Repair of complex CNS circuitry requires newly incorporated neurons to become appropriately, functionally integrated. One approach is to direct differentiation of endogenous progenitors in situ, or ex vivo followed by transplantation. Prior studies find that newly incorporated neurons can establish long-distance axon projections, form synapses and functionally integrate in evolutionarily old hypothalamic energy-balance circuitry. We now demonstrate that postnatal neocortical connectivity can be reconstituted with point-to-point precision, including cellular integration of specific, molecularly identified projection neuron subtypes into correct positions, combined with development of appropriate long-distance projections and synapses. Using optogenetics-based electrophysiology, experiments demonstrate functional afferent and efferent integration of transplanted neurons into transcallosal projection neuron circuitry. Results further indicate that ‘primed’ early postmitotic neurons, including already fate-restricted deep-layer projection neurons and/or plastic postmitotic neuroblasts with partially fate-restricted potential, account for the predominant population of neurons capable of achieving this optimal level of integration.
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We thank P. Davis, D. Schuback and K. Yee for superb technical assistance; B. Goetze at the Harvard Center for Biological Imaging for excellent assistance with confocal spectral unmixing; M. Eichner at the Institute for Clinical Epidemiology and Applied Biometry at University of Tübingen for statistical advice; C. Lois at Caltech, R. Hevner at University of Washington, M. Hibi at Nagoya University) and S.K. McConnell at Stanford University for generous sharing of mice, antibodies and reagents; and current and past members of the Macklis laboratory for helpful suggestions. This work was partially supported by grants to J.D.M. from the National Institutes of Health-NINDS NS041590 and NS049553, with additional infrastructure support from NS045523 and NS075672; from the ALS Association to J.D.M.; from The Regeneration Project to J.D.M. and from DFG grant Wu 590/2-1 to T.V.W. F.M. was partially supported from a grant to V.N.M. (DC011291). H.P. was partially supported by an International Brain Research Organization (IBRO) Research Fellowship and by a fellowship from The Regeneration Project.
Integrated supplementary information
Supplementary Figure 1 Time course of subcerebral and transcallosal axon extension by transplanted neurons.
Transplanted neurons progressively extend long-distance axon projections and reach pons and more distal subcerebral targets as well as the contralateral cortex (f and g) by 5-6 days post-transplantation (dpt). Subcerebral axon projections: 4 dpt (a [striatum] and b [cerebral peduncle]), 5 dpt (c [pons] and d [proximal medulla oblongata]), 6 dpt (e [distal medulla oblongata]). Trans-callosal axon projections: 4 dpt (h), 5 dpt (i), 6 dpt (j). eGFP (green); L1 (red). Scale bars: a – e, 200 µm; h, – j, 100 µm.
Supplementary Figure 2 (related to Fig. 2) Transplanted neurons assuming positions in cortical layers II–-VI progressively mature and successfully extend transcallosal long-distance axon projections.
(a) Schematic illustrating the experimental outline. Micro-transplantation of eGFP+ single cell suspension into P0/P1 recipient sensorimotor cortex. Stereotaxic injection of retrograde axonal tracer (CTB555) into the contralateral hemisphere at P6. Analysis at P7 and P14. (b, c) A subset of transplanted neurons project trans-callosally. These neurons progressively mature from a DCX+ and NeuN-/low (b) to a DCX-/low and NeuN+ state (c). Compare the DCX expression of the recipient cortex at both time points. DCX decreases dramatically on a very similar time scale in the recipient cortex as for the transplanted neurons. The positions of the boxes in low magnification panels (b, c) correspond to the cortical area examined in high magnification panels Arrows indicate transplanted neurons. eGFP (green), CTB555 (red), DCX (immature neuronal marker; purple), NeuN (mature neuronal marker; blue). Scale bars: b, c (low magnification), 50 µm; b, c (high magnification), 10 µm.
Supplementary Figure 3 (related to Fig. 2) Long-distance axon projections of transplanted neurons remain stable long-term.
(a) Schematic illustrating the experimental outline. Micro-transplantation of eGFP+ single cell suspension into P0/P1 recipient sensorimotor cortex. Stereotaxic injection of retrograde axonal tracer (CTB555) into the contralateral hemisphere at P35 and analysis at P37/P38. (b) A subset of transplanted neurons establish trans-callosal axon projections, remaining stable long-term, without pruning or retraction for periods as long as analyzed (5 weeks). These neurons progressively mature to a DCX- and NeuN+ state. The position of the box in the low magnification panel (b) corresponds to the cortical area examined in high magnification panels. Arrows indicate a transplanted neuron. eGFP (green), CTB555 (red), DCX (immature neuronal marker; purple), NeuN (mature neuronal marker; blue). Scale bars: b (low magnification), 50 µm; b (high magnification), 10 µm.
