Thyroid hormone regulates proximodistal patterning in fin rays

Abstract

Processes that regulate size and patterning along an axis must be highly integrated to generate robust shapes; relative changes in these processes underlie both congenital disease and evolutionary change. Fin length mutants in zebrafish have provided considerable insight into the pathways regulating fin size, yet signals underlying patterning have remained less clear. The bony rays of the fins possess distinct patterning along the proximodistal axis, reflected in the location of ray bifurcations and the lengths of ray segments, which show progressive shortening along the axis. Here, we show that thyroid hormone (TH) regulates aspects of proximodistal patterning of the caudal fin rays, regardless of fin size. TH promotes distal gene expression patterns, coordinating ray bifurcations and segment shortening with skeletal outgrowth along the proximodistal axis. This distalizing role for TH is conserved between development and regeneration, in all fins (paired and medial), and between Danio species as well as distantly related medaka. During regenerative outgrowth, TH acutely induces Shh-mediated skeletal bifurcation. Zebrafish have multiple nuclear TH receptors, and we found that unliganded Thrab—but not Thraa or Thrb—inhibits the formation of distal features. Broadly, these results demonstrate that proximodistal morphology is regulated independently from size-instructive signals. Modulating proximodistal patterning relative to size—either through changes to TH metabolism or other hormone-independent pathways—can shift skeletal patterning in ways that recapitulate aspects of fin ray diversity found in nature.

Growth patterns of caudal fin rays are informed by both external signals from the regenerating organ and remembered identity autonomous to the local tissue

“…Intact fins show transcriptomic differences across the proximodistal axis (Harper et al, 2023;Rabinowitz et al, 2017), and here we identified a robust suite of gene expression patterns that shift as the fin regenerated different proximodistal regions of the organ. Previous transcriptomic analyses of regenerating fins have focused on the early shifts in expression as the tissue initiates regenerative regrowth (Li et al, 2021;Nauroy et al, 2019); we found that there are substantial shifts in expression patterns even after regeneration is underway, as different stages of outgrowth proceed.…”

“…scpp7 is proximally enriched during regeneration. Since distal-specific morphologies are regulated by TH (Harper et al, 2023), genes that are expressed in a TH-dependent proximodistal differential are strong candidates for involvement in distal patterning. Of the transcripts showing TH-dependent proximal enrichment, we selected secretory calcium-binding phosphoprotein 7 (scpp7).…”

“…Thyroid hormone maintains the proximodistal expression of many genes. Developmental hypothyroidism proximalizes both transcriptional expression and ray morphology in intact fins (Harper et al, 2023), and we asked if fins regenerating in a hypothyroid context also showed proximalized gene expression patterns. Analyzing the transcriptomes holistically, the major axes of variation robustly captured proximodistal location (dimension 1) and TH condition (dimension 2), but there was little apparent correlation between the two (Fig.…”

“…Any damaged rays were excluded from analysis. Fin ray morphology was quantified with the StereoMorph R package (Olsen & Westneat, 2015) as described in (Harper et al, 2023). We used paired or unpaired Welch two-sample t tests or a paired repeated samples ANOVA followed by pairwise t tests to account for the two rays from a single fin or multiple time points assessed.…”

“…Intact fin rays exhibit morphological differences along the proximodistal axis. At the proximal base, ray segments are longest and widest, tapering and shortening progressively towards the distal edge; rays also form bifurcations at specific locations along the axis (Harper et al, 2023). Components of proximodistal patterning are regulated by thyroid hormone (TH), which induces distal features (Harper et al, 2023).…”