Non-cell-autonomous regulation of GABAergic neuron development by neurotrophins and the p75 receptor

Acetylcholine regulation of GnRH neuronal activity: A circuit in the medial septum

In vertebrates, gonadotropin-releasing hormone (GnRH)-secreting neurons control fertility by regulating gonadotrophs in the anterior pituitary. While it is known that acetylcholine (ACh) influences GnRH secretion, whether the effect is direct or indirect, and the specific ACh receptor (AChR) subtype(s) involved remain unclear. Here, we determined 1) whether ACh can modulate GnRH cellular activity and 2) a source of ACh afferents contacting GnRH neurons. Calcium imaging was used to assay GnRH neuronal activity. With GABAergic and glutamatergic transmission blocked, subtype-specific AChR agonists and antagonists were applied to identify direct regulation of GnRH neurons. ACh and nicotine caused a rise in calcium that declined gradually back to baseline after 5-6 min. This response was mimicked by an alpha3-specific agonist. In contrast, muscarine inhibited GnRH calcium oscillations, and blocking M2 and M4 together prevented this inhibition. Labeling for choline acetyltransferase (ChAT) and GnRH revealed ChAT fibers contacting GnRH neurons, primarily in the medial septum (MS), and in greater number in females than males. ChAT positive cells in the MS are known to express p75NGFRs. Labeling for p75NGFR, ChAT and GnRH indicated that ChAT fibers contacting GnRH cells originate from cholinergic cells within these same rostral areas. Together, these results indicate that cholinergic cells in septal areas can directly regulate GnRH neurons.

Small molecule modulation of the p75 neurotrophin receptor suppresses age- and genotype-associated neurodegeneration in HIV gp120 transgenic mice

The persistence of HIV in the central nervous system leads to cognitive deficits in up to 50% of people living with HIV even with systemic suppression by antiretroviral treatment. The interaction of chronic inflammation with age-associated degeneration places these individuals at increased risk of accelerated aging and other neurodegenerative diseases and no treatments are available that effectively halt these processes. The adverse effects of aging and inflammation may be mediated, in part, by an increase in the expression of the p75 neurotrophin receptor (p75^NTR) which shifts the balance of neurotrophin signaling toward less protective pathways. To determine if modulation of p75^NTR could modify the disease process, we treated HIV gp120 transgenic mice with a small molecule ligand designed to engage p75^NTR and downregulate degenerative signaling. Daily treatment with 50 mg/kg LM11A-31 for 4 months suppressed age- and genotype-dependent activation of microglia, increased microtubule associated protein-2 (MAP-2), reduced dendritic varicosities and slowed the loss of parvalbumin immunoreactive neurons in the hippocampus. An age related accumulation of microtubule associated protein Tau was identified in the hippocampus in extracellular clusters that co-expressed p75^NTR suggesting a link between Tau and p75^NTR. Although the significance of the relationship between p75^NTR and Tau is unclear, a decrease in Tau-1 immunoreactivity as gp120 mice entered old age (>16 months) suggests that the Tau may transition to more pathological modifications; a process blocked by LM11A-31. Overall, the effects of LM11A-31 are consistent with strong neuroprotective and anti-inflammatory actions that have significant therapeutic potential.

All-trans retinoic acid induces reprogramming of canine dedifferentiated cells into neuron-like cells

The specification of cell identity depends on the exposure of cells to sequences of bioactive ligands. All-trans retinoic acid (ATRA) affects neuronal development in the early stage, and it is involved in neuronal lineage reprogramming. We previously established a fibroblast-like dedifferentiated fat cells (DFATs) derived from highly homogeneous mature adipocytes, which are more suitable for the study of cellular reprogramming. Canine cognitive dysfunction is similar to human cognitive dysfunction, suggesting that dogs could be a pathological and pharmacological model for human neuronal diseases. However, the effect of ATRA on neuronal reprogramming in dogs has remained unclear. Therefore, in this study, we investigated the effect of ATRA on the neuronal reprogramming of canine DFATs. ATRA induced the expression of neuronal marker mRNA/protein. The neuron-like cells showed Ca²⁺ influx with depolarization (50 mM KCl; 84.75 ± 4.05%) and Na⁺ channel activation (50 μM veratridine; 96.02 ± 2.02%). Optical imaging of presynaptic terminal activity and detection of neurotransmitter release showed that the neuron-like cells exhibited the GABAergic neuronal property. Genome-wide RNA-sequencing analysis shows that the transcriptome profile of canine DFATs is effectively reprogrammed towards that of cortical interneuron lineage. Collectively, ATRA can produce functional GABAergic cortical interneuron-like cells from canine DFATs, exhibiting neuronal function with > 80% efficiency. We further demonstrated the contribution of JNK3 to ATRA-induced neuronal reprogramming in canine DFATs. In conclusion, the neuron-like cells from canine DFATs could be a powerful tool for translational research in cell transplantation therapy, in vitro disease modeling, and drug screening for neuronal diseases.

