Fragile X Mental Retardation Protein (FMRP) controls diacylglycerol kinase activity in neurons.

Fragile X syndrome (FXS) is caused by the absence of the Fragile X Mental Retardation Protein (FMRP) in neurons. In the mouse, the lack of FMRP is associated with an excessive translation of hundreds of neuronal proteins, notably including postsynaptic proteins. This local protein synthesis deregulation is proposed to underlie the observed defects of glutamatergic synapse maturation and function and to affect preferentially the hundreds of mRNA species that were reported to bind to FMRP. How FMRP impacts synaptic protein translation and which mRNAs are most important for the pathology remain unclear. Here we show by cross-linking immunoprecipitation in cortical neurons that FMRP is mostly associated with one unique mRNA: diacylglycerol kinase kappa (Dgkκ), a master regulator that controls the switch between diacylglycerol and phosphatidic acid signaling pathways. The absence of FMRP in neurons abolishes group 1 metabotropic glutamate receptor-dependent DGK activity combined with a loss of Dgkκ expression. The reduction of Dgkκ in neurons is sufficient to cause dendritic spine abnormalities, synaptic plasticity alterations, and behavior disorders similar to those observed in the FXS mouse model. Overexpression of Dgkκ in neurons is able to rescue the dendritic spine defects of the Fragile X Mental Retardation 1 gene KO neurons. Together, these data suggest that Dgkκ deregulation contributes to FXS pathology and support a model where FMRP, by controlling the translation of Dgkκ, indirectly controls synaptic proteins translation and membrane properties by impacting lipid signaling in dendritic spine.

G-quadruplex RNA structure as a signal for neurite mRNA targeting.

Targeting of messenger RNAs (mRNAs) in neuron processes relies on cis-acting regulatory elements, the nature of which is poorly understood. Here, we report that approximately 30% of the best-known dendritic mRNAs contain a guanine (G)-quadruplex consensus in their 3'-untranslated region. Among these mRNAs, we show by using RNA structure probing that a G-quadruplex is present in the mRNAs of two key postsynaptic proteins: PSD-95 and CaMKIIa. The G-quadruplex structure is necessary and sufficient for the potent and fast localization of mRNAs in cortical neurites and this occurs in a metabotropic glutamate receptor-responsive manner. Thus, G-quadruplex seems to be a common neurite localization signal.

Corrigendum: IL-3 produced by T cells is crucial for basophil extravasation in hapten- induced allergic contact dermatitis.

[This corrects the article DOI: 10.3389/fimmu.2023.1151468.].

IL-3 produced by T cells is crucial for basophil extravasation in hapten-induced allergic contact dermatitis.

Basophils have been recognized as a characterized cellular player for Th2 immune responses implicated in allergic diseases, but the mechanisms responsible for basophil recruitment to allergic skin remain not well understood. Using a hapten fluorescein isothiocyanate (FITC)-induced allergic contact dermatitis (ACD) mouse model, we show that basophils in FITC-treated IL-3-knockout mice are defective in crossing the vascular endothelium to enter the inflamed skin. By generating mice in which IL-3 is selectively ablated in T cells, we further demonstrate that IL-3 produced by T cells mediates basophil extravasation. Moreover, basophils sorted from FITC-treated IL-3-knockout mice exhibit a decreased expression of integrins Itgam, Itgb2, Itga2b and Itgb7, which are potentially implicated in extravasation process. Interestingly, we observed that these basophils had a reduced expression of retinaldehyde dehydrogenase 1 family member A2 (Aldh1a2), an enzyme responsible for the production of retinoic acid (RA), and administration of all-trans RA restored partially the extravasation of basophils in IL-3-knockout mice. Finally, we validate that IL-3 induces the expression of ALDH1A2 in primary human basophils, and provide further evidence that IL-3 stimulation induces the expression of integrins particularly ITGB7 in an RA-dependent manner. Together, our data propose a model that IL-3 produced by T cells activates ALDH1A2 expression by basophils, leading to the production of RA, which subsequently induces the expression of integrins crucially implicated in basophil extravasation to inflamed ACD skin.

Context-dependent function of TSLP and IL-1ß in skin allergic sensitization and atopic march.

