Blockade of TrkB but not p75NTR activates a subpopulation of quiescent neural precursor cells and enhances neurogenesis in the adult mouse hippocampus

Brain‐derived neurotrophic factor (BDNF) signaling plays a major role in the regulation of hippocampal neurogenesis in the adult brain. While the majority of studies suggest that this is due to its effect on the survival and differentiation of newborn neurons, it remains unclear whether this signaling directly regulates neural precursor cell (NPC) activity and which of its two receptors, TrkB or the p75 neurotrophin receptor (p75^NTR) mediates this effect. Here, we examined both the RNA and protein expression of these receptors and found that TrkB but not p75^NTR receptors are expressed by hippocampal NPCs in the adult mouse brain. Using a clonal neurosphere assay, we demonstrate that pharmacological blockade of TrkB receptors directly activates a distinct subpopulation of NPCs. Moreover, we show that administration of ANA‐12, a TrkB‐selective antagonist, in vivo either by systemic intraperitoneal injection or by direct infusion within the hippocampus leads to an increase in the production of new neurons. In contrast, we found that NPC‐specific knockout of p75^NTR had no effect on the proliferation of NPCs and did not alter neurogenesis in the adult hippocampus. Collectively, these results demonstrate a novel role of TrkB receptors in directly regulating the activity of a subset of hippocampal NPCs and suggest that the transient blockade of these receptors could be used to enhance adult hippocampal neurogenesis.     

The Interesting Interplay Between Interneurons and Adult Hippocampal Neurogenesis

Adult neurogenesis is a unique form of plasticity found in the hippocampus, a brain region key to learning and memory formation. While many external stimuli are known to modulate the generation of new neurons in the hippocampus, little is known about the local circuitry mechanisms that regulate the process of adult neurogenesis. The neurogenic niche in the hippocampus is highly complex and consists of a heterogeneous population of cells including interneurons. Because interneurons are already highly integrated into the hippocampal circuitry, they are in a prime position to influence the proliferation, survival, and maturation of adult-generated cells in the dentate gyrus. Here, we review the current state of our understanding on the interplay between interneurons and adult hippocampal neurogenesis. We focus on activity- and signaling-dependent mechanisms, as well as research on human diseases that could provide better insight into how interneurons in general might add to our comprehension of the regulation and function of adult hippocampal neurogenesis.     

Neuritin Attenuates Cognitive Function Impairments in Tg2576 Mouse Model of Alzheimer's Disease

Neuritin, also known as CPG15, is a neurotrophic factor that was initially discovered in a screen to identify genes involved in activity-dependent synaptic plasticity. Neuritin plays multiple roles in the process of neural development and synaptic plasticity, although its binding receptor(s) and downstream signaling effectors remain unclear. In this study, we found that the cortical and hippocampal expression of neuritin is reduced in the brains of Alzheimer''s disease (AD) patients and demonstrated that viral-mediated expression of neuritin in the dentate gyrus of 13-month-old Tg2576 mice, an AD animal model, attenuated a deficit in learning and memory as assessed by a Morris water maze test. We also found that neuritin restored the reduction in dendritic spine density and the maturity of individual spines in primary hippocampal neuron cultures prepared from Tg2576 mice. It was also shown that viral-mediated expression of neuritin in the dentate gyrus of 7-week-old Sprague-Dawley rats increased neurogenesis in the hippocampus. Taken together, our results demonstrate that neuritin restores the reduction in dendritic spine density and the maturity of individual spines in primary hippocampal neurons from Tg2576 neurons, and also attenuates cognitive function deficits in Tg2576 mouse model of AD, suggesting that neuritin possesses a therapeutic potential for AD.     

