Abstract
Lysosomes are membrane-bound organelles mainly involved in catabolic processes. In addition, lysosomes can expel their contents outside of the cell via lysosomal exocytosis. Some of the key steps involved in these important cellular processes, such as vesicular fusion and trafficking, require calcium (Ca2+) signaling. Recent data show that lysosomal functions are transcriptionally regulated by transcription factor EB (TFEB) through the induction of genes involved in lysosomal biogenesis and exocytosis. Given these observations, we investigated the roles of TFEB and lysosomes in intracellular Ca2+ homeostasis. We studied the effect of transient modulation of TFEB expression in HeLa cells by measuring the cytosolic Ca2+ response after capacitative Ca2+ entry activation and Ca2+ dynamics in the endoplasmic reticulum (ER) and directly in lysosomes. Our observations show that transient TFEB overexpression significantly reduces cytosolic Ca2+ levels under a capacitative influx model and ER re-uptake of calcium, increasing the lysosomal Ca2+ buffering capacity. Moreover, lysosomal destruction or damage abolishes these TFEB-dependent effects in both the cytosol and ER. These results suggest a possible Ca2+ buffering role for lysosomes and shed new light on lysosomal functions during intracellular Ca2+ homeostasis.
Highlights
Lysosomes are membrane-bound organelles mainly involved in catabolic processes
The ER Ca2+ store is replenished via the sarco-/endoplasmic reticulum Ca2+-ATPase (SERCA) pump[8,9,10,11] in a process known as capacitative Ca2+ entry or store-operated Ca2+ entry (SOCE)
We transiently overexpressed TFEB (TFEB-3xflag) for 48 h in human cervical carcinoma (HeLa) cells (Supplementary Fig. S1A), revealing an increase in protein expression compared with control cells transfected with pcDNA3
Summary
Lysosomes are membrane-bound organelles mainly involved in catabolic processes. In addition, lysosomes can expel their contents outside of the cell via lysosomal exocytosis. Recent data show that lysosomal functions are transcriptionally regulated by transcription factor EB (TFEB) through the induction of genes involved in lysosomal biogenesis and exocytosis. Given these observations, we investigated the roles of TFEB and lysosomes in intracellular Ca2+ homeostasis. We studied the effect of transient modulation of TFEB expression in HeLa cells by measuring the cytosolic Ca2+ response after capacitative Ca2+ entry activation and Ca2+ dynamics in the endoplasmic reticulum (ER) and directly in lysosomes. Lysosomal destruction or damage abolishes these TFEB-dependent effects in both the cytosol and ER These results suggest a possible Ca2+ buffering role for lysosomes and shed new light on lysosomal functions during intracellular Ca2+ homeostasis. A reduction in the lysosomal Ca2+ content caused by mutations of the human TRPML1 gene is considered to be the primary pathogenic cause underlying some lysosomal storage diseases and common neurodegenerative diseases[13,24], such as type IV Mucolipidosis[25]
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