Max Slevogt - Landschaftsskizze bei Neukastel (1920)
Frans van Mieris (Leiden, 1635 - 1681); Brothel Scene, 1658; oil on panel, 33,3 x 42,8 cm; Mauritshuis, The Hague
Willem Van Hasselt (Dutch; 1882–1963)
Hommage à Vuillard = Homage to Vuillard
Oil on canvas, 1943
Musée des Beaux-Arts, Bordeaux, Francs. © ADAGP. Photo: Lysiane Gauthier
From left, clockwise: Edouard Vuillard, Pierre Bonnard (holding dog), Maurice Denis (rolling a cigarette), Aristide Maillol (with long beard), Jacques Salomon (in profile, leaning forward), and Ker-Xavier Roussel (seated).
Yuri Pimenov (Moscow, 1903 - 1977); New Moscow, 1937; oil on canvas, 170 x 140 cm; Tretyakov Gallery, Moscow
Francisco Goya (Francisco José de Goya y Lucientes; Fuendetodos 1746 - Bordeaux 1828); Corral de Locos (Yard with Lunatics), 1793/94; oil on tin plated iron, 33 x 44 cm; Meadows Museum, Dallas
Jean-Baptiste-Camille Corot (Paris 1796 - Ville d’Avray 1875); Ronde de Nimphes - Effet du Matin (Circle of Nymphs - Morning Effect), c. 1857; oil on canvas, 58 x 37 cm
Giovanni Francesco Romanelli (called Raffaellino or Il Viterbese; Viterbo, 1610 - 1662); Galatea, n/d; oil on canvas, 298 x 198 cm
Existence of new neuron repair pathway discovered
Most of your neurons can’t be replaced.
Other parts of your body – such as skin and bone – can be replaced by the body growing new cells, but when you injure your neurons, you can’t just grow new ones; instead, the existing cells have to repair themselves.
In the case of axon injury, the neuron is able to repair or sometimes even fully regenerate its axon. But neurons have two sides – the axon (which sends signals to other cells) and the dendrite (which receives signals from other cells).
“We know that the axon side can repair itself,” says Rolls, “and we know a bunch of the molecular players. But we really didn’t know whether neurons have the same capacity to regenerate their dendrites, and so that’s what we set out to find in this study.”
“Our lab uses a Drosophila model system, where the dendrites are very accessible to manipulation,” she says, “so we decided that we would start by removing all the dendrites from the neurons to see if they could regenerate. We didn’t start with anything subtle, like taking off just a few dendrites. We said ‘Let’s just push the system to its maximum and see if this is even possible.’ And we were surprised because we found that not only is it possible, it’s actually much faster than axon regeneration: at least in the cells that we’re using, axon regeneration takes a day or two to initiate, while dendrite regeneration typically initiates within four to six hours and it works really well. All the cells where we removed the dendrites grew new dendrites – none of them died; so it’s clear that these cells have a way to both detect dendrite injury and initiate regrowth of the injured part.”
Having confirmed the neurons’ ability to initiate dendrite regeneration, the researchers then set out to find the injury surveillance system used by the cells to detect dendrite injury.
“We wanted to know,” says Rolls, “whether these cells were using the same kind of injury surveillance system for both the axons and the dendrites. In the case of axon injury, there’s a kinase – a signaling molecule – called DLK that is known to be used to initiate axon regeneration in everything from soil nematodes to Drosophila to mammals. We confirmed that in the cells we were studying, the DLK signaling molecule is required for axon regeneration, and then we did the same experiment to see whether DLK was required for dendrite regeneration – and it turns out that it’s not.”
Using a technique known as RNA interference (RNAi) to block the synthesis of the DLK kinase, and also by using Drosophila strains where the DLK gene is mutated, the researchers observed that the dendrites regrow normally without DLK. The researchers also used reporter strains to observe activity in the DLK pathway, and various loss-of-function approaches – all of which showed DLK’s involvement in axon regeneration, but never in dendrite regeneration.
“Since this paper came out,” says Rolls, “we’ve tested a bunch of other pathways that are known to be required for axon regeneration, and none of them are required for dendrite regeneration – which is kind of cool, because it means that these cells have a way that we didn’t even know about to respond to dendrite injury, and it means we essentially know nothing about that process. We don’t know how they sense the injury, or what they have to do to regrow dendrites. And so that’s something we’re now trying to figure out.”
Ilya Repin (Chuguyev 1844 - Kuokkala, now Repino, 1930); Portrait of Vera Repina (the artist’s wife), 1892; oil on canvas, 65 x 111 cm; Tretyakov Gallery, Moscow
Jeffrey Smart (Adelaide, 1921 - Montevarchi, Arezzo, 2013); Ballgame, Athens (1981)