Neurotrophic actions initiated by proNGF in adult sensory neurons may require peri-somatic glia to drive local cleavage to NGF

The nerve growth factor (NGF) precursor, proNGF, is implicated in various neuropathological states. ProNGF signals apoptosis by forming a complex with the receptors p75 and sortilin, however, it can also induce neurite growth, proposed to be mediated by the receptor of mature NGF, tyrosine kinase receptor A (TrkA). The way in which these dual effects occur in adult neurons is unclear. We investigated the neurotrophic effects of proNGF on peptidergic sensory neurons isolated from adult mouse dorsal root ganglia and found that proNGF stimulated neurite extension and branching, requiring p75, sortilin and TrkA. Neurite growth rarely occurred in sortilin-expressing neurons but was commonly observed in TrkA-positive, sortilin-negative neurons that associated closely with sortilin-positive glia. ProNGF was unable to induce local trophic effects at growth cones where sortilin-positive glia was absent. We propose that in adult sensory neurons the neurotrophic response to proNGF is mediated by NGF and TrkA, and that peri-somatic glia may participate in sortilin- and p-75 dependent cleavage of proNGF. The potential ability of local glial cells to provide a targeted supply of NGF may provide an important way to promote trophic (rather than apoptotic) outcomes under conditions where regeneration or sprouting is required.     

Neurogenesis in mouse models of Alzheimer's disease

The brains of the adult mouse and human possess neural stem cells (NSCs) that retain the capacity to generate new neurons through the process of neurogenesis. They share the same anatomical locations of stem cell niches in the brain, as well as the prominent feature of rostral migratory stream formed by neuroblasts migrating from the lateral ventricles towards the olfactory bulb. Therefore the mouse possesses some fundamental features that may qualify it as a relevant model for adult human neurogenesis. Adult born young hippocampal neurons in the mouse display the unique property of enhanced plasticity, and can integrate physically and functionally into existing neural circuits in the brain. Such crucial properties of neurogenesis may at least partially underlie the improved learning and memory functions observed in the mouse when hippocampal neurogenesis is augmented, leading to the suggestion that neurogenesis induction may be a novel therapeutic approach for diseases with cognitive impairments such as Alzheimer's disease (AD). Research towards this goal has benefited significantly from the use of AD mouse models to facilitate the understanding in the impact of AD pathology on neurogenesis. The present article reviews the growing body of controversial data on altered neurogenesis in mouse models of AD and attempts to assess their relative relevance to humans.     

Prolactin stimulates precursor cells in the adult mouse hippocampus

In the search for ways to combat degenerative neurological disorders, neurogenesis-stimulating factors are proving to be a promising area of research. In this study, we show that the hormonal factor prolactin (PRL) can activate a pool of latent precursor cells in the adult mouse hippocampus. Using an in vitro neurosphere assay, we found that the addition of exogenous PRL to primary adult hippocampal cells resulted in an approximate 50% increase in neurosphere number. In addition, direct infusion of PRL into the adult dentate gyrus also resulted in a significant increase in neurosphere number. Together these data indicate that exogenous PRL can increase hippocampal precursor numbers both in vitro and in vivo. Conversely, PRL null mice showed a significant reduction (approximately 80%) in the number of hippocampal-derived neurospheres. Interestingly, no deficit in precursor proliferation was observed in vivo, indicating that in this situation other niche factors can compensate for a loss in PRL. The PRL loss resulted in learning and memory deficits in the PRL null mice, as indicated by significant deficits in the standard behavioral tests requiring input from the hippocampus. This behavioral deficit was rescued by direct infusion of recombinant PRL into the hippocampus, indicating that a lack of PRL in the adult mouse hippocampus can be correlated with impaired learning and memory.     

