Coordination of cell signaling, trafficking and proteostasis in health and disease
Background. Ran-binding protein 2 (RanBP2) is a large 358 kDa scaffold protein composed of several distinct structural modules. Structure-function studies of RanBP2 have led to the identification of numerous partners with seemingly disparate functions and specific binding activities toward selective domains of RanBP2 (A). Studies from multiple laboratories, including ours, support the idea that RanBP2 plays critical roles in cell-stage and cell type-dependent biological and pathophysiological processes. These include nucleocytoplasmic and microtubule-based intracellular trafficking, nuclear envelope breakdown and mitosis, modulation of protein homeostasis by the ubiquitin-proteasome system, mitochondrial function and trafficking, regulation of protein-protein interactions and localizations by SUMOylation, and gene-environment interactions affecting glucose and lipid metabolism.
Although complex and not well understood, RanBP2's pleiotropic functions that are cell type-dependent reflect (1) the association of RanBP2's structural modules with proteins that possess distinct functional properties, (2) the multifunctional properties associated with single domains of RanBP2, and (3) the cross-talk between domains of RanBP2 and its partners. Human semi-dominant mutations affecting selective domains of RANBP2 have severe impacts on the central nervous system and cause familial necrotic encephalopathies. Furthermore, RanBP2 is essential for organism survival, while RanBP2 insufficiency plays critical roles in neurodegeneration, neuroprotection, and gene-environment interactions.
Current projects. We are engaged in several synergistic topics and interdisciplinary approaches that range from single molecule-level analyses of the interplay between RanBP2, kinesin-1, and other partners, to cellular assays aimed at uncovering the biological effects of the modular activities of RanBP2 and its partners. Additionally, we are defining the physiological and pathological activities of RanBP2 and its structural modules in cell types susceptible to various diseases using genetic mouse models of RanBP2.
1. A central theme of our work is to uncover the mechanisms regulating kinesin-1 motor activities by RanBP2 (B). RanBP2 was the first cargo shown to activate a kinesin in a minimal in vitro system of purified components. Current studies aim to establish direct correlations between single molecule activity of kinesin and the regulation of motor-driven subcellular processes, such as mitochondrial motility and function. These studies are yielding novel mechanistic insights into several competitive and cooperative modes of regulating kinesin motor activities, RanBP2 functions, and the dynamics of mitochondrial motility and the transfer of cargoes exiting nuclear pores. This understanding will provide opportunities for therapeutic interventions in numerous neurodegenerative disorders where these molecular processes are severely impaired.
2. Another topic of study is the regulation of proteostasis by RanBP2 and some of its partners. Our laboratory found that at least two distinct domains of RanBP2 control the proteostasis of selective substrates expressed in distinct tissues or cell types. The modulation of proteostasis by RanBP2 is achieved through chaperone or foldase activities harbored by specific domains of RanBP2 that directly or indirectly affect unique functions of the ubiquitin-proteasome system (UPS). Current studies are uncovering proteostatic mechanisms and targets controlled by RanBP2, as well as the implications of these processes for substrates involved in various pathological processes affecting different cell types or tissues. These studies will pave the way for the development of novel pharmacological strategies to harness and unveil the therapeutic potential of RanBP2 and its partners in the onset, progression, and manifestation of various neurodegenerative diseases.
3. A third theme of our work aims to reveal and define gene-environment responses mediated by RanBP2 to disease stressors. Currently, we are investigating a number of extrinsic and intrinsic disease stressors, such as phototoxicity, Parkinsonian neurotoxic insults, and disease mutations. For example, our data support that deficits in RanBP2 overall play antagonistic roles in neuroprotection among distinct neuronal or supporting cell types, such as dopaminergic and photoreceptor neurons, and glial cells against various disease stressors (C). These studies also aim to define the modulation of extrinsic and intrinsic disease pathways by RanBP2 and its partners, along with new therapeutic prospects.
4. A hallmark manifestation of RanBP2 insufficiency is a change in cellular and tissue metabolic profiles, such as glucose and lipid metabolites (e.g., free fatty acids and cholesterol). Some of these metabolic changes appear to confer age-dependent neuroprotection to selective neurons with high susceptibility to oxidative stress, while other cells lack such protection. Hence, efforts are underway to define the molecular bases of the regulation of metabolic pathways by RanBP2 and its partners, and their roles in neuronal cell death and neuroprotection.
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