granule cell axons of the cerebellar cortex, at the time when they are elongating. Purkinje cell dendrites were not labelled. Profilin immunostaining was present in presynaptic varicosities, but not in dendritic spines within the molecular layer of juvenile and adult rats. The profilin concentration was higher in synaptosomes than in the total cerebellum during the second and third postnatal weeks, a period of intense synaptogenesis. Thus, profilin may help regulate actin polymerization and depolymerization during axonal elongation and synaptogenesis. Its restriction to the presynaptic site in the adult suggests that it may also be involved in the regulation of the release of synaptic vesicles.

Brain-specific tropomyosins TMBr-1 and TMBr-3 have distinct patterns of expression during development and in adult brain

In this study we report on the developmental and regional expression of two brain-specific isoforms of tropomyosin, TMBr-1 and TMBr-3, that are generated from the rat alpha-tropomyosin gene via the use of alternative promoters and alternative RNA splicing. Western blot analysis using an exon-specific peptide polyclonal antibody revealed that the two isoforms are differentially expressed in development with TMBr-3 appearing in the embryonic brain at 16 days of gestation, followed by the expression of TMBr-1 at 20 days after birth. TMBr-3 was detected in all brain regions examined, whereas TMBr-1 was detected predominantly in brain areas that derived from the prosencephalon. Immunocytochemical studies on mixed primary cultures made from rat embryonic midbrain indicate that expression of the brain-specific epitope is restricted to neurons. The developmental pattern and neuronal localization of these forms of tropomyosin suggest that these isoforms have a specialized role in the development and plasticity of the nervous system.

The organization of F-actin and microtubules in growth cones exposed to a brain-derived collapsing factor

In previous work we characterized a brain derived collapsing factor that induces the collapse of dorsal root ganglion growth cones in culture (Raper and Kapfhammer, 1990). To determine how the growth cone cytoskeleton is rearranged during collapse, we have compared the distributions of F-actin and microtubules in normal and partially collapsed growth cones. The relative concentration of F-actin as compared to all proteins can be measured in growth cones by rationing the intensity of rhodamine-phalloidin staining of F-actin to the intensity of a general protein stain. The relative concentration of F-actin is decreased by about one half in growth cones exposed to collapsing factor for five minutes, a time at which they are just beginning to collapse. During this period the relative concentration of F-actin in the leading edges of growth cones decreases dramatically while the concentration of F-actin in the centers decreases little. These results suggest that collapse is associated with a net loss of F-actin at the leading edge. The distributions of microtubules in normal and collapsing factor treated growth cones were examined with antibodies to tyrosinated and detyrosinated isoforms of alpha-tubulin. The tyrosinated form is found in newly polymerized microtubules while the detyrosinated form is not. The relative proximal-distal distributions of these isoforms are not altered during collapse, suggesting that rates of microtubule polymerization and depolymerization are not greatly affected by the presence of collapsing factor. An analysis of the distributions of microtubules before and after collapse suggests that microtubules are rearranged, but their polymerization state is unaffected during collapse. These results are consistent with the hypothesis that the brain derived collapsing factor has little effect on microtubule polymerization or depolymerization. Instead it appears to induce a net loss of F-actin at the leading edge of the growth cone.

The expression of tropomyosin genes in pure cultures of rat neurons, astrocytes and oligodendrocytes is highly cell-type specific and strongly regulated during development

Transcripts from the alpha-, beta- and delta-tropomyosin genes were studied during development of pure cultures of rat neurons, astrocytes and oligodendrocytes. The three cell types contained five alpha-tropomyosin messengers, produced using both alternative promoters and splicing; one was specific for mature neurons. The beta-tropomyosin gene is expressed only in astrocytes and the delta-tropomyosin gene in all three cell types, especially in immature cells. Most of the tropomyosin isoforms are highly cell-specific. Their developmental regulation involves either differential expression of genes, in neurons and oligodendrocytes, and/or changes in alternative splicing, in astrocytes, delta-Tropomyosin (TM-4) may be important during the growth of neuronal and glial cell processes, while specialized isoforms such as the neuron-specific alpha-tropomyosin TMBr-3 may be involved in the function or plasticity of the mature cells.

