Generation of telencephalon-eye organoids comprising concentric zones of anterior ectodermal progenitors (CONCEPT). (A)

A scheme of the procedure. (B) Diagrams of developing CONCEPT organoids showing concentric zones of the anterior ectodermal progenitors. A summary of Figs.1, 2, 7, S1-S3, S15. (C) Morphology of cysts at day 2 showing the epithelial structure indicated by apical localization of the reporter TJ::GFP at the lumen. (D) Morphology of CONCEPT organoids at day 26. (E-G) Expression of telencephalon (Tel) marker Foxg1, neuroretinal (NR) markers Vsx2 and Pax6 in mouse eyes at E10-10.5. Rostral optic stalk (OS) connected the telencephalic vesicle to the optic cup. (H-O) FOXG1+ telencephalic progenitors, VSX2+ and/or PAX6+ retinal progenitors formed concentric zones in CONCEPT organoids. N > 5 experiments. (P-W) In CONCEPT organoids, morphogens FGF8, BMP4, and BMP7 mRNA expression started at early stages and subsequently formed circular gradients. N > 5 experiments. Scale bars, 100 µm (C, E, M, O, P, S, T), 200 µm (I, K), 500 µm (Q, U), 1 mm (D, H, J, L, N, R, V, W).

Retinal ganglion cells (RGCs) grow axons toward and then along a path defined by PAX2+ cell populations in CONCEPT telencephalon-eye organoids.

N > 5 experiments. (A-D) POU4F2+ RGCs grew TUBB3+ axons toward and then along a path with a circular or a portion of circular shape. (E, F) In mice, Pax2 was expressed in central regions of the retina and optic stalk at E10.5 (E) and in the optic disc and optic stalk at E13.5 (F). Tubb3+ axons from the initial RGCs grew toward the optic disc, exited the eye, and navigated within the optic stalk (G). (H-L) In CONCEPT organoids at day 26, TUBB3+ RGC axons grew toward and then along a path defined by an adjacent PAX2+ VSX2+ cell population (arrowhead in H, brackets in I, J); the PAX2+ VSX2-cell population sets up an inner boundary of RGC axon growth. (L) A diagram summarizing RGC axon growth, PAX2+ optic disc (OD), and PAX2+ optic stalk (OS) in CONCEPT organoids. The area labeled by the asterisk may appear as false signals in a low-resolution printout but it is clearly a background in digital display. (M) A count of CONCEPT organoids showing directional retinal ganglion cell axons. Scale bars, 50 µm (E, F, G), 100 µm (B), 200 µm (A, H, I).

CONCEPT telencephalon-eye organoids contain lens cells that undergo terminal differentiation.

N > 5 experiments. (A-L) In CONCEPT organoids, lens markers CRYAA and beta crystallin (CRY B) were expressed at day 22 (A, B) and day 39 (C, D, K; a count in H). Lens cells were not stained by DAPI (E, F); they exhibited a crystal-like shape (G). When CONCEPT organoids were detached using Dispase at around day 28 and grown in suspension, crystal-like clusters, named as lentoid bodies, were found (I) and survived for months (J). These lentoid bodies highly expressed gamma crystallin (CRY G), as revealed by Western blot (L). (M-N) These lentoid bodies were free of organelles and exhibited ball-and-socket structures (K, L), as revealed by electron microscopy. Scale bars, 100 µm (A, B, C, D, I), 200 µm (J), 5 µm (K), 200 nm (L).

scRNA-seq of CONCEPT organoids identifies telencephalic and ocular cells, including PAX2+ VSX2+ optic disc cells, PAX2+ VSX2-optic stalk cells, and CNTN2+ RGCs.

CONCEPT organoids at day 24 were used for profiling. (A) Identification of 14 cell clusters. (B) Cell cycle phases revealed by cell cycle scores. (C) FOXG1 expression marked telencephalic cells. (D, E) The expression of PAX6 and/or VSX2 marked retinal cells. (F) PAX2+ cells were found in two major cell populations: PAX2+ VSX2+ cells were assigned as the optic disc (OD), whereas PAX2+ VSX2-FOXG1+ cells were assigned as the optic stalk (OS). (G-L) The expression of major DEGs in cluster 2, the major cell population that mimics the optic disc. (M-P) The expression of major gene markers for PAX2+ VSX2-optic stalk cells. (Q-T) Identification of CNTN2 as a specific marker for early human RGCs. A large portion of cluster 11 differentially expressed neurogenic retinal progenitor marker ATOH7 and RGC markers POU4F2 and SNCG. The expression of CNTN2 and POU4F2 largely overlapped. (U-X) Two small portions of cluster 11 differentially expressed early photoreceptor cell markers (OTX2 and CRX, U, V) and amacrine/horizontal cell markers (TFAP2C and PTF1A, W, X), respectively.

Cell counts for clusters in the scRNA-seq dataset of CONCEPT organoids at day 24.

Expression signatures of human fetal retinas HGW9 are similar to those of CONCEPT organoids. (A)

Cell clustering of human fetal retinas HGW9 (GSE138002) identified 21 clusters. (B-F) Cluster 18 differentially expressed PAX2, COL9A3, CYP1B1, SEMA5A, and FGF9, which were top DEGs of cluster 2 in CONCEPT organoids. (G) VSX2 expression marked retinal progenitor cells. (H-M) Identification of neurogenic retinal progenitor cells (H), early photoreceptor cells (I), amacrine/horizontal cells (J, K), and early RGCs (L, M). POU4F2 and CNTN2 were largely co-expressed in early RGCs, consistent with their expression profiles in CONCEPT organoids. (N-R) When cells in cluster 18 and retinal progenitors from HGW9 were combined with cells in clusters 2, 4, 5, 7 from CONCEPT organoids (CR24) for Seurat anchor-based clustering, cells in cluster 18 from HGW9 (H18) were grouped with cluster 2 from CONCEPT organoids (C2, assigned optic disc; N), and these cells expressed both PAX2 and VSX2 (arrowheads in N-R). A small portion of H18 cells were grouped with C4 cells (assigned optic stalk; N), and these cells expressed PAX2 but not VSX2 (arrows in N-R).

