Neocortical neurons lacking the protein-tyrosine kinase B receptor display abnormal differentiation and process elongation in vitro and in vivo

Extracellular molecular signals shaping dendrite architecture during brain development

Proper growth and branching of dendrites are crucial for adequate central nervous system (CNS) functioning. The neuronal dendritic geometry determines the mode and quality of information processing. Any defects in dendrite development will disrupt neuronal circuit formation, affecting brain function. Besides cell-intrinsic programmes, extrinsic factors regulate various aspects of dendritic development. Among these extrinsic factors are extracellular molecular signals which can shape the dendrite architecture during early development. This review will focus on extrinsic factors regulating dendritic growth during early neuronal development, including neurotransmitters, neurotrophins, extracellular matrix proteins, contact-mediated ligands, and secreted and diffusible cues. How these extracellular molecular signals contribute to dendritic growth has been investigated in developing nervous systems using different species, different areas within the CNS, and different neuronal types. The response of the dendritic tree to these extracellular molecular signals can result in growth-promoting or growth-limiting effects, and it depends on the receptor subtype, receptor quantity, receptor efficiency, the animal model used, the developmental time windows, and finally, the targeted signal cascade. This article reviews our current understanding of the role of various extracellular signals in the establishment of the architecture of the dendrites.

Receptor-ligand supplementation via a self-cleaving 2A peptide-based gene therapy promotes CNS axonal transport with functional recovery

Gene replacement approaches are leading to a revolution in the treatment of previously debilitating monogenic neurological conditions. However, the application of gene therapy to complex polygenic conditions has been limited. Down-regulation or dysfunction of receptor expression in the disease state or in the presence of excess ligand has been shown to compromise therapeutic efficacy. Here, we offer evidence that combined overexpression of both brain-derived neurotrophic factor and its receptor, tropomyosin receptor kinase B, is more effective in stimulating axonal transport than either receptor administration or ligand administration alone. We also show efficacy in experimental glaucoma and humanized tauopathy models. Simultaneous administration of a ligand and its receptor by a single gene therapy vector overcomes several problems relating to ligand deficiency and receptor down-regulation that may be relevant to multiple neurodegenerative diseases. This approach shows promise as a strategy to target intrinsic mechanisms to improve neuronal function and facilitate repair.

Effect of Physical Activity on Cognitive Function and Neurogenesis: Roles of BDNF and Oxidative Stress

Context: Cognitive disorders are one of the most common neurological problems that can be caused by lifestyle patterns, especially sedentary lifestyle, poor nutrition, exposure to a variety of toxins or diseases. Evidence Acquisition: There are various strategies recommended for the prevention and treatment of these disorders, including drug therapy, psychological therapy, dietary pattern changes, and physical activity. Results: It seems that physical activity with biological mechanisms can have beneficial effects on the central nervous system and improve cognitive function, including enhanced learning and memory, as well as reduced depression and anxiety. Conclusions: Of the major mechanisms that physical activity can affect cognitive function include increased neurogenic factors, decreased oxidative stress, decreased inflammatory mediators, and mitochondrial biogenesis. Therefore, it is recommended that people with cognitive impairments can use physical activity as an appropriate strategy to prevent and treat cognitive impairment problems.

Inhibition of TrkB- and TrkC-Signaling Pathways Affects Neurogenesis in the Opossum Developing Neocortex

We have previously reported that the blockage of TrkB and TrkC signaling in primary culture of opossum neocortical cells affects neurogenesis that involves a range of processes including cell proliferation, differentiation, and survival. Here, we studied whether TrkB and TrkC activity specifically affects various types of progenitor cell populations during neocortex formation in the Monodelphis opossum in vivo. We found that the inhibition of TrkB and TrkC activities affects the same proliferative cellular phenotype, but TrkC causes more pronounced changes in the rate of cell divisions. Additionally, inhibition of TrkB and TrkC does not affect apoptosis in vivo, which was found in cell culture experiments. The lack of TrkB and TrkC receptor activity caused the arrest of newly generated neurons; therefore, they could not penetrate the subplate zone. We suggest that at this time point in development, migration consists of 2 steps. During the initial step, neurons migrate and reach the base of the subplate, whereas during the next step the migration of neurons to their final position is regulated by TrkB or TrkC signaling.

[Intrinsic and extrinsic mechanisms regulating neuronal dendrite morphogenesis]

Neurons are the structural and functional unit of the nervous system. Precisely regulated dendrite morphogenesis is the basis of neural circuit assembly. Numerous studies have been conducted to explore the regulatory mechanisms of dendritic morphogenesis. According to their action regions, we divide them into two categories: the intrinsic and extrinsic regulators of neuronal dendritic morphogenesis. Intrinsic factors are cell type-specific transcription factors, actin polymerization or depolymerization regulators and regulators of the secretion or endocytic pathways. These intrinsic factors are produced by neuron itself and play an important role in regulating the development of dendrites. The extrinsic regulators are either secreted proteins or transmembrane domain containing cell adhesion molecules. They often form receptor-ligand pairs to mediate attractive or repulsive dendritic guidance. In this review, we summarize recent findings on the intrinsic and external molecular mechanisms of dendrite morphogenesis from multiple model organisms, including Caenorhabditis elegans, Drosophila and mice. These studies will provide a better understanding on how defective dendrite development and maintenance are associated with neurological diseases.

Connectomics of Morphogenetically Engineered Neurons as a Predictor of Functional Integration in the Ischemic Brain

Recent advances in cell reprogramming technologies enable the in vitro generation of theoretically unlimited numbers of cells, including cells of neural lineage and specific neuronal subtypes from human, including patient-specific, somatic cells. Similarly, as demonstrated in recent animal studies, by applying morphogenetic neuroengineering principles in situ, it is possible to reprogram resident brain cells to the desired phenotype. These developments open new exciting possibilities for cell replacement therapy in stroke, albeit not without caveats. Main challenges include the successful integration of engineered cells in the ischemic brain to promote functional restoration as well as the fact that the underlying mechanisms of action are not fully understood. In this review, we aim to provide new insights to the above in the context of connectomics of morphogenetically engineered neural networks. Specifically, we discuss the relevance of combining advanced interdisciplinary approaches to: validate the functionality of engineered neurons by studying their self-organizing behavior into neural networks as well as responses to stroke-related pathology in vitro; derive structural and functional connectomes from these networks in healthy and perturbed conditions; and identify and extract key elements regulating neural network dynamics, which might predict the behavior of grafted engineered neurons post-transplantation in the stroke-injured brain.

