Introduction
All cells require efficient mRNA translation, but protein synthesis capacity varies by cell type [1]. Cell type-specific translation kinetics determine proliferation potential and maintain unique cellular properties. Cancer cells may alter translation initiation to globally increase protein synthesis and sustain proliferation [2]. A delicate balance of translation activity is also critical for hematopoietic stem cell (HSC) homeostasis, with increased or decreased global translation impairing HSC function [3]. Variation in ribosome concentration is one mechanism cells use to control translation output [4]. In addition to influencing growth and proliferative capacity, ribosome concentration can affect cell fate through biased translation of certain mRNAs [4, 5]. Calibration of ribosome abundance and altered protein synthesis has been described in several developmental contexts. Ribosomal protein levels decrease when mouse embryonic stem cells differentiate to embryoid bodies while global translation efficiency increases [6]. Similarly, in the Drosophila female germline production of ribosomal RNA (rRNA) and ribosome assembly factors decreases along the differentiation pathway [7, 8] yet germline stem cell differentiation is associated with increased global protein synthesis [8]. These studies suggest that the relationship between ribosome abundance and cell fate is more complex than simply meeting the metabolic needs of progenitors versus differentiated progeny.
Regulation of ribosomal RNA synthesis is one way of tuning global translation capacity. Decreased rRNA synthesis can affect developmental transitions in the Drosophila female germline [7] and mammalian tissue culture cells [9]. In eukaryotes, rRNA is transcribed by RNA polymerase I from tandem repeats of a gene encoding precursor rRNA (pre-rRNA). Pre-rRNA is processed into individual 28S, 18S, and 5.8S rRNAs while a fourth rRNA, 5S rRNA, is transcribed by RNA polymerase III. Ribosomal RNA transcription occurs at a specialized site in the nucleus called the nucleolus. Differences in rRNA transcription can be recognized via changes in nucleolus size (a larger nucleolus usually indicates more rRNA transcription) and via direct detection of nascent rRNA. For example, Drosophila female germline stem cells have large nucleoli and high rates of rRNA transcription but their differentiated progeny have smaller nucleoli and reduced rRNA synthesis [7]. Downregulation of rRNA synthesis also occurs during differentiation in the mammalian forebrain [10]. Decreased ribosome abundance appears to be a common feature of neural differentiation: additional studies have shown that ribosomal protein production [11] and rRNA synthesis [12] decreases in post-mitotic neurons. Compartment-specific changes, such as loss of ribosomes from mature axons, also occurs during neural development [13, 14]. These findings raise interesting questions regarding how ribosome biogenesis is regulated to meet the mRNA translation needs of neurons. One possibility is that components of the translation machinery, including rRNAs, are primarily synthesized in progenitors then passed to neurons during differentiating divisions. The absence of cytokinesis in neurons and the long half-life of ribosomes (days to weeks) could establish a pool of ribosomes sufficient to meet the protein synthesis needs of neurons in the absence of any autonomous ribosome production [15].
Drosophila neural stem cells, called neuroblasts, undergo multiple rounds of asymmetric self-renewing divisions to ultimately produce neurons and glia. There are two main types of neuroblast in the larval brain: type I neuroblasts produce a transient progenitor, the ganglion mother cell (GMC), at each division while type II neuroblasts produce intermediate neural progenitors (INPs) that undergo multiple rounds of asymmetric divisions, self-renewing and producing a GMC [16]. In both lineages the GMC divides once to produce post-mitotic progeny. Previous work has shown that the nucleolus is smaller in differentiated cells (INPs, GMCs, neurons) compared to neuroblasts [17, 18], suggesting that rRNA synthesis is restricted upon differentiation. The transcription factor Myc is likely a crucial regulator of this restriction. Myc is expressed at high levels in neuroblasts but is absent from INPs, GMCs and neurons [18]. Myc promotes cell growth and proliferation via several pathways, including transcriptional activation of genes encoding RNA polymerase I subunits [19].
While decreased nucleolus size suggests rRNA synthesis is restricted upon neural differentiation in Drosophila , multiple questions remain. First, does direct measurement of rRNA synthesis confirm this prediction? Second, to what degree is rRNA synthesis restricted along the differentiation pathway? Third, since the absence of Myc is predicted to limit rRNA synthesis, how do differentiated progenitors and neurons obtain the necessary amount of rRNA to support their translation needs? Here we show that high levels of nascent rRNAs are present in neuroblasts, INPs and GMCs but rRNA synthesis is severely restricted in neurons. Our data reveal that neural progenitors pass rRNA to their progeny during cytokinesis and suggest that the rRNA in INPs and GMCs is at least partly derived from their neuroblast parent. Ultimately, the rRNA in GMCs is inherited by neurons at cytokinesis. Finally, we show that progenitor-derived rRNAs are sufficient to support brain development and normal protein synthesis in neurons. This work supports a model in which rRNA inheritance establishes cell type-specific translation programs along the neural differentiation pathway.