Neuron Review Motor Circuits in Action: Specification, Connectivity, and Function Silvia Arber1,2,* 1Biozentrum, Department of Cell Biology, University of Basel, Klingelbergstrasse 70, 4056 Basel, Switzerland 2Friedrich Miescher Institute for Biomedical Research, Maulbeerstrasse 66, 4058 Basel, Switzerland *Correspondence: [email protected] DOI 10.1016/j.neuron.2012.05.011 Mammalian motor behavior is enabled by a hierarchy of interleaved circuit modules constructed by interneu- rons in the spinal cord, sensory feedback loops, and bilateral communication with supraspinal centers. Neuronal subpopulations are specified through a process of precisely timed neurogenesis, acquisition of transcriptional programs, and migration to spatially confined domains. Developmental and genetic programs instruct stereotyped and highly specific connectivity patterns, binding functionally distinct neuronal subpop- ulations into motor circuit modules at all hierarchical levels. Recent work demonstrates that spatial organi- zation of motor circuits relates to precise connectivity patterns and that these patterns frequently correlate with specific behavioral functions of motor output. This Review highlights key examples of how develop- mental specification dictates organization of motor circuit connectivity and thereby controls movement. Introduction Through specific perturbations of functional or genetic differen- Movement is generated by the activity of neuronal circuits tiation programs in defined neuronal populations, recent studies collecting and integrating information, ultimately leading to have successfully probed models of motor circuit organization precisely timed skeletal muscle contractions. Work over many and output. Studies on spinal interneurons, sensory-motor years has demonstrated that the motor control system exhibits connectivity, descending motor control through cortical and a multitude of interleaved layers of organization. It produces basal ganglia circuits, as well as ascending pathways from the an enormous repertoire of behaviors including routine actions spinal cord to the cerebellum, provide evidence that common such as walking, as well as sophisticated movements like playing organizational and mechanistic principles guide connectivity a violin or dancing. Independent of the action type performed, and function across diverse neuronal circuits controlling motor the interplay of three main components is important and adds behavior. modularity and flexibility to the system. First, neurons with projections confined to the spinal cord are essential to produce Genetic, Developmental, and Functional Diversification rhythmic and patterned motor activity as well as to support many of Spinal Interneurons other activities (Jankowska, 2001; Kiehn, 2011; Orlovsky et al., Diversification of spinal neurons has its origin at early develop- 1999). These include highly diverse neuronal populations glob- mental stages. This process establishes functional spinal circuits ally referred to as spinal interneurons. Second, spinal circuits that are needed to generate and maintain rhythmic motor output, are dependent on interactions with supraspinal centers in brain- including repetitive alternation of left-right and extensor-flexor stem and higher brain areas (Grillner et al., 2005; Orlovsky et al., muscle contractions as key motor output behaviors. Recent 1999). Communication is bidirectional and includes many de- studies have begun to address the important question of scending and ascending channels intersecting with local spinal how diversification programs established during development circuits. Third, sensory feedback systems constantly monitor control the emergence of functionally distinct neuronal sub- consequences of motor action (Brown, 1981; Rossignol et al., populations required to support these tasks. They highlight the 2006; Windhorst, 2007). The modular and interconnected nature importance of genetic programs and time of neurogenesis in of these three systems lies at the core of motor behavioral setting up a spatial matrix in which terminally differentiated repertoire diversification but, at the same time, also makes neuronal subpopulations are interconnected in highly precise understanding connectivity and function of the motor system patterns. a challenging task. Progenitor Domain Origin Contributes to Spinal Neuron The combined efforts of studies on motor circuits using Diversification functional approaches, anatomical morphological analysis, as Neurons with cell bodies positioned in the spinal cord are derived well as more recent developmental and genetic entry points, from local progenitors. Spinal progenitor cells are arrayed at now allow for a synthesized look at the overall logic of motor conserved dorsoventral positions along