Growth cone responses to growth and chemotropic factors

Somatostatin-Mediated Regulation of Retinoic Acid-Induced Differentiation of SH-SY5Y Cells: Neurotransmitters Phenotype Characterization

During brain development, neurite formation plays a critical role in neuronal communication and cognitive function. In the present study, we compared developmental changes in the expression of crucial markers that govern the functional activity of neurons, including somatostatin (SST), choline acetyltransferase (ChAT), tyrosine hydroxylase (TH), brain nitric oxide synthase (bNOS), gamma-aminobutyric acid (GABA), glutamic acid decarboxylase (GAD-65) and synaptic vesicle protein synaptophysin (SYP) in non-differentiated and retinoic acid (RA)-induced differentiated SH-SY5Y cells. We further determined the role of SST in regulating subcellular distribution and expression of neurotransmitters. Our results indicate that SST potentiates RA-induced differentiation of SH-SY5Y cells and involves regulating the subcellular distribution and expression of neurotransmitter markers and synaptophysin translocation to neurites in a time-dependent manner, anticipating the therapeutic implication of SST in neurodegeneration.

Growth Factors as Axon Guidance Molecules: Lessons From in vitro Studies

Growth cones at the tips of extending axons navigate through developing organisms by probing extracellular cues, which guide them through intermediate steps and onto final synaptic target sites. Widespread focus on a few guidance cue families has historically overshadowed potentially crucial roles of less well-studied growth factors in axon guidance. In fact, recent evidence suggests that a variety of growth factors have the ability to guide axons, affecting the targeting and morphogenesis of growth cones in vitro. This review summarizes in vitro experiments identifying responses and signaling mechanisms underlying axon morphogenesis caused by underappreciated growth factors.

A Putative Mechanism of Age-Related Synaptic Dysfunction Based on the Impact of IGF-1 Receptor Signaling on Synaptic CaMKIIα Phosphorylation

Insulin-like growth factor 1 receptor (IGF-1R) signaling regulates the activity and phosphorylation of downstream kinases linked to inflammation, neurodevelopment, aging and synaptic function. In addition to the control of Ca2+ currents, IGF-1R signaling modulates the activity of calcium-calmodulin-dependent kinase 2 alpha (CaMKIIα) and mitogen activated protein kinase (MAPK/ErK) through multiple signaling pathways. These proteins (CaMKIIα and MAPK) regulate Ca2+ movement and long-term potentiation (LTP). Since IGF-1R controls the synaptic activity of Ca2+, CaMKIIα and MAPK signaling, the possible mechanism through which an age-dependent change in IGF-1R can alter the synaptic expression and phosphorylation of these proteins in aging needs to be investigated. In this study, we evaluated the relationship between an age-dependent change in brain IGF-1R and phosphorylation of CaMKIIα/MAPK. Furthermore, we elucidated possible mechanisms through which dysregulated CaMKIIα/MAPK interaction may be linked to a change in neurotransmitter processing and synaptic function. Male C57BL/6 VGAT-Venus mice at postnatal days 80 (P80), 365 and 730 were used to study age-related neural changes in two brain regions associated with cognitive function: hippocampus and prefrontal cortex (PFC). By means of high throughput confocal imaging and quantitative immunoblotting, we evaluated the distribution and expression of IGF-1, IGF-1R, CaMKIIα, p-CaMKIIα, MAPK and p-MAPK in whole brain lysate, hippocampus and cortex. Furthermore, we compared protein expression patterns and regional changes at P80, P365 and P730. Ultimately, we determined the relative phosphorylation pattern of CaMKIIα and MAPK through quantification of neural p-CaMKIIα and p-MAPK/ErK, and IGF-1R expression for P80, P365 and P730 brain samples. In addition to a change in synaptic function, our results show a decrease in neural IGF-1/IGF-1R expression in whole brain, hippocampus and cortex of aged mice. This was associated with a significant upregulation of phosphorylated neural MAPK (p-MAPK) and decrease in total brain CaMKIIα (i.e., CaMKIIα and p-CaMKIIα) in the aged brain. Taken together, we showed that brain aging is associated with a change in neural IGF-1/IGF-1R expression and may be linked to a change in phosphorylation of synaptic kinases (CaMKIIα and MAPK) that are involved in the modulation of LTP.