Prefrontal cortical trkB, glucocorticoids, and their interactions in stress and developmental contexts

The tropomyosin/tyrosine receptor kinase B (trkB) and glucocorticoid receptor (GR) regulate neuron structure and function and the hormonal stress response. Meanwhile, disruption of trkB and GR activity (e.g., by chronic stress) can perturb neuronal morphology in cortico-limbic regions implicated in stressor-related illnesses like depression. Further, several of the short- and long-term neurobehavioral consequences of stress depend on the developmental timing and context of stressor exposure. We review how the levels and activities of trkB and GR in the prefrontal cortex (PFC) change during development, interact, are modulated by stress, and are implicated in depression. We review evidence that trkB- and GR-mediated signaling events impact the density and morphology of dendritic spines, the primary sites of excitatory synapses in the brain, highlighting effects in adolescents when possible. Finally, we review the role of neurotrophin and glucocorticoid systems in stress-related metaplasticity. We argue that better understanding the long-term effects of developmental stressors on PFC trkB, GR, and related factors may yield insights into risk for chronic, remitting depression and related neuropsychiatric illnesses.

The Virtuous Cycle of Axon Growth: Axonal Transport of Growth-Promoting Machinery as an Intrinsic Determinant of Axon Regeneration

Injury to the brain and spinal cord has devastating consequences because adult central nervous system (CNS) axons fail to regenerate. Injury to the peripheral nervous system (PNS) has a better prognosis, because adult PNS neurons support robust axon regeneration over long distances. CNS axons have some regenerative capacity during development, but this is lost with maturity. Two reasons for the failure of CNS regeneration are extrinsic inhibitory molecules, and a weak intrinsic capacity for growth. Extrinsic inhibitory molecules have been well characterized, but less is known about the neuron-intrinsic mechanisms which prevent axon re-growth. Key signaling pathways and genetic/epigenetic factors have been identified which can enhance regenerative capacity, but the precise cellular mechanisms mediating their actions have not been characterized. Recent studies suggest that an important prerequisite for regeneration is an efficient supply of growth-promoting machinery to the axon; however, this appears to be lacking from non-regenerative axons in the adult CNS. In the first part of this review, we summarize the evidence linking axon transport to axon regeneration. We discuss the developmental decline in axon regeneration capacity in the CNS, and comment on how this is paralleled by a similar decline in the selective axonal transport of regeneration-associated receptors such as integrins and growth factor receptors. In the second part, we discuss the mechanisms regulating selective polarized transport within neurons, how these relate to the intrinsic control of axon regeneration, and whether they can be targeted to enhance regenerative capacity. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 00: 000-000, 2018.

Mechanisms regulating dendritic arbor patterning

The nervous system is populated by diverse types of neurons, each of which has dendritic trees with strikingly different morphologies. These neuron-specific morphologies determine how dendritic trees integrate thousands of synaptic inputs to generate different firing properties. To ensure proper neuronal function and connectivity, it is necessary that dendrite patterns are precisely controlled and coordinated with synaptic activity. Here, we summarize the molecular and cellular mechanisms that regulate the formation of cell type-specific dendrite patterns during development. We focus on different aspects of vertebrate dendrite patterning that are particularly important in determining the neuronal function; such as the shape, branching, orientation and size of the arbors as well as the development of dendritic spine protrusions that receive excitatory inputs and compartmentalize postsynaptic responses. Additionally, we briefly comment on the implications of aberrant dendritic morphology for nervous system disease.

Cellular Repair of Complex Cortical Circuitry

The mammalian neocortex is a highly complex structure that consists of a large number of distinct cell types and connections unparalleled throughout the rest of the mammalian central nervous system (CNS). The neocortex is responsible for high-level functions including sensorimotor integration, associative behavior, and cognition. Although many studies demonstrate that embryonic tissue or precursor cells transplanted into the embryonic or neonatal cortex can integrate and connect with a wide variety of targets, such connectivity in most studies using adult recipients has been quite limited (1, 2). In contrast, relatively recent advances in transplantation paradigms from our laboratory and other laboratories have made apparent the possibility of successful reconstruction of complex cortical circuitry in the adult mammalian brain by transplantation or by manipulation of endogenous precursors in situ. These advances include the ability to isolate increasingly specific populations of immature neurons and precursors from donor animals, and the establishment of a highly selective model of targeted degeneration in the neocortex (3-10, Leavitt, unpublished observations). Such advances now allow both the transplantation of more defined and specific populations of immature neocortical neuroblasts and precursors, and also make it possible to study the cellular, anatomic, and functional efficacy of both transplantation and manipulation of endogenous precursors in situ (9, 11) in increasingly refined models of neocortical degeneration. The neocortex has become a model system for studying circuitry formation, regional specification, cell specification, and cell autonomous versus environmental influences on neuronal development and function in the CNS. Although neocortical transplantation paradigms have proven very useful for elucidating how complex circuits and CNS cytoarchitecture evolve in the mammalian brain, more recent work using cell transplantation has made apparent the possibility of cell replacement and complex circuit repair in the neocortex. Our most recent results (9, 11) suggest that neuronal replacement therapies for neurodegenerative disease and other CNS injury may even some day be possible via molecular manipulation of endogenous neural precursors in situ, without transplantation. Although only fanciful just several years ago, it now appears that studies of cellular repopulation and circuitry reconstruction in the neocortex may provide a foundation toward developing cellular therapies for degenerative or acquired disease in neocortex and other regions of the CNS.

