Mike Lorenz

Molecular mechanisms of invadopodium formation: the role of the N-WASP–Arp2/3 complex pathway and cofilin

Invadopodia are actin-rich membrane protrusions with a matrix degradation activity formed by invasive cancer cells. We have studied the molecular mechanisms of invadopodium formation in metastatic carcinoma cells. Epidermal growth factor (EGF) receptor kinase inhibitors blocked invadopodium formation in the presence of serum, and EGF stimulation of serum-starved cells induced invadopodium formation. RNA interference and dominant-negative mutant expression analyses revealed that neural WASP (N-WASP), Arp2/3 complex, and their upstream regulators, Nck1, Cdc42, and WIP, are necessary for invadopodium formation. Time-lapse analysis revealed that invadopodia are formed de novo at the cell periphery and their lifetime varies from minutes to several hours. Invadopodia with short lifetimes are motile, whereas long-lived invadopodia tend to be stationary. Interestingly, suppression of cofilin expression by RNA interference inhibited the formation of long-lived invadopodia, resulting in formation of only short-lived invadopodia with less matrix degradation activity. These results indicate that EGF receptor signaling regulates invadopodium formation through the N-WASP–Arp2/3 pathway and cofilin is necessary for the stabilization and maturation of invadopodia.

Spatial regulation of β-actin translation by Src-dependent phosphorylation of ZBP1

Localization of β-actin messenger RNA to sites of active actin polymerization modulates cell migration during embryogenesis, differentiation and possibly carcinogenesis. This localization requires the oncofetal protein ZBP1 (Zipcode binding protein 1), which binds to a conserved 54-nucleotide element in the 3′-untranslated region of the β-actin mRNA known as the ‘zipcode’. ZBP1 promotes translocation of the β-actin transcript to actin-rich protrusions in primary fibroblasts and neurons. It is not known how the ZBP1–RNA complex achieves asymmetric protein sorting by localizing β-actin mRNA. Here we show that chicken ZBP1 modulates the translation of β-actin mRNA. ZBP1 associates with the β-actin transcript in the nucleus and prevents premature translation in the cytoplasm by blocking translation initiation. Translation only occurs when the ZBP1–RNA complex reaches its destination at the periphery of the cell. At the endpoint of mRNA transport, the protein kinase Src promotes translation by phosphorylating a key tyrosine residue in ZBP1 that is required for binding to RNA. These sequential events provide both temporal and spatial control over β-actin mRNA translation, which is important for cell migration and neurite outgrowth.

Imaging sites of N-WASP activity in lamellipodia and invadopodia of carcinoma cells

Cell migration is crucial for many biological and pathological processes such as chemotaxis of immune cells, fibroblast migration during wound healing, and tumor cell invasion and metastasis. Cells migrate forward by extending membrane protrusions. The formation of these protrusions is driven by assembly of actin filaments at the leading edge. Neural Wiskott-Aldrich syndrome protein (N-WASP), a ubiquitous member of the WASP family, induces actin polymerization by activating Arp2/3 complex and is thought to regulate the formation of membrane protrusions. However, it is totally unclear how N-WASP activity is spatially and temporally regulated inside migrating cells. To detect and image sites of N-WASP activity during cell motility and invasion in carcinoma cells, we designed an N-WASP fluorescence resonance energy transfer (FRET) biosensor that distinguishes between the active and inactive conformations and mimics the function of endogenous N-WASP. Our data show that N-WASP is involved in lamellipodia extension, where it is activated at the leading edge, as well as in invadopodia formation of invasive carcinoma cells, where it is activated at the base. This is the first time that the activity of full-length N-WASP has been visualized in vivo, and this has lead to new insights for N-WASP function.

Recent advances in FRET: distance determination in protein-DNA complexes.

