One-dimensional acoustic potential landscapes guide the neurite outgrowth and affect the viability of B35 neuroblastoma cells

On the way towards neuronal stimulation and signalling, standing surface acoustic waves (SSAW) have become a widely used technique to create well-defined networks of living cells in vitro during the past years. An overall challenge in this research area is to maintain cell viability in long-term treatments long enough to observe changes in cellular functions. To close this gap, we here investigate SSAW-directed neurite outgrowth of B35 (neuroblastoma) cells in microchannels on LiNbO3 chips, employing one-dimensional pulsed and continuous MHz-order SSAW signals at different intensities for up to 40 hours. To increase the efficiency of future investigations, we explore the limits of applicable SSAW parameters by quantifying their viability and proliferation behaviour in this long-term setup. While cell viability is impaired for power levels above 15 dBm (32 mW), our investigations on SSAW-directed neurite outgrowth reveal a significant increase of neurites growing in preferential directions by up to 31.3 % after 30 hours of SSAW treatment.

TRPV1 Agonist, Capsaicin, Induces Axon Outgrowth after Injury via Ca2+/PKA Signaling

Preconditioning nerve injuries activate a pro-regenerative program that enhances axon regeneration for most classes of sensory neurons. However, nociceptive sensory neurons and central nervous system neurons regenerate poorly. In hopes of identifying novel mechanisms that promote regeneration, we screened for drugs that mimicked the preconditioning response and identified a nociceptive ligand that activates a preconditioning-like response to promote axon outgrowth. We show that activating the ion channel TRPV1 with capsaicin induces axon outgrowth of cultured dorsal root ganglion (DRG) sensory neurons, and that this effect is blocked in TRPV1 knockout neurons. Regeneration occurs only in NF200-negative nociceptive neurons, consistent with a cell-autonomous mechanism. Moreover, we identify a signaling pathway in which TRPV1 activation leads to calcium influx and protein kinase A (PKA) activation to induce a preconditioning-like response. Finally, capsaicin administration to the mouse sciatic nerve activates a similar preconditioning-like response and induces enhanced axonal outgrowth, indicating that this pathway can be induced in vivo. These findings highlight the use of local ligands to induce regeneration and suggest that it may be possible to target selective neuronal populations for repair, including cell types that often fail to regenerate.

Autoantibodies in Human Diabetic Depression Inhibit Adult Neural Progenitor Cells In vitro and Induce Depressive-Like Behavior in Rodents

AIM

Diabetic depression increases in association with microvascular complications. We tested a hypothesis that circulating autoantibodies having anti-endothelial and anti-neuronal properties increase in subsets of diabetes with co-morbid depression.

METHODS

Protein-A eluates from plasma of 20 diabetic depression patients and 30 age-matched controls were tested for effects on endothelial cell survival, neurite outgrowth in rat pheochromocytoma (PC12) cells, or process extension and survival in adult rat dentate gyrus neural progenitor cells. The protein-A eluates from depressed or non-depressed, diabetic patients were injected (via intracerebroventricular route) into mice and 7-10 days later behavioral tests (sucrose preference, and tail suspension tests) were conducted to determine whether the autoantibodies induced anhedonia or despair.

RESULTS

Diabetic depression (n=20) autoantibodies caused a significant inhibition of PC12 cell neurite outgrowth (P<0.001) or endothelial cell proliferation compared to autoantibodies in control, diabetic (n=20) or non-diabetic (n=10) patients without depression. Process extension and survival in adult rat dentate gyrus neural progenitor cells was significantly reduced (P<0.001) by diabetic depression autoantibodies (n= 11) compared to the effects from similar concentrations (5-7 μg/mL) of autoantibodies in diabetic (n=12) or non-diabetic patients without depression (n=7). Ten micromolar concentrations of Y27632, a selective Rho-Associated Protein Kinase (ROCK) inhibitor, significantly prevented (P<0.0001) neural progenitor cell process retraction induced by diabetes depression autoantibodies (n=5). Mice treated with diabetic depression autoantibodies (n=16 from two different patients' autoantibodies) exhibited significantly reduced (P=0.027) sucrose preference (anhedonia) compared to mice treated with diabetic control autoantibodies (n=16 from two different patients' autoantibodies).

CONCLUSION

These data suggest that autoantibodies in a subset of older adult diabetic depression inhibit endothelial cell survival, and impair process extension and survival in adult dentate gyrus neural progenitor cells in vitro.

