The earthworm, Eudrilus eugeniae, has a prodigious ability to regenerate lost segments. The skin of the worm has an outermost epidermal layer followed by a thick circular muscle layer and an innermost thin longitudinal cell layer. During the process of regeneration, the circular muscle layer decreased in thickness, and longitudinal cell layer increased. The histological analysis of the regenerated worm shows that the longitudinal cell layer forms the regeneration blastema. BrdU-labeling retention assay confirmed that the circular muscle and longitudinal cell layers have BrdU-positive cells, which migrate from the adjacent segments to the regeneration blastema. In addition, it was noted that the cells of the earthworm, E. eugeniae, have the property of autofluorescence. Autofluorescence was found in the cytoplasm, but not in the nucleus. It has been also found that the major source for autofluorescence is riboflavin. Further, it was also demonstrated that supplementation with riboflavin increases the rate of regeneration, while regeneration was hampered by reduced levels of riboflavin. The importance of riboflavin in regeneration was also confirmed by rescue assay. In addition, it was also identified that BrdU-positive cells are highly fluorescent compared to the surrounding cells.

Lamin A is an intermediate filament protein found in most of the differentiated vertebrate cells but absent in stem cells. It shapes the skeletal frame structure beneath the inner nuclear membrane of the cell nucleus. As there are few studies of the expression of lamin A in invertebrates, in the present work, we have analyzed the sequence, immunochemical conservation and expression pattern of lamin A protein in the earthworm Eudrilus eugeniae, a model organism for tissue regeneration. The expression of lamin A has been confirmed in E. eugeniae by immunoblot. Its localization in the nuclear membrane has been observed by immunohistochemistry using two different rabbit anti-sera raised against human lamin A peptides, which are located at the C-terminus of the lamin A protein. These two antibodies detected 70 kDa lamin A protein in mice and a single 65 kDa protein in the earthworm. The Oct-4 positive undifferentiated blastemal tissues of regenerating earthworm do not express lamin A, while the Oct-4 negative differentiated cells express lamin A. This pattern was also confirmed in the earthworm prostate gland. The present study is the first evidence for the immunochemical identification of lamin A and Oct-4 in the earthworm. Along with the partial sequence obtained from the earthworm genome, the present results suggest that lamin A protein and its expression pattern is conserved from the earthworm to humans.

Lamin A is an intermediate filament protein which is cleaved by the enzyme, FACE 1 at VTRSY↓L. The cleavage is the final step in the production of the mature lamin A protein. The mature lamin A protein localizes in the inner membrane of the nucleus. The mutation in the lamin A gene causes many diseases, including accelerated aging. It is known that the protein is not expressed in neuronal cells of the brain. Many splicing variants of the lamin A gene have been reported. In this study, the amino acid sequence VTRSY (a penta-peptide repeat) was found in three different sites of the C-terminal end of the lamin A protein, the protein expressed in cells of ear cartilage tissues is shorter than the protein expressed in cells of the skin tissues. Using two lamin A antibodies, it was found that the amino acid sequence between penta-peptide 2 and 3 is missing in lamin A protein that was expressed in the cells of mouse ear cartilage tissue, besides the RT-PCR data confirmed that the corresponding coding sequence between the penta repeat 2 and 3 is intact. Cleavage may occur at the penta-peptide (VTRSY) at site 3 in the lamin A tail of mouse ear cartilage.

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