A new study published in Nature Neuroscience on 27 August, led by Prof Ed Lein and his team from the University of Szeged in Szeged, Hungary along with colleagues from the Allen Institute and J. Craig Venter Institute in the United States, has revealed a possible clue towards answering what has possibly been one of the most difficult questions for scientists to answer: What makes the human and animal brain different? The researchers discovered a new type of human brain cell not seen in mice and other well-studied laboratory animals. The new neuron has been named the “rosehip neuron” since the axons form dense bundles around the center of the small compact cell making it look just like a rose that has shed its petals.
The study looked at postmortem brain tissue of two men in their late fifties and focused specifically on the first layer of the cortex. This is the outermost region of the brain responsible for consciousness and many other functions though to be individual to humans ― those functions which make us “human” ― and also the most complex part of the brain.
Combining both microscopic studies on slices of brain tissue and genetic analyses of individual cells, rosehip cells were identified and found to have a unique a genetic signature not seen in any mouse brain cell type. The cells were also shown to form synapses with pyramidal neurons, another type of neuron located in a different part of the human cortex. The findings also show that rosehip neurons only attach to one specific part of their cellular partner, suggesting they may control the flow of information in a highly specialized way.
The newly discovered neurons seem to make up only 10% to 15% of inhibitory neurons in the first layer of cortex and are probably more sparsely distributed in other parts of the brain. In addition, the locations of their points of contact on other neurons indicate they may play a role in the complex circuits of neurons that activate one another throughout the brain by halting incoming, excitatory signals since rosehip neurons are inhibitory neurons responsible for making postsynaptic neurons much less likely to generate action potentials. However, the exact function of these cells has yet to be determined.
The fact that the set of genes expressed in rosehip neurons does not match the genetic signature of any previously identified cell in the mouse suggests there is no equivalent cell type in rodents. This may have important implications since rodents are often used to model humans. This doesn’t prove the cell type is unique to humans but it can be been added to the list of specialized cells that are potentially only found in the human brain.
The next step for the researchers will be to look for rosehip neurons in other parts of the brain and how rosehip neurons fit into the larger brain circuitry. Owing to their inhibitory nature and capacity to prevent cell-cell communications, scientists will also seek to identify whether rosehip neurons are associated with neurological disorders and whether the cells play a role in human disease.
(1) Boldog, E. et al. Transcriptomic and morphophysiological evidence for a specialized human cortical GABAergic cell type. Nature Neuroscience (2018). DOI: 10.1038/s41593-018-0205-2
Image: Boldog E. et al., Nature Neuroscience (2018)