Metabolic basis of the astrocyte-synapse interaction governs dopaminergic-motor connection

Reviewer #1 (Public review):

This study investigates how astrocyte metabolic state influences astrocyte-synapse interactions and the organization of the dopaminergic circuit in the Drosophila CNS. Using a creative split-GFP-based contact reporter ("PEAPOD"), combined with genetic perturbations of glycolytic enzymes, synaptic labeling, EM, transsynaptic tracing, single-cell transcriptomics, and behavioral assays, the authors propose that disruption of astrocyte glycolysis enhances astrocyte-dopamine neuron contacts, promotes synaptogenesis, and biases dopaminergic-motor circuit connectivity through a mechanism involving altered Neuroligin 2 trafficking.

The work is conceptually ambitious and technically broad. The development and application of a contact reporter for astrocyte-neuronal interfaces is potentially valuable to the field, and the convergence of multiple glycolytic perturbations on similar phenotypes is a notable strength. However, several central conclusions currently extend beyond the direct evidence presented. Clarification and calibration of these claims would substantially strengthen the manuscript.

Major Points:

(1) Astrocyte glycolytic impairment is inferred rather than directly demonstrated

The central premise of the manuscript is that reduced astrocyte glycolysis drives the observed phenotypes. While multiple glycolytic enzymes (e.g., pfk, eno, pyk) are genetically perturbed and produce similar increases in PEAPOD signal, the manuscript does not directly demonstrate altered glycolytic flux or metabolic state in astrocytes under these conditions. Reduced enzyme levels or genetic mutation do not necessarily establish functional metabolic deficiency, particularly given potential compensatory mechanisms.

Because glycolytic impairment is foundational to the proposed mechanism, the conclusions should either be supported by direct metabolic readouts in astrocytes or framed more cautiously as perturbations of glycolytic enzymes rather than confirmed metabolic deficiency.

(2) Interpretation of the PEAPOD signal requires clearer calibration

The PEAPOD system is an innovative tool to detect membrane proximity between astrocytes and dopamine neurons. However, the manuscript frequently interprets increased PEAPOD intensity and volume as increased "ensheathment" or increased synaptic contact. A split-GFP-based reporter measures membrane apposition within a defined spatial range but does not directly quantify structural ensheathment, synapse number, or functional synaptic engagement.

Although the authors show an association of the PEAPOD signal with presynaptic markers in some regions, the distinction between increased membrane contact, altered membrane organization, and true changes in perisynaptic coverage should be more explicitly discussed. Several conclusions would benefit from clearer wording that distinguishes contact proximity from ultrastructural or functional synapse remodeling.

(3) Evidence for biased dopaminergic-motor circuit wiring is indirect

The manuscript proposes that disruption of astrocyte glycolysis biases dopaminergic-motor connectivity. This conclusion relies heavily on trans-Tango labeling intensity and downstream cell-type composition analysis via FACS and single-cell RNA sequencing.

Transsynaptic labeling approaches can be influenced by expression levels, reporter trafficking, labeling efficiency, and differential recovery during dissociation and FACS. Changes in labeled cell abundance or reporter intensity do not necessarily equate to altered synaptic wiring. Given that this conclusion represents a major conceptual advance of the study, the manuscript should either provide additional orthogonal support or temper the claim to reflect that altered labeling efficiency or synaptic engagement, rather than definitive rewiring, has been demonstrated.

(4) Mechanistic claims regarding Neuroligin 2 trafficking are suggestive but not definitive

The authors propose that astrocyte glycolytic disruption alters Neuroligin 2 (Nlg2) trafficking, leading to ER retention and downstream synaptogenic effects. The observation of Nlg2-positive intracellular bodies colocalizing with ER markers is intriguing. However, quantitative analysis, additional compartment markers, and/or biochemical support would be necessary to firmly establish altered ER exit or glycosylation status.

At present, the mechanistic model is plausible but should be presented more explicitly as a working model supported by suggestive evidence rather than a fully established trafficking defect.

(5) Behavioral phenotypes are not yet causally linked to dopaminergic circuit changes

The locomotor phenotypes observed upon astrocyte glycolytic perturbation are clear. However, the manuscript attributes these changes to altered dopaminergic-motor connectivity. A direct causal linkage between astrocyte metabolic state, dopaminergic circuit remodeling, and behavior is not conclusively demonstrated. The discussion should either clarify the inferential nature of this link or provide additional evidence supporting dopamine-specific dependence.

Minor Points:

(1) Statistical analyses across multi-group comparisons should be more clearly justified, particularly where multiple pairwise tests are performed. A clarification of the multiple-comparison correction and the exact comparison strategy would improve rigor.

(2) The temporal interpretation of activity-dependent remodeling experiments would benefit from a clearer explanation of what timescale is being tested.

