No abstract

Chromatin proteins are covalently modified by at least five different processes; in no case has the precise physiological function been established. One of these post-synthetic, covalent modifications is effected by the enzyme poly(ADP-ribose) polymerase, which uses the coenzyme NAD+ to ADP-ribosylate chromatin proteins. The modification consists largely of mono(ADP-ribose), but long, homopolymer chains of (ADP-ribose) are also present. Various physiological functions have been suggested for (ADP-ribose)n. Here we demonstrate that one function of (ADP-ribose)n is to participate in the cellular recovery from DNA damage. Specific inhibitors of poly(ADP-ribose) polymerase prevent rejoining of DNA strand breaks caused by dimethyl sulphate and cytotoxicity is enhanced thereby. The rejoining of strand breaks is prevented also by nutritionally depleting the cells of NAD.

We show that transplantation of adult bone marrow-derived cells expressing c-kit reduces hyperglycemia in mice with streptozotocin-induced pancreatic damage. Although quantitative analysis of the pancreas revealed a low frequency of donor insulin-positive cells, these cells were not present at the onset of blood glucose reduction. Instead, the majority of transplanted cells were localized to ductal and islet structures, and their presence was accompanied by a proliferation of recipient pancreatic cells that resulted in insulin production. The capacity of transplanted bone marrow-derived stem cells to initiate endogenous pancreatic tissue regeneration represents a previously unrecognized means by which these cells can contribute to the restoration of organ function.

The interferon-inducible, double-stranded RNA-dependent protein kinase PKR has been implicated in anti-viral, anti-tumor, and apoptotic responses. Others have attempted to examine the requirement of PKR in these roles by targeted disruption at the amino terminal-encoding region of the Pkr gene. By using a strategy that aims at disruption of the catalytic domain of PKR, we have generated mice that are genetically ablated for functional PKR. Similar to the other mouse model of Pkr disruption, we have observed no consequences of loss of PKR on tumor suppression. Anti-viral response to influenza and vaccinia also appeared to be normal in mice and in cells lacking PKR. Cytokine signaling in the type I interferon pathway is normal but may be compromised in the erythropoietin pathway in erythroid bone marrow precursors. Contrary to the aminoterminal targeted Pkr mouse, tumor necrosis factor ␣-induced apoptosis and the anti-viral apoptosis response to influenza is not impaired in catalytic domain-targeted Pkrnull cells. The observation of intact eukaryotic initiation factor-2␣ phosphorylation in these Pkr-null cells provides proof of rescue by another eukaryotic initiation factor-2␣ kinase(s).

Lipofuscin is membrane-bound cellular waste that can be neither degraded nor ejected from the cell but can only be diluted through cell division and subsequent growth. The fate of postmitotic cells is to accumulate lipofuscin, which as an "aging pigment" has been considered a reliable biomarker for the age of cells such as neurons and, by extension, their hosts. In the aging human brain, deposits of lipofuscin are not uniformly distributed but are concentrated in specific regions of functional interest. The prevailing thought is that the major source of lipofuscin is incomplete lysosomal degradation of damaged mitochondria. Accumulating evidence suggests that lipofuscin is not benign but can impair the functioning of seemingly unrelated cellular systems, including the ubiquitin/proteasome pathway. A damaging feedback loop of lysosomal and proteasomal inhibition may occur as lipofuscin accumulates, leading to what has been appropriately named a "garbage catastrophe." Reversing this catastrophe presents a formidable challenge.