Mois : novembre 2024

In Aging Cell, researchers have described the specific cell types that give exercise protective effects against motor nerve degeneration.

A different level of back problems

With aging, the prevalence of nerves connecting to muscle tissue dwindles, a phenomenon known as denervation [1]. This has been linked to a depopulation of the alpha motor neurons in the spinal cord [2]. Unless interventions are undertaken, the muscles themselves dwindle as a result [3], being gradually replaced by non-muscle fibrosis instead [4].

This fibrotic replacement is driven by meschenchymal fibroblasts [5] instead of the muscle fibroblasts that would replenish the normal tissue [6]. Fibroblasts have also been reported to secrete factors that encourage nerve regrowth [7], as have muscle stem cells [8].

While it is disputed, some work even suggests that, in middle age, human beings’ dwindling nerves are replaced by these natural processes [9] and that this results in more complex nerve centers than younger people have, particularly in people who exercise regularly [10]. While the accelerated decline caused by runaway aging processes leads to degradation without regeneration of the nervous tissue [11], intensive exercise has been found to efficiently protect against this [12].

The precise molecular mechanisms and cells involved, however, had not been fully described. This research focuses on muscle fibroblasts and stem cells in an effort to discover precisely what makes exercise such a powerful treatment for motor nerve degradation.

Cell type matters

In the first experiment, primary motor neurons were cultured from rat embryos, while muscle fibroblasts and stem cells were taken from human muscle biopsies. Despite the species difference, these cells were found to interact in a compatible way. Four of the human volunteers were young, four were old and sedentary, and six were old people who had exercised throughout their lives.

Muscle stem cells and fibroblasts were found to have starkly different gene expression profiles and stimulate the rat neurons in starkly different ways: a total of 11% of the neurons’ genes were expressed differently between the two groups, with roughly equal amounts of upregulation and downregulation. Considering the functions of these genes, culturing with fibroblasts seemed to encourage neural growth in a way that culturing with stem cells did not, including two genes that code for synapse transmission and a gene for the formation of new neurons (neurogenesis).

Culturing the rat neurons with conditioned media from these cell types, instead of directly, yielded somewhat similar results. Here, the researchers found that motor neurons are affected in complementary ways by each of these cell types, with fibroblasts still appearing to be more important for growth and development.

Lifelong exercisers have multiple advantages

The researchers then started comparing the cells of the different groups. Interestingly, they found that, while the young people were still better at exerting force than the older people and had slightly more lean mass, the older exercisers’ muscles appeared to be more efficient than younger people and older sedentary people. CAF, a key biomarker of neurological impairment, was not elevated in the older exercisers compred to the younger people, while older sedenary people had significantly elevated amounts.

Culturing the motor neurons with cells grown from older sedentary people had substantially negative effects: a full 53% more of the neurons survived after being cultured with cells derived from older exercisers versus their sedentary counterparts. The older exercisers’ cells even appeared to promote neuronal health more than those taken from younger people, although there was no statistically significant difference found with this small group.

This was a relatively small study, and it did not delve into the precise molecular mechanisms involved in this communication between muscle fibroblasts, muscle stem cells, and motor neurons. However, it has clearly identified the cell types involved. An analysis of such factors as extracellular vesicles and other intercellular communication will be necessary to translate these findings into a useful treatment.

Despite its limitations, however, this study makes it absolutely clear that exercise is necessary for long-term muscle health and defense against motor neuron loss with age. It is unknown when life-changing biological interventions will enter the clinic; right now, according to this and many other studies, exercise remains one of the best treatments that is broadly available.

Literature

[1] Soendenbroe, C., Andersen, J. L., & Mackey, A. L. (2021). Muscle-nerve communication and the molecular assessment of human skeletal muscle denervation with aging. American Journal of Physiology-Cell Physiology, 321(2), C317-C329.

[2] McNeil, C. J., Doherty, T. J., Stashuk, D. W., & Rice, C. L. (2005). Motor unit number estimates in the tibialis anterior muscle of young, old, and very old men. Muscle & Nerve: Official Journal of the American Association of Electrodiagnostic Medicine, 31(4), 461-467.

[3] McPhee, J. S., Cameron, J., Maden-Wilkinson, T., Piasecki, M., Yap, M. H., Jones, D. A., & Degens, H. (2018). The contributions of fiber atrophy, fiber loss, in situ specific force, and voluntary activation to weakness in sarcopenia. The Journals of Gerontology: Series A, 73(10), 1287-1294.

[4] Madaro, L., Passafaro, M., Sala, D., Etxaniz, U., Lugarini, F., Proietti, D., … & Puri, P. L. (2018). Denervation-activated STAT3–IL-6 signalling in fibro-adipogenic progenitors promotes myofibres atrophy and fibrosis. Nature cell biology, 20(8), 917-927.

