Notch2 paves the way to mast cells by Hes1 and Gata3

Once regarded only as a nuisance cell that mediates allergy, mast cells (MCs) have recently gained increasing recognition as a significant regulator of the physiologic and pathologic immune response. For example, MCs not only protect the host against parasites but also play an important antibacterial role in murine models of peritonitis in which MC-deficient mice are acutely sensitive to septic shock-induced death (1, 2). Furthermore, in pathologic immune processes, MCs have been implicated in hypersensitivity and in several autoimmune diseases (3).

Mast cells leave the bone marrow as committed mast cell progenitors and only undergo full maturation in target tissues. In response to certain types of inflammation, new recruitment of MC progenitors to inflamed tissues and proliferation of existing mature MCs result in robust hyperplasia of MCs in mucosal tissues. MC differentiation and expansion processes are only beginning to be characterized, and the work of Sakata-Yanagimoto et al. (4) in this issue of PNAS suggests that Notch signaling may be involved.

Notch is a transmembrane receptor that regulates numerous cellular and developmental processes in multicellular organisms. Four Notch receptors (Notch1–4) and twoNotch ligand families (Delta and Jagged) have been described in mammals. Upon ligand binding, the Notch receptor undergoes cleavage, releasing the intracellular domain of Notch (ICN). ICN translocates to the nucleus where it forms a complex with CSL (CBF-1 Suppressor of Hairless-Lag1, or RBP-J) and a MAML family member, which results in transcriptional activation.

Evidence that the Notch pathway may play a role in MC development and/or inflammation-induced MC hyperplasia includes expression of low levels of Notch1, high levels of Notch2, and Jagged-1 in bone marrow cultured MCs (5, 6). Additionally, recent work by Taghon and colleagues (7–9) demonstrates that thymic progenitor cells that have received Notch signals can be redirected to the mast cell fate by overexpression of GATA3, which is a direct Notch transcriptional target in some contexts.

In their article, Sakata-Yanagimoto et al. (4) studied the effects of the Notch ligand Delta-1 on common myeloid progenitors (CMPs) and granulocyte monocyte progenitors (GMPs). Both of these cell types can give rise to MCs but also give rise to other myeloid lineages, such as granulocytes and macrophages. In the presence of plate-bound Delta-1, CMPs and GMPs preferentially gave rise to MCs at the expense of granulocytes and macrophages. Furthermore, this enhancement of MC development depends on the expression of Notch2, because Notch2-deficient CMPs or GMPs do not potentiate MC development.

To identify the signals that mediate Notch2-directed MC development, Sakata-Yanagimoto et al. (4) introduced two putative downstream targets of Notch2, Hes-1 and GATA3, into CMPs and GMPs. Coexpression of both Hes-1 and GATA3 promoted MC development, whereas neither Hes-1 nor GATA3 alone generated mast cells from CMPs or GMPs. Sakata-Yanagimoto et al. also demonstrated that retroviral expression of Hes-1 in CMPs and GMPs down-regulated C/EBPα mRNA. These data are significant because previous studies showed that excision of C/EBPα enhanced MC development, whereas C/EBPα overexpression inhibited MC development (10).

Together, these results suggest that Notch signaling and its downstream components play a role in mast cell development as summarized in Fig. 1. These results may appear inconsistent with studies from Taghon et al. (7) who demonstrated that introduction of GATA3 alone into thymocytes results in mast cell generation. However, several possibilities can explain this paradox: (i) lymphoid progenitors and myeloid progenitors operate on different transcriptional networks, with distinct requirements for Notch activity; (ii) the specific Notch receptors expressed in these cells differ (MCs express Notch 1 and Notch2, whereas thymocytes express Notch1 and Notch3); (iii) the intensity of Notch signaling required for MC development may be lower than that required for T cell development; and (iv) the pro-T cells that were redirected to MCs had been exposed to Notch signaling. The results of both Taghon et al. and Sakata-Yanagimoto et al. (4) indicate a need for further characterization of the role for Notch in mast cell development to address the aforementioned as well as other unanswered questions. It will be interesting to determine whether GATA3 has a physiologic role in MC development (which to date has not been shown) or whether Notch can also up-regulate GATA1 or GATA2 in vivo. Furthermore, Notch1 directly binds the proximal GATA3 promoter in peripheral CD4⁺ T cells (9, 11); however, it remains to be determined whether GATA3 is a direct target of Notch2 in CMPs, GMPs, or more mature MCs.

Fig. 1.

The hematopoietic stem cell (HSC) can differentiate into a common myeloid progenitor (CMP), which further differentiates into a granulocyte monocyte progenitor (GMP). Sakata-Yanagimoto et al. (4) show that Notch signaling increases mast cell development by increasing expression of GATA3 and Hes1. Recent data from Taghon et al. (7) demonstrate that thymic progenitors, which have already received Notch signaling (such as ETP/DN1 and DN2), but not those that express the pre-T cell receptor, such as DN3 can develop into mast cells when placed in cytokines conducive to mast cell development. Redirection to mast cells requires down-regulation of Notch signals whereas progression to DN3 cells requires maintenance of Notch signaling.

It is also unclear whether Notch2 is essential for MC development. Sakata-Yanagimoto et al. (4) mentioned that conditional deletion of Notch2 resulted in normal steady state development of mast cells in Notch2^−/− mice. Although Notch2 may not have an essential function in MC development, it is also possible that compensatory expression of other Notch receptors may conceal the Notch2-dependent developmental defects. Alternatively, Hes1 and Gata3 up-regulation may occur independently of Notch signaling; for example, E2A can directly up-regulate Hes1 expression (12). Other questions that remain open to study are the identification of the role and specific mediators of Notch signaling both in the generation of tissue-specific MCs and in inflammation-induced MC hyperplasia. In summary, the identification of a connection between Notch signaling and MC development by Hes1 and Gata3 provides new clues to unraveling the mechanisms behind mast cell differentiation and function.