Constitutive activation of Ras prevents lesion-induced neuronal degeneration and promotes proliferation of synapses

R Heumann PhD^a, F Thor^a, G Seeger PhD^c, G Barmashenko PhD^b, K Palm MD^c, T Mittmann PhD^b, L Yan MD^b, M Hümmeke^b, J Behrbohm MD^c, R Kamdem MD^c, MK Brückner PhD^c, U Gärtner PhD^c, M Holzer PhD^c, T Arendt PhD^c

^aMolecular Neurobiochemistry, ^bNeurophysiology; Ruhr-University, Bochum, Germany, ^cDepartment of Neuroanatomy, Paul Flechsig Institute of Brain Research, University Leipzig, Leipzig, Germany

AIM: Ras proteins are universally expressed intracellular membrane anchored proteins cycling between the inactive GDP-bound and signalling competent GTP-bound conformations. In order to study the function of Ras in brain neurons we have developed a synRas animal model expressing activated Val12 Ha-Ras selectively in neurons (Heumann et al,.2000 J Cell Biol 151, 1537-1548). METHODS: As shown by Ras pull down assays and Westernblot analysis neuronal Ras was constitutively active and resulted in a corresponding activating phosphorylation of mitogen activated kinase.Perycaryal volume of cortical neurons was determined on Nissl stained sections by the physical dissector method. The size and ramifications of dendritic arborization was analysed after iontophoretic application of lucifer yellow. The number and size of synaptic contacts was established in the somatosensory cortex using a Zeiss EM 912 electron microscope. RESULTS: Ras-transgene protein expression increased postnatally, reaching 4-5 fold elevation at day 40 persisting at this level, thereafter. In synRas mice degeneration of motorneurons was completely prevented after facial nerve lesion. Furthermore, neurotoxin-induced degeneration of dopaminergic substantia nigra neurons and their striatal projections was greatly attenuated. In the intact brain we investigated the role of Ras for neuronal connectivity: Adult cortical pyramidal neurons showed complex structural rearrangements associated with an increased size and ramification of dendritic arborisation and with constitutive activation of the Rho-family protein Cdc42. Dendritic spine density was elevated and correlated with an increased number of synapses. This was associated with an enhancement of amplitudes in the extracellular field potential recorded from neocortical layer II/III in brain slices. Furthermore, theta burst-induced long term potentiation was enhanced indicating that Ras is able to modify changes in synaptic transmission that are discussed as correlatives for learning processes. CONCLUSIONS: We conclude that neuronal Ras is a common switch of plasticity in adult mammalian brain sculpturing neuronal architecture in concert with tuning synaptic efficacies perhaps even in normal brai

Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Nice, France; February 7 - 10, 2004; in oral session 994 (Signaling pathways - Part II).