Path 9: Stem Cell Exhaustion

Rationale: Running on empty: rejuvenating the body’s repair capacity. Stem cells – the progenitors that replenish tissues – decline in function and number with age. Tissues like blood, muscle, bone, and the immune system rely on stem cells, and in older organisms these cells become quiescent, less effective, or die off. Protect, rejuvenate, or replace these stem cells, and tissues could retain youthful regenerative capacity.

Prerequisites: Developmental biology, stem cell biology, regenerative medicine.

Dependencies: Connects with Path 8 (senescence) since accumulation of senescent cells in niches can impair stem cells. Ties to Path 4 (epigenetics) because stem cell aging involves epigenetic changes.

Signs of Progress: Stem-cell based therapies that demonstrably reverse aging in an organ. Drugs coaxing old stem cells to act young again. Safer iPSC techniques for autologous young stem cell transplantation. Maintenance of telomeres in stem cells to prevent exhaustion without cancer risk.

BC9.1: Adult Stem Cells and Their Niches

Scope: Major stem cells relevant to aging: HSCs, muscle satellite cells, intestinal stem cells, neural stem cells, MSCs. Concept of niche.

• Rando, Thomas A. “Stem cells, ageing and the quest for immortality.” Nature, 2006. – How stem cells age and whether that limits organism aging.
• Jones, D.L. & Rando, T.A. “Emerging models and paradigms for stem cell ageing.” Nat. Cell Biol., 2011. – Intrinsic vs extrinsic changes in stem cell aging.
• Yousef, H. et al. “Aging and rejuvenation of the bone-marrow microenvironment.” Dev. Cell, 2015. – Concrete examples of niche changes with age.

BC9.2: Evidence of Stem Cell Decline with Age

Scope: HSCs biased output, satellite cell quiescence, neural stem cell decline, thymic involution.

• López-Otín, C. et al. “Stem cell aging and age-related pathology.” Cell Stem Cell, 2023. – How aged stem cells contribute to disease.
• Chang, J. et al. “Individual muscle stem cells exhibit heterogeneity altered with age.” Stem Cell Reports, 2016. – Not all stem cells age equally.
• Beerman, I. & Rossi, D.J. “Hematopoietic stem cells and the aging hematopoietic system.” Curr. Opin. Immunol., 2015. – Aging changes blood-forming stem cells and immune consequences.

BC9.3: Systemic Factors and Rejuvenation

Scope: Heterochronic parabiosis experiments. Candidate factors: GDF11, Oxytocin, young CSF.

• Conboy, Irina M. & Michael J. “Bloodborne rejuvenation: young blood for old bodies?” Cell, 2010. – Discussing 2005 parabiosis results.
• Rebo, J. et al. “A single heterochronic blood exchange reveals rapid inhibition of multiple tissues by old blood.” Nat. Commun., 2016. – Old blood quickly induces inflammatory and aging markers.
• Castello, P.R. & Browns, R. “Parabiosis and systemic factors: rejuvenation of the aging brain.” Gerontology, 2021. – Listed factors and recent breakthroughs.

BC9.4: Stem Cell Therapies and Regenerative Medicine

Scope: Stem cell transplants, boosting endogenous stem cells, reprogramming old stem cells, gene therapy, thymus regeneration (TRIIM trial).

• Fahy, G.M. et al. “Reversal of epigenetic aging and immunosenescent trends in humans.” Aging Cell, 2019. – TRIIM trial: GH/DHEA/metformin regenerated thymus.
• Katsimpardi, L. et al. “Systemic GDF11 stimulates secretion of adiponectin.” Aging Cell, 2014. – Single factor therapy attempt.
• Borisov, V. & Shenson, J. “Clinical trials of stem cell therapy for age-related frailty.” J. Gerontol. A, 2020. – Human studies using stem cells to treat general aging frailty.

Bibliography (Path 9)

• Rando, Thomas A. & Howard Y. Chang. “Aging, Rejuvenation, and Epigenetic Reprogramming.” Cell 148(1-2): 46–57, 2012. (Systemic and intrinsic changes causing stem cell aging)
• Oh, Jiankun et al. “Decline of the Muscular Niche in Aged Regeneration-Limited Salamanders.” Aging Cell 12(4): 633–642, 2013. (Niche environment deterioration in aged salamanders)
• Schultz, Michael B. & David A. Sinclair. “When Stem Cells Grow Old.” Development 143(1): 3–14, 2016. (How tissue-specific stem cells age)
• Conboy, Irina M. et al. “Heterochronic Parabiosis.” Aging Cell 12(3): 525–530, 2013. (Guide to parabiosis experiments and interpretations)
• Rebo, Jamie et al. “A Single Heterochronic Blood Exchange.” Nat. Commun. 7: 13363, 2016. (Old blood factors quickly induce aging in young mice)