Neurogenesis

The discovery of neural stem cells in the adult central nervous system (CNS) has overturned the long-held dogma that neurons are formed exclusively before birth. Furthermore, this discovery has raised hopes that self renewal leading to repair may be possible in the mature CNS. The two adult neurogenic regions include the subventricular zone (SVZ, also called SEL) and the subgranular zone (SGZ) of the hippocampal dentate gyrus. The SVZ contains the largest pool of dividing neural progenitors in the adult brain. These cells migrate to the olfactory bulb where they differentiate into interneurons. Uncovering the fundamental mechanisms governing adult neurogenesis has two major outcomes: (1) It will improve understanding of brain development in utero, assuming that similar concepts can be applied during embryonic and adult neurogenesis. (2) It will help us achieve self-repair in clinical applications by understanding the recruitment of endogenous neural stem cells and progenitors from the SVZ to the olfactory bulb, examining how they proliferate, migrate, and differentiate into functional glia or neurons, and properly integrate and survive. Recruitment strategies are conceivable because neural progenitors with the characteristics of stem cells have been isolated from regions along the lateral ventricular wall of humans. Nevertheless, recruitment approaches will require a thorough understanding of the factors and signals regulating neurogenesis. Signals can be endogenous, including local signals in the neurogenic niche as well as signals from elsewhere within the body. Signals can also come from the outside world, such as olfactory sensory cues that drive neural activity. Understanding how intrinsic signals control neurogenesis may provide insights to improve the efficiency of cell transplantation. Finally, understanding how the environment and the activity states of the olfactory bulb and hippocampal networks control adult neurogenesis may provide further clues about its function.

 

HISTORICAL PERSPECTIVE ( For references: see Bordey A. (2006) Cell Cycle 5:722 )

Early in the past century, Allen (1912) identified mitotic cells in the SVZ of the lateral wall of the lateral ventricle in adult rats. Later in the 1960s, Altman (1969) proposed that immature SVZ cells migrate to the olfactory bulb where they differentiate into mature neurons and glia. However, according to Smart (1961), cell migration was thought to be minimal in adult. Not until the 1990s did focal labeling of the SVZ in the neonate and adult indicate that proliferating SVZ cells migrate through the SVZ and along the rostral migratory stream (RMS, also called SVZ rostral extension) into the olfactory bulb where they differentiate into interneurons. Nearly simultaneously, the presence of cells with stem cell attributes (i.e., self-renewal and multipotency) were identified in the SVZ. These findings opened a new field of investigation that spawned a multitude of studies from research groups throughout the world, all focused on neurogenesis in the adult SVZ.

 

CELLULAR ARCHITECTURE OF THE ADULT SVZ

The SVZ ependymal region contains at least four different cell types defined by their morphology, ultrastructure, and molecular markers (see figure below). The migrating neuroblasts (referred to as type A cells, class 1 cells or neuronal progenitors) migrate in chains to the olfactory bulb along the rostral migratory stream (RMS). A particular type of protoplasmic astrocyte (also called type B cells or class 2 cells) ensheathes the chains of migrating neuroblasts. More spherical and highly proliferative progenitors (transit amplifying cells or type C cells) form clusters next to the chains of migrating neuroblasts. The SVZ is largely separated from the ventricular cavity by a layer of ependymal cells (also called type E cells). The neuroblasts and astrocytes are the two main progenitor types lying between the ependymal cell layer and the striatal parenchyma.Neuroblasts can be selectively detected by antibodies raised against the polysialylated form of the neural cell adhesion molecule (PSA-NCAM), class III beta-tubulin (TuJ1), and doublecortin, while astrocytes can be marked with antibodies against glial fibrillary acidic protein (GFAP), the glutamate transporter GLAST, and the GABA transporter GAT4. Immunohistochemical studies have revealed that a cluster of 3–5 neuroblasts is tightly encapsulated by one or two astrocytes in the coronal plane. Based on electron microscopy data, the extracellular space between neuroblasts or between neuroblasts and astrocytes is ~20–50 nm. Although this distance is on the same order as that of the synaptic cleft, synapses are absent in the SVZ. This arrangement and cell:cell proximity lends itself to non-synaptic neuroblast:neuroblast and neuroblast:astrocyte signaling (see the following manuscripts in our reference list: Liu et al 2005 and Bolteus and Bordey, 2004). In the embryonic brain, newly born progenitors migrate along the processes of radial glia that secrete extracellular matrix molecules controlling cell migration. The close proximity between radial glia and migrating cells suggests the existence of a tight communication via diffusible messengers, as postulated in the SVZ. Although SVZ and RMS neuroblasts do not migrate along a radial process, the glial processes form a tube around the neuroblasts and may provide guidance cues as shown in the developing brain. Blood vessels may also play an important role in the neurogenic niche. The processes of SVZ astrocytes and neuroblasts are engulfed by the vascular basal lamina. The proximity of stem cells/progenitors to blood vessels/endothelial cells suggests a highly regulated communication between these two cell types. In the embryonic brain, angiogenic sprouts initially extend along radial glia from the pial surface toward the ventricle. Later, both radial glia and migrating cells make specialized contacts with blood vessels.

