nkx3.2mutant zebrafish accommodate jaw joint loss through a phenocopy of the head shapes of Paleozoic jawless fish

Abstract

The vertebrate jaw is a versatile feeding apparatus that facilitated explosive diversification. To function, it requires a joint between the upper and lower jaws, so jaw joint defects - such as osteoarthritis or even ankylosis - are often highly disruptive and difficult to study. To describe consequences of jaw-joint dysfunction, we engineered two independent null alleles of a single jaw-joint marker gene, nkx3.2, in zebrafish. These mutations caused zebrafish to become functionally jawless via fusion of the upper and lower jaw cartilages (ankylosis). Despite lacking jaw joints, nkx3.2 mutants survive to adulthood and accommodate this defect by: a) remodeling their skulls; and b) altering their behavior from suction feeding to ram feeding. As a result of remodeling, nkx3.2 mutants developed superficial similarities to the skull shapes observed in two lineages of ancient jawless vertebrates (anaspids and furcacaudiid thelodonts), including: a fixed open gape, reduced snout, and enlarged branchial region. However, no homology exists in individual skull elements between these taxa, and most of the modified elements in the mutant zebrafish occur outside known expression domains of nkx3.2. Therefore, we interpret the adult nkx3.2 phenotype not as a reversal to an ancestral state, but as convergence due to similar functional requirement of feeding without moveable jaws. This remarkable convergence strongly suggests that jaw movements themselves dramatically influence the development of jawed vertebrate skulls, which implies that functionally viable skull morphologies are finite, with or without functional jaws. Because nkx3.2 null zebrafish display prominent joint ankylosis, drastically modified skull shape, and altered feeding behaviors, these mutants provide a unique model with which to investigate mechanisms of skeletal remodeling and joint diseases.

Knockdown of hspg2 is associated with mandibular jaw joint fusion and neural crest cell dysfunction in zebrafish

“…This could be one of the mechanisms by which perlecan indirectly mediates the expression of nkx3.2 and it would prove to be a novel discovery. Recently, it was shown that nkx3.2 null animals are viable, making it possible to study this gene in relation to hspg2 without early lethality (49).…”

Knockdown of hspg2 is associated with abnormal mandibular joint formation and neural crest cell dysfunction in zebrafish

“…This could be one of the mechanisms by which perlecan indirectly mediates the expression of nkx3.2 and it would prove to be a novel discovery. Recently, it was shown that nkx3.2 null animals are viable, making it possible to study this gene in relation to hspg2 without early lethality (45).…”

The power of zebrafish models for understanding the co‐occurrence of craniofacial and limb disorders

“…2 ( bapx1 ) is an additional homeobox gene that regulates genes involved in jaw joint formation. Its expression is limited to the intermediate region of the pharyngeal arch, and it is repressed by Hand2 in the ventral region, Fgf8 in the oral epithelium, and Bmp4 in the distal pharyngeal arch (Miller, Yelon, Stainier, & Kimmel, 2003; Miyashita et al, 2020; Wilson & Tucker, 2004). DLX5/DLX6 and NKX3 .…”