The congenic BALB/c-crup (‘crup’ meaning ‘cruza-pernas’ in Portuguese or cross legs in English), a homozygous recessive mutant mouse derived from N-ethyl-N-nitrosourea mutagenesis, exhibits a unique phenotype characterized by hindlimb crossing when the mouse is suspended by its tail, along with age-related neuromotor issues. The study aimed to identify the genetic mutation causing the BALB/c-crup phenotype and evaluate the behavioral responses of the mice. Open field test, elevated plus maze, and elevated beam experiments were conducted to evaluate general activity, motor function, as well as sensorimotor and autonomic nervous systems. Genetic mapping and exome analysis identified a nonsynonymous (missense mutation), a single nucleotide variant, in the Taf15 gene. This mutation results in the p.G55S substitution, where glycine is replaced by serine at position 55 in the gene product. Longitudinal assessment by the open field test revealed altered locomotion, decreased mobility, and reduced rearing and grooming frequency in mutant mice. Sensorimotor function declines were observed through reduced surface righting reflex scores, grip strength, and increased hindquarter angle. In the elevated beam test, mutants exhibited tail hypotonia and aversion to traversing the beam. The elevated plus maze revealed altered behavior in closed arms, suggesting increased anxiety-like behavior or sensorimotor impairment. Our findings provide insights into neurologic and behavioral anomalies associated with a Taf15 gene mutation. The altered locomotion, sensory impairments, and disorientation observed in the crup phenotype indicate a progressive neuromotor condition, potentially serving as a novel mouse model for neurodegenerative diseases.
Experimental timeline and design for behavioral studies in BALB/cJ and BALB/c-crup male mice. (Top) 8-wk-old male BALB/cJ (n = 8) and BALB/c-crup (n = 8) mice were tested in the open field over 4 consecutive days. (Bottom) Male BALB/cJ (n = 14) and BALB/c-crup (n = 14) mice, aged 4, 8, 12, 24, and 48 wk, were assessed in a longitudinal study over 5 d, with behavioral assessments conducted at designated intervals as shown in the timeline. The study included open field, elevated beam, and elevated plus maze behavioral tests represented in the diagram by different icons. Created with BioRender.com.
Comparative performance analysis of BALB/cJ and BALB/c-crup mice (n = 8/group) in the open field test over 4 d: (A) distance traveled (cm); (B) average speed (cm/s); and (C) mobility mean (%). Results are presented as mean and SE. Significance at P < 0.05.
Behavior analysis in the open field test: (A) distance traveled (cm); (B) average speed (cm/s); (C) rearing frequency; and (D) grooming frequency in BALB/cJ and BALB/c-crup mice at 4, 8, 12, 24, and 48 wk of age (n = 5 to 14/group). Results are presented as mean, SE, and ranges (minimum to maximum) with individual data points. Significance at P < 0.05.
Evaluation of sensorimotor system parameters: (A) surface-righting reflex; (B) grip strength; and (C) hindquarter angle in BALB/cJ and BALB/c-crup mice at 4, 8, 12, 24, and 48 wk of age (n = 5 to 14/group). Results are presented as mean and SE. Significance at P < 0.05.
Autonomic nervous system parameters: (A) micturition (number of urine spots) and (B) defecation (number of fecal boli) in BALB/cJ and BALB/c-crup mice at 4, 8, 12, 24, and 48 wk of age (n = 5 to 14/group). Results are presented as mean and SE. Significance at P < 0.05.
Elevated beam test performance in BALB/cJ and BALB/c-crup mutant mice. (A and B) BALB/cJ mouse exhibiting normal performance on the beam, demonstrating good balance and coordination while successfully traversing the beam. (C and D) BALB/c-crup mutant mouse displaying impaired performance, including hesitation, difficulty navigating the beam, and tail hypotonia, characterized by a limp tail that droops downward, a distinctive feature of the BALB/c-crup phenotype.
Elevated plus maze parameters: (A) closed arms entries; (B) time spent in the central platform; and (C) number of central platform crosses of BALB/cJ and BALB/c-crup mice at 4, 8, 12, 24, and 48 wk of age (n = 5 to 14/group). Results are presented as mean and SE. Significance at P < 0.05.
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