For example, inhibition of ERK by the MEK inhibitor, PD98059, in fetal thymic organ cultures showed no defects in either anti-CD3-mediated or HY TCR male antigen-mediated negative selection 12. On the contrary, another group using the P14 TCR transgenic fetal thymic organ cultures showed defects in negative
selleck chemical selection with the same inhibitor 8 and was confirmed in at least two other transgenic TCR models 6. More recently, Hedrick’s group showed that there was no negative selection defect in ERK1/2 double knockout OT-I CD8+ transgenic TCR thymocytes both in vitro and in vivo13. Our results with KSR1-deficient mice showing a mild negative selection defect in HY-thymocytes is consistent with a role for ERK in negative selection but could be due to some idiosyncrasy with the HY TCR transgenic system. It is also possible find more that the role of ERK in negative selection is dependent on differences in the affinity of the pMHC:TCR complex. Although all the previous studies show that the absence of KSR1 leads to the general attenuation of ERK activation, we were surprised to find that the role of KSR1 was more important for PMA than for CD3 stimulation. To our knowledge, these are the first data that implicate a scaffold in one but not another similar pathway.
One possible explanation is that PMA stimulates a second pathway that enhances KSR1 recruitment to the membrane. Since PMA stimulates Ras exclusively via RasGRP and CD3 stimulates Ras through both RasGRP and SOS 38, another possibility
is that KSR1 might function specifically in the RasGRP but not the SOS pathway. We are currently exploring between these possibilities and others using a variety of biological and computational approaches. We also noted that the magnitude of the ERK defect varied by thymocyte subset. After CD3 stimulation, the ERK defect was greatest in the SP subsets and less in the DP and DN subsets. The relatively small defect in ERK activation after CD3 stimulation in the DP subset could explain the absence of a developmental defect in KSR1-deficient thymocytes. What explains the differences of KSR1 function in thymocyte subsets is unclear, Interleukin-2 receptor but it is interesting to speculate that this is due to differences in the signaling potential between SP versus DN and DP cells. DN and DP cells exhibit low-level expression of both the TCR and the RasGRP that would lead one to speculate they have a low signaling potential 39. Lower overall levels of Ras activation might result in changes between the ratio of activated Ras and the number of KSR1 molecules, which are known to influence the efficiency of KSR1-mediated ERK activation 35, 40. It is also possible that the decreased signaling potential influences the feedback loops between RasGRP and SOS, leading to unexpected changes in levels of ERK activation 41.