One of the earliest steps during the development of the nervous system is the establishment of neuronal polarity and the formation of an axon. The intrinsic mechanisms that promote axon formation have been extensively analyzed. However, much less is known about the extrinsic signals that initiate axon formation. One of the candidates for these signals is Insulin-like growth factor 1 (Igf1) that acts through the Igf1 (Igf1R) and insulin receptors (InsR). Since Igf1R and InsR may act redundantly we analyzed conditional cortex-specific knockout mice that are deficient for both Igf1r and Insr to determine if they regulate the development of the cortex and the formation of axons in vivo . Our results show that Igf1R/InsR function is required for the normal development of the embryonic hippocampus and cingulate cortex while the lateral cortex does not show apparent defects in the Igf1r ; Insr knockout. In the cingulate cortex, the number of intermediate progenitors and deep layer neurons is reduced and the corpus callosum is absent at E17. However, cortical organization and axon formation are not impaired in knockout embryos. In culture, cortical and hippocampal neurons from Igf1r ; Insr knockout embryos extend an axon but the length of this axon is severely reduced. Our results indicate that Igf1R/InsR function is required for brain development in a region-specific manner and promotes axon growth but is not essential for neuronal polarization and migration in the developing brain.

Highlights d Ankyrin B acts upstream of the kinases SadA and SadB and activates them d The loss of AnkB or SadA/B increases mitochondria length in cortical axons d SadA/B are required for the balance of fission and fusion in cortical axons d Loss of tau phosphorylation by SadA/B excessively stabilizes actin filaments

The highly conserved GTPase Cdc42 is an essential regulator of cell polarity and promotes exocytosis through the exocyst complex in budding yeast andDrosophila. In mammals, this function is performed by the closely related GTPase TC10, whereas mammalian Cdc42 does not interact with the exocyst. Axon formation is facilitated by the exocyst complex that tethers vesicles before their fusion to expand the plasma membrane. This function depends on the recruitment of the Exo70 subunit to the plasma membrane. Alternative splicing generates two Cdc42 isoforms that differ in their C-terminal 10 amino acids. Our results identify an isoform-specific function of Cdc42 in neurons. We show that the brain-specific Cdc42b isoform, in contrast to the ubiquitous isoform Cdc42u, can interact with Exo70. Inactivation of Arhgef7 or Cdc42b interferes with the exocytosis of post-Golgi vesicles in the growth cone. Cdc42b regulates exocytosis and axon formation downstream of its activator Arhgef7. Thus, the function of Cdc42 in regulating exocytosis is conserved in mammals but specific to one isoform.

Neurons are highly polarized cells that display characteristic differences in the organization of their organelles in axons and dendrites. Mitochondria are of particular importance for neuronal homeostasis due to their high metabolic demand. The kinases SadA and SadB (SadA/B) promote the formation of distinct axonal and dendritic extensions during the development of cortical and hippocampal neurons. Here, we show that SadA/B are required for the axon-specific dynamics of mitochondria. The interaction with Ankyrin B (AnkB) stimulates the activity of SadA/B that function as regulators of mitochondrial dynamics through the phosphorylation of Tau. Suppression of SadA/B or AnkB in cortical neurons induces the elongation of mitochondria by disrupting the balance of fission and fusion. The normal dynamics of axonal mitochondria could be restored by mild actin destabilization. Thus, the elongation after a loss of SadA/B results from an excessive stabilization of actin filaments and reduction of Drp1 recruitment to mitochondria.

The highly conserved GTPase Cdc42 is an essential regulator of cell polarity and promotes exocytosis through the exocyst complex in budding yeast and Drosophila. In mammals, this function is performed by the closely related GTPase TC10 while mammalian Cdc42 does not interact with the exocyst. Axon formation is facilitated by the exocyst complex that tethers vesicles prior to their fusion to expand the plasma membrane. This function depends on the recruitment of the Exo70 subunit to the plasma membrane. Alternative splicing generates two Cdc42 isoforms that differ in their C-terminal 10 amino acids. Our results identify an isoform-specific function of Cdc42 in neurons. We show that the brain-specific Cdc42b isoform, in contrast to the ubiquitous isoform Cdc42u, can interact with Exo70. Cdc42b regulates exocytosis and axon formation downstream of its activator Arhgef7. Thus, the function of Cdc42 in regulating exocytosis is conserved in mammals but specific to one isoform.