tPA regulates neurite outgrowth by phosphorylation of LRP5/6 in neural progenitor cells

Despite the important role of tissue plasminogen activator (tPA) as a neuromodulator in neurons, microglia, and astrocytes, its role in neural progenitor cell (NPC) development is not clear yet. We identified that tPA is highly expressed in NPCs compared with neurons. Inhibition of tPA activity or expression using tPA stop, PAI-1, or tPA siRNA inhibited neurite outgrowth from NPCs, while overexpression or addition of exogenous tPA increased neurite outgrowth. The expression of Wnt and β-catenin as well as phosphorylation of LRP5 and LRP6, which has been implicated in Wnt-β-catenin signaling, was rapidly increased after tPA treatment and was decreased by tPA siRNA transfection. Knockdown of β-catenin or LRP5/6 expression by siRNA prevented tPA-induced neurite extension. NPCs obtained from tPA KO mice showed impaired neurite outgrowth compared with WT NPCs. In ischemic rat brains, axon density was higher in the brains transplanted with WT NPCs than in those with tPA KO NPCs, suggesting increased axonal sprouting by NPC-derived tPA. tPA-mediated regulation of neuronal maturation in NPCs may play an important role during development and in regenerative conditions.

BDNF in fragile X syndrome

Fragile X syndrome (FXS) is a monogenic disorder that is caused by the absence of FMR1 protein (FMRP). FXS serves as an excellent model disorder for studies investigating disturbed molecular mechanisms and synapse function underlying cognitive impairment, autism, and behavioral disturbance. Abnormalities in dendritic spines and synaptic transmission in the brain of FXS individuals and mouse models for FXS indicate perturbations in the development, maintenance, and plasticity of neuronal network connectivity. However, numerous alterations are found during the early development in FXS, including abnormal differentiation of neural progenitors and impaired migration of newly born neurons. Several aspects of FMRP function are modulated by brain-derived neurotrophic factor (BDNF) signaling. Here, we review the evidence of the role for BDNF in the developing and adult FXS brain. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Prevention and intervention studies with telmisartan, ramipril and their combination in different rat stroke models

Objectives

The effects of AT1 receptor blocker, telmisartan, and the ACE inhibitor, ramipril, were tested head-to head and in combination on stroke prevention in hypertensive rats and on potential neuroprotection in acute cerebral ischemia in normotensive rats.

Methods

Prevention study: Stroke-prone spontaneously hypertensive rats (SHR-SP) were subjected to high salt and randomly assigned to 4 groups: (1) untreated (NaCl, n = 24), (2) telmisartan (T; n = 27), (3) ramipril (R; n = 27) and (4) telmisartan +ramipril (T+R; n = 26). Drug doses were selected to keep blood pressure (BP) at 150 mmHg in all groups. Neurological signs and stroke incidence at 50% mortality of untreated SHR-SP were investigated.

Intervention study: Normotensive Wistar rats were treated s.c. 5 days prior to middle cerebral artery occlusion (MCAO) for 90 min with reperfusion. Groups (n = 10 each): (1) sham, (2) vehicle (V; 0,9% NaCl), (3) T (0,5 mg/kg once daily), (4) R (0,01 mg/kg twice daily), (5) R (0,1 mg/kg twice daily) or (6) T (0,5 mg/kg once daily) plus R (0,01 mg/kg twice daily). Twenty-four and 48 h after MCAO, neurological outcome (NO) was determined. Forty-eight h after MCAO, infarct volume by MRI, neuronal survival, inflammation factors and neurotrophin receptor (TrkB) were analysed.

Results

Stroke incidence was reduced, survival was prolonged and neurological outcome was improved in all treated SHR-SP with no differences between treated groups. In the acute intervention study, T and T+R, but not R alone, improved NO, reduced infarct volume, inflammation (TNFα), and induced TrkB receptor and neuronal survival in comparison to V.

Conclusions

T, R or T+R had similar beneficial effects on stroke incidence and NO in hypertensive rats, confirming BP reduction as determinant factor in stroke prevention. In contrast, T and T+R provided superior neuroprotection in comparison to R alone in normotensive rats with induced cerebral ischemia.

BDNF and TrkB in neuronal differentiation of Fmr1-knockout mouse

Fragile X syndrome (FXS) is a common cause of inherited mental retardation and the best characterized form of autistic spectrum disorders. FXS is caused by the loss of functional fragile X mental retardation protein (FMRP), which leads to abnormalities in the differentiation of neural progenitor cells (NPCs) and in the development of dendritic spines and neuronal circuits. Brain-derived neurotrophic factor (BDNF) and its TrkB receptors play a central role in neuronal maturation and plasticity. We studied BDNF/TrkB actions in the absence of FMRP and show that an increase in catalytic TrkB expression in undifferentiated NPCs of Fmr1-knockout (KO) mice, a mouse model for FXS, is associated with changes in the differentiation and migration of neurons expressing TrkB in neurosphere cultures and in the developing cortex. Aberrant intracellular calcium responses to BDNF and ATP in subpopulations of differentiating NPCs combined with changes in the expression of BDNF and TrkB suggest cell subtype-specific alterations during early neuronal maturation in the absence of FMRP. Furthermore, we show that dendritic targeting of Bdnf mRNA was increased under basal conditions and further enhanced in cortical layer V and hippocampal CA1 neurons of Fmr1-KO mice by pilocarpine-induced neuronal activity represented by convulsive seizures, suggesting that BDNF/TrkB-mediated feedback mechanisms for strengthening the synapses were compromised in the absence of FMRP. Pilocarpine-induced seizures caused an accumulation of Bdnf mRNA transcripts in the most proximal segments of dendrites in cortical but not in hippocampal neurons of Fmr1-KO mice. In addition, BDNF protein levels were increased in the hippocampus but reduced in the cortex of Fmr1-KO mice in line with regional differences of synaptic plasticity in the brain of Fmr1-KO mice. Altogether, the present data suggest that alterations in the BDNF/TrkB signaling modulate brain development and impair synaptic plasticity in FXS.

