Stimulation inhibits the mobility of recycling peptidergic vesicles in astrocytes

Abstract

Astrocytes are increasingly viewed as playing many roles in the integration of brain function. These cells store among other gliotransmitters also neuroactive peptides in membrane bound vesicles, the trafficking and release of which, may be changed in altered conditions, therefore affecting the physiological status of neurons. In general, peptidergic membrane-bound secretory vesicles fuse with the plasma membrane in the process of exocytosis. Some of them are retrieved from the plasma membrane to be recycled back into the cytosol. The mobility of retrieving vesicles in astrocytes was not studied yet, however, understanding the mechanisms of such trafficking would highlight the communication paths between astrocytes and neurons. We labeled vesicles with antibodies against the vesicle atrial natriuretic peptide (ANP), which is stored inside secretory vesicles. ANP-vesicles in astrocytes have been proposed to enter Ca2+-dependent secretion and here we show that they are associated with synaptotagmin IV (SytIV), a regulator of exocytosis in astrocytes. Moreover, the results show that recycling ANP-vesicles are to a significant extent acidic. Their velocity (0.06+/-0.001 microm/s) is one order of magnitude lower than the velocity of vesicles trafficking to the plasma membrane (Potokar et al. (2005) Biochem Biophys Res Commun 329:678-683; Potokar et al. (2007) Traffic 8:12-20). Interestingly, ionomycin or ATP application further attenuated ANP-vesicle mobility to 0.02+/-0.002 and to 0.03+/-0.001 microm/s, respectively. In summary, the mobility of recycling peptidergic vesicles appears to be slower than the vesicle traffic to the plasma membrane and it requires an intact cytoskeleton. Physiological implications of attenuated traffic of ANP-vesicles are considered in the discussion.