Considerable evidence indicates that mRNA associates with structural filaments in the cell (cytoskeleton). This relationship would be an important mechanism to effect mRNA sorting since specific mRNAs could be sequestered at sites within the cell. In addition, it can provide a mechanism for spatial regulation of mRNA expression. However, the precise structural interactions between mRNA and the cytoskeleton have yet to be defined. An objective of this work was to visualize 'individual' poly(A) mRNA molecules in situ by electron microscopy to identify their relationship to individual filaments. Poly(A) RNA and filaments were identified simultaneously using antibodies to detect hybridized probe and filaments or actin-binding proteins. In human fibroblasts, most of the poly(A) mRNA (72%) was localized within 5 nm of orthogonal networks of F-actin filaments. Poly(A) mRNA also colocalized with vimentin filaments (29%) and microtubules (<10%). The sites of mRNA localization were predominantly at filament intersections. The majority of poly(A) mRNA and polysomes colocalized with the actin crosslinking proteins, filamin, and α-actinin, and the elongation factor, EF-1α (actin-binding protein; ABP-50). Evidence that intersections contained single mRNA molecules was provided by using a labeled oligo dT probe to prime the synthesis of cDNA in situ using reverse transcriptase. Both the poly(A) and cis sequences of the same mRNA molecule could then be visualized independently. We propose that the cytoskeletal intersection is a mRNA receptor and serves as a 'microdomain' where mRNA is attached and functionally expressed.
The localization of β-actin mRNA to the leading lamellae of chicken fibroblasts and neurite growth cones of developing neurons requires a 54-nt localization signal (the zipcode) within the 3′ untranslated region. In this study we have identified and isolated five proteins binding to the zipcode. One of these we previously identified as zipcode binding protein (ZBP)1, a 4-KH domain protein. A second is now investigated in detail: a 92-kD protein, ZBP2, that is especially abundant in extracts from embryonic brain. We show that ZBP2 is a homologue of the human hnRNP protein, KSRP, that appears to mediate premRNA splicing. However, ZBP2 has a 47-amino acid (aa) sequence not present in KSRP. Various portions of ZBP2 fused to GFP indicate that the protein most likely shuttles between the nucleus and the cytoplasm, and that the 47-aa insert promotes the nuclear localization. Expression of a truncated ZBP2 inhibits the localization of β-actin mRNA in both fibroblast and neurons. These data suggest that ZBP2, although predominantly a nuclear protein, has a role in the cytoplasmic localization of β-actin mRNA.
The intracellular distribution of HIV-1 RNA transcripts in infected cells was studied using in situ hybridization detected by electron microscopy and cellular fractionation. Although viral RNA and core protein could be detected throughout the cytoplasm and nucleus, viral RNA was found in significantly increased amounts in mitochondria relative to the cytoplasm and nucleus. In contrast, cellular poly(A) RNA or viral gag proteins were not increased in the mitochondria. A cell line containing an integrated latent genome that could be induced to express viral RNA after phorbol ester stimulation showed an increase in viral RNA accumulation in mitochondria parallel with the increase in HIV expression levels. Concomitant with HIV expression, there was a decrease in mitochondrial viability. Using immunofluorescent markers to detect probes to HIV RNA transcripts and antibodies to mitochondrial proteins simultaneously in single cells, there was an inverse relationship between the amount of viral RNA and mitochondrial integrity. High levels of viral RNA in mitochondria were found in acutely (but not chronically) infected cells. We propose that HIV RNA import into mitochondria can compromise mitochondrial function.
Neutrophils exposed to chemoattractants polarize and accumulate polymerized actin at the leading edge. In neutrophil-like HL-60 cells, this asymmetry depends on a positive feedback loop in which accumulation of a membrane lipid, phosphatidylinositol (PI) 3,4,5-trisphosphate (PI[3,4,5]P3), leads to activation of Rac and/or Cdc42, and vice versa. We now report that Rac and Cdc42 play distinct roles in regulating this asymmetry. In the absence of chemoattractant, expression of constitutively active Rac stimulates accumulation at the plasma membrane of actin polymers and of GFP-tagged fluorescent probes for PI(3,4,5)P3 (the PH domain of Akt) and activated Rac (the p21-binding domain of p21-activated kinase). Dominant negative Rac inhibits chemoattractant-stimulated accumulation of actin polymers and membrane translocation of both fluorescent probes and attainment of morphologic polarity. Expression of constitutively active Cdc42 or of two different protein inhibitors of Cdc42 fails to mimic effects of the Rac mutants on actin or PI(3,4,5)P3. Instead, Cdc42 inhibitors prevent cells from maintaining a persistent leading edge and frequently induce formation of multiple, short lived leading edges containing actin polymers, PI(3,4,5)P3, and activated Rac. We conclude that Rac plays a dominant role in the PI(3,4,5)P3-dependent positive feedback loop required for forming a leading edge, whereas location and stability of the leading edge are regulated by Cdc42.
