Sebastian Jessberger

Milestones of neuronal development in the adult hippocampus

Adult hippocampal neurogenesis originates from precursor cells in the adult dentate gyrus and results in new granule cell neurons. We propose a model of the development that takes place between these two fixed points and identify several developmental milestones. From a presumably bipotent radial-glia-like stem cell (type-1 cell) with astrocytic properties, development progresses over at least two stages of amplifying lineage-determined progenitor cells (type-2 and type-3 cells) to early postmitotic and to mature neurons. The selection process, during which new neurons are recruited into function, and other regulatory influences differentially affect the different stages of development.

Wnt signalling regulates adult hippocampal neurogenesis

The generation of new neurons from neural stem cells is restricted to two regions of the adult mammalian central nervous system: the subventricular zone of the lateral ventricle, and the subgranular zone of the hippocampal dentate gyrus. In both regions, signals provided by the microenvironment regulate the maintenance, proliferation and neuronal fate commitment of the local stem cell population. The identity of these signals is largely unknown. Here we show that adult hippocampal stem/progenitor cells (AHPs) express receptors and signalling components for Wnt proteins, which are key regulators of neural stem cell behaviour in embryonic development. We also show that the Wnt/β-catenin pathway is active and that Wnt3 is expressed in the hippocampal neurogenic niche. Overexpression of Wnt3 is sufficient to increase neurogenesis from AHPs in vitro and in vivo. By contrast, blockade of Wnt signalling reduces neurogenesis from AHPs in vitro and abolishes neurogenesis almost completely in vivo. Our data show that Wnt signalling is a principal regulator of adult hippocampal neurogenesis and provide evidence that Wnt proteins have a role in adult hippocampal function.

A Functional Role for Adult Hippocampal Neurogenesis in Spatial Pattern Separation

Neurogenesis and Spatial Memory The dentate gyrus of the hippocampus is one of two sites in the brain where new neurons are produced throughout life. Adult-born neurons integrate into the dentate gyrus circuitry and are thought to play a role in learning and memory. However, their contribution to hippocampal function remains unclear. Clelland et al. (p. 210) disrupted neurogenesis in mice and used two behavioral tasks to test for impairment in the formation of uncorrelated episodic memory representations. In one task, two arms were presented and the mice were rewarded for choosing the most recently visited arm in an earlier sequence; in the second task, animals were rewarded for choosing a certain location on a touch screen. Ablation of neurogenesis affected discrimination performance in both tasks but only when the arms or screen locations were close to one another. Neurogenesis is thus necessary for spatial pattern separation in the dentate gyrus. Disruption of neurogenesis in a neuron-forming site in the brain impairs spatial memory functions in mice. The dentate gyrus (DG) of the mammalian hippocampus is hypothesized to mediate pattern separation—the formation of distinct and orthogonal representations of mnemonic information—and also undergoes neurogenesis throughout life. How neurogenesis contributes to hippocampal function is largely unknown. Using adult mice in which hippocampal neurogenesis was ablated, we found specific impairments in spatial discrimination with two behavioral assays: (i) a spatial navigation radial arm maze task and (ii) a spatial, but non-navigable, task in the mouse touch screen. Mice with ablated neurogenesis were impaired when stimuli were presented with little spatial separation, but not when stimuli were more widely separated in space. Thus, newborn neurons may be necessary for normal pattern separation function in the DG of adult mice.

In vivo demonstration that alpha-synuclein oligomers are toxic.

The aggregation of proteins into oligomers and amyloid fibrils is characteristic of several neurodegenerative diseases, including Parkinson disease (PD). In PD, the process of aggregation of α-synuclein (α-syn) from monomers, via oligomeric intermediates, into amyloid fibrils is considered the disease-causative toxic mechanism. We developed α-syn mutants that promote oligomer or fibril formation and tested the toxicity of these mutants by using a rat lentivirus system to investigate loss of dopaminergic neurons in the substantia nigra. The most severe dopaminergic loss in the substantia nigra is observed in animals with the α-syn variants that form oligomers (i.e., E57K and E35K), whereas the α-syn variants that form fibrils very quickly are less toxic. We show that α-syn oligomers are toxic in vivo and that α-syn oligomers might interact with and potentially disrupt membranes.