Actin-depolymerizing factor (ADF) in the cerebellum of the developing rat: a quantitative and immunocytochemical study

A specific antiserum against actin-depolymerizing factor (ADF) was used in a quantitative and immunocytochemical study of ADF in the cerebellum of developing rats. The Triton-soluble ADF concentration remained stable throughout development. Light and electron microscopic immunocytochemistry showed that ADF was not detected in all cerebellar cells. ADF immunoreactivity was found in Purkinje cells, but not in granule cells. It was found in the Bergmann astrocytes and the astrocytes of the white matter, but not in the oligodendrocytes. The cell bodies and dendrites of Purkinje cells were immunoreactive for ADF but the axons were not. In contrast, the other axons of the white matter (mossy and climbing fibres) were labeled. Thus, ADF was not restricted to either the dendritic or axonal compartments. However, dendritic spines and postsynaptic densities were immunoreactive, whereas presynaptic varicosities were unlabeled. The immunoreactivities for ADF and actin were compared. ADF staining was uniformly distributed throughout the entire dendritic arborization of the Purkinje cell, while filamentous actin is highly concentrated in the dendritic spines, indicating that ADF activity might vary according to its cellular localization.

Actin cytoskeletal network in aging and cancer

The cytoskeleton is being recognized as an important modulator of metabolic functions of the cell. The actin cytoskeletal network, in particular, is involved in events regulating cell proliferation and differentiation. The state of actin in a variety of cell types is regulated by signals arising from the cell surface through a wide spectrum of interactions. In this review, we explore the role of actin cytoskeletal network in a series of events which are known to influence cell proliferation and differentiation. These include interaction of actin network with extracellular matrix proteins, cell surface membranes, second messengers, cytoplasmic enzymes and the nucleus. Because of the involvement of the actin network in such diverse interactions, we propose that alterations in the actin cytoskeletal function may be an important aspect of generalized decrease in cellular functions associated with aging. Preliminary data indicate that alterations in the cytoskeletal network do occur in cells obtained from older individuals. Alterations in actin state are also reported during malignant transformation of cells in culture, and in naturally occurring tumors. Taken together, the existing data seem to suggest that changes in the actin cytoskeletal network may be a part of the aging process as well as malignant transformation. Therefore, the study of the actin cytoskeletal network and its regulation has the potential to yield important information regarding cellular senescence and neoplastic transformation.

Expression of tropomyosin genes during the development of the rat cerebellum

The expression of the tropomyosin genes in the rat nervous system was examined during the postnatal development of the cerebellum, using human-specific alpha-, beta-, gamma-, and delta-tropomyosin cDNA probes and rat-specific alpha-, beta-, and delta-tropomyosin oligonucleotide probes. The beta- and gamma-genes do not seem to be expressed in the rat brain. The delta-tropomyosin gene produces two mRNAs: a major one of 2.4 kb, which is highly concentrated during the first postnatal week and then decreases fourfold in level until the age of 35 days, and a minor one of 2 kb, with the same developmental profile as the 2.4-kb mRNA. A 3-kb mRNA is expressed by the alpha-tropomyosin gene and is characteristic of the mature rat. The expression of the tropomyosin genes during the development of the rat cerebellum does not seem to be regulated through alternative splicing but rather implies the differential expression of two different isogenes. The multiple isoforms of tropomyosin produced during neuronal differentiation may be intimately involved in the regulation of the organization and function of actin microfilaments.