Supplementary Figure 4 (related to Fig. 3) Double birthdating of donor neurons prior to transplantation identifies the stage and origin of cells undergoing successful cortical integration.
(a) Recipient cortex after transplantation and retrograde labeling from the cp. The eGFP+ transplanted neurons (boxed areas in low magnification panels) and recipient SCPN are retrogradely labeled with CTB555 from the cp. There are three additional eGFP+ birthdated, transplanted, but retrograde CTB555 negative neurons (inset in high magnification CldU panel, top row, displays the CldU signal) in deeper layers of the same displayed section. These three neurons were double-positive for IdU and CldU, demonstrating that both birthdating and ICC were successful, and indicating that these three neurons were likely to be still mitotic or only very early postmitotic (less than 16 hours) at the time of transplantation. Analysis of eGFP+ and CTB555+ transplanted neurons shows that they are positive for IdU, but not CldU (arrows), demonstrating that these neurons were postmitotic for more than 16 hours before transplantation. The position of the boxes in the low magnification panels corresponds to the cortical area examined in the high magnification panels. eGFP (green), CTB555 (red), IdU (blue), CldU (purple). (b) Quantification from transplanted neurons analyzed with combined double IdU/CldU birthdating plus retrograde labeling (n=4). Double birthdating data are presented for three transplanted and retrogradely labeled SCPN (IdU+/CldU-; green in graphical representation); primary data from one additional neuron (shown here in black) is presented in Fig. 3. All analyzed neurons were IdU+/CldU-. These data indicate that early postmitotic neurons, encompassing already fate-restricted deep layer projection neurons and/or plastic post-mitotic neuroblasts with partially fate-restricted potential, account for the predominant population of neurons capable of achieving optimal level integration, and establishing subcerebral axon projections after transplantation.
Supplementary Figure 5 (related to Fig. 4) Newly incorporated neurons of combinatorial subcerebral molecular identity establish anatomical subcerebral output connectivity with high fidelity.
(a) High power confocal images of 3 eGFP+ (green) transplanted neurons retrogradely labeled with LumaFluor microspheres or CTB555 (red) from the cerebral peduncle demonstrate SCPN identity by high-level CTIP2 (purple) expression (as opposed to low-level expression, a characteristic of corticothalamic projection neurons) and by only barely detectable or completely absent callosal projection neuron identifier SATB2 (blue). Insets display retrogradely labeled layer V recipient SCPN within the same sections, and their respective CTIP2 (purple) expression levels for comparison. (b) High power confocal images of 3 additional eGFP+ (green) transplanted neurons retrogradely labeled with CTB555 (red) from the cerebral peduncle confirm SCPN identity and exclude identity of closely related corticofugal projection neuron populations by being negative for the corticothalamic identifier FOG2 (purple). DAPI (blue). Scale bars: a, b, 10 µm. (c) Quantification from transplanted neurons analyzed with combined molecular projection neuron subtype identity markers plus retrograde labeling (n=13). Molecular identity data are presented for six transplanted and retrogradely labeled SCPN (green in graphical representation); primary data from seven additional neurons (shown here in black) are presented in Figs. 4 and 6; Suppl. Figs. 6, 8, and 12. Each row of dots matches with the same row molecular descriptions to their right. Taken together, these data reveal a remarkable level of molecular-to-projection fidelity, even compared with the very closely related corticofugal CThPN, since neither SATB2+ nor FOG2+ transplanted neurons projected subcerebrally.
Supplementary Figure 6 (related to Fig. 4) Combined epifluorescence and confocal CTIP2 expression level analysis confirms SCPN identity of transplanted and subcerebrally projecting neurons.