Prostacyclin prevents pericyte loss and demyelination induced by lysophosphatidylcholine in the central nervous system

Pericytes play pivotal roles in physiological and pathophysiological conditions in the central nervous system. As pericytes prevent vascular leakage, they can halt neuronal damage stemming from a compromised blood-brain barrier. Therefore, pericytes may be a good target for the treatment of neurodegenerative disorders, although evidence is lacking. In this study, we show that prostacyclin attenuates lysophosphatidylcholine (LPC)-mediated vascular dysfunction through pericyte protection in the adult mouse spinal cord. LPC decreased the number of pericytes in an in vitro blood-brain barrier model, and this decrease was prevented by iloprost treatment, a prostacyclin analog. Intrathecal administration of iloprost attenuated vascular barrier disruption after LPC injection in the mouse spinal cord. Furthermore, iloprost treatment diminished demyelination and motor function deficits in mice injected with LPC. These results support the notion that prostacyclin acts on pericytes to maintain vascular barrier integrity.

Structure-function analyses of the small GTPase Rab35 and its effector protein centaurin-β2/ACAP2 during neurite outgrowth of PC12 cells

The small GTPase Rab35 is a molecular switch for membrane trafficking that regulates a variety of cellular events. We previously showed that Rab35 promotes neurite outgrowth of nerve growth factor-stimulated PC12 cells through interaction with centaurin-β2 (also called ACAP2). Centaurin-β2 is the only Rab35-binding protein reported thus far that exclusively recognizes Rab35 and does not recognize any of the other 59 Rabs identified in mammals, but the molecular basis for the exclusive specificity of centaurin-β2 for Rab35 has remained completely unknown. In this study, we performed deletion and mutation analyses and succeeded in identifying the residues of Rab35 and centaurin-β2 that are crucial for formation of a Rab35·centaurin-β2 complex. We found that two threonine residues (Thr-76 and Thr-81) in the switch II region of Rab35 are responsible for binding centaurin-β2 and that the same residues are dispensable for Rab35 recognition by other Rab35-binding proteins. We also determined the minimal Rab35-binding site of centaurin-β2 and identified two asparagine residues (Asn-610 and Asn-691) in the Rab35-binding site as key residues for its specific Rab35 recognition. We further showed by knockdown-rescue approaches that neither a centaurin-β2 binding-deficient Rab35(T76S/T81A) mutant nor a Rab35 binding-deficient centaurin-β2(N610A/N691A) mutant supported neurite outgrowth of PC12 cells, thereby demonstrating the functional significance of the Rab35/centaurin-β2 interaction during neurite outgrowth of PC12 cells.

Activation of microtubule dynamics increases neuronal growth via the nerve growth factor (NGF)- and Gαs-mediated signaling pathways

Signals that activate the G protein Gαs and promote neuronal differentiation evoke Gαs internalization in rat pheochromocytoma (PC12) cells. These agents also significantly increase Gαs association with microtubules, resulting in an increase in microtubule dynamics because of the activation of tubulin GTPase by Gαs. To determine the function of Gαs/microtubule association in neuronal development, we used real-time trafficking of a GFP-Gαs fusion protein. GFP-Gαs concentrates at the distal end of the neurites in differentiated living PC12 cells as well as in cultured hippocampal neurons. Gαs translocates to specialized membrane compartments at tips of growing neurites. A dominant-negative Gα chimera that interferes with Gαs binding to tubulin and activation of tubulin GTPase attenuates neurite elongation and neurite number both in PC12 cells and primary hippocampal neurons. This effect is greatest on differentiation induced by activated Gαs. Together, these data suggest that activated Gαs translocates from the plasma membrane and, through interaction with tubulin/microtubules in the cytosol, is important for neurite formation, development, and outgrowth. Characterization of neuronal G protein dynamics and their contribution to microtubule dynamics is important for understanding the molecular mechanisms by which G protein-coupled receptor signaling orchestrates neuronal growth and differentiation.

Quantitative analysis of receptor tyrosine kinase-effector coupling at functionally relevant stimulus levels

