Beyond Molecular Codes: Simple Rules to Wire Complex Brains

Abstract

Summary Molecular codes, like postal zip codes, are generally considered a robust way to ensure the specificity of neuronal target selection. However, a code capable of unambiguously generating complex neural circuits is difficult to conceive. Here, we re-examine the notion of molecular codes in the light of developmental algorithms. We explore how molecules and mechanisms that have been considered part of a code may alternatively implement simple pattern formation rules sufficient to ensure wiring specificity in neural circuits. This analysis delineates a pattern-based framework for circuit construction that may contribute to our understanding of brain wiring.

An Engineered orco Mutation Produces Aberrant Social Behavior and Defective Neural Development in Ants

“…Furthermore, neural stem cells (NSCs) that contribute to adult neurogenesis in two brain regions, the hippocampus and the olfactory bulb (OB), are regulated by environmental input throughout life (Suh et al, 2009). In contrast, the insect brain is widely believed to be more hardwired, and to depend less on activity patterning than the mammalian brain (Hassan and Hiesinger, 2015; Jefferis et al, 2001). For example, in the moth Manduca sexta , odorant-induced ORN activity appears after the development of the 64 glomeruli (Oland and Tolbert, 1996) and thus, is unlikely to play a role.…”

Wiring visual systems: common and divergent mechanisms and principles

“…Similar design principles among disparate visual circuit ensembles may be best appreciated in the context of shared developmental processes that orchestrate iterative patterns of synaptic connectivity [4]. Synaptic specification is determined by two core processes: (1) precision of wiring prior to initial synapse formation ( pre-specification ); and (2) pruning and fine-tuning of connections ( post-specification ).…”

“…In contrast, visual system wiring in Drosophila appears to be predominantly determined by a genetic program, highlighting pre-specification (Fig. 2) [4,8,9]. However, in both systems pre- and post-specification likely work hand in hand: initial axonal and dendritic targeting to distinct columnar or laminar structures provides important milestones along the road to mature synaptic specificity [1,4,10,11].…”

Contribution of Genoarchitecture to Understanding Hippocampal Evolution and Development

“…Thus, developmental studies can offer relevant information for understanding the organizing principles of the brain connectome and predict both fiber tract trajectories and targets . As elegantly discussed by Hassan and Hiesinger [2015], the developmental perspective may also provide general rules for understanding the formation of brain networks, and goes beyond the most common and narrower focus on deciphering the molecular mechanisms that execute those rules.…”