Neutral or Detrimental Effects of TREM2 Agonist Antibodies in Preclinical Models of Alzheimer's Disease and Multiple Sclerosis.

Human genetics and preclinical studies have identified key contributions of TREM2 to several neurodegenerative conditions, inspiring efforts to modulate TREM2 therapeutically. Here, we characterize the activities of three TREM2 agonist antibodies in multiple mixed-sex mouse models of Alzheimer's disease (AD) pathology and remyelination. Receptor activation and downstream signaling are explored in vitro, and active dose ranges are determined in vivo based on pharmacodynamic responses from microglia. For mice bearing amyloid-ß (Aß) pathology (PS2APP) or combined Aß and tau pathology (TauPS2APP), chronic TREM2 agonist antibody treatment had limited impact on microglia engagement with pathology, overall pathology burden, or downstream neuronal damage. For mice with demyelinating injuries triggered acutely with lysolecithin, TREM2 agonist antibodies unexpectedly disrupted injury resolution. Likewise, TREM2 agonist antibodies limited myelin recovery for mice experiencing chronic demyelination from cuprizone. We highlight the contributions of dose timing and frequency across models. These results introduce important considerations for future TREM2-targeting approaches.

Loss of MAFB Function in Humans and Mice Causes Duane Syndrome, Aberrant Extraocular Muscle Innervation, and Inner-Ear Defects.

Duane retraction syndrome (DRS) is a congenital eye-movement disorder defined by limited outward gaze and retraction of the eye on attempted inward gaze. Here, we report on three heterozygous loss-of-function MAFB mutations causing DRS and a dominant-negative MAFB mutation causing DRS and deafness. Using genotype-phenotype correlations in humans and Mafb-knockout mice, we propose a threshold model for variable loss of MAFB function. Postmortem studies of DRS have reported abducens nerve hypoplasia and aberrant innervation of the lateral rectus muscle by the oculomotor nerve. Our studies in mice now confirm this human DRS pathology. Moreover, we demonstrate that selectively disrupting abducens nerve development is sufficient to cause secondary innervation of the lateral rectus muscle by aberrant oculomotor nerve branches, which form at developmental decision regions close to target extraocular muscles. Thus, we present evidence that the primary cause of DRS is failure of the abducens nerve to fully innervate the lateral rectus muscle in early development.

TREM2 Regulates Microglial Cholesterol Metabolism upon Chronic Phagocytic Challenge.

Loss-of-function (LOF) variants of TREM2, an immune receptor expressed in microglia, increase Alzheimer's disease risk. TREM2 senses lipids and mediates myelin phagocytosis, but its role in microglial lipid metabolism is unknown. Combining chronic demyelination paradigms and cell sorting with RNA sequencing and lipidomics, we find that wild-type microglia acquire a disease-associated transcriptional state, while TREM2-deficient microglia remain largely homeostatic, leading to neuronal damage. TREM2-deficient microglia phagocytose myelin debris but fail to clear myelin cholesterol, resulting in cholesteryl ester (CE) accumulation. CE increase is also observed in APOE-deficient glial cells, reflecting impaired brain cholesterol transport. This finding replicates in myelin-treated TREM2-deficient murine macrophages and human iPSC-derived microglia, where it is rescued by an ACAT1 inhibitor and LXR agonist. Our studies identify TREM2 as a key transcriptional regulator of cholesterol transport and metabolism under conditions of chronic myelin phagocytic activity, as TREM2 LOF causes pathogenic lipid accumulation in microglia.

Brain delivery and activity of a lysosomal enzyme using a blood-brain barrier transport vehicle in mice.

