Regulation of serum response factor activation by the B cell antigen receptor
Open Access
- Author:
- Hao, Shengli
- Graduate Program:
- Integrative Biosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 09, 2003
- Committee Members:
- Avery August, Committee Chair/Co-Chair
Robert Paulson, Committee Member
Andrew Thomas Henderson, Committee Member
Pamela Hankey Giblin, Committee Member
Sally Johnson, Committee Member - Keywords:
- BCR
NFAT
Ca2+
DAG
actin
IP3R
Lyn
Syk
Btk
PLCg
ERK
BCR signal
SRF - Abstract:
- ABSTRACT How B cell antigen receptor (BCR) signals lead to different cell responses is one of the central questions in immunology. In this thesis, we examined the transcription factors Serum Response Factor (SRF) and NFAT as model systems to study how antigen receptor signal specifically regulates SRF vs NFAT. SRF and NFAT are important for cell proliferation, cell survival, and cytokine production. However, how SRF is regulated by antigen receptor signal is unknown. Here we show that SRF is activated by BCR stimulation via an Src-Syk-Tec-PLCg-Ca2+ (Lyn-Syk-Btk-PLCg2-Ca2+) pathway. Although this pathway is also used to activate NFAT, SRF and NFAT are differently regulated by the downstream Ca2+ and diacylglycerol (DAG) signals. SRF responds to lower levels of Ca2+ and is less dependent on IP3R expression than NFAT. In addition, the Ca2+-dependent phosphatase calcineurin plays a partial role in SRF activation, while it is fully required for NFAT activation. Signals from the DAG effectors PKC, Ras and Rap1, and the downstream MEK-ERK pathway are required for both SRF and NFAT, however, NFAT but not SRF is also dependent on JNK signals. We also show that Ca2+ and DAG signals act in concert to regulate transcription factors in that the nuclear association of ERK is dependent on Ca2+ signals, although its activation is Ca2+ independent. Actin cytoskeleton is another regulator of SRF in various adherent cells. In lymphocytes, the actin cytoskeleton has been found essential for antigen receptor signaling. Filamentous actin (F-actin) or actin polymerization process has been proposed to play positive roles in T cell activation stimulated by antigen presenting cells. We found that actin polymerization or F-actin plays negative roles as well in transducing BCR signals. Our results demonstrate that induction of actin depolymerization enhances and stablization of F-actin inhibits BCR induced SRF and NFAT activation and the sustained phase of Ca2+ and ERK signals. Furthermore, a rapid induction of actin depolymerization is detected by BCR stimulation prior to polarized actin repolymerization, and the degree of actin depolymerization correlates with the strength of BCR stimulation. We further show that actin depolymerization by BCR stimulation is triggered by intracellular Ca2+ increase. These results suggest that the actin cytoskeleton serves as a transducer for the strength of antigen receptor stimulation and act as a coupler between early BCR signals to the late signaling events. We also found that the actin cytoskeleton modulates BCR signals by regulating the dynamics of lipid rafts, the special lipid domains on the plasma membrane that serve as signaling platform. Forced actin depolymerization delays the internalization of lipid rafts after BCR stimulation. Surprisingly, actin depolymerization by itself activates ERK and induces lipid raft clustering. We therefore propose that actin cytoskeleton has two novel roles in modulating BCR signals by regulating lipid raft clustering. First, actin cytoskeleton at the resting state keeps lipid rafts apart, thus preventing non-specific cell activation. Second, actin cytoskeleton regulates the extent and duration of the lipid raft clustering in response to different strength of BCR stimulation. In conclusion, this work describes how a single BCR can differently regulate SRF and NFAT. This work also shows that actin superstructure modulates the strength of the BCR signals.