Regulated reprogramming in the regeneration of sensory receptor cells

Abstract

Vision, olfaction, hearing, and balance are mediated by receptors that reside in specialized sensory epithelial organs. Age-related degeneration of the photoreceptors in the retina and the hair cells in the cochlea, caused by macular degeneration and sensorineural hearing loss, respectively, affect a growing number of individuals. Although sensory receptor cells in the mammalian retina and inner ear show only limited or no regeneration, in many nonmammalian vertebrates, these sensory epithelia show remarkable regenerative potential. We summarize the current state of knowledge of regeneration in the specialized sense organs in both nonmammalian vertebrates and mammals and discuss possible areas where new advances in regenerative medicine might provide approaches to successfully stimulate sensory receptor cell regeneration. The field of regenerative medicine is still in its infancy, but new approaches using stem cells and reprogramming suggest ways in which the potential for regeneration may be restored in individuals suffering from sensory loss.

BOX Figure

Similarities in developmental mechanisms in the different specialized sensory epithelia relevant to studies on regeneration in these systems. The sensory receptor cells arise from specialized Sox2 expressing regions of the nasal and inner ear epithelia. The progenitor cells in these regions go on to express proneural bHLH transcription factors necessary to produce the receptor cells. A similar series of early events occurs in the retina, though it is derived from the neural tube. Notch signaling, and its downstream effectors Hes1 and Hes5, promote support cells and Müller glial fates in the inner ear and retina.

Figure 1

Simplified schematic diagrams of the specialized sensory organs that have been most studied for their regeneration potential. (A) The main olfactory epithelium (MOE) and the vomeronasal organ (VNO) contain specialized sensory receptor neurons (ORNs) and supporting cells, called sustentacular cells. The axons of the ORNs project directly to the olfactory bulb (OB) in the brain. (B) The inner ear of vertebrates (top) contains two basic types of sensory receptor organs (red), the auditory sensory epithelia in the cochlea, known as the organ of Corti in mammals, and the vestibular epithelia-the three cristae, the utricle and the saccule. The vestibular epithelia (middle) are organized with alternating hair cells (red) and support cells (blue). In the organ of Corti (bottom), one row of inner hair cells (IHCs) and three rows of outer hair cells (OHCs) alternate with various types of supporting cells, the pillar cells (PCs) and the Deiters’ cells (DCs). The hair cells are innervated by afferent fibers from associated ganglia (spiral ganglia) and from efferent fibers from the CNS. (C) The neurosensory retina (red) lines the back of the eye; it contains the sensory receptors, the rods and cones (red), supporting Müller glia (blue), and other neurons (light red) that process and relay the light responses of the photoreceptors to the brain via the optic nerve.

Figure 2

There is a very good correlation between the process of ongoing sensory receptor cell generation throughout life and the capacity for regeneration in the epithelium. (A) In the normal olfactory epithelium, vestibular system and retina in some vertebrates, there is a continual generation of new sensory receptor cells. The globose basal cells (GBCs; yellow) and Horizontal basal cells (HBCs; yellow) act as progenitors and stem cells for the other cells in the epithelium, the olfactory receptor neurons (red) and the sustentacular cells (blue). Similarly, in the vestibular epithelium of some vertebrates, the support cells (SC; blue) can enter the mitotic cell cycle (yellow) to generate additional support cells and hair cells (red). In the retina of fish, new rod photoreceptors (red) are continually added from mitotic divisions of the rod precursors (yellow), while the Müller glia (MG; blue) are quiescent. (B) The correlation between robust regeneration and ongoing sensory cell genesis is diagrammed as red (regeneration) and green (ongoing sensory cell production) bars. Exceptions to the general rule occur in the bird auditory epithelium (a) where there is no ongoing production of sensory receptor cells, but there is robust regeneration, whereas in the bird vestibular organs (v), both processes occur. The lighter red bar for the bird retina indicates that whereas there is robust proliferation of Müller glia after damage, much like that observed in fish, the number of new receptor cells generated is small. (C) The changes in these epithelia during regeneration are diagramed. In the olfactory epithelium, the GBCs increase in their proliferation after damage, and the HBCs are recruited when the damage is extensive. In the auditory system of birds, the support cells express Atoh1 and other developmental genes, and in some cases directly transdifferentiate into hair cells. If the retina is damaged in fish, the Müller glia re-enter the mitotic cell cycle and produce all types of retinal neurons, including both rods and cones.