Figure 1.

Retrograde Labeling of Caudally Projecting Neurons through Cre-Mediated Expression in TdTomato-Expressing Mouse Neural Progenitor Cell Grafts

Wild-type mice received grafts of NPCs expressing Cre-dependent TdTomato into a site of C5 hemisection, and injections of AAV9-Cre into the caudal host spinal cord 2 weeks later.

(A) Overview of one section double labeled for anti-TdTomato and anti-Cre to demonstrate position of AAV9-Cre injection (arrow) 2 mm caudal to NPC graft (g). Graft is from donor transgenic mouse E12 spinal cord that conditionally expresses TdTomato. Horizontal section, rostral to left. Graft occupies right C5 lateral hemisection lesion cavity. All TdTomato labeling in graft arises from cells in graft bearing axons that project to the caudal site of AAV9-Cre injection, since TdTomato expression is Cre dependent. Individual neurons in graft are shown in Figure 2.

(B) A different section from the same animal exhibits a large number of TdTomato-expressing axons extending from the graft into the caudal host spinal cord.

(C) Higher magnification of boxed area in (B). Axons extend through both white and gray matter. Dashed lines indicate border of white matter (WM) and gray matter (GM).

(D) Neurofilament-200 (NF), an axonal marker, and TdTomato are frequently co-expressed, indicating the axonal nature of TdTomato-labeled processes extending into the caudal host spinal cord. Image is 2 mm caudal from the graft.

Scale bars represent 550 μm (A), 530 μm (B), 63 μm (C), and 19 μm (D).