Project description:Transcriptome analysis was used to investigate the global stress response of the Gram-positive bacterium Bacillus subtilis caused by overproduction of the well-secreted AmyQ α-amylase from Bacillus amyloliquefaciens. Analyses of the control and overproducing strains were carried out at the end of exponential growth and in stationary phase, when protein secretion from B. subtilis is optimal. Among the genes that showed increased expression were htrA and htrB, which are part of the CssRS regulon that responds to high-level protein secretion and heat stress. The analysis of the transcriptome profiles of a cssS mutant compared to the wild-type, under identical secretion stress conditions, revealed several genes with altered transcription in a CssRS-dependent manner, for example citM, ylxF, yloA, ykoJ and several genes of the flgB operon. However, a high affinity CssR-binding was only observed for htrA and htrB, and possibly for citM. In addition, the DNA macroarray approach reveal that several genes of the sporulation pathway are downregulated by AmyQ overexpression, and a group of motility-specific (σD-dependent) transcripts were clearly upregulated. Subsequent flow cytometric analyses demonstrate that upon overproduction of AmyQ as well as a non-secretable variant of the α-amylase, the process of sporulation is severely inhibited. The same experiments were implemented to investigate the expression levels of the hag promoter, a well-established reporter for σD-dependent gene expression. This approach confirmed the observations based on our DNA macroarray analyses and led us to conclude that expression levels of several genes involved in motility are maintained at high levels under all conditions of α-amylase overproduction. Secretion stress was applied by overproducing the well-secreted AmyQ α-amylase (pKTH10 vector) from B. amyloliquefaciens. Besides examining secretion stress in wild-type cells, we compared transcriptome profiles of a cssS mutant strain under conditions of high-level AmyQ production. Samples for transcriptome analyses were collected at the late exponential growth stage (one hour before the transition point) and 3 hours upon entry in the stationary growth phase. Three independent cultures of each strain were used and cells were sampled for macroarray experiments. Duplicate spots were averaged in Array-Pro software (Media Cybernetics, Inc.) and the signal was normalized after background subtraction by calculation of the percentage of total signal per gene using Microsoft Excel.

Project description:Protein secretion is an essential process in cell biology. The conventional secretion pathway is well known, and involves proteins with a signal sequence being directed into the endoplasmic reticulum. Once in the lumen of the endoplasmic reticulum, proteins are trafficked through the Golgi and remain separate from the cytosol throughout their journey to the outside of the cell. More recently, proteins without a signal sequence have been found outside the cell. These proteins are secreted unconventionally, and avoid the ER-Golgi route. For both pathways, some aspects remain unclear, particularly surrounding the regulation of protein secretion. Galectin-3, an unconventionally secreted protein, was used here to investigate both conventional and unconventional protein secretion. The unconventional protein secretion of galectin-3 is poorly understood. To address this, genetic manipulation of galectin-3 was used to gain insights into the mechanism of galectin-3 secretion. It was found that galectin-3 does not require unfolding or binding to cell surface glycoproteins to be secreted. Galectin-3 was also used to assess protein secretion using a genome-wide CRISPR screen. As galectin‑3 binds to cell surface glycoproteins, galectin-3 on the cell surface was used to assess secretion of these glycoproteins. Hits were validated by a glycoprotein secretion assay. Using this pipeline, 93 hits were identified as involved in protein secretion, of which 51 were novel hits not previously associated with protein secretion. Many of the hits identified also resulted in a phenotype of altered Golgi morphology, and five of these were investigated further. Two novel hits that localised to the Golgi were identified, TMEM220 and GPR161. GPR161 likely mediates its effect on Golgi morphology by its interaction with golgin A5. This thesis presents new insights into both conventional and unconventional protein secretion. Insights into the mechanism for galaectin-3 secretion were revealed. A novel Golgi-localised regulator of secretion, GRP161, was identified, with a mechanism for action suggested. Importantly, the list of 51 novel genes identified will serve as a useful resource for other researchers investigating protein secretion.

Project description:Appropriate glucagon secretion from pancreatic alpha cells in response to hypoglycemia is an important component of maintaining glucose homeostasis. Dysregulated glucagon secretion leads to the delayed recovery from a hypoglycemic attack in type 1 diabetes patients which can be lethal. Although elucidating the precise mechanism of glucagon secretion in hypoglycemia is warranted, the underlying mechanism remains poorly understood.The present study provides evidence of the role of autophagy in glucagon secretion in hypoglycemia by demonstrating that autophagy regulates adrenergic stimulation of glucagon secretion downstream of beta2 adrenergic receptor. First, from the analyses of T1D human islets and the published database of scRNA-seq of T1D human alpha cells, we described autophagy pathways altered in T1D alpha cells. Second, we generated alpha cell-specific Atg7KO mice (alphaAtg7KO) and clarified that the lack of autophagy in alpha cells impairs the reactive glucagon secretion in acute hypoglycemia. Third, to interrogate the molecular mechanism of autophagy-mediated glucagon regulation, we analyzed top genes downregulated inT1D and T2D alpha cells and found the expression of beta2 adrenergic receptor showed significant down-regulation in T1D alpha cells. We confirmed the decreased expression of beta2 adrenergic receptor in alpha cells of the T1D pancreas section, the islets of alphaAtg7KO mice, and a murine alpha cell line with stable knockdown of Atg7. Furthermore, in vivo and ex vivo studies of alphaAtg7KO mice exhibited that lack of autophagy led to the loss of stimulatory effect of beta2-adrenergic signaling on glucagon secretion. Finally, we established the Atg7 knockdown model in sorted human alpha cells and confirmed the effect of autophagy on the expression of the beta2 adrenergic receptor. Together, our study provides novel insights into the regulatory role of autophagy in glucagon secretion in response to hypoglycemia and provides therapeutic options to achieve stable glycemic control in T1D patients.

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Perturbation of gene expression for increased protein secretion in yeast.

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