Making Ramón y Cajal proud: Development of cell identity and diversity in the cerebral cortex.

Since the beautiful images of Santiago Ramón y Cajal provided a first glimpse into the immense diversity and complexity of cell types found in the cerebral cortex, neuroscience has been challenged and inspired to understand how these diverse cells are generated and how they interact with each other to orchestrate the development of this remarkable tissue. Some fundamental questions drive the field's quest to understand cortical development: what are the mechanistic principles that govern the emergence of neuronal diversity? How do extrinsic and intrinsic signals integrate with physical forces and activity to shape cell identity? How do the diverse populations of neurons and glia influence each other during development to guarantee proper integration and function? The advent of powerful new technologies to profile and perturb cortical development at unprecedented resolution and across a variety of modalities has offered a new opportunity to integrate past knowledge with brand new data. Here, we review some of this progress using cortical excitatory projection neurons as a system to draw out general principles of cell diversification and the role of cell-cell interactions during cortical development.

Temporally divergent regulatory mechanisms govern neuronal diversification and maturation in the mouse and marmoset neocortex.

Mammalian neocortical neurons span one of the most diverse cell type spectra of any tissue. Cortical neurons are born during embryonic development, and their maturation extends into postnatal life. The regulatory strategies underlying progressive neuronal development and maturation remain unclear. Here we present an integrated single-cell epigenomic and transcriptional analysis of individual mouse and marmoset cortical neuron classes, spanning both early postmitotic stages of identity acquisition and later stages of neuronal plasticity and circuit integration. We found that, in both species, the regulatory strategies controlling early and late stages of pan-neuronal development diverge. Early postmitotic neurons use more widely shared and evolutionarily conserved molecular regulatory programs. In contrast, programs active during later neuronal maturation are more brain- and neuron-specific and more evolutionarily divergent. Our work uncovers a temporal shift in regulatory choices during neuronal diversification and maturation in both mice and marmosets, which likely reflects unique evolutionary constraints on distinct events of neuronal development in the neocortex.

Instructing Perisomatic Inhibition by Direct Lineage Reprogramming of Neocortical Projection Neurons.

During development of the cerebral cortex, local GABAergic interneurons recognize and pair with excitatory projection neurons to ensure the fine excitatory-inhibitory balance essential for proper circuit function. Whether the class-specific identity of projection neurons has a role in the establishment of afferent inhibitory synapses is debated. Here, we report that direct in vivo lineage reprogramming of layer 2/3 (L2/3) callosal projection neurons (CPNs) into induced corticofugal projection neurons (iCFuPNs) increases inhibitory input onto the converted neurons to levels similar to that of endogenous CFuPNs normally found in layer 5 (L5). iCFuPNs recruit increased numbers of inhibitory perisomatic synapses from parvalbumin (PV)-positive interneurons, with single-cell precision and despite their ectopic location in L2/3. The data show that individual reprogrammed excitatory projection neurons extrinsically modulate afferent input by local PV(+) interneurons, suggesting that projection neuron class-specific identity can actively control the wiring of the cortical microcircuit.

RUSH and CRUSH: a rapid and conditional RNA interference method in mice.

RNA interference (RNAi) is a powerful approach to phenocopy mutations in many organisms. Gold standard conventional knock-out mouse technology is labor- and time-intensive; however, off-target effects may confound transgenic RNAi approaches. Here, we describe a rapid method for conditional and reversible gene silencing in RNAi transgenic mouse models and embryonic stem (ES) cells. RUSH and CRUSH RNAi vectors were designed for reversible or conditional knockdown, respectively, demonstrated using targeted replacement in an engineered ROSA26(lacZ) ES cell line and wildtype V6.5 ES cells. RUSH was validated by reversible knockdown of Dnmt1 in vitro. Conditional mouse model production using CRUSH was expedited by deriving ES cell lines from Cre transgenic mouse strains (nestin, cTnnT, and Isl1) and generating all-ES G0 transgenic founders by tetraploid complementation. A control CRUSH(GFP) RNAi mouse strain showed quantitative knockdown of GFP fluorescence as observed in compound CRUSH(GFP) , Ds-Red Cre-reporter transgenic mice, and confirmed by Western blotting. The capability to turn RUSH and CRUSH alleles off or on using Cre recombinase enables this method to rapidly address questions of tissue-specificity and cell autonomy of gene function in development.

Transcriptional program of the endocrine pancreas in mice and humans.

The Endocrine Pancreas Consortium was formed in late 1999 to derive and sequence cDNA libraries enriched for rare transcripts expressed in the mammalian endocrine pancreas. Over the past 3 years, the Consortium has generated 20 cDNA libraries from mouse and human pancreatic tissues and deposited >150,000 sequences into the public expressed sequence tag databases. A special effort was made to enrich for cDNAs from the endocrine pancreas by constructing libraries from isolated islets. In addition, we constructed a library in which fetal pancreas from Neurogenin 3 null mice, which consists of only exocrine and duct cells, was subtracted from fetal wild-type pancreas to enrich for the transcripts from the endocrine compartment. Sequence analysis showed that these clones cluster into 9,464 assembly groups (approximating unique transcripts) for the mouse and 13,910 for the human sequences. Of these, >4,300 were unique to Consortium libraries. We have assembled a core clone set containing one cDNA for each assembly group for the mouse and have constructed the corresponding microarray, termed "PancChip 4.0," which contains >9,000 nonredundant elements. We show that this PancChip is highly enriched for genes expressed in the endocrine pancreas. The mouse and human clone sets and corresponding arrays will be important resources for diabetes research.

Direct lineage tracing reveals the ontogeny of pancreatic cell fates during mouse embryogenesis.

Lineage tracing follows the progeny of labeled cells through development. This technique identifies precursors of mature cell types in vivo and describes the cell fate restriction steps they undergo in temporal order. In the mouse pancreas, direct cell lineage tracing reveals that Pdx1- expressing progenitors in the early embryo give rise to all pancreatic cells. The progenitors for the mature pancreatic ducts separate from the endocrine/exocrine tissues before E12.5. Expression of Ngn3 and pancreatic polypeptide marks endocrine cell lineages during early embryogenesis, and these cells behave as transient progenitors rather than stem cells. In adults, Ngn3 is expressed within the endocrine islets, and the NGN3+ cells seem to contribute to pancreatic islet renewal. These results indicate the stage at which each progenitor population is restricted to a particular fate and provide markers for isolating progenitors to study their growth, differentiation, and the genes necessary for their development.