AP-4 mediates vesicular transport of the 2-AG endocannabinoid producing enzyme DAGLB

The adaptor protein complex AP-4 mediates anterograde axonal transport and is essential for axon health. AP-4-deficient patients suffer from a severe neurological disorder, which encompasses neurodevelopmental and neurodegenerative features. While impaired autophagy has been suggested to account for axon degeneration in AP-4 deficiency, axon growth defects occur through an unknown mechanism. Here we use orthogonal proteomic and imaging methods to identify DAGLB (diacylglycerol lipase-beta) as a cargo of AP-4 vesicles. DAGLB is a key enzyme for the generation of 2-AG (2-arachidonoylglycerol), the most abundant endocannabinoid in brain. During normal development, DAGLB is targeted to the axon, where 2-AG signalling drives axonal growth. We show that DAGLB accumulates at the TGN of AP-4-deficient cells, including in iPSC-derived neurons from a patient with AP-4 deficiency syndrome. Our data thus support that AP-4 mediates axonal targeting of DAGLB, and we propose that axon growth defects in AP-4 deficiency may arise through spatial dysregulation of endocannabinoid signalling.