Isolation and Quantification of Axonal mRNAs Using Porous Membrane Inserts and RTddPCR

The spatial dynamics of mRNA localization and translation within neurons are essential for various mechanisms of neuronal function, including neuronal connectivity, synaptic plasticity, and response to injury. Due to the extreme polarity of neurons, many of these functions rely on the ability of axons to locally translate specific transcripts. However, quantifying these subcellular RNA populations remains technically challenging. Here, we describe a reproducible approach for obtaining separate somatic and axonally enriched compartments from cultured rodent neurons and for quantifying compartment-specific mRNA expression. Primary rodent embryonic or adult neurons were cultured on inserts with a porous membrane of size 1-3 µm. These membranes are only permissive to axons, allowing physical separation of the somatic and axonal compartments. RNA was then isolated from the whole neuron and axon-enriched fractions separately, which were further used for reverse transcriptase droplet digital PCR (RTddPCR) with gene-specific primers. This system offers an absolute quantitative comparison between subcellular compartments, enabling high-sensitivity detection of localized transcripts. This approach measures steady-state RNA abundance and facilitates examination of axonal RNA changes over time in response to neurotrophic factors, stress, or injury models. The combination of physical compartmentalization and RTddPCR analysis reduces cross-contamination and gives exact copy numbers of rare transcripts, offering high sensitivity, reproducibility, and detection of low-copy-number mRNAs that control axon growth and regeneration. This method also works with downstream tests, such as measuring protein synthesis, studying RNA stability, and doing perturbation experiments using siRNA or drugs that block certain proteins. Importantly, this technique can be adapted for different neuronal subtypes, developmental stages, or injury models. In general, this approach is a flexible, sensitive, and reproducible way to study the molecular basis of axonal mRNA localization and how it affects neuronal function and disease mechanisms.