New Site: Senescent Cells, Aging, & Senolytics

 From causes of aging to death from COVID-19

 COVID-19 is not deadly early in life, but mortality increases exponentially with age, which is the strongest predictor of mortality. Mortality is higher in men than in women, because men age faster, and it is especially high in patients with age-related diseases, such as diabetes and hypertension, because these diseases are manifestations of aging and a measure of biological age. At its deepest level, aging (a program-like continuation of developmental growth) is driven by inappropriately high cellular functioning. The hyperfunction theory of quasi-programmed aging explains why COVID-19 vulnerability (lethality) is an age-dependent syndrome, linking it to other age-related diseases. It also explains inflammaging and immunosenescence, hyperinflammation, hyperthrombosis, and cytokine storms, all of which are associated with COVID-19 vulnerability. Anti-aging interventions, such as rapamycin, may slow aging and age-related diseases, potentially decreasing COVID-19 vulnerability. 

Lewis-McDougall, Kirkland, 2019.

D+Q animal study.

"Like other organs, the adult mammalian hear has the capacity, albeit low, to self-renew cardiomyocytes over the human lifespan."

The self-renewing cells are called "multipotent cardiac stem cells" and progenitor cells" hereafter abbreviated CPCs. About 1% of heart cells are CPCs.

Cardiac aging and pathology affects the activity and potency of CPCs which translates into diminished capacity of the aging and diseased myocardium to repair and regenerate. 

In the old and failing heart there is the accumulation of senescent CPCs. Senescent CPCs can't divide and have very diminished self-renewal, differentiation and regenerative potential. Furthermore, senescent CPCs have a SASP that negatively affects non-senescent cycling-competent CPCs, rendering them senescent.

This study is about effect of eliminating senescent CPCs and restoring capacity to remaining CPCs.

1. CPCs exhibit a senescent phenotype with increased age. 

Fig 1: "Over half of CPCs in the aged human heart are senescent."

2. CPCs from old subjects show impaired cell growth and differentiation. Only about half of CPCs old myocardium are senescent. However, non-senescent indistinguishable from young subjects.

3. Aged-senescent CPCs lose their regenerative capacity in vivo.

4. Aged-senescent CPCs have a senescent associated secretory pheotype (SASP).

5. Elimination of senescent CPCs using senolytic drugs abrogates the SASP in vitro.

Study showed Dasatinib effectively cleared CPCs, whereas Fisetin was less effective.

6. Elimination of senescent cells in vivo activates resident CPCs and increases number of small, proliferating cardiomyocytes in the aged heart."

To determine effect:

D+Q treatment administered in 4 cycles for 3 consequtive days, 12 days apart, to 24-32 month mice.

D+Q increased numbers CPCs.

D+Q treated had small newly formed proliferating myocytes in contrast to non-treated controls only old hypertrophic cells.

D+Q mice had decrease in fibrosis in left ventricle. 

No D+Q effect in young mice.

"These finding show that clearance of senescent cells leads to stimulation of CPCs and cardiomyocytes with increased DNA-synthesizing activity and that this strategy is specific to the aged heart."

"Our study shows that CPCs isolated from the failing human heart develop a senescent phenotype...Aged human hearts with dilated cardiomyopathy showed greater numbers of senescent cells.

Our finding demonstrate that CPCs accumulate in the failing hearts of elderly subject (>70 years) and are dysfunctional, compared to CPCs isolated from the hearts of middle-aged (32-66 years) subjects. 

As the adult heart possesses very low numbers (<1%) of CPCs, furthermore by 80 years of age >50% of resident CPCs are senescent.

The presence of SASP in the aged and failing heart impairs the function of the remaining nonsenescent CPCs.

Study showed D+Q increased numbers of immature and newly formed myocytes.

The frequency of resident cardiac stem and progenitor cells in healthy myocardium including humans is approximately one per every 1000-2,000 myocytes depending on age. We detected a 23-foldi ncrease in the number of CPCs following elimination of senescent cells with D+Q. Moreover 10% of CPCs after elimination of senescent cells were activated young cells showing a 9 fold increase wit D+Q. 

The number of DNA-synthesizing cardiomyocytes in young mous is 0.12% and in old 22 months old was 0.07% oftotal cardiomyocytes. Elimination of senescent cells led to triple the number in old hearts. 

Therefore, the present data represent a significant and physiologically relevant increase in cardiomyocyte DNA synthetic activity following clearance of senescent cells. These finding are consistent with the hypothesis that clearing senescent cells can beneficially alter stem and progenitor cell function across multiple tissues.

"In conclusion, the present work demonstrates that eliminating senescent cells may be useful for treating age-related cardiac deterioration and rejuvenating the regenerative capacity of the aged heart."

