Ndfip1 represses cell proliferation by controlling Pten localization and signaling specificity.

Pten controls a signaling axis that is implicated to regulate cell proliferation, growth, survival, migration, and metabolism. The molecular mechanisms underlying the 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.

Nedd4 family interacting protein 1 (Ndfip1) is required for ubiquitination and nuclear trafficking of BRCA1-associated ATM activator 1 (BRAT1) during the DNA damage response.

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.

PTEN secretion in exosomes.

PTEN was discovered as a membrane-associated tumor suppressor protein nearly two decades ago, but the concept that it can be secreted and taken up by recipient cells is revolutionary. Since then, various laboratories have reported that PTEN is indeed secreted and available for uptake by other cells in at least two different guises. First, PTEN may be packaged and exported within extracellular vesicles (EV) called exosomes.

Rab5 and Ndfip1 are involved in Pten ubiquitination and nuclear trafficking.

The spatial regulation of Pten is critical for its role as a tumour suppressor with both nuclear and cytoplasmic locations being implicated with distinct functions. In the cytoplasm, Pten plays a central role in opposing PI3K/Akt cell signalling, whereas in the nucleus, Pten is important for maintaining genome stability and enhancing the tumour suppressor activity of APC-CDH1. Despite this diversity in protein function at different subcellular locations, there is limited knowledge on how Pten is able to find different cellular niches.

Nuclear trafficking of Pten after brain injury leads to neuron survival not death.

There is controversy whether accumulation of the tumor suppressor PTEN protein in the cell nucleus under stress conditions such as trauma and stroke causes cell death. A number of in vitro studies have reported enhanced apoptosis in neurons possessing nuclear PTEN, with the interpretation that its nuclear phosphatase activity leads to reduction of the survival protein phospho-Akt. However, there have been no in vivo studies to show that nuclear PTEN in neurons under stress is detrimental.

Ndfip1 expression in developing neurons indicates a role for protein ubiquitination by Nedd4 E3 ligases during cortical development.

During development, protein substrates need to be removed and degraded when they are no longer required. The E3 ubiquitin ligases, including Nedd4 family proteins, are a major group of enzymes responsible for adding ubiquitin chains to protein substrates prior to their degradation. Ndfip1 (Nedd4 family-interacting protein 1) is an adaptor and activator for Nedd4-family ubiquitin ligases for increasing substrate specificity.

Ndfip1 is required for the development of pyramidal neuron dendrites and spines in the neocortex.

Ubiquitin ligases of the Nedd4 family are important for axon and dendrite development, but little is known about their adaptor, Nedd4 family-interacting protein 1 (Ndfip1), that is responsible for their enzymatic activation. To study the function of Ndfip1 in cortical development, we generated a conditional knock-out (conditional KO) in neurons. The Ndfip1 conditional KO mice were viable; however, cortical neurons in the adult brain exhibited atrophic characteristics, including stunted dendritic arbors, blebbing of dendrites, and fewer dendritic spines.

The tumor suppressor PTEN is exported in exosomes and has phosphatase activity in recipient cells.

Exosomes are microvesicles of endosomal origin that are secreted, and their contents (proteins, lipids, DNA, or microRNAs) can alter the physiological states of recipient cells. We demonstrated that phosphatase and tensin homolog deleted on chromosome 10 (PTEN), a tumor suppressor protein normally localized in the cytoplasm and nucleus, was secreted in exosomes. Secreted PTEN was internalized by recipient cells with resultant functional activity, which resulted in reduced phosphorylation of the serine and threonine kinase Akt and reduced cellular proliferation.