Progeria is one of the better known accelerated aging conditions. It isn't actually accelerated aging, but rather one specific runaway form of cell damage that gives rise to general dysfunction in cells throughout the body. Since degenerative aging is also a matter of general dysfunction in cells throughout the body, there is some overlap in the observed results, even though the root causes are completely different. So progeria patients appear, superficially at least, to be prematurely aged, and die from heart disease early in life.
The cause of progeria was discovered to be a mutation in the Lamin A (LMNA) gene, resulting in a malformed protein now called progerin. This protein is an important structural component of the cell nucleus. If it doesn't function correctly, the nucleus becomes misshapen, and near all processes involving nuclear DNA maintenance and gene expression - the production of needed proteins at the right time from their genetic blueprints - cease to work correctly. The cell becomes dysfunctional. When near all cells are in this state, the prognosis for the individual is dire. Interestingly, in the years since this discovery, it has become clear that progerin is also present in small amounts in genetically normal older individuals. There is some debate over whether or not this is important in the progression of aging. Does it cause enough damage, or is it insignificant in comparison to other harmful processes?
In this context, we can consider cellular senescence, a mechanism closely connected to DNA damage, by which a small number of problem cells can cause outsized amounts of harm. Another possibility is damage to stem cells, as they are also small in number but highly influential on tissue function. Cells become senescent in response to internal damage, including that produced by progerin, and then either self-destruct or linger to secrete a potent mix of inflammatory and other signals. It is these signals, the senescence-associated secretory phenotype, that allows a comparatively small number of cells to produce comparatively large problems. It is known that senescent cells are important in many age-related conditions, particular those with a strong inflammatory component. Is generation of progerin significant as a cause of cellular senescence in normal aging, however? In this open access paper, researchers consider some of the mechanisms involved.
The LMNA gene encodes lamin and lamin C, which are major components of the nuclear lamina. Mutations in the LMNA gene have been implicated in premature aging disorders, including Hutchinson-Gilford progeria syndrome (HGPS). HGPS is caused by splicing defect and consequent generation of progerin, mutant-truncated lamin protein. Cells of HGPS patients exhibit an abnormal nuclear structure, increased DNA damage and premature senescence. In addition to the effects of progerin, accumulation of prelamin A, precursor of lamin A, induces defects in nuclear structures. ZMPSTE24 is an enzyme that produces mature lamin by cleavage of amino acids in prelamin A.
Zmpste24 knock-out mice have been widely used to study the mechanisms of aging and progeria. Depletion of Zmpste24 causes premature senescence in mice, including decreases in life span and bone density. Increased prelamin expression caused by ZMPSTE24 deficiency causes defective DNA repair. Zmpste24 knock-out mice have been extensively studied because of their impaired DNA damage response (DDR). Lamin also functions as a structural barrier to DDR. Altogether, these findings indicate that defects in the nuclear structure induced by progerin or prelamin lead to the accumulation of DNA damage, which results in accelerated aging.
It has been reported that exogenous expression of progerin in human mesenchymal stem cells (hMSCs) can impair their differentiation potential. Furthermore, production of induced pluripotent stem cells (iPSCs) from HGPS patients has revealed that the progerin expression levels are the highest in MSCs, vascular smooth muscle cells, and fibroblasts. These results indicate that MSCs are a specific target cell type of progerin-induced senescence. Like progerin, excessive accumulation of prelamin induces premature senescence in MSCs, including wrinkled nuclei. Downregulation of ZMPSTE24 in hMSCs also induces the senescence phenotype. These investigations imply that both progerin and prelamin can induce senescence in hMSCs with change in nuclear morphology.
Senescent cells secrete a group of factors that induce senescence in neighboring cells, a phenomenon termed senescence-associated secretory phenotype (SASP). The secreted inflammatory factors propagate senescence and recruit immune cells to senescent tissues by the generation of a pro-inflammatory environment. Among the factors reported to regulate the SASP, GATA4 has been recently identified as a regulator of senescence and inflammation. GATA4 is expressed during oncogene- and irradiation-induced senescence in fibroblasts in response to DNA damage. During the process of cellular senescence, GATA4 has a regulatory role in the SASP of fibroblasts. Because GATA4-dependent cellular senescence is closely associated with DDR, the role of GATA4 in other senescence models and other cell types may reveal a new mechanism.
Senescent hMSCs also induce senescence in neighboring cells. Monocyte chemoattractant protein-1 (MCP-1) secreted from senescent human umbilical cord blood-derived mesenchymal stem cells (hUCB-MSCs) induces premature senescence in neighboring cells. Insulin-like growth factor binding proteins are also produced by senescent hMSCs, and they trigger senescence in adjacent normal cells. These studies investigated the mechanisms of the SASP by inducing senescence in hMSCs through prolonged passaging. However, cellular senescence of MSCs can be regulated by various factors other than passaging. In our previous report, we have demonstrated that depletion of ZMPSTE24 and introduction of progerin induce premature senescence in hUCB-MSCs. It remains to be determined whether defective lamin triggers paracrine senescence via inflammatory factors in hMSCs.
In this study, we identified that paracrine senescence is triggered in senescent hMSCs with abnormal nuclear structures by increasing the expression of MCP-1 and that inhibition of MCP-1 decreases the SASP. Furthermore, we found that GATA4 mediates the senescence of hMSCs induced by defective lamin A. We assessed whether down-regulation of GATA4 disturbs the progerin- or prelamin A-dependent senescence phenotype. Elucidating how GATA4 regulates senescence in hMSCs with nuclear defects may aid in understanding the etiology of complex aging disorders. We show that inhibition of GATA4 expression protects hMSCs from cellular senescence, implying unique therapeutic opportunity against progeroid syndromes and physiological aging.