A major goal of current signaling research is to develop a quantitative understanding of how receptor activation is coupled to downstream signaling events and to functional cellular responses. Here, we measure how activation of the RET receptor tyrosine kinase on mouse neuroblastoma cells by the neurotrophin artemin (ART) is quantitatively coupled to key downstream effectors. We show that the efficiency of RET coupling to ERK and Akt depends strongly on ART concentration, and it is highest at the low (∼100 pM) ART levels required for neurite outgrowth. Quantitative discrimination between ERK and Akt pathway signaling similarly is highest at this low ART concentration. Stimulation of the cells with 100 pM ART activated RET at the rate of ∼10 molecules/cell/min, leading at 5-10 min to a transient peak of ∼150 phospho-ERK (pERK) molecules and ∼50 pAkt molecules per pRET, after which time the levels of these two signaling effectors fell by 25-50% while the pRET levels continued to slowly rise. Kinetic experiments showed that signaling effectors in different pathways respond to RET activation with different lag times, such that the balance of signal flux among the different pathways evolves over time. Our results illustrate that measurements using high, super-physiological growth factor levels can be misleading about quantitative features of receptor signaling. We propose a quantitative model describing how receptor-effector coupling efficiency links signal amplification to signal sensitization between receptor and effector, thereby providing insight into design principles underlying how receptors and their associated signaling machinery decode an extracellular signal to trigger a functional cellular outcome.

Disrupted-in-schizophrenia 1 (DISC1) regulates dysbindin function by enhancing its stability

Dysbindin and DISC1 are schizophrenia susceptibility factors playing roles in neuronal development. Here we show that the physical interaction between dysbindin and DISC1 is critical for the stability of dysbindin and for the process of neurite outgrowth. We found that DISC1 forms a complex with dysbindin and increases its stability in association with a reduction in ubiquitylation. Furthermore, knockdown of DISC1 or expression of a deletion mutant, DISC1 lacking amino acid residues 403-504 of DISC1 (DISC1(Δ403-504)), effectively decreased levels of endogenous dysbindin. Finally, the neurite outgrowth defect induced by knockdown of DISC1 was partially reversed by coexpression of dysbindin. Taken together, these results indicate that dysbindin and DISC1 form a physiologically functional complex that is essential for normal neurite outgrowth.

SH2B1 and IRSp53 proteins promote the formation of dendrites and dendritic branches

SH2B1 is an adaptor protein known to enhance neurite outgrowth. In this study, we provide evidence suggesting that the SH2B1 level is increased during in vitro culture of hippocampal neurons, and the β isoform (SH2B1β) is the predominant isoform. The fact that formation of filopodia is prerequisite for neurite initiation suggests that SH2B1 may regulate filopodium formation and thus neurite initiation. To investigate whether SH2B1 may regulate filopodium formation, the effect of SH2B1 and a membrane and actin regulator, IRSp53 (insulin receptor tyrosine kinase substrate p53), is investigated. Overexpressing both SH2B1β and IRSp53 significantly enhances filopodium formation, neurite outgrowth, and branching. Both in vivo and in vitro data show that SH2B1 interacts with IRSp53 in hippocampal neurons. This interaction depends on the N-terminal proline-rich domains of SH2B1. In addition, SH2B1 and IRSp53 co-localize at the plasma membrane, and their levels increase in the Triton X-100-insoluble fraction of developing neurons. These findings suggest that SH2B1-IRSp53 complexes promote the formation of filopodia, neurite initiation, and neuronal branching.

Anchoring of protein kinase A by ERM (ezrin-radixin-moesin) proteins is required for proper netrin signaling through DCC (deleted in colorectal cancer)

Netrin-1, acting through its principal receptor DCC (deleted in colorectal cancer), serves as an axon guidance cue during neural development and also contributes to vascular morphogenesis, epithelial migration, and the pathogenesis of some tumors. Several lines of evidence suggest that netrin-DCC signaling can regulate and be regulated by the cAMP-dependent protein kinase, PKA, although the molecular details of this relationship are poorly understood. Specificity in PKA signaling is often achieved through differential subcellular localization of the enzyme by interaction with protein kinase A anchoring proteins (AKAPs). Here, we show that AKAP function is required for DCC-mediated activation of PKA and phosphorylation of cytoskeletal regulatory proteins of the Mena/VASP (vasodilator-stimulated phosphoprotein) family. Moreover, we show that DCC and PKA physically interact and that this association is mediated by the ezrin-radixin-moesin (ERM) family of plasma membrane-actin cytoskeleton cross-linking proteins. Silencing of ERM protein expression inhibits DCC-PKA interaction, DCC-mediated PKA activation, and phosphorylation of Mena/VASP proteins as well as growth cone morphology and neurite outgrowth. Finally, although expression of wild-type radixin partially rescued growth cone morphology and tropism toward netrin in ERM-knockdown cells, expression of an AKAP-deficient mutant of radixin did not fully rescue growth cone morphology and switched netrin tropism from attraction to repulsion. These data support a model in which ERM-mediated anchoring of PKA activity to DCC is required for proper netrin/DCC-mediated signaling.