the midline and prolif- circuit organization at multiple hierarchical levels. This Review erate to give rise to postmitotic neurons during temporally will focus on emerging understanding of developmental and restricted periods. Early action of ventral sonic hedgehog (shh) genetic programs that regulate neuronal diversification and in and dorsal bone morphogenetic protein (BMP) signaling sources turn anatomical and functional connectivity in the motor system. leads to spatial subdivision of progenitor domain territory along Neuron 74, June 21, 2012 ª2012 Elsevier Inc. 975 Neuron Review ABdorsal of ventral origin mostly by genetic lineage-tracing analysis to Dbx1 fate map neurons derived from a progenitor domain transiently transcription factor dI1 expressing a defined transcription factor (Figure 1B). With the V0 exception of the progenitor domain-generating motor neurons (pMNs), the other domains probably give rise to more than one dI6 progenitor generic neuronal type, as several well-documented examples V0 postmitotic neurons illustrate (Figures 1C–1F). V0 interneurons are derived from V1 Dbx1Cre postmitotic Dbx1-expressing progenitors and make up a diverse set of x V2 V0 mostly commissural neurons including excitatory (V0e) and MN Ubiq promoter:: progenitor domain V3 lox-STOP-lox inhibitory (V0i) populations (Lanuza et al., 2004), as well as the -reporter or XYZ minor fraction of V0c neurons of cholinergic partition cells in ventral mice (Zagoraiou et al., 2009)(Figure 1C). A recent study in zebra- C mouse zebrafish fish demonstrates diversification of V0e neurons into ascending (V0eA), descending (V0eD), and bifurcating (V0eB) populations or: V0 V0d V0e V0e V0eA based on projection patterns (Satou et al., 2012)(Figure 1C). V1 interneurons are defined by the expression of the transcrip- V0v V0i V0i V0eB Dbx1 Evx1/2 Dbx1 tion factor Engrailed-1. They are inhibitory and contain Renshaw V0cI V0c Pitx2 V0c V0eD cells, Ia inhibitory interneurons (Alvarez et al., 2005), and several ChAT V0cB as-yet-uncharacterized subpopulations (Figure 1D). The case of d: dorsal e: excitatory A: ascending v: ventral i: inhibitory I: ipsilateral B: bifurcating Ia inhibitory interneurons illustrates that not all functionally c: cholinergic c: cholinergic B: bilateral D: descending defined neuronal subpopulations derive from a single progenitor V0: left-right V0c: task-dependent modulation domain. Mice lacking V1 interneurons still show functional Ia inhibitory interneurons, suggesting that at least one additional DE F excitatory progenitor domain contributes to their generation (Wang et al., V1 RC Oc1/2 V2 V2a Chx10 V3 ? Sox14 2008). V2 interneurons (Lhx3 labeled, excluding motor neurons) Notch En1 IaIN Pvalb Lhx3 V2b Gata3 Sim1 include ipsilaterally projecting excitatory V2a neurons (Chx10 inhibitory labeled) (Crone et al., 2008) and inhibitory V2b (GATA3 labeled) ? V2c inhibitory ~80% Sox1 excitatory and V2c (Sox1 labeled) neurons (Panayi et al., 2010)(Fig- ure 1E), each with likely additional subtype diversification. Notch V1: locomotor speed V2a: left-right; robustness V3: robustness signaling through the regulation of the transcriptional cofactor Lmo4 tilts the balance between V2a-V2b subtypes and contrib- Figure 1. Progenitor Domain Origin and Transcriptional Programs utes to diversification (Del Barrio et al., 2007; Joshi et al., 2009; Control Neuronal Diversification (A) Division of spinal neurons into 11 cardinal populations based on develop- Lee et al., 2008). Similar V2 neuron diversification occurs in mental progenitor domain origin. These include six populations with dorsal zebrafish (Batista et al., 2008; Kimura et al., 2008). Finally, little origin (dI1–dI6; d, dorsal; I, interneurons) and five populations with ventral is known about diversification of excitatory and predominant- origin (MN, motor neurons; V0–V3, four ventral interneuron populations). (B) Permanent labeling of spinal subpopulations in the mouse using Cre ly commissural V3 interneurons (Sim1 labeled) (Zhang et al., expression driven from a genomic locus transiently expressed in a defined 2008). In summary, subtype diversification for neurons derived Cre progenitor domain. The cross between Dbx1 mice and a ubiquitous reporter from most of the 11 cardinal progenitor domains is likely. The mouse strain leads to the expression of a reporter or XYZ protein upon Cre recombination. This strategy initiates Cre recombination in Dbx1-expressing extent of neuronal diversification still remains to be fully eluci- progenitor cells and permanently labels V0 postmitotic neurons as they dated and is likely to vary for different progenitor domains. differentiate. Caution should be taken since very few examples exist with (C–F) Diversification of V0 (C): mouse and zebrafish; dark and