Loss of Wt1 in the murine spinal cord alters interneuron composition and locomotion

Abstract

Rhythmic and patterned locomotion is driven by spinal cord neurons that form neuronal circuits, referred to as central pattern generators (CPGs). Recently, dI6 neurons were suggested to participate in the control of locomotion. The dI6 neurons can be subdivided into three populations, one of which expresses the Wilms tumor suppressor gene Wt1. However, the role that Wt1 exerts on these cells is not understood. Here, we aimed to identify behavioral changes and cellular alterations in the spinal cord associated with Wt1 deletion. Locomotion analyses of mice with neuron-specific Wt1 deletion revealed that these mice ran slower than controls with a decreased stride frequency and an increased stride length. These mice showed changes in their fore-hindlimb coordination, which were accompanied by a loss of contralateral projections in the spinal cord. Neonates with Wt1 deletion displayed an increase in uncoordinated hindlimb movements and their motor neuron output was arrhythmic with a decreased frequency. The population size of dI6, V0 and V2a neurons in the developing spinal cord of conditional Wt1 mutants was significantly altered. These results show that the development of particular dI6 neurons depends on Wt1 expression and loss of Wt1 is associated with alterations in locomotion.

Spinal cords: Symphonies of interneurons across species

“…Originating from the dp6 progenitor domain, they additionally express a combination of DBX2 and PAX transcription factors (Andersson et al, 2012;Hernandez-Miranda et al, 2017). The dI6 inhibitory neurons fall into three subtypes based on the expression of DMRT3 and WT1: those that express one, the other or both (Goulding, 2009;Andersson et al, 2012;Schnerwitzki et al, 2018). In zebrafish and mice, dI6 neurons connect to other dI6 neurons and project commissurally to contact motor neurons on the contralateral side to regulate left-right alternation, rhythm generation and locomotor pattern (Soffe et al, 1984;Gross et al, 2002;Müller et al, 2002;Birinyi et al, 2003;Lanuza et al, 2004;Goulding, 2009;Rabe et al, 2009;Dyck et al, 2012;Griener et al, 2017;Haque et al, 2018;Perry et al, 2019;Satou et al, 2020;Uemura et al, 2020).…”

“…However, expression of WT1, together with GABAergic and glycinergic neurotransmitters, define a broader population of dI6 neurons in the mouse (Figure 6B; Goulding, 2009;Andersson et al, 2012;Haque et al, 2018). This population can be further divided into subtypes based on morphology, electrophysiology, neurotransmitters, birth order, transcription factors and axon guidance gene expression (Andersson et al, 2012;Griener et al, 2017;Schnerwitzki et al, 2018;Perry et al, 2019;Kishore et al, 2020;Iglesias González et al, 2021).…”

“…To examine the function of the rest of the dI6 population, Schnerwitzki et al evaluated Wt1-knockout mice (Schnerwitzki et al, 2018). Neonatal mice with this deletion displayed uncoordinated and variable locomotor activity: a slower walk with a decreased stride frequency and increased stride length, and loss of left-right and fore−/hindlimb coordination.…”

“…The anatomical projection pattern of WT1-dI6 neurons is consistent with these defects. Whereas the only known targets of the DMRT3-dI6 are the motor neurons on both sides of the spinal cord (Andersson et al, 2012), the WT1-dI6 neurons have commissural projections and terminate close to, and likely onto, the DMRT3-dI6 and V0 neurons (Haque et al, 2018;Schnerwitzki et al, 2018). V0 neurons are proposed to excite contralateral inhibitory interneurons, which in turn contact motor neurons (Talpalar et al, 2013;Danner et al, 2017), conferring an indirect function in contralateral inhibition onto this WT1 population.…”

“…These anatomical differences seem to confer each subpopulation with different roles in locomotion in the mouse. Work in horses, mice and zebrafish suggest that the DMRT3-dI6 neurons, with their monosynaptic connections onto motor neurons, are important for the coordination of body bend and gaits, likely via sensory integration (Andersson et al, 2012;Schnerwitzki et al, 2018;Del Pozo et al, 2020) and potentially through flexor-extensor regulation as has been demonstrated in zebrafish (Uemura et al, 2020). WT1-dI6 neurons, in contrast, are proposed to gate the rhythm-generating circuitry in general by integrating supraspinal and proprioceptive input, as well as releasing motor neurons on the opposite side of the spinal cord from same-side inhibition, although it is yet to be shown experimentally whether this is indeed the case (Schnerwitzki et al, 2018).…”