"Role of the neuronal protein Cap23 in the maturation and maintenance of dendritic arbors in-vivo"

Dendrites in the central nervous system are the postsynaptic counterparts in the neural circuitry, and the principal sites of excitatory synaptic inputs. Little is known about the genetic elements regulating the specification, formation, development, and maintenance of these structures. They are formed early in development, and barring small changes in structure, remain essentially unchanged throughout life. More than 90% of the synapses in the brain are located on the heads and necks of dendritic protuberances called spines. Since synapses are the functional units of brain function, a detailed study of their anatomical and morphological features is important for understanding the functioning of the brain, both in health and disease. But the high density and the structural complexity of neurons, and the small size of the spines in the brain impedes a detailed examination of spine morphology and synaptic structure. We used a line of transgenic mice expressing membrane targeted GFP (m-GFP) under the Thy-1 promoter to study dendritic morphology. These lines of mice express GFP in a subset of neurons, and lights up their entire arbor, enabling visualization of the proverbial “tree from the forest”. Expression of GFP in the membrane gets rid of all artifacts associated with volume and intensity, and enables visualization of fine structure of dendrites with an unprecedented clarity and resolution. Using deconvolution confocal microscopy, we are able to detect dendritic structures hitherto visible only in electron micrographs, and are able to resolve spines that are below the theoretical limit of resolution of a light microscope. This line of mice has the potential to become an invaluable assay tool for detecting early onset defects in neuropsychiatric disorders, as it is increasingly becoming apparent that changes in synapses (i.e. spines) are the first markers of all neural diseases. We next used the increased clarity offered by the m-GFP mice to address the role of the neuronal protein Cap23 in the maintenance of dendrites. Cap23 is a major cortical-cytoskeleton associated and calmodulin protein binding protein that is widely and abundantly expressed during development, maintained in selected brain regions in adults, and re-induced during nerve regeneration. Mice deficient in Cap23 start out having normal dendritic structure and arborization, but subsequently start decreasing in their arborization from around the time corresponding to synapse elimination in the CNS. This decrease in branching is progressive in nature, and correlates with the levels of the protein expressed. Since dendrites are normal to start with, but decrease in their arborization subsequently and in a steady manner, we refer to this novel phenomenon as “Dendritic Atrophy”, and implicate Cap23 in the maintenance of dendrites. The atrophy starts in the higher-order branches and proceeds towards the lower-order ones, and the remaining branches develop ‘complex spines’. Deficiency of Cap23 leads to the misregulation of a number of important genes in the proteome of the brain, but not in the transcriptome, suggesting the role of Cap23 in regulating dendritic structure by modulating the levels of several important dendritic proteins. Interestingly, in the background of a deficiency of Cap23, the transcriptome of the brain shows the predominant upregulation of a number of non-coding RNAs of unknown function that show important similarities with microRNAs. At least one miRNA (miR-128) is starkly downregulated in Cap23 mutants. This leads to the interesting possibility that Cap23 might be involved in the maintenance of dendrites through miRNA mediated regulation of protein levels. Since defects in dendritic structure and arborization is a hallmark of all neuronal diseases, it becomes interesting to speculate whether aberrations in RNA mediated control is a general mechanism underlying neuropsychiatric diseases in general.