Two classes of actin microfilaments are associated with the inner cytoskeleton of axons

The distribution and length of actin microfilaments (MF) was determined in axoplasm extruded from the giant axons of the squid (Loligo pealeii). Extruded axoplasm that was separated from the axonal cortex contains approximately 92% of the total axonal actin, and 60% of this actin is polymerized (Morris, J., and R. Lasek. 1984. J. Cell Biol. 98:2064-2076). Localization of MF with rhodamine-phalloidin indicated that the MF were organized in fine columns oriented longitudinally within the axoplasm. In the electron microscope, MF were surrounded by a dense matrix and they were associated with the microtubule domains of the axoplasm. The surrounding matrix tended to obscure the MF which may explain why MF have rarely been recognized before in the inner regions of the axon. The axoplasmic MF are relatively short (number average length of 0.55 micron). Length measurements of MF prepared either in the presence or absence of the actin-filament stabilizing drug phalloidin indicate that axoplasm contains two populations of MF: stable MF (number average length of 0.79 micron) and metastable MF (number average length of 0.41 micron). Although individual axonal MF are much shorter than axonal microtubules, the combined length of the total MF is twice that of the total microtubules. Apparently, these numerous short MF have an important structural role in the architecture of the inner axonal cytoskeleton.

Disruption of microfilament organization and deregulation of disk membrane morphogenesis by cytochalasin D in rod and cone photoreceptors

Morphogenesis of photoreceptor outer segment disks appears to occur by an evagination of the ciliary plasma membrane (Steinberg et al., J Comp Neurol 190:501-519, '80). We tested if polymerized actin (F-actin) was necessary for the regulation of this postulated process by incubating Xenopus eyecups with 5 or 25 microM cytochalasin D for 6-28 hours. During the second hour, the incubation medium contained 3H-leucine. Both concentrations of cytochalasin resulted in: 1) dissolution of the rhodamine-phalloidin labeling pattern of photoreceptors, and 2) collapse of the calycal processes (which are normally filled with actin filaments) and disappearance of the inner segment microfilaments. In addition, the few most basal rod and cone outer segment disks appeared several times their normal diameter. These oversized disks had incorporated 3H-leucine and extended along the margin of the outer or inner segment. The nature of the overgrown disks is consistent only with a morphogenetic process involving evaginations of the ciliary plasma membrane. Deregulation by cytochalasin D was manifest by excessive growth of a few nascent disks rather than normal growth of many. Therefore, the normal network of actin filaments is apparently not necessary for continued evagination of the membrane, but it does seem to be an essential part of the mechanism that initiates the evagination of the ciliary plasma membrane and/or the mechanism that controls how far nascent disks grow.

A lower proportion of filamentous to monomeric actin in the developing cerebellum of thyroid-deficient rats

Using a DNase I inhibition assay, in the presence or the absence of guanidine hydrochloride (which depolymerizes the actin filaments), developmental changes in total and filamentous actin were determined in the cerebellum of normal and hypothyroid rats. The total actin content per mg protein was not modified by hypothyroidism. As in normal animals, it reached a maximum around the age of 8 days and then decreased until adulthood. In contrast, the proportion of filamentous actin, which increased after the first postnatal week during normal development, was significantly reduced in the thyroid-deficient rats, only reaching normal values at 35 days. Thyroxine treatment for at least 4 days returned the filamentous actin content to normal at 14 days. The present study shows that the morphogenetic action of thyroid hormone is exerted not only on the microtubular apparatus, as previously described, but also in part through a control of actin monomer-polymer equilibrium.

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

In this review I have discussed the ultrastructure of the growth cone in relation to two aspects of growth cone behaviour; motility and membrane recycling. There are obvious and severe limitations in studying such a dynamic entity as the growth cone with the static images produced by the electron microscope, but these notwithstanding, electron microscopy, as I have tried to show here, has made important contributions in this area. Notable amongst these contributions is the fairly complete catalogue we now have of the organelles within the growth cone and their spatial relations, in particular the cytoskeletal and membrane bounded elements. Among the important questions that remain unanswered are those relating to the source and destiny of plasma membrane components, especially those concerned with recognising extrinsic cues, and the control of the cytoskeleton in relation to neurite extension and growth cone guidance. These questions can be approached using electron microscopy especially the rapid freezing and deep-etching methods used in conjunction with specific probes such as antibodies and we can look forward to progress in these areas in the near future.

