A methodology for specific disruption of microtubule polymerization into dendritic spines

Dendritic spines, the mushroom-shaped extensions along dendritic shafts of excitatory neurons, are critical for synaptic function and are one of the first neuronal structures disrupted in neurodevelopmental and neurodegenerative diseases. Microtubule (MT) polymerization into dendritic spines is an activity-dependent process capable of affecting spine shape and function. Studies have shown that MT polymerization into spines occurs specifically in spines undergoing plastic changes. However, discerning the function of MT invasion of dendritic spines requires the specific inhibition of MT polymerization into spines, while leaving MT dynamics in the dendritic shaft, synaptically connected axons and associated glial cells intact. This is not possible with the unrestricted, bath application of pharmacological compounds. To specifically disrupt MT entry into spines we coupled a MT elimination domain (MTED) from the Efa6 protein to the actin filament-binding peptide LifeAct. LifeAct was chosen because actin filaments are highly concentrated in spines and are necessary for MT invasions. Temporally controlled expression of this LifeAct-MTED construct inhibits MT entry into dendritic spines, while preserving typical MT dynamics in the dendrite shaft. Expression of this construct will allow for the determination of the function of MT invasion of spines and more broadly, to discern how MT-actin interactions affect cellular processes.

A Methodology for Specific Disruption of Microtubules in Dendritic Spines

Dendritic spines, the mushroom-shaped extensions along dendritic shafts of excitatory neurons, are critical for synaptic function and are one of the first neuronal structures disrupted in neurodevelopmental and neurodegenerative diseases. Microtubule (MT) polymerization into dendritic spines is an activity-dependent process capable of affecting spine shape and function. Studies have shown that MT polymerization into spines occurs specifically in spines undergoing plastic changes. However, discerning the function of MT invasion of dendritic spines requires the specific inhibition of MT polymerization into spines, while leaving MT dynamics in the dendritic shaft, synaptically connected axons and associated glial cells intact. This is not possible with the unrestricted, bath application of pharmacological compounds. To specifically disrupt MT entry into spines we coupled a MT elimination domain (MTED) from the Efa6 protein to the actin filament-binding peptide LifeAct. LifeAct was chosen because actin filaments are highly concentrated in spines and are necessary for MT invasions. Temporally controlled expression of this LifeAct-MTED construct inhibits MT entry into dendritic spines, while preserving typical MT dynamics in the dendrite shaft. Expression of this construct will allow for the determination of the function of MT invasion of spines and more broadly, to discern how MT-actin interactions affect cellular processes.

Handling Difficult Cryo-ET Samples: A Study with Primary Neurons from Drosophila melanogaster

Cellular neurobiology has benefited from recent advances in the field of cryo-electron tomography (cryo-ET). Numerous structural and ultrastructural insights have been obtained from plunge-frozen primary neurons cultured on electron microscopy grids. With most primary neurons having been derived from rodent sources, we sought to expand the breadth of sample availability by using primary neurons derived from 3rd instar Drosophila melanogaster larval brains. Ultrastructural abnormalities were encountered while establishing this model system for cryo-ET, which were exemplified by excessive membrane blebbing and cellular fragmentation. To optimize neuronal samples, we integrated substrate selection, micropatterning, montage data collection, and chemical fixation. Efforts to address difficulties in establishing Drosophila neurons for future cryo-ET studies in cellular neurobiology also provided insights that future practitioners can use when attempting to establish other cell-based model systems.

Handling difficult cryo-ET samples: A study with primary neurons from Drosophila melanogaster

Cellular neurobiology has benefited from recent advances in the field of cryo-electron tomography (cryo-ET). Numerous structural and ultrastructural insights have been obtained from plunge-frozen primary neurons cultured on electron microscopy grids. With most primary neurons been derived from rodent sources, we sought to expand the breadth of sample availability by using primary neurons derived from 3rd instar Drosophila melanogaster larval brains. Ultrastructural abnormalities were encountered while establishing this model system for cryo-ET, which were exemplified by excessive membrane blebbing and cellular fragmentation. To optimize neuronal samples, we integrated substrate selection, micropatterning, montage data collection, and chemical fixation. Efforts to address difficulties in establishing Drosophila neurons for future cryo-ET studies in cellular neurobiology also provided insights that future practitioners can use when attempting to establish other cell-based model systems.

Contributions of microtubule dynamics and transport to presynaptic and postsynaptic functions

Microtubules (MT) are elongated, tubular, cytoskeletal structures formed from polymerization of tubulin dimers. They undergo continuous cycles of polymerization and depolymerization, primarily at their plus ends, termed dynamic instability. Although this is an intrinsic property of MTs, there are a myriad of MT-associated proteins that function in regulating MT dynamic instability and other dynamic processes that shape the MT array. Additionally, MTs assemble into long, semi-rigid structures which act as substrates for long-range, motor-driven transport of many different types of cargoes throughout the cell. Both MT dynamics and motor-based transport play important roles in the function of every known type of cell. Within the last fifteen years many groups have shown that MT dynamics and transport play ever-increasing roles in the neuronal function of mature neurons. Not only are neurons highly polarized cells, but they also connect with one another through synapses to form complex networks. Here we will focus on exciting studies that have illuminated how MTs function both pre-synaptically in axonal boutons and post-synaptically in dendritic spines. It is becoming clear that MT dynamics and transport both serve important functions in synaptic plasticity. Thus, it is not surprising that disruption of MTs, either through hyperstabilization or destabilization, has profound consequences for learning and memory. Together, the studies described here suggest that MT dynamics and transport play key roles in synaptic function and when disrupted result in compromised learning and memory.

A Single Transcript Knockdown-Replacement Strategy Employing 5' UTR Secondary Structures to Precisely Titrate Rescue Protein Translation

One overarching goal of gene therapy is the replacement of faulty genes with functional ones. A significant hurdle is presented by the fact that under- or over-expression of a protein may cause disease as readily as coding mutations. There is a clear and present need for pipelines to translate experimentally validated gene therapy strategies to clinical application. To address this we developed a modular, single-transgene expression system for replacing target genes with physiologically expressed variants. In order to accomplish this, we first designed a range of 5' UTR "attenuator" sequences which predictably diminish translation of the paired gene. These sequences provide wide general utility by allowing control over translation from high expression, ubiquitous promoters. Importantly, we demonstrate that this permits an entirely novel knockdown and rescue application by pairing microRNA-adapted shRNAs alongside their respective replacement gene on a single transcript. A noteworthy candidate for this corrective approach is the degenerative and uniformly fatal motor neuron disease ALS. A strong proportion of non-idiopathic ALS cases are caused by varied mutations to the SOD1 gene, and as clinical trials to treat ALS are being initiated, it is important to consider that loss-of-function mechanisms contribute to its pathology as strongly as any other factor. As a generalized approach to treat monogenic diseases caused by heterogeneous mutations, we demonstrate complete and predictable control over replacement of SOD1 in stable cell lines by varying the strength of attenuators.

