A Dlx2- and Pax6-Dependent Transcriptional Code for Periglomerular Neuron Specification in the Adult Olfactory Bulb

Abstract

Distinct olfactory bulb (OB) interneurons are thought to become specified depending on from which of the different subregions lining the lateral ventricle wall they originate, but the role of region-specific transcription factors (TFs) in the generation of OB interneurons diversity is still poorly understood. Despite the crucial roles of the Dlx family of TFs for patterning and neurogenesis in the ventral telencephalon during embryonic development, their role in adult neurogenesis has not yet been addressed. Here we show that in the adult brain, Dlx 1 and Dlx2 are expressed in progenitors of the lateral but not the dorsal subependymal zone (SEZ), thus exhibiting a striking regional specificity. Using retroviral vectors to examine the function of Dlx2 in a cell-autonomous manner, we demonstrate that this TF is necessary for neurogenesis of virtually all OB interneurons arising from the lateral SEZ. Beyond its function in generic neurogenesis, Dlx2 also plays a crucial role in neuronal subtype specification in the OB, promoting specification of adult-born periglomerular neurons (PGNs) toward a dopaminergic fate. Strikingly, Dlx2 requires interaction with Pax6, because Pax6 deletion blocks Dlx2-mediated PGN specification. Thus, Dlx2 wields a dual function by first instructing generic neurogenesis from adult precursors and subsequently specifying PGN subtypes in conjunction with Pax6.

NeuroD1 induces terminal neuronal differentiation in olfactory neurogenesis

“…At 15 dpe, Pax6-expressing neurons were found in the OB indistinguishable from those in the controls (25). An increase in TH-expressing neurons, as described before (11,12), was not observed at this early time point. Later time points were not analyzed because of the transitory expression of the transgene.…”

“…S3). Only Dlx2 showed a moderate increase in the PGN lineage outgoing, however, from an already considerable baseline level in migrating precursors (12) (Fig. 1C).…”

“…We studied the neurogenic potential of NeuroD1 in primary cultured neural stem cells using the neurosphere assay. In parallel to NeuroD1, we performed all experiments under the same conditions using the transcription factor Pax6, a well-described neurogenic signal in the system (9,11,12), to control for specificity of the observed effects. Neurosphere cells were coelectroporated with NeuroD1 or Pax6 expression vectors and GFP immediately before plating in differentiation conditions.…”

“…For example, coincident presence of Pax6 and Dlx2 is necessary for the specification of dopaminergic neurons in GL. However, coexpression of both factors is found only in the RMS and not in the periventricular region (9,12). Thus, it appears that neuronal cells in the olfactory system have to pass through subsequent maturation steps, during which they acquire the competence (for example, by the generation of the correct transcription factor context) to express the full spectrum of neuronal characteristics.…”

“…Furthermore, it appears that interneuron diversity relies on the combinatorial expression of such transcription factors. This is exemplified by Pax6 and Dlx2, which have been shown to interact in the determination of adult generated neuronal precursors toward a dopaminergic fate (9,11,12). All of these transcriptional regulators are expressed early during the neurogenic process and remain present until terminal differentiation occurs.…”