In vivo association of the stability control protein alphaCP with actively translating mRNAs

RNA-Binding Proteins PCBP1 and PCBP2 Are Critical Determinants of Murine Erythropoiesis

We previously demonstrated that the two paralogous RNA-binding proteins PCBP1 and PCBP2 are individually essential for mouse development: Pcbp1-null embryos are peri-implantation lethal, while Pcbp2-null embryos lose viability at midgestation. Midgestation Pcbp2^-/- embryos revealed a complex phenotype that included loss of certain hematopoietic determinants. Whether PCBP2 directly contributes to erythropoietic differentiation and whether PCBP1 has a role in this process remained undetermined. Here, we selectively inactivated the genes encoding these two RNA-binding proteins during differentiation of the erythroid lineage in the developing mouse embryo. Individual inactivation of either locus failed to impact viability or blood formation. However, combined inactivation of the two loci resulted in midgestational repression of erythroid/hematopoietic gene expression, loss of blood formation, and fetal demise. Orthogonal ex vivo analyses of primary erythroid progenitors selectively depleted of these two RNA-binding proteins revealed that they mediate a combination of overlapping and isoform-specific impacts on hematopoietic lineage transcriptome, impacting both mRNA representation and exon splicing. These data lead us to conclude that PCBP1 and PCBP2 mediate functions critical to differentiation of the erythroid lineage.

Reciprocal regulation of hnRNP C and CELF2 through translation and transcription tunes splicing activity in T cells

RNA binding proteins (RBPs) frequently regulate the expression of other RBPs in mammalian cells. Such cross-regulation has been proposed to be important to control networks of coordinated gene expression; however, much remains to be understood about how such networks of cross-regulation are established and what the functional consequence is of coordinated or reciprocal expression of RBPs. Here we demonstrate that the RBPs CELF2 and hnRNP C regulate the expression of each other, such that depletion of one results in reduced expression of the other. Specifically, we show that loss of hnRNP C reduces the transcription of CELF2 mRNA, while loss of CELF2 results in decreased efficiency of hnRNP C translation. We further demonstrate that this reciprocal regulation serves to fine tune the splicing patterns of many downstream target genes. Together, this work reveals new activities of hnRNP C and CELF2, provides insight into a previously unrecognized gene regulatory network, and demonstrates how cross-regulation of RBPs functions to shape the cellular transcriptome.

The RNA-binding protein TcUBP1 up-regulates an RNA regulon for a cell surface-associated Trypanosoma cruzi glycoprotein and promotes parasite infectivity

The regulation of transcription in trypanosomes is unusual. To modulate protein synthesis during their complex developmental stages, these unicellular microorganisms rely largely on post-transcriptional gene expression pathways. These pathways include a plethora of RNA-binding proteins (RBPs) that modulate all steps of the mRNA life cycle in trypanosomes and help organize transcriptomes into clusters of post-transcriptional regulons. The aim of this work was to characterize an RNA regulon comprising numerous transcripts of trypomastigote-associated cell-surface glycoproteins that are preferentially expressed in the infective stages of the human parasite Trypanosoma cruzi. In vitro and in vivo RNA-binding assays disclosed that these glycoprotein mRNAs are targeted by the small trypanosomatid-exclusive RBP in T. cruzi, U-rich RBP 1 (TcUBP1). Overexpression of a GFP-tagged TcUBP1 in replicative parasites resulted in >10 times up-regulated expression of transcripts encoding surface proteins and in changes in their subcellular localization from the posterior region to the perinuclear region of the cytoplasm, as is typically observed in the infective parasite stages. Moreover, RT-quantitative PCR analysis of actively translated mRNAs by sucrose cushion fractionation revealed an increased abundance of these target transcripts in the polysome fraction of TcUBP1-induced samples. Because these surface proteins are involved in cell adherence or invasion during host infection, we also carried out in vitro infections with TcUBP1-transgenic trypomastigotes and observed that TcUBP1 overexpression significantly increases parasite infectivity. Our findings provide evidence for a role of TcUBP1 in trypomastigote stage-specific gene regulation important for T. cruzi virulence.

A cytosine-rich splice regulatory determinant enforces functional processing of the human α-globin gene transcript

The establishment of efficient and stable splicing patterns in terminally differentiated cells is critical to maintenance of specific functions throughout the lifespan of an organism. The human α-globin (hα-globin) gene contains 3 exons separated by 2 short introns. Naturally occurring α-thalassemia mutations that trigger aberrant splicing have revealed the presence of cryptic splice sites within the hα-globin gene transcript. How cognate (functional) splice sites are selectively used in lieu of these cryptic sites has remained unexplored. Here we demonstrate that the preferential selection of a cognate splice donor essential to functional splicing of the hα-globin transcript is dependent on the actions of an intronic cytosine (C)-rich splice regulatory determinant and its interacting polyC-binding proteins. Inactivation of this determinant by mutation of the C-rich element or by depletion of polyC-binding proteins triggers a dramatic shift in splice donor activity to an upstream, out-of-frame, cryptic donor. The essential role of the C-rich element in hα-globin gene expression is supported by its coevolution with the cryptic donor site in primate species. These data lead us to conclude that an intronic C-rich determinant enforces functional splicing of the hα-globin transcript, thus acting as an obligate determinant of hα-globin gene expression.

PolyC-binding proteins enhance expression of the CDK2 cell cycle regulatory protein via alternative splicing

The PolyC binding proteins (PCBPs) impact alternative splicing of a subset of mammalian genes that are enriched in basic cellular functions. Here, we focus our analysis on PCBP-controlled cassette exon-splicing within the cell cycle control regulator cyclin-dependent kinase-2 (CDK2) transcript. We demonstrate that PCBP binding to a C-rich polypyrimidine tract (PPT) preceding exon 5 of the CDK2 transcript enhances cassette exon inclusion. This splice enhancement is U2AF65-independent and predominantly reflects actions of the PCBP1 isoform. Remarkably, PCBPs' control of CDK2 ex5 splicing has evolved subsequent to mammalian divergence via conversion of constitutive exon 5 inclusion in the mouse CDK2 transcript to PCBP-responsive exon 5 alternative splicing in humans. Importantly, exclusion of exon 5 from the hCDK2 transcript dramatically represses the expression of CDK2 protein with a corresponding perturbation in cell cycle kinetics. These data highlight a recently evolved post-transcriptional pathway in primate species with the potential to modulate cell cycle control.

