Tuba Activates Cdc42 during Neuronal Polarization Downstream of the Small GTPase Rab8a

Abstract

The acquisition of neuronal polarity is a complex molecular process that depends on changes in cytoskeletal dynamics and directed membrane traffic, regulated by the Rho and Rab families of small GTPases, respectively. However, during axon specification, a molecular link that couples these protein families has yet to be identified. In this paper, we describe a new positive feedback loop between Rab8a and Cdc42, coupled by Tuba, a Cdc42-specific guanine nucleotide-exchange factor (GEF), that ensures a single axon generation in rodent hippocampal neurons from embryos of either sex. Accordingly, Rab8a or Tuba gain-of-function generates neurons with supernumerary axons whereas Rab8a or Tuba loss-of-function abrogated axon specification, phenocopying the well-established effect of Cdc42 on neuronal polarity. Although Rab8 and Tuba do not interact physically, the activity of Rab8 is essential to generate a proximal to distal axonal gradient of Tuba in cultured neurons. Tuba-associated and Rab8a-associated polarity defects are also evidenced in vivo, since dominant negative (DN) Rab8a or Tuba knock-down impairs cortical neuronal migration in mice. Our results suggest that Tuba coordinates directed vesicular traffic and cytoskeleton dynamics during neuronal polarization.

Crosstalk between the Rho and Rab family of small GTPases in neurodegenerative disorders

“…In the neuroblastoma cell line, it was demonstrated that Rab8a activity promotes Cdc42 activation through Tuba. Tuba and Rab8a also regulate the morphology of migrating neurons (Urrutia et al, 2021; Figure 3B). Aside from Rab8a, Rab35 has also been demonstrated to modulate Cdc42 activity in neurons, as Rab35-induced neurite growth dependent on Cdc42 and Rac1.…”

“…Similarly, suppression of Rab11 also resulted in delayed neuronal migration (Kawauchi, 2011). Both exocytic and endocytic pathways are critical for neuron differentiation, elongation of neuronal processes, and their migration (Kawauchi, 2011;Urrutia et al, 2021). Hence Cdc42 and Rab GTPases might crosstalk in modulating neuronal morphology, migration, and polarity (Kawauchi, 2011;Villarroel-Campos et al, 2016;Urrutia et al, 2021).…”

“…Both exocytic and endocytic pathways are critical for neuron differentiation, elongation of neuronal processes, and their migration (Kawauchi, 2011;Urrutia et al, 2021). Hence Cdc42 and Rab GTPases might crosstalk in modulating neuronal morphology, migration, and polarity (Kawauchi, 2011;Villarroel-Campos et al, 2016;Urrutia et al, 2021). Cdc42 induces filopodia formation through its effector protein Wiskott-Aldrich syndrome protein (WASP).…”

“…Additionally, in a study using mouse hippocampal neurons and N1E-115 neuroblastoma cell line, it was demonstrated that Tuba, a Cdc42 GEF, was required for neuronal polarization, as shRNA knockdown of Tuba increased the number of unpolarized neurons as well as decreased axonal length (Urrutia et al, 2021). Rab8a activity results in an increased density of Tuba proteins at the distal end of the axon.…”

Perspectives on Mechanisms Supporting Neuronal Polarity From Small Animals to Humans

“…In fact, the small GTPase Rap1b, an upstream activator of CDC-42 is enriched in the axonal tip of stage 3 neurons, suggesting a local regulation ( Schwamborn and Püschel, 2004 ). Along this line of evidence, a recent article reported that the dynamin-binding protein (DNMBP) TUBA segregates into the axon working as GEF for CDC-42 ( Urrutia et al, 2021 ). Collectively, these observations suggest local regulations favoring CDC-42 activation in axons.…”