Adult Neurogenesis: Reading for the Curious and a Breakthrough in Human Brain Imaging
As Jane Austen might have said: “It is a truth universally acknowledged, that a biological process in possession of a good molecular definition, must be in want of a disease.” In this case, the process in question is adult neurogenesis: the generation of new neurons at the inner face of the dentate gyrus of the hippocampus. A recent Science paper forms the core of this commentary as the work described within has the potential to transform our understanding of the field in two ways (Manganas et al., 2007). Firstly, it offers the first real chance of making definitive links to disease by permitting study of the phenomenon in human subjects, both well and unwell. This will strengthen the currently inferred links to psychiatric and other conditions. Secondly, the paper highlights a previously unexplored class of molecules that appear to be biomarkers for, and perhaps critical participants in the function of, the neuronal precursor cells of the hippocampus.

It is only a decade or so since the replenishing pool of neurons in the adult subgranular zone of the hippocampal dentate gyrus was identified in the rodent. After a period of intense debate, a chemical marker, BrdU, which labels replicating DNA in the new neuronal precursors, and a set of immunocytochemical markers for differentiation stages have provided entirely convincing confirmation and definition of the neurogenesis process. For reviews on the subject, I have found these to be particularly helpful: Elder et al., 2006, a good general review; Grote and Hannan, 2007, a review featuring a more neurobiological/pharmacological take on the subject; Ehninger and Kempermann, 2007, a discussion of cell types including the possible astrocytic qualities of neural precursor cells; von Bohlen Und Halbach, 2007, for a detailed description of stage markers used to delineate the neuronal differentiation process; Gould, 2007, for a brief history of the subject and discussion of possible neurogenesis in other brain regions.

In essence, adult neurogenesis can be thought of as a “slight return” to the developmental neurogenesis and neuronal migration originating within the subventricular region and giving rise to the patterning of the cortical layers. Many of the same processes are present: regulated division of a stem cell population, programmed cell fate determination, cellular migration, survival/cell death decisions, and full maturation into functional neurons. In the adult rodent dentate gyrus, maturing neurons move into the granule cell layer from its boundary with the hilus and begin to produce axonal (mossy fiber) and dendritic projections. The result, within a timescale of days to weeks, is that these cells can become incorporated into the hippocampal trisynaptic circuitry so extensively studied in the context of learning and memory. The adoption of transgenic labeling and imaging techniques has greatly facilitated the observation and timing of this process in rodents.

In their current paper, Manganas and colleagues began with the assumption that neural precursor cells would possess a unique molecular fingerprint that would allow their identification within the living brain. They used proton nuclear resonance spectroscopy in the identification stage—profiling purified neural progenitor cells alongside mature neurons, oligodendrocytes, and astrocytes. A peak, designated “1.28ppm,” was identified as specific to the progenitor population. This was confirmed by a variety of approaches including pinpointing its enrichment in nestin-expressing cells (nestin being a marker for early neuronal differentiation events), observing its upregulation after stimuli known to increase neurogenesis and showing it decreased as cell differentiation progressed.

Small animal magnetic resonance imaging (MRI) was then employed to see if this biomarker was detectable in the living rat brain, not forgetting that the target cells are only likely to be a small percentage of the total cells in the hippocampus. It was, but only after a new signal-to-noise ratio processing algorithm—“singular value decomposition”—was applied to the raw imaging data. This led directly on to the examination of human subjects in medical MRI scanners. Again, using the more advanced imaging processing algorithm, 1.28ppm was detectable in the human hippocampus. The levels of 1.28ppm were relatively constant over the medium term for any individual, but when a wider sweep of the population was studied it became very clear that, just as in rodents, there is a steep decline in new neuron production as a function of aging. Looking at their graph I can see that my eldest daughter has 10 times the levels of neurogenesis that I do, which is a sobering fact—but one which, apparently, I may not remember tomorrow!

Finally, it is worth highlighting the potential origin of the 1.28ppm signal. The authors conclude that it is most likely the result of a pool of saturated or monounsaturated fatty acids or related molecules. Its specificity to the precursor population may imply either particular membrane composition, an increase in fatty acid-based secondary signaling molecules or altered energy metabolism.

The link between altered neurogenesis and human disease is tenuous only inasmuch as it has never, until now, been directly measurable in order to make correlations with pathology. In my opinion, the current state of the field is moderately suggestive that schizophrenia and major depression may be connected with a reduced level of hippocampal neurogenesis (Reif et al., 2007; Toro and Deakin, 2007). The administration of antipsychotic, mood-stabilizing, and antidepressant drugs in animal models all seem to have a pronounced effect on the generation of new neurons (see citations below). Admittedly, neurogenesis seems a particularly labile and sensitive process, and so results have to be interpreted critically, particularly for those experimental paradigms, such as eliciting seizures, which may not bear a strong resemblance to steady-state physiology. However, here is a selection of reviews that will give the reader a taste of adult neurogenesis in the context of disease: Lie et al., 2004, a general review of neurogenesis and disease; Reif et al., 2007, for evidence for neurogenesis deficits in schizophrenia; Drew and Hen, 2007, for neurogenesis deficits in depression and potential neurogenesis-based therapies; Sahay and Hen, 2007, an excellent review on neurogenesis and depression; and Toro and Deakin, 2007, for schizophrenia and neurogenesis and using adult neurogenesis to investigate developmental neurogenesis.

