Frailty Assessment in an Aging Mouse Model

Aging involves progressive loss of normal functions and differs across individuals. Frailty reflects the inability to maintain function with stressors. Studying frailty can enhance understanding of aging variability and progression.

Despite the frequent use of frailty assessments clinically, many preclinical aging projects don't include frailty as a variable of interest, potentially because of obstacles to implementing a frailty assessment tool. This frailty index requires no specialized equipment or stressful testing for mice. It is customizable, allowing investigators to tailor assessments to specific dimensions of frailty relevant to their research focus.

We hope that presenting the feasibility of simplifying and implementing a frailty index will encourage others to use frailty as a variable of interest, making for more clinically relevant projects. To begin, divide the assessment into open-field examination, manual examination, and weighing. For the open-field examination, transfer the mouse of interest into an open environment.

Examine physical characteristics including alopecia, loss of fur color, dermatitis, poor coat condition, kyphosis, tail stiffening, gait disorders, tremor, and abnormal breathing. For the manual examination, gently scruff the mouse. Examine for alopecia, loss of fur color, dermatitis, tumors, distended abdomen, cataracts, eye discharge, rectal prolapse, vaginal or penile prolapse, and poor body condition score.

Assess weight following open-field and manual examinations at a consistent time of day within two to four hours of the beginning of the inactive period. During open-field examination, assess for gait disorders. Dragging limbs, circling, wobbling, or a wide stance are signs of gait disorders.

To further evaluate gait disorders, place the mouse on a large-cage lid and tilt it. Lightly brush the mouse's rear to encourage climbing. Next, allow the mouse to acclimatize, and observe the mouse at rest for changes to breathing rate and depth.

Gently stroke the tail with a finger to assess tail stiffness. During the open-field examination, assess integumentary criteria including alopecia, loss of fur color, dermatitis, and poor coat condition. Alopecia is commonly present on the back of the neck and will be visible as the mouse moves freely.

Kyphosis is best assessed by observing the mouse's spinal curvature during the open-field examination, both at rest and while moving. Kyphosis is scored based on the extent of abnormal curvature of the spine visible during movement and hunched posture at rest. Manually examine by scruffing the mouse.

Use fingers to assess the extent of fat and muscle loss around the pubic bone and lower spine, which are used to evaluate body condition score. While holding the mouse, examine the eyes for signs of eye discharge, crusting, or discoloration, and for cataracts. Observe the thoracic and abdominal areas for additional signs of alopecia, which occur commonly around joints, change in fur, color, and dermatitis.

Gently rub a finger along the abdomen to assess for the presence of tumors or distended abdomen. Carefully tilt the mouse head down to assess for the presence of rectal and vaginal or penile prolapse. If there is suspected prolapse, gently lower the mouse to the open-field area.

Once it has returned to rest, gently lift the tail to confirm prolapse in the absence of any strain caused by handling. Following the open-field and manual examinations, weigh the mouse. Zero the scale between each animal, and record the weight to the nearest 0.1 gram.

During evaluation of each mouse, assign a score of 0, 0.5, or 1.0 to each frailty measure. Record the weight and calculate the average of all 16 measures to generate a final frailty score. Take notes on any health or behavioral changes requiring follow-up, as well as any abnormal post-handling behaviors.

Frailty index values increased progressively with age across all measured groups at 12, 18, 24, and 30 months. At 18 months, alopecia, kyphosis, and loss of fur color were the most frequently observed deficits, particularly in male mice. At 24 months, body condition score and poor coat condition deficits became common.

Longitudinal data showed that frailty rose with age for both sexes, with male mice having higher frailty values at similar ages. Very few males were observed beyond 120 weeks of age, indicating reduced survival in males. Mice that experienced more than 10%body weight loss were of advanced age and had frailty index values exceeding 0.15.

Most mice had minimal frailty index changes of less than 0.05 over two-week intervals. Larger frailty increases were more frequent at older ages and accompanied by notable weight loss. Four phenotypes of frailty weight change were observed, with the most common being frailty increase without weight change, present in 46%of tracked animals.

30%exhibited no change in either frailty or weight. 12%of mice showed either frailty increase with weight gain or frailty increase with weight loss.