Birth time/order-dependent neuron type specification

Abstract

Neurons derived from the same progenitor may acquire different fates according to their birth timing/order. To reveal temporally guided cell fates, we must determine neuron types as well as their lineage relationships and times of birth. Recent advances in genetic lineage analysis and fate mapping are facilitating such studies. For example, high-resolution lineage analysis can identify each sequentially derived neuron of a lineage and has revealed abrupt temporal identity changes in diverse Drosophila neuronal lineages. In addition, fate mapping of mouse neurons made from the same pool of precursors shows production of specific neuron types in specific temporal patterns. The tools used in these analyses are helping to further our understanding of the genetics of neuronal temporal identity.

Figure 1

Patterns of neural progenitor proliferation in Drosophila. (A) The classic mode of neural proliferation involves the generation of an intermediate precursor, GMC, from a neural progenitor. A GMC divides only once to yield two post-mitotic neurons. The serially derived GMCs may make post-mitotic neurons sequentially, such that only one GMC per lineage divides at any given developmental time (as indicated by a blue stroke). (B) Distinctive from classical GMC, specialized intermediate precursors, known as transit-amplifying precursors, are generated by those posterior Asense-negative Nbs in the Drosophila central brain. This type of intermediate precursors can undergo a limited number of cell divisions, not only regenerating itself but also giving rise to a GMC for each cell cycle. Consequently, multiple pairs of post-mitotic neurons can be generated at a given developmental time from precursors of distinct temporal origins (as indicated by a blue stroke).

Figure 2

(A) A schematic representation of genetic elements used in the twin-spot MARCM system. With the adoption of two sets of reporters and corresponding silencers (miRNA-based suppressors against two reporter genes in the current design) that have been placed on opposing homologous chromosome arms and distal to the recombination site, the paired Nb and two-cell clones can be labeled differentially at the same time in a mosaic brain after FRT/FLP-mediated mitotic recombination. Symbol “X” indicates the suppression of reporter gene expression. (B) Temporal cell fate mapping with twin-spot MARCM analysis. For a given neuronal lineage, a selected GAL4 driver was used to label the sequentially produced neural subtypes (A–J subtypes shown in the figure). By including this GAL4 driver in twin-spot MARCM, the temporal identity of individual marked two-cell clones generated at different developmental time points can be unambiguously determined based on the size and composition of their sister Nb clones, allowing high-resolution birth order mapping. (C) A schematic representation of genetic elements used in the inducible genetic fate mapping. Two transgenes are involved. One encodes an inducible site-specific recombinase CreER, which is a fusion protein of Cre recombinase with a tamoxifen-responsive Estrogen Receptor ligand-binding domain. Only after tamoxifen administration, CreER activity is turned on, providing a temporal control over the recombination event. Moreover, CreER expression under the control of a distinct promoter (or enhancer) element is restricted to specific precursors. The other transgenic element encodes an inheritable reporter whose expression depends on Cre-mediated excision of a stop signal and persistently marks the progeny of CreER-positive precursors. (D) Temporal cell fate mapping with the inducible genetic fate mapping in mouse. For a given neuronal precursor lineage, the sequentially derived neural subtypes (A–F types labeled by a non-inducible Cre activity) can be selectively labeled by the CreER activity depending on the time window of tamoxifen administration. Their birth order could be subsequently deduced.

Figure 3

A model of birth time/order-dependent neuron type specification in Drosophila CNS. As neurogenesis proceeds, a distinct set of temporal identity factors (as indicated with different colors), which sequentially express in the Nb (circle) and its GMC progeny (square), determines their individual temporal identities at specific time points. On top of the temporal identity factors, certain timing factors are used to regulate their temporal expression profiles, controlling the switch of temporal identity windows. Upon birth of post-mitotic neurons (triangle), temporal fate master genes start to express, dictating the terminal cell fates. The molecular asymmetries during neuron-producing mitoses may confer different fates on sister neurons born from the same GMCs. Together, multiple layers of temporal modulations underlie the production of many distinct neurons from a common progenitor.