The p75 neurotrophin receptor is localized to primary cilia in adult murine hippocampal dentate gyrus granule cells

The densely ciliated granule cell layer of the adult murine hippocampal dentate gyrus is one of two sites of adult neurogenesis. The granule cells have already been proven to localize their SSTR3 (somatostatin receptor 3) receptors to their so-called primary cilia. Here we show for the first time that 70-90% of these cells in 7-18 months-old wild-type and 3×Tg-AD (Alzheimer disease transgenic) mice also load p75^NTR receptors into the structures containing SSTR3, i.e., their primary cilia. On the other hand, p75^NTR’s TrkA co-receptors were not localized to cilia but conventionally distributed throughout the cell surface. Significantly fewer cells (20-40%) in the hippocampal CA1 and CA3 regions and cerebral cortex have p75^NTR containing cilia. While we don’t know what the impact of the cilial localization of p75^NTR on dentate gyral adult neurogenesis and memory encoding might be, the cilia’s amyloid β-activatable p75^NTR receptors could be damaging or lethal to the hippocampal functioning of amyloid β-accumulating Alzheimer brain.     

Nerve growth factor and its precursor differentially regulate hair cycle progression in mice.

Nerve growth factor (NGF) promotes proliferation via its high affinity receptor (TrkA). Its precursor proNGF promotes apoptosis via the pan-neurotrophin-receptor p75. Recently, we have identified NGF and p75 as important hair growth terminators. However, if proNGF is involved or if NGF can also promote hair growth via TrkA is unclear. By RT-PCR we found that NGF/proNGF mRNA levels peak during early anagen in murine back skin, whereas NGF/proNGF protein levels peak during catagen, indicating high turnover in early anagen and protein accumulation in catagen. By immunohistochemistry, NGF and TrkA are found in the proliferating compartments of the epidermis and hair follicle throughout the cycle. In contrast, strong proNGF is found in the highly differentiated inner root sheath and adjacent to the p75+ regressing epithelial strand in catagen. Commercial 7S NGF, which contains both NGF and proNGF, promotes anagen development in organ-cultured early anagen mouse skin, whereas it promotes catagen development in late anagen skin. Together, our findings suggest an anagen-promoting or anagen-supporting role for NGF/TrkA, and a catagen-promoting role for proNGF/p75 interactions. This has important implications for the future design of specific neurotrophin receptor ligands as novel pharmaceuticals in the modification of tissue remodeling processes such as hair growth or wound healing.     

Dominance-related changes in spatial memory are associated with changes in hippocampal cell proliferation rates in mountain chickadees

It is well established that spatial memory is dependent on the hippocampus in both mammals and birds. As memory capacity can fluctuate on a temporal basis, it is important to understand the mechanisms mediating such changes. It is known that early memory-dependent experiences in young animals result in hippocampal enlargement and in increased neurogenesis, including cell proliferation and neuron survival. It is less clear, however, whether temporal changes in spatial memory are also associated with changes in hippocampal anatomy and cell proliferation in fully grown and experienced adult animals. In a previous study, we experimentally demonstrated that socially subordinate mountain chickadees (Poecile gambeli) showed inferior spatial memory performance compared to their dominant group mates, in the absence of significant differences in baseline corticosterone levels. Here we investigated whether these differences in memory between dominant and subordinate birds were associated with changes in the hippocampus. Following memory tests, chickadees were injected with 5-bromo-2'-deoxyuridine to label dividing cells and sacrificed 2 days after the injections. We found no significant differences in volume or the total number of neurons in the hippocampal formation between dominant and subordinate chickadees, but subordinate birds had significantly lower cell proliferation rates in the ventricular zone adjacent to both the hippocampus and mesopallium compared to the dominants. Individuals, which performed better on spatial memory tests tended to have higher levels of cell proliferation. These results suggest that social status can affect cell proliferation rates in the ventricular zone and support the hypothesis that neurogenesis might be involved in memory function in adult animals.     

Deficient neurogenesis in forebrain-specific presenilin-1 knockout mice is associated with reduced clearance of hippocampal memory traces.

To examine the in vivo function of presenilin-1 (PS1), we selectively deleted the PS1 gene in excitatory neurons of the adult mouse forebrain. These conditional knockout mice were viable and grew normally, but they exhibited a pronounced deficiency in enrichment-induced neurogenesis in the dentate gyrus. This reduction in neurogenesis did not result in appreciable learning deficits, indicating that addition of new neurons is not required for memory formation. However, our postlearning enrichment experiments lead us to postulate that adult dentate neurogenesis may play a role in the periodic clearance of outdated hippocampal memory traces after cortical memory consolidation, thereby ensuring that the hippocampus is continuously available to process new memories. A chronic, abnormal clearance process in the hippocampus may conceivably lead to memory disorders in the mammalian brain.     

