Hematopoietic stem cell transplantation chemotherapy causes microglia senescence and peripheral macrophage engraftment in the brain.

Hematopoietic stem cell transplantation (HSCT) is a therapy used for multiple malignant and nonmalignant diseases, with chemotherapy used for pretransplantation myeloablation. The post-HSCT brain contains peripheral engrafted parenchymal macrophages, despite their absence in the normal brain, with the engraftment mechanism still undefined. Here we show that HSCT chemotherapy broadly disrupts mouse brain regenerative populations, including a permanent loss of adult neurogenesis. Microglial density was halved, causing microglial process expansion, coinciding with indicators of broad senescence. Although microglia expressed cell proliferation markers, they underwent cell cycle arrest in S phase with a majority expressing the senescence and antiapoptotic marker p21. In vivo single-cell tracking of microglia after recovery from chemical depletion showed loss of their regenerative capacity, subsequently replaced with donor macrophages. We propose that HSCT chemotherapy causes microglial senescence with a gradual decrease to a critical microglial density, providing a permissive niche for peripheral macrophage engraftment of the brain.

Youth Comes But Once in a Lifetime for Adult-Born Neurons.

Using new methods to functionally dissect circuits, two papers from 2015 found enhanced synaptic properties of the inputs and outputs of hippocampal adult-born neurons specifically during a critical period of their development. These studies provided a circuit-level view of unique roles for new neurons and how they cope with the ever-changing environment.

Nicotine reverses hypofrontality in animal models of addiction and schizophrenia.

The prefrontal cortex (PFC) underlies higher cognitive processes that are modulated by nicotinic acetylcholine receptor (nAChR) activation by cholinergic inputs. PFC spontaneous default activity is altered in neuropsychiatric disorders, including schizophrenia-a disorder that can be accompanied by heavy smoking. Recently, genome-wide association studies (GWAS) identified single-nucleotide polymorphisms (SNPs) in the human CHRNA5 gene, encoding the α5 nAChR subunit, that increase the risks for both smoking and schizophrenia. Mice with altered nAChR gene function exhibit PFC-dependent behavioral deficits, but it is unknown how the corresponding human polymorphisms alter the cellular and circuit mechanisms underlying behavior. Here we show that mice expressing a human α5 SNP exhibit neurocognitive behavioral deficits in social interaction and sensorimotor gating tasks. Two-photon calcium imaging in awake mouse models showed that nicotine can differentially influence PFC pyramidal cell activity by nAChR modulation of layer II/III hierarchical inhibitory circuits. In α5-SNP-expressing and α5-knockout mice, lower activity of vasoactive intestinal polypeptide (VIP) interneurons resulted in an increased somatostatin (SOM) interneuron inhibitory drive over layer II/III pyramidal neurons. The decreased activity observed in α5-SNP-expressing mice resembles the hypofrontality observed in patients with psychiatric disorders, including schizophrenia and addiction. Chronic nicotine administration reversed this hypofrontality, suggesting that administration of nicotine may represent a therapeutic strategy for the treatment of schizophrenia, and a physiological basis for the tendency of patients with schizophrenia to self-medicate by smoking.

Adult neurogenesis beyond the niche: its potential for driving brain plasticity.

Adult neurogenesis emerges as a tremendous form of plasticity with the continuous addition and loss of neurons in the adult brain. It is unclear how preexisting adult circuits generated during development are capable of modifying existing connections to accommodate the thousands of new synapses formed and exchanged each day. Here we first make parallels with sensory deprivation studies and its ability to induce preexisting non-neurogenic adult circuits to undergo massive reorganization. We then review recent studies that show high structural and synaptic plasticity in circuits directly connected to adult-born neurons. Finally, we propose future directions in the field to decipher how host circuits can accommodate new neuron integration and to determine the impact of adult neurogenesis on global brain plasticity.

Persistent Structural Plasticity Optimizes Sensory Information Processing in the Olfactory Bulb.

In the mammalian brain, the anatomical structure of neural circuits changes little during adulthood. As a result, adult learning and memory are thought to result from specific changes in synaptic strength. A possible exception is the olfactory bulb (OB), where activity guides interneuron turnover throughout adulthood. These adult-born granule cell (GC) interneurons form new GABAergic synapses that have little synaptic strength plasticity. In the face of persistent neuronal and synaptic turnover, how does the OB balance flexibility, as is required for adapting to changing sensory environments, with perceptual stability? Here we show that high dendritic spine turnover is a universal feature of GCs, regardless of their developmental origin and age. We find matching dynamics among postsynaptic sites on the principal neurons receiving the new synaptic inputs. We further demonstrate in silico that this coordinated structural plasticity is consistent with stable, yet flexible, decorrelated sensory representations. Together, our study reveals that persistent, coordinated synaptic structural plasticity between interneurons and principal neurons is a major mode of functional plasticity in the OB.

