Transgenic Activation of Ras in Neurons Promotes Hypertrophy and Protects from Lesion-Induced Degeneration

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-XL. 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.

Modulation of Presynaptic Plasticity and Learning by the H-ras/Extracellular Signal-Regulated Kinase/Synapsin I Signaling Pathway

“…The H-ras G12V transgene is a potent upstream activator of ERK in postmitotic neurons (Rosen et al, 1994;Heumann et al, 2000;Koh et al, 2002). In addition, the late postnatal onset of the ␣CaMKII promoter restricts transgene expression to postmitotic neurons, thereby minimizing putative developmental effects caused by the oncogenic properties of the transgene.…”

“…Ras -mediated activation of neuronal ERK (Rosen et al, 1994;Heumann et al, 2000;Iida et al, 2001;Adams and Sweatt, 2002;Koh et al, 2002;Komiyama et al, 2002), H-ras G12V mice had increased levels of phosphorylated ERK1 and ERK2 without a change in total ERK1/2 expression (Fig. 3 A, B) (pERK1, F (1,9) ϭ 24.9, p Ͻ 0.001; pERK2, F (1,9) ϭ 10.2, p Ͻ 0.01; total ERK1, F (1,9) ϭ 0.26, p ϭ 0.62; total ERK2, F (1,9) ϭ 0.16, p ϭ 0.74).…”

Scaling properties of pyramidal neurons in mice neocortex

“…We used data derived from experiments with three synRas mice aged nine months (see [3] for the establishment of the synRas mouse line), as well as with three wildtype mice of the same age. The details of experimental procedures have been described elsewhere [4].…”

“…In a specific mouse mutant (introduced by [3] and referred to as synRas mice), a permanently active Ras protein (Val12-Ha-Ras) in post-mitotic neurons is expressed. In this mutant the expression of Ras starts post-natally around day 15, when neurons are post-mitotic and the majority of synaptic contacts has been established.…”

“…In particular cortical pyramidal neurons express the synRas construct at high levels resulting in a dramatically enlarged volume of the cortical pyramidal cells which is mainly caused by increased dendritic diameter and tree degree [4,5]. The number of neurons, however, remains unchanged [3].…”