The Drosophila 7SK snRNP complex is required for synaptic growth and function of motoneurons
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Abstract
The 7SK snRNP is a ribonucleoprotein complex composed of the abundant non-coding nuclear RNA 7SK, the RNA binding proteins Methylphosphate Capping Enzyme (MePCE), La-related protein 7 (Larp7) and Hexamethylene bis-acetamide-inducible (HEXIM). In higher eukaryotes, the 7SK snRNP complex plays a major role in preventing the premature entry of paused RNA Pol II into the elongation phase by sequestering the positive transcription elongation factor (P-TEFb). Intriguingly, despite this general function characterized essentially from cell culture studies, the LARP7 loss of function in human is viable. Nevertheless, the patients suffer from several defects including restricted growth and intellectual disability, also known as the Alazami syndrome. Currently, it is unclear how the absence of LARP7 specifically gives rise to this syndrome. In order to gain insights into this question and to more globally assess the function of promoter-proximal pausing in a developmental context, I used Drosophila as a model organism to generate mutants of several 7SK snRNP complex subunits. I found that the knockout of Larp7 or 7SK RNA in Drosophila does not affect viability but alters fly locomotion. Consistently, alteration of the 7SK snRNP complex specifically reduces axonal growth at neuromuscular junctions (NMJ) of developing larvae. I showed that Larp7 is enriched in a few subtypes of motoneurons and acts autonomously in these cells to promote axonal growth. In addition, electrophysiology analysis of synaptic activity shows mild alteration of synaptic transmission. Interestingly, decreasing the level of P-TEFb fully restores the axonal growth and partially the locomotion, indicating that the 7SK snRNP complex regulates growth via a transcriptional function. Our transcriptomic analysis of mutant motoneurons revealed that the 7SK snRNP complex regulates genes that contain high GC content at their promoter as well as long introns. Altogether, our work adds new insights into the specificity of the 7SK snRNP complex during the development of a multicellular organism and highlights the importance of promoter-proximal pausing in the development of motoneurons.