# Deficiency of Tissue Nonspecific Alkaline Phosphatase Dysregulates Microglial Morphology and Function in a Mouse Model of Infantile Hypophosphatasia

## TL;DR

This study shows that a lack of TNAP in mice leads to microglial changes and health issues, suggesting TNAP is important for brain function and disease prevention.

## Contribution

The study reveals TNAP's role in regulating microglial morphology, metabolism, and signaling in a sex- and cell-specific manner.

## Key findings

- TNAP deficiency causes microglial morphological changes like enlarged cell bodies and shortened processes.
- Loss of TNAP shifts microglial metabolism toward neurotoxicity and increases neuroinflammatory and senescence markers.
- TNAP depletion in microglia alters purinergic signaling and modulates inflammatory responses.

## Abstract

Tissue‐nonspecific alkaline phosphatase (TNAP) has emerged as a crucial regulator of neuronal circuit formation and maintenance; however, the complexities of its sex‐ and cell type‐specific roles within microglia remain largely unexplored. To address this critical knowledge gap, this study examined how TNAP deficiency differentially affects microglial morphology, function, and signaling in both male and female mice, and investigated its broader implications for neurodevelopment and disease susceptibility. Using Alpl

+/+
(wild‐type) and Alpl

−/−
(TNAP knockout) mice, we conducted behavioral assessments at postnatal Days 13–14 to evaluate early neurobehavioral outcomes. Microglia were subsequently isolated for molecular, metabolic, and morphological analyses. TNAP‐deficient mice of both sexes exhibited profound physiological deficits, including stunted growth and significant sensorimotor impairments, confirming effective TNAP knockout and indicating that systemic TNAP loss affects multiple cell types beyond microglia. At the cellular level, TNAP loss induced notable morphological changes in microglia, characterized by enlarged cell soma and shortened processes, hallmarks of microglial activation. Molecular profiling revealed upregulation of neuroinflammatory and phagocytic markers, implicating TNAP as a modulator of the innate immune response. Furthermore, metabolic analyses uncovered a dramatic shift in tryptophan‐kynurenine metabolism, with increased quinolinic acid production signifying a transition to a neurotoxic, pro‐inflammatory state. Additionally, TNAP‐deficient microglia displayed extensive dysregulation in purinergic signaling pathways, exemplified by increased expression of key purinergic receptors, and acquired a senescent phenotype evidenced by elevated canonical senescence gene expression. Given the influence of TNAP deficiency on multiple cell populations, some observed microglial phenotypes may result from altered intercellular signaling or indirect effects. To delineate cell‐autonomous effects, siRNA‐mediated TNAP knockdown was performed in primary microglia isolated from wild‐type (WT) mice. TNAP depletion modulated inflammatory responses, suggesting an intrinsic role for TNAP in microglial regulation; however, these effects may not fully recapitulate the extent of deficiency observed in vivo. Overall, TNAP emerges as a key modulator of microglial structure and function, with its dysfunction potentially increasing susceptibility to neurodevelopmental and neurodegenerative disorders. This highlights the potential of TNAP as a therapeutic target for central nervous system health and disease.

Using male and female tissue‐nonspecific alkaline phosphatase (TNAP) knockout (KO) and wild type (WT) mice, we show TNAP loss impairs growth and sensorimotor function and induces marked microglial morphological changes (enlarged soma, retracted processes). TNAP‐deficient microglia upregulate neuroinflammatory, phagocytic, and senescence markers; shift kynurenine metabolism toward neurotoxicity; and exhibit dysregulated purinergic signaling. These findings reveal TNAP as a key regulator of microglial structure, metabolism, and homeostasis, with broad implications for central nervous system (CNS) health and disease.