Potential teratogenic effects of ultrasound on corticogenesis: implications for autism

The phenotypic expression of autism, according to the Triple Hit Hypothesis, is determined by three factors: a developmental time window of vulnerability, genetic susceptibility, and environmental stressors. In utero exposure to thalidomide, valproic acid, and maternal infections are examples of some of the teratogenic agents which increase the risk of developing autism and define a time window of vulnerability. An additional stressor to genetically susceptible individuals during this time window of vulnerability may be prenatal ultrasound. Ultrasound enhances the genesis and differentiation of progenitor cells by activating the nitric oxide (NO) pathway and related neurotrophins. The effects of this pathway activation, however, are determined by the stage of development of the target cells, local concentrations of NO, and the position of nuclei (basal versus apical), causing consequent proliferation at some stages while driving differentiation and migration at others. Ill-timed activation or overactivation of this pathway by ultrasound may extend proliferation, increasing total cell number, and/or may trigger precipitous migration, causing maldistribution of neurons amongst cortical lamina, ganglia, white matter, and germinal zones. The rising rates of autism coincident with the increased use of ultrasound in obstetrics and its teratogenic/toxic effects on the CNS demand further research regarding a putative correlation.

MeCP2 functions largely cell-autonomously, but also non-cell-autonomously, in neuronal maturation and dendritic arborization of cortical pyramidal neurons

Rett syndrome is a human neurodevelopmental disorder presenting almost exclusively in female infants; it is the second most common cause of mental retardation in girls, after Down's syndrome. The identification in 1999 that mutation of the methyl-CpG-binding protein 2 (MECP2) gene on the X chromosome causes Rett syndrome has led to a rapid increase in understanding of the neurobiological basis of the disorder. However, much about the functional role of MeCP2, and the cellular phenotype of both patients with Rett syndrome and mutant Mecp2 mouse models, remains unclear. Building on prior work in which we demonstrated that cortical layer 2/3 pyramidal neurons (primarily interhemispheric "callosal projection neurons" (CPN)) have reduced dendritic complexity and smaller somata in Mecp2-null mice, here we investigate whether Mecp2 loss-of-function affects neuronal maturation cell-autonomously and/or non-cell-autonomously by creating physical chimeras. We transplanted Mecp2-null or wild-type (wt) E17-18 cortical neuroblasts and immature neurons from mice constitutively expressing enhanced green fluorescent protein (eGFP) into wt P2-3 mouse cortices to generate chimeric cortices. Mecp2-null layer 2/3 pyramidal neurons in both Mecp2-null and wt neonatal cortices exhibit equivalent reduction in dendritic complexity, and are smaller than transplanted wt neurons, independent of recipient environment. These results indicate that the phenotype of Mecp2-null pyramidal neurons results largely from cell-autonomous mechanisms, with additional non-cell-autonomous effects. Dysregulation of MeCP2 target genes in individual neuronal populations such as CPN is likely centrally involved in Rett syndrome pathogenesis. Our results indicating MeCP2 function in the centrally affected projection neuron population of CPN themselves provide a foundation and motivation for identification of transcriptionally regulated MeCP2 target genes in developing CPN.

Candesartan but not ramipril pretreatment improves outcome after stroke and stimulates neurotrophin BNDF/TrkB system in rats

OBJECTIVES

Drugs interfering with the renin-angiotensin system (RAS) have been shown to reduce the incidence of stroke in patients at risk and to afford neuroprotection in experimental brain ischemia. This study aimed to compare potential neuroprotective effects of systemic pretreatment with the angiotensin receptor blocker, candesartan, and the angiotensin-converting enzyme (ACE)-inhibitor, ramipril, in normotensive Wistar rats after focal cerebral ischemia, with special emphasis on the regulation of neurotrophins.

METHODS

Equipotent subcutaneous doses of candesartan and ramipril were determined via inhibition of pressor responses to intravenously injected angiotensin II (Ang II) or angiotensin I (Ang I), respectively. Accordingly, animals were treated with candesartan (0.1 mg/kg body weight, twice daily), ramipril (0.01 and 0.1 mg/kg body weight, twice daily) or vehicle (0.9% saline, twice daily), respectively, 5 days prior to middle cerebral artery occlusion (MCAO) with reperfusion. Severity of stroke was estimated via infarct size [magnetic resonance imaging (MRI) 48 h after MCAO] and neurological outcome (24 h, 48 h after MCAO). Measurements of neurotrophins/receptors in brain tissue were performed 48 h after MCAO.

RESULTS

Pretreatment with candesartan and ramipril (low dose) did not reduce blood pressure during MCAO, whereas ramipril high dose did. Candesartan, but not ramipril at any dose, significantly reduced stroke volume and improved neurological outcome. Poststroke mRNA and protein of the neurotrophin receptor, TrkB, were significantly elevated in animals treated with candesartan, but not ramipril.

CONCLUSIONS

Systemic pretreatment with a sub-hypotensive, RAS-blocking dose of candesartan affords neuroprotection after focal ischemia, associated with increased activity of the neurotrophin BDNF/TrkB system. Ramipril at sub-hypotensive and hypotensive, RAS-blocking doses showed no significant neuroprotective effects.

Extensive genotyping of the BDNF and NTRK2 genes define protective haplotypes against obsessive-compulsive disorder

BACKGROUND

Family, twin and molecular studies provide increasing evidence for the importance of genetic factors in obsessive-compulsive disorder (OCD). Recent work suggests that brain-derived neurotrophic factor (BDNF) may be involved in OCD pathophysiology. We used a linkage disequilibrium (LD)-mapping approach to investigate the role that BDNF and its specific receptor neurotrophic tyrosine kinase receptor type 2 (NTRK2) may play in increasing susceptibility to OCD.

