Anita C. Hall

Axonal Remodeling and Synaptic Differentiation in the Cerebellum Is Regulated by WNT-7a Signaling

Synapse formation requires changes in cell morphology and the upregulation and localization of synaptic proteins. In the cerebellum, mossy fibers undergo extensive remodeling as they contact several granule cells and form complex, multisynaptic glomerular rosettes. Here we show that granule cells secrete factors that induce axon and growth cone remodeling in mossy fibers. This effect is blocked by the WNT antagonist, sFRP-1, and mimicked by WNT-7a, which is expressed by granule cells. WNT-7a also induces synapsin I clustering at remodeled areas of mossy fibers, a preliminary step in synaptogenesis. Wnt-7a mutant mice show a delay in the morphological maturation of glomerular rosettes and in the accumulation of synapsin I. We propose that WNT-7a can function as a synaptogenic factor.

OLIGODENDROCYTE POPULATION DYNAMICS AND THE ROLE OF PDGF IN VIVO

Oligodendrocyte progenitors originate near the floor plate of the spinal cord, then proliferate and migrate throughout the cord before giving rise to oligodendrocytes. Progenitor cell proliferation stops before birth because the cell cycle slows down, linked to an increase in differentiation and death. Experiments with transgenic mice show that platelet-derived growth factor (PDGF) drives progenitor cell division and suggest that slowing of and exit from the cycle reflects a decline in PDGF signaling. Overexpressing PDGF induces hyperproliferation of progenitor cells and excessive, ectopic production of oligodendrocytes. However, the superfluous oligodendrocytes die at an immature stage of differentiation, leaving a normal complement of myelin-forming cells. Therefore, cell survival controls override proliferation controls for determining the final number and distribution of mature oligodendrocytes.

A Wnt1-regulated genetic network controls the identity and fate of midbrain-dopaminergic progenitors in vivo

Midbrain neurons synthesizing the neurotransmitter dopamine play a central role in the modulation of different brain functions and are associated with major neurological and psychiatric disorders. Despite the importance of these cells, the molecular mechanisms controlling their development are still poorly understood. The secreted glycoprotein Wnt1 is expressed in close vicinity to developing midbrain dopaminergic neurons. Here, we show that Wnt1 regulates the genetic network, including Otx2 and Nkx2-2, that is required for the establishment of the midbrain dopaminergic progenitor domain during embryonic development. In addition, Wnt1 is required for the terminal differentiation of midbrain dopaminergic neurons at later stages of embryogenesis. These results identify Wnt1 as a key molecule in the development of midbrain dopaminergic neurons in vivo. They also suggest the Wnt1-controlled signaling pathway as a promising target for new therapeutic strategies in the treatment of Parkinsons disease.

Signaling across the synapse: a role for Wnt and Dishevelled in presynaptic assembly and neurotransmitter release

Proper dialogue between presynaptic neurons and their targets is essential for correct synaptic assembly and function. At central synapses, Wnt proteins function as retrograde signals to regulate axon remodeling and the accumulation of presynaptic proteins. Loss of Wnt7a function leads to defects in the localization of presynaptic markers and in the morphology of the presynaptic axons. We show that loss of function of Dishevelled-1 (Dvl1) mimics and enhances the Wnt7a phenotype in the cerebellum. Although active zones appear normal, electrophysiological recordings in cerebellar slices from Wnt7a/Dvl1 double mutant mice reveal a defect in neurotransmitter release at mossy fiber–granule cell synapses. Deficiency in Dvl1 decreases, whereas exposure to Wnt increases, synaptic vesicle recycling in mossy fibers. Dvl increases the number of Bassoon clusters, and like other components of the Wnt pathway, it localizes to synaptic sites. These findings demonstrate that Wnts signal across the synapse on Dvl-expressing presynaptic terminals to regulate synaptic assembly and suggest a potential novel function for Wnts in neurotransmitter release.