Direct Regulation of Alternative Splicing by SMAD3 through PCBP1 Is Essential to the Tumor-Promoting Role of TGF-β

In advanced stages of cancers, TGF-β promotes tumor progression in conjunction with inputs from receptor tyrosine kinase pathways. However, mechanisms that underpin the signaling cooperation and convert TGF-β from a potent growth inhibitor to a tumor promoter are not fully understood. We report here that TGF-β directly regulates alternative splicing of cancer stem cell marker CD44 through a phosphorylated T179 of SMAD3-mediated interaction with RNA-binding protein PCBP1. We show that TGF-β and EGF respectively induce SMAD3 and PCBP1 to colocalize in SC35-positive nuclear speckles, and the two proteins interact in the variable exon region of CD44 pre-mRNA to inhibit spliceosome assembly in favor of expressing the mesenchymal isoform CD44s over the epithelial isoform CD44E. We further show that the SMAD3-mediated alternative splicing is essential to the tumor-promoting role of TGF-β and has a global influence on protein products of genes instrumental to epithelial-to-mesenchymal transition and metastasis.

αCP binding to a cytosine-rich subset of polypyrimidine tracts drives a novel pathway of cassette exon splicing in the mammalian transcriptome

Alternative splicing (AS) is a robust generator of mammalian transcriptome complexity. Splice site specification is controlled by interactions of cis-acting determinants on a transcript with specific RNA binding proteins. These interactions are frequently localized to the intronic U-rich polypyrimidine tracts (PPT) located 5′ to the majority of splice acceptor junctions. αCPs (also referred to as polyC-binding proteins (PCBPs) and hnRNPEs) comprise a subset of KH-domain proteins with high affinity and specificity for C-rich polypyrimidine motifs. Here, we demonstrate that αCPs promote the splicing of a defined subset of cassette exons via binding to a C-rich subset of polypyrimidine tracts located 5′ to the αCP-enhanced exonic segments. This enhancement of splice acceptor activity is linked to interactions of αCPs with the U2 snRNP complex and may be mediated by cooperative interactions with the canonical polypyrimidine tract binding protein, U2AF65. Analysis of αCP-targeted exons predicts a substantial impact on fundamental cell functions. These findings lead us to conclude that the αCPs play a direct and global role in modulating the splicing activity and inclusion of an array of cassette exons, thus driving a novel pathway of splice site regulation within the mammalian transcriptome.

Insights into the Regulation of mRNA Processing of Polycistronic Transcripts Mediated by DRBD4/PTB2, a Trypanosome Homolog of the Polypyrimidine Tract-Binding Protein

Trypanosomes regulate gene expression mostly by posttranscriptional mechanisms, including control of mRNA turnover and translation efficiency. This regulation is carried out via certain elements located at the 3'-untranslated regions of mRNAs, which are recognized by RNA-binding proteins. In trypanosomes, trans-splicing is of central importance to control mRNA maturation. We have previously shown that TcDRBD4/PTB2, a trypanosome homolog of the human polypyrimidine tract-binding protein splicing regulator, interacts with the intergenic region of one specific dicistronic transcript, referred to as TcUBP (and encoding for TcUBP1 and TcUBP2, two closely kinetoplastid-specific proteins). In this work, a survey of TcUBP RNA processing revealed certain TcDRBD4/PTB2-regulatory elements within its intercistronic region, which are likely to influence the trans-splicing rate of monocistronic-derived transcripts. Furthermore, TcDRBD4/PTB2 overexpression in epimastigote cells notably decreased both UBP1 and UBP2 protein expression. This type of posttranscriptional gene regulatory mechanism could be extended to other transcripts as well, as we identified several other RNA precursor molecules that specifically bind to TcDRBD4/PTB2. Altogether, these findings support a model in which TcDRBD4/PTB2-containing ribonucleoprotein complexes can prevent trans-splicing. This could represent another stage of gene expression regulation mediated by the masking of trans-splicing/polyadenylation signals.

The Poly(C) Binding Protein Pcbp2 and Its Retrotransposed Derivative Pcbp1 Are Independently Essential to Mouse Development

RNA-binding proteins participate in a complex array of posttranscriptional controls essential to cell type specification and somatic development. Despite their detailed biochemical characterizations, the degree to which each RNA-binding protein impacts mammalian embryonic development remains incompletely defined, and the level of functional redundancy among subsets of these proteins remains open to question. The poly(C) binding proteins, PCBPs (αCPs and hnRNP E proteins), are encoded by a highly conserved and broadly expressed gene family. The two major Pcbp isoforms, Pcbp2 and Pcbp1, are robustly expressed in a wide range of tissues and exert both nuclear and cytoplasmic controls over gene expression. Here, we report that Pcbp1-null embryos are rendered nonviable in the peri-implantation stage. In contrast, Pcbp2-null embryos undergo normal development until midgestation (12.5 to 13.5 days postcoitum), at which time they undergo a dramatic loss in viability associated with combined cardiovascular and hematopoietic abnormalities. Mice heterozygous for either Pcbp1 or Pcbp2 null alleles display a mild and nondisruptive defect in initial postpartum weight gain. These data reveal that Pcbp1 and Pcbp2 are individually essential for mouse embryonic development and have distinct impacts on embryonic viability and that Pcpb2 has a nonredundant in vivo role in hematopoiesis. These data further provide direct evidence that Pcbp1, a retrotransposed derivative of Pcpb2, has evolved an essential function(s) in the mammalian genome.

Specific enrichment of the RNA-binding proteins PCBP1 and PCBP2 in chief cells of the murine gastric mucosa

RNA-binding proteins and corresponding post-transcriptional controls play critical roles in gene expression. The poly-(C) binding proteins, PCBPs (αCPs, hnRNPEs), comprise a well-characterized family of abundant RNA-binding proteins that impact on RNA processing in the nucleus as well as mRNA stability and translation in the cytoplasm. Here we demonstrate that PCBP1 and PCBP2 are abundantly expressed in the gastric epithelium with prominent enrichment in specific cell types within the gastric glandular mucosa. The spatial and intracellular patterns of PCBP1 and PCBP2 expression in these regions are highly correlated. Remarkably, we observe that these proteins are present in the nuclear and cytoplasmic compartments of zymogenic chief cells while they are restricted to the nuclear compartment in acid-secreting parietal cells and poorly expressed in pit cells that line the gland exit. This specificity of expression patterns and subcellular localization of PCBP1 and PCBP2, along with their appearance in the precursor tissues of the gastric epithelium during early postnatal development, suggests these RNA-binding proteins play specific roles in cell differentiation and organismal development within the gastric glandular epithelium.

