Walker Scot Jackson

Nociceptors, primary afferent nerve fibers that signal noxious stimuli, are broadly divided into slowly conducting unmyelinated C fibers and fast-conducting myelinated A fibers. Whereas C-nociceptors have been extensively studied, considerably less is known about the function of A-nociceptors. Here we demonstrate, combining genetic targeting of these fibers in mice with observations in human participants, a key involvement of A-nociceptors in mechanical nociceptive withdrawal reflexes and affective pain. In mice, optogenetic stimulation induced rapid and precise withdrawal reflexes as well as place aversion and facial expression changes consistent with pain affect, while inhibition strongly impaired mechanical nociceptive withdrawal reflexes. Prolonged A-nociceptor activation induced mechanical pain hypersensitivity and central sensitization. In a rare individual lacking thickly myelinated Aβ fibers, and in healthy participants during preferential Aβ-fiber nerve block, mechanical withdrawal reflexes were absent and mechanical pain perception reduced. Together, these findings identify fast-conducting mechano-nociceptors as essential drivers of nocifensive behaviors in mice and humans.

Neuroinflammation, particularly that involving reactive microglia, the brain’s resident immune cells, is implicated in the pathogenesis of major neurodegenerative diseases (NDs). Multiple studies have reported changes in ribosomal protein (RP) expression during neurodegeneration, but the significance of these changes remains unclear. Ribosomes are evolutionarily conserved protein-synthesizing machines; although commonly viewed as invariant, accumulating evidence suggests functional ribosome specialization through variation in their protein composition. Among RPs, S24, encoded by RPS24 in humans and Rps24 in mice, is unique as its transcripts undergo alternative splicing to produce protein variants with different C-terminal sequences that are differentially expressed across tissues and cell types. Understanding heterogeneous RP expression patterns across brain regions and cell types could reveal mechanisms underlying selective vulnerability in NDs and provide new biomarkers for neuroinflammatory responses. To characterize RP expression patterns across brain regions in neurons, astrocytes and microglia, we analysed cell type-specific translating mRNAs from mice. To investigate Rps24 isoform-specific expression, we performed cell type-resolved transcript analysis and developed antibodies specific for the S24-PKE protein variant encoded by mRNA isoform Rps24c. We examined Rps24c/S24-PKE expression in brains from mouse models of ageing and neurodegeneration, as well as in human post-mortem tissue from patients with Alzheimer’s disease (AD), Parkinson’s disease (PD) and Huntington’s disease (HD). This work revealed distinct RP expression patterns across brain regions and between neurons, astrocytes and microglia, including neuron-enriched RPs Rpl13a and Rps10. Analysis of RP paralogues revealed complex expression relationships with their canonical counterparts, suggesting regulated mechanisms for generating heterogeneous ribosomes. Across brain regions and cell types, Rplp0 and Rpl13a, commonly used normalization references, showed heterogeneous expression, raising important methodological considerations for gene expression studies. Rps24 isoforms exhibited striking cell type-specific expression patterns. Rps24c was predominantly expressed in microglia and was increased by neuroinflammation caused by ageing, neurodegeneration or inflammatory chemicals. Using S24-PKE-specific antibodies, we verified increased expression of this protein variant in brains with AD, PD and HD, and in relevant mouse models. These findings establish heterogeneous RP expression as a feature of brain cell types which may enable cell type-specific translation regulation via specialized ribosomes. This work also identifies Rps24c/S24-PKE as a potential novel marker for neuroinflammation and neurodegeneration and provides new tools for monitoring these responses.

Chronic wasting disease (CWD) is a strictly fatal and highly contagious prion disease of wild and farmed cervids currently expanding in North America. Prion diseases are caused by conversion of the cellular prion protein to its pathological isoform PrP ^Sc . Vaccination is considered a promising strategy to contain CWD, even though prion diseases do not show classical immune responses. For CWD containment, it is important that vaccines reduce shedding of prions in excreta, a major contributor to transmission. Here, we tested the effect of vaccines on prion shedding in feces and urine by vaccinating and prion infecting knock-in mice that recapitulate CWD pathogenesis as found in cervids. Vaccination reduced or even prevented CWD shedding in feces and urine collected between 30–90% of incubation time to disease. This is the first report showing that prion shedding can be blocked in a prion disease. For CWD specifically it may reduce the environmental prion burden and break the disease transmission cycle.

