Recombinant Danio rerio Mitochondrial import inner membrane translocase subunit TIM50 (timm50)

What is the developmental expression pattern of timm50 in zebrafish?

Zebrafish timm50 exhibits a specific temporal and spatial expression pattern during development:

• mRNA is first detectable at approximately 7 hours post-fertilization (hpf)

• At 10 and 24 hpf, expression is ubiquitous with highest levels in the head region

• By 51 hpf, expression becomes predominantly cardiac-restricted

This developmental pattern suggests important roles for TIM50 in early zebrafish development, particularly in cardiac tissue. Research has shown that knockdown of Tim50 in zebrafish embryos results in neurodegeneration, dysmorphic heart features, and reduced motility due to increased apoptosis , confirming its crucial role in normal development.

How does Tim50 function within the mitochondrial protein import machinery?

Tim50 serves as a critical receptor component of the TIM23 complex in the mitochondrial inner membrane . Its functional role includes:

• Recognizing and binding presequences of proteins destined for import into mitochondria

• Facilitating the transfer of precursor proteins from the TOM complex to the TIM23 complex across the intermembrane space

• Maintaining the permeability barrier of the mitochondrial inner membrane

The mechanism involves Tim50's large IMS domain interacting with both the presequence of imported proteins and with Tim23 . Studies using crosslinking have demonstrated that Tim50 can be found in close proximity to segments of precursor proteins present in the intermembrane space, even when they are only bound to the TOM complex . This indicates Tim50 plays an early role in the recognition and guidance of proteins during the import process.

What methods are effective for analyzing Tim50 protein-protein interactions in zebrafish mitochondria?

Several methodological approaches have proven successful for investigating Tim50 interactions:

Crosslinking and Co-immunoprecipitation

• Chemical crosslinking: Use membrane-permeable crosslinkers (e.g., DSP, DFDNB) to stabilize transient interactions

• Protocol steps:

  • Isolate intact mitochondria from zebrafish tissues

  • Create import intermediate by incubating radiolabeled precursor proteins with isolated mitochondria

  • Apply crosslinker at appropriate concentration (0.1-0.5 mM) for 30 min at 4°C

  • Quench reaction with excess glycine

  • Lyse mitochondria with detergent (typically 1% digitonin)

  • Perform immunoprecipitation using antibodies against Tim50 or interaction partners

Yeast Two-Hybrid and Split-GFP Assays

These techniques can identify direct protein interactions but require validation in the mitochondrial context.

Proximity-Based Labeling

BioID or APEX2 fusion proteins can identify the proximity interactome of Tim50 in living cells .

Table 1. Experimental conditions for analyzing Tim50 interactions

How can researchers effectively generate zebrafish models with Tim50 mutations?

Several approaches are available for generating Tim50-deficient or mutant zebrafish:

Morpholino-Based Knockdown

• Antisense morpholino oligonucleotides targeting timm50 mRNA can be injected into 1-4 cell stage embryos

• Recommended dosage: 1-10 ng per embryo, with careful titration to avoid off-target effects

• Critical validation: Rescue experiments by co-injecting wild-type timm50 mRNA to confirm specificity

• Advantages: Rapid, transient knockdown useful for studying early developmental roles

CRISPR-Cas9 Genome Editing

• More precise method for generating stable mutant lines

• Target selection: Multiple sgRNAs targeting different exons should be tested

• Protocol outline:

  • Design sgRNAs targeting conserved regions of timm50

  • Inject Cas9 protein with sgRNAs into one-cell stage embryos

  • Screen F0 founders for germline transmission

  • Characterize F1 heterozygotes and establish F2 homozygous lines if viable

Tol2 Transgenesis for Rescue or Overexpression

• The tol2 transposon system allows efficient integration of transgenes

• Can be used to express wild-type or mutant forms of Tim50

• Success rate: 50-70% of injected fish transmit the transgene to offspring

For disease-modeling research, consider introducing specific mutations that correspond to human pathogenic variants, such as the R113C or R114Q mutations that have been associated with mitochondrial disorders .

Zebrafish Tim50 has proven valuable for modeling mitochondrial diseases:

Tim50 Deficiency Phenotypes:

• Developmental and growth retardation

• Neurodegeneration

• Cardiac abnormalities (bradycardia, pericardial effusions)

• Reduced motility

• Increased apoptosis due to disrupted mitochondrial membrane potential

These phenotypes parallel aspects of human mitochondrial disorders, particularly those affecting the nervous system and heart.

Tim50 and Disease Mechanisms:

• Impaired protein import: Knockdown affects import of presequence-containing proteins but not proteins using alternative import pathways

• Mitochondrial membrane potential: Both over and under-expression of Tim50 disrupt membrane potential and promote apoptosis

• Oxidative phosphorylation: Tim50 deficiency affects assembly and function of respiratory complexes

Modeling Human Mutations:
Specific human TIMM50 mutations causing mitochondrial disorders have been identified:

• R113C mutation affects the IMS-facing domain after the transmembrane segment

• R114Q and R114W mutations in adjacent residues cause similar phenotypes

• G190A mutation in the transmembrane domain causes severe encephalopathy

Researchers can recreate these specific mutations in zebrafish timm50 using CRISPR-Cas9 genome editing to study their pathophysiology and test potential therapeutic approaches.