Cytoplasmic poly(A) binding protein C4 serves a critical role in erythroid differentiation

The expression of an mRNA is strongly impacted by its 3' poly(A) tail and associated poly(A)-binding proteins (PABPs). Vertebrates encode six PABP isoforms that vary in abundance, distribution, developmental control, and subcellular localization. Here we demonstrate that the minor PABP isoform PABPC4 is expressed in erythroid cells and impacts the steady-state expression of a subset of erythroid mRNAs. Motif analyses reveal a high-value AU-rich motif in the 3' untranslated regions (UTRs) of PABPC4-impacted mRNAs. This motif enhances the association of PABPC4 with mRNAs containing critically shortened poly(A) tails. This association may serve to protect a subset of mRNAs from accelerated decay. Finally, we demonstrate that selective depletion of PABPC4 in an erythroblast cell line inhibits terminal erythroid maturation with corresponding alterations in the erythroid gene expression. These observations lead us to conclude that PABPC4 plays an essential role in posttranscriptional control of a major developmental pathway.

Trypanosome MKT1 and the RNA-binding protein ZC3H11: interactions and potential roles in post-transcriptional regulatory networks

The trypanosome zinc finger protein ZC3H11 binds to AU-rich elements in mRNAs. It is essential for survival of the mammalian-infective bloodstream form, where it stabilizes several mRNAs including some encoding chaperones, and is also required for stabilization of chaperone mRNAs during the heat-shock response in the vector-infective procyclic form. When ZC3H11 was artificially 'tethered' to a reporter mRNA in bloodstream forms it increased reporter expression. We here show that ZC3H11 interacts with trypanosome MKT1 and PBP1, and that domains required for both interactions are necessary for function in the bloodstream-form tethering assay. PBP1 interacts with MKT1, LSM12 and poly(A) binding protein, and localizes to granules during parasite starvation. All of these proteins are essential for bloodstream-form trypanosome survival and increase gene expression in the tethering assay. MKT1 is cytosolic and polysome associated. Using a yeast two-hybrid screen and tandem affinity purification we found that trypanosome MKT1 interacts with multiple RNA-binding proteins and other potential RNA regulators, placing it at the centre of a post-transcriptional regulatory network. A consensus interaction sequence, H(E/D/N/Q)PY, was identified. Recruitment of MKT1-containing regulatory complexes to mRNAs via sequence-specific mRNA-binding proteins could thus control several different post-transcriptional regulons.

Identification of a secondary promoter within the human B cell receptor component gene hCD79b

The human B cell-specific protein, CD79b (also known as Igβ and B29) constitutes an essential signal transduction component of the B cell receptor. Although its function is central to the triggering of B cell terminal differentiation in response to antigen stimulation, the transcriptional determinants that control CD79b gene expression remain poorly defined. In the present study, we explored these determinants using a series of hCD79b transgenic mouse models. Remarkably, we observed that the previously described hCD79b promoter along with its associated enhancer elements and first exon could be deleted without appreciable loss of hCD79b transcriptional activity or tissue specificity. In this deletion setting, a secondary promoter located within exon 2 maintained full levels and specificity of hCD79b transcription. Of note, this secondary promoter was also active, albeit at lower levels, in the wild-type hCD79b locus. The activity of the secondary promoter was dependent on the action(s) of a conserved sequence element mapping to a chromatin DNase I hypersensitive site located within intron 1. mRNA generated from this secondary promoter is predicted to encode an Igβ protein lacking a signal sequence and thus unable to serve normal B cell receptor function. Although the physiologic role of the hCD79b secondary promoter and its encoded protein remain unclear, the current data suggest that it has the capacity to play a role in normal as well as pathologic states in B cell proliferation and function.

αCP Poly(C) binding proteins act as global regulators of alternative polyadenylation

We have previously demonstrated that the KH-domain protein αCP binds to a 3' untranslated region (3'UTR) C-rich motif of the nascent human alpha-globin (hα-globin) transcript and enhances the efficiency of 3' processing. Here we assess the genome-wide impact of αCP RNA-protein (RNP) complexes on 3' processing with a specific focus on its role in alternative polyadenylation (APA) site utilization. The major isoforms of αCP were acutely depleted from a human hematopoietic cell line, and the impact on mRNA representation and poly(A) site utilization was determined by direct RNA sequencing (DRS). Bioinformatic analysis revealed 357 significant alterations in poly(A) site utilization that could be specifically linked to the αCP depletion. These APA events correlated strongly with the presence of C-rich sequences in close proximity to the impacted poly(A) addition sites. The most significant linkage was the presence of a C-rich motif within a window 30 to 40 bases 5' to poly(A) signals (AAUAAA) that were repressed upon αCP depletion. This linkage is consistent with a general role for αCPs as enhancers of 3' processing. These findings predict a role for αCPs in posttranscriptional control pathways that can alter the coding potential and/or levels of expression of subsets of mRNAs in the mammalian transcriptome.

RNA-binding protein PCBP2 modulates glioma growth by regulating FHL3

PCBP2 is a member of the poly(C)-binding protein (PCBP) family, which plays an important role in posttranscriptional and translational regulation by interacting with single-stranded poly(C) motifs in target mRNAs. Several PCBP family members have been reported to be involved in human malignancies. Here, we show that PCBP2 is upregulated in human glioma tissues and cell lines. Knockdown of PCBP2 inhibited glioma growth in vitro and in vivo through inhibition of cell-cycle progression and induction of caspase-3-mediated apoptosis. Thirty-five mRNAs were identified as putative PCBP2 targets/interactors using RIP-ChIP protein-RNA interaction arrays in a human glioma cell line, T98G. Four-and-a-half LIM domain 3 (FHL3) mRNA was downregulated in human gliomas and was identified as a PCBP2 target. Knockdown of PCBP2 enhanced the expression of FHL3 by stabilizing its mRNA. Overexpression of FHL3 attenuated cell growth and induced apoptosis. This study establishes a link between PCBP2 and FHL3 proteins and identifies a new pathway for regulating glioma progression.

