The endoplasmic reticulum (ER) confronts a challenge to accommodate long, smooth ER tubules into the structural complexity of the axonal compartment. Here, we describe a morphological feature for the axonal ER network in developing neurons we termed the ER ladder. Axonal ER ladders are composed of rungs that wrap tightly around the microtubule bundle and dynamic rails, which slide across microtubules. We found that the ER-shaping protein Reticulon 2 determines the architecture and dynamics of the axonal ER ladder by modulating its interaction with microtubules. Moreover, we show that ER ladder depletion impairs the trafficking of associated vesicular axonal cargoes. Finally, we demonstrate that stromal interaction molecule 1 (Stim1) localizes to ER rungs and translocates to ER-plasma membrane contact sites upon depletion of luminal Ca²⁺. Our findings uncover fundamental insights into the structural and functional organization of the axonal ER network in developing mammalian neurons.

内质网 (ER) 面临着将长而光滑的 ER 小管容纳到轴突隔室的结构复杂性中的挑战。在这里，我们描述了发育中神经元中轴突 ER 网络的形态特征，我们称之为 ER 阶梯。轴突 ER 梯由紧紧包裹在微管束周围的梯级和滑过微管的动态导轨组成。我们发现 ER 整形蛋白 Reticulon 2 通过调节其与微管的相互作用来决定轴突 ER 梯的结构和动力学。此外，我们表明 ER 阶梯耗尽会损害相关囊泡轴突货物的运输。最后，我们证明基质相互作用分子 1 (Stim1) 定位于 ER 梯级，并在腔 Ca 耗尽后转移到 ER 质膜接触位点。²⁺。我们的研究结果揭示了在哺乳动物神经元发育过程中轴突 ER 网络的结构和功能组织的基本见解。