Protein kinase A rescues microtubule affinity-regulating kinase 2-induced microtubule instability and neurite disruption by phosphorylating serine 409

Microtubule affinity-regulating kinase 2 (MARK2)/PAR-1b and protein kinase A (PKA) are both involved in the regulation of microtubule stability and neurite outgrowth, but whether a direct cross-talk exists between them remains unclear. Here, we found the disruption of microtubule and neurite outgrowth induced by MARK2 overexpression was blocked by active PKA. The interaction between PKA and MARK2 was confirmed by coimmunoprecipitation and immunocytochemistry both in vitro and in vivo. PKA was found to inhibit MARK2 kinase activity by phosphorylating a novel site, serine 409. PKA could not reverse the microtubule disruption effect induced by a serine 409 to alanine (Ala) mutant of MARK2 (MARK2 S409A). In contrast, mutation of MARK2 serine 409 to glutamic acid (Glu) (MARK2 S409E) did not affect microtubule stability and neurite outgrowth. We propose that PKA functions as an upstream inhibitor of MARK2 in regulating microtubule stability and neurite outgrowth by directly interacting and phosphorylating MARK2.

Cohen syndrome-associated protein COH1 physically and functionally interacts with the small GTPase RAB6 at the Golgi complex and directs neurite outgrowth

Postnatal microcephaly, intellectual disability, and progressive retinal dystrophy are major features of autosomal recessive Cohen syndrome, which is caused by mutations in the gene COH1 (VPS13B). We have recently identified COH1 as a Golgi-enriched scaffold protein that contributes to the structural maintenance and function of the Golgi complex. Here, we show that association of COH1 with the Golgi complex depends on the small GTPase RAB6. RNAi-mediated knockdown of RAB6A/A' prevents the localization of COH1 to the Golgi complex. Expression of the constitutively inactive RAB6_T27N mutant led to an increased solubilization of COH1 from lipid membrane preparations. Co-IP experiments confirmed the physical interaction of COH1 with RAB6 that preferentially occurred with the constitutively active RAB6_Q72L mutants. Depletion of COH1 in primary neurons negatively interfered with neurite outgrowth, indicating a causal link between the integrity of the Golgi complex and axonal outgrowth. We conclude that COH1 is a RAB6 effector protein and that reduced brain size in Cohen syndrome patients likely results from impaired COH1 function at the Golgi complex, causing decreased neuritogenesis.

Involvement of the Na+/Ca2+ exchanger isoform 1 (NCX1) in neuronal growth factor (NGF)-induced neuronal differentiation through Ca2+-dependent Akt phosphorylation

NGF induces neuronal differentiation by modulating [Ca(2+)]i. However, the role of the three isoforms of the main Ca(2+)-extruding system, the Na(+)/Ca(2+) exchanger (NCX), in NGF-induced differentiation remains unexplored. We investigated whether NCX1, NCX2, and NCX3 isoforms could play a relevant role in neuronal differentiation through the modulation of [Ca(2+)]i and the Akt pathway. NGF caused progressive neurite elongation; a significant increase of the well known marker of growth cones, GAP-43; and an enhancement of endoplasmic reticulum (ER) Ca(2+) content and of Akt phosphorylation through an early activation of ERK1/2. Interestingly, during NGF-induced differentiation, the NCX1 protein level increased, NCX3 decreased, and NCX2 remained unaffected. At the same time, NCX total activity increased. Moreover, NCX1 colocalized and coimmunoprecipitated with GAP-43, and NCX1 silencing prevented NGF-induced effects on GAP-43 expression, Akt phosphorylation, and neurite outgrowth. On the other hand, the overexpression of its neuronal splicing isoform, NCX1.4, even in the absence of NGF, induced an increase in Akt phosphorylation and GAP-43 protein expression. Interestingly, tetrodotoxin-sensitive Na(+) currents and 1,3-benzenedicarboxylic acid, 4,4'-[1,4,10-trioxa-7,13-diazacyclopentadecane-7,13-diylbis(5-methoxy-6,12-benzofurandiyl)]bis-, tetrakis[(acetyloxy)methyl] ester-detected [Na(+)]i significantly increased in cells overexpressing NCX1.4 as well as ER Ca(2+) content. This latter effect was prevented by tetrodotoxin. Furthermore, either the [Ca(2+)]i chelator(1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) (BAPTA-AM) or the PI3K inhibitor LY 294002 prevented Akt phosphorylation and GAP-43 protein expression rise in NCX1.4 overexpressing cells. Moreover, in primary cortical neurons, NCX1 silencing prevented Akt phosphorylation, GAP-43 and MAP2 overexpression, and neurite elongation. Collectively, these data show that NCX1 participates in neuronal differentiation through the modulation of ER Ca(2+) content and PI3K signaling.