Active stabilization of human endothelial nitric oxide synthase mRNA by hnRNP E1 protects against antisense RNA and microRNAs

Human endothelial nitric oxide synthase (eNOS) mRNA is highly stable in endothelial cells (ECs). Posttranscriptional regulation of eNOS mRNA stability is an important component of eNOS regulation, especially under hypoxic conditions. Here, we show that the human eNOS 3' untranslated region (3' UTR) contains multiple, evolutionarily conserved pyrimidine (C and CU)-rich sequence elements that are both necessary and sufficient for mRNA stabilization. Importantly, RNA immunoprecipitations and RNA electrophoretic mobility shift assays (EMSAs) revealed the formation of heterogeneous nuclear ribonucleoprotein E1 (hnRNP E1)-containing RNP complexes at these 3'-UTR elements. Knockdown of hnRNP E1 decreased eNOS mRNA half-life, mRNA levels, and protein expression. Significantly, these stabilizing RNP complexes protect eNOS mRNA from the inhibitory effects of its antisense transcript sONE and 3'-UTR-targeting small interfering RNAs (siRNAs), as well as microRNAs, specifically, hsa-miR-765, which targets eNOS mRNA stability determinants. Hypoxia disrupts hnRNP E1/eNOS 3'-UTR interactions via increased Akt-mediated serine phosphorylation (including serine 43) and increased nuclear localization of hnRNP E1. These mechanisms account, at least in part, for the decrease in eNOS mRNA stability under hypoxic conditions. Thus, the stabilization of human eNOS mRNA by hnRNP E1-containing RNP complexes serves as a key protective mechanism against the posttranscriptional inhibitory effects of antisense RNA and microRNAs under basal conditions but is disrupted under hypoxic conditions.

PCBP2 siRNA reverses the alcohol-induced pro-fibrogenic effects in hepatic stellate cells

PURPOSE

Type I collagen accumulates during liver fibrosis primarily because α-complex protein-2 (αCP(2)), encoded by the poly(rC) binding protein 2 (PCBP2) gene, binds to the 3' end of the collagen mRNA and increases its half-life. This study aimed to reverse the pro-fibrogenic effect of alcohol on hepatic stellate cells (HSCs) by silencing the PCBP2 gene with siRNA.

METHODS

The silencing effects of a series of predesigned PCBP2 siRNAs were evaluated in the rat hepatic stellate cell line, HSC-T6. The pro-fibrogenic effects of alcohol on the expression levels of PCBP2 and type-I collagen were examined by several methods. The effect of PCBP2 siRNA on the stability of type I collagen α1(I) mRNA was investigated by an in vitro mRNA decay assay.

RESULTS

We identified one potent PCBP2 siRNA that reversed the alcohol-induced expression of PCBP2 in HSCs. The decay rate of the collagen α1(I) mRNA increased significantly in HSCs treated with the PCBP2 siRNA.

CONCLUSION

This study provides the first evidence that alcohol up-regulates the expression of PCBP2, which subsequently increases the half-life of collagen α1(I) mRNA. Silencing of PCBP2 using siRNA may provide a promising strategy to reverse the alcohol-induced pro-fibrogenic effects in HSCs.

An RNA-protein complex links enhanced nuclear 3' processing with cytoplasmic mRNA stabilization

Post-transcriptional controls are critical to gene regulation. These controls are frequently based on sequence-specific binding of trans-acting proteins to cis-acting motifs on target RNAs. Prior studies have revealed that the KH-domain protein, αCP, binds to a 3' UTR C-rich motif of hα-globin mRNA and contributes to its cytoplasmic stability. Here, we report that this 3' UTR αCP complex regulates the production of mature α-globin mRNA by enhancing 3' processing of the hα-globin transcript. We go on to demonstrate that this nuclear activity reflects enhancement of both the cleavage and the polyadenylation reactions and that αCP interacts in vivo with core components of the 3' processing complex. Consistent with its nuclear processing activity, our studies reveal that αCP assembles co-transcriptionally at the hα-globin chromatin locus and that this loading is selectively enriched at the 3' terminus of the gene. The demonstrated linkage of nuclear processing with cytoplasmic stabilization via a common RNA-protein complex establishes a basis for integration of sequential controls critical to robust and sustained expression of a target mRNA.

The poly(rC)-binding protein alphaCP2 is a noncanonical factor in X. laevis cytoplasmic polyadenylation

Post-transcriptional control of mRNA stability and translation is central to multiple developmental pathways. This control can be linked to cytoplasmic polyadenylation in certain settings. In maturing Xenopus oocytes, specific mRNAs are targeted for polyadenylation via recruitment of the Cytoplasmic Polyadenylation Element (CPE) binding protein (CPEB) to CPE(s) within the 3' UTR. Cytoplasmic polyadenylation is also critical to early embryonic events, although corresponding determinants are less defined. Here, we demonstrate that the Xenopus ortholog of the poly(rC) binding protein αCP2 can recruit cytoplasmic poly(A) polymerase activity to mRNAs in Xenopus post-fertilization embryos, and that this recruitment relies on cis sequences recognized by αCP2. We find that the hα-globin 3' UTR, a validated mammalian αCP2 target, constitutes an effective target for cytoplasmic polyadenylation in Xenopus embryos, but not during Xenopus oocyte maturation. We further demonstrate that the cytoplasmic polyadenylation activity is dependent on the action of the C-rich αCP-binding site in conjunction with the adjacent AAUAAA. Consistent with its ability to target mRNA for poly(A) addition, we find that XαCP2 associates with core components of the Xenopus cytoplasmic polyadenylation complex, including the cytoplasmic poly(A) polymerase XGLD2. Furthermore, we observe that the C-rich αCP-binding site can robustly enhance the activity of a weak canonical oocyte maturation CPE in early embryos, possibly via a direct interaction between XαCP2 and CPEB1. These studies establish XαCP2 as a novel cytoplasmic polyadenylation trans factor, indicate that C-rich sequences can function as noncanonical cytoplasmic polyadenylation elements, and expand our understanding of the complexities underlying cytoplasmic polyadenylation in specific developmental settings.

Research resource: T-antigen transformation of pituitary cells captures three novel cell lines in the Pit-1 lineage

We report the establishment of three distinct pituitary-derived murine cell lines generated by targeted T-antigen-induced transformation. The Pit1/0 line expresses pituitary-specific transcription factor-1 (Pit-1) but lacks expression of GH, prolactin (Prl), or TSH, and the Pit1/Prl line is selectively positive for Pit-1 and Prl. The third line, Pit1/Triple, expresses Pit-1 and all three of the Pit-1-dependent hormones: GH, Prl, and TSHβ/glycoprotein hormone α-subunit. The three corresponding transformation events appear to have captured pituitary cells representing: 1) an initial step in the Pit-1(+) lineage, 2) a cell line that corresponds to the differentiated lactotrope, and 3) a novel tri-hormone intermediate that may represent a pivotal step in Pit-1(+) cell lineage differentiation. The documented dependence of the tri-hormone expression in the Pit-1/Triple line on Pit-1 activity supports its potential role in the pathway of pituitary cell differentiation. The presence of a 123-kb human transgene encompassing the hGH locus (hGH/bacterial artificial chromosome) in two of these lines, Pit1/0 and Pit1/Prl, further expands their potential utility to the analysis of gene activation within the hGH gene cluster.

