Identification of a novel interaction of 14-3-3 with p190RhoGEF

Activation of Rho GTPases by guanine nucleotide exchange factors (GEFs) mediates a broad range of cytoskeletal alterations that determine cell shape. In the nervous system, Rho GTPases are essential for establishing highly asymmetrical neuronal forms and may fine-tune the shape of dendrites in differentiated neurons. p190RhoGEF is a brain-enriched, RhoA-specific GEF whose highly interactive C-terminal domain provides potential linkage to multiple pathways in the cell. In the present study, a yeast two-hybrid screen was used to identify 14-3-3eta and 14-3-3epsilon as additional binding partners of p190RhoGEF. Interactions between p190RhoGEF and 14-3-3eta were confirmed biochemically and by colocalization of the respective proteins when fused to fluorescent markers and transfected in neuronal cells. We also mapped a unique phosphorylation-independent binding site (I(1370)QAIQNL) in p190RhoGEF. Deletion of the binding site abolished interactions in vitro as well as the ability of 14-3-3eta to alter the cytoplasmic aggregation of p190RhoGEF in cotransfected cells. The findings suggest a potential role for 14-3-3 in modulating p190RhoGEF activity or in linking p190RhoGEF to the activities of other pathways in the neuron.

p190RhoGEF Binds to a destabilizing element in the 3' untranslated region of light neurofilament subunit mRNA and alters the stability of the transcript

Stabilization of neurofilament (NF) mRNAs plays a major role in regulating levels of NF expression and in establishing axonal size and rate of axonal conduction. Previous studies have identified a 68-nucleotide destabilizing element at the junction of the coding region and 3' untranslated region of the light NF subunit (NF-L) mRNA. The present study has used the destabilizing element (probe A) to screen a rat brain cDNA library for interactive proteins. A cDNA clone encoding 1068 nucleotides in the C-terminal domain of p190RhoGEF (clone 39) was found to bind strongly and specifically to the RNA probe. The interaction was confirmed using a glutathione S-transferase/clone 39 fusion protein in Northwestern, gel-shift, and cross-linkage studies. The glutathione S-transferase/clone 39 fusion protein also enhanced the cross-linkage of a major 43-kDa protein in brain extract to the destabilizing element. Functional studies on stably transfected neuronal cells showed that p190RhoGEF expression increased the half-life of a wild-type NF-L mRNA but did not alter the half-life of a mutant NF-L mRNA lacking the destabilizing element. The findings reveal a novel interactive feature of p190RhoGEF that links the exchange factor with NF mRNA stability and regulation of the axonal cytoskeleton.

Similar poly(C)-sensitive RNA-binding complexes regulate the stability of the heavy and light neurofilament mRNAs

The potential role of RNA processing in regulating neurofilament (NF) subunit expression and in mediating the neuropathic effects of NF transgenes was explored by determining whether similar regulatory elements and cognate binding factors are present in NF mRNAs. Gel-shift studies were used to compare RNA-binding complexes that assemble on the 3'UTR of the heavy (NF-H), mid-sized (NF-M) and light (NF-L) NF mRNAs when radioactive RNA probes are incubated with high-speed supernatants (S100) of rat brain homogenates. RNA-binding complexes were characterized by their rate of migration in non-denaturing gels and by their ability to be competed with specific homoribopolymers. Similar RNA-binding complexes formed on probes to the 3'UTRs of NF-L and NF-H mRNAs. The complexes were competed with poly(C) and are referred to as poly(C)-sensitive complexes. Their binding sites were localized to a 36 nt sequence in the mid-distal region of the NF-H 3'UTR and to a 45 nt sequence at the proximal edge of the 3'UTR of the NF-L transcript. Although the binding sites showed limited sequence homology, the complexes were cross-competed with unlabeled probes and radioactivity in either probe was cross-linked to a 43 kDa protein. The 43 kDa protein also bound directly to NF-L and NF-H probes in Northwestern blots. Functional studies showed that deletion of the binding sites markedly increased expression of a luciferase reporter gene containing the 3'UTR of NF-L or NF-H by stabilizing the fusion transcripts. Point mutations in the NF-H binding site which prevented formation of the poly(C)-sensitive complex also stabilized the fusion mRNA. The findings reveal a common destabilizing element in the 3'UTR of NF-L and NF-H mRNAs that may be important in coordinating NF subunit expression and in mediating the neuropathic effects of the NF-L and NF-H transgenes in transgenic mice.

Mutation in neurofilament transgene implicates RNA processing in the pathogenesis of neurodegenerative disease

A mouse neurofilament light subunit (NF-L) transgene with a 36 bp c-myc insert at the end of the coding region was found to have neuropathic effects on enteric and motor neurons of transgenic mice. The severity of phenotype was related directly to the levels of transgenic mRNA expression. High levels of transgene expression were lethal to newborn pups, causing profound alterations in the development of the enteric nervous system and extensive vacuolar changes in motor neurons. Lower levels of transgene expression led to a transient stunting of growth and focal alterations of enteric and motor neurons. Because the positioning of the c-myc insert coincided with the location of the major stability determinant of the NF-L mRNA (Cañete-Soler et al., 1998a,b), additional studies were undertaken. These studies showed that the c-myc insert alters the ribonucleoprotein (RNP) complexes that bind to the stability determinant and disrupts their ability to regulate the stability of the transcripts. The findings indicate that expression of an NF-L transgene with a mutant mRNA stability determinant is highly disruptive to enteric and motor neurons and implicate alterations in RNA processing in the pathogenesis of a neurodegenerative condition.