Fluorescence resonance energy transfer (FRET) provides information on the distance between a donor and an acceptor dye in the range 10 to 100 A. Knowledge of the exact positions of some dyes with respect to nucleic acids now enables us to translate these data into precise structural information using molecular modeling. Advances in the preparation of dye-labeled nucleic acid molecules and in new techniques, such as the measurement of FRET in polyacrylamide gels or in vivo, will lead to an increasingly important role of FRET in structural and molecular biology.

SR Protein Family Members Display Diverse Activities in the Formation of Nascent and Mature mRNPs In Vivo

The SR proteins are a family of pre-mRNA splicing factors with additional roles in gene regulation. To investigate individual family members in vivo, we generated a comprehensive panel of stable cell lines expressing GFP-tagged SR proteins under endogenous promoter control. Recruitment of SR proteins to nascent FOS RNA was transcription dependent and RNase sensitive, with unique patterns of accumulation along the gene specified by the RNA recognition motifs (RRMs). In addition, all SR protein interactions with Pol II were RNA dependent, indicating that SR proteins are not preassembled with Pol II. SR protein interactions with RNA were confirmed in situ by FRET/FLIM. Interestingly, SC35-GFP also exhibited FRET with DNA and failed to associate with cytoplasmic mRNAs, whereas all other SR proteins underwent nucleocytoplasmic shuttling and associated with specific nuclear and cytoplasmic mRNAs. Because different constellations of SR proteins bound nascent, nuclear, and cytoplasmic mRNAs, mRNP remodeling must occur throughout an mRNAs lifetime.

A peptide motif in Raver1 mediates splicing repression by interaction with the PTB RRM2 domain

Polypyrimidine tract–binding protein (PTB) is a regulatory splicing repressor. Raver1 acts as a PTB corepressor for splicing of α-tropomyosin (Tpm1) exon 3. Here we define a minimal region of Raver1 that acts as a repressor domain when recruited to RNA. A conserved [S/G][I/L]LGxxP motif is essential for splicing repressor activity and sufficient for interaction with PTB. An adjacent proline-rich region is also essential for repressor activity but not for PTB interaction. NMR analysis shows that LLGxxP peptides interact with a hydrophobic groove on the dorsal surface of the RRM2 domain of PTB, which constitutes part of the minimal repressor region of PTB. The requirement for the PTB-Raver1 interaction that we have characterized may serve to bring the additional repressor regions of both proteins into a configuration that allows them to synergistically effect exon skipping.

RhoA/ROCK-mediated switching between Cdc42- and Rac1-dependent protrusion in MTLn3 carcinoma cells

Rho GTPases are versatile regulators of cell shape that act on the actin cytoskeleton. Studies using Rho GTPase mutants have shown that, in some cells, Rac1 and Cdc42 regulate the formation of lamellipodia and filopodia, respectively at the leading edge, whereas RhoA mediates contraction at the rear of moving cells. However, recent reports have described a zone of RhoA/ROCK activation at the front of cells undergoing motility. In this study, we use a FRET-based RhoA biosensor to show that RhoA activation localizes to the leading edge of EGF-stimulated cells. Inhibition of Rho or ROCK enhanced protrusion, yet markedly inhibited cell motility; these changes correlated with a marked activation of Rac-1 at the cell edge. Surprisingly, whereas EGF-stimulated protrusion in control MTLn3 cells is Rac-independent and Cdc42-dependent, the opposite pattern is observed in MTLn3 cells after inhibition of ROCK. Thus, Rho and ROCK suppress Rac-1 activation at the leading edge, and inhibition of ROCK causes a switch between Cdc42 and Rac-1 as the dominant Rho GTPase driving protrusion in carcinoma cells. These data describe a novel role for Rho in coordinating signaling by Rac and Cdc42.

