Mechanisms of Motor Superperformance

MECHANISMS OF MOTOR SUPERPERFORMANCE: ABSTRACT Clinical experience and world population-level data indicate that most neurological disability stems from motor dysfunction. Yet, spontaneous superperformer mutations occur in a variety of persons and animals, illustrating that the intrinsic motor capacity of the organism can be augmented. We set out to identify similar mutations by rotarod screening of 32,726 laboratory mice harboring chemically induced random mutations with the goal of mechanistically explaining motor superperformance. In this context, we have discovered that a point mutation in an unsuspected gene, Rif1 (Replication Timing Regulatory Factor 1), converts a single mouse residue to the primate one and confers supernormal motor ability. Using clustered regularly interspaced short palindromic repeats (CRISPR) Rif1-mutant mice, we have determined that this superperformance is a motor-selective phenotype manifest upon several motor tasks but devoid of other effects upon various rigorous behavioral and longevity analyses. Rif1-mutant mice also exhibit enhanced recovery from stroke in the motor cerebral cortex. The superperformance mechanism is unknown: although Rif1 participates in DNA repair and in transcriptional regulation via G4 folded DNA structural stabilization, little is known about its function in the nervous system. There is precedent that DNA repair may be associated with synaptic transmission strength, while DNA G4 regulation could enhance the transcription of genes active in the motor system. We have strengthened this hypothetical framework by identifying several consequences of the Rif1 mutation: a) Altered Purkinje cell firing regularity and local field potential changes in mouse cerebellum, which can influence movement precision, with change of these neurophysiological parameters upon locomotion on a treadmill; b) Increased cellular resistance to DNA-damaging radiation; c) Increased cell resistance to G4 stabilization; d) Overexpression of a fraction of the cerebellar (but not forebrain or spinal cord) synaptic transcriptome including potential Rif1 mutation mediators such as Kcnma1, Kif5c and Nab2; e) These transcripts may be relevant to the phenotype because we show that their loss of function degrades motor performance, whereas f) Cerebellar injection of adenovirus-containing Nab2 induces superperformance. Thus, we will expand current cerebellar learning conceptions by postulating that the Rif1 mutation facilitates DNA repair and/or loosens G4 DNA folding leading to upregulation of synaptic transcripts, with either one or both mechanisms modifying the range or precision of cerebellar synapse activity that underlies movement control. To this effect, we will conduct neurophysiological studies, study the function of native RIF1, manipulate Nab2, Kcnma1 and Kif5c expression levels, and investigate DNA repair and DNA G4 regulation to test which of these mechanisms enable the superperformance phenotype. We will also investigate if enhanced stroke recovery also partakes from the same cerebellar neurophysiological mechanism to enable future work on this important observation.