EOLA1, a novel mitochondria-localized protein critical for heart functions via regulating mitochondrial translation

Reviewer #1 (Public review):

Summary:

Mitochondria encode a small set of proteins that are made inside the organelle by specialized ribosomes. When this mitochondrial translation system fails, oxidative phosphorylation is impaired, an outcome that is particularly harmful to energy-demanding tissues such as the heart. In this manuscript, the authors use a targeted CRISPR/Cas9 screen in cultured cells grown on galactose (a condition that forces reliance on oxidative phosphorylation) to identify genes required for mitochondrial activity. They highlight EOLA1, previously studied mainly in inflammatory contexts, as a top candidate.

Strengths:

The authors present data suggesting that EOLA1 is imported into mitochondria via an N-terminal targeting sequence and resides in the mitochondrial matrix. Loss of EOLA1 reduces oxygen consumption and is associated with altered mitochondrial ultrastructure. Mechanistically, affinity purification suggests interaction with mitochondrial elongation factors TUFM (mtEF-Tu), and RNA immunoprecipitation experiments enrich 12S mt-rRNA, consistent with a relationship to the small ribosomal subunit. Multiple assays, including sucrose-gradient profiling, reduced abundance of selected mtDNA-encoded proteins, and a click-chemistry labeling approach, support the conclusion that mitochondrial protein synthesis is decreased in EOLA1-deficient cells. Finally, whole-body Eola1 knockout mice show echocardiographic findings consistent with dilated cardiomyopathy and reduced levels of representative mitochondrially encoded proteins in cardiac tissue.

How to interpret the work:

The data support a role for EOLA1 in maintaining mitochondrial gene expression and oxidative phosphorylation capacity, and they plausibly implicate mitochondrial translation.

Weaknesses:

The main caveat is that the study does not yet establish how EOLA1 acts, whether it directly modulates translation elongation through TUFM, whether it is primarily required for mitoribosome biogenesis/rRNA stability, or whether it influences translation indirectly through mitochondrial stress pathways. The in vivo phenotype is intriguing, but without tissue-specific deletion/rescue and deeper cardiac pathology/mitochondrial functional measurements, it remains uncertain how directly the heart phenotype reflects a cardiomyocyte-autonomous defect in mitochondrial translation.

Reviewer #2 (Public review):

Summary:

In this study, the authors identify a previously uncharacterised regulator of mitochondrial function using a genetic screen and propose a role for this protein in supporting mitochondrial protein production. They provide evidence that the protein localises to mitochondria, interacts with components of the mitochondrial translation machinery, and is required for normal heart function in an animal model.

Strengths:

A major strength of the work is the use of multiple independent approaches to assess mitochondrial activity and protein production, which together provide support for the central conclusions. The in vivo data linking loss of this factor to impaired heart function are particularly compelling and elevate the relevance of the study beyond a purely cell-based context.

Weaknesses:

Given prior reports placing this protein outside mitochondria, its mitochondrial localisation would benefit from more rigorous and quantitative validation, and the proposed mechanism of the interaction with the mitochondrial translation machinery remains only partially explored. In addition, the physiological analysis is largely limited to the heart, leaving open questions about how broadly this pathway operates across tissues.

Major comments:

(1) Evidence for mitochondrial localization of EOLA1
EOLA1 has previously been reported as a nuclear and cytosolic protein and is not annotated in MitoCarta 3.0, making rigorous validation of its mitochondrial localization particularly important. Although the authors provide several lines of evidence, interpretation is complicated by the use of different cell lines across localization, interaction, and functional experiments. Greater consistency in the cellular models used would strengthen the conclusions. The immunofluorescence analysis of tagged EOLA1 would also benefit from quantification across more cells and the inclusion of an additional mitochondrial marker (e.g., an outer membrane marker such as TOM20), as HSP60 staining can vary with mitochondrial state.

(2) Normalization of OCR measurements
Clarification of how Seahorse oxygen consumption rate measurements were normalized (e.g., cell number or protein content) would aid interpretation, particularly given potential effects of Eola1 loss on cell growth.

(3) Linking interaction data to functional phenotypes
Loss-of-function analyses are performed in mouse cell lines, whereas localization and interactome studies are conducted in human HEK293T cells. The absence of a human EOLA1 knockout model makes it difficult to directly connect the interaction data to the observed functional phenotypes. Additional validation or discussion of species conservation would improve clarity.

(4) Mechanistic interpretation of the EOLA1-TUFM-12S rRNA interaction
The identification of TUFM and 12S mt-rRNA as EOLA1 interactors is an interesting finding; however, the basis for prioritizing TUFM among the many mitochondrial proteins identified in the interactome is not fully explained. Providing enrichment statistics and functional categorization of mitochondrial interactors would increase transparency. In addition, the proposed role of the ASCH domain in RNA binding would be strengthened by structure-informed or mutational analysis of the conserved RNA-binding motif.

