Parkinson’s disease (PD) is a degenerative neurological condition that causes tremors, rigidity in the muscles, an uneven gait, and disturbance in balance and coordination. The loss of dopaminergic neurons in ‘Substantia Nigra pars compacta (SNpc)’ of the midbrain region leads to dopamine deficiency leading to disrupted transmission in nigrostriatal pathways and uncoordinated movements. Globally, 85% of the PD cases are sporadic, whereas mutations associated with genes including LRRK2, DJ-1, PINK1, Parkin, and α-synuclein are observed in the remaining 15% of familial cases of PD (Tran et al., 2020). Both genetic and sporadic forms of PD are associated with mitochondrial dysfunction, the prime cause of other neurodegenerative pathophysiology.
Alterations in the electron transport chain (ETC) lead to leakage of electrons, decreased ATP production and ROS generation leading to mitochondrial dysfunction in different cell types including neurons (Kausar et al., 2018). Interestingly, impairment in mitochondrial activity can also lead to atypical neurite branching, synaptic transmission, and decreased regeneration, ultimately resulting in neurodegeneration (Guo et al., 2017, Rangaraju et al., Oct. 2019). Dysfunctional mitochondria are degraded via a selective process of autophagy called mitophagy (Palikaras et al., 2018). During intrinsic mitochondrial stress and depolarization, PINK1 translocates to mitochondria and recruits PARKIN on the outer mitochondrial membrane (OMM), ubiquitinating a wide range of OMM proteins for degradation (Sarraf et al., 2013). Interestingly, typical ubiquitin topology on target protein determines the specific degradation pathways for degradation. K48 linked ubiquitination of target proteins leads to degradation by the ubiquitin-proteasome system (UPS) (Chan et al., 2011, Manohar et al., 2019, Du et al., 2022), whereas K63-linked ubiquitinated proteins on damaged mitochondria are targeted to the lysosomal pathway for degradation (Richard et al., 2020a, McKeon et al., 2015).
Ubiquitination is generally a three-step enzymatic process and involves: E1, ubiquitin-activating enzyme, E2, ubiquitin-conjugating enzyme, and E3, ubiquitin-ligases. The transfer of ubiquitin moieties from the E2 conjugating enzyme to lysine (K) residues on the target protein is mediated by E3 ligase by interaction with both the target protein and the E2 conjugating enzyme (Komander and Rape, 2012). E3 ligases have been categorized based on their distinctive domains: RING (Really Interesting New Gene), HECT (Homologous to the E6AP Carboxyl Terminus), U-box E3 ligases, and RBR (RING-IBR-RING) E3 ligases (Yang et al., 2021).
The TRIM (Tri-partite Motif) family is the largest family of RING E3 ubiquitin ligases, and more than 70 members have been identified in humans (Tomar and Singh, 2015). TRIM proteins have a conserved N-terminal RING domain, one or two B-boxes, and a Coiled-coil domain followed by a particular C-terminal domain, which is referred to as the RBCC domain. TRIMs play a role in a variety of biological processes, including inflammation, autophagy, innate immune response, and cell proliferation (Tomar and Singh, 2015, Rajsbaum et al., 2014, Yang et al., 2019, Tomar et al., 2012a). TRIM71 belongs to the C-VII subfamily having a peculiar NHL (NCL-1/H2TA/LIN-41) domain on its C-terminus. TRIM71 plays a major role in embryonic development, embryonic stem cell differentiation, and reprogramming (Goyani et al., 2024), however, its involvement in the regulation of the cell death pathway is not known. Recently, we demonstrated that TRIM32, another TRIM-NHL member, regulates mitochondrial functions and sensitizes cells to oxidative stress-induced cell death (Goyani et al., 2024, Prajapati et al., 2020), however, the role of other TRIM-NHL proteins in the regulation of mitochondrial functions is not well understood. In the present study, we explore the subcellular localization and role of TRIM71 in the regulation of mitochondrial functions and neuronal survival under rotenone and 6-OHDA-induced PD stress conditions.