PI-3 kinase and IP3: partners in NT3-induced synaptic transmission

Neural specialization for 'visual' concepts emerges in the absence of vision

Vision provides a key source of information about many concepts, including 'living things' (e.g., tiger) and visual events (e.g., sparkle). According to a prominent theoretical framework, neural specialization for different conceptual categories is shaped by sensory features, e.g., living things are neurally dissociable from navigable places because living things concepts depend more on visual features. We tested this framework by comparing the neural basis of 'visual' concepts across sighted (n=22) and congenitally blind (n=21) adults. Participants judged the similarity of words varying in their reliance on vision while undergoing fMRI. We compared neural responses to living things nouns (birds, mammals) and place nouns (natural, manmade). In addition, we compared visual event verbs (e.g., 'sparkle') to non-visual events (sound emission, hand motion, mouth motion). People born blind exhibited distinctive univariate and multivariate responses to living things in a temporo-parietal semantic network activated by nouns, including the precuneus (PC). To our knowledge, this is the first demonstration that neural selectivity for living things does not require vision. We additionally observed preserved neural signatures of 'visual' light events in the left middle temporal gyrus (LMTG+). Across a wide range of semantic types, neural representations of sensory concepts develop independent of sensory experience.

Atlastin regulates store-operated calcium entry for nerve growth factor-induced neurite outgrowth

Homotypic membrane fusion of the endoplasmic reticulum (ER) is mediated by a class of dynamin-like GTPases known as atlastin (ATL). Depletion of or mutations in ATL cause an unbranched ER morphology and hereditary spastic paraplegia (HSP), a neurodegenerative disease characterized by axon shortening in corticospinal motor neurons and progressive spasticity of the lower limbs. How ER shaping is linked to neuronal defects is poorly understood. Here, we show that dominant-negative mutants of ATL1 in PC-12 cells inhibit nerve growth factor (NGF)-induced neurite outgrowth. Overexpression of wild-type or mutant ATL1 or depletion of ATLs alters ER morphology and affects store-operated calcium entry (SOCE) by decreasing STIM1 puncta formation near the plasma membrane upon calcium depletion of the ER. In addition, blockage of the STIM1-Orai pathway effectively abolishes neurite outgrowth of PC-12 cells stimulated by NGF. These results suggest that SOCE plays an important role in neuronal regeneration, and mutations in ATL1 may cause HSP, partly by undermining SOCE.

Neurotrophin Signaling and Stem Cells-Implications for Neurodegenerative Diseases and Stem Cell Therapy

Neurotrophins (NTs) are members of a neuronal growth factor protein family whose action is mediated by the tropomyosin receptor kinase (TRK) receptor family receptors and the p75 NT receptor (p75NTR), a member of the tumor necrosis factor (TNF) receptor family. Although NTs were first discovered in neurons, recent studies have suggested that NTs and their receptors are expressed in various types of stem cells mediating pivotal signaling events in stem cell biology. The concept of stem cell therapy has already attracted much attention as a potential strategy for the treatment of neurodegenerative diseases (NDs). Strikingly, NTs, proNTs, and their receptors are gaining interest as key regulators of stem cells differentiation, survival, self-renewal, plasticity, and migration. In this review, we elaborate the recent progress in understanding of NTs and their action on various stem cells. First, we provide current knowledge of NTs, proNTs, and their receptor isoforms and signaling pathways. Subsequently, we describe recent advances in the understanding of NT activities in various stem cells and their role in NDs, particularly Alzheimer's disease (AD) and Parkinson's disease (PD). Finally, we compile the implications of NTs and stem cells from a clinical perspective and discuss the challenges with regard to transplantation therapy for treatment of AD and PD.

Aberrant resting-state functional connectivity in limbic and salience networks in treatment--naïve clinically depressed adolescents

BACKGROUND

Depression is prevalent and typically has its onset in adolescence. Resting-state fMRI could help create a better understanding of the underlying neurobiological mechanisms during this critical period. In this study, resting-state functional connectivity (RSFC) is examined using seed regions-of-interest (ROIs) associated with three networks: the limbic network, the default mode network (DMN) and the salience network.

