The Microtubule-Associated Protein Tau Mediates the Organization of Microtubules and Their Dynamic Exploration of Actin-Rich Lamellipodia and Filopodia of Cortical Growth Cones.

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 (Mesocricetus auratus) 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.SIGNIFICANCE STATEMENT 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.

Branch management: mechanisms of axon branching in the developing vertebrate CNS.

The remarkable ability of a single axon to extend multiple branches and form terminal arbors enables vertebrate neurons to integrate information from divergent regions of the nervous system. Axons select appropriate pathways during development, but it is the branches that extend interstitially from the axon shaft and arborize at specific targets that are responsible for virtually all of the synaptic connectivity in the vertebrate CNS. How do axons form branches at specific target regions? Recent studies have identified molecular cues that activate intracellular signalling pathways in axons and mediate dynamic reorganization of the cytoskeleton to promote the formation of axon branches.

Wnt5a evokes cortical axon outgrowth and repulsive guidance by tau mediated reorganization of dynamic microtubules.

Wnt5a guides cortical axons in vivo by repulsion and in vitro evokes cortical axon outgrowth and repulsion by calcium signaling pathways. Here we examined the role of microtubule (MT) reorganization and dynamics in mediating effects of Wnt5a. Inhibiting MT dynamics with nocodazole and taxol abolished Wnt5a evoked axon outgrowth and repulsion of cultured hamster cortical neurons. EGFP-EB3 labeled dynamic MTs visualized in live cell imaging revealed that growth cone MTs align with the nascent axon. Wnt5a increased axon outgrowth by reorganization of dynamic MTs from a splayed to a bundled array oriented in the direction of axon extension, and Wnt5a gradients induced asymmetric redistribution of dynamic MTs toward the far side of the growth cone. Wnt5a gradients also evoked calcium transients that were highest on the far side of the growth cone. Calcium signaling and the reorganization of dynamic MTs could be linked by tau, a MT associated protein that stabilizes MTs. Tau is phosphorylated at the Ser 262 MT binding site by CaMKII, and is required for Wnt5a induced axon outgrowth and repulsive turning. Phosphorylation of tau at Ser262 is known to detach tau from MTs to increase their dynamics. Using transfection with tau constructs mutated at Ser262, we found that this site is required for the growth and guidance effects of Wnt5a by mediating reorganization of dynamic MTs in cortical growth cones. Moreover, CaMKII inhibition also prevents MT reorganization required for Wnt5a induced axon outgrowth, thus linking Wnt/calcium signaling to tau mediated MT reorganization during growth cone behaviors.

Wnt-induced calcium signaling mediates axon growth and guidance in the developing corpus callosum.

Wnt5a gradients guide callosal axons by repulsion through Ryk receptors in vivo. We recently found that Wnt5a repels cortical axons and promotes axon outgrowth through calcium signaling in vitro. Here, using cortical slices, we show that Wnt5a signals through Ryk to guide and promote outgrowth of callosal axons after they cross the midline. Calcium transient frequencies in callosal growth cones positively correlate with axon outgrowth rates in vitro. In cortical slices, calcium release through inositol 1,4,5-trisphosphate (IP(3)) receptors and calcium entry through transient receptor potential channels modulate axon growth and guidance. Knocking down Ryk inhibits calcium signaling in cortical axons, reduces rates of axon outgrowth subsequent to midline crossing, and causes axon guidance defects. Calcium- and calmodulin-dependent protein kinase II (CaMKII) is required downstream of Wnt-induced calcium signaling for postcrossing callosal axon growth and guidance. Taken together, these results suggest that growth and guidance of postcrossing callosal axons by Wnt-Ryk-calcium signaling involves axon repulsion through CaMKII.

Signaling mechanisms in cortical axon growth, guidance, and branching.

