Over the past 20 years great progress has been made in defining most of the key signalling pathways that functionally regulate immune cells. Recently, it has become clear that scaffold proteins have a crucial role in regulating many of these signalling cascades. By binding two or more components of a signalling pathway, scaffold proteins can help to localize signalling molecules to a specific part of the cell or to enhance the efficacy of a signalling pathway. Scaffold proteins can also affect the thresholds and the dynamics of signalling reactions by coordinating positive and negative feedback signals. In this Review, we focus on recent progress in the understanding of the function of scaffold proteins in immune cells.

Because mutations in RAS and BRAF represent the most common mutations found in human tumors, identification of inhibitors has been a major goal. Surprisingly, new oncogenic BRAF specific inhibitors inhibit cells transformed with mutated BRAF but paradoxically stimulate the growth of cells transformed with RAS. Here, we show that the mechanism for activation is via drug-induced dimer formation between CRAF and kinase suppressor of Ras (KSR)1. To understand the function of KSR1, we generated a KSR1 mutant that cannot bind ATP but stabilizes the closed, active conformation of KSR1. Molecular modeling suggested that the mutant stabilizes the two hydrophobic spines critical for the closed active conformation. We, therefore, could use the mutant to discriminate between the scaffold versus kinase functions of KSR1. The KSR1 mutant bound constitutively to RAF and mitogen-activated protein kinase kinase (MEK) but could not reconstitute activity suggesting that the catalytic activity of KSR1 is required for its function. Analogous mutations in BRAF and CRAF allowed us to test the generality of the model. The mutation induced changes consistent with the active, closed conformation of both kinases and confirmed that BRAF functions distinctly from CRAF in the MAP kinase pathway. Not only does this work suggest that KSR1 may function as a kinase, we anticipate that the mutation that we generated may be broadly applicable to stabilize the closed conformation of other kinases many of which may also form dimers.cancer | protein kinase | signal transduction

Growth hormone (GH) is an underappreciated but important regulator of T cell development that can reverse age-related declines in thymopoiesis in rodents. Here, we report findings of a prospective randomized study examining the effects of GH on the immune system of HIV-1-infected adults. GH treatment was associated with increased thymic mass. In addition, GH treatment enhanced thymic output, as measured by both the frequency of T cell receptor rearrangement excision circles in circulating T cells and the numbers of circulating naive and total CD4 + T cells. These findings provide compelling evidence that GH induces de novo T cell production and may, accordingly, facilitate CD4 + T cell recovery in HIV-1-infected adults. Further, these randomized, prospective data have shown that thymic involution can be pharmacologically reversed in humans, suggesting that immune-based therapies could be used to enhance thymopoiesis in immunodeficient individuals.

PEA-15 is a death effector domain-containing phosphoprotein that binds ERK and restricts it to the cytoplasm. PEA-15 also binds to FADD and thereby blocks apoptosis induced by death receptors. Abnormal expression of PEA-15 is associated with type II diabetes and some cancers; however, its physiological function remains unclear. To determine the function of PEA-15 in vivo, we used C57BL/6 mice in which the PEA-15 coding region was deleted. We thereby found that PEA-15 regulates T-cell proliferation. PEA-15-null mice did not have altered thymic or splenic lymphocyte cellularity or differentiation. However, PEA-15 deficient T cells had increased CD3/CD28-induced nuclear translocation of ERK and increased activation of IL-2 transcription and secretion in comparison to control wild-type littermates. Indeed, activation of the T-cell receptor in wild-type mice caused PEA-15 release of ERK. In contrast, overexpression of PEA-15 in Jurkat T cells blocked nuclear translocation of ERK and IL-2 transcription. Finally, PEA-15-null T cells showed increased IL-2 dependent proliferation on stimulation. No differences in T cell susceptibility to apoptosis were found. Thus, PEA-15 is a novel player in T-cell homeostasis. As such this work may have far reaching implications in understanding how the immune response is controlled.

Polarization of T cells involves reorientation of the microtubule-organizing center (MTOC). Because activated ERK is localized at the immunological synapse, we investigated its role by showing that ERK activation is important for MTOC polarization. Suspecting that ERK phosphorylates a regulator of microtubules, we next focused on stathmin, a known ERK substrate. Our work indicates that during T cell activation, ERK is recruited to the synapse allowing it to phosphorylate stathmin molecules near the immunological synapse. Supporting an important role of stathmin phosphorylation in T cell activation, we showed that T cell activation results in increased microtubule growth rate dependent on the presence of stathmin. The significance of this finding was demonstrated by results showing that CTL from stathmin−/− mice displayed defective MTOC polarization and defective target cell cytolysis. These data implicate stathmin as a regulator of the microtubule network during T cell activation.