SV2 modulates the size of the readily releasable pool of secretory vesicles

Abstract

The exocytosis of neurotransmitters is regulated by calcium and is plastic — features that suggest specialized regulation of the basic membrane trafficking process. Here we show that Synaptic Vesicle Protein 2 (SV2), a protein specific to neurons and endocrine cells, is required to maintain a pool of vesicles available for calcium-stimulated exocytosis. Direct measures of exocytosis in adrenal chromaffin cells showed that the calcium-induced exocytotic burst, which operationally defines the readily releasable pool of vesicles, was significantly reduced in mice lacking SV2A. Burst kinetics were normal in cells from SV2A knockout animals, however, indicating that SV2 functions before the final events of fusion. Analyses of SDS-resistant SNARE (soluble NSF (N-ethylmaleimide-sensitive fusion) attachment protein receptor) complexes in brain tissue showed that loss of SV2A was associated with fewer SDS-resistant complexes. Our observations indicate that SV2 may modulate the formation of protein complexes required for fusion and therefore the progression of vesicles to a fusion-competent state.

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Figure 1: SV2C expression increases in the adrenal medulla of SV2A knockout mice.
Figure 2: Exocytosis is reduced in chromaffin cells from SV2A knockout mice.
Figure 3: Granule number and size are normal in chromaffin cells from SV2A knockout mice.
Figure 4: The calcium dependence of release is not altered in chromaffin cells from SV2A knockout mice.
Figure 5: Resting calcium concentrations and calcium decay rates were not altered in chromaffin cells from SV2 knockout mice.
Figure 6: Both fast and slow components of the exocytotic burst are present in SV2A knockout cells.
Figure 7: The proportion of SDS-resistant SNARE complexes is reduced in SV2A knockout brains.

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Acknowledgements

We thank B. Hille for his gracious and generous support of this work. We also thank L. Baldwin for maintaining the mouse colony and genotyping animals; R. Janz for anti-SV2C antibody; R. Scheller for anti-n-sec1 antibody; Z. Sheng for anti-syntaphilin antibody; L. Pallanck, N. Stella and W. Catterall for reading the manuscript; D. Babcock for help with figures; P. Hanson for helpful discussions; and R. Collman of the University of Washington Department of Pathology for sample processing and photography for morphometric analysis. This work was supported by grants from the NIMH (to S.M.B.) and NIH (to B. Hille) and a grant from the Whitehall Foundation (to S.M.B).

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Correspondence to Sandra M. Bajjalieh.

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