Axonal Length Determines Distinct Homeostatic Phenotypes in Human iPSC Derived Motor Neurons on a Bioengineered Platform

Stem cell-based experimental platforms for neuroscience can effectively model key mechanistic aspects of human development and disease. However, conventional culture systems often overlook the engineering constraints that cells face in vivo. This is particularly relevant for neurons covering long range connections such as spinal motor neurons (MNs). Their axons extend up to 1m in length and require a complex interplay of mechanisms to maintain cellular homeostasis. However, shorter axons in conventional cultures may not faithfully capture important aspects of their longer counterparts. Here this issue is directly addressed by establishing a bioengineered platform to assemble arrays of human axons ranging from micrometers to centimeters, which allows systematic investigation of the effects of length on human axonas for the first time. This approach reveales a link between length and metabolism in human MNs in vitro, where axons above a threshold size induce specific molecular adaptations in cytoskeleton composition, functional properties, local translation, and mitochondrial homeostasis. The findings specifically demonstrate the existence of a length-dependent mechanism that switches homeostatic processes within human MNs. The findings have critical implications for in vitro modeling of several neurodegenerative disorders and reinforce the importance of modeling cell shape and biophysical constraints with fidelity and precision in vitro.

Automated summary

Stem cell-based experimental platforms in neuroscience can effectively mimic key aspects of human development and disease. However, conventional culture systems may not accurately represent the engineering constraints faced by cells in vivo, especially for neurons with long axons like spinal motor neurons. The establishment of a bioengineered platform to assemble arrays of human axons of various lengths has revealed a link between axon length and metabolism in human motor neurons, shedding light on a length-dependent mechanism that influences homeostatic processes within these cells. These findings have important implications for modeling neurodegenerative disorders in vitro and emphasize the importance of accurately modeling cell shape and biophysical constraints in experimental settings.