Arf6 guanine nucleotide exchange factor cytohesin-2 binds to CCDC120 and is transported along neurites to mediate neurite growth

The mechanism of neurite growth is complicated, involving continuous cytoskeletal rearrangement and vesicular trafficking. Cytohesin-2 is a guanine nucleotide exchange factor for Arf6, an Arf family molecular switch protein, controlling cell morphological changes such as neuritogenesis. Here, we show that cytohesin-2 binds to a protein with a previously unknown function, CCDC120, which contains three coiled-coil domains, and is transported along neurites in differentiating N1E-115 cells. Transfection of the small interfering RNA (siRNA) specific for CCDC120 into cells inhibits neurite growth and Arf6 activation. When neurites start to extend, vesicles containing CCDC120 and cytohesin-2 are transported in an anterograde manner rather than a retrograde one. As neurites continue extension, anterograde vesicle transport decreases. CCDC120 knockdown inhibits cytohesin-2 localization into vesicles containing CCDC120 and diffuses cytohesin-2 in cytoplasmic regions, illustrating that CCDC120 determines cytohesin-2 localization in growing neurites. Reintroduction of the wild type CCDC120 construct into cells transfected with CCDC120 siRNA reverses blunted neurite growth and Arf6 activity, whereas the cytohesin-2-binding CC1 region-deficient CCDC120 construct does not. Thus, cytohesin-2 is transported along neurites by vesicles containing CCDC120, and it mediates neurite growth. These results suggest a mechanism by which guanine nucleotide exchange factor for Arf6 is transported to mediate neurite growth.

Amigo adhesion protein regulates development of neural circuits in zebrafish brain

The Amigo protein family consists of three transmembrane proteins characterized by six leucine-rich repeat domains and one immunoglobulin-like domain in their extracellular moieties. Previous in vitro studies have suggested a role as homophilic adhesion molecules in brain neurons, but the in vivo functions remain unknown. Here we have cloned all three zebrafish amigos and show that amigo1 is the predominant family member expressed during nervous system development in zebrafish. Knockdown of amigo1 expression using morpholino oligonucleotides impairs the formation of fasciculated tracts in early fiber scaffolds of brain. A similar defect in fiber tract development is caused by mRNA-mediated expression of the Amigo1 ectodomain that inhibits adhesion mediated by the full-length protein. Analysis of differentiated neural circuits reveals defects in the catecholaminergic system. At the behavioral level, the disturbed formation of neural circuitry is reflected in enhanced locomotor activity and in the inability of the larvae to perform normal escape responses. We suggest that Amigo1 is essential for the development of neural circuits of zebrafish, where its mechanism involves homophilic interactions within the developing fiber tracts and regulation of the Kv2.1 potassium channel to form functional neural circuitry that controls locomotion.

RNA protein granules modulate tau isoform expression and induce neuronal sprouting

The neuronal microtubule-associated protein Tau is expressed in different variants, and changes in Tau isoform composition occur during development and disease. Here, we investigate a potential role of the multivalent tau mRNA-binding proteins G3BP1 and IMP1 in regulating neuronal tau expression. We demonstrate that G3BP1 and IMP1 expression induces the formation of structures, which qualify as neuronal ribonucleoprotein (RNP) granules and concentrate multivalent proteins and mRNA. We show that RNP granule formation leads to a >30-fold increase in the ratio of high molecular weight to low molecular weight tau mRNA and an ∼12-fold increase in high molecular weight to low molecular weight Tau protein. We report that RNP granule formation is associated with increased neurite formation and enhanced process growth. G3BP1 deletion constructs that do not induce granule formation are also deficient in inducing neuronal sprouting or changing the expression pattern of tau. The data indicate that granule formation driven by multivalent proteins modulates tau isoform expression and suggest a morphoregulatory function of RNP granules during health and disease.

Myelin basic protein cleaves cell adhesion molecule L1 and promotes neuritogenesis and cell survival

The cell adhesion molecule L1 is a Lewis(x)-carrying glycoprotein that plays important roles in the developing and adult nervous system. Here we show that myelin basic protein (MBP) binds to L1 in a Lewis(x)-dependent manner. Furthermore, we demonstrate that MBP is released by murine cerebellar neurons as a sumoylated dynamin-containing protein upon L1 stimulation and that this MBP cleaves L1 as a serine protease in the L1 extracellular domain at Arg(687) yielding a transmembrane fragment that promotes neurite outgrowth and neuronal survival in cell culture. L1-induced neurite outgrowth and neuronal survival are reduced in MBP-deficient cerebellar neurons and in wild-type cerebellar neurons in the presence of an MBP antibody or L1 peptide containing the MBP cleavage site. Genetic ablation of MBP in shiverer mice and mutagenesis of the proteolytically active site in MBP or of the MBP cleavage site within L1 as well as serine protease inhibitors and an L1 peptide containing the MBP cleavage site abolish generation of the L1 fragment. Our findings provide evidence for novel functions of MBP in the nervous system.