Double UP: A Dual Color, Internally Controlled Platform for in utero Knockdown or Overexpression

In utero electroporation (IUE) is a powerful tool for testing the role of genes in neuronal migration and function, but this technique suffers from high degrees of variability. Such variability can result from inconsistent surgery, developmental gradients along both rostral-caudal and medial-lateral axes, differences within littermates and from one litter to another. Comparisons between control and experimental electroporations rely on section matching, which is inherently subjective. These sources of variability are cumulative, leading to difficult to interpret data and an increased risk of both false positives and false negatives. To address these limitations, we developed two tools: (1) a new plasmid, termed Double UP, which combines LoxP-flanked reporters and limiting Cre dosages to generate internal controls, and (2) an automated program for unbiased and precise quantification of migration. In concert, these tools allow for more rigorous and objective experiments, while decreasing the mice, time, and reagents required to complete studies.

Novel α-Lipoic Acid/3-n-Butylphthalide Conjugate Enhances Protective Effects against Oxidative Stress and 6-OHDA Induced Neuronal Damage

Neurodegenerative diseases are irreversible conditions that result in progressive degeneration and death of nerve cells. Although the underlying mechanisms may vary, oxidative stress is considered to be one of the major causes of neuronal loss. Importantly, there are still no comprehensive treatments to completely cure these diseases. Therefore, protecting neurons from oxidative damage may be the most effective therapeutic strategy. Here we report a neuroprotective effects of a novel hybrid compound (dlx-23), obtained by conjugating α-lipoic acid (ALA), a natural antioxidant agent, and 3-n-butylphthalide (NBP), a clinical anti-ischemic drug. Dlx-23 protected against neuronal death induced by both H₂O₂ induced oxidative stress in Cath.-a-differentiated (CAD) cells and 6-OHDA, a toxin model of Parkinson's disease (PD) in SH-SY5Y cells. These activities proved to be more potent than the parent compound (ALA) alone. Dlx-23 scavenged free radicals, increased glutathione levels, and prevented mitochondria damage. In addition, live imaging of primary cortical neurons demonstrated that dlx-23 protected against neuronal growth cone damage induced by H₂O₂. Taken together these results suggest that dlx-23 has substantial potential to be further developed into a novel neuroprotective agent against oxidative damage and toxin induced neurodegeneration.

Opposing functions of F-BAR proteins in neuronal membrane protrusion, tubule formation, and neurite outgrowth

Neurite formation is a fundamental antecedent to axon and dendrite formation, but the mechanisms that underlie this important process are poorly characterized. Here, we demonstrate that two F-BAR proteins, CIP4 and FBP17, have opposing functions in early cortical neuron development.

The F-BAR family of proteins play important roles in many cellular processes by regulating both membrane and actin dynamics. The CIP4 family of F-BAR proteins is widely recognized to function in endocytosis by elongating endocytosing vesicles. However, in primary cortical neurons, CIP4 concentrates at the tips of extending lamellipodia and filopodia and inhibits neurite outgrowth. Here, we report that the highly homologous CIP4 family member, FBP17, induces tubular structures in primary cortical neurons and results in precocious neurite formation. Through domain swapping and deletion experiments, we demonstrate that a novel polybasic region between the F-BAR and HR1 domains is required for membrane bending. Moreover, the presence of a poly-PxxP region in longer splice isoforms of CIP4 and FBP17 largely reverses the localization and function of these proteins. Thus, CIP4 and FBP17 function as an antagonistic pair to fine-tune membrane protrusion, endocytosis, and neurite formation during early neuronal development.

Of microtubules and memory: implications for microtubule dynamics in dendrites and spines

Microtubules (MTs) are cytoskeletal polymers composed of repeating subunits of tubulin that are ubiquitously expressed in eukaryotic cells. They undergo a stochastic process of polymerization and depolymerization from their plus ends termed dynamic instability. MT dynamics is an ongoing process in all cell types and has been the target for the development of several useful anticancer drugs, which compromise rapidly dividing cells. Recent studies also suggest that MT dynamics may be particularly important in neurons, which develop a highly polarized morphology, consisting of a single axon and multiple dendrites that persist throughout adulthood. MTs are especially dynamic in dendrites and have recently been shown to polymerize directly into dendritic spines, the postsynaptic compartment of excitatory neurons in the CNS. These transient polymerization events into dendritic spines have been demonstrated to play important roles in synaptic plasticity in cultured neurons. Recent studies also suggest that MT dynamics in the adult brain function in the essential process of learning and memory and may be compromised in degenerative diseases, such as Alzheimer's disease. This raises the possibility of targeting MT dynamics in the design of new therapeutic agents.

Transport of a kinesin-cargo pair along microtubules into dendritic spines undergoing synaptic plasticity

Synaptic plasticity often involves changes in the structure and composition of dendritic spines. Vesicular cargos and organelles enter spines either by exocytosing in the dendrite shaft and diffusing into spines or through a kinesin to myosin hand-off at the base of spines. Here we present evidence for microtubule (MT)-based targeting of a specific motor/cargo pair directly into hippocampal dendritic spines. During transient MT polymerization into spines, the kinesin KIF1A and an associated cargo, synaptotagmin-IV (syt-IV), are trafficked in unison along MTs into spines. This trafficking into selected spines is activity-dependent and results in exocytosis of syt-IV-containing vesicles in the spine head. Surprisingly, knockdown of KIF1A causes frequent fusion of syt-IV-containing vesicles throughout the dendritic shaft and diffusion into spines. Taken together, these findings suggest a mechanism for targeting dendritic cargo directly into spines during synaptic plasticity and indicate that MT-bound kinesins prevent unregulated fusion by sequestering vesicular cargo to MTs.

Transport of cargo into dendritic spines is required for synaptic plasticity. McVicker et al. describe a method of activity-dependent transport of a kinesin KIF1A and its cargo synaptotagmin-IV along microtubules that are transiently polymerized into dendritic spines.

