Defective Neuronal Positioning Correlates With Aberrant Motor Circuit Function in Zebrafish

Abstract

Precise positioning of neurons resulting from cell division and migration during development is critical for normal brain function. Disruption of neuronal migration can cause a myriad of neurological disorders. To investigate the functional consequences of defective neuronal positioning on circuit function, we studied a zebrafish frizzled3a (fzd3a) loss-of-function mutant off-limits (olt) where the facial branchiomotor (FBM) neurons fail to migrate out of their birthplace. A jaw movement assay, which measures the opening of the zebrafish jaw (gape), showed that the frequency of gape events, but not their amplitude, was decreased in olt mutants. Consistent with this, a larval feeding assay revealed decreased food intake in olt mutants, indicating that the FBM circuit in mutants generates defective functional outputs. We tested various mechanisms that could generate defective functional outputs in mutants. While fzd3a is ubiquitously expressed in neural and non-neural tissues, jaw cartilage and muscle developed normally in olt mutants, and muscle function also appeared to be unaffected. Although FBM neurons were mispositioned in olt mutants, axon pathfinding to jaw muscles was unaffected. Moreover, neuromuscular junctions established by FBM neurons on jaw muscles were similar between wildtype siblings and olt mutants. Interestingly, motor axons innervating the interhyoideus jaw muscle were frequently defasciculated in olt mutants. Furthermore, GCaMP imaging revealed that mutant FBM neurons were less active than their wildtype counterparts. These data show that aberrant positioning of FBM neurons in olt mutants is correlated with subtle defects in fasciculation and neuronal activity, potentially generating defective functional outputs.

Zebrafish Erc1b mediates motor innervation and organization of craniofacial muscles in control of jaw movement

“…2.1 | kimble (kim m533 ) mutant zebrafish display reduced mouth gape magnitude and jaw opening frequency Zebrafish begin active feeding and increase jaw opening frequency between 5 and 7 days post-fertilization (dpf). 30 We evaluated lower jaw movements of 6 dpf zebrafish larvae embedded laterally, with excess agarose removed to allow free movement of the head. To assess jaw movements, we measured mouth gape, or the distance between the lower and upper jaw during minimal (Figure 1A) and maximal jaw displacement (Figure 1B), and calculated the change in jaw displacement as gape magnitude.…”

“…WT zebrafish displayed two distinct patterns of jaw movement during the video recordings (Movie S1A): bursts of jaw movements with frequent opening events and more evenly distributed jaw openings, as has been previously reported. 30 From our collection of craniofacial zebrafish mutants, we selected the kimble (kim m533 ) 28 line that shows jaw movement defects. Compared to WT, kim m533 mutants displayed defects specifically in jaw closure, with the lower jaw not contacting the upper jaw (Movie S1B).…”

“…34 We examined WT and erc1b À/À zebrafish larvae at two developmental timepoints: early in jaw movement at 4 dpf and at 6 dpf, when movements are more prevalent. 30 In WT larvae, the intermandibularis anterior (IMA), intermandibularis posterior (IMP), and interhyal (IH) muscles, which are involved in lower jaw movement, were well-organized with condensed MTJs at the tips of each muscle group, as shown by Tsp4b labeling (Figure 2A, white arrows). In erc1b À/À larvae, Tsp4b labeling revealed less organized muscle groups and tendon attachments, including the presence of ectopic fibers at both developmental stages (open yellow arrowheads, Figure 2C,D 0 ).…”

“…The motor branch of the trigeminal nerve innervates the muscles responsible for jaw movement 35,36 . It was previously shown that zebrafish mutations in a gene causing defects in facial (VIIth cranial) nerve cell body migration can cause altered jaw movements in zebrafish 30 . During development, cell bodies of cranial nerves are specified in the hindbrain and migrate to their respective rhombomeres 37,38 .…”

Celsr1 suppresses WNT5A-mediated chemoattraction to prevent incorrect rostral migration of facial branchiomotor neurons

“…The branchiomotor neurons originating in the vertebrate hindbrain are of extreme clinical significance being that these neurons innervate the muscles that drive respiratory and feeding behaviors. Based on their stereotyped migration, the facial branchiomotor (FBM) neurons have specifically proved to be an efficient model for studies into how deficient migration can ultimately result in reduced or abnormal behavior (Asante et al, 2021;Allen et al, 2017;.…”