Developmentally informed cellular repopulation by transplantation
A central contribution of our lab’s early work was in the area of cellular CNS / neocortical circuit repair by transplantation of immature neocortical neurons and neural precursors. We used an innovative, newly developed approach of noninvasive, optically-biophysically-targeted, population-specific apoptotic neuronal degeneration, via exogenous long-wavelength chromophore targeting to specific populations by retrograde microsphere transport. This enabled investigation of transplantation of developmentally primed and appropriate immature neurons to enable them to integrate into newly available synaptic space, mimicking what was then only recently identified as ongoing adult neurogenesis in the dentate gyrus and olfactory bulb; this enabled transplantation-based “adult neurogenesis”. Results included: 1) first reports of neuronal migration and integration in adult mammalian cortex; 2) evidence that signals directing neuronal migration and specific differentiation of immature neurons and progenitors in neocortex can be re-expressed in adult mammals well beyond corticogenesis; 3) demonstration that anatomic / cellular reconstruction of even highly complex cortical circuitry is possible, if appropriate immature neurons or progenitors are provided a correct combination of signals within an appropriately permissive environment; 4) related work using the powerful system of hypothalamic energy balance circuitry in mutant mice demonstrated the first true functional neuronal circuit integration in the field, including immature neuron molecular and morphologic subtype development and maturation, cellular integration, neurotransmitter development, anatomic and EM level synapses, downstream signal transduction, metabolic signaling, bi-directional patch-clamp electrophysiological synaptic integration, behavioral amelioration of energy imbalance; 5) more recently, we demonstrated 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.
- Wuttke, T. V. et al. Developmentally primed cortical neurons maintain fidelity of differentiation and establish appropriate functional connectivity after transplantation. Nat Neurosci (2018). doi:10.1038/s41593-018-0098-0 Pubmed
- Czupryn, A. et al. Transplanted hypothalamic neurons restore leptin signaling and ameliorate obesity in db/db mice. Science 334, 1133–1137 (2011). Pubmed
- Fricker-Gates, R. A., Shin, J. J., Tai, C. C., Catapano, L. A. & Macklis, J. D. Late-stage immature neocortical neurons reconstruct interhemispheric connections and form synaptic contacts with increased efficiency in adult mouse cortex undergoing targeted neurodegeneration. J Neurosci 22, 4045–4056 (2002). Pubmed
- Shin, J. J. et al. Transplanted neuroblasts differentiate appropriately into projection neurons with correct neurotransmitter and receptor phenotype in neocortex undergoing targeted projection neuron degeneration. J Neurosci 20, 7404–7416 (2000). Pubmed
- Sheen, V. L. & Macklis, J. D. Targeted neocortical cell death in adult mice guides migration and differentiation of transplanted embryonic neurons. J Neurosci 15, 8378–8392 (1995).Macklis, J. D. Pubmed
- Transplanted neocortical neurons migrate selectively into regions of neuronal degeneration produced by chromophore-targeted laser photolysis. J Neurosci 13, 3848–3863 (1993). Pubmed
We were the first to manipulate endogenous neural progenitors / precursors / “stem cells” in situ (in adult mouse) to undergo induced neurogenesis, birth of new neurons in normally “non-neurogenic” cortex. We demonstrated that newborn neurons progressively migrate, differentiate layer- and region-specifically, and some extend appropriate long-distance projections, with re-formation de novo of targeted, degenerated circuitry in adult mouse neocortex to thalamus, and a few years later to spinal cord. This was without transplantation. In collaborative work, we also induced behaviorally functional neurogenesis in situ in zebrafinch from homologous endogenous progenitors. We also published the first identification of a function of normally adult-born mammalian neurons (in olfactory bulb)– they uniquely and specifically provide a new form of synaptic plasticity at the cellular level, and undergo specific response enhancement to novel odorant stimuli (experience-dependent modification) during a neuronal critical period, implicating them in olfactory learning and memory, not simply “replacements”.
- Magavi, S. S. P., Mitchell, B. D., Szentirmai, O., Carter, B. S. & Macklis, J. D. Adult-born and preexisting olfactory granule neurons undergo distinct experience-dependent modifications of their olfactory responses in vivo. J Neurosci 25, 10729–10739 (2005). Pubmed
- Chen, J., Magavi, S. S. P. & Macklis, J. D. Neurogenesis of corticospinal motor neurons extending spinal projections in adult mice. Proc Natl Acad Sci U S A 101, 16357–16362 (2004). Pubmed
- Magavi, S. S., Leavitt, B. R. & Macklis, J. D. Induction of neurogenesis in the neocortex of adult mice. Nature 405, 951–955 (2000). Pubmed
- Scharff, C., Kirn, J. R., Grossman, M., Macklis, J. D. & Nottebohm, F. Targeted neuronal death affects neuronal replacement and vocal behavior in adult songbirds. Neuron 25, 481–492 (2000). Pubmed
Directed differentiation of endogenous cortical progenitors
The evidence that active and quiescent progenitors exist in the adult brain, and the demonstration that new neurons can integrate into preexisting neural circuitry (either from endogenous progenitors or via transplantation of immature neurons), even in the normally non-neurogenic adult neocortex, support the feasibility of cellular repair in the CNS. Complementing these efforts, work from our lab and others has begun to identify central molecular controls over development of broad classes and specific subtypes of cortical projection neurons, in particular CSMN.
The next step toward future therapeutic functional repair of cortical output circuitry is the identification of endogenous cortical progenitors already integrating cortical identity, and their molecular manipulation for directed differentiation into CSMN. Toward this goal, we investigate the endogenous progenitors exist in the adult cortex -their diversity and lineage competency- as well as development of tools and refinement of molecular controls toward generation of new cortical projection neurons/CSMN. Successful integration of new neurons, even at low levels, might help to partially restore function, and ameliorate disease symptoms.