Oligodendrocyte Lineage and the Motor Neuron Connection

One of the more surprising recent discoveries in glial biology has been that oligodendrocytes (OLs) originate from very restricted regions of the embryonic neural tube. This was surprising because myelinating OLs are widespread in the mature central nervous system, so there was no reason to suspect that their precursors should be restricted. What we now know about early OL development suggests that they might have as much (or more) in common with ventral neurons—specifically motor neurons (MNs)—as with other types of glia. This has implications for the way we think about glial development, function, and evolution. In this article we review the evidence for a shared MN‐OL lineage and debate whether this is the only lineage that generates OLs. We decide in favour of a single embryonic lineage with regional variations along the anterior‐posterior neuraxis. GLIA 29:136–142, 2000.

Valproate regulates GSK-3-mediated axonal remodeling and synapsin I clustering in developing neurons.

Valproate (VPA) and lithium have been used for many years in the treatment of manic depression. However, their mechanisms of action remain poorly understood. Recent studies suggest that lithium and VPA inhibit GSK-3beta, a serine/threonine kinase involved in the insulin and WNT signaling pathways. Inhibition of GSK-3beta by high concentrations of lithium has been shown to mimic WNT-7a signaling by inducing axonal remodeling and clustering of synapsin I in developing neurons. Here we have compared the effect of therapeutic concentrations of lithium and VPA during neuronal maturation. VPA and, to a lesser extent, lithium induce clustering of synapsin I. In addition, lithium and VPA induce similar changes in the morphology of axons by increasing growth cone size, spreading, and branching. More importantly, both mood stabilizers decrease the level of MAP-1B-P, a GSK-3beta-phosphorylated form of MAP-1B in developing neurons, suggesting that therapeutic concentrations of these mood stabilizers inhibit GSK-3beta. In vitro kinase assays show that therapeutic concentrations of VPA do not inhibit GSK-3beta but that therapeutic concentrations of lithium partially inhibit GSK-3beta activity. Our results support the idea that both mood stabilizers inhibit GSK-3beta in developing neurons through different pathways. Lithium directly inhibits GSK-3beta in contrast to VPA, which inhibits GSK-3beta indirectly by an as-yet-unknown pathway. These findings may have important implications for the development of new strategies to treat bipolar disorders.

Origins of Spinal Cord Oligodendrocytes: Possible Developmental and Evolutionary Relationships with Motor Neurons

Spinal cord oligodendrocytes develop from migratory glial progenitor cells that are generated by a small subset of neuroepithelial cells in the ventral part of the neural tube. Specification of these neuroepithelial oligodendrocyte precursors, in common with other ventral cells such as motor neurons, depends on morphogenetic signals from the notochord and/or floor plate. The ventrally derived signals can be mimicked in vitro by purified Sonic hedgehog (Shh) protein. Oligodendrocytes and motor neurons are induced over the same range of concentrations of Shh, consistent with the idea that Shh might specify a common precursor of motor neurons and oligodendrocytes. A lineage relationship between motor neurons and oligodendrocytes has previously been suggested by clonal analysis in the embryonic chick spinal cord. We propose a lineage diagram that connects oligodendrocytes and motor neurons and that takes into account the fact that motor neurons and oligodendrocyte precursors are generated at different times during development. Oligodendrocytes might originally have evolved to ensheath motor axons and facilitate a rapid escape response. If so, oligodendrocyte ontogeny and phylogeny might share a common basis.

Liver X Receptors and Oxysterols Promote Ventral Midbrain Neurogenesis In Vivo and in Human Embryonic Stem Cells

Control over progenitor proliferation and neurogenesis remains a key challenge for stem cell neurobiology and a prerequisite for successful stem cell replacement therapies for neurodegenerative diseases like Parkinsons disease (PD). Here, we examined the function of two nuclear receptors, liver X receptors (Lxralpha and beta) and their ligands, oxysterols, as regulators of cell division, ventral midbrain (VM) neurogenesis, and dopaminergic (DA) neuron development. Deletion of Lxrs reduced cell cycle progression and VM neurogenesis, resulting in decreased DA neurons at birth. Activation of Lxrs with oxysterol ligands increased the number of DA neurons in mouse embryonic stem cells (ESCs) and in wild-type but not Lxralphabeta(-/-) VM progenitor cultures. Likewise, oxysterol treatment of human ESCs (hESCs) during DA differentiation increased neurogenesis and the number of mature DA neurons, while reducing proliferating progenitors. Thus, Lxr ligands may improve current hESC replacement strategies for PD by selectively augmenting the generation of DA neurons.