Neurotrophins play an essential role in the regulation of actin- dependent changes in growth cone shape and motility. We have studied whether neurotrophin signaling can promote the localization of β-actin mRNA and protein within growth cones. The regulated localization of specific mRNAs within neuronal processes and growth cones could provide a mechanism to modulate cytoskeletal composition and growth cone dynamics during neuronal development. We have previously shown that β-actin mRNA is localized in granules that were distributed throughout processes and growth cones of cultured neurons. In this study, we demonstrate that the localization of β- actin mRNA and protein to growth cones of forebrain neurons is stimulated by neurotrophin-3 (NT-3). A similar response was observed when neurons were exposed to forskolin or db-cAMP, suggesting an involvement of a cAMP signaling pathway. NT-3 treatment resulted in a rapid and transient stimulation of PKA activity that preceded the localization of β-actin mRNA. Localization of β-actin mRNA was blocked by prior treatment of cells with Rp-cAMP, an inhibitor of cAMP-dependent protein kinase A. Depolymerization of microtubules, but not microfilaments, inhibited the NT-3-induced localization of β-actin mRNA. These results suggest that NT-3 activates a cAMP-dependent signaling mechanism to promote the microtubule-dependent localization of β- actin mRNA within growth cones.
by
Dianna E. Willis;
Erna A. van Niekerk;
Yukio Sasaki;
Mariano Mesngon;
Tanuja T. Merianda;
Gervan G. Williams;
Marvin Kendall;
Deanna S. Smith;
Gary Bassell;
Jeffery L. Twiss
Subcellular regulation of protein synthesis requires the correct localization of messenger RNAs (mRNAs) within the cell. In this study, we investigate whether the axonal localization of neuronal mRNAs is regulated by extracellular stimuli. By profiling axonal levels of 50 mRNAs detected in regenerating adult sensory axons, we show that neurotrophins can increase and decrease levels of axonal mRNAs. Neurotrophins (nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3) regulate axonal mRNA levels and use distinct downstream signals to localize individual mRNAs. However, myelin-associated glycoprotein and semaphorin 3A regulate axonal levels of different mRNAs and elicit the opposite effect on axonal mRNA levels from those observed with neurotrophins. The axonal mRNAs accumulate at or are depleted from points of ligand stimulation along the axons. The translation product of a chimeric green fluorescent protein-β-actin mRNA showed similar accumulation or depletion adjacent to stimuli that increase or decrease axonal levels of endogenous β-actin mRNA. Thus, extracellular ligands can regulate protein generation within subcellular regions by specifically altering the localized levels of particular mRNAs.
The mechanisms by which catenins regulate cadherin function are not fully understood, and the precise function of p120 catenin (p120ctn) has remained particularly elusive. In microvascular endothelial cells, p120ctn colocalized extensively with cell surface VE-cadherin, but failed to colocalize with VE-cadherin that had entered intracellular degradative compartments. To test the possibility that p120ctn binding to VE-cadherin regulates VE-cadherin internalization, a series of approaches were undertaken to manipulate p120ctn availability to endogenous VE-cadherin. Expression of VE-cadherin mutants that competed for p120ctn binding triggered the degradation of endogenous VE-cadherin. Similarly, reducing levels of p120ctn using siRNA caused a dramatic and dose-related reduction in cellular levels of VE-cadherin. In contrast, overexpression of p120ctn increased VE-cadherin cell surface levels and inhibited entry of cell surface VE-cadherin into degradative compartments. These results demonstrate that cellular levels of p120ctn function as a set point mechanism that regulates cadherin expression levels, and that a major function of p120ctn is to control cadherin internalization and degradation.