Common effects of attractive and repulsive signaling: Further analysis of Mical-mediated F-actin disassembly and regulation by Abl

To change their size, shape, and connectivity, cells require actin and tubulin proteins to assemble together into long polymers – and numerous extracellular stimuli have now been identified that alter the assembly and organization of these cytoskeletal structures. Yet, there remains a lack of defined signaling pathways from the cell surface to the cytoskeleton for many of these extracellular signals, and so we still know little of how they exert their precise structural effects. These extracellular cues may be soluble or substrate-bound and have historically been classified into two independently acting and antagonistic groups: growth-promoting/attractants (inducing turning toward the source of the factor/positive chemotropism) or growth-preventing/repellents (turning away from the source of the factor/negative chemotropism). Paradoxically, our recent results directly link the action of growth factors/chemoattractants and their signaling pathways to the promotion of the disassembly of the F-actin cytoskeleton (a defined readout of repellents/repulsive signaling). Herein, we add to this by simply driving a constitutively active form of Mical, which strongly disassembles F-actin/remodels cells in vivo independent of repulsive cues – and find that loss of Abl, which mediates growth factor signaling in these cells, decreases Mical's F-actin disassembly/cellular remodeling effects. Thus, our results are consistent with a hypothesis that cues defined as positive effectors of movement (growth factors/chemoattractants) can at least in some contexts enhance the F-actin disassembly and remodeling activity of repellents.

The Role of Hypothalamic Neuropeptides in Neurogenesis and Neuritogenesis

The hypothalamus is a source of neural progenitor cells which give rise to different populations of specialized and differentiated cells during brain development. Newly formed neurons in the hypothalamus can synthesize and release various neuropeptides. Although term neuropeptide recently undergoes redefinition, small-size hypothalamic neuropeptides remain major signaling molecules mediating short- and long-term effects on brain development. They represent important factors in neurite growth and formation of neural circuits. There is evidence suggesting that the newly generated hypothalamic neurons may be involved in regulation of metabolism, energy balance, body weight, and social behavior as well. Here we review recent data on the role of hypothalamic neuropeptides in adult neurogenesis and neuritogenesis with special emphasis on the development of food intake and social behavior related brain circuits.

Physiology, signaling, and pharmacology of galanin peptides and receptors: three decades of emerging diversity

Galanin was first identified 30 years ago as a "classic neuropeptide," with actions primarily as a modulator of neurotransmission in the brain and peripheral nervous system. Other structurally-related peptides-galanin-like peptide and alarin-with diverse biologic actions in brain and other tissues have since been identified, although, unlike galanin, their cognate receptors are currently unknown. Over the last two decades, in addition to many neuronal actions, a number of nonneuronal actions of galanin and other galanin family peptides have been described. These include actions associated with neural stem cells, nonneuronal cells in the brain such as glia, endocrine functions, effects on metabolism, energy homeostasis, and paracrine effects in bone. Substantial new data also indicate an emerging role for galanin in innate immunity, inflammation, and cancer. Galanin has been shown to regulate its numerous physiologic and pathophysiological processes through interactions with three G protein-coupled receptors, GAL1, GAL2, and GAL3, and signaling via multiple transduction pathways, including inhibition of cAMP/PKA (GAL1, GAL3) and stimulation of phospholipase C (GAL2). In this review, we emphasize the importance of novel galanin receptor-specific agonists and antagonists. Also, other approaches, including new transgenic mouse lines (such as a recently characterized GAL3 knockout mouse) represent, in combination with viral-based techniques, critical tools required to better evaluate galanin system physiology. These in turn will help identify potential targets of the galanin/galanin-receptor systems in a diverse range of human diseases, including pain, mood disorders, epilepsy, neurodegenerative conditions, diabetes, and cancer.