Selective localization of PCBP2 to cytoplasmic processing bodies

Processing bodies (P-bodies) are cytoplasmic domains that have been implicated in critical steps of the regulation of gene expression, including mRNA decay and post-transcriptional gene silencing. Previously, we reported that PCBP2 (Poly-(rC) Binding Protein 2), a facilitator of IRES-mediated translation, is a novel P-body component. Interestingly, PCBP2 is recruited to only a subset of Dcp1a-positive P-bodies, which may reflect functional diversity among these structures. In this study, we examined the selective P-body localization of PCBP2 in detail. Co-localization studies between Dcp1a and PCBP2 revealed that PCBP2 is present in approximately 40% of P-bodies. While PCBP2 was more likely to reside in larger P-bodies, P-body size did not seem to be the sole determinant, and puromycin-induced enlargement of P-bodies only modestly increased the percentage of PCBP2-positive P-bodies. Photobleaching experiments demonstrated that the accumulation of PCBP2 to specific P-bodies is a dynamic process, which does not involve the protein's transcription-dependent nucleo-cytoplasmic shuttling activity. Finally, we found that PCBP1, a close relative of PCBP2, localizes to P-bodies in a similar manner to PCBP2. Taken together, these results establish the compositional diversity among P-bodies, and that PCBP2, probably in complex with other mRNP factors, may dynamically recognize such differences and accumulate to specific P-bodies.

PERK-dependent regulation of IAP translation during ER stress

Exposure of cells to endoplasmic reticulum (ER) stress leads to activation of phosphatidylinositol 3-kinase (PI3K)-Akt signaling pathway and transcriptional induction of the inhibitor of apoptosis family of proteins. One of the proximal effectors of the ER stress response, the PKR-like ER kinase (PERK), leads to cellular adaptation to stress by multiple mechanisms, including attenuation of protein synthesis and transcriptional induction of pro-survival genes. Although PERK activity leads to cellular adaptation to ER stress, we now demonstrate that PERK activity also inhibits the ER stress-induced apoptotic program through the induction of cellular inhibitor of apoptosis (cIAP1 and cIAP2) proteins. This induction of IAPs occurs through both transcriptional and translational responses that are PERK dependent. Reintroduction of cIAP1 or cIAP2 expression into PERK-/- murine embryonic fibroblasts during ER stress delays the early onset of ER stress-induced caspase activation and apoptosis observed in these cells. Furthermore, we demonstrate that the activation of the PI3K-Akt pathway by ER stress is dependent on PERK, suggesting additional ways in which PERK activity protects cells from ER stress-induced apoptosis.

Hypoxia-mediated selective mRNA translation by an internal ribosome entry site-independent mechanism

Although it is advantageous for hypoxic cells to inhibit protein synthesis and conserve energy, it is also important to translate mRNAs critical for adaptive responses to hypoxic stress. Because internal ribosome entry sites (IRES) have been postulated to mediate this preferential synthesis, we analyzed the 5 '-untranslated regions from a panel of stress-regulated mRNAs for m(7)GTP cap-independent translation and identified putative IRES elements in encephalomyocarditis virus, vascular endothelial growth factor, hypoxia-inducible factors (HIFs) 1alpha and 2alpha, glucose transporter-like protein 1, p57(Kip2), La, BiP, and triose phosphate isomerase transcripts. However, when capped and polyadenylated dicistronic RNAs were synthesized in vitro and transfected into cells, cellular IRES-mediated translation accounted for less than 1% that of the level of cap-dependent translation. Moreover, hypoxic stress failed to activate cap-independent synthesis, indicating that it is unlikely that this is the primary mechanism for the maintenance of the translation of these mRNAs under low O(2). Furthermore, although HIF-1alpha is frequently cited as an example of an mRNA that is preferentially translated, we demonstrate that under different levels and durations of hypoxic stress, changes in newly synthesized HIF-1alpha and beta-actin protein levels mirror alterations in corresponding mRNA abundance. In addition, our data suggest that cyclin-dependent kinase inhibitor p57(Kip2) and vascular endothelial growth factor mRNAs are selectively translated by an IRES-independent mechanism under hypoxic stress.

Regulation and localization of TOB and IFR1 in differentiating red cells

In differentiating red blood cells (RBCs) of the chick embryo, the synthesis of carbonic anhydrase (CAII) and pyrimidine 5'-nucleotidase (P5N-I) is triggered by the hypoxic mediators norepinephrine and adenosine via receptor-mediated cAMP formation. The process is accompanied by the induction of IFR1 and TOB which are putative regulators of transcription or translation in different cell types. The present investigation studied the erythroid TOB and IFR1 expression: mRNA and protein are up-regulated in post-mitotic RBCs from D11-19 treated with cAMP-elevating agonists. In contrast, immature RBCs of early embryos (D5-7) fail to synthesize a significant amount of IFR1/TOB. In D11 RBCs, TOB and IFR1 are cytosolic proteins with different half-lives (TOB<4h, IFR1>12h). Cytosolic fractionation characterized TOB as a free soluble protein while the abundant IFR1 (c(max) approximately 3microM) is completely associated with the ribosomal fraction. A putative function of both proteins as translational regulators is discussed.

Iron/IRP-1-dependent regulation of mRNA expression for transferrin receptor, DMT1 and ferritin during human erythroid differentiation

OBJECTIVE

We investigated iron regulatory protein (IRP)-dependent expression of transferrin receptor (TfR), divalent metal transporter-1 (DMT1) and ferritin during erythroid differentiation system using an in vitro three-phase liquid culture.

METHOD

Peripheral blood hematopoietic progenitor cells were cultured with interleukin-3 and stem cell factor (SCF) for 7 days (first phase), subsequently with SCF, erythropoietin (EPO) and insulin-like growth factor-I (IGF-I) for 5 days (second phase), and finally with EPO and IGF-I for 3 days (third phase). Cells were subjected to colony assay, flow-cytometric analysis, mRNA assessment, electrophoretic mobility shift assay (EMSA), immunoblotting, and immunoprecipitation.