Chronic wasting disease (CWD) is a strictly fatal and highly contagious prion disease of wild and farmed cervids currently expanding in North America. Prion diseases are caused by conversion of the cellular prion protein to its pathological isoform PrP ^Sc . Vaccination is considered a promising strategy to contain CWD, even though prion diseases do not show classical immune responses. For CWD containment, it is important that vaccines reduce shedding of prions in excreta, a major contributor to transmission. Here, we tested the effect of vaccines on prion shedding in feces and urine by vaccinating and prion infecting knock-in mice that recapitulate CWD pathogenesis as found in cervids. Vaccination reduced or even prevented CWD shedding in feces and urine collected between 30-90% of incubation time to disease. This is the first report showing that prion shedding can be blocked in a prion disease. For CWD specifically it may reduce the environmental prion burden and break the disease transmission cycle.

Nociceptors, primary afferent nerve fibers that signal noxious stimuli, are broadly divided into slowly conducting unmyelinated C fibers and fast-conducting myelinated A fibers. Whereas C-nociceptors have been extensively studied, considerably less is known about the function of A-nociceptors. To address this gap, we developed an intersectional genetic approach for robust and selective interrogation and manipulation of mechanically responsive A-nociceptors (A-MNs) in mice. Optogenetic A-MN stimulation induced rapid and precise withdrawal reflexes as well as place aversion and facial expression changes consistent with pain affect, while inhibition strongly impaired mechanical nociceptive withdrawal reflexes, demonstrating that A-MNs are necessary and sufficient for rapid avoidance of noxious mechanical stimuli. Prolonged A-MN activation induced mechanical allodynia and central sensitization. In a rare individual lacking thickly myelinated Aβ fibers, mechanical withdrawal reflexes were completely absent, and pain perception reduced. Together, these findings identify fast-conducting mechano-nociceptors as essential drivers of nocifensive behaviors in mice and humans.

Neuroinflammation, particularly that involving reactive microglia, the brain’s resident immune cells, is implicated in the pathogenesis of major neurodegenerative diseases. However, early markers of this process are in high demand. Multiple studies have reported changes in ribosomal protein (RP) expression during neurodegeneration, but the significance of these changes remains unclear. Ribosomes are evolutionarily conserved protein synthesizing machines, and although commonly viewed as invariant, accumulating evidence suggest functional ribosome specialization through variation in their protein composition. By analyzing cell type-specific translating mRNAs from mouse brains, we identify distinct RP expression patterns between neurons, astrocytes, and microglia, including neuron-specific RPs, Rpl13a and Rps10 . We also observed complex expression relationships between RP paralogs and their canonical counterparts, suggesting regulated mechanisms for generating heterogeneous ribosomes. Analysis across brain regions revealed that Rplp0 and Rpl13a , commonly used normalization references, show heterogeneous expression, raising important methodological considerations for gene expression studies. Importantly, we show that Rps24 , an essential ribosome component that undergoes alternative splicing to produce protein variants with different C-termini, exhibits striking cell type-specific isoform expression in brain. The Rps24c isoform is predominantly expressed in microglia and is increased by neuroinflammation caused by aging, neurodegeneration, or inflammatory chemicals. We verify increased expression of S24-PKE, the protein variant encoded by Rps24c , in brains with Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease, and relevant mouse models, using isoform-specific antibodies. These findings establish heterogeneous RP expression as a feature of brain cell types and identify Rps24c /S24-PKE as a novel marker for neuroinflammation and neurodegeneration.

Primary afferent neurons that convey somatosensory modalities comprise two large, heterogeneous populations: small-diameter neurons that give rise to slowly conducting unmyelinated axonal C fibers and medium-to-large diameter neurons with fast myelinated A fibers. Despite these two major groupings, tools to differentiate between unmyelinated and myelinated primary afferent fibers by genetic targeting have not been available; in particular, whereas numerous mouse driver lines exist to target different C fiber populations, genetic tools that target myelinated primary afferent populations are scarce. Here we describe a knock-in mouse line expressing tamoxifen-dependent CreERT2 under control of the Nefh gene, which encodes neurofilament heavy chain (NFH or NF200), a protein that is highly enriched in myelinated fibers. This mouse enables highly selective and efficient recombination of Cre-dependent reporters for functional and anatomical interrogation of myelinated fibers while excluding unmyelinated C fibers. In combination with other recombinase-expressing mouse lines, this genetic tool will be valuable for intersectional targeting of subpopulations of myelinated primary afferent fibers.

