Highlights d Dedicated genes govern paligenosis, a conserved cellular regeneration program d DDIT4 first blocks mTORC1, inducing massive autophagy to downscale the cell d p53 activation continues mTORC1 suppression to maintain cell quiescence d IFRD1 suppresses p53 to reinduce mTORC1 and license progression into the cell cycle

BackgroundBrush border microvilli are ∼1-µm long finger-like projections emanating from the apical surfaces of certain, specialized absorptive epithelial cells. A highly symmetric hexagonal array of thousands of these uniformly sized structures form the brush border, which in addition to aiding in nutrient absorption also defends the large surface area against pathogens. Here, we present a molecular model of the protein cytoskeleton responsible for this dramatic cellular morphology.Methodology/Principal FindingsThe model is constructed from published crystallographic and microscopic structures reported by several groups over the last 30+ years. Our efforts resulted in a single, unique, self-consistent arrangement of actin, fimbrin, villin, brush border myosin (Myo1A), calmodulin, and brush border spectrin. The central actin core bundle that supports the microvillus is nearly saturated with fimbrin and villin cross-linkers and has a density similar to that found in protein crystals. The proposed model accounts for all major proteinaceous components, reproduces the experimentally determined stoichiometry, and is consistent with the size and morphology of the biological brush border membrane.Conclusions/SignificanceThe model presented here will serve as a structural framework to explain many of the dynamic cellular processes occurring over several time scales, such as protein diffusion, association, and turnover, lipid raft sorting, membrane deformation, cytoskeletal-membrane interactions, and even effacement of the brush border by invading pathogens. In addition, this model provides a structural basis for evaluating the equilibrium processes that result in the uniform size and structure of the highly dynamic microvilli.

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The primary, secondary, and tertiary structures of spectrin are reasonably well defined, but the structural basis for the known dramatic molecular shape change, whereby the molecular length can increase three-fold, is not understood. In this study, we combine previously reported biochemical and high-resolution crystallographic data with structural mass spectroscopy and electron microscopic data to derive a detailed, experimentally-supported quaternary structure of the spectrin heterotetramer. In addition to explaining spectrin’s physiological resting length of ~55-65 nm, our model provides a mechanism by which spectrin is able to undergo a seamless three-fold extension while remaining a linear filament, an experimentally observed property. According to the proposed model, spectrin’s quaternary structure and mechanism of extension is similar to a Chinese Finger Trap: at shorter molecular lengths spectrin is a hollow cylinder that extends by increasing the pitch of each spectrin repeat, which decreases the internal diameter. We validated our model with electron microscopy, which demonstrated that, as predicted, spectrin is hollow at its biological resting length of ~55-65 nm. The model is further supported by zero-length chemical crosslink data indicative of an approximately 90 degree bend between adjacent spectrin repeats. The domain-domain interactions in our model are entirely consistent with those present in the prototypical linear antiparallel heterotetramer as well as recently reported inter-strand chemical crosslinks. The model is consistent with all known physical properties of spectrin, and upon full extension our Chinese Finger Trap Model reduces to the ~180-200 nm molecular model currently in common use.

Villin-type headpiece domains are compact motifs that have been used extensively as model systems for protein folding. Although the majority of headpiece domains bind actin, there are some that lack this activity. Here, we present the first NMR solution structure and 15 N-relaxation analysis of a villintype headpiece domain natively devoid of F-actin binding activity, that of supervillin headpiece (SVHP). The structure was found to be similar to other headpiece domains that bind F-actin. Our NMR analysis demonstrates that supervillin headpiece lacks a conformationally flexible region (Vloop), present in all other villin-type headpiece domains, and which is essential to the phosphoryl regulation of dematin headpiece. In comparing the electrostatic surface potential map of SVHP, to that of other villin-type headpiece domains with significant affinity for F-actin, we identified a positive surface potential conserved among headpiece domains that bind F-actin, but absent from SVHP. A single point mutation (L38K) in SVHP, which creates a similar positive surface potential, endowed SVHP with specific affinity for F-actin that is within an order of magnitude of the tightest binding headpiece domains. We propose that this effect is likely conferred by a specific buried saltbridge between headpiece and actin. As no high-resolution structural information exists for the villintype headpiece: F-actin complex, our results demonstrate that through positive mutagenesis, it is possible to design binding activity into homologous proteins without structural information of the counterpart's binding surface.

Our data represent the first report of a salivary component exerting specific antimicrobial activity against an enteric pathogen and suggest that histatin-5 and related peptides might be exploited for prophylactic and/or therapeutic uses. Numerous viruses, bacteria, and fungi traverse the oropharynx to cause disease, so there is considerable opportunity for various salivary components to neutralize these pathogens prior to arrival at their target organ. Identification of additional salivary components with unexpectedly broad antimicrobial spectra should be a priority.

PAGE 8317:The images of actin microfilaments shown in panels B-D of Fig. 4 were not correct. Specifically, panels B and C were mistakenly made from images of the tD mutant micrograph at different magnifications when panel B should have been prepared from an rD-core micrograph. The micrograph published as rD-S381E in the original figure was also prepared from the wrong micrograph. In the corrected figure, panel B has been replaced with a micrograph of rD-core that was made by rescanning the original micrographs, and panel D has been replaced with an image from the correct micrograph. These corrections do not change the interpretation of the results or the conclusions of this work.

Complex multicellular organisms have evolved specific mechanisms to replenish cells in homeostasis and during repair. Here, we discuss how emerging technologies (e.g., single-cell RNA sequencing) challenge the concept that tissue renewal is fueled by unidirectional differentiation from a resident stem cell. We now understand that cell plasticity, i.e., cells adaptively changing differentiation state or identity, is a central tissue renewal mechanism. For example, mature cells can access an evolutionarily conserved program (paligenosis) to reenter the cell cycle and regenerate damaged tissue. Most tissues lack dedicated stem cells and rely on plasticity to regenerate lost cells. Plasticity benefits multicellular organisms, yet it also carries risks. For one, when long-lived cells undergo paligenotic, cyclical proliferation and redifferentiation, they can accumulate and propagate acquired mutations that activate oncogenes and increase the potential for developing cancer. Lastly, we propose a new framework for classifying patterns of cell proliferation in homeostasis and regeneration, with stem cells representing just one of the diverse methods that adult tissues employ. Expected final online publication date for the Annual Review of Physiology, Volume 84 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.