Analysis of osteoarthritis in a mouse model of the progeroid human DNA repair syndrome trichothiodystrophy

Ageing-associated phenotypes in mice

Ageing is a continuous process in life featuring progressive damage accumulation that leads to physiological decline, functional deterioration and ultimately death of an organism. Based on the relatively close anatomical and physiological similarity to humans, the mouse has been proven as a valuable model organism in ageing research over the last decades. In this review, we survey methods and tools currently in use to assess ageing phenotypes in mice. We summarize a range of ageing-associated alterations detectable at two major levels of analysis: (1) physiology and pathophysiology and (2) molecular biomarkers. Age-sensitive phenotypes provided in this article may serve to inform future studies targeting various aspects of organismal ageing in mice. In addition, we discuss conceptual and technical challenges faced by previous ageing studies in mice and, where possible, provide recommendations on how to resolve some of these issues.

Induction of Accelerated Aging in a Mouse Model

With the global increase of the elderly population, the improvement of the treatment for various aging-related diseases and the extension of a healthy lifespan have become some of the most important current medical issues. In order to understand the developmental mechanisms of aging and aging-related disorders, animal models are essential to conduct relevant studies. Among them, mice have become one of the most prevalently used model animals for aging-related studies due to their high similarity to humans in terms of genetic background and physiological structure, as well as their short lifespan and ease of reproduction. This review will discuss some of the common and emerging mouse models of accelerated aging and related chronic diseases in recent years, with the aim of serving as a reference for future application in fundamental and translational research.

Prematurely aging mitochondrial DNA mutator mice display subchondral osteopenia and chondrocyte hypertrophy without further osteoarthritis features

Mitochondrial mutations and dysfunction have been demonstrated in several age-related disorders including osteoarthritis, yet its relative contribution to pathogenesis remains unknown. Here we evaluated whether premature aging caused by accumulation of mitochondrial DNA mutations in Polg^D275A mice predisposes to the development of knee osteoarthritis. Compared with wild type animals, homozygous Polg^D275A mice displayed a specific bone phenotype characterized by osteopenia of epiphyseal trabecular bone and subchondral cortical plate. Trabecular thickness was significantly associated with osteocyte apoptosis rates and osteoclasts numbers were increased in subchondral bone tissues. While chondrocyte apoptosis rates in articular and growth plate cartilage were similar between groups, homozygous mitochondrial DNA mutator mice displayed elevated numbers of hypertrophic chondrocytes in articular calcified cartilage. Low grade cartilage degeneration, predominantly loss of proteoglycans, was present in all genotypes and the development of osteoarthritis features was not found accelerated in premature aging. Somatically acquired mitochondrial DNA mutations predispose to elevated subchondral bone turnover and hypertrophy in calcified cartilage, yet additional mechanical or metabolic stimuli would seem required for induction and accelerated progression of aging-associated osteoarthritis.

Osteoarthritis: Trauma vs Disease

Osteoarthritis (OA) is the most prevalent joint disease characterized by pain and degenerative lesions of the cartilage, subchondral bone, and other joint tissues. The causes of OA remain incompletely understood. Over the years, it has become recognized that OA is a multifactorial disease. In particular, aging and trauma are the main risk factors identified for the development of OA; however, other factors such as genetic predisposition, obesity, inflammation, gender and hormones, or metabolic syndrome contribute to OA development and lead to a more severe outcome. While this disease mainly affects people older than 60 years, OA developed after joint trauma affects all range ages and has a particular impact on young individuals and people who have highest levels of physical activity such as athletes. Traumatic injury to the joint often results in joint instability or intra-articular fractures which lead to posttraumatic osteoarthritis (PTOA). In response to injury, several molecular mechanisms are activated, increasing the production and activation of different factors that contribute to the progression of OA.In this chapter, we have focused on the interactions and contribution of the multiple factors involved in joint destruction and progression of OA. In addition, we overview the main changes and molecular mechanisms related to OA pathogenesis.