METHODS

Twenty-six treatment-naïve, clinically depressed adolescents of whom 18 had comorbid anxiety, and 26 pair-wise matched healthy controls underwent resting-state fMRI. The three networks were investigated using a seed-based ROI approach with seeds in the bilateral amygdala (limbic network), bilateral dorsal anterior cingulate cortex (dACC; salience network) and bilateral posterior cingulate cortex (default mode network).

RESULTS

Compared to healthy controls, clinically depressed adolescents showed increased RSFC of the left amygdala with right parietal cortical areas, and decreased right amygdala RSFC with left frontal cortical areas including the ACC, as well as with right occipito-parietal areas. The bilateral dACC showed decreased RSFC with the right middle frontal gyrus, frontal pole, and inferior frontal gyrus in clinically depressed adolescents. No abnormalities in DMN RSFC were found, and differences in RSFC did not correlate with clinical measures.

CONCLUSIONS

The aberrant RSFC of the amygdala network and the dACC network may be related to altered emotion processing and regulation in depressed adolescents. Our results provide new insights into RSFC in clinically depressed adolescents and future models on adolescent depression may include abnormalities in the connectivity of salience network.

Genetic association of the Phosphoinositide-3 kinase in schizophrenia and bipolar disorder and interaction with a BDNF gene polymorphism

Phosphoinositide-3-kinase, class III (PIK3C3) is a member of the phosphoinosite-3-kinases family, involved in cell signaling, membrane trafficking, and neurodevelopment. Previous studies have indeed shown an association between PIK3C3 gene variants and both bipolar disorder (BD) and schizophrenia (SZ). Brain-derived neurotrophic factor (BDNF) is a neurodevelopmental factor, which can regulate the PI3K signaling pathway. Associations have been reported between BDNF gene polymorphisms and affective and psychotic disorders. The aim of the present study was to replicate an association between PIK3C3 and BDNF gene variants in SZ and BD and a putative epistasis between the two genes. Patients meeting the DSM-IV criteria of BD and SZ were included in this study (98 BD and 79 SZ) as well as 158 healthy controls. Blood DNA was extracted and genotyping was performed either by the polymerase chain reaction (PCR) technique followed by enzymatic digestion or by the high-resolution melt (HRM) method. Genotype and haplotype association was assessed with the UNPHASED statistical program.The results showed one nominal association with BD (P < 0.02) and two risk haplotypes in both SZ (P < 0.001) and BP (P < 0.0005), which survived multiple testing correction. A modest interaction between a BDNF variant and PI3KC3 polymorphism was observed (P < 0.04).These preliminary results confirm the genetic association of PI3K gene variants with both SZ and BD, and support the hypothesis that SZ and BD share a genetic background.

Signaling pathways of the early differentiation of neural stem cells by neurotrophin-3

Neurotrophin-3 (NT-3) is well known to play an important role in facilitating neuronal survival and differentiation during development. However, the mechanisms by which neurotrophin-3 promotes prolonged Akt/MAPK signaling at an early stage are not well understood. Here, we report that NT-3 works at an early stage of neuronal differentiation in mouse neural stem cells (NSCs). After treatment with NT-3 for 12h, more NSCs differentiated into neurons than did untreated cells. These findings demonstrated that stimulation with NT-3 causes NSCs to differentiate into neurons through a phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway and the phosphorylated extracellular signal-regulated kinase (ERK) pathway. In addition, treatment with NT-3 induced neurite outgrowth by specific phosphorylation of p38 MAPK, which was accompanied by neuronal differentiation. Taken together, these results suggest that NT-3, along with the Trk C receptors in NSCs, might lead to the survival and neuronal differentiation of NSCs via two distinct downstream signaling pathways at an early stage of neuronal differentiation.

