Programmed cell senescence in the mouse developing spinal cord and notochord

Abstract

Programmed cell senescence is a cellular process that seems to contribute to morphogenesis during embryo development, in addition to cell proliferation, migration, differentiation and programmed cell death, and has been observed in evolutionary distant organisms like mammals, amphibians and fish. Programmed cell senescence is a phenotype similar to stress-induced cellular senescence, characterized by the expression of cell cycle inhibitors such as CDKN1A/p21, increased activity of a lysosomal enzyme with beta-galactosidase activity (coined senescence-associated beta-galactosidase) and, most importantly, secretion of growth factors, interleukins, chemokines, metalloproteases, etc., collectively known as a senescent-associated secretory phenotype that instructs surrounding tissue. How wide is the distribution of programmed cell senescence during mouse development and its specific mechanisms to shape the embryo are still poorly understood. Here, we investigated whether markers of programmed cell senescence are found in the developing mouse spinal cord and notochord. We found discrete areas and developmental windows with high senescence-associated beta galactosidase in both spinal cord and notochord; expression of CDKN1A/p21 was documented in epithelial cells of the spinal cord and the notochord. Treatment of mice embryos developed ex-utero in the presence of the senolytic ABT-263 resulted in decrease senescence-associated beta-galactosidase activity and number of motoneurons. Our data suggest that several cell types undergo programmed cell senescence in developing spinal cord and notochord contributing to morphogenesis. Contribution to the Field StatementCellular senescence is a state in which cells no longer divide but have the remarkable ability to secrete signaling molecules that alter the tissue where they reside. In adults, this state is typically induced by stress situations that cause DNA damage so cells with altered genome do not multiply. Senescent cells also form when a tissue is injured; they help to regenerate damaged tissue and contribute to wound healing. Phagocytic cells eliminate them when their function is done, having a transient existence. During vertebrate development some cells acquire a very similar phenotype, coined programmed cell senescence, and interestingly they have been found in regions that organize the pattern of development of some organs. How wide is the distribution of programmed cell senescence during development and how they help to shape the embryo are still poorly understood. We discovered in mice embryos different types of cells with senescent features located in particular regions of the developing nervous system: where motoneurons form and in a region that secrete molecules that instruct the embryo where different types of neurons will be created. We propose that programed cell senescence contributes to the morphogenesis of the nervous system.