Targeted Apoptosis of Senescent Cells Restores Tissue Homeostasis in Response to Chemotoxicity and Aging

The accumulation of irreparable cellular damage restricts healthspan after acute stress or natural aging. Senescent cells are thought to impair tissue function, and their genetic clearance can delay features of aging. Identifying how senescent cells avoid apoptosis allows for the prospective design of anti-senescence compounds to address whether homeostasis can also be restored. Here, we identify FOXO4 as a pivot in senescent cell viability. We designed a FOXO4 peptide that perturbs the FOXO4 interaction with p53. In senescent cells, this selectively causes p53 nuclear exclusion and cell-intrinsic apoptosis. Under conditions where it was well tolerated in vivo, this FOXO4 peptide neutralized doxorubicin-induced chemotoxicity. Moreover, it restored fitness, fur density, and renal function in both fast aging Xpd^TTD/TTD and naturally aged mice. Thus, therapeutic targeting of senescent cells is feasible under conditions where loss of health has already occurred, and in doing so tissue homeostasis can effectively be restored.

Pathology of Mouse Models of Accelerated Aging

Progeroid mouse models display phenotypes in multiple organ systems that suggest premature aging and resemble features of natural aging of both mice and humans. The prospect of a significant increase in the global elderly population within the next decades has led to the emergence of "geroscience," which aims at elucidating the molecular mechanisms involved in aging. Progeroid mouse models are frequently used in geroscience as they provide insight into the molecular mechanisms that are involved in the highly complex process of natural aging. This review provides an overview of the most commonly reported nonneoplastic macroscopic and microscopic pathologic findings in progeroid mouse models (eg, osteoporosis, osteoarthritis, degenerative joint disease, intervertebral disc degeneration, kyphosis, sarcopenia, cutaneous atrophy, wound healing, hair loss, alopecia, lymphoid atrophy, cataract, corneal endothelial dystrophy, retinal degenerative diseases, and vascular remodeling). Furthermore, several shortcomings in pathologic analysis and descriptions of these models are discussed. Progeroid mouse models are valuable models for aging, but thorough knowledge of both the mouse strain background and the progeria-related phenotype is required to guide interpretation and translation of the pathology data.

The age-related changes in cartilage and osteoarthritis

Osteoarthritis (OA) is closely associated with aging, but its underlying mechanism is unclear. Recent publications were reviewed to elucidate the connection between aging and OA. With increasing OA incidence, more senior people are facing heavy financial and social burdens. Age-related OA pathogenesis is not well understood. Recently, it has been realized that age-related changes in other tissues besides articular cartilage may also contribute to OA development. Many factors including senescence-related secretory phenotypes, chondrocytes' low reactivity to growth factors, mitochondrial dysfunction and oxidative stress, and abnormal accumulation of advanced glycation end products (AGEs) may all play key roles in the pathogenesis of age-related OA. Lately, epigenetic regulation of gene expression was recognized for its impact on age-related OA pathogenesis. Up to now, few studies have been reported about the role of miRNA and long-noncoding RNA (lncRNA) in age-related OA. Research focusing on this area may provide valuable insights into OA pathogenesis. OA-induced financial and social burdens have become an increasingly severe threat to older population. Age-related changes in noncartilage tissue should be incorporated in the understanding of OA development. Growing attention on oxidative stress and epigenetics will provide more important clues for the better understanding of the age-related OA.

An overview of underlying causes and animal models for the study of age-related degenerative disorders of the spine and synovial joints

As human lifespan increases so does the incidence of age-associated degenerative joint diseases, resulting in significant negative socioeconomic consequences. Osteoarthritis (OA) and intervertebral disc degeneration (IDD) are the most common underlying causes of joint-related chronic disability and debilitating pain in the elderly. Current treatment methods are generally not effective and involve either symptomatic relief with non-steroidal anti-inflammatory drugs and physical therapy or surgery when conservative treatments fail. The limitation in treatment options is due to our incomplete knowledge of the molecular mechanism of degeneration of articular cartilage and disc tissue. Basic understanding of the age-related changes in joint tissue is thus needed to combat the adverse effects of aging on joint health. Aging is caused at least in part by time-dependent accumulation of damaged organelles and macromolecules, leading to cell death and senescence and the eventual loss of multipotent stem cells and tissue regenerative capacity. Studies over the past decades have uncovered a number of important molecular and cellular changes in joint tissues with age. However, the precise causes of damage, cellular targets of damage, and cellular responses to damage remain poorly understood. The objectives of this review are to provide an overview of the current knowledge about the sources of endogenous and exogenous damaging agents and how they contribute to age-dependent degenerative joint disease, and highlight animal models of accelerated aging that could potentially be useful for identifying causes of and therapies for degenerative joint diseases.

