, 2001 and Takamori et al., 2006). Therefore, in order to examine the presynaptic fusion machinery that underlies this effect, we tested the effect of Reelin on neurons deficient in the canonical synaptic SNARE proteins SNAP-25 and syb2 (Figure 4A). Although neurons from their wild-type littermates showed swift responses to Reelin application (Figure 4B), neurons lacking SNAP-25 (SNAP25−/−) buy SB431542 showed no response to Reelin and had an overall lower mEPSC frequency (Figure 4C) (Bronk et al., 2007). Here, it is important to note that SNAP-25 deficient synapses respond to other secretagogues such as hypertonic sucrose, ionomycin, or α-latrotoxin (Bronk et al., 2007 and Deák et al., 2009).

These data indicate that Reelin causes an increase in SCH 900776 purchase SV fusion frequency that requires the function of the plasma membrane-associated SNARE, SNAP-25, in agreement with an earlier study suggesting a SNAP-25-dependent role for Reelin in presynaptic function (Hellwig et al., 2011). To test if the SV SNARE syb2 is also required for the Reelin-dependent augmentation in transmission, we added Reelin to neurons deficient in syb2 (syb2−/−) (Figures 4A and 4D). Surprisingly, neurons lacking syb2 still responded to Reelin despite their low basal mEPSC frequency (Figure 4D). Taken together, these results suggest that

the Reelin-dependent increase in spontaneous transmission requires a SNARE complex that contains SNAP-25 but does Thymidine kinase not require the vesicle-associated protein syb2. The ability of Reelin to increase spontaneous release in the absence of syb2 but not SNAP-25 suggests that the presynaptic Reelin effect requires an alternative vesicular SNARE. This observation is rather surprising as we detected a modest Reelin-dependent increase in presynaptic Ca2+ levels in presynaptic terminals identified via coexpression of syb2-mOrange in our Ca2+ imaging experiments (Figures 3M and 3N). These findings suggest that Reelin can signal to presynaptic terminals expressing syb2 but its effect on neurotransmitter release does not require syb2 function. To identify the alternative vesicular

SNARE that mediates the observed Reelin-elicited exocytosis, we monitored the fluorescence of wild-type neurons expressing one of four vesicular SNAREs (syb2, VAMP4, vti1a, or VAMP7) tagged with pHluorin at their C-terminal ends in the SV lumen. Using the same setting as in Figure 2G, we took advantage of the vacuolar ATPase inhibitor, folimycin, to prevent SV re-acidification at rest and monitored spontaneous fusion of vesicles tagged with the four vesicular SNAREs. In this setting, we measured the increase in fluorescence after 10 min of Reelin application. Under these conditions, syb2-pHluorin (Figure 5A), VAMP4-pHluorin (Figure 5B) or vti1a-pHluorin (Figure 5C) trafficking did not respond to Reelin when compared to vehicle.