[5] Rebolledo, D. L., González, D., Faundez-Contreras, J., Contreras, O., Vio, C. P., Murphy-Ullrich, J. E., … & Brandan, E. (2019). Denervation-induced skeletal muscle fibrosis is mediated by CTGF/CCN2 independently of TGF-β. Matrix Biology, 82, 20-37.

[6] Wosczyna, M. N., & Rando, T. A. (2018). A muscle stem cell support group: coordinated cellular responses in muscle regeneration. Developmental cell, 46(2), 135-143.

[7] Theret, M., Rossi, F. M., & Contreras, O. (2021). Evolving roles of muscle-resident fibro-adipogenic progenitors in health, regeneration, neuromuscular disorders, and aging. Frontiers in Physiology, 12, 673404.

[8] Liu, W., Klose, A., Forman, S., Paris, N. D., Wei-LaPierre, L., Cortes-Lopez, M., … & Chakkalakal, J. V. (2017). Loss of adult skeletal muscle stem cells drives age-related neuromuscular junction degeneration. Elife, 6, e26464.

[9] Deschenes, M. R. (2011). Motor unit and neuromuscular junction remodeling with aging. Curr Aging Sci 4 (3): 209–220.

[10] Jones, E. J., Piasecki, J., Ireland, A., Stashuk, D. W., Atherton, P. J., Phillips, B. E., … & Piasecki, M. (2021). Lifelong exercise is associated with more homogeneous motor unit potential features across deep and superficial areas of vastus lateralis. GeroScience, 1-11.

[11] Snow, L. M., Mcloon, L. K., & Thompson, L. V. (2005). Adult and developmental myosin heavy chain isoforms in soleus muscle of aging Fischer Brown Norway rat. The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology: An Official Publication of the American Association of Anatomists, 286(1), 866-873.

[12] Soendenbroe, C., Heisterberg, M. F., Schjerling, P., Kjaer, M., Andersen, J. L., & Mackey, A. L. (2022). Human skeletal muscle acetylcholine receptor gene expression in elderly males performing heavy resistance exercise. American Journal of Physiology-Cell Physiology, 323(1), C159-C169.

An analysis of data from over twenty thousand people has indicated that greater dietary diversity is associated with slower biological aging [1].

Your health is what you eat

Good dietary habits are linked to many health benefits, and different diets were previously reported to impact the speed of aging and senescence. For example, adherence to the Mediterranean diet is positively associated with increased lifespan and healthspan.

We have also previously reported on some health benefits linked to different dietary patterns, such as associations linking an anti-inflammatory diet and the Mediterranean diet with a reduced risk of dementia, the positive impact of a ketogenic diet on symptoms of multiple sclerosis, the impact of Mediterranean, keto, and plant-based diets on cancer risk and progression, and the metabolic benefits of a ketogenic diet and the Mediterranean diet in pre-diabetes and Type 2 diabetes patients.

The authors of this study did not focus on any specific diet; instead, they focused on the diversity of food consumed by the study participants. They discuss the impact of a diverse diet, which is rich in macronutrients, micronutrients, antioxidants, and bioactive compounds, on the speed of aging.

Biological age is not just a number

Compared to chronological age alone, the relationship between biological age and chronological age is a better estimate of health and the risk of developing age-related diseases. A higher biological age suggests a higher possibility of developing age-related diseases and a higher chance of dying.

The researchers analyzed data from 22,600 participants (49.3% male) with an average age of 48 years from the National Health and Nutrition Examination Survey (NHANES), a cross-sectional survey conducted in the United States. People under 20 years of age, pregnant, and those with no available food intake or biological age data were excluded from the analysis.

The researchers in this study used phenotypic age and Klemera–Doubal method (KDM) biological age to represent the biological age of study participants. Those measures are based on the composite clinical biomarkers.

They used systolic blood pressure, blood creatinine, urea nitrogen, albumin, total cholesterol, glycosylated hemoglobin A1c, percentage of lymphocytes, mean erythrocyte volume, leukocyte count, and alkaline phosphatase as biomarkers for their assessment.

The more diverse, the better

The researchers assessed the dietary diversity score (DDS), which was described as simple, effective, and validated in clinical trials. It measures the number of food groups in one’s diet, based on five major food groups and 18 subgroups. “A higher DDS is generally indicative of a more varied diet and is associated with a broader intake of essential nutrients.” Previous research had reported an association between a higher DDS and a lower risk of chronic diseases such as diabetes mellitus [2] and cardiovascular diseases (CVD) [3].

In the analyzed group, the researchers measured DDS based on the average score from two self-reported 24-hour dietary recalls.

