The T-cell costimulatory receptors, CD28 and the inducible costimulator (ICOS), are required for the generation of follicular B helper T cells (T FH) and germinal center (GC) reaction. A common signal transducer used by CD28 and ICOS is the phosphoinositide 3-kinase (PI3K). Although it is known that CD28-mediated PI3K activation is dispensable for GC reaction, the role of ICOS-driven PI3K signaling has not been defined. We show here that knock-in mice that selectively lost the ability to activate PI3K through ICOS had severe defects in T FH generation, GC reaction, antibody class switch, and antibody affinity maturation. In preactivated CD4 ؉ T cells, ICOS delivered a potent PI3K signal that was critical for the induction of the key T FH cytokines, IL-21 and IL-4. Under the same settings, CD28 was unable to activate PI3K but supported a robust secondary expansion of T cells. Thus, our results demonstrate a nonredundant function of ICOS-PI3K pathway in the generation of T FH and suggest that CD28 and ICOS play differential roles during a multistep process of T FH differentiation.CD28 ͉ follicular B helper T-cell ͉ germinal center ͉ ICOS ͉ PI3K

The myocyte enhancer factor 2 (MEF2) family of transcription factors is not only important for controlling gene expression in normal cellular programs, like muscle differentiation, T-cell apoptosis, neuronal survival, and synaptic differentiation, but has also been linked to cardiac hypertrophy and other pathological conditions. Lysine acetylation has been shown to modulate MEF2 function, but it is not so clear which deacetylase(s) is involved. We report here that treatment of HEK293 cells with trichostatin A or nicotinamide upregulated MEF2D acetylation, suggesting that different deacetylases catalyze the deacetylation. Related to the trichostatin A sensitivity, histone deacetylase 4 (HDAC4) and HDAC5, two known partners of MEF2, exhibited little deacetylase activity towards MEF2D. In contrast, HDAC3 efficiently deacetylated MEF2D in vitro and in vivo. This was specific, since HDAC1, HDAC2, and HDAC8 failed to do so. While HDAC4, HDAC5, HDAC7, and HDAC9 are known to recognize primarily the MEF2-specific domain, we found that HDAC3 interacts directly with the MADS box. In addition, HDAC3 associated with the acetyltransferases p300 and p300/CBP-associated factor (PCAF) to reverse autoacetylation. Furthermore, the nuclear receptor corepressor SMRT (silencing mediator of retinoid acid and thyroid hormone receptor) stimulated the deacetylase activity of HDAC3 towards MEF2 and PCAF. Supporting the physical interaction and deacetylase activity, HDAC3 repressed MEF2-dependent transcription and inhibited myogenesis. These results reveal an unexpected role for HDAC3 and suggest a novel pathway through which MEF2 activity is controlled in vivo.Protein lysine acetylation refers to transfer of the acetyl moiety from acetyl coenzyme A (acetyl-CoA) to the ε-amino group of a lysine residue and is an important posttranslational modification that has recently emerged and rivals phosphorylation (41, 61). Proteins known to be subject to lysine acetylation include histones, over 50 transcription factors, and various other proteins (10,40,41,61,77). This dynamic modification is controlled by the opposing actions of acetyltransferases and deacetylases in vivo. Histones were the first substrates identified, so these two families of enzymes are known as histone acetyltransferases (HATs) and histone deacetylases (HDACs), although most of them also act on nonhistone proteins. In the past decade, many proteins have been shown to possess HDAC activity (4,22,39,66,78). On the basis of homology to budding yeast counterparts, human HDACs are grouped into four classes, with HDAC1, -2, -3, and -8, homologs of yeast Rpd3, forming class I. Class II comprises HDAC4, -5, -6, -7, -9, and -10, which possess deacetylase domains highly related to that of yeast Hda1. HDAC4, -5, -7, and -9 have similar domain organization and thus belong to a subgroup known as class IIa.Class III consists of SIRT1 and other Sir2-related proteins. A recent phylogenetic analysis revealed that HDAC11 represents class IV (21). Members of classes I, II, and IV are zinc-depend...

The development of more intricate devices for the analysis of small molecules and protein activity in single cells would advance our knowledge of cellular heterogeneity and signaling cascades. Therefore, in this study, a nanokit was produced by filling a nanometersized capillary with a ring electrode at the tip with components from traditional kits, which could be egressed outside the capillary by electrochemical pumping. At the tip, femtoliter amounts of the kit components were reacted with the analyte to generate hydrogen peroxide for the electrochemical measurement by the ring electrode. Taking advantage of the nanotip and small volume injection, the nanokit was easily inserted into a single cell to determine the intracellular glucose levels and sphingomyelinase (SMase) activity, which had rarely been achieved. High cellular heterogeneities of these two molecules were observed, showing the significance of the nanokit. Compared with the current methods that use a complicated structural design or surface functionalization for the recognition of the analytes, the nanokit has adapted features of the well-established kits and integrated the kit components and detector in one nanometer-sized capillary, which provides a specific device to characterize the reactivity and concentrations of cellular compounds in single cells.

How extracellular cues are transduced to the nucleus is a fundamental issue in biology. The paralogous WW-domain proteins YAP (Yes-associated protein) and TAZ (transcriptional coactivator with PDZ-binding motif; also known as WWTR1, for WW-domain containing transcription regulator 1) constitute a pair of transducers linking cytoplasmic signaling events to transcriptional regulation in the nucleus. A cascade composed of mammalian Ste20-like (MST) and large tumor suppressor (LATS) kinases directs multisite phosphorylation, promotes 14-3-3 binding, and hinders nuclear import of YAP and TAZ, thereby inhibiting their transcriptional coactivator and growth-promoting activities. A similar cascade regulates the trafficking and function of Yorkie, the fly orthologue of YAP. Mammalian YAP and TAZ are expressed in various tissues and serve as coregulators for transcriptional enhancer factors (TEFs; also referred to as TEADs, for TEA-domain proteins), runt-domain transcription factors (Runxs), peroxisome proliferator-activated receptor gamma (PPARgamma), T-box transcription factor 5 (Tbx5), and several others. YAP and TAZ play distinct roles during mouse development. Both, and their upstream regulators, are intimately linked to tumorigenesis and other pathogenic processes. Here, we review studies on this family of signal-responsive transcriptional coregulators and emphasize how relative sequence conservation predicates their function and regulation, to provide a conceptual framework for organizing available information and seeking new knowledge about these signal transducers.