JCB_201606065 2179..2200

Neurons are highly polarized cells and typically contain multiple dendrites and a single long axon, crucial for conveying input and output electrical signals, respectively. Intrinsic mechanisms governing neuronal polarity have been extensively investigated (Namba et al., 2015; Bentley and Banker, 2016). However, a neuron cannot survive and properly function alone. Besides chemical and electrical communications, it physically interacts with its microenvironment constantly. The interaction depends on the forces acting on and exerted by the neuron, their mechanical properties, and coupling. Although it is widely accepted that mechanical forces are involved in normal neural development, such as neurogenesis, neuron–glia interactions, neuronal migration, axonal outgrowth, growth cone motility, synapse and neural circuit formation, and brain folding (Van Essen, 1997; Gilmour et al., 2004; Engler et al., 2006; Lu et al., 2006; Elkin et al., 2007; Franze et al., 2009; Betz et al., 2011; Amack and Manning, 2012; Cámpas et al., 2014), the underlying mechanisms are poorly understood. In particular, little is known about mechanical regulation of morphological and functional polarity of central neurons. Such possible regulation may represent a novel form of neuronal plasticity. Mechanical impact can alter the morphology and function of neurons in the central nervous system (CNS). A traumatic brain injury (TBI) includes the primary injury that occurs at the moment of traumatic impact and the secondary injury that occurs afterward and often involves excitotoxicity and inflammation. For neurons, both axons and dendrites are found to be damaged in a TBI. As a prominent feature of TBI, a diffuse axonal injury displays characteristic axonal varicosities (swelling or beading; Smith et al., 2013). In mild TBI (mTBI), axons are not uniformly injured, and unmyelinated axons appear more vulnerable (Reeves et al., 2005). On the other hand, dendritic varicosities and degenerating spines were also observed in the mTBI mouse model (Gao et al., 2011). However, it is not clear which neuronal compartment is more likely damaged first during the initial primary injury. Moreover, abundant axonal varicosities are a key sign for irreversible neurodegeneration in Alzheimer’s and Parkinson’s diseases and multiple sclerosis (Nikić et al., 2011; Yang et al., 2013). In fact, even in the normal CNS, not all axons appear like perfect long tubes with uniform diameters. Axonal varicosities often form before axonal pruning and synapse formation during development of the CNS and persist at a low level in the normal adult brain (Shepherd and Harris, 1998; Luo and O’Leary, 2005; Nikić et al., 2011; Smith et al., 2013). Although varicosities can profoundly affect action potential propagation and synaptic transmission (Debanne, 2004), how varicosities are induced in axons under various normal and abnormal conditions remains a mystery. Little is known about mechanical regulation of morphological and functional polarity of central neurons. In this study, we report that mechanical stress specifically induces varicosities in the axons but not the dendrites of central neurons by activating TRPV4, a Ca2+/Na+-permeable mechanosensitive channel. This process is unexpectedly rapid and reversible, consistent with the formation of axonal varicosities in vivo induced by mechanical impact in a mouse model of mild traumatic brain injury. In contrast, prolonged stimulation of glutamate receptors induces varicosities in dendrites but not in axons. We further show that axonal varicosities are induced by persistent Ca2+ increase, disassembled microtubules (MTs), and subsequently reversible disruption of axonal transport, and are regulated by stable tubulin-only polypeptide, an MT-associated protein. Finally, axonal varicosity initiation can trigger action potentials to antidromically propagate to the soma in retrograde signaling. Therefore, our study demonstrates a new feature of neuronal polarity: axons and dendrites preferentially respond to physical and chemical stresses, respectively. Polarity of varicosity initiation in central neuron mechanosensation