Protein phosphorylation events during T cell receptor (TCR) signaling control the formation of complexes among proteins proximal to the TCR, the activation of kinase cascades, and the activation of transcription factors; however, the mode and extent of the influence of phosphorylation in coordinating the diverse phenomena associated with T cell activation are unclear. Therefore, we used the human Jurkat T cell leukemia cell line as a model system and performed large-scale quantitative phosphoproteomic analyses of TCR signaling. We identified 10,665 unique phosphorylation sites, of which 696 showed TCR-responsive changes. In addition, we analyzed broad trends in phosphorylation data sets to uncover underlying mechanisms associated with T cell activation. We found that, upon stimulation of the TCR, phosphorylation events extensively targeted protein modules involved in all of the salient phenomena associated with T cell activation: patterning of surface proteins, endocytosis of the TCR, formation of the F-actin cup, inside-out activation of integrins, polarization of microtubules, production of cytokines, and alternative splicing of messenger RNA. Further, case-by-case analysis of TCR-responsive phosphorylation sites on proteins belonging to relevant functional modules together with network analysis allowed us to deduce that serine-threonine (S-T) phosphorylation modulated protein-protein interactions (PPIs) in a system-wide fashion. We also provide experimental support for this inference by showing that phosphorylation of tubulin on six distinct serine residues abrogated PPIs during the assembly of microtubules. We propose that modulation of PPIs by stimulus-dependent changes in S-T phosphorylation state is a widespread phenomenon applicable to many other signaling systems.

The position of the centrosome is actively maintained at the cell center, but the mechanisms of the centering force remain largely unknown. It is known that centrosome positioning requires a radial array of cytoplasmic microtubules (MTs) that can exert pushing or pulling forces involving MT dynamics and the activity of cortical MT motors. It has also been suggested that actomyosin can play a direct or indirect role in this process. To examine the centering mechanisms, we introduced an imbalance of forces acting on the centrosome by local application of an inhibitor of MT assembly (nocodazole), and studied the resulting centrosome displacement. Using this approach in combination with microinjection of function-blocking probes, we found that a MT-dependent dynein pulling force plays a key role in the positioning of the centrosome at the cell center, and that other forces applied to the centrosomal MTs, including actomyosin contractility, can contribute to this process.

The fish melanophore has been considered the exemplar of microtubule-based organelle transport. In this system, a radial array of uniformly polarized microtubules [1] provides a framework on which dynein-related and kinesin-related motors drive pigment granules toward the minus or plus ends, respectively [2-4]. Stimulation of minus-end motors accounts satisfactorily for aggregation of granules at the cell center. Rapid dispersion is clearly microtubule-dependent; however, the uniform distribution of granules throughout the cytoplasm is paradoxical because stimulation of plus-end motors is predicted to drive the granules to the cell margin. This paradox suggested that the transport system was incompletely understood. Here, we report the discovery of a microtubule-independent motility system in fish melanophores. The system is based on actin filaments and is required for achieving uniform distribution of pigment granules. When it is abrogated, granules accumulate at the cell's margin as predicted for microtubule plus-end motors acting alone. The results presented here demonstrate the functional coordination of microtubule and actin filament systems, a finding that may be of general significance for organelle motility in cytoplasm.

Although microtubules (MTs) are generally thought to originate at the centrosome, a number of cell types have significant populations of MTs with no apparent centrosomal connection. The origin of these noncentrosomal MTs has been unclear. We applied kinetic analysis of MT formation in vivo to establish their mode of origin. Time-lapse f luorescence microscopy demonstrated that noncentrosomal MTs in cultured epithelial cells arise primarily by constitutive nucleation at, and release from, the centrosome. After release, MTs moved away from the centrosome and tended to depolymerize. Laser-marking experiments demonstrated that released MTs moved individually with their plus ends leading, suggesting that they were transported by minus end-directed motors. Released MTs were dynamic. The laser marking experiments demonstrated that plus ends of released MTs grew, paused, or shortened while the minus ends were stable or shortened. The centrosome, consisting of two centrioles surrounded by an amorphous cloud of pericentriolar material, is thought to be the primary site of nucleation for cytoplasmic microtubules (MTs) in animal cells. Nonetheless, a number of cell types contain significant proportions of noncentrosomal MTs (for review, see ref. 1). Several mammalian cell types, including neurons (2) and skeletal muscle cells (3) as well as epithelial cells from liver (4), kidney (5), intestine (6), and cochlea (7), have large arrays of noncentrosomal MTs that are important for the cells' specialized functions. In polarized epithelial cells in particular, MTs run in parallel arrays, with the minus ends at the apical surface and the plus ends at the basal surface (8), an arrangement that supports polarized sorting of membrane components and vesicle traffic (6). Although the centrosome is in the apical region, the MT minus ends are not evidently associated with the centrosome but terminate along the apical surface.A priori, four classes of mechanism could generate noncentrosomal MTs: self-assembly of MTs in the cytoplasm, nucleation of MTs at noncentrosomal sites, breakage or severing of centrosomal MTs along their length, and release of MTs from the centrosome. Work performed in a variety of systems has provided indirect evidence for release of MTs from the centrosome. In sea urchin eggs (9, 10) and Dictyostelium cells (11), reorganization of the interphase MT array into the mitotic spindle is preceded by what appears to be release of nearly all of the MTs from the centrosome or spindle pole body, respectively. In epithelial cells during regrowth of the interphase MT array after complete depolymerization, MTs are initially found only at the centrosome but at later times become noncentrosomally located (12-14), suggesting that MTs are first nucleated at the centrosome and are then translocated to other regions of the cell. In neurons, several lines of evidence indicate that axonal MTs arise from released centrosomal MTs (15-17). Nonetheless, the results discussed above all have been obtained from populations of cells ...

Human neuronal models of hereditary spastic paraplegias (HSP) that recapitulate disease-specific axonal pathology hold the key to understanding why certain axons degenerate in patients and to developing therapies. SPG4, the most common form of HSP, is caused by autosomal dominant mutations in the SPAST gene, which encodes the microtubule-severing ATPase spastin. Here, we have generated a human neuronal model of SPG4 by establishing induced pluripotent stem cells (iPSCs) from an SPG4 patient and differentiating these cells into telencephalic glutamatergic neurons. The SPG4 neurons displayed a significant increase in axonal swellings, which stained strongly for mitochondria and tau, indicating the accumulation of axonal transport cargoes. In addition, mitochondrial transport was decreased in SPG4 neurons, revealing that these patient iPSC-derived neurons recapitulate disease-specific axonal phenotypes. Interestingly, spastin protein levels were significantly decreased in SPG4 neurons, supporting a haploinsufficiency mechanism. Furthermore, cortical neurons derived from spastin-knockdown human embryonic stem cells (hESCs) exhibited similar axonal swellings, confirming that the axonal defects can be caused by loss of spastin function. These spastin-knockdown hESCs serve as an additional model for studying HSP. Finally, levels of stabilized acetylated-tubulin were significantly increased in SPG4 neurons. Vinblastine, a microtubule-destabilizing drug, rescued this axonal swelling phenotype in neurons derived from both SPG4 iPSCs and spastin-knockdown hESCs. Thus, this study demonstrates the successful establishment of human pluripotent stem cell-based neuronal models of SPG4, which will be valuable for dissecting the pathogenic cellular mechanisms and screening compounds to rescue the axonal degeneration in HSP.