Threonine 680 phosphorylation of FLJ00018/PLEKHG2, a Rho family-specific guanine nucleotide exchange factor, by epidermal growth factor receptor signaling regulates cell morphology of Neuro-2a cells

FLJ00018/PLEKHG2 is a guanine nucleotide exchange factor for the small GTPases Rac and Cdc42 and has been shown to mediate the signaling pathways leading to actin cytoskeleton reorganization. The function of FLJ00018 is regulated by the interaction of heterotrimeric GTP-binding protein Gβγ subunits or cytosolic actin. However, the details underlying the molecular mechanisms of FLJ00018 activation have yet to be elucidated. In the present study we show that FLJ00018 is phosphorylated and activated by β1-adrenergic receptor stimulation-induced EGF receptor (EGFR) transactivation in addition to Gβγ signaling. FLJ00018 is also phosphorylated and activated by direct EGFR stimulation. The phosphorylation of FLJ00018 by EGFR stimulation is mediated by the Ras/mitogen-activated protein kinase (MAPK) pathway. Through deletion and site-directed mutagenesis studies, we have identified Thr-680 as the major site of phosphorylation by EGFR stimulation. FLJ00018 T680A, in which the phosphorylation site is replaced by alanine, showed a limited response of the Neuro-2a cell morphology to EGF stimulation. Our results provide evidence that stimulation of the Ras/MAPK pathway by EGFR results in FLJ00018 phosphorylation at Thr-680, which in turn controls changes in cell shape.

Gas7b (growth arrest specific protein 7b) regulates neuronal cell morphology by enhancing microtubule and actin filament assembly

Neurons undergo several morphological changes as a part of normal neuron maturation process. Alzheimer disease is associated with increased neuroproliferation and impaired neuronal maturation. In this study, we demonstrated that Gas7b (growth arrest specific protein 7b) expression in a neuronal cell line, Neuro 2A, induces cell maturation by facilitating formation of dendrite-like processes and/or filopodia projections and that Gas7b co-localizes with neurite microtubules. Molecular analysis was performed to evaluate whether Gas7b associates with actin filaments and microtubules, and the data revealed two novel roles of Gas7b in neurite outgrowth: we showed that Gas7b enhances bundling of several microtubule filaments and connects microtubules with actin filaments. These results suggest that Gas7b governs neural cell morphogenesis by enhancing the coordination between actin filaments and microtubules. We conclude that lower neuronal Gas7b levels may impact Alzheimer disease progression.

TROY interacts with Rho guanine nucleotide dissociation inhibitor α (RhoGDIα) to mediate Nogo-induced inhibition of neurite outgrowth

TROY can functionally substitute p75 to comprise the Nogo receptor complex, which transduces the inhibitory signal of myelin-associated inhibitory factors on axon regeneration following CNS injury. The inhibition of neurite extension relies on TROY-dependent RhoA activation, but how TROY activates RhoA remains unclear. Here, we firstly identified Rho guanine nucleotide dissociation inhibitor α (RhoGDIα) as a binding partner of TROY using GST pull-down combined with two-dimensional gel electrophoresis and mass spectra analysis. The interaction was further confirmed by coimmunoprecipitation in vitro and in vivo. Deletion mutagenesis revealed that two regions of the TROY intracellular domain (amino acids 234-256 and 321-350) were essential for the interaction with RhoGDIα. Secondly, TROY and RhoGDIα were coexpressed in postnatal dorsal root ganglion neurons, cortex neurons, and cerebellar granule neurons (CGNs). Thirdly, TROY/RhoGDIα association was potentiated by Nogo-66 and was independent of p75/RhoGDIα interaction. Fourthly, TROY/RhoGDIα interaction was still able to activate RhoA when p75 was deficient. Furthermore, RhoA activation was decreased dramatically when TROY was knocked down in p75-deficient CGNs cells. Finally, RhoGDIα overexpression abolished RhoA activation and following neurite outgrowth inhibition by Nogo-66 in both wild-type and p75-deficient CGNs. These results showed that the association of RhoGDIα with TROY contributed to TROY-dependent RhoA activation and neurite outgrowth inhibition after Nogo-66 stimulation.

