Aberrant early growth of individual trigeminal sensory and motor axons in a series of mouse genetic models of 22q11.2 deletion syndrome

Abstract

We identified divergent modes of initial axon growth that prefigure disrupted differentiation of the trigeminal nerve (CN V), a cranial nerve essential for suckling, feeding and swallowing (S/F/S), a key innate behavior compromised in multiple genetic developmental disorders including DiGeorge/22q11.2 Deletion Syndrome (22q11.2 DS). We combined rapid in vivo labeling of single CN V axons in LgDel+/− mouse embryos, a genomically accurate 22q11.2DS model, and 3-dimensional imaging to identify and quantify phenotypes that could not be resolved using existing methods. We assessed these phenotypes in three 22q11.2-related genotypes to determine whether individual CN V motor and sensory axons wander, branch and sprout aberrantly in register with altered anterior–posterior hindbrain patterning and gross morphological disruption of CN V seen in LgDel+/−. In the additional 22q11.2-related genotypes: Tbx1+/−, Ranbp1−/−, Ranbp1+/−, and LgDel+/−:Raldh2+/−; axon phenotypes are seen when hindbrain patterning and CN V gross morphology is altered, but not when it is normal or restored toward WT. This disordered growth of CN V sensory and motor axons, whose appropriate targeting is critical for optimal S/F/S, may be an early, critical determinant of imprecise innervation leading to inefficient oropharyngeal function associated with 22q11.2 deletion from birth onward.

Why Does the Face Predict the Brain? Neural Crest Induction, Craniofacial Morphogenesis, and Neural Circuit Development

“…We have begun to assess interactions between neural crest and placodal cells underlying development of cranial somatosensory ganglia. The dual origin of cranial ganglion sensory neurons, as well as their divergent fates -primarily mechanoreceptive for placode descendants, nociceptive for those from the neural crest (D' Amico- Martel and Noden, 1983) -is well established, and our analyses in the mouse (Karpinski et al, 2016;Maynard et al, 2020a;Motahari et al, 2020) confirmed and extended earlier studies. Using transcriptional lineage tracing, we identified diversity within the neural crest population (Figure 4).…”

“…Individual trigeminal motor axons, as well as primarily placodal derived trigeminal sensory axons, respond differently as they interact with neural crest derived mesenchymal substrates in the periphery whose A-P identity has been presumably altered by enhanced RA signaling in the anterior rhombomeres. December 2020 | Volume 11 | Article 610970 the posterior shift of rhombomere patterning, based upon ectopic RA-regulated gene expression, is far more prominent than that in LgDel, as is disruption of CN V differentiation (Motahari et al, 2020). We confirmed the RA-dependence of this hindbrain patterning change in LgDel and its relationship to initial cranial nerve dysmorphology via genetic rescue of the anomalous RA-dependent shift in patterning.…”

“…We diminished RA signaling by approximately 20% (Maynard et al, 2013) using a heterozygous null allele of the rate limiting RA synthetic enzyme Raldh2 (Zhao et al, 1996;Niederreither et al, 1999). In these compound, LgDel:Raldh2 +/− embryos at E9.5, hindbrain patterning, and RA-dependent gene expression, detected by in situ hybridization (Figure 7) approximates the WT pattern (Figure 7), as does CN V differentiation and appropriately directed axon growth (Karpinski et al, 2014;Motahari et al, 2020). To confirm this impression based upon in situ hybridization, we performed qPCR for Cyp26b1 message, as well as that of three other RA-regulated genes: Gli1, Rarα and Hoxa2, in microdissected E9.5 hindbrains from E9.5 WT, LgDel, and LgDel:Raldh2 +/− embryos.…”

“…Instead, 22q11DS phenotypes may result from altered neural crest-mediated interactions with mesodermal or endodermal targets that express Tbx1. Our evidence suggest that a significant portion of the 22q11DS phenotypic spectrum reflects the coordinated expression of multiple 22q11 genes and their dosagesensitive influence on neural crest-mediated M/E induction beyond that of Tbx1 (Maynard et al, 2013(Maynard et al, , 2020aKarpinski et al, 2014;Motahari et al, 2020). These 22q11 genes have reciprocal regulatory interactions with cardinal inductive signaling pathways critical for optimal M/E induction at each of the phenotypic sites.…”

“…We found an apparent RA-mediated "posteriorization" of anterior rhombomeres, altered expression of additional rhombomere-specific genes, and apparent increased RA signaling in posterior rhombomeres by E9.5 in the hindbrain of LgDel embryos (Karpinski et al, 2014;Motahari et al, 2020;Yitsege et al, 2020). This early disruption of hindbrain A-P patterning was accompanied by a 22q11 deletion-specific changes in position and initial axon outgrowth of the trigeminal nerve (CN V) in LgDel embryos (Karpinski et al, 2014;Maynard et al, 2020a;Motahari et al, 2020;Yitsege et al, 2020).…”