This study provides an overview of the fate of Neurog1-expressing progenitors throughout the brain. To identify cells in the adult brain arising from Neurog1-expressing progenitors it was necessary to use genetic approaches since Neurog1 expression is transient and is absent in mature neurons. To interpret in vivo fate-map studies using Cre recombinase paradigms as used here, it is important to note that this strategy is not 100% efficient and it depends on the level of Cre expression and efficiency of recombination of the Cre-reporter alleles. Therefore, at any given time only a subset of a given population will be labeled, and those with lowest Neurog1 (Cre) may not be efficiently detected. However, the matched expression patterns of Neurog1 and Cre in the transgenic models used here (), and in accordance with previous studies for several lineages, strongly support the validity of using these models for identifying Neurog1 lineages throughout the brain.
There are multiple general principles that arise from combining the results here with previous literature on Neurog1 expression and function. 1) Neurog1 exclusively marks neuronal restricted progenitor cells in the brain. 2) Neurog1 is present in progenitor cells as they are progressing from mitotic to postmitotic states. 3) Neurog1 neuronal lineages are often restricted to glutamatergic types particularly in the rostral CNS, but there are plenty of exceptions in more caudal regions where GABAergic neurons are generated in the lineage. And 4) Neurog1 lineages are nonoverlapping with lineages derived from Ascl1 and Atoh1-expressing progenitors with a few notable exceptions.
The finding that Neurog1 marks neuronal restricted progenitor cells is not unexpected from Neurog1 functional studies, but it is in contrast with recent fate-mapping studies with another neural bHLH factor Ascl1. In similar in vivo genetic fate-mapping studies with Ascl1, it was found that Ascl1 is also in neuronal lineage restricted progenitors in the neural tube, but later it is present in progenitors that will give rise to oligodendrocytes (Parras et al., 2004
; Battiste et al., 2007
; Kim et al., 2008
). Indeed, forced expression of Ascl1 in neural stem cells promoted neurons and oligodendrocytes (Sugimori et al., 2007
) while loss of Ascl1 function disrupts neurons and oligodendrocyte development (Casarosa et al., 1999
; Horton et al., 1999
; Parras et al., 2004
; Sugimori et al., 2008
). In contrast, forced expression of Neurog1 in cortical progenitor cells or in neural stem cells induced neuronal differentiation while suppressing glial differentiation, consistent with Neurog1 lineage restriction to neurons (Sun et al., 2001
; Sugimori et al., 2007
). Restriction of Neurog1-derived cells to neuronal lineages is more similar to that seen with Atoh1-expressing progenitors in the CNS, which are also restricted to neuronal over glial fates, possibly reflecting a closer evolutionary relationship between Neurog1 and Atoh1 compared to Ascl1 (Bertrand et al., 2002
The second principle highlighted with these studies is that Neurog1 is present in progenitor cells as they are progressing from mitotic to postmitotic states. This is inferred from the observation that in every brain region the pattern of neurons labeled with the lineage marker X-gal after tamoxifen administration at a specific embryonic stage follows the same pattern of neurons known through birthdating studies with 3
H-thymidine (Altman and Bayer, 1981a
). Thus, these observations using temporal control of the genetic fate-mapping paradigm support the definition of Neurog1 as an inducer of neuronal differentiation. An exception to this concept is seen in the cortex with tamoxifen injections at E10.5. In this case, there are apparent clones that are labeled with columns of cells from lower layers to the upper layers of the cortex (see , inset). This implies that, at least at this early stage, Neurog1 is present in progenitors cells that undergo multiple rounds of the cell cycle before differentiating.
One early characteristic of the Neurog1 lineage was that it was found in progenitors to glutamatergic neurons (Fode et al., 2000
; Schuurmans et al., 2004
). The fate-mapping studies here confirm this bias for Neurog1 lineages derived from progenitors in the telencephalon and diencephalon. However, in more caudal lineages, such as those arising from hindbrain and spinal cord, some GABAergic cell types are clearly in the Neurog1 lineage. A clear demonstration of this is in the cerebellum, where the Neurog1 lineage contributes to two GABAergic cell populations with some Purkinje neurons and some Golgi or Lugaro neurons arising from Neurog1 progenitors. It will be important in future studies to determine the downstream targets of Neurog1 transcription activity in gluta-matergic neurons versus nonglutamatergic neurons to understand what role this essential factor is playing in neuronal subtype specification.
Previous studies with detailed examination of Neurog1 expression compared to that of other bHLH factors have shown that in multiple regions of the developing nervous system, Neurog1 is nonoverlapping with Ascl1 and Atoh1 (Sommer et al., 1996
; Gowan et al., 2001
; Landsberg et al., 2005
; Nakatani et al., 2007
; Zordan et al., 2008
). These distinct expression patterns support the idea that discrete bHLH transcription factor codes determine diverse neuronal cell types in the CNS. The nonoverlap in lineages from Neurog1, Ascl1
, and Atoh1
holds true for many of the Neurog1
lineages using the genetic fate-mapping paradigm (Machold and Fishell, 2005
; Wang et al., 2005
; Vue et al., 2007
; Kim et al., 2008
; Quinones et al., 2010
). However, there are some regions that contain neurons from both Ascl1
lineages, although with this analysis it cannot be determined whether Ascl1 and Neurog1 are marking the same neurons or distinct subpopulations of neurons. One of the regions with a contribution from both Neurog1 and Ascl1 lineages are the excitatory pyramidal and granule neurons in the hippocampal formation. Hippocampus is formed from progenitors located in the medial wall of dorsal telencephalon (Altman and Bayer, 1990
; Tole and Grove, 2001
). A detailed examination of Neurog1 and Ascl1 expression in the hippocampal neuroepithelium has not been reported, so it remains unclear if they are marking different lineages, or are possibly expressed sequentially in the same lineage. The latter relationship has been determined for Ascl1 and Neurog1 in the development of the olfactory epithelium (Cau et al., 2002
). A second neuronal population with contributions from both lineages is the Purkinje cell layer in the cerebellum. In the lineage studies, Ascl1 and Neurog1 fate mapping only labels subsets of the Purkinje neurons but it has not been determined if they are overlapping. If they are different subsets of neurons in the hippocampus and cerebellum, they may define functionally distinct populations. For reference, a summary of the comparison between Neurog1
lineage cells contributing to each CNS structure is provided in .
Neuronal Derivatives of Neurog1 or Ascl1 Lineages in CNS