The activity and expression of adenylosuccinate lyase were reduced during modern human evolution, affecting brain and behavior

Abstract

Adenylosuccinate lyase (ADSL), an enzyme that is crucial for purine biosynthesis, carries an amino acid substitution that is present in almost all humans today but absent in Neandertals and Denisovans. This substitution reduces the stability of the enzyme, but what functional consequences it has are unknown. Here, we show that when introduced into mice, this substitution causes substrates of the enzyme to accumulate in amounts that correlate negatively with ADSL expression levels. In the brain, where the expression of the enzyme is low, the substitution results in particularly high substrate levels. When the behavior of the mice is analyzed, female mice expressing the modern human-like version of ADSL access water more efficiently for drinking than their wild-type littermates. In addition to the amino acid substitution, a haplotype in the ADSL gene occurs at a carrier frequency of >97% in present-day humans and exhibits evidence of positive selection. It is associated with less ADSL expression as well as with increased concentrations of succinyladenosine, one of the substrates of the enzyme, in cerebrospinal fluid. Thus, two genetic changes have reduced ADSL activity in human tissues since modern and archaic humans separated, affecting purine biosynthesis, particularly in the brain.

Keywords: adenylosuccinate lyase; human evolution; purine biosynthesis.

Fig. 1.

ADSL enzymatic activities and protein sequence. (A) The ADSL enzyme catalyzes two reactions in purine biosynthesis. The levels of the dephosphorylated forms of the ADSL substrates (red), SAICAr and S-Ado, are elevated when ADSL activity is decreased. SAICAR: succinylaminoimidazole carboxamide ribotide; SAICAr: succinylaminoimidazole carboxamide riboside; AICAR: aminoimidazole carbozamide ribotide; IMP: inosine monophosphate; SAMP: adenylosuccinate; S-Ado: succinyladenosine; AMP: adenine monophosphate. (B) Partial amino acid sequences surrounding position 429 in the ADSL protein.

Fig. 2.

SAICAr and S-Ado concentrations in mice humanized for ADSL and their wild-type littermates. (A) Upper-quartile normalized SAICAr and S-Ado concentrations (y-axis) in various tissues of humanized Adsl mice (hAdsl, red) and their wild-type (WT, black) littermates (F: females. M: males). Tissues (Left to Right): forebrain, heart, lung, liver, spleen, kidney, and skeletal muscle. (B) Upper-quartile normalized SAICAr concentrations (y-axis) in 8 brain regions of hAdsl (red) mice and their WT (black) littermates. OB: olfactory bulb, FC: Frontal cortex, CC: cerebral cortex, HP: hippocampus, STR: striatum, TH/HY, thalamus and hypothalamus, AMG: amygdala, CB: cerebellum. For each group of mice, mean concentrations ± 95% CI as well as individual values are given. Student’s t test: *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001.

Fig. 3.

Female mice humanized for ADSL access water quicker after water restriction. (A) The proportion of visits by hAdsl (humanized) mice among the first 10 visits at the four corners on each day as the water was progressively restricted (12, 8, 6, 4, 3 h a day). Upper and Lower panels correspond to female and male cages (5 each). Mean ± SD across cages are shown. Green dots indicate significant differences from shuffled datasets at α = 0.05 level (two-tailed test with Bonferroni correction). A dotted horizontal line at 50% indicates equal access by hAdsl and WT (wild-type) mice. (B) The proportion of days on which hAdsl mice visited the four corners first when water became available (at 22:00). The total number of days was 9 d for each cage. (C) The percentage of days upon which hAdsl mice visited the indicated corners after water became available at 22:00. The total number of days analyzed was 14 d for each cage.

Fig. 4.

A haplotype associated with lower ADSL expression in present-day humans. (A) ADSL exon map and the four variants forming a 7.8-kb haplotype (shadowed area). (B) Frequencies of the rs8135371 alleles across five continental populations: AFR (African), AMR (Admixed American), CSA (Central/South Asian), EAS (East Asian), and EUR (European), with ALL indicating the combined dataset. (C) Tajima’s D (excess of rare alleles) in different populations in the region of the ADSL gene (shadowed area). The dotted line indicates the rs8135371 variant position.

Fig. 5.

Associations of rs8135371 with 440 metabolites in 2,602 human cerebrospinal fluid samples. Dashed line: P-value = 1.9 × e⁻⁶. Data from the ONTIME web portal (https://ontime.wustl.edu/).