Reissner Carsten, Stahn Johanna, Breuer Dorothee, Klose Martin, Pohlentz Gottfried, Mormann Michael, Missler Markus
From the Institute of Anatomy and Molecular Neurobiology, Westfälische Wilhelms-University, Vesaliusweg 2-4, 48149 Münster, Germany.
Institute of Medical Physics and Biophysics, Westfälische Wilhelms-University, Robert-Koch Strasse 31, 48149 Münster, Germany, and.
J Biol Chem. 2014 Oct 3;289(40):27585-603. doi: 10.1074/jbc.M114.595413. Epub 2014 Aug 25.
α-Neurexins (α-Nrxn) are mostly presynaptic cell surface molecules essential for neurotransmission that are linked to neuro-developmental disorders as autism or schizophrenia. Several interaction partners of α-Nrxn are identified that depend on alternative splicing, including neuroligins (Nlgn) and dystroglycan (αDAG). The trans-synaptic complex with Nlgn1 was extensively characterized and shown to partially mediate α-Nrxn function. However, the interactions of α-Nrxn with αDAG, neurexophilins (Nxph1) and Nlgn2, ligands that occur specifically at inhibitory synapses, are incompletely understood. Using site-directed mutagenesis, we demonstrate the exact binding epitopes of αDAG and Nxph1 on Nrxn1α and show that their binding is mutually exclusive. Identification of an unusual cysteine bridge pattern and complex type glycans in Nxph1 ensure binding to the second laminin/neurexin/sex hormone binding (LNS2) domain of Nrxn1α, but this association does not interfere with Nlgn binding at LNS6. αDAG, in contrast, interacts with both LNS2 and LNS6 domains without inserts in splice sites SS#2 or SS#4 mostly via LARGE (like-acetylglucosaminyltransferase)-dependent glycans attached to the mucin region. Unexpectedly, binding of αDAG at LNS2 prevents interaction of Nlgn at LNS6 with or without splice insert in SS#4, presumably by sterically hindering each other in the u-form conformation of α-Nrxn. Thus, expression of αDAG and Nxph1 together with alternative splicing in Nrxn1α may prevent or facilitate formation of distinct trans-synaptic Nrxn·Nlgn complexes, revealing an unanticipated way to contribute to the identity of synaptic subpopulations.
α-神经连接蛋白（α-Nrxn）大多是神经传递所必需的突触前细胞表面分子，与自闭症或精神分裂症等神经发育障碍有关。已鉴定出α-Nrxn的几个相互作用伙伴，它们依赖于可变剪接，包括神经连接蛋白（Nlgn）和肌营养不良聚糖（αDAG）。与Nlgn1的跨突触复合物已得到广泛表征，并显示部分介导α-Nrxn功能。然而，α-Nrxn与αDAG、亲神经素（Nxph1）和Nlgn2（这些配体特异性出现在抑制性突触处）之间的相互作用尚未完全了解。我们通过定点诱变，证明了αDAG和Nxph1在Nrxn1α上的确切结合表位，并表明它们的结合是相互排斥的。Nxph1中异常的半胱氨酸桥模式和复合型聚糖的鉴定确保了其与Nrxn1α的第二个层粘连蛋白/神经连接蛋白/性激素结合（LNS2）结构域的结合，但这种结合并不干扰Nlgn在LNS6处的结合。相比之下，αDAG与LNS2和LNS6结构域相互作用，在剪接位点SS#2或SS#4处没有插入片段，主要通过附着在粘蛋白区域的依赖于LARGE（类乙酰葡糖胺基转移酶）的聚糖进行。出乎意料的是，αDAG在LNS2处的结合会阻止Nlgn在LNS6处的相互作用，无论SS#4处有无剪接插入片段，这可能是因为在α-Nrxn的u型构象中它们在空间上相互阻碍。因此，αDAG和Nxph1与Nrxn1α中的可变剪接共同表达可能会阻止或促进不同的跨突触Nrxn·Nlgn复合物的形成，揭示了一种意想不到的方式来影响突触亚群的特性。
