You-Wen He

Correlating Notch Signaling with Thymocyte Maturation

The Notch receptor and its ligands are involved in many developmental processes. They are highly expressed in the thymus and have been implicated in the CD4 versus CD8 lineage decision. We identified the constitutively active intracellular fragment of murine Notch-1 as capable of rendering thymomas resistant to glucocorticoid-induced apoptosis. This effect was confirmed in other T cell lines and in CD4+ CD8+ DP thymocytes. Activation of the Notch signaling pathway also upregulated a number of other markers that, like steroid resistance, correlate with DP maturation into both the CD4 and CD8 lineages. These results suggest that Notch signaling is critically involved in the maturation of DP thymocytes into both CD4+ and CD8+ SP thymocytes.

Autophagy Is Essential for Mitochondrial Clearance in Mature T Lymphocytes

Macroautophagy plays an important role in the regulation of cell survival, metabolism, and the lysosomal degradation of cytoplasmic material. In the immune system, autophagy contributes to the clearance of intracellular pathogens, MHCII cross-presentation of endogenous Ags, as well as cell survival. We and others have demonstrated that autophagy occurs in T lymphocytes and contributes to the regulation of their cellular function, including survival and proliferation. Here we show that the essential autophagy gene Atg7 is required in a cell-intrinsic manner for the survival of mature primary T lymphocytes. We also find that mitochondrial content is developmentally regulated in T but not in B cells, with exit from the thymus marking a transition from high mitochondrial content in thymocytes to lower mitochondrial content in mature T cells. Macroautophagy has been proposed to play an important role in the clearance of intracellular organelles, and autophagy-deficient mature T cells fail to reduce their mitochondrial content in vivo. Consistent with alterations in mitochondrial content, autophagy-deficient T cells have increased reactive oxygen species production as well as an imbalance in pro- and antiapoptotic protein expression. With much recent interest in the possibility of autophagy-dependent developmentally programmed clearance of organelles in lens epithelial cells and erythrocytes, our data demonstrate that autophagy may have a physiologically significant role in the clearance of superfluous mitochondria in T lymphocytes as part of normal T cell homeostasis.

The autophagy gene ATG5 plays an essential role in B lymphocyte development

Macroautophagy (herein autophagy) is an evolutionarily conserved process, requiring the gene ATG5, by which cells degrade cytoplasmic constituents and organelles. Here we show that ATG5 is required for efficient B cell development and for the maintenance of B-1a B cell numbers. Deletion of ATG5 in B lymphocytes using Cre-LoxP technology or repopulation of irradiated mice with ATG5-/- fetal liver progenitors resulted in a dramatic reduction in B-1 B cells in the peritoneum. ATG5-/- progenitors exhibited a significant defect in B cell development at the pro- to pre-B cell transition, although a proportion of pre-B cells survived to populate the periphery. Inefficient B cell development in the bone marrow was associated with increased cell death, indicating that ATG5 is important for B cell survival during development. In addition, B-1a B cells require ATG5 for their maintenance in the periphery. We conclude that ATG5 is differentially required at discrete stages of development in distinct, but closely related, cell lineages.

Regulation of the TCRalpha repertoire by the survival window of CD4(+)CD8(+) thymocytes.

T cell receptor (TCR) α alleles undergo primary and secondary rearrangement in double-positive (DP) thymocytes. By analyzing TCRα rearrangement in orphan nuclear receptor RORγ-deficient mice, in which the DP lifespan is shorter, and in Bcl-xL–transgenic mice, in which the DP lifespan is extended, we show that the progression of secondary Vα to Jα rearrangements is controlled by DP thymocyte survival. In addition, because Bcl-xL induces a bias towards 3′ Jα usage in peripheral T cells, we conclude that the programmed cell death of DP thymocytes is not simply a consequence of failed positive selection. Rather, it limits the progression of rearrangement along the Jα locus and the opportunities for positive selection, thereby regulating the TCRα repertoire.

RORγt, a Novel Isoform of an Orphan Receptor, Negatively Regulates Fas Ligand Expression and IL-2 Production in T Cells

We have identified RORgamma t, a novel, thymus-specific isoform of the orphan nuclear receptor RORgamma that is expressed predominantly in CD4+ CD8+ double-positive thymocytes. Ectopic expression of RORgamma t protects T cell hybridomas from activation-induced cell death by inhibiting the upregulation of Fas ligand. Following hybridoma stimulation, RORgamma t also inhibits IL-2 production but does not affect the induction of Nur-77 and Egr-3 nor the upregulation of CD69. Both the ligand-binding and DNA-binding domains of RORgamma t are required for this effect. We propose that the role of RORgamma t expression in immature thymocytes is to inhibit Fas ligand expression and cytokine secretion following engagement of their TCR during positive or negative selection.

The crosstalk between autophagy and apoptosis: where does this lead?

Recent advances in the understanding of the molecular processes contributing to autophagy have provided insight into the relationship between autophagy and apoptosis. In contrast to the concept of “autophagic cell death,” accumulating evidence suggests that autophagy serves a largely cytoprotective role in physiologically relevant conditions. The cytoprotective function of autophagy is mediated in many circumstances by negative modulation of apoptosis. Apoptotic signaling, in turn, serves to inhibit autophagy. While the mechanisms mediating the complex counter-regulation of apoptosis and autophagy are not yet fully understood, important points of crosstalk include the interactions between Beclin-1 and Bcl-2/Bcl-xL and between FADD and Atg5, caspase- and calpain-mediated cleavage of autophagy-related proteins, and autophagic degradation of caspases. Continued investigation of these and other means of crosstalk between apoptosis and autophagy is necessary to elucidate the mechanisms controlling the balance between survival and death both under normal conditions and in diseases including cancer.