Sequential appearance of cytoskeletal components during the early stages of neurite outgrowth from cerebellar granule cells in vitro

Cerebellar granule cells derived from one-week-old rat pups have been placed into culture to examine the expression of microfilament, microtubular and neurofilament proteins in developing neurites up to 2 days in vitro. Immunofluorescence and rhodamine-phalloidin studies have shown that the first cytoskeletal element to appear in the developing neurite was the microfilament network which was present in cell processes and expansions from at least 8 minutes in vitro. Rhodamine-phalloidin fluorescence was maintained in the developing neurite over 2 days and was enriched in some growth cones. At between 30 and 60 min in culture microtubules (reacting with anti alpha-tubulin antibodies) appeared in the neurites but did not ramify throughout the growth cone at this or later time points. The expression of tyrosylated alpha-tubulin in microtubules of the developing neurite appeared to precede that of acetylated alpha-tubulin. Only one of the neurofilament subunits, the 200 kDa subunit, could be positively identified in granule cell processes in these cultures. The developmental pattern of cytoskeletal protein expression in granule cell neurites in vitro may reflect a process of stabilisation of the growing neurite behind an active, motile growth cone.

The cytoskeletons of isolated, neuronal growth cones

We have examined by electron microscopy the cytoskeletons of growth cones isolated from neonatal rat forebrain by the method of Gordon-Weeks and Lockerbie [Gordon-Weeks and Lockerbie (1984) Neuroscience 13, 119-136]. When fixed in suspension with conventional fixatives, isolated growth cones contain a central region filled with a branching system of smooth endoplasmic reticulum and a cortical region immediately beneath the plasma membrane that is relatively free of organelles and is composed of an amorphous granular cytoplasm. The filopodia of isolated growth cones are also devoid of organelles and contain a cytoplasm that is similar in appearance to that in the cortical region. No microtubules or neurofilaments have been found in these growth cones. When isolated growth cones were prepared for electron microscopy by a method which preserves actin filaments [Boyles, Anderson and Hutcherson (1985) J. Histochem. Cytochem. 33, 1116-1128], microfilaments were found throughout the cortical cytoplasm. In the filopodia, the microfilaments were bundled together and oriented longitudinally. Filopodial microfilament bundles often extended into the body of the growth cone and could traverse it completely. Inclusion of Triton X-100 (1% v/v) in the fixative solubilized the membranes and soluble cytoplasmic proteins of growth cones, allowing an unobscured view of the microfilament cytoskeleton including the core bundle of microfilaments in filopodia. Suspended within the cytoskeleton were the coats of coated vesicles. These were particularly numerous at the broad bases of filopodia. Microfilaments bound heavy meromyosin and were cytochalasin B (2.0 X 10(-7) M) sensitive. Individual microfilaments branched and within filopodia they were extensively cross-linked by thin (7 nm) filaments. Microtubules and neurofilaments were not seen in these cytoskeletons despite the fact that the fixative contained a Ca2+ chelator. When growth cones were preincubated in taxol (14 microM) their cytoskeletons were found to contain microtubules. These were located mainly in the centre of the growth cone, were absent from the filopodia and were contiguous with microfilaments. We conclude that the cytoskeletons of isolated neuronal growth cones from neurones of the central nervous system are mainly composed of actin microfilaments. Although microtubules are not normally present, there is a pool of soluble tubulin which will form microtubules in the presence of taxol. This may imply that those microtubule-associated proteins that promote tubulin polymerization are absent in the growth cone or are below the concentration threshold for polymerization.