New GABAergic Neurogenesis in the Hippocampal CA1 Region of a Gerbil Model of Long-Term Survival after Transient Cerebral Ischemic Injury

We investigated the probability of newly generated neurons that could survive and mature in the ischemic hippocampal CA1 region (CA1) of a gerbil model of transient cerebral ischemia. Neuronal death was shown in the stratum pyramidale (SP) from 4 days post-ischemia, and a significant increase in NeuN-positive ((+) ) neurons was found in the SP at 180 days post-ischemia. 5-Bromo-2-deoxyuridine (BrdU)(+) cells were co-stained with NeuN and glutamic decarboxylase 67 (GAD67). Brain-derived neurotrophic factor (BDNF) immunoreactivity and protein level was shown in nonpyramidal cells from 4 days post-ischemia, and the immunoreactivity was strong at 30 days post-ischemia and not significantly changed until 180 days post-ischemia. Furthermore, TrkB immunoreactivity was co-stained with GAD67 when we examined at 180 days post-ischemia. Myelin basic protein (MBP)(+) nerve fibers were reduced at 4 days post-ischemia and maintained until 60 days post-ischemia, and MBP immunoreactivity and levels were significantly increased at 180 days post-ischemia. In the passive avoidance test, cognitive dysfunction was improved at 180 days post-ischemia. These results suggest that the differentiation of neural progenitor cells into new GABAergic neurons may be promoted via BDNF in the ischemic CA1 and that the neurogenesis may partially mediate the recovery of cognitive impairments via increasing myelinated nerve fibers.

Crosstalk among electrical activity, trophic factors and morphogenetic proteins in the regulation of neurotransmitter phenotype specification

Morphogenetic proteins are responsible for patterning the embryonic nervous system by enabling cell proliferation that will populate all the neural structures and by specifying neural progenitors that imprint different identities in differentiating neurons. The adoption of specific neurotransmitter phenotypes is crucial for the progression of neuronal differentiation, enabling neurons to connect with each other and with target tissues. Preliminary neurotransmitter specification originates from morphogen-driven neural progenitor specification through the combinatorial expression of transcription factors according to morphogen concentration gradients, which progressively restrict the identity that born neurons adopt. However, neurotransmitter phenotype is not immutable, instead trophic factors released from target tissues and environmental stimuli change expression of neurotransmitter-synthesizing enzymes and specific vesicular transporters modifying neuronal neurotransmitter identity. Here we review studies identifying the mechanisms of catecholaminergic, GABAergic, glutamatergic, cholinergic and serotonergic early specification and of the plasticity of these neurotransmitter phenotypes during development and in the adult nervous system. The emergence of spontaneous electrical activity in developing neurons recruits morphogenetic proteins in the process of neurotransmitter phenotype plasticity, which ultimately equips the nervous system and the whole organism with adaptability for optimal performance in a changing environment.

Interplay of environmental signals and progenitor diversity on fate specification of cortical GABAergic neurons

Cortical GABAergic interneurons constitute an extremely diverse population of cells organized in a well-defined topology of precisely interconnected cells. They play a crucial role regulating inhibitory-excitatory balance in brain circuits, gating sensory perception, and regulating spike timing to brain oscillations during distinct behaviors. Dysfunctions in the establishment of proper inhibitory circuits have been associated to several brain disorders such as autism, epilepsy, and schizophrenia. In the rodent adult cortex, inhibitory neurons are generated during the second gestational week from distinct progenitor lineages located in restricted domains of the ventral telencephalon. However, only recently, studies have revealed some of the mechanisms generating the heterogeneity of neuronal subtypes and their modes of integration in brain networks. Here we will discuss some the events involved in the production of cortical GABAergic neuron diversity with focus on the interaction between intrinsically driven genetic programs and environmental signals during development.