A brain-specific Grb2-associated regulator of extracellular signal-regulated kinase (Erk)/mitogen-activated protein kinase (MAPK) (GAREM) subtype, GAREM2, contributes to neurite outgrowth of neuroblastoma cells by regulating Erk signaling

Grb2-associated regulator of Erk/MAPK1 (GAREM) is an adaptor molecule in the EGF-mediated signaling pathway. GAREM is expressed ubiquitously in human organs and cultured cells. Two GAREM homologues are encoded by the human genome. Therefore, previously identified GAREM is named GAREM1. Here we characterized a new subtype of GAREM, GAREM2, that is specifically expressed in the mouse, rat, and human brain. Three GAREM2 tyrosines (Tyr-102, Tyr-429, and Tyr-551) are phosphorylated upon EGF stimulation and are necessary for binding to Grb2. Furthermore, GAREM2 and Shp2 regulate Erk activity in EGF-stimulated cells. These characteristics are similar to those of GAREM1. GAREM2 is expressed in some neuroblastoma cell lines and is also tyrosine-phosphorylated and bound to Grb2 after treatment with EGF. Eventually, GAREM2 regulates Erk activation in the presence of EGF or insulin like growth factor 1. GAREM2 also regulates insulin-like growth factor 1-induced neuronal differentiation of the SH-SY5Y neuroblastoma cell line. Although the structure and function of both GAREM subtypes are similar, GAREM1 is recruited into the nucleus and GAREM2 is not. Nuclear localization of GAREM1 might be controlled by a GAREM1-specific nuclear localization sequence and 14-3-3ε binding. The N-terminal 20 amino acids of GAREM1 make up its nuclear localization sequence that is also a 14-3-3ε binding site. The GAREM family is a new class of adaptor molecules with subtype-specific biological functions.

Low-density lipoprotein receptor-related protein 1 (LRP1)-dependent cell signaling promotes axonal regeneration

Low-density lipoprotein receptors (LRPs) are present extensively on cells outside of the nervous system and classically exert roles in lipoprotein metabolism. It has been reported recently that LRP1 activation could phosphorylate the neurotrophin receptor TrkA in PC12 cells and increase neurite outgrowth from developing cerebellar granule cells. These intriguing findings led us to explore the hypothesis that LRP1 activation would activate canonical neurotrophic factor signaling in adult neurons and promote axonal regeneration after spinal cord injury. We now find that treatment of adult rat dorsal root ganglion neurons in vitro with LRP1 agonists (the receptor binding domain of α-2-macroglobulin or the hemopexin domain of matrix metalloproteinase 9) induces TrkC, Akt, and ERK activation; significantly increases neurite outgrowth (p < 0.01); and overcomes myelin inhibition (p < 0.05). These effects require Src family kinase activation, a classic LRP1-mediated Trk transactivator. Moreover, intrathecal infusions of LRP1 agonists significantly enhance sensory axonal sprouting and regeneration after spinal cord injury in rats compared with control-infused animals (p < 0.05). A significant role is established for lipoprotein receptors in sprouting and regeneration after CNS injury, identifying a novel class of therapeutic targets to explore for traumatic neurological disorders.

Differential role of PTEN phosphatase in chemotactic growth cone guidance

Negatively targeting the tumor suppressor and phosphoinositide phosphatase PTEN (phosphatase and tensin homologue) promotes axon regrowth after injury. How PTEN functions in axon guidance has remained unknown. Here we report the differential role of PTEN in chemotactic guidance of axonal growth cones. Down-regulating PTEN expression in Xenopus laevis spinal neurons selectively abolished growth cone chemorepulsion but permitted chemoattraction. These findings persisted during cAMP-dependent switching of turning behaviors. Live cell imaging using a GFP biosensor revealed rapid PTEN-dependent depression of phosphatidylinositol 3,4,5-trisphosphate levels in the growth cone induced by the repellent myelin-associated glycoprotein. Moreover, down-regulating PTEN expression blocked negative remodeling of β1-integrin adhesions triggered by myelin-associated glycoprotein, yet permitted integrin clustering by a positive chemotropic treatment. Thus, PTEN negatively regulates growth cone phosphatidylinositol 3,4,5-trisphosphate levels and mediates chemorepulsion, whereas chemoattraction is PTEN-independent. Regenerative therapies targeting PTEN may therefore suppress growth cone repulsion to soluble cues while permitting attractive guidance, an essential feature for re-forming functional neural circuits.

