Review articleMerging advanced technologies with classical methods to uncover dendritic spine dynamics: A hot spot of synaptic plasticity
Introduction
Almost a hundred billion neurons and an estimated hundred trillion (1014) synapses make the human brain the most complex structure known (Williams and Herrup, 1988, Nimchinsky et al., 2004). These neurons are involved in maintenance of basic brain functions as well as learning, memory, and higher-order thought processes. Maintaining healthy synaptic structure is therefore critical to the preservation of normal brain functions. Importantly, all higher-order neuronal communications are mediated by dendritic spines – specialized knob-like structures protruding from dendritic shafts (Hering and Sheng, 2001, Nimchinsky et al., 2002, Bourne and Harris, 2007). They are considered specialized, semi-autonomous postsynaptic compartments on which most excitatory synapses (over 95% in vertebrate brain) impinge (Hering and Sheng, 2001, Nimchinsky et al., 2002, Bourne and Harris, 2007). The dendritic spines are of multiple shapes and sizes, with diverse functions depending on type and activity of the neurons (Jones and Powell, 1969, Harris et al., 1992, Hering and Sheng, 2001, Nimchinsky et al., 2002, Bourne and Harris, 2007). Most importantly, spine morphology determines the strength and stability of the synapse, and can be significantly altered in neurodevelopmental and neurodegenerative diseases (Fiala et al., 2002, van Spronsen and Hoogenraad, 2010). Experimental evidence suggests that abnormal spine morphology is a principal cause of synaptic dysfunction in a number of neurological and neuropsychiatric disorders (Fiala et al., 2002, Bredesen et al., 2006, Rubinsztein, 2006). However, in order to understand the role of dendritic spines in synaptic plasticity and disease, it is first vital to characterize them accurately – not only their numbers, but also their three-dimensional (3D) structure (Calabrese et al., 2006, Kasai et al., 2010). Because of diffraction limitations and lack of spatial resolution in light microscopy, the dynamics and nanoscale structure of spine necks and distributions of spine proteins remained unexplored for many years. Recently, new technologies have facilitated significant advances in our understanding of the basic structure and function of dendritic spines. Notably, the development of super-resolution fluorescence microscopes has enabled capture of nanoscale-level spine structures in living neurons non-invasively. However, a great deal still needs to be done, particularly with respect to dendritic spine morphology and its role in impaired synaptic plasticity in neurological disorders. In this review we will discuss how classical methods and novel approaches can be used in a complimentary fashion to discover the detailed structures and functions of dendritic spines.
Section snippets
Why is it important to study dendritic spines?
There are several reasons to study the structure, function, genesis, and loss of dendritic spines. In addition to developmental changes, it is important to explore spine dynamics when the brain is under stress or in an injury or disease state because structural abnormalities in dendritic spines are thought to underlie symptomatology in many neuropathological states (Fiala et al., 2002). Changes associated with impaired cognitive function include loss or decrease in spine number or density,
Golgi stain
Golgi impregnation, also called ‘black reaction’, is a powerful classical histochemical technique. It has long been the gold standard with respect to neuronal and dendritic morphology (Mazzarello, 1999) (Fig. 1A–D). It is appropriate not only for visualization and investigation of neuronal anatomy in experimental animals, but also for autopsied human brain tissue (Millhouse, 1969, Glaser and Van der Loos, 1981, Spacek, 1989, Gibb and Kolb, 1998, Zhang et al., 2003). The Italian scientist
Fluorescent labeling of neurons and dendritic spines
The classical staining methods described above can be use to investigate the anatomy of neurons in vivo but are not appropriate for visualization of cultured living neurons. Several methods have been developed to overcome the limitations of Golgi stains for spine study, including the use of various commercially available tracer dyes, fluorochrome-labeled antibodies, and genetically encoded fluorescent proteins such as green fluorescent protein (GFP) or yellow fluorescent protein (YFP; Staffend
Advanced microscopic tools to elucidate dendritic spine architecture
Light microscopes do not provide sufficient resolution for the study of dendritic spines. The discovery of transmission electron microscopy (TEM) allowed us to see the ultrastructure of dendritic spines for the first time (Harris and Stevens, 1989, Papa et al., 1995). However, TEM cannot elucidate the activity-dependent spine dynamics in living neurons. To resolve this issue scientists developed a number of advanced microscopic tools, including confocal laser scanning microscopy (CLSM) and
Conclusion
Dendritic spines are the hot sites for synaptic plasticity, a vital mechanism in the establishment of long-term memory. Alteration of dendritic spine architecture significantly influences synaptic strength and efficacy. Abnormal spines are strongly linked with synaptic failure in several brain diseases, so it is necessary to characterize them accurately. However, despite a century of research, the precise role of dendritic spines in synapse formation and overall brain function remains unknown.
Author's contribution
P.M. conducted research and P.M., J.M. and M.P.M. wrote the paper.
Conflict of interest
Authors declare no conflict of interest to publish this review article.
Acknowledgements
Supports from Defence Research and Development Organization, Ministry of Defence, Govt. of India, the Alzheimer's Association, National Institutes of Health/National Institute of Aging are acknowledged. We are thankful to Dr. Gal Bitan, University California Los Angeles for his support. We are also thankful to Prof. Kaushik Parthasarathi, University of Tennessee Health Science Center for his critical comments.
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