Itk, a Tec family tyrosine kinase, plays an important but as yet undefined role in T cell receptor (TCR) signaling. Here we show that T cells from Itk-deficient mice have a TCR-proximal signaling defect, resulting in defective interleukin 2 secretion. Upon TCR stimulation, Itk−/− T cells release normal amounts of calcium from intracellular stores, but fail to open plasma membrane calcium channels. Since thapsigargin-induced store depletion triggers normal calcium entry in Itk−/− T cells, an impaired biochemical link between store depletion and channel opening is unlikely to be responsible for this defect. Biochemical studies indicate that TCR-induced inositol 1,4,5 tris-phosphate (IP3) generation and phospholipase C γ1 tyrosine phosphorylation are substantially reduced in Itk−/− T cells. In contrast, TCR-ζ and ZAP-70 are phosphorylated normally, suggesting that Itk functions downstream of, or in parallel to, ZAP-70 to facilitate TCR-induced IP3 production. These findings support a model in which quantitative differences in cytosolic IP3 trigger distinct responses, and in which only high concentrations of IP3 trigger the influx of extracellular calcium.

Biochemical studies of signaling mediated by many cytokine and growth factor receptors have implicated members of the Jak family of tyrosine kinases in these pathways. Specifically, Jak3 has been shown to be associated with the interleukin-2 (IL-2) receptor gamma chain, a component of the receptors for IL-2, IL-4, IL-7, IL-9, and IL-15. Mice lacking Jak3 showed a severe block in B cell development at the pre-B stage in the bone marrow. In contrast, although the thymuses of these mice were small, T cell maturation progressed relatively normally. In response to mitogenic signals, peripheral T cells in Jak3-deficient mice did not proliferate and secreted small amounts of IL-2. These data demonstrate that Jak3 is critical for the progression of B cell development in the bone marrow and for the functional competence of mature T cells.

Profilins are thought to be essential for regulation of actin assembly. However, the functions of profilins in mammalian tissues are not well understood. In mice profilin I is expressed ubiquitously while profilin II is expressed at high levels only in brain. In extracts from mouse brain, profilin I and profilin II can form complexes with regulators of endocytosis, synaptic vesicle recycling and actin assembly. Using mass spectrometry and database searching we characterized a number of ligands for profilin I and profilin II from mouse brain extracts including dynamin I, clathrin, synapsin, Rho-associated coiled-coil kinase, the Racassociated protein NAP1 and a member of the NSF/ sec18 family. In vivo, profilins co-localize with dynamin I and synapsin in axonal and dendritic processes. Our findings strongly suggest that in brain profilin I and profilin II complexes link the actin cytoskeleton and endocytic membrane flow, directing actin and clathrin assembly to distinct membrane domains.

Neuronal plasticity is an important process for learning, memory and complex behaviour. Rapid remodelling of the actin cytoskeleton in the postsynaptic compartment is thought to have an important function for synaptic plasticity. However, the actin-binding proteins involved and the molecular mechanisms that in vivo link actin dynamics to postsynaptic physiology are not well understood. Here, we show that the actin filament depolymerizing protein n-cofilin is controlling dendritic spine morphology and postsynaptic parameters such as late long-term potentiation and long-term depression. Loss of n-cofilin-mediated synaptic actin dynamics in the forebrain specifically leads to impairment of all types of associative learning, whereas exploratory learning is not affected. We provide evidence for a novel function of n-cofilin function in synaptic plasticity and in the control of extrasynaptic excitatory AMPA receptors diffusion. These results suggest a critical function of actin dynamics in associative learning and postsynaptic receptor availability.