Niche, Notch and Nudge

This post relates to the Stem Cell Supply Chain Breakdown theory of aging, and is about getting somatic stem cells in mature individuals to keep up their rate of differentiation with aging.  The central issue is how safely to nudge multipotent stem cells (Type B cells in my classification, like hematopoietic stem cells) which live in niches using the Notch signaling pathway so they continue to differentiate and provide a stream of new mature (Type D) cells, such as working blood or immune system cells.  I was stimulated to look into this topic by a long comment to the blog post The stem cell supply chain – closing the loop for very long lives posted by eric25001.  I suggest readers to review that comment as background before going further into this post.  The comment incidentally is a news item that should have been attributed to UCBerkeley News. “A study led by researchers at the University of California, Berkeley, has identified critical biochemical pathways linked to the aging of human muscle. By manipulating these pathways, the researchers were able to turn back the clock on old human muscle, restoring its ability to repair and rebuild itself.”

The grist of this post deals with both new research and a couple of complicated cell signal-transduction pathways that have been extensively studied for over 15 years now, known as Notch and MAPK.  I also discuss a new and interesting role of old friends – antioxidants.  I will start with rudimentary introductions to Notch and MAPK, although these are subjects that could fill up a 4-year graduate school program in biochemistry and molecular biology.

About Notch and MAPK

Notch is an ancient signaling pathway that has been inherited from primitive multi-cellular organisms and has to do with signaling between cells, such as when stem cells decide to differentiate.  Ever wonder how whole bunches of cells work together to generate new blood vessels or new nerve tissue?  Look into Notch. “Because Notch often acts in concert with other signaling pathways, it is able to regulate a diverse set of biological processes in a cell-context dependent manner(ref). “  Notch protein receptors (there are 4 different ones) sit on the surfaces of cells and communicate between adjacent cells via Notch ligands.   Ligand binding to a receptor alters the chemical conformation, that is the three dimensional shape of the receptor protein(ref).” Intracellular proteins transmit Notch signals into the cell’s nucleus where they can activate genes, including ones that initiate differentiation in stem cells. Notch signaling can play an important role in determining the morphology of organs.  For example see ref. Also Notch plays several important roles in stem and progenitor cell differentiation, particularly ones that maintain balance during development. “Notch signaling is a powerful means of turning adult CNS precursor cells into astrocytes(ref).”   “In the developing nervous system, the balance between proliferation and differentiation is critical to generate the appropriate numbers and types of neurons and glia. Notch signaling maintains the progenitor pool throughout this process(ref).”

MAPK/ERK is another very complicated signal transduction pathway way that couples intracellular responses to the binding of growth factors to cell surface receptors.  MAPK signaling is important for cell growth and differentiation, inflammation and apoptosis.  A diagram showing all the ways MAPK signaling can work would fill a large wall.  For example this diagram shows four different MAPK cascades.  Clicking on the individual bubbles in the diagram reveal more-detailed diagrams, showing cascades such as for growth, differentiation and inflammation.

Both Notch and MAPK signaling are deeply involved in embryogenesis and stem cell differentiation.  It is no surprise that there is crosstalk between the Notch and MAPK pathways.  For example, this report  states: “Here we show that Notch signaling activation in C2C12 cells suppresses the activity of p38 MAPK to inhibit myogenesis. Our results show that Notch specifically induces expression of MKP-1, a member of the dual-specificity MAPK phosphatase, which directly inactivates p38 to negatively regulate C2C12 myogenesis.” 

So much for background. 

Going back to the recent UCBerkeley news item, the idea of the research was to regenerate old muscle cells by using Notch and MAPK signaling to re-invigorate old pools of stem cells.  “The researchers further examined the response of the human muscle to biochemical signals. They learned from previous studies that adult muscle stem cells have a receptor called Notch, which triggers growth when activated. Those stem cells also have a receptor for the protein TGF-beta that, when excessively activated, sets off a chain reaction that ultimately inhibits a cell’s ability to divide. — The researchers said that aging in mice is associated in part with the progressive decline of Notch and increased levels of TGF-beta, ultimately blocking the stem cells’ capacity to effectively rebuild the body. — This study revealed that the same pathways are at play in human muscle, but also showed for the first time that mitogen-activated protein (MAP) kinase was an important positive regulator of Notch activity essential for human muscle repair, and that it was rendered inactive in old tissue. MAP kinase (MAPK) is familiar to developmental biologists since it is an important enzyme for organ formation in such diverse species as nematodes, fruit flies and mice. — For old human muscle, MAPK levels are low, so the Notch pathway is not activated and the stem cells no longer perform their muscle regeneration jobs properly, the researchers said. — In practical terms, we now know that to enhance regeneration of old human muscle and restore tissue health, we can either target the MAPK or the Notch pathways. The ultimate goal, of course, is to move this research toward clinical trials.”

