User:Olivialiautaud/sandbox

SPREAD (Spatial Propagation of Radial ERK Activity Distribution)
ERK is a highly conserved phosphorylation pathway that activates the ERK enzyme, which contains many substrates including transcription factors and cytoskeleton regulators. ERK catalyzes phosphorylation of proteins in many signal cascades that control a large variety of cellular functions and processes including proliferation and differentiation. (6)

Kinetics of signaling of molecules need to be tightly controlled because small changes in the kinetics can create significant changes downstream for cell structure and function. In absence of abnormalities or mutations, the sensitivity of kinetic changes in molecular signaling pathways are beneficial, particularly in complex biology systems, as it directs the cell towards the appropriate outcome. (check) Effective changes in the kinetics of ERK signaling include effect of duration of sustained signal on cell function of pulse frequency on gene expression via transcription factors activation frequencies.

EGFR (extracellular growth factor receptor) induces ERK in a stochastic manner (Albeck et al.). Precusors of EGFR cognate ligands, heparin EGF and TFN-a, are expressed on the plasma membrane and are released through matrix metalloproteinases-mediated cleavage generating EGF. This mechanism contributes to the strict regulation of ERK MAPK signaling that is required for homeostasis in the epidermis, as well as in other tissues. (4) Activated EGFR induces Ras; basal Ras activity is a prerequisite for stochastic pulses of ERK activity, though Ras itself just recruits and activates Raf and is not. Activated EGFR also activates Rac1, a Rho-family GTPase, which is involved in E-cadherin-mediated cell proliferation and specifically, activates Raf as well. XX and al. found that the Raf protein generates or regenerates, if activated from propagation, the pulse, that may have been lost upstream. Low doses (0.1 mM) of SB590885, a chemical inhibitor of Raf, leads to upregulation of ERK activity. At the high end of SB590885 doses (1 mM), ERK activity is suppressed. Raf shows biphasic dose response in ERK activity and is the pulse generator between propagated signal receiving and ERK activation. ERK activity follows a biphasic model; switch like ERK activation is derived from positive feedback loop of EGFR-ERK signaling, which propels into cells for downstream signaling pathways. (1)

Transient activation of extracellular-signal-regulated kinase generates specific patterns of cell fates in developing tissues. Changes in the parameters of these essential transients, although not yet fully known and understood, can result in abnormalities in developing tissues. (8)

-      What is SPREAD?

ERK activity spatial propagation relays one cell’s ERK activation to a nearby cell to activate its own ERK pathway, that will also send out ERK activation signals to its neighbor cells.

EGRK-ERK signaling for cell migration, differentiation and proliferation follows switch-like ERK activation, not a graded type cell processes regulation. (2) This stochastic firing of signal is seen in the subsequent ERK activity propagation patterns, but any propagation pattern of ERK activity requires MPP-mediated cleavage of pro-EFGR ligands.

ERK activity pulses occur then from either stochastic, non-ordered, firing or from lateral propagation of ERK activity signal from adjacent cell. Lateral propagation of these signals is distributed radially; cross-correlation analyses indicate a delay of 3-6 minutes for the time course of ERK activity propagation. (1) Importantly, in SPREAD, activation of ERK in the cell receiving propagated signal is always transient regardless of the duration and intensity of the ERK activity in the propagating cell. (1)

Cell Density in ERK activity pulses and SPREAD

Cell density affects the frequency of pulses. As cell density increases, basal ERK activity steadily decreases (5). However, plotting ERK activity pulse frequency as a function of cell density gives a bell-shaped graph, with peak frequency at intermediate cell density, where the majority of cells are in contact with one or two other cells. Intermediate cell density showed propagation of signals in a radially distributing-like manner (lower and higher cell density did not exhibit this behavior). This density also correlated with peak cell proliferation observed. At low cellular density, where most cells are in little to zero contact with another cell, cell replication rate was slow. At above-intermediate cell density, cells approach confluence and contact inhibition of cell growth occurs, indicating other types of cell-cell communication to maintain cell population integrity and health. Research on the fundamental mechanisms and parameters of SPREAD, particularly in vitro, have been largely worked with cells at this intermediate density. This correlation between cell density and ERK activity pulse frequency in capacity of cell to propagate and in its effect on upregulation or downregulation of cell proliferation rates has encouraged new ideas for potential therapeutic approaches to solving different cell division defects and anomalies in humans.