Tumorigenicity of hypoxic respiring cancer cells revealed by a hypoxia-cell cycle dual reporter.

Although aerobic glycolysis provides an advantage in the hypoxic tumor microenvironment, some cancer cells can also respire via oxidative phosphorylation. These respiring ("non-Warburg") cells were previously thought not to play a key role in tumorigenesis and thus fell from favor in the literature. We sought to determine whether subpopulations of hypoxic cancer cells have different metabolic phenotypes and gene-expression profiles that could influence tumorigenicity and therapeutic response, and we therefore developed a dual fluorescent protein reporter, HypoxCR, that detects hypoxic [hypoxia-inducible factor (HIF) active] and/or cycling cells. Using HEK293T cells as a model, we identified four distinct hypoxic cell populations by flow cytometry. The non-HIF/noncycling cell population expressed a unique set of genes involved in mitochondrial function. Relative to the other subpopulations, these hypoxic "non-Warburg" cells had highest oxygen consumption rates and mitochondrial capacity consistent with increased mitochondrial respiration. We found that these respiring cells were unexpectedly tumorigenic, suggesting that continued respiration under limiting oxygen conditions may be required for tumorigenicity.

Seamless reconstruction of intact adult-born neurons by serial end-block imaging reveals complex axonal guidance and development in the adult hippocampus.

In the adult mammalian hippocampus, newborn dentate granule cells are continuously integrated into the existing circuitry and contribute to specific brain functions. Little is known about the axonal development of these newborn neurons in the adult brain due to technological challenges that have prohibited large-scale reconstruction of long, thin, and complex axonal processes within the mature nervous system. Here, using a new serial end-block imaging (SEBI) technique, we seamlessly reconstructed axonal and dendritic processes of intact individual retrovirus-labeled newborn granule cells at different developmental stages in the young adult mouse hippocampus. We found that adult-born dentate granule cells exhibit tortuous, yet highly stereotyped, axonal projections to CA3 hippocampal subregions. Primary axonal projections of cohorts of new neurons born around the same time organize into laminar patterns with staggered terminations that stack along the septo-temporal hippocampal axis. Analysis of individual newborn neuron development further defined an initial phase of rapid axonal and dendritic growth within 21 d after newborn neuron birth, followed by minimal growth of primary axonal and whole dendritic processes through the last time point examined at 77 d. Our results suggest that axonal development and targeting is a highly orchestrated, precise process in the adult brain. These findings demonstrate a striking regenerative capacity of the mature CNS to support long-distance growth and guidance of neuronal axons. Our SEBI approach can be broadly applied for analysis of intact, complex neuronal projections in limitless tissue volume.

Secreted frizzled-related protein 3 regulates activity-dependent adult hippocampal neurogenesis.

Adult neurogenesis, the process of generating mature neurons from adult neural stem cells, proceeds concurrently with ongoing neuronal circuit activity and is modulated by various physiological and pathological stimuli. The niche mechanism underlying the activity-dependent regulation of the sequential steps of adult neurogenesis remains largely unknown. Here, we report that neuronal activity decreases the expression of secreted frizzled-related protein 3 (sFRP3), a naturally secreted Wnt inhibitor highly expressed by adult dentate gyrus granule neurons. Sfrp3 deletion activates quiescent radial neural stem cells and promotes newborn neuron maturation, dendritic growth, and dendritic spine formation in the adult mouse hippocampus. Furthermore, sfrp3 reduction is essential for activity-induced adult neural progenitor proliferation and the acceleration of new neuron development. Our study identifies sFRP3 as an inhibitory niche factor from local mature dentate granule neurons that regulates multiple phases of adult hippocampal neurogenesis and suggests an interesting activity-dependent mechanism governing adult neurogenesis via the acute release of tonic inhibition.

A dense poly(ethylene glycol) coating improves penetration of large polymeric nanoparticles within brain tissue.