Effects of glossopharyngeal nerve section on the expression of neurotrophins and their receptors in lingual taste buds of adult mice

The expression of neurotrophins and neurotrophin receptors is essential for the proper establishment and function of many sensory systems. To determine which neurotrophins and neurotrophin receptors are expressed in taste buds, and in taste buds of mice following denervation, antibodies directed against the neurotrophins and their receptors were applied to adult mouse gustatory tissue. Immunohistochemistry reveals that nerve growth factor (NGF)-like immunoreactive (LIR), tyrosine kinase (trk) A-LIR, trkB-LIR, and p75-LIR elongated, differentiated taste cells are present within all lingual taste buds, whereas neither neurotrophin (NT)-3- nor trkC-LIR was detected in taste cells. Double-label immunohistochemistry using markers of different taste cell types in brain-derived neurotrophic factor (BDNF)LacZ mice reveals that BDNF (beta-gal) and trkB colocalize, mainly in type III taste cells. NGF, pro-NGF, and trkA coexist in type II taste cells, i.e., those expressing phospholipase Cbeta2 (PLCbeta2). p75-LIR also is present in both BDNF and NGF taste cell populations. To determine the neural dependence of neurotrophin expression in adult taste buds, glossopharyngeal nerves were cut unilaterally. During the period of denervation (10 days to 3 weeks), taste buds largely disappear, and few neurotrophin-expressing cells are present. Three weeks after nerve transection, nerve fascicles on the operated side of the tongue exhibit BDNF-LIR, NGF-LIR, and ubiquitin carboxyl terminal hydrolase (PGP 9.5)-LIR. However, BDNF-LIR staining intensity but not NGF-LIR or PGP 9.5-LIR is increased in nerve fascicles on the operated compared with the unoperated side. Five weeks following nerve transection, NT and NT receptor expression resumes and appears normal in taste buds and nerves. These results indicate that neurotrophin expression in taste buds is dependent on gustatory innervation, but expression in nerves is not dependent on contact with taste buds.

Phosphatidylinositol 3-kinase activates ERK in primary sensory neurons and mediates inflammatory heat hyperalgesia through TRPV1 sensitization

Although the PI3K (phosphatidylinositol 3-kinase) pathway typically regulates cell growth and survival, increasing evidence indicates the involvement of this pathway in neural plasticity. It is unknown whether the PI3K pathway can mediate pain hypersensitivity. Intradermal injection of capsaicin and NGF produce heat hyperalgesia by activating their respective TRPV1 (transient receptor potential vanilloid receptor-1) and TrkA receptors on nociceptor sensory nerve terminals. We examined the activation of PI3K in primary sensory DRG neurons by these inflammatory agents and the contribution of PI3K activation to inflammatory pain. We further investigated the correlation between the PI3K and the ERK (extracellular signal-regulated protein kinase) pathway. Capsaicin and NGF induce phosphorylation of the PI3K downstream target AKT (protein kinase B), which is blocked by the PI3K inhibitors LY294002 and wortmannin, indicative of the activation of PI3K by both agents. ERK activation by capsaicin and NGF was also blocked by PI3K inhibitors. Similarly, intradermal capsaicin in rats activated PI3K and ERK in C-fiber DRG neurons and epidermal nerve fibers. Injection of PI3K or MEK (ERK kinase) inhibitors into the hindpaw attenuated capsaicin- and NGF-evoked heat hyperalgesia but did not change basal heat sensitivity. Furthermore, PI3K, but not ERK, inhibition blocked early induction of hyperalgesia. In acutely dissociated DRG neurons, the capsaicin-induced TRPV1 current was strikingly potentiated by NGF, and this potentiation was completely blocked by PI3K inhibitors and primarily suppressed by MEK inhibitors. Therefore, PI3K induces heat hyperalgesia, possibly by regulating TRPV1 activity, in an ERK-dependent manner. The PI3K pathway also appears to play a role that is distinct from ERK by regulating the early onset of inflammatory pain.