Analysis and comparison of CTIP2 expression levels (by epifluorescence and confocal microscopy) of CTB555 (red) retrogradely labeled recipient layer V SCPN and an eGFP+ (green) transplanted and subcerebrally projecting neuron, being localized in close vicinity at the border between layers IV and V. (a) Epifluorescence montage of upper cortical layers and adjacent layer V. The eGFP+ transplanted neuron and a recipient SCPN (indicated by arrow) with their respective nuclei are boxed and magnified in (b, c and g, h). Imaging was performed at the same exposure settings, demonstrating comparable high-level CTIP2 expression, in contrast to low-level CTIP2 expression as typical for related CFuPN populations including CThPN. (d – f) Single optical confocal section of the eGFP+ (green) transplanted neuron (boxed in (a)), unequivocally demonstrating that the analyzed high-level CTIP2 positive nucleus (purple) belongs to the transplanted and CTB555 (red) retrogradely labeled neuron. Scale bars: a, 50 µm; b – h, 10 µm. (i) Quantification from transplanted neurons analyzed with combined molecular projection neuron subtype identity markers plus retrograde labeling (n=13). Molecular identity data are presented for one transplanted and retrogradely labeled SCPN (green in graphical representation); primary data from 12 additional neurons (shown here in black) are presented in Figs. 4 and 6; Suppl. Figs. 5, 8, and 12. Each row of dots matches with the same row molecular descriptions to their right. Taken together, these data indicate specific SCPN, rather than broad corticofugal projection neuron (CFuPN), identity.
Supplementary Figure 7 (related to Fig. 5) Newly incorporated neurons of combinatorial callosal molecular identity establish anatomic interhemispheric output connectivity with high fidelity.
(a) High power confocal images of 3 eGFP+ (green) transplanted neurons retrogradely labeled with CTB555 (red) from the contralateral hemisphere demonstrate CPN identity by expression of the CPN identifier SATB2 (blue) and being negative for or only exhibiting very low-level expression of CTIP2 (purple). (b) High power confocal images of 3 additional eGFP+ (green) transplanted and with CTB555 (red) from the contralateral hemisphere retrogradely labeled neurons further confirm CPN identity by SATB2 (blue) expression and being negative for FOG2 (purple). Scale bars: a, b, 10 µm. (c) Quantification from transplanted neurons analyzed with combined molecular projection neuron subtype identity markers plus retrograde labeling (n=9). Molecular identity data are presented for six transplanted and retrogradely labeled CPN (green in graphical representation); primary data from three additional neurons (shown here in black) are presented in Fig. 5. Each row of dots matches with the same row molecular descriptions to their right. Taken together, all analyzed transplanted and trans-callosally projecting neurons possess CPN molecular identity, and no transplanted neurons of SCPN or CThPN molecular identity were retrogradely labeled from the contralateral hemisphere.
Supplementary Figure 8 Transplanted primed immature neurons rebuild subcerebral axon projections in Fezf2-knockout recipient cortex.
(a) Micro-transplantation of eGFP+ single cell suspension into Fezf2-/- P0/P1 recipient sensorimotor cortex. Ultrasound-guided injection of retrograde axonal tracer (CTB555) into the cerebral peduncle (cp) at P6. Analysis at P8. (b) Epifluorescence montage of a sagittal section of recipient Fezf2-/- cortex, including the adjacent part of the striatum (indicated by asterisk). There are 3 eGFP+ transplanted neurons in cortical layer VI (boxed area in in layer VI). CTB555 retrogradely labels the center neuron (arrow) from the cerebral peduncle (cp) and medium spiny neurons (MSNs; asterisk) in the striatum (due to the anatomical close proximity of the cp and MSN projection targets in the substantia nigra). Note the absence of retrograde label and high-level CTIP2 expression in layer V, demonstrating the absence of endogenous layer V SCPN in the Fezf2-/- recipient cortex (for comparison: the 3-ways split inset in the right upper corner displays control cortex after retrograde labeling from the cp; note the high-level CTIP2 staining and CTB555 labeling of layer V neurons (SCPN) and striatal MSN (asterisk)). Retrograde label of MSN in the Fezf2-/- recipient ascertains correct targeting of the cp with CTB555. The position of the box in layer VI corresponds to the cortical area examined in the adjacent confocal panel (first panel, top row; merge: eGFP, CTB555, CTIP2, SATB2) and to the inset in the right upper corner of the confocal panel (CTIP2 only) demonstrating high-level CTIP2 expression of the transplanted and retrogradely labeled neuron (arrow) in comparison to adjacent recipient-derived layer VI neurons. The position of the box in the confocal panel (first panel, top row) corresponds to the cortical area examined in high magnification panels, indicating high-level CTIP2 expression and absence of SATB2 expression in the retrogradely labeled transplanted neuron (arrow) (note: one of the two flanking (not retrogradely labeled) transplanted neurons also exhibits high-level CTIP2 expression, while the other one is negative for CTIP2). eGFP (green), CTB555 (red), SATB2 (blue), CTIP2 (purple). Scale bars: b (epifluorescence montage and first confocal panel, top row), 50 µm; b (high magnification confocal panels), 10 µm. (c) Quantification from transplanted neurons analyzed with combined molecular projection neuron subtype identity markers plus retrograde labeling (n=13). Molecular identity data are presented for one retrogradely labeled SCPN after transplantation into Fezf2-/- recipient cortex (green in graphical representation); primary data from 12 additional neurons (shown here in black) are presented in Figs. 4 and 6; Suppl. Figs. 5, 6, and 12. Each row of dots matches with the same row molecular descriptions to their right. Taken together, these data not only reveal a remarkable level of molecular-to-projection fidelity, but, quite strikingly, demonstrate that newly incorporated (transplanted) neurons of SCPN molecular subtype identity extend subcerebral axon projections even in the absence of the endogenous pyramidal tract, thereby reconstituting otherwise entirely absent long-distance connectivity in the recipient brain.
