FBF represses the Cip/Kip cell-cycle inhibitor CKI-2 to promote self-renewal of germline stem cells in C. elegans.
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ABSTRACT:
SUBMITTER: Kalchhauser I
PROVIDER: S-EPMC3173791 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:Stem cells support tissue maintenance, but the mechanisms that coordinate the rate of stem cell self-renewal with differentiation at a population level remain uncharacterized. We find that two PUF family RNA-binding proteins FBF-1 and FBF-2 have opposite effects on Caenorhabditis elegans germline stem cell dynamics: FBF-1 restricts the rate of meiotic entry, while FBF-2 promotes both cell division and meiotic entry rates. Antagonistic effects of FBFs are mediated by their distinct activities toward the shared set of target mRNAs, where FBF-1-mediated post-transcriptional control requires the activity of CCR4-NOT deadenylase, while FBF-2 is deadenylase-independent and might protect the targets from deadenylation. These regulatory differences depend on protein sequences outside of the conserved PUF family RNA-binding domain. We propose that the opposing FBF-1 and FBF-2 activities serve to modulate stem cell division rate simultaneously with the rate of meiotic entry.
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Project description:The mammalian CIP/KIP family of cyclin-dependent kinase (CDK) inhibitors (CKIs) comprises three proteins--p21(Cip1/WAF1), p27(Kip1), and p57(Kip2)--that bind and inhibit cyclin-CDK complexes, which are key regulators of the cell cycle. CIP/KIP CKIs have additional independent functions in regulating transcription, apoptosis and actin cytoskeletal dynamics. These divergent functions are performed in distinct cellular compartments and contribute to the seemingly contradictory observation that the CKIs can both suppress and promote cancer. Multiple ubiquitin ligases (E3s) direct the proteasome-mediated degradation of p21, p27 and p57. This review analyzes recent data highlighting our current understanding of how distinct E3 pathways regulate subpopulations of the CKIs to control their diverse functions.
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Project description:The cortactin oncoprotein is frequently overexpressed in head and neck squamous cell carcinoma (HNSCC), often due to amplification of the encoding gene (CTTN). While cortactin overexpression enhances invasive potential, recent research indicates that it also promotes cell proliferation, but how cortactin regulates the cell cycle machinery is unclear. In this article we report that stable short hairpin RNA-mediated cortactin knockdown in the 11q13-amplified cell line FaDu led to increased expression of the Cip/Kip cyclin-dependent kinase inhibitors (CDKIs) p21(WAF1/Cip1), p27(Kip1), and p57(Kip2) and inhibition of S-phase entry. These effects were associated with increased binding of p21(WAF1/Cip1) and p27(Kip1) to cyclin D1- and E1-containing complexes and decreased retinoblastoma protein phosphorylation. Cortactin regulated expression of p21(WAF1/Cip1) and p27(Kip1) at the transcriptional and posttranscriptional levels, respectively. The direct roles of p21(WAF1/Cip1), p27(Kip1), and p57(Kip2) downstream of cortactin were confirmed by the transient knockdown of each CDKI by specific small interfering RNAs, which led to partial rescue of cell cycle progression. Interestingly, FaDu cells with reduced cortactin levels also exhibited a significant diminution in RhoA expression and activity, together with decreased expression of Skp2, a critical component of the SCF ubiquitin ligase that targets p27(Kip1) and p57(Kip2) for degradation. Transient knockdown of RhoA in FaDu cells decreased expression of Skp2, enhanced the level of Cip/Kip CDKIs, and attenuated S-phase entry. These findings identify a novel mechanism for regulation of proliferation in 11q13-amplified HNSCC cells, in which overexpressed cortactin acts via RhoA to decrease expression of Cip/Kip CDKIs, and highlight Skp2 as a downstream effector for RhoA in this process.
