Rap GTPase-mediated adhesion and migration: A target for limiting the dissemination of B-cell lymphomas?

Pax5 mediates the transcriptional activation of the CD81 gene

CD81 is an integral membrane protein of the tetraspanin family and forms complexes with a variety of other cell surface membrane proteins. CD81 is involved in cell migration and B cell activation. However, the mechanism of the transcriptional regulation of the CD81 gene remains unclear. Here, we revealed that CD81 transcriptional activation was required for binding of the transcription factor Pax5 at the Pax5-binding sequence (-54)GCGGGAC(-48) located upstream of the transcriptional start site (TSS) of the CD81 gene. The reporter assay showed that the DNA sequence between - 130 and - 39 bp upstream of the TSS of the CD81 gene had promoter activity for CD81 transcription. The DNA sequence between - 130 and - 39 bp upstream of TSS of CD81 harbors two potential Pax5-binding sequences (-87)GCGTGAG(-81) and (-54)GCGGGAC(-48). Reporter, electrophoresis mobility shift, and chromatin immunoprecipitation (ChIP) assays disclosed that Pax5 bound to the (-54)GCGGGAC(-48) in the promoter region of the CD81 gene in order to activate CD81 transcription. Pax5 overexpression increased the expression level of CD81 protein, while the Pax5-knockdown by shRNA decreased CD81 expression. Moreover, we found that the expression level of CD81 was positively correlated with Pax5 expression in human tumor cell lines. Because CD81 was reported to be involved in cell migration, we evaluated the effects of Pax5 overexpression by wound healing and transwell assays. The data showed that overexpression of either Pax5 or CD81 promoted the epithelial cell migration. Thus, our findings provide insights into the transcriptional mechanism of the CD81 gene through transcription factor Pax5.

Rap2 and TNIK control Plexin-dependent tiled synaptic innervation in C. elegans

During development, neurons form synapses with their fate-determined targets. While we begin to elucidate the mechanisms by which extracellular ligand-receptor interactions enhance synapse specificity by inhibiting synaptogenesis, our knowledge about their intracellular mechanisms remains limited. Here we show that Rap2 GTPase (rap-2) and its effector, TNIK (mig-15), act genetically downstream of Plexin (plx-1) to restrict presynaptic assembly and to form tiled synaptic innervation in C. elegans. Both constitutively GTP- and GDP-forms of rap-2 mutants exhibit synaptic tiling defects as plx-1 mutants, suggesting that cycling of the RAP-2 nucleotide state is critical for synapse inhibition. Consistently, PLX-1 suppresses local RAP-2 activity. Excessive ectopic synapse formation in mig-15 mutants causes a severe synaptic tiling defect. Conversely, overexpression of mig-15 strongly inhibited synapse formation, suggesting that mig-15 is a negative regulator of synapse formation. These results reveal that subcellular regulation of small GTPase activity by Plexin shapes proper synapse patterning in vivo.

Genes do more than just direct the color of our hair or eyes. They produce proteins that are involved in almost every process in the body. In humans, the majority of active genes can be found in the brain, where they help it to develop and work properly – effectively controlling how we move and behave.

The brain’s functional units, the nerve cells or neurons, communicate with each other by releasing messenger molecules in the gap between them, the synapse. These molecules are then picked up from specific receptor proteins of the receiving neuron.

In the nervous system, neurons only form synapses with the cells they need to connect with, even though they are surrounded by many more cells. This implies that they use specific mechanisms to stop neurons from forming synapses with incorrect target cells. This is important, because if too many synapses were present or if synapses formed with incorrect target cells, it would compromise the information flow in the nervous system. This would ultimately lead to various neurological conditions, including Autism Spectrum Disorder.

In 2013, researchers found that in the roundworm Caenorhabditis elegans, a receptor protein called Plexin, is located at the surface of the neurons and can inhibit the formation of nearby synapses. Now, Chen et al. – including one author involved in the previous research – wanted to find out what genes Plexin manipulates when it stops synapses from growing. Knowing what each of those genes does can help us understand how neurons can inhibit synapses.

The results revealed that Plexin appears to regulate two genes, Rap2 and TNIK. Plexin reduced the activity of Rap2 in the neuron that released the messenger, which hindered the formation of synapses. The gene TNIK and its protein on the other hand, have the ability to modify other proteins and could so inhibit the growth of synapses. When TNIK was experimentally removed, the number of synapses increased, but when its activity was increased, the number of synapses was strongly reduced.

