A flow-cytometric method for quantification of neurolipofuscin and comparison with existing histological and biochemical approaches.

The ability to measure lipofuscin accumulation accurately is essential for understanding its role in physiological ageing and human disease, and for its recent use as an ecological tool for age determination. Existing quantification methods are problematic. In situ histological measurement by microscopy can be very precise but is labour intensive. Spectrofluorimetric measurement of whole lipid extracts is rapid but not sufficiently specific. A recent HPLC assay for the retinal pigment epithelium lipofuscin fluorophore, A2-E, is potentially both precise and rapid but not applicable to lipofuscin in other tissues, or from fixed samples. In this study, I explore the use of flow cytometry or fluorescence activated cell sorting (FACS) for specific quantification of lipofuscin granules in formalin-fixed CNS homogenates from lobsters (Homarus gammarus). Free neurolipofuscin granules were discriminated in FACS samples by their size distribution (forward scatter), distinctive orange autofluorescence (FL3) and refractive internal structure (side scatter). A quantitative neurolipofuscin index was developed, which was highly correlated with the microscopically measured neurolipofuscin concentration in the same tissue. Sample-processing rate was at least an order of magnitude greater for FACS than for quantitative microscopy but the latter yielded a much more precise estimate of neurolipofuscin concentration. While the FACS approach may be ideal where rapid handling and only semiquantitative results are required, loss of precision will preclude use in many ecological studies where the highest available resolution is needed. Further refinements to the FACS approach are possible but advanced histological methods for neurolipofuscin quantification remain the most reliable at this time.