Atopic diseases, including atopic dermatitis (AD) and asthma, affect a large proportion of the population, with increasing prevalence worldwide. AD often precedes the development of asthma, known as the atopic march. Allergen sensitization developed through the barrier-defective skin of AD has been recognized to be a critical step leading to asthma, in which thymic stromal lymphopoietin (TSLP) was previously shown to be critical. In this study, using a laser-assistant microporation system to disrupt targeted skin layers for generating micropores at a precise anatomic depth of mouse skin, we model allergen exposure superficially or deeply in the skin, leading to epicutaneous sensitization or dermacutaneous sensitization that is associated with a different cytokine microenvironment. Our work shows a differential requirement for TSLP in these two contexts, and identifies an important function for IL-1ß, which is independent of TSLP, in promoting allergen sensitization and subsequent allergic asthma.

Keratinocyte-derived cytokine TSLP promotes growth and metastasis of melanoma by regulating the tumor-associated immune microenvironment.

Malignant melanoma is a major public health issue displaying frequent resistance to targeted therapy and immunotherapy. A major challenge lies in better understanding how melanoma cells evade immune elimination and how tumor growth and metastasis is facilitated by the tumor microenvironment. Here, we show that expression of the cytokine thymic stromal lymphopoietin (TSLP) by epidermal keratinocytes is induced by cutaneous melanoma in both mice and humans. Using genetically engineered models of melanoma and tumor cell grafting combined with TSLP-KO or overexpression, we defined a crosstalk between melanoma cells, keratinocytes, and immune cells in establishing a tumor-promoting microenvironment. Keratinocyte-derived TSLP is induced by signals derived from melanoma cells and subsequently acts via immune cells to promote melanoma progression and metastasis. Furthermore, we show that TSLP signals through TSLP receptor-expressing (TSLPR-expressing) DCs to play an unrecognized role in promoting GATA3+ Tregs expressing a gene signature including ST2, CCR8, ICOS, PD-1, CTLA-4, and OX40 and exhibiting a potent suppressive activity on CD8+ T cell proliferation and IFN-γ production. An analogous population of GATA3-expressing Tregs was also identified in human melanoma tumors. Our study provides insights into the role of TSLP in programming a protumoral immune microenvironment in cutaneous melanoma.

Cells lacking the fragile X mental retardation protein (FMRP) have normal RISC activity but exhibit altered stress granule assembly.

The fragile X mental retardation protein (FMRP) is an RNA-binding protein involved in the mRNA metabolism. The absence of FMRP in neurons leads to alterations of the synaptic plasticity, probably as a result of translation regulation defects. The exact molecular mechanisms by which FMRP plays a role in translation regulation have remained elusive. The finding of an interaction between FMRP and the RNA interference silencing complex (RISC), a master of translation regulation, has suggested that both regulators could be functionally linked. We investigated here this link, and we show that FMRP exhibits little overlap both physically and functionally with the RISC machinery, excluding a direct impact of FMRP on RISC function. Our data indicate that FMRP and RISC are associated to distinct pools of mRNAs. FMRP, unlike RISC machinery, associates with the pool of mRNAs that eventually goes into stress granules upon cellular stress. Furthermore, we show that FMRP plays a positive role in this process as the lack of FMRP or a point mutant causing a severe fragile X alter stress granule formation. Our data support the proposal that FMRP plays a role in controlling the fate of mRNAs after translation arrest.

Induction of somatic hypermutation in immunoglobulin genes is dependent on DNA polymerase iota.

Somatic hypermutation of immunoglobulin genes is a unique, targeted, adaptive process. While B cells are engaged in germinal centres in T-dependent responses, single base substitutions are introduced in the expressed Vh/Vl genes to allow the selection of mutants with a higher affinity for the immunizing antigen. Almost every possible DNA transaction has been proposed to explain this process, but each of these models includes an error-prone DNA synthesis step that introduces the mutations. The Y family of DNA polymerases--pol eta, pol iota, pol kappa and rev1--are specialized for copying DNA lesions and have high rates of error when copying a normal DNA template. By performing gene inactivation in a Burkitt's lymphoma cell line inducible for hypermutation, we show here that somatic hypermutation is dependent on DNA polymerase iota.

Cutting edge: DNA polymerases mu and lambda are dispensable for Ig gene hypermutation.

Mutations arising in Ig V genes during an immune response are most likely introduced by one or several error-prone DNA polymerases. Many of the recently described nonreplicative DNA polymerases have an intrinsic fidelity compatible with such an activity, the strongest candidates being polymerase (pol) eta, pol iota, pol zeta, and pol mu. We report in this work that mice inactivated for either of the two polymerases related to pol beta (i.e., pol mu and pol lambda) are viable and fertile and display a normal hypermutation pattern.