What methodological considerations are important for working with recombinant Danio rerio Tim50 protein?

Protein Expression and Purification

The recombinant Tim50 protein available commercially is typically produced in bacterial expression systems . For researchers preparing their own recombinant protein:

• Expression system selection:

  • E. coli systems may be suitable for the soluble IMS domain

  • Insect cell or mammalian systems recommended for full-length protein with TMD

• Purification strategy:

  • Initial capture using affinity tag (His, GST)

  • Ion exchange chromatography (Tim50 has predicted pI ~5.31)

  • Size exclusion chromatography as final polishing step

• Buffer considerations:

  • Tris-based buffer with 50% glycerol for stability

  • pH range 7.2-7.4 for optimal stability

  • Addition of reducing agent (DTT or β-mercaptoethanol) to prevent oxidation

Storage and Handling Recommendations

Based on commercial product information and research protocols:

• Store at -20°C; for extended storage, -80°C is recommended

• Avoid repeated freeze-thaw cycles

• Working aliquots can be stored at 4°C for up to one week

• Optimize concentration based on experimental needs (typical working range: 1-100 μg/ml)

Functional Assays

To verify activity of recombinant Tim50:

• Presequence binding assay: Using fluorescence anisotropy with labeled presequence peptides

• Interaction studies: Surface plasmon resonance or microscale thermophoresis to measure binding to Tim23

• Phosphatase activity assay: If testing human TIMM50 ortholog with conserved phosphatase activity

How does Tim50 contribute to mitochondrial dynamics beyond protein import?

Recent research has revealed multifunctional roles for Tim50 beyond its canonical function in protein import:

Regulation of Apoptosis

• Tim50 levels correlate with mitochondrial membrane potential

• Both overexpression and depletion of Tim50 can trigger apoptosis via cytochrome c release

• The precise mechanism may involve regulation of inner membrane integrity

Steroid Hormone Synthesis

• Tim50 interacts with 3β-hydroxysteroid dehydrogenase type 2 (3βHSD2)

• This interaction is essential for DHEA and androstenedione synthesis

• Knockdown of Tim50 reduces 3βHSD2 activity even when expression is restored, suggesting a role in proper docking and orientation

Cardiac Function

• Downregulation of Tim50 is observed in mouse models of dilated cardiomyopathy

• Tim50 overexpression protects against induced cardiac hypertrophy

• Knockout mice show increased ROS levels and reduced respiratory complex activity in heart tissue

Cancer Biology

• Elevated Tim50 levels have been observed in breast cancer and non-small cell lung carcinoma

• Tim50 upregulation correlates with increased cyclin D1 levels and cell proliferation

• The link between mitochondrial function and signaling events like ERK and P90RSK phosphorylation appears to be mediated by Tim50

These diverse functions make Tim50 an interesting target for research beyond basic mitochondrial biology, with potential implications for various disease processes.

What experimental approaches can assess Tim50 function in zebrafish mitochondrial respiratory chain activity?

Mitochondrial Isolation and Respiratory Analysis

• Isolation protocol:

  • Homogenize zebrafish embryos or adult tissues in isolation buffer (250 mM sucrose, 20 mM HEPES, 1 mM EDTA, pH 7.4)

  • Perform differential centrifugation to separate mitochondria

  • Assess purity using Western blot for mitochondrial markers

• Respiratory analysis:

  • Oxygen consumption rate (OCR) using Seahorse XF analyzer or Clark-type electrode

  • Measure state 3 (ADP-stimulated) and state 4 (ADP-limited) respiration

  • Calculate respiratory control ratio (RCR = state 3/state 4)

Blue Native PAGE for Respiratory Complex Analysis

This technique allows assessment of respiratory complex assembly and supercomplex formation:

• Solubilize isolated mitochondria with digitonin (mild) or DDM (more stringent)

• Separate complexes using blue native PAGE

• Perform in-gel activity assays for complexes I-V

• Western blot using antibodies against complex subunits

• Reduced levels of complexes I and IV

• Persistence of complex III dimers

• Disruption of supercomplex formation

Mitochondrial Membrane Potential Measurement

• In isolated mitochondria:

  • Use fluorescent dyes like TMRM, JC-1, or Rhodamine 123

  • Quantify using spectrofluorometry or flow cytometry

• In intact zebrafish embryos:

  • Use vital dyes permeable to embryos

  • Visualize using confocal microscopy

  • Quantify signal intensity as measure of membrane potential

ATP Production Assay

• Use luciferase-based assays to measure ATP levels

• Compare ATP production with different substrates (pyruvate/malate vs. succinate) to assess specific complex deficiencies

• Inhibitor studies using oligomycin (ATP synthase), rotenone (complex I), antimycin A (complex III)

Table 2. Expected findings in Tim50-deficient zebrafish models