Autophagy Regulates Endoplasmic Reticulum Homeostasis and Calcium Mobilization in T Lymphocytes

Macroautophagy (hereafter referred to as autophagy) is an evolutionarily conserved intracellular bulk degradation pathway that plays critical roles in eliminating intracellular pathogens, presenting endogenous Ags, and regulating T lymphocyte survival and proliferation. In this study, we have investigated the role of autophagy in regulating the endoplasmic reticulum (ER) compartment in T lymphocytes. We found that ER content is expanded in mature autophagy-related protein (Atg) 7-deficient T lymphocytes. Atg7-deficient T cells stimulated through the TCR display impaired influx, but not efflux, of calcium, and ER calcium stores are increased in Atg7-deficient T cells. Treatment with the ER sarco/ER Ca2+-ATPase pump inhibitor thapsigargin rescues the calcium influx defect in Atg7-deficient T lymphocytes, suggesting that this impairment is caused by an intrinsic defect in ER. Furthermore, we found that the stimulation-induced redistribution of stromal interaction molecule-1, a critical event for the store-operated Ca2+ release-activated Ca2+ channel opening, is impaired in Atg7-deficient T cells. Together, these findings indicate that the expanded ER compartment in Atg7-deficient T cells contains increased calcium stores, and the inability of these stores to be depleted causes defective calcium influx in these cells. Our results demonstrate that autophagy plays an important role in maintaining ER and calcium homeostasis in T lymphocytes.

Temporal Regulation of Intracellular Organelle Homeostasis in T Lymphocytes by Autophagy

The highly conserved self-degradation pathway known as autophagy plays important roles in regulating T lymphocyte homeostasis. Recently, we found that T lymphocytes lacking the autophagy-related gene Atg5 or Atg7 have defective survival and contain expanded mitochondria and endoplasmic reticulum (ER); however, whether these defects are caused by impaired autophagy or by defects in their autophagy-independent signaling capacity of Atg5 or Atg7 in T lymphocytes remains unknown. Furthermore, the function of the microtubule-associated protein L chain 3 (LC3) conjugation system in T lymphocytes remains unclear. To address these questions, we generated conditional knockout mice with specific deletion of Atg3, a ubiquitin enzyme E2-like molecule involved in the LC3 conjugation system, in T lymphocytes. Atg3-deficient T lymphocytes displayed a phenotype similar to those of Atg7- and Atg5-deficient T cells. The survival of Atg3-deficient naive CD4+ and CD8+ T cells was defective. Furthermore, the mitochondria and ER were expanded in Atg3-deficient T cells. Interestingly, mitochondrial and ER content did not change instantly upon inducible deletion of Atg3 in mature T lymphocytes in vitro. Instead, it began to expand 10 d after inducible deletion of Atg3 in mature T lymphocytes, and mitochondrial content continued to increase on day 18. Cell death began to increase 24 d after inducible deletion of Atg3. These data show that the LC3 conjugation system is essential for autophagy in T lymphocytes. Our data suggest that autophagy promotes T lymphocyte survival by regulating organelle homeostasis and that the decreased survival of autophagy-deficient T cells is due to the temporal accumulation of these autophagy-related defects.

The extracellular matrix protein mindin is a pattern-recognition molecule for microbial pathogens

Microbial pathogens use a variety of their surface molecules to bind to host extracellular matrix (ECM) components to establish an effective infection. However, ECM components can also serve as an integral part of the innate immunity. Mice lacking expression of mindin (spondin 2), a highly conserved ECM protein, have an impaired ability to clear bacterial infection, and mindin-deficient macrophages show defective responses to a broad spectrum of microbial stimuli. Moreover, mindin binds directly to bacteria and their components and functions as an opsonin for macrophage phagocytosis of bacteria. Thus, mindin is essential in the initiation of the innate immune response and represents a unique pattern-recognition molecule in the ECM for microbial pathogens.

Deletion of PIK3C3/Vps34 in sensory neurons causes rapid neurodegeneration by disrupting the endosomal but not the autophagic pathway

The lipid kinase PIK3C3 (also called Vps34) regulates both the endosomal and autophagic pathways. However, the effect of inactivating PIK3C3 on neuronal endosomal versus autophagic processes in vivo has not been studied. We generated mice in which Pik3c3 was conditionally deleted in differentiated sensory neurons. Within a few days after Pik3c3 deletion, mutant large-diameter myelinated neurons accumulated numerous enlarged vacuoles and ubiquitin-positive aggregates and underwent rapid degeneration. By contrast, Pik3c3-deficient small-diameter unmyelinated neurons accumulated excessive numbers of lysosome-like organelles and degenerated more slowly. These differential degenerative phenotypes are unlikely caused by a disruption in the autophagy pathway, because inhibiting autophagy alone by conditional deletion of Atg7 results in a completely distinct phenotype in all sensory neurons (i.e., formation of very large intracellular inclusion bodies and slow degeneration over a period of several months). More surprisingly, a noncanonical PIK3C3-independent LC3-positive autophagosome formation pathway was activated in Pik3c3-deficient small-diameter neurons. Analyses of Pik3c3/Atg7 double mutant neurons revealed that this unconventional initiation pathway still depends on ATG7. Our studies represent in vivo characterization of PIK3C3 functions in mammals and provide insights into the complexity of neuronal endo-lysosomal and autophagic pathways.