RESULTS

In the second/third phases, erythroid cells serially differentiated. Expression of TfR and DMT1 mRNA, which have iron-responsive elements (IREs) at 3'-UTR, reached a maximum on second phase, and thereafter decreased, while expression of ferritin mRNA, which has an IRE at the 5'-UTR, decreased reciprocally on second phase. IRP in the cytosol after precipitation of polysome decreased on second phase, suggesting that IRP bound to IREs of these mRNAs in the polysome. When cells were incubated with (59)FeCl(3), (59)Fe-bound IRP-1 immunoprecipitated with anti-IRP-1 antibodies was detected on first phase and third phase, but was not detected on second phase.

CONCLUSION

These results suggest that IRP-1/IRE interactions, which are supposedly induced after sensing a decrease of the intracellular non-Heme iron levels, play a crucial role on the posttranscriptional regulation of TfR, DMT1, and ferritin mRNAs during differentiation of normal human erythropoietic cells.

A cell type-restricted mRNA surveillance pathway triggered by ribosome extension into the 3' untranslated region

The accuracy of eukaryotic gene expression is monitored at multiple levels. Surveillance pathways have been identified that degrade messenger RNAs containing nonsense mutations, harboring stalled ribosomes or lacking termination codons. Here we report a previously uncharacterized surveillance pathway triggered by ribosome extension into the 3' untranslated region. This ribosome extension-mediated decay, REMD, accounts for marked repression of protein synthesis from a human alpha-globin gene containing a prevalent antitermination mutation. REMD can be mechanistically distinguished from other surveillance pathways by its functional linkage to accelerated deadenylation, by its independence from the NMD factor Upf1 and by cell-type restriction. This unusual pathway of mRNA surveillance is likely to act as a modifier of additional genetic defects and may reflect post-transcriptional controls particular to erythroid and other differentiated cell lineages.

MicroRNA silencing through RISC recruitment of eIF6

MicroRNAs (miRNAs) are a class of small RNAs that act post-transcriptionally to regulate messenger RNA stability and translation. To elucidate how miRNAs mediate their repressive effects, we performed biochemical and functional assays to identify new factors in the miRNA pathway. Here we show that human RISC (RNA-induced silencing complex) associates with a multiprotein complex containing MOV10--which is the homologue of Drosophila translational repressor Armitage--and proteins of the 60S ribosome subunit. Notably, this complex contains the anti-association factor eIF6 (also called ITGB4BP or p27BBP), a ribosome inhibitory protein known to prevent productive assembly of the 80S ribosome. Depletion of eIF6 in human cells specifically abrogates miRNA-mediated regulation of target protein and mRNA levels. Similarly, depletion of eIF6 in Caenorhabditis elegans diminishes lin-4 miRNA-mediated repression of the endogenous LIN-14 and LIN-28 target protein and mRNA levels. These results uncover an evolutionarily conserved function of the ribosome anti-association factor eIF6 in miRNA-mediated post-transcriptional silencing.

The 3' untranslated region complex involved in stabilization of human alpha-globin mRNA assembles in the nucleus and serves an independent role as a splice enhancer

Posttranscriptional controls, mediated primarily by RNA-protein complexes, have the potential to alter multiple steps in RNA processing and function. Human alpha-globin mRNA is bound at a C-rich motif in the 3' untranslated region (3'UTR) by the KH domain protein alpha-globin poly(C)-binding protein (alphaCP). This "alpha-complex" is essential to cytoplasmic stability of alpha-globin mRNA in erythroid cells. Here we report that the 3'UTR alpha-complex also serves an independent nuclear role as a splice enhancer. Consistent with this role, we find that alphaCP binds alpha-globin transcripts prior to splicing. Surprisingly, this binding occurs at C-rich sites within intron I as well as at the 3'UTR C-rich determinant. The intronic and 3'UTR alphaCP complexes appear to have distinct effects on splicing. While intron I complexes repress intron I excision, the 3'UTR complex enhances splicing of the full-length transcript both in vivo and in vitro. In addition to its importance to splicing, nuclear assembly of the 3'UTR alphaCP complex may serve to "prepackage" alpha-globin mRNA with its stabilizing complex prior to cytoplasmic export. Linking nuclear and cytoplasmic controls by the action of a particular RNA-binding protein, as reported here, may represent a modality of general importance in eukaryotic gene regulation.

Dynamic association with polysomes during P19 neuronal differentiation and an untranslated-region-dependent translation regulation of the tau mRNA by the tau mRNA-associated proteins IMP1, HuD, and G3BP1

Regulation of mRNA translation is a key step in mediating neuronal polarity during differentiation, insofar as neuronal polarity is partially determined by local translation of specific mRNA molecules as dendrites and axons are emanating. The multiplicity of mRNA-binding proteins in neurons plays an essential role in controlling mRNA translation. These proteins are associated with ribosomes and translation factors, thereby regulating both temporally and spatially the translation process. In a previous study, we have shown an association among the tau mRNA-binding proteins HuD, IMP1, and G3BP1 with translating polysomes in P19 neurons. In the present study, we determined the dynamics of the association among G3BP1, IMP1, and HuD with polysomes through P19 neuronal differentiation as well as the functional effect of these proteins on tau mRNA translation. We show a novel, differentiation-dependent association of these proteins with polysomes. In addition, we show a strong, negative effect on translation of the tau mRNA by IMP1, G3BP1, and HuD proteins in HEK-293 cells. To our knowledge this is the first observation of a direct translational role of G3BP1 for any mRNA and the first report of a translation inhibition by IMP1 and HuD on the tau mRNA in a cell system. The translation inhibition is shown to be mediated by the tau mRNA 3'untranslated regions (UTRs), thus giving a new, translational role for these sequences, which were previously implicated in mRNA stabilization. We also define a novel mechanism for IMP1 binding to tau mRNA, which suggests a conformational binding, which is not sequence dependent.