Higher diversity, lower biological age

The researchers used a few models to analyze the data. In the first model, they didn’t include any confounding variables. The second and third models were adjusted for different factors. Model two included demographic factors. The third model also included health metrics, such as cancer, smoking, alcohol consumption, and metabolic data. The researchers performed multiple modeling analyses using different variables (continuous and categorical) and corrected for multiple confounders.

Their results suggested an association between higher DDS and slower biological aging. They note that this relationship is both highly significant (overall p of under 0.001) and linear.

Analysis of the participants’ subgroups divided by different health or demographic factors suggested an inverse relationship between DDS and phenotypic age acceleration across subgroups; however, these results were mainly not statistically significant.

The researchers also performed a sensitivity analysis that ensured the robustness of their observations. They did this analysis using multiple adjustments and concluded that the consistency of all three models suggests “a higher dietary diversity is significantly associated with lower phenotypic age acceleration, regardless of the adjustment methods employed.”

The researchers also explored the idea of oxidative stress being the factor mediating the relationship between dietary diversity and aging. They observed that “the oxidative stress indicator GGT had a significant mediating effect on the association of DDS and phenotypic age acceleration.”

Glutamyltransferase (GGT) was one of the proteins that was significantly lower in people with higher DDS. White blood cell count and neutrophil-lymphocyte ratio, two indicators of inflammation, were also significantly reduced. In contrast, levels of albumin, a potential indicator of anti-inflammatory capacity, and serum klotho, a protein with anti-aging properties, were higher.

Robust results, but without mechanistic understanding

Since this study is based on observational data, it cannot determine the mechanism behind the observed association. Still, the researchers proposed some hypotheses. They believe that oxidative stress and inflammation could be key processes mediating the effect of diet on aging, as a more diverse diet includes more antioxidants and anti-inflammatory compounds that protect cells from aging-related processes. They also consider the possible role of gut microbiota since a diverse diet can help maintain microbial diversity, an important factor for gut health. However, that particular aspect was not explored in this study.

The researchers claim that their results are robust and can be extrapolated to multi-ethnic and otherwise varied populations due to this data’s consistency and analysis.

While this analysis suggested that some associations are mediators, it cannot establish causality, and potential confounding factors (beyond what was tested) cannot be ruled out. The reporting of food intake should also be optimized for future studies.

Overall, this study’s results align with previous research, which linked reduced food diversity to an increased risk of age-related chronic diseases and mortality [2-5]. As the authors note, “promoting dietary diversity may facilitate healthy aging, which has significant implications for public health.”

Literature

[1] Liao, W., & Li, M. Y. (2024). Dietary diversity contributes to delay biological aging. Frontiers in medicine, 11, 1463569.

[2] Zheng, G., Cai, M., Liu, H., Li, R., Qian, Z., Howard, S. W., Keith, A. E., Zhang, S., Wang, X., Zhang, J., Lin, H., & Hua, J. (2023). Dietary Diversity and Inflammatory Diet Associated with All-Cause Mortality and Incidence and Mortality of Type 2 Diabetes: Two Prospective Cohort Studies. Nutrients, 15(9), 2120.

[3] Chalermsri, C., Ziaei, S., Ekström, E. C., Muangpaisan, W., Aekplakorn, W., Satheannopakao, W., & Rahman, S. M. (2022). Dietary diversity associated with risk of cardiovascular diseases among community-dwelling older people: A national health examination survey from Thailand. Frontiers in nutrition, 9, 1002066.

[4] Zheng, G., Xia, H., Lai, Z., Shi, H., Zhang, J., Wang, C., Tian, F., & Lin, H. (2024). Dietary Inflammatory Index and Dietary Diversity Score Associated with Sarcopenia and Its Components: Findings from a Nationwide Cross-Sectional Study. Nutrients, 16(7), 1038.

[5] Chalermsri, C., Rahman, S. M., Ekström, E. C., Ziaei, S., Aekplakorn, W., Satheannopakao, W., & Muangpaisan, W. (2023). Dietary diversity predicts the mortality among older people: Data from the fifth Thai national health examination survey. Archives of gerontology and geriatrics, 110, 104986.

A novel therapy based on induced neuronal stem cells shows promise in a mouse model of Alzheimer’s disease – and it can be administered intranasally [1].

Who needs cells?

Stem cell therapies have made great strides in recent decades. They have been successfully employed against numerous diseases, from cancer to osteoarthritis. However, storing, transporting, and administering cells is not easy. Moreover, in some cases, cells, with their extraordinarily complex metabolisms, can produce unwanted side effects. For instance, neural stem/progenitor cells (NSCs) have been shown to improve symptoms of certain brain diseases [2] but can also trigger pathological changes in the brain [3].

However, the whole cell might not always be necessary. Cells have been known to communicate with each other by excreting extracellular vesicles (EVs), tiny membrane-bound bubbles that can carry various cargoes, such as RNA molecules and proteins. Scientists have learned to harvest vesicles produced by cells and administer them locally or systematically, often recapitulating much of the effect of cell administration.