Supplementary Figure 9 Spines of transplanted neurons are juxtaposed to recipient-derived presynaptic synaptophysin, suggesting synaptic integration with the recipient cortex.
(a) Confocal stack 3D reconstructions of a transplanted eGFP+ neuron surrounded by synaptophysin-positive recipient-derived presynaptic terminals. (b) Epifluorescence image of the transplanted neuron. The boxed areas are magnified on the right (merge 3D reconstruction of a confocal stack; and high power magnification of single optical sections), showing a synaptophysin-positive recipient-derived presynaptic terminal flanked from two sides by eGFP+ synaptic spines on the apical dendrite of the transplanted neuron (indicated by arrows). These data are highly suggestive of synaptic integration of the transplanted neuron into the recipient cortex. eGFP (green), synaptophysin (red). Scale bars: a, 20 µm; b (epifluorescence image), 20 µm; b (merge 3D reconstruction), 10 µm; b (high power single optical sections), 10 µm.
Supplementary Figure 10 (related to Fig. 7 and Fig. 8) ChR2-Venus expression in recipient-derived neurons or donor neurons renders neurons light-sensitive.
(a) tdTomato is used to visualize in utero-electroporated layer II/III neurons by epifluorescence in an acute slice preparation without exciting co-electroporated channelrhodopsin-2-Venus (gray scale panel). One neuron is patched (indicated by the arrows) and subsequently photo-stimulated with a blue LED. A biocytin-containing internal solution allowed for post hoc ICC analysis. A low power collapsed confocal stack (first panel) gives an overview of in utero-electroporated recipient-derived neurons in layers II/III (td-tomato+ and channelrhodopsin-2-Venus+). The position of the box corresponds to the cortical area examined in the high magnification panels giving a detailed view of the biocytin filled neuron. Note this neuron is positive for biocytin, cytoplasmic tdTomato and membrane-bound channelrhodopsin-2-Venus. Representative traces are shown on the right. Blue bars indicate light pulses. Top trace: Each light pulse robustly elicits a series of action potentials, thereby demonstrating sufficient expression of channelrhodopsin-2-Venus by in utero-electroporated neurons. Bottom trace: Corresponding action currents mediated by channlerhodopsin-2-Venus. The arrow points out an action current at a smaller time scale. Channelrhodopsin-2-Venus (green, membrane bound), tdTomato (red, cytoplasmic), biocytin (blue, cytoplasmic). Scale bars: a, 50 µm. (b, c) Two examples of patch clamp recordings of transplanted and channelrhodopsin-2-Venus and tdTomato in vitro Amaxa electroporated neurons are shown; transplanted neurons are visualized by epifluorescence of tdTomato in the acute slice preparation (gray scale panels). Depolarization of the transplanted neurons by current injection through the patch pipette elicits action potential trains, thereby confirming neuronal identity (middle panels). Photo-stimulation (blue bars) results in a robust depolarization and elicits several action potential spikes, thereby demonstrating sufficient expression of channelrhodopsin-2-Venus by in vitro Amaxa electroporated and transplanted neurons (right panels; top traces). Corresponding channelrhodopsin-2-Venus mediated action currents (right panels; bottom traces). Arrows point out action currents at a smaller time scale. Scale bars: b, c, 50 µm.
