The Multi‐functional Enzyme CHIP Regulates Microtubule Dynamics

Abstract

The Carboxy‐terminus of Hsc70 Interacting Protein (CHIP) controls several aspects of protein quality control machinery through its ability to function as a co‐chaperone, autonomous chaperone, and ubiquitin ligase. CHIP is ubiquitously expressed in all tissues, but interestingly, CHIP expression is higher in certain tissues such as heart, muscle and brain, suggesting an increased dependence on CHIP function in these organ systems. This concept is supported by studies that show how altered CHIP function associates with several ailments such as cardiac diseases, neurological disorders, and cancer. Microtubules (MTs) are a part of the cellular cytoskeleton that provides shape and structure to cells. In addition to these roles, MTs play a crucial role in cell division, migration, intracellular organization and trafficking. Dynamic assembly and disassembly of MTs upon various stimuli is important for the proper functioning of differentiated cells and is a process orchestrated by multiple proteins. Dysregulation in the MT reorganization machinery is implicated in a similar spectrum of diseases associated with the loss of CHIP function, however, there has yet to be a direct connection between CHIP and MT dynamics. We performed a proteomics screen on wild type and CHIP−/− mouse embryonic fibroblasts and identified differential regulation of proteins that regulate several aspects of MT function, such as STMN1 and PDLIM. Given the importance of CHIP in protein homeostasis, we hypothesized that CHIP is necessary for proper MT dynamics in response to cellular stimulations. We found that CHIP is required for the phosphorylation of stathmin, a regulator of MT assembly, and for the tubulin polymerization in C2C12 myoblast cells following insulin and glucose stimulation. This stimulation triggers an important MT‐dependent process to mediate glucose uptake. Furthermore, ex vivo studies using muscle fibers isolated from CHIP−/− mice failed to form MT lattices after glucose stimulation, consistent with a new role for CHIP in regulating MT in muscle. Altered MT dynamics, as well as CHIP loss of function, are also implicated in neurodegenerative diseases, particularly those associated with proteinopathies. To determine if CHIP contributes to MT regulation in neurons, we used an immunoprecipitation‐mass spectrometry approach and identified changes in interactions between CHIP and MT proteins during proteotoxic stress. Future experiments will focus on validating the interaction data and further dissecting the specific functions of CHIP that contribute to the regulation of MT dynamics using biochemical approaches and imaging. In conclusion, for the first‐time, our data couple CHIP to MT dynamics, and suggest that CHIP may contribute to cell protection by regulating the proteins that are vital to MT dynamics.Support or Funding InformationFunding was provided by a grant to J.C.S. from the National Institute of Aging (R01‐GM061728).