The amyloid precursor protein (APP) triplicated gene impairs neuronal precursor differentiation and neurite development through two different domains in the Ts65Dn mouse model for Down syndrome

Intellectual disability in Down syndrome (DS) appears to be related to severe proliferation impairment during brain development. Recent evidence shows that it is not only cellular proliferation that is heavily compromised in DS, but also cell fate specification and dendritic maturation. The amyloid precursor protein (APP), a gene that is triplicated in DS, plays a key role in normal brain development by influencing neural precursor cell proliferation, cell fate specification, and neuronal maturation. APP influences these processes via two separate domains, the APP intracellular domain (AICD) and the soluble secreted APP. We recently found that the proliferation impairment of neuronal precursors (NPCs) from the Ts65Dn mouse model for DS was caused by derangement of the Shh pathway due to overexpression of patched1(Ptch1), its inhibitory regulator. Ptch1 overexpression was related to increased levels within the APP/AICD system. The overall goal of this study was to determine whether APP contributes to neurogenesis impairment in DS by influencing in addition to proliferation, cell fate specification, and neurite development. We found that normalization of APP expression restored the reduced neuronogenesis, the increased astrogliogenesis, and the reduced neurite length of trisomic NPCs, indicating that APP overexpression underpins all aspects of neurogenesis impairment. Moreover, we found that two different domains of APP impair neuronal differentiation and maturation in trisomic NPCs. The APP/AICD system regulates neuronogenesis and neurite length through the Shh pathway, whereas the APP/secreted AP system promotes astrogliogenesis through an IL-6-associated signaling cascade. These results provide novel insight into the mechanisms underlying brain development alterations in DS.

The atypical guanine nucleotide exchange factor Dock4 regulates neurite differentiation through modulation of Rac1 GTPase and actin dynamics

Precise regulation of neurite growth and differentiation determines accurate formation of synaptic connections, whose disruptions are frequently associated with neurological disorders. Dedicator of cytokinesis 4 (Dock4), an atypical guanine nucleotide exchange factor for Rac1, is found to be associated with neuropsychiatric diseases, including autism and schizophrenia. Nonetheless, the neuronal function of Dock4 is only beginning to be understood. Using mouse neuroblastoma (Neuro-2a) cells as a model, this study identifies that Dock4 is critical for neurite differentiation and extension. This regulation is through activation of Rac1 and modulation of the dynamics of actin-enriched protrusions on the neurites. In cultured hippocampal neurons, Dock4 regulates the establishment of the axon-dendrite polarity and the arborization of dendrites, two critical processes during neural differentiation. Importantly, a microdeletion Dock4 mutant linked to autism and dyslexia that lacks the GEF domain leads to defective neurite outgrowth and neuronal polarization. Further analysis reveals that the SH3 domain-mediated interaction of Dock4 is required for its activity toward neurite differentiation, whereas its proline-rich C terminus is not essential for this regulation. Together, our findings reveal an important role of Dock4 for neurite differentiation during early neuronal development.

Pituitary adenylate cyclase-activating polypeptide (PACAP) promotes both survival and neuritogenesis in PC12 cells through activation of nuclear factor κB (NF-κB) pathway: involvement of extracellular signal-regulated kinase (ERK), calcium, and c-REL

The pituitary adenylate cyclase-activating polypeptide (PACAP) is a trophic factor that promotes neuronal survival and neurite outgrowth. However, the signaling pathways and the transcriptional mechanisms involved are not completely elucidated. Our previous studies aimed at characterizing the transcriptome of PACAP-differentiated PC12 cells revealed an increase in the expression of nuclear factor κB2 (NF-κB2) gene coding for p100/p52 subunit of NF-κB transcription factor. Here, we examined the role of the NF-κB pathway in neuronal differentiation promoted by PACAP. We first showed that PACAP-driven survival and neuritic extension in PC12 cells are inhibited following NF-κB pathway blockade. PACAP stimulated both c-Rel and p52 NF-κB subunit gene expression and nuclear translocation, whereas c-Rel down-regulation inhibited cell survival and neuritogenesis elicited by the neuropeptide. PACAP-induced c-Rel nuclear translocation was inhibited by ERK1/2 and Ca(2+) blockers. Furthermore, the neuropeptide stimulated NF-κB p100 subunit processing into p52, indicative of activation of the NF-κB alternative pathway. Taken together, our data show that PACAP promotes both survival and neuritogenesis in PC12 cells by activating NF-κB pathway, most likely via classical and alternative signaling cascades involving ERK1/2 kinases, Ca(2+), and c-Rel/p52 dimers.