Early asymmetry of gene transcription in embryonic human left and right cerebral cortex

Abstract

The human left and right cerebral hemispheres are anatomically and functionally asymmetric. To test whether human cortical asymmetry has a molecular basis, we studied gene expression levels between the left and right embryonic hemispheres using serial analysis of gene expression (SAGE). We identified and verified 27 differentially expressed genes, which suggests that human cortical asymmetry is accompanied by early, marked transcriptional asymmetries. LMO4 is consistently more highly expressed in the right perisylvian human cerebral cortex than in the left and is essential for cortical development in mice, suggesting that human left-right specialization reflects asymmetric cortical development at early stages.

Fig. 1

Dissection of human embryonic brain tissues and generation of human SAGE libraries. (A) A top view of a 14-week human embryonic brain. Tissues were dissected from perisylvian regions in the left (L) and right (R) hemispheres. (B) A side view of a 14-week human embryonic right hemisphere. Tissues in the frontal (f) (red) and occipital (o) (blue) ventricular regions containing dividing cells were dissected. The dorsal cortex (d) is on the top. (C) The left side view of a real 14-week human embryonic brain. The perisylvian region is circled. (D) Summary of human brain SAGE libraries. The male (M) and female (F) brains are listed. At least 55,000 tags were sequenced in each library. (E) Summary of differentially expressed genes detected by SAGE analysis between the left-right hemispheres. Numbers of genes that are highly expressed in the left (L>R) or right (L<R) are listed with confidence levels p>99% and 95%<p<99%.

Fig. 2

Human LMO4 was highly expressed in the right hemisphere as detected by SAGE, real-time RT-PCR and in situ hybridization. (A) The human LMO4 expression levels in the perisylvian regions measured by SAGE (tag frequencies) in 12 (12 wk), 14 (14 wk) and 19 week (19 wk) brains. (B) The LMO4 expression levels between the left (L) and right (R) hemispheres were verified by real-time RT-PCR in eight human embryonic brains (12-19 weeks). Two data points from duplicated experiments for each sample are illustrated. (C-F) LMO4 expression in coronal sections cut from the frontal (f) to occipital (o) lobes of a human embryonic 12-week brain. The medial-lateral extent of LMO4 expression in the cortical plate between the left (white arrowheads) and right (black arrowheads) hemispheres was defined by a red line connecting the corticostriatal sulcus (cs) and the lateral border of the putamen (arrow). Numbers of sections illustrated in (C) are labeled in sections shown. Human LMO4 was more highly expressed in the cortical plates in the right hemispheres than the left in coronal sections of a 14-week brain (G-J) and 16-week brain (K-L). (H, J and L) are high power views of selected areas in panel G, I and K, respectively. The dorsal (d) and ventral (v) cortex is labeled.

Fig. 3

The dynamic and asymmetric expression of Lmo4 in mouse brains. The patchy and asymmetric expression patterns of Lmo4 in the coronal sections in the left (L) and right (R) hemispheres of E11.5 (A), E15.5 (B) and P5 (E) mouse brains. Its expression appears uniform in the cortical plate in a sagittal section of a P17 brain (G). (C) and (F) High power views of Lmo4 expression in the cortical plates (cp) (red stars) in selected areas in panel (B) and (E), respectively. The dorsal cortex is on the left side. (D) The dorsal view of whole mount in situ hybridization of a P1 mouse brain. (I) Lmo4 expression was divided into the anterior (a) (Expression I), the posterior (p) (Expression II) and the Non-expression region in a sagittal section (H) of a P1 mouse cortex. The dorsal (d) and ventral (v) cortex is labeled. (J) and (K) Asymmetric expression of Lmo4 in the left-right hemispheres in representative P1 mouse cortices (totally 6 brains were similar to that in J and 4 brains to that in K).