Identification of midbrain floor plate radial glia-like cells as dopaminergic progenitors.

The floor plate (FP), a signaling center and a structure rich in radial glia‐like cells, has been traditionally thought to be devoid of neurons and neuronal progenitors. However, in the midbrain, the FP contains neurons of the dopaminergic (DA) lineage that require contact with radial glia‐like cells for their induction. We, therefore, decided to explore the interaction relationship between radial glia and neurons during DA neurogenesis. Taking advantage of a novel FP radial glia‐like cell culture system and retroviruses, DA neurons were lineage traced in vitro. In utero BrdU pulse‐chases extensively labeled the midbrain FP and traced DA neurons both in vivo and in FP cultures. Moreover, from E9.5 to E13.5 the midbrain FP contained dividing cells only in the most apical part of the neuroepithelium, in cells identified as radial glia‐like cells. We, therefore, hypothesized that midbrain FP radial glia‐like cells could be DA progenitors and tested our hypothesis in vivo. Lineage tracing of DA progenitors with EGFP in Tis21‐EGFP knock‐in mice, and genetic fate mapping in GLAST::CreERT2/ZEG mice identified the neuroepithelium of the midbrain FP, and specifically, GLAST+ radial glia‐like cells as DA progenitors. Combined, our experiments support the concept that the midbrain FP differs from other FP regions and demonstrate that FP radial glia‐like cells in the midbrain are neurogenic and give rise to midbrain DA neurons.

Wnt5a Regulates Ventral Midbrain Morphogenesis and the Development of A9–A10 Dopaminergic Cells In Vivo

Wnt5a is a morphogen that activates the Wnt/planar cell polarity (PCP) pathway and serves multiple functions during development. PCP signaling controls the orientation of cells within an epithelial plane as well as convergent extension (CE) movements. Wnt5a was previously reported to promote differentiation of A9–10 dopaminergic (DA) precursors in vitro. However, the signaling mechanism in DA cells and the function of Wnt5a during midbrain development in vivo remains unclear. We hereby report that Wnt5a activated the GTPase Rac1 in DA cells and that Rac1 inhibitors blocked the Wnt5a-induced DA neuron differentiation of ventral midbrain (VM) precursor cultures, linking Wnt5a-induced differentiation with a known effector of Wnt/PCP signaling. In vivo, Wnt5a was expressed throughout the VM at embryonic day (E)9.5, and was restricted to the VM floor and basal plate by E11.5–E13.5. Analysis of Wnt5a−/− mice revealed a transient increase in progenitor proliferation at E11.5, and a precociously induced NR4A2+ (Nurr1) precursor pool at E12.5. The excess NR4A2+ precursors remained undifferentiated until E14.5, when a transient 25% increase in DA neurons was detected. Wnt5a−/− mice also displayed a defect in (mid)brain morphogenesis, including an impairment in midbrain elongation and a rounded ventricular cavity. Interestingly, these alterations affected mostly cells in the DA lineage. The ventral Sonic hedgehog-expressing domain was broadened and flattened, a typical CE phenotype, and the domains occupied by Ngn2+ DA progenitors, NR4A2+ DA precursors and TH+ DA neurons were rostrocaudally reduced and laterally expanded. In summary, we hereby describe a Wnt5a regulation of Wnt/PCP signaling in the DA lineage and provide evidence for multiple functions of Wnt5a in the VM in vivo, including the regulation of VM morphogenesis, DA progenitor cell division, and differentiation of NR4A2+ DA precursors.