Synapse Formation: From Cellular and Molecular Mechanisms to Neurodevelopmental and Neurodegenerative Disorders.

The precise patterns of neuronal assembly during development determine all functional outputs of a nervous system; these may range from simple reflexes to learning, memory, cognition etc. To understand how brain functions and how best to repair it after injury, disease or trauma, it is imperative that we first seek to define fundamental steps mediating this neuronal assembly. To acquire the sophisticated ensemble of highly specialized networks seen in a mature brain, all proliferated and migrated neurons must extend their axonal and dendritic processes towards targets, which are often located at some distance. Upon contact with potential partners, neurons must undergo dramatic structural changes to become either a pre-or a postsynaptic neuron. This connectivity is cemented through specialized structures termed synapses. Both structurally and functionally, the newly formed synapses are however, not static as they undergo consistent changes in order for an animal to meet its behavioural needs in a changing environment. These changes may either be in the form of new synapses or an enhancement of their synaptic efficacy - referred to as synaptic plasticity. Thus, synapse formation is not restricted to neurodevelopment; it is a process that remains active throughout life. As the brain ages, either the lack of neuronal activity or cell death render synapses dysfunctional thus giving rise to neurodegenerative disorders. This review seeks to highlight salient steps that are involved in a neuron's journey - starting with the establishment, maturation and consolidation of synapses; we particularly focus on identifying key players involved in the synaptogenic program.