Physical models have gender-specific effects on student understanding of protein structure-function relationships

Understanding how basic structural units influence function is identified as a foundational/core concept for undergraduate biological and biochemical literacy. It is essential for students to understand this concept at all size scales, but it is often more difficult for students to understand structure-function relationships at the molecular level, which they cannot as effectively visualize. Students need to develop accurate, 3-dimensional mental models of biomolecules to understand how biomolecular structure affects cellular functions at the molecular level, yet most traditional curricular tools such as textbooks include only 2-dimensional representations. We used a controlled, backward design approach to investigate how hand-held physical molecular model use affected students' ability to logically predict structure-function relationships. Brief (one class period) physical model use increased quiz score for females, whereas there was no significant increase in score for males using physical models. Females also self-reported higher learning gains in their understanding of context-specific protein function. Gender differences in spatial visualization may explain the gender-specific benefits of physical model use observed. © 2016 The Authors Biochemistry and Molecular Biology Education published by Wiley Periodicals, Inc. on behalf of International Union of Biochemistry and Molecular Biology, 44(4):326-335, 2016.

Beyond the cytoskeleton: The emerging role of organelles and membrane remodeling in the regulation of axon collateral branches

The generation of axon collateral branches is a fundamental aspect of the development of the nervous system and the response of axons to injury. Although much has been discovered about the signaling pathways and cytoskeletal dynamics underlying branching, additional aspects of the cell biology of axon branching have received less attention. This review summarizes recent advances in our understanding of key factors involved in axon branching. This article focuses on how cytoskeletal mechanisms, intracellular organelles, such as mitochondria and the endoplasmic reticulum, and membrane remodeling (exocytosis and endocytosis) contribute to branch initiation and formation. Together this growing literature provides valuable insight as well as a platform for continued investigation into how multiple aspects of axonal cell biology are spatially and temporally orchestrated to give rise to axon branches. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1293-1307, 2016.

BAR-SH3 sorting nexins are conserved interacting proteins of Nervous wreck that organize synapses and promote neurotransmission

Nervous wreck (Nwk) is a conserved F-BAR protein that attenuates synaptic growth and promotes synaptic function in Drosophila. In an effort to understand how Nwk carries out its dual roles, we isolated interacting proteins using mass spectrometry. We report a conserved interaction between Nwk proteins and BAR-SH3 sorting nexins, a family of membrane-binding proteins implicated in diverse intracellular trafficking processes. In mammalian cells, BAR-SH3 sorting nexins induce plasma membrane tubules that localize NWK2, consistent with a possible functional interaction during the early stages of endocytic trafficking. To study the role of BAR-SH3 sorting nexins in vivo, we took advantage of the lack of genetic redundancy in Drosophila and employed CRISPR-based genome engineering to generate null and endogenously tagged alleles of SH3PX1. SH3PX1 localizes to neuromuscular junctions where it regulates synaptic ultrastructure, but not synapse number. Consistently, neurotransmitter release was significantly diminished in SH3PX1 mutants. Double-mutant and tissue-specific-rescue experiments indicate that SH3PX1 promotes neurotransmitter release presynaptically, at least in part through functional interactions with Nwk, and might act to distinguish the roles of Nwk in regulating synaptic growth and function.

Neurite guidance and three-dimensional confinement via compliant semiconductor scaffolds

Neurons are often cultured in vitro on a flat, open, and rigid substrate, a platform that does not reflect well the native microenvironment of the brain. To address this concern, we have developed a culturing platform containing arrays of microchannels, formed in a crystalline-silicon nanomembrane (NM) resting on polydimethylsiloxane; this platform will additionally enable active sensing and stimulation at the local scale, via devices fabricated in the silicon. The mechanical properties of the composite Si/compliant substrate nanomaterial approximate those of neural tissue. The microchannels, created in the NM by strain engineering, demonstrate strong guidance of neurite outgrowth. Using plasma techniques, we developed a means to coat just the inside surface of these channels with an adhesion promoter (poly-d-lysine). For NM channels with openings larger than the cross-sectional area of a single axon, strong physical confinement and guidance of axons through the channels are observed. Imaging of axons that grow in channels with openings that approximate the size of an axon suggests that a tight seal exists between the cell membrane and the inner surface of the channel, mimicking a myelin sheath. Such a tight seal of the cell membrane with the channel surface would make this platform an attractive candidate for future neuronal repair. Results of measurements of impedance and photoluminescence of bare NM channels are comparable to those on a flat NM, demonstrating electrical and optical modalities of our platform and suggesting that this scaffold can be expanded for active sensing and monitoring of neuron cellular processes in conditions in which they exist naturally.

Toward intelligent synthetic neural circuits: directing and accelerating neuron cell growth by self-rolled-up silicon nitride microtube array

In neural interface platforms, cultures are often carried out on a flat, open, rigid, and opaque substrate, posing challenges to reflecting the native microenvironment of the brain and precise engagement with neurons. Here we present a neuron cell culturing platform that consists of arrays of ordered microtubes (2.7-4.4 μm in diameter), formed by strain-induced self-rolled-up nanomembrane (s-RUM) technology using ultrathin (<40 nm) silicon nitride (SiNx) film on transparent substrates. These microtubes demonstrated robust physical confinement and unprecedented guidance effect toward outgrowth of primary cortical neurons, with a coaxially confined configuration resembling that of myelin sheaths. The dynamic neural growth inside the microtube, evaluated with continuous live-cell imaging, showed a marked increase (20×) of the growth rate inside the microtube compared to regions outside the microtubes. We attribute the dramatic accelerating effect and precise guiding of the microtube array to three-dimensional (3D) adhesion and electrostatic interaction with the SiNx microtubes, respectively. This work has clear implications toward building intelligent synthetic neural circuits by arranging the size, site, and patterns of the microtube array, for potential treatment of neurological disorders.

Signaling to the microtubule cytoskeleton: an unconventional role for CaMKII

Synaptic plasticity is a hallmark of the nervous system and is thought to be integral to higher brain functions such as learning and memory. Calcium, acting as a second messenger, and the calcium/calmodulin dependent kinase CaMKII are key regulators of neuronal plasticity. Given the importance of the actin and microtubule (MT) cytoskeleton in dendritic spine morphology, composition and plasticity, it is not surprising that many regulators of these cytoskeletal elements are downstream of the CaMKII pathway. In this review, we discuss the emerging role of calcium and CaMKII in the regulation of MTs and cargo unloading during synaptic plasticity.

Microtubules in neurons as information carriers

Microtubules in neurons consist of highly dynamic regions as well as stable regions, some of which persist after bouts of severing as short mobile polymers. Concentrated at the plus ends of the highly dynamic regions are microtubule plus end tracking proteins called +TIPs that can interact with an array of other proteins and structures relevant to the plasticity of the neuron. It is also provocative to ponder that short mobile microtubules might similarly convey information with them as they transit within the neuron. Thus, beyond their known conventional functions in supporting neuronal architecture and organelle transport, microtubules may act as 'information carriers' in the neuron.