Prevailing opinion suggests that only substances up to 64 nm in diameter can move at appreciable rates through the brain extracellular space (ECS). This size range is large enough to allow diffusion of signaling molecules, nutrients, and metabolic waste products, but too small to allow efficient penetration of most particulate drug delivery systems and viruses carrying therapeutic genes, thereby limiting effectiveness of many potential therapies. We analyzed the movements of nanoparticles of various diameters and surface coatings within fresh human and rat brain tissue ex vivo and mouse brain in vivo. Nanoparticles as large as 114 nm in diameter diffused within the human and rat brain, but only if they were densely coated with poly(ethylene glycol) (PEG). Using these minimally adhesive PEG-coated particles, we estimated that human brain tissue ECS has some pores larger than 200 nm and that more than one-quarter of all pores are ≥ 100 nm. These findings were confirmed in vivo in mice, where 40- and 100-nm, but not 200-nm, nanoparticles spread rapidly within brain tissue, only if densely coated with PEG. Similar results were observed in rat brain tissue with paclitaxel-loaded biodegradable nanoparticles of similar size (85 nm) and surface properties. The ability to achieve brain penetration with larger nanoparticles is expected to allow more uniform, longer-lasting, and effective delivery of drugs within the brain, and may find use in the treatment of brain tumors, stroke, neuroinflammation, and other brain diseases where the blood-brain barrier is compromised or where local delivery strategies are feasible.

Interaction between FEZ1 and DISC1 in regulation of neuronal development and risk for schizophrenia.

Disrupted-in Schizophrenia 1 (DISC1), a susceptibility gene for major mental disorders, encodes a scaffold protein that has a multifaceted impact on neuronal development. How DISC1 regulates different aspects of neuronal development is not well understood. Here, we show that Fasciculation and Elongation Protein Zeta-1 (FEZ1) interacts with DISC1 to synergistically regulate dendritic growth of newborn neurons in the adult mouse hippocampus, and that this pathway complements a parallel DISC1-NDEL1 interaction that regulates cell positioning and morphogenesis of newborn neurons. Furthermore, genetic association analysis of two independent cohorts of schizophrenia patients and healthy controls reveals an epistatic interaction between FEZ1 and DISC1, but not between FEZ1 and NDEL1, for risk of schizophrenia. Our findings support a model in which DISC1 regulates distinct aspects of neuronal development through its interaction with different intracellular partners and such epistasis may contribute to increased risk for schizophrenia.

Chordin-induced lineage plasticity of adult SVZ neuroblasts after demyelination.

The mechanisms that regulate the developmental potential of adult neural progenitor populations under physiological and pathological conditions remain poorly defined. Glutamic acid decarboxylase 65 (GAD65)- and Doublecortin (Dcx)-expressing cells constitute major progenitor populations in the adult mouse subventricular zone (SVZ). Under normal physiological conditions, SVZ-derived GAD65-positive and Dcx-positive cells expressed the transcription factor Pax6 and migrated along the rostral migratory stream to the olfactory bulb to generate interneurons. After lysolecithin-induced demyelination of corpus callosum, however, these cells altered their molecular and cellular properties and migratory path. Demyelination upregulated chordin in the SVZ, which redirected GAD65-positive and Dcx-positive progenitors from neuronal to glial fates, generating new oligodendrocytes in the corpus callosum. Our findings suggest that the lineage plasticity of SVZ progenitor cells could be a potential therapeutic strategy for diseased or injured brain.

Development of hippocampal mossy fiber synaptic outputs by new neurons in the adult brain.

New neurons are continuously generated in restricted regions of the adult mammalian brain. Although these adult-born neurons have been shown to receive synaptic inputs, little is known about their synaptic outputs. Using retrovirus-mediated birth-dating and labeling in combination with serial section electron microscopic reconstruction, we report that mossy fiber en passant boutons of adult-born dentate granule cells form initial synaptic contacts with CA3 pyramidal cells within 2 weeks after their birth and reach morphologic maturity within 8 weeks in the adult hippocampus. Knockdown of Disrupted-in-Schizophrenia-1 (DISC1) in newborn granule cells leads to defects in axonal targeting and development of synaptic outputs in the adult brain. Together with previous reports of synaptic inputs, these results demonstrate that adult-born neurons are fully integrated into the existing neuronal circuitry. Our results also indicate a role for DISC1 in presynaptic development and may have implications for the etiology of schizophrenia and related mental disorders.

Synaptic integration and plasticity of new neurons in the adult hippocampus.

