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Pten controls a signaling axis that is implicated to regulate cell proliferation, growth, survival, migration, and metabolism. The molecular mechanisms underlyingthe specificity of Pten responses to such diverse cellular functions are currently poorly understood. Here we report the control of Pten activity and signaling specificity during the cell cycle by Ndfip1 regulation of Pten spatial distribution. Genetic deletion of Ndfip1 resulted in a loss of Pten nuclear compartmentalization and increased cell proliferation, despite cytoplasmic Pten remaining active in regulating PI3K/Akt signaling. Cells lacking nuclear Pten were found to have dysregulated levels of Plk1 and cyclin D1 that could drive cell proliferation. In vivo, transgene expression of Ndfip1 in the developing brain increased nuclear Pten and lengthened the cell cycle of neuronal progenitors, resulting in microencephaly. Our results show that local partitioning of Pten from the cytoplasm to the nucleus represents a key mechanism contributing to the specificity of Pten signaling during cell proliferation. [ABSTRACT FROM AUTHOR]

During injury, cells are vulnerable to apoptosis from a variety of stress conditions including DNA damage causing double-stranded breaks. Without repair, these breaks lead to aberrations in DNA replication and transcription, leading to apoptosis. A major response to DNA damage is provided by the protein kinase ATM (ataxia telangiectasia mutated) that is capable of commanding a plethora of signaling networks for DNA repair, cell cycle arrest, and even apoptosis. A key element in the DNA damage response is the mobilization of activating proteins into the cell nucleus to repair damaged DNA. BRAT1 is one of these proteins, and it functions as an activator of ATM by maintaining its phosphorylated status while also keeping other phosphatases at bay. However, it is unknown how BRAT1 is trafficked into the cell nucleus to maintain ATM phosphorylation. Here we demonstrate that Ndfip1-mediated ubiquitination of BRAT1 leads to BRAT1 trafficking into the cell nucleus. Without Ndfip1, BRAT1 failed to translocate to the nucleus. Under genotoxic stress, cells showed increased expression of both Ndfip1 and phosphorylated ATM. Following brain injury, neurons show increased expression of Ndfip1 and nuclear translocation of BRAT1. These results point to Ndfip1 as a sensor protein during cell injury and Ndfip1 up-regulation as a cue for BRAT1 ubiquitination by Nedd4 E3 ligases, followed by nuclear translocation of BRAT1. [ABSTRACT FROM AUTHOR]

PTEN (phosphatase and tensin homologue deleted on chromosome TEN) is the major negative regulator of phosphatidylinositol 3-kinase signaling and has cell-specific functions including tumor suppression. Nuclear localization of PTEN is vital for tumor suppression; however, outside of cancer, the molecular and physiological events driving PTEN nuclear entry are unknown. In this paper, we demonstrate that cytoplasmic Pten was translocated into the nuclei of neurons after cerebral ischemia in mice. Critically, this transport event was dependent on a surge in the Nedd4 family-interacting protein 1 (Ndfip1), as neurons in Ndfip1-deficient mice failed to import Pten. Ndfip1 binds to Pten, resulting in enhanced ubiquitination by Nedd4 E3 ubiquitin ligases. In vitro, Ndfip1 overexpression increased the rate of Pten nuclear import detected by photobleaching experiments, whereas Ndfip1-/- fibroblasts showed negligible transport rates. In vivo, Ndfip1 mutant mice suffered larger infarct sizes associated with suppressed phosphorylated Akt activation. Our findings provide the first physiological example of when and why transient shuttling of nuclear Pten occurs and how this process is critical for neuron survival. [ABSTRACT FROM AUTHOR]