These findings could help scientists understand how mutations in Rap2 or TNIK can lead to various neurological conditions. A next step will be to test if these genes also affect the formation of synapses in other species such as mice, which have a more complex nervous system that is structurally and functionally more similar to that of humans.

C3G knock-down enhances migration and invasion by increasing Rap1-mediated p38α activation, while it impairs tumor growth through p38α-independent mechanisms

C3G, a Guanine nucleotide Exchange Factor (GEF) for Rap1 and R-Ras, has been shown to play important roles in development and cancer. Previous studies determined that C3G regulates cell death through down-regulation of p38α MAPK activity. Here, we found that C3G knock-down in MEFs and HCT116 cells promotes migration and invasion through Rap1-mediated p38α hyper-activation. These effects of C3G were inhibited by Rap1 knock-down or inactivation. The enhanced migration observed in C3G depleted HCT116 cells was associated with reduction in E-cadherin expression, internalization of ZO-1, actin cytoskeleton reorganization and decreased adhesion. We also found that matrix metalloproteases MMP2 and MMP9 are involved in the pro-invasive effect of C3G down-regulation. Additionally, our studies revealed that both C3G and p38α collaborate to promote growth of HCT116 cells in vitro and in vivo, possibly by enhancing cell survival. In fact, knocking-down C3G or p38α individually or together promoted cell death in vitro, although only the double C3G-p38α silencing was able to increase cell death within tumors. Notably, we found that the pro-tumorigenic function of C3G does not depend on p38α or Rap1 activation. Altogether, our studies uncover novel mechanisms by which C3G controls key aspects of tumorigenesis.

Loss of has-miR-337-3p expression is associated with lymph node metastasis of human gastric cancer

Background

Metastasis is the major cause of cancer-related death in patients with gastric cancer, and aberrant expression of various microRNAs (miRNAs) is associated with cancer metastasis.

Methods

Profiling of differentially expressed miRNAs was performed in three cases of primary gastric cancer and the corresponding metastatic lymph node tissues. Then, the five most altered miRNAs were further verified in 16 paired samples. Two of these five miRNAs were further assessed for their effects on the regulation of gastric cancer cell growth and invasion.

Results

The miRNA profile data showed 151 upregulated miRNAs (≥ 1.5-fold) and 285 downregulated miRNAs (≤ 0.67-fold) in the metastatic tissues compared to the primary gastric cancer tissues. Among these five miRNAs (i.e., hsa-miR-508-5p, hsa-miR-30c, hsa-miR-337-3p, hsa-miR-483-5p, and hsa-miR-134), expression of hsa-miR-337-3p and hsa-miR-134 was significantly downregulated in these 16 lymph node metastatic tissues compared to their primary tumor tissues (P<0.05) and in nine gastric cancer cell lines compared to the nonmalignant GES cell line. Furthermore, induction of hsa-miR-134 or hsa-miR-337-3p expression did not dramatically affect gastric cancer cell proliferation, but transfection of the hsa-miR-337-3p mimic did reduce gastric cancer cell invasion capacity.

Conclusions

These findings indicate that hsa-miR-337-3p plays a role in the reduction of gastric cancer cell invasion capacity, and further studies on the mechanism of hsa-miR-337-3p in gastric cancer metastasis are warranted.

The gap junction protein Cx43 regulates B-lymphocyte spreading and adhesion

The gap junction protein connexin43 (Cx43) is widely expressed in mammalian cells and forms intercellular channels for the transfer of small molecules between adjacent cells, as well as hemichannels that mediate bidirectional transport of molecules between the cell and the surrounding environment. Cx43 regulates cell adhesion and migration in neurons and glioma cells, and we now show that Cx43 influences BCR-, LFA-1- and CXCL12-mediated activation of the Rap1 GTPase. Using shRNA knockdown of Cx43 in WEHI 231 cells, we show that Cx43 is required for sustained Rap1 activation and BCR-mediated spreading. To determine the domains of Cx43 that are important for this effect, Cx43-null J558 μm3 B cells (which express a wild-type IgM BCR) were transfected with wild-type Cx43-GFP or a C-terminal-truncated Cx43 (Cx43ΔT-GFP). Expression of wild-type Cx43-GFP, but not Cx43ΔT-GFP, was sufficient to restore sustained, BCR-mediated Rap1 activation and cell spreading. Cx43, and specifically the C-terminal domain, was also important for LFA-1- and CXCL12-mediated Rap1 activation, spreading and adhesion to an endothelial cell monolayer. These data show that Cx43 has an important and previously unreported role in B-cell processes that are essential to normal B-cell development and immune responses.