Adult neurogenesis, a developmental process encompassing the birth of new neurons from adult neural stem cells and their integration into the existing neuronal circuitry, highlights the plasticity and regenerative capacity of the adult mammalian brain. Substantial evidence suggests essential roles of newborn neurons in specific brain functions; yet it remains unclear how these new neurons make their unique contribution. Recently, a series of studies have delineated the basic steps of the adult neurogenesis process and shown that many of the distinct steps are dynamically regulated by the activity of the existing circuitry. Here we review recent findings on the synaptic integration and plasticity of newborn neurons in the adult hippocampus, including the basic biological process, unique characteristics, critical periods, and activity-dependent regulation by the neurotransmitters GABA and glutamate. We propose that adult neurogenesis represents not merely a replacement mechanism for lost neurons, but also an ongoing developmental process in the adult brain that offers an expanded capacity for plasticity for shaping the existing circuitry in response to experience throughout life.

Adult neurogenesis and hippocampal memory function: new cells, more plasticity, new memories?

The discovery of active adult neurogenesis in mammals, a process of generating functional neurons from neural stem cells, suggests that the adult brain is more dynamic than once imagined. The coincidence of this phenomenon occurring in the hippocampus, a region critical to the learning process, begs the question of whether adult neurogenesis is involved in memory formation. Here, the authors review rapidly accumulating evidence showing a strong correlation between certain types of memory functions and adult neurogenesis in the hippocampus. Establishment of the potential link between memory formation and adult neurogenesis is instrumental, at a basic science level, to understand the function of neural networks and is essential, at a clinical level, to develop effective therapies for various cognitive dysfunctions.

Monocyte chemoattractant protein-1 plays a critical role in neuroblast migration after focal cerebral ischemia.

Transient focal ischemia is known to induce proliferation of neural progenitors in adult rodent brain. We presently report that doublecortin positive neuroblasts formed in the subventricular zone (SVZ) and the posterior peri-ventricle region migrate towards the cortical and striatal penumbra after transient middle cerebral artery occlusion (MCAO) in adult rodents. Cultured neural progenitor cells grafted into the non-infarcted area of the ipsilateral cortex migrated preferentially towards the infarct. As chemokines are known to induce cell migration, we investigated if monocyte chemoattractant protein-1 (MCP-1) has a role in post-ischemic neuroblast migration. Transient MCAO induced an increased expression of MCP-1 mRNA in the ipsilateral cortex and striatum. Immunostaining showed that the expression of MCP-1 was localized in the activated microglia and astrocytes present in the ischemic areas between days 1 and 3 of reperfusion. Furthermore, infusion of MCP-1 into the normal striatum induced neuroblast migration to the infusion site. The migrating neuroblasts expressed the MCP-1 receptor CCR2. In knockout mice that lacked either MCP-1 or its receptor CCR2, there was a significant decrease in the number of migrating neuroblasts from the ipsilateral SVZ to the ischemic striatum. These results show that MCP-1 is one of the factors that attract the migration of newly formed neuroblasts from neurogenic regions to the damaged regions of brain after focal ischemia.

Neurogenesis as a potential therapeutic strategy for neurodegenerative diseases.

In the adult mammalian brain, new neurons are continuously generated from a proliferating population of neural progenitor/stem cells and become incorporated into the existing neuronal circuitry via a process termed adult neurogenesis. The existence of active functional adult neurogenesis raises the exciting possibility that manipulating endogenous neural progenitors, or transplanting the progeny of exogenously expanded neural progenitors, may lead to successful cell replacement therapies for various degenerative neurological diseases. Significant effort is being made to decipher the mechanisms regulating adult neurogenesis, which may allow us to translate this endogenous neuronal replacement system into therapeutic interventions for neurodegenerative diseases. This review focuses on adult neurogenesis as a strategy to derive potential therapies, and discusses future directions in the field.

Insulin-like growth factor-1 is an endogenous mediator of focal ischemia-induced neural progenitor proliferation.

