Establishing a role for LTK as a regulator of the proteostasis network in response to folded proteins

The proteostasis network is a system that safeguards our proteome by maintaining the balance between synthesis, trafficking and degradation of proteins. Because 70% of cellular energy is devoted to protein synthesis, it is conceivable that proteostasis must be carefully regulated to ensure homeostatic balance. A major site for proteostasis is the endoplasmic reticulum (ER) and thus this organelle is equipped with sensors that measure proteostatic imbalance and initiate a set of responses that re-establish homeostasis. It is relatively well understood how the ER deals with the presence of improperly folded or mutated proteins. However, much less is known on how proteostasis of the ER deals with an overload of proteins. Certainly, it is clear that an excess of proteins must be met with a response that increases the exit of these excessive proteins out of the ER. In our previous work we identified a signaling protein called LTK that can adjust the ability of the ER to export proteins. This project follows the hypothesis that LTK is the sensor that detects an overload of proteins in the ER and allows it to deal with this overload. Our results will illuminate a new area of proteostasis research and help us understand how cells cope with an excess of folded proteins. This could be relevant for diseases where such protein overload is known to occur, most notably cancers such as multiple myeloma.

• Impact on fundamental biology: Better understanding of the regulation of ER-proteostasis and its role in disease. Furthermore, we provided additional results on top of that about the role of mechanobiology in the regulation of ER-proteostasis, which we publsihed in 2022 in EMBO Journal. Due to a failure of the system, I was not able to include this paper into the Results page (PMID: 35938214). • Impact for medical research: we are devoting considerable efforts towards establishing a role for LTK in multiple myeloma. I therefore foresee that our research will set the basis for LTK inhibitors such as crizotinib to quickly enter clinical trials in the future. As a sign of this success, the use of LTK inihibitors was patented for the use in myeloma as well as in viral infection (US patent 18247498; US patent 17772192). • Societal impact: multiple myeloma is the second most common hematologic malignancy and accounts for 2,4% of cancer associated deaths. Our research will expand the arsenal of therapeutic options against myeloma.

The proteostasis network is a system that safeguards our proteome by maintaining the balance between synthesis, trafficking and degradation of proteins. Because 70% of cellular energy is devoted to protein synthesis, it is conceivable that proteostasis must be carefully regulated to ensure homeostatic balance. A major site for proteostasis is the endoplasmic reticulum (ER) and thus this organelle is equipped with sensors that measure proteostatic imbalance and initiate a set of responses that re-establish homeostasis. Disruption of ER homeostasis by accumulation of too many unfolded or misfolded proteins results in activation of the unfolded protein response (UPR), which increases the capacity to fold and degrade these potentially toxic proteins. However, much less is known on how proteostasis is regulated in response to an excess of folded proteins. Certainly, such a condition must also be sensed and followed by an increase in the rate of export of proteins from the ER. We have recently identified leukocyte tyrosine kinase (LTK) as the first ER resident receptor tyrosine kinase that regulates ER export (Centonze et al, J Cell Biol). In addition, our preliminary data show that LTK is regulated by the load of folded proteins, but not by misfolded proteins. Thus, we are in a unique position to test how the proteostasis network responds through LTK to a stress imposed by too many folded proteins. We will mechanistically dissect pathways leading to LTK activation and elucidate mechanisms terminating its action. To understand the physiologic role of LTK, we will test whether this kinase helps cells transition from a state of low to high proteostatic demand. Finally, we will investigate the role of LTK as a secretion regulator in multiple myeloma, a cancer characterised by hypersecretion. Our results will illuminate a new area of proteostasis research and help us understand how cells cope with an excess of folded proteins