Neurotrophins in spinal cord nociceptive pathways

Neurotrophins are a well-known family of growth factors for the central and peripheral nervous systems. In the course of the last years, several lines of evidence converged to indicate that some members of the family, particularly NGF and BDNF, also participate in structural and functional plasticity of nociceptive pathways within the dorsal root ganglia and spinal cord. A subpopulation of small-sized dorsal root ganglion neurons is sensitive to NGF and responds to peripheral NGF stimulation with upregulation of BDNF synthesis and increased anterograde transport to the dorsal horn. In the latter, release of BDNF appears to modulate or even mediate nociceptive sensory inputs and pain hypersensitivity. We summarize here the status of the art on the role of neurotrophins in nociceptive pathways, with special emphasis on short-term synaptic and intracellular events that are mediated by this novel class of neuromessengers in the dorsal horn. Under this perspective we review the findings obtained through an array of techniques in naïve and transgenic animals that provide insight into the modulatory mechanisms of BDNF at central synapses. We also report on the results obtained after immunocytochemistry, in situ hybridization, and monitoring intracellular calcium levels by confocal microscopy, that led to hypothesize that also NGF might have a direct central effect in pain modulation. Although it is unclear whether or not NGF may be released at dorsal horn endings of certain nociceptors in vivo, we believe that these findings offer a clue for further studies aiming to elucidate the putative central effects of NGF and other neurotrophins in nociceptive pathways.

Signalling pathways involved in the short-term potentiation of dopamine release by BDNF

Brain-derived neurotrophic factor (BDNF) has been shown to modulate synaptic plasticity in the corpus striatum in vitro by activation of the tyrosine kinase linked receptor, TrkB. However, the signalling pathways that mediate this modulation of plasticity are poorly understood. Three proteins mediating signalling pathways are activated by the binding of BDNF to TrkB: phosphoinositol-3 kinase (PI3K); Ras-MEK and phospholipase C-gamma (PLCgamma). The present study investigates which of these pathways are necessary for BDNF-mediated potentiation of synaptic output of dopamine from slices and synaptosomes of rat corpus striatum. The results indicate that activation of the PI3K and Ras-MEK pathways, but not PLCgamma, are involved. Inhibitors of transcription and translation had no effect on the potentiation of depolarisation-stimulated (15 mM KCl) dopamine release mediated by BDNF.

Diversity of neurotrophin action in the postnatal spinal cord

The expression of neurotrophins and their receptors in the adult spinal cord indicates that they have postnatal actions in addition to their well-known prenatal ones on axonal growth and cell survival. In this review we summarize evidence in support of mechanisms by which neurotrophins acutely modulate the response both of sensory neurons and of synapses within the spinal cord. The selective action of neurotrophins is achieved via restricted expression of high affinity trk receptors through which the neurotrophins act. Activation of trk receptors enhances the response of the vanilloid VR-1 receptor in nociceptive neurons leading to peripheral sensitization of the response to capsaicin or noxious heat. At synapses on motoneurons trk receptor activation enhances the response of NMDA receptors that in turn can increase the response of AMPA/kainate receptors on the same cell. Both of these sensitizing actions have a very rapid onset that is contrasted with slower neurotrophin effects on growth of axotomized afferents. It is likely that these different functional effects of neurotrophins reflect activation of different intracellular signaling pathways. These studies suggest mechanisms by which neurotrophins might be used to improve function of the damaged spinal cord.

Costorage and coexistence of neuropeptides in the mammalian CNS

The term neuropeptides commonly refers to a relatively large number of biologically active molecules that have been localized to discrete cell populations of central and peripheral neurons. I review here the most important histological and functional findings on neuropeptide distribution in the central nervous system (CNS), in relation to their role in the exchange of information between the nerve cells. Under this perspective, peptide costorage (presence of two or more peptides within the same subcellular compartment) and coexistence (concurrent presence of peptides and other messenger molecules within single nerve cells) are discussed in detail. In particular, the subcellular site(s) of storage and sorting mechanisms within neurons are thoroughly examined in the view of the mode of release and action of neuropeptides as neuronal messengers. Moreover, the relationship of neuropeptides and other molecules implicated in neural transmission is discussed in functional terms, also referring to the interactions with novel unconventional transmitters and trophic factors. Finally, a brief account is given on the presence of neuropeptides in glial cells.

