Expression pattern and neurotrophic role of the c-fms proto-oncogene M-CSF receptor in rodent Purkinje cells

Colony-stimulating factor 1 receptor signaling in the central nervous system and the potential of its pharmacological inhibitors to halt the progression of neurological disorders

Colony Stimulating Factor-1 (CSF-1)/Colony Stimulating Factor-1 Receptor (CSF-1R) signaling axis plays an essential role in the development, maintenance, and proliferation of macrophage lineage cells. Within the central nervous system, CSF-1R signaling primarily maintains microglial homeostasis. Microglia, being the resident macrophage and first responder to any neurological insults, plays critical importance in overall health of the human brain. Aberrant and sustained activation of microglia along with continued proliferation and release of neurotoxic proinflammatory cytokines have been reported in various neurological and neurodegenerative diseases. Therefore, halting the neuroinflammatory pathway via targeting microglial proliferation, which depends on CSF-1R signaling, has emerged as a potential therapeutic target for neurological disorders. However, apart from regulating the microglial function, recently it has been discovered that CSF-1R has much broader role in central nervous system. These findings limit the therapeutic utility of CSF-1R inhibitors but also highlight the need for a complete understanding of CSF-1R function within the central nervous system. Moreover, it has been found that selective inhibitors of CSF-1R may be more efficient in avoiding non-specific targeting and associated side effects. Short-term depletion of microglial population in diseased conditions have also been found to be beneficial; however, the dose and therapeutic window for optimum effects may need to be standardized further.This review summarizes the present understanding of CSF-1R function within the central nervous system. We discuss the CSF-1R signaling in the context of microglia function, crosstalk between microglia and astroglia, and regulation of neuronal cell function. We also discuss a few of the neurological disorders with a focus on the utility of CSF-1R inhibitors as potential therapeutic strategy for halting the progression of neurological diseases.

Mechanisms and significance of microglia-axon interactions in physiological and pathophysiological conditions

Microglia are the resident immune cells of the central nervous system, and are important for cellular processes. In addition to their classical roles in pathophysiological conditions, these immune cells also dynamically interact with neurons and influence their structure and function in physiological conditions. Microglia have been shown to contact neurons at various points, including the dendrites, cell bodies, synapses, and axons, and support various developmental functions, such as neuronal survival, axon elongation, and maturation of the synaptic circuit. This review summarizes the current knowledge regarding the roles of microglia in brain development, with particular emphasis on microglia-axon interactions. We will review recent findings regarding the functions and signaling pathways involved in the reciprocal interactions between microglia and neurons. Moreover, as these interactions are altered in disease and injury conditions, we also discuss the effect and alteration of microglia-axon interactions in disease progression and the potential role of microglia in developmental brain disorders.

Rh-CSF1 Attenuates Oxidative Stress and Neuronal Apoptosis via the CSF1R/PLCG2/PKA/UCP2 Signaling Pathway in a Rat Model of Neonatal HIE

Oxidative stress (OS) and neuronal apoptosis are major pathological processes after hypoxic-ischemic encephalopathy (HIE). Colony stimulating factor 1 (CSF1), binding to CSF1 receptor (CSF1R), has been shown to reduce neuronal loss after hypoxic-ischemia- (HI-) induced brain injury. In the present study, we hypothesized that CSF1 could alleviate OS-induced neuronal degeneration and apoptosis through the CSF1R/PLCG2/PKA/UCP2 signaling pathway in a rat model of HI. A total of 127 ten-day old Sprague Dawley rat pups were used. HI was induced by right common carotid artery ligation with subsequent exposure to hypoxia for 2.5 h. Exogenous recombinant human CSF1 (rh-CSF1) was administered intranasally at 1 h and 24 h after HI. The CSF1R inhibitor, BLZ945, or phospholipase C-gamma 2 (PLCG2) inhibitor, U73122, was injected intraperitoneally at 1 h before HI induction. Brain infarct volume measurement, cliff avoidance test, righting reflex test, double immunofluorescence staining, western blot assessment, 8-OHdG and MitoSOX staining, Fluoro-Jade C staining, and TUNEL staining were used. Our results indicated that the expressions of endogenous CSF1, CSF1R, p-CSF1R, p-PLCG2, p-PKA, and uncoupling protein2 (UCP2) were increased after HI. CSF1 and CSF1R were expressed in neurons and astrocytes. Rh-CSF1 treatment significantly attenuated neurological deficits, infarct volume, OS, neuronal apoptosis, and degeneration at 48 h after HI. Moreover, activation of CSF1R by rh-CSF1 significantly increased the brain tissue expressions of p-PLCG2, p-PKA, UCP2, and Bcl2/Bax ratio, but reduced the expression of cleaved caspase-3. The neuroprotective effects of rh-CSF1 were abolished by BLZ945 or U73122. These results suggested that rh-CSF1 treatment attenuated OS-induced neuronal degeneration and apoptosis after HI, at least in part, through the CSF1R/PLCG2/PKA/UCP2 signaling pathway. Rh-CSF1 may serve as therapeutic strategy against brain damage in patients with HIE.

