Summary During development, wound repair and disease-related processes, such as cancer, normal, or neoplastic cell types traffic through the extracellular matrix (ECM), the complex composite of collagens, elastin, glycoproteins, proteoglycans, and glycosaminoglycans that dictate tissue architecture. Current evidence suggests that tissue-invasive processes may proceed by protease-dependent or protease-independent strategies whose selection is not only governed by the characteristics of the motile cell population, but also by the structural properties of the intervening ECM. Herein, we review the mechanisms by which ECM dimensionality, elasticity, crosslinking, and pore size impact patterns of cell invasion. This summary should prove useful when designing new experimental approaches for interrogating invasion programs as well as identifying potential cellular targets for next-generation therapeutics.

1,25-Dihydroxyvitamin D3 (D3) exerts its effects by binding to and activating nuclear vitamin D3 receptors (VDRs) that regulate transcription of target genes. We have investigated regulation of VDR levels in human skin in vivo and in cultured human keratinocytes. Quantitative ligand-binding analysis revealed that human skin expressed approximately 220 VDRs per cell, which bound D3 with high affinity [(dissociation constant (Kd) = 0.22 nM]. In human skin nuclear extracts, VDR exclusively bound to DNA containing vitamin D3 response elements as heterodimers with retinoid X receptors. Topical application of D3 to human skin elevated VDR protein levels 2-fold, as measured by both ligand-binding and DNA-binding assays. In contrast, the D3 analog calcipotriene had no effect on VDR levels. Topical D3 had no effect on VDR mRNA, indicating that D3 either stimulated synthesis and/or inhibited degradation of VDRs. To investigate this latter possibility, recombinant VDRs were incubated with skin lysates in the presence or absence of D3. The presence of D3 substantially protected VDRs against degradation by human skin lysates. VDR degradation was inhibited by proteasome inhibitors, but not lysosome or serine protease inhibitors. In cultured keratinocytes, D3 or proteasome inhibitors increased VDR protein without affecting VDR mRNA levels. In cells, VDR was ubiquitinated and this ubiquitination was inhibited by D3. Proteasome inhibitors in combination with D3 enhanced VDR-mediated gene expression, as measured by induction of vitamin D3 24-hydroxylase mRNA in cultured keratinocytes. Taken together, our findings indicate that low VDR levels are maintained, in part, through ubiquitin/proteasome-mediated degradation and that low VDR levels limit D3 signaling. D3 exerts dual positive influences on its nuclear receptor, simultaneously stimulating VDR transactivation activity and retarding VDR degradation.

An ability to delay aging or to reverse the negative effects of aging could prevent age-related disease and greatly enhance quality of life in old age. However, whether it is possible to globally reverse the physiological effects of aging in order to extend healthspan is unknown. The freshwater planarian Schmidtea mediterranea has been considered immortal due to its exceptional tissue regeneration capabilities. Here, we report that a sexually reproducing lineage of S. mediterranea exhibits age-associated physiological decline 12 months after birth. Age-associated changes include alterations in sensory organs, loss of neurons and muscle, loss of fertility, and impaired motility, but no reduction in stem cells at the age of 3 years. Differential gene expression analysis, comparing young and old planarian cells, furthermore revealed cell-type-specific changes in transcription as well as changes in classical aging pathways (e.g., insulin signaling). Remarkably, amputation followed by regeneration of lost tissues led to a global reversal of these age-associated changes. Older individuals that underwent regeneration showed restored youthful patterns of gene expression, stem cell states, tissue composition and rejuvenation of whole-animal physiology. Our work reveals a naturally evolved solution to age reversal in planaria that may provide insights into anti-aging strategies in humans.