(5) Interpretation of mitochondrial translation and protein abundance data
Several assays supporting impaired mitochondrial translation would benefit from additional controls and quantification. The de novo mitochondrial translation assay (Fig. 3h) is not quantified, making it difficult to assess the magnitude and reproducibility of the effect. In addition, western blots showing reduced levels of mitochondrially encoded OXPHOS subunits (Figure 3g) lack a mitochondrial loading control (e.g., TOM20 or VDAC). Since loss of EOLA1 may affect mitochondrial mass, normalization to a mitochondrial marker is necessary. Relatedly, it would be informative to assess whether steady-state levels of mitoribosomal proteins (e.g., MRPS15, MRPL37) and nuclear-encoded OXPHOS subunits are altered upon Eola1 loss, both in knockout cell lines and in the knockout mouse.

(6) Physiological scope of the in vivo analysis
The cardiac phenotype observed in the whole-body Eola1 knockout mouse is compelling, but the focus on a single tissue limits interpretation of EOLA1's broader physiological role. Examination of additional high-energy-demand tissues would help clarify whether the observed effects are heart-specific or more general. In addition, the presence of residual EOLA1 protein bands in western blots (Figure 4a) and remaining Eola1 transcripts in qRT-PCR analyses (Extended Figure 4e) from knockout tissues should be addressed. The authors should clarify whether these signals reflect incomplete knockout, alternative isoforms, antibody cross-reactivity, or technical background.

(7) Relationship to previously reported MT2A interaction
Given prior reports of EOLA1 interaction with MT2A, a brief comment on whether MT2A was detected in the authors' co-immunoprecipitation experiments and how this relates to the proposed mitochondrial role would be useful.

Reviewer #3 (Public review):

The authors identified EOLA1 in a CRISPR/Cas9 screen for essential mitochondrial genes in a mouse B16-F10 cell line; however, no information on the library used for this screen or the list of all identified essential genes is provided. What was the p-value for EOLA1 in Figure 1b?

The authors show that EOLA1 is indeed a mitochondrial protein (using both mouse and human cell lines). It is valuable that the authors use different cell lines to investigate the function of this protein; however, this also presents a challenge, as four different cell lines (two mouse and two human) are used across individual experiments, with no consistency between them. Knock-out (KO) experiments were performed in mouse cell lines only, and human cell lines were used in overexpression experiments, in which EOLA1 was tagged with FLAG-HA. It would be beneficial if a knock-out were also generated in a human cell line to confirm the effect on the expression of mitochondria-encoded proteins, along with a rescue experiment in which the EOLA1 protein is reintroduced into KO cells.

Functional analysis of EOLA1: The authors performed affinity immunoprecipitation of FLAG-HA-tagged EOLA1 from stably overexpressing cells, and identified 202 co-immunoprecipitating proteins, of which 71 were known mitochondrial proteins; however, no list of these proteins is provided. Why did the authors choose TUFM? Were any mitochondrial ribosomal proteins co-immunoprecipitated, if EOLA1 is suggested to regulate translation? Were levels of TUFM affected in EOLA1-KO cells?

The authors continued to analyze mitochondrial ribosomes using sucrose gradient fractionation and in-vitro mitochondrial translation. However, there are several technical problems with the presented data: It has been established that mitochondrial ribosomes do not form polysomes in mammalian cells but rather perform translation as monosomes. The authors indirectly confirm this: almost no 12S or 16S rRNA (Fig. 3f) or MRP proteins (Extended data 3c) are present in "polysome" fractions. Although indeed 12S and 16S rRNAs are decreased in monosome fractions, the levels of mRNAs are not different between KO and WT cells, and neither is the migration of mitochondrial ribosomal proteins. As there is no loading control provided for the sucrose gradients blots (such as SDHA, VDAC), it is not possible to assess the overall levels of mitochondrial ribosomes. The gel presented for mitochondrial translation is of poor quality, as it is impossible to identify any of the expected 13 polypeptides. Although the intensity of the signal is weaker for KO, so is the intensity in the portion of Coomassie stained gel. A better-quality gel and quantification need to be provided to support the claims.

What is the difference between endogenous and exogenous RIP-qPCR? EOLA1 pulled down 12S rRNA without cross-linking (Figure 3d) or with UV-crosslinking (Figure 3e), however, both 12S and 16S rRNAs were enriched in UV-crosslinked cells (Figure 3c) and by UV-RIP-seq (Extended data 3b; although no control is provided here). Is no discussion offered for this observation? Is it possible that EOLA1 plays a role in the maturation of the mito-ribosome, rather than translation? Does EOLA1 co-migrate with the mito-ribosome on sucrose gradients?

Altogether, there is insufficient evidence to support the conclusion that EOLA1 plays a role in mitochondrial translation.

To investigate EOLA1 biological function, the authors created a whole-body EOLA1-/- mouse that exhibited no overall developmental abnormalities; however presented with an abnormal cardiac function. This is an ideal model to confirm prior observations in cellular models; however, apart from one western-blot for three mitochondrial encoded subunits, no other experiments were provided (such as measurements of the levels of 12S, or 16S rRNA, TUFM levels, ribosomes profile, mitochondrial translation, OXPHOS assembly, respirometry).

In Figure 2 g-i: TEM images are presented, but the method is not described, nor is any information on the cells used provided, nor is it clear how the circularity was determined. KO cells certainly look abnormal; however, are the authors sure that the indicated structures are mitochondria? They rather resemble autophagosomes/lysosomes with lamellar inclusions.