Effects of sustained proNGF blockade on attentional capacities in aged rats with compromised cholinergic system

Disruption in nerve growth factor (NGF) signaling via tropomyosin-related kinase A (trkA) receptors compromises the integrity of the basal forebrain (BF) cholinergic system, yielding cognitive, specifically attentional, impairments in Alzheimer's disease (AD). Although normal aging is considered a risk factor for AD, the mechanisms underlying the selective vulnerability of the aging cholinergic system to trkA disruption is not clear. The levels of proNGF, a proneurotrophin that possesses higher affinity for p75 receptors, increase in aging. The present study was designed to test the hypothesis that cholinergic and attentional dysfunction in aged rats with reduced BF trkA receptors occurs due to the overactivation of endogenous proNGF signaling. We employed a viral vector that produced trkA shRNA to suppress trkA receptors in the corticopetal cholinergic neurons of aged rats. BF trkA suppression impaired animals' performance on signal trials in both the sustained attention task (SAT) and the cognitively taxing distractor version of SAT (dSAT) and these deficits were normalized by chronic intracerebroventricular administration of proNGF antibody. Moreover, depolarization-evoked acetylcholine (ACh) release and the density of cortical cholinergic fibers were partially restored in these animals. However, SAT/dSAT scores reflecting overall performance did not improve following proNGF blockade in trkA knockdown rats due to impaired performance in non-signal trials. Sustained proNGF blockade alone did not alter baseline attentional performance but produced moderate impairments during challenging conditions. Collectively, our findings indicate that barring proNGF-p75 signaling may exert some beneficial effects on attentional capacities specifically when BF trkA signaling is abrogated. However, endogenous proNGF may also possess neurotrophic effects and blockade of this proneurotrophin may not completely ameliorate attentional impairments in AD and potentially hinder performance during periods of high cognitive load in normal aging.

Aβ40 modulates GABA(A) receptor α6 subunit expression and rat cerebellar granule neuron maturation through the ERK/mTOR pathway

In addition to their neurotoxic role in Alzheimer's disease (AD), β-amyloid peptides (Aβs) are also known to play physiological roles. Here, we show that recombinant Aβ40 significantly increased the outward current of the GABA(A) receptor containing (GABA(A)α6) in rat cerebellar granule neurons (CGNs). The Aβ40-mediated increase in GABA(A)α6 current was mediated by an increase in GABA(A)α6 protein expression at the translational rather than the transcriptional level. The exposure of CGNs to Aβ40 markedly induced the phosphorylation of ERK (pERK) and mammalian target of rapamycin (pmTOR). The increase in GABA(A)α6 current and expression was attenuated by specific inhibitors of ERK or mTOR, suggesting that the ERK and mTOR signaling pathways are required for the effect of Aβ40 on GABA(A)α6 current and expression in CGNs. A pharmacological blockade of the p75 neurotrophin receptor (p75(NTR)), but not the insulin or α7-nAChR receptors, abrogated the effect of Aβ40 on GABA(A)α6 protein expression and current. Furthermore, the expression of GABA(A)α6 was lower in CGNs from APP(-/-) mice than in CGNs from wild-type mice. Moreover, the internal granule layer (IGL) in APP(-/-) mice was thinner than the IGL in wild-type mice. The injection of Aβ40 into the cerebellum reversed this effect, and the application of p75(NTR) blocking antibody abolished the effects of Aβ40 on cerebellum morphology in APP(-/-) mice. Our results suggest that low concentrations of Aβ40 play a role in regulating CGN maturation through p75(NTR).

