Generation and functional characterization of a conditional Pumilio2 null allele

PUM1 represses CDKN1B translation and contributes to prostate cancer progression

Posttranscriptional regulation of cancer gene expression programs plays a vital role in carcinogenesis; identifying the critical regulators of tumorigenesis and their molecular targets may provide novel strategies for cancer diagnosis and therapeutics. Highly conserved RNA-binding protein Pumilio-1 (PUM1) regulates mouse growth and cell proliferation, propelling us to examine its role in cancer. We found human PUM1 is highly expressed in a diverse group of cancer, including prostate cancer; enhanced PUM1 expression is also correlated with reduced survival among prostate cancer patients. Detailed expression analysis in twenty prostate cancer tissues showed enhanced expression of PUM1 at mRNA and protein levels. Knockdown of PUM1 reduced prostate cancer cell proliferation and colony formation, and subcutaneous injection of PUM1 knockdown cells led to reduced tumor size. Downregulation of PUM1 in prostate cancer cells consistently elevated cyclin-dependent kinase inhibitor 1B (CDKN1B) protein expression through increased translation but did not impact its mRNA level, while overexpression of PUM1 reduced CDKN1B protein level. Our finding established a critical role of PUM1 mediated translational control, particularly the PUM1-CDKN1B axis, in prostate cancer cell growth and tumorigenesis. We proposed that PUM1-CDKN1B regulatory axis may represent a novel mechanism for the loss of CDKN1B protein expression in diverse cancers and potential targets for therapeutics development.

Mammalian Pum1 and Pum2 Control Body Size via Translational Regulation of the Cell Cycle Inhibitor Cdkn1b

Body and organ size regulation in mammals involves multiple signaling pathways and remains largely enigmatic. Here, we report that Pum1 and Pum2, which encode highly conserved PUF RNA-binding proteins, regulate mouse body and organ size by post-transcriptional repression of the cell cycle inhibitor Cdkn1b. Binding of PUM1 or PUM2 to Pumilio binding elements (PBEs) in the 3^’ UTR of Cdkn1b inhibits translation, promoting G1-S transition and cell proliferation. Mice with null mutations in Pum1 and Pum2 exhibit gene dosage-dependent reductions in body and organ size, and deficiency for Cdkn1b partially rescues postnatal growth defects in Pum1^−/− mice. We propose that coordinated tissue-specific expression of Pum1 and Pum2, which involves auto-regulatory and reciprocal post-transcriptional repression, contributes to the precise regulation of body and organ size. Hence PUM-mediated post-transcriptional control of cell cycle regulators represents an additional layer of control in the genetic regulation of organ and body size.

Lin et al. show that the RNA-binding proteins PUM1 and PUM2 regulate translation of cell cycle proteins such as CDKN1B by binding to their 3^’ UTR and achieve precise control of organ and body size in a gene dosage-sensitive manner via auto and reciprocal gene expression regulation.

Diversity of RNA-Binding Proteins Modulating Post-Transcriptional Regulation of Protein Expression in the Maturing Mammalian Oocyte

The oocyte faces a particular challenge in terms of gene regulation. When oocytes resume meiosis at the end of the growth phase and prior to ovulation, the condensed chromatin state prevents the transcription of genes as they are required. Transcription is effectively silenced from the late germinal vesicle (GV) stage until embryonic genome activation (EGA) following fertilisation. Therefore, during its growth, the oocyte must produce the mRNA transcripts needed to fulfil its protein requirements during the active period of meiotic completion, fertilisation, and the maternal-to zygote-transition (MZT). After meiotic resumption, gene expression control can be said to be transferred from the nucleus to the cytoplasm, from transcriptional regulation to translational regulation. Maternal RNA-binding proteins (RBPs) are the mediators of translational regulation and their role in oocyte maturation and early embryo development is vital. Understanding these mechanisms will provide invaluable insight into the oocyte's requirements for developmental competence, with important implications for the diagnosis and treatment of certain types of infertility. Here, we give an overview of post-transcriptional regulation in the oocyte, emphasising the current knowledge of mammalian RBP mechanisms, and develop the roles of these mechanisms in the timely activation and elimination of maternal transcripts.

Essential requirement of mammalian Pumilio family in embryonic development

Mouse PUMILIO1 (PUM1) and PUMILIO2 (PUM2) belong to the PUF (Pumilio/FBF) family, a highly conserved RNA binding protein family whose homologues play critical roles in embryonic development and germ line stem cell maintenance in invertebrates. However, their roles in mammalian embryonic development and stem cell maintenance remained largely uncharacterized. Here we report an essential requirement of the Pum gene family in early embryonic development. A loss of both Pum1 and Pum2 genes led to gastrulation failure, resulting in embryo lethality at E8.5. Pum-deficient blastocysts, however, appeared morphologically normal, from which embryonic stem cells (ESCs) could be established. Both mutant ESCs and embryos exhibited reduced growth and increased expression of endoderm markers Gata6 and Lama1, making defects in growth and differentiation the likely causes of gastrulation failure. Furthermore, ESC Gata6 transcripts could be pulled down via PUM1 immunoprecipitation and mutation of conserved PUM-binding element on 3'UTR (untranslated region) of Gata6 enhanced the expression of luciferase reporter, implicating PUM-mediated posttranscriptional regulation of Gata6 expression in stem cell development and cell lineage determination. Hence, like its invertebrate homologues, mouse PUM proteins are conserved posttranscriptional regulators essential for embryonic and stem cell development.

Pumilio2 regulates synaptic plasticity via translational repression of synaptic receptors in mice

PUMILIO 2 (PUM2) is a member of Pumilio and FBF (PUF) family, an RNA binding protein family with phylogenetically conserved roles in germ cell development. The Drosophila Pumilio homolog is also required for dendrite morphogenesis and synaptic function via translational control of synaptic proteins, such as glutamate receptors, and recent mammalian studies demonstrated a similar role in neuronal culture with associated motor and memory abnormalities in vivo. Importantly, transgenic mice with PUM2 knockout show prominent epileptiform activity, and patients with intractable temporal lobe epilepsy and mice with pilocarpine-induced seizures have decreased neuronal PUM2, possibly leading to further seizure susceptibility. However, how PUM2 influences synaptic function in vivo and, subsequently, seizures is not known. We found that PUM2 is highly expressed in the brain, especially in the temporal lobe, and knockout of Pum2 (Pum2^-/- ) resulted in significantly increased pyramidal cell dendrite spine and synapse density. In addition, multiple proteins associated with excitatory synaptic function, including glutamate receptor 2 (GLUR2), are up-regulated in Pum2^-/- mice. The expression of GLUR2 protein but not mRNA is increased in the Pum2^-/- mutant hippocampus, Glur2 transcripts are increased in mutant polysome fractions, and overexpression of PUM2 led to repression of reporter expression containing the 3'Untranslated Region (3'UTR) of Glur2, suggesting translation of GLUR2 was increased in the absence of Pum2. Overall, these studies provide a molecular mechanism for the increased temporal lobe excitability observed with PUM2 loss and suggest PUM2 might contribute to intractable temporal lobe epilepsy.