Formation of a Protocadherin-19–N-cadherin complex facilitates adhesion by Protocadherin-19 while suppressing homophilic interactions by N-cadherin.

Protocadherin-19 and N-cadherin act synergistically during brain morphogenesis in zebrafish.

Proper organization and dynamics of the actin and microtubule (MT) cytoskeleton are essential for growth cone behaviors during axon growth and guidance. The MT-associated protein tau is known to mediate actin/MT interactions in cell-free systems but the role of tau in regulating cytoskeletal dynamics in living neurons is unknown. We used cultures of cortical neurons from postnatal day (P)0-P2 golden Syrian hamsters () of either sex to study the role of tau in the organization and dynamics of the axonal growth cone cytoskeleton. Here, using super resolution microscopy of fixed growth cones, we found that tau colocalizes with MTs and actin filaments and is also located at the interface between actin filament bundles and dynamic MTs in filopodia, suggesting that tau links these two cytoskeletons. Live cell imaging in concert with shRNA tau knockdown revealed that reducing tau expression disrupts MT bundling in the growth cone central domain, misdirects trajectories of MTs in the transition region and prevents single dynamic MTs from extending into growth cone filopodia along actin filament bundles. Rescue experiments with human tau expression restored MT bundling, MT penetration into the growth cone periphery and close MT apposition to actin filaments in filopodia. Importantly, we found that tau knockdown reduced axon outgrowth and growth cone turning in Wnt5a gradients, likely due to disorganized MTs that failed to extend into the peripheral domain and enter filopodia. These results suggest an important role for tau in regulating cytoskeletal organization and dynamics during growth cone behaviors. Growth cones are the motile tips of growing axons whose guidance behaviors require interaction of the dynamic actin and microtubule cytoskeleton. Tau is a microtubule-associated protein that stabilizes microtubules in neurons and in cell-free systems regulates actin-microtubule interaction. Here, using super resolution microscopy, live-cell imaging, and tau knockdown, we show for the first time in living axonal growth cones that tau is important for microtubule bundling and microtubule exploration of the actin-rich growth cone periphery. Importantly tau knockdown reduced axon outgrowth and growth cone turning, due to disorganized microtubules that fail to enter filopodia and co-align with actin filaments. Understanding normal tau functions will be important for identifying mechanisms of tau in neurodegenerative diseases such as Alzheimer's.

One of the earliest stages of brain morphogenesis is the establishment of the neural tube during neurulation. While some of the cellular mechanisms responsible for neurulation have been described in a number of vertebrate species, the underlying molecular processes are not fully understood. We have identified the zebrafish homolog of protocadherin-19, a member of the cadherin superfamily, which is expressed in the anterior neural plate and is required for brain morphogenesis. Interference with Protocadherin-19 function with antisense morpholino oligonucleotides leads to a severe disruption in early brain morphogenesis. Despite these pronounced effects on neurulation, axial patterning of the neural tube appears normal, as assessed by in situ hybridization for otx2, pax2.1 and krox20. Characterization of embryos early in development by in vivo 2-photon timelapse microscopy reveals that the observed disruption of morphogenesis results from an arrest of cell convergence in the anterior neural plate. These results provide the first functional data for protocadherin-19, demonstrating an essential role in early brain development.

Interference with Pcdh18b function results in impaired arborization of motor axons in the developing zebrafish. Pcdh18b interacts with Nap1, a regulator of actin assembly. Time-lapse imaging indicates that both Pcdh18b and Nap1 may affect axon arborization by regulating the density of axonal filopodia.