SPEx: Compartment-Resolved Proteomics via Expansion Microscopy-Guided Microdissection

The paper introduces SPEx, a cost-effective method that combines expansion microscopy with laser microdissection and mass spectrometry to achieve high-resolution subcellular proteomics, successfully identifying known and novel components of organelles like the Golgi, nucleus, and nucleoli.

The Gist

Imagine a bustling city inside every single cell of your body. This city is filled with different neighborhoods: the Nucleus (the city hall where the blueprints are kept), the Golgi (the post office that packages and ships goods), and the Nucleolus (a busy factory inside city hall making ribosomes).

For a long time, scientists trying to study these neighborhoods had a major problem. To see what was inside, they had to tear the whole city apart, mix everything into a giant smoothie, and then try to guess which ingredients came from the post office and which came from city hall. It was like trying to figure out what was in the bakery by tasting a smoothie made of the whole city. You'd get the flavors, but you'd lose the location.

Other methods tried to use "proximity tags" (like putting a sticker on a specific building), but these were expensive, slow, and often missed the tiny details.

Enter SPEx: The "Expansion and Zoom" Method

The scientists in this paper developed a clever new trick called SPEx (Subcellular spatial Proteomics coupled to Expansion). Think of it as a three-step magic trick to see the city's neighborhoods in high definition without destroying the city.

Step 1: The "Popcorn" Effect (Expansion)

First, they take a cell and treat it with a special gel. Imagine the cell is a tiny, dense piece of popcorn. When you heat it up, it explodes and expands to 10 times its original size.

• The Analogy: It's like taking a tiny, crumpled map of a city and blowing it up until the streets are wide enough to drive a truck on. Suddenly, the tiny "Golgi post office" isn't a speck anymore; it's a huge, easy-to-see building.

Step 2: The "Laser Scalpel" (Microdissection)

Now that the city is huge, the scientists use a super-precise laser to cut out only the neighborhood they want to study.

• The Analogy: Imagine you have a giant, expanded map. You use a laser cutter to slice out only the Post Office, leaving the rest of the city (the cytoplasm) behind. You can even cut out the Post Office, then cut out the rest of the building to see what's left. This is crucial because it lets them compare "Post Office only" vs. "Everything else."

Step 3: The "Inventory Check" (Mass Spectrometry)

Once they have the laser-cut piece of the Post Office (or City Hall), they put it into a machine that lists every single protein (the workers and machines) inside.

• The Analogy: They take the cut-out Post Office, dump it into a scanner, and get a perfect list of every employee working there. Because they have the "Everything else" list from the other cut, they can instantly see: "Aha! This protein is only in the Post Office, not in the rest of the city!"

Why is this a Big Deal?

1. It's Cheap and Simple: Unlike other high-tech methods that require million-dollar machines and complex genetic engineering, SPEx uses tools many labs already have. It's like upgrading from a bicycle to a car using parts you can buy at a local hardware store.
2. It Finds the "Hidden" Workers: Because the method is so precise, they didn't just find the famous proteins everyone knew about. They found new proteins living in the Nucleus, the Nucleolus, and the Golgi that no one knew were there before. It's like discovering a new department in the Post Office that nobody knew existed.
3. It Works on "Ghost" Buildings: Some parts of the cell, like the Nucleolus, don't have walls (they are "membraneless"). They are more like a crowd of people swirling together. Traditional methods struggle to separate these crowds. But because SPEx expands the space, the scientists can laser-cut right through the crowd and grab just the Nucleolus, proving it's possible to study these "fuzzy" neighborhoods.
4. It's a "Subtraction" Superpower: The scientists showed you can figure out what's inside a room by looking at what's missing from the rest of the house. If you cut out the Nucleus and look at the rest of the cell, you can see exactly what proteins left the room. This double-check system makes their results incredibly accurate.

The Bottom Line

SPEx is like giving scientists a pair of glasses that not only make the cell look huge but also let them surgically remove specific rooms to see exactly who lives there. It turns the blurry, mixed-up smoothie of cell biology into a clear, high-definition photo of the city's neighborhoods, helping us understand how our bodies work, adapt, and sometimes get sick.

Technical Summary

1. The Problem

Subcellular spatial proteomics aims to map protein localization, concentration, and interactions within specific cellular compartments to understand cellular function and homeostasis. However, existing methods face significant limitations:

• Proximity Labeling (e.g., BioID, APEX): While powerful, these lack precise spatial resolution and rely on proximity rather than actual physical position.
• Fractionation/Immunoprecipitation: These require cell lysis, which destroys all spatial information. They also rely on known markers and often suffer from cross-contamination between organelles.
• Microscoop®: A newer technique allowing laser-guided biotinylation of specific regions. While high-resolution, it is expensive, time-consuming, requires massive automation (machine learning), and needs specialized equipment not widely available.
• Laser Microdissection (LMD): A standard technique for isolating tissue or cell regions, but its resolution is limited by the laser beam diameter (0.5–1 µm). This prevents the isolation of small subcellular organelles (e.g., nucleoli, Golgi) from standard-sized cells, as the organelles are often smaller than the laser spot or the collection area.

2. Methodology: SPEx (Subcellular Spatial Proteomics coupled to Expansion)

The authors developed SPEx, a novel workflow that combines Expansion Microscopy (ExM), Laser Microdissection (LMD), and Mass Spectrometry (LC-MS) to overcome resolution limits without requiring expensive new hardware.

