Vertebrate vestibular circuits use sensory signals derived from the inner ear to guide both corrective and volitional movements. A major challenge in the neuroscience of balance is to link the synaptic and cellular substrates that encode body tilts to specific behaviors that stabilize posture and enable efficient locomotion. Here we address this problem by measuring the development, synaptic architecture, and behavioral contributions of vestibulospinal neurons in the larval zebrafish. First, we find that vestibulospinal neurons are born and are functionally mature before larvae swim freely, allowing them to act as a substrate for postural regulation. Next, we map the synaptic inputs to vestibulospinal neurons that allow them to encode posture. Further, we find that this synaptic architecture allows them to respond to linear acceleration in a directionally-tuned and utricle-dependent manner; they are thus poised to guide corrective movements. After loss of vestibulospinal neurons, larvae adopted eccentric postures with disrupted movement timing and weaker corrective kinematics. We used a generative model of swimming to demonstrate that together these disruptions can account for the increased postural variability. Finally, we observed that lesions disrupt vestibular-dependent coordination between the fins and trunk during vertical swimming, linking vestibulospinal neurons to navigation. We conclude that vestibulospinal neurons turn synaptic representations of body tilt into defined corrective behaviors and coordinated movements. As the need for stable locomotion is common and the vestibulospinal circuit is highly conserved our findings reveal general mechanisms for neuronal control of balance.

Central nervous system (CNS) development is regulated by regionally expressed transcription factors that impart initial cell identity, connectivity, and function to neural circuits through complex molecular genetic cascades. genomic screen homeobox 1 and 2 (gsx1 and gsx2) encode homeobox transcription factors expressed in the developing CNS in multiple vertebrates examined to date. However, we have limited knowledge of the expression of these transcription factors and the gene networks that they regulate across developing brain regions in zebrafish. The objective of this study was to comprehensively examine gsx1 and gsx2 expression throughout neurodevelopment and characterize gsx1 and gsx2 mutants to study the essential roles of these closely related transcription factors. Using RT-PCR, whole-mount in situ hybridization (WISH), and fluorescence in situ hybridization, we examine gsx1 and gsx2 expression from early embryonic to late larval stages. gsx1 is expressed initially in the hindbrain and diencephalon and later in the optic tectum, pretectum, and cerebellar plate. Comparatively, gsx2 is expressed in the early telencephalon and later in the pallium and olfactory bulb. gsx1 and gsx2 are regionally co-expressed in the hypothalamus, preoptic area, and hindbrain, however rarely co-localize in the same cells. To identify forebrain target genes, we utilize mutants made with Transcription activator-like effector nucleases (TALEN). gsx1 mutant zebrafish exhibit stunted growth, however, they survive through adulthood and are fertile. gsx2 mutant zebrafish experience swim bladder inflation failure that prevents survival past larval stage. Using WISH and RT-qPCR we demonstrate altered expression of genes including, distal-less homeobox genes and forkhead box gene foxp2. This work provides novel tools with which other target genes and functions of Gsx1 and Gsx2 can be characterized across the CNS to better understand the unique and overlapping roles of these highly conserved transcription factors.

Background Homeobox transcription factor encoding genes, genomic screen homeobox 1 and 2 (gsx1 and gsx2), are expressed during neurodevelopment in multiple vertebrates. However, we have limited knowledge of the dynamic expression of these genes through developmental time and the gene networks that they regulate in zebrafish. Results We confirmed that gsx1 is expressed initially in the hindbrain and diencephalon and later in the optic tectum, pretectum, and cerebellar plate. gsx2 is expressed in the early telencephalon and later in the pallium and olfactory bulb. gsx1 and gsx2 are co‐expressed in the hypothalamus, preoptic area, and hindbrain, however, rarely co‐localize in the same cells. gsx1 and gsx2 mutant zebrafish were made with TALENs. gsx1 mutants exhibit stunted growth, however, they survive to adulthood and are fertile. gsx2 mutants experience swim bladder inflation failure that prevents survival. We also observed significantly reduced expression of multiple forebrain patterning distal‐less homeobox genes in mutants, and expression of foxp2 was not significantly affected. Conclusions This work provides novel tools with which other target genes and functions of Gsx1 and Gsx2 can be characterized across the central nervous system to better understand the unique and overlapping roles of these highly conserved transcription factors.