Fig. 8: Conserved alteration of the secretory pathway in mettl9-MO X. laevis neuralised animal caps (a.c.).
From: METTL9 sustains vertebrate neural development primarily via non-catalytic functions
a Schematic depicting the preparation of neuralised animal caps (a.c.) from mettl9-MO or ctrl-MO embryos in X. laevis. b Most down-regulated GO terms in the mettl-MO a.c. proteome. c, d Most down-regulated Molecular Function (c) and Cellular Component (d) GO terms in mettl9-MO neuralised a.c. RNA-seq data (see Methods). GO terms related to Cytoskeleton and Secretory pathways were highlighted in green and blue, respectively. (b–d): hypergeometric test; colour scale shows adjusted p values, Benjamini-Hochberg correction. e Representative anterior and dorsal views of X. laevis embryos injected with either ctrl-MO or mettl9-MO, or co-injected with mettl9-MO and either mettl9WT or mettl9CatD mRNAs, showing elrC mRNA expression (purple), by whole-mount RNA in situ hybridisation. Arrowheads indicate intermediate (i) neuron precursors and trigeminal placodes (tp) affected in the treated side of mettl9-MO injected embryos. Inj.: Injected side. (m) medial and (l) lateral stria. Embryos numbers are shown above each panel. f Bar graph showing the quantification of embryos: ctrl-MO, mettl9-MO or mettl9-MO co-injected with either mettl9WT or mettl9CatD mRNAs, screened for altered elrC expression (% embryos are reported in the graph; χ2 test). Error bars: mean ± SD. g Schematic depicting our working model: METTL9 has an evolutionary conserved role in vertebrates in sustaining early neural development, mainly through catalytic independent functions (in green). Among these, we identified one related to the maintenance of the secretory pathway. This function is mediated by protein-protein interactions occurring most likely at the peripheral side of the Golgi (magenta), where METTL9 is localised in mNSCs. We envisage that METTL9 binding to STMN1 and RAB2 regulates their functions related to the cytoskeleton, cargo motility and Golgi structure. Therefore, in METTL9-deficient NSCs, cellular trafficking and Golgi morphology are perturbed, and this is detrimental to the establishment of neural polarity, cell signalling, axon development and ultimately to neural differentiation. METTL9 methyltransferase activity may cooperate with the maintenance of the secretory pathway through histidine methylation (and thus, probable regulation) of Golgi-related substrates, like MYO18A or zinc transporters like SLC30A5/7 and SLC39A7 but might be marginal for neural development.