Genetics is not far behind the cell biological studies. Mapping the loci behind mouse strain differences in neurogenesis levels is ongoing and a number of mouse knockout or knockdown lines have abnormal neurogenesis. Of these, mice with reduced Npas3 (Pieper et al., 2005) and Disc1 (Duan et al., 2007) expression are particularly notable because these two genes were originally discovered through their chromosomal disruption in humans with psychiatric illness.

The study by Manganas et al. has the potential to allow future researchers to phenotype after genetic stratification or genotype after (endo-)phenotype stratification. It will also allow drug action time course and responsiveness to be assessed, including answering the question of whether SSRI response lag in depression is correlated with neurogenesis. Perhaps the currently rather messy field of trying to link particular animal learning paradigms with neurogenesis will also benefit from the live recordings now possible, and this may highlight particular subregions along the septo-temporal axis of the hippocampus linked with specific behaviors and stressors. Finally, the chemical composition of 1.28ppm is likely to stimulate research into the (therapeutic?) role of lipids in neuronal precursor cell biology in the normal and diseased states.

References:

Drew MR, Hen R (2007) Adult hippocampal neurogenesis as target for the treatment of depression. CNS Neurol Disord Drug Targets 6:205-218. Abstract

Duan X, Chang JH, Ge S, Faulkner RL, Kim JY, Kitabatake Y, Liu XB, Yang CH, Jordan JD, Ma DK, Liu CY, Ganesan S, Cheng HJ, Ming GL, Lu B, Song H (2007) Disrupted-In-Schizophrenia 1 regulates integration of newly generated neurons in the adult brain. Cell 130:1146-1158. Abstract

Ehninger D, Kempermann G (2007) Neurogenesis in the adult hippocampus. Cell Tissue Res. Abstract

Elder GA, De Gasperi R, Gama Sosa MA (2006) Research update: neurogenesis in adult brain and neuropsychiatric disorders. Mt Sinai J Med 73:931-940. Abstract

Gould E (2007) How widespread is adult neurogenesis in mammals? Nat Rev Neurosci 8:481-488. Abstract

Grote HE, Hannan AJ (2007) Regulators of adult neurogenesis in the healthy and diseased brain. Clin Exp Pharmacol Physiol 34:533-545. Abstract

Lie DC, Song H, Colamarino SA, Ming GL, Gage FH (2004) Neurogenesis in the adult brain: new strategies for central nervous system diseases. Annu Rev Pharmacol Toxicol 44:399-421. Abstract

Manganas LN, Zhang X, Li Y, Hazel RD, Smith SD, Wagshul ME, Henn F, Benveniste H, Djuric PM, Enikolopov G, Maletic-Savatic M (2007) Magnetic resonance spectroscopy identifies neural progenitor cells in the live human brain. Science 318:980-985. Abstract

Pieper AA, Wu X, Han TW, Estill SJ, Dang Q, Wu LC, Reece-Fincanon S, Dudley CA, Richardson JA, Brat DJ, McKnight SL (2005) The neuronal PAS domain protein 3 transcription factor controls FGF-mediated adult hippocampal neurogenesis in mice. Proc Natl Acad Sci U S A 102:14052-14057. Abstract

Reif A, Schmitt A, Fritzen S, Lesch KP (2007) Neurogenesis and schizophrenia: dividing neurons in a divided mind? Eur Arch Psychiatry Clin Neurosci 257:290-299. Abstract

Sahay A, Hen R (2007) Adult hippocampal neurogenesis in depression. Nat Neurosci 10:1110-1115. Abstract

Toro CT, Deakin JF (2007) Adult neurogenesis and schizophrenia: a window on abnormal early brain development? Schizophr Res 90:1-14. Abstract

von Bohlen Und Halbach O (2007) Immunohistological markers for staging neurogenesis in adult hippocampus. Cell Tissue Res 329:409-420. Abstract

View all comments by Ben Pickard

Several recent studies on disruptions of the DISC1 gene in mice illustrate the great potential of genetic approaches to studying functions of putative schizophrenia susceptibility genes but also signal the complexity of the problem. An initial rationale for studying the effects of mutations in DISC1 came from the discovery of the chromosomal translocation, resulting in a breakpoint in the DISC1 gene that co-segregated with major mental illness in a Scottish family (reviewed by Porteous et al., 2006). These clinical findings were followed by a number of association studies, which reported that numerous SNPs across the gene were associated with schizophrenia and mood disorders and a variety of intermediate phenotypes, suggesting that other problems in the DISC1 gene may exist in other subjects/populations.

