Differentiation signals from glia are fine-tuned to set neuronal numbers during development

Abstract

Neural circuit formation and function require that diverse neurons are specified in appropriate numbers. Known strategies for controlling neuronal numbers involve regulating either cell proliferation or survival. We used the Drosophila visual system to probe how neuronal numbers are set. Photoreceptors from the eye-disc induce their target field, the lamina, such that for every unit eye there is a corresponding lamina unit (column). Although each column initially contains ~6 post-mitotic lamina precursors, only 5 differentiate into neurons, called L1-L5; the 'extra' precursor, which is invariantly positioned above the L5 neuron in each column, undergoes apoptosis. Here, we showed that a glial population called the outer chiasm giant glia (xgO), which resides below the lamina, secretes multiple ligands to induce L5 differentiation in response to EGF from photoreceptors. By forcing neuronal differentiation in the lamina, we uncovered that though fated to die, the 'extra' precursor is specified as an L5. Therefore, two precursors are specified as L5s but only one differentiates during normal development. We found that the row of precursors nearest to xgO differentiate into L5s and, in turn, antagonise differentiation signalling to prevent the 'extra' precursors from differentiating, resulting in their death. Thus, an intricate interplay of glial signals and feedback from differentiating neurons defines an invariant and stereotyped pattern of neuronal differentiation and programmed cell death to ensure that lamina columns each contain exactly one L5 neuron

Photoreceptors generate neuronal diversity in their target field through a Hedgehog morphogen gradient in Drosophila

“…Photoreceptors are the sole source of Hh to the optic lobes at these developmental stages (Huang and Kunes, 1996). We showed that Hh derived from photoreceptors patterns lamina cell identities, thus implying that in addition to inducing LPCs, promoting their assembly into columns and triggering their differentiation through glial signalling relays (Fernandes et al, 2017; Huang et al, 1998; Huang and Kunes, 1998, 1996; Prasad et al, 2021; Umetsu et al, 2006), photoreceptors also determine the neuronal diversity of the lamina. This is the first example of a morphogen patterning neuronal fates in Drosophila; it highlights a remarkable similarity with the ventral neural tube of vertebrates where a gradient of the morphogen Sonic Hedgehog, a Hh homologue, diversifies cell types (Placzek and Briscoe, 2018).…”

“…Indeed, many vertebrate neuronal types express Shh (Dakubo et al, 2003;Farmer et al, 2016;Garcia et al, 2010;Gonzalez-Reyes et al, 2012;Harwell et al, 2012;Petralia et al, 2011;Wallace, 1999), however, their ability to generate instructive gradients to pattern distant neural fields has not been determined thus far. (Fernandes et al, 2017;Prasad et al, 2021). Lamina neuron types are positioned stereotypically along the distal to proximal axis of columns: L2, L3, L1, L4, L5; the LPCs between L4s and L5s undergo apoptosis.…”

“…ensembles of stacked LPC cell bodies associated with photoreceptor axon bundles; Figure 1A, B) (Huang and Kunes, 1998, 1996; Sugie et al, 2010; Umetsu et al, 2006). Later in development, photoreceptor-derived EGF is required for LPCs to initiate neuronal differentiation (indicated by expression of the pan-neuronal marker Embryonic lethal abnormal vision [Elav]; Figure 1A, B) (Huang et al, 1998; Yogev et al, 2010); however, this communication is indirect and involves a signalling relay through multiple glial populations, which ultimately induce lamina neuron differentiation through Mitogen Activated Protein Kinase (MAPK) signalling (Figure 1A) (Fernandes et al, 2017; Prasad et al, 2021). Lamina neuron identities are invariant and stereotyped according to their position along the distal-proximal axis of columns; LPCs in the distal-most positions of columns will differentiate into L2s, followed by L3s, then L1s, then L4s, and finally L5s are positioned at the proximal end of columns (Figure 1A, C) (Pecot et al, 2014).…”

“…The LPCs positioned between L4s and L5s do not differentiate but instead are fated to die by apoptosis (Figure 1A). Given that MAPK signalling drives differentiation in the lamina indiscriminately for all neuronal types (Fernandes et al, 2017; Prasad et al, 2021), how these neurons acquire their individual identities has remained elusive.…”

“…Photoreceptor axons release Hh into the optic lobe which guides lamina development by driving neuroepithelial cells to develop into lamina precursor cells (LPCs), their terminal divisions and column assembly. Once assembled into columns of six LPCs each, LPCs differentiate into five distinct neuron types (L1-L5) in response to MAPK activity that is induced by signals from glia (Fernandes et al, 2017; Prasad et al, 2021). Lamina neuron types are positioned stereotypically along the distal to proximal axis of columns: L2, L3, L1, L4, L5; the LPCs between L4s and L5s undergo apoptosis.…”