Despite substantial advances in genomics, transcriptomics, and proteomics, the molecular basis of most cellular dysfunctions remains poorly understood, limiting development of targeted molecular therapies. Genetically encoded fluorescent proteins have transformed molecular imaging through super-resolution techniques; however, mapping the nanoscale distribution of key molecular components—proteins, mRNA, and signaling processes—remains fundamentally challenging when cellular ultrastructure must be preserved and individual cells must be visualized within multicellular networks across millimeter-scale tissue volumes.
Electron microscopy is the most established technique for imaging such volumes with nanometer resolution when operated in "volume EM" mode with sequential mechanical or ion beam sectioning. EM provides dense labeling of cellular membranes, organelles, DNA, and ribosomes through heavy-metal stains, whereas fluorescence microscopy requires active inclusion of these anatomical references, severely limiting available spectral channels for molecular multiplexing.
Current connectomics initiatives mapping neuronal wiring diagrams capture only static structural snapshots that cannot resolve signaling molecules, proteomes, or transcriptomes—the molecular machinery governing synaptic plasticity that changes long before morphological correlates become visible. This missing information must be obtained through labor-intensive correlative measures involving fluorescence imaging, electrophysiology, and transcriptomics, presenting severe technical challenges in co-registering data across vastly different spatial scales.
Existing semi-genetic EM labeling methods—including immunogold labeling, enzymatic DAB polymerization via HRP or APEX2, and photooxidation via miniSOG—have critical limitations: epitope masking, limited antibody diffusion, spatial blurring of contrast that precludes geometric multiplexing, and dependence on exogenous compounds impeding high-throughput serial EM. Ferritin, while fully genetic, produces particles (~12 nm) too small for reliable detection at resolutions used for high-throughput volume EM.
Understanding the relationship between cellular ultrastructure and function contributes directly to mechanistic comprehension of cellular processes with implications for therapeutic strategies. In the brain, connectomics aims to map neuronal wiring diagrams to identify architectural motifs supporting specific computations. However, structural data alone lacks molecular identity information essential for interpreting circuit function. Beyond neuroscience, this methodology enables deciphering structure-function relationships in other cellular networks including the immune system, with applications in cellular therapies, organoid generation, tissue engineering, and synthetic biology.
The overarching goal of EMcapsulins is to accelerate morphometric analysis in biomedicine—quantitative analysis of cellular ultrastructure in relation to tissue function—by creating genetically controlled EM markers and reporters that geometrically encode cellular identity and activation history readable by electron microscopy.