Dynamic regulation of neurotransmitter specification: relevance to nervous system homeostasis

During nervous system development the neurotransmitter identity changes and coexpression of several neurotransmitters is a rather generalized feature of developing neurons. In the mature nervous system, different physiological and pathological circumstances recreate this phenomenon. The rules of neurotransmitter respecification are multiple. Among them, the goal of assuring balanced excitability appears as an important driving force for the modifications in neurotransmitter phenotype expression. The functional consequences of these dynamic revisions in neurotransmitter identity span a varied range, from fine-tuning the developing neural circuit to modifications in addictive and locomotor behaviors. Current challenges include determining the mechanisms underlying neurotransmitter phenotype respecification and how they intersect with genetic programs of neuronal specialization. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

NMDA-dependent switch of proBDNF actions on developing GABAergic synapses

The brain-derived neurotrophic factor (BDNF) has emerged as an important messenger for activity-dependent development of neuronal network. Recent findings have suggested that a significant proportion of BDNF can be secreted as a precursor (proBDNF) and cleaved by extracellular proteases to yield the mature form. While the actions of proBDNF on maturation and plasticity of excitatory synapses have been studied, the effect of the precursor on developing GABAergic synapses remains largely unknown. Here, we show that regulated secretion of proBDNF exerts a bidirectional control of GABAergic synaptic activity with NMDA receptors driving the polarity of the plasticity. When NMDA receptors are activated during ongoing synaptic activity, regulated Ca(2+)-dependent secretion of proBDNF signals via p75(NTR) to depress GABAergic synaptic activity, while in the absence of NMDA receptors activation, secreted proBDNF induces a p75(NTR)-dependent potentiation of GABAergic synaptic activity. These results revealed a new function for proBDNF-p75(NTR) signaling in synaptic plasticity and a novel mechanism by which synaptic activity can modulate the development of GABAergic synaptic connections.

Induction of the GABA cell phenotype: an in vitro model for studying neurodevelopmental disorders

Recent studies of the hippocampus have suggested that a network of genes is associated with the regulation of the GAD₆₇ (GAD1) expression and may play a role in γ-amino butyric acid (GABA) dysfunction in schizophrenia (SZ) and bipolar disorder (BD). To obtain a more detailed understanding of how GAD₆₇ regulation may result in GABAergic dysfunction, we have developed an in vitro model in which GABA cells are differentiated from the hippocampal precursor cell line, HiB5. Growth factors, such as PDGF, and BDNF, regulate the GABA phenotype by inducing the expression of GAD₆₇ and stimulating the growth of cellular processes, many with growth cones that form appositions with the cell bodies and processes of other GAD₆₇-positive cells. These changes are associated with increased expression of acetylated tubulin, microtubule-associated protein 2 (MAP2) and the post-synaptic density protein 95 (PSD95). The addition of BDNF, together with PDGF, increases the levels of mRNA and protein for GAD₆₇, as well as the high affinity GABA uptake protein, GAT1. These changes are associated with increased concentrations of GABA in the cytoplasm of "differentiated" HiB5 neurons. In the presence of Ca²⁺ and K⁺, newly synthesized GABA is released extracellularly. When the HiB5 cells appear to be fully differentiated, they also express GAD₆₅, parvalbumin and calbindin, and GluR subtypes as well as HDAC1, DAXX, PAX5, Runx2, associated with GAD₆₇ regulation. Overall, these results suggest that the HiB5 cells can differentiate into functionally mature GABA neurons in the presence of gene products that are associated with GAD₆₇ regulation in the adult hippocampus.

Genetically induced cholinergic hyper-innervation enhances taste learning

Acute inhibition of acetylcholine (ACh) has been shown to impair many forms of simple learning, and notably conditioned taste aversion (CTA). The most adhered-to theory that has emerged as a result of this work – that ACh increases a taste’s perceived novelty, and thereby its associability – would be further strengthened by evidence showing that enhanced cholinergic function improves learning above normal levels. Experimental testing of this corollary hypothesis has been limited, however, by side-effects of pharmacological ACh agonism and by the absence of a model that achieves long-term increases in cholinergic signaling. Here, we present this further test of the ACh hypothesis, making use of mice lacking the p75 pan-neurotrophin receptor gene, which show a resultant over-abundance of cholinergic neurons in sub-regions of the basal forebrain (BF). We first demonstrate that the p75−/− abnormality directly affects portions of the CTA circuit, locating mouse gustatory cortex (GC) using a functional assay and then using immunohistochemisty to demonstrate cholinergic hyper-innervation of GC in the mutant mice – hyper-innervation that is unaccompanied by changes in cell numbers or compensatory changes in muscarinic receptor densities. We then demonstrate that both p75−/− and wild-type (WT) mice learn robust CTAs, which extinguish more slowly in the mutants. Further testing to distinguish effects on learning from alterations in memory retention demonstrate that p75−/− mice do in fact learn stronger CTAs than WT mice. These data provide novel evidence for the hypothesis linking ACh and taste learning.