Comparisons of enriched GO terms in DEGs (top 200 genes) of cluster 2 in CONCEPT organoids and DEGs (113 genes) of cluster 18 in human fetal retinas HGW9.

A large number of GO terms were enriched in both samples.

RGCs grow CNTN2+ axons toward and then along a defined path in CONCEPT telencephalon-eye organoids and can be isolated in one step via CNTN2 in a native condition.

N > 5 experiments. (A-F) PAX2+ cells formed two concentric zones mimicking the optic stalk (OS) and optic disc (OD), respectively (A-C; high magnifications in D, E). POU4F2+ RGCs grew CNTN2+ axons toward and then along a path defined by adjacent PAX2+ optic-disc cells (A-E). RGCs at a few hundreds of micrometers away from PAX2+ optic-disc cells grew axons centrifugally (arrow in C). At regions where there was a gap in PAX2+ optic-disc cells, CNTN2+ RGC axons exited the circular path and grew centrifugally (diamond arrowhead in A, double arrowheads in B and F). PAX2+ optic-stalk cells set up an inner boundary for RGC axon growth. (G, H) PAX2+ optic-disc cells did not express ALDH1A3; the cells that set up the boundaries of the path highly expressed ALDH1A3. (I, J) Cells that set up the inner boundary for RGC axon growth expressed VAX1/VAX2 (the antibody recognizes both VAX1 and VAX2). (K) In E13.5 mouse eye, Aldh1a3 expression was high in the peripheral retina and was low or nearly absent in the central retina. (L-P) One-step isolation of RGCs. RGCs from floating retinal organoids at day 41 (L, M) and day 70 (N-O) were dissociated into single cells using Accutase and then isolated using MACS via CNTN2 for 10-day growth. Isolated RGCs expressed POU4F2 and grew TUBB3+ neurites in random directions (L). RGCs also expressed CNTN2 (M), ISL1 (N), RBPMS (N), and SNCG (O); positive cells were counted (P). Scale bars, 200 µm (A,G,I), 100 µm (D-F,H,J,K), 50 µm (L).

Electrophysiological features of RGCs. RGCs from retinal organoids on day 48 were isolated using MACS via a CNTN2 antibody and then grown on polymer coverslips in a chamber slide for 20-25 days before whole-cell patch clamp recordings. (A) Resting membrane potential of RGCs. Black triangle: average; black line: median; box: interquartile range, n=9. (B) RGCs can fire action potentials. Cells were patched in current-clamp mode. A steady current (Im) was injected to maintain the membrane potential at –70mV and depolarizing current steps of 10ms (left), 100ms (middle) or 1s (right) were injected to elicit action potentials (Vm). (C) RGCs show functional voltage-gated currents. Cells were recorded in voltage-clamp (holding=-80mV) and depolarizing voltage steps (200ms, +10mV steps up to 80mV, Vm) were applied to record inward and outward voltage-gated currents (Im). Inset: zoom on inward currents. (D) Outward currents are primarily due to voltage-gated potassium channels. Left, representative example of a current-voltage experiment performed in presence of 20mM Tetraethylammonium (TEA), a blocker of voltage-gated potassium channels. Inset: zoom on inward currents. Right, amplitude of potassium current as a function of membrane potential (mean±SEM; ncontrol=5, nTEA=5). (E) Inward currents result from activity of voltage-gated sodium channels. Left, representative example of a current-voltage experiment performed in presence of 1µM Tetrodotoxin (TTX), a blocker of voltage-gated sodium channels. Inset: zoom on inward currents. Right, amplitude of sodium current as a function of membrane potential (mean±SEM; ncontrol=5, nTTX=3).

FGF signaling mediated by FGFRs is required for early RGC differentiation and directional axon growth. (A)

PAX2, FGF9, and FGF8 are differentially expressed in cluster 2, the major component of PAX2+ optic-disc cells. (B) PAX2 mRNA expression in CONCEPT organoids on day 25. Two PAX2+ concentric zones corresponding to the optic stalk (OS) and optic disc (OD) are labeled. (C) Dual-color immunocytochemistry indicates the co-localization of FGF8 and PAX2 in the optic-disc zone of CONCEPT organoids on day 25. (D-E) TUBB3+ axons grew towards and then along the cells that expressed high levels of FGF8 (D) and FGF9 mRNA (E) in CONCEPT organoids on day 25. Immunocytochemistry of TUBB3 was performed after in situ hybridization. (F-K) After the inhibition of FGF signaling with FGFR inhibitor PD 161570 during days 17-24, FGF8 expression still largely remained, but the number of RGC somas drastically reduced (J, K). Notably, remaining RGCs nearly did not grow directional axons (arrowheads in K), and a few remaining axons wandered around (arrow in K). CNTN2 immunocytochemistry before (F, H) and after FGF8 immunocytochemistry (G, I, J, K) are shown. N = 3/3 experiments. Scale bar, 250 µm (B), 100 µm (C-E), 1 mm (F), 200 µm (K).