On-tissue tryptic digestion was carried out as described by Huber et al . After lipid wash, trypsin (0.05 µg/µL in 10 mM NH4HCO3) was deposited using a TM HTX3 sprayer (HTX imaging, Chapel Hill, USA) using 15 cycles at 10 µL/min. Between each cycle, the tissue was placed in an incubator at 37°C and 100% relative humidity for 10 minutes. After the final cycle, the tissue was incubated for 2 hours.