Functional amyloids as natural storage of peptide hormones in pituitary secretory granules.

Plethora of Secretory Amyloids Protein aggregation and the formation of amyloids are associated with several dozen pathological conditions in humans, including Alzheimers disease, Parkinsons disease, and type II diabetes. In addition, a few functional amyloid systems are known: the prions of fungi, the bacterial protein curli, the protein of chorion of the eggshell of silkworm, and the amyloid protein Pmel-17 involved in mammalian skin pigmentation. Now Maji et al. (p. 328, published online 18 June) propose that endocrine hormone peptides and proteins are stored in an amyloid-like state in secretory granules. Thus, the amyloid fold may represent a fundamental, ancient, and evolutionarily conserved protein structural motif that is capable of performing a wide variety of functions contributing to normal cell and tissue physiology. Peptide and protein hormones are stored in secretory granules in a nonpathological amyloid conformation. Amyloids are highly organized cross–β-sheet–rich protein or peptide aggregates that are associated with pathological conditions including Alzheimer’s disease and type II diabetes. However, amyloids may also have a normal biological function, as demonstrated by fungal prions, which are involved in prion replication, and the amyloid protein Pmel17, which is involved in mammalian skin pigmentation. We found that peptide and protein hormones in secretory granules of the endocrine system are stored in an amyloid-like cross–β-sheet–rich conformation. Thus, functional amyloids in the pituitary and other organs can contribute to normal cell and tissue physiology.

Dentate gyrus-specific knockdown of adult neurogenesis impairs spatial and object recognition memory in adult rats

New granule cells are born throughout life in the dentate gyrus of the hippocampal formation. Given the fundamental role of the hippocampus in processes underlying certain forms of learning and memory, it has been speculated that newborn granule cells contribute to cognition. However, previous strategies aiming to causally link newborn neurons with hippocampal function used ablation strategies that were not exclusive to the hippocampus or that were associated with substantial side effects, such as inflammation. We here used a lentiviral approach to specifically block neurogenesis in the dentate gyrus of adult male rats by inhibiting WNT signaling, which is critically involved in the generation of newborn neurons, using a dominant-negative WNT (dnWNT). We found a level-dependent effect of adult neurogenesis on the long-term retention of spatial memory in the water maze task, as rats with substantially reduced levels of newborn neurons showed less preference for the target zone in probe trials >2 wk after acquisition compared with control rats. Furthermore, animals with strongly reduced levels of neurogenesis were impaired in a hippocampus-dependent object recognition task. Social transmission of food preference, a behavioral test that also depends on hippocampal function, was not affected by knockdown of neurogenesis. Here we identified a role for newborn neurons in distinct aspects of hippocampal function that will set the ground to further elucidate, using experimental and computational strategies, the mechanism by which newborn neurons contribute to behavior.

Adult-born hippocampal neurons mature into activity-dependent responsiveness.

The mammalian hippocampus is a highly plastic brain structure in which new neurons are generated throughout adulthood. Do these new neurons qualitatively and quantitatively participate in the activity‐dependent gene expression after challenging the hippocampus in a learning task? Adult mice were injected with bromodeoxyuridine (BrdU) to label dividing cells and indeed, individual BrdU‐labelled new neurons expressed the immediate early gene protein (IEGp) c‐fos during the acquisition phase of the Morris water maze task to the same extend as older granule cells. To assess the responsiveness of all new neurons we also analysed the expression patterns of three IEGps (c‐fos, zif268 and Homer1A) after kainic acid (KA)‐ or pentylenetetrazol (PTZ)‐induced seizures. We found that after a maturation period of between 2 and 5 weeks a comparable ratio of adult‐generated granule cells participated in IEGp‐expression as in the population of older granule cells. Thus, new neurons appear quantitatively integrated into hippocampal circuits, suggestive of a relevant contribution to hippocampal function.

