Department of Developmental Neurobiology, St. Jude Children's Research Hospital, Memphis, TN 38105-3678, USA.
J Neurochem. 2012 Jun;121(5):717-29. doi: 10.1111/j.1471-4159.2012.07648.x. Epub 2012 Feb 6.
Cerebellin precursor protein (Cbln1) is essential for synapse integrity in cerebellum through assembly into complexes that bridge pre-synaptic β-neurexins (Nrxn) to post-synaptic GluRδ2. However, GluRδ2 is largely cerebellum-specific, yet Cbln1 and its little studied family members, Cbln2 and Cbln4, are expressed throughout brain. Therefore, we investigated whether additional proteins mediate Cbln family actions. Whereas Cbln1 and Cbln2 bound to GluRδ2 and Nrxns1-3, Cbln4 bound weakly or not at all, suggesting it has distinct binding partners. In a candidate receptor-screening assay, Cbln4 (but not Cbln1 or Cbln2) bound selectively to the netrin receptor, (deleted in colorectal cancer (DCC) in a netrin-displaceable fashion. To determine whether Cbln4 had a netrin-like function, Cbln4-null mice were generated. Cbln4-null mice did not phenocopy netrin-null mice. Cbln1 and Cbln4 were likely co-localized in neurons thought to be responsible for synaptic changes in striatum of Cbln1-null mice. Furthermore, complexes containing Cbln1 and Cbln4 had greatly reduced affinity to DCC but increased affinity to Nrxns, suggesting a functional interaction. However, Cbln4-null mice lacked the striatal synaptic changes seen in Cbln null mice. Thus, Cbln family members interact with multiple receptors/signaling pathways in a subunit composition-dependent manner and have independent functions with Cbln4 potentially involved in the less well-characterized role of netrin/DCC in adult brain.
脑钙黏蛋白前体蛋白（Cbln1）通过组装成复合物，将前突触β-神经连接蛋白（Nrxn）与后突触 GluRδ2 桥接，从而对小脑突触完整性至关重要。然而，GluRδ2 主要存在于小脑，而 Cbln1 及其研究较少的家族成员 Cbln2 和 Cbln4，则在整个大脑中表达。因此，我们研究了是否有其他蛋白介导 Cbln 家族的作用。虽然 Cbln1 和 Cbln2 与 GluRδ2 和 Nrxns1-3 结合，但 Cbln4 结合较弱或根本不结合，表明其具有独特的结合伙伴。在候选受体筛选测定中，Cbln4（但不是 Cbln1 或 Cbln2）以 netrin 受体（结直肠癌缺失（DCC）的方式选择性结合 netrin，以 netrin 置换的方式。为了确定 Cbln4 是否具有类似 netrin 的功能，生成了 Cbln4 缺失小鼠。Cbln4 缺失小鼠没有表现出 netrin 缺失小鼠的表型。Cbln1 和 Cbln4 可能在神经元中共同定位，这些神经元被认为是 Cbln1 缺失小鼠纹状体突触变化的原因。此外，含有 Cbln1 和 Cbln4 的复合物对 DCC 的亲和力大大降低，但对 Nrxns 的亲和力增加，表明存在功能相互作用。然而，Cbln4 缺失小鼠缺乏 Cbln 缺失小鼠在纹状体中观察到的突触变化。因此，Cbln 家族成员以亚基组成依赖的方式与多种受体/信号通路相互作用，并具有独立的功能，Cbln4 可能参与 netrin/DCC 在成年大脑中的作用尚不清楚。