The research related to MAPK and Notch described below suggests to me that this avenue of pursuit for invigorating old stem cells is likely to be very worthwhile but also very tricky.  Some studies relate to the hopefulness of the approach:The 2006 study report Notch signaling regulates stem cell numbers in vitro and in vivo states:  “In both murine somatic and human embryonic stem cells, these positive signals are opposed by a control mechanism that involves the p38 mitogen-activated protein kinase. (MAPK) Transient administration of Notch ligands to the brain of adult rats increases the numbers of newly generated precursor cells and improves motor skills after ischaemic injury. These data indicate that stem cell expansion in vitro and in vivo, two central goals of regenerative medicine, may be achieved by Notch ligands through a pathway that is fundamental to development and cancer.”No surprise,  Notch and MAPK signaling are also intimately involved in the development of cancers, and herein lay the rub.

A November 2009 (e-publication in advance) research report Emerging role of Notch signaling in epidermal differentiation and skin cancer states “Signaling mediated by the Notch receptor governs tissue development during embryonal organogenesis, while in adult tissues it contributes to maintenance of cellular differentiation, proliferation and apoptosis. In addition, control by the Notch pathway of stem cell self-renewal and multi-potency points to an expanding role of Notch signaling in the progression of solid tumors. — Notch signaling has a dual action (either as an oncogene or as a tumor suppressor), depending on the tumor cell type and the synchronous activation of other intracellular signaling mechanisms.” Notch signaling that promotes stem cell differentiation can also promote tumorgenesis.  Beware!

This June 2009 study Emerging role of Notch in stem cells and cancer relates a similar message: “The Notch signaling pathway is known to be responsible for maintaining a balance between cell proliferation and death and, as such, plays important roles in the formation of many types of human tumors. Recently, Notch signaling pathway has been shown to control stem cell self-renewal and multi-potency. As many cancers are thought to be developed from a number of cancer stem-like cells, which are also known to be linked with the acquisition of epithelial-mesenchymal transition (EMT); and thus suggesting an expanding role of Notch signaling in human tumor progression.”

A 2008 study relates to the role of Notch signaling in promoting the differentiation of cancer stem cells: Notch activation promotes cell proliferation and the formation of neural stem cell-like colonies in human glioma cells.  “We hypothesized that  Notch signaling might play roles in cancer stem cells and cancer cells with a stem cell phenotype. In this study, we accessed potential functions of the Notch pathway in the formation of cancer stem cells using human glioma. — These data suggest that Notch signaling promote the formation of cancer stem cell-like cells in human glioma.”

This 2008 report again emphasizes the dual role of Notch signaling, this time with respect to the brain, and points out an additional interesting fact:  Notch, neural stem cells, and brain tumors.  “During neocortical development, Notch signaling inhibits neuronal differentiation and maintains the neural stem/progenitor cell pool to permit successive waves of neurogenesis, which are followed by gliogenesis. In addition, recent evidence suggests that Notch signaling is not uniformly used among distinct proliferative neural cells types, with the canonical cascade functional in neural stem cells but attenuated in neurogenic progenitors. Although the role of Notch in neural development is increasingly well understood, it has recently become evident that Notch also has a role in brain tumor biology. Notch receptors are overexpressed in many different brain tumor types, and they may have an initiating role in some. Stem-like cells in brain tumors share many similarities with neural stem/progenitor cells and may require Notch for their survival and growth.”   The additional fact is that Notch signaling can serve to inhibit rather than promote differentiation in the interest of maintaining healthy pools of stem cells.  Its action can be selectively to promote or inhibit stem cell differentiation.

The same point, that Notch activity is involved in both cell differentiation and tumorgenesis, appears in many other studies, for example Notch signaling at the crossroads of T cell development and leukemogenesis.

An interesting discovery for me was that Reactive oxygen species act through p38 MAPK to limit the lifespan of hematopoietic stem cells (HSCs)  but, as pointed out in the same report  “prolonged treatment with an antioxidant or an inhibitor of P38 MAPK extended the lifespan of HSCs from wild-type mice in serial transplantation experiments.”  These seem to be key points from an anti-aging viewpoint linking up the Oxidative Damage and the Stem Cell Supply Chain Breakdown  theories of aging and pointing to a different role for antioxidants than those discussed before.

The idea of inhibiting P38 MAPK via antioxidants to treat vascular smooth muscle cell hypertrophy goes back to 2001(ref).  Other studies have also implicated activated p38 MAPK in disease progression and suggest that its inhibition may represent a rational strategy for therapeutic intervention(ref)(ref).  However, the linkage of inhibiting p38 MAPK to health of stem cell pools appears to be fairly new. A 2004 in-vitro study of rabbit cells telegraphed the punch:  “We show that the inactivation of p38 kinase leads to the stimulation of proliferation, the extension of life span, and a delay in the onset of senescence, thus implying that p38 kinase limits the life span of rabbit articular chondrocytes in vitro(ref).”

For me, bottom-lines from the above are:

1.    Stimulating Notch signaling as a way of increasing stem cell differentiation in older folks is likely to turn out to be tricky for three reasons, first because the stimulation can also stimulate cancer stem cells and carcinogenesis, second because too much stimulation can cause exhaustion of stem cell pools and third and most basic: because the multipotent Type B cells in stem cell pools are subject to replicative senescence, a more basic approach to stem cell renewal is probably needed.  See my recent post The stem cell supply chain – closing the loop for very long lives.

2.    There appears to be another positive role for antioxidants beyond those commonly discussed: inhibiting p38 MAPK to help preserve the life spans of hematopoietic and possibly other multipotent stem cells and to assist in the prevention of a number of disease processes.