SENSORY INNERVATION

Establishment and refinement of sensory innervation in the lateral-line system of the zebrafish

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 Obiettivo del progetto (Objective)

'The establishment of sensory-organ innervation is crucial for behaviour, social interactions and survival. Neuronal arborisation is dictated by signals that guide neurites to their appropriate targets in combination with the spatial distribution of the target cells. To date, most emphasis has been placed on the genetic bases of neuronal-type specification and organ formation during embryonic development and, consequently, there is a serious gap in our knowledge about the long-term dynamics of organ innervation. To advance towards this goal, we will need to elucidate whether and how neuronal activity and sensory organ architecture can modulate neural arborisation. Vertebrate sensory organs represent an excellent model to study the establishment and maintenance of organ innervation. To investigate these processes, I have chosen the zebrafish mechanosensory “lateral line” as a model system. In general, planar polarized sensory hair cells in each unit of the lateral line, called neuromast, receive innervation by two afferent neurons. Hair cells can regenerate and re-innervate very rapidly, providing a simple model of sensory neurons arborisation whose dynamics can be visualized in vivo over long periods and under normal or altered physiological conditions. To investigate the mechanisms that govern the acquisition and maintenance of hair-cell innervation, I will conduct a systematic and comprehensive study of the processes that underlie the elaboration and remodelling of sensory-neuronal architecture in the lateral-line organ. I will perform high-resolution live imaging combined with sophisticated techniques to permit spatio-temporal control of protein expression. The findings derived from these studies shall reveal the mechanisms by which animals maintain sensory abilities throughout their entire lives.'

Introduzione (Teaser)

An EU-funded project has focused efforts on studying the zebrafish in order to improve our understanding of the mechanisms behind appropriate sensory organ functioning.

Descrizione progetto (Article)

Established sensory-organ innervation, the supply of nerves to a part of the body, is critical for behaviour, survival and quality of life. Although much research on related issues has focused on embryonic development, there is relatively little understanding of the long-term dynamics of organ innervation.

The 'Establishment and refinement of sensory innervation in the lateral-line system of the zebrafish' (Sensory innervation) project set out to address this gap in knowledge. The first step towards achieving such a goal relies on learning what roles neuronal activity and sensory organ architecture play in the way neurons branch out. This neuronal arborisation is dictated by signals guiding neurites to the intended targets, together with spatial distribution of the target cells.

Given various difficulties in studying these processes in humans, prior research has established that the vertebrate sensory organs offer an excellent model for analysing how organ innervation is established and maintained. Sensory innervation researchers therefore chose the zebrafish mechanosensory lateral line as a model system.

The lateral line, the structure and physiology of which are the same as those of the mammalian inner ear, is a functionally sophisticated but anatomically simple mechanosensory organ. It is formed by units called neuromasts, made up of a core of mechanosensory hair cells innervated by a minimum of two afferent neurons, which conduct impulses to the brain or spinal cord. Hair cells are able to regenerate and re-innervate quickly, even after the neuromasts have fully developed. This simple model of sensory neuronal arborisation allows for the processes to be studied in vivo for long periods and under various conditions.

Researchers carried out an intensive study of the processes underlying the elaboration and remodelling of sensory-neuronal architecture in the lateral line organ in order to examine what influences the proper innervation of hair cells. Results showed that during hair-cell regeneration, afferent neurons are strict selectors of polarity which enable synapses to be re-established with identically oriented targets. To understand more about this selection mechanism, a simple method was devised to collect important information of specific axonal terminals and thus correlate hair-cell orientation to single afferent neurons at sub-cellular resolution.

In deaf zebrafish mutants, lateralis afferents arborised profusely in the periphery and were observed to display less stability as well as make improper target selections. This points to the major role played by hair cell activity in developmental mechanisms related to peripheral arborisation and target recognition.

Findings of the Sensory innervation studies have the potential to generate enhanced knowledge on how animals maintain sensory abilities throughout their lives.