TRIM32: A Multifunctional Protein Involved in Muscle Homeostasis, Glucose Metabolism, and Tumorigenesis

Abstract

Human tripartite motif family of proteins 32 (TRIM32) is a ubiquitous multifunctional protein that has demonstrated roles in differentiation, muscle physiology and regeneration, and tumor suppression. Mutations in TRIM32 result in two clinically diverse diseases. A mutation in the B-box domain gives rise to Bardet-Biedl syndrome (BBS), a disease whose clinical presentation shares no muscle pathology, while mutations in the NHL (NCL-1, HT2A, LIN-41) repeats of TRIM32 causes limb-girdle muscular dystrophy type 2H (LGMD2H). TRIM32 also functions as a tumor suppressor, but paradoxically is overexpressed in certain types of cancer. Recent evidence supports a role for TRIM32 in glycolytic-mediated cell growth, thus providing a possible mechanism for TRIM32 in the accumulation of cellular biomass during regeneration and tumorigenesis, including in vitro and in vivo approaches, to understand the broad spectrum of TRIM32 functions. A special emphasis is placed on the utility of the Drosophila model, a unique system to study glycolysis and anabolic pathways that contribute to the growth and homeostasis of both normal and tumor tissues.

Keywords: cancer; costamere; muscle; muscular dystrophy.

Figure 1

Classification of the tripartite motif family of proteins 32 (TRIM). (A) General schematic of a TRIM protein. The N-terminus is comprised of a really interesting new gene (RING) domain, one or two B-box regions, and a coiled-coil domain followed by a variable C-terminus. (B) Table representing different categories of TRIM proteins. (C) Different NHL (NCL-1, HT2A, LIN-41) proteins encoded in vertebrates and invertebrates (adapted from WILLIAMS et al. 2019).

Figure 2

Limb-girdle muscular dystrophy type 2H (LGMD2H)-causing TRIM32 mutations in the NHL domains. (A) Schematic of the TRIM32 protein. (B) Ribbon diagram showing the face (top panel) and 90° rotation (bottom panel) of human NHL domains. Colored amino acids are listed in panel B. (C) Table illustrating mutations causative for LGMD2H. Known point mutations are in color, with blue representing amino acids strictly conserved in the Drosophila TRIM32 sequence.

Figure 3

Mechanisms of TRIM32 function. (A) Polyubiquitination and proteasomal turnover of target substrates is a well-characterized role of TRIM32. (B) TRIM32 also appends single ubiquitin (Ub) molecules to proteins, resulting in either monoubiquitination or postulated multi-monoubiquitination. (C) Alternatively, TRIM32 may act as a scaffold protein to mediate protein localization within cells or to limit diffusion of proteins away for their sites of action.

Figure 4

Classification of TRIM32 interacting proteins according to biological function and tissue specificity. The generalized role of TRIM32 in various biological processes is depicted in the inner circle followed by TRIM32 binding partners and/or substrates of E3 activity in the middle ring. Proteins with muscle-related functions are colored according to biological role.

Figure 5

Compromising TRIM32 function results in the abnormal accumulation of costamere components and a decrease in glycolytic enzyme protein levels and/or localization within muscle tissue. (A) Large protein complexes called costameres connect the contractile apparatus at the Z-disc and sarcolemma to the extracellular matrix (ECM). (B) Both dystrophin glycoprotein complex (DGC) and integrin complexes are present at higher levels and lose their costamere location upon loss of TRIM32 or upon expression of LGMD2H alleles. (C) The glycolytic enzymes Ald and Pglym are found at the Z-disc in close association with costameres in normal muscle tissue. (D) Loss of TRIM32 reduces the levels of these proteins, either due to loss of their normal localization or lack of protein stability. WT, wild type.

Figure 6

The Drosophila model has been instrumental in understanding how TRIM32 and glycolysis regulate cell growth and cell death. (A) Glycolytic enzyme localization to the Z-disc and M-line in Drosophila adult indirect flight muscles (IFMs) is required for flight maintenance. TRIM32 is also required to maintain the structural integrity of the IFMs, possibly by mediating the subcellular distribution and/or stability of glycolytic enzymes. (B) Insulin acts upstream of glycolysis to promote muscle growth by fusion during embryonic development. (C) TRIM32 directly acts on glycolytic enzymes to regulate protein levels and/or protein localization, which in turn allows for high glycolytic flux in larval tissues undergoing massive cell growth. (D) Much like that of XIAP, TRIM32 regulates the levels of DIAP1 to control the apoptotic death of a set of muscles that undergo histolysis during pupal development. DEOM, dorsal external oblique muscle. FCM, fusion competent myoblasts. FC, founder cells.