On the functional overlap between two Drosophila POU homeo domain genes and the cell fate specification of a CNS neural precursor.

Abstract

The -200 distinct neurons comprising each hemisegment of the Drosophila embryonic CNS are derived from a stereotypic array of -30 progenitor stem cells, called neuroblasts (NBs). Each NB undergoes repeated asymmetric divisions to produce several smaller ganglion mother cells (GMCs), each of which, in turn, divides to produce two neurons and/or glia cells. To understand the process by which cell type diversity is generated in the CNS, we are focusing on identifying genes that affect cell identity in the NB4-2 lineage from which the RP2 motoneuron is derived. We show here that within the early part of the NB4-2 lineage, two closely linked and structurally related POU homeo domain genes, pdm-2 (dPOU28) and pdm-1 (dPOU19), both encode proteins that accumulate to high levels only in the first GMC (GMC4-2a) and not in its progeny, the RP2 motoneuron. Our results from the genetic and developmental analysis of pdm-1 and pdm-2 demonstrate that these genes are not required for the birth of GMC4-2a; however, they are both involved in specifying the identity of GMC4-2a and, ultimately, in the genesis of RP2 neurons, with pdm-2 being the more dominant player in this process. In mutant animals where both pdm-1 and pdm-2 functions are removed, GMC4-2a fails to express markers consistent with a GMC4-2a identity and no mature (Eve protein expressing) RP2 neurons are produced. We demonstrate that in some mutant combinations in which no mature RP2 neurons are produced, some GMC4-2a cells can nevertheless divide. Hence, the failure of the POU mutants to produce mature RP2 neurons is not attributable to a block in GMC4-2a cell division per se but, rather, because the GMC4-2a cells fail to acquire their correct cellular identity.[Key Words: Drosophila; POU homeo domain; CNS cell fate; redundancy] Received January 27, 1995; revised version accepted March 29, 1995.The Drosophila CNS is generated from a stereospecific set of stem cells, neuroblasts (NBs), which are formed by delamination of -20% of the cells from the neuroectodermal region (for review, see Artavanis-Tsakonas and Simpson 1991; Campos-Ortega 1993; Jimenez and Modolell 1993). Most of the genetic elements regulating the formation of -30 NBs/hemisegment have been identified and analyzed extensively; they are comprised of proneural genes, which impart the potential for neurectodermal cells to adopt the neural path, and the neurogenic genes, which act via a process called lateral inhibition to ensure that the correct number of neuroblasts are formed. Each NB divides assymetrically and repeatedly to give rise to a set of ganglion mother cells (GMCs), and these each divide once to produce neuronal and glia progZCorresponding author.eny. In this way, each neuroblast that occupies a unique position within a hemisegment possesses a unique identity and will produce a characteristic family of neurons and glia cells {for review, see Doe 1992a; Goodman and Doe 1993).To understand how the diversity of neuronal and glia cell types is generated, we need to delineate neuronal cell lineages...

Regulation of POU genes by castor andhunchback establishes layered compartments in theDrosophila CNS

“…The dynamic expression of Hb in early sublineages may allow for pdm expression at specific stages during sublineage development. For example, the pdm genes are known to collaborate in the S2 NB4-2 → GMC-1 → RP2 neuron lineage by specifying GMC-1 identity (Bhat et al 1995;Yeo et al 1995). Interestingly, pdm expression is detected only in GMC-1 and not in its NB.…”

“…The structurally related and coexpressed pdm genes are expressed in most, if not all, neuroectoderm cells during NB delaminations. Their expression during lineage development, however, is dynamic and maintained only in a subset of cells that make up each of the CNS ganglia (Billin et al 1991;Dick et al 1991;Bhat et al 1994Bhat et al , 1995Yeo et al 1995). Unlike the identical expression patterns of the pdm genes, drifter (drf) and I-POU are activated relatively late in overlapping subsets of NB sublineages (Treacy et al 1991;Anderson et al 1995).…”

The Caenorhabditis elegans gene sem-4 controls neuronal and mesodermal cell development and encodes a zinc finger protein.

“…POU homeo domain proteins have been shown to be required for neuronal development in both Drosophila (Bhat et al 1995;Yeo et al 1995) and mammals (for review, see Sharp and Morgan 1996). Mutation of the C. elegans gene sere-4 (sex muscle defective) causes the same spectrum of defects in HSN development as that caused by mutation of unc-86 (Desai et al 1988).…”

Mutational analysis of a Drosophila neuroblast enhancer governing nubbin expression during CNS development

“…During embryonic CNS development, the sequential NB expression of the TFs Hunchback→ Krüppel→ Nubbin and Pdm‐2→ Castor→ Grainyhead coordinates specification of neuronal temporal identity (reviewed by Brody & Odenwald, 2002, 2005; Chai, Liu, Chia, & Cai, 2013; Kuzin et al, 2012; Syed, Mark, & Doe, 2017). Loss of any of these TFs, including nubbin and pdm‐2 , triggers abnormal CNS development and consequently embryonic lethality (Grosskortenhaus, Pearson, Marusich, & Doe, 2005; Kohwi & Doe, 2013; Tran & Doe, 2008; Yang, Yeo, Dick, & Chia, 1993; Yeo et al, 1995). The pdm genes are expressed in overlapping but non‐identical patterns within intermediate neuroblast sublineages.…”