SNAP23 deficiency causes severe brain dysplasia through the loss of radial glial cell polarity

In the developing brain, the polarity of neural progenitor cells, termed radial glial cells (RGCs), is important for neurogenesis. Intercellular adhesions, termed apical junctional complexes (AJCs), at the apical surface between RGCs are necessary for cell polarization. However, the mechanism by which AJCs are established remains unclear. Here, we show that a SNARE complex composed of SNAP23, VAMP8, and Syntaxin1B has crucial roles in AJC formation and RGC polarization. Central nervous system (CNS)-specific ablation of SNAP23 (NcKO) results in mice with severe hypoplasia of the neocortex and no hippocampus or cerebellum. In the developing NcKO brain, RGCs lose their polarity following the disruption of AJCs and exhibit reduced proliferation, increased differentiation, and increased apoptosis. SNAP23 and its partner SNAREs, VAMP8 and Syntaxin1B, are important for the localization of an AJC protein, N-cadherin, to the apical plasma membrane of RGCs. Altogether, SNARE-mediated localization of N-cadherin is essential for AJC formation and RGC polarization during brain development.

Reduction in SNAP-23 Alters Microfilament Organization in Myofibrobastic Hepatic Stellate Cells

Hepatic stellate cells (HSC) are critical effector cells of liver fibrosis. In the injured liver, HSC differentiate into a myofibrobastic phenotype. A critical feature distinguishing myofibroblastic from quiescent HSC is cytoskeletal reorganization. Soluble NSF attachment receptor (SNARE) proteins are important in trafficking of newly synthesized proteins to the plasma membrane for release into the extracellular environment. The goals of this project were to determine the expression of specific SNARE proteins in myofibroblastic HSC and to test whether their alteration changed the HSC phenotype in vitro and progression of liver fibrosis in vivo. We found that HSC lack the t-SNARE protein, SNAP-25, but express a homologous protein, SNAP-23. Downregulation of SNAP-23 in HSC induced reduction in polymerization and disorganization of the actin cytoskeleton associated with loss of cell movement. In contrast, reduction in SNAP-23 in mice by monogenic deletion delayed but did not prevent progression of liver fibrosis to cirrhosis. Taken together, these findings suggest that SNAP-23 is an important regular of actin dynamics in myofibroblastic HSC, but that the role of SNAP-23 in the progression of liver fibrosis in vivo is unclear.

Snap29 mutant mice recapitulate neurological and ophthalmological abnormalities associated with 22q11 and CEDNIK syndrome

Synaptosomal-associated protein 29 (SNAP29) encodes a member of the SNARE family of proteins implicated in numerous intracellular protein trafficking pathways. SNAP29 maps to the 22q11.2 region and is deleted in 90% of patients with 22q11.2 deletion syndrome (22q11.2DS). Moreover, bi-allelic SNAP29 mutations in patients are responsible for CEDNIK (cerebral dysgenesis, neuropathy, ichthyosis, and keratoderma) syndrome. A mouse model that recapitulates abnormalities found in these syndromes is essential for uncovering the cellular basis of these disorders. In this study, we report that mice with a loss of function mutation of Snap29 on a mixed CD1;FvB genetic background recapitulate skin abnormalities associated with CEDNIK, and also phenocopy neurological and ophthalmological abnormalities found in CEDNIK and a subset of 22q11.2DS patients. Our work also reveals an unanticipated requirement for Snap29 in male fertility and supports contribution of hemizygosity for SNAP29 to the phenotypic spectrum of abnormalities found in 22q11.2DS patients.

SNAP23 regulates BAX-dependent adipocyte programmed cell death independently of canonical macroautophagy

The t-SNARE protein SNAP23 conventionally functions as a component of the cellular machinery required for intracellular transport vesicle fusion with target membranes and has been implicated in the regulation of fasting glucose levels, BMI, and type 2 diabetes. Surprisingly, we observed that adipocyte-specific KO of SNAP23 in mice resulted in a temporal development of severe generalized lipodystrophy associated with adipose tissue inflammation, insulin resistance, hyperglycemia, liver steatosis, and early death. This resulted from adipocyte cell death associated with an inhibition of macroautophagy and lysosomal degradation of the proapoptotic regulator BAX, with increased BAX activation. BAX colocalized with LC3-positive autophagic vacuoles and was increased upon treatment with lysosome inhibitors. Moreover, BAX deficiency suppressed the lipodystrophic phenotype in the adipocyte-specific SNAP23-KO mice and prevented cell death. In addition, ATG9 deficiency phenocopied SNAP23 deficiency, whereas ATG7 deficiency had no effect on BAX protein levels, BAX activation, or apoptotic cell death. These data demonstrate a role for SNAP23 in the control of macroautophagy and programmed cell death through an ATG9-dependent, but ATG7-independent, pathway regulating BAX protein levels and BAX activation.

Deletion of SNAP-23 results in pre-implantation embryonic lethality in mice

SNARE-mediated membrane fusion is a pivotal event for a wide-variety of biological processes. SNAP-25, a neuron-specific SNARE protein, has been well-characterized and mouse embryos lacking Snap25 are viable. However, the phenotype of mice lacking SNAP-23, the ubiquitously expressed SNAP-25 homolog, remains unknown. To reveal the importance of SNAP-23 function in mouse development, we generated Snap23-null mice by homologous recombination. We were unable to obtain newborn SNAP-23-deficient mice, and analysis of pre-implantation embryos from Snap23^Δ/wt matings revealed that Snap23-null blastocysts were dying prior to implantation at embryonic day E3.5. Thus these data reveal a critical role for SNAP-23 during embryogenesis.