2,5-Hexanedione induced apoptosis in rat spinal cord neurons and VSC4.1 cells via the proNGF/p75NTR and JNK pathways

Increasing evidence suggests that n-hexane induces nerve injury via neuronal apoptosis induced by its active metabolite 2,5-hexanedione (HD). However, the underlying mechanism remains unknown. Studies have confirmed that pro-nerve growth factor (proNGF), a precursor of mature nerve growth factor (mNGF), might activate apoptotic signaling by binding to p75 neurotrophin receptor (p75NTR) in neurons. Therefore, we studied the mechanism of the proNGF/p75NTR pathway in HD-induced neuronal apoptosis. Sprague–Dawley (SD) rats were injected with 400 mg/kg HD once a day for 5 weeks, and VSC4.1 cells were treated with 10, 20, and 40 mM HD in vitro. Results showed that HD effectively induced neuronal apoptosis. Moreover, it up-regulated proNGF and p75NTR levels, activated c-Jun N-terminal kinase (JNK) and c-Jun, and disrupted the balance between B-cell lymphoma-2 (Bcl-2) and Bcl-2-associated X protein (Bax). Our findings revealed that the proNGF/p75NTR signaling pathway was involved in HD-induced neuronal apoptosis; it can serve as a theoretical basis for further exploration of the neurotoxic mechanisms of HD.     

Proteolytic processing of proNGF is necessary for mature NGF regulated secretion from neurons

Nerve growth factor mediates neuronal survival, synaptogenesis, and synaptic remodeling. We utilized primary hippocampal cultures to investigate the intrinsic motifs of proNGF that might contribute to its processing and subsequent allocation to a regulated versus constitutive secretory pathway. The addition of a carboxypeptidase E motif to proNGF did not alter the secretion of NGF. However, mutagenesis of proNGF proteolytic processing sites had significant effects on the final NGF product and its secretion. The furin recognition site (R118-S-K-R121) is essential for the proper processing of proNGF to its 13.5kDa mature product and mutating the furin site exposed an alternative processing site resulting in an intermediate NGF product of approximately 22kDa. Finally, inhibiting the processing of proNGF abolished regulated secretion of the resulting NGF product. These experiments demonstrate that hippocampal neurons harbor multiple pathways to process proNGF of which the furin consensus sequence is the preferred processing site.     

Hippocampal learning,memory, and neurogenesis: Effects of sex and estrogens across the lifespan in adults

This article is part of a Special Issue “Estradiol and Cognition”.There are sex differences in hippocampus-dependent cognition and neurogenesis suggesting that sex hormones are involved. Estrogens modulate certain forms of spatial and contextual memory and neurogenesis in the adult female rodent, and to a lesser extent male, hippocampus. This review focuses on the effects of sex and estrogens on hippocampal learning, memory, and neurogenesis in the young and aged adult rodent. We discuss how factors such as the type of estrogen, duration and dose of treatment, timing of treatment, and type of memory influence the effects of estrogens on cognition and neurogenesis. We also address how reproductive experience (pregnancy and mothering) and aging interact with estrogens to modulate hippocampal cognition and neurogenesis in females. Given the evidence that adult hippocampal neurogenesis plays a role in long-term spatial memory and pattern separation, we also discuss the functional implications of regulating neurogenesis in the hippocampus.     