Key Workflow Steps:

1. Fixation and Labeling: HeLa cells are fixed and labeled.
   • Specific organelles (Golgi) are labeled with antibodies (e.g., anti-GM130).
   • A non-specific fluorescent NHS-ester dye is used to label all proteins, providing high contrast for the nucleus and nucleoli.
2. Expansion (TREx Protocol):
   • Cells are embedded in a hydrogel matrix.
   • Protein Denaturation: Instead of protease digestion (which destroys the proteome for MS), the authors optimized the TREx protocol using SDS denaturation at 95°C. This disrupts non-covalent interactions to allow expansion while preserving peptide integrity for downstream MS.
   • Expansion Factor: This modification achieved a consistent ~9-fold expansion (linear) in the hydrated state.
3. Drying and LMD Preparation:
   • Hydrated gels are difficult to cut due to surface tension and laser diffraction. The authors dried the gels completely onto PPS-membrane slides.
   • Crucial Finding: Drying caused shrinkage only in the Z-axis (height), preserving the ~7-fold expansion in the X-Y plane. This increased the surface area by ~50x, making subcellular structures large enough for standard LMD.
4. Laser Microdissection:
   • Using a standard Leica LMD7 system, researchers dissected specific regions of interest (ROIs):
     • Whole Nuclei
     • Nucleoli (membraneless)
     • Golgi Apparatus
     • Controls: "CellsMinusNucleus," "CellsMinusGolgi," and "NucleiMinusNucleoli" were cut from the same expanded cells to serve as matched controls.
5. Proteomic Analysis:
   • Dissected samples (as few as 25–100 organelles) were processed using a low-input protocol.
   • Analysis was performed on a high-sensitivity timsTOF Ultra mass spectrometer using DIA-PASEF (Data-Independent Acquisition with Parallel Accumulation Serial Fragmentation) for deep proteome coverage.

3. Key Contributions

• Novel Methodology: SPEx is the first method to successfully combine expansion microscopy with LMD and LC-MS for in situ subcellular proteomics.
• Accessibility: It utilizes existing, widely available technologies (standard LMD, ExM reagents, and LC-MS) rather than requiring specialized, expensive, or automated platforms like Microscoop®.
• Dual Analysis Strategy: The authors demonstrated two powerful analytical approaches:
  1. Standard: Comparing the isolated organelle against the remaining cell fraction.
  2. Subtraction: Comparing the cell without the organelle against the whole cell (or vice versa) to identify proteins depleted from the organelle.
• Membraneless Organelle Analysis: Successfully applied to membraneless structures (nucleoli), which are notoriously difficult to purify biochemically.

4. Key Results

The method was validated on three distinct targets in HeLa cells:

• Nuclear Proteome:

  • Analyzed 25 nuclei. Identified ~2,200 proteins with >95% specificity for nuclear localization.
  • Discovery: Identified 23 proteins not previously annotated as nuclear (e.g., SPOUT1, C7orf50). Confirmed nuclear localization of C7orf50 and SPOUT1 via immunofluorescence.
  • Depletion: Successfully identified proteins depleted from the nucleus (e.g., mitochondrial proteins, cytoplasmic microtubule binders like CLIP1), demonstrating the method's ability to detect absence as well as presence.

• Nucleolar Proteome:

  • Analyzed 100 nucleoli. Due to the dynamic exchange of proteins between nucleoli and nucleoplasm, direct enrichment was low (~11% nucleolar annotation).
  • Subtraction Method: By analyzing "NucleiMinusNucleoli," specificity for nucleolar proteins jumped to >92%.
  • Discovery: Confirmed SPOUT1 as a nucleolar protein and identified components of the 90S pre-ribosome.
  • Specificity: SPEx achieved >90% specificity for nucleolar proteins, significantly outperforming classical density gradient centrifugation (~50%).

• Golgi Proteome:

  • Analyzed 35 Golgi structures. Identified 356 significantly enriched proteins, with ~81% annotated to the Golgi or extracellular space.
  • Detected known markers (GM130, GOLGAs, Rab6A) and trafficking machinery (SNAREs, COG, Coatomer).
  • Discovery: Identified KIAA2013 as a novel Golgi-localized protein, confirmed by immunofluorescence.
  • Comparison: Achieved proteome coverage and specificity comparable to specialized Golgi immunoprecipitation methods but without the need for genetic manipulation or complex purification steps.

5. Significance

• High Specificity with Low Input: SPEx can generate high-quality spatial proteomic data from very small sample inputs (dozens of organelles), making it suitable for rare cell types or limited clinical samples.
• Versatility: It works for both membrane-bound (Golgi) and membraneless (nucleoli) organelles and does not require transfection or fusion proteins; standard antibodies or dyes are sufficient.
• Cell-to-Cell Heterogeneity: Because LMD allows the selection of specific cells within a population, SPEx enables the study of organelle proteomes in morphologically distinct cells (e.g., dividing vs. non-dividing) within the same dish, a capability lacking in bulk lysis methods.
• Future Applications: The authors suggest the workflow could be extended to lipidomics, RNA/DNA sequencing of specific condensates, and application to non-model organisms or tissues where genetic manipulation is difficult.

In summary, SPEx democratizes high-resolution spatial proteomics by transforming standard LMD into a subcellular resolution tool through simple sample expansion, offering a cost-effective, high-specificity alternative to current state-of-the-art methods.