Wnt5a evokes cortical axon outgrowth and repulsive guidance by tau mediated reorganization of dynamic microtubules

Wnt5a guides cortical axons in vivo by repulsion and in vitro evokes cortical axon outgrowth and repulsion by calcium signaling pathways. Here we examined the role of microtubule (MT) reorganization and dynamics in mediating effects of Wnt5a. Inhibiting MT dynamics with nocodazole and taxol abolished Wnt5a evoked axon outgrowth and repulsion of cultured hamster cortical neurons. EGFP-EB3 labeled dynamic MTs visualized in live cell imaging revealed that growth cone MTs align with the nascent axon. Wnt5a increased axon outgrowth by reorganization of dynamic MTs from a splayed to a bundled array oriented in the direction of axon extension, and Wnt5a gradients induced asymmetric redistribution of dynamic MTs toward the far side of the growth cone. Wnt5a gradients also evoked calcium transients that were highest on the far side of the growth cone. Calcium signaling and the reorganization of dynamic MTs could be linked by tau, a MT associated protein that stabilizes MTs. Tau is phosphorylated at the Ser 262 MT binding site by CaMKII, and is required for Wnt5a induced axon outgrowth and repulsive turning. Phosphorylation of tau at Ser262 is known to detach tau from MTs to increase their dynamics. Using transfection with tau constructs mutated at Ser262, we found that this site is required for the growth and guidance effects of Wnt5a by mediating reorganization of dynamic MTs in cortical growth cones. Moreover, CaMKII inhibition also prevents MT reorganization required for Wnt5a induced axon outgrowth, thus linking Wnt/calcium signaling to tau mediated MT reorganization during growth cone behaviors. © 2013 The Authors. Developmental Neurobiology Published by Wiley Periodicals, Inc.Develop Neurobiol 74: 797–817, 2014

CIP4 coordinates with phospholipids and actin-associated proteins to localize to the protruding edge and produce actin ribs and veils

Cdc42-interacting protein 4 (CIP4), a member of the F-BAR family of proteins, plays important roles in a variety of cellular events by regulating both membrane and actin dynamics. In many cell types, CIP4 functions in vesicle formation, endocytosis and membrane tubulation. However, recent data indicate that CIP4 is also involved in protrusion in some cell types, including cancer cells (lamellipodia and invadopodia) and neurons (ribbed lamellipodia and veils). In neurons, CIP4 localizes specifically to extending protrusions and functions to limit neurite outgrowth early in development. The mechanism by which CIP4 localizes to the protruding edge membrane and induces lamellipodial/veil protrusion and actin rib formation is not known. Here, we show that CIP4 localization to the protruding edge of neurons is dependent on both the phospholipid content of the plasma membrane and the underlying organization of actin filaments. Inhibiting phosphatidylinositol (3,4,5)-trisphosphate (PIP3) production decreases CIP4 at the membrane. CIP4 localization to the protruding edge is also dependent on Rac1/WAVE1, rather than Cdc42/N-WASP. Capping actin filaments with low concentrations of cytochalasin D or by overexpressing capping protein dramatically decreases CIP4 at the protruding edge, whereas inactivating Arp2/3 drives CIP4 to the protruding edge. We also demonstrate that CIP4 dynamically colocalizes with Ena/VASP and DAAM1, two proteins known to induce unbranched actin filament arrays and play important roles in neuronal development. Together, this is the first study to show that the localization of an F-BAR protein depends on both actin filament architecture and phospholipids at the protruding edge of developing neurons.

Adhesive micro-line periodicity determines guidance of axonal outgrowth

Adhesive micro-lines of various sub-cellular geometries were created using a non-traditional micro stamping technique. This technique employed the use of commercially available diffraction gratings as the molds for the micro stamps, a method which is quick and inexpensive, and which could easily be adopted as a patterning tool in a variety of research efforts. The atypical saw-tooth profile of the micro stamps enabled a unique degree of control and flexibility over patterned line and gap widths. Cortical neurons cultured on patterned poly-lysine micro-lines on PDMS exhibit a startling transition in axonal guidance: From the expected parallel guidance to an unexpected perpendicular guidance that becomes dominant as patterned lines and gaps become sufficiently narrow. This transition is most obvious when the lines are narrow relative to gaps, while the periodicity of the pattern is reduced. Axons growing perpendicular to micro-lines exhibited 'vinculated' growth, a unique morphological phenotype consisting of periodic orthogonal extensions along the axon.

Dynamic microtubules promote synaptic NMDA receptor-dependent spine enlargement

Most excitatory synaptic terminals in the brain impinge on dendritic spines. We and others have recently shown that dynamic microtubules (MTs) enter spines from the dendritic shaft. However, a direct role for MTs in long-lasting spine plasticity has yet to be demonstrated and it remains unclear whether MT-spine invasions are directly influenced by synaptic activity. Lasting changes in spine morphology and synaptic strength can be triggered by activation of synaptic NMDA receptors (NMDARs) and are associated with learning and memory processes. To determine whether MTs are involved in NMDAR-dependent spine plasticity, we imaged MT dynamics and spine morphology in live mouse hippocampal pyramidal neurons before and after acute activation of synaptic NMDARs. Synaptic NMDAR activation promoted MT-spine invasions and lasting increases in spine size, with invaded spines exhibiting significantly faster and more growth than non-invaded spines. Even individual MT invasions triggered rapid increases in spine size that persisted longer following NMDAR activation. Inhibition of either NMDARs or dynamic MTs blocked NMDAR-dependent spine growth. Together these results demonstrate for the first time that MT-spine invasions are positively regulated by signaling through synaptic NMDARs, and contribute to long-lasting structural changes in targeted spines.

Semiconductor nanomembrane tubes: three-dimensional confinement for controlled neurite outgrowth

In many neural culture studies, neurite migration on a flat, open surface does not reflect the three-dimensional (3D) microenvironment in vivo. With that in mind, we fabricated arrays of semiconductor tubes using strained silicon (Si) and germanium (Ge) nanomembranes and employed them as a cell culture substrate for primary cortical neurons. Our experiments show that the SiGe substrate and the tube fabrication process are biologically viable for neuron cells. We also observe that neurons are attracted by the tube topography, even in the absence of adhesion factors, and can be guided to pass through the tubes during outgrowth. Coupled with selective seeding of individual neurons close to the tube opening, growth within a tube can be limited to a single axon. Furthermore, the tube feature resembles the natural myelin, both physically and electrically, and it is possible to control the tube diameter to be close to that of an axon, providing a confined 3D contact with the axon membrane and potentially insulating it from the extracellular solution.