 

DO SVZ ASTROCYTES RESEMBLE MATURE ASTROCYTES? (for references, see Liu et al., 2006)

The identity of these stem cells has been the subject of intense research and it is now well-accepted that some of the GFAP-expressing cells are stem cells in rodent and human SVZ. The expression of GFAP in SVZ stem cells has lead investigators to call them astrocytes , a generic term for glia expressing GFAP. These GFAP-expressing cells express several features in common with astrocytes; these include the presence of glycogen granules, close contacts with blood vessels , ensheathment of neuronal elements (neuroblasts in the SVZ and synapses elsewhere in the brain) , and glutamate transporters . However, these features are common to perinatal radial glia that transform into GFAP-expressing cells in the postnatal SVZ . Furthermore, ependymal cells, which are not astrocytes, express GFAP and derive from radial glia but are postmitotic . Here for nomenclature purposes, ependymal cells are not called GFAP-expressing cells. In addition, GFAP-expressing cells outside the SVZ will be called astrocytes. It is thus important to determine whether GFAP-expressing cells in the SVZ express functional features in common with astrocytes beyond the expression of common anatomical and molecular markers. Astrocytes have recently been found to be heterogeneous. However, they are thought to possess gap junctions, K + channels opened at rest, glutamate transporters, but lack AMPA-type receptors in acute slices. It remains unknown whether GFAP-expressing cells in the SVZ express K + currents, functional glutamate transporters, and AMPA-type glutamate receptors.

We found that both GFAP-expressing cells and ependymal cells display classical characteristics of glia (common to both astrocytes and neonatal radial glia) but differ in their functional K + channel expression. The figure below displays a diagram illustrating the lineage and some of the hypothetical functions of SVZ astrocytes and ependymal cells in the SVZ.

Diagram illustrating the lineage and function of GFAP-expressing cells and ependymal cells the postnatal SVZ. Radial glia (light yellow) have been shown to transform into both GFAP-expressing cells (light orange) and ependymal cells (light green) in the SVZ, and astrocytes (dark orange) elsewhere in the brain. GFAP-expressing cells are known to give rise to neuroblasts (blue) and glioblasts via bipotential progenitor (dark green). Glioblasts could then differentiate into astrocytes (red) or oligodendrocytes (not shown) elsewhere in the brain during neonatal development or following injury. In the SVZ, both GFAP-expressing cells and ependymal cells have the ability to take up K+ and glutamate. GFAP-expressing cells may take up K+ released from neuroblasts and then transfer K+ to a low K+ region via the glial syncitium. K+ released by the GFAP-expressing cells could be taken up by ependymal cells and shunted to the ventricle. Similar transfer of glutamate from unknown sources could occur.

IMPACT OF THE OUTSIDE WORLD ON ADULT NEUROGENESIS (For references: see Bordey A. (2006) Cell Cycle 5:722)

A similar title was used over 30 years ago by Goldman and Rakic who discussed the notion of environmental control of brain development. It is now well known that the environment has a profound impact on the developing brain during prenatal and neonatal ages, termed the “critical period”. This impact is reflected in social, cognitive, and motor abilities due to changes in dendritic branches, wiring, synaptogenesis, gene expression and neurogenesis. Furthermore, a more complex environment has attenuated or prevented the sequelae of early brain injury. It is thus not surprising that the environment profoundly affects adult neurogenesis as it does prenatal and neonatal neurogenesis. Captivity environmental minimalism as opposed to an enriched environment and physical inactivity significantly reduce hippocampal neurogenesis in the adult brain. Olfactory bulb neurogenesis is also positively affected by enriched odor exposure. Although the environment influences both hippocampal and olfactory bulb neurogenesis, it may differentially affect neurogenesis in these two regions. For example, an enriched environment and physical activity did not stimulate olfactory bulb neurogenesis. The positive effect of an enriched environment on adult neurogenesis in the hippocampus led us to suspect that animals in the lab setting are in an artificial state resulting in reduced neurogenesis. However, it is unknown whether an enriched environment replicates a more natural situation. It is thus critical to continue to study neurogenesis and its regulation in animals in their normal environment as previously reported.

The cellular and molecular mechanisms responsible for the environmental control of adult neurogenesis remain elusive. Hippocampal neurogenesis is though to be affected by both excitatory and inhibitory inputs. While GABAergic inputs have been reported on immature SGZ neurons suggesting activity-driven control of neurogenesis, a direct link of hippocampal activity remains to be shown in vivo. One attractive mechanism responsible for the olfactory activity-driven increase in olfactory neurogenesis is the odor exposure- induced increase in hormones that may control neurogenesis. For example, it is known that exposure of rats to predator odor elicits a stress response characterized by increased adrenal steroid levels known to affect hippocampal neurogenesis.

 

WHY DOES NEUROGENESIS PERSIST?

Since the discovery of adult neurogenesis one central question continually recurs in the minds of those studying it: Why does neurogenesis persist? Altman (1967) suggested that the newly generated interneurons may be responsible for neural plasticity or may be the substrate of memory. In 1966, he refined his hypothesis and proposed that the plasticity or memory mediated by the interneurons formed postnatally in the cerebellum, dentate gyrus, or olfactory bulb is associated with a circumscribed class of functions, namely the acquisition of locomotor memory (cerebellum) and the fixation of behavior patterns relating to affective, need-catering functions (olfactory bulb, hippocampus) and not with memory processes related to cognitive instrumental functions. This concept has recently been revisited. First, neurogenesis persists in regions involved in certain types of learning and memory. Second, regulators of both hippocampal and olfactory bulb neurogenesis alter hippocampal-dependent learning odor memory, and olfactory discrimination. These correlations implicate neurogenesis in certain types of learning and memory. Furthermore, to fully understand why neurogenesis persists, it is important to determine why old neurons die and need to be replaced. Are the old cells activity-deprived or alternatively overloaded by neuronal activity leading to cell death? Hyperexcitability, as observed during epileptic seizures, could lead to excitotoxicity and cell death. This is an attractive hypothesis because increased neuronal activity increases neurogenesis therefore synchronizing cell death and replacement. It is also possible that newly born cells induce the death of old neurons.