CD28 is recognized as the primary costimulatory molecule involved in the activation of naïve T cells. However, the biochemical signaling pathways that are activated by CD28 and how these pathways are integrated with TCR signaling are still not understood. We have recently shown that there are at least two independent activation pathways induced by CD28 costimulation. One is integrated with TCR signaling in the context of the immunological synapse and is mediated through transcriptional enhancement and the second is mediated through the induction of mRNA stability. Here, we review the immunological consequences and biochemical mechanisms associated with CD28 costimulation and discuss the major questions that need to be resolved to understand the molecular mechanisms that transduce CD28 costimulation.

One emerging paradigm of cellular organization of RNA and RNA binding proteins is the formation of membraneless organelles (MLOs). Examples of MLOs include several types of ribonucleoprotein granules that form via phase separation. A variety of intracellular pH changes and post-translational modifications, as well as extracellular stresses can stimulate the condensation of proteins into granules. For example, the assembly of stress granules induced by oxidative stress, osmotic stress, and heat stress has been well-characterized in a variety of somatic cell types. In the germ line, similar stress-induced condensation of proteins occurs; however, less is known about the role of phase separation during gamete production. Researchers who study phase transitions often make use of fluorescent reporters to study the dynamics of RNA binding proteins during live-cell imaging. In this report, we demonstrate that common conditions of live-imaging C. elegans can cause an inadvertent stress and trigger phase transitions of RNA binding proteins. We show this imaging-associated stress stimulates decondensation of multiple germ granule proteins, and condensation of several P-body proteins. Proteins within larger RNP granules in meiotically-arrested oocytes do not appear to be as sensitive to the stress as proteins in diakinesis oocytes of young hermaphrodites, with the exception of the germ granule protein PGL-1. Our results have important methodological implications for all researchers using live-cell imaging techniques. The data also suggest that the RNA binding proteins within large RNP granules of arrested oocytes may have distinct phases which we characterize in our companion paper.

The germ line provides an excellent in vivo system to study the regulation and function of RNP granules. Germ granules are conserved germ line-specific RNP granules that are positioned in the C. elegans adult gonad to function in RNA maintenance, regulation, and surveillance. In C. elegans, when oogenesis undergoes extended meiotic arrest, germ granule proteins and other RNA binding proteins assemble into much larger RNP granules whose hypothesized function is to regulate RNA metabolism and maintain oocyte quality. To gain insight into the function of oocyte RNP granules, in this report we characterize distinct phases for four protein components of RNP granules in arrested oocytes. We find the RNA binding protein PGL-1 is dynamic and has liquid-like properties, while the intrinsically disordered protein MEG-3 has gel-like properties, similar to the properties of the two proteins in small germ granules of embryos. We find that MEX-3 exhibits several gel-like properties but is more dynamic than MEG-3, while CGH-1 is dynamic but does not consistently exhibit liquid-like characteristics and may be an intermediate phase within RNP granules. These distinct phases of RNA binding proteins correspond to, and may underlie, differential responses to stress. Interestingly, in oocyte RNP granules MEG-3 is not required for the condensation of PGL-1 or other RNA binding proteins, which differs from the role of MEG-3 in small, embryonic germ granules. Lastly, we show the PUF-5 translational repressor appears to promote MEX-3 and MEG-3 condensation into large RNP granules; however, this role may be associated with regulation of oogenesis.

Cyst formation is a key feature of the T. gondii life cycle but the genetic networks that drive this process are not yet fully characterized. To identify new components of this network, we compared T. gondii to its nearest extant relative Hammondia hammondi given the critical differences between these species in the timing and efficiency of cyst formation. Using transcriptional data from critical developmental and pH exposure time points from both species, we identified the gene TGVEG_311100, which we named Regulator of Cystogenesis 1 (ROCY1), as being both necessary and sufficient for cyst formation in T. gondii. Compared to WT parasites, TGVEGΔROCY1 parasites formed significantly fewer tissue cysts in response to alkaline pH stress in vitro and cysts were nearly undetectable in mouse brains for up to 9 weeks post-infection. Overexpression of tagged ROCY1 in WT parasites was sufficient to induce cyst formation in vitro in both WT and ROCY1-deficient parasites, demonstrating that ROCY1 is both necessary and sufficient for cyst formation. Moreover this induction of cyst formation required at least 1 of 3 predicted CCCH Zinc finger domains. Mice chronically infected with ΔROCY1 parasites had detectable tachyzoites in the brain for up to 37 days post-infection (while mice infected with WT parasites did not), and CNS transcriptional analyses at day 30 post-infection throughout the chronic phase of infection revealed inflammatory signatures consistent with acute infection in ΔROCY1 parasites compared to WT. Despite our inability to detect brain cysts in infected mice, both WT and ΔROCY1 knockout parasites reactivated after dexamethasone treatment with similar timing and magnitude for up to 5 months post infection, challenging the paradigm that long term parasite persistence in the CNS requires cyst formation. These data identify a new regulator of cyst formation in T. gondii that is both necessary and sufficient for cyst formation, and whose function relies on its conserved nucleic acid binding motif.