The role of DNA damage and repair in aging through the prism of Koch-like criteria

Since the first publication on Somatic Mutation Theory of Aging (Szilárd, 1959), a great volume of knowledge in the field has been accumulated. Here we attempted to organize the evidence "for" and "against" the hypothesized causal role of DNA damage and mutation accumulation in aging in light of four Koch-like criteria. They are based on the assumption that some quantitative relationship between the levels of DNA damage/mutations and aging rate should exist, so that (i) the longer-lived individuals or species would have a lower rate of damage than the shorter-lived, and (ii) the interventions that modulate the level of DNA damage and repair capacity should also modulate the rate of aging and longevity and vice versa. The analysis of how the existing data meets the proposed criteria showed that many gaps should still be filled in order to reach a clear-cut conclusion. As a perspective, it seems that the main emphasis in future studies should be put on the role of DNA damage in stem cell aging.

Age-related skeletal dynamics and decrease in bone strength in DNA repair deficient male trichothiodystrophy mice

Accumulation of DNA damage caused by oxidative stress is thought to be one of the main contributors of human tissue aging. Trichothiodystrophy (TTD) mice have a mutation in the Ercc2 DNA repair gene, resulting in accumulation of DNA damage and several features of segmental accelerated aging. We used male TTD mice to study the impact of DNA repair on bone metabolism with age. Analysis of bone parameters, measured by micro-computed tomography, displayed an earlier decrease in trabecular and cortical bone as well as a loss of periosteal apposition and a reduction in bone strength in TTD mice with age compared to wild type mice. Ex vivo analysis of bone marrow differentiation potential showed an accelerated reduction in the number of osteogenic and osteoprogenitor cells with unaltered differentiation capacity. Adipocyte differentiation was normal. Early in life, osteoclast number tended to be increased while at 78 weeks it was significantly lower in TTD mice. Our findings reveal the importance of genome stability and proper DNA repair for skeletal homeostasis with age and support the idea that accumulation of damage interferes with normal skeletal maintenance, causing reduction in the number of osteoblast precursors that are required for normal bone remodeling leading to a loss of bone structure and strength.

Aging and osteoarthritis

PURPOSE OF REVIEW

Osteoarthritis is strongly linked to aging but the mechanisms for this link are incompletely understood. The recent literature was reviewed to find studies providing new insights into the connection between aging and osteoarthritis.

RECENT FINDINGS

Basic aging studies in nonarticular cells suggest that a cell stress or cell damage response contributes to chronic inflammation that promotes age-related diseases. This cellular response results in the senescence-associated secretory phenotype which has many of the characteristics of an osteoarthritic chondrocyte in terms of the cytokines, chemokines, and proteases produced. Oxidative stress can promote cell senescence and studies have shown a role for oxidative stress in altering cell signaling pathways in chondrocytes that can disrupt the response to growth factors. Mitochondria are an important source of reactive oxygen species and studies continue to support a role for the mitochondria in osteoarthritis, including work suggesting changes in energy production. Cell death occurs in osteoarthritic cartilage and recent studies suggest autophagy may play a role in determining if a cell lives or dies when stressed.

SUMMARY

Continued progress is being made on characterizing aging-related changes in cartilage. Additional studies are needed that focus on the tissues outside of the articular cartilage that play a role in osteoarthritis. Because osteoarthritis occurs in older adults who also have age-related changes in muscle, bone, fat, and the nervous system, it is likely that a more general and systemic approach will be needed to better understand the link between aging and osteoarthritis.