Competitive dynamics during resource-driven neurite outgrowth

Neurons form networks by growing out neurites that synaptically connect to other neurons. During this process, neurites develop complex branched trees. Interestingly, the outgrowth of neurite branches is often accompanied by the simultaneous withdrawal of other branches belonging to the same tree. This apparent competitive outgrowth between branches of the same neuron is relevant for the formation of synaptic connectivity, but the underlying mechanisms are unknown. An essential component of neurites is the cytoskeleton of microtubules, long polymers of tubulin dimers running throughout the entire neurite. To investigate whether competition between neurites can emerge from the dynamics of a resource such as tubulin, we developed a multi-compartmental model of neurite growth. In the model, tubulin is produced in the soma and transported by diffusion and active transport to the growth cones at the tip of the neurites, where it is assembled into microtubules to elongate the neurite. Just as in experimental studies, we find that the outgrowth of a neurite branch can lead to the simultaneous retraction of its neighboring branches. We show that these competitive interactions occur in simple neurite morphologies as well as in complex neurite arborizations and that in developing neurons competition for a growth resource such as tubulin can account for the differential outgrowth of neurite branches. The model predicts that competition between neurite branches decreases with path distance between growth cones, increases with path distance from growth cone to soma, and decreases with a higher rate of active transport. Together, our results suggest that competition between outgrowing neurites can already emerge from relatively simple and basic dynamics of a growth resource. Our findings point to the need to test the model predictions and to determine, by monitoring tubulin concentrations in outgrowing neurons, whether tubulin is the resource for which neurites compete.

Galanin stimulates neurite outgrowth from sensory neurons by inhibition of Cdc42 and Rho GTPases and activation of cofilin

We and others have previously shown that the neuropeptide galanin modulates neurite outgrowth from adult sensory neurons via activation of the second galanin receptor; however, the intracellular signalling pathways that mediate this neuritogenic effect have yet to be elucidated. Here, we demonstrate that galanin decreases the activation state in adult sensory neurons and PC12 cells of Rho and Cdc42 GTPases, both known regulators of filopodial and growth cone motility. Consistent with this, activated levels of Rho and Cdc42 levels are increased in the dorsal root ganglion of adult galanin knockout animals compared with wildtype controls. Furthermore, galanin markedly increases the activation state of cofilin, a downstream effector of many of the small GTPases, in the cell bodies and growth cones of sensory neurons and in PC12 cells. We also demonstrate a reduction in the activation of cofilin, and alteration in growth cone motility, in cultured galanin knockout neurons compared with wildtype controls. These data provide the first evidence that galanin regulates the Rho family of GTPases and cofilin to stimulate growth cone dynamics and neurite outgrowth in sensory neurons. These findings have important therapeutic implications for the treatment of peripheral sensory neuropathies.

GABAergic terminals are a source of galanin to modulate cholinergic neuron development in the neonatal forebrain

The distribution and (patho-)physiological role of neuropeptides in the adult and aging brain have been extensively studied. Galanin is an inhibitory neuropeptide that can coexist with γ-aminobutyric acid (GABA) in the adult forebrain. However, galanin's expression sites, mode of signaling, impact on neuronal morphology, and colocalization with amino acid neurotransmitters during brain development are less well understood. Here, we show that galaninergic innervation of cholinergic projection neurons, which preferentially express galanin receptor 2 (GalR2) in the neonatal mouse basal forebrain, develops by birth. Nerve growth factor (NGF), known to modulate cholinergic morphogenesis, increases GalR2 expression. GalR2 antagonism (M871) in neonates reduces the in vivo expression and axonal targeting of the vesicular acetylcholine transporter (VAChT), indispensable for cholinergic neurotransmission. During cholinergic neuritogenesis in vitro, GalR2 can recruit Rho-family GTPases to induce the extension of a VAChT-containing primary neurite, the prospective axon. In doing so, GalR2 signaling dose-dependently modulates directional filopodial growth and antagonizes NGF-induced growth cone differentiation. Galanin accumulates in GABA-containing nerve terminals in the neonatal basal forebrain, suggesting its contribution to activity-driven cholinergic development during the perinatal period. Overall, our data define the cellular specificity and molecular complexity of galanin action in the developing basal forebrain.

