Opposing effects of an F-box protein and the HSP90 chaperone network on microtubule stability and neurite growth inCaenorhabditis elegans

Abstract

31Molecular chaperones often work collaboratively with the ubiquitination-proteasome 32 system (UPS) to facilitate the degradation of misfolded proteins, which typically safeguards 33 cellular differentiation and protects cells from stress. In this study, however, we report that the 34 Hsp70/Hsp90 chaperone machinery antagonizes the activity of F-box protein MEC-15 to 35 negatively regulate neuronal differentiation. Using the touch receptor neurons (TRNs) of C. 36 elegans, we find that mec-15(-) mutants display defects in MT formation, neurite growth, 37 synaptic development, and neuronal functions, and these defects can be rescued by the loss of 38 Hsp70/Hsp90 chaperones and cochaperones. MEC-15 likely functions in a SCF complex to 39 degrade DLK-1, which is an Hsp90 client protein stabilized by the chaperones. The abundance 40 of DLK-1 and likely other Hsp90 substrates are fine-tuned by the antagonism between MEC-41 15 and chaperones, which regulates TRN development and synaptic functions of GABAergic 42 motor neurons. Therefore, a balance between UPS and chaperones tightly controls neuronal 43 differentiation. 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61Molecular chaperones, including the heat shock proteins (Hsps), play essential roles 62 in protein maturation, refolding, and degradation. Although the function of Hsps in the 63 response to stress has been extensively characterized, their roles in neuronal differentiation are 64 much less understood. Ishimoto et al. (1998) found that Hsp90 promotes neurite extension for 65 the chick telencephalic neurons and spinal neurons in vitro. More recently, pharmacological 66 inhibition of Hsp90 by 17-demethoxygeldanamycin (17-AAG) disturbed neuronal polarization 67 and axonal elongation of cultured hippocampal neurons (Benitez et al., 2014). Hsp90 inhibition 68 decreased expression of two Hsp90 client proteins, Akt and GSK3, which have diverse 69 functions in cell differentiation. Thus, Hsp90 may regulate axon specification and growth by 70 affecting specific signaling pathways through its chaperone activity. Given that the Hsp70 and 71 Hsp90 chaperones interact with numerous client proteins, including transcription factors, 72 kinases, and signaling molecules (Wayne et al., 2011), the regulation of neuronal 73 morphogenesis by the chaperones is likely to be context-dependent. Whether Hsp70 and Hsp90 74 chaperones and their co-chaperones can also negatively regulate neurite growth, however, is 75 unclear. 76The ubiquitination-proteasome system (UPS) often works in concert with the 77 chaperone-mediated refolding machinery for protein quality control, which promotes 78 degradation of numerous misfolded proteins in a chaperone-dependent manner (Buchberger et 79 al., 2010). In developing neurons, this process safeguards the protein quality of important 80 guidance molecules. For example, the C. elegans BC-box protein EBAX-1, the substrate-81 recognition subunit of the Elongin BC-containing Cullin-RING ubiquitin ligase (CRL), and 82 HSP-90/Hsp90 c...