Wnt7a overexpression delays beta-tubulin III expression in transgenic mouse embryos

Cell–Cell Contact Mediates Gene Expression and Fate Choice of Human Neural Stem/Progenitor Cells

Transplantation of Neural Stem/Progenitor Cells (NPCs) is a promising regenerative strategy to promote neural repair following injury and degeneration because of the ability of these cells to proliferate, migrate, and integrate with the host tissue. Precise in vitro control of NPC proliferation without compromising multipotency and differentiation ability is critical in stem cell maintenance. This idea was highlighted in recent clinical trials, where discrepancies in NPC culturing protocols produced inconsistent therapeutic benefits. Of note, cell density plays an important role in regulating the survival, proliferation, differentiation, and fate choice of stem cells. To determine the extent of variability produced by inconsistent culturing densities, the present study cultured human-induced pluripotent NPCs (hiPSC-NPCs) at either a low or high plating density. hiPSC-NPCs were then isolated for transcriptomic analysis or differentiation in vitro. Following sequencing analysis, genes involved in cell–cell contact-mediated pathways, including Hippo-signaling, NOTCH, and WNT were differentially expressed. Modulation of these pathways was highly associated with the regulation of pro-neuronal transcription factors, which were also upregulated in response to higher-density hiPSC-NPC culture. Moreover, higher plating density translated into a greater neuronal and less astrocytic differentiation in vitro. This study highlights the importance of precisely controlling culture conditions during the development of NPC transplantation therapies.

Diverse roles for Wnt7a in ventral midbrain neurogenesis and dopaminergic axon morphogenesis

During development of the central nervous system, trophic, together with genetic, cues dictate the balance between cellular proliferation and differentiation. Subsequent to the birth of new neurons, additional intrinsic and extrinsic signals regulate the connectivity of these cells. While a number of regulators of ventral midbrain (VM) neurogenesis and dopaminergic (DA) axon guidance are known, we identify a number of novel roles for the secreted glycoprotein, Wnt7a, in this context. We demonstrate a temporal and spatial expression of Wnt7a in the VM, indicative of roles in neurogenesis, differentiation, and axonal growth and guidance. In primary VM cultures, and validated in Wnt7a-deficient mice, we show that the early expression within the VM is important for regulating VM progenitor proliferation, cell cycle progression, and cell survival, thereby dictating the number of midbrain Nurr1 precursors and DA neurons. During early development of the midbrain DA pathways, Wnt7a promotes axonal elongation and repels DA neurites out of the midbrain. Later, Wnt7a expression in the VM midline suggests a role in preventing axonal crossing while expression in regions flanking the medial forebrain bundle (thalamus and hypothalamus) ensured appropriate trajectory of DA axons en route to their forebrain targets. We show that the effects of Wnt7a in VM development are mediated, at least in part, by the β-catenin/canonical pathways. Together, these findings identify Wnt7a as a new regulator of VM neurogenesis and DA axon growth and guidance.

Transgenic increase of Wnt7b in neural progenitor cells decreases expression of T-domain transcription factors and impairs neuronal differentiation

Wnt/beta-catenin signaling plays an important role in neural development, instructing both progenitor cell division and differentiation. During early corticogenesis, Wnt7b is expressed in a restricted expression pattern in the ventricular zone progenitor cells. However, its influence on progenitor cell behavior has not been fully studied. We report that transgenic overexpression of Wnt7b in neural progenitor cells impairs neuronal differentiation and the development of forebrain structures at embryonic day 10.5 (E10.5). This was accompanied by a decreased expression of T-domain transcription factors Tbr1 and Tbr2, in both progenitor cells and post-mitotic neurons. However, proliferation, apoptosis and the overall proportion of pax6(+) neural progenitor cells were similar to wild-type litter mates. These results suggest that Wnt signaling may affect early neural progenitor differentiation by regulating the expression of pro-neural transcription factors.

Dynamic expression of secreted Frizzled-related protein 3 (sFRP3) in the developing mouse spinal cord and dorsal root ganglia

Wnt proteins have been implicated in regulating a variety of developmental processes in the CNS. Secreted Frizzled-related protein 3 (sFRP3) is a member of the sFRP family that can inhibit the Wnt signaling by binding directly to Wnts via their regions of homology to the Wnt-binding domain of Frizzleds. Recent studies suggested that sFRP3 plays an important role in cell proliferation and differentiation in various tissues. To understand the role of sFRP3 in neural development, we carried out detailed studies on the expression of sFRP3 in the developing nervous system. Our results revealed that sFRP3 is initially expressed in the ventricular zone of the spinal cord and dorsal root ganglia (DRG), and later in the dorsal horn of spinal cord and subpopulation of DRG neurons. The spatiotemporally dynamic expression ofsFRP3 strongly suggests that sFRP3 has potential functions in the sensory neuron genesis and sensory circuitry formation.

Dynamic changes in Wnt signaling are required for neuronal differentiation of mouse embryonic stem cells

Embryonic stem cells (ESC) and the epiblast share a similar gene expression profile and an attenuated cell cycle, making them an accessible and tractable model system to study lineage choice at gastrulation. Differentiation of the epiblast and ESC to the mesendodermal lineage has been shown to rely on Wnt/β-catenin signaling; which counterintuitively, is also required to inhibit differentiation and maintain pluripotency. To examine these seemingly contradictory roles, we developed a mouse ESC (ESC) line that inducibly expresses a dominant negative Tcf4 (dnTcf4) protein to block canonical Wnt signaling. Cells expressing the dnTcf4 protein differentiated largely to Sox3 positive neural precursors but were unable to progress to βIII tubulin positive neurons unless Wnt signaling was derepressed, demonstrating a sequential requirement for Wnt signaling in lineage differentiation. To determine if Wnt/β-catenin signaling is similarily required at sequential stages of neural differentiation in the intact embryo, we delivered shRNA targeting β-catenin to pregnant mice on E5.5 of development. Blocking canonical Wnt signaling during post-implantation development increased the number of neural precursors which failed to differentiate to mature neurons, and produced defects of embryonic axis elongation, neurulation and neural tube closure that phenocopy the β-catenin null embryo. These results demonstrate that lineage differentiation relies on sequential repression and derepression of critical signaling pathways involved in maintaining pluripotency versus differentiation.