Group IVA phospholipase A₂ is necessary for growth cone repulsion and collapse

The repellent semaphorin 3A (Sema3A) causes growth cone turning or collapse by triggering cytoskeletal rearrangements and detachment of adhesion sites. Growth cone detachment is dependent on eicosanoid activation of protein kinase C epsilon (PKCε), but the characterization of the phospholipase A(2) (PLA(2) ) that releases arachidonic acid (AA) for eicosanoid synthesis has remained elusive. Here, we show, in rat dorsal root ganglion (DRG) neurons, that Sema3A stimulates PLA(2) activity, that Sema3A-induced growth cone turning and collapse are dependent on the release of AA, and that the primary PLA(2) involved is the group IV α isoform (GIVA). Silencing GIVA expression renders growth cones resistant to Sema3A-induced collapse, and GIVA inhibition reverses Sema3A-induced repulsion into attraction. These studies identify a novel, early step in Sema3A-signaling and a PLA(2) necessary for growth cone repulsion and collapse.

Neuronal growth cones respond to laser-induced axonal damage

Although it is well known that damage to neurons results in release of substances that inhibit axonal growth, release of chemical signals from damaged axons that attract axon growth cones has not been observed. In this study, a 532 nm 12 ns laser was focused to a diffraction-limited spot to produce site-specific damage to single goldfish axons in vitro. The axons underwent a localized decrease in thickness ('thinning') within seconds. Analysis by fluorescence and transmission electron microscopy indicated that there was no gross rupture of the cell membrane. Mitochondrial transport along the axonal cytoskeleton immediately stopped at the damage site, but recovered over several minutes. Within seconds of damage nearby growth cones extended filopodia towards the injury and were often observed to contact the damaged site. Turning of the growth cone towards the injured axon also was observed. Repair of the laser-induced damage was evidenced by recovery of the axon thickness as well as restoration of mitochondrial movement. We describe a new process of growth cone response to damaged axons. This has been possible through the interface of optics (laser subcellular surgery), fluorescence and electron microscopy, and a goldfish retinal ganglion cell culture model.

Factors promoting neurite outgrowth during deer antler regeneration

Every year male deers completely regenerate their antlers. During this process, antlers are reinnervated by sensory fibers, growing at the highest rate recorded for any adult mammal. Despite its clinical potential, only a few studies have dealt with this fascinating phenomenon. Among the possible factors underlying fast growth of the antler's innervation, the effects of the antler's endocrine and paracrine factors were evaluated, using an in vitro assay for sensory neurite growth. We found that soluble molecules secreted by the velvet, the modified skin that covers the antler, strongly promote neurite outgrowth. Using specific blocking antibodies, we demonstrated that nerve growth factor is partially responsible for these effects, although other unidentified molecules are also involved. On the contrary, neither endocrine serum factors nor antler substrates promoted neurite outgrowth, although antler substrata from deep velvet layers cause neurite outgrowth orientation. Taken together, our results point to the existence in the deep velvet of an environment that promotes oriented axon growth, in agreement with the distribution of the antler innervation.