The adult mammalian brain contains resident neural progenitors in the subgranular zone of the dentate gyrus (DG) and the subventricular zone (SVZ) of the lateral ventricles. The proliferation of neural progenitors increases after focal cerebral ischemia in both of these regions, but the mechanisms that promote ischemia-induced neural progenitor proliferation are not yet understood. We hypothesize that diffusible factors from the ischemic area play a role in this process as the DG is remote from the area of infarction. In this study, we observed that the peak of neural progenitor proliferation in the ipsilateral DG was between day 2 and day 4 of reperfusion after transient middle cerebral artery occlusion in adult spontaneously hypertensive rats. GeneChip and real-time PCR analysis showed a three- to 102-fold increase in the expression of 15 diffusible, mitogenic factors in the ischemic cortex at 3 days of reperfusion. Of these, insulin-like growth factor-1 (IGF-1) showed increased protein expression in the activated astrocytes in the ischemic penumbra. In addition, the progenitors in both the SVZ and DG showed IGF-1 receptor expression. Inhibiting IGF-1 activity by introcerebroventricular infusion of IGF-1 antibody significantly prevented the ischemia-induced neural progenitor proliferation. These results indicate that IGF-1 formed in the ischemic penumbra might be one of the diffusible factors that mediate post-ischemic neural progenitor proliferation.

GABA regulates synaptic integration of newly generated neurons in the adult brain.

Adult neurogenesis, the birth and integration of new neurons from adult neural stem cells, is a striking form of structural plasticity and highlights the regenerative capacity of the adult mammalian brain. Accumulating evidence suggests that neuronal activity regulates adult neurogenesis and that new neurons contribute to specific brain functions. The mechanism that regulates the integration of newly generated neurons into the pre-existing functional circuitry in the adult brain is unknown. Here we show that newborn granule cells in the dentate gyrus of the adult hippocampus are tonically activated by ambient GABA (gamma-aminobutyric acid) before being sequentially innervated by GABA- and glutamate-mediated synaptic inputs. GABA, the major inhibitory neurotransmitter in the adult brain, initially exerts an excitatory action on newborn neurons owing to their high cytoplasmic chloride ion content. Conversion of GABA-induced depolarization (excitation) into hyperpolarization (inhibition) in newborn neurons leads to marked defects in their synapse formation and dendritic development in vivo. Our study identifies an essential role for GABA in the synaptic integration of newly generated neurons in the adult brain, and suggests an unexpected mechanism for activity-dependent regulation of adult neurogenesis, in which newborn neurons may sense neuronal network activity through tonic and phasic GABA activation.

EGF and FGF-2 infusion increases post-ischemic neural progenitor cell proliferation in the adult rat brain.

OBJECTIVE: Epidermal growth factor (EGF) and fibroblast growth factor-2 (FGF-2) play a critical role in neurogenesis. In the present study, we evaluated the additive effect of administering these two factors on post-ischemic progenitor cell proliferation, survival, and phenotypic maturation in the hippocampal dentate gyrus (DG) and the subventricular zone (SVZ) in the adult rat brain after transient middle cerebral artery occlusion. METHODS: A combination of EGF+FGF-2 (each 1.44 ng/d) was continuously administered into the lateral ventricles for 3 days, 5-bromodeoxyuridine (BrdUrd) was injected (50 mg/Kg) twice daily for 3 days starting on Day 1 of reperfusion, and cohorts of rats were sacrificed on Day 5 and Day 21 of reperfusion. RESULTS: Compared with sham controls, ischemic rats showed a significantly higher number of newly proliferated cells in both the DG (by 766 +/- 37%, P < 0.05) and the SVZ (by 650 +/- 43%, P < 0.05). Of the progenitor cells proliferated on Day 5 after ischemia, 41 +/- 6% in the DG and 28 +/- 5% in the SVZ survived to 3 weeks. Compared with vehicle control, the EGF + FGF-2 infusion significantly increased the post-ischemic progenitor cell proliferation (by 319 +/- 40%, P < 0.05 in the DG and by 366 +/- 32%, P < 0.05 in the SVZ) and survival (by 40 +/- 12%, P < 0.05 in the DG and by 522 +/- 47%, P < 0.05 in the SVZ) studied at 5 and 21 days, respectively. Furthermore, of the newly proliferated cells survived to 3 weeks after ischemia, EGF + FGF-2 infusion caused a significantly higher number of neuronal nuclear protein-BrdUrd double-positive mature neurons in the DG (46 +/- 9%, P < 0.05) compared with vehicle control. Neuronal nuclear protein and BrdUrd double-positive mature neurons were also found in the DG. Glial fibrillary acidic protein-positive astrocytes did not show double-positive staining in either region. CONCLUSION: Specific growth factor infusion enhances post-ischemic progenitor cell proliferation by 5 days of reperfusion and neuronal maturation by 21 days of reperfusion in both the DG and SVZ in the adult rat brain.