Structural protein transport in elongating motor axons after sciatic nerve crush. Effect of a conditioning lesion

In elongating motor axons of the rat sciatic nerve, the maximum outgrowth rate increased from 4.6 to 5.3 mm/d (5.3-6.1 X 10(-8) m/s) when a testing lesion of spinal nerves L4 and L5 was preceded 2 wk earlier by a conditioning lesion of the sciatic nerve. Axonal outgrowth was examined by measuring the transport of 35[S]methionine-labeled structural proteins (tubulin, actin, and neurofilament triplet) from "parent" axon stumps into "daughter" axon sprouts. Since these proteins are conveyed by the slow component of axonal transport at 1-5 mm/d (1.2-6.0 X 10(-8) m/s), the isotope was injected into the spinal cord 1 wk before the testing lesion. Nerves were removed 8 d after the testing lesion, sectioned into 3-mm segments, and homogenized; soluble proteins were separated by polyacrylamide gel electrophoresis. Fluorographs were used as templates to identify gel segments for removal, solubilization, and liquid scintillation counting. Distributions of mean radioactivity for tubulin, actin, and neurofilament triplet were plotted for animals receiving a conditioning vs sham-conditioning lesion. Greater amounts of tubulin and actin were transported into daughter axons in the conditioned group. Tubulin was mainly increased in axon shafts, whereas actin was mainly increased in axon tips. These findings suggest that the axonal transport of tubulin and actin governs the rate of elongation.

Polarized compartmentalization of organelles in growth cones from developing optic tectum

We have used computer-assisted reconstructions of continuous serial sections to study the cytoplasmic organization of growth cones in vivo. Optic tecta from 6.25-6.5-d-old chicken embryos were quick-frozen and then freeze-substituted in acetone-osmium tetroxide or, for comparison, prepared by conventional fixation. Images of eight freeze-substituted and two conventionally fixed growth cones were reconstructed from aligned serial micrographs. After freeze-substitution, numerous lumenless membrane-bound sacs arrayed in multilamellar stacks appear to replace the abundant smooth endoplasmic reticulum found after chemical fixation. Microtubule fascicles progressively diverge from their typical fascicular organization in the initial segment of the growth cone and are absent in the varicosity and the more distal segment. Mitochondria, in contrast, are concentrated in the proximal segment of the varicosity; multilamellar stacks and endosome-like vacuoles are in the distal segment; and coated pits and vesicles are concentrated near the terminal filopodium, which is the most distal and organelle-poor domain of the growth cone. These observations suggest that dilation and fusion of the lumenless, membrane-bound sacs that occurs during chemical fixation give rise to the network of smooth endoplasmic reticulum. The three-dimensional reconstructions show that the cytoplasmic components of growth cones, including the membrane-bound sacs and multilamellar stacks revealed by freeze substitution, are polarized along the axis of these growth cones, which suggests that they have a role in recycling of membrane during elongation of the growth cone.

Proteins transported in slow components a and b of axonal transport are distributed differently in the transverse plane of the axon

The distribution of the proteins migrating with the slow components a (SCa) and b (SCb) of axonal transport were studied in cross-sections of axons with electron microscope autoradiography. Radiolabeled amino acids were injected into the hypoglossal nucleus of rabbits and after 15 d, the animals were killed. Hypoglossal nerves were processed either for SDS-polyacrylamide gel electrophoresis fluorography to identify and locate the two components of slow transport, or for quantitative electron microscope autoradiography. Proteins transported in SCa were found to be uniformly distributed within the cross-section of the axon. Labeled SCb proteins were also found throughout the axonal cross-section, but the subaxolemmal region of the axon contained 2.5 times more SCb radioactivity than any comparable area in the remainder of the axon.

