Axon collateral branching enables single neurons to innervate multiple targets and is therefore a central determinant of neuronal circuit architecture. Local remodelling of actin within the axon shaft initiates branch formation, but mechanisms that restrict actin assembly to prevent excessive branching are incompletely understood. The authors identify Kaptin (Kptn) as a conserved inhibitor of axon collateral formation and characterise its role in controlling actin dynamics during neuronal development.
The study reports that Kptn functions as a negative regulator of axon collateral branching. Loss of Kptn increases the maturation of axonal actin patches into filopodial protrusions, thereby promoting the formation of additional collateral branches. This effect was observed in primary neuronal cultures where quantitative live-cell imaging revealed enhanced conversion of actin-rich patches on the axon shaft into protrusive structures in the absence of Kptn.
The authors combined several experimental systems to define Kptn's role. Primary neuronal cultures and high-resolution live imaging were used to quantify actin patch behaviour and the dynamics of filopodium emergence. Complementary in vivo analysis employed zebrafish genetics to assess consequences of Kptn loss at the level of axon arborisation, synapse density, and motor behaviour. Together these approaches link cell-biological changes in actin-driven branch initiation to altered circuit structure and function in an intact vertebrate model.
Mechanistic experiments indicate that Kptn antagonises the actin elongation factor Formin-2 (Fmn2). By opposing Fmn2 activity, Kptn regulates barbed-end dynamics of actin filaments within axonal patches, thereby limiting elongation and the probability that a patch will mature into a filopodium and a stable collateral branch. This antagonism of a formin-dependent elongation pathway establishes a molecular brake on productive branch initiation through direct regulation of actin filament behaviour.
In vivo loss of Kptn in zebrafish resulted in increased motor axon arborisation and elevated density of neuromuscular junctions. These anatomical changes were accompanied by impaired motor behaviour, linking excessive collateral branching to functional deficits in motor circuit performance. The zebrafish data therefore provide organismal-level evidence that Kptn-mediated restraint of actin patch maturation is necessary for correct neuronal circuit assembly and motor function.
Kptn has an established role in regulating mTORC1 signalling. Importantly, the branching phenotypes described here were reported to occur independently of that signalling role, indicating a distinct physiological function of Kptn during neuronal development. The authors separate the actin-regulatory activity that controls axon collateral formation from Kptn's previously described influence on mTORC1.
By identifying Kptn as a molecular brake on actin-driven branch maturation, the study provides a mechanistic framework for how mutations in KPTN associated with intellectual disability and epilepsy could disrupt neuronal connectivity. Excessive collateral branching and altered synapse distribution could contribute to circuit-level dysfunction in such conditions. The work thus connects cell-biological mechanisms of branch regulation to potential pathological consequences of KPTN perturbation.
The work was supported by the Department of Biotechnology grant listed in the source and intramural funding from the Indian Institute of Science Education and Research Pune. The authors declared no competing interests. This report is a preprint posted to bioRxiv and has not been certified by peer review; details of experimental design, datasets, and additional methods are those provided in the source preprint.
All statements in this summary are drawn from the cited preprint. As a preprint, the findings have not yet been peer reviewed; the source does not report additional confirmatory data, clinical correlations, or therapeutic implications beyond the mechanistic and developmental observations described above.