BIOL 3204: Super-resolution Light Microscopy (PALM & STORM)

Introduction to Super-resolution Light Microscopy

Resolution Comparison of Imaging Technologies:
- Human eye: $\sim200\mu m$
- Light microscope: $\sim200nm$
- Super-resolution fluorescence microscopy: $\sim10nm$
- Electron Microscope: $\sim0.2nm$
Nobel Recognition (2014 Nobel Prize in Chemistry): Awarded to Eric Betzig, Stefan W. Hell, and William E. Moerner "for the development of super-resolved fluorescence microscopy." Key techniques recognized include Stimulated Emission Depletion (STED), Single-Molecule Tracking, and Photoactivated Localization Microscopy (PALM).
Visual Impact: Super-resolution fluorescence microscopy significantly enhances detail compared to traditional laser scanning confocal microscopy, allowing for clearer visualization of cellular structures.

Types of Super-resolution Light Microscopy

Major Techniques Discussed:
- PALM (Photoactivated localization microscopy)
- STORM (Stochastic optical reconstruction microscopy) – developed by Xiaowei Zhuang, Harvard
- STED (Stimulated emission depletion) microscopy
Categorization of Super-resolution Techniques: Based on their working principles, they fall into two broad groups:
1. Patterned Illumination Microscopy (e.g., RESOLFT, STED):
  - Involves an excitation beam which creates an Airy spot, followed by a STED pattern (depletion beam) that narrows the effective excitation area.
  - This process is scanned across the sample.
2. Localization-based Super-resolution Microscopy (e.g., PALM, FPALM, STORM):
  - Relies on activating a small, sparse (stochastic) subset of fluorophores at any given time.
  - Each activated single molecule is excited and its position precisely localized (e.g., by fitting a Gaussian to its point spread function).
  - The activated molecules are then deactivated (photobleached or switched off).
  - This cycle is repeated with different subsets of fluorophores.
  - A final 'image' is reconstructed by compiling a density map of all localized positions.
  - This category is the primary focus of this class.

Key Questions for Understanding Super-resolution Microscopy

To grasp PALM & STORM, specifically single-molecule localization microscopy, it's crucial to address:

When can a light microscope be classified as a super-resolution microscope?
What constitutes single-molecule biology?
What is single-molecule localization microscopy?
What is the fundamental working principle for STORM/PALM?

The Diffraction Limit of Conventional Light Microscopy

Underlying Principle: The resolution of a conventional light microscope is fundamentally limited by the wave nature of light, a phenomenon known as diffraction.
Abbe's Diffraction Limit Formula: Resolution $=0.612\times\lambda/NA$
- Where $\lambda$ is the wavelength of light and $NA$ is the numerical aperture of the objective lens.
Quantitative Limit: For visible light, the diffraction limit is approximately $200-250nm$ . This means structures closer than this distance cannot be resolved as separate entities by traditional optical microscopes.
Components of a Fluorescence Microscope: A typical fluorescence microscope includes eyepieces, a vertical illuminator, filter/optical block turret, objective, stage, condenser, field lens, base, arc-discharge lamphouse (e.g., Mercury HBO lamp), transmitted light filters, and a Peltier-cooled CCD camera.

Definition of Super-resolution Light Microscopy

Beyond the Limit: Super-resolution light microscopy encompasses advanced imaging techniques that overcome the diffraction limit of conventional light microscopy.
Enhanced Resolution: These techniques achieve significantly higher resolution, often down to $10-20nm$ or even smaller, surpassing the traditional $200-250nm$ limit.
Biological Significance: This breakthrough enables researchers to visualize molecular and cellular structures at the nanometer scale with unprecedented detail. It's transformative for biological sciences, allowing for in-depth observation of cellular components, molecular interactions, and dynamic processes within living cells.