A quantitative study of growth cone filopodial extension

The extension of filopodia from growth cones of regenerating neurites from rat superior cervical ganglion neurons in tissue culture was studied. Cultures were grown on a thin layer of fibrous collagen and maintained in a medium containing serum and nerve growth factor. Time-lapse cinematography and computer-assisted morphometry were used to observe and measure the kinetics of extension of individual filopodia. Filopodia extended from the growth cone margin, trailing neurite, or from each other. Frequently, extension was preceded by the appearance at the cone margin of a nodule of cytoplasm which appeared dense in phase-contrast optics. Branch points between adjacent extending filopodia remained fixed with respect to the growth cone while the filopodia lengthened. The rate of extension was maximum just after initiation (0.12 +/- 0.4 micron/sec; mean +/- SD; n = 36) and declined thereafter until the filopodium collapsed. This initial rate of extension was directly correlated with the eventual length of the filopodium (r = 0.67). Filopodia of growth cones arising from embryonic neurons exhibited higher initial extension rates (range: 0.07 to 0.20 micron/sec; mean = 0.13 micron/sec) than those of postnatal neurons (range: 0.01 to 0.13 micron/sec; mean = 0.09 micron/sec). These data are discussed in relation to a model proposed by Tilney and Inoue [1982] for the extension, by distal addition of G-actin to growing filaments, of another type of elongating process filled with microfilaments, the acrosomal process of Thyone sperm.

Cytoplasmic morphology weaver (wv) mouse cerebellar neurons at the culture substratum

The purpose of this study was to determine the structural basis for the hypermotility and impaired growth cone elongation of the homozygous weaver (wv/wv) mouse cerebellar granule cell neurons in culture. Two-day cultures of dissociated week-old normal (+/+) and wv/wv cerebellum were processed for electron microscopy of intact cells and cytoskeleton. Serial sections parallel to and starting from the substrate were examined. Fine-caliber neurites of normal granule cells are packed with parallel arrays of microtubules at all levels. Microfilament-packed microspikes are present at substrate level emanating from a cortical microfilament lattice at the terminus of neurites of varying length. Homozygous weaver granule cells at substrate level have lateral cytoplasmic extensions along the neurite. Microtubules that curve throughout the neurite are separated by cytoplasm. The lateral extensions and growth cone cytoplasmic projections contain microfilaments and occasionally microtubules. Microfilament-packed microspikes are not observed. Immunofluorescent detection of actin confirms the ultrastructural picture. A hallmark of the wv/wv cytopathology is the presence of large numbers of coated vesicles throughout the neurite shaft at the cell-substratum interface. These are rare at similar locations in +/+ neurites. We hypothesize that reduced tension in the growth cone and neurite owing to the presence of lateral extensions and absence of stable microspikes are responsible for the impaired elongation and hypermotility of mutant neurons.

Isolation and partial characterisation of neuronal growth cones from neonatal rat forebrain

We have devised a method for the isolation of viable neuronal growth cones from neonatal rat forebrain. The method involves differential and density gradient centrifugation and exploits the relatively low buoyant density (approximately 1.018 g/cm3) of growth cones. There are no known biochemical markers for growth cones and it was necessary therefore to monitor for their presence during the isolation using transmission electron microscopy. Several criteria were used to identify isolated growth cones including the presence of filopodia, an extensive system of branching, tubular smooth endoplasmic reticulum and a region rich in microfilaments subjacent to the plasma membrane. These morphological features are similar to those of growth cones identified unequivocally in intact developing brain and in tissue culture. Electron microscopical analysis showed that greater than 90% of membrane-bound, identifiable objects in one fraction were growth cones by these criteria. The major contaminant consisted of membrane sacs and vesicles of unidentified origin. There were only small amounts of isolated rough endoplasmic reticulum and mitochondria. Isolated growth cones were roughly spherical in shape with a diameter of 1.9 +/- 0.5 micron (mean +/- 1 SD). They usually contained mitochondria, large granular vesicles and small vesicles, and occasionally contained coated vesicles, lysosomes, lamellar bodies and multivesicular bodies, and only very rarely, intermediate filaments. Occasionally, growth cones had rudimentary synapses on them. The viability of isolated growth cones was investigated by observing their behaviour in short-term culture. After a few hours in culture on poly-D-lysine-coated coverslips, growth cones flattened down and extended filopodia-like processes. This behaviour was inhibited by cytochalasin B and reversibly by cold (4 degrees C). We conclude that physiologically active growth cones can be isolated rapidly and in large numbers by the method described here.