Precise wiring of cortical circuits during development depends upon axon extension, guidance, and branching to appropriate targets. Motile growth cones at axon tips navigate through the nervous system by responding to molecular cues, which modulate signaling pathways within axonal growth cones. Intracellular calcium signaling has emerged as a major transducer of guidance cues but exactly how calcium signaling pathways modify the actin and microtubule cytoskeleton to evoke growth cone behaviors and axon branching is still mysterious. Axons must often pause their extension in tracts while their branches extend into targets. Some evidence suggests a competition between growth of axons and branches but the mechanisms are poorly understood. Since it is difficult to study growing axons deep within the mammalian brain, much of what we know about signaling pathways and cytoskeletal dynamics of growth cones comes from tissue culture studies, in many cases, of non-mammalian species. Consequently it is not well understood how guidance cues relevant to mammalian neural development in vivo signal to the growth cone cytoskeleton during axon outgrowth and guidance. In this review we describe our recent work in dissociated cultures of developing rodent sensorimotor cortex in the context of the current literature on molecular guidance cues, calcium signaling pathways, and cytoskeletal dynamics that regulate growth cone behaviors. A major challenge is to relate findings in tissue culture to mechanisms of cortical development in vivo. Toward this goal, we describe our recent work in cortical slices, which preserve the complex cellular and molecular environment of the mammalian brain but allow direct visualization of growth cone behaviors and calcium signaling. Findings from this work suggest that mechanisms regulating axon growth and guidance in dissociated culture neurons also underlie development of cortical connectivity in vivo.

Wnt/calcium signaling mediates axon growth and guidance in the developing corpus callosum.

It has been shown in vivo that Wnt5a gradients surround the corpus callosum and guide callosal axons after the midline (postcrossing) by Wnt5a-induced repulsion via Ryk receptors. In dissociated cortical cultures we showed that Wnt5a simultaneously promotes axon outgrowth and repulsion by calcium signaling. Here to test the role of Wnt5a/calcium signaling in a complex in vivo environment we used sensorimotor cortical slices containing the developing corpus callosum. Plasmids encoding the cytoplasmic marker DsRed and the genetically encoded calcium indicator GCaMP2 were electroporated into one cortical hemisphere. Postcrossing callosal axons grew 50% faster than pre-crossing axons and higher frequencies of calcium transients in axons and growth cones correlated well with outgrowth. Application of pharmacological inhibitors to the slices showed that signaling pathways involving calcium release through IP3 receptors and calcium entry through TRP channels regulate post-crossing axon outgrowth and guidance. Co-electroporation of Ryk siRNA and DsRed revealed that knock down of the Ryk receptor reduced outgrowth rates of postcrossing but not precrossing axons by 50% and caused axon misrouting. Guidance errors in axons with Ryk knockdown resulted from reduced calcium activity. In the corpus callosum CaMKII inhibition reduced the outgrowth rate of postcrossing (but not precrossing) axons and caused severe guidance errors which resulted from reduced CaMKII-dependent repulsion downstream of Wnt/calcium. We show for the first time that Wnt/Ryk calcium signaling mechanisms regulating axon outgrowth and repulsion in cortical cultures are also essential for the proper growth and guidance of postcrossing callosal axons which involve axon repulsion through CaMKII.

Wnt5a induces simultaneous cortical axon outgrowth and repulsive turning through distinct signaling mechanisms.

Wnt5a is thought to propel cortical axons down the corticospinal tract and through the corpus callosum by repulsive mechanisms. We cultured dissociated early postnatal cortical neurons from hamsters and exposed them to a gradient of Wnt5a as a model for studying the mechanism of Wnt5a effects. Turning assays indicated that cortical axons were repelled away from a point source of Wnt5a. Surprisingly, during the 1-hour turning assay, axons exposed to Wnt5a also increased their growth rates by almost 50%. Ryk receptors but not Frizzled (Fz) receptors were required for Wnt5a-promoted axon outgrowth, whereas both Ryk and Fz receptors were required for repulsive growth-cone turning. Both Ryk and Fz receptors mediated calcium (Ca(2+)) signaling, which is required for axon outgrowth and repulsive turning. Treatments with pharmacological inhibitors revealed that distinct Ca(2+) signaling mechanisms were involved in Wnt5a-dependent axon outgrowth versus repulsive guidance. Ca(2+) release from intracellular stores through inositol 1,4,5-trisphosphate receptors was required for Wnt5a-induced axon outgrowth but not for repulsive turning. In contrast, Ca(2+) entry through transient receptor potential channels was required for both repulsive growth-cone turning and Wnt5a-increased axon outgrowth. Taken together, these results showed that a guidance cue can induce increased rates of axon outgrowth simultaneously with repulsive guidance and may provide an understanding of how cortical axons may be repelled down the spinal cord in vivo. Moreover, we demonstrate that previously unidentified Wnt signaling pathways differentially mediate these growth-cone behaviors.

