Ian4 is required for mitochondrial integrity and T cell survival

Abstract

Apoptosis is a regulated cell death program controlled by extrinsic and intrinsic signaling pathways. The intrinsic pathway involves stress signals that activate pro-apoptotic members of the Bcl-2 family, inducing permeabilization of mitochondria and release of apoptogenic factors. These proteins localize to the outer mitochondrial membrane. Ian4, a mitochondrial outer membrane protein with GTP-binding activity, is normally present in thymocytes, T cells, and B cells. We and others have recently discovered that a mutation in the rat Ian4 gene results in severe T cell lymphopenia that is associated with the expression of autoimmune diabetes. The mechanism by which Ian4 controls T cell homeostasis is unknown. Here we show that the absence of Ian4 in T cells causes mitochondrial dysfunction, increased mitochondrial levels of stress-inducible chaperonins and a leucine-rich protein, and T cell-specific spontaneous apoptosis. T cell activation and caspase 8 inhibition both prevented apoptosis, whereas transfection of T cells with Ian4-specific small interfering RNA recapitulated the apoptotic phenotype. The findings establish Ian4 as a tissuespecific regulator of mitochondrial integrity. Apoptosis is a regulated cell death program that can be initiated by two different signaling pathways. The extrinsic pathway involves ligation of cell surface death receptors and recruitment of proteins to a death-inducing signaling complex (1). The intrinsic pathway is death receptor-independent and involves stress signals that activate pro-apoptotic members of the Bcl-2 family; this action in turn induces permeabilization of mitochondria and the release of apoptogenic factors (2). These proteins are localized to the outer mitochondrial membrane, but the mechanisms responsible for regulating mitochondrial homeostasis localize to the inner membrane (3). How the two systems interact is unknown.Immune-associated nucleotide-binding protein 4 (Ian4) was originally identified as a highly expressed protein in Bcr͞Abl-transformed 32D cells (4). It localized to the mitochondrial outer membrane and displayed GTP-binding activity in vitro (4). Cells transfected with mutated Bcr͞Abl constructs lacked oncogenic potential and displayed lower Ian4 expression. Homologues of Ian4 are present in mouse, rat, and human (5). More recently, Ian4 was found to be disrupted in diabetes-prone BB (BBDP) rats (5, 6), which develop severe T lymphopenia due to apoptosis of recent thymic emigrants (7). We hypothesized that mitochondrial Ian4 (also known in the rat as Ian4l1 and Ian5) plays an important role in regulating T cell survival through control of T cell apoptosis. MethodsAnimals. Diabetes-resistant BB (BBDR) rats and BBDP rats were obtained from Biomedical Research Models (Worcester, MA). Approximately 90% of BBDP rats develop spontaneous autoimmune diabetes; they are Ian4 Ϫ/Ϫ and congenitally lymphopenic (8). BBDR rats are Ian4 ϩ/ϩ and never become spontaneously diabetic (8). Wistar Furth (WF) rats were obtained from Harlan-Sprague-Dawley. A congenic, no...

hIan5: the human ortholog to the rat Ian4/Iddm1/lyp is a new member of the Ian family that is overexpressed in B-cell lymphoid malignancies

“…The orthologs thus far described (mIAN4 and rIAN4) were about 30 kDa and are localized in the mitochondria. 6,11 Since the proteins are highly similar (murine Ian4 76%, rat Ian4 68%), there is currently no explanation for this finding. In our study, IAN5 migrated at about 35 kDa and was located in the heavy membrane fraction/ mitochondria, with identical behavior as bcl-2.…”

“…9,10 The mutation results in a truncated protein in which the COOHterminal 215 amino acids are replaced by 19 other amino acids. 9,10 Very recently, Pandarpurkar et al 11 found that lymphocytes lacking rIan4 have a defect in mitochondrial integrity and substantially increased propensity to undergo apoptosis. In humans, three Ian members have been characterized so far: the HIMAP1 (human immunity-associated protein 1/hIan2) 34 kDa protein is localized at the endoplasmic reticulum (ER).…”

“…14 Taken together, the investigation of the function of Ian family members suggests that Ian proteins are a distinct group of GTP-binding proteins that play a role in immune responses (plant: hypersensitivity; mammals: thymocyte development (autoimmunity)) and are involved in the control of lymphocyte apoptosis (lymphopenia). 4,6,7,[9][10][11][12][13][14] Here we describe the characterization of hIan5, the human ortholog to rIan4/Iddm1/lyp. HIan5 consists of 307 amino acids with a predicted molecular weight of 35 kDa.…”

Structural basis of oligomerization in septin-like GTPase of immunity-associated protein 2 (GIMAP2)

“…GIMAP3, GIMAP4, and GIMAP5 were shown to interact with members of the Bcl2 family and regulate apoptosis (8,10). Several studies have established a role for GIMAPs in the late stages of lymphocyte development and in lymphocyte maintenance.…”

Animal models have little to teach us about Type 1 diabetes: 2. In opposition to this proposal

“…In the BB diabetes-prone rat, the diabetes-susceptible environment is established in part by a defect in the Ian4L1 gene. This T-cell-specific gene regulates mitochondrial integrity and is required for T cell survival [8]. In the KDP rat, diabetogenesis is associated with defects in yet another gene, Cblb [9], the normal function of which, like that of Ctla4 in the mouse, is to downregulate T cell activation.…”