Mosaic analysis of gene function in postnatal mouse brain development by using virus-based Cre recombination

Targeted approaches to delineate neuronal morphology during early development

Understanding the developmental changes that affect neurons is a key step in exploring the assembly and maturation of neural circuits in the brain. For decades, researchers have used a number of labeling techniques to visualize neuronal morphology at different stages of development. However, the efficiency and accuracy of neuronal labeling technologies are limited by the complexity and fragility of neonatal brains. In this review, we illustrate the various labeling techniques utilized for examining the neurogenesis and morphological changes occurring during the early stages of development. We compare the advantages and limitations of each technique from different aspects. Then, we highlight the gaps remaining in our understanding of the structure of neurons in the neonatal mouse brain.

In vivo cell tracking with viral vector mediated genetic labeling

Cell tracking is a useful technique to monitor specific cell populations for their morphology, development, proliferation, migration, interaction, function, and other properties, both in vitro and in vivo. Using different materials and methodologies to label the target cells directly or indirectly, the dynamic biological processes in living organisms can be visualized with appropriate detection techniques. Viruses, with the unique ability to deliver exogenous genes into host cells, have been used as vectors to mediate gene transfer. Genetic labeling of target cells by viral vectors endows the cells to express reporter genes with high efficiency and specificity. In conjunction with corresponding imaging techniques, cells labeled with different genetic reporters mediated by different viral vectors can be monitored across spatial and temporal scales to fulfill various purposes and address different questions. In the present review, we introduce the basic principle of viral vectors in cell tracking and highlight the examples of cell tracking in various research areas.

PDK1 Regulates the Maintenance of Cell Body and the Development of Dendrites of Purkinje Cells by pS6 and PKCγ

3-Phosphoinositide-dependent protein kinase-1 (PDK1) plays a critical role in the development of mammalian brain. Here, we investigated the role of PDK1 in Purkinje cells (PCs) by generating the PDK1-conditional knock-out mice (cKO) through crossing PV-cre or Pcp2-cre mice with Pdk1^fl/fl mice. The male mice were used in the behavioral testing, and the other experiments were performed on mice of both sexes. These PDK1-cKO mice displayed decreased cerebellar size and impaired motor balance and coordination. By the electrophysiological recording, we observed the reduced spontaneous firing of PCs from the cerebellar slices of the PDK1-cKO mice. Moreover, the cell body size of PCs in the PDK1-cKO mice was time dependently reduced compared with that in the control mice. And the morphologic complexity of PCs was also decreased after PDK1 deletion. These effects may have contributed to the reduction of the rpS6 (reduced ribosomal protein S6) phosphorylation and the PKCγ expression in PDK1-cKO mice since the upregulation of pS6 by treatment of 3-benzyl-5-((2-nitrophenoxy) methyl)-dihydrofuran-2(3H)-1, the agonist of mTOR1, partly rescued the reduction in the cell body size of the PCs, and the delivery of recombinant adeno-associated virus-PKCγ through cerebellar injection rescued the reduced complexity of the dendritic arbor in PDK1-cKO mice. Together, our data suggest that PDK1, by regulating rpS6 phosphorylation and PKCγ expression, controls the cell body maintenance and the dendritic development in PCs and is critical for cerebellar motor coordination.SIGNIFICANCE STATEMENT Here, we show the role of 3-phosphoinositide-dependent protein kinase-1 (PDK1) in Purkinje cells (PCs). The ablation of PDK1 in PCs resulted in a reduction of cell body size, and dendritic complexity and abnormal spontaneous firing, which attributes to the motor defects in PDK1-conditional knock-out (cKO) mice. Moreover, the ribosomal protein S6 (rpS6) phosphorylation and the expression of PKCγ are downregulated after the ablation of PDK1. Additionally, upregulation of rpS6 phosphorylation by3-benzyl-5-((2-nitrophenoxy) methyl)-dihydrofuran-2(3H)-1 partly rescued the reduction in cell body size of PCs, and the overexpression of PKCγ in PDK1-KO PCs rescued the reduction in the dendritic complexity. These findings indicate that PDK1 contributes to the maintenance of the cell body and the dendritic development of PCs by regulating rpS6 phosphorylation and PKCγ expression.

