Transgenic activation of Ras in neurons promotes hypertrophy and protects from lesion-induced degeneration.

Rolf Heumann,Petra Wahle,Karl Schellander,Bastian Hengerer,Hartmut Berns,Rigobert H Kamdem,Christoph Goemans,Daniela Bartsch,Erwin F Wagner,Peter R Allegrini,Frank Narz,Thomas Arendt,Kirstin Obst-Pernberg,Peter C Waldmeier,Kurt Lingenhöhl

doi:10.1083/jcb.151.7.1537

Abstract

Ras is a universal eukaryotic intracellular protein integrating extracellular signals from multiple receptor types. To investigate its role in the adult central nervous system, constitutively activated V12-Ha-Ras was expressed selectively in neurons of transgenic mice via a synapsin promoter. Ras-transgene protein expression increased postnatally, reaching a four- to fivefold elevation at day 40 and persisting at this level, thereafter. Neuronal Ras was constitutively active and a corresponding activating phosphorylation of mitogen-activated kinase was observed, but there were no changes in the activity of phosphoinositide 3-kinase, the phosphorylation of its target kinase Akt/PKB, or expression of the anti-apoptotic proteins Bcl-2 or Bcl-X(L). Neuronal Ras activation did not alter the total number of neurons, but induced cell soma hypertrophy, which resulted in a 14.5% increase of total brain volume. Choline acetyltransferase and tyrosine hydroxylase activities were increased, as well as neuropeptide Y expression. Degeneration of motorneurons was completely prevented after facial nerve lesion in Ras-transgenic mice. Furthermore, neurotoxin-induced degeneration of dopaminergic substantia nigra neurons and their striatal projections was greatly attenuated. Thus, the Ras signaling pathway mimics neurotrophic effects and triggers neuroprotective mechanisms in adult mice. Neuronal Ras activation might become a tool to stabilize donor neurons for neural transplantation and to protect neuronal populations in neurodegenerative diseases.

Highlights

The Ras/mitogen-activated protein kinase (MAPK)1 pathway is an evolutionary conserved signaling pathway found in all eukaryotic cells, including yeast (Marshall, 1996)
The transgene encoded for a dicistronic mRNA, which directs the synthesis of two separate proteins, V12-Ha-Ras and LacZ (Fig. 1 A), and was tested for its Ras activity by transfection and induction of fiber outgrowth in PC12 cells
By using the synapsin I promoter, expression of the Ras-TG was restricted to neurons, giving rise to healthy transgenic mice with a normal life span

Summary

Introduction

The Ras/mitogen-activated protein kinase (MAPK) (extracellular signal–regulated kinase [ERK]) pathway is an evolutionary conserved signaling pathway found in all eukaryotic cells, including yeast (Marshall, 1996). The major pathways induced by Ras are Raf-1/B-raf/MAPK, Ral-GDS/phospholipase D/Rho, and phosphoinositide 3-kinase (PI 3-kinase)/Akt-1(phosphoinositide-dependent protein kinase [PKB]) (Katz and McCormick, 1997). The latter signaling branch has been shown to promote survival in many neuronal systems (Williams and Doherty, 1999). In addition to PI 3-kinase, MAPK (ERK) activation is necessary for neuronal survival, but this pathway may regulate distinct apoptotic signaling cascades (Xue et al, 2000). MAPK (ERK) activates the ribosomal Rskkinase, which, in turn, phosphorylates and thereby inactivates the proapoptotic activity of Bad (Bonni et al, 1999) The latter belongs to the Bcl-2 family of apoptosis-regulating proteins, forming homotypic or heterotypic dimers. We asked if neuronal Ras activation could mimic typical neurotrophic effects, such as neuronal hypertrophy, induction of neuropeptide Y (NPY), or increase in neurotransmitter synthesizing enzyme activities

Materials and Methods

Results

Discussion