The growth cone cytoskeleton in axon outgrowth and guidance

Axon outgrowth and guidance to the proper target requires the coordination of filamentous (F)-actin and microtubules (MTs), the dynamic cytoskeletal polymers that promote shape change and locomotion. Over the past two decades, our knowledge of the many guidance cues, receptors, and downstream signaling cascades involved in neuronal outgrowth and guidance has increased dramatically. Less is known, however, about how those cascades of information converge and direct appropriate remodeling and interaction of cytoskeletal polymers, the ultimate effectors of movement and guidance. During development, much of the communication that occurs between environmental guidance cues and the cytoskeleton takes place at the growing tip of the axon, the neuronal growth cone. Several articles on this topic focus on the "input" to the growth cone, the myriad of receptor types, and their corresponding cognate ligands. Others investigate the signaling cascades initiated by receptors and propagated by second messenger pathways (i.e., kinases, phosphatases, GTPases). Ultimately, this plethora of information converges on proteins that associate directly with the actin and microtubule cytoskeletons. The role of these cytoskeletal-associated proteins, as well as the cytoskeleton itself in axon outgrowth and guidance, is the subject of this article.

Controlling Neuronal Growth on Au Surfaces by Directed Assembly of Proteins

Studying how individual neuronal cells grow and interact with each other is of fundamental importance for understanding the functions of the nervous system. However, the mechanism of axonal navigation to their target region and their specific interactions with guidance factors such as membrane-bound proteins, chemical and temperature gradients, mechanical guidance cues, etc. are not well understood. Here we describe a new approach for controlling the adhesion, growth and interconnectivity of cortical neurons on Au surfaces. Specifically, we use Atomic Force Microscopy (AFM) nanolithography to immobilize growth-factor proteins at well-defined locations on Au surfaces. These surface-immobilized proteins act as a) adhesion proteins for neuronal cells (i.e. well-defined locations where the cells “stick” to the surface), and b) promoters/inhibitors for the growth of neurites. Our results show that protein patterns can be used to confine neuronal cells and to control their growth and interconnectivity on Au surfaces. We also show that AFM nanolithography presents unique advantages for this type of work, such as high degree of control over location and shape of the protein patterns, and application of proteins in aqueous solutions (protein buffers), such that the proteins are very likely to retain their folding conformation/bioactivity.

Nucleofection and primary culture of embryonic mouse hippocampal and cortical neurons

Hippocampal and cortical neurons have been used extensively to study central nervous system (CNS) neuronal polarization, axon/dendrite outgrowth, and synapse formation and function. An advantage of culturing these neurons is that they readily polarize, forming distinctive axons and dendrites, on a two dimensional substrate at very low densities. This property has made them extremely useful for determining many aspects of neuronal development. Furthermore, by providing glial conditioning for these neurons they will continue to develop, forming functional synaptic connections and surviving for several months in culture. In this protocol we outline a technique to dissect, culture and transfect embryonic mouse hippocampal and cortical neurons. Transfection is accomplished by electroporating DNA into the neurons before plating via nucleofection. This protocol has the advantage of expressing fluorescently-tagged fusion proteins early in development (~4-8hrs after plating) to study the dynamics and function of proteins during polarization, axon outgrowth and branching. We have also discovered that this single transfection before plating maintains fluorescently-tagged fusion protein expression at levels appropriate for imaging throughout the lifetime of the neuron (> 2 months in culture). Thus, this methodology is useful for studying protein localization and function throughout CNS development with little or no disruption of neuronal function.

Distance dependence of neuronal growth on nanopatterned gold surfaces

Understanding network development in the brain is of tremendous fundamental importance, but it is immensely challenging because of the complexity of both its architecture and function. The mechanisms of axonal navigation to target regions and the specific interactions with guidance factors such as membrane-bound proteins, chemical gradients, mechanical guidance cues, etc., are largely unknown. A current limitation for the study of neural network formation is the ability to control precisely the connectivity of small groups of neurons. A first step in designing such networks is to understand the "rules" central nervous system (CNS) neurons use to form functional connections with one another. Here we begin to delineate novel rules for growth and connectivity of small numbers of neurons patterned on Au substrates in simplified geometries. These studies yield new insights into the mechanisms determining the organizational features present in intact systems. We use a previously reported atomic force microscopy (AFM) nanolithography method to control precisely the location and growth of neurons on these surfaces. By examining a series of systems with different geometrical parameters, we quantitatively and systematically analyze how neuronal growth depends on these parameters.

Positioning and guidance of neurons on gold surfaces by directed assembly of proteins using Atomic Force Microscopy

We demonstrate that Atomic Force Microscopy nanolithography can be used to control effectively the adhesion, growth and interconnectivity of cortical neurons on Au surfaces. We demonstrate immobilization of neurons at well-defined locations on Au surfaces using two different types of patterned proteins: 1) poly-d-lysine (PDL), a positively charged polypeptide used extensively in tissue culture and 2) laminin, a component of the extracellular matrix. Our results show that both PDL and laminin patterns can be used to confine neuronal cells and to control their growth and interconnectivity on Au surfaces, a significant step towards the engineering of artificial neuronal assemblies with well-controlled neuron position and connections.

Ena/VASP Is Required for neuritogenesis in the developing cortex

Mammalian cortical development involves neuronal migration and neuritogenesis; this latter process forms the structural precursors to axons and dendrites. Elucidating the pathways that regulate the cytoskeleton to drive these processes is fundamental to our understanding of cortical development. Here we show that loss of all three murine Ena/VASP proteins, a family of actin regulatory proteins, causes neuronal ectopias, alters intralayer positioning in the cortical plate, and, surprisingly, blocks axon fiber tract formation during corticogenesis. Cortical fiber tract defects in the absence of Ena/VASP arise from a failure in neurite initiation, a prerequisite for axon formation. Neurite initiation defects in Ena/VASP-deficient neurons are preceded by a failure to form bundled actin filaments and filopodia. These findings provide insight into the regulation of neurite formation and the role of the actin cytoskeleton during cortical development.

Touch and go: guidance cues signal to the growth cone cytoskeleton

Growth cones, the highly motile tips of growing axons, guide axons to their targets by responding to molecular cues. Growth cone behaviors such as advancing, retracting, turning and branching are driven by the dynamics and reorganization of the actin and microtubule cytoskeleton through signaling pathways linked to guidance cue receptors. Actin filaments play a major part in growth cone motility, and because of their peripheral locations were thought to be the primary target of molecular cues. However, recent studies have shown that dynamic microtubules can penetrate the growth cone periphery where guidance molecules can influence them directly. Moreover, guidance cues can regulate growth cone steering by modulating dynamic actin-microtubule interactions.