Wnt5a induces simultaneous cortical axon outgrowth and repulsive axon guidance through distinct signaling mechanisms.

Wnts are morphogens that also function as axon guidance molecules. In vivo Wnt5a gradients via Ryk receptors were found to repel cortical axons into developing callosal and corticospinal pathways. Here, using dissociated cortical cultures, we found that bath-applied Wnt5a increased axon outgrowth. In turning assays, Wnt5a gradients simultaneously increased axon outgrowth and induced repulsive turning, a potential mechanism for propelling cortical axons in vivo. We found that axon outgrowth is mediated by Ryk, whereas axon repulsion requires both Ryk and Frizzled receptors. Both receptors mediate Wnt-evoked fluctuations in intracellular calcium, which is required for increased axon outgrowth and repulsion by Wnt5a. However, whereas increased axon outgrowth involves calcium release from stores through IP3 receptors as well as calcium influx through TRP channels, axon repulsion is mediated by TRP channels without involvement of IP3 receptors. These results reveal distinct signaling mechanisms underlying Wnt5a-induced axon outgrowth and repulsive guidance.

Differential outgrowth of axons and their branches is regulated by localized calcium transients.

During development axon outgrowth and branching are independently regulated such that axons can stall or retract while their interstitial branches extend toward targets. Previous studies have shown that guidance cues and intracellular signaling components can promote branching of cortical axons without affecting axon outgrowth. However, the mechanisms that regulate differential outgrowth of axons and their branches are not well understood. Based on our previous work showing the importance of localized repetitive calcium transients in netrin-1-induced cortical axon branching, we sought to investigate the role of calcium signaling in regulating differential outgrowth of axons and their branches. Using fluorescence calcium imaging of dissociated developing cortical neurons, we show that localized spontaneous calcium transients of different frequencies occur in restricted regions of axons and their branches. Higher frequencies occur in more rapidly extending processes whereas lower frequencies occur in processes that stall or retract. Direct induction of localized calcium transients with photolysis of caged calcium induced rapid outgrowth of axonal processes. Surprisingly outgrowth of one axonal process was almost invariably accompanied by simultaneous retraction of another process belonging to the same axon, suggesting a competitive mechanism for differential process outgrowth. Conversely, reducing frequencies of calcium transients with nifedipine and TTX reduced the incidence of differential process outgrowth. Together these results suggest a novel activity-dependent mechanism whereby intrinsic localized calcium transients regulate the competitive growth of axons and their branches. These mechanisms may also be important for the development of cortical connectivity in vivo.

Touch and go: guidance cues signal to the growth cone cytoskeleton.

Growth cones, the highly motile tips of growing axons, guide axons to their targets by responding to molecular cues. Growth cone behaviors such as advancing, retracting, turning and branching are driven by the dynamics and reorganization of the actin and microtubule cytoskeleton through signaling pathways linked to guidance cue receptors. Actin filaments play a major part in growth cone motility, and because of their peripheral locations were thought to be the primary target of molecular cues. However, recent studies have shown that dynamic microtubules can penetrate the growth cone periphery where guidance molecules can influence them directly. Moreover, guidance cues can regulate growth cone steering by modulating dynamic actin-microtubule interactions.

Netrin-1 induces axon branching in developing cortical neurons by frequency-dependent calcium signaling pathways.

A single axon can innervate multiple targets by collateral branching. Axon branching is thus essential for establishing CNS connectivity. However, surprisingly little is known about the mechanisms by which branching is regulated. Axons often stop elongating before branches develop and anatomical and molecular data suggest that axon branching occurs independent of axon outgrowth. We found that netrin-1 dramatically increases cortical axon branching. Here, we sought to identify intracellular signaling components involved in netrin-1-induced axon branching. Using live cell imaging of dissociated developing cortical neurons, we show that netrin-1 rapidly increases the frequency of repetitive calcium transients. These transients are often restricted to small regions of the axon. Simultaneous imaging of calcium activity and development of axon branches revealed that Ca2+ transients coincide spatially and temporally with protrusion of branches from the axon. Remarkably, fully formed branches with motile growth cones could develop de novo within 20 min. Netrin-1-induced Ca2+ transients involve release from intracellular stores and Ca2+ signaling is essential for netrin-1-induced axon branching. Using techniques to overexpress or suppress kinase activity, we find that calcium/calmodulin-dependent protein kinase II (CaMKII) and mitogen-activated protein kinase (MAPK) are major downstream targets of the netrin-1 calcium signaling pathway and are required for axon branching. CaMKII, but not MAPK, is also involved in axon outgrowth. The role of CaMKII and MAPKs in axon branching is consistent with the sensitivity of these kinases to changes in the frequency Ca2+ transients. Together, these novel findings define calcium signaling mechanisms required for development of new axon branches promoted by a guidance cue.