Microglia permit climbing fiber elimination by promoting GABAergic inhibition in the developing cerebellum

Circuit refinement during postnatal development is finely regulated by neuron–neuron interactions. Recent studies suggest participation of microglia in this process but it is unclear how microglia cooperatively act with neuronal mechanisms. To examine roles of microglia, we ablate microglia by microglia-selective deletion of colony-stimulating factor 1 receptor (Csf1r) by crossing floxed-Csf1r and Iba1-iCre mice (Csf1r-cKO). In Csf1r-cKO mice, refinement of climbing fiber (CF) to Purkinje cell (PC) innervation after postnatal day 10 (P10)–P12 is severely impaired. However, there is no clear morphological evidence suggesting massive engulfment of CFs by microglia. In Csf1r-cKO mice, inhibitory synaptic transmission is impaired and CF elimination is restored by diazepam, which suggests that impairment of CF elimination is caused by a defect of GABAergic inhibition on PCs, a prerequisite for CF elimination. These results indicate that microglia primarily promote GABAergic inhibition and secondarily facilitate the mechanism for CF elimination inherent in PCs.

In the mammalian cerebellum, surplus synapses between climbing fibers (CF) and Purkinje cells (PC) are developmentally pruned. Here, Nakayama and colleagues show that ablation of microglia impairs pruning of CF-PC synapses because of dysfunction of GABAergic inhibition prerequisite for pruning.

Csf1R inhibition attenuates experimental autoimmune encephalomyelitis and promotes recovery

Multiple sclerosis (MS) is a chronic autoimmune disease of the central nervous system (CNS) characterized by progressive neuronal demyelination and degeneration. Much of this damage can be attributed to microglia, the resident innate immune cells of the CNS, as well as monocyte-derived macrophages, which breach the blood-brain barrier in this inflammatory state. Upon activation, both microglia and macrophages release a variety of factors that greatly contribute to disease progression, and thus therapeutic approaches in MS focus on diminishing their activity. We use the CSF1R inhibitor PLX5622, administered in mouse chow, to ablate microglia and macrophages during the course of experimental autoimmune encephalomyelitis (EAE), an animal model of MS. Here, we show that ablation of these cells significantly improves animal mobility and weight gain in EAE. Further, we show that this treatment addresses the pathological hallmarks of MS, as it reduces demyelination and immune activation. White matter lesion areas in microglia/macrophage-depleted animals show substantial preservation of mature, myelinating oligodendrocytes in comparison to control animals. Taken together, these findings suggest that ablation of microglia/macrophages during the symptomatic phase of EAE reduces CNS inflammation and may also promote a more permissive environment for remyelination and recovery. This microglia and macrophage-targeted therapy could be a promising avenue for treatment of MS.

MCSF orchestrates branching morphogenesis in developing submandibular gland tissue

The importance of macrophages in tissue development and regeneration have been strongly emphasized. However, the specific roles of macrophage colony-stimulating factor (MCSF), the key regulator of macrophage differentiation, in glandular tissue development have been unexplored. Here, we disclose new macrophage-independent roles of MCSF in tissue development. We initially found that MCSF is markedly upregulated at embryonic day E13.5, at a stage preceding the colonization of macrophages (at E15.5) in mouse submandibular gland (SMG) tissue. Surprisingly, MCSF-induced branching morphogenesis was based on a direct effect on epithelial cells, as well as indirectly, by modulating the expression of major growth factors of SMG growth, FGF7 and FGF10, via the phosphoinositide 3-kinase (PI3K) pathway. Additionally, given the importance of neurons in SMG organogenesis, MCSF-induced SMG growth was associated with regulation of neurturin expression and neuronal network development during early SMG development in an in vitro organogenesis model as well as in vivo. These results indicate that MCSF plays pleiotropic roles and is an important regulator of early SMG morphogenesis.

