CNS deletion of Pten in the mouse has revealed its roles in controlling cell size and number, thus providing compelling etiology for macrocephaly and Lhermitte-Duclos disease. PTEN mutations in individuals with autism spectrum disorders (ASD) have also been reported, although a causal link between PTEN and ASD remains unclear. In the present study, we deleted Pten in limited differentiated neuronal populations in the cerebral cortex and hippocampus of mice. Resulting mutant mice showed abnormal social interaction and exaggerated responses to sensory stimuli. We observed macrocephaly and neuronal hypertrophy, including hypertrophic and ectopic dendrites and axonal tracts with increased synapses. This abnormal morphology was associated with activation of the Akt/mTor/S6k pathway and inactivation of Gsk3beta. Thus, our data suggest that abnormal activation of the PI3K/AKT pathway in specific neuronal populations can underlie macrocephaly and behavioral abnormalities reminiscent of certain features of human ASD.

The signal transduction cascade comprising Raf, mitogen-activated protein (MAP) kinase kinase (MEK) and MAP kinase is a Ras effector pathway that mediates diverse cellular responses to environmental cues and contributes to Ras-dependent oncogenic transformation. Here we report that the Ras effector protein Impedes Mitogenic signal Propagation (IMP) modulates sensitivity of the MAP kinase cascade to stimulus-dependent activation by limiting functional assembly of the core enzymatic components through the inactivation of KSR, a scaffold/adaptor protein that couples activated Raf to its substrate MEK. IMP is a Ras-responsive E3 ubiquitin ligase that, on activation of Ras, is modified by auto-polyubiquitination, which releases the inhibition of Raf-MEK complex formation. Thus, Ras activates the MAP kinase cascade through simultaneous dual effector interactions: induction of Raf kinase activity and derepression of Raf-MEK complex formation. IMP depletion results in increased stimulus-dependent MEK activation without alterations in the timing or duration of the response. These observations suggest that IMP functions as a threshold modulator, controlling sensitivity of the cascade to stimulus and providing a mechanism to allow adaptive behaviour of the cascade in chronic or complex signalling environments.

The Ras effector and E3 ligase family member IMP (impedes mitogenic signal propagation) acts as a steady-state resistor within the Raf-MEK-ERK kinase module. IMP concentrations are directly regulated by Ras, through induction of autoubiquitination, to permit productive Raf-MEK complex assembly. Inhibition of Raf-MEK pathway activation by IMP occurs through the inactivation of KSR, a scaffold/adapter protein that couples activated Raf to its substrate MEK1. The capacity of IMP to inhibit signal propagation through Raf to MEK is, in part, a consequence of disrupting KSR1 homo-oligomerization and c-Raf-B-Raf hetero-oligomerization. These observations suggest that IMP functions as a threshold modulator, controlling sensitivity of the cascade to stimulus by directly limiting the assembly of functional KSR1-dependent Raf-MEK complexes.The Raf-MEK 2 -ERK kinase cascade is a fundamental component of both normal and pathological cell regulatory networks. ERK activation ultimately results in modulation of gene transcription, and its amplitude, duration, and subcellular compartmentalization are critical determinants of the biological response (1, 2). Non-catalytic scaffold proteins can generate higher order molecular organization to modulate the assembly, activation, and compartmentalization of MAPK cascades (3-5). Specificity and fidelity may be achieved not only through pre-assembled complexes but also by locally assigning those complexes to distinct receptors or other activators for stimulus-specific induction of the appropriate pathway.We have described a Ras effector, IMP (impedes mitogenic signal propagation), which negatively regulates ERK activation by limiting formation of Raf-MEK complexes (6). The mechanism of inhibition appears to be through inactivation of KSR1, a scaffold protein that couples activated Raf to its substrate MEK. IMP is a Ras-responsive E3 ubiquitin ligase. Upon Ras activation, IMP is modified by autopolyubiquitination, which relieves its inhibitory effects on KSR. Thus, Ras activates the Raf-MAPK cascade through dual effector interactions: induction of Raf protein kinase activity concomitant with liberation of KSR-dependent Raf-MEK complex assembly. This relationship potentially provides a mechanism to tether MAPK mobilization to appropriate Ras activation thresholds. Domain Organization and Sequence Conservation of IMPIMP is a unique protein in terms of its predicted functional domains, domain structure, and high degree of conservation across species. The primary amino acid sequence of IMP predicts a RING-H2 domain, followed by a UBP-ZnF and leucine heptad repeats predicted to form a coiled-coil (SMART, smart.embl-heidelberg.de). This domain architecture is very similar to the RBCC family of proteins that include the proto-oncogenes PML and TIF-1 (7), with the exception of a UBP-ZnF in place of a B-box zinc finger. IMP is the only identifiable protein in the current data bases that has the RING-UBP-ZnF-coiled-coil structure.The sequential tripartite domain organization of RBCC proteins has be...

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