Acute sensitization by NGF of the response of small-diameter sensory neurons to capsaicin

We investigated acute sensitization by nerve growth factor (NGF) of the response of small-diameter (<30 microm) dissociated dorsal root ganglion (DRG) cells to brief repeated puffs of capsaicin as a model for thermal hyperalgesia induced by NGF. We have previously shown that placing NGF in the bath after an initial puff of capsaicin can completely overcome the tachyphylaxis normally observed in response to a second puff 10 min later, and this response is often substantially larger than the first. If tachyphylaxis is abolished by carrying out the experiment in Ca2+-free solution, NGF still elicits potentiation of the second puff. However, the amount of potentiation is considerably less than that observed when tachyphylaxis also takes place. Thus it is concluded that NGF has two effects: overcoming tachyphylaxis and potentiation. With three puffs of capsaicin separated by 10 min, we have found that the potentiation established after 10 min exposure to NGF is no longer evident 10 min after removal of NGF. In Ca2+-free solution the potentiation can last up to 1 h after removal of NGF. These results suggest that the initial behavioral sensitization elicited by NGF could result from a direct effect on the sensory neuron but that its later components most likely involve other mechanisms. We have also investigated the contribution of various second-messenger pathways in these actions of NGF by treating the cells with blockers of MAP kinase (PD98059), protein kinase A (PKA; PKAI14-22, H89), and PKC (Bisindolylmaleimide I). Surprisingly, PD98059, which previously has been shown to diminish the enhancement of capsaicin responses of dissociated neurons when exposed to NGF for several days, had no effect on the acute response to NGF; nor did the PKC inhibitor. However, PKA inhibitors reduced the capsaicin response of the cells to NGF (as determined from the NGF effect on tachyphylaxis). Consistent with these findings we confirmed that forskolin, a PKA activator, enhances the effect of NGF on the capsaicin response. The percentage of small cells sensitized by NGF under these conditions, as determined by its ability to reduce tachyphylaxis, was 64%. This suggests that about two-thirds of DRG cells <30 microm and sensitive to capsaicin express a functional trkA receptor.

Removal of NMDA receptor Mg(2+) block extends the action of NT-3 on synaptic transmission in neonatal rat motoneurons

NT-3 has previously been reported to enhance AMPA/kainate receptor-mediated synaptic responses in motoneurons via an effect on the N-methyl-D-aspartate (NMDA) receptor. To investigate neurotrophin-3 (NT-3) action further, we measured the NMDA receptor (NMDAR)-mediated synaptic response directly by intracellular recording in motoneurons after blocking AMPA/kainate, GABA(A), GABA(B) and glycine receptor-mediated responses pharmacologically. Two pathways were stimulated, the segmental dorsal root (DR) and the descending ventrolateral fasciculus (VLF). The DR-evoked NMDAR-mediated response in motoneurons of rats younger than 1 wk has two components, the initial one of which is generated monosynaptically. NT-3 strongly potentiated both NMDA components in a rapidly reversible manner. No NMDAR-mediated responses were present at VLF connections and at DR connections in older (1- to 2-wk-old) neonates. Bath-applied NT-3-induced potentiation of the AMPA/kainate receptor-mediated response occurred only at connections that exhibit a synaptic NMDA receptor-mediated response. Reducing Mg(2+) concentration in the bathing solution restored the NMDAR-mediated response elicited by DR stimulation in older neonates and by VLF throughout the neonatal period (0-2 wk). In low-Mg(2+), NT-3 enhanced AMPA/kainate receptor-mediated responses elicited by inputs normally not influenced by NT-3. Thus a major reason for the loss of NT-3 action on AMPA/kainate synaptic responses is the reduced activity of the NMDA receptor due to developing Mg(2+) block of NMDA receptor-channel complex as the animal matures, and both can be re-established by reducing Mg(2+) concentration in fluid bathing the spinal cord.