T-box transcription factor 21 is expressed in terminal Schwann cells at the neuromuscular junction

Terminal Schwann cells (tSCs) are nonmyelinating Schwann cells present at the neuromuscular junction (NMJ) with multiple integral roles throughout their lifespan. There is no known gene differentiating tSCs from myelinating Schwann cells, making their isolation and investigation challenging. In this work we investigated genes expressed within tSCs.

A novel dissection technique was utilized to isolate the tSC-containing NMJ band from the sternomastoid muscles of S100-GFP mice. RNA was isolated from samples containing: (a) NMJ bands (tSCs with nerve and muscle), (b) nerve, and (c) muscle, and microarray genetic expression analysis was conducted. Data were validated by quantitative real-time polymerase chain reaction (qRT-PCR) and immunofluorescent staining. To identify genes specific to tSCs compared with other NMJ components, analysis of variance and rank-order analysis were performed using the Partek Genomic Suite.

Microarray analysis of the tSC-enriched NMJ band revealed upregulation (by 4- to 12-fold) of several genes unique to the NMJ compared with muscle or nerve parts alone (P < .05). Among these genes, Tbx21 (or T-bet) was identified, which showed a 12-fold higher expression at the NMJ compared with sciatic nerve (P < .002). qRT-PCR analysis showed Tbx21 mRNA expression was over ninefold higher (P < .05) in the NMJ relative to muscle and nerve. Tbx21 protein colocalized with tSCs and was not noted in myelinating SCs from sciatic nerve.

We found TBX21 to be expressed in tSCs. Additional studies will be performed to determine the functional significance of TBX21 in tSCs. These studies may enhance the investigative tools available to modulate tSCs to improve motor recovery after nerve injury.