"Aging leads to increased cellular senescence and is associated with decreased potency of tissue-specific stem/progenitor cells. Here, we have done an extensive analysis of cardiac progenitor cells (CPCs) isolated from human subjects with cardiovascular disease, aged 32-86 years. In aged subjects (>70 years old), over half of CPCs are senescent and unble to replicate and restore cardiac function. SASP factors secreted by senescent CPCs render otherwise healthy CPCs to senescence. Elimination of senescnt CPCs using senolytics (D+Q) abrogates the SASP and its debilatative effects in vitro. Global elimination of senescent cells in aged mice using D+Q senolytics in vivoactivates resident CPCs and increased the numbers of new myocytes.

"Therapeutic approaches that eliminate senescent cells may alleviate cardiac deterioration with aging and restore the regenertive capacity of the heart."

This paper gives an extraordinary insight into the aging heart and aging in general. Putting aside specific cardiac disease such as ASHD, the heart must replace the work force of cardiomyocytes. CPCs form the new cells. If half the CPCs. don't work because they are senescent and these senescent cells render healthy CPCs senescent and non-functional, the heart will soon not have enough working cardiomyocytes and fail.

In medicine, this failure to rejuvenate new cells leading to organ failure is viewed as normal aging and not a disease. However, this paper shows that this process can be viewed as senescent cell disease; a treatable condition.

"Chronic senolytic treatment alleviates established vasomotor dysfunction in aged or atherosclerotic mice". Roos. Kirkland, 2016, Mayo Clinic, Rochester, MN.

Study to determine whether long-term senolytic treatment with D+Q improves vasomotor function, vascular stiffness, and intimal plaque size and composition in aged and hypercholesterolemic mice with established disease.

Standard mice, fed on regular chow, 24 months old; were then initiated on D+Q once monthly for 3 months.

In the aorta, D+Q markedly reduced senescent cells in both endothelial and smooth muscle layer.

D+Q improved vascular relaxation in response to acetycholine. Interpretation was improved NO signaling.

D+Q also reduced medial vascular calcification in aorta and protein levels of osterix (early bone).

ApoE-/- mice were fed a Western atherogenic diet for 4 months to allow development of established atherosclerosis.

Mice then fed D+Q once weekly for next two months. 

Senescent cells were present in media and intimal plaques  of aorta from control mice. 

In D+Q treated mice, senescent cells burden reduced in medial segments of vessels; but not from established intimal atherosclerotic plaques. 

D+Q modestly improved vascular relaxation to acetycholine, markedly improved relaxation to sodium nitroprusside and had markers of improved NO signaling.

In accordance with no reduction in senescent cell burden in intimalplaque; there was no reduction in size of plaque or lipid composition.

There was significant reduction of intimal calcification by D+Q. No change in intimal plaque fibrosis.

Calcification and osterix (bone) in media was reduced.

The interpretation was there was limited plaque penetration by D+Q. However, significant reduction in plaque calcification was unusual as change in calcification very refractory to standard treatment.

"Collectively, this study shows that chronic  pharmacological clearance of senescent cells alleviates vasomotor dysfunction in naturally aging mice and mice with established aherosclerosis. Furthermore, senescent cell clearance reduces markers ofosteogenesis in advanced intimal plaques, ultimately reducing intimal plaque calcification. Based on these finding, we conclude that senescent cell clearance may be an effective complemantary therapy to classical risk factor management to reduce morbidity and mortality with age-related cardiovascular disease." 

Looking at these 3 papers together; long-term D+Q looks like an excellent addition to management of cardiovascular disease. 

"Senolytic therapy alleviates AB-associated oligodendrocytes progenitor cell senescence and cognitive deficits in an Alzheimer's disease model." Zhang, 2019.

Paper identifies an Oligodendrocyte progenitor glial cells that becomes senescent in presence of beta-amyloid deposits in AD. These cells then drive AD disease pathology. These destruction with D+Q ameliorates AD pathology and cognitive deficits. 

The glial cells in the brain are the oligodendrocytes, astrocytes and microglia. The oligodrodrocytes arise from oligodendrocyte progenitor cells (OPCs). OPCs are a major population of cells in the brain which are mobilized in response to neuronal injury and demyelination and then proliferate into mature oligodrodrocytes to aid in repair. However, direct exposure of OPCs to beta-amyloid  (AB) causes them to become senescent cells. Then their SASP profile drives AD.

7.5 month olf transgenetic mice were administer D+Q daily for 9 days. Short-term treatment removed the senescent OPC cells and reduced the proinflammatory cytokine activity and reduced microglia activation.

Long-term treatment was D+Q was administered once a week for 11 weeks beginning at 3.5 month old transgenic mice. At both 6 and 11 weeks the D+Q treated mice performed significantly better in the hippocampus-dependent learning and memory tests.

There was a significant reduction in the AB-plaque load, plaque size, proinflammatory cytokines and great reduction in senescent cells. 

Cellular senescence has been most extensively studied in peripharal tissues, where cell turnover (proliferation,differentiation and cell death is robust. In such tissues, senescent cells accumulate during normal aging and their SASP contributes to age-related disease. In contrast to proliferative tissues, the major cell population in the brain are either postmitotic (neurons) or quiescent (astrocytes, oligodendrocytes and microglia). OPCs are the major proliferative cell in most brain regions. In response to ischemic and traumatic brain injuries, OPCs migrate into damage tissues where they can differentiate into oligodendrocytes and remyelinate damages axons. 

