Protein kinase D (PKD) is known to participate in various cellular functions, including secretory vesicle fission from theGolgi and plasma membrane-directed transport. Here, we report on expression and function of PKD in hippocampal neurons. Expression of an enhanced green fluorescent protein (EGFP)-tagged PKD activity reporter in mouse embryonal hippocampal neurons revealed high endogenous PKD activity at the Golgi complex and in the dendrites, whereas PKD activity was excluded from the axon in parallel with axonal maturation. Expression of fluorescently tagged wild-type PKD1 and constitutively active PKD1 S738/742E (caPKD1) in neurons revealed that both proteins were slightly enriched at the trans-Golgi network (TGN) and did not interfere with its thread-like morphology. By contrast, expression of dominant-negative kinase inactive PKD1 K612W (kdPKD1) led to the disruption of the neuronal Golgi complex, with kdPKD1 strongly localized to the TGN fragments. Similar findings were obtained from transgenic mice with inducible, neuron-specific expression of kdPKD1-EGFP. As a prominent consequence of kdPKD1 expression, the dendritic tree of transfected neurons was reduced, whereas caPKD1 increased dendritic arborization. Our results thus provide direct evidence that PKD activity is selectively involved in the maintenance of dendritic arborization and Golgi structure of hippocampal neurons. INTRODUCTIONNeurons are nondividing and extremely polarized cells, with enormous membrane surface compared with the size of the soma. These features assume a highly specialized secretory machinery, which resembles in certain parts the directed transport mechanisms described in polarized nonneuronal cells (Winckler and Mellman, 1999;Horton and Ehlers, 2004). The vast neuronal membrane surface is functionally and structurally divided into axonal and somatodendritic compartments, possessing specific lipid and protein components required for the spatially different functions, e.g., for pre-or postsynaptic activity (Dresbach et al., 2001;Sheng, 2001;Bresler et al., 2004). The constitutive and precisely directed supply of surface membrane components is indispensable for the function of neurons, especially when considering that postmitotic neurons normally serve throughout the life span of the organism. Up to now, we are far from understanding how the extreme polarization has been evolved and is maintained in neurons.Despite a likely central role in neuronal morphogenesis and membrane trafficking, little is known about the special structure and transport features of the neuronal Golgi apparatus. A thread-like and reticular structure of the Golgi apparatus has been already described in many neuronal types (Takamine et al., 2000;Fujita and Okamoto, 2005;Horton et al., 2005). Central neuronal Golgi complex is localized in the soma and often extends into the principal dendrites, but Golgi elements were also found as discontinuous structures in the distal dendrites, often near synaptic contacts or in dendritic spines (Gardiol et al., 1999;Pierc...

† Both authors contributed equally to this work.The protein kinase D (PKD) family comprises multifunctional serine/threonine-specific protein kinases with three mammalian isoforms: PKD1, PKD2 and PKD3. A prominent PKD function is the regulation of basolateraltargeted transport carrier fission from the trans-Golgi network (TGN). To visualize site-specific PKD activation at this organelle, we designed a molecular reporter consisting of a PKD-specific substrate sequence fused to enhanced green fluorescent protein (EGFP), specifically targeted to the TGN via the p230 GRIP domain. Quantitative analyses using a phosphospecific antibody and ratiometric fluorescence imaging revealed that Golgi-specific phosphorylation of the reporter was strictly dependent on stimulation of endogenous PKD or transient expression of active PKD constructs. Conversely, PKDspecific pharmacological inhibitors and siRNA-mediated PKD knockdown suppressed reporter phosphorylation. Using this reporter we investigated a potential role for PKD in the regulation of Golgi complex morphology. Interestingly, nocodazole-induced Golgi complex break-up and dispersal was associated with local PKD activation as measured by reporter phosphorylation and this was efficiently blocked by expression of a dominantnegative PKD mutant or PKD depletion. Our data thus identify a novel link between PKD activity and the microtubule cytoskeleton, whereby Golgi complex integrity is regulated.

CD8+ T cells directed against beta cell autoantigens are considered relevant for the pathogenesis of type 1 diabetes. Using single cell T cell receptor sequencing of CD8+ T cells specific for the IGRP265-273 epitope, we examined whether there was expansion of clonotypes and sharing of T cell receptor chains in autoreactive CD8+ T cell repertoires. HLA-A*0201 positive type 1 diabetes patients (n = 19) and controls (n = 18) were analysed. TCR α- and β-chain sequences of 418 patient-derived IGRP265-273-multimer+ CD8+ T cells representing 48 clonotypes were obtained. Expanded populations of IGRP265-273-specific CD8+ T cells with dominant clonotypes that had TCR α-chains shared across patients were observed. The SGGSNYKLTF motif corresponding to TRAJ53 was contained in 384 (91.9%) cells, and in 20 (41.7%) patient-derived clonotypes. TRAJ53 together with TRAV29/DV5 was found in 15 (31.3%) clonotypes. Using next generation TCR α-chain sequencing, we found enrichment of one of these TCR α-chains in the memory CD8+ T cells of patients as compared to healthy controls. CD8+ T cell clones bearing the enriched motifs mediated antigen-specific target cell lysis. We provide the first evidence for restriction of T cell receptor motifs in the alpha chain of human CD8+ T cells with specificity to a beta cell antigen.