Hermansky-Pudlak Protein Complexes, AP-3 and BLOC-1, Differentially Regulate Presynaptic Composition in the Striatum and Hippocampus

Abstract

Endosomal sorting mechanisms mediated by AP-3 and BLOC-1 are perturbed in Hermansky-Pudlak Syndrome, a human genetic condition characterized by albinism and prolonged bleeding (OMIM #203300). Additionally, mouse models defective in either one of these complexes possess defective synaptic vesicle biogenesis (Newell-Litwa et al., 2009). These synaptic vesicle phenotypes were presumed uniform throughout the brain. However, here we report that AP-3 and BLOC-1 differentially regulate the composition of presynaptic terminals in the striatum and dentate gyrus of the hippocampus. Quantitative immunoelectron microscopy demonstrated that the majority of AP-3 immunoreactivity in both wild-type striatum and hippocampus localizes to presynaptic axonal compartments, where it regulates synaptic vesicle size. In the striatum, loss of AP-3 (Ap3d mh/mh ) resulted in decreased synaptic vesicle size. In contrast, loss of AP-3 in the dentate gyrus increased synaptic vesicle size, thus suggesting anatomically specific AP-3-regulatory mechanisms. Loss-offunction alleles of BLOC-1, Pldn pa/pa , and Muted mu/mu revealed that this complex acts as a brain-region-specific regulator of AP-3. In fact, BLOC-1 deficiencies selectively reduced AP-3 and AP-3 cargo immunoreactivity in presynaptic compartments within the dentate gyrus both at the light and/or electron microscopy level. However, the striatum did not exhibit these BLOC-1-null phenotypes. Our results demonstrate that distinct brain regions differentially regulate AP-3-dependent synaptic vesicle biogenesis. We propose that anatomically restricted mechanisms within the brain diversify the biogenesis and composition of synaptic vesicles.

Presynaptic Membrane Retrieval and Endosome Biology: Defining Molecularly Heterogeneous Synaptic Vesicles

“…This proposition is compatible with evidence indicating that synaptic vesicle recycling can proceed even in the absence of AP-2 (Gu et al 2008;Kim and Ryan 2009a,b;Willox and Royle 2012). The adaptorcomplexes AP-1 and AP-3 localize to nerve terminals and copurify with synaptic vesicles, suggesting that these complexes participate in their life cycle (Salazar et al 2005;Seong et al 2005;Takamori et al 2006;Newell-Litwa et al 2009;Glyvuk et al 2010;Gronborg et al 2010;Newell-Litwa et al 2010). Moreover, AP-3 decorates synaptic vesicles by immunoelectron microscopy (Newman et al 1995;Newell-Litwa et al 2010).…”

“…The adaptorcomplexes AP-1 and AP-3 localize to nerve terminals and copurify with synaptic vesicles, suggesting that these complexes participate in their life cycle (Salazar et al 2005;Seong et al 2005;Takamori et al 2006;Newell-Litwa et al 2009;Glyvuk et al 2010;Gronborg et al 2010;Newell-Litwa et al 2010). Moreover, AP-3 decorates synaptic vesicles by immunoelectron microscopy (Newman et al 1995;Newell-Litwa et al 2010). Predictably, AP-1 and AP-3 null mice are characterized by one or more of the following phenotypes: alterations in synaptic vesicle size, composition or acidification; impaired intraterminal trafficking of neurotransmitter transporters; and defective neurotransmission (Blumstein et al 2001;Nakatsu et al 2004;Salazaret al 2004;Seong et al 2005;Voglmaier et al 2006;NewellLitwa et al 2009NewellLitwa et al , 2010Glyvuk et al 2010).…”

Mutations in the BLOC-1 Subunits Dysbindin and Muted Generate Divergent and Dosage-dependent Phenotypes

“…VAMP7 monoclonal antibody was a generous gift of Dr. Andrew Peden (University of Sheffield, UK). Dysbindin 1A and 1C were detected with the antibody PA3111 (32) Mouse mutants have been previously described (17,33,34). Bloc1s5 mu/mu mice in CHMU background (CHMU/LeJ, stock number 000293) were backcrossed by at least six generations with C57B6 mice obtained from The Jackson Laboratory (Bar Harbor, ME).…”

“…Brain Sections, Immunohistochemistry, and MicroscopyDetailed procedures for mouse tissue preparation, indirect immunofluorescence microscopy, and quantification procedures were described in our previous work (17,33,36). Briefly, brains were obtained from mice 6 -8 weeks postnatal.…”

Defective release of α granule and lysosome contents from platelets in mouse Hermansky-Pudlak syndrome models

“…11 Because BLOC-1 and AP-3 regulate SNARE distribution in neurons, [62][63][64] we tested whether these SNARE components were depleted from HPS model platelets. However, levels of each SNARE component were not significantly different in lysates from WT, pearl, pallid, or light ear platelets ( Figure 6E).…”