Heterochromatin Structures Disperse as Somatic Cells Move to Pluripotency
University of Toronto ♦ Hospital for Sick Children ♦ Ontario Human iPS Cell Facility ♦ Sprott Centre for Stem Cell Research
Published in EMBO Journal, May 4, 2011
Cellular reprogramming of adult cells is achieved through the introduction of genetic factors that make widespread changes to the genome. A hallmark of this process is the remodeling of the epigenome which establishes repressive epigenetic marks at specific locations of genetic code. The presence of repressive marks prevents the transcription of DNA, silencing genes, while their absence leaves genes free for transcription and expression at the protein level. Reprogramming occurs in gradual steps, allowing for both partially and fully reprogrammed states. Partially reprogrammed cells are pseudo-stem cells, possessing some of the qualities of true stem cells, but not all. The exact nature of the structural changes that occur during reprogramming are largely unknown.
A recent study used spectroscopic imaging to identify the changes that occur in heterochromatin during remodeling of the epigenome. Researchers found that heterochromatin was densely packed in the centre of the chromosome in somatic cells and partially reprogrammed iPS cells. On the contrary, there were no clearly defined boundaries of heterochromatin in embryonic stem cells or fully reprogrammed cells. Instead, heterochromatin was structured irregularly into 10nm fibres dispersed across the chromosome. To determine whether chromatin reorganization was a characteristic of achieving the fully reprogrammed state researchers brought partially reprogrammed iPS cells to the fully reprogrammed state using a a cocktail of cell signaling inhibitors. The results showed that reorganization does indeed occur in the latter portion of the reprogramming process.
Enzymatic Modification of Glycoproteins in the Lab
University of Guelph ♦ Published in PNAS, May 3, 2011
Proteins can be powerful therapeutics for the treatment of human disease, however, their use is often hampered by processes within the body that reduce their half-lives and circulation time. Instability, break down by enzymes, neutralization by antibodies, and clearance from the bloodstream are known factors that contribute to reduced half-life. One method that can significantly increase the circulation time of a therapeutic protein, and its resulting effect on the body, is the addition of modifiers that make proteins just a little more rugged. Two such modifiers are polyethylene glycol (PEG), and polysialic acid (PSA).
PSA is biodegradable, and unlike PEG, it is non-immunogenic; the development of antibodies against PEGylated therapeutic proteins has raised some concerns in the scientific community. As a result, PSA is an attractive modifier for future therapies. Although PSA has been successfully added to proteins in previous studies by chemical means, this recent study from the University of Guelph marks the first time PSA has been added to specific sites on therapeutic proteins in vitro, using enzymes.
To accomplish this task researchers utilized two transferase enzymes, a -sialtransferase derived from Campylobacter jejuni and a -polysialtransferase derived from Neisseria meningitidis. In order to prove that the enzymatic approach works, two different human therapeutic proteins were subject to treatment: alpha-1-antitrypsin (A1AT, used to prevent uncontrolled tissue breakdown) and factor IX (used to treat hemophilia B). The experiment was a success and both proteins were modified as expected. Researchers took a closer look at A1AT, and the effect that polysialylation had on its activity and stability; their findings were positive. Polysialylation did not affect A1AT’s in vitro inhibition of human neutrophil elastase. After injecting modified A1AT into mice, its pharmacokinetic profile was significantly improved.