Epigenetic Modulation of Seizure-Induced Neurogenesis and Cognitive Decline

The conceptual understanding of hippocampal function has been challenged recently by the finding that new granule cells are born throughout life in the mammalian dentate gyrus (DG). The number of newborn neurons is dynamically regulated by a variety of factors. Kainic acid-induced seizures, a rodent model of human temporal lobe epilepsy, strongly induce the proliferation of DG neurogenic progenitor cells and are also associated with long-term cognitive impairment. We show here that the antiepileptic drug valproic acid (VPA) potently blocked seizure-induced neurogenesis, an effect that appeared to be mainly mediated by inhibiting histone deacetylases (HDAC) and normalizing HDAC-dependent gene expression within the epileptic dentate area. Strikingly, the inhibition of aberrant neurogenesis protected the animals from seizure-induced cognitive impairment in a hippocampus-dependent learning task. We propose that seizure-generated granule cells have the potential to interfere with hippocampal function and contribute to cognitive impairment caused by epileptic activity within the hippocampal circuitry. Furthermore, our data indicate that the effectiveness of VPA as an antiepileptic drug may be partially explained by the HDAC-dependent inhibition of aberrant neurogenesis induced by seizure activity within the adult hippocampus.

Differential regulation of gliogenesis in the context of adult hippocampal neurogenesis in mice.

In adult hippocampal neurogenesis, new neurons appear to originate from a cell with astrocytic properties expressing glial fibrillary acidic protein (GFAP). Also, new astrocytes are generated in the adult dentate gyrus. Whereas the putative astrocyte‐like progenitor cells are consistently S‐100β‐negative, many new astrocytes are S‐100β‐positive. Thus, it is unclear whether the GFAP‐positive progenitor cells are astrocytes in a general sense or rather neural progenitor cells with certain astrocytic characteristics. We therefore investigated the development of GFAP‐expressing cells in the context of adult hippocampal neurogenesis. Proliferating cells could be either GFAP‐positive or doublecortin‐positive (DCX), but never both, indicating two independent populations of dividing cells in the glial and neuronal lineages. Two distinct populations of cells with astroglial properties were detected—one expressing GFAP, the other co‐expressing GFAP and S‐100β. We never found S‐100β‐cells to be in S‐phase. No overlap between neuronal and glial markers was seen at any time point. Thus, astrogenesis occurred in parallel and to some degree independent of adult neurogenesis. The uninterrupted GFAP expression in this lineage, and neuronal markers in the other lineage, argue against a late common precursor for neurogenesis and gliogenesis in the adult hippocampus. Very few newly generated microglia and no new oligodendrocytes were detected. Environmental enrichment and voluntary wheel running—two experimental paradigms with robust stimulatory effects on adult hippocampal neurogenesis—affected hippocampal astrogenesis differentially: Running, but not enrichment, strongly induced net astrogenesis (GFAP/S‐100β), but also GFAP‐positive S‐100β‐negative cells, which thus appear to be a transiently amplifiable intermediate population within the glial lineage.

Signaling through BMPR-IA Regulates Quiescence and Long-Term Activity of Neural Stem Cells in the Adult Hippocampus

Neural stem cells (NSCs) in the adult hippocampus divide infrequently, and the molecules that modulate their quiescence are largely unknown. Here, we show that bone morphogenetic protein (BMP) signaling is active in hippocampal NSCs, downstream of BMPR-IA. BMPs reversibly diminish proliferation of cultured NSCs while maintaining their undifferentiated state. In vivo, acute blockade of BMP signaling in the hippocampus by intracerebral infusion of Noggin first recruits quiescent NSCs into the cycle and increases neurogenesis; subsequently, it leads to decreased stem cell division and depletion of precursors and newborn neurons. Consistently, selective ablation of Bmpr1a in hippocampal NSCs, or inactivation of BMP canonical signaling in conditional Smad4 knockout mice, transiently enhances proliferation but later leads to a reduced number of precursors, thereby limiting neuronal birth. BMPs are therefore required to balance NSC quiescence/proliferation and to prevent loss of the stem cell activity that supports continuous neurogenesis in the mature hippocampus.