Figure 4. BK upregulation promotes lysosomal Ca²⁺ release in ML4 fibroblasts carrying F408Δ mutation.

(A) BK overexpression increased ML-SA1 (10 μM)-induced GECO-TRPML1-F408Δ response in HEK293T cells. (B) BK overexpression did not alter ML-SA1 (10 μM)-induced GECO-TRPML1-R403C response in HEK293T cells. (C) Statistical analysis of GECO response to ML-SA1 (10 μM) showing that BK overexpression significantly increased GECO-TRPML1-F408Δ signal but not others. (D) BK overexpression did not alter GPN (200 μM)-induced GECO responses. (E) BK overexpression did not alter Ionomycin (1 μM)-induced GECO responses. (F,G) BK overexpression enhanced GECO-TRPML1-F408Δ response to ML-SA1 (10 μM) in TRPML1-F408Δ human fibroblasts, suggesting that BK upregulation facilitates TRPML1-F408Δ activity. (H,I) GECO-TRPML1-F408Δ responses to GPN (200 μM) (H) and Ionomycin (1 μM) (I) were not altered in TRPML1-F408Δ human fibroblasts by BK overexpession, suggesting BK overexpression did not alter lysosomal Ca²⁺ content and GECO- TRPML1-F408Δ expression level in TRPML1-F408Δ human fibroblasts, respectively.