Netrin-1 and semaphorin 3A promote or inhibit cortical axon branching, respectively, by reorganization of the cytoskeleton

In many CNS pathways, target innervation occurs by axon branching rather than extension of the primary growth cone into targets. To investigate mechanisms of branch formation, we studied the effects of attractive and inhibitory guidance cues on cortical axon branching. We found that netrin-1, which attracts cortical axons, and FGF-2 increased branching by >50%, whereas semaphorin 3A (Sema3A), which repels cortical axons, inhibited branching by 50%. Importantly, none of the factors affected axon length significantly. The increase in branching by FGF-2 and the inhibition of branching by Sema3A were mediated by opposing effects on the growth cone (expansion vs collapse) and on the cytoskeleton. FGF-2 increased actin polymerization and formation of microtubule loops in growth cones over many hours, whereas Sema3A depolymerized actin filaments, attenuated microtubule dynamics, and collapsed microtubule arrays within minutes. Netrin-1 promoted rapid axon branching, often without involving the growth cone. Branches formed de novo on the axon shaft within 30 min after local application of netrin-1, which induced rapid accumulation of actin filaments in filopodia. Importantly, increased actin polymerization and microtubule dynamics were necessary for axon branching to occur. Taken together, these results show that guidance factors influence the organization and dynamics of the cytoskeleton at the growth cone and the axon shaft to promote or inhibit axon branching. Independent of axon outgrowth, axon branching in response to guidance cues can occur over different time courses by different cellular mechanisms.

Hot +tips

Plus end tracking proteins (+TIPS) bind to microtubules. Results reported by Lee et al. and Zhou et al. in this issue of Neuron demonstrate that, via intracellular pathways signaling to +TIPs, microtubules can be a direct target of guidance cues in steering growth cones and regulating axon elongation.

Critical role of Ena/VASP proteins for filopodia formation in neurons and in function downstream of netrin-1

Ena/VASP proteins play important roles in axon outgrowth and guidance. Ena/VASP activity regulates the assembly and geometry of actin networks within fibroblast lamellipodia. In growth cones, Ena/VASP proteins are concentrated at filopodia tips, yet their role in growth cone responses to guidance signals has not been established. We found that Ena/VASP proteins play a pivotal role in formation and elongation of filopodia along neurite shafts and growth cone. Netrin-1-induced filopodia formation was dependent upon Ena/VASP function and directly correlated with Ena/VASP phosphorylation at a regulatory PKA site. Accordingly, Ena/VASP function was required for filopodial formation from the growth cone in response to global PKA activation. We propose that Ena/VASP proteins control filopodial dynamics in neurons by remodeling the actin network in response to guidance cues.

Ena/VASP proteins: regulators of the actin cytoskeleton and cell migration

Ena/VASP proteins are a conserved family of actin regulatory proteins made up of EVH1, EVH2 domains, and a proline-rich central region. They have been implicated in actin-based processes such as fibroblast migration, axon guidance, and T cell polarization and are important for the actin-based motility of the intracellular pathogen Listeria monocytogenes. Mechanistically, these proteins associate with barbed ends of actin filaments and antagonize filament capping by capping protein (CapZ). In addition, they reduce the density of Arp2/3-dependent actin filament branches and bind Profilin at sites of actin polymerization. Vertebrate Ena/VASP proteins are substrates for PKA/PKG serine/threonine kinases. Phosphorylation by these kinases appears to modulate Ena/VASP function within cells, although the mechanism underlying this regulation remains to be determined.

Axon guidance by growth cones and branches: common cytoskeletal and signaling mechanisms

Growing axons are guided to appropriate targets by responses of their motile growth cones to environmental cues. Interstitial axon branching is also an important form of axon guidance in the mammalian CNS. Visualization of growing axons in cortical slices and in dissociated cortical cultures showed that growth cone pausing behaviors demarcate sites of future axon branching. Studies of vertebrate and invertebrate growth cones suggest common mechanisms that regulate growth cone behaviors and axon branching. These include reorganization of the actin and microtubule cytoskeleton, dynamic interactions between microtubules and actin filaments, effects of axon guidance molecules, actions of actin regulatory proteins, and dynamic changes in intracellular calcium signaling. Future challenges will be to extend high-resolution imaging of single neurons to studies of intracellular events in the intact nervous system and to apply knowledge of developmental mechanisms to the promotion of axon sprouting after injury in the adult CNS.

Cytoskeletal dynamics and transport in growth cone motility and axon guidance

Recent studies indicate the actin and microtubule cytoskeletons are a final common target of many signaling cascades that influence the developing neuron. Regulation of polymer dynamics and transport are crucial for the proper growth cone motility. This review addresses how actin filaments, microtubules, and their associated proteins play crucial roles in growth cone motility, axon outgrowth, and guidance. We present a working model for cytoskeletal regulation of directed axon outgrowth. An important goal for the future will be to understand the coordinated response of the cytoskeleton to signaling cascades induced by guidance receptor activation.

Spontaneous calcium transients in developing cortical neurons regulate axon outgrowth

Growth cones of cortical axons pause for many hours in preparation for axon branching. They become large and complex compared with small advancing growth cones. We wanted to investigate whether calcium transients regulate the advance of mammalian CNS growth cones. We found that spontaneous calcium transients in developing cortical neurons have characteristic patterns, frequencies, and amplitudes. Importantly, neurons with large paused growth cones exhibit high-frequency spontaneous calcium transients, which are rare in those with small advancing growth cones. The incidence, frequencies, and amplitudes of calcium transients are inversely related to rates of axon outgrowth. The transients are mediated primarily by L-type voltage-gated calcium channels, and silencing them with channel blockers promotes axon outgrowth. Thus calcium transients regulate growth cone advance by direct effects on the growth cone.

Axon branching requires interactions between dynamic microtubules and actin filaments

Cortical neurons innervate many of their targets by collateral axon branching, which requires local reorganization of the cytoskeleton. We coinjected cortical neurons with fluorescently labeled tubulin and phalloidin and used fluorescence time-lapse imaging to analyze interactions between microtubules and actin filaments (F-actin) in cortical growth cones and axons undergoing branching. In growth cones and at axon branch points, splaying of looped or bundled microtubules is accompanied by focal accumulation of F-actin. Dynamic microtubules colocalize with F-actin in transition regions of growth cones and at axon branch points. In contrast, F-actin is excluded from the central region of the growth cone and the axon shaft, which contains stable microtubules. Interactions between dynamic microtubules and dynamic actin filaments involve their coordinated polymerization and depolymerization. Application of drugs that attenuate either microtubule or F-actin dynamics also inhibits polymerization of the other cytoskeletal element. Importantly, inhibition of microtubule or F-actin dynamics prevents axon branching but not axon elongation. However, these treatments do cause undirected axon outgrowth. These results suggest that interactions between dynamic microtubules and actin filaments are required for axon branching and directed axon outgrowth.