The data here show that OPCs undergo senescence in the AB plaque environment and then contribute to local inflammation and damage to healthy neurons. 

The finding suggest following steps:

1. Senescent cells that express OPC marker proteins are present in the AB plaque environment in the brins of patients with AD and the AD mouse model.

2. In AB plaques, OPCs become senescent with a proinflammatory phenotype.

3. Aggregating AB can act directly on OPCs to induce cellular senescence.

"Senolytic treatment of AD mice with D+Q selectively removed senescent cells from the plaque environment, reduced neuroinflammation, lessened AB load, and ameliorated cognitive deficits. Our finding suggest a role for AB-induced OPC cell senescence in neuroinflammation and cognitive deficits in AD, and a potential therapeutic benefit of senolytic treatments" with D+Q.

Cell senescence appears to be the missing link explaining the cascade of events in which AB accumulation causes accelerated cognitive impairment in AD.

Ogrodnik, Kirkland, 2019. 

D+Q senolytic study

Obesity drives senescence in glial cells in Lateral ventricles of mouse brain.

Senescent cells Impair Neurogenesis and drives Anxiety.

Clearance of senescent cells with D+Q restores neurogenesis and ameliorates anxiety.

"Obesity is associated with increased senescent cell burden and neuropsychiatric disorders, including anxiety and depression. To investigate role of senescent cells we used D+Q to eliminate senescent cells. We found that obesity results in the accumulation of senescent cells in proximity to the lateral ventricles, a region in which adult neurogenesis occurs...Clearing senescent cells in obese mice restored neurogenesis and alleviated anxiety-related behavior. Our study provides proof-of-concept evidence that senescent cells are major contributors to obesity-induced anxiety and that senolytics (D+Q) are a potential new therapeutic avenue for treating neuropsychiatric disorders."

1. Obese Mice show Increased Anxiety-like Behavior Not related to Body Mass.

2. Clearance of Senescent Cells with D+Q Alleviates Obesity related Behavioral Changes.

3. Clearance senescent Cells with D+Q reduces Circulating Cytokine levels

4. Senolytic Treatment with D+Q Reduces Senescent cells in the Amygdala and Hypothalmus, but not in Other Regions of the Brain.

5. Impaired Neurogenesis in Obese mice is rescued by Clearance of Senescent Cells with D+Q. 

The following is my analysis of this very interesting study. For readers who want the analysis by the authors of the study; they can download the study.

1. As shown in following study, obesity causes metabolic dysfunction and insulin resistance.

2. Insulin resistance causes high insulin, glucose intolerance which results in high mTOR1. 

3. High insulin, high mTOR, high glucose causes development of senescents cells in amygdala. Amygdala is emotion center and dysfunction can result in anxiety.

4. High Insulin, high mTOR causes development of senescent glial cells in the proximity of lateral ventricles an area in which ADULT  Neurogenesis occurs.

5. D+Q cross the blood brain barrier and cleared senescent cells which ameliorated anxiety from Amygdala dysfunction due to senescent cells and restored neurogenesis.

Conclusion by the authors of study: Anxiety, depression, suicide is associated with obesity. "Here, we show that cellular senescence provides a potential explanation for increased anxiety in obese individuals and that targeting senescence cells holds promise as a therapeutic strategy."

Farr, James Kirkland, 2017.

This study investigates the role of senescent cells in age-related bone loss by multiple approaches. 

The study includes the use of dasatinib and quercetin to prevent bone loss in an approach applicable to humans. 

Age-related osteoporosis resulting in pathological fractures with minimal trauma is an enormous public health problem in the aging population. In addition to the morbidity associated with fractures, up to 25% of the elderly hip fractures patients die within a year of this devastating event.

Senescent osteocytes cause the following effects:

1. Senescent osteocytes accumulate in old age 

2. Senescent osteocytes secrete SASP.

3. SASP results in the activation and increase in number of osteoclasts which destroy bone.

4. SASP results in the decrease in activity of osteoblasts which make new bone.

5. The net effect of senescent osteocytes is loss of trabecular bone and thinning of cortical bone resulting in fractures with minimal trauma.

After 18 months of age mice have a marked accumulation of senescent osteocytes and develop osteoporosis.

In This study D+Q was administered orally to 20-month old mice once monthly for 4 months. At the end of 4 month treatment D+Q results in decrease in osteoclast numbers, increase in osteoblasts numbers, increase in bone formation, higher femur cortical thickness, increase trabecular thickness, better bone microarchitecture, increased bone strength. 

"Senolytic (D+Q) reduce senescent cells, which in turn suppress bone resorption with either an increase in cortical bone or maintenance of trabecular bone formation"... "Our study provides a critical proof-of-concept for the feasibility of this approach for treatment of age-related bone loss."