Test of the immortal strand hypothesis: label-dilution in adult mice as evidenced with the NanoSIMS indicates random segregation of DNA strands and permits rejecting this hypothesis.
In this NATURE article the authors test the ‘immortal strand hypothesis’, which predicts that during asymmetric stem cell division chromosomes containing older template DNA are segregated to the daughter destined to remain a stem cell, thus insuring lifetime genetic stability. After labelling mice with
15N-thymidine from gestation until post-natal week 8, the authors found no 15N label retention by dividing small intestinal crypt cells after a four-week chase. In adult mice administered
15N-thymidine pulse-chase, they found that proliferating crypt cells dilute the
15N label, consistent with random strand segregation.
Top image:
a, One of the numerous protocols of the article:
15N-thymidine administered for 2 weeks to adult mice then BrdU for 24 h before euthanasia.
b, analysis after protocol a): parallel imaging of
14N (proteins),
31P,
32S,
81Br (from BrdU, signing dividing cells), and
15N/
14N ratio (labeled DNA strands, with natural level in dark blue).
THE DIVIDING CELLS (BRDU1) DILUTE
15N-THYMIDINE LABEL (SMALL ARROWS) RELATIVE TO UNDIVIDED CELLS (BRDU2) (LARGE ARROWS).
Note two CBC cells with elongated nuclei at the crypt base. Scale bar, 10 µm.
Below:
c, Divided crypt cells (BrdU1), residing in CBC or 14–10 positions, demonstrated 15N DILUTION CONSISTENT WITH ONE OR TWO ROUNDS OF DIVISION DURING THE CHASE.
d, Mitotic crypt cell. Segregating chromosomes are visible in
14N and
31P images.
15N-label and BrdU were measured in both segregating chromosomal complements consistent with symmetric chromosomal segregation. Scale bar, 2 µm.
From: Multi-isotope imaging mass spectrometry quantifies stem cell division and metabolism. Matthew L. Steinhauser, Andrew P. Bailey, Samuel E. Senyo, Christelle Guillermier, Todd S. Perlstein, Alex P. Gould, Richard T. Lee & Claude P. Lechene. NATURE, vol 481, 26 January 2012