METHODS

Eight tag single nucleotide polymorphisms (tagSNPs) covering the BDNF gene region and 46 tagSNPs in the NTRK2 region were genotyped in 215 OCD patients and 342 control subjects. Single nucleotide polymorphism association and haplotype analysis were performed. The possible relationship between genetic factors and clinical characteristics including age of OCD onset, tic disorders, clinical dimensions, and family history of OCD were investigated.

RESULTS

Haplotype analysis revealed a significant association between OCD and a five-marker protective haplotype located toward the 5' of the BDNF gene (odds ratio [OR] = .80; 95% confidence interval [CI] = .69-.92; permutation p value = .006) containing the functional valine (Val)66-to-methionine (Met) variant. A significant association between a NTRK2 intronic SNP (rs2378672) and OCD was identified (p < .0001) in female patients under an additive model. A protective haplotype located in intron 19 of NTRK2 was also associated with OCD (OR = .76; 95% CI = .66-.87; permutation p value = .001).

CONCLUSIONS

These findings support a role for the BDNF/NTRK2 signaling pathway in genetic susceptibility to OCD.

Cholesterol loss enhances TrkB signaling in hippocampal neurons aging in vitro

Binding of the neurotrophin brain-derived neurotrophic factor (BDNF) to the TrkB receptor is a major survival mechanism during embryonic development. In the aged brain, however, BDNF levels are low, suggesting that if TrkB is to play a role in survival at this stage additional mechanisms must have developed. We here show that TrkB activity is most robust in the hippocampus of 21-d-old BDNF-knockout mice as well as in old, wild-type, and BDNF heterozygous animals. Moreover, robust TrkB activity is evident in old but not young hippocampal neurons differentiating in vitro in the absence of any exogenous neurotrophin and also in neurons from BDNF -/- embryos. Age-associated increase in TrkB activity correlated with a mild yet progressive loss of cholesterol. This, in turn, correlated with increased expression of the cholesterol catabolic enzyme cholesterol 24-hydroxylase. Direct cause-effect, cholesterol loss-high TrkB activity was demonstrated by pharmacological means and by manipulating the levels of cholesterol 24-hydroxylase. Because reduced levels of cholesterol and increased expression of choleseterol-24-hydroxylase were also observed in the hippocampus of aged mice, changes in cellular cholesterol content may be used to modulate receptor activity strength in vivo, autonomously or as a way to complement the natural decay of neurotrophin production.

Developmental and activity-dependent regulation of ARMS/Kidins220 in cultured rat hippocampal neurons

Neurotrophin activation of Trk receptors elicits diverse effects on neuronal survival, differentiation, and synaptic plasticity. One of the central questions is how specificity is encoded in neurotrophin receptor signaling and actions. A unique downstream protein is the Ankyrin-Repeat Rich Membrane Spanning (ARMS)/Kinase D-interacting substrate-220 kDa (Kidins220), a very abundant scaffold protein in the hippocampus. To determine the roles of ARMS/Kidins220 in hippocampal neurons, we have analyzed the effects of synaptic activity upon the regulation and distribution of ARMS/Kidins220. At early times in vitro (<7 DIV), synaptic activity was low and ARMS/Kidins220 levels were high. As synaptic activity and markers for synapse maturation, such as PSD-95, increased, ARMS/Kidins220 significantly decreased to a plateau by later times in vitro (>12 DIV). Immunocytochemistry showed ARMS/Kidins220 to be concentrated at the tips of growing processes in immature cultures, and more diffusely distributed in older cultures. To examine the apparent inverse relationship between activity and ARMS/Kidins220 levels, neuronal firing was manipulated pharmacologically. Chronic exposure to TTX increased ARMS/Kidins220 levels, whereas bicuculline caused the opposite effect. Moreover, using shRNA to decrease ARMS/Kidins220 levels produced a corresponding increase in synaptic activity. We find that ARMS/Kidins220 may function in neuronal development as an indicator and potentially as a homeostatic regulator of overall synaptic strength in hippocampal neurons.

Trk signaling regulates neural precursor cell proliferation and differentiation during cortical development

Increasing evidence indicates that development of embryonic central nervous system precursors is tightly regulated by extrinsic cues located in the local environment. Here, we asked whether neurotrophin-mediated signaling through Trk tyrosine kinase receptors is important for embryonic cortical precursor cell development. These studies demonstrate that inhibition of TrkB (Ntrk2)and/or TrkC (Ntrk3) signaling using dominant-negative Trk receptors, or genetic knockdown of TrkB using shRNA, caused a decrease in embryonic precursor cell proliferation both in culture and in vivo. Inhibition of TrkB/C also caused a delay in the generation of neurons, but not astrocytes, and ultimately perturbed the postnatal localization of cortical neurons in vivo. Conversely, overexpression of BDNF in cortical precursors in vivo promoted proliferation and enhanced neurogenesis. Together, these results indicate that neurotrophin-mediated Trk signaling plays an essential, cell-autonomous role in regulating the proliferation and differentiation of embryonic cortical precursors and thus controls cortical development at earlier stages than previously thought.

Developmental changes of neurotrophin mRNA expression in the layers of rat visual cortex

Neurotrophins are essential factors for the structural, neurochemical and functional maturation of the brain including developmental and adult plasticity. Northern blots and polymerase chain reaction revealed the expression of neurotrophin 4 (NT4), neurotrophin 3 (NT3), brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) in the cortex. The cellular producers of NT3 and BDNF have been characterized by anatomical methods as being mostly pyramidal, and the tyrosine kinase B (TrkB) receptor is expressed by many cortical neurons. However, these methods have so far failed to identify the cells producing NT4 and NGF mRNA. These factors are much lower in expression than, e.g. BDNF, and apparently remain below detection levels of in situ hybridization. Given their specific actions on cell types and afferent systems, knowledge about the producing cell types is highly desirable. To narrow down on the producing cell types, we quantified by reverse transcriptase-polymerase chain reaction (RT-PCR) the developmental changes of BDNF, NT3, NT4, NGF and TrkB mRNA expression in total visual cortex lysates, and in the cortical layers dissected by tangential cryostat sectioning. We found dramatic changes in laminar expression of NT3 and NGF, mild changes of NT4, and no changes of BDNF and TrkB mRNA. For instance, NT3 is important early on for thalamocortical axons, and we found transient peaks of NT3 mRNA expression first in layer VI, then in layer IV. NT4 mRNA was in layers IV and VI, suggesting NT4 protein production in thalamorecipient layers, but peak expression gradually shifted to upper layers as did NGF expression. The layer-specific developmental expression shifts of neurotrophin mRNAs correlate with morphogenetic processes.

