The proteostasis network (PN) is comprised of a large consortium of cellular processes that drive the spatial and temporal control and maintenance of cell-type specific proteomes. The translation machinery includes the ribosome and associated proteins that form an integral part of the PN. Until the 1990s, ribosomes were thought to be homogeneous protein synthesis organelles that were not selective in which proteins they translated. However, emerging evidence from several organisms, including Drosophila, has challenged this dogma giving rise to the concept of specialized ribosomes which posits that ribosomes are, in fact, heterogeneous in composition and this heterogeneity regulates which mRNAs are actively translated. While the significance of local protein translation in neurons have has primarily focused on their physiological roles in memory formation and synaptic plasticity, there remains a critical gap in our understanding of the role of local translation in regulating diverse neuronal architectures. To better ascertain the requirement of ribosomal proteins in regulating dendritic morphology, we conducted a neurogenetic RNAi knockdown (KD) screen of a subset of 30 RPs in CI and CIV multi-dendritic (md) neurons subtypes in Drosophila. While most RP knockdowns exhibited dendritic defects in both CI and CIV neurons, select RPs showed cell-type-specific dendritic requirements. Further, loss of RPs that affected dendritic morphology also led to defects in ribosome trafficking and localization. Previous studies have shown that stoichiometric differences in ribosomal RPs can determine which pool of mRNAs are preferentially translated. We previously characterized the role of the cytoskeletal modulator Formin 3 (Form3) in regulating dendritic morphology in CIV md neurons. Loss of form3 leads to severe reduction in dendritic morphology similar to that observed for the knockdown of RpL7 and RpL36A. Immunohistochemical (IHC) analysis revealed that compared to control, RpL7 KD leads to a significant reduction in Form3 protein levels while RpL36A KD did not. To further investigate the putative effects of ribosome heterogeneity on subtype-specific protein translation, we performed multi-omics analysis including ribosome profiling under control and experimental conditions focused on RPs that were distinctively required for subtype-specific CI (RpL23) vs. CIV (RpL31) dendritogenesis, as well as RP (RpL7) that was required in both neuron subtypes. Collectively, these data provide evidence of differential RP requirements in regulating dendritic morphological diversity, as well as distinct roles of RPs in preferentially translating certain mRNAs compared to others (e.g., Form3), and in regulating ribosomal trafficking and global protein translation.
