Cellular Architecture from the Outside-In
By Kristin Lamont, Ph.D.
The ECM plays a dynamic role in intracellular biology
Intracellular organization intimately affects cellular homeostasis and function. The cytoskeleton provides supportive superstructure to maintain proper cell shape and serves as infrastructure for transport and compartmentalization within the cell. The nuclear lamina acts as both a scaffold for the organization of the chromatin and a barrier to protect the genetic material. Similarly, the extracellular matrix (ECM) is an extracellular molecular framework that mediates cell-cell or cell-matrix adhesion, communication, and motility within a tissue system. Defects in any of these molecular frameworks can disrupt basic cellular functions and lead to a number of pathologies. In a 2015 study published in The EMBO Journal, a team led by Dr. Carlos López-Otín at Universidad de Oviedo examined how a deficit in an ECM remodeling enzyme, MMP14 (MTI-MMP), alters the intracellular cytoskeleton and nuclear lamina, culminating in the induction of senescence. Treatment with retinoic acid reversed these effects. This study brings into focus the interconnectedness of these organizational structures, and may suggest targets for future therapies for pathologies whose underlying etiologies involve cellular or nuclear disorganization, such as premature aging, or progeria.
Aberrant ECM Remodeling Causes Cellular Senescence
Previous studies demonstrated that a deficit in an ECM matrix metalloproteinase leads to changes in intracellular cytoskeleton and nuclear morphology. In order to examine the effects of a more general inability to effectively maintain ECM homeostasis, the López-Otín team created an Mmp14-/- mutant mouse, which lacks MMP14 (MT1-MMP), a collagenolytic enzyme required for ECM maintenance. To generate this strain, the researchers inserted loxP sites to flank exons 4 and 5 in the Mmp14 gene, then bred the targeted mutant founder mice to B6.C-Tg(CMV-cre)1Cgn/J (006054) to generate germline knockouts. The resulting Mmp14-/- mice were then backcrossed to C57BL/6J (000664) for five generations.
Mmp14-/- mice are phenotypic dwarfs, have a number of bone abnormalities, and reduced lifespans. Deeper investigation revealed enlargement of airways and alveoli in the lungs, accumulation of collagen fibers in the skeletal muscle, 30% shorter muscle fibers, and thickened cardiac muscle compared to wild-type mice. In addition to these anatomical differences, the researchers discovered that fibroblasts derived from Mmp14-/- mice were less proliferative and showed increased expression of senescence markers HP1γ and p16INK4A in vitro. Further, nuclear lamin structure was disrupted in the Mmp14-/--derived fibroblasts: they have irregular, filamentous depositions of Lamin A rather than a uniform peripheral organization characteristic of wild-type fibroblasts. This disruption in nuclear architecture was associated with increased DNA damage, which likely contributed to senescent signaling and, possibly, the premature death observed in Mmp14-/- mice. Additionally, cytoskeletal actin and tubulin filaments were disorganized in the Mmp14-/- -derived fibroblasts. Together, these cytostructural abnormalities indicate that an inability to efficiently remodel the ECM has profound consequences on the internal organization of the nucleus and cytoplasm and thus the biology of the cell.
In a final set of experiments, the researchers demonstrated that treatment of Mmp14-/- fibroblasts with all-trans retinoic acid (a known inducer of MMP activity) reversed many of the phenotypes associated with the MMP14 knockout. These experiments may point toward potential treatment strategies for individuals affected by diseases or conditions caused by MMP deficiency, such as progeria.