The roles of afadin in motor neuron development and assembly of spinal motor circuits
Final Report Abstract
All motor neurons controlling the function of a single muscle are found grouped together in anatomical structures termed pools whose location in the spinal cord is highly stereotyped. Motor neurons belonging to the same pool not only connect to the same muscle target but also receive similar neuronal input. Thus, motor pool organization is a strategy to simplify the problem of wiring the motor system. By using a combination of molecular, anatomical tracing and mouse genetic approaches we defined the roles of afadin, a scaffold protein regulating cell adhesive properties, in controlling the development of motor pools and spinal motor circuits. We show that motor pool positional organization is achieved by combining two independent phases of neuronal migration that rely on the partially overlapping functions of afadin and catenin signalling. The first phase results in the positioning of motor neurons on the medio-lateral axis of the spinal cord. The second phase organizes motor pools on the dorso-ventral axis, does not require afadin activity but only on the function of type II classical cadherin. Next, we studied the role of afadin for the assembly of spinal motor circuits. We found that afadin inactivation in motor neurons results in a peculiar locomotor phenotype, alternation of limb movements normally observed during walking is substituted synchronous activation, resulting in a rabbit-like hopping movements. In order to understand the mechanism underlying this phenotype, we studied spinal cord development and the assembly of motor circuits in afadin mutant mice. Our data show that afadin expression at the motor neuron progenitor zone in the neuroepithelium is necessary for lumen formation during development and its elimination results in the generation of two central canals. As a consequence, the structural integrity of the spinal cord midline is compromised and wiring of motor circuits altered. We identified aberrant connectivity patterns in different cardinal classes of spinal interneurons, thus highlighting their importance for spinal circuits that control alternation of limb movements. Altogether, our studies identify two important roles for afadin during spinal cord development. The first, in the control of motor pool spatial organization by precisely orchestrating motor neuron migration. The second, for the correct development of the spinal cord midline, which is in turn critical for the correct wiring of spinal circuits for movement.
Publications
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(2018). Nuclear organization in the spinal cord depends on motor neuron lamination orchestrated by catenin and afadin function. Cell Reports 22 (7), 1681–1694
Dewitz, C., Pimpinella, S., Hackel, P., Akalin, A., Jessell, T.M., and Zampieri, N.
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(2019). Organization of motor pools depends on the combined function of N-cadherin and type II cadherins. Development 146, dev180422
Dewitz, C., Duan, X., and Zampieri, N.
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(2019). Teashirt1 (Tshz1) is essential for the development, survival and function of hypoglossal and phrenic motor neurons. Development 146, dev174045
Chaimowicz, C., Ruffault, P., Chéret, C., Woehler, A., Zampieri, N., Fortin, G., Garratt, A.N., and Birchmeier, C.
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(2020). Afadin signaling at the spinal neuroepithelium regulates central canal formation and gait selection. Cell Reports 31 (10), 107741
Skarlatou, S., Hérent, C., Toscano, E., Mendes, C., Bouvier, J., and Zampieri, N.
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(2020). Phrenic-specific transcriptional programs shape respiratory motor output. Elife 9, e52859
Vagnozzi, A.N., Garg, K., Dewitz, C., Moore, M.T., Cregg, J.M., Jeannotte, L., Zampieri, N., Landmesser, L.T., and Philippidou, P.