Concerted action of CB1 cannabinoid receptor and deleted in colorectal cancer in axon guidance

Endocannabinoids (eCBs) are retrograde neurotransmitters that modulate the function of many types of synapses. The presence of eCBs, their CB1 receptor (CB1R), and metabolizing enzymes at embryonic and early postnatal periods have been linked to developmental processes such as neuronal proliferation, differentiation, and migration, axon guidance, and synaptogenesis. Here, we demonstrate the presence of a functional eCB system in the developing visual system and the role of CB1R during axon growth and retinothalamic development. Pharmacological treatment of retinal explants and primary cortical neuron cultures with ACEA, a selective CB1R agonist, induced a collapse of the growth cone (GC). Furthermore the application of AM251, a CB1R inverse agonist, to the neuronal cultures increased the surface area of GC. In vivo, intraocular injection of ACEA diminished retinal projection growth, while AM251 promoted growth and caused aberrant projections. In addition, compared with their wild-type littermates, CB1R-deficient adult mice revealed a lower level of eye-specific segregation of retinal projections in the dorsal lateral geniculate nucleus. Finally, we found that pharmacological modulation of CB1R affected the trafficking of Deleted in colorectal cancer (DCC) receptor to the plasma membrane in a PKA-dependent manner. Moreover, pharmacological inhibition or genetic inactivation of DCC abolished the CB1R-induced reorganization of the GC. Overall, these findings establish a mechanism by which the CB1R influences GC behavior and nervous system development in concerted action with DCC.

Dynamic plasticity of axons within a cutaneous milieu

The skin is a repository of sensory axons immersed within the turnover of epidermal, follicular, and dermal cellular constituents. We show that epidermal and perifollicular axons within intact hairy skin of mice possess a remarkable dynamic plasticity linked to their microenvironment. For example, the majority of epidermal axons express the growth protein GAP43. Unexpectedly, we induced new cutaneous axogenesis by simple and noninvasive hair clipping, a response linked to a series of changes in their cutaneous neighbors. In thy-1 YFP transgenic mice with fluorescent axons, superficial epidermal and perifollicular cells newly acquired YFP, indicating diffuse activation by clipping despite the absence of skin injury. At 48 h after clipping, this activation was accompanied by a rise in the number of epidermal cells, transient rises in mRNA of Sox2, a marker of follicular stem cells, and a rise in mRNA of glial fibrillary acidic protein, a marker of glial cells. Axons responded with rises in their numbers in the epidermis and around dermal hair follicles. Linking these responses were early, large, and selective rises in hepatic growth factor (HGF) mRNA, with its protein identified in epidermal cells, perifollicular cells, and sensory axons. Moreover, these elements also expressed the HGF receptor c-Met, especially in small caliber sensory neurons. Finally, we identified concurrent rises in Rac1 activation, a downstream target of ligated c-Met. Together, these results confirm critical linkages between sensory axons and their cutaneous milieu. We believe that the plasticity is provoked by follicular-originating cutaneous activation with HGF and Rac1 signaling, allowing cross talk and axonal remodeling.

Galanin and consummatory behavior: special relationship with dietary fat, alcohol and circulating lipids

Galanin (GAL) plays an integral role in consummatory behavior. In particular, hypothalamic GAL has a positive, reciprocal relationship with dietary fat and alcohol. In this relationship, GAL increases the consumption of fat or alcohol which, in turn, stimulates the expression of GAL, ultimately leading to overconsumption. Through actions in the amygdala, this relationship may become especially important in stress-induced food or drug intake. These effects of GAL in promoting overconsumption may involve various neurotransmitters, with GAL facilitating intake by stimulating norepinephrine and dopamine and reducing satiety by decreasing serotonin and acetylcholine. In addition, GAL in the hypothalamus stimulates the opioid, enkephalin, throughout the brain, which also promotes overconsumption. The relationship between GAL, fat, and alcohol may involve triglycerides, circulating lipids that are released by fat or alcohol and that correlate positively with hypothalamic GAL expression. In females, levels of endogenous GAL also fluctuate across the reproductive cycle, driven by a rise in the ovarian steroids, estrogen, and progesterone. They peak during the proestrous phase and also at puberty, simultaneous to a sharp increase in preference for fat to meet energy demands. Prenatal exposure to a high-fat diet also enhances hypothalamic expression of GAL into adulthood because of an increase in neurogenesis and proliferation of GAL-expressing neurons in this region. This organizational change may reflect the role of GAL in neuronal development, including neurite growth in adulthood, cell survival in aging, and cell stability in the disease state. By responding positively to fat and alcohol and guiding further neuronal development, GAL potentiates a long-term propensity to overconsume fat and alcohol.