GABAergic neurons expressing p75 in rat substantia innominata and nucleus basalis

In vitro findings suggested a role for the p75 neurotrophin receptor in the maturation of GABAergic neurons residing in the basal forebrain (BF), a brain area known to have p75 expression only on cholinergic neurons. We document here the presence of GABAergic neurons which express p75 in the BF in vivo. Colocalization of p75 with the cholinergic marker choline-acetyltransferase (ChAT) and/or the GABAergic marker glutamic acid decarboxylase-67 (GAD67) was investigated in the BF at birth, at two weeks, and in adulthood. A subset of GAD67(+) neurons was p75(+) (p75(+)/GAD67(+)) but ChAT(-) in the substantia innominata and nucleus basalis magnocellularis at birth, whereas all p75(+)/GAD67(+) neurons were also ChAT(+) from two weeks onward. These phenotypic features suggest that a subpopulation of GABAergic neurons could be sensitive to neurotrophins during brain maturation. To unravel this issue, we then pursued a functional analysis by assessing p75 expression profile, and its modulation by nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF) in primary BF cell cultures. NGF increased p75 expression exclusively in cholinergic neurons, whereas BDNF induced p75 expression only in a subset of GABAergic neurons (p75(+)/GAD67(+)/ChAT(-)) through a p75- and tyrosine-kinase-dependent mechanism. The latter findings point to a selective role of BDNF in the induction of p75 expression in BF GABAergic neurons. Altogether these results confirm the role of neurotrophins in the developing and mature circuitry of GABAergic neurons in the BF regions.

BMP9 (bone morphogenetic protein 9) induces NGF as an autocrine/paracrine cholinergic trophic factor in developing basal forebrain neurons

Acetylcholine (ACh) synthesis and release from basal forebrain cholinergic neurons (BFCN) innervating the cerebral cortex and hippocampus are essential processes for normal learning, memory and attention. Bone morphogenetic protein (BMP) 9 is a cholinergic differentiation factor in the developing septum that increases ACh synthesis and choline acetyltransferase (Chat) gene expression both in vivo and in vitro. We investigated the possible induction of cholinergic trophic factors by BMP9 in murine septal cells. Nerve growth factor (NGF) protein expression and secretion into the medium was increased in cultured embryonic septal cells treated with BMP9, and partially mediated BMP9-induced acetylcholine production and Chat gene expression. BMP9-induced Ngf gene expression was detected in postmitotic cells, required new protein synthesis and was blocked by BMP type I receptor inhibition. Cholinergic neurons were isolated by fluorescence-activated cell sorting based on either transgenic expression of green fluorescent protein driven by the Chat promoter or NGF receptor (p75) immunostaining. Although both noncholinergic and cholinergic neurons in untreated cultures expressed similar low levels of Ngf, increased Ngf gene expression was restricted to Chat-positive neurons in BMP9-treated cultures. Likewise, similar levels of Ngf mRNA were detected in p75-negative and p75-positive septal cells, yet only p75-positive BFCN increased their Ngf gene expression when treated with BMP9, and only these cells expressed the Alk1 BMP receptor. The data suggest an autocrine/paracrine role for NGF in the development and/or maintenance of BFCN and imply that the stimulation of NGF production and release contributes to the cholinergic-supportive properties of BMP9.