(3) Developmental compensation versus the acute effects of glycolytic perturbation are not fully distinguished and should be discussed.

(4) The orthology and functional equivalence of Drosophila Nlg2 should be described with greater precision to avoid potential confusion.

Reviewer #2 (Public review):

Summary:

This manuscript presents a significant advance in our understanding of how metabolic states in astrocytes directly influence the structural assembly and functional output of neural circuits. By focusing on the Drosophila larval dopaminergic system, the authors uncover an interesting mechanism: astrocyte glycolysis acts as a negative regulator of PEAPODs, ultimately altering locomotor behavior. Metabolic fluctuations (e.g., due to diet, development, or disease) could fundamentally reshape neural connectivity, with broad implications for neurodevelopmental and metabolic disorders.

Strengths:

The manuscript offers a compelling narrative linking astrocyte metabolism to DA-MN circuit wiring and behavior. For the field, this study serves as an important prompt to investigate how metabolic states might dynamically tune neural connectivity during development and in disease.

Weaknesses:

The definitive acceptance of the proposed linear mechanism depends on future validation through genetic interaction tests and rescue experiments.

Reviewer #3 (Public review):

Summary:

The authors are trying to demonstrate how astrocytes influence the connections within neural circuits that control behavior.

Strengths:

The data presented in the manuscript are thorough and well-executed, using advanced Drosophila approaches (Ca2+ imaging, GRASP, clonal analysis, trans-Tango) in new ways (PEAPODS) and with new tools (pyk mutants, anti-pyk Ab, LexAop2-pykRNAi). Use of two RNAi lines for each of three glycolytic enzymes is strong evidence that perturbation of glycolysis is responsible, though it does not rule out that inappropriate build-up of intermediates, or shunting to alternative pathways, may play a role here. Subsequent focus on Pyk alone is understandable.

Weaknesses:

As strong as the data is, it does not always support some of the stated claims, and this should be addressed in any revision. In addition, there seems to be an oversimplification of the possible effects of Pyk RNAi, and some missing pieces that could fill in gaps and align the proposed mechanism with observed phenotypes.

Where the data does not support stated claims:

(1) The authors claim larvae executed more reorientation actions during locomotion "as a result" of attenuated astrocyte-to-DAergic neuron signaling through neuroligin 2 (astrocyte) and neurexin 1 (DA Neuron). They correlated these, but did not make a direct connection.

(2) There is a claim that "at the circuit level, behavioral alterations were found to arise from increased DAergic neuronal synaptogenesis and DAergic-motor connection" (sic). However, the work does not build a causal relationship between behavior and synaptogenesis or connectivity. At present, the manuscript does not directly address whether increased DA-motor neuron synapses are sufficient to explain the increased orientation reactions observed.

(3) It is asserted that (line 182, and elsewhere) "astrocyte glycolysis deficiency increased PEAPODs and DAergic neuron synaptogenesis". While astrocyte Pyk KD increased PEAPODS (Figure 2), and it also increased endogenous Brp-GFP in DA neurons (via STaR, Figure 3F), the added Brp-GFP was not localized to synapses under these conditions (pyk KD), to unequivocally demonstrate that the increased PEAPODS are at the sites of DAergic synapses. Also seen in 6I-J.

(4) It may be premature to refer to this strictly as synaptogenesis, as alternative explanations (e.g., stabilization or impaired pruning) could also account for the observations.

(5) The use of trans-Tango is an elegant way to support the idea that extra DAergic synapses are formed onto motor neurons, with potential impact on motor circuits. But again, the claim (line 215, and elsewhere) that this "Biased DAergic-motor wiring" is what "alters motor output", would benefit from additional evidence.

(6) Oversimplification of the possible effects of Pyk RNAi: Because Pyk knockdown is likely to alter glycolytic flux rather than abolish glycolysis entirely, it may be clearer to describe the manipulation as 'Pyk loss' rather than 'glycolysis-deficient' in most contexts.

(7) Filling gaps to align the proposed mechanism with observed phenotypes:

a) Figure 6K-M - the ER retention of Nlg2 should also be tested using Pyk-RNAi, in addition to the pyk mutants shown. This would confirm the astrocyte-specific nature of this effect and close the loop to align the phenotypes.

b) From the mechanism proposed (ER retention of Nlg, presumably leading to loss of Nlg function in astrocytes), one might expect that the effects of loss of Nlg2 from astrocytes could phenocopy the behavioral deficits seen in pyk KD (from astrocytes). Ackerman et al (2021) knocked down Nlg2 from astrocytes and examined motor behavior with FIMTrack. They saw increased accumulated distance but did not see the effects seen upon pyk KD in this manuscript (increased pausing, sweeping). The authors could perform this experiment themselves or alternatively should address this inconsistency in the discussion.