Recent animal models designed to mimic partial loss of DISC1 function suggested that DISC1 is necessary to support development of the cerebral cortex as its loss resulted in impaired neurite outgrowth and the spectrum of behavioral abnormalities characteristic of major mental disorders ( Kamiya et al., 2005; Koike et al., 2006; Clapcote et al., 2007; Hikida et al. 2007). Unexpectedly, however, the paper by Duan et al., 2007, is showing that DISC1 may also function as a brake and master regulator of neuronal development, and that its partial loss could lead to the opposite effects than previously described, i.e., dendritic overgrowth and accelerated synapse formation and faster maturation of newly generated neurons. In contrast to previous studies, they have used the DISC1 knockdown model achieved by RNA interference in a subpopulation of single cells of the dentate gyrus. Other emerging studies continue to reveal the highly complex nature of the DISC1 gene with multiple isoforms exhibiting different functions, perhaps depending on localization, timing, and interactions with a multitude of other genes’ products, some of which confer susceptibility to mental illness independent of DISC1. Similar molecular complexity has also emerged in other susceptibility genes for schizophrenia: GRM3 (Sartorius et al., 2006), NRG1 (Tan et al., 2007), and COMT (Tunbridge et al., 2007). With the growing knowledge about transcript complexity, it becomes increasingly clear that subtle disturbances of isoform(s) of susceptibility gene products and disruptions of intricate interactions between the susceptibility genes may account for the etiology of neuropsychiatric disorders. Research in animals will have a critical role in disentangling this web of interwoven genetic pathways.

View all comments by Barbara K. Lipska

I am very glad that our colleagues at Johns Hopkins University have published a very intriguing paper in Cell, showing a novel role for DISC1 in adult hippocampus. This is very consistent with previous publications (Miyoshi et al., 2003; Kamiya et al., 2005; and others; reviewed by Ishizuka et al., 2006), and adds a new insight into a key role for DISC1 during neurodevelopment. In short, DISC1 is a very important regulator in various phases of neurodevelopment, which is reinforced in this study. Specifically, DISC1 is crucial for regulating neuronal migration and dendritic development—for acceleration in the developing cerebral cortex, and for braking in the adult hippocampus.

There is precedence for signaling molecules playing the same role in different contexts, with the resulting molecular activity going in different directions. For example, FOXO3 (a member of the Forkhead transcription factor family) plays a role in cell survival/death in a bidirectional manner (Brunet et al., 2004). FOXO3 endows cells with resistance to oxidative stress in some contexts, and induces apoptosis in other contexts. SIRT1 (known as a key modulator of organismal lifespan) deacetylates FOXO3 and tips FOXO3-dependent responses away from apoptosis and toward stress resistance. In analogy to FOXO3, context-dependent post-translational modifications, such as phosphorylation, may be an underlying mechanism for DISC1 to function in a bidirectional manner. Indeed, a collaborative team at Johns Hopkins, including Pletnikov's lab, Song's lab, and ours, has started exploring, in both cell and animal models, the molecular switch that makes DISC1's effects bidirectional.

References:

Brunet A, Sweeney LB, Sturgill JF, Chua KF, Greer PL, Lin Y, Tran H, Ross SE, Mostoslavsky R, Cohen HY, Hu LS, Cheng HL, Jedrychowski MP, Gygi SP, Sinclair DA, Alt FW, Greenberg ME. Stress-dependent regulation of FOXO transcription factors by the SIRT1 deacetylase. Science. 2004 Mar 26;303(5666):2011-5. Abstract

View all comments by Akira Sawa

Recent findings, including the interactome study by Camargo et al., 2007, and this beautiful study by Duan and colleagues, implicate DISC1 (a leading candidate schizophrenia susceptibility gene) in synaptic function, consistent with prevailing ideas of the disorder as one of the synapse and connectivity (see Stephan et al., 2006). As we learn more about DISC1 and its protein partners, evidence demonstrating the importance of microtubules in the regulation of several neuronal processes (see Eastwood et al., 2006, for review) suggests that DISC1’s interactions with microtubule associated proteins (MAPs) may underpin its pathogenic influence.