Long-term, stable differentiation of human embryonic stem cell-derived neural precursors grafted into the adult mammalian neostriatum

Stem cell grafts have been advocated as experimental treatments for neurological diseases by virtue of their ability to offer trophic support for injured neurons and, theoretically, to replace dead neurons. Human embryonic stem cells (HESCs) are a rich source of neural precursors (NPs) for grafting, but have been questioned for their tendency to form tumors. Here we studied the ability of HESC-derived NP grafts optimized for cell number and differentiation stage prior to transplantation, to survive and stably differentiate and integrate in the basal forebrain (neostriatum) of young adult nude rats over long periods of time (6 months). NPs were derived from adherent monolayer cultures of HESCs exposed to noggin. After transplantation, NPs showed a drastic reduction in mitotic activity and an avid differentiation into neurons that projected via major white matter tracts to a variety of forebrain targets. A third of NP-derived neurons expressed the basal forebrain-neostriatal marker dopamine-regulated and cyclic AMP-regulated phosphoprotein. Graft-derived neurons formed mature synapses with host postsynaptic structures, including dendrite shafts and spines. NPs inoculated in white matter tracts showed a tendency toward glial (primarily astrocytic) differentiation, whereas NPs inoculated in the ventricular epithelium persisted as nestin(+) precursors. Our findings demonstrate the long-term ability of noggin-derived human NPs to structurally integrate tumor-free into the mature mammalian forebrain, while maintaining some cell fate plasticity that is strongly influenced by particular central nervous system (CNS) niches.

ProNGF induces TNFalpha-dependent death of retinal ganglion cells through a p75NTR non-cell-autonomous signaling pathway

Neurotrophin binding to the p75 neurotrophin receptor (p75(NTR)) activates neuronal apoptosis following adult central nervous system injury, but the underlying cellular mechanisms remain poorly defined. In this study, we show that the proform of nerve growth factor (proNGF) induces death of retinal ganglion cells in adult rodents via a p75(NTR)-dependent signaling mechanism. Expression of p75(NTR) in the adult retina is confined to Müller glial cells; therefore we tested the hypothesis that proNGF activates a non-cell-autonomous signaling pathway to induce retinal ganglion cell (RGC) death. Consistent with this, we show that proNGF induced robust expression of tumor necrosis factor alpha (TNFalpha) in Müller cells and that genetic or biochemical ablation of TNFalpha blocked proNGF-induced death of retinal neurons. Mice rendered null for p75(NTR), its coreceptor sortilin, or the adaptor protein NRAGE were defective in proNGF-induced glial TNFalpha production and did not undergo proNGF-induced retinal ganglion cell death. We conclude that proNGF activates a non-cell-autonomous signaling pathway that causes TNFalpha-dependent death of retinal neurons in vivo.

Differential modulation of nerve growth factor receptor (p75) and cholinergic gene expression in purified p75-expressing and non-expressing basal forebrain neurons by BMP9

The synthesis of acetylcholine and its release from basal forebrain cholinergic neurons (BFCN) that innervate the cerebral cortex and hippocampus are considered essential processes for normal learning, memory and attention. We have developed a purification and cell culture method of BFCN in order to examine the regulation of their cholinergic phenotype. Cells isolated from the septal region of late embryonic mice were purified by fluorescence-activated cell sorting based on their expression of the nerve growth factor receptor (p75), a surface marker for mature BFCN. Consistent with previous reports, p75-positive (p75+) cells were enriched in choline acetyltransferase (ChAT) and the high-affinity choline transporter (ChT), as measured by reverse transcriptase PCR. In culture, these cells maintained their gene expression of p75, ChAT and ChT, while p75-negative (p75-) cells had a low expression of these genes. Incubation of the cells with BMP9 not only increased p75 and ChAT gene expression in p75- cells, but also augmented the expression of these genes in p75+ cells. Conversely, BMP9 decreased ChT gene expression in p75+ cells and had no such effect in p75- cells. Immunostaining confirmed that p75 protein expression was modulated by BMP9 in a similar way as p75 mRNA, and also revealed that only a subset of p75- cells respond to BMP9 in this manner. These data suggest that mature BFCN in culture may express their cholinergic phenotype in the absence of exogenous trophic input, but that BMP9 can further modulate this phenotype. Moreover, BMP9 induces the cholinergic phenotype in a set of basal forebrain non-cholinergic neurons or precursor cells.