Transplantations of either control eGFP+/Fezf2+/- neuronal suspensions (contain all projection neuron subtypes) or eGFP+/Fezf2-/- suspensions (contain all projection neuron subtypes but SCPN, which were genetically removed from the mix) were performed and the axon projection pattern was analyzed. While transplanted Fezf2+/- neurons (a) send axons to corticofugal/subcortical targets (b [striatum], c [thalamus], d [tectum], e [pons]), Fezf2-/- neurons (f) reach (g) the striatum but fail to innervate more distal subcortical targets including (h) thalamus, (i) tectum and (j) pons (arguing against potential cell fusion events between transplanted and recipient derived SCPN, in which case a similar axon projection pattern would have been postulated). The inset in the top right corner in (f) demonstrates that Fezf2-/- neurons are capable of sending trans-callosal axons (the hemisphere contralateral to the transplanted cortex is shown on the right side; the boxed area is magnified in the left part of the inset, visualizing eGFP+ axons projecting from the corpus callosum up into the cortex). eGFP (green), L1 (red), NeuN (blue). Scale bars: a, f, 1000 µm; b – e and g – j, 250 µm; inset (right panel), 1000 µm; inset (left panel), 100 µm.
(a) Micro-transplantation of eGFP+ single cell suspension into P0/P1 Fezf2+/LacZ recipient sensorimotor cortex. Ultrasound-guided injection of retrograde axonal tracer (CTB555) into the cerebral peduncle (cp) at P6. Five-channel confocal (with spectral unmixing) analysis at P8. (b) The eGFP+ transplanted neuron (arrow) in layer V and adjacent recipient SCPN (arrowheads) in the same field of view are retrogradely labeled with CTB555 from the cerebral peduncle and express comparable high levels of CTIP2; the transplanted neuron is negative for Satb2 and Fezf2-b-gal while a large number of recipient SCPN in the same optical plane are Fezf2-b-gal positive. The position of the boxes corresponds to the cortical area examined in high magnification panels. The position of the inset boxes in the low magnification panels is magnified and shown as an inset in the high magnification Fezf2-b-gal panel for comparison. Fezf2-b-gal negativity of the transplanted neuron in comparison to positivity of recipient-derived neurons argues against fusion of eGFP+ transplant-derived cells with recipient-derived SCPN. eGFP (green), CTB555 (red), SATB2 (blue), CTIP2 (cyan), Fezf2-b-gal (purple). Scale bars: b (low magnification), 50 µm; b (high magnification), 10 µm; b (inset in the high magnification Fezf2-b-gal panel), 10 µm. (c) Quantification from transplanted neurons analyzed with combined molecular projection neuron subtype identity markers plus retrograde labeling (n=13). Molecular identity data are presented for one transplanted and retrogradely labeled SCPN (green in graphical representation); primary data from 12 additional neurons (shown here in black) are presented in Figs. 4 and 6; Suppl. Figs. 5, 6, and 8. Each row of dots matches with the same row molecular descriptions to their right. Taken together, these data reveal a remarkable level of molecular-to-projection fidelity, even compared with the very closely related corticofugal CThPN, since neither SATB2+ nor FOG2+ transplanted neurons projected subcerebrally.
For analysis of molecular identity (CTIP2, SATB2, FOG2 ICC) combined with retrograde labeling (from cerebral peduncle or contralateral hemisphere), all neurons identified throughout all experiments and complying with the a priori criteria (neurons located in cortical gray matter separated from white matter by at least one layer of recipient-derived cortical neurons, and retrogradely labeled from target sites) were included in analysis; no neurons meeting these criteria were excluded. Representative neurons are depicted in the main figures (Figs. 4, 5, 6), and all additional analyzed neurons in the Supplementary Figures (Suppl. Figures 5, 6, 7, 8, 12). Each of these neurons is represented by a solid dot in the graphical representation. In total, n=13 transplanted neurons projecting to subcerebral targets (CSMN/SCPN) and n=9 neurons projecting to the contralateral hemisphere (CPN) (projection target is indicated at the bottom) were analyzed, and all display appropriate molecular identity for SCPN or CPN, respectively (molecular identity are indicated on the right side). Each row of dots matches with the same row molecular descriptions to their right. No transplanted and retrogradely labeled neurons were identified to have mismatching projection target and molecular identity (indicated by Ø). Fisher’s exact test was applied for statistical analysis, comparing the frequency of expression of a marker combination (SCPN or CPN) by neurons projecting to expected targets (cerebral peduncle or contralateral hemisphere) with the frequency of expression of a marker combination (SCPN or CPN) by neurons projecting to the alternative (mismatching) targets (contralateral hemisphere or cerebral peduncle): p<0.00001, demonstrating an extremely high level of molecular-to-projection fidelity. Fisher’s exact test is explicitly valid when a chi-squared test-based approximation is inadequate, since sample sizes are small, or the data are very unequally distributed among the cells of the table.
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