# Phenotypical, Behavioral, and Systemic Hallmarks in End-Point Mouse Scenarios

## TL;DR

This study identifies physical and behavioral signs of frailty in aging mice, distinguishing end-point status from normal aging.

## Contribution

The study introduces structural kyphosis and piloerection as key indicators of end-point frailty in mice.

## Key findings

- Structural kyphosis and piloerection are hallmarks of end-point frailty in 16-month-old mice.
- Hepatomegaly and splenomegaly occur alongside WAT loss in frail mice, contradicting typical end-of-life traits.
- Behavioral indicators like increased rearing latency correlate with physical frailty signs.

## Abstract

Husbandry homecage behavior and a physical frailty phenotype point to end-point status in mice.Piloerection is the best gross examination hallmark of 16-month-old mice at end-point.Structural kyphosis characterizes end-point mice, while postural kyphosis indicates normal aging.WAT loss and hepatic and splenomegaly indexes can be indirect measures of sarcopenia.Carcass index correlates with piloerection and kyphosis in end-point mice.

Husbandry homecage behavior and a physical frailty phenotype point to end-point status in mice.

Piloerection is the best gross examination hallmark of 16-month-old mice at end-point.

Structural kyphosis characterizes end-point mice, while postural kyphosis indicates normal aging.

WAT loss and hepatic and splenomegaly indexes can be indirect measures of sarcopenia.

Carcass index correlates with piloerection and kyphosis in end-point mice.

This study examines frailty in 16-month-old C57BL/6J mice, revealing physical signs such as kyphosis and alopecia alongside behavioral indicators like poor emotional responses. Unlike typical end-of-life traits, frail mice exhibited unexpected muscle weight changes, hepatomegaly, and splenomegaly. These findings suggest a complex relationship between physical deterioration and emotional distress in frailty, offering insights into mechanisms relevant to aging populations. Recognizing both physical and behavioral signs in preclinical models is crucial for future research on interventions to address frailty-related decline.

The state of frailty is a clinical–biological syndrome that affects the older population with a higher risk of functional dependence. Animal models can provide a tool to study this complex scenario. In the present work, we analyzed the physical and behavioral hallmarks of end-point status in 16-month-old mice (C57BL/6J) according to animal welfare regulations compared to age-matched counterparts with normal aging. A group of 6-month-old mice was added to control for age bias. First, we identified ‘structural kyphosis’ (visible and unmodifiable deformation in locomotion) correlated with piloerection as the hallmarks of the physical frailty phenotype compared to the ‘postural kyphosis’ (adjustment to counteract increased visceral volume but attenuated during locomotion) of old mice with normal aging. Alopecia (barbering) was presented in both old groups. Normal levels of exploratory activity in the corner test for neophobia and triceps surae muscle weight but an increased latency of rearing indicated the poorest emotional phenotype, with a possible contribution of structural kyphosis. The presence of hepatomegaly and splenomegaly counteracted the significant WAT loss commonly associated with end-of-life traits, which should have a normal body weight but preserved muscle mass.