Unlike stem cells, EVs can be frozen and thawed without compromising their therapeutic efficacy. Furthermore, cells of different types can now be easily produced from induced pluripotent stem cells (iPSCs). Those are somatic cells reverted into pluripotency by applying certain molecules, such as the original reprogramming cocktail of Yamanaka factors (OSKM).

Less inflammation

In a new study published in the Journal of Extracellular Vesicles, researchers from Texas A&M University took human iPSCs and re-differentiated them into NSCs. They then harvested the extracellular vesicles produced by the cells, purified them, and administered them intranasally to a mouse model of familial Alzheimer’s disease (5xFAD mice). Despite decades of research and tens of billions of dollars spent, a cure for Alzheimer’s remains elusive.

While mouse models of Alzheimer’s have their limitations, as mice naturally don’t develop this disease, 5xFAD mice have been widely used. These animals start displaying Alzheimer’s-like pathologies, such as amyloid beta (Aβ) plaques and increased neuroinflammation at the age of three months, which is when the treatment was administered. About two months later, the mice underwent cognitive and neuropathological assessments.

The researchers confirmed that the EVs were indeed taken up by the brain’s resident macrophages (microglia). In Alzheimer’s, these cells surround Aβ plaques, presumably in an attempt to remove them. They display increased activation and inflammation, which has been linked to disease progression.

“Prolonged activation causes microglia to lose their normal function and begin to harm neurons, leading to progressive neuron loss,” explains Ashok K. Shetty, Ph.D., a University Distinguished Professor and associate director at the Institute for Regenerative Medicine in the Department of Cell Biology and Genetics, and the corresponding author on the study.

RNA sequencing revealed that the treatment downregulated multiple inflammation-related pathways that were significantly upregulated in 5xFAD mice compared to healthy controls. Notably, this occurred without compromising the microglia’s phagocytosis function: their ability to engulf and destroy pathogens.

The treatment also led to a significant reduction in the burden of Aβ plaques and phosphorylated tau protein, two major hallmarks of Alzheimer’s. While both sexes showed improvements, males demonstrated a more robust response to the treatment.

By the age of five months, 5xFAD mice typically demonstrate significant cognitive decline, which was also observed in this study. The EV treatment, however, appeared to effectively block this decline. Tests included the object location test, in which cognitively healthy animals are expected to spend more time exploring an object in a novel place than in a familiar place, and the pattern recognition test, which measures the ability to discern novel objects from familiar ones.

Interestingly, the researchers also assessed the mice’s mood. Mood changes are increasingly gaining recognition as a clinically important aspect of Alzheimer’s disease. Just like many human Alzheimer’s patients, untreated 5xFAD mice exhibited anhedonia, the inability to enjoy things – in this case, sweetened water. The EV treatment restored the rodents’ joie de vivre – or at least their preference for sugar.

Similar effects in human cells

While the researchers did not test their treatment in human patients, they pursued the next best alternative: applying NSC-derived EVs to human microglia in vitro. When challenged with Aβ-42, an isoform of Aβ known for its high aggregation propensity and central role in the pathology of Alzheimer’s disease, microglia exhibited overactivation and an inflammatory phenotype. These effects, however, were reversed by the EV treatment.

According to the paper, EVs are superior to NSCs in that they do not replicate and readily cross the blood-brain barrier. Intranasal administration is easy, non-invasive, and characterized by rapid action. While the study was conducted on animals at early stages of the disease, constant advances in diagnostics make this less of a limitation.

“This approach is effective because the cargo carried by these extracellular vesicles could reduce the neuropathological changes in the brain,” says Shetty, who has filed a patent on the intranasal application of neural stem cell-derived extracellular vesicles for treating Alzheimer’s and other neurological disorders. “Our journey to advance the application of this therapy for Alzheimer’s disease is just beginning.”

Literature

[1] Madhu, L. N., Kodali, M., Upadhya, R., Rao, S., Somayaji, Y., Attaluri, S., … & Shetty, A. K. (2024). Extracellular vesicles from human‐induced pluripotent stem cell‐derived neural stem cells alleviate proinflammatory cascades within disease‐associated microglia in Alzheimer’s disease. Journal of Extracellular Vesicles, 13(11), e12519.

[2] Temple, S. (2023). Advancing cell therapy for neurodegenerative diseases. Cell stem cell, 30(5), 512-529.

[3] Abdi, S., Javanmehr, N., Ghasemi-Kasman, M., Bali, H. Y., & Pirzadeh, M. (2022). Stem cell-based therapeutic and diagnostic approaches in Alzheimer’s disease. Current Neuropharmacology, 20(6), 1093-1115.