A proposed mechanism for spatial propagation of ERK activity:

Stochastic activation of ERK promotes activation of ADAMs. This releases EGF proteins, enabling them to freely interact with EGF receptors on adjacent cells, thereby initiating the signal cascade that will ultimately reproduce ERK activity pulse in this second cell. Because the original ERK activity pulse is stochastic, the release of EGF proteins, their activation of EGFR on the adjacent cell and downstream signaling in EGFR-ERK pathway should also exhibit stochastic behavior. However, stochastic activation of Ras, upstream of Raf, does not occur, contradicting this cell-cell propagation model. Potential explanations in support of this proposed mechanism include the PKC and PI3K-activated PAK that can activate Raf (phosphorylation) in absence of Ras or that level of Ras activation in the cell-cell interface region only is too low for FRET detection.

Differences of ERK activity pulse propagation in vivo and in vitro:

-      IN VITRO:

1.    Amplitudes of ERK activation are relatively constant during propagation

2.    ERK activity propagation to neighboring cells is discrete and random

3.    SPREAD emerges from many cells

-      IN VIVO (SPREADS):

1.    Amplitudes of ERK activation decay as propagation progresses

2.    ERK activity propagation is continuous

3.    SPREAD emerges from a small fraction of cells

Role of SPREAD in the cell cycle

SPREAD has an essential role in cell division. ERK-deficient cells in vitro showed cell cycle arrest in the G2-M transition, highlighting the importance of cell coordination in the propagation of activation signal and cell-cell communication in cell proliferation and differentiation. Other evidence to support the role of SPREAD in cell division is that in imaging propagating ERK activity pulses across tissue cells, SPREAD often emerges repeatedly in certain spots suggesting a potential correlation between follicular and interfollicular stem cells and SPREAD emergence.

Wound Healing

-      Discuss organ of skin being one of the four major organs of the body and its role as primary physical barrier to infection, diseases and invaders so want to repair damage to epidermis fast and properly (cells in the layers right under are the first to respond to any potentially harmful outsider)

It has been shown previously that ERK activity is discrete, and that in single cell analysis, a pulse of ERK activity will propagate to its neighboring cells, through spatial propagation. The distance between cells for activity propagation affects the success of signal transmission.

In wound healing, epithelial cell movements occur by cell sheet coordinated migration as a unit. Wounds induce ERK activity signal waves that propagates across the cell sheet during migration, causing force generation. Lesions in the epidermis initiate many wave-like signal cascades, one of which is ERK activation signaling. Injury to the skin induces two waves of ERK, the first is short-lived ERK activity pulse, and the second wave has longer duration and is sustained in the surrounding cells of the wound. Cells that have higher phosphorylated ERK (active) have higher motility, which shows in their elongated cell shape, and these are often detected at the edge of the wound. This correlates to Hiratsuka’s results, as the cells on the boundary of the wound first initiate a propagating ERK activity signal parallel to the wound margin to communicate to sub-marginal cells of the injury and the need for coordinated repair. These cells then re-propagate ERK activity pulses radially, which will directly or indirectly reach back to marginal cells. Around the open wound, ERK activity is high as migration of existing epithelial cells and reconstruction of the epidermis is still needed. In areas where two ends of a wound merge to close, ERK is deactivated through contact inhibition, demonstrating activated ERK is highly associated in migratory cells. (7)

Wound Healing in Diabetic Individuals

Adiponectin is an essential mediator in diabetes. Adiponectin is involved in cutaneous wound healing by enhancing proliferation and migration of keratinocytes, epidermis cells that express keratin and adiponectin receptors. AdipoR1/AdipoR2 and the ERK signaling pathway act as mediators of these adiponectin effector functions. Research has demonstrated adiponectin deficiency in vivo results in significant delay of wound closure and re-epithelialization of keratinocytes.

Through proliferative and migratory effects on keratinocytes, adiponectin promotes skin wound healing. However, these effects are only mediated through ERK activity and its initiation of signaling cascades inside the cell. Current research on therapeutic approaches in wound healing in individuals with diabetes and other autoimmune diseases or immunodeficiencies that do not repair skin lesions easily, focus heavily on adiponectin, whose function relies on ADIPO1/2 and ERK signaling. ERK pathways are required for keratinocyte proliferation and migration in wound healing in certain cells under high glucose conditions (a), and previous studies have also provided strong evidence for injury or stretching of keratinocytes being activators of ERK themselves (b,c). ERK activity propagation on the edge of the wound and its induction of SPREAD in the surrounding epidermal cells are critical mechanisms for wound healing in these particularly vulnerable individuals. (3)