DISC1 has been shown to bind to several MAPs (e.g., MAP1A, MIPT3) and other proteins important in regulating microtubule function (see Kamiya et al., 2005; Porteous et al., 2006). As a key component of the cell cytoskeleton, microtubules are involved in many cellular processes including mitosis, motility, vesicle transport, and morphology, and their dynamics are regulated by MAPs, which modulate microtubule polymerization, stability, and arrangement. Decreased microtubule stability in mutant mice for one MAP, stable tubule only polypeptide (STOP; MAP6), results in behavioral changes relevant to schizophrenia and altered synaptic protein expression (Andrieux et al., 2002; Eastwood et al., 2006), indicating the importance of microtubules in synaptic function and suggesting that they may be a molecular mechanism contributing to the pathogenesis of schizophrenia. Likewise, DISC1 mutant mice exhibit behavioral alterations characteristic of psychiatric disorders (e.g., Clapcote et al., 2007), and altered microtubule dynamics are thought to underlie perturbations in cerebral cortex development and neurite outgrowth caused by decreased DISC1 expression or that of a schizophrenia-associated DISC1 mutation (Kamiya et al., 2005).

Our interpretation of the possible functions of DISC1 has been complicated by the unexpected findings of Duan and colleagues that DISC1 downregulation during adult hippocampal neurogenesis leads to overextended neuronal migration and accelerated dendritic outgrowth and synaptic formation. In terms of neuronal positioning, they suggest that their results indicate that DISC1 may relay positional signals to the intracellular machinery, rather than directly mediate migration. In this way, decreased DISC1 expression may result in the mispositioning of newly formed neurons rather than a simple decrease or increase in their migratory distance. Of note, MAP1B, a neuron-specific MAP important in regulating microtubule stability and the crosstalk between microtubules and actin, is required for neurons to correctly respond to netrin 1 signaling during neuronal migration and axonal guidance (Del Rio et al., 2004), and DISC1 may function similarly during migration. Reconciling differences between the effect of decreased DISC1 expression upon neurite outgrowth during neurodevelopment and adult neurogenesis is more difficult, but could be due to differences in the complement of MAPs expressed by different neuronal populations at different times. Regardless, the results of Duan and colleagues have provided additional evidence implicating DISC1 in neuronal functions thought to go awry in schizophrenia. Further characterization of DISC1’s interactions with microtubules and MAPs may lead to a better understanding of the role of DISC1 in the pathogenesis of psychiatric disorders.

References:

Andrieux A, Salin PA, Vernet M, Kujala P, Baratier J, Gory Faure S, Bosc C, Pointu H, Proietto D, Schweitzer A, Denarier E, Klumperman J, Job D (2002). The suppression of brain cold-stable microtubules in mice induces synaptic deficits associated with neuroleptic-sensitive behavioural disorders. Genes Dev. 16: 2350-2364. Abstract

Camargo LM, Collura V, Rain JC, Mizuguchi K, Hermjakob H, Kerrien S, Bonnert TP, Whiting PJ, Brandon NJ (2007). Disrupted in Schizophrenia 1 Interactome: evidence for the close connectivity of risk genes and a potential synaptic basis for schizophrenia. Mol. Psychiatry 12: 74-86. Abstract

Clapcote SJ, Lipina TV, Millar JK, Mackie S, Christie S, Ogawa F, Lerch JP, Trimble K, Uchiyama M, Sakuraba Y, Kaneda H, Shiroishi T, Houslay MD, Henkelman RM, Sled JG, Gondo Y, Porteous DJ, Roder JC (2007). Behavioral phenotypes of Disc1 missense mutations in mice. Neuron 54: 387-402. Abstract

Del Rio, J.A., Gonzalez-Billault, C., Urena, J.M., Jimenez, E.M., Barallobre, M.J., Pascual, M., Pujadas, L., Simo, S., La Torre, A., Wandosell, F., Avila, J. and Soriano, E. (2004). MAP1B is required for netrin 1 signaling in neuronal migration and axonal guidance. Cur. Biol. 14: 840-850. Abstract

Eastwood SL, Lyon L, George L, Andrieux A, Job D, Harrison PJ (2006). Altered expression of synaptic protein mRNAs in STOP (MAP6) mutant mice. J. Psychopharm. 21: 635-644. Abstract

Kamiya A, Kubo K, Tomoda T, Takaki M, Youn R, Ozeki Y, Sawamura N, Park U, Kudo C, Okawa M, Ross CA, Hatten ME, Nakajima K, Sawa A. A schizophrenia-associated mutation of DISC1 perturbs cerebral cortex development. Nat Cell Biol. 2005 Dec;7(12):1167-78. Epub 2005 Nov 20. Erratum in: Nat Cell Biol. 2006 Jan;8(1):100. Abstract

Porteous DJ, Thomson P, Brandon NJ, Millar JK (2006). The genetics and biology of DISC1-an emerging role in psychosis and cognition. Biol. Psychiatry 60: 123-131. Abstract

Stephan KE, Baldeweg T, Friston KJ (2006). Synaptic plasticity and disconnection in schizophrenia. Biol. Psychiatry 59: 929-939. Abstract

View all comments by Sharon Eastwood