Ubiquitin–Synaptobrevin Fusion Protein Causes Degeneration of Presynaptic Motor Terminals in Mice

• DOI: 10.1523/jneurosci.5288-14.2015
• Publication Date: 2015-08-19T16:28:40Z
• Authors: Yun Liu; Hongqiao Li; Yoshie Sugiura; Weiping Han; Gilbert Gallardo; Mikhail Khvotchev; Yinan Zhang; Ege T. Kavalali; Thomas C. Südhof; Weichun Lin
• Keywords: Synaptobrevin; SNAP25; Synaptotagmin 1; Axoplasmic transport; Synaptophysin

Protein aggregates containing ubiquitin (Ub) are commonly observed in neurodegenerative disorders, implicating the involvement of proteasome system (UPS) their pathogenesis. Here, we aimed to generate a mouse model for monitoring UPS function using green fluorescent protein (GFP)-based substrate that carries "noncleavable" N-terminal moiety (Ub^G76V). We engineered transgenic mice expressing fusion protein, consisting following: (1) Ub^G76V, GFP, and synaptic vesicle synaptobrevin-2 (Ub^G76V-GFP-Syb2); (2) GFP-Syb2; or (3) Ub^G76V-GFP-Syntaxin1, all under control neuron-specific <i>Thy-1</i> promoter. As expected, Ub^G76V-GFP-Syb2, GFP-Syb2, Ub^G76V-GFP-Sytaxin1 were highly expressed neurons, such as motoneurons motor nerve terminals neuromuscular junction (NMJ). Surprisingly, Ub^G76V-GFP-Syb2 developed progressive adult-onset degeneration terminals, whereas GFP-Syb2 Ub^G76V-GFP-Syntaxin1 normal. The was preceded by impairment transmission at NMJs. Biochemical analyses demonstrated interacted with SNAP-25 Syntaxin1, SNARE partners synaptobrevin. Ultrastructural revealed marked reduction density, accompanying an accumulation tubulovesicular structures presynaptic terminals. These morphological defects largely restricted ultrastructure motoneuron somata appeared be normal stages when degenerated. Furthermore, endocytosis membrane trafficking impaired mice. findings indicate may compete endogenous synaptobrevin, acting gain-of-function mutation impedes function, resulting depletion vesicles <b>SIGNIFICANCE STATEMENT</b> Degeneration occurs amyotrophic lateral sclerosis (ALS) patients well models ALS, leading paralysis. What causes terminal degenerate remains unknown. Here report on ubiquitinated (Ub^G76V-GFP-Syb2) develop serve further elucidating underlying cellular molecular mechanisms degeneration.