Assessment of DNA degradation in live spermatozoon using laser tweezers raman microspectrometry

Abstract

Purpose: Sperm nuclear proteins and DNA integrity have been implicated in infertility and treatment failures. High stallion to stallion variability is observed in sperm cryopreservation protocols. The cells are destroyed with harsh chemicals prior to using biochemical assays to test sperm DNA quality. The feasibility of using Raman spectrometry in combination with a laser trap for non-destructive micromanipulation and characterization of DNA damage in motile stallion and human sperm is experimentally investigated in this thesis. Methods: Live stallion sperms were subjected to controlled cellular damage: (a) four grades of chemically induced oxidative stress using Xanthine – Xanthine Oxidase (b) three grades of osmotic stress using PBS and (c) membrane damage using thermal shock. Live human sperm DNA disintegration with time and oxidative stress were explored on fresh, cryopreserved and swim-up categories. The specimens ranged from sub-fertile patients to fertile donors in a limited study. Post-treatment sperms resuspended in sperm media, placed on a quartz coverslip were trapped with a 785 nm, 25 mW laser, using a 1.4 NA, 60X, water immersion microscope objective. A Raman spectrum of a trapped cell was acquired for 20 – 30 seconds. The spectra from 20 – 40 cells from each specimen were analysed in the 630 cm-1 – 1630 cm-1 region using statistical variance and PCA. Results: The Raman spectra from trapped motile sperm head contain intense peaks that did not require smoothing prior to analysis. PCA of the Raman spectra could not resolve the different grades of applied osmotic and oxidative stress in stallion cells. PCA showed high variability between specimens from the same stallion and between stallions, with distinct clustering by ejaculate. Membrane damage study and spectra from extended trapping also showed distinct specimen to specimen difference within and between stallions. Specimen to specimen variability is observed in motility and viability tests on 1000s of stallion cells using CASA and flow cytometry. Human sperms showed some clustering by category, time, stress and motility and appeared more sensitive to the tests than stallion sperms. Conclusions: Raman spectra originate from the dense region of the trapped sperm head and resemble the fingerprint of dense calf thymus DNA. The cells show species specific response to the applied stress/damage. Stallion sperms show high variability between ejaculates that could not be differentiated by stallions. Human cells appear more sensitive to the applied processes. LTRS of live sperms needs further detailed research, cross correlated with other established complementary techniques, to identify spectral bands that are most sensitive to the various grades of induced DNA and membrane damage.