The Elusive Varicose Astrocytes

Astrocytes are one of the major glial cells found in the central nervous system and have multiple functions in brain development, physiology, and pathophysiology [1]. However, there is still much to be known, and the evolutionary properties of these majestic cells within and among species warrant further investigation. In a fascinating study published in Glia, Falcone et al. [2] investigated an elusive form of astrocyte — the varicose projection astrocyte (VPA) — in cortical tissue from various primates, ranging from marmosets to humans. In brief, the authors revealed that this astrocyte is exclusively found in hominoid brains — humans and other apes while being absent from all other primate species’ brains analyzed in the study. Strikingly, VPAs were not found in tissue from all hominoid brains but exhibited notable inter-subject variability, seemingly defying the idea that this form of astrocyte comprises a distinct cell type.

VPAs were first identified and described in human and chimpanzee tissue over a decade ago [3]. Residing in cortical layers 5 and 6, these astrocytes have short spiny processes and typically between one and five varicose-containing approximately millimeter-long processes which extend in all directions and contact blood vessels. Noticeably, VPAs could not be found in rodents, macaques, and monkeys [3], suggesting that these might be exclusive to selected primates. However, a comprehensive analysis of VPAs in primates has been lacking. Equally important, the study in which VPAs were first identified utilized tissue resected from patients with neurological pathologies [3]. Therefore, the questions remained of whether these astrocytes emerge in response to pathological conditions and whether they are present in the healthy brain.

In their work, Falcone et al. harnessed tissue from an impressive range of primates, including apes, monkeys, and humans. Notably, the human tissue was from subjects with no known prior neurological disorders. In their investigations, the authors focused on the prefrontal cortex — a brain region often implicated in higher-order cognitive functions [4]. To visualize cells, the authors employed immunohistochemistry and fluorescence microscopy, and included several criteria for identifying VPAs: the cell needed to stain positive for the glial fibrillary acidic protein, have its cell body residing in layers 5 or 6, have short spiny processes, and display one or more long processes containing varicosities.

One of the major findings by Falcone et al. is that VPAs are exclusively found in brains from humans and other apes but not in other primates, suggesting that these astrocytes are closely linked to hominoid evolution. Yet, contrary to previous work [3], Falcone et al. did not find VPAs in chimpanzees. What might explain this discrepancy is difficult to nail down, but animal age and the small sample size in both studies could have played a role. Hence, whether chimpanzees constitute a special hominoid case by lacking VPAs, for now, remains unresolved. Another finding by Falcone et al. was that VPAs comprised only about 0.01% of all cortical astrocytes. What are the implications of this extremely low number (Box 1)? One possibility is that even a few of these astrocytes are sufficient for impacting brain functioning. Another possibility, perhaps less exciting but equally feasible is that functionally, these cells play a limited role (if at all). Yet, based on their numerous and far-reaching processes, it is conceivable that despite being sparse, VPAs can communicate with many neurons and hence impact neural circuit activity. An earlier, foundational study that could help shed light on VPAs’ possible roles explored the functional consequences of engrafting human glial progenitor cells into the rodent brain [5]. These cells developed into (among other cells) mature human-like astrocytes, and which one could argue resemble morphologically VPAs. Although this study did not specifically assess the role of VPAs, it offers several insights that may point to it. Specifically, human astrocytes propagated intracellular calcium signals faster than their rodent counterpart, synaptic potentiation was enhanced, and mice with human astrocytes displayed enhanced learning and memory. This suggests the fascinating possibility that VPAs are involved in regulating neuronal communication and synaptic plasticity in particular ways. However, much more research on this matter is needed. In the future, it will be essential to identify the specific roles of VPAs in regulating neuronal activity in the healthy and diseased brain. For this, chimeric animals engrafted with human cells [5,6] or organoids [7] may be powerful model systems. To this end, it will be critical to determine molecular markers that distinguish VPAs from the remaining astrocyte types by harnessing single-cell RNA sequencing [8].

Interestingly, Falcone et al. found that VPAs were not present in all human brains analyzed. In fact, in around half of the samples, these cells could not be identified. Based on this observation, the authors have speculated that VPAs may represent a specific astrocyte state rather than a distinct cell type, and that these cells’ varicosities could be induced by stress, aging, or unknown pathological conditions. The authors, however, provided evidence against the possibility that the heterogeneity in VPAs within species was due to age, cause of death, tissue processing, or brain inflammation. Lastly, it cannot be ruled out that the glial fibrillary acidic protein expression level might be variable across VPAs, and that VPAs with low expression levels might not have been detected. Determining what factors regulate VPAs in the hominoid brain thus remain unresolved, but the study by Falcone et al. lays the foundation for addressing this and related questions (Box 1).

While the physiological role of VPAs, for now, remains enigmatic, the work by Falcone et al. offers a first broad overview of the pattern of this astrocyte across the evolutionary branch of primates. These findings suggest that VPAs emerged in the hominoid brain and answering the questions of how and why this occurred remains a major mission for future research.