Dynamic changes in the relationship of microglia to cardiovascular neurons in response to increases and decreases in blood pressure

Microglia are present throughout the central nervous system (CNS) and express receptors for every known neurotransmitter. During inflammation, microglia change into a state that either promotes removal of debris (M1), or into a state that promotes soothing (M2). Caudal- and rostral- ventrolateral medullary regions (CVLM and RVLM, respectively) of the brainstem are key nuclei involved in all aspects of the cardiovascular system. In this study, we investigate a novel role for microglia in cardiovascular control in the brainstem of adult male Sprague-Dawley (SD) rat. Here we show, that increases and decreases in blood pressure (BP) triggers alertness in the physiology of microglia in the brainstem region; inducing changes in microglial spatial distribution and the number of synapses in contact with microglial end processes. Following 6h of acute hypertension, the number of synapses in contact with microglia increased by ≈30% in both regions of the brainstem, CVLM and RVLM. Induction of acute hypotension for 6h causes microglia to reduce the number of synaptic contacts by >20% in both, CVLM and RVLM, nuclei of the brainstem. Our analysis of the morphological characteristics of microglia, and expression levels of M1 and M2, reveals that the changes induced in microglial behavior do not require any obvious dramatic changes in their morphology. Taken together, our findings suggest that microglia play a novel, unexpected, physiological role in the uninjured autonomic nuclei of CNS; we therefore speculate that microglia act cooperatively with brainstem cardiovascular neurons to maintain them in a physiologically receptive state.

Emerging Roles for CSF-1 Receptor and its Ligands in the Nervous System

The colony-stimulating factor-1 receptor (CSF-1R) kinase regulates tissue macrophage homeostasis, osteoclastogenesis, and Paneth cell development. However, recent studies in mice have revealed that CSF-1R signaling directly controls the development and maintenance of microglia, and cell autonomously regulates neuronal differentiation and survival. While the CSF-1R-cognate ligands, CSF-1 and interleukin-34 (IL-34) compete for binding to the CSF-1R, they are expressed in a largely non-overlapping manner by mature neurons. The recent identification of a dominantly inherited, adult-onset, progressive dementia associated with inactivating mutations in the CSF-1R highlights the importance of CSF-1R signaling in the brain. We review the roles of the CSF-1R and its ligands in microglial and neural development and function, and their relevance to our understanding of neurodegenerative disease.

Colony-stimulating factor 1 receptor (CSF1R) signaling in injured neurons facilitates protection and survival

Colony-stimulating factor 1 and IL-34 protect against and partially reverse neurodegeneration in mice in part via promoting CREB signaling.

Colony-stimulating factor 1 (CSF1) and interleukin-34 (IL-34) are functional ligands of the CSF1 receptor (CSF1R) and thus are key regulators of the monocyte/macrophage lineage. We discovered that systemic administration of human recombinant CSF1 ameliorates memory deficits in a transgenic mouse model of Alzheimer’s disease. CSF1 and IL-34 strongly reduced excitotoxin-induced neuronal cell loss and gliosis in wild-type mice when administered systemically before or up to 6 h after injury. These effects were accompanied by maintenance of cAMP responsive element–binding protein (CREB) signaling in neurons rather than in microglia. Using lineage-tracing experiments, we discovered that a small number of neurons in the hippocampus and cortex express CSF1R under physiological conditions and that kainic acid–induced excitotoxic injury results in a profound increase in neuronal receptor expression. Selective deletion of CSF1R in forebrain neurons in mice exacerbated excitotoxin-induced death and neurodegeneration. We conclude that CSF1 and IL-34 provide powerful neuroprotective and survival signals in brain injury and neurodegeneration involving CSF1R expression on neurons.

The CSF-1 receptor ligands IL-34 and CSF-1 exhibit distinct developmental brain expression patterns and regulate neural progenitor cell maintenance and maturation

