Sensory perception in the inner ear relies on the hair bundle, the highly polarized brush of movement detectors that crowns hair cells. We previously showed that, in the mouse cochlea, the edge of the forming bundle is defined by the 'bare zone', a microvilli-free sub-region of apical membrane specified by the Insc-LGN-Gαi protein complex. We now report that LGN and Gαi also occupy the very tip of stereocilia that directly abut the bare zone. We demonstrate that LGN and Gαi are both essential for promoting the elongation and differential identity of stereocilia across rows. Interestingly, we also reveal that total LGN-Gαi protein amounts are actively balanced between the bare zone and stereocilia tips, suggesting that early planar asymmetry of protein enrichment at the bare zone confers adjacent stereocilia their tallest identity. We propose that LGN and Gαi participate in a long-inferred signal that originates outside the bundle to model its staircase-like architecture, a property that is essential for direction sensitivity to mechanical deflection and hearing.

GPSM2-GNAI Specifies the Tallest Stereocilia and Defines Hair Bundle Row Identity Highlights d GPSM2, GNAI, WHRN, MYO15A, and EPS8 work in the same pathway to shape hair bundles d GPSM2-GNAI-WHRN is a late module added to MYO15A-EPS8 at row 1 stereocilia tips only d GPSM2-GNAI defines the identity of the tallest, first-row stereocilia d Mutant bundles comprise generic stereocilia lacking differential row identity

Bardet-Biedl syndrome (BBS) is a pleiotropic, heterogeneous human disease whose etiology lies primarily in dysfunctional basal bodies and/or cilia. Both BBS patients and several BBS mouse models exhibit impaired olfactory function. To explore the nature of olfactory defects in BBS, a genetic ablation of the mouse Bbs8 gene that incorporates a fluorescent reporter protein was created. The endogenous BBS8 protein and reporter are particularly abundant in olfactory sensory neurons (OSNs), and specific BBS8 antibodies reveal staining in the dendritic knob in a shell-like structure that surrounds the basal bodies. Bbs8-null mice have reduced olfactory responses to a number of odorants, and immunohistochemical analyses reveal a near-complete loss of cilia from OSNs and mislocalization of proteins normally enriched in cilia. To visualize altered protein localization in OSNs, we generated a SLP3 eGFP knock-in mouse and imaged the apical epithelium, including dendritic knobs and proximal cilia, in ex vivo tissue preparations. Additionally, protein reagents that reflect the characteristic neuronal activity of each OSN revealed altered activity in Bbs8-null cells. In addition to previously known defects at the ciliary border, we also observed aberrant targeting of OSN axons to the olfactory bulb; axons expressing the same receptor display reduced fasciculation and project to multiple targets in the olfactory bulb. We suggest that loss of BBS8 leads to a dramatic and variable reduction in cilia, the essential signaling platform for olfaction, which alters the uniformity of responses in populations of OSNs expressing the same receptor, thereby contributing to the observed axon-targeting defects.ciliopathy | olfactory activity | protein trafficking B ardet-Biedl syndrome (BBS), a heterogeneous human disease, encompasses pleiotropic phenotypes including obesity, polydactyly, retinal degeneration, and renal anomalies. The disease, associated with mutations in at least 16 genes, shows complex inheritance. The BBS8 gene was identified via shared homology with BBS4 and was recognized to bear similarity to bacterial pilF; pilF is thought to be involved in the assembly of pili, which are thin, hairlike extensions on prokaryotic cells (1). This prompted the hypothesis that BBS is primarily a disease of the basal body, a microtubule-based modified centriole that nucleates the ciliary axoneme. Subsequent work supports this common etiology of BBS (2). Characterized BBS genes are highly conserved exclusively among ciliated eukaryotes, and most BBS proteins localize to the basal body, centrosome, and/or cilium in ciliated cell-culture models and in ciliated tissues. BBS proteins are not thought to be essential structural proteins as the basal body and cilium remain largely intact in most mutant BBS models (3-5). Disruption of individual BBS genes leads to defects in intraflagellar transport (IFT), a process essential for protein trafficking within the cilium (6-8).Recent studies have found that seven BBS proteins-BBS1, -2, -4, -5, -7, -8,...

Defeating peripheral neuropathy The mechanisms underlying peripheral neuropathies are not well understood. Spaulding et al . studied mouse models of the inherited Charcot-Marie-Tooth (CMT) disease, which is caused by mutations in transfer RNA (tRNA) synthetases. Changes in gene expression and the rate of protein synthesis in neurons in the spinal cord triggered the cell stress response activated by the protein sensor GCN2. When GCN2 was genetically deleted or inhibited with drugs, the stress response was blocked, and the neuropathy was much milder. Zuko et al . found that mutant glycyl-tRNA synthetases bind tRNA Gly but fail to release it, thus depleting the cellular tRNA Gly pool. This process caused stalling of translating ribosomes on glycine codons and activated the integrated stress response. Transgenic tRNA Gly overexpression prevented peripheral neuropathy and protein synthesis defects in mouse and fruit fly models. Thus, elevating tRNA Gly levels or targeting GCN2 may have therapeutic potential for this currently untreatable disease (see the Perspective by Mellado and Willis). —SMH