Single-molecule Biology

Fundamental Contrast: Single-molecule biology contrasts with measurements performed on an ensemble or bulk collection of molecules. In ensemble studies, only average characteristics can be measured, and individual molecular behaviors are obscured.
Advantages of Single-molecule Studies:
- Individual behavior of molecules can be distinguished, providing insights not accessible through ensemble averaging.
- Reveals new details on measured processes.
- Allows for observation of heterogeneity within a population of molecules.
Emergence and Impact: Single-molecule fluorescence techniques, alongside optical tweezers and scanning probe microscopy, emerged in the 1990s.
- These tools enable the manipulation and measurement of single biological molecules, whether in live cells or in culture.
- They are considered powerful tools for elucidating biological function, uncovering molecular mechanisms, and tracking molecular behavior in living systems.
Challenge of Intrinsic Fluorescence: Many biomolecules do not naturally fluoresce or exhibit very dim fluorescence, necessitating the use of external labels.
Applications: Single-molecule techniques allow for detailed imaging and localization of various biological components, such as lipids, proteins, and nucleic acids, as demonstrated in fibroblast cells.

Fluorescent Probes and Labels

To make biomolecules visible under fluorescence microscopy, various probes are used:

Fluorophores (Organic Dyes): Examples include members of the cyanine and rhodamine families, such as Cy3, Cy5, Alexa Fluor dyes, Rhodamine 6G, and Texas Red.
Fluorescent Proteins: Green Fluorescent Protein (GFP) and its diverse derivatives (e.g., RFP, BFP, YFP, mCherry) are widely used due to their genetic compatibility.
Quantum Dots: Semiconductor nanocrystals that emit light.
Dyed Polymer Particles: Polymeric beads impregnated with dyes.
Example of Dye-Labeled Sample: Mouse fibroblasts can be labeled to visualize specific structures: F-Actin (Green) using FITC, Tubulin (Red) using Cy5, and Nuclei (Blue) using DAPI.

Green Fluorescent Protein (GFP)

Origin and Characteristics:
- A unique protein isolated from the jellyfish Aequorea victoria.
- Emits green light when excited by blue or UV light.
- Composed of $238\text{ amino acids}$ .
- Its structure features a beta-barrel enclosing an alpha-helix, with the chromophore (formed by Tyr66, Ser65, Gly57) at its core.
Nobel Prize (2008 Nobel Prize in Chemistry): Awarded to Osamu Shimomura, Martin Chalfie, and Roger Y. Tsien "for the discovery and development of the green fluorescent protein, GFP." This recognition highlights its profound impact on biological sciences.
Applications:
- GFP can be genetically fused to a protein of interest (e.g., EGFP fused to Human Alpha-Tubulin) to make it fluorescent within living cells without external staining.
- Genetic mutations have led to a vast diversity of fluorescent proteins, enabling multi-color imaging, as exemplified by bacteria expressing 8 different colors to create complex patterns.
- Used in both in vitro studies (e.g., individual organic dyes in buffer) and in vivo applications (e.g., GFP-HIV infecting live cells).

Single-molecule Localization Microscopy (SMLM) and its Principle

Core Idea: SMLM aims to precisely determine the position (localization) of individual fluorescent molecules and track their movement ( $x<em>c, y</em>c$ coordinates) over time, ultimately creating a detailed density map.
Historical Development: The field evolved through several stages:
- Wide-field imaging (1995)
- Single-molecule imaging (2002)
- Emitter localization and SMLM image rendering (2006)
- Quantitative SMLM (2010 and beyond)
Working Principle (PALM/STORM):
1. Conventional Microscopy Limitation: In traditional fluorescence, all fluorophores are activated simultaneously, leading to overlapping diffraction-limited spots and poor resolution of closely spaced molecules.
2. SMLM Strategy:
  - Sparse Activation: The key is to activate only a sparse subset of fluorophores at any given moment, ensuring their diffraction-limited images do not overlap.
  - Localization: Each isolated, bright spot corresponding to a single activated molecule is then precisely localized by fitting its point spread function (often assumed to be a Gaussian) to determine its sub-diffraction-limit center.
  - Deactivation: The activated molecules are subsequently photobleached or switched off.
  - Repetition: This cycle of sparse activation, imaging, localization, and deactivation is repeated numerous times.
  - Reconstruction: The precise coordinates from all localization events are then compiled and rendered to form a super-resolution image, overcoming the diffraction limit. This results in a much higher density of localized molecules compared to conventional imaging.
Fluorophore ON/OFF Cycles: This stochastic activation and deactivation (photocleavage, photoswitching, blinking) of fluorophores is critical for PALM and STORM, allowing for sequential imaging and precise localization of a small, non-overlapping population of molecules at each step.