Dissecting MECP2 function in the central nervous system

Rett syndrome is a neurodevelopmental disorder and an important cause of mental retardation and autistic behavior in girls and in a small group of boys. In 1999, mutation of the methyl-CpG binding protein 2 (MECP2) gene encoding a transcriptional repressor on the X chromosome was found to cause Rett syndrome. Since this discovery, significant research has focused on the elucidation of its specific role in the central nervous system. Recent studies revealed that MECP2 is expressed in more differentiated neurons rather than in less differentiated neuroblasts and that MECP2 is involved in the maturation and maintenance of neurons, including dendritic arborization and axonal projections, rather than in early cell fate decisions in the mammalian brain. In this review, we summarize recent findings regarding regional, temporal, and cell type-specific MECP2 expression in the central nervous system; neurobiologic abnormalities in MECP2 -mutant mice; and MECP2 target genes in the central nervous system.

MECP2 is progressively expressed in post-migratory neurons and is involved in neuronal maturation rather than cell fate decisions

Rett syndrome is a neurodevelopmental disorder and one of the causes of mental retardation and autistic behavior in girls, as well as in a small group of boys. It was recently discovered that mutation of the methyl-CpG-binding protein 2 (MECP2) gene encoding a transcriptional repressor on the X chromosome causes Rett syndrome. Although it is evident that phenotypes of MECP2 mutant mice that resemble those of Rett syndrome are attributable to lack of the MECP2 gene in the central nervous system (CNS), there is little understanding of the neuropathological abnormalities in the CNS of MECP2-null mice. Here, we investigated the developmental regulation and specific cellular expression of MECP2 during neural development both in vitro and in vivo. MECP2 is expressed in mature neurons, but not in astroglia or oligodendroglia, and is increasingly expressed during development of the mouse neocortex. In addition, in vitro culture studies suggest that MECP2 is expressed in more differentiated neurons rather than in less differentiated neuroblasts. Under in vitro conditions using neural precursor cultures, we find that MECP2 mutant neural precursors differentiate into morphologically mature neurons and glia, and no significant differences in differentiation are detected between cells from wild-type and MECP2 mutant mice, suggesting that MECP2 may play a different role in mice than it does in Xenopus embryos. In agreement with this hypothesis, neocortical projection layers in MECP2 -/y mice are thinner than those in wild-type mice, and pyramidal neurons in layer II/III in MECP2 -/y mice are smaller and less complex than those in wild-type mice. Taken together, our results indicate that MECP2 is involved in the maturation and maintenance of neurons, including dendritic arborization, rather than in cell fate decisions.

Inhibition of tyrosine kinase receptor type B synthesis blocks axogenic effect of estradiol on rat hypothalamic neurones in vitro

17-beta-estradiol (E2) increases axonal growth and tyrosine kinase receptor (Trk)B levels of male-derived hypothalamic neurones in vitro. To investigate whether the axogenic response depends on the upregulation of TrkB, we analysed neuritic growth and neuronal polarization in cultures treated with an antisense oligonucleotide against TrkB mRNA. In cultures without E2, treatment with 7.5 or 10 micro m antisense reduced TrkB levels and the percentage of neurones showing an identifiable axon; the number and length of minor processes were increased. In cultures treated with 5 micro m antisense, morphometric parameters were normal although total TrkB levels were reduced. The same dose prevented the E2-dependent increase of TrkB levels and suppressed the axogenic effect of E2. These results indicate that TrkB is necessary for normal neuronal growth and maturation and further suggest that an increase in TrkB is necessary for E2 to exert its axogenic effect in male-derived neurones.

TrkB regulates neocortex formation through the Shc/PLCgamma-mediated control of neuronal migration

The generation of complex neuronal structures, such as the neocortex, requires accurate positioning of neurons and glia within the structure, followed by differentiation, formation of neuronal connections, and myelination. To understand the importance of TrkB signaling during these events, we have used conditional and knockin mutagenesis of the TrkB neurotrophin receptor, and we now show that this tyrosine kinase receptor, through docking sites for the Shc/FRS2 adaptors and phospholipase Cgamma (PLCgamma), coordinates these events in the cerebral cortex by (1) controlling cortical stratification through the timing of neuronal migration during cortex formation, and (2) regulating both neuronal and oligodendrocyte differentiation. These results provide genetic evidence that TrkB regulates important functions throughout the formation of the cerebral cortex via recruitment of the Shc/FRS2 adaptors and PLCgamma.

Stage-specific and opposing roles of BDNF, NT-3 and bFGF in differentiation of purified callosal projection neurons toward cellular repair of complex circuitry

Cellular repair of neuronal circuitry affected by neurodegenerative disease or injury may be approached in the adult neocortex via transplantation of neural precursors ("neural stem cells") or via molecular manipulation and recruitment of new neurons from endogenous precursors in situ. A major challenge for potential future approaches to neuronal replacement will be to specifically direct and control progressive differentiation, axonal projection and connectivity of neural precursors along a specific neuronal lineage. This goal will require a progressively more detailed understanding of the molecular controls over morphologic differentiation of specific neuronal lineages, including neurite outgrowth and elongation, in order to accurately permit and direct proper neuronal integration and connectivity. Here, we investigate controls over the morphologic differentiation of a specific prototypical lineage of cortical neurons: callosal projection neurons (CPN). We highly enriched CPN to an essentially pure population, and cultured them at three distinct stages of development from embryonic and postnatal mouse cortex by retrograde fluorescence labelling, followed by fluorescence-activated cell sorting. We find that specific peptide growth factors exert direct stage-specific positive and negative effects over the morphologic differentiation and process outgrowth of CPN. These effects are distinct from the effects of these growth factors on CPN survival [Catapano et al. (2001)J. Neurosci., 21, 8863-8872]. These data may be critical for the future goal of directing lineage-specific neuronal differentiation of transplanted or endogenous precursors/"stem cells" toward cellular repair of complex cortical circuitry.

