Coactivator-associated arginine methyltransferase 1 regulates fetal hematopoiesis and thymocyte development.

Coactivator-associated arginine methyltransferase 1 (CARM1) is a protein arginine methyltransferase that methylates histones and transcriptional regulators. We previously reported that the absence of CARM1 partially blocks thymocyte differentiation at embryonic day 18.5 (E18.5). In this study, we find that reduced thymopoiesis in Carm1(-/-) mice is due to a defect in the fetal hematopoietic compartment rather than in the thymic stroma. To determine the cellular basis for impaired thymopoiesis, we examined the number and function of fetal liver (FL) and bone marrow cells. Despite markedly reduced cellularity of hematopoietic progenitors in E18.5 bone marrow, the number of long-term hematopoietic stem cells and downstream subsets was not reduced in Carm1(-/-) E14.5 or E18.5 FL. Nevertheless, competitive reconstitution assays revealed a deficit in the ability of Carm1(-/-) FL cells to contribute to hematopoiesis. Furthermore, impaired differentiation of Carm1(-/-) FL cells in a CARM1-sufficient host showed that CARM1 is required cell autonomously in hematopoietic cells. Coculture of Carm1(-/-) FL cells on OP9-DL1 monolayers showed that CARM1 is required for survival of hematopoietic progenitors under conditions that promote differentiation. Taken together, this report demonstrates that CARM1 is a key epigenetic regulator of hematopoiesis that affects multiple lineages at various stages of differentiation.

Enzymatic activity is required for the in vivo functions of CARM1.

CARM1 is one of nine protein arginine methyltransferases that methylate arginine residues in proteins. CARM1 is recruited by many different transcription factors as a positive regulator. Gene targeting of CARM1 in mice has been performed, and knock-out mice, which are smaller than their wild-type littermates, die just after birth. It has been proposed that CARM1 has functions that are independent of its enzymatic activity. Indeed, CARM1 is found to interact with a number of proteins and may have a scaffolding function in this context. However, CARM1 methylates histone H3, PABP1, AIB1, and a number of splicing factors, which strongly suggests that its impact on transcription and splicing is primarily through its ability to modify these substrates. To unequivocally establish the importance of CARM1 enzymatic activity in vivo, we generated an enzyme-dead knock-in of this protein arginine methyltransferase. We determined that knock-in cells and mice have defects similar to those seen in their knock-out counterparts with respect to the time of embryo lethality, T cell development, adipocyte differentiation, and transcriptional coactivator activity. CARM1 requires its enzymatic activity for all of its known cellular functions. Thus, small molecule inhibitors of CARM1 will incapacitate all of the enzyme's cellular functions.

IL7-hCD25 and IL7-Cre BAC transgenic mouse lines: new tools for analysis of IL-7 expressing cells.

IL-7 is a cytokine that is required for T-cell development and homeostasis as well as for lymph node organogenesis. Despite the importance of IL-7 in the immune system and its potential therapeutic relevance, questions remain regarding the sites of IL-7 synthesis, specific cell types involved and molecular mechanisms regulating IL-7 expression. To address these issues, we generated two bacterial artificial chromosome (BAC) transgenic mouse lines in which IL-7 regulatory elements drive expression of either Cre recombinase or a human CD25 (hCD25) cell surface reporter molecule. Expression of the IL-7.hCD25 BAC transgene, detected by reactivity with anti-hCD25 antibody, mimicked endogenous IL-7 expression. Fetal and adult tissues from crosses between IL-7.Cre transgenic mice and Rosa26R or R26-EYFP reporters demonstrated X-gal or YFP staining in tissues known to express endogenous IL-7 at some stage during development. These transgenic lines provide novel genetic tools to identify IL-7 producing cells in various tissues and to manipulate gene expression selectively in IL-7 expressing cells.

Increased thymus- and decreased parathyroid-fated organ domains in Splotch mutant embryos.

Embryos that are homozygous for Splotch, a null allele of Pax3, have a severe neural crest cell (NCC) deficiency that generates a complex phenotype including spina bifida, exencephaly and cardiac outflow tract abnormalities. Contrary to the widely held perception that thymus aplasia or hypoplasia is a characteristic feature of Pax3(Sp/Sp) embryos, we find that thymic rudiments are larger and parathyroid rudiments are smaller in E11.5-12.5 Pax3(Sp/Sp) compared to Pax3(+/+) embryos. The thymus originates from bilateral third pharyngeal pouch primordia containing endodermal progenitors of both thymus and parathyroid glands. Analyses of Foxn1 and Gcm2 expression revealed a dorsal shift in the border between parathyroid- and thymus-fated domains at E11.5, with no change in the overall cellularity or volume of each shared primordium. The border shift increases the allocation of third pouch progenitors to the thymus domain and correspondingly decreases allocation to the parathyroid domain. Initial patterning in the E10.5 pouch was normal suggesting that the observed change in the location of the organ domain interface arises during border refinement between E10.5 and E11.5. Given the well-characterized NCC defects in Splotch mutants, these findings implicate NCCs in regulating patterning of third pouch endoderm into thymus- versus parathyroid-specified domains, and suggest that organ size is determined in part by the number of progenitor cells specified to a given fate.

Loss of CARM1 results in hypomethylation of thymocyte cyclic AMP-regulated phosphoprotein and deregulated early T cell development.

The coactivator-associated arginine methyltransferase, CARM1, is a positive regulator of transcription. Using high density protein arrays, we have previously identified in vitro substrates for CARM1. One of these substrates, TARPP (thymocyte cyclic AMP-regulated phosphoprotein), is expressed specifically in immature thymocytes. Here, we have demonstrated that TARPP is arginine-methylated at a single residue, Arg(650), both in vitro and in vivo. In addition, recombinant TARPP is not methylated by extracts from Carm1(-/-) cells, indicating that there is no redundancy in this pathway. We show that thymi from Carm1(-/-) embryos (E18.5) have a 5-10-fold reduction in cellularity compared with wild type littermates. Flow cytometric analysis of thymocytes revealed a decrease in the relative proportion of double negative thymocytes in Carm1(-/-) embryos because of a partial developmental arrest in the earliest thymocyte progenitor subset. These results demonstrate that CARM1 plays a significant role in promoting the differentiation of early thymocyte progenitors, possibly through its direct action on TARPP.

Cutting edge: thymocyte-independent and thymocyte-dependent phases of epithelial patterning in the fetal thymus.

Thymic epithelial cells (TECs) in adult mice have been classified into distinct subsets based on keratin expression profiles. To explore the emergence of TEC subsets during ontogeny, we analyzed keratin 8 and keratin 5 expression at several stages of fetal development in normal C57BL/6J mice. In addition, thymic epithelial development and compartmentalization were explored in recombination-activating gene 2/common cytokine receptor gamma-chain-deficient and Ikaros-null mice that sustain early and profound blocks in thymocyte differentiation. The results demonstrate that initial patterning of the thymic epithelial compartment as defined by differential keratin expression does not depend on inductive signals from hematopoietic cells. However, thymocyte-derived signals are required during late fetal stages for continued development and maintenance of TEC subsets in the neonate and adult.