A fraction of mouse sperm chromatin is organized in nucleosomal hypersensitive domains enriched in retroposon DNA

We have characterized a nuclease hypersensitive chromatin fraction from murine spermatozoa. Endogenous nuclease activity can be induced in mouse epididymal spermatozoa by appropriate stimuli and cause the localized degradation of chromosomal DNA. Based on these observations, we have isolated nuclease hypersensitive chromatin regions released from spermatozoa in the supernatant of pelleted sperm cells, and have cloned and characterized the DNA. Gel electrophoresis of end-labelled released DNA fragments showed a typical nucleosomal distribution. Peripherally distributed nucleohistones were visualized by immunofluorescence in sperm nuclei, and histones were identified by western blot in sperm chromatin. Moreover, the released DNA is enriched in retroposon DNA from a variety of families. FISH and immunofluorescence analysis showed that retroposon DNA and nucleohistone chromatin co-localize and are both peripherically distributed in nuclei of spermatozoa. In contrast, a major satellite DNA probe, used for control, co-localizes with highly condensed chromatin in the central region of sperm nuclei. The nuclear Ran and RCC1 proteins were also visualized in the dorsal margin of sperm nuclei, and were abundantly released with the hypersensitive chromatin fraction. Together, these results indicate that nucleohistone chromatin fraction(s) with typical features of 'active' chromatin are present in murine spermatozoa, are hypersensitive to nuclease cleavage, enriched in retroposon DNA and organized in nucleosomal domains. These observations suggest that nucleohistone domains identify a fraction of the sperm genome which may be functional during early embryogenesis.