Sunday, May 25, 2025
5/25/2025
Noncanonical roles of Bim in heat shock-induced cell death - PROJECT SUMMARY/ABSTRACT Severe hyperthermia (heatstroke) is due to a failure of thermoregulation that can lead to cell death in various tissues resulting in multisystem organ failure. Hyperthermia generally results from overexertion due to severe physical activity and/or prolonged exposure to elevated temperatures. Patients who suffer from anhidrosis or hypohidrosis, due to disease or certain medications, such as anesthetics, diuretics, anticholinergics, and amphetamines, are likewise at greater risk. Apart from the negative effects of hyperthermia, in the clinic a variety of approaches are being utilized to induce localized hyperthermia in the treatment of various cancers, as many tumors exhibit enhanced sensitivity to heat. Despite the general importance of heat shock responses, however, the mechanisms by which heat induces cell death remain unclear. Heat shock reportedly induces cell death through the activation of various canonical pathways involving the initiator caspases-2, -8, and -9; however, we and others find that while these pathways can serve to amplify the cell death signal, they are largely dispensable for heat shock-induced cell death, particularly at higher temperatures or following longer exposures. Instead, in a surprising discovery, we have found that Bim mediates heat shock-induced cell death, independently of its BH3 domain, through an interaction with the LC8 dynein light chain subunit of the dynein motor complex (DMC). In binding to LC8, Bim promotes anterograde trafficking of lysosomes from the perinuclear region to the cell periphery, where they become sensitive to the effects of heat shock. Additional data indicate that peripheral lysosomes undergo direct membrane permeabilization (LMP) or exocytotic release following heat shock, resulting in the release of cathepsins into the cytoplasm or extracellular space, respectively. Moreover, knockout or depletion of cathepsin L (CatL) renders cells highly resistant to cell death. Lastly, while cathepsins released into different cellular compartments play roles in mediating cell death, targeted localization of the cathepsin inhibitor, cystatin B (CSTB), to the nucleus results in profound suppression of heat shock-induced cell death. Thus, our overall hypothesis is that Bim plays a critical noncanonical role in regulating lysosomal number and positioning within cells, and as a result, it determines the susceptibility of peripheral lysosomes to heat shock- induced LMP/exocytosis, cathepsin release, and cell death. In four specific aims, we will determine: (1) the roles of Bim in lysosome number and positioning; (2) the roles of Bim in heat shock-induced LMP and lysosome exocytosis; (3) the roles of nuclear CatL in heat shock-induced cell death; and (4) the roles of Bim in two mouse models of localized and whole-body hyperthermia. Collectively, the proposed studies will provide important insights into an unexpected role for Bim in regulating lysosomal trafficking with far reaching implications for Bim biology in general and heat shock-induced cell death in particular.
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