A channel for degeneration

Mutations in the transient receptor potential cation channel subfamily V member 4 (TRPV4) gene can cause degenerative disorders of the peripheral nervous system, but the mechanisms are not clear. A new study shows that mutant TRPV4 impairs mitochondrial transport and causes axonal degeneration and neuronal dysfunction in flies.

In humans, TPRV4 mutations are associated with early life disease and the onset of progressive symptoms later in life. Here, the authors generated flies that expressed TRPV4^R269C — a neuropathy-causing TRPV4 variant — and showed that they were unable to expand their wings properly after emerging from their pupae. By contrast, flies expressing TRPV4^R269C with an additional, engineered mutation that blocked the ion-conducting pore of TRPV4 (TRPV4^R269C+M680K) showed normal wing expansion. Inducing TRPV4 neuronal expression in young adult flies revealed that TRPV4^R269C flies were poorer at climbing than flies expressing wild-type (WT) TRPV4 or TRPV4^R269C+M680K. These findings suggest that TRPV4^R269C can cause early-onset and later-onset neural dysfunction in flies but only if it has a functional ion channel pore.

Credit: Yuichiro Chino/Getty

TRPV4^R269C expression led to a marked loss of the axonal projections of C4da neurons — sensory neurons with dendrites in the body wall of third instar fly larvae — and a reduction in C4da dendritic arborizations. This suggests that TRPV4^R269C causes neural dysfunction and neurodegeneration.

The authors used screening approaches to identify fly genes that modified the effects of TRPV4^R269C on wing expansion. Knocking down Ca²⁺/calmodulin-dependent kinase II (CaMKII) inhibited this and other TRPV4^R269C-associated phenotypes, suggesting it is required for TRPV4^R269C-induced neurotoxicity.

Crustacean cardioactive peptide-expressing neurons are key regulators of wing expansion in flies. TRPV4^R269C expression led to increases in the spontaneous firing rate of these neurons and their intrinsic excitability. Ca²⁺ chelation, TRPV4 antagonism or CaMKII knockdown blocked these effects, suggesting that TRPV4^R269C causes hyperexcitability via a mechanism involving CaMKII.

TRPV4^R269C C4da neurons showed a faster Ca²⁺ response in the neuronal somata to a TRPV4 agonist and higher levels of spontaneous Ca²⁺ transients in axonal projections than did TRPV4^WT C4da neurons. Cultured mouse trigeminal neurons expressing reporter-tagged TRPV4^R269C also showed a faster and greater Ca²⁺ response to the TRPV4 agonist than those expressing TRPV4^WT. Inhibiting CaMKII attenuated these effects. These data suggest that the R269C mutation sensitizes TRPV4 in flies and mice and that CaMKII is required for TRPV4-mediated Ca²⁺ influx.

Mitochondrial axonal transport is highly regulated by intracellular Ca²⁺ levels. Expression of TRPV4^R269C, but not TRPV4^WT, markedly impaired such transport in Cd4a neurons. However, TRPV4^WT Cd4a neurons did show mitochondrial axonal transport deficits if they were treated with a TRPV4 agonist. Thus, TRPV4 activation, via mutation or an agonist, impairs this process.

These data suggest that the R269C mutation sensitizes TRPV4

Together, these data indicate that, in flies, CaMKII-dependent Ca²⁺ influx via mutant TRPV4 impairs mitochondrial axonal transport and causes axonal degeneration and neuronal dysfunction.