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Project description:Control over cell cycle exit is fundamental to the normal generation of the wide array of distinct cell types that comprise the mature vertebrate CNS. Here, we demonstrate a critical role for Cip/Kip class cyclin-kinase inhibitory (CKI) proteins in regulating this process during neurogenesis in the embryonic spinal cord. Using immunohistochemistry, we show that all three identified Cip/Kip CKI proteins are expressed in both distinct and overlapping populations of nascent and post-mitotic neurons during early neurogenesis, with p27(Kip1) having the broadest expression, and both p57(Kip2) and p21(Cip1) showing transient expression in restricted populations. Loss- and gain-of-function approaches were used to establish the unique and redundant functions of these proteins in spinal cord neurogenesis. Using genetic lineage tracing, we provide evidence that, in the absence of p57, nascent neurons re-enter the cell cycle inappropriately but later exit to begin differentiation. Analysis of p57(Kip2);p27(Kip1) double mutants, where p21 expression is confined to only a small population of interneurons, demonstrates that Cip/Kip CKI-independent factors initiate progenitor cell cycle exit for the majority of interneurons generated in the developing spinal cord. Our studies indicate that p57 plays a critical cell-autonomous role in timing cell cycle exit at G1/S by opposing the activity of Cyclin D1, which promotes cell cycle progression. These studies support a multi-step model for neuronal progenitor cell cycle withdrawal that involves p57(Kip2) in a central role opposing latent Cyclin D1 and other residual cell cycle promoting activities in progenitors targeted for differentiation.
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Project description:RATIONALE:Mammalian heart has minimal regenerative capacity. In response to mechanical or pathological stress, the heart undergoes cardiac remodeling. Pressure and volume overload in the heart cause increased size (hypertrophic growth) of cardiomyocytes. Whereas the regulatory pathways that activate cardiac hypertrophy have been well-established, the molecular events that inhibit or repress cardiac hypertrophy are less known. OBJECTIVE:To identify and investigate novel regulators that modulate cardiac hypertrophy. METHODS AND RESULTS:Here, we report the identification, characterization, and functional examination of a novel cardiac Isl1-interacting protein (CIP). CIP was identified from a bioinformatic search for novel cardiac-expressed genes in mouse embryonic hearts. CIP encodes a nuclear protein without recognizable motifs. Northern blotting, in situ hybridization, and reporter gene tracing demonstrated that CIP is highly expressed in cardiomyocytes of developing and adult hearts. Yeast two-hybrid screening identified Isl1, a LIM/homeodomain transcription factor essential for the specification of cardiac progenitor cells in the second heart field, as a cofactor of CIP. CIP directly interacted with Isl1, and we mapped the domains of these two proteins, which mediate their interaction. We show that CIP represses the transcriptional activity of Isl1 in the activation of the myocyte enhancer factor 2C. The expression of CIP was dramatically reduced in hypertrophic cardiomyocytes. Most importantly, overexpression of CIP repressed agonist-induced cardiomyocyte hypertrophy. CONCLUSIONS:Our studies therefore identify CIP as a novel regulator of cardiac hypertrophy.
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Project description:FBF-1 and FBF-2 (collectively FBF) are two nearly identical Puf-domain RNA-binding proteins that regulate the switch from mitosis to meiosis in the C. elegans germline. In germline stem cells, FBF prevents premature meiotic entry by inhibiting the expression of meiotic regulators, such as the RNA-binding protein GLD-1. Here, we demonstrate that FBF also directly inhibits the expression of structural components of meiotic chromosomes. HIM-3, HTP-1, HTP-2, SYP-2 and SYP-3 are components of the synaptonemal complex (SC) that forms between homologous chromosomes during meiotic prophase. In wild-type germlines, the five SC proteins are expressed shortly before meiotic entry. This pattern depends on FBF binding sites in the 3' UTRs of the SC mRNAs. In the absence of FBF or the FBF binding sites, SC proteins are expressed precociously in germline stem cells and their precursors. SC proteins aggregate and SC formation fails at meiotic entry. Precocious SC protein expression is observed even when meiotic entry is delayed in fbf mutants by reducing GLD-1. We propose that parallel regulation by FBF ensures that in wild-type gonads, meiotic entry is coordinated with just-in-time synthesis of synaptonemal proteins.
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