Hot +TIPS: guidance cues signal directly to microtubules.

Plus end tracking proteins (+TIPS) bind to microtubules. Results reported by Lee et al. and Zhou et al. in this issue of Neuron demonstrate that, via intracellular pathways signaling to +TIPs, microtubules can be a direct target of guidance cues in steering growth cones and regulating axon elongation.

Netrin-1 and semaphorin 3A promote or inhibit cortical axon branching, respectively, by reorganization of the cytoskeleton.

In many CNS pathways, target innervation occurs by axon branching rather than extension of the primary growth cone into targets. To investigate mechanisms of branch formation, we studied the effects of attractive and inhibitory guidance cues on cortical axon branching. We found that netrin-1, which attracts cortical axons, and FGF-2 increased branching by >50%, whereas semaphorin 3A (Sema3A), which repels cortical axons, inhibited branching by 50%. Importantly, none of the factors affected axon length significantly. The increase in branching by FGF-2 and the inhibition of branching by Sema3A were mediated by opposing effects on the growth cone (expansion vs collapse) and on the cytoskeleton. FGF-2 increased actin polymerization and formation of microtubule loops in growth cones over many hours, whereas Sema3A depolymerized actin filaments, attenuated microtubule dynamics, and collapsed microtubule arrays within minutes. Netrin-1 promoted rapid axon branching, often without involving the growth cone. Branches formed de novo on the axon shaft within 30 min after local application of netrin-1, which induced rapid accumulation of actin filaments in filopodia. Importantly, increased actin polymerization and microtubule dynamics were necessary for axon branching to occur. Taken together, these results show that guidance factors influence the organization and dynamics of the cytoskeleton at the growth cone and the axon shaft to promote or inhibit axon branching. Independent of axon outgrowth, axon branching in response to guidance cues can occur over different time courses by different cellular mechanisms.

Axon guidance by growth cones and branches: common cytoskeletal and signaling mechanisms.

Growing axons are guided to appropriate targets by responses of their motile growth cones to environmental cues. Interstitial axon branching is also an important form of axon guidance in the mammalian CNS. Visualization of growing axons in cortical slices and in dissociated cortical cultures showed that growth cone pausing behaviors demarcate sites of future axon branching. Studies of vertebrate and invertebrate growth cones suggest common mechanisms that regulate growth cone behaviors and axon branching. These include reorganization of the actin and microtubule cytoskeleton, dynamic interactions between microtubules and actin filaments, effects of axon guidance molecules, actions of actin regulatory proteins, and dynamic changes in intracellular calcium signaling. Future challenges will be to extend high-resolution imaging of single neurons to studies of intracellular events in the intact nervous system and to apply knowledge of developmental mechanisms to the promotion of axon sprouting after injury in the adult CNS.

Spontaneous calcium transients in developing cortical neurons regulate axon outgrowth.

Growth cones of cortical axons pause for many hours in preparation for axon branching. They become large and complex compared with small advancing growth cones. We wanted to investigate whether calcium transients regulate the advance of mammalian CNS growth cones. We found that spontaneous calcium transients in developing cortical neurons have characteristic patterns, frequencies, and amplitudes. Importantly, neurons with large paused growth cones exhibit high-frequency spontaneous calcium transients, which are rare in those with small advancing growth cones. The incidence, frequencies, and amplitudes of calcium transients are inversely related to rates of axon outgrowth. The transients are mediated primarily by L-type voltage-gated calcium channels, and silencing them with channel blockers promotes axon outgrowth. Thus calcium transients regulate growth cone advance by direct effects on the growth cone.