Single-stranded nucleic acids promote SAMHD1 complex formation

SAM domain and HD domain-containing protein 1 (SAMHD1) is a dGTP-dependent triphosphohydrolase that degrades deoxyribonucleoside triphosphates (dNTPs) thereby limiting the intracellular dNTP pool. Mutations in SAMHD1 cause Aicardi–Goutières syndrome (AGS), an inflammatory encephalopathy that mimics congenital viral infection and that phenotypically overlaps with the autoimmune disease systemic lupus erythematosus. Both disorders are characterized by activation of the antiviral cytokine interferon-α initiated by immune recognition of self nucleic acids. Here we provide first direct evidence that SAMHD1 associates with endogenous nucleic acids in situ. Using fluorescence cross-correlation spectroscopy, we demonstrate that SAMHD1 specifically interacts with ssRNA and ssDNA and establish that nucleic acid-binding and formation of SAMHD1 complexes are mutually dependent. Interaction with nucleic acids and complex formation do not require the SAM domain, but are dependent on the HD domain and the C-terminal region of SAMHD1. We finally demonstrate that mutations associated with AGS exhibit both impaired nucleic acid-binding and complex formation implicating that interaction with nucleic acids is an integral aspect of SAMHD1 function.

ZBP1 enhances cell polarity and reduces chemotaxis

The interaction of β-actin mRNA with zipcode-binding protein 1 (ZBP1) is necessary for its localization to the lamellipod of fibroblasts and plays a crucial role in cell polarity and motility. Recently, we have shown that low ZBP1 levels correlate with tumor-cell invasion and metastasis. In order to establish a cause and effect relationship, we expressed ZBP1 in a metastatic rat mammary adenocarcinoma cell line (MTLn3) that has low endogenous ZBP1 levels and delocalized β-actin mRNA. This leads to localization of β-actin mRNA, and eventually reduces the chemotactic potential of the cells as well as their ability to move and orient towards vessels in tumors. To determine how ZBP1 leads to these two apparently contradictory aspects of cell behavior – increased cell motility but decreased chemotaxis – we examined cell motility in detail, both in cell culture and in vivo in tumors. We found that ZBP1 expression resulted in tumor cells with a stable polarized phenotype, and reduced their ability to move in response to a gradient in culture. To connect these results on cultured cells to the reduced metastatic ability of these cells, we used multiphoton imaging in vivo to examine tumor cell behavior in primary tumors. We found that ZBP1 expression actually reduced tumor cell motility and chemotaxis, presumably mediating their decreased metastatic potential by reducing their ability to respond to signals necessary for invasion.

The mechanism of CSF-1-induced Wiskott-Aldrich syndrome protein activation in vivo: a role for phosphatidylinositol 3-kinase and Cdc42.

A role for Wiskott-Aldrich syndrome protein (WASP) in chemotaxis to various agents has been demonstrated in monocyte-derived cell types. Although WASP has been shown to be activated by multiple mechanisms in vitro, it is unclear how WASP is regulated in vivo. A WASP biosensor (WASPbs), which uses intramolecular fluorescence resonance energy transfer to report WASP activation in vivo, was constructed, and following transfection of macrophages, activation of WASPbs upon treatment with colony-stimulating factor-1 (CSF-1) was detected globally as early as 30 s and remained localized to protrusive regions at later time points. Similar results were obtained when endogenous WASP activation was determined using conformation-sensitive antibodies. In vivo CSF-1-induced WASP activation was fully Cdc42-dependent. Activation of WASP in response to treatment with CSF-1 was also shown to be phosphatidylinositol 3-kinase-dependent. However, treatment with the Src family kinase inhibitors PP2 or SU6656 or disruption of the major tyrosine phosphorylation site of WASPbs (Y291F mutation) did not reduce the level of CSF-1-induced WASP activation. Our results indicate that WASP activation downstream of CSF-1R is phosphatidylinositol 3-kinase- and Cdc42-dependent consistent with an involvement of these molecules in macrophage migration. However, although tyrosine phosphorylation of WASP has been proposed to stimulate WASP activity, we found no evidence to indicate that this occurs in vivo.