Characterization of Dmrt3-Derived Neurons Suggest a Role within Locomotor Circuits

Abstract

Neuronal networks within the spinal cord, collectively known as the central pattern generator (CPG), coordinate rhythmic movements underlying locomotion. The transcription factor doublesex and mab-3-related transcription factor 3 (DMRT3) is involved in the differentiation of the dorsal interneuron 6 class of spinal cord interneurons. In horses, a non-sense mutation in the Dmrt3 gene has major effects on gaiting ability, whereas mice lacking the Dmrt3 gene display impaired locomotor activity. Although the Dmrt3 gene is necessary for normal spinal network formation and function in mice, a direct role for Dmrt3-derived neurons in locomotor-related activities has not been demonstrated. Here we present the characteristics of the Dmrt3-derived spinal cord interneurons. Using transgenic mice of both sexes, we characterized interneurons labeled by their expression of Cre driven by the endogenous Dmrt3 promoter. We used molecular, retrograde tracing and electrophysiological techniques to examine the anatomical, morphological, and electrical properties of the Dmrt3-Cre neurons. We demonstrate that inhibitory Dmrt3-Cre neurons receive extensive synaptic inputs, innervate surrounding CPG neurons, intrinsically regulate CPG neuron's electrical activity, and are rhythmically active during fictive locomotion, bursting at frequencies independent to the ventral root output. The present study provides novel insights on the character of spinal Dmrt3-derived neurons, data demonstrating that these neurons participate in locomotor coordination.

Current Principles of Motor Control, with Special Reference to Vertebrate Locomotion

“…Knockout of the dmrt3 gene produced marked changes in the frequency and coordination of drug-induced activity in vitro (14,187,458). These studies in horse and mice have revealed conserved functions of the DRMT3 gene that might be carried out by dI6 interneurons to control the flexibility of coordi- (377) and seem to contact motoneuron on the contralateral side (14).…”

Functional Diversity of Glycinergic Commissural Inhibitory Neurons in Larval Zebrafish

“…V0d neurons are all commissural (Lanuza et al, 2004), while dI6 inhibitory neurons are divided into two types on the basis of the expression of either the dmrt3 or wt1 gene (Andersson et al, 2012;Vallstedt and Kullander, 2013). dmrt3 neurons have both contralateral and ipsilateral projections, but contralateral projections are predominant (Andersson et al, 2012;Perry et al, 2019). Recently, wt1 neurons were also characterized and shown to be commissural neurons (Haque et al, 2018).…”

“…Here, we addressed this question by characterizing genetically defined classes of interneurons in the zebrafish spinal cord. We focused on V0 inhibitory neurons (hereafter called V0d neurons) and a dmrt3a-expressing subtypes of dI6 neurons (hereafter called dI6dmrt3a neurons; dmrt3a is a zebrafish homolog of mammalian dmrt3) because these cell populations have been implicated in commissural inhibitory pathways in mammalian CPGs (Andersson et al, 2012;Bellardita and Kiehn, 2015;Kiehn, 2016;Lanuza et al, 2004;Perry et al, 2019;Talpalar et al, 2013;Vallstedt and Kullander, 2013) and have been shown to be commissural neurons in zebrafish (Andersen, 2017;Satou et al, 2012). We revealed that both V0d and dI6dmrt3a neurons made monosynaptic inhibitory connections onto contralateral CPG and motor neurons.…”

Mapping Connectivity Amongst Interneuronal Components of the Locomotor CPG

“…dI6 neurons express the transcription factors WT1 or DMRT3 at postmitotic time points (Goulding, 2009), and settle in lamina VII/VIII of the postnatal spinal cord (Dyck et al, 2012; Griener et al, 2017). The DMRT3 subset of dI6 neurons is an inhibitory population (Perry et al, 2019) which projects axons to both ipsilateral and contralateral targets. Retrograde tracing with PRV-152 demonstrated that the DMRT3 neurons project to motoneurons on either side of the spinal cord (Andersson et al, 2012) but again, given the nature of these experiments, the proportion of the DMRT3 population with this projection pattern is unknown.…”

“…Retrograde tracing with PRV-152 demonstrated that the DMRT3 neurons project to motoneurons on either side of the spinal cord (Andersson et al, 2012) but again, given the nature of these experiments, the proportion of the DMRT3 population with this projection pattern is unknown. Recent work has also identified axon terminals from DMRT3-expressing neurons in close proximity to Renshaw cells as well as cholinergic neurons (possibly V0 C interneurons) located near the central canal (Perry et al, 2019). This population has been predicted to play a critical role during locomotion based on data indicating that they are rhythmically active during stepping (Perry et al, 2019), as well as defects observed in left-right alternation in their absence (Andersson et al, 2012).…”