Dynamic posttranscriptional regulation of epsilon-globin gene expression in vivo

Functional studies of embryonic epsilon-globin indicate that individuals with beta thalassemia or sickle cell disease are likely to benefit from therapeutic, transcriptional derepression of its encoding gene. The success of epsilon-globin gene-reactivation strategies, however, will be tempered by the stability that epsilon-globin mRNA exhibits in developmental stage-discordant definitive erythroid progenitors. Using cell culture and transgenic mouse model systems, we demonstrate that epsilon-globin mRNA is modestly unstable in immature, transcriptionally active erythroid cells, but that this characteristic has relatively little impact on the accumulation of epsilon-globin mRNA at subsequent stages of terminal differentiation. Importantly, the constitutive stability of epsilon-globin mRNA increases in transgenic mouse models of beta thalassemia, suggesting that epsilon- and beta-globin mRNAs are coregulated through a shared posttranscriptional mechanism. As anticipated, relevant cis-acting determinants of epsilon-globin mRNA stability map to its 3' UTR, consistent with the positioning of functionally related elements in other globin mRNAs. These studies demonstrate that posttranscriptional processes do not pose a significant practical barrier to epsilon-globin gene reactivation and, moreover, indicate that related therapeutic strategies may be particularly effective in individuals carrying beta-thalassemic gene defects.

Nitric oxide activation of Erk1/2 regulates the stability and translation of mRNA transcripts containing CU-rich elements

Nitric oxide (NO*) can stabilize mRNA by activating p38 mitogen-activated protein kinase (MAPK). Here, transcript stabilization by NO* was investigated in human THP-1 cells using microarrays. After LPS pre-stimulation, cells were treated with actinomycin D and then exposed to NO* without or with the p38 MAPK inhibitor SB202190 (SB). The decay of 220 mRNAs was affected; most were stabilized by NO*. Unexpectedly, SB often enhanced rather than antagonized transcript stability. NO* activated p38 MAPK and Erk1/2; SB blocked p38 MAPK, but further activated Erk1/2. RT-PCR confirmed that NO* and SB could additively stabilize certain mRNA transcripts, an effect abolished by Erk1/2 inhibition. In affected genes, these responses were associated with CU-rich elements (CURE) in 3'-untranslated regions (3'-UTR). NO* stabilized the mRNA of a CURE-containing reporter gene, while repressing translation. Dominant-negative Mek1, an Erk1/2 inhibitor, abolished this effect. NO* similarly stabilized, but blocked translation of MAP3K7IP2, a natural CURE-containing gene. NO* increased hnRNP translocation to the cytoplasm and binding to CURE. Over-expression of hnRNP K, like NO*, repressed translation of CURE-containing mRNA. These findings define a sequence-specific mechanism of NO*-triggered gene regulation that stabilizes mRNA, but represses translation.

Cerebral ischemia induces neuronal expression of novel VL30 mouse retrotransposons bound to polyribosomes

Mammalian genomes are burdened with a large heterogeneous group of endogenous replication defective retroviruses (retrotransposons). Previously, we identified a transcript resembling a virus-like 30S (VL30) retrotransposon increasing in mouse brain following transient cerebral ischemia. Paradoxically, this non-coding RNA was found bound to polyribosomes. Further analysis revealed that multiple retrotransposon species (BVL-1-like and mVL30-1-like) were bound to polyribosomes and induced by ischemia. These VL30 transcripts remained associated with polyribosomes in the presence of 0.5 M KCl, indicating that VL30 mRNA was tightly associated with ribosomal subunits. Furthermore, the profile of BVL-1 distribution on polyribosomal profiles was distinct from those of translated and translationally repressed mRNA. Consistent with expectations, 5.0 kb VL30 transcripts were detected in ischemic brain with a temporal pattern of expression that was distinct from c-fos. Expression of VL30 was localized in neurons using a combination of in situ hybridization and immunocytochemistry. 3'-RACE-PCR experiments yielded two unique sequences (VL30x-1 and VL30x-2) that were homologous to known VL30 genes. Phylogenetic analysis of VL30 promoter sequence (U3 region) resulted in the identification of two large VL30 subgroups. VL30x-1 and VL30x-2 were closely related and classified in a group that was distinct from the well-characterized VL30 genes BVL-1 and mVL30-1. The promoter regions of VL30x-1 and VL30x-2 did not possess the consensus sequences for either hypoxia or anoxia response elements, suggesting an alternative mechanism for induction. This is the first report that demonstrates ischemia-induced, neuronal expression of unique VL30 retrotransposons in mouse brain.

Haploinsufficiency due to deletion within the 3′-UTR of C1-INH-gene associated with hereditary angioedema

PURPOSE

Sequences within the non-coding 3'UTR (untranslated region) of genes were reported to be involved in the regulation of gene expression by modifying pathways of (co)transcription, post-transcriptional processing and RNA transport. However, direct biological evidence (i.e., knock-out models) is sparse. This report intends to correlate the first reported alteration within the 3'UTR of the C1 inhibitor (C1-INH) gene with clinical presentation of hereditary angioedema (HAE).

METHODS AND RESULTS

Direct sequencing of genomic DNA revealed in all affected members of a family suffering from HAE a heterozygous 155 bp deletion 100 bp downstream of the physiological stop-codon in exon 8. A substantial reduction of both mRNA as well as C1-INH protein expression was revealed by RT-PCR and nephelometry, respectively.

CONCLUSION

We suppose that the mutation within the 3'UTR interferes with integral pathways of gene expression leading to pathogenic haploinsufficiency in this family.

Partial hepatectomy induced liver proteome changes in mice

Acceleration of liver regeneration could be of great clinical benefit in various liver-associated diseases. However, at present little is known about therapeutic interventions to enhance this regenerative process. Our limited understanding and the complexity of the mechanisms involved have prevented the identification of new targets for treatment. Here we propose a broad-range proteomic approach to this problem that makes possible the simultaneous study of different signaling and metabolic pathways on the liver proteome. Changes in protein expression in mouse livers (n = 5 per group) at 6 h and 12 h after partial hepatectomy and sham operation, as compared to untreated controls, were analyzed using two-dimensional gel electrophoresis, mass spectrometry (MS), and mass fingerprinting. Twelve proteins, identified by MS, were up-regulated by at least 2-fold after partial hepatectomy. These included adipose differentiation-related protein, gamma-actin, enoyl coenzyme A hydratase 1, serum amyloid A and eukaryotic translation initiation factor 3. These results indicate that liver regeneration following partial hepatectomy affects various signaling and metabolic pathways.

