Biological function and molecular mechanisms of intramembrane proteoylsis by Signal Peptide Peptidase Like-3 (SPPL3)

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Signal Peptide Peptidase-like 3 (SPPL3) is an aspartyl intramembrane protease of the SPP/SPPL family. Within the first funding period of this project, we identified SPPL3 as a regulator of protein glycosylation. By cleavage of glycan-modifying enzymes in the Golgi, SPPL3 reduces their catalytic activity and, therefore, overexpression of SPPL3 results in hypoglycosylation of many cellular proteins, while reduced SPPL3 expression induces hyperglycosylation. Thus, adapting the expression level of SPPL3 enables cells to rapidly change the glycan pattern of proteins within the secretory pathway. Consequently, we asked in the second funding period how expression of SPPL3 is regulated. We found that expression of SPPL2c, another member that is exclusively expressed in spermatids, cleaves SNARE proteins and, thus, alters protein transport in the secretory pathway. By that SPPL3 expression as well as expression of glycan modifying enzymes is altered since they are retained in the endoplasmic reticulum. However, this is only of physiological relevance in testis, while regulation of SPPL3 expression in all other tissues occurs differently. Interestingly, insulin suppresses SPPL3 expression and we have evidence that the mTORC pathway is involved in this regulation. Glucose starvation strongly increases SPPL3 expression and, as expected, results in hypoglycosylation of cellular glycoproteins. Of note, stabilization of SPPL3 under conditions of glucose starvation seems to be independent of mTORC signaling. Based on this, we will now identify the precise signaling events that regulate SPPL3 expression upon glucose starvation and decipher the link between insulin induced mTORC activation and downregulation of SPPL3 expression. Additionally, we will elucidate the impact of other nutrients on SPPL3 expression and identify the underlying signaling cascades. Finally, we will try to link the findings on the mechanisms of SPPL3 regulation from cell culture model systems to the phenotypes we observe in SPPL3 -/- mice. To this end, we have already started to establish conditional SPPL3 -/- mice that will allow us to analyze the observed phenotypes individually.The results of this project will finally provide a detailed picture of SPPL3 regulation and help to fully understand its physiological function. This may provide the basis to develop new strategies for treatment of for instance obesity or Diabetes type 2.