Fibroblast growth factor-2 promotes axon branching of cortical neurons by influencing morphology and behavior of the primary growth cone

Interstitial branching is an important mechanism for target innervation in the developing CNS. A previous study of cortical neurons in vitro showed that the terminal growth cone pauses and enlarges in regions from which interstitial axon branches later develop (Szebenyi et al., 1998). In the present study, we investigated how target-derived signals affect the morphology and behaviors of growth cones leading to development of axon branches. We used bath and local application of a target-derived growth factor, FGF-2, on embryonic pyramidal neurons from the sensorimotor cortex and used time-lapse digital imaging to monitor effects of FGF-2 on axon branching. Observations of developing neurons over periods of several days showed that bath-applied FGF-2 significantly increased growth cone size and slowed growth cone advance, leading to a threefold increase in axon branching. FGF-2 also had acute effects on growth cone morphology, promoting rapid growth of filopodia within minutes. Application of FGF-2-coated beads promoted local axon branching in close proximity to the beads. Branching was more likely to occur when the FGF-2 bead was on or near the growth cone, suggesting that distal regions of the axon are more responsive to FGF-2 than other regions of the axon shaft. Together, these results show that interstitial axon branches can be induced locally through the action of a target-derived growth factor that preferentially exerts effects on the growth cone. We suggest that, in target regions, growth factors such as FGF-2 and other branching factors may induce formation of collateral axon branches by enhancing the pausing and enlargement of primary growth cones that determine future branch points.

Common mechanisms underlying growth cone guidance and axon branching

During development, growth cones direct growing axons into appropriate targets. However, in some cortical pathways target innervation occurs through the development of collateral branches that extend interstitially from the axon shaft. How do such branches form? Direct observations of living cortical brain slices revealed that growth cones of callosal axons pause for many hours beneath their cortical targets prior to the development of interstitial branches. High resolution imaging of dissociated living cortical neurons for many hours revealed that the growth cone demarcates sites of future axon branching by lengthy pausing behaviors and enlargement of the growth cone. After a new growth cone forms and resumes forward advance, filopodial and lamellipodial remnants of the large paused growth cone are left behind on the axon shaft from which interstitial branches later emerge. To investigate how the cytoskeleton reorganizes at axon branch points, we fluorescently labeled microtubules in living cortical neurons and imaged the behaviors of microtubules during new growth from the axon shaft and the growth cone. In both regions microtubules reorganize into a more plastic form by splaying apart and fragmenting. These shorter microtubules then invade newly developing branches with anterograde and retrograde movements. Although axon branching of dissociated cortical neurons occurs in the absence of targets, application of a target-derived growth factor, FGF-2, greatly enhances branching. Taken together, these results demonstrate that growth cone pausing is closely related to axon branching and suggest that common mechanisms underlie directed axon growth from the terminal growth cone and the axon shaft.

Reorganization and movement of microtubules in axonal growth cones and developing interstitial branches

Local changes in microtubule organization and distribution are required for the axon to grow and navigate appropriately; however, little is known about how microtubules (MTs) reorganize during directed axon outgrowth. We have used time-lapse digital imaging of developing cortical neurons microinjected with fluorescently labeled tubulin to follow the movements of individual MTs in two regions of the axon where directed growth occurs: the terminal growth cone and the developing interstitial branch. In both regions, transitions from quiescent to growth states were accompanied by reorganization of MTs from looped or bundled arrays to dispersed arrays and fragmentation of long MTs into short MTs. We also found that long-term redistribution of MTs accompanied the withdrawal of some axonal processes and the growth and stabilization of others. Individual MTs moved independently in both anterograde and retrograde directions to explore developing processes. Their velocities were inversely proportional to their lengths. Our results demonstrate directly that MTs move within axonal growth cones and developing interstitial branches. Our findings also provide the first direct evidence that similar reorganization and movement of individual MTs occur in the two regions of the axon where directed outgrowth occurs. These results suggest a model whereby short exploratory MTs could direct axonal growth cones and interstitial branches toward appropriate locations.

Cortical neurite outgrowth and growth cone behaviors reveal developmentally regulated cues in spinal cord membranes

Corticospinal axon outgrowth in vivo and the ability to sprout or regenerate after injury decline with age. This developmental decline in growth potential has been correlated with an increase in inhibitory myelin-associated proteins in older spinal cord. However, previous results have shown that sprouting of corticospinal fibers after contralateral lesions begins to diminish prior to myelination, suggesting that a decrease in growth promoting and/or an increase in inhibitory molecules in spinal gray matter may also regulate corticospinal axon outgrowth. To address this possibility, we carried out in vitro experiments to measure neurite outgrowth from explants of 1-day-old hamster forelimb sensorimotor cortex that were plated onto membrane carpets or membrane stripe assays prepared from white or gray matter of 1-to 22-day-old cervical spinal cord. On uniform carpets and in the stripe assays cortical neurites grew robustly on young but not older membranes from both white and gray matter. Mixtures of membranes from 1- and 15-day spinal cord inhibited neurite outgrowth, suggesting that the presence of inhibitory molecules in the 15-day cord overwhelmed permissive or growth promoting molecules in membranes from 1-day cord. Video microscopic observations of growth cone behaviors on membrane stripe assays transferred to glass coverslips supported this view. Cortical growth cones repeatedly collapsed at borders between permissive substrates (laminin or young membrane stripes) and nonpermissive substrates (older membrane stripes). Growth cones either turned away from the older membranes or reduced their growth rates. These results suggest that molecules in both the gray and white matter of the developing spinal cord can inhibit cortical neurite outgrowth.

Interstitial branches develop from active regions of the axon demarcated by the primary growth cone during pausing behaviors

Interstitial branches arise from the axon shaft, sometimes at great distances behind the primary growth cone. After a waiting period that can last for days after extension of the primary growth cone past the target, branches elongate toward their targets. Delayed interstitial branching is an important but little understood mechanism for target innervation in the developing CNS of vertebrates. One possible mechanism of collateral branch formation is that the axon shaft responds to target-derived signals independent of the primary growth cone. Another possibility is that the primary growth cone recognizes the target and demarcates specific regions of the axon for future branching. To address whether behaviors of the primary growth cone and development of interstitial branches are related, we performed high-resolution time-lapse imaging on dissociated sensorimotor cortical neurons that branch interstitially in vivo. Imaging of entire cortical neurons for periods of days revealed that the primary growth cone pauses in regions in which axon branches later develop. Pausing behaviors involve repeated cycles of collapse, retraction, and extension during which growth cones enlarge and reorganize. Remnants of reorganized growth cones are left behind on the axon shaft as active filopodial or lamellar protrusions, and axon branches subsequently emerge from these active regions of the axon shaft. In this study we propose a new model to account for target innervation in vivo by interstitial branching. Our model suggests that delayed interstitial branching results directly from target recognition by the primary growth cone.

