Editors' ChoiceNeuroscience

Branching with Ubiquitin

Science Signaling  16 Feb 2010:
Vol. 3, Issue 109, pp. ec52
DOI: 10.1126/scisignal.3109ec52

Neurons form two kinds of processes—axons, the signal-sending processes, and dendrites, the signal-receiving processes. Branching of each of these types of neurite extensions is critical for the proper development of neuronal circuits. Two groups report that the E3 ubiquitin ligase Nedd4 contributes to both dendritic (Kawabe et al.) and axonal (Drinjakovic et al.) branching. Kawabe et al. examined the function of Nedd4-1 in mice, whereas Drinjakovic et al. investigated its function in Xenopus. In mice, Nedd4-1 was found to ubiquitylate the guanosine triphosphatase Rap1 to inhibit its activity and thereby promote dendritic arborization. In Xenopus, Nedd4 targeted the lipid and protein phosphatase PTEN to promote axonal branching.

Knockout mice exhibited embryonic lethality; therefore, to examine Nedd4-1 in vivo, Kawabe et al. generated conditional knockout mice in cerebral and hippocampal glutamatergic neurons. The cerebrum was smaller than that of control mice, and dendritic branching of pyramidal cells of the hippocampus was also reduced. To understand the mechanism by which Nedd4-1 affected dendritic branching, Kawabe et al. examined the properties of cortical neurons cultured from wild-type or Nedd4-1–knockout embryos. Neurons lacking Nedd4-1 were smaller and exhibited decreased dendrite size and complexity. The Nedd4-1–deficient neurons exhibited smaller evoked excitatory postsynaptic current (EPSC) amplitudes and reduced frequency of miniature EPSCs (mEPSCs) without any change in mEPSC amplitude and without any reduction in the density of glutamate receptors, consistent with a reduction in the number of synapses. Staining for markers of synapses confirmed that there were fewer synapses in the Nedd4-1–deficient neuronal cultures. The kinase TNIK, which influences actin dynamics through its interaction with the guanosine triphosphatase Rap1, was identified by affinity chromatography as a protein that interacted with the WW domain of Nedd4-1. TNIK, Nedd4-1, and Rap1 colocalized in neurons and coimmunoprecipitated, which suggests that they form a complex. This interaction promoted the ubiquitylation of Rap2A, which ubiquitin-linkage specific antibodies indicated was the K63-linked mono- or di-ubiquitin involved in regulation, not proteasome-mediated degradation. Overexpression of dominant-negative Rap1 in the Nedd4-1–deficient neurons rescued the synaptic density and dendritogenesis defects, whereas knockdown of TNIK phenocopied the neurite development defect seen with Nedd4-1 deficiency. Thus, in mice, Nedd4-1 appears to ubiquitylate Rap1 to inhibit its activity toward TNIK and thus promote dendritic arborization.

Drinjakovic et al. examined the role of the ubiquitin-proteasome system in axonal branching of retinal ganglion cells (RGCs). Electroporation of a mutated form of ubiquitin that cannot form K48-linked polyubiquitin chains into the embryonic retina did not affect axonal pathfinding but inhibited terminal branching of RGCs upon reaching the tectum. Drinjakovic et al. found that Nedd4 and its substrate PTEN were present in the axons of the RGCs at the appropriate time to influence branching. Furthermore, introduction of a dominant-negative (catalytically inactive) Nedd4 mutant or knockdown of Nedd4 reduced RGCs’ axonal branching, and both treatments also increased the abundance of PTEN in the RGCs. Overexpression of PTEN also reduced axonal branching of the RGCs, and knockdown of PTEN rescued the branching defects seen when Nedd4 was knocked down. Netrin-1 is an extracellular factor that affects terminal branching of retinal axons. Application of netrin-1 to cultured retinal ganglion cells decreased the abundance of PTEN and triggered growth cone collapse, both of which were blocked by coapplication of an inhibitor of the proteasome. Thus, in Xenopus retina, Nedd4 appears to mediate ubiquitination of PTEN that triggers its degradation and thus promotes axonal branching.

H. Kawabe, A. Neeb, K. Dimova, S. M. Young Jr., M. Takeda, S. Katsurabayashi, M. Mitkovski, O. A. Malakhova, D.-E. Zhang, M. Umikawa, K.-i. Kariya, S. Goebbels, K.-A. Nave, C. Rosenmund, O. Jahn, J. Rhee, N. Brose, Regulation of Rap2A by the ubiquitin ligase Nedd4-1 controls neurite development. Neuron 65, 358–372 (2010). [Online Journal]

J. Drinjakovic, H. Jung, D. S. Campbell, L. Strochlic, A. Dwivedy, C. E. Holt, E3 ligase Nedd4 promotes axon branching by downregulating PTEN. Neuron 65, 341–357 (2010). [Online Journal]

A. DiAntonio, Nedd4 branches out. Neuron 65, 293–294 (2010). [Online Journal]