Rab46-dependent trafficking in endothelial cells requires calcium release from NAADP-sensitive channels

The endothelium plays a vital role in haemostasis yet is susceptible to dysfunction. This vascular bed contains a monolayer of cells capable of modulating inflammation. Endothelial cells (ECs) contain potent, bioactive cargo stored within specific storage organelles termed Weibel-Palade bodies (WPBs). These contain the pro-thrombotic and pro-inflammatory mediators, angiopoietin-2 (Ang2) and P-selectin which are differentially released in response to vasoactive stimuli. Thrombin (released after vascular injury) or histamine (immunogenic assault) promote WPB cargo release and WPB dysregulation can contribute to an adverse environment, as evidenced in cardiovascular disease.
Rab46 has been shown to localise to Ang2-positive WPBs and is implicated in the regulation of differential cargo release. Rab46 is necessary for the acute immunogenic histamine, but not thrombin, trafficking of these WPBs to the Microtubule Organising Centre (MTOC). This retrograde movement of Rab46 is dependent on dynein motor interactions yet occurs independently of Ca2+. The detachment of Rab46 from the MTOC is a Ca2+-dependent process where Ca2+ binds the EF-hand domain of Rab46 and this source of Ca2+ has been the subject of ongoing investigation. Histamine stimulation elicits a rapid upregulation of nicotinic acid adenine dinucleotide phosphate (NAADP) within ECs, which is a potent second Ca2+ messenger that targets small, acidic intracellular Ca2+ stores on the endo-lysosomal system.
Nicotinic acid adenosine dinucleotide phosphate (NAADP) has been shown to indirectly bind via accessory proteins to various lysosomal Ca2+ channels, Two-pore channel 1 (TPC1), Two-pore channel 2 (TPC2) and Transient potential receptor mucolipin 1 (TPRML1) and are candidates for the modulation of Ca2+ signals generated via NAADP to refill or empty the lysosomal Ca2+ pool. Activation of these channels can elicit global Ca2+ rises in cells through Ca2+-induced- Ca2+ release (CICR) exploiting the ER Ca2+ pool as a mechanism to potentiate Ca2+ efflux from lysosomes. Understanding the Ca2+ channels involved in Rab46 dispersal from the MTOC is crucial for elucidating how ECs modulate their secretory response, particularly Ang2 secretion.