Steven W. Sherwood

Uncoupling proteins (UCPs) are a family of mitochondrial transporter proteins that have been implicated in thermoregulatory heat production and maintenance of the basal metabolic rate. We have identified and partially characterized a novel member of the human uncoupling protein family, termed uncoupling protein-4 (UCP4). Protein sequence analyses showed that UCP4 is most related to UCP3 and possesses features characteristic of mitochondrial transporter proteins. Unlike other known UCPs, UCP4 transcripts are exclusively expressed in both fetal and adult brain tissues. UCP4 maps to human chromosome 6p11.2-q12. Consistent with its potential role as an uncoupling protein, UCP4 is localized to the mitochondria and its ectopic expression in mammalian cells reduces mitochondrial membrane potential. These findings suggest that UCP4 may be involved in thermoregulatory heat production and metabolism in the brain.

Cell growth and division require the doubling of cellular constituents followed by their equal distribution to the two daughter cells. Within a growing population, the ratio of mitochondrial to cellular volume is maintained, as is the number of mitochondrial genomes per cell. The mechanisms responsible for coordinating nuclear and mitochondrial DNA synthesis, and for balancing increases in cell and mitochondrial size are not well understood. In studies of the fission yeast Schizosaccharomyces pombe we quantified cellular and mitochondrial DNA content by both Southern blot analysis and flow cytometry of cells stained with a variety of DNA-binding fluorochromes, which we show are able to detect nuclear and mitochondrial DNA with different efficiencies. In the conditional cell division cycle mutant cdc10, which is unable to initiate nuclear DNA synthesis, we found that there was an increase in the mitochondrial DNA content in the absence of nuclear DNA replication. This demonstrates that mitochondrial and nuclear DNA synthesis are not obligately linked. We also show that mitochondrial DNA replication is not required for the increase in mitochondrial size that occurs as cells elongate, although this results in a decrease in the ratio of mitochondrial DNA to mitochondrial volume.

We propose that a fundamental problem in the faithful replication of complex chromosomes of higher eukaryotes is the proper control of both the number and timing of the multiple initiations of replication on single chromosomes. When replication patterns are disrupted by any of a variety of agents, overreplication of DNA can occur. We propose a model that accounts for the generation of a wide variety of chromosomal aberrations-rearrangements, resulting from the various ways in which the overreplicated strands can undergo recombination. We also discuss certain implications of the generation of chromosomal alterations in higher eukaryotes as they may relate to cancer chemotherapy, cancer progression, aging, and rapid speciation-evolution.

This paper reports that there are major differences between mammalian cell lines in the propensity to progress into subsequent cell cycles when mitosis is inhibited with agents that disrupt the assembly of the mitotic spindle apparatus (Colcemid, nocodazole, and taxol). Human HeLa S3 cells, which represent one extreme, remain arrested in mitosis, with elevated levels of cyclin B and p34cdc2 kinase activity. In Chinese hamster ovary cells, at the other extreme, the periodic rise and fall of cyclin B levels and p34cdc2 kinase activity is only transiently inhibited in the absence of mitosis. The cells progress into subsequent cell cycles, without dividing, resulting in serial doublings of cellular DNA content. In general, the propensity to progress into subsequent cell cycles in the absence of mitosis appears to be species related, such that human cell lines remain permanently blocked in a mitotic state, whereas rodent cell lines are only transiently inhibited when spindle assembly is disrupted. We interpret these results to indicate that in mammalian cell lines there exists a checkpoint which serves to couple cell cycle progression to the completion of certain karyokinetic events. Furthermore, either such a checkpoint exists in some cell lines but not in others or the stringency of the control mechanism varies among different cell lines.