Innervation of the masseter requires Mllt11 (Af1q/Tcf7c) function during trigeminal ganglion development

The development of cranial nerves, including the trigeminal nerve, and the formation of neuromuscular junctions (NMJs) are crucial processes for craniofacial motor function. Mllt11/Af1q/Tcf7c (hereafter Mllt11), a novel type of cytoskeletal-interacting protein, has been implicated in neuronal migration and neuritogenesis during central nervous system development. However, its role in peripheral nerve development and NMJ formation remains poorly understood. This study investigates the function of Mllt11 during trigeminal ganglion development and its impact on motor innervation of the masseter muscle. We report Mllt11 expression in the developing trigeminal ganglia, suggesting a potential role in cranial nerve development. Using a conditional knockout mouse model to delete <i>Mllt11</i> in Wnt1-expressing neural crest cells, we assessed trigeminal ganglion development and innervation of the masseter muscle in the jaw. Surprisingly, we found that <i>Mllt11</i> loss does not affect the initial formation of the trigeminal ganglion but disrupts its placodal vs. neural crest cellular composition. Furthermore, we showed that conditional inactivation of <i>Mllt11</i> using <i>Wnt1^Cre2</i> led to a reduction of neurofilament density and NMJs within the masseter muscle, along with a reduction of Phox2b⁺ branchiomotor neurons in rhombomere 2, indicating altered trigeminal motor innervation. This was due to the surprising finding that the <i>Wnt1^Cre2/+</i> mouse driver promoted aberrant recombination and reporter gene expression within branchiomotor neuron pools in rhombomere 2, as well as targeting neural crest cell populations. Our findings show that Mllt11 regulates the cellular composition of the trigeminal ganglion and is essential for proper trigeminal motor innervation in the masseter muscle.