Most lysosomal storage diseases (LSDs) involve progressive central nervous system (CNS) impairment, resulting from deficiency of a lysosomal enzyme. Treatment of neuronopathic LSDs remains a considerable challenge, as approved intravenously administered enzyme therapies are ineffective in modifying CNS disease because they do not effectively cross the blood-brain barrier (BBB). We describe a therapeutic platform for increasing the brain exposure of enzyme replacement therapies. The enzyme transport vehicle (ETV) is a lysosomal enzyme fused to an Fc domain that has been engineered to bind to the transferrin receptor, which facilitates receptor-mediated transcytosis across the BBB. We demonstrate that ETV fusions containing iduronate 2-sulfatase (ETV:IDS), the lysosomal enzyme deficient in mucopolysaccharidosis type II, exhibited high intrinsic activity and degraded accumulated substrates in both IDS-deficient cell and in vivo models. ETV substantially improved brain delivery of IDS in a preclinical model of disease, enabling enhanced cellular distribution to neurons, astrocytes, and microglia throughout the brain. Improved brain exposure for ETV:IDS translated to a reduction in accumulated substrates in these CNS cell types and peripheral tissues and resulted in a complete correction of downstream disease-relevant pathologies in the brain, including secondary accumulation of lysosomal lipids, perturbed gene expression, neuroinflammation, and neuroaxonal damage. These data highlight the therapeutic potential of the ETV platform for LSDs and provide preclinical proof of concept for TV-enabled therapeutics to treat CNS diseases more broadly.

Mutant α2-chimaerin signals via bidirectional ephrin pathways in Duane retraction syndrome.

Duane retraction syndrome (DRS) is the most common form of congenital paralytic strabismus in humans and can result from α2-chimaerin (CHN1) missense mutations. We report a knockin α2-chimaerin mouse (Chn1KI/KI) that models DRS. Whole embryo imaging of Chn1KI/KI mice revealed stalled abducens nerve growth and selective trochlear and first cervical spinal nerve guidance abnormalities. Stalled abducens nerve bundles did not reach the orbit, resulting in secondary aberrant misinnervation of the lateral rectus muscle by the oculomotor nerve. By contrast, Chn1KO/KO mice did not have DRS, and embryos displayed abducens nerve wandering distinct from the Chn1KI/KI phenotype. Murine embryos lacking EPH receptor A4 (Epha4KO/KO), which is upstream of α2-chimaerin in corticospinal neurons, exhibited similar abducens wandering that paralleled previously reported gait alterations in Chn1KO/KO and Epha4KO/KO adult mice. Findings from Chn1KI/KI Epha4KO/KO mice demonstrated that mutant α2-chimaerin and EphA4 have different genetic interactions in distinct motor neuron pools: abducens neurons use bidirectional ephrin signaling via mutant α2-chimaerin to direct growth, while cervical spinal neurons use only ephrin forward signaling, and trochlear neurons do not use ephrin signaling. These findings reveal a role for ephrin bidirectional signaling upstream of mutant α2-chimaerin in DRS, which may contribute to the selective vulnerability of abducens motor neurons in this disorder.

Human CFEOM1 mutations attenuate KIF21A autoinhibition and cause oculomotor axon stalling.

The ocular motility disorder "Congenital fibrosis of the extraocular muscles type 1" (CFEOM1) results from heterozygous mutations altering the motor and third coiled-coil stalk of the anterograde kinesin, KIF21A. We demonstrate that Kif21a knockin mice harboring the most common human mutation develop CFEOM. The developing axons of the oculomotor nerve's superior division stall in the proximal nerve; the growth cones enlarge, extend excessive filopodia, and assume random trajectories. Inferior division axons reach the orbit but branch ectopically. We establish a gain-of-function mechanism and find that human motor or stalk mutations attenuate Kif21a autoinhibition, providing in vivo evidence for mammalian kinesin autoregulation. We identify Map1b as a Kif21a-interacting protein and report that Map1b⁻/⁻ mice develop CFEOM. The interaction between Kif21a and Map1b is likely to play a critical role in the pathogenesis of CFEOM1 and highlights a selective vulnerability of the developing oculomotor nerve to perturbations of the axon cytoskeleton.

Human disorders of axon guidance.

Axon pathfinding is essential for the establishment of proper neuronal connections during development. Advances in neuroimaging and genomic technologies, coupled with animal modeling, are leading to the identification of an increasing number of human disorders that result from aberrant axonal wiring. In this review, we summarize the recent clinical, genetic and molecular advances with regard to three human disorders of axon guidance: Horizontal gaze palsy with progressive scoliosis, Congenital mirror movements, and Congenital fibrosis of the extraocular muscles, Type III.