Mitochondrial Ca2+ homeostasis in trypanosomes

Abstract

Mitochondrial calcium ion (Ca²⁺) uptake is important for buffering cytosolic Ca²⁺ levels, for regulating cell bioenergetics, and for cell death and autophagy. Ca²⁺ uptake is mediated by a mitochondrial Ca²⁺ uniporter (MCU) and the discovery of this channel in trypanosomes has been critical for the identification of the molecular nature of the channel in all eukaryotes. However, the trypanosome uniporter, which has been studied in detail in Trypanosoma cruzi, the agent of Chagas disease, and T. brucei, the agent of human and animal African trypanosomiasis, has lineage-specific adaptations which include the lack of some homologues to mammalian subunits, and the presence of unique subunits. Here, we review newly emerging insights into the role of mitochondrial Ca²⁺ homeostasis in trypanosomes, the composition of the uniporter, its functional characterization, and its role in general physiology.

Keywords: Acidocalcisome; Calcium; Cell bioenergetics; Inositol phosphate; Mitochondria; Mitochondrial calcium uniporter; Polyphosphate; Trypanosomatids.

Figure 1.

Scheme of metabolic pathway in T. brucei bloodstream forms. Rectangles indicate steps of glucose and threonine metabolism; dashed arrows indicate steps for which no evidence of flux is available. A, ATPase; AcCoA, acetyl-CoA; FAS II, type II fatty-acid biosynthesis pathway; DHAP, dihydroxyacetone phosphate; GAP, glyceraldehyde 3-phosphate; G3P, glycerol 3-phosphate; GPDH, glycerol 3-phosphate dehydrogenase; PEP, phosphoenolpyruvate; Pyr, pyruvate; UQ, ubiquinone; TAO, trypanosome alternative oxidase. Enzymes are: 1. Pyruvate dehydrogenase; 7, threonine dehydrogenase. Activity stimulated by Ca²⁺ is in yellow. Figure 2. Phylogenetic tree of trypanosomatid and human MCU complex subunits. Thee scale bar corresponds to a distance of 20 changes per 100 amino acid positions. Reproduced with permission from reference (Huang et al., 2013b).

Figure 2.

Phylogenetic tree of trypanosomatid and human MCU complex subunits. The TriTrpDB and GenBank accession numbers for 30 MCUC subunits were described in (Huang, Docampo, 2018). The scale bar corresponds to a distance of 20 changes per 100 amino acid positions. Reproduced with permission from reference (Huang, Docampo, 2018).

Figure 3.

A. Scheme depicting the putative organization and composition of a hetero-hexameric TbMCU complex. Reproduced with permission from reference (Huang, Docampo, 2018). B. conserved WDXXEPXTY motif in TcMCU complex subunits. Alignment of the C-terminal fragment of the first transmembrane domain (TM1, in blue) and the N-terminal fragment of the second transmembrane domain (TM2, in green), including conserved WDXXEPXTY motif (in red), of TcMCU complex subunits with human MCU and MCUb. Conserved putative critical acidic residues in or near the WDXXEPXTY selectivity filter are indicated. TcMCU, TcMCUb, TcMCUc and TcMCUd exhibit a substitution of the Ru360-sensitive residue S to D/G in the pore region. Residues in each TcMCU complex subunit that were subjected to substitutions are indicated with the corresponding number in their sequence. Reproduced with permission from reference (Chiurillo et al., 2019).

Figure 4.

Models showing organization of putative MCU complex-ATP synthasome megacomplex in trypanosomes. A. The MCU complex physically interacts with the ATP synthasome (ATP synthase, ANT, and PiC) via the c-ring of the F₀ ATP synthase. In trypanosomes, ATP synthase consists of F₁ region with the central stalk (α3β3, γ, δ and ε) for ATP synthesis, F₀ region with the putative stator (c_n, a, p18, Tb1, Tb2, and OSCP) for proton (H⁺) translocation, and trypanosome-specific associated proteins (ap), while the molecular identity of the peripheral stalk is unknown. OSCP, oligomycin sensitivity-conferring protein; ANT, adenine nucleotide translocator; PiC, phosphate carrier. B. Cross section model to hypothetical T. brucei c_n-ring-MCU complex. The T. brucei heterohexameric MCU complex consisting of 4 different subunits (MCU, MCUb, MCUc, and MCUd), with a molecular weight of approximately 145 kDa, is within the c-ring of ATP synthase. TMH1 of each MCU complex subunit (excluding MCUb) interacts with TMH1 of ATPc. The c-ring rotates in counterclockwise direction and translocates H⁺ from the intermembrane space to matrix during ATP synthesis. Reproduced with permission from reference (Huang, Docampo, 2020).