Spinal nociceptive withdrawal reflexes are widely believed to rely on unmyelinated and thinly myelinated nociceptive fibers that also signal affective and motivational aspects of pain. Here we discover a population of myelinated mechanoreceptive nociceptor that forms free nerve endings as well as circumferential endings around hair follicles, and exclusively terminate in the deep spinal dorsal horn. Optogenetic activation of these fibers triggers rapid withdrawal reflexes that are precise and selective for the targeted limb, while silencing increases the threshold of mechanical nociceptive withdrawal reflexes. By contrast, optogenetic stimulation of the fibers is not associated with place aversion nor with changes in facial expression. Thus, we conclude that this nerve fiber population is uniquely positioned to rapidly respond to mechanical threats via selective withdrawal of the targeted body part, whereas other fast and slow nociceptive pathways are required for affective-motivational aspects of pain.

Numerous mouse models have been engineered to carry alterations to the Disrupted in Schizophrenia 1 ( Disc1 ) gene, thought to be involved in neurodevelopmental conditions. However, most Swiss and probably all 129 mouse substrains, which are widely used in biological research, naturally carry a 25 bp deletion in Disc1 exon 6. Despite the prevalence of these strains, little has been done to characterize the extent to which this natural mutation affects behavioral output and may unintentionally impact studies. Here, we report on experiments to test the effects of this deletion on social and exploratory behaviors. To model natural conditions, we designed a seminatural environment to house groups of mice (4 females and 1 male; 3 groups per strain) for prolonged periods and then employed this model to study social behaviors. First, we compared behavioral phenotypes in C57Bl/6Jrj (B6) mice and 129S4 (S4 ^Disc1−/− ) natural mutants to validate our setup. Then, to assess the contribution of the naturally mutated Disc1 to social behavior differences, the wild-type (WT) Disc1 allele was crossed into S4 mice (S4 ^Disc1+/+ ). S4 and B6 lines were drastically different, with S4 mice being hypoactive, less explorative, and less social than B6 mice. However, S4 mice expressing WT Disc1 only marginally differed from S4 ^Disc1−/− mice, showing little to no contribution of Disc1 to their behavioral phenotype. Thus, this mutation holds little significance for natural exploratory and social behaviors in the seminatural environment.

Genetic targeting of select populations of cells in the mouse nervous system is often hampered by a lack of selectivity, as candidate genes for such targeting are commonly expressed by multiple cell populations, also in the same region. Intersectional targeting using two or more genes has been enabled by the development of reporter tools dependent on more than one recombinase or gene regulator. Still, widespread adoption of intersectional tools is complicated by a scarcity of driver mice expressing recombinases otherthan Cre. Here we report the generation and characterization of a new driver mouse that expresses the FlpO recombinase from the endogenous locus of the Scn10a gene encoding Na _V 1.8, a voltage-gated sodium channel that is almost exclusively expressed in the afferent limb of the peripheral nervous system. Moreover, among sensory neurons the channel is preferentially expressed in nociceptors and in low-threshold C-fiber mechanoreceptors (C-LTMRs). The mouse showed high recombination efficiency (97 %) and selectivity (93 %) in dorsal root ganglia. Reporter-expressing fibers were observed in a variety of peripheral tissues, including skin, skeletal muscle, genitalia, bladder and intestines. To validate the suitability of the FlpO mouse line for intersectional targeting, we crossed it with a mouse line expressing CreERT2 from the Th (tyrosine hydroxylase) locus. This approach resulted in strikingly selective and efficient targeting of C-LTMRs, showing robust visualization of nerve endings of these fibers in skin and spinal cord at the light and electron microscopic level. Thus, the Na _V 1.8 ^Flpo mouse line presented here constitutes a selective and versatile tool for intersectional genetic targeting of Na _V 1.8 expressing primary afferent neurons.