Syntaxin 1b Is Important for Mouse Postnatal Survival and Proper Synaptic 1 Function at the Mouse Neuromuscular Junctions. 2 3 4 Authors 5

51 52 Syntaxin 1 (Stx1) is a major neuronal Stx protein located at the plasma membrane of 53 the neuronal tissues. Rodent Stx1 has two highly similar paralogs, Stx1A and Stx1B, 54 which are thought to be functionally redundant. Interestingly, some studies have 55 shown that the distribution patterns of Stx1A and Stx1B at the central and peripheral 56 nervous systems only partially overlapped, implying that there might be differential 57 functions between these paralogs. In the current study, we generated an Stx1B 58 knockout (KO) mouse line and studied the impact of Stx1B removal in neurons of 59 several brain regions and the neuromuscular junction (NMJ). We found that either 60 complete removal of Stx1B or selective removal of it from forebrain excitatory 61 neurons in mice caused premature death. Autaptic hippocampal and striatal cultures 62 derived from Stx1B KO mice still maintained efficient neurotransmission compared 63 to neurons from Stx1B wildtype and heterozygous mice. Interestingly, examining 64 high-density cerebellar cultures revealed a decrease in the spontaneous GABAergic 65 transmission frequency, which was most likely due to a lower number of neurons in 66 the Stx1B KO cultures, suggesting that Stx1B is essential for neuronal survival in 67 vitro. Moreover, our study also demonstrated that although Stx1B is dispensable for 68 the formation of the mouse NMJ, it is required to maintain the efficiency of 69 neurotransmission at the nerve-muscle synapse. 70 71 72