Differences in the organization of actin in the growth cones compared with the neurites of cultured neurons from chick embryos

Sensory neurons from chick embryos were cultured on substrata that support neurite growth, and were fixed and prepared for both cytochemical localization of actin and electron microscopic observation of actin filaments in whole-mounted specimens. Samples of cells were treated with the detergent Triton X-100 before, during, or after fixation with glutaraldehyde to determine the organization of actin in simpler preparations of extracted cytoskeletons. Antibodies to actin and a fluorescent derivative of phallacidin bound strongly to the leading margins of growth cones, but in neurites the binding of these markers for actin was very weak. This was true in all cases of Triton X-100 treatment, even when cells were extracted for 4 min before fixation. In whole-mounted cytoskeletons there were bundles and networks of 6-7-nm filaments in leading edges of growth cones but very few 6-7-n filaments were present among the microtubules and neurofilaments in the cytoskeletons of neurites. These filaments, which are prominent in growth cones, were identified as actin because they were stabilized against detergent extraction by the presence of phallacidin or the heavy meromyosin and S1 fragments of myosin. In addition, heavy meromyosin and S1 decorated these filaments as expected for binding to F-actin. Microtubules extended into growth cone margins and terminated within the network of actin filaments and bundles. Interactions between microtubule ends and these actin filaments may account for the frequently observed alignment of microtubules with filopodia at the growth cone margins.

Basic aspects of development and maturation of the brain: embryological contributions to neuroendocrinology

The interpretation of studies on the development of neuroendocrine function presupposes a thorough knowledge of the complementary phenomena of morphogenesis and histogenesis of the brain. A short analysis of the morphogenesis of the diencephalic floor is given. The pituitary Anlage can be identified early in the neural plate stage. The hypophysis cerebri appears to be a key structure in the morphogenesis of both the head and the brain. The spatiotemporal pattern of histogenesis within the brain can be analysed by a study of the proliferative activity of the neuroepithelial matrix cell layer; a heterochrony of matrix (ventricular) layer mitotic activity and of mantle (intermediate) layer differentiation can be demonstrated. The process of neuron differentiation shows an articulate sequence of phenomena, among them migration, axon growth, dendrite growth, synapse formation and myelination. Dendritogenesis and the development of synapses in a particular area are strongly influenced by the ingrowth of axon nerve terminals from elsewhere. A number of structures observed in the developing central nervous system are only temporary phenomena that go into regression during subsequent stages; dendrites and synapses especially show a high degree of plasticity. Cell death occurs as a normal concomitant of development. Monoaminergic neuron systems originate early and show a positive histofluorescence shortly afterwards; their target areas are retarded as far as differentiation is concerned. The development of these target regions probably is influenced by the monoaminergic cells. The monoaminergic neurons are not subject to feedback regulation for some time, because of the typical late development of the dendritic receptive apparatus in these cells. Steroid receptors may play a role in the development of intersexual dimorphism of the brain. Probably a modulation of neurotransmitter synthesis is the intermediate between steroid receptor stimulation and a change in synaptogenesis in the target area of the neuron. Neuropeptide systems appear to possess a distribution beyond the limits of the hypophysiotropic area. The early presence of some of the neuropeptides within the embryonic brain suggests a role in histogenesis that is different from the usually presumed neurotransmitter or neuromodulator function of the neuropeptides.

Translocation of the neuronal cytoskeleton and axonal locomotion

Recent studies of axonal transport indicate that cytoskeletal proteins are assembled into polymers in the neuron cell body and that these polymers move from the cell body toward the end of the axon. On the other hand, membranous elements appear to be inserted into the axonal plasma membrane preferentially at the end of the axon. These new observations are explored in relation to our current understanding of axonal elongation.