The Expression of Shmt Genes in Amphioxus Suggests a Role in Tissue Proliferation Rather than in Neurotransmission

Abstract

Serine hydroxymethyltransferases (SHMTs) are key enzymes in one-carbon metabolism, with vertebrates possessing two paralogs, cytosolic SHMT1 and mitochondrial SHMT2, implicated in nucleotide biosynthesis and glycine metabolism. In this study, we investigate the evolutionary history of animal Shmt genes and analyze the expression patterns of Shmt genes in developing amphioxus (Branchiostoma lanceolatum). Phylogenetic analyses indicate the presence of Shmt1 and Shmt2 orthologs in deuterostomes, spiralians and placozoans, which is consistent with an ancient Shmt gene duplication event predating bilaterian diversification. Gene expression analyses in developing amphioxus show that Shmt2 expression is confined to the somites and absent from neural tissues. In contrast, Shmt1 is broadly expressed across germ layers, but its transcription is restricted to tissues characterized by strong cell proliferation. Notably, Shmt1 expression in the nervous system does not match the distribution of glycinergic neuron populations, implying a negligible role in glycine neurotransmitter synthesis. Instead, the spatial correlation of Shmt1 expression with mitotically active domains suggests a primary function in nucleotide biosynthesis via one-carbon metabolism. These findings indicate that SHMTs predominantly support cell proliferation rather than neurotransmission in amphioxus.

Keywords: development; evolution; glycine biosynthesis; glycinergic neurons; one-carbon metabolism; serine hydroxymethyltransferase.

Figure 1

Phylogeny of metazoan SHMT proteins. Neighbor Joining and Maximum Likelihood methods were used, and the Neighbor Joining tree is shown with branch support values for the Neighbor Joining and Maximum Likelihood analyses, respectively, shown as X/Y. “-” indicates branches not recovered in the Maximum Likelihood tree. Branch lengths are expressed as amino acid substitutions per site. The tree was rooted using the sponge SHMT sequences. The Branchiostoma lanceolatum sequences, whose expression has been analyzed in this study, are shown in bold. List of animal species featured in the tree: Ame: Antedon mediterranea (sea lily, crinoid echinoderm); Aqu: Amphimedon queenslandica (sponge); Bfl: Branchiostoma floridae (Florida amphioxus, cephalochordate); Bla: Branchiostoma lanceolatum (European amphioxus, cephalochordate); Cel: Caenorhabditis elegans (roundworm, nematode); Cin: Ciona intestinalis (sea squirt, ascidian tunicate); Cte: Capitella teleta (polychaete annelid); Dma: Daphnia magna (water flea, crustacean arthropod); Dme: Drosophila melanogaster (fruit fly, insect arthropod); Dre: Danio rerio (zebrafish, teleost fish); Hpa: Halichondria panicea (sponge); Hsa: Homo sapiens (human, mammal); Mgi: Magallana gigas (Pacific oyster, bivalve mollusk); Mmu: Mus musculus (house mouse, mammal); Obi: Octopus bimaculoides (Californian two-spot octopus, cephalopod mollusk); Pca: Pomacea caniculata (apple snail, gastropod mollusk); Pmi: Patiria miniata (sea star, echinoderm); Pte: Parasteatoda tepidariorum (common house spider, arachnid arthropod); Rno: Rattus norvegicus (rat, mammal); Scl: Styela clava (sea squirt, ascidian tunicate); Sko: Saccoglossus kowalevskii (acorn worm, hemichordate); Tad: Trichoplax adhaerens (placozoan); Xtr: Xenopus tropicalis (western clawed frog, amphibian).

Figure 2

Expression of Shmt1 in developing amphioxus (Branchiostoma lanceolatum). (A) N0 neurula in lateral (top) and dorsal (bottom) views. Arrowhead highlights expression in the posterior neural plate. (B) N3 neurula in lateral (top) and dorsal (bottom) views. Arrow and arrowhead, respectively, highlight expression in the cerebral vesicle and the posterior neural plate. (C,D) Transverse sections of the N3 neurula at the levels indicated by the corresponding lowercase letters in (B). (E) T1 stage embryo in lateral view. Arrow and arrowheads, respectively, highlight expression in the cerebral vesicle and the posterior rhombospinal region. (F–H) Transverse sections of the T1 embryo at the levels indicated in (E) by lowercase letters. (I) L1 larva in lateral view. Arrow and arrowhead, respectively, highlight expression in the club-shaped gland and the first pharyngeal gill slit. All whole mounts are oriented with the anterior to the left. Scale bar is 50 µm for whole mounts and 25 µm for sections. st.: stage; cv: cerebral vesicle; en: endoderm; no: notochord; np: neural plate; nt: neural tube; ph: pharynx; so: somite; tb: tail bud.

Figure 3

Expression of Shmt2 in developing amphioxus (Branchiostoma lanceolatum). (A) N0 neurula whole mount in dorsal (top) and lateral (bottom) views. (B) N3 neurula in dorsal (left) and lateral (right) views. (C) Transverse section of the N3 neurula at the level indicated by the lowercase letter in (B). (D) T1 stage embryo in lateral view. (E–G) Transverse sections of the T1 embryo at the levels indicated by the corresponding lowercase letters in (D). (H) L1 larva in lateral view. All whole mounts are oriented with the anterior to the left. Scale bar is 50 µm for whole mounts and 25 µm for sections. st.: stage; cv: cerebral vesicle; en: endoderm; no: notochord; np: neural plate; nt: neural tube; so: somite.