Cell Guidance by 3D-Gradients in Hydrogel Matrices: Importance for Biomedical Applications

Concentration gradients of soluble and matrix-bound guidance cues in the extracellular matrix direct cell growth in native tissues and are of great interest for design of biomedical scaffolds and on implant surfaces. The focus of this review is to demonstrate the importance of gradient guidance for cells as it would be desirable to direct cell growth onto/into biomedical devices. Many studies have been described that illustrate the production and characterization of surface gradients, but three dimensional (3D)-gradients that direct cellular behavior are not well investigated. Hydrogels are considered as synthetic replacements for native extracellular matrices as they share key functions such as 2D- or 3D-solid support, fibrous structure, gas- and nutrition permeability and allow storage and release of biologically active molecules. Therefore this review focuses on current studies that try to implement soluble or covalently-attached gradients of growth factors, cytokines or adhesion sequences into 3D-hydrogel matrices in order to control cell growth, orientation and migration towards a target. Such gradient architectures are especially desirable for wound healing purposes, where defined cell populations need to be recruited from the blood stream and out of the adjacent tissue, in critical bone defects, for vascular implants or neuronal guidance structures where defined cell populations should be guided by appropriate signals to reach their proper positions or target tissues in order to accomplish functional repair.

The trip of the tip: understanding the growth cone machinery

The central component in the road trip of axon guidance is the growth cone, a dynamic structure that is located at the tip of the growing axon. During its journey, the growth cone comprises both 'vehicle' and 'navigator'. Whereas the 'vehicle' maintains growth cone movement and contains the cytoskeletal structural elements of its framework, a motor to move forward and a mechanism to provide traction on the 'road', the 'navigator' aspect guides this system with spatial bias to translate environmental signals into directional movement. The understanding of the functions and regulation of the vehicle and navigator provides new insights into the cell biology of growth cone guidance.

NPY and its involvement in axon guidance, neurogenesis, and feeding

OBJECTIVES

The role of neuropeptides in nervous system function is still in many cases undefined. In the present study we examined a possible role of the 36-amino acid neuropeptide Y (NPY) with regard to three functions: axon guidance and attraction/repulsion, adult neurogenesis, and control of food intake.

METHODS

Growth cones from embryonic dorsal root ganglion neurons were studied in culture during asymmetrical gradient application of NPY. Growth cones were monitored over a 60-min period, and final turning angle and growth rate were recorded. In the second part the NPY Y(1) and Y(2) receptors were studied in the subventricular zone, the rostral migratory stream, and the olfactory bulb in normal mice and mice with genetically deleted NPY Y(1) or Y(2) receptors. In the third part an anorectic mouse was analyzed with immunohistochemistry.

RESULTS

1) NPY elicited an attractive turning response and an increase in growth rate, effects exerted via the NPY Y(1) receptor. 2) The NPY Y(1) receptor was expressed in neuroblasts in the anterior rostral migratory stream. Mice deficient in the Y(1) or Y(2) receptor had fewer proliferating precursor cells and neuroblasts in the subventricular zone and rostral migratory stream and fewer neurons in the olfactory bulb expressing calbindin, calretinin or tyrosine hydroxylase. 3) In the anorectic mouse markers for microglia were strongly upregulated in the arcuate nucleus and in projection areas of the NPY/agouti gene-related protein arcuate system.

CONCLUSION

NPY participates in several mechanisms involved in the development of the nervous system and is of importance in the control of food intake.