Amot and Yap1 regulate neuronal dendritic tree complexity and locomotor coordination in mice

The angiomotin (Amot)-Yes-associated protein 1 (Yap1) complex plays a major role in regulating the inhibition of cell contact, cellular polarity, and cell growth in many cell types. However, the function of Amot and the Hippo pathway transcription coactivator Yap1 in the central nervous system remains unclear. We found that Amot is a critical mediator of dendritic morphogenesis in cultured hippocampal cells and Purkinje cells in the brain. Amot function in developing neurons depends on interactions with Yap1, which is also indispensable for dendrite growth and arborization in vitro. The conditional deletion of Amot and Yap1 in neurons led to a decrease in the complexity of Purkinje cell dendritic trees, abnormal cerebellar morphology, and impairments in motor coordination. Our results indicate that the function of Amot and Yap1 in dendrite growth does not rely on interactions with TEA domain (TEAD) transcription factors or the expression of Hippo pathway-dependent genes. Instead, Amot and Yap1 regulate dendrite development by affecting the phosphorylation of S6 kinase and its target S6 ribosomal protein.

Rapid and Sparse Labeling of Neurons Based on the Mutant Virus-Like Particle of Semliki Forest Virus

Sparse labeling of neurons contributes to uncovering their morphology, and rapid expression of a fluorescent protein reduces the experiment range. To achieve the goal of rapid and sparse labeling of neurons in vivo, we established a rapid method for depicting the fine structure of neurons at 24 h post-infection based on a mutant virus-like particle of Semliki Forest virus. Approximately 0.014 fluorescent focus-forming units of the mutant virus-like particle transferred enhanced green fluorescent protein into neurons in vivo, and its affinity for neurons in vivo was stronger than for neurons in vitro and BHK21 (baby hamster kidney) cells. Collectively, the mutant virus-like particle provides a robust and convenient way to reveal the fine structure of neurons and is expected to be a helper virus for combining with other tools to determine their connectivity. Our work adds a new tool to the approaches for rapid and sparse labeling of neurons in vivo.

Neurodevelopment. Dendrite morphogenesis depends on relative levels of NT-3/TrkC signaling

Neurotrophins regulate diverse aspects of neuronal development and plasticity, but their precise in vivo functions during neural circuit assembly in the central brain remain unclear. We show that the neurotrophin receptor tropomyosin-related kinase C (TrkC) is required for dendritic growth and branching of mouse cerebellar Purkinje cells. Sparse TrkC knockout reduced dendrite complexity, but global Purkinje cell knockout had no effect. Removal of the TrkC ligand neurotrophin-3 (NT-3) from cerebellar granule cells, which provide major afferent input to developing Purkinje cell dendrites, rescued the dendrite defects caused by sparse TrkC disruption in Purkinje cells. Our data demonstrate that NT-3 from presynaptic neurons (granule cells) is required for TrkC-dependent competitive dendrite morphogenesis in postsynaptic neurons (Purkinje cells)--a previously unknown mechanism of neural circuit development.

Dendrite self-avoidance requires cell-autonomous slit/robo signaling in cerebellar purkinje cells

Dendrites from the same neuron usually develop nonoverlapping patterns by self-avoidance, a process requiring contact-dependent recognition and repulsion. Recent studies have implicated homophilic interactions of cell surface molecules, including Dscams and Pcdhgs, in self-recognition, but repulsive molecular mechanisms remain obscure. Here, we report a role for the secreted molecule Slit2 and its receptor Robo2 in self-avoidance of cerebellar Purkinje cells (PCs). Both molecules are highly expressed by PCs, and their deletion leads to excessive dendrite self-crossing without affecting arbor size and shape. This cell-autonomous function is supported by the boundary-establishing activity of Slit in culture and the phenotype rescue by membrane-associated Slit2 activities. Furthermore, genetic studies show that they act independently from Pcdhg-mediated recognition. Finally, PC-specific deletion of Robo2 is associated with motor behavior alterations. Thus, our study uncovers a local repulsive mechanism required for self-avoidance and demonstrates the molecular complexity at the cell surface in dendritic patterning.