Brain-derived neurotrophic factor is required for the maintenance of cortical dendrites

Brain-derived neurotrophic factor (BDNF) is thought to be involved in neuronal survival, migration, morphological and biochemical differentiation, and modulation of synaptic function in the CNS. In the rodent cortex, postnatal BDNF expression is initially low but subsequently increases to reach maximal levels around weaning. Thus, BDNF expression peaks at a time when both structural and functional maturation of cortical circuitry occurs. Although the function of BDNF has been probed using many approaches, its requirements during this phase of life have not previously been examined genetically. To test the in vivo requirements for BDNF during this important phase of development we generated early-onset forebrain-specific BDNF mutant mice. Although these mice undergo forebrain-restricted deletion of BDNF by Cre-mediated recombination during embryogenesis, they are healthy, and we did not detect the loss of specific cortical excitatory or inhibitory neurons. However, the neocortex of 5-week-old mice was thinner, attributable at least partly to neuronal shrinkage. Importantly, although visual cortical layer 2/3 neurons in the mutants initially developed normal dendrite structure, dendritic retraction became apparent by 3 weeks of age. Thus, our observations suggest that cortically expressed BDNF functions to support the maintenance of cortical neuron size and dendrite structure rather than the initial development of these features. This is consistent with a role for BDNF in stabilizing the "survival" of circuitry during the phase of activity-dependent reorganization of cortical connectivity.

Interactions between TrkB signaling and serotonin excess in the developing murine somatosensory cortex: a role in tangential and radial organization of thalamocortical axons

Mice lacking monoamine oxidase A (MAOA) display high levels of brain serotonin during the first postnatal week, causing an exuberant outgrowth of thalamocortical axons (TCAs) in layer IV of the somatosensory cortex (S1). We asked whether this exuberance is attributable to abnormal TrkB signaling, because modulation of TrkB signaling during a critical period dramatically influences the segregation and the morphology of TCAs in layer IV of the visual cortex. Using in situ hybridization and ELISA immunoassays, we showed that the levels of trkB mRNA and BDNF and neurotrophin-4 (NT-4) proteins are normal in the thalamus and the cortex of mice lacking MAOA during barrel field formation. Because the release of BDNF and NT-4 could be abnormal in MAOA knock-out (KO) mice, we tested whether abnormal TrkB signaling is required for TCA exuberance in MAOA-KO mice by generating mice lacking both trkB and MAOA. Surprisingly, these mice exhibited more severe phenotypes than those found in MAOA-KO mice: a widespread tangential expansion of TCAs in layer IV of the cortex, resulting in a fusion of all sensory representations and a radial expansion of TCAs in layers II-III of the cortex. Careful examination of mice lacking trkB alone revealed subtle alterations of TCAs, with abnormal invasion of layer III. This study reveals the following: (1) expression of trkB, BDNF, and NT-4 are not modulated by an excess of serotonin during barrel formation, (2) TrkB signaling limits branching of TCAs in inappropriate supragranular cortical layers, and (3) serotonin and TrkB signaling act together to cluster thalamocortical axons in layer IV.

Late-stage immature neocortical neurons reconstruct interhemispheric connections and form synaptic contacts with increased efficiency in adult mouse cortex undergoing targeted neurodegeneration

In the neocortex, the effectiveness of potential cellular repopulation therapies for diseases involving neuronal loss may depend critically on whether newly incorporated cells can differentiate appropriately into precisely the right kind of neuron, re-establish precise long-distance connections, and reconstruct complex functional circuitry. Here, we test the hypothesis that increased efficiency of connectivity could be achieved if precursors could be more fully differentiated toward desired phenotypes. We compared embryonic neuroblasts and immature murine neurons subregionally dissected from either embryonic day 17 (E17) (Shin et al., 2000) or E19 primary somatosensory (S1) cortex and postnatal day 3 (P3) purified callosal projection neurons (CPNs) with regard to neurotransmitter and receptor phenotype and afferent synapse formation after transplantation into adult mouse S1 cortex undergoing targeted apoptotic degeneration of layer II/III and V CPNs. Two weeks after transplantation, neurons from all developmental stages were found dispersed within layers II/III and V, many with morphological features typical of large pyramidal neurons. Retrograde labeling with FluoroGold revealed that 42 +/- 2% of transplanted E19 immature S1 neurons formed connections with the contralateral S1 cortex by 12 weeks after transplantation, compared with 23 +/- 7% of E17 neurons. A greater percentage of E19-derived neurons received synapses (77 +/- 1%) compared with E17-derived neurons (67 +/- 2%). Similar percentages of both E17 and E19 donor-derived neurons expressed neurotransmitters and receptors [glutamate, aspartate, GABA, GABA receptor (GABA-R), NMDA-R, AMPA-R, and kainate-R] appropriate for endogenous adult CPNs progressively over a period of 2-12 weeks after transplantation. Although P3 fluorescence-activated cell sorting-purified neurons also expressed these mature phenotypic markers after transplantation, their survival in vivo was poor. We conclude that later-stage and region-specific immature neurons develop a mature CPN phenotype and make appropriate connections with recipient circuitry with increased efficiency. However, at postnatal stages of development, limitations in survival outweigh this increased efficiency. These results suggest that efforts to direct the differentiation of earlier precursors precisely along specific desired neuronal lineages could potentially make possible the highly efficient reconstruction of complex neocortical and other CNS circuitry.