Butyrate increases the efficiency of translation of gamma-globin mRNA

Fetal hemoglobin (Hb F) levels increase in most patients with sickle cell disease following intermittent butyrate therapy. Although the full effects of butyrate on Hb F levels usually require multiple treatment cycles, in some patients a peak level is achieved after a few days of butyrate therapy. Our investigation of the mechanism(s) responsible for this rapid induction of Hb F by butyrate showed that reticulocyte gamma-globin chain synthesis markedly increased within 24 hours of butyrate exposure, without concomitant changes in reticulocyte gamma-globin mRNA levels. This suggests that butyrate might induce Hb F by increasing the efficiency of translation of gamma-globin mRNA. This hypothesis was confirmed by ribosome loading studies that demonstrated enrichment of the polysomal fraction of reticulocytes with gamma-globin mRNA following butyrate exposure. Thus, the induction of Hb F by butyrate may be mediated by translational effects in addition to its well-known effects on transcription of the gamma-globin genes.

Bystander gene activation by a locus control region

Random assortment of genes within mammalian genomes establishes the potential for interference between neighboring genes with distinct transcriptional specificities. Long-range transcriptional controls further increase this potential. Exploring this problem is of fundamental importance to understanding gene regulation. In the human genome, the Igbeta (CD79b) gene is situated between the pituitary-specific human growth hormone (hGH) gene and its locus control region (hGH LCR). Igbeta protein is considered B-cell specific; its only known role is in B-cell receptor signaling. Unexpectedly, we found that hIgbeta is transcribed at high levels in the pituitary. This Igbeta transcription is dependent on pituitary-specific epigenetic modifications generated by the hGH LCR. In contrast, expression of Igbeta at its native site in B cells is independent of hGH LCR activity. These studies demonstrated that a gene with tissue-restricted transcriptional determinants (B cell) can be robustly activated in an unrelated tissue (pituitary) due to fortuitous positioning within an active chromatin domain. This 'bystander' gene activation pathway impacts on current concepts of tissue specificity and models of active chromatin domains.

Nonsense mutations in close proximity to the initiation codon fail to trigger full nonsense-mediated mRNA decay

Nonsense-mediated mRNA decay (NMD) is a surveillance mechanism that degrades mRNAs containing premature translation termination codons. In mammalian cells, a termination codon is ordinarily recognized as "premature" if it is located greater than 50-54 nucleotides 5' to the final exon-exon junction. We have described a set of naturally occurring human beta-globin gene mutations that apparently contradict this rule. The corresponding beta-thalassemia genes contain nonsense mutations within exon 1, and yet their encoded mRNAs accumulate to levels approaching wild-type beta-globin (beta(WT)) mRNA. In the present report we demonstrate that the stabilities of these mRNAs with nonsense mutations in exon 1 are intermediate between beta(WT) mRNA and beta-globin mRNA carrying a prototype NMD-sensitive mutation in exon 2 (codon 39 nonsense; beta 39). Functional analyses of these mRNAs with 5'-proximal nonsense mutations demonstrate that their relative resistance to NMD does not reflect abnormal RNA splicing or translation re-initiation and is independent of promoter identity and erythroid specificity. Instead, the proximity of the nonsense codon to the translation initiation AUG constitutes a major determinant of NMD. Positioning a termination mutation at the 5' terminus of the coding region blunts mRNA destabilization, and this effect is dominant to the "50-54 nt boundary rule." These observations impact on current models of NMD.

A novel set of nuclear localization signals determine distributions of the alphaCP RNA-binding proteins

AlphaCPs comprise a subfamily of KH-domain-containing RNA-binding proteins with specificity for C-rich pyrimidine tracts. These proteins play pivotal roles in a broad spectrum of posttranscriptional events. The five major alphaCP isoforms are encoded by four dispersed loci. Each isoform contains three repeats of the RNA-binding KH domain (KH1, KH2, and KH3) but lacks other identifiable motifs. To explore the complexity of their respective functions, we examined the subcellular localization of each alphaCP isoform. Immunofluorescence studies revealed three distinct distributions: alphaCP1 and alphaCP2 are predominantly nuclear with specific enrichment of alphaCP1 in nuclear speckles, alphaCP3 and alphaCP4 are restricted to the cytoplasm, and alphaCP2-KL, an alphaCP2 splice variant, is present at significant levels in both the nucleus and the cytoplasm. We mapped nuclear localization signals (NLSs) for alphaCP isoforms. alphaCP2 contains two functionally independent NLS. Both NLSs appear to be novel and were mapped to a 9-amino-acid segment between KH2 and KH3 (NLS I) and to a 12-amino-acid segment within KH3 (NLS II). NLS I is conserved in alphaCP1, whereas NLS II is inactivated by two amino acid substitutions. Neither NLS is present in alphaCP3 or alphaCP4. Consistent with mapping studies, deletion of NLS I from alphaCP1 blocks its nuclear accumulation, whereas NLS I and NLS II must both be inactivated to block nuclear accumulation of alphaCP2. These data demonstrate an unexpected complexity in the compartmentalization of alphaCP isoforms and identify two novel NLS that play roles in their respective distributions. This complexity of alphaCP distribution is likely to contribute to the diverse functions mediated by this group of abundant RNA-binding proteins.

Identification of mRNAs associated with alphaCP2-containing RNP complexes

Posttranscriptional controls in higher eukaryotes are central to cell differentiation and developmental programs. These controls reflect sequence-specific interactions of mRNAs with one or more RNA binding proteins. The alpha-globin poly(C) binding proteins (alphaCPs) comprise a highly abundant subset of K homology (KH) domain RNA binding proteins and have a characteristic preference for binding single-stranded C-rich motifs. alphaCPs have been implicated in translation control and stabilization of multiple cellular and viral mRNAs. To explore the full contribution of alphaCPs to cell function, we have identified a set of mRNAs that associate in vivo with the major alphaCP2 isoforms. One hundred sixty mRNA species were consistently identified in three independent analyses of alphaCP2-RNP complexes immunopurified from a human hematopoietic cell line (K562). These mRNAs could be grouped into subsets encoding cytoskeletal components, transcription factors, proto-oncogenes, and cell signaling factors. Two mRNAs were linked to ceroid lipofuscinosis, indicating a potential role for alphaCP2 in this infantile neurodegenerative disease. Surprisingly, alphaCP2 mRNA itself was represented in alphaCP2-RNP complexes, suggesting autoregulatory control of alphaCP2 expression. In vitro analyses of representative target mRNAs confirmed direct binding of alphaCP2 within their 3' untranslated regions. These data expand the list of mRNAs that associate with alphaCP2 in vivo and establish a foundation for modeling its role in coordinating pathways of posttranscriptional gene regulation.