Limitations of SMLM (PALM & STORM)

Speed Constraint: SMLM techniques generally work best for immobilized molecules and face significant limitations when imaging rapidly moving molecules within a live cell.
Resolution-Speed Trade-off: There is an inherent trade-off between the achievable detection speed and spatial resolution.
Acquisition Time Example: Obtaining an image of $28\mu m\times28\mu m$ with a single-molecule localization precision of $20nm$ and an overall image resolution of $\sim60nm$ can take between $25\text{ and }60\text{ seconds}$

Comparison of Microscope Resolutions

Conventional Light Microscope: Resolution $\sim200nm$
Phase Contrast Microscope: Resolution $\sim200nm$
Laser Scanning Confocal Microscope: Resolution $\sim140-180nm$
Super-resolution Light Microscopy: Resolution $\sim1-100nm$ (significantly below the diffraction limit)
Other Microscopes/Imaging Tech Spatial Resolutions (from detailed chart):
- Electron Microscopy (EM): <1nm
  Near-field Scanning Optical Microscopy (NSOM): down to a few nanometers
- PALM, FPALM, STORM: tens of nanometers
- STED, GSD, SSIM: several tens of nanometers
- High Resolution Structured Illumination, 4 Pi or I5M: in the range of $100-200nm$
- Confocal Microscopy: $140-180nm$
- Widefield and TIRF Microscopy: >200nm
  Optical Coherence Tomography (OCT): micrometers
- MRI, CT, PET Scan: millimeters to centimeters
Applications Contrast: Traditional bright-field and phase contrast microscopy provide limited detail for living cells compared to the detailed nanometer-scale insights offered by super-resolution techniques. Confocal microscopy offers optical sectioning by rejecting out-of-focus light with a pinhole, improving contrast and axial resolution over widefield fluorescence, but it remains diffraction-limited compared to super-resolution methods.

Here is a list of key terms and their definitions from the provided notes on Super-resolution Light Microscopy:

Resolution: The smallest distance between two points that can be distinguished as separate entities. For a human eye, it's $\sim 200 \mu m$ , for a light microscope $\sim 200 nm$ , and for super-resolution fluorescence microscopy, it's $\sim 10 nm$ down to $\sim 0.2 nm$ for electron microscopes.
Diffraction Limit: The fundamental limit to the resolution of a conventional light microscope, caused by the wave nature of light. Quantitatively, it is approximately $\sim 200-250 nm$ for visible light, meaning structures closer than this cannot be resolved as separate. It is described by Abbe's Diffraction Limit Formula: Resolution $= 0.612 \times \lambda / NA$ .
Super-resolution Light Microscopy: Advanced imaging techniques that overcome the diffraction limit of conventional light microscopy, achieving significantly higher resolution, often down to $\approx 10-20 nm$ or even smaller.
STED (Stimulated Emission Depletion): A patterned illumination super-resolution technique that uses an excitation beam and a depletion beam to narrow the effective excitation area, thus improving resolution.
PALM (Photoactivated localization microscopy): A localization-based super-resolution technique that relies on activating a sparse subset of fluorophores, localizing each precisely, deactivating them, and repeating the cycle to reconstruct a super-resolution image.
STORM (Stochastic optical reconstruction microscopy): Similar to PALM, it is also a localization-based super-resolution microscopy technique that uses sparse activation and localization of individual fluorophores.
Single-molecule Biology: The study of individual molecules, contrasting with ensemble measurements, allowing insights into individual molecular behaviors, heterogeneity, and dynamics not visible in bulk studies.
Fluorophores: Molecules that absorb light at one wavelength and emit it at a longer wavelength, used as labels in fluorescence microscopy. Examples include organic dyes (e.g., Cy3, Alexa Fluor), fluorescent proteins (e.g., GFP), and quantum dots.
Green Fluorescent Protein (GFP): A protein isolated from jellyfish that emits green light when excited. It is widely used to genetically label proteins of interest within living cells.
Single-molecule Localization Microscopy (SMLM): An overarching principle for techniques like PALM and STORM that precisely determine the position of individual fluorescent molecules over time by sequentially activating and localizing sparse subsets of fluorophores to build a high-resolution image.
Numerical Aperture (NA): A measure of the ability of an objective lens to gather light and resolve fine specimen detail. It is a key factor in the Abbe's Diffraction Limit Formula.