Characterization of the mouse neurofilament light (NF-L) gene promoter by in vitro transcription

We have used in vitro transcription to access the basic sequences and factors required for the transcription of the mouse neurofilament light promoter (pNF-L) in the absence of chromatin structure. Deletion from -1.7 to -154 results in little change in NF-L promoter activity using nuclear extracts from either brain (expressing) or liver (non-expressing) tissues. Further deletion to -29 results in a gradual five-fold drop in promoter activity in both extracts. Only replacement of the entire -148 to -29 region results in a drop in NF-L promoter activity to basal levels. Thus, the NF-L promoter differs from the mouse NF heavy (NF-H) and mid-sized (NF-M) promoters in that no specific sequence within the immediate upstream NF-L promoter region (-154 to -29) appears to be responsible for enhancement or brain-specific transcription. We show that the order of strength of the three NF promoters is NF-H>NF-M>NF-L and identify sequences that can increase or reduce transcription when placed in front of heterologous NF promoters. We conclude that the NF-L promoter is a modular, weak and promiscuous promoter whose regulation differs from NF-H or NF-M. Our data suggest that chromatin structure may play an important role in the regulation of the NF-L promoter.

Stability determinants are localized to the 3'-untranslated region and 3'-coding region of the neurofilament light subunit mRNA using a tetracycline-inducible promoter

The tetracycline-responsive expression system of Bujard was used to compare rates of decay of wild-type and mutant neurofilament (NF) light subunit (NF-L) mRNAs. Optimal conditions for activation and inactivation of the target transgene were determined using a luciferase reporter gene. Analyses of mRNA stability were thereupon conducted on cells that were doubly transfected with transactivator and inducible target genes and derived from pooled clones of transfected cells. Rates of mRNA decay were compared upon inactivation of the transgenes after high levels of mRNA had been induced. Deletion of the 445-nucleotide (nt) 3'-untranslated region (3'-UTR) (L/++(+)-) or 527 nt of the 3'-coding region (3'-CR) (L/++-+) increased the stability of NF-L mRNA compared with the full-length (L/++(++)) transcript in neuronal (N2a and P19 cells) and non-neuronal (L cells) lines. Deletion of both the 3'-UTR and 3'-CR (L/++--) led to a further stabilization of the transcript. A major stability determinant was then localized to a 68-nt sequence that forms the junction between the 3'-CR and 3'-UTR of NF-L and is the binding site of a unique ribonucleoprotein complex (Cañete-Soler, R., Schwartz, M. L., Hua, Y., and Schlaepfer, W. W. (1998) J. Biol. Chem. 273, 12655-12661). The studies establish a novel system for mapping determinants of mRNA stability and have applied the system to localize determinants that regulate the stability of the NF-L mRNA.

Characterization of ribonucleoprotein complexes and their binding sites on the neurofilament light subunit mRNA

Levels of neurofilament (NF) gene expression are important determinants of basic neuronal properties, but overexpression can lead to motoneuron degeneration in transgenic mice. In a companion study (Cañete-Soler, R., Schwartz, M. L., Hua, Y., and Schlaepfer, W. W. (1998) J. Biol. Chem. 273, 12650-12654), we show that levels of NF expression are regulated by altering mRNA stability and that stability determinants are present in the 3'-coding region (3'-CR) and 3'-untranslated region (3'-UTR) of the NF light subunit (NF-L) transcript. This study characterizes the ribonucleoprotein complexes that bind to the NF-L mRNA when cytoplasmic brain extracts are incubated with radioactive probes. Gel retardation assays reveal ribonucleoprotein complexes that are selectively competed with poly(C) or poly(U))/poly(A) homoribopolymers and are referred to as C-binding and U/A-binding complexes, respectively. The C-binding complex forms on the proximal 45 nucleotides of 3'-UTR, but its assembly is markedly enhanced by 23 nucleotides of flanking 3'-CR sequence. U/A-binding complexes form at multiple binding sites in the 3'-CR and 3'-UTR. A pattern of reciprocal binding suggests that the C-binding and U/A-binding complexes interact and may compete for common components or binding sites. Cross-linking studies reveal unique polypeptides in the C-binding and U/A-binding complexes. The findings provide the basis for probing mechanisms regulating NF-L mRNA stability and the relationship between NF overexpression and motoneuron degeneration in transgenic mice.

Developmental expression of MMP-9 (gelatinase B) mRNA in mouse embryos

Considerable remodeling of the extracellular matrix as well as cellular migration takes place during embryogenesis. Since the metalloproteinase MMP-9 is implicated in these functions in cancer cells, we studied the patterns of expression of MMP-9 mRNA during the development of post-implantation mouse embryos. MMP-9 mRNA was detected using the ribonuclease protection assay in poly A+RNA from 13 to 17 day embryos, but not at 11 days. In order to localize these transcripts, in situ hybridization was performed on sections of murine embryos from 7.5 to 15 days of gestation. At the time of implantation, MMP-9 mRNA was localized to the invading trophoblast cells. Strong signals were also seen in the yolk sac. No signal for MMP-9 mRNA was seen by in situ hybridization in the embryo until day 11 when detectable reaction was seen in the central nervous system. By day 15 strong signals were seen in the liver, in the developing bronchial epithelium of the lungs and in the primordial alveoli, in the epithelium of the thyroid gland, in the thymus, in the endochondrial plates of the bone, and in neural cells. The liver from day 15 embryos contained gelatinase activity at 105 kDa consistent with MMP-9. Thus, MMP-9 expression appears to be expressed in specific organs in a precise temporal sequence during development.