Distribution of phosphorylated GAP-43 (neuromodulin) in growth cones directly reflects growth cone behavior

Phosphorylation of GAP-43 (neuromodulin) by protein kinase C (PKC) occurs at a single site, serine41. In vivo, phosphorylation is induced after initiation of axonogenesis and is confined to distal axons and growth cones. Within individual growth cones, phosphorylation is nonuniformly distributed. Here, we have used high-resolution video-enhanced microscopy of cultured dorsal root ganglia neurons together with immunocytochemistry with a monoclonal antibody that recognizes PKC-phosphorylated GAP-43 to correlate the distribution of phosphorylated GAP-43 with growth cone behavior. In "quiescent," nontranslocating growth cones, phosphorylated GAP-43 was confined to the proximal neurite and the central organelle-rich region, and was low in organelle-poor lamellae. However, levels in lamellae were elevated when they became motile. Conversely, levels of phosphorylated GAP-43 were low in either lamellae that were actively retracting or in the central organelle-rich region and proximal neurite of growth cones that had totally collapsed. The results suggest a mechanism whereby phosphorylation of GAP-43 by PKC, potentially in response to extracellular signals, could direct the functional behavior of the growth cone.

Modulation of actin filament behavior by GAP-43 (neuromodulin) is dependent on the phosphorylation status of serine 41, the protein kinase C site

Synthesis of GAP-43 (also known as neuromodulin) in neurons is induced during axon growth, and high concentrations (estimated between 50 and 100 microM) accumulate in the growth cone. GAP-43 is tightly associated with the growth cone membrane skeleton, the structure that transduces extracellular guidance cues into alterations in morphology by spatially regulating polymerization of actin filaments, thereby causing directional changes in axon growth. GAP-43 cosediments with actin filaments, and its phosphorylation on serine 41 by PKC, too, is spatially regulated so that phosphorylated GAP-43 is found in areas where growth cones make productive, stable contacts with other cells. In contrast, unphosphorylated GAP-43, which binds calmodulin, is always found in parts of the growth cone that are retracting. Here we have used a cell-free assay to investigate how the phosphorylation status of GAP-43 affects its interactions with actin and show that both phosphorylated and unphosphorylated GAP-43 have different, independent effects on actin filament structure. Phosphorylated GAP-43 stabilizes long actin filaments (Kd = 161 nM), and antibodies to phosphorylated GAP-43 inhibit binding of actin to phalloidin, implying a lateral interaction with filaments. In contrast, unphosphorylated GAP-43 reduces filament length distribution (Kd = 1.2 microM) and increases the critical concentration for polymerization. Prebinding calmodulin potentiates this effect. The results show that spatially regulated post-translational modifications of GAP-43 within the growth cone, which can be regulated in response to extracellular signals, have the ability to directly influence the structure of the actin cytoskeleton.

Altered GAP-43 immunoreactivity in regenerating sciatic nerve of diabetic rats

Experimental diabetes in the rat is associated with impaired axon regeneration. Successful regeneration depends on the construction of axonal growth cones and establishment of appropriate target connections. The growth-associated protein (GAP)-43 is a major component of the axonal growth cone, and its synthesis and axonal transport are markedly increased during regeneration. The purpose of this study was to determine the effect of experimental diabetes on the synthesis and axonal transport of GAP-43 in regenerating sciatic nerves. Rats were rendered diabetic with 50 mg/kg streptozotocin i.p. Four weeks later, the rats were anesthetized, and one sciatic nerve was crushed to induce regeneration. After 2 weeks, nerves were ligated, and 6 h later, nerve pieces proximal to the ligature and dorsal root ganglia were removed, and proteins were separated by PAGE. Western blots of gels were probed with antibody 10E8/E7 against GAP-43. The presence of GAP-43 was confirmed by immunohistochemistry of nerve sections. Densitometric analysis of the blots showed a 45% reduction in native GAP-43 immunoreactivity in nerve pieces proximal to the ligature (P < 0.05; n = 7). Northern blots of total RNA extracted from pooled dorsal root ganglia were probed with a 32P-radiolabeled cDNA probe for GAP-43. There was no significant difference in the amount of GAP-43 mRNA between diabetic and nondiabetic rats. Immunohistochemistry of sciatic nerve confirmed the reduction in GAP-43 immunoreactivity. We conclude that a defect in turnover or axonal transport of GAP-43 may contribute to the impaired peripheral nerve regeneration in diabetes.

Mutagenesis of ser41 to ala inhibits the association of GAP-43 with the membrane skeleton of GAP-43-deficient PC12B cells: effects on cell adhesion and the composition of neurite cytoskeleton and membrane

To investigate the molecular basis for GAP-43 function in axon outgrowth, we produced a mutant, GAP-43 (Ala41), whose interaction with calmodulin in vitro was unaffected by increasing Ca2+ concentrations, and stably transfected it into GAP-43-deficient PC12B cells. Several lines that expressed wild-type or mutant protein at levels that resembled endogenous GAP-43 expression in PC12 controls were subcloned and characterized. GAP-43 (Ala41) was significantly more extractable with Nonidet P-40 and less tightly associated with the membrane skeleton than the wild-type protein. Furthermore, GAP-43 (Ala41) expression by PC12B cells profoundly affected their phenotype: First, observation of living cells using video-enhanced microscopy revealed irregular plasma membranes with numerous blebs and protrusions and neurites that appeared thin and varicose. Second, both the cells' ability to remain attached to laminin substrates and the amount of alpha 1 beta 1 integrin expressed on the cell surface was significantly decreased. Finally, peripherin transport, which is abnormal in PC12B cells, could be rescued by transfection of wild-type GAP-43 but not the GAP-43 (Ala41) mutant. The phenotypic abnormalities resemble other cell types in which membrane skeleton/plasma membrane interactions have been functionally decoupled, and our results are consistent with the notion that these interactions may be abnormal in GAP-43 (Ala41)-expressing PC12B cells, either as a direct consequence of the mutation or arising secondarily to the altered availability of calmodulin in the growing neurite.