The fragile X syndrome protein represses activity-dependent translation through CYFIP1, a new 4E-BP.

Strong evidence indicates that regulated mRNA translation in neuronal dendrites underlies synaptic plasticity and brain development. The fragile X mental retardation protein (FMRP) is involved in this process; here, we show that it acts by inhibiting translation initiation. A binding partner of FMRP, CYFIP1/Sra1, directly binds the translation initiation factor eIF4E through a domain that is structurally related to those present in 4E-BP translational inhibitors. Brain cytoplasmic RNA 1 (BC1), another FMRP binding partner, increases the affinity of FMRP for the CYFIP1-eIF4E complex in the brain. Levels of proteins encoded by known FMRP target mRNAs are increased upon reduction of CYFIP1 in neurons. Translational repression is regulated in an activity-dependent manner because BDNF or DHPG stimulation of neurons causes CYFIP1 to dissociate from eIF4E at synapses, thereby resulting in protein synthesis. Thus, the translational repression activity of FMRP in the brain is mediated, at least in part, by CYFIP1.

Nedd4-2 regulates surface expression and may affect N-glycosylation of hyperpolarization-activated cyclic nucleotide-gated (HCN)-1 channels.

HCN channels are important regulators of neuronal excitability. The proper function of these channels is governed by various mechanisms, including post-translational modifications of channel subunits. Here, we provide evidence that ubiquitination via a ubiquitin ligase, neuronal precursor cell expressed developmentally downregulated (Nedd)-4-2, is involved in the regulation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. We identified a PY motif (L/PPxY), the characteristic binding motif for Nedd4-2 in the C terminus of the HCN1 subunit, and showed that HCN1 and Nedd4-2 interacted both in vivo (rat hippocampus, neocortex, and cerebellum) and in vitro [human embryonic kidney 293 (HEK293) cells], resulting in increased HCN1 ubiquitination. Elimination of the PY motif reduced, but did not abolish, Nedd4-2 binding, which further involved a stretch of ∼100 aa downstream in the HCN1 C terminus. Coexpression of Nedd4-2 and HCN1 drastically reduced the HCN1-mediated h-current amplitude (85-92%) in Xenopus laevis oocytes and reduced surface expression (34%) of HCN1 channels in HEK293 cells, thereby opposing effects of tetratricopeptide repeat-containing Rab8b interacting protein (TRIP8b)-(1a-4), an auxiliary subunit that promotes HCN1 surface expression. Regulation may further include N-glycosylation of HCN1 channels, which is significantly enhanced by TRIP8b(1a-4), but may be reduced by Nedd4-2. Taken together, our data indicate that Nedd4-2 plays an important role in the regulation of HCN1 trafficking and may compete with TRIP8b(1a-4) in this process.

Makorin ring zinc finger protein 1 (MKRN1), a novel poly(A)-binding protein-interacting protein, stimulates translation in nerve cells.

The poly(A)-binding protein (PABP), a key component of different ribonucleoprotein complexes, plays a crucial role in the control of mRNA translation rates, stability, and subcellular targeting. In this study we identify RING zinc finger protein Makorin 1 (MKRN1), a bona fide RNA-binding protein, as a binding partner of PABP that interacts with PABP in an RNA-independent manner. In rat brain, a so far uncharacterized short MKRN1 isoform, MKRN1-short, predominates and is detected in forebrain nerve cells. In neuronal dendrites, MKRN1-short co-localizes with PABP in granule-like structures, which are morphological correlates of sites of mRNA metabolism. Moreover, in primary rat neurons MKRN1-short associates with dendritically localized mRNAs. When tethered to a reporter mRNA, MKRN1-short significantly enhances reporter protein synthesis. Furthermore, after induction of synaptic plasticity via electrical stimulation of the perforant path in vivo, MKRN1-short specifically accumulates in the activated dendritic lamina, the middle molecular layer of the hippocampal dentate gyrus. Collectively, these data indicate that in mammalian neurons MKRN1-short interacts with PABP to locally control the translation of dendritic mRNAs at synapses.

Interaction of rat poly(A)-binding protein with poly(A)- and non-poly(A) sequences is preferentially mediated by RNA recognition motifs 3+4.

Vasopressin (VP) mRNA and the non-coding BC200 RNA are sorted to neuronal dendrites. Among proteins interacting specifically with both RNAs is the multifunctional poly(A)-binding protein (PABP) consisting of four RNA recognition motifs (RRMs) and a C-terminal auxiliary domain. The protein/RNA interaction studies presented here reveal that PABPs association with VP- and BC200 RNA is exclusively mediated by RRMs 3+4. Quantitative binding studies with PABP deletion mutants demonstrate preferential binding of RRMs 3+4 even to poly(A)-homopolymers, while RRMs 1+2 exhibit a lower affinity for those sequences. An optimal interaction with both poly(A)- and non-poly(A) sequences is only achieved by full-size PABP.

Diversity of mRNAs in the Axonal Compartment of Peptidergic Neurons in the Rat.

Vasopressin and oxytocin mRNAs, which are normally translated in the perikarya of magnocellular neurons, have recently been demonstrated to be also present in axons and nerve terminals which are located in the posterior pituitary. The physiological significance of this observation has not yet been resolved. In order to gain further insight into the function and plasticity of the peptidergic neuron the question was addressed whether axonal localization is a unique feature of the above-mentioned transcripts. Biochemical evidence is presented that magnocellular axons and nerve terminals also contain mRNA species encoding a member of the neurofilament protein family and the prodynorphin precursor. These data imply that axons may harbour a variety of additional protein-encoding transcripts. Furthermore, it is shown that in the mutant (Brattleboro) rat, which lacks detectable levels of vasopressin but which still transcribes the corresponding gene, axonal vasopressin but not oxytocin mRNA contents are dramatically reduced. Most likely, vasopressin transcripts are absent from the nerve terminals as a consequence of the impaired precursor biosynthesis in the cytoplasm of the mutant rat.