Ethanol exposure enhances cell death in the developing cerebral cortex: role of brain-derived neurotrophic factor and its signaling pathways

Exposure to ethanol during fetal development induces brain damage, causing cell loss in several brain areas and affecting synaptic connections. Because neurotrophin signaling plays an important role in neuronal survival and differentiation, we have investigated the effect of ethanol exposure on cell death in the developing cerebral cortex and whether this effect correlates with alterations in brain-derived neurotrophic factor (BDNF) levels, expression of its receptors, TrkB, and its signaling. We report that chronic ethanol intake during gestation and lactation enhances natural cell death and induces cell necrosis, decreases BDNF levels, and increases the ratio of the truncated to full-length TrkB mRNA receptors during postnatal developing cerebral cortex. Furthermore, we provide evidence that during brain development BDNF activates the extracellular signal-regulated kinases (ERK1 and ERK2) and the phosphoinoside-3-kinase (PI-3-K/Akt) pathways. However, BDNF-induced cell signaling throughout the above-mentioned survival pathways is significantly reduced by ethanol exposure. These findings suggest that ethanol-induced alterations in BDNF availability and in its receptor function might impair intracellular signaling pathways involved in cell survival, growth, and differentiation, leading to enhanced natural cell death during cerebral cortex development.

Alterations in trkB mRNA in the human prefrontal cortex throughout the lifespan

Signalling through tyrosine kinase receptor B (trkB) influences neuronal survival, differentiation and synaptogenesis. trkB exists in a full-length form (trkB(TK+)), which contains a catalytic tyrosine kinase (TK) domain, and a truncated form (trkB(TK-)), which lacks this domain. In the rodent brain, expression of trkB(TK+) decreases and trkBTK- increases during postnatal life. We hypothesized that both forms of trkB receptor mRNA would be present in the human neocortex and that the developmental profile of trkB gene expression in human may be distinct from that in rodent. We detected both trkB(TK+) and trkB(TK-) mRNA in RNA extracted from multiple human brain regions by Northern blot. Using in situ hybridization, we found trkB(TK+) mRNA in all cortical layers, with highest expression in layer IV and intermediate-to-high expression in layers III and V of the human dorsolateral prefrontal cortex. trkB(TK+) mRNA was present in neurons with both pyramidal and nonpyramidal shapes in the dorsolateral prefrontal cortex. trkB(TK+) mRNA levels were significantly increased in layer III in young adults as compared with infants and the elderly. In the elderly, trkB(TK+) mRNA levels were reduced markedly in all cortical layers. Unlike the mRNA encoding the full-length form of trkB, trkB(TK-) mRNA was distributed homogeneously across the grey matter, and trkB(TK-) mRNA levels increased only slightly during postnatal life. The results suggest that neurons in the human dorsolateral prefrontal cortex are responsive to neurotrophins throughout postnatal life and that this responsiveness may be modulated during the human lifespan.

Specific neurotrophic factors support the survival of cortical projection neurons at distinct stages of development

Repair of specific neuronal circuitry in the neocortex may be possible via neural precursor transplantation or manipulation of endogenous precursors in situ. These approaches will almost certainly require a detailed understanding of the mechanisms that control survival and differentiation of specific neuronal lineages. Such analysis has been hampered by the overwhelming diversity of neuronal types intermixed in neocortex and the inability to isolate individual lineages. To elucidate stage-specific controls over the survival of individual lineages of cortical neurons, we purified immature callosal projection neurons (CPN) at distinct stages of development from embryonic and postnatal mouse cortex by retrograde fluorescence labeling, followed by fluorescence-activated cell sorting. Purified CPN survive well in culture, acquire stage-specific projection neuron morphologies, and express appropriate neurotransmitters and growth factor receptors. Purified CPN are dependent on exogenous trophic support for survival in a stage-specific manner. Survival of postnatal day 2 (P2) to P3 and P6-P7 CPN is promoted by overlapping but distinct sets of neurotrophic factors, whereas embryonic day 19 CPN show less specificity of dependence on peptide factors. These studies demonstrate for the first time the stage-specific control by peptide growth factors over the survival of a specific cortical neuronal lineage. Such information may be critical for the future goal of directed differentiation of transplanted or endogenous precursors toward cellular repair of complex cortical circuitry.

Differential cellular expression of neurotrophins in cortical tubers of the tuberous sclerosis complex

Neurotrophins and their receptors modulate cerebral cortical development. Tubers in the tuberous sclerosis complex (TSC) are characterized histologically by disorganized cortical cytoarchitecture and thus, we hypothesized that expression of neurotrophin mRNAs and proteins might be altered in tubers. Using in situ transcription and mRNA amplification to probe cDNA arrays, we found that neurotrophin-3 (NT3) and trkB mRNA expression were reduced whereas neurotrophin-4 (NT4) and trkC mRNA expression were increased in whole tuber sections. Alterations in mRNA abundance were defined in single microdissected dysplastic neurons (DNs) and giant cells (GCs). NT3 mRNA expression was reduced in GCs and trkB mRNA expression was reduced in DNs. NT4 mRNA expression was increased in DNs and trkC mRNA expression was increased in both DNs and GCs. In three patients, TSC2 locus mutations were confirmed and the mean tuberin mRNA expression levels was reduced across all nine cases. Consistent with these observations, NT3 mRNA expression was reduced but trkC mRNA expression was increased in vitro in human NTera2 neurons (NT2N) transfected with a tuberin antisense construct that reduced tuberin expression. Western analysis of tuber homogenates and computer-assisted densitometry of immunolabeled sections confirmed the neurotrophin mRNA expression data in whole sections and single neurotrophin immunoreactive cells. We conclude that alterations in NT4/trkB and NT3/trkC expression may contribute to tuber formation during brain development as downstream effects of the hamartin and tuberin pathway in TSC.