The microtubule destabilizing protein stathmin controls the transition from dividing neuronal precursors to postmitotic neurons during adult hippocampal neurogenesis

The hippocampus is one of the two areas in the mammalian brain where adult neurogenesis occurs. Adult neurogenesis is well known to be involved in hippocampal physiological functions as well as pathophysiological conditions. Microtubules (MTs), providing intracellular transport, stability, and transmitting force, are indispensable for neurogenesis by facilitating cell division, migration, growth, and differentiation. Although there are several examples of MT‐stabilizing proteins regulating different aspects of adult neurogenesis, relatively little is known about the function of MT‐destabilizing proteins. Stathmin is such a MT‐destabilizing protein largely restricted to the CNS, and in contrast to its developmental family members, stathmin is also expressed at significant levels in the adult brain, notably in areas involved in adult neurogenesis. Here, we show an important role for stathmin during adult neurogenesis in the subgranular zone of the mouse hippocampus. After carefully mapping stathmin expression in the adult dentate gyrus (DG), we investigated its role in hippocampal neurogenesis making use of stathmin knockout mice. Although hippocampus development appears normal in these animals, different aspects of adult neurogenesis are affected. First, the number of proliferating Ki‐67+ cells is decreased in stathmin knockout mice, as well as the expression of the immature markers Nestin and PSA‐NCAM. However, newborn cells that do survive express more frequently the adult marker NeuN and have a more mature morphology. Furthermore, our data suggest that migration in the DG might be affected. We propose a model in which stathmin controls the transition from neuronal precursors to early postmitotic neurons. © 2014 Wiley Periodicals, Inc. Develop Neurobiol 74: 1226–1242, 2014     

Deleterious impact of nerve growth factor precursor (proNGF) on bladder urothelial and smooth muscle cells

The nerve growth factor precursor (proNGF) activates p75^NTR receptor and promotes cell death in different tissues, yet this pathophysiological effect is not fully described in the bladder. The aim of this study was to identify the biological effect of proNGF/p75^NTR activation on urothelial and smooth muscle (SM) cells of rodents' bladder. Cell viability was assessed by MTT assay which showed a significant reduction in urothelial viability after 24 h of incubation with proNGF in culture medium [5 or 10 nM], an effect not seen in SM cells. Western blot analysis on cellular protein extracts showed increased expression of the transmembrane TNF-α and activation of RhoA in urothelial cells exposed to proNGF with no evidence of a nuclear translocation of NF-κB assessed by western blotting on nuclear extracts and immunofluorescence. The activation of p75^NTR-death domain related pathways in urothelial cells such as TNF-α or RhoA had a downstream effect on NO release and the junctional protein occludin, as estimated respectively by colorimetric and western blotting. On the other hand, proNGF did not induce TNF-α or RhoA expression in SM cells, but induced a significant NF-κB nuclear translocation. ProNGF had a different impact on SM as evidenced by a significant dose- and time-dependent increase in SM proliferation and migration examined by MTT test and cell migration assay. Together, our results indicate that activation of proNGF/p75^NTR axis induces degenerative changes to the urothelial layer impacting its barrier and signaling integrity, while promoting adaptive proliferative changes in detrusor SM cells that can interfere with the contractile phenotype essential for proper bladder function.     

An inducible transgenic Cre mouse line for the study of hippocampal development and adult neurogenesis

The hippocampus is crucial for higher brain functions, such as learning, memory, and emotion. Many diseases like epilepsy and Down's syndrome are associated with abnormalities in early hippocampal development. In addition, adult dentate neurogenesis is thought to be defective in several classes of psychiatric disorders. However, the mechanisms regulating hippocampal development and adult neurogenesis remain unclear. One of the limitations to studying these processes is the scarcity of available specific mouse tools. Here, we report an inducible transgenic Cre mouse line, Frizzled 9‐CreER?, in which tamoxifen administration induces Cre recombinant. Our data show that Cre is expressed in the developing hippocampal primordium, confined to the granule cell layer at P20 and further limited to the subgranular zone in the adult dentate gyrus. Cre recombinase shows very high activity in all of these regions. Thus, this transgenic line will be a powerful tool in understanding the mechanisms of hippocampal development, adult neurogenesis, and associated diseases. genesis 49:919–926, 2011. © 2011 Wiley Periodicals, Inc.