Local synthesis of the rat Vasopressin precursor in dendrites of in vitro cultured nerve cells.

Vasopressin (VP) mRNA is subject to dendritic targeting both in vivo and in primary cultured neurons microinjected with an appropriate expression vector. We have constructed a vector encoding the mutant Brattleboro rat VP precursor which is non-diffusable, because it cannot leave the site of its synthesis, the rough endoplasmic reticulum. Expression of this construct in cultured nerve cells shows that the mutant protein is readily detectable in dendrites when mRNA transport has occurred, while dendrites devoid of the mRNA lack the protein. These results demonstrate that neurons have the capacity to locally synthesize secretory proteins in the dendritic compartment.

Rat vasopressin mRNA: a model system to characterize cis-acting elements and trans-acting factors involved in dendritic mRNA sorting.

The genes encoding the vasopressin (VP) and oxytocin (OT) precursors are expressed in magnocellular neurons of the hypothalamo-neurohypophyseal system. The neuropeptides have a dual function: (1) they are secreted from the nerve terminals into the systemic circulation to act as hormones on various peripheral target organs; and (2) VP and OT are also released from the dendrites into the central nervous system where they presumably play a role as either neurotransmitters or as modulators of the classical transmitters. Substantial amounts of VP and OT mRNAs are sorted to both axons and dendrites. Since the latter are equipped with components of the translation machinery, the peptide hormone precursors are likely to be locally synthesized in dendrites of magnocellular neurons. Evidence for axonal precursor synthesis, on the other hand, has not been obtained. Subcellular mRNA localization is a complex pathway. It is determined by sequences (cis-acting elements) within the RNA and proteins (trans-acting factors) which interact with these elements in order to guide the molecules to their ultimate destination. We have investigated the mechanisms involved in mRNA targeting in neurons by using VP mRNA as a model system. Recombinant eukaryotic expression vectors harboring the VP cDNA have been microinjected into the cell nuclei of cultured superior cervical ganglion (SCG) neurons. The subcellular distribution of the vector-expressed mRNAs was determined by non-radioactive in situ hybridization techniques. This revealed transport of VP mRNA to the dendrites, but not to the axonal compartment of SCG neurons. A complex dendritic localizer sequence (DLS) that spans part of the coding region as well as the 3'-untranslated region was identified by microinjecting constructs encoding partial sequences of the VP mRNA. In order to characterize trans-acting factors interacting with this element, protein/RNA binding experiments with radiolabeled in vitro synthesized VP RNA probes and proteins extracted from rat brain have been carried out. A protein specifically interacts with the DLS of the VP mRNA but not with sequences that obviously lack a role in subcellular RNA transport. Biochemical purification revealed that this protein is the multifunctional poly(A)-binding protein (PABP). It is well known for its ability to bind with high affinity to poly(A) tails of mRNAs, prerequisite for mRNA stabilization and stimulation of translational initiation, respectively. With lower affinities, PABP can also associate with non-poly(A) sequences. The physiological consequences of these PABP/RNA interactions include functions such as translational silencing. The translational state of mRNAs subject to dendritic sorting is most likely influenced by external stimuli. Consequently, PABP could represent one of several components necessary to regulate local synthesis of the VP precursor and possibly of other proteins.

Posttranscriptional regulation of the p85α adapter subunit of phosphatidylinositol 3-kinase in human leukemia cells.

Constitutive activation of phosphatidylinositol 3-kinase (PI3K)/Akt signaling has been observed in up to 70% of acute myeloid leukemia. Class I(A) PI3K consists of a catalytic subunit (p110α, p110ß, p110δ) and an adapter subunit (p85α, p55α, p50α, p85ß, p55γ). The p85α adapter subunit stabilizes the catalytic p110 subunit and recruits p110 to the plasma membrane. In addition, p85α inhibits the basal activity of p110α and can negatively regulate signal transduction, as shown for insulin and GM-CSF receptor signaling. Here, we describe that the expression of p85α is posttranscriptionally regulated in several human and murine leukemia cell lines and in a Hodgkin lymphoma cell line (CO) by translational repression. A detailed analysis of CO cells revealed that both wild type and a mutated p85α mRNA are detectable at similar ratios in the nucleus and polysomes. However, while the mutated p85α protein is expressed in CO cells, translation of the wild type p85α mRNA is completely inhibited. Ectopic expression of wild type p85α from a retroviral vector is suppressed in CO cells and in five out of six leukemia cell lines. Our data indicate that leukemia cells can regulate the expression of p85α by posttranscriptional regulation.

Molecular determinants and physiological relevance of extrasomatic RNA localization in neurons.

Specific sorting of mRNA molecules to subcellular microdomains is an evolutionarily conserved mechanism by which the polarized nature of eukayotic cells may be established and maintained. The molecular composition of the RNA localization machinery is complex. Sequence motifs within RNA molecules to be transported, called cis-acting elements, and proteins, referred to as trans-acting factors, are essential components. Transport of the resulting ribonucleoprotein complexes to distinct cytoplasmic regions occurs along the cytoskeletal network. The pathway is observed in organisms as diverse as yeast and human and it plays a critical role in development and cell differentiation. Moreover, RNA localization takes place in differentiated cell types including neurons. There is ample evidence to suggest that sorting of defined mRNA species to the neurites of nerve cells and on-site translation has an impact on various aspects of nerve cell biology.