The CSF-1 receptor (CSF-1R) regulates CNS microglial development. However, the localization and developmental roles of this receptor and its ligands, IL-34 and CSF-1, in the brain are poorly understood. Here we show that compared to wild type mice, CSF-1R-deficient (Csf1r-/-) mice have smaller brains of greater mass. They further exhibit an expansion of lateral ventricle size, an atrophy of the olfactory bulb and a failure of midline crossing of callosal axons. In brain, IL-34 exhibited a broader regional expression than CSF-1, mostly without overlap. Expression of IL-34, CSF-1 and the CSF-1R were maximal during early postnatal development. However, in contrast to the expression of its ligands, CSF-1R expression was very low in adult brain. Postnatal neocortical expression showed that CSF-1 was expressed in layer VI, whereas IL-34 was expressed in the meninges and layers II-V. The broader expression of IL-34 is consistent with its previously implicated role in microglial development. The differential expression of CSF-1R ligands, with respect to CSF-1R expression, could reflect their CSF-1R-independent signaling. Csf1r-/- mice displayed increased proliferation and apoptosis of neocortical progenitors and reduced differentiation of specific excitatory neuronal subtypes. Indeed, addition of CSF-1 or IL-34 to microglia-free, CSF-1R-expressing dorsal forebrain clonal cultures, suppressed progenitor self-renewal and enhanced neuronal differentiation. Consistent with a neural developmental role for the CSF-1R, ablation of the Csf1r gene in Nestin-positive neural progenitors led to a smaller brain size, an expanded neural progenitor pool and elevated cellular apoptosis in cortical forebrain. Thus our results also indicate novel roles for the CSF-1R in the regulation of corticogenesis.

Multiple receptor tyrosine kinases are expressed in adult rat retinal ganglion cells as revealed by single-cell degenerate primer polymerase chain reaction

BACKGROUND

To achieve a better understanding of the repertoire of receptor tyrosine kinases (RTKs) in adult retinal ganglion cells (RGCs) we performed polymerase chain reaction (PCR), using degenerate primers directed towards conserved sequences in the tyrosine kinase domain, on cDNA from isolated single RGCs univocally identified by retrograde tracing from the superior colliculi.

RESULTS

All the PCR-amplified fragments of the expected sizes were sequenced, and 25% of them contained a tyrosine kinase domain. These were: Axl, Csf-1R, Eph A4, Pdgfrbeta, Ptk7, Ret, Ros, Sky, TrkB, TrkC, Vegfr-2, and Vegfr-3. Non-RTK sequences were Jak1 and 2. Retinal expression of Axl, Csf-1R, Pdgfrbeta, Ret, Sky, TrkB, TrkC, Vegfr-2, and Vegfr-3, as well as Jak1 and 2, was confirmed by PCR on total retina cDNA. Immunodetection of Csf-1R, Pdgfralpha/beta, Ret, Sky, TrkB, and Vegfr-2 on retrogradely traced retinas demonstrated that they were expressed by RGCs. Co-localization of Vegfr-2 and Csf-1R, of Vegfr-2 and TrkB, and of Csf-1R and Ret in retrogradely labelled RGCs was shown. The effect of optic nerve transection on the mRNA level of Pdgfrbeta, Csf-1R, Vegfr-2, Sky, and Axl, and of the Axl ligands Gas6 and ProteinS, was analysed. These analyses show transection-induced changes in Axl and ProteinS mRNA levels.

CONCLUSIONS

The repertoire of RTKs expressed by RGCs is more extensive than previously anticipated. Several of the receptors found in this study, including Pdgfrbeta, Csf-1R, Vegfr-2, Sky, and Axl, and their ligands, have not previously been primarily associated with retinal ganglion cells.

An evolutionary perspective on FoxP2: strictly for the birds?

FoxP2 mutations in humans are associated with a disorder that affects both the comprehension of language and its production, speech. This discovery provided the first opportunity to analyze the genetics of language with molecular and neurobiological tools. The amino acid sequence and the neural expression pattern of FoxP2 are extremely conserved, from reptile to man. This suggests an important role for FoxP2 in vertebrate brains, regardless of whether they support imitative vocal learning or not. Its expression pattern pinpoints neural circuits that might have been crucial for the evolution of speech and language, including the basal ganglia and the cerebellum. Recent studies in songbirds show that during times of song plasticity FoxP2 is upregulated in a striatal region essential for song learning. This suggests that FoxP2 plays important roles both in the development of neural circuits and in the postnatal behaviors they mediate.

Modulation of CSF-1-regulated post-natal development with anti-CSF-1 antibody

Colony-stimulating factor-1 (CSF-1) regulates the survival, proliferation and differentiation of macrophages. CSF-1-deficient mice are osteopetrotic due to a lack of osteoclasts, while their tissue macrophage deficiencies and an absence of CSF-1 regulation of CSF-1 receptor-expressing cells in the female reproductive tract contribute to their pleiotropic phenotype. To further understand CSF-1 regulation of macrophages in vivo, we developed a neutralizing anti-mouse CSF-1 antibody which was expressed as a recombinant Fab' fragment and coupled to 40 kDa polyethylene glycol. As developmental regulation by CSF-1 is highest during the early post-natal period, the ability of this anti-CSF-1 reagent to inhibit development was tested by regular subcutaneous injection of mice from post-natal days 0.5-57.5. Antibody treatment decreased growth rate, decreased osteoclast number, induced osteopetrosis, decreased macrophage density in bone marrow, liver, dermis, synovium and kidney and decreased adipocyte size in adipose tissue, thereby inducing phenotypes shared by CSF-1- and CSF-1 receptor-deficient mice. While the antibody blocked macrophage development in some tissues, macrophage densities in other tissues were initially high and were reduced by treatment, proving that the antibody also blocked macrophage maintenance. Since cell surface CSF-1 is sufficient for the maintenance of normal synovial macrophage densities, these studies suggest that anti-CSF-1 Fab'-PEG efficiently neutralizes all three CSF-1 isoforms in vivo, namely the secreted proteoglycan, secreted glycoprotein and cell surface glycoprotein. Since CSF-1 has been shown to enhance chronic disease development in a number of mouse model systems, these studies demonstrate the feasibility of neutralizing CSF-1 effects in these models with an anti-CSF-1 antibody.

Collaborative action of M-CSF and CTGF/CCN2 in articular chondrocytes: possible regenerative roles in articular cartilage metabolism

It is known that expression of the macrophage colony-stimulating factor (M-CSF) gene is induced in articular chondrocytes upon inflammation. However, the functional role of M-CSF in cartilage has been unclear. In this study, we describe possible roles of M-CSF in the protection and maintenance of the articular cartilage based on the results of experiments using human chondrocytic cells and rat primary chondrocytes. Connective tissue growth factor (CTGF/CCN2) is known to be a potent molecule to regenerate damaged cartilage by promoting the growth and differentiation of articular chondrocytes. Here, we uncovered the fact that M-CSF induced the mRNA expression of the ctgf/ccn2 gene in those cells. Enhanced production of CTGF/CCN2 protein by M-CSF was also confirmed. Furthermore, M-CSF could autoactivate the m-csf gene, forming a positive feed-back network to amplify and prolong the observed effects. Finally, promotion of proteoglycan synthesis was observed by the addition of M-CSF. These findings taken together indicate novel roles of M-CSF in articular cartilage metabolism in collaboration with CTGF/CCN2, particularly during an inflammatory response. Such roles of M-CSF were further supported by the distribution of M-CSF producing chondrocytes in experimentally induced rat osteoarthritis cartilage in vivo.

Macrophage colony stimulating factor prevents NMDA-induced neuronal death in hippocampal organotypic cultures

Macrophage colony stimulating factor (M-CSF) and its receptor are up-regulated in the brain in Alzheimer's disease (AD), in transgenic mouse models for AD, and experimental models for traumatic and ischemic brain injury. M-CSF induces activation and proliferation of microglial cells and expression of proinflammatory cytokines. We examined the role of M-CSF in excitotoxic neuronal cell death in organotypic hippocampal cultures. NMDA treatment induced neuronal apoptosis and caspase-3 activation in organotypic hippocampal cultures, whereas treatment with M-CSF protected hippocampal neurons from NMDA-induced apoptosis. Caspase-3 activation was inhibited by M-CSF treatment to the same degree as with the caspase inhibitor Z-VAD-FMK. These results suggest that M-CSF has neuroprotective properties through inhibition of caspase-3 that could promote neuronal survival after excitotoxic insult. The role of M-CSF in neurological disease should be reevaluated as a microglial activator with potentially neuroprotective effects.

Targeted disruption of the mouse colony-stimulating factor 1 receptor gene results in osteopetrosis, mononuclear phagocyte deficiency, increased primitive progenitor cell frequencies, and reproductive defects

The effects of colony-stimulating factor 1 (CSF-1), the primary regulator of mononuclear phagocyte production, are thought to be mediated by the CSF-1 receptor (CSF-1R), encoded by the c-fms proto-oncogene. To investigate the in vivo specificity of CSF-1 for the CSF-1R, the mouse Csf1r gene was inactivated. The phenotype of Csf1(-)/Csf1r(-) mice closely resembled the phenotype of CSF-1-nullizygous (Csf1(op)/Csf1(op)) mice, including the osteopetrotic, hematopoietic, tissue macrophage, and reproductive phenotypes. Compared with their wild-type littermates, splenic erythroid burst-forming unit and high-proliferative potential colony-forming cell levels in both Csf1(op)/Csf1(op) and Csf1(-)/Csf1r(-) mice were significantly elevated, consistent with a negative regulatory role of CSF-1 in erythropoiesis and the maintenance of primitive hematopoietic progenitor cells. The circulating CSF-1 concentration in Csf1r(-)/Csf1r(-) mice was elevated 20-fold, in agreement with the previously reported clearance of circulating CSF-1 by CSF-1R-mediated endocytosis and intracellular destruction. Despite their overall similarity, several phenotypic characteristics of the Csf1r(-)/Csf1r(-) mice were more severe than those of the Csf1(op)/Csf1(op) mice. The results indicate that all of the effects of CSF-1 are mediated via the CSF-1R, but that subtle effects of the CSF-1R could result from its CSF-1-independent activation.

Chemical-induced hippocampal neurodegeneration and elevations in TNFalpha, TNFbeta, IL-1alpha, IP-10, and MCP-1 mRNA in osteopetrotic (op/op) mice

The osteopetrotic (op/op) mouse, deficient in biologically active colony stimulating factor 1 (CSF-1), was used to examine the role of microglia in chemical-induced trauma. Op/op mice and normal phenotype littermates (non-op/op) received an acute i.p. injection of the hippocampal toxicant, trimethyltin hydroxide (TMT; 1.5 or 2.0 mg/kg). At 2.0 mg/kg, both mice displayed severe degeneration of dentate granule neurons. At 1.5 mg/kg, non-op/op mice showed a limited punctate pattern of neuronal death while op/op mice showed prominent neuronal death. TMT-induced astrocyte reactivity was similar in both groups. RNase protection assays were conducted on hippocampal tissue at 24 hr post-TMT. Elevations were seen in mRNA levels for the host response genes: intercellular cell adhesion molecule (ICAM-1; non-op/op 80%, op/op 85%), the protease inhibitor EB22 (non-op/op 60%, op/op 300%), and glial fibrillary acidic protein (GFAP; non-op/op 300%, op/op 480%) within 24 hr. Macrophage-1 antigen (Mac-1) mRNA levels were lower in all op/op mice and were not induced by TMT exposure. Macrophage inflammatory protein (MIP)-1alpha and MIP-1beta mRNA levels were elevated in non-op/op mice while mRNA levels for interferon inducible protein (IP-10) and monocyte chemoattractant protein (MCP-1) were elevated in op/op mice. Tumor necrosis factor alpha (TNFalpha) mRNA levels were significantly elevated in both non-op/op (100%) and op/op (600%) mice. TNFbeta mRNA levels in op/op mice were elevated 200% and interleukin 1alpha (IL-1alpha) 150%. Reverse transcriptase polymerase chain reaction (RT-PCR) showed a TMT-induced elevation in INFalpha and INFbeta mRNA levels and no elevation of INFgamma. mRNA levels of the CSF-1 receptor, c-fms, were unaltered.

Purkinje cell fate in staggerer mutants: agenesis versus cell death

Staggerer (sg/sg) is an autosomal recessive mutation in an orphan nuclear hormone receptor gene, RORalpha, that causes a cell-autonomous, lineage-specific block in the development of the Purkinje cell. Purkinje cell number is reduced by about 75-90% in adult mutants, and many of the surviving cells are small and ectopically positioned. To determine whether Purkinje cell numbers are reduced owing to either agenesis or cell death, cohorts of Purkinje cells were labeled with the birth-date marker bromodeoxyuridine (BrdU) at embryonic day (E) 10.5 or E11.5. The total number of BrdU-labeled profiles was then compared between cerebella from wild-type mice, heterozygous staggerer, and staggerer mutants at E17.5 and postnatal day (P)5. There was no significant difference between sg/sg mutants and +/sg or +/+ controls in the number of BrdU-labeled profiles or in cerebellar volumes in the E17 embryos. By P5, however, cerebellar volume was significantly reduced in the sg/sg mutants compared to controls (p <.005) and the number of BrdU-labeled profiles was reduced by 33% following E11.5 BrdU injections (p <.02). The results suggest that Purkinje cell genesis is not affected by the staggerer mutation and that Purkinje cell loss begins some time after E17. RORalpha is highly expressed in Purkinje